Search

An upscalable perovskite film deposition method combining raster ultrasonic spray coating and chemical vapor deposition is reported. This method overcomes the coating size limitation of the existing stationary spray, single-pass spray, and spin-coating methods. In contrast with the spin-coating method (>90% Pb waste), negligible Pb waste during PbI2 deposition makes this method more environmentally friendly. Outstanding film uniformity across the entire area of 5 cm x 5 cm is confirmed by both large-area compatible characterization methods (electroluminescence and scattered light imaging) and local characterization methods (atomic force microscopy, scanning electron microscopy, photoluminescence mapping, UV-vis, and X-ray diffraction measurements on multiple sample locations), resulting in low solar cell performance decrease upon increasing device area. With the FAPb(I0.85Br0.15)(3) (FA = formamidinium) perovskite layer deposited by this method, champion solar modules show a power conversion efficiency of 14.7% on an active area of 12.0 cm(2) and an outstanding shelf stability (only 3.6% relative power conversion efficiency decay after 3600 h aging). Under continuous operation (1 sun light illumination, maximum power point condition, dry N-2 atmosphere with <5% relative humidity, no encapsulation), the devices show high light-soaking stability corresponding to an average T-80 lifetime of 535 h on the small-area solar cells and 388 h on the solar module.

2,2 ',7,7 '-tetrakis(N,N-di-p-methoxyphenylamine)-9,9 '-spirobifluorene (spiro-MeOTAD) has been widely employed as a hole transport layer (HTL) in perovskite-based solar cells. Despite high efficiencies, issues have been reported regarding solution processed spiro-MeOTAD HTL such as pinholes and the strong dependence of electrical properties upon air exposure, which poses challenges for solar cell stability and reproducibility. In this work, we perform a systematic study to unravel the fundamental mechanisms for the generation of pinholes in solution-processed spiro-MeOTAD films. The formation of pinholes is closely related to the presence of small amounts of secondary solvents (e.g. H2O, 2-methyl-2-butene or amylene employed as a stabilizer, absorbed moisture from ambient, etc), which have low miscibility in the primary solvent generally used to dissolve spiro-MeOTAD (e.g. chlorobenzene). The above findings are not only applicable for spiro-MeOTAD (a small organic molecule), but also applicable to polystyrene (a polymer). The influence of secondary solvents in the primary solvents is the main cause for the generation of pinholes on film morphology. Our findings are of direct relevance for the reproducibility and stability in perovskite solar cells and can be extended to many other spin-coated or drop-casted thin films.

Hybrid lead halide perovskites have emerged as promising active materials for photovoltaic cells. Although superb efficiencies have been achieved, it is widely recognized that long-term stability is a key challenge intimately determining the future development of perovskite-based photovoltaic technology. Herein, we present reversible and irreversible photodecomposition reactions of methylammonium lead iodide (MAPbI(3)). Simulated sunlight irradiation and temperature (40-80 degrees C) corresponding to solar cell working conditions lead to three degradation pathways: (1) CH3NH2 + HI (identified as the reversible path), (2) NH3 + CH3I (the irreversible or detrimental path), and (3) a reversible Pb(0) + I-2(g) photodecomposition reaction. If only the reversible reactions (1) and (3) take place and reaction (2) can be avoided, encapsulated MAPbI(3) can be regenerated during the off-illumination timeframe. Therefore, to further improve operational stability in hybrid perovskite solar cells, detailed understanding of how to mitigate photodegradation and thermal degradation processes is necessary. First, encapsulation of the device is necessary not only to avoid contact of the perovskite with ambient air, but also to prevent leakage of volatile products released from the perovskite. Second, careful selection of the organic cations in the compositional formula of the perovskite is necessary to avoid irreversible reactions. Third, selective contacts must be as chemically inert as possible toward the volatile released products. Finally, hybrid halide perovskite materials are speculated to undergo a dynamic formation and decomposition process; this can gradually decrease the crystalline grain size of the perovskite with time; therefore, efforts to deposit highly crystalline perovskites with large crystal sizes may fail to increase the long-term stability of photovoltaic devices.

Thermal gravimetric and differential thermal analysis (TG-DTA) coupled with quadrupole mass spectrometry (MS) and first principles calculations were employed to elucidate the chemical nature of released gases during the thermal decomposition of CH3NH3PbI3. In contrast to the common wisdom that CH3NH3PbI3 is decomposed into CH3NH2 and HI, the major gases were methyliodide (CH3I) and ammonia (NH3). We anticipate that our findings will provide new insights into further formulations of the perovskite active material and device design that can prevent methylammonium decomposition and thus increase the long-term stability of perovskite-based opto-electronic devices.

The use of a solid hole transport layer (HTL) was transformational for the recent perovskite solar cell (PSC) revolution in solar energy technology. Often high efficiency PSC devices employ heavily doped hole transport materials such as spiro-MeOTAD. Independent of HTL chemistry, lithium-bis-trifluoromethanesulfonyl-imide (LiTFSI) and tert-butylpyridine (TBP) are commonly used as additives in HTL formulations for PSCs. LiTFSI and TBP were originally optimized for dye sensitized solar cells, where their roles have been extensively studied. However, in the case of PSCs, the function of TBP is not clearly understood. In this study, properties of the HTL composite deposited on flat silicon substrates were systematically measured at several length scales, e.g., macroscopically (profilometry, 4-point probe conductivity, and thermogravimetrydifferential thermal analysis), microscopically, and at the nanoscale to investigate film morphology, conductivity, and dopant distribution. Microscopic distributions of spiro-MeOTAD, LiTFSI, and TBP were determined using 2D Fourier transform infrared (FTIR) microscopy and electrostatic atomic force microscopy (EFM). Our findings reveal that the main role of TBP is to prevent phase segregation of LiTFSI and Spiro-MeOTAD, resulting in a homogeneous hole transport layer. These properties are critical for charge transport in the HTL bulk film as well as at the perovskite/HTL and HTL/electrode interfaces and for efficient solar cell performance.