Defining a Novel Figure of Merit for Qualifying Tco Layers in Solar Cell Applications and Simulating the Effects of Them

Abstract

Transparent conductive oxides are widely used in solar cell applications. These layers have low electrical resistance that can reduce the top contact losses by reducing series resistance and increasing the fill factor of the cell. On the contrary, due to their conductivity, these layers have higher absorption coefficient than non conductive anti reflection coatings. Extensive interests in preparing of high quality TCO films lead us to defining a novel figure of merit Q for qualifying these layers and predicting the influence of using TCO films on solar cells operation. In this paper, all of the top contact effects such as shadows of contacts, contact resistance, lateral resistance and metal resistance were analytically calculated. TCO layers mainly affect the lateral and contact resistance. Optical properties of layers are modeled for solar cell applications by defining a modified average transmission coefficient using the air mass 1.5 and IQE as the weight factors. Considering the electro-optical effects of TCO layers, a novel figure of merit is introduced which is proportional to the efficiency of the cell. Using this figure of merit we are able to optimize the top contact pattern and choose the best values for finger spacing and busbar width. The defined Q is used to qualifying the ITO layers prepared by RF sputtering in TFL laboratory and it shows that the films with RF power of 300w and the mixture pressure 27 mTorr (90% Ar-10% O2) and the annealing temperature of 400°C has the most amount of Q in optimize top contact pattern.