Jiajun Peng

Structure and Growth Control of Organic–Inorganic Halide Perovskites for Optoelectronics: From Polycrystalline Films to Single Crystals

Recently, organic–inorganic halide perovskites have sparked tremendous research interest because of their ground‐breaking photovoltaic performance. The crystallization process and crystal shape of perovskites have striking impacts on their optoelectronic properties. Polycrystalline films and single crystals are two main forms of perovskites. Currently, perovskite thin films have been under intensive investigation while studies of perovskite single crystals are just in their infancy. This review article is concentrated upon the control of perovskite structures and growth, which are intimately correlated for improvements of not only solar cells but also light‐emitting diodes, lasers, and photodetectors. We begin with the survey of the film formation process of perovskites including deposition methods and morphological optimization avenues. Strategies such as the use of additives, thermal annealing, solvent annealing, atmospheric control, and solvent engineering have been successfully employed to yield high‐quality perovskite films. Next, we turn to summarize the shape evolution of perovskites single crystals from three‐dimensional large sized single crystals, two‐dimensional nanoplates, one‐dimensional nanowires, to zero‐dimensional quantum dots. Siginificant functions of perovskites single crystals are highlighted, which benefit fundamental studies of intrinsic photophysics. Then, the growth mechanisms of the previously mentioned perovskite crystals are unveiled. Lastly, perspectives for structure and growth control of perovskites are outlined towards high‐performance (opto)electronic devices.

Efficient and Balanced Charge Transport Revealed in Planar Perovskite Solar Cells

Hybrid organic-inorganic perovskites have emerged as novel photovoltaic materials and hold great promise for realization of high-efficiency thin film solar modules. In this study, we unveil the ambipolar characteristics of perovskites by employing the transport measurement techniques of charge extraction by linearly increasing voltage (CELIV) and time-of-flight (TOF). These two complementary methods are combined to quantitatively determine the mobilities of hole and electron of CH3NH3PbI3 perovskite while revealing the recombination process and trap states. It is revealed that efficient and balanced transport is achieved in both CH3NH3PbI3 neat film and CH3NH3PbI3/PC61BM bilayer solar cells. Moreover, with the insertion of PC61BM, both hole and electron mobilities of CH3NH3PbI3 are doubled. This study offers a dynamic understanding of the operation of perovskite solar cells.

2D Ruddlesden–Popper Perovskites for Optoelectronics

Conventional 3D organic-inorganic halide perovskites have recently undergone unprecedented rapid development. Yet, their inherent instabilities over moisture, light, and heat remain a crucial challenge prior to the realization of commercialization. By contrast, the emerging 2D Ruddlesden-Popper-type perovskites have recently attracted increasing attention owing to their great environmental stability. However, the research of 2D perovskites is just in their infancy. In comparison to 3D analogues, they are natural quantum wells with a much larger exciton binding energy. Moreover, their inner structural, dielectric, optical, and excitonic properties remain to be largely explored, limiting further applications. This review begins with an introduction to 2D perovskites, along with a detailed comparison to 3D counterparts. Then, a discussion of the organic spacer cation engineering of 2D perovskites is presented. Next, quasi-2D perovskites that fall between 3D and 2D perovskites are reviewed and compared. The unique excitonic properties, electron-phonon coupling, and polarons of 2D perovskites are then be revealed. A range of their (opto)electronic applications is highlighted in each section. Finally, a summary is given, and the strategies toward structural design, growth control, and photophysics studies of 2D perovskites for high-performance electronic devices are rationalized.

Triple-cation mixed-halide perovskites: towards efficient, annealing-free and air-stable solar cells enabled by Pb(SCN) 2 additive

Organo-metal halide perovskites have suffered undesirably from structural and thermal instabilities. Moreover, thermal annealing is often indispensable to the crystallization of perovskites and removal of residual solvents, which is unsuitable for scalable fabrication of flexible solar modules. Herein, we demonstrate the non-thermal annealing fabrication of a novel type of air-stable triple-cation mixed-halide perovskites, FA0.7MA0.2Cs0.1Pb(I5/6Br1/6)3 (FMC) by incorporation of Pb(SCN)2 additive. It is found that adding Pb(SCN)2 functions the same as thermal annealing process by not only improving the crystallinity and optical absorption of perovskites, but also hindering the formation of morphological defects and non-radiative recombination. Furthermore, such Pb(SCN)2-treated FMC unannealed films present micrometer-sized crystal grains and remarkably high moisture stability. Planar solar cells built upon these unannealed films exhibit a high PCE of 14.09% with significantly suppressed hysteresis phenomenon compared to those of thermal annealing. The corresponding room-temperature fabricated flexible solar cell shows an impressive PCE of 10.55%. This work offers a new avenue to low-temperature fabrication of air-stable, flexible and high-efficiency perovskite solar cells.

Solid-State Ligand-Exchange Fabrication of CH3NH3PbI3 Capped PbS Quantum Dot Solar Cells

CH3NH3PbI3 capped PbS colloidal quantum dots have been successfully fabricated by solid‐state ligand exchange from oleate and oleylamine capped PbS. The optimal solar cells made by layer‐by‐layer solution deposition give a high power conversion efficiency of 4.25% with an impressive short‐circuit photocurrent density of 24.83 mA cm−2.

Correlating Molecular Structures with Transport Dynamics in High-Efficiency Small-Molecule Organic Photovoltaics

Efficient charge transport is a key step toward high efficiency in small-molecule organic photovoltaics. Here we applied time-of-flight and organic field-effect transistor to complementarily study the influences of molecular structure, trap states, and molecular orientation on charge transport of small-molecule DRCN7T (D1) and its analogue DERHD7T (D2). It is revealed that, despite the subtle difference of the chemical structures, D1 exhibits higher charge mobility, the absence of shallow traps, and better photosensitivity than D2. Moreover, charge transport is favored in the out-of-plane structure within D1-based organic solar cells, while D2 prefers in-plane charge transport.

Composition Engineering in Two-Dimensional Pb–Sn-Alloyed Perovskites for Efficient and Stable Solar Cells

Environmentally friendly tin (Sn)-based metallic halide perovskites suffer from oxidation and morphological issues. Here, we demonstrate the composition engineering of Pb-Sn-alloyed two-dimensional (2D) Ruddlesden-Popper perovskites, (BA)2(MA)3Pb4- xSn xI13, for efficient and stable solar cell applications. Smooth thin films with high surface coverage are readily formed without using any additive owing to the self-assembly characteristic of 2D perovskites. It is found that Sn plays a significant role in improving the crystallization and crystal orientation while narrowing the bandgap of Pb-Sn 2D perovskites. Photophysical studies further reveal that the optimal Sn ratio (25 mol %) based sample exhibits both minimized trap density and weakened quantum confinement for efficient charge separation. Consequently, the optimized (BA)2(MA)3Pb3SnI13-based solar cells yield the best power conversion efficiency close to 6% with suppressed hysteresis.