title: Conversion efficiency limits and bandgap designs for multi-junction solar cells with internal radiative efficiencies below unity.
abstract: We calculated the conversion-efficiency limit ηsc and the optimized subcell bandgap energies of 1 to 5 junction solar cells without and with intermediate reflectors under 1-sun AM1.5G and 1000-sun AM1.5D irradiations, particularly including the impact of internal radiative efficiency (ηint) below unity for realistic subcell materials on the basis of an extended detailed-balance theory. We found that the conversion-efficiency limit ηsc significantly drops when the geometric mean ηint* of all subcell ηint in the stack reduces from 1 to 0.1, and that ηsc degrades linearly to logηint* for ηint* below 0.1. For ηint*<0.1 differences in ηsc due to additional intermediate reflectors became very small if all subcells are optically thick for sun light. We obtained characteristic optimized bandgap energies, which reflect both ηint* decrease and AM1.5 spectral gaps. These results provide realistic efficiency targets and design principles.
authors.forename: Lin
authors.initials: L
authors.lastname: Zhu
authors.forename: Toshimitsu
authors.initials: T
authors.lastname: Mochizuki
authors.forename: Masahiro
authors.initials: M
authors.lastname: Yoshita
authors.forename: Shaoqiang
authors.initials: S
authors.lastname: Chen
authors.forename: Changsu
authors.initials: C
authors.lastname: Kim
authors.forename: Hidefumi
authors.initials: H
authors.lastname: Akiyama
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Kanemitsu