Cu(In,Ga)Se-2 thin film solar cells with solution processed silver nanowire composite window layers: Buffer/window junctions and their effects

Abstract

We quantitatively and analytically investigate the properties of buffer/window junctions and their effects on the energy band alignment and the current-voltage characteristics of Cu(In,Ga)Se-2 (CIGS) thin film solar cells with solution processed silver nanowire (AgNW) composite window layers. AgNWs are generally embedded in a moderately conductive matrix layer to ensure lateral collection efficiency of charge carriers photogenerated in the lateral gaps present between AgNWs. Studies on the junctions between a buffer and AgNW-composite window layers and their effects on the performances of CIGS thin film solar cells have seldom been addressed. Here, we show that solution processed AgNW-composite window layers could induce defect states at the buffer/window interface, resulting in poor energy band alignment impeding carrier transport in the solar cells. On the basis of our analysis, we suggest an analytical expression of n(matrix)/D-i(2) >= 3046x10(-5)/epsilon to avoid losses in the power conversion efficiency of the solar cells. n(matrix) is the carrier concentration in a matrix layer embedding AgNWs, D-i is the negative defect density at the buffer/window interface, and epsilon is the relative dielectric constant of the matrix layer embedding AgNWs.