Theoretical estimation to double the performance of perovskite solar cells using a graded absorber layer†

Metal halide perovskite solar cells (PSCs) have shown a remarkable increase in efficiency, with the latest record of 26.7% for a single bandgap absorber. According to the Shockley–Queisser limit, single-junction PSCs are predicted to achieve a maximum efficiency of ≈33%. However, open circuit voltage (V_OC) losses originating from non-radiative recombination at the absorber/charge transporting layer (CTL) interfaces due to band-level mismatches and defect states cause a lag in achieving the actual limit of PSCs. Composition-dependent bandgap tuning in halide perovskites offers a great advantage in tuning the optical properties of the absorber layer. In this article, we introduce a novel scheme for absorber band grading by altering the metallic or B-site composition of the FAPb_1−ySn_yI₃ perovskite absorber. By replacing the single absorber layer (FAPb_0.5Sn_0.5I₃) in the device configuration ITO/PEDOT:PSS/FAPb_0.5Sn_0.5I₃/PCBM/Ag using a graded bandgap absorber (FAPb_1−ySn_yI₃) with y varying between 0 and 1, a full range grading, the efficiency limit of the device is extended by 95%. Besides, a more convenient partial grading scheme with y of a smaller range can yield satisfactory results. A systematic study of both these grading schemes and simulations reveals that such an architectural design strategy with precise execution could be the next step in overcoming the practical limits of conventional single absorber PSCs.