wangzhaowei

Organic–inorganic halide perovskite single crystals possess many outstanding properties conducive for photovoltaic and optoelectronic applications. However, a clear photophysics picture is still elusive, particularly, their surface and bulk photophysics are inexorably convoluted by the spectral absorbance, defects, coexisting photoexcited species, etc. In this work, an all-optical study is presented that clearly distinguishes the surface kinetics from those of the bulk in the representative methylammonium-lead bromide (MAPbBr₃) and -lead iodide (MAPbI₃) single crystals. It is found that the bulk recombination lifetime of the MAPbBr₃ single crystal is shortened significantly by approximately one to two orders (i.e., from ≈34 to ≈1 ns) at the surface with a surface recombination velocity of around 6.7 × 10³ cm s⁻¹. The surface trap density is estimated to be around 6.0 × 10¹⁷ cm⁻³, which is two orders larger than that of the bulk (5.8 × 10¹⁵ cm⁻³). Correspondingly, the diffusion length of the surface excited species is ≈130–160 nm, which is considerably reduced compared to the bulk value of ≈2.6–4.3 μm. Furthermore, the surface region has a wider bandgap that possibly arises from the strong lattice deformation. The findings provide new insights into the intrinsic photophysics essential for single crystal perovskite photovoltaics and optoelectronic devices.

The surface and bulk photophysics of perovskite single crystals are clearly separated in an all-optical approach. Their surface photophysics is similar to that of polycrystalline thin films, but is significantly different in the bulk region. Key properties from both regions (e.g., recombination lifetimes, trap densities, surface recombination velocity, diffusion lengths, etc.) essential for optoelectronic applications are obtained in this comprehensive study.

During the past 6 years, perovskite solar cells have experienced a rapid development and shown great potential as the next-generation photovoltaics. For the perovskite solar cells with regular structure (n-i-p structure), device efficiency has reached over 20% after the intense efforts of researchers from all over the world. Recently, perovskite solar cells with the inverted structure (p-i-n structure) have been becoming more and more attractive, owing to their easy-fabrication, cost-effectiveness, and suppressed hysteresis characteristics. Some recent progress in their device performance and stability has indicated their promising future. Here, recent developments and future perspectives about inverted perovskite solar cells are reviewed. Interface engineering, film morphology control, device stability, hysteresis phenomena and other research hotspots are discussed to present the roadmap for the development of inverted perovskite solar cells.

Inverted perovskite solar cells are attracting much attention owing to their superior characteristics. Recent developments and future perspectives about inverted perovskite solar cells are reviewed. Interface engineering, film morphology control, device stability, hysteresis phenomena and other research hotspots are discussed to present the promising future of inverted perovskite solar cells.

Replacing toxic solvents with nonhazardous solvents is one of the key challenges for industrial scale commercialization of thin film perovskite photovoltaics. Here, nonhazardous solvent/alcohol/acid systems are presented for the single-step deposition of pinhole-free perovskite layers with combined lead halide precursors of Pb(CH₃CO₂)₂·3H₂O, PbCl₂, and CH₃NH₃I. Comparable performance to standard hazardous inks is achieved: devices with 15.1% power conversion efficiency are demonstrated and maintain 13.5% tracked for 5 min at maximum power point. Blade coated 4 cm² solar modules fabricated with highest performing device ink attain 11.9% in power conversion efficiency.

Nonhazardous solvent-alcohol-acid systems are developed for single-step deposition of perovskite photovoltaic layers, based on Hansen solubility parameters. The blade coating of 4 cm² modules demonstrates the high yield and uniformity of the ink's coating capability.

The critical role of grain boundaries for (CH(NH₂)₂PbI₃)_0.85(CH₃NH₃PbBr₃)_0.15 perovskite solar cells studied by Kelvin probe force microscopy under bias voltage and illumination is reported. Ion migration is enhanced at the grain boundaries. Under illumination, the light-induced potential causes ion migration leading to a rearranged ion distribution. Such a distribution favors photogenerated charge-carrier collection at the grain boundaries.

Hybrid organic–inorganic halide-perovskite-based solar cells have achieved notable progress. A hot topic in this field is exploring inexpensive, stable and effective hole-transporting materials (HTMs) in order to improve the device performance and be favorable for large-scale production in the future. The HTMs have been proven to be an important component of perovskite solar cells, which can form selective contact being favorable for reducing charge recombination and effective hole collection, thus resulting in the enhancement of the open-circuit voltage and the fill factor. Here, an overview of the design and development of HTMs is given, mainly divided into conductive polymers, inorganic p-type semiconductors in inverted-structure-based planar perovskite solar cells. The influences of their mobility, work function and film property on device performance are discussed.

Hole-transporting materials in inverted planar perovskite solar cells have been widely studied in the past years. Most commonly these are p-type wide band-gap semiconductors which can be mainly divided into conductive polymers and inorganic p-type semiconductors. Their energy levels and chemical structures are summarized and the effects of their properties on the device performance are discussed in detail.

A new fluorinated polythiophene (PT) derivative, PBDD-ff4T, is designed and synthesized. The PBDD-ff4T/PC₇₁BM-based device without any extra treatment exhibits a high efficiency of 9.2%, under the irradiation of AM 1.5G, 100 mW cm⁻², which is the highest power conversion efficiency reported for PT derivative-based polymer solar cells.

A triphenylamine based molecular “butterfly” is developed as dopant-free hole-transport material for perovskite solar cells exhibiting excellent power conversion efficiency of 16.3% which is comparable to the state-of-the-art doped 2,2′,7,7′-tetrakis(N,N′-di-p-methoxy-phenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Moreover, the device is much more stable than that of spiro-OMeTAD based device under light irradiation.

The idea of regulating the hydrodynamics of incoming perovskite-containing droplets — that is whether the droplets undergo coalesce, merge and spread or pinning, and individual drying — sheds light on the scalable production of printable perovskite photovoltaics with reproducibility and high efficiency. Hidetaka Ishihara and co-workers investigate these hydrodynamics for a high throughput production of highly stable perovskite photovoltaics in article 1500762.

Three small molecules with different substituents on bithienyl-benzo[1,2-b:4,5-b′]dithiophene (BDTT) units, BDTT-TR (meta-alkyl side chain), BDTT-O-TR (meta-alkoxy), and BDTT-S-TR (meta-alkylthio), are designed and synthesized for systematically elucidating their structure–property relationship in solution-processed bulk heterojunction organic solar cells. Although all three molecules show similar molecular structures, thermal properties and optical band gaps, the introduction of meta-alkylthio-BDTT as the central unit in the molecular backbone substantially results in a higher absorption coefficient, slightly lower highest occupied molecular orbital level and significantly more efficient and balanced charge transport property. The bridging atom in the meta-position to the side chain is found to impact the microstructure formation which is a subtle but decisive way: carrier recombination is suppressed due to a more balanced carrier mobility and BDTT based devices with the meta-alkylthio side chain (BDTT-S-TR) show a higher power conversion efficiency (PCE of 9.20%) as compared to the meta-alkoxy (PCE of 7.44% for BDTT-TR) and meta-alkyl spacer (PCE of 6.50% for BDTT-O-TR). Density functional density calculations suggest only small variations in the torsion angle of the side chains, but the nature of the side chain linkage is further found to impact the thermal as well as the photostability of corresponding devices. The aim is to provide comprehensive insight into fine-tuning the structure–property interrelationship of the BDTT material class as a function of side chain engineering.

A comprehensive insight into fine-tuning the structure–property interrelationship (a trade-off beyond efficiency) of the bithienyl-benzo[1,2-b:4,5-b′]dithiophene material class as a function of side chain engineering is provided.