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

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 phi(b0) is held constant, the effect of increasing the band gap of the emitter layer E(mu)(p) is to depress the open-circuit voltage V-oc, fill factor (FF), and cell efficiency eta, although the short-circuit current J(sc) increases as expected. The main reason for the decrease of V-oc, FF, and eta at constant phi(b0) is that as E(mu)(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 phi(b0), we find that J(sc) in these structures is practically independent of this parameter. However, phi(b0) exerts considerable influence on V-oc, FF, and eta. 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 phi(b0) 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 J(sc), 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 similar to 80 Angstrom 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. (C) 1996 American Institute of Physics.