Article

Controlled Growth of CH3NH3PbI3 Using a Dynamically Dispensed Spin-Coating Method: Improving Efficiency with a Reproducible PbI2 Blocking Layer

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Abstract

It is commonly believed that excess PbI2 has beneficial effects for perovskite solar cells due to the modification of charge transport behavior at interfaces, by surface passivation and by blocking electron-hole recombination. Here, we introduce a dynamically dispensed spin coating technique in a two-step deposition to form a perovskite layer with controllable quantities of crystalline PbI2. Using this technique, the concentration of CH3NH3I solution is kept constant at the reaction interface, ensuring smooth growth of films. By changing the spinning rate during the reaction, the PbI2 conversion ratio and perovskite cuboid size can be optimized, resulting in a power conversion efficiency improvement over control devices. This dynamically dispensed technique represents a repeatable method for compositional control in perovskite solar cells and improves our understanding of how a PbI2 blocking layer improves the performance of perovskite solar cells.

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... The cleaned substrates are treated with UV-Ozone for 25 min to enhance the wettability of the substrates. The obtained suspension of AgNWs in ethanol is used to prepare the AgNW network layers on glass substrates using a dynamic dispensed spin-coating method [53]. For this, 100 µL of AgNW suspension is dropped onto a clean and UV-Ozone-treated glass substrate while spinning the substrate at 1000 rpm ( Figure 2a). ...
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... [5][6][7][8][9][10] Among them, metal halide perovskite nanocrystals, such as CsPbX 3 , MAPbX 3 , FAPbX 3 (FA = formamidinium, MA = methylammonium, X = Cl, Br, I), have emerged as a new class of most promising candidates for optoelectronic applications. 9,[11][12][13][14][15] In particular, due to the pure inorganic structure of CsPbX 3 nanocrystals, high thermal stability can be achieved alongside their characteristic advantages of near unity PLQY and low processing cost. 12,[16][17][18] Therefore, research into other possible inorganic perovskite structures, such as Cs 4 PbBr 6 and Rb y Cs 1−y PbBr 3 , has intensified. ...
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... In addition to the commonly used fluorescence spectrum technique [92,93,95,99], light-modulated scanning tunneling microscopy (LMSTM) enables spatially resolved mapping of the photoinduced interfacial band bending of valence and conduction bands, and of the photo-generated electron and hole carriers at the hetero-interfaces of perovskite crystal grains. Shih et al. explored the interfacial electronic structures of individual perovskite grains, and directly observed enhanced charge separation and reduced back recombination when interfacial PbI 2 passivation layers were applied to the perovskite crystal grains [100]. ...
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Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3) have emerged as a revolutionary class of light-absorbing semiconductors that has demonstrated a rapid increase in efficiency within a few years of active research. Controlling perovskite morphology and composition has been found critical to developing high-performance perovskite solar cells. The recent development of solution chemistry engineering has led to fabrication of greater than 15–17%-efficiency solar cells by multiple groups, with the highest certified 17.9% efficiency that has significantly surpassed the best-reported perovskite solar cell by vapor-phase growth. In this Perspective, we review recent progress on solution chemistry engineering processes and various control parameters that are critical to the success of solution growth of high-quality perovskite films. We discuss the importance of understanding the impact of solution-processing parameters and perovskite film architectures on the fundamental charge carrier dynamics in perovskite solar cells. The cost and stability issues of perovskite solar cells will also be discussed.
Article
The band gap of formamidinium lead iodide (FAPbI3) perovskites allows broader absorption of the solar spectrum compared to conventional methylammonium lead iodide (MAPbI3). The optoelectronic properties of perovskite films are closely related to the film-quality, so depositing dense and uniform films is crucial for fabricating high-performance perovskite solar cells (PSCs). We report an approach for depositing high-quality FAPbI3 films, involving FAPbI3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide. This process produces FAPbI3 films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using films prepared by this technique, FAPbI3-based PSCs with maximum power conversion efficiency of over 20% were fabricated. Copyright © 2015, American Association for the Advancement of Science.
Article
The past two years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic-inorganic halide perovskite. Grain boundaries are present in polycrystalline thin film solar cell and they play an important role which could be benign or detrimental to solar cell performance. Here we present efficient charge separation and collection at the grain boundaries measured by KPFM and c-AFM in CH3NH3PbI3 film in a CH3NH3PbI3/TiO2/FTO/glass hetero-junction structure. We observe the presence of a potential barrier along the grain boundaries in dark condition and higher photovoltage along the grain boundaries compare to grain interior under the illumination. Also, c-AFM measurement presents higher short-circuit current collection near grain boundaries confirming the beneficial roles grain boundaries play in collecting carriers efficiently.
Article
Of the many materials and methodologies aimed at producing low-cost, efficient photovoltaic cells, inorganic-organic lead halide perovskite materials appear particularly promising for next-generation solar devices owing to their high power conversion efficiency. The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium lead halide materials. Here we combine the promising-owing to its comparatively narrow bandgap-but relatively unstable formamidinium lead iodide (FAPbI3) with methylammonium lead bromide (MAPbBr3) as the light-harvesting unit in a bilayer solar-cell architecture. We investigated phase stability, morphology of the perovskite layer, hysteresis in current-voltage characteristics, and overall performance as a function of chemical composition. Our results show that incorporation of MAPbBr3 into FAPbI3 stabilizes the perovskite phase of FAPbI3 and improves the power conversion efficiency of the solar cell to more than 18 per cent under a standard illumination of 100 milliwatts per square centimetre. These findings further emphasize the versatility and performance potential of inorganic-organic lead halide perovskite materials for photovoltaic applications.
Article
Material crystallinity is paramount in determining the electronic properties in both organic and inorganic electronic materials and the performance of electronic devices. [1-4]Thermal annealing is the most broadly applied technique to increase the crystallinity of materials, especially thin films, because of its simplicity. Complementary to thermal-annealing, solventannealing, where solvent vapor is introduced during the crystallization of the bulk or thin film materials, [2,3]has been found to be an effective method to increase the crystallinity of some very specifi c organic semiconductors; however, it has never been demonstrated in inorganic semiconductors. Here we show that solvent-annealing can be applied to a new family of organic- inorganic hybrid materials-organometaltrihalide perovskites -to increase the crystallinity and grain size.
Article
CH3NH3PbI3 perovskite layered films deposited on substrates with and without a titania support structure have been prepared and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450-800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. In addition, for the first time we could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red-shifted at ∼780 nm. We show that an additional bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to estimate relative amounts of PbI2 in the samples. Kinetic analysis reveals that perovskite films with less PbI2 show faster relaxation rates than those containing more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples containing PbI2 are due to a passivation effect, in line with other recently reported work.
Article
Thin-film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two-step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one-step, solvent-induced, fast crystallization method involving spin-coating of a DMF solution of CH3NH3PbI3 followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution-processed thin films yielded an average power conversion efficiency of 13.9±0.7 % and a steady state efficiency of 13 % under standard AM 1.5 conditions.
Article
Modulated charge separation across (MO)/CH3NH3PbI3 and (MO)/PbI2/CH3NH3PbI3 (MO = TiO2, MoO3) interfaces was investigated by surface photovoltage (SPV) spectroscopy. Perovskite layers were deposited by solution-based one-step preparation and two-step preparation methods. An unreacted PbI2 layer remained at the interface between the metal oxide and CH3NH3PbI3 for two-step preparation. For the two-step preparation on TiO2, the SPV signal related to absorption in CH3NH3PbI3 increased in comparison to the one-step preparation due to electron transfer from CH3NH3PbI3 via PbI2 into TiO2 whereas the SPV signal related to defect transitions decreased. For the one-step preparation on MoO3, holes photogenerated in CH3NH3PbI3 recombined with electrons in MoO3. In contrast, a hole transfer from CH3NH3PbI3 towards MoO3 was blocked by the PbI2 interlayer for the two-step preparation on MoO3. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
Article
Organolead trihalide perovskite materials have been successfully used as light absorbers in efficient photovoltaic cells. Two different cell structures, based on mesoscopic metal oxides and planar heterojunctions have already demonstrated very impressive advances in performance. Here, we report a bilayer architecture comprising the key features of mesoscopic and planar structures obtained by a fully solution-based process. We used CH3NH3 Pb(I1 - xBrx)3 (x = 0.1-0.15) as the absorbing layer and poly(triarylamine) as a hole-transporting material. The use of a mixed solvent of γ-butyrolactone and dimethylsulphoxide (DMSO) followed by toluene drop-casting leads to extremely uniform and dense perovskite layers via a CH3NH3I-PbI2-DMSO intermediate phase, and enables the fabrication of remarkably improved solar cells with a certified power-conversion efficiency of 16.2% and no hysteresis. These results provide important progress towards the understanding of the role of solution-processing in the realization of low-cost and highly efficient perovskite solar cells.
Article
To improve the performance of the polycrystalline thin film devices, it requires a delicate control of its grain structures. As one of the most promising candidates among current thin film photovoltaic techniques, the organic/inorganic hybrid perovskites generally inherit polycrystalline nature, and exhibit compositional/structural dependence in regard to their optoelectronic properties. Here, we demonstrate a controllable passivation technique for perovskite films, which enables their compositional change, and allows substantial enhancement in corresponding device performance. By releasing the organic species during annealing, PbI2 phase is presented in perovskite grain boundaries and at the relevant interfaces. The consequent passivation effects and underlying mechanisms are investigated with complementary characterizations, including SEM, XRD, TRPL, SKPM and UPS. This controllable self-induced passivation technique represents an important step to understand the polycrystalline nature of hybrids perovskites thin films, and contributes to the development of perovskite solar cells judiciously.
Article
Organic-inorganic hybrid solar cells that combine a mesoporous scaffold, a perovskite light absorber and an organic hole transporter have emerged at the forefront of solution-processable photovoltaic devices; however, they require processing temperatures of up to 500 °C to sinter the mesoporous metal-oxide support. Here, we report the use of a thin film of ZnO nanoparticles as an electron-transport layer in CH3NH3PbI3-based solar cells; in contrast to mesoporous TiO2, the ZnO layer is both substantially thinner and requires no sintering. We took advantage of these facts to prepare flexible solar cells with power-conversion efficiencies in excess of 10%. The use of ZnO also results in improvements to device performance for cells prepared on rigid substrates. Solar cells based on this design exhibit power-conversion efficiencies as high as 15.7% when measured under AM1.5G illumination, which makes them some of the highest-performing perovskite solar cells reported to date.
Article
Hybrid organic-inorganic lead halide perovskite APbX3 pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid-state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite-based photovoltaics is to extend their optical-absorption onset further into the red to enhance solar-light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN=CHNH3 (+) , FA) and methylammonium (CH3 NH3 (+) , MA) cations in the A position of the APbI3 perovskite structure. This combination leads to an enhanced short-circuit current and thus superior devices to those based on only CH3 NH3 (+) . This concept has not been applied previously in perovskite-based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials.
Article
Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.
Article
A plethora of solution processed materials have been developed for solar cell applications. Hybrid solar cells based on light absorbing semiconducting polymers infiltrated into mesoporous TiO2 are an interesting concept, but generating charge at the polymer:metal oxide heterojunction is challenging. Metal-organic Perovskite absorbers have recently shown remarkable efficiencies but currently lack the range of color tunability of organics. Here, we have combined a fullerene self-assembled monolayer (C60SAM) functionalised mesoporous titania, a perovskite absorber (CH3NH3PbI3-xClx) and a light absorbing polymer hole-conductor, P3HT, to realise a 6.7% hybrid solar cell. We find that photoexcitations in both the perovskite and the polymer undergo very efficient electron transfer to the C60SAM. The C60SAM acts as an electron acceptor, but inhibits further electron transfer into the TiO2 mesostructure due to energy level misalignment and poor electronic coupling. Thermalized electrons from the C60SAM are then transported through the perovskite phase. This strategy allows a reduction of energy loss, whilst still employing a "mesoporous electron acceptor", representing an exciting and versatile route forward for hybrid photovoltaics incorporating light absorbing polymers. Finally, we show that we can use the C60SAM functionalization of mesoporous TiO2 to achieve an 11.7% perovskite-sensitized solar cell using Spiro-OMeTAD as a transparent hole transporter.
Article
We use transient photovoltage and differential charging experiments, complemented by transient absorption data, to determine charge carrier lifetimes and densities in a poly(3-hexylthiophene): methanofullerene solar cell at Voc as a function of white light-bias intensity. For a typical device, the charge carrier decay dynamics are observed to exhibit an approximately third order dependence on charge density (dn/dt∝n3).
Article
The spin coating of thin (> 200 nm thick) and ultrathin (< 200 nm thick) polymer films is examined in several solvents of varying volatility over a broad range of polymer solution concentrations and spin speeds. Experimentally measured film thicknesses are compared with a simple model proposed by Bornside, Macosko, and Scriven, which predicts film thickness based on the initial properties of the polymer solution, solvent, and spin speed. This model is found to predict film thickness values within 10% over the entire range of conditions explored, which gave film thicknesses from 10 nm to 33 μ:m. The model underpredicts film thickness for cases in which a very volatile solvent is used or the initial concentration of polymer is high, while overpredicting film thickness for cases in which a low volatility solvent is used or the initial polymer concentration is very low. These deviations are a consequence of how the model decouples fluid flow and solvent evaporation.
Article
A new dynamic dispersing technique (DDT) is proposed and demonstrated to improve the photoresist (PR) coating process in planar lightwave circuit (PLC) fabrication. In this technique, the PR is dispensed during the wafer's spin cycle on the spin coater instead of the conventional static dispensing before spinning. In comparison to the conventional static dispersing technique (SDT), DDT has significantly improved several properties of the PR layer including its thinning behavior, coating uniformity, and PR amount. Using DDT, the PR thickness averagely reduces by ∼0.2 μm. The radial uniformity of the PR from the centre is also improved from the average difference of 0.28 μm in SDT to 0.14 μm in DDT. Furthermore, the PR amount required for the coating process is reduced from 3.00 ml (SDT) to 0.25 ml (DDT). Two different DDTs are also compared in this paper including the application of PR during the spin coater's acceleration step (1st DDT) and the application of PR during the spin coater's constant speed (2nd DDT). © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1993–1995, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22609
Article
Two organolead halide perovskite nanocrystals, CH(3)NH(3)PbBr(3) and CH(3)NH(3)PbI(3), were found to efficiently sensitize TiO(2) for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO(2) films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH(3)NH(3)PbI(3)-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH(3)NH(3)PbBr(3)-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.
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