Analysis of Optical and Recombination Losses in Solar Cells

Abstract

For efficient optimization of solar-cell device structures and processing, it is essential to reveal the performance-limiting optical and physical factors in solar cells. Quite fortunately, a global solar-cell characterization method has recently been developed from which the parasitic light absorption and carrier recombination in the devices are evaluated systematically based on external quantum efficiency (EQE) analysis. In this new method, the optical and recombination losses in complex solar cell structures are readily determined within the framework of a rather simple optical admittance method. The EQE analysis method described in this chapter is appropriate for a wide variety of photovoltaic devices, including crystalline Si (c-Si), hydrogenated amorphous silicon (a-Si:H), Cu(In,Ga)Se2 chalcopyrite, Cu2ZnSn(S,Se)4 kesterite, CdTe zincblende and hybrid perovskite solar cells, and provides excellent fitting to numerous EQE spectra obtained experimentally. This chapter introduces the basic concept of the global EQE analysis method in which the effects of (i) light scattering by submicron textures and (ii) carrier recombination in light absorbers are fully incorporated. As examples, the EQE analyses of Cu(In,Ga)Se2, Cu2ZnSnSe4, Cu2ZnSnS4, CdTe, a-Si:H and c-Si solar cells are described. Based on the analysis results, we will further discuss the carrier loss mechanisms in different types of photovoltaic devices.