Suitable junction partners for promising solar absorbers

In traditional photovoltaics, a junction of p- and n-type semiconductors creates an intrinsic voltage that ultimately drives a current of liberated photoelectrons. In silicon solar cells, such a junction is created by doping two regions of silicon with atoms that can donate either electrons or holes. Unfortunately, many of the materials that researchers are eyeing for next-generation, high-efficiency photovoltaics resist this kind of doping, and an alternative method is needed to create p-n junctions with these materials.

One possible alternative is to create a heterojunction between an absorber that is sensitive to solar radiation and another material with suitable electrical properties. Wallace et al. used numerical calculations to guide a search for materials to partner with two promising and naturally occurring solar absorbers: enargite and bournonite. Enargite is a greyish-black metallic-looking mineral with the chemical formula Cu₃AsS₄, and bournonite is another mineral whose chemical formula is PbCuSbS₃.

They identified four promising candidates — SnS₂, ZnTe, WO₃ and Bi₂O₃ — out of a total of 173 tested materials.

“The aim of this work was to reduce the search space for optimal devices to accelerate the optimization of new solar cell technologies,” said Suzanne Wallace, the paper’s first author.

One of the hurdles to finding good heterojunction partners is finding a good match between electron conduction and valence bands at the interface. Wallace and her collaborators calculated the ionization potentials for different surface structures of enargite and bournonite and chose partner candidates with favourable band gaps and band offsets. They also filtered for partner materials that would introduce as little strain at the boundary as possible, which can make epitaxy easier and limit any negative strain-based effects on performance.

Source: “Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching,” by Suzanne K. Wallace, Keith T. Butler, Yoyo Hinuma, and Aron Walsh, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5079485 .