Article

# Energy transfer mechanisms in layered 2D perovskites

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

## Abstract

Two-dimensional (2D) perovskite quantum wells are generating broad scientific interest because of their potential for use in optoelectronic devices. Recently, it has been shown that layers of 2D perovskites can be grown in which the average thicknesses of the quantum wells increase from the back to the front of the film. This geometry carries implications for light harvesting applications because the bandgap of a quantum well decreases as its thickness increases. The general structural formula for the 2D perovskite systems under investigation in this work is (PEA)2(MA)n−1[PbnI3n+1] (PEA = phenethyl ammonium, MA = methyl ammonium). Here, we examine two layered 2D perovskites with different distributions of quantum well thicknesses. Spectroscopic measurements and model calculations suggest that both systems funnel electronic excitations from the back to the front of the film through energy transfer mechanisms on the time scales of 100’s of ps (i.e., energy transfer from thinner to thicker quantum wells). In addition, the model calculations demonstrate that the transient absorption spectra are composed of a progression of single exciton and biexciton resonances associated with the individual quantum wells. We find that exciton dissociation and/or charge transport dynamics make only minor contributions to the transient absorption spectra within the first 1 ns after photo-excitation. An analysis of the energy transfer kinetics indicates that the transitions occur primarily between quantum wells with values of n that differ by 1 because of the spectral overlap factor that governs the energy transfer rate. Two-dimensional transient absorption spectra reveal a pattern of resonances consistent with the dominance of sequential energy transfer dynamics.

## No full-text available

... This observation has been attributed to lower formation energies and faster growth of low n-value phases, as well as local diffusion limitations of larger spacer cations (e.g., butylammonium). 16,26,[34][35][36][37]42,43,53 In situ investigations of 2D RP film growth have found that precursor-solvent intermediate states, comparable to lead-solvate complexes observed during growth of 3D MAPbI 3 , 54−64 affect the distribution of RP phases and defects. 26,28,38 These observations point to the role of solvent on growth; films formed from lead halide RP precursors dissolved in a solvent mixture of dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) show distinct structural evidence of n = 1−3 RP phases. ...
... Several recent works have demonstrated through transient optical spectroscopies that photoexcited carriers in RP thin films tend to recombine in the lowest-gap phase. 27,[35][36][37][40][41][42]51 Upon excitation, phases with n ≲ 4 will form excitonsevident from the excitonic peaks observed in absorbance spectra (Figures 7−8)and higher n phases may form free charge carriers. Emission from the lowgap phase has been attributed to both exciton transfer and charge formation, 27,[35][36][37][40][41][42]51 but one expects the mechanism to vary with the exact phase composition and sizes of the domains. ...
... 27,[35][36][37][40][41][42]51 Upon excitation, phases with n ≲ 4 will form excitonsevident from the excitonic peaks observed in absorbance spectra (Figures 7−8)and higher n phases may form free charge carriers. Emission from the lowgap phase has been attributed to both exciton transfer and charge formation, 27,[35][36][37][40][41][42]51 but one expects the mechanism to vary with the exact phase composition and sizes of the domains. Figure 7b schematically illustrates the role that spin speed (i.e., solvent evaporation rate) plays in domain growth. ...
Article
The growth of Ruddlesden-Popper perovskite thin films of organic lead halides is complicated by the existence of multiple crystallization pathways available to precursors in solution. During thin-film growth processes, such as spin-coating or blade-coating, solvents can evaporate too quickly to clearly resolve different reaction intermediates and products that form during crystallization. Here, we resolve multiple reaction products and intermediates that form during growth of (C4H9NH3)2(CH3NH3)n-1PbnI3n+1 Ruddlesden-Popper compounds by studying drop-cast precursor solutions through the evolution of X-ray diffraction, photoluminescence, and optical micrographs in situ over long timescales in a thin-film geometry. We found that methylammonium-rich solvate intermediates play a crucial role in directing the bulk optical properties of the films and form simultaneously with smaller regions of Ruddlesden-Popper phases during growth. The microstructure and optical properties of these sub-phases were characterized during growth and after annealing, revealing that discrepancies between thin-film and single-crystal optical properties originate from solvate intermediates. These lower-band-gap minority phases dominate the optical emission spectrum by means of rapid energy migration and contribute to sub-band-gap electronic states in photovoltaic devices. Processing routes to yield thin films with optical properties similar to single crystals of Ruddlesden-Popper phases were developed by tuning the precursor stoichiometry and deposition kinetics.
... Ruddlesden-Popper (RP) organic-inorganic halide perovskites have been attracting increasing attention for applications in high efficiency light emitting diodes (LEDs) due to their wider range of tunability, improved environmental stability, and higher exciton binding energy compared to more commonly studied 3D organic-inorganic halide perovskites. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] They have a general formula Aʹ 2 A n-1 B n X 3n+1 , where Aʹ is the bulky organic spacer cation, A is Cs + or a smaller organic cation capable of forming a 3D perovskite ABX 3 , B is a divalent metal cation (Pb, Sn), X is a halide anion, while n is the number of perovskite sheets between Aʹ spacer cations. A number of different RP perovskite materials have been reported to date, with the most commonly studied ones using butylammonium (BA) or phenethylammonium (PEA) spacer cations. ...
... [5] This phenomenon has been extensively studied for different quasi-2D RP perovskites. [3,[5][6][7][8][10][11][12][13][14] Various experimental procedures for the perovskite film preparation, which include the use of additives, [6] stoichiometry optimization, [13,14] different solvents, [11,13] different antisolvents, [7] etc. have been investigated. While the energy funneling process indeed results in bright and efficient emission from the lowest bandgap domain in the film, control over the domain distribution and energy landscape is difficult, resulting in poor reproducibility. ...
... [5] This phenomenon has been extensively studied for different quasi-2D RP perovskites. [3,[5][6][7][8][10][11][12][13][14] Various experimental procedures for the perovskite film preparation, which include the use of additives, [6] stoichiometry optimization, [13,14] different solvents, [11,13] different antisolvents, [7] etc. have been investigated. While the energy funneling process indeed results in bright and efficient emission from the lowest bandgap domain in the film, control over the domain distribution and energy landscape is difficult, resulting in poor reproducibility. ...
Article
Significant enhancement of the light emission in Ruddlesden–Popper organic–inorganic halide perovskites is obtained by antisolvent induced spontaneous formation of nanocrystals in an amorphous matrix. This morphology change results in the passivation of defects and significant enhancement of light emission and 16 times higher photoluminescence quantum yield (PLQY), and it is applicable to different spacer cations. The use of trioctylphosphine oxide results in further defect passivation leading to an increase in PLQY (≈2.3 times), the suppression of lower energy emission in low temperature photoluminescence spectra, the dominance of radiative recombination, and the disappearance of thermal quenching of the luminescence. The proposed method offers a reproducible, controllable, and antisolvent‐insensitive alternative to energy landscape engineering to utilize energy funneling phenomenon to achieve bright emission. Instead of facilitating fast energy transfer from lower to higher number of perovskite sheets to prevent nonradiative losses, it is demonstrated that defects can be effectively passivated via morphology control and the use of a passivating agent, so that bright emission can be obtained from single phase nanocrystals embedded in amorphous matrix, resulting in light emitting diodes with a maximum external quantum efficiency of 2.25%. Bright emission in Ruddlesden–Popper organic–inorganic halide perovskite is achieved by morphology modification and defect passivation. The antisolvent‐induced spontaneous formation of nanocrystals in amorphous matrix results in the strong localization and bright light emission without thermal quenching. The emission enhancement is highly reproducible and applicable to different antisolvents and different spacer cations.
... 14−21 Disentangling these mechanisms with conventional transient absorption spectroscopy has proven quite challenging due to ambiguities in signal interpretation. 14,15,[17][18][19][20]22 Moreover, the electronic states and processes important for the function of a photovoltaic cell do not necessarily yield spectroscopic signatures with significant intensities. The complexity of layered perovskite materials has motivated applications of "action spectroscopies" capable of directly targeting the functionally relevant processes induced by light absorption. ...
... In solution-processed films, the smallest and largest quantum wells are primarily concentrated near the glass and air interfaces, respectively (i.e., near opposing electrodes in a photovoltaic device). 14,17,18,36 As indicated in Figure 1, the average band gaps and conduction band levels decrease from the glass to air sides of a film, thereby promoting energy and electron transport in the same direction. A consensus has not been reached regarding the preferential direction of hole transport. ...
... Photoexcitation induces a multitude of relaxation processes in layered perovskites, which have proven difficult to disentangle by transient absorption spectroscopy (e.g., energy transfer, charge transfer, spontaneous emission, many-body recombination). 5,14,15,[17][18][19][20][21]24 Although dominant carrier funneling dynamics were suggested in early work, it is now clear that energy transfer processes have significant yields for the smallest quantum wells due to their higher binding energies and transition dipole couplings. 38 Transient absorption cross-peaks were previously suggested to represent extraordinary hole transfer processes; however, we have shown these resonances to be natural consequences of transition dipole couplings 24 similar to those observed in molecular aggregates and photosynthetic complexes. ...
Article
Interest in layered perovskite quantum wells is motivated by their potential for use in optoelectronic devices. In these systems, the smallest and largest quantum wells are most concentrated near opposing electrodes in photovoltaic cells. Coincident gradients in the energy levels and quantum well concentrations promote the funneling of electronic excitations and charge carriers through space. In this Perspective, we describe the development of several nonlinear optical techniques designed to elucidate the relaxation processes induced by light absorption in layered perovskite systems. Transient absorption microscopy provides insight into carrier diffusion and two-body recombination processes, whereas two-dimensional action spectroscopies are used to correlate elementary relaxation mechanisms to practical metrics of photovoltaic device performance. Our experiments suggest that charge carrier funneling processes do not facilitate long-range transport due to trapping. Rather, the bulklike phases of the films absorb light and transport carriers without participation of the smallest quantum wells.
... Previous studies reported a decay process of either hundreds of fs or a few to hundreds of ps in the small n RPP phases, and a corresponding rise in the large n RPP phases via time-resolved spectroscopy measurements, which is attributed to the carrier transport between the mixed perovskite phases via charge or energy transfer. 13,[24][25][26][27][28][29] Recent theoretical study suggests that both type-I and type-II band alignments could coexist in 2D perovskite heterostructures due to the "soness" of inorganic layers and the electron-phonon coupling at room temperature, which makes the intrinsic carrier transport mechanism even more complicated. 30 Although there are also reports on the fabrication of phase pure 2D perovskite lms, 31,32 to date, 2D PSCs with the highest PCEs are still based on mixed phase lms. ...
... 25,28 Nevertheless, Williams et al. suggested that energy transfer proceeds on the time scale of 30-120 ps in (PEA) 2 (MA) nÀ1 Pb n I 3n+1 lms (n ¼ 2 and 3). 27 Yuan et al. suggested that cascade energy transfer occurs on the 100 fs to 100 ps time scale from small n to large n phases in (PEA) 2 (MA) nÀ1 Pb n I 3n+1 lms (n ¼ 3). 13 According to the TA experimental and global analysis results in our study, the TA spectra of the 2D layered perovskite (PEA) 2 (MA) 2 Pb 3 I 10 are complicated by the strong spectral overlap between the absorption of hot carriers and the GSB signals of multiple RPP phases, which makes it difficult to draw a denite conclusion based on single wavelength kinetics analysis. ...
... It is worthy to mention that we did not observe a cascade energy transfer between small n RPPs as proposed previously. 13,27 PL signals from all small n RPPs build up with only hundreds of fs hot carrier relaxation time, while the ps rise is solely observed in the PL kinetics of large n RPPs. This suggests that efficient energy transfer proceeds directly from the individual RPP phase with small n (n ¼ 1, 2, 3.) to the large n (n 10) RPPs. ... Article Quasi-two-dimensional (2D) layered perovskites have lately attracted extensive research interest due to their superior environmental stability to their three-dimensional (3D) counterparts. While solar cells based on 2D layered perovskites have achieved remarkably high efficiencies over 18%, their intrinsic carrier dynamics after photoexcitation remain obscure due to the presence of bound excitons and the coexistence of multiple layered perovskite phases. Discrepancies were found in the interpretations of the reported ultrafast processes with hundreds of femto- or pico-second time scales in previous literature via transient absorption measurements. Here, carrier dynamics in (PEA)2(MA)2Pb3I10 (PEA = C6H5(CH2)2NH3; MA = CH3NH3) 2D layered perovskite was systematically studied through a combination of transient absorption and time-resolved photoluminescence measurements with femtosecond time resolution. The ultrafast sub-ps (100–400 fs) carrier dynamics is unambiguously demonstrated due to hot carrier relaxation. Carrier transport occurs from small n layered perovskites to their surrounding 3D like (large n) perovskite via energy transfer on the time scale of 2–300 ps. These results provide valuable information on the intrinsic carrier dynamics in 2D layered perovskites which would be helpful in further improving the device performance. ... Excitation energy transfer has been suggested to occur between 2D perovskite layers with different sheet numbers. [6,[14][15][16] On the contrary, Shang et al. [17] and Liu et al. [18] concluded that electron and hole transfer is the key process in such a system. Particularly, Liu et al. came to the conclusion that charge carriers tend to separate in mixed quasi-2D perovskites, when electrons localize in the thickest 2D layers while holes diffuse to thin layers. ... ... Majority of recent publications consider that quasi-2D perovskites with different layer numbers n form Type I structures. [14][15][16]40,41] These assignments were mainly based on observation of gradual redistribution of luminescence originating from low-n to large-n domains. On the other hand, Type II heterojunctions were also suggested, where authors considered that hole localization takes place in domains with low-n and electrons in domains with high-n. ... Article Full-text available Heterogeneous organic‐inorganic halide perovskites possess inherent non‐uniformities in bandgap that are sometimes engineered and exploited on purpose, like in quasi‐2D perovskites. In these systems, charge carrier and excitation energy migration to lower‐bandgap sites are key processes governing luminescence. The question, which of them dominates in particular materials and under specific experimental conditions, still remains unanswered, especially when charge carriers comprise excitons. In this study transient absorption (TA) and transient photoluminescence (PL) techniques are combined to address the excited state dynamics in quasi‐2D and other heterogeneous perovskite structures in broad temperature range, from room temperature down to 15 K. The data provide clear evidence that charge carrier transfer rather than energy migration dominates in heterogeneous quasi‐2D perovskite films. ... For each film, we observed several excitonic peaks (Fig. 2a), corresponding to a distribution of perovskite phases with different n. This observation indicates that there are multiple perovskite phases in these 2D OIHP films having a nominal n = 4 [46][47][48][49] . However, there are subtle differences when comparing these absorption spectra. ... ... These observations indicate in PEA, mF1PEA, or pF1PEA based 2D OIHP films, a vertical phase distribution of multiple 2D OIHP phases occurs, where phases having smaller n values dominate on the glass side of the substrate while phases with larger n or 3D appear more on the air side. As suggested by others, this sequential phase distribution could benefit energy transfer (and/or charge transfer) across the film and lead to high device performance (Supplementary Figure 6a) 37,[46][47][48][49]52 . However, for oF1PEA 2D OIHP, we only observed a strong and dominating emission peak from the 3D perovskite phase for both back side and the front side excitation (Fig. 2b, c), and these two emission spectra for oF1PEA 2D OIHP are almost identical. ... Article Full-text available Two-dimensional perovskites have emerged as more intrinsically stable materials for solar cells. Chemical tuning of spacer organic cations has attracted great interest due to their additional functionalities. However, how the chemical nature of the organic cations affects the properties of two-dimensional perovskites and devices is rarely reported. Here we demonstrate that the selection of spacer cations (i.e., selective fluorination of phenethylammonium) affects the film properties of two-dimensional perovskites, leading to different device performance of two-dimensional perovskite solar cells (average n = 4). Structural analysis reveals that different packing arrangements and orientational disorder of the spacer cations result in orientational degeneracy and different formation energies, largely explaining the difference in film properties. This work provides key missing information on how spacer cations exert influence on desirable electronic properties and device performance of two-dimensional perovskites via the weak and cooperative interactions of these cations in the crystal lattice. ... Indeed, a photon cascade has already been observed in 2D perovskite films having different numbers of layers, and multiple emission peaks were visible. 51,55,56 This is supported by the decay associated spectrum (DAS) (Supporting Information, Figure S17) obtained by globally fitting the FLUPS signal, in which all the amplitudes have their maximum at the same wavelength. Because photon recycling is faster than the IRF, it cannot explain the observed short-lived emission (τ = 300 fs) when the NPl concentration is kept low enough. ... Article Full-text available Strongly confined, fully inorganic cesium lead halide perovskite nanocrystals are of great promise for light-emitting devices in the blue spectral range owing to their high photoluminescence quantum yields. A combination of broadband fluorescence up-conversion and transient absorption spectroscopies was used to study early exciton dynamics in quasi-1D CsPbBr3 nanoplatelets (3 × 4 × 23 nm³, NPls). This allowed to reveal emitting band-edge states in the NPls that form instantaneously upon photoexcitation and then relax within the first picosecond to lower energy confined hole states (CHSs). The influence of the pump excitation intensity on the latter process was further scrutinized. The band-edge population lifetime was found to increase with the rise of the photon fluence due to CHS filling. When the concentration of NPls in solution becomes very high, nanoparticles overlap, resulting in a decrease in their external absorption cross-section and an increase in the emission Stokes shift due to photon reabsorption. ... On the other hand, the real-world devices employing (F-PEA) 2 MA 4 Pb 5 I 16 (n = 5) or (F-PEA) 2 MA 3 Pb 4 I 13 (n = 4), where films contain mixed dimensional phases (n = 2, 3, 4, 5 and N), can benefit from ordered phase distribution depending on the position of fluorine on PEA. 65,160 4.2 BA cations as organic spacers in the RP phase ... Article Full-text available Astounding development of organic-inorganic halide perovskite solar cells (PSCs) in the past decade has been led by three-dimensional (3D) perovskites. Nevertheless, the concern over the stability of 3D PSCs casts a shadow on their real-world applications. By adopting various technological and scientific approaches, some progress has been made in improving the stability of 3D perovskite devices. Nonetheless, the best, definitive solution consists in improving the inherent chemical stability of the halide perovskite itself. Two-dimensional (2D) perovskites, on the other hand, display excellent stability under ambient conditions and have been recognized as an alternative to their 3D analogs. Although the first generation 2D PSCs have shown relatively lower photovoltaic performance, recent reports suggest that they are also capable of achieving high power conversion efficiency well beyond 20%. In the wake of the recent resurgence of 2D halide perovskite materials, we review their structural and optoelectronic properties, followed by an extensive analysis of recent progress in 2D PSCs. ... 39,40 Transient absorption spectroscopy indicates that the electrons and holes can incur a funnel effect, where energy can transfer from low n-number to high n ones. 41,42 Different nanoplatelet thickness distributions would affect the funnel effect. Besides the phase distribution issue, it has been found that when the crystal growth orientation is parallel to the substrate, it hinders charge transport in the out-of-plane direction due to the insulating organic layers, which affects device performance. ... Article Full-text available Quasi-two-dimensional (2D) perovskites promise the intrinsically stable solar cell performance. However, the crystal orientation and phase distribution in 2D solution processed perovskite are difficult to be manipulated, which restricts the device efficiency as well as its reproducibility. Here, we simply incorporate potassium ion (K+) into quasi-2D precursor solution, which can dramatically change the nucleation steps during perovskite films spin-coating process probed by in-situ synchrotron-based grazing incident X-ray diffraction (GIXRD). It is notable that a desired vertical oriented 2D phase without intermediate compound can be easily formed after spin-coating, which simultaneously reduces the distribution of low dimensional 2D perovskite phases in association with suppressed trap states. Therefore, the power conversion efficiency of doped 10% K+ 2D perovskite solar cells can yield up to 11.3% as well as long-term stable performance with high reproducibility. This work paves a key path to control the quasi-2D nucleation and crystallization processing via chemical additives. ... With varied n value, the structure can be determined to be 3D (n → ∞) or quantum well (n = 1, 2, 3, etc.). [134][135][136] Many of the repeating units can stack together through van der Waals forces to form bulk crystals. [137,138] Apart from the RP 2D perovskites with long-chain "L" cations, 2D perovskites can also be acquired by decreasing the thickness of 3D perovskites. ... Article Full-text available Photo(electro)catalysis has triggered ripples of excitement in environment protection and energy conversion such as degradation of organic pollutants, evolution of H2 and O2 from H2O splitting, and reduction of CO2 by utilizing solar energy. Over the past three years, halide perovskites, which render extraordinary charge transport capability in solar cells, have witnessed burgeoning development in photocatalysis over the conventional oxide perovskites. The latter demonstrates a small surface area, limited light utilization and high carrier recombination, resulting in inadequate reactant contact on catalyst surfaces and decreased catalytic activity. In this topical review, the progress of halide perovskites will be presented starting from fundamental understanding (i.e. synthesis and structure) to applications in light-driven reactions with the focus on crystal dimensions, toxicity and stability. In addition, computational studies on halide perovskites from electronic properties to catalytic mechanisms are presented to lay a foundation for the future research and advancement in this pacey field. Lastly, we will provide critical insights into the existing limitations and cast favorable prospects for further investigation of halide perovskites in the near future. ... For example, both BA 2 MA n−1 Pb n I 3n+1 and PEA 2 MA n−1 Pb n I 3n+1 RP perovskites have been reported to be featured with type-I band alignment in some literatures but reported to show type-II band alignment in other literatures. [70,[77][78][79][80][81] To reconcile the incompatible observations of both type-I and type-II alignments in the literatures, Sargent's group carried out a large number of experimental and computational researches, verifying that both type-I and type-II alignments avoidably coexisted in untreated solution-processed RP perovskites. The inadvertent misalignments in RP perovskites were ascribed to the inhomogeneous distribution of ligands across the RP perovskite films. ... Article Full-text available In the last decade, perovskite solar cells (PSCs) have undergone unprecedented rapid development and become a promising candidate for a new‐generation solar cell. Among various PSCs, typical 3D halide perovskite‐based PSCs deliver the highest efficiency but they suffer from severe instability, which restricts their practical applications. By contrast, the low‐dimensional Ruddlesden–Popper (RP) perovskite‐based PSCs have recently raised increasing attention due to their superior stability. Yet, the efficiency of RP perovskite‐based PSCs is still far from that of the 3D counterparts owing to the difficulty in fabricating high‐quality RP perovskite films. In pursuit of high‐efficiency RP perovskite‐based PSCs, it is critical to manipulate the film formation process to prepare high‐quality RP perovskite films. This review aims to provide comprehensive understanding of the high‐quality RP‐type perovskite film formation by investigating the influential factors. On this basis, several strategies to improve the RP perovskite film quality are proposed via summarizing the recent progress and efforts on the preparation of high‐quality RP perovskite film. This review will provide useful guidelines for a better understanding of the crystallization and phase kinetics during RP perovskite film formation process and the design and development of high‐performance RP perovskite‐based PSCs, promoting the commercialization of PSC technology. ... Growth of the 3D- like perovskite bleach peak and fast decay dynamics at 572 nm and 604 nm suggests an energy/charge transfer process between small n number 2D phase and 3D-like perovskite phase. According to previous studies 24,25 , we speculate that the energy transfer is more dominant in this system, because the transfer process happens at a very early delay time scale (far before 500 ps). We speculate energy transfer from the larger bandgap layered perovskites to lower bandgap 3D-like perovskites across the ligands. ... Article Full-text available Layered perovskites have been shown to improve the stability of perovskite solar cells while its operation mechanism remains unclear. Here we investigate the process for the conversion of light to electrical current in high performance layered perovskite solar cells by examining its real morphology. The layered perovskite films in this study are found to be a mixture of layered and three dimensional (3D)-like phases with phase separations at micrometer and nanometer scale in both vertical and lateral directions. This phase separation is explained by the surface initiated crystallization process and the competition of the crystallization between 3D-like and layered perovskites. We further propose that the working mechanisms of the layered perovskite solar cells involve energy transfer from layered to 3D-like perovskite network. The impact of morphology on efficiency and stability of the hot-cast layered perovskite solar cells are also discussed to provide guidelines for the future improvement. ... Yan and coworkers observed the similar vertical phase separation phenomenon for a post-annealed (C 4 H 9 NH 3 ) 2 MA 3 Pb 4 I 13 thin film and found that the efficient energy transfer and possible charge transfer account for the high device performance [165]. The mechanism of energy transfer from low-n to highn quasi-2D perovskite was studied by Williams and workers, who suggested the excitonic funneling from one side to another side of the film could happen in a time scale of 100 ps [166]. Other than PVs, 2D organic metal halide hybrids were also found to have applications in LEDs. ... Article Organic-inorganic metal halide hybrids are an important class of crystalline materials with exceptional structural and property tunability. Recently metal halide perovskites with ABX3 structure have been extensively investigated as new generation semiconductors for various optoelectronic devices, including photovoltaic cells, light emitting diodes, photodetectors, and lasers, for their exceptional optical and electronic properties. By controlling the morphological dimensionality, low dimensional metal halide perovskites, including 2D perovskite nanoplatelets, 1D perovskite nanowires, and 0D perovskite quantum dots, have been developed to exhibit distinct properties from their bulk counterparts, due to quantum size effects. Besides ABX3 perovskites, organic-inorganic metal halide hybrids, containing the same fundamental building block of metal halide octahedra (BX6), can also be assembled to possess other types of crystallographic structures. Using appropriate organic and inorganic components, low dimensional organic-inorganic metal halide hybrids with 2D, quasi-2D, corrugated-2D, 1D, and 0D structures at the molecular level have been developed and studied. Due to the strong quantum confinement and site isolation, these low dimensional metal halide hybrids at the molecular level exhibit remarkable and unique properties that are significantly different from those of ABX3 perovskites. In light of the rapid development of low dimensional metal halide perovskites and hybrids, it is indeed timely to review the recent progress in these areas. Also, there is a need to clarify the difference between morphological low dimensional metal halide perovskites and molecular level low dimensional metal halide hybrids, as currently the terminologies of low dimensional perovskites are not appropriately used in many cases. In this review article, we discuss the synthesis, characterization, application, and computational studies of low dimensional metal halide perovskites and hybrids. ... We did not observe PL emission close to the absorption peak of n = 3 and 2 materials in Figure S9b; also, the PL spectral shape does not depend on the excitation intensity at both 80 and 300 K ( Figure S10); therefore, the energy transfer from a small n to large n nanoplatelet is effective. 19,44 However, the PL cannot be attributed simply to specific n materials in (PEA) 2 (CH 3 NH 3 ) n−1 Pb n I 3n+1 films. The PL intensity as the function of excitation intensity for PMPI 3 is shown in Figure 3b at 80 K and at room temperature. ... ... This means there exists efficient energy funneling from 2D to 3D phases in which region the light is emitted. [37,38] Only for the perovskite film made with BABr:PbBr 2 = 1:2, which contains a significant amount of 2D components with excitonic absorption around 435 nm, we observed an additional PL emission peak at 437 nm. We note that an increase in the excitation intensity by a factor of two or more, PL emission from the 2D phases is also visible for perovskite with BABr:PbBr 2 ratio 1:4, together with a strong amplified spontaneous emission ( Figure S1b, Supporting Information). ... Article Mixed 2D/3D perovskite films with self‐assembled quantum wells have significantly improved the performance of perovskite light emitting diodes (PeLEDs). In this work, such films are fabricated through a two‐step interdiffusion method that is widely employed in processing of perovskite solar cells, however, remains rarely explored for PeLEDs. The effects of incorporating large‐cation ligand, i.e., butylammonium bromide (BABr) into formamidinium lead bromide (FAPbBr3) based perovskites, in terms of film composition, morphology, optoelectronic properties as well as device performance are thoroughly investigated in this method. By modulating BABr:PbBr2 ratio in the precursor solution, the optimal device shows a maximum external quantum efficiency (EQE) of 7.36% at 147.7 mA cm⁻² and a brightness of 37 720 Cd m⁻² at 5 V. The performance is remarkably higher than a reference device without BABr that shows a maximum EQE of 2.53% and a brightness of 6190 Cd m⁻² at 5 V. The versatility of this method is further extended to another large‐cation ligand, 4‐fluoro‐benzylammonium bromide (F‐BZABr), which leads to maximum EQE of 8.55%. This work indicates two‐step processed mixed 2D/3D perovskites are promising for bright green PeLEDs. Article Two-dimensional coherent photocurrent spectroscopies directly probe the electronic states and processes that are relevant to the performance of a photovoltaic device. In this letter, we apply a two-pulse nonlinear photocurrent spectroscopy to a photovoltaic device based on layered perovskite quantum wells. The method effectively decomposes the photovoltaic response into contributions from separate quantum wells and excited state species (i.e., either single excitons or biexcitons). Our experiments show that the efficiency of photocurrent generation increases with the size of the quantum well. Overall, the results suggest that energy funneling processes in layered perovskites, which are most prominent in transient absorption spectroscopies, are largely irrelevant to the function of a photovoltaic cell. Article Two-dimensional (2D) hybrid perovskites are generating broad scientific interest because of their potential for use in photovoltaics and microcavity lasers. It has recently been demonstrated that mixtures of quantum wells with different thicknesses can be assembled in films with heterogeneous quantum well distributions. Large (small) quantum wells are concentrated at the air-side (substrate-side) of the films, thereby promoting directional energy and/or electron transfer. However, profiles of the quantum well concentrations have not been directly measured throughout the full thicknesses of the films. Similarly, the lateral motions of the excitations in these systems are not well-characterized. In this work, we perform focused ion beam milling tests to establish quantum well concentrations as a function of depth in layered 2D perovskite films. In addition, transient absorption microscopy is used to investigate carrier diffusion and two-body recombination processes. Comparisons of the layered films with phase-pure single crystals reveal that diffusion is suppressed by grain boundaries in the films, which in turn promotes two-body recombination. Similar behaviors were previously observed in bulk perovskite films and single crystals. These studies suggest that the morphology of the film, rather than the identity of the material, is the primary factor that governs the two-body recombination dynamics. Enhancement of two-body recombination processes is desirable for applications such as microcavity lasers. Article Metal halide perovskites constitute a new type of semiconducting materials with long charge carrier lifetimes and efficient light-harvesting. The performance of perovskite solar cells and related devices is limited by nonradiative charge and energy losses, facilitated by defects. Combining nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that charge losses depend strongly on the defect chemical state. By considering an extra Pb atom in CH3NH3PbI3, which is a common defect in lead halide perovskites, we investigate its influence on charge trapping and recombination. In a chemically inert form as a Pb interstitial, the extra Pb atom has only a mild influence on charge recombination. However, if the extra Pb atom binds to a native Pb atom to form a dimer, the charge trapping and recombination are greatly accelerated because the Pb-dimer creates a localized midgap trap state that couples strongly to the perovskite valence band edge. Holes disappear from the valence band two orders of magnitude faster than in the pristine perovskite and recombine with conduction band electrons one order of magnitude faster. The simulations identify the phonon modes involved in the nonradiative charge trapping and recombination and highlight the importance of rapid decoherence within the electronic subsystem for long carrier lifetimes. The detailed atomistic analysis of the charge trapping and recombination mechanisms enriches the understanding of defect properties and provides theoretical guidance for improving perovskite performance. Article Formamidinium (FA)-based quasi-two dimensional (Q-2D) perovskite normally has better environmental stability than methylammonium (MA)-based one, however, it is difficult to form larger n (n > 2) phases of layered FA-based perovskite due to their larger formation energies, resulting in insufficient absorption of sunlight and then poor performance of its solar cell. Here we partially replace FA with MA to form (BA)2(FA1-xMAx)3Pb4I13 mixture, which contains enriched larger n (n = 3, 4, and larger) phases, where BA is butylammonium. Particularly, the (BA)2(FA0.8MA0.2)3Pb4I13 perovskite film show the best stability under ambient conditions. Furthermore, using inverted planar heterojunction structure, the photovoltaic device based on (BA)2(FA0.8MA0.2)3Pb4I13 demonstrates power conversion efficiency of 14.2%, which is among the highest efficiencies for the FA-rich Q-2D perovskites with n ≤ 4. This work highlights that FA-rich Q-2D perovskites have great promise for optoelectronic devices. Article Formation of heterostructures is often inevitable in two-dimensional (2D) halide perovskites and band alignment in 2D perovskite heterostructures is of central importance to their applications. However, controversies abound in literature on the band alignment of the 2D perovskite heterostructures. While external factors have been sought to reconcile the controversies, we show that the 2D perovskite heterostructures are in fact intrinsically prone to band “misalignment”, driven by thermal fluctuations. Owing to the “softness” of inorganic layers in the perovskites, electron-phonon coupling at room temperature could be strong enough to override band offsets at zero temperature, leading to oscillational band alignment between type-I and type-II at 300 K. We further demonstrate that by tuning the inorganic layers, one can increase the band offsets and stabilize the band alignment, paving the way for optoelectronic applications of the 2D perovskite heterostructures. Article Layered perovskites are hybrid 2D materials, formed through the self-assembly of inorganic lead halide networks separated by organic ammonium cation layers. In these natural quantum-well structures, quantum and dielectric confinement lead to strongly bound excitonic states that depend sensitively on the material composition. In this article, we review current understanding of exciton photophysics in layered perovskites and highlight the many ways in which their excitonic properties can be tuned. In particular, we focus on the coupling of exciton dynamics to lattice motion and local distortions of the soft and deformable hybrid lattice. These effects lead to complex excited-state dynamics, presenting new opportunities for design of optoelectronic materials and exploration of fundamental photophysics in quantum confined systems. Article The properties of mid-bandgap electronic states are central to the potential application of self-assembled, hybrid organic-inorganic perovskite-like quantum wells in opto-electronic technologies. In this study we investigate broadband light emission from mid-bandgap states in fast-forming hybrid organic lead iodide quantum wells at room temperature. By comparing temperature and intensity dependent photoluminescence (PL) spectra emitted from butyl ammonium spaced inorganic layers, we propose structural defects in a meta-stable material phase trap excitons and cause broadband light emission spanning wavelengths between 600 nm and 1000 nm. We use temperature dependent THz time-domain spectroscopy to correlate changes in the sub-gap PL emission with changes in the chemical bonding of the inorganic octahedral layer. Our results provide new fundamental physical insights into the array of mechanisms capable of inducing broadband light emission from low dimensional perovskite-like materials central to their application in future opto-electronic technologies and novel spectroscopic tools to characterize these states. Article Reported here is the low temperature photoluminescence, energy transfer mechanism and exciton dynamics of low dimensional Mn²⁺ doped 2D perovskites that show interesting differences from their 3D doped counterpart. Dopant emission in 2D system shows increased PL intensity and shortened lifetime with increase of temperature with strong dopant emission even at low temperatures. Transient absorption (TA) spectroscopy reveals the dominant role of ‘hot’ excitons in dictating the fast energy transfer timescale. The operative dynamics of the generated 'hot' excitons include filling up of existing trap states (shallow and deep) and energy transfer channel from 'hot' excitons to dopant states. Global analysis and target modeling of TA data provides an estimate of excitons (‘hot’ and band edge) to dopant energy transfer timescale of ~330 ps, which is much faster than the band edge exciton lifetime (~2 ns). Such fast energy transfer timescale arises due to enhanced carrier exchange interaction resulting from, higher exciton confinement, increased covalency, and involvement of ‘hot’ excitons in the 2D perovskites. In stark contrast to 3D systems, the fast energy transfer rate in 2D system results high dopant emission intensity even at low temperatures. Increased intrinsic vibronic coupling at higher temperatures further supports efficient Mn²⁺ sensitization that ultimately dictates the observed temperature dependence of the dopant emission (intensity, lifetime). Article Organic metal halide Ruddlesden-Popper layered perovskite phases combine the excellent optoelectronic properties of three-dimensional, bulk hybrid perovskites with superior material stability under ambient conditions. However, the thin film structure of these layered perovskites is still poorly understood, as phase purity is typically determined solely by specular X-ray diffraction. The thin film structure of these Ruddlesden-Popper phases was examined by increasingly local characterization techniques. From the comparison of grazing-incidence wide-angle X-ray scattering patterns of cast films to expected scattering from single-crystal structures, significant in-plane disorder was observed. Spatially localized photoluminescence measurements show that films do not phase separate on the micrometer scale. Selected area electron diffraction measurements show the intergrowth of different phases within the same thin film, consistent with previous observations seen in epitaxially grown Ruddlesden-Popper complex oxides. Despite the presence of phase impurities that would typically be detrimental for device performance, fits to photothermal deflection spectroscopy measurements show relatively low Urbach energies of 33 meV for (C4H9NH3)2(CH3NH3)2Pb3I10 and 32 meV for (C4H9NH3)2(CH3NH3)3Pb4I13, indicating that the electronic properties are insensitive to the phase impurities. Article Two-dimensional organic-inorganic hybrid perovskites (OIHPs) have showed impressive stability compared to their 3D counterparts. However, tuning the chemical structure of the organic cations to simultaneously improve the device performance and stability of 2D OIHP solar cells is rarely reported. Here, we demonstrate that by introducing a classic non-covalent aryl-perfluoroaryl interaction, 2D OIHP solar cells with 1:1 mixed phenethylammonium (PEA) and perfluorophenethylammonium (F5-PEA) can achieve efficiency over 10% with much enhanced stability using a simple deposition at low temperature without using any additives. The competing effects of surface morphology and crystal orientation with increased amount of F5-PEA result in highest efficiency at 1:1 ratio, while single crystal studies reveal the expected aryl-perfluoroaryl interaction, accounting for the highest device stability of 2D OIHP solar cell at 1:1 ratio as well. This work provides an example where tuning the interactions of organic cations via molecular engineering can have a profound effect on device performance and stability of 2D OIHP solar cells. Article Interest in layered organohalide perovskites is motivated by their potential for use in optoelectronic devices. In these systems, the smallest and largest quantum wells are primarily concentrated near the glass and air interfaces of a film, thereby establishing a gradient in the average values of the bandgaps. It has been suggested that this layered architecture promotes the funneling of electronic excitations through space in a manner similar to light-harvesting processes in photosynthetic antennae. Whereas energy and charge transfer are difficult to distinguish by conventional transient absorption techniques, it has recently been shown that these competing relaxation mechanisms may be separately targeted with nonlinear fluorescence (NLFL) and photocurrent "action spectroscopies."Here, we present perturbative rate functions to describe NLFL experiments conducted on layered perovskite systems. The formulas reproduce the patterns of resonances observed in experimental measurements and show how signatures of energy transfer manifest in two-dimensional spectra. Overall, this work suggests that NLFL spectroscopy may be used to fully reveal the trajectories of electronic excitations by correlating ultrafast energy transfer pathways to fluorescence emission from the thickest quantum wells. Article Interest in photovoltaic devices based on layered perovskites is motivated by their tunable optoelectronic properties and stabilities in humid conditions. In these systems, quantum wells with different sizes are organized to direct energy and charge transport between electrodes; however, these relaxation mechanisms are difficult to distinguish based on conventional transient absorption techniques. Here, two-dimensional “action spectroscopies” are employed to separately target processes that lead to the production of photocurrent and energy loss due to fluorescence emission. These measurements show that energy transfer between quantum wells dominates the sub-ns time scale, whereas electron transfer occurs at later times. Overall, this study suggests that while the intense exciton transitions promote light harvesting, much of the absorbed energy is lost by way of spontaneous emission. This limitation may be overcome with alternate layered perovskite systems that combine smaller exciton binding energies with large absorbance cross sections in the visible spectral range. Article Two-dimensional hybrid metal halide perovskites (2D perovskites) are attractive for light-emitting devices and other applications because their emission is tunable across the visible spectrum. The emission profile of 2D perovskites can be broadened via a variety of mechanisms and is further complicated by the presence of impurities. Here, the challenge of making phase-pure films in Ruddlesden-Popper phases [(A′)2(A)n−1BnX3n+1structure] is overcome by using a single A/A′-site cation, ethylammonium (EA), whose optimal size also prohibits the formation of off-target phases. In the (EA)2(EA)n−1PbnBr3n+1family, the low-energy, broad emission observed in bulk crystals is reduced in spin-cast, polycrystalline films. This decrease in broad emission, attributed to phonon-mediated processes, is correlated with the strain in polycrystalline films that is observed by X-ray scattering. Photothermal deflection spectroscopy shows that strain also increases the electronic disorder near the free exciton absorbance. Broad emission in films can be recovered by slowing growth kinetics, which removes the strain acquired from spin-casting and increases the domain size. These results help extend the utility of 2D perovskites by suggesting design rules for the growth of thin films with the targeted phase and emission. Article 2D hybrid perovskites are attractive for optoelectronic devices. In thin films, the color of optical emission and the texture of crystalline domains are often difficult to control. Here, a method for extinguishing or enhancing different emission features is demonstrated for the family of 2D Ruddlesden–Popper perovskites (EA1−xFAx)4Pb3Br10 (EA = ethylammonium, FA = formamidinium). When grown from aqueous hydrobromic acid, crystals of (EA1−xFAx)4Pb3Br10 retain all the emission features of their parent compound, (EA)4Pb3Br10. Surprisingly, when grown from dimethylformamide (DMF), an emission feature, likely self‐trapped exciton (STE), near 2.7 eV is missing. Extinction of this feature is correlated with DMF being incorporated between the 2D Pb‐Br sheets, forming (EA1−xFAx)4Pb3Br10∙(DMF)y. Without FA, films grown from DMF form (EA)4Pb3Br10, retain little solvent, and have strong emission near 2.7 eV. Slowing the kinetics of film growth strengthens a different emission feature, likely a different type of STE, which is much broader and present in all compositions. Films of (EA1−xFAx)4Pb3Br10∙(DMF)y have large, micron‐sized domains and homogeneous orientation of the semiconducting sheets, resulting in low electronic disorder near the absorption edge. The ability to selectively strengthen or extinguish different emission features in films of (EA1−xFAx)4Pb3Br10∙(DMF)y reveals a pathway to tune the emission color in these compounds. A method for extinguishing or enhancing different emission features is demonstrated for the family of 2D Ruddlesden–Popper perovskites (EA1−xFAx)4Pb3Br10 (EA = ethylammonium, FA = formamidinium). Incorporation of dimethylformamide in compositions with FA leads to removal of phonon‐coupled emission and also to growth of large micron‐sized domains in thin films. Article On hot substrates with a temperature of 100 °C, the qualities of two-dimensional perovskite PEA2MA2Pb3I10 (PEA = phenethylammonium, MA = methylammonium) films have been explored which are constructed with different spin-casting speeds. These films are performed at the speed of 1000, 2000, 4000, and 6000 revolution per minute (RPM). Below 4000 r, a higher RPM results in higher crystalline quality with more uniform morphology. Correspondingly, 4000 r devices show better performance on average (4.3% power conversion efficiency) and less hysteresis in the J-V curve than 1000 r (3.6%) and 2000 r devices (3.4%). However, for devices that were fabricated at 6000 r, inferior performance (2.8% on average) may not be predicted simply by the morphology characterization or optical measurement results at room temperature; instead, carrier trapping states can occur that result in thermally activated PL below 200 K with an activation energy of 18 meV, which do not occur in the 1000 r, 2000 r, and 4000 r films. Our results suggest that for evaluating 2D perovskite films prior to fabricating optimal devices, multiple morphology characterizations and optical measurements, including of low-temperature PL, will be helpful. Integrated PL intensity as a function of the inverse temperature of 1000 r, 2000 r, 4000 r and 6000 r, respectively, shows that optical measurements (especially low-temperature PL) are needed if we want to directly linked the optical properties of perovskite materials to the performances of corresponding devices. Article Two-dimensional (2D) perovskites have been attracting extensive attention due to their intrinsic stability compared with their three-dimensional (3D) counterparts. These materials are widely tailorable in composition, structure, and bandgap, and provide an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In the field of photovoltaic, the fabricated 2D perovskite solar cells (PSCs) have achieved high stability as well as sustainable breakthrough in power conversion efficiency (PCE). However, the PCE of 2D PSCs still lags far behind their 3D counterpart, which is attributed to the special physicochemical properties of organic ligands. This review focuses the 2D halide perovskites from a structural perspective, namely the Ruddlesden-Popper (RP) phases, Dion-Jacobson (DJ) phases, alternating cation in the interlayer space (ACI) phases and mixed organic ligands phases, which stems from the diversity and versatility of spacers. Then the impacts of the species, chemical compositions, and physical characteristics of spacers on 2D perovskites, especially on the structure, carrier behavior, and the specific properties of solar cells, were discussed. Finally, several strategies on the rational selection of novel spacers are elucidated, and an outlook toward high-performance of 2D PSCs is presented. Article As research on both bulk and low dimensional metal halide perovskites (MHPs) continues to grow, the tools necessary to gain insights into their exotic and highly convoluted optoelectronic features must... Article Experimental methods based on a wide range of physical principles are used to determine carrier mobilities for light-harvesting materials in photovoltaic cells. For example, in a time-of-flight experiment, a single laser pulse photoexcites the active layer of a device, and the transit time is determined by the arrival of carriers at an acceptor electrode. With inspiration from this conventional approach, we present a multidimensional time-of-flight technique in which carrier transport is tracked with a second intervening laser pulse. Transient populations of separate material components of an active layer may then be established by tuning the wavelengths of the laser pulses into their respective electronic resonances. This experimental technique is demonstrated using photovoltaic cells based on mixtures of organohalide perovskite quantum wells. In these “layered perovskite” systems, charge carriers are funneled between quantum wells with different thicknesses because of staggered band alignments. Multidimensional time-of-flight measurements show that these funneling processes do not support long-range transport because of carrier trapping. Rather, our data suggest that the photocurrent is dominated by processes in which the phases of the thickest quantum wells absorb light and transport carriers without transitions into domains occupied by quantum wells with smaller sizes. These same conclusions cannot be drawn using conventional one-dimensional techniques for measuring carrier mobilities. Advantages and disadvantages of multidimensional time-of-flight experiments are discussed in the context of a model for the signal generation mechanism. Article Two-dimensional (2D) organic-inorganic hybrid perovskite is a re-emerging material with strongly excitonic absorption and emission properties that are attractive for photonics and optoelectronics. Here we report experimental observation of excitonic energy transfer (ET) in van der Waals heterostructures consisting of 2D hybrid perovskite (C6H5C2H4NH3)2PbI4 (PEPI) and monolayer WS2. Photoluminescence excitation spectroscopy reveals a distinct ground exciton resonance feature of perovskite, evidencing ET from perovskite to WS2. We find unexpectedly high photoluminescence enhancement factors of up to ~8, which cannot be explained by single-interface ET. Our analysis reveals that interlayer ET across the bulk of layered perovskite also contributes to the large enhancement factor. Further, from the weak temperature dependence of the ET rate, which we found to be ~3 ns-1, we conclude that Förster-type mechanism is responsible. Article Recently, hybrid perovskites have gained attention as sensitizers for molecular triplet generation. Layered, two-dimensional (2D) perovskites are especially well-suited for this purpose because the triplet donor (inorganic framework) and triplet acceptor (organic layer) are self-assembled into adjacent sheets, so that with the appropriate energetics, triplets can be driven across the interface. Here we examine interlayer energy transfer in a series of mixed-halide Dion-Jacobson 2D perovskites containing divalent naphthalene cations. We find that the sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the inorganic exciton is tuned near resonance with the naphthalene triplet, naphthalene monomer phosphorescence competes with triplet excimer formation. The interlayer energy-transfer process is further revealed by ultrafast transient absorption spectroscopy through kinetic variations in triplet excimer formation times. Ultimately, gaining control over interlayer interactions in 2D perovskites through cation design will help uncover new functions and applications for these materials. Article Two-dimensional (2D) semiconductors are attractive candidates for a variety of optoelectronic applications owing to the unique electronic properties that arise from quantum confinement along a single dimension. Incorporating nonradiative mechanisms that enable directed migration of bound charge carriers, such as Förster resonance energy transfer (FRET), could boost device efficiencies provided that FRET rates outpace undesired relaxation pathways. However, predictive models for FRET between distinct 2D states are lacking, particularly with respect to the distance d between a donor and acceptor. We approach FRET in systems with binary mixtures of donor and acceptor 2D perovskite quantum wells (PQWs), and we synthetically tune distances between donor and acceptor by varying alkylammonium spacer cation lengths. FRET rates are monitored using transient absorption spectroscopy and ultrafast photoluminescence, revealing rapid picosecond lifetimes that scale with spacer cation length. We theoretically model these binary mixtures of PQWs, describing the emitters as classical oscillating dipoles. We find agreement with our empirical lifetimes and then determine the effects of lateral extent and layer thickness, establishing fundamental principles for FRET in 2D materials. Article Quasi two-dimensional halide perovskites (also known as Ruddlesden-Popper or RPs) are the most recent and exciting evolution in the perovskite field. Possessing a unique combination of enhanced moisture and material stability, whilst retaining the excellent optoelectronic properties, RPs are poised to be a game changer in the perovskite field. Spurred by their recent achievements in solar cells, light-emitting diodes and spintronic devices, these materials have garnered a mounting interest. Herein, we critically review the photophysics of RPs and distill the science behind their structure–property relations. We first focus on their structure and morphology by highlighting the crucial role of large cations: dictating the RPs' layered structure and the statistical distribution of thicknesses (i.e., n-phases). Next, we discuss how optoelectronic properties of RPs differ from conventional halide perovskites. Structural disorder, stronger excitonic, and polaronic interaction shape the nature of photo-excitations and their fate. For example, faster recombinations and hindered transport are expected for charge carriers in thinner n-phases. However, the complex energetic landscape of RPs, which originates from the coexistence of different n-phases, allows for funneling of energy and charges. Presently, the photophysics of RPs is still nascent, with many recent exciting discoveries from coherence effects in the above-mentioned funneling cascade to spin effects. Giant Rashba spin–orbit coupling, also observed in RPs, dictates their spin dynamics and provides exciting spintronics opportunities. To leverage these propitious RPs, future research must entail a cross-disciplinary approach. While materials engineering will unlock new chiral RPs and Dion-Jacobson variants, novel characterization techniques such as in situ synchrotron-based x-ray diffraction, ultrafast electron microscopy, and multidimensional electronic spectroscopy, etc., are essential in unraveling their secrets and unleashing their full potential. Article Highly robust, swiftly reversible thermochromic nature of a two-dimensional (2D) perovskite of (PDMA)(CH3NH3)n-1Pb n I3n+1, nominally prepared as n = 2 is found, where PDMA = C6H4(CH2NH3)2. A wide band gap variation from 700 to 430 nm is observed between room temperature and >60 °C under ambient conditions, resulting from moisture absorption and desorption. X-ray diffraction and Fourier-transform infrared spectroscopy are performed to analyze the hydrated and dehydrated states. Furthermore, the (PDMA)(CH3NH3)n-1Pb n I3n+1 film is demonstrated as an active material for smart windows and thermochromic solar cells, which could lower the inside air temperature in an enclosed space and supply a power conversion efficiency of more than 0.5% at a high ambient temperature, respectively. Overall, we may pave a pathway for exploring the novel phenomena and applications of Dion-Jacobson 2D perovskites. Article Layered organohalide perovskite films consist of quantum wells with concentration distributions tailored to enhance long-range charge transport. Whereas cascaded energy and charge funneling behaviors have been detected with conventional optical spectroscopies, it is not clear that such dynamics contribute to the efficiencies of photovoltaic cells. In this Letter, we use nonlinear photocurrent spectroscopy to selectively target charge transport processes within devices based on layered perovskite quantum wells. The photocurrent induced by a pair of laser pulses is directly measured in this "action" spectroscopy to remove ambiguities in signal interpretation. By varying the external bias, we determine carrier mobilities for quantum-well-specific trajectories taken through the active layers of the devices. The results suggest that the largest quantum wells are primarily responsible for photocurrent production, whereas the smallest quantum wells trap charge carriers and are a major source of energy loss in photovoltaic cells. Article Full-text available Using substrates at room temperature or at 110 °C, two-dimensional (2D) perovskite films and solar cells using mixed butylammonium (BA) and formamidinium (FA) cations, i.e., (BA)2(FA)n-1PbnI3n+1 (n = 3 nominally), with the same molar ratio of components, but two concentrations of 0.25 mol/L (M) and 1.0 mol/L (M) based on PbI2, are fabricated with adding thiourea in the precursor solution, respectively. No matter what the substrate temperature is, the 1.0 M concentration of solution results in more preferential out of plane alignment with respect to the contacts and larger crystalline grain size than that from the 0.25 M solution. However, devices from the 0.25 M solution using the hot-cast technique present better performance averagely, with the best power conversion efficiency (PCE) of 7.33%. In incident-photon-to-current efficiency (IPCE) spectra of devices from the 0.25 M solution, we observe the dominant contribution from exciton absorption, which is lacking in devices from the 1 M solution. Our work demonstrates the highest PCE reported for mixed BA and FA based 2D perovskite solar cells with n < 5 and manifests that the effective exciton dissociation will be crucial for FA based 2D perovskite solar cells. Article Solution-processable two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) quantum wells naturally self-assemble through weak van der Waals forces. In this study, we investigate the structural and optoelectronic properties of 2D-layered butylammonium (C4H9NH3+, BA+) methylammonium (CH3NH3+, MA) lead iodide, (BA)2(MA)n-1Pb n I3n+1 quantum wells with varying n from 1 to 4. Through conventional structural characterization, (BA)2(MA)n-1Pb n I3n+1 thin films showcase high-quality phase (n) purity. However, while investigating the optoelectronic properties, it is clear that these van der Waals heterostructures consist of multiple quantum well thicknesses coexisting within a single thin film. We utilized electroabsorption spectroscopy and Liptay theory to develop an analytical tool capable of deconvoluting the excitonic features that arise from different quantum well thicknesses (n) in (BA)2(MA)n-1Pb n I3n+1 thin films. To obtain a quantitative assessment of exciton heterogeneities within a thin film comprising multiple quantum well structures, exciton resonances quantified by absorption spectroscopy were modeled as Gaussian features to yield various theory-generated electroabsorption spectra, which were then fit to our experimental electroabsorption features. In addition to identifying the quantum well heterostructures present within a thin film, this novel analytical tool provides powerful insights into the exact exciton composition and can be utilized to analyze the optoelectronic properties of many other mixed-phase quantum well heterostructures beyond those formed by OIHPs. Our findings may help in designing more efficient and reproducible light-emitting diodes based on 2D mixed-phase metal-organic multiple quantum wells. Article Using in situ photoluminescence measurements during the spin‐coating and annealing steps, this study probes the formation of 2D layers on 3D triple cation perovskite films comparing phenylethylammonium and 2‐thiophenemethylammonium iodide bulky cations. This study elucidates the formation mechanisms of the surface layers for both cases and reveals two regimes during 2D layer formation: a kinetic‐driven and a thermodynamic‐driven process. These driving forces result in different compositions of the 2D/3D interface for each treatment, namely, different ratios of pure 2D (n = 1) and quasi‐2D (n > 1) structures. This study shows that a higher ratio of quasi‐2D phases is more beneficial for device performance, as pure‐2D layers may hamper current extraction. Due to a more evenly distributed formation energy profile among 2D and quasi‐2D phases, highly concentrated 2‐thiophenemethylammonium iodide appears to be more suited for effective surface passivation than its phenylethylammonium analog. This study uses in situ photoluminescence measurements to elucidate the formation dynamics of 2D Ruddlesden‐Popper (RP) phases on triple cation perovskite films using two different molecules (phenylethylammonium iodide and 2‐thiophenemethylammonium iodide). This study finds that the formation dynamics and final composition of the RP‐phases depend on the molecule used, opening opportunities to optimize the 2D‐passivation mechanisms individually, and improve solar cell efficiency. Article Two-dimensional (2D) perovskites, with a formula of (RNH3)2MAn-1PbnI3n+1, have shown impressive photovoltaic device efficiency with improved stability. The operating mechanism of such photovoltaic devices is under debate and the scope of incorporated organic cations (RNH3⁺) is limited. We report a general post-annealing method to incorporate a variety of organic cations into 2D perovskites, which demonstrate significant device efficiencies (7% - 12%). A detailed investigation of the archetypical (C4H9NH3)2MA3Pb4I13 (n=4) discloses that such perovskites thin films contain multiple 2D phases (i.e., 2D quantum wells, n=2, 3, 4…). These phases appear to be distributed with decreasing n values from the top to the bottom of the 2D perovskites thin film, enabling efficient energy transfer in the first 500 ps and possible charge transfer at longer time scale, thereby accounting for high device efficiencies. Our post-annealing method is compatible with ambient condition, and only requires relatively low annealing temperature at a very short period of time, offering significant prospects for scalable manufacturing of 2D perovskites solar cells. Article Full-text available Highly photoluminescent organo-lead halide perovskite nanoparticles have recently attracted a wide interest in the context of high-stake applications, such as light emitting diodes (LEDs), light emitting transistors and lasers. In addition, they constitute ideal model systems to explore charge and energy transport phenomena occurring at the boundaries of nanocrystalline grains forming thin films in high-efficiency perovskite solar cells (PSCs). Here we report a complete photophysical study of CH3NH3PbBr3 perovskite nanoparticles suspended in chlorobenzene and highlight some important interaction properties. Colloidal suspension under study were constituted of dispersed aggregates of quasi-2D platelets of a range of thicknesses, decorated with 3D-like spherical nanoparticles. These types of nanostructures possess different optical properties that afford a handle for probing them individually. The photophysics of the colloidal particles was studied by femtosecond pump-probe spectroscopy and time-correlated single-photon counting. We show here that a cascade of charge and energy transfer occurs between nanostructures: Upon photoexcitation, localized excitons within one nanostructure can either recombine on a ps timescale, yielding a short-lived emission, or form charge-transfer states (CTSs) across adjacent domains, resulting in longer-lived photoluminescence in the millisecond timescale. Furthermore, CTSs exhibit a clear signature in the form of a strong photoinduced electroabsorption evidenced in femtosecond transient absorption measurements. Charge transfer dynamics at the surface of the nanoparticles have been studied with various quenchers in solution. Efficient hole transfer to N,N,N’,N’-tetrakis(4-methoxyphenyl)benzidine (MeO-TPD) and 1,4-bis(diphenylamino)benzene (BDB) donors was attested by the quenching of the nanoparticles emission. The charge transfer rate was limited by the organic layer used to stabilize the nanoparticles, which acted as a wide spacer between reactants. The forward charge transfer was found to take place in the sub-microsecond time-scale in competition with slow carrier recombination, while back transfer was shown to occur with a time-constant τ = 25 ms. Article Full-text available The hybrid two-dimensional (2D) halide perovskites have recently drawn significant interest because they can serve as excellent photoabsorbers in perovskite solar cells. Here we present the large scale synthesis, crystal structure, and optical characterization of the 2D (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 1, 2, 3, 4, ∞) perovskites, a family of layered compounds with tunable semiconductor characteristics. These materials consist of well-defined inorganic perovskite layers intercalated with bulky butylammonium cations that act as spacers between these fragments, adopting the crystal structure of the Ruddlesden-Popper type. We find that the perovskite thickness (n) can be synthetically controlled by adjusting the ratio between the spacer cation and the small organic cation, thus allowing the isolation of compounds in pure form and large scale. The orthorhombic crystal structures of (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 (n = 2, Cc2m; a = 8.9470(4), b = 39.347(2) Å, c = 8.8589(6)), (CH3(CH2)3NH3)2(CH3NH3)2Pb3I10 (n = 3, C2cb; a = 8.9275(6), b = 51.959(4) Å, c = 8.8777(6)), and (CH3(CH2)3NH3)2(CH3NH3)3Pb4I13 (n = 4, Cc2m; a = 8.9274(4), b = 64.383(4) Å, c = 8.8816(4)) have been solved by single-crystal X-ray diffraction and are reported here for the first time. The compounds are noncentrosymmetric, as supported by measurements of the nonlinear optical properties of the compounds and density functional theory (DFT) calculations. The band gaps of the series change progressively between 2.43 eV for the n = 1 member to 1.50 eV for the n = ∞ adopting intermediate values of 2.17 eV (n = 2), 2.03 eV (n = 3), and 1.91 eV (n = 4) for those between the two compositional extrema. DFT calculations confirm this experimental trend and predict a direct band gap for all the members of the Ruddlesden-Popper series. The estimated effective masses have values of mh = 0.14 m0 and me = 0.08 m0 for holes and electrons, respectively, and are found to be nearly composition independent. The band gaps of higher n members indicate that these compounds can be used as efficient light absorbers in solar cells, which offer better solution processability and good environmental stability. The compounds exhibit intense room-temperature photoluminescence with emission wavelengths consistent with their energy gaps, 2.35 eV (n = 1), 2.12 eV (n = 2), 2.01 eV (n = 3), and 1.90 eV (n = 4) and point to their potential use in light-emitting diodes. In addition, owing to the low dimensionality and the difference in dielectric properties between the organic spacers and the inorganic perovskite layers, these compounds are naturally occurring multiple quantum well structures, which give rise to stable excitons at room temperature. Article Full-text available The past two years have seen the unprecedentedly rapid emergence of a new class of solar cell based on mixed organic-inorganic halide perovskites. Although the first efficient solid-state perovskite cells were reported only in mid-2012, extremely rapid progress was made during 2013 with energy conversion efficiencies reaching a confirmed 16.2% at the end of the year. This increased to a confirmed efficiency of 17.9% in early 2014, with unconfirmed values as high as 19.3% claimed. Moreover, a broad range of different fabrication approaches and device concepts is represented among the highest performing devices-this diversity suggests that performance is still far from fully optimized. This Review briefly outlines notable achievements to date, describes the unique attributes of these perovskites leading to their rapid emergence and discusses challenges facing the successful development and commercialization of perovskite solar cells. Article Full-text available The electronic structures of three-dimensional and two-dimensional lead-halide-based crystals CH3NH3PbI3 and (C4H9NH3)2PbI4 are investigated by photoelectron spectroscopy and band calculations using the linear combination of atomic orbitals within the density-functional theory. For both crystals, the top of the valence band is found to consist mainly of the σ-antibonding states of Pb 6s and I 5p orbitals, and the bottom of the conduction band to be composed primarily of the σ-antibonding states of Pb 6p and I 5s orbitals. Photoelectron spectra of the valence-band region indicate that the electronic structures change depending on the dimensionality of the crystals. Based on the calculation results, the differences observed in the spectra are rationalized in terms of narrowing bandwidth as the dimensionality decreases from three to two dimensions. It is shown that the bandwidth narrowing of the two-dimensional crystal is due to zero dispersion in the vertical direction and the Jahn-Teller effect in the layered structure. These effects lead to a wideband gap and high exciton stability in (C4H9NH3)2PbI4. Article Full-text available The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This “meso-superstructured solar cell” exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite. Article Full-text available We report on the first observation of the one-exciton to two-exciton transition in J aggregates. A theoretical analysis supports our interpretation. Article Full-text available Organic-inorganic hybrid materials promise both the superior carrier mobility of inorganic semiconductors and the processability of organic materials. A thin-film field-effect transistor having an organic-inorganic hybrid material as the semiconducting channel was demonstrated. Hybrids based on the perovskite structure crystallize from solution to form oriented molecular-scale composites of alternating organic and inorganic sheets. Spin-coated thin films of the semiconducting perovskite (C(6)H(5)C(2)H(4)NH(3))(2)SnI(4) form the conducting channel, with field-effect mobilities of 0.6 square centimeters per volt-second and current modulation greater than 10(4). Molecular engineering of the organic and inorganic components of the hybrids is expected to further improve device performance for low-cost thin-film transistors. Article Full-text available Femtosecond visible and infrared analogues of multiple-pulse nuclear magnetic resonance techniques provide novel snapshot probes into the structure and electronic and vibrational dynamics of complex molecular assemblies such as photosynthetic antennae, proteins, and hydrogen-bonded liquids. A classical-oscillator description of these spectroscopies in terms of interacting quasiparticles (rather than transitions among global eigenstates) is developed and sets the stage for designing new pulse sequences and inverting the multidimensional signals to yield molecular structures. Considerable computational advantages and a clear physical insight into the origin of the response and the relevant coherence sizes are provided by a real-space analysis of the underlying coherence-transfer pathways in Liouville space. Article Quasi two-dimensional Ruddlesden-Popper perovskites driving carrier self-separation has rapidly advanced the development of high performance optoelectronic devices. However, insightful understanding of carrier dynamics in the perovskites is still inadequate. The distribution of multiple perovskite phases, crucial important for carrier separation, is in controversy. Here we report a systematic study on carrier dynamics of spin-coated (C6H5CH2CH2NH3)2(CH3NH3)n-1PbnI3n+1 (n = 3 and n = 5) perovskite thin films. Efficient electrons transfer from small-n to large-n perovskite phases and holes transfer reversely with time scales from ~ 0.3 to 30.0 ps. The multiple perovskites phases are arranged perpendicularly to substrate from small to large n, and also co-exist randomly in the same horizontal planes. Further, the carrier separation dynamics is tailored by engineering crystalline structure of perovskite film, which leads to controllable emission properties. These results have important significance for the design of optoelectronic devices from solar cell, light emitting diode, laser, etc. Article Quasi-two-dimensional lead halide perovskites, MAn-1PbnX3n+1, are quantum confined materials, with an ever-developing range of optoelectronic device applications. Like other semiconductors, the correlated motion of electrons and holes dominates the material’s response to optical excitation influencing its electrical and optical properties such as charge formation and mobility. However, the effects of many-particle correlation have been relatively unexplored in perovskite due to the difficultly of probing these states directly. Here, we use double quantum coherence spectroscopy to explore the formation and localization of multi-exciton states. Between the most confined domains, we demonstrate the presence of an inter-well, two-exciton excited state. This demonstrates that the four-body Coulomb interaction electronically couples neighboring wells despite weak electron/hole hybridization in these materials. Additionally, in contrast with inorganic semiconductor quantum wells, we demonstrate a rapid decrease in the dephasing time as wells become thicker, indicating that exciton delocalization is not limited by structural inhomogeneity in low dimensional perovskite. Article Recently, metal halide perovskite solar cells (PSCs) of the general formular ABX3 where A is a monovalent cation, that is, methylammonium (MA) CH3NH3+•, formamidinium CH2(NH2)2⁺, Cs⁺, or Rb⁺, B stands for Pb(II) or Sn(II), and X for iodide or bromide have achieved solar to electric power conversion efficiencies (PCEs) above 22%, exceeding the efficiency of the present market leader polycrystalline silicon while using 1000 times less light harvesting material and simple solution processing for their fabrication. The top performing devices all employ formulations containing a mixture of up to four A cations and iodide as well as a small fraction of bromide as anion, whose emergence will be described in this Commentary. Apart from leading the current PV efficiency race, these new perovskite materials exhibit intense electroluminescence and an extraordinarily high stability under heat and light stress. Article Understanding and controlling charge and energy flow in state-of-the-art semiconductor quantum-wells has enabled high-efficiency optoelectronic devices. Two-dimensional Ruddlesden-Popper perovskites are solution-processed quantum-wells wherein the band gap can be tuned by varying the perovskite layer thickness, which modulates the effective electron-hole confinement. We report that, counterintuitive to classical quantum-confined systems where photo-generated electrons and holes are strongly bound by Coulomb interactions or excitons, the photo-physics of thin films made of Ruddlesden-Popper perovskites with a thickness exceeding two perovskite crystal-units (>1.3 nanometers) is dominated by lower energy states associated with the local intrinsic electronic structure of the edges of the perovskite layers. These states provide a direct pathway for dissociating excitons into longer-lived free-carriers that significantly improve the performance of optoelectronic devices. Article Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multi-layered (BA)2(MA)n−1PbnI3n+1 (BA = CH3(CH2)3NH3+, MA = CH3NH3+) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as “n = 4”, are found to be mixture of multiple perovskite phases, with n = 2, 3, 4 and ≈ ∞, that naturally align in the order of n along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-n to large-n phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ~358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices. Article Two-dimensional (2D) halide perovskites with the formula of A2MIVX4VII are now emerging as a new family of 2D materials and promising candidates for nanoelectronics and optoelectronics. Potentially, there could be abundance of 2D halide perovskites by varying the compositions of A, M and X and their properties can be widely tuned to satisfy the requirements of the practical applications. While several samples have been experimentally realized, most of them are currently unexplored and their chemical trends in relation to the chemical compositions are yet not well understood, which thus drags down the exploration of their potential applications. In this work, using first-principles calculation methods, we systematically investigate the properties of 2D halide perovskites, including their structural stabilities, electronic, optical, and transport properties. The chemical trends in this novel family of 2D materials are established and we find that the bandgaps increase with increased lattice distortions by changing A ion from Cs+ to CH3NH3+, increase with MIV ion changing from Sb to Pb, and decrease with X changing from Cl to Br to I. Some of the studied systems like Cs2SnI4 are identified with good optical properties for photovoltaics and most of the systems have good motilities suitable for electric devices like transistors. The abundance of potential 2D halide perovskites not only enriches current 2D families but also offers more possibility for electrical and optoelectrical applications. Our work is expected to provide theoretical understanding and guidance for the further study of these 2D halide perovskites. Article Organometal halide perovskites can be processed from solutions at low temperatures to form crystalline direct-bandgap semiconductors with promising optoelectronic properties. However, the efficiency of their electroluminescence is limited by non-radiative recombination, which is associated with defects and leakage current due to incomplete surface coverage. Here we demonstrate a solution-processed perovskite light-emitting diode (LED) based on self-organized multiple quantum wells (MQWs) with excellent film morphologies. The MQW-based LED exhibits a very high external quantum efficiency of up to 11.7%, good stability and exceptional high-power performance with an energy conversion efficiency of 5.5% at a current density of 100 mA cm⁻². This outstanding performance arises because the lower bandgap regions that generate electroluminescence are effectively confined by perovskite MQWs with higher energy gaps, resulting in very efficient radiative decay. Surprisingly, there is no evidence that the large interfacial areas between different bandgap regions cause luminescence quenching. Article A recent theoretical study proposed that two-quantum (2Q) two-dimensional (2D) electronic spectroscopy should be a background-free probe of post-Hartree–Fock electronic correlations. Testing this theoretical prediction requires an instrument capable of not only detecting multiple transitions among molecular excited states but also distinguishing molecular 2Q signals from nonresonant response. Herein we describe a 2Q 2D spectrometer with a spectral range of 300 nm that is passively phase stable and uses only beamsplitters and mirrors. We developed and implemented a dual-chopping balanced-detection method to resolve the weak molecular 2Q signals. Experiments performed on cresyl violet perchlorate and rhodamine 6G revealed distinct 2Q signals convolved with nonresonant response. Density functional theory computations helped reveal the molecular origin of these signals. The experimental and computational results demonstrate that 2Q electronic spectra can provide a singular probe of highly excited electronic states. Article Organometal halide perovskites exhibit large bulk crystal domain sizes, rare traps, excellent mobilities and carriers that are free at room temperature-properties that support their excellent performance in charge-separating devices. In devices that rely on the forward injection of electrons and holes, such as light-emitting diodes (LEDs), excellent mobilities contribute to the efficient capture of non-equilibrium charge carriers by rare non-radiative centres. Moreover, the lack of bound excitons weakens the competition of desired radiative (over undesired non-radiative) recombination. Here we report a perovskite mixed material comprising a series of differently quantum-size-tuned grains that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence non-radiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 W sr(-1) m(-2). These represent the brightest and most efficient solution-processed near-infrared LEDs to date. Article Photovoltaic (PV) devices that harvest the energy provided by the sun have great potential as renewable energy sources, yet uptake has been hampered by the increased cost of solar electricity compared with fossil fuels. Hybrid metal halide perovskites have recently emerged as low-cost active materials in PV cells with power conversion efficiencies now exceeding 20%. Rapid progress has been achieved over only a few years through improvements in materials processing and device design. In addition, hybrid perovskites appear to be good light emitters under certain conditions, raising the prospect of applications in low-cost light-emitting diodes and lasers. Article We report the first coherent multidimensional spectroscopy study of a MoS2 film. A four-layer sample of MoS2 was synthesized on a silica substrate by a simplified sulfidation reaction and characterized by absorption and Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. State-selective coherent multidimensional spectroscopy (CMDS) on the as-prepared MoS2 film resolved the dynamics of a series of diagonal and cross-peak features involving the spin-orbit split A and B excitonic states and continuum states. The spectra are characterized by striped features that are similar to those observed in CMDS studies of quantum wells where the continuum states contribute strongly to the initial excitation of both the diagonal and cross-peak features, while the A and B excitonic states contributed strongly to the final output signal. The strong contribution from the continuum states to the initial excitation shows that the continuum states are coupled to the A and B excitonic states and that fast intraband relaxation is occurring on a sub-70 fs time scale. A comparison of the CMDS excitation signal and the absorption spectrum shows that the relative importance of the continuum states is determined primarily by their absorption strength. Diagonal and cross-peak features decay with a 680 fs time constant characteristic of exciton recombination and/or trapping. The short time dynamics are complicated by coherent and partially coherent pathways that become important when the excitation pulses are temporally overlapped. In this region, the coherent dynamics create diagonal features involving both the excitonic states and continuum states, while the partially coherent pathways contribute to cross-peak features. Article The dynamic optical properties of perovskite CH3NH3PbI3 single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon excitation exhibits redshift with the elapse of time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emis-sions from the localized states because of strong band-to-band ab-sorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron-hole radiative recombination and carrier diffusion effects. Article We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals. Article Accurate mapping of the electronic and vibrational structure of a molecular system is a basic goal of chemistry as it underpins reactivity and function. Experimentally, the challenge is to uncover the intramolecular interactions and ensuing dynamics that define this structure. Multidimensional coherent spectroscopy can map such interactions analogous to the way in which nuclear magnetic resonance provides access to the nuclear spin structure. Here we present two-dimensional coherent spectra measured using few-cycle continuum light. Critically, our approach instantaneously maps the energy landscape of a complex molecular system in a single laser pulse across 350 nm of bandwidth, thereby making it suitable for rapid molecular fingerprinting. We envision few-cycle supercontinuum spectroscopy based on the nonlinear optical response as a powerful tool to examine molecules in the condensed phase at the extremes of time, space, and energy. Article The diode behavior of photovoltaic devices based on methylammonium lead iodide (MAPbI3 ) perovskites is examined, in combination with electroluminescence measurements. From this analysis the diode ideality factors are extracted, which indicate that non-radiative trap-assisted recombination is the dominant recombination pathway, primarily caused by the presence of electron traps. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Article Femtosecond optical pump-probe spectroscopy with 10 fs visible pulses is employed to elucidate the earliest ultrafast carrier dynamics in few-layer MoS2. A nonthermal carrier distribution is generated immediately after the photoexcitation. Carrier thermalization is observed to occur on the <20 fs time scale via carrier and phonon scatterings. The excitation pump fluence-dependence study provides evidence for the existence of non-Markovian quantum kinetic behavior in the carrier-carrier scattering process. Subsequent cooling of the hot Fermi-Dirac carrier distribution occurs on the ~0.6 ps time scale via phonon emission on account of the observed hot-phonon effect. Nonadiabatic ab initio molecular dynamics simulations predict time constants of 30 fs and 0.5 ps, respectively, for carrier-carrier and carrier-phonon scatterings, consistent with the assignment of the observed carrier dynamics. 8th Biennial Meeting of The Materials Research Society of Singapore (MRS-S) Article The unique and promising properties of semiconducting organometal halide perovskites have brought these materials to the forefront of solar energy research. Here, we present new insights into the excited-state properties of CH3NH3PbI3 thin films through femtosecond transient absorption spectroscopy measurements. The photoinduced bleach recovery at 760 nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via recombination of free electrons and holes. Additionally, charge accumulation in the perovskite films leads to an increase in the intrinsic bandgap that follows the Burstein–Moss band filling model. Both the recombination mechanism and the band-edge shift are studied as a function of the photogenerated carrier density and serve to elucidate the behaviour of charge carriers in hybrid perovskites. These results offer insights into the intrinsic photophysics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications. Article Using time-resolved photoluminescence and transient absorption measurements at room temperature, we report excitation-intensity-dependent photocarrier recombination processes in thin films made from the organo-metal halide perovskite semiconductor CH3NH3PbI3 for solar-cell applications. The photocarrier dynamics are well described by a simple rate equation including single-carrier trapping and electron-hole radiative recombination. This result provides clear evidence that the free-carrier picture is better than the exciton picture for interpreting the optical properties of CH3NH3PbI3. The observed large two-carrier recombination rate suggests the promising potential of perovskite semiconductors for optoelectronic device applications. Our findings provide the information about the dynamical behaviors of photoexcited carriers that is needed for developing high-efficiency perovskite solar cells. Book This work brings together quantum theory and spectroscopy to convey excitation processes to advanced students and specialists wishing to conduct research and understand the entire fi eld rather than just single aspects. Written by experienced authors and recognized authorities in the field, this text covers numerous applications and offers examples taken from different disciplines. As a result, spectroscopists, molecular physicists, physical chemists, and biophysicists will all fi nd this a must-have for their research. Also suitable as supplementary reading in graduate level courses. Article Reflection, luminescence and absorption spectra are measured in an exciton region of (C10H21NH3)2PbI4. Crystal structure is determined to be of layered perovskite type by X-ray diffraction. PbI4 layers are sandwiched by barrier layers consisting of alkylammonium chains. The exciton state has a large binding energy of 370 meV and is very stable even at room temperature. Its oscillator strength is estimated to be about 0.7 per a formula unit. The binding energy is too large to be explained only by the two-dimensional character of the exciton and is ascribed to the small dielectric constant of the barrier layer, which makes Coulomb interaction between an electron and a hole stronger. Article This paper presents a calculation of the lifetimes (\tau $) of excess electrons and holes in a semiconductor assuming the Auger effect between bands (electron-electron and hole-hole collisions) to be the only recombination mechanism. If pair annihilation, and the corresponding reverse process of pair creation, are counted separately, there are four classes of processes to be considered. The suitably weighted algebraic sum of the rates of these processes yields a net recombination rate R. If N be the non-equilibrium number of pairs, then$\tau $= N/R. In the calculation the effect of traps is neglected, and the group of electrons in the conduction band and the group in the valence band are each assumed to be in equilibrium among themselves, but not with each other, by the use of quasi-Fermi levels. Bloch functions$\psi _{{\bf k}}$= u(${\bf k}$,${\bf r}$) exp (i${\bf k}$.${\bf r}$) are used. The matrix element of the Coulomb interaction is obtained as a multiple sum over reciprocal lattice vectors. Most of these terms correspond to Umklapp-type processes whose probability of occurrence is shown to be small. The dominant term, after integration over all initial and final states, yields the dependence of lifetime on temperature, carrier concentration, energy gap and other parameters. The absolute value of the lifetime depends also on an overlap intergral of the form$\int $u$^{\ast}$(${\bf k}$,${\bf r}$) u(${\bf k}^{\prime}$,${\bf r}$) d${\bf r}$where k,${\bf k}^{\prime}$are in different bands. This integral is estimated on the basis of a one-dimensional model. The theory is compared with experimental lifetimes in InSb, and shows that the mechanism envisaged may dominate radiative recombination above 240 degrees K and accounts for the order of magnitude of the observed lifetimes ($\sim $10$^{-8}\$ s) in the neighbourhood of the highest temperature (330 degrees K) at which recombination in InSb has so far been studied.
Article
A form of two-dimensional (2D) vibrational spectroscopy, which uses two ultrafast IR laser pulses, is used to examine the structure of a cyclic penta-peptide in solution. Spectrally resolved cross peaks occur in the off-diagonal region of the 2D IR spectrum of the amide I region, analogous to those in 2D NMR spectroscopy. These cross peaks measure the coupling between the different amide groups in the structure. Their intensities and polarizations relate directly to the three-dimensional structure of the peptide. With the help of a model coupling Hamiltonian, supplemented by density functional calculations, the spectra of this penta-peptide can be regenerated from the known solution phase structure. This 2D-IR measurement, with an intrinsic time resolution of less than 1 ps, could be used in all time regimes of interest in biology.
Article
Optical spectra in the visible and uv regions are investigated in layer-type perovskite compounds (CnH2n+1NH3)PbI4 with n=4, 6, 8, 9, 10, and 12. The spacing between the PbI4 layers changes from 15.17 Å for n=4 to 24.51 Å for n=12. In spite of these different spacings, the optical spectra are almost the same for these compounds, which means that the interaction between the layers is weak. The lowest exciton is located at 2.56 eV at 1.6 K, and its oscillator strength and binding energy are 0.7 per formula unit and 320 meV, respectively. These values are very large compared with those in a three-dimensional analog PbI2. The large oscillator strength and binding energy can be explained by the small dielectric constant of the alkylammonium ‘‘barrier layer,’’ which strengthens the Coulomb interaction between an electron and a hole.
Article
By varying the dielectric environment in new PbI4-based layer-type perovskite compounds, we have demonstrated directly the contribution by dielectric confinement to the exciton binding energy in three such ‘‘natural-quantum-well’’ semiconductors. With different dielectric environment, exciton binding energies of 320, 220, and 170 meV have been observed, dominated by the dielectric confinement. In terms of the conventional size-related electronic confinement, two of the materials represent monolayer PbI4 quantum wells while the third corresponds to a bilayer case, with a corresponding reduction in the electronic confinement. From theory, including the dielectric confinement effect, the effective mass of the exciton in a PbI4-based dielectric quantum well has been determined to be 0.09me; the corresponding quasi-two-dimensional exciton Bohr radii were 15.5, 17.0, and 20.5 Å for the three cases, respectively.
Article
Tryptophan is commonly used to study protein structure and dynamics, such as protein folding, as a donor in fluorescence resonant energy transfer (FRET) studies. Using ultra-broadband, ultrafast two-dimensional (2D) spectroscopy in the ultraviolet and transient absorption in the visible range, we have disentangled the excited-state decay pathways of the tryptophan amino acid residues in ferric myoglobins (MbCN and metMb). Whereas the more distant tryptophan (Trp(7)) relaxes by energy transfer to the heme, Trp(14) excitation predominantly decays by electron transfer to the heme. The excited Trp(14)→heme electron transfer occurs in <40 picoseconds with a quantum yield of > 60%, over an edge-to-edge distance below ~10 Å, out-competing the FRET process. Our results raise the question of whether such electron transfer pathways occur in a larger class of proteins.
Article
Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si3N4 substrate-supported monolayer and few-layer MoS2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band-gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS2. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS2 along with controlling their dimensions.
Article
The short pulse durations and broad frequency spectra of femtosecond laser pulses allow coherent superpositions of states to be prepared and probed. Two-dimensional electronic spectroscopy (2D ES) has the potential to identify more clearly the origin and evolution of such coherences. In this report we examine how electronic and vibrational coherences can be distinguished by decomposing the total 2D ES signal into rephasing and nonrephasing components. We investigate and identify differences between the cross peak oscillations measured in two laser dyes with those measured in the PC645 light-harvesting antenna protein of the cryptophyte alga Chroomonas sp. strain CCMP270 at ambient temperature.Keywords: two-dimensional spectroscopy; quantum biology; quantum coherence; vibrational wavepacket; phycobiliproteins; cryptophyte algae
Auger recombination in direct-gap semiconductors is reinvestigated taking into account a realistic band structure instead of using the usual parabolic approximation. It is found that the direct conduction band process is negligible in large-gap semiconductors (Eg>or=0.6 eV), contrary to what has been found using the parabolic approximation. The new values for the probabilities of the valence band process and of the phonon-assisted Auger processes are also found to be smaller, but not by so much. Altogether, smaller values for the total Auger recombination are obtained and these are in agreement with experimental results. In addition, the effect of carrier degeneracy is shown to be weak, apart from where carrier concentrations are extremely high (n>or=1020 cm-3). Finally, the uncertainties involved in the quantitative values of the Auger coefficients are discussed.
Chapter
Multidimensional optical spectroscopy in the visible and infrared is a rapidly developing technique enabling direct observation of complex dynamics of molecules in complex environments such as liquids and proteins. Measuring the correlation between excited and detected frequencies with sub‐picosecond resolution has enabled the resolution of long‐standing problems such as energy transfer in photosynthesis and the life‐sustaining structural rearrangements of liquid water. This chapter aims to provide a bridge between the concepts familiar in the AMO physics community and how those ideas and experimental methods are applied to condensed phase molecular spectroscopy. We outline the technical challenges of these powerful methods while considering a few examples of experiments that showcase the unique perspective offered by 2D electronic and vibrational spectroscopy.
Article
In Weiterentwicklung früherer Theorien von J. und F. Perrin und klassischphysikalischer Überlegungen des Verfassers wird eine quantenmechanische Behandlung des Übergangs von Elektronenanregungsenergie zwischen gleichartigen Molekülen in Lösung gegeben. Der kritische Molekülabstand, unterhalb dessen der übergang während der Anregungsdauer stattfindet, läßt sich aus den Absorptions- und Fluoreszenzspektren und der Anregungsdauer der Moleküle berechnen. Für Fluorescein und Chlorophyll a ergeben sich Werte von 50 bzw. 80 ÅE, entsprechend den mittleren Molekülabständen in Lösungen von 3,2 · 10−3 bzw. 7,7 · 10−4 Molen/Liter. Für die Bereiche oberhalb und unterhalb der kritischen Konzentration werden Formeln zur Berechnung der Energieabwanderung vom Primärmolekül angegeben, die mit den vorliegenden Messungen der Konzentrationsdepolarisation der Fluoreszenz gut übereinstimmen. Die Anwendung auf analoge Energiewanderungsprobleme in Molekülkristallen und im Assimilationsapparat der Pflanze wird diskutiert.
Article
Multiresonant coherent multidimensional spectroscopy is a frequency-domain method that uses tunable excitation pulses to excite multiple quantum coherences (MQCs) and/or state populations using fully coherent or partially coherent excitation pathways. Pairs of states that are coupled by intra- and intermolecular interactions re-emit light at their frequency differences. The MQCs are coherent and interfere constructively to create phase-matched output beams. Scanning the excitation frequencies with fixed-excitation-pulse time delays creates multidimensional spectra, whereas scanning the time delays with fixed excitation frequencies measures the MQCs' coherent and incoherent dynamics. Multiresonant methods can excite any combination of vibrational and/or electronic states and use any coherence pathway. Cross-peaks occur between states when the excitation of one perturbs the other. This requirement for coupling acts to eliminate spectral congestion. Spectral resolution is increased because multiresonant methods narrow inhomogeneous broadening, enhance peaks from specific components, and spread the resolution over multiple dimensions.
Article
The electronic structure and photoinduced relaxation dynamics of the cyanobacterial light harvesting protein, C-Phycocyanin (CPC), are examined using transient grating and two-dimensional (2D) photon echo spectroscopies possessing sub-20 fs time resolution. In combination with linear absorption and fluorescence measurements, these time-resolved experiments are used to constrain the parameters of a Frenkel exciton Hamiltonian. Particular emphasis is placed on elucidating the nature of excited states involving the alpha84 and beta84 phycocyanobilin pigment dimers of CPC. This paper obtains new experimental evidence suggesting that electronic relaxation proceeds by way of incoherent energy transfer between the alpha84 and beta84 pigment sites (i.e., the weak coupling limit of energy transfer). Transient absorption anisotropies simulated in the weak coupling limit agree well with measurements, whereas signals computed in an exciton basis possess short-lived (electronic) coherent components not present in the experimental data. In addition, 2D photon echo spectra for CPC show no sign of the interfering nonlinearities predicted by a theoretical model to be characteristic of exciton formation. Another important new observation is that the sub-100 fs dynamics in the transient absorption anisotropy are dominated by an impulsively excited hydrogen out-of-plane wagging mode similar to those observed in phytochrome and retinal. Detection of this 795 cm(-1) coherence is of particular interest because our recent study of a closely related protein, Allophycocyanin (APC), assigns a similar coordinate as a promoting mode enabling ultrafast internal conversion. Together, the experiments conducted for APC and CPC suggest that interactions between the pigments and environment are the key to understanding why electronic relaxation in CPC is more than three times slower than APC despite the nearly identical geometries of the pigment dimers. Most important in reaching this conclusion is the present finding that relaxation of the 2D photon echo line shapes of CPC is approximately two times faster than that measured for APC. Overall, the present results underscore the ability of phycobiliproteins to control light harvesting dynamics through solvation and variation in the conformations of open-chain tetrapyrrole chromophores.
Article
Femtosecond transient grating and photon echo spectroscopies with a sub-20 fs time resolution are applied to allophycocyanin (APC), a protein located at the base of the phycobilisome antenna of cyanobacteria. Coupling between pairs of phycocyanobilin pigments with nondegenerate energy levels gives rise to the four-level exciton electronic structure of APC. Spectroscopic signals obtained in multiple experiments (e.g., linear absorption, fluorescence, transient grating, 2D Fourier transform photon echo) are used to constrain the parameters of a Frenkel exciton Hamiltonian. Comparison between experiment and theory yields a robust microscopic understanding of the electronic and nuclear relaxation dynamics. In agreement with previous work, transient absorption anisotropy establishes that internal conversion between the exciton states of the dimer occurs with time constants of 35, 220, and 280 fs. The sub-100 fs dynamics are decomposed into three distinct relaxation processes: electronic population transfer, intramolecular vibrational energy redistribution, and the dephasing of electronic and nuclear coherences. Model calculations show that the sub-100 fs red-shift in the transient absorption signal spectrum reflects interference between stimulated emission (ESE) and excited state absorption (ESA) signal components. It is also established that the pigment fluctuations in the dimer are not well-correlated, although further experiments will be required to precisely quantify the amount of correlation. The findings of this paper suggest that the light harvesting function of APC is enhanced by nondegeneracy of the pigments comprising the dimer and strong vibronic coupling of intramolecular modes on the phycocyanobilins. We find that the exciton states are 96% localized to the individual molecular sites within a particular dimer. Localization of the transition densities, in turn, is suggested to promote significant vibronic coupling which serves to both broaden the absorption line shape and open channels for fast internal conversion. The dominant internal conversion channel is assigned to a promoting mode near 800 cm(-1) involving hydrogen out-of-plane (HOOP) wagging motion similar to that observed in phytochrome and retinal. This rate enhancement ensures that all photoexcitations quickly and efficiently relax to the electronic origin of the lower energy exciton state from which energy transfer to the reaction center occurs.
Article
We study the optical response of Frenkel excitons in molecular J aggregates with a cylindrical geometry. Such aggregates have recently been prepared for a class of cyanine dyes and are akin to the rod- and ring-shaped light-harvesting systems found in certain bacteria. The linear absorption spectrum exhibits two lines with perpendicular polarization that are separated by a “ring energy scale”, which is set by the circumference of the cylinder and the intermolecular transfer interaction in the circumferential direction. On the other hand, the pump-probe spectrum shows bleaching and induced absorption features that are separated by a much smaller energy scale, which is determined by an effective Pauli gap imposed by the length of the cylinder and the transfer interaction in its longitudinal direction. We show that this can be well-understood from the approximate separation of the set of two-exciton states, into classes of inter-ring and intra-ring two-exciton states. Our calculations show that the experimental linear absorption spectrum may be used to estimate the cylinder circumference, while the pump-probe spectrum yields information on the length of the cylinder or on the delocalization length of the excitons in its longitudinal direction. We apply this method to cylindrical aggregates of cyanine dyes.