Multiphonon recombination processes : Yutaka Toyozawa. Solid-St. Electron.21, 1313 (1978)

Abstract

Thin-film, polycrystalline, CIGS solar cells with localized and whole back contacts were modeled and simulated in two dimensions (2D) in order to investigate their potential for achieving high performance. The simulation results show that the short-circuit current density (Jsc) of the CIGS cells with localized back contacts increases with a decreasing contacted area due to the decreased cumulative surface recombination rate at the back side of the CIGS layer. The conversion efficiency (η) of these thin-film, CIGS cells can be increased by 15% relatively compared to CIGS cells with traditional whole back contacts if the surface-recombination velocity of the electrons and the holes (sn/p) between two adjacent fingers of the back contact is zero and the density of the acceptor defect states (NtA) inside the CIGS layer is low (NtA<1013 cm−3). The polycrystalline properties of the CIGS were modeled by setting vertical grain boundaries (GBs) within the CIGS absorbent layer. The simulation results show that a valence-band offset (VBO) above 0.2 eV at the GBs increases the Jsc, but it also significantly reduces the fill factor (FF) of the CIGS cells with localized back contacts. The results show that the efficiency of the CIGS cells with localized back contacts can be increased by 7% relatively if the density of the acceptor defect states at the GBs (NtA,GB) equals 1017 cm−3. In contrast, the efficiency of the CIGS cells with localized back contacts can be increased by only 1% relatively at the same NtA,GB if the VBO is 0.5 eV.