Kesterite Solar Cells: Insights into Current Strategies and Challenges

Earth-abundant and environmentally benign kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising alternative to its cousin chalcopyrite Cu(In,Ga)(S,Se)2 (CIGS) for photovoltaic applications. However, the power conversion efficiency of CZTSSe solar cells has been stagnant at 12.6% for years, still far lower than that of CIGS (23.35%). In this report, insights into the latest cutting-edge strategies for further advance in the performance of kesterite solar cells is provided, particularly focusing on the postdeposition thermal treatment (for bare absorber, heterojunction, and completed device), alkali doping, and bandgap grading by engineering graded cation and/or anion alloying. These strategies, which have led to the step-change improvements in the power conversion efficiency of the counterpart CIGS solar cells, are also the most promising ones to achieve further efficiency breakthroughs for kesterite solar cells. Herein, the recent advances in kesterite solar cells along these pathways are reviewed, and more importantly, a comprehensive understanding of the underlying mechanisms is provided, and promising directions for the ongoing development of kesterite solar cells are proposed.

Rapid Microwave-Annealing Process of Hybrid Perovskites to Eliminate Miscellaneous Phase for High Performance Photovoltaics

Rapid processing technologies of perovskite solar cells (PSCs) offer an exciting approach to raise the rate of production. Herein, a rapid microwave-annealing process (MAP) is reported to replace the traditional hotplate annealing process (HAP) and the processing period of perovskite is reduced to less than 1 min. Benefiting from the penetrability and simultaneity of microwave irradiation, the MAP method can effectively eliminate miscellaneous phases and thus achieve >1 µm large-size crystal grains in perovskite films. These MAP treated perovskite films exhibit pure crystalline phase, long charge-carrier lifetime, and low defect density, which can substantially improve the PSC efficiency without requiring an additional enhancer/passivation layer. The inverted planar PSCs present enhanced power conversion efficiency from 18.33% (HAP) to 21.59% (MAP) and good stability of >1000 h lifetime without encapsulation under ambient conditions. In addition, MAP can be applied to a large-size (10 cm × 10 cm) perovskite film fabrication as well as a broader tolerance in environmental temperature and precursor concentration, making it a reliable method for repeatably practical fabrication of perovskite photovoltaics.

Solution-Processed Faraday Rotators Using Single Crystal Lead Halide Perovskites

Lead halide perovskites (LHPs) have become a promising alternative for a wide range of optoelectronic devices, thanks to their solution-processability and impressive optical and electrical properties. More recently, LHPs have been investigated in magneto-optic studies and have exhibited spin-polarized emission, photoinduced magnetization, and long spin lifetimes. Here, the viability of methylammonium lead bromide (MAPbBr3) single crystals as solution-processed Faraday rotators is demonstrated. Compared to terbium gallium garnet, the industry standard in the visible, it is found that MAPbBr3 exhibits Verdet constants (i.e., strength of Faraday effect) of similar or greater magnitude (up to 2.5x higher), with lower temperature dependence. Due to its low trap absorption, it is calculated that an optical isolator made from MAPbBr3, with appropriate antireflection coatings, should reach ≈95% transmission and achieve 40 dB isolation for incoming powers of over 2 W. It is also shown that the Verdet constant of MAPbBr3 can be calculated accurately from its dispersion in refractive index, allowing the possibility to predict similar effects in other perovskite materials.

Acetic Acid Assisted Crystallization Strategy for High Efficiency and Long-Term Stable Perovskite Solar Cell

Improving the quality of perovskite poly-crystalline film is essential for the performance of associated solar cells approaching their theoretical limit efficiency. Pinholes, unwanted defects, and nonperovskite phase can be easily generated during film formation, hampering device performance and stability. Here, a simple method is introduced to prepare perovskite film with excellent optoelectronic property by using acetic acid (Ac) as an antisolvent to control perovskite crystallization. Results from a variety of characterizations suggest that the small amount of Ac not only reduces the perovskite film roughness and residual PbI2 but also generates a passivation effect from the electron-rich carbonyl group (C=O) in Ac. The best devices produce a PCE of 22.0% for Cs0.05FA0.80MA0.15Pb(I0.85Br0.15)3 and 23.0% for Cs0.05FA0.90MA0.05Pb(I0.95Br0.05)3 on 0.159 cm2 with negligible hysteresis. This further improves device stability producing a cell that maintained 96% of its initial efficiency after 2400 h storage in ambient environment (with controlled relative humidity (RH) <30%) without any encapsulation.