No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Energy transfer mechanisms in layered 2D perovskites
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.
... For example, quantum wells with n=2, 3, 4, and 5 have exciton resonances near 570, 600, 640, and 680 nm. 2,3,[16][17][18] In addition, the exciton binding energies, which range from 250-125 meV for n=2-5, 4 decrease as the thicknesses of the quantum wells increase. Incorporation of insulating organic spacer cations into the structures (e.g., butylammonium) also has significant implications for carrier transport in these materials. ...
... Energy and charge funneling behaviors are promoted by the prototypical light-harvesting antenna structure; [16][17][18][21][22][23][24][25][26][27] however, our recent studies suggest that long-range charge transport primarily occurs within the phases of the thickest quantum wells rather than exploiting a cascade Accepted to J. Chem. Phys. ...
... 29 This interpretation is based on the analysis of a wide range of nonlinear optical and photocurrent spectroscopies applied to systems composed of quantum wells with similar size distributions. 17,20,23,28 While these measurements are self-consistent, the understanding of transport mechanisms in solution-processed films is challenged by heterogeneity. In-plane and out-of-plane transport cannot be distinguished because of quasirandom quantum well orientations. ...
Mixtures of layered perovskite quantum wells with different sizes form prototypical light-harvesting antenna structures in solution-processed films. Gradients in the band gaps and energy levels are established by concentrating the smallest and largest quantum wells near opposing electrodes in photovoltaic devices. Whereas short-range energy and charge carrier funneling behaviors have been observed in layered perovskites, our recent work suggests that such light-harvesting processes do not assist long-range charge transport due to carrier trapping at interfaces between quantum wells and interstitial organic spacer molecules. Here we apply a two-pulse time-of-flight technique to a family of layered perovskite systems to explore the effects that interstitial organic molecules have on charge carrier dynamics. In these experiments, the first laser pulse initiates carrier drift within the active layer of a photovoltaic device, whereas the second pulse probes the transient concentrations of photoexcited carriers as they approach the electrodes. The instantaneous drift velocities determined with this method suggest that the rates of trap-induced carrier deceleration increase with the concentrations of organic spacer cations. Overall, our experimental results and model calculations suggest that the layered perovskite device efficiencies primarily reflect the dynamics of carrier trapping at interfaces between quantum wells and interstitial organic phases.
... 68 Knowledge of photoinduced relaxation mechanisms in layered perovskites has largely been gathered from transient absorption experiments. 63,64,[69][70][71][72][73][74] It is now understood that the energy transfer concentrates electronic excitations in the thickest quantum wells on the sub-ns timescale, whereas the charge transfer between quantum wells occurs at later times. 22,64,70,73,74 Here, we suggest a sub-ns timescale for the energy funneling processes in systems with heterogeneous concentrations of quantum wells [see Fig. 7 ...
... 63,64,[69][70][71][72][73][74] It is now understood that the energy transfer concentrates electronic excitations in the thickest quantum wells on the sub-ns timescale, whereas the charge transfer between quantum wells occurs at later times. 22,64,70,73,74 Here, we suggest a sub-ns timescale for the energy funneling processes in systems with heterogeneous concentrations of quantum wells [see Fig. 7 ...
... At sub-ns delay times, the layered system exhibits exciton resonances; however, the linewidths are much broader than those detected with transient absorption and nonlinear fluorescence spectroscopies. 63,64,69,70,72,73,76 We attribute these differences to enhanced contributions from continuum states in photocurrent spectroscopies. 21 In comparison to transient absorption data, the responses of excitons in the smallest quantum wells are suppressed by spontaneous emission due to their large binding energies and transition dipoles. ...
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.
... 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. ...
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 "soness" 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 denite 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. ...
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.
... [29][30][31][32] It has recently been discovered that a mixture of layered 2D perovskites can be engineered to better balance light absorption and/or charge carriers transport in solar cells in order to increase the conversion efficiency. [33] Femtosecond Transient Absorption Spectroscopy (FTAS), a pump-probe technique, has proven to be an important tool to investigate the ultrafast physics of halide perovskites with different dimensionalities, [34][35][36] morphologies and architectures. [37] FTAS gives information on the charge carrier generation, recombination, and many-body interaction by studying the evolution of the transient signal as a function of the time delay between the pump and the probe. ...
... [38][39][40] Previous investigations on the charge carrier dynamics in 2D perovskites by ultrafast time-resolved spectroscopies provide information on the lifetime and the diffusion length of the charge carriers, [41,42] on the hot carriers cooling, [43,44] on the role of the organic cations and the number of inorganic layers in governing the ultrafast charge carrier dynamics [45] and on the charge transfer processes. [33,44] A transient absorption microscopy experiment has shown the occurrence in 2D perovskites of a slow exciton-exciton annihilation (EEA), [46] which is a non-radiative many-body interaction, a form of Auger recombination where the kinetic energy of an exciton is transferred to another exciton. [47] This non-radiative channel, which is still not completely and extensively studied, is strictly connected to the exciton energetics and more in general to the optoelectronic properties of 2D perovskite materials. ...
Femtosecond transient absorption spectroscopy (FTAS) is an important tool to investigate the physics of halide perovskites having different dimensionality, morphology, and architectures, giving insights into the electronic and excitonic optical transitions. Here, FTAS on monolayer (n = 1) and multilayer (n = 2, 3) quasi‐2D perovskites in the Ruddlesden–Popper phase: DA2MAn‐1PbnI3n+1 (DAMAPI) is presented, with the dodecylammonium (DA = CH3‐(CH2)11‐NH3+) as the spacer and methylammonium (MA = CH3NH3+) as the organic cation for samples with n > 1. The measurements, performed at 77 K and room temperature using several pump energies and excitation densities, allow the observation of different absorption bleaching energies. Those energies are compared with the results of first‐principles theoretical simulations based on density functional theory, the GW method, and the Bethe–Salpeter equation and assigned to transitions involving excitons with principal quantum numbers 1s and 2s. The temporal analysis of the absorption bleaching indicates the exciton–exciton annihilation as the main relaxation mechanism in the first picoseconds after excitation, while exciton radiative recombination is observed at longer time delays (>100 ps). Therefore, FTAS allows the study of the carrier dynamics and, given its high sensitivity to carrier density changes, the observation of spectral features not observable with steady‐state measurements. Ultrafast transient measurements on photoexcited monolayer (n = 1) and multilayer (n = 2, 3) quasi‐2D perovskites (DA2MAn–1PbnI3n+1) show different electronic transitions assigned to the formation of excitons with principal quantum numbers 1s and 2s in agreement with first‐principles theoretical simulations. The temporal analysis indicates the exciton–exciton annihilation as the main relaxation mechanism in the first picoseconds after the excitation.
... 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. ...
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.
... In Figure 6, we summarized the transient absorption spectra (TAS) of four films at various delay times excited at 3.1 eV (400 nm). For OA 2 PbI 4 of RP phase, although the delayed maximum of photo-bleaching (PB) peak may suggest the slow thermalization happens in OA 2 PbI 4 since the pump photon energy is 0.65 eV higher than exitonic transition (2.44 eV in Figure 1b), the photoinduced features around band edge can be plausibly explained by a combination of "band filling" due to the excitonic transition around 2.45 eV [63], photoinduced symmetry-breaking process (such as spatially inhomogeneous strain or photoinduced electric field [64,65]), and transient broadening [66]. From the dominant exciton PB band in the Figure 6a for OA 2 PbI 4 , we conclude that the exciton is formed after photon's absorption within 1 ps, and remains as major photoexcitations within the time domain shown in Figure 6a. ...
We have studied four 2D layered perovskites, including OA2PbI4 (RP phase), ODAPbI4 and BDAPbI4 (DJ phase), (GA)MAPbI4 (ACI phase), where OA is [(C m H2m+1)NH3](m = 8), ODA is [NH3(CH2) m NH3](m = 8), BDA is [NH3(CH2) m NH3](m = 4), and GA is [C(NH2)3]; RP, DJ, and ACI means Ruddlesden–Popper, Dion–Jacobson and alternating cations in the interlayer, respectively. The temperature dependence of absorption and photoluminescence (PL) spectra have been measured. From which the average phonon energy (electron-phonon interaction strength) is analyzed as around 34 (80), 47 (184), 50 (402), and 63 (758) with the unit of meV for OA2PbI4, ODAPbI4, BDAPbI4, and (GA)MAPbI4, respectively. Larger phonon energy indicates the involvement of more phonons in organic spacer layer, with the corresponding stronger electron-phonon interaction. Furthermore, ultrafast transient absorption spectroscopy proves that, when the excitation photon energy is serval hundred meV higher than bandgap, the excitons still are the major photoexcitations in OA2PbI4, but polarons are major one in ODAPbI4, BDAPbI4, and (GA)MAPbI4 films, no matter the excitonic transitions dominate the absorption at their band edges. This work proves the organic spacers can regulate electron–phonon interaction then optoelectronic properties in 2D perovskites profoundly, which have implications toward future rational design for relevant devices.
... The alternating arrangement of inorganic-organic layers forms multiple quantum well (MQW) structures that are generated through self-assembly owing to differences in formation energy, resulting in slow charge transfer and reduced carrier extraction efficiency [35][36][37] . Multiphase quasi-2D perovskites, which follow the chemical structural formula L 2 A n−1 B n X 3n+1 with thickness n > 1, comprise a random width distribution of different specific single QW, which, as in the case of impurity phases and structural defects, aggravate trap-assisted recombination and thus degrade device performance [38][39][40][41] . In addition, the mixture of multiple phases results in inefficient charge injection to the target emission centres, which also leads to the coexistence of several photoluminescence (PL) peaks and of several unstable emission characteristics under bias voltage [42][43][44] . ...
Two-dimensional layered metal-halide perovskites (2D-LMHPs) are a promising family of organic–inorganic hybrid semiconductor materials because of their superior electronic and optical properties and high stability. To date, solution-processed 2D-LMHP thin films have a multiple quantum wells (QWs) structure, which has seriously impeded further progress in optoelectronics. Compared with 2D-LMHPs with multiple QWs, 2D-LMHPs with phase-pure QWs have a flattened energy landscape, resulting in less energy or charge-transfer losses and making them less susceptible to degradation. They would thus be attractive to promote the development of perovskite-based devices. In this Perspective article, we first elucidate the structure and optoelectronic properties of phase-pure 2D-LMHP films. Second, we systematically discuss their precursor engineering, focusing on stoichiometry, ligand design, chemical compositional engineering and formation energy aspects. Third, we comprehensively summarize the intermediate phase growth mechanism, in situ dynamic transformation observation and methodologies for the formation of quasi-2D perovskites with phase-pure structures. Finally, we deliberate the prospects and challenges of phase-pure 2D perovskites as a new family of semiconductors.
... 17 In general, during one-step deposition and annealing processes, a small-n phase tends to form on the bottom of the perovskite film and parallel to the substrate, whereas a large-n phase tends to form at the top part of the film and perpendicular to the substrate. 18,19 It is generally assumed that electrons are transmitted from the small-n phases to large-n phases and that holes are transmitted from large-n phases to small-n phases. 20,21 Hence, this gradient QW distribution with type-II band alignment is favorable for self-driven charge separation in inverse (p-i-n) devices but inhibits the charge carrier transport of normal (n-i-p) devices. ...
One‐step deposition approaches have been widely applied and developed in the fabrication of quasi‐2D perovskites. However, the regulation of quantum wells (QWs) and crystalline orientation is difficult and complicated when using this methodology. Sequential deposition is another widespread synthetic approach for preparing perovskite films and perovskite dimension engineering. In this article, δ‐CsPbI3 intermediate phase assisted sequential (IPAS) deposition is successfully carried out to fabricate MA‐free quasi‐2D ACI perovskites. The amount of the δ‐CsPbI3 intermediate phase in the PbI2 layer and the concentration of GAI molecule in the IPA solution both play important roles in the production of MA‐free quasi‐2D ACI perovskite films. The n value of the MA‐free quasi‐2D ACI perovskites can be adjusted, which affects the photovoltaic performance and device stability. Compared with one‐step deposition, the MA‐free quasi‐2D ACI perovskites prepared via IPAS deposition have opposite reverse‐graded QW distribution and improved vertical orientation, leading to a remarkable PEC of up to 18.86% and allowing the preparation of unpackaged devices with prominent working stability (80%, ~400 h). The underlying mechanism and crystallization pathway of IPAS deposition confirm that sequential deposition has unique superiority in regulating the QW distribution and crystalline orientation of quasi‐2D perovskites.
image
... In RPP, A and ABX 3 with different dielectric constants are stacked alternately to form a quantum well-like structure [3]. In theory, this structure greatly reduces the non-radiative recombination of excitons and produces high photoluminescence quantum yields (PLQY) [4][5][6]. However, the chaotic phase in the thin film caused by varying values of n in A 2 A n−1 B n X 3n+1 samples leads to irregular quantum well widths, limiting further enhancement of its PLQY [7][8][9]. ...
The intrinsic chaotic phase distribution in Ruddlesden-Popper Perovskite (RPP) hinders its further improvement of photoluminescence (PL) emission and limits its application in optical devices. In this work, we achieve the phase distribution regulation of RPP by varying the composition ratio of organic bulky spacer cations 1-naphthylmethylamine (NMA) and phenylethyl-ammonium (PEA), which is controllable and nondestructive for structures of RPP. By suppressing the small n-phase, the PL intensity emission of RPP is further improved. Through the time-resolved PL (TRPL) measurements, we find the PL lifetime of the sample with 66% PEA concentration increases with the temperature initially and possesses the highest values of τ1 and τ2 at ~255 K, indicating the immediate state assisting exciton radiative recombination, and it can be modulated by phase manipulation in RPP. The immediate state may outcompete other non-radiative decay channels for excited carriers, leading to the PL enhancement in RPP, and broadening its further application.
... A dipole-dipole energy transfer has been seen with stacks of 2D perovskite material with varying quantum well thicknesses. 53 It was discovered in another study that the radiative energy transfer occurred more quickly than 50 ps within the heterostructures of two distinct species of the organicinorganic hybrid perovskite. 54 It is hypothesized that in these material systems, the organic insulating layers block interlayer charge transfer enables the energy transfer to happen with high efficiencies. ...
Two-dimensional (2D) van der Waals (vdW) heterostructured transition metal dichalcogenides (TMDs) open up new possibilities for a wide range of optoelectronic applications. Interlayer couplings are responsible for several fascinating physics phenomena, which are in addition to the multifunctionalities that have been discovered in the field of optoelectronics. These couplings can influence the overall charge, or the energy transfer processes via stacking, separation, and dielectric angles. This focused review article summarizes the most recent and promising strategies for interlayer exciton emission in 2D or integrated perovskites and TMD heterostructures. These types of devices require a thorough comprehension and effective control of interlayer couplings in order to realize their functionalities and improve performance, which is demonstrated in this article with the energy or charge transfer mechanisms in the individual devices. An ideal platform for examining the interlayer coupling and the related physical processes is provided by a summary of the recent research findings in 2D perovskites and TMDs. Furthermore, it would encourage more investigation into the comprehension and regulation of excitonic effects and the related optoelectronic applications in vdW heterostructures over a broad spectral response range. Finally, the current challenges and prospects are summarized in this paper.
... 58 Two-body recombination is assumed in this model because transient absorption experiments conducted on our solution-processed perovskite films revealed dominant quadratic dependence of recombination rates on the carrier densities. 54,55,59 Quadratic scaling is consistent with radiative and trap-assisted Auger recombination, which are both accounted for with the β parameter. 60 Trap-assisted band-to-band recombination scales linearly in the carrier density; 60,61 however, it is fundamentally a two-body process and may represent a significant contribution to β. ...
Conventional time-of-flight (TOF) measurements yield charge carrier mobilities in photovoltaic cells with time resolution limited by the RC time constant of the device, which is on the order of 0.1–1 µs for the systems targeted in the present work. We have recently developed an alternate TOF method, termed nonlinear photocurrent spectroscopy (NLPC), in which carrier drift velocities are determined with picosecond time resolution by applying a pair of laser pulses to a device with an experimentally controlled delay time. In this technique, carriers photoexcited by the first laser pulse are “probed” by way of recombination processes involving carriers associated with the second laser pulse. Here, we report NLPC measurements conducted with a simplified experimental apparatus in which synchronized 40 ps diode lasers enable delay times up to 100 µs at 5 kHz repetition rates. Carrier mobilities of ∼0.025 cm ² /V/s are determined for MAPbI 3 photovoltaic cells with active layer thicknesses of 240 and 460 nm using this instrument. Our experiments and model calculations suggest that the nonlinear response of the photocurrent weakens as the carrier densities photoexcited by the first laser pulse trap and broaden while traversing the active layer of a device. Based on this aspect of the signal generation mechanism, experiments conducted with co-propagating and counter-propagating laser beam geometries are leveraged to determine a 60 nm length scale of drift velocity dispersion in MAPbI 3 films. Contributions from localized states induced by thermal fluctuations are consistent with drift velocity dispersion on this length scale.
... Qing et al. proposed adding DMSO and MACl to the precursor solution to obtain singlecrystal-like films which had a small n value for vertical growth at the bottom and a large n value for the top growth, resulting in a self-driving charge-separated type-II band arrangement of (PEA) 2 (MA) n−1 Pb n I 3n+1 . [116] Type-I band arrangement scripts have also been explored in recent years; [117,118] however, we will not discuss these in detail. The types of band arrangement in the heterostructure of different 2DRP perovskites are being explored but have not been clearly determined as yet. ...
In recent years, two‐dimensional Ruddlesden‐Popper (2DRP) perovskite materials have been explored as emerging semiconductor materials in solar cells owing to their excellent stability and structural diversity. Although 2DRP perovskites have achieved photovoltaic efficiencies exceeding 19%, their widespread use is hindered by their inferior charge‐carrier transport properties in the presence of diverse organic spacer cations, compared to that of traditional 3D perovskites. Hence, a systematic understanding of the 2D perovskite's carrier transport mechanism is critical for the development of high‐performance 2D perovskite solar cells (PSCs). In this review, we summarize the recent advances in the carrier behavior of 2DRP PSCs and provide guidelines for successfully enhancing carrier transport. First, we discuss the composition and crystal structure of 2DRP perovskite materials that affect carrier transport. Then, we evaluate the features of 2DRP perovskite films (phase separation, grain orientation, crystallinity kinetics, etc.), which are closely related to carrier transport. Next, we reveal the principal direction of carrier transport guiding the selection of the transport layer. Finally, we propose an outlook and rationalize strategies for enhancing carrier transport in high‐performance PSCs. This article is protected by copyright. All rights reserved
... 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. ...
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.
... 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. ...
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.
... For stacks of the same species of 2D perovskite layers having different QW thicknesses, dipole−dipole ET in the time scale of hundreds of picoseconds has been reported. 27 Another study on the heterostructures of two different species of organic−inorganic hybrid perovskite (OIHP) reported radiative energy transfer faster than 50 ps. 28 In these materials systems, the insulating organic layers are believed to prevent interlayer charge transfer (CT), allowing ET to occur with high efficiencies. ...
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.
In the past decade, metal halide perovskite polycrystalline films have witnessed significant advancements in the field of high‐performance optoelectronic devices, including photodetectors, solar cells, light‐emitting diodes, and lasers. Perovskite films with periodic micro/nanoarrays have garnered substantial attention due to their capability not only to improve the efficiency of individual devices but also to hold great promise for future commercialization. Surpassing their polycrystalline counterparts, perovskite single crystals typically exhibit longer carrier diffusion lengths, extended carrier lifetimes, and enhanced carrier mobility due to the absence of grain boundaries and reduced defects, positioning them as promising candidates for both fundamental studies and advanced optoelectronic devices. To this end, significant endeavors have been dedicated to the development of diverse methodologies for synthesizing large‐scale perovskite single crystals, including bulk single crystals and single‐crystal thin films. Furthermore, aiming to integrate the distinctive functionality with single crystals, considerable efforts have been directed toward the design of certain patterns on single‐crystal surfaces. Herein, this review presents recent progress in technologies for the preparation of large‐scale single crystals and the approaches to patterning their surfaces, highlights the unique advantages of each method, and presents their promising advances in various optoelectronic applications as well as the potential challenges.
It is generally difficult to achieve high quality in low-n-value (MA-free) FA-based Dion-Jacobson (DJ) 2D perovskites due to their low crystallinity, random well-width distribution of multiple quantum wells (QWs) with disordered orientation, which has seriously hindered further advances in photovoltaics. Meanwhile, pure-phase DJ 2D perovskites with uniform QWs width and fixed low-n values are highly desirable for high efficiency and stability. Herein we first present a novel 4APP(FA)3Pb4I13-based quasi-DJ-2D perovskite, where 4APP is 4-aminopiperidinium. The remarkable rigidity of 4APP and its strong hydrogen bonding with the perovskites prolong the perovskite intrinsic stability. We further establish the tri-solvent engineering approach by matching solvent properties to achieve pure-phase DJ-2D perovskites with uniform QWs width structure and finely-oriented crystal grains, which enhance the power conversion efficiency of unencapsulated solar cells by 48%, representing one of the highest values in MA-free DJ 2D perovskite with n≤4. Remarkably, the unencapsulated devices retain 95% of their initial PCE after 2000 hr of operation under 1-sun illumination at 40 °C, and 3000 hr exposure in the air at 85 °C and 60-90% relative humidity (RH). The simultaneous control of rigidity and phase purity in DJ-2D perovskites will open up new directions for fundamental research and application exploration.
Conventional time-of-flight methods can be used to determine carrier mobilities for photovoltaic cells in which the transit time between electrodes is greater than the RC time constant of the device. To measure carrier drift on sub-ns timescales, we have recently developed a two-pulse time-of-flight technique capable of detecting drift velocities with 100-ps time resolution in perovskite materials. In this method, the rates of carrier transit across the active layer of a device are determined by varying the delay time between laser pulses and measuring the magnitude of the recombination-induced nonlinearity in the photocurrent. Here, we present a related experimental approach in which diffractive optic-based transient grating spectroscopy is combined with our two-pulse time-of-flight technique to simultaneously probe drift and diffusion in orthogonal directions within the active layer of a photovoltaic cell. Carrier density gratings are generated using two time-coincident pulse-pairs with passively stabilized phases. Relaxation of the grating amplitude associated with the first pulse-pair is detected by varying the delay and phase of the density grating corresponding to the second pulse-pair. The ability of the technique to reveal carrier diffusion is demonstrated with model calculations and experiments conducted using MAPbI3 photovoltaic cells.
Producing efficient blue and deep blue perovskite LEDs (PeLEDs) still represents a significant challenge in optoelectronics. Blue PeLEDs still have problems relating to color, luminance, and structural and electrical stability so new materials are needed to achieve better performance. Recent reports suggest using low n states (n = 1, 2, 3) to achieve blue electroluminescence in Ruddlesden–Popper (RP) perovskite films. However, there are fewer reports on the other quasi‐2D structure, Dion–Jacobson (DJ) perovksites, despite their highly desirable optical properties, due to the difficulty in achieving charge injection. To resolve this issue, herein, w e have mixed DJ phase precursors, propane‐1,3‐diammonium (PDA) bromide into RP phase perovskites and fabricated low‐dimensional PeLEDs. It is found that these specific precursors aid in suppressing both the low n (n = 1) and high n (n ≥ 4) quasi‐2D RP phases and is an effective strategy in blue‐shifting sky‐blue RP perovskites into the sub‐470 nm region. With optimization of the PDA concentration and device layers, it is achieved an external quantum efficiency of 1.5% at 469 nm and stable electroluminescence for the first deep blue PeLED to be reported using DJ perovskites.
Electroluminescence efficiencies of deep-blue quasi-two-dimensional (2D) perovskite are limited by a lack of post-treatment strategies that can both construct an ideal energy-transfer tunnel structure minimizing the exciton losses and passivate...
Quasi two-dimensional (2D) (C(NH2)3)(CH3NH3)nPbnI3n+1 (nominally, n = 4) perovskite solar cell with alternating cations in the interlayer space (ACI) is fabricated using methylammonium chloride (MACl) or guanidinium chloride (GACl) as an additive in precursor solution. With an appropriate molar ratio of MACl or GACl, the 2D perovskite films show better crystallinity, smaller defect density and higher carrier mobility, resulting in more than 30% enhancement of power conversion efficiency (PCE) compared to that of without additives. Particularly, an optimal concentration of GACl eventually leads to the best PCE of 14.3% here, and importantly, eliminating current density-voltage (J-V) curves hysteresis which is prominent in devices from precursor solutions with various concentration of MACl. Thus, GACl is demonstrated firstly as a better additive compared to commonly used MACl in synthesizing ACI perovskite, providing a possible constructive route to fabricate highly efficient and J-V hysteresis-free solar cells.
Two‐dimensional Ruddlesden‐Popper perovskites have risen to prominence as stable and efficient photovoltaic materials because of their structural diversity, rich photophysics and low moisture ingression. However, thin films processed from stoichiometric precursor solutions possess a broad phase distribution of different number of inorganic layers with random crystal orientation, crippling device performance. We investigate the effect of methylammonium chloride (MACl) and 3‐Amino 4‐Phenol Sulphonic Acid (APSA) on the fabrication of perpendicularly oriented (PEA)2MA4Pb5I16 films with narrow phase distribution using anti‐solvent and hot‐casting processing techniques. MACl plays a critical role in suppressing parasitic n ≤ 2 and 3D‐like phases. APSA performs the dual function of trap passivation and further narrowing phase polydispersity through strong coordination with Pb2+. Ex‐situ GIWAXS and ultrafast spectroscopic characterisation reveal uniformly mixed‐phase distribution with disordered orientation in anti‐solvent treated films, while additive‐assisted hot‐casting treatment results in oriented, reverse‐graded phase distribution, i.e., small‐n on the film surface and large‐n at the bottom. Arising thin films enable efficient p‐i‐n solar cells with an efficiency of 14.34%, and a Voc of 1.20 V, retaining 96% initial efficiency after 1440 h under ambient conditions (RH = 50‐60%) without encapsulation. This article is protected by copyright. All rights reserved.
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.
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.
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.
While the constraints on the choice of organic cations are greatly relaxed for layered two-dimensional perovskites compared to three-dimensional perovskites, the shape of the spacer cation is still subject to limitations due to the size of the inorganic pocket between four adjacent corner-sharing octahedra. To investigate the effect of the spacer cation branching on the formation of Ruddlesden-Popper (RP) structures, we performed a comprehensive investigation of structures formed using tert-butyl ammonium (t-BA). We demonstrate that in contrast to pure bromides and pure iodides, the use of mixed halides enables the formation of the t-BA2PbBr2I2 RP perovskite structure with the specific ordering of the bromide and iodide anions. The t-BA spacer, despite its branched and bulky shape that prevents its deeper penetration, is able to form significant H-bonds that lead to the stabilization of the RP assembly if the inorganic pocket is designed in such a way that the bromide anions occupy terminal axial positions, while the iodides occupy equatorial positions. We obtain excellent agreement between experimentally determined and theoretically predicted structures using global optimization via a minima hopping algorithm for layered perovskites, illustrating the ability to predict the structure of RP perovskites and to manipulate the perovskite structure by the rational design of the inorganic pocket.
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.
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.
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...
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
We report on the first observation of the one-exciton to two-exciton transition in J aggregates. A theoretical analysis supports our interpretation.
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.
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.
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.
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.
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.
How perovskites have the edge
Two-dimensional Ruddlesden-Popper perovskites form quantum wells by sandwiching inorganic-organic perovskite layers used in photovoltaic devices between organic layers. Blancon et al. show that if the perovskite layer is more than two unit cells thick, photogenerated excitons undergo an unusual but highly efficient process for creating free carriers that can be harvested in photovoltaic devices (see the Perspective by Bakr and Mohammed). Lower-energy local states at the edges of the perovskite layer facilitate dissociation into electrons and holes that are well protected from recombination.
Science , this issue p. 1288 ; see also p. 1260
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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)
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.