Understanding the Origin of Thermal Annealing Effects in Low-Temperature Amorphous Silicon Films and Solar Cells

Abstract

A detailed investigation of the effects of prolonged postdeposition annealing on the performance of amorphous silicon (a-Si:H) solar cells and the properties of individual a-Si:H layers that are fabricated at low temperature of 120 degrees C is presented. A substantial improvement in all parameters of the current-voltage curves of these solar cells is observed upon annealing, consistent with an improvement in the collection voltage of the solar cells. Modifications of p-type layers during deposition of the solar cells are found to make no significant contribution to the annealing behavior of solar cells, while variations in the properties of n-type and intrinsic layers contribute substantially. The results indicate that the largest contribution to the annealing effect originates from changes in the electron mu tau-product in the intrinsic absorber layer upon annealing, while changes in hole mu tau-products have a minor contribution to the annealing effect in the solar cell. Besides a lack of significant changes in the number of recombination centers upon annealing, an improvement in the external quantum efficiency curves upon annealing may be accurately reproduced in computer simulations by assuming an increase in the band mobilities of both electrons and holes.