Flexible Perovskite Solar Cells

In a short time of 7 years, perovskite solar cells (PSCs) have achieved an amazing power conversion efficiency (PCE) of 24.2%, which exceeds the PCEs of multi-crystalline Si (22.3%), thin-film crystalline Si (21.2%), copper indium gallium selenide (22.6%), and CdTe-based thin-film SCs (22.1%). Owing to low process temperature, mechanical durability, and the potential for the solution-based roll-to-roll (R2R) process, the PSC has a strong potential of being utilized in the form of flexible solar cell based on a plastic substrate. This flexible-PSC (F-PSC) is expected to be used in niche applications such as portable electric chargers, electronic textiles, large-scale industrial roofing, and power sources for unmanned aerial vehicles (UAVs). However, the champion-cell efficiency of the F-PSC is 19.11%, which is apparently lower than that of the rigid cell (24.2%). Also, the world-best perovskite module efficiency on a rigid substrate is 17.1%, outstripping the efficiency of flexible perovskite module (11.7%). Moreover, the F-PSCs have not shown superior long-term stability to rigid cells. To commercialize the F-PSC, the efficiency needs to be comparable to the glass-based rigid PSC as well as the long-term stability. In this review paper, we investigate the fundamental challenges of F-PSCs such as the optical transmittance of flexible substrates and electrical conductivity of flexible transparent conducting oxides, uniform coating technology with a large area on flexible substrates, the high moisture permeability of plastic flexible substrates, and super flexibility. We also introduce recent efforts for overcoming the aforementioned issues as well as for facilitating the commercialization of F-PSCs. As a perspective, we suggest the future direction of research and development of F-PSCs such as the module technology involving assembling multiple subcells and the flexible tandem devices including flexible PSC/CIGS or flexible PSC/organic photovoltaics (OPVs).