A comparison of the effect of CdCl2 and MgCl2 processing on the transport properties of n-CdS/p-CdTe solar cells and a simple approach to determine their back contact barrier height

A simple approach, which can estimate the barrier height of non-Ohmic back contacts for CdS/CdTe solar cell by using its temperature dependent forward biased current-voltage data, is explained. The method involves modelling the forward J-V characteristics using a double exponential expression for the main junction and by a reverse biased Schottky barrier for the back contact. Cells processed with both CdCl2 and MgCl2 are compared, with the current transport phenomena in both kinds of cells also being analysed. Performance loss due to limitation of the forward bias hole current, and its dependence on the post-deposition chloride processing, is discussed. The forward current transport is mainly dominated by recombination at CdS/CdTe interfacial region with pronounced tunnelling effects. Classical Schottky-type conduction, as described by the Richardson-Schottky formula, is a good fit to the reverse biased current-voltage behaviour of an Au/CdTe junction above ∼240 K. Below this temperature, the current limiting effect due to the increasing contribution from interfacial defect states can be satisfactorily explained by Bardeen's model for a modified Schottky type barrier at back contact interface.