Theoretical impacts of single band gap grading of perovskite and valence band offset of perovskite/hole transport layer interface on its solar cell performances

Abstract

Single bandgap (Eg)-grading in CH3NH3(SnxPb1−x)(I1−yBry)3 perovskites and valence band offset (VBO) of a perovskite/hole transport layer (HTL) interface of perovskite solar cells (PSCs) are theoretically scrutinized. Eg values at front and back is defined as Eg values of perovskite at light incident side and back side, respectively, which are changed from 1.10 eV to 1.80 eV based on Sn and Br compositions for different Eg-grading. It is disclosed that optimized single Eg-grading (1.47 / 1.33 eV at front/back) results in enhancement of cell performances because of effective utilization of broader solar spectrum and formation of electric field in direction of promoting charge separation. Furthermore, optimized carrier diffusion length of 2.5 μm and perovskite thickness of 0.75 μm yields the highest power conversion efficiency of 26.0%. CH3NH3(Sn0.07Pb0.93)I3 at front for front Eg of 1.47 eV and CH3NH3(Sn0.38Pb0.62)I3 at back for back Eg of 1.33 eV are thus suggested for optimized single Eg-grading. Moreover, optimized VBO is in a range from − 0.1 eV to + 0.2 eV regardless of Eg-grading to improve cell performances. The results indicated that PEDOT:PSS, Spiro-OMeTAD, and PTAA as HTL are ultimately suitable for CH3NH3 (SnxPb1−x)I3 based PSCs, whereas NiOx and CuOx as HTL is appropriate for CH3NH3 (SnxPb1−x)I3 and CH3NH3Pb(I1−yBry)3 based PSCs.