A computer analysis of the effect of a wide-band-gap emitter layer on the performance of a-Si:H-based heterojunction solar cells

Abstract

A first principles computer model for simulating the performance of amorphous-silicon-based solar cells has been applied to study the effect on solar cell performance of using an emitter layer with band gap larger than that of the intrinsic absorber. We surprisingly find that if the transparent conducting oxide/p-layer contact barrier height φb0 is held constant, the effect of increasing the band gap of the emitter layer Eμ(p) is to depress the open-circuit voltage Voc, fill factor (FF), and cell efficiency η, although the short-circuit current Jsc increases as expected. The main reason for the decrease of Voc, FF, and η at constant φb0 is that as Eμ(p) expands, the field corresponding to the gradient in the electron affinity at the p/i interface increases also, leading more and more to a collapse of the field over the intrinsic absorber layer. Considering the effect of φb0, we find that Jsc in these structures is practically independent of this parameter. However, φb0 exerts considerable influence on Voc, FF, and η. In fact, the observed improvement in a-Si:H-based single junction solar cell performance when a wider-band-gap p-a-SiC:H layer is introduced can only be explained by assuming that φb0 in this case is larger than that for a p-i-n homojunction structure a-Si:H solar cell. A study of the p/i interface states induced by the heterojunction reveals that these mainly limit Jsc, FF, and the efficiency of such cells. A reduction in the number of these interface states by using a graded band-gap buffer layer at this junction has been found to enhance the conversion efficiency by more than 25%. We further find that cells with p-layer thickness ∼80 Å have the highest conversion efficiency when this layer has a wider band gap than the intrinsic absorber. Finally, we show that when a buffer layer is introduced at the p/i heterojunction, the energy location of the p-layer/i-layer band-gap discontinuity does not have a crucial impact on heterojunction device performance.