GaInP/GaAs three-terminal heterojunction bipolar transistor solar cell

Abstract

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.

GaInP/GaAs three-terminal heterojunction bipolar transistor solar cell

“…Due to the irregular etching of GaInP and the possibility of underetching, 34 large areas are etched around the base contacts (the two horizontal metal pads in Figure 1e). This can be avoided in future prototypes by detaching the epitaxial semiconductor structure from the substrate and contacting the base layer with an interdigitated bottom contact 35 (SI, Fig. S12).…”

“…More discussion can be found in the SI, Section S3. We have shown elsewhere 35 that HBTSC devices can be fabricated with interdigitated bottom contacts (base/collector) instead of implementing top contacts for the base if the device is removed from the epitaxial substrate. Therefore, in optimized HBTSC devices the shadowing factor is expected to be similar to that of conventional MJSCs and the efficiency to be a more straightforward determination.…”

“…As mentioned before, in future devices the epitaxial structure will be detached from the substrate, enabling the implementation of an interdigitated bottom contact for the base terminal as demonstrated in Ref. 35 . This strategy circumvents the problem related to lateral series resistance, reduces the shadowing factor to the range which is characteristic of two-terminal devices and enables the scalation of the HBTSC concept to larger areas (see SI, Fig.…”

III-V-on-silicon triple-junction based on the heterojunction bipolar transistor solar cell concept

“…1c) enabling that the top-junction open-circuit voltage (VOC top ) is larger than the bottom-junction open-circuit voltage (VOC bot ) [3]. This has been recently demonstrated in a GaInP/GaAs HBTSC fabricated by our group in collaboration with NREL [6], [7]. In that proof-of-concept device the base layer was 800 nm thick.…”

“…AM1.5G VOCs of 0.95 and 1.33 V were recorded at the bottom and top junctions, respectively, and the overall efficiency reached 28%. More recently, we have made the first inverted GaInP/GaAs HBTSC, transferring it successfully onto a Si carrier [8], opening the path to the practical implementation of a III-V-HBTSC-on-Si triplejunction device.…”