The three-terminal heterojunction bipolar transistor solar cell (HBTSC) concept enables the realization of a monolithic double-junction device with individual current extraction. We present an HBTSC realized by a heterojunction of GaInP and GaAs. The one-sun open-circuit voltage (VOC ) of the top and bottom junctions are 1.33 V and 0.99 V, respectively, while fill factors (FF) are above 80%. At one-sun illumination, reducing one junction's bias from VOC to maximum power point degrades the performance of the other junction only slightly (< 0.5% efficiency loss). These results demonstrate the potential of the HBTSC concept to produce high-efficiency independently connected double-junction solar cells.

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 opencircuit 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.Index Terms -Drift-diffusion model, heterojunction bipolar transistor solar cell, multi-junction solar cell, novel photovoltaic concept.

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We demonstrate a novel multijunction architecture, the heterojunction bipolar transistor solar cell (HBTSC), which exhibits the performance of a double-junction solar cell in a more compact npn (or pnp) semiconductor structure. The HBTSC concept has the advantages of being a three-terminal device, such as low spectral sensitivity and high tolerance to non-optimal band gap energies, while it reduces the fabrication and operation complexity with respect to other multi-terminal devices because, for example, it can produce independent power extraction from the two junctions without the need for extra layers for their isolation or inter-connection. The top and bottom junctions in our proof-of-concept HBTSC prototype, which is made of epitaxial GaInP/GaAs, exhibit independent current-voltage characteristics under AM1.5G illumination, with respective open-circuit voltages of 1.33 and 0.95 V. The voltage difference between the two junctions is notable considering that they share a thin (< 600 nm) GaInP layer which contributes to the photogeneration of both junctions. This can be explained by a gradient in the minority carrier quasi-Fermi level within the base layer, which is compatible with a high fill factor. We also offer a technological solution for contacting the intermediate layer and study the effect of series resistance on the device performance. The HBTSC opens a new perspective in the understanding of multi-junction devices and it is an excellent candidate for the application of low-cost fabrication techniques, and for the implementation of III-V on silicon tandems with parallel/series interconnection for high energy yield.