Considerations for the Design of a Heterojunction Bipolar Transistor Solar Cell

Independent current extraction in multijunction solar cells has gained attention in recent years because it can deliver higher annual energy yield and can work for more semiconductor material combinations than the more established series-connected multijunction technology. The heterojunction bipolar transistor solar cell concept (HBTSC) was recently proposed as a simple, compact, and cost-effective multiterminal device structure that allows independent current extraction. It consists of only three main layers: emitter, base, and collector. In this article, we use a drift-diffusion model to analyze important aspects in the design of an HBTSC structure based on typical III-V semiconductor materials. We find that carrier injection from the emitter into the collector (transistor effect) degrades the open-circuit voltage of the top sub-cell, but this risk can be eliminated by optimizing the base design. We find requirements for the base layer which are, in principle, achievable in the context of current III-V semiconductor technology.