  Independent current extraction in multi-junction 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 multi-junction technology. The heterojunction bipolar
transistor solar cell concept (HBTSC) was recently proposed as a simple,
compact and cost-effective multi-terminal device structure that allows
independent current extraction. It consists of only three main layers: emitter,
base and collector. In this work 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.
