High-quality Perovskite Thin Films Research Articles

High stability planar perovskite solar cells with inorganic charge transport layers

Perovskite solar cells based on methylammonium lead halide perovskites (abbreviated as perovskites) maintain great promise for low-cost solar-to-electricity energy conversion, due to the excellent photoelectric properties of perovskites, abundance of raw materials to synthesize perovskites, and the capability to deposit perovskites by a solution process. Significant effort has been taken to deposit high-quality perovskite thin films and, thereafter, to fabricate highly efficient perovskite solar cells. Nevertheless, the degradation of perovskite solar cells greatly limits their chance for mass production and deployment. Here, we present a device structure that can significantly improve the stability of perovskite solar cells using all inorganic charge transport layers. Specifically, solution-processed inorganic thin films, lithium (Li)-doped NiOx (Li : NiOx), and ZnO nanoparticles are used as a hole transport layer and an electron transport layer, respectively. The mechanisms behind the enhanced stability are analyzed. Solutions for further improvement in the stability and the performance of perovskite solar cells are suggested.

Preparation of CH3NH3PbI3 thin films for solar cells via Vapor Transfer Method

Organometal halide perovskite based solar cells have emerged as one of the most promising candidates for low-cost and high-efficiency solar cell technologies. Here a Vapor Transfer Method (VTM) is used to fabricate high quality perovskite thin films in a balanced vacuum capsule. By adjusting the reaction temperature, CH3NH3I saturated vapor which then reacts with PbI2 films can be controlled and the formation process of CH3NH3PbI3 perovskite films can be further influenced. Prepared perovskite films which exhibit pure phase, smooth surface and high crystallinity are assembled into planar heterojunction inverted solar cells. The whole fabrication process of solar cell devices is organic solution free. Finally, the champion cell achieved power conversion efficiency (PCE) of 13.08% with negligible current–voltage hysteresis under fully open-air conditions. The photovoltaic performance could be further enhanced by optimizing perovskite composition and the device structure.

A Self-Powered and Flexible Organometallic Halide Perovskite Photodetector with Very High Detectivity.

Flexible and self-powered photodetectors (PDs) are highly desirable for applications in image sensing, smart building, and optical communications. In this paper, a self-powered and flexible PD based on the methylammonium lead iodide (CH3 NH3 PBI3 ) perovskite is demonstrated. Such a self-powered PD can operate even with irregular motion such as human finger tapping, which enables it to work without a bulky external power source. In addition, with high-quality CH3 NH3 PBI3 perovskite thin film fabricated with solvent engineering, the PD exhibits an impressive detectivity of 1.22 × 1013 Jones. In the self-powered voltage detection mode, it achieves a large responsivity of up to 79.4 V mW-1 cm-2 and a voltage response of up to ≈90%. Moreover, as the PD is made of flexible and transparent polymer films, it can operate under bending and functions at 360 ° of illumination. As a result, the self-powered, flexible, 360 ° omnidirectional perovskite PD, featuring high detectivity and responsivity along with real-world sensing capability, suggests a new direction for next-generation optical communications, sensing, and imaging applications.

Vapor-Assisted Solution Approach for High-Quality Perovskite CH3NH3PbBr3 Thin Films for High-Performance Green Light-Emitting Diode Applications.

The vapor-assisted solution method was developed to prepare high-quality organic-inorganic halide perovskite CH3NH3PbBr3 (MAPbBr3) thin films. We detailedly investigated the effect of evaporation time and temperature of MABr powder on the microstructure, crystallinity, and optical characterizations of MAPbBr3 thin films, and a controllable morphology evolution with varying surface coverage was observed. Temperature-dependent and time-resolved photoluminescence measurements were carried out to investigate the optical transition mechanisms and carrier recombination dynamics of MAPbBr3 thin films. Our results revealed that no structural phase transition occurred within the heating process (10-300 K). In addition to the exciton-related emission, a trapped charge-carrier emission appeared at a critical temperature of 140 K. The corresponding temperature sensitivity coefficient of band gap, exciton binding energy, and optical phonon energy of the MAPbBr3 thin films were extracted from the experimental data. Furthermore, planar perovskite light-emitting diodes (PeLEDs) based on a Al/LiF/TPBi/MAPbBr3/NiO/ITO structure were fabricated, and a high-purity green emission at ∼532 nm with a low line width (25 nm) was achieved. The devices demonstrated remarkable performances with high luminance (6530 cd/m2), current efficiency (8.16 cd/A), external quantum efficiency (4.36%), and power efficiency (4.49 lm/W). This research will provide valuable information for the preparation of high-quality perovskite thin films, facilitating their future applications in novel high-performance PeLEDs.

High-Performance Flexible Photodetectors based on High-Quality Perovskite Thin Films by a Vapor-Solution Method.

Organometal halide perovskites are new light-harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low-temperature process capability. However, the preparation of high-quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor-solution method is developed to achieve uniform and pinhole-free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as-prepared high-quality perovskite thin films, high-performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W-1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin-film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high-quality perovskite thin films opens up a new avenue to develop high-performance flexible optoelectronic devices.

Solvent-induced crystallization for hybrid perovskite thin-film photodetector with high-performance and low working voltage

Organometal trihalide perovskites have emerged as a class of solution-processed semiconductors exhibiting remarkable optoelectronic properties. Using a high-quality perovskite thin film prepared by solvent-induced crystallization method and adopting a novel device configuration based on photon recycling effect, a perovskite thin-film photodetector has been constructed with the highest external quantum efficiency of 4.1 × 104% and responsivity of 219 A W−1 at a low bias of 1 V so far. The device working mechanism was further disclosed based on energy band bending model. The high-performance and low working-voltage perovskite thin-film photodetector will find potential applications in photodetection and optoelectronic integrated circuits.

Improvement of CH3NH3PbI3 thin film using the additive 1,8-diiodooctane for planar heterojunction perovskite cells

The thin-film quality is critical for obtaining high-performance perovskite solar cells (PSCs). The additive 1,8-diiodooctane (DIO) was used to control the morphology and structure of CH3NH3PbI3 perovskite thin films, and planar heterojunction (PHJ) PSCs with an architecture of ITO/PEDOT: PSS/CH3NH3PbI3/PCBM/Al was fabricated. It was found that the DIO played an important role on CH3NH3PbI3 thin-film quality and the performance of PHJ-PSCs. With the optimal volume ratio of 2%, the compact and uniform high-quality CH3NH3PbI3 thin films with enhanced crystallinity and less roughness were achieved, resulting in the great improvement of power conversion efficiency (PCE) from about 4.5% to over 9.0%. The research results indicate that the additive DIO is a simple and effective method to produce high-quality perovskite thin film and accordingly develop high-performance PHJ-PSCs.

Low-Temperature Soft-Cover Deposition of Uniform Large-Scale Perovskite Films for High-Performance Solar Cells.

Large-scale high-quality perovskite thin films are crucial to produce high-performance perovskite solar cells. However, for perovskite films fabricated by solvent-rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low-temperature soft-cover deposition (LT-SCD) method is presented, where the thermal convection-induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection-induced-defects-free perovskite films are obtained on an area of 12 cm2 , which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm2 . This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low-temperature processing. Hence, the present LT-SCD technology provides a new non-spin-coating route to the deposition of large-area uniform perovskite films for both rigid and flexible perovskite devices.

Large grained and high charge carrier lifetime CH3NH3PbI3 thin-films: implications for perovskite solar cells

Spin coated perovskite thin films are known to have an issue of pinholes & poor morphology control which lead to poor device-to-device repeatability, that is an impediment to scale-up. In this work, Methylamine vapor annealing process is demonstrated which consistently leads to high-quality perovskite thin-films with an average grain-size of 10–15 μm. The improvement in film morphology enables improvement in effective carrier recombination lifetime, from 21 μs in as-deposited films to 54 μs in vapor-annealed films. The annealed films with large-grains are also more stable in ambient conditions. Devices made on annealed perovskite films are very consistent, with a standard deviation of only 0.7%. Methylamine vapor annealing process is a promising method of depositing large-grain CH3NH3PbI3 films with high recombination lifetime and the devices with improved performance.

Air-Induced High-Quality CH3NH3PbI3 Thin Film for Efficient Planar Heterojunction Perovskite Solar Cells

Efficient planar heterojunction perovskite solar cells (PHJ–PSCs) with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al were fabricated using air-induced high-quality CH3NH3PbI3 perovskite thin films, in which the air-inducing process was controlled with a humidity of ∼40%. The air exposure of CH3NH3PbI3 thin films could dramatically improve their properties with large grains and smooth surface, as well as uniform morphology, resulting in an impressive enhancement in carrier lifetime. The ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy results proved that the CH3NH3PbI3 film was n-doped by the absorption of H2O on the surface but was very stable without obvious degradation for 10 days’ exposure in air. The power conversion efficiency (PCE) of PHJ–PSCs with an air exposure process showed a significant increase up to 16.21% as compared to reference PHJ–PSCs with a PCE of 12.02%. The research work demonstrated that an air-exposure process with a suitable humidity could produce high...

Efficient planar heterojunction perovskite solar cells fabricated by in-situ thermal-annealing doctor blading in ambient condition

The in-situ thermal-annealing doctor blading was developed to fabricate high-quality perovskite CH3NH3PbI3 thin film and efficient planar heterojunction perovskite solar cells (PHJ-PSCs) in ambient condition with humidity of ∼45%. The morphology of CH3NH3PbI3 thin film fabricated by in-situ thermal-annealing doctor blading varied from random nanowires to oriented domains as increasing the substrate temperature, and the domain size became larger and larger with increasing substrate temperature. The PHJ-PSCs with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Ag was fabricated based on in-situ thermal-annealing doctor-bladed CH3NH3PbI3 thin film in ambient condition, resulting in the power conversion efficiency up to 11.29% without obvious hysteresis under different scanning directions and speeds. The performance is comparable to that of PHJ-PSCs fabricated by spin-coating deposition in glovebox with the same structure. The research results suggested that efficient PHJ-PSCs could be fabricated by large-scale in-situ thermal-annealing doctor blading in ambient condition, which is matchable with large-scale, roll-to-roll process and shows potential application in industrial production.

Dual-Source Precursor Approach for Highly Efficient Inverted Planar Heterojunction Perovskite Solar Cells.

The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium and formamidinium mixed cations. Currently, high-quality mixed-cation perovskite thin films are normally made by use of antisolvent protocols. However, the widely used "antisolvent"-assisted fabrication route suffers from challenges such as poor device reproducibility, toxic and hazardous organic solvent, and incompatibility with scalable fabrication process. Here, a simple dual-source precursor approach is developed to fabricate high-quality and mirror-like mixed-cation perovskite thin films without involving additional antisolvent process. By integrating the perovskite films into the planar heterojunction solar cells, a power conversion efficiency of 20.15% is achieved with negligible current density-voltage hysteresis. A stabilized power output approaching 20% is obtained at the maximum power point. These results shed light on fabricating highly efficient perovskite solar cells via a simple process, and pave the way for solar cell fabrication via scalable methods in the near future.

Improved photovoltaic performance from high quality perovskite thin film grown with the assistance of PC71BM

The strategy of sequentially spin-coating a perovskite film from the perovskite precursor and an electron transporting layer of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is developed to simplify the fabrication procedure of perovskite solar cells. X-ray diffraction and scanning electron microscopy indicate that PC71BM film on perovskite layer can retard the evaporation of dimethyl sulfoxide (DMSO) efficiently, thus prolonging the transformation of intermediate phase to perovskite crystals, leading to a high quality perovskite thin film. The solar cells with the structure of indium tin oxides (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/CH3NH3PbI3/PC71BM/bathocuproine (BCP)/Ag made from this simplified method exhibit a higher efficiency (12.68%) than those from the conventional one-step method (9.49%).

High-quality CH3NH3PbI3 thin film fabricated via intramolecular exchange for efficient planar heterojunction perovskite solar cells

The high-quality CH3NH3PbI3 perovskite thin film with excellent coverage and uniformity was prepared using an intramolecular exchange technology via a low-temperature, two-step sequential deposition process. The PbI2(DMSO) complex was synthesized at room temperature without any additives and was deposited, then the CH3NH3I solution was deposited subsequently. The further controllable thermal annealing process resulted in the complete formation of flat and uniform CH3NH3PbI3 thin film with large-size grains and (110) preferred crystallographic orientation. The perovskite solar cells (PSCs) with a very simple inverted planar heterojunction structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al and without other buffer layers, e.g., C60, LiF, BCP, etc., were fabricated, resulting in a power conversion efficiency (PCE) as high as 14.26%. The results suggest that the low-temperature, two-step sequential deposition process with intramolecular exchange technology provides a good route to fabricate high-quality perovskite thin film and efficient PSCs, which would match with large-scale, high-output roll-to-roll (R2R) printing/coating techniques.

The Additive Coordination Effect on Hybrids Perovskite Crystallization and High-Performance Solar Cell.

The coordination effects of additives during perovskite crystal growth are investigated, and a novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.

Facile and Scalable Fabrication of Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells in Air Using Gas Pump Method.

Control of the perovskite film formation process to produce high-quality organic-inorganic metal halide perovskite thin films with uniform morphology, high surface coverage, and minimum pinholes is of great importance to highly efficient solar cells. Herein, we report on large-area light-absorbing perovskite films fabrication with a new facile and scalable gas pump method. By decreasing the total pressure in the evaporation environment, the gas pump method can significantly enhance the solvent evaporation rate by 8 times faster and thereby produce an extremely dense, uniform, and full-coverage perovskite thin film. The resulting planar perovskite solar cells can achieve an impressive power conversion efficiency up to 19.00% with an average efficiency of 17.38 ± 0.70% for 32 devices with an area of 5 × 2 mm, 13.91% for devices with a large area up to 1.13 cm(2). The perovskite films can be easily fabricated in air conditions with a relative humidity of 45-55%, which definitely has a promising prospect in industrial application of large-area perovskite solar panels.

Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement.

Here we demonstrate a radically different chemical route for the creation of HC(NH2)2PbI3 (FAPbI3) perovskite thin films. This approach entails a simple exposure of as-synthesized CH3NH3PbI3 (MAPbI3) perovskite thin films to HC(═NH)NH2 (formamidine or FA) gas at 150 °C, which leads to rapid displacement of the MA(+) cations by FA(+) cations in the perovskite structure. The resultant FAPbI3 perovskite thin films preserve the microstructural morphology of the original MAPbI3 thin films exceptionally well. Importantly, the myriad processing innovations that have led to the creation of high-quality MAPbI3 perovskite thin films are directly adaptable to FAPbI3 through this simple, rapid chemical-conversion route. Accordingly, we show that efficiencies of perovskite solar cells fabricated with FAPbI3 thin films created using this route can reach ∼18%.

Electrohydrodynamically Assisted Deposition of Efficient Perovskite Photovoltaics

Organic–inorganic perovskites that combine the strength of both chemical worlds have emerged as tantalizing candidates for next generation photovoltaics. Here, the electrohydrodynamically assisted continuous liquid interface propagation as a general, and potentially scalable nanomanufacturing route toward synthesizing high quality perovskite thin films in a rapid and high throughput fashion is reported. This strategy conceptually mimics the advantageous self‐organizing features of emulsion droplets where the use of a binary solvent system, concurrently and continuously, initiates a three‐stage process of coalescence, spreading, and merging, thus optimizing thin film morphology upon deposition without the needs for additional engineering steps. The resulting perovskite thin film not only exhibits a smooth topology with the root mean square roughness of only a few nm but also reveals hybrid morphology where micrometer‐sized grains intersperse between interconnected and continuous crystalline networks. This gives rise to the highest power conversion efficiency of 16.50% and average 14.68%; representing a nearly twofold increase compared to that of conventional spray‐pyrolysis approach. As a final critical aspect, the proposed strategy contributes new insights to efficiently managing the environmentally hazardous lead during processing, significantly reducing the amount by two orders of magnitude compared to that of spin‐coating to achieve the same thin film thickness.

Preparation of planar CH3NH3PbI3 thin films with controlled size using 1-ethyl-2-pyrrolidone as solvent

Recently, planar perovskite solar cells based on CH3NH3PbI3 have attracted many researcher's interest due to their unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction comparing to the initial mesoporous structure. However, the preparation of planar perovskite films with high quality is still in challenge. In this paper, we developed a vapor-assisted solution process using a novel and green solvent of 1-Ethyl-2-pyrrolidone (NEP) instead of the traditional N, N-dimethylformamide (DMF) to construct a high-quality perovskite CH3NH3PbI3 thin film with pure phase, high compactness, small surface roughness and controlled size. The phase evolution and growth mechanism of the perovskite films are also discussed. Utilizing the NEP of low volatility and moderate boiling point as solvent, we dried the PbI2-NEP precursor films at different temperature under vacuum and then obtained PbI2 thin films with different crystalline degree from amorphous to highly crystalline. The perovskite films with crystal size ranged from hundreds of nanometers to several micrometers can be prepared by reacting the PbI2 films of different crystalline degree with CH3NH3I vapor. Moreover, planar-structured solar cells combining the perovskite film with TiO2 and spiro-OMeTAD as the electron and holes transporting layer achieves a power conversion efficiency of 10.2%.

Making and Breaking of Lead Halide Perovskites.

A new front-runner has emerged in the field of next-generation photovoltaics. A unique class of materials, known as organic metal halide perovskites, bridges the gap between low-cost fabrication and exceptional device performance. These compounds can be processed at low temperature (typically in the range 80-150 °C) and readily self-assemble from the solution phase into high-quality semiconductor thin films. The low energetic barrier for crystal formation has mixed consequences. On one hand, it enables inexpensive processing and both optical and electronic tunability. The caveat, however, is that many as-formed lead halide perovskite thin films lack chemical and structural stability, undergoing rapid degradation in the presence of moisture or heat. To date, improvements in perovskite solar cell efficiency have resulted primarily from better control over thin film morphology, manipulation of the stoichiometry and chemistry of lead halide and alkylammonium halide precursors, and the choice of solvent treatment. Proper characterization and tuning of processing parameters can aid in rational optimization of perovskite devices. Likewise, gaining a comprehensive understanding of the degradation mechanism and identifying components of the perovskite structure that may be particularly susceptible to attack by moisture are vital to mitigate device degradation under operating conditions. This Account provides insight into the lifecycle of organic-inorganic lead halide perovskites, including (i) the nature of the precursor solution, (ii) formation of solid-state perovskite thin films and single crystals, and (iii) transformation of perovskites into hydrated phases upon exposure to moisture. In particular, spectroscopic and structural characterization techniques shed light on the thermally driven evolution of the perovskite structure. By tuning precursor stoichiometry and chemistry, and thus the lead halide charge-transfer complexes present in solution, crystallization kinetics can be tailored to yield improved thin film homogeneity. Because degradation of the as-formed perovskite film is in many ways analogous to its initial formation, the same suite of monitoring techniques reveals the moisture-induced transformation of low band gap methylammonium lead iodide (CH3NH3PbI3) to wide band gap hydrate compounds. The rate of degradation is increased upon exposure to light. Interestingly, the hydration process is reversible under certain conditions. This facile formation and subsequent chemical lability raises the question of whether CH3NH3PbI3 and its analogues are thermodynamically stable phases, thus posing a significant challenge to the development of transformative perovskite photovoltaics. Adequately addressing issues of structural and chemical stability under real-world operating conditions is paramount if perovskite solar cells are to make an impact beyond the benchtop. Expanding our fundamental knowledge of lead halide perovskite formation and degradation pathways can facilitate fabrication of stable, high-quality perovskite thin films for the next generation of photovoltaic and light emitting devices.