Carrier selective contacts:a selection of high efficiency silicon solar cells

Abstract

Solar cell has two basic units:the photon absorption layer and the contact layer. The contact layer is a region between the highly recombination-active metal interface and the photon absorption layer. It is vital to reduce the recombination loss between the photon absorption layer and the contact layer in pursuit of the higher conversion efficiency of silicon solar cell. In recent years, carrier selective contact is arousing research interest in photovoltaic industry because it is deemed as one of the last remaining obstacles in approaching to the theoretical efficiency limit of silicon solar cell. In this paper, three different types of carrier selective contacts are analyzed, which includes:1) sandwiching a heavily doped thin layer between the photon absorption layer and the metal interface, which is the so-called emitter or back surface field; 2) aligning the conduction bands or the valence bands of two materials; 3) inducing the band bending through a high work function metal oxide contacting crystalline silicon. Based on one-dimensional solar cell simulation software wxAMPS, three different silicon solar cell structures are numerically simulated, which includes:1) diffused homojunction silicon solar cell[(p+)c-Si/(n)c-Si/(n+)c-Si]; 2) silicon heterojunction solar cell with amorphous silicon thin films[(p+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n+)a-Si]; 3) silicon heterojunction solar cell with metal oxide thin films[(n)MoOx/(n)c-Si/(n)TiOx], then the energy band structures and the spatial distributions of carrier concentrations of solar cells in the dark are discussed. The simulation results show that the key factor of carrier selective contacts is the asymmetric spatial distribution of the carrier concentrations, i.e. the asymmetric conductivities of electrons and holes. This leads to the formation of high resistance to electrons and low resistance to holes, or high resistance to holes and low resistance to electrons, so the holes will go through the contact easily and the electrons will be blocked simultaneously, or the electrons will go through the contact easily and the holes will be blocked simultaneously. Therefore a hole selective contact or a electron selective contact is formed, respectively.