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Simple Solution‐Processed Approach for Nanoscale Coverage of Perovskite on Textured Silicon Surface Enabling Highly Efficient Perovskite/Si Tandem Solar Cells

Nowadays, crystalline silicon (c-Si) solar cells account for more than 90% of the photovoltaic market due to the advantages of nonpollution, popularization of raw materials, low levelized cost, and high power conversion efficiencies (PCEs). The efficiency of c-Si solar cells based on the heterojunction back contact structure has reached up to 26.6%. For single-junction solar cells, it is difficult to further improve their PCEs attributed to the Shockley–Queisser limit, which is 29.4% for c-Si solar cells. Recently, perovskite/c-Si monolithic tandem solar cells (TSCs) have attracted extensive attention. Perovskite is considered to be the ideal top cell material because of its tunable bandgaps, high absorption coefficient, and various preparation methods. The efficiency of devices has quickly increased from 3.8% to 25.2%. Si solar cells have the advantages of intense short-circuit current and high absorption in longwave, suitable as the bottom cell. Perovskite/ c-Si TSCs combine Si cells with a bandgap of 1.12 eV and perovskite cells with a tunable bandgap of 1.5–2.3 eV to achieve spectral distribution absorption. In theory, the ultimate efficiency of two-junction cells with full-spectrum matching can reach 43%, which is the dawn of the future photovoltaic industry. In the past 5 years, the PCEs of the perovskite/c-Si monolithic TSCs have sharply increased from an initial 13.7% to a recently certified 29.15%, exhibiting the enormous potential of TSCs. The early studies on perovskite/Si TSCs were carried out on the front surfacepolished silicon cells as the perovskite preparation process was mostly based on flat substrates. However, nontextured surface could cause severe optical losses. To maximize the absorption of the solar spectrum and avoid the surface polishing step, many researchers have now joined in studying the growth of perovskite films on the fully textured silicon surfaces. The incompatibility of silicon and perovskite solar cell (PSC) manufacturing processes has brought new challenges and opportunities for photovoltaic community. The difficulty lies in the large undulations of the silicon surface, and the ordinary solution route will cause serious enrichment of the perovskite precursor in the Si pyramid valley, which results in exposure of the Si pyramid tip. Moreover, due to the different wettability of perovskite precursor solutions to different substrate morphologies, the Si pyramid induces new difficulties for the growth of perovskite controllable film thickness. Up to now, the growth of perovskite on the fully pyramidtextured silicon surface can be summarized into three types: 1) Micrometer-thick perovskite full coverage; 2) Dual-source coevaporation method combined with solution method; and 3) Mechanical stacking. For the first scenario, Hou et al. used J. Wang, C. Gao, X. Wang, Y. Wang, Z. Cheng, Dr. H. Liu, Prof. W. Shen Institute of Solar Energy, and Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240, P. R. China E-mail: wzshen@sjtu.edu.cn