Perovskite solar cells (PSCs) emerging as a promising photovoltaic technology with high efficiency and low manufacturing cost have attracted the attention from all over the world. Both the efficiency and stability of PSCs have increased steadily in recent years, and the research on reducing lead leakage and developing eco-friendly lead-free perovskites pushes forward the commercialization of PSCs step by step. This review summarizes the main progress of PSCs in 2020 and 2021 from the aspects of efficiency, stability, perovskite-based tandem devices, and lead-free PSCs. Moreover, a brief discussion on the development of PSC modules and its challenges toward practical application is provided.
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Besides the efficiency progress, the long-term stability of PSCs against damp, light, and heat also improved significantly in recent years, which could be attributed to the construction of diffusion barrier against ion migration, additive engineering, design of chemically inert carbon-based electrodes, and development of cell encapsulation technique to reduce the lead leakage from a broken PSC module [11,12,13,14,15,16,17,18]. It was demonstrated that the printable PSCs had passed the most popular international standards of IEC61215:2016 for mature PV technology [19].
With the continuous progress of PSCs toward commercialization, exploiting eco-friendly lead-free perovskite materials has also become a hot research topic in this field in view of the toxicity of Pb element in Pb-containing PSCs giving rise to the concern of environmental pollution [20,21,22,23]. So far, the highest certified efficiency of lead-free PSCs has reached 11.22%, enabled by minimizing the defect density in tin halide perovskite films via a template-growth deposition method [24].
With the continuous movement of PSCs to commercialization, the toxicity of Pb element in perovskite absorber arouses the concern of environmental problems [66]. In recent years, a growing number of studies have aimed at developing the eco-friendly lead-free PSCs to directly avoid the use of lead in metal halide perovskite layers. So far, a number of lead-free perovskites based on tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), germanium (Ge), and copper (Cu) have been exploited for the application of solar cells [67,68,69,70,71].
Besides the tin PSCs, other types of lead-free PSCs based on wide-bandgap Bi and Sb perovskites also showed a large efficiency progress. Hu et al. fabricated the bulk-heterojunction perovskite active layers consisting of phase-separated Cs3Bi2I9 and Ag3Bi2I9 to improve the grain orientation and interface band alignment [77], achieving a record efficiency of about 3.6% with a high VOC reaching 0.89 V for Bi-based PSCs. Singh et al. used the indacenodithiophene-based organic acceptor with Lewis base groups to improve the morphology of wide-bandgap Cs3Sb2I9 perovskites and enhance the electron-extraction ability in Sb-based PSCs [78], yielding a PCE of 3.25% for the inverted-structure device.
Furthermore, recent study showed that organic solvents used in the mass production of PSC modules such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), and gamma-butyrolactone (GBL) are toxic to the human reproductive systems [80]. Therefore, development of green solvent systems or the solvent-free deposition technology for fabricating large-area perovskite film will be an important research topic in the future [81, 82].
Development of highly efficient lead-free PSCs is also an alternative choice to extend their application range in the PV markets, especially for the indoor power generation like wearable power sources that have a strict limit on lead content [86,87,88]. It was demonstrated that tin PSCs could be the next generation of PSCs for realizing over 20% efficiency and the strategies for their mass production have been investigated [20]. Moreover, developing reducing solvent system for Sn2+ precursor, constructing antioxidant capping layer during the crystallization of tin perovskite film, and the investigation of compatible device encapsulation approaches also led to a large stability improvement of tin PSCs [89, 90]. In our opinion, the application of tin PSCs for indoor PV products could be prior to lead PSCs when their efficiencies reach over 15%. 2ff7e9595c
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