To identify new promising wide-band gap (1.6-2.0 eV) photabsorbers to be used in tandem solar cells, we have computationally screened 705 compounds with the ABS3 formula (A,B = metals; S = sulfur). Within the final list of 15 theoretically promising candidates, we have synthesized and characterized three novel ABS3 compounds in polycrystalline, thin film form by means of a two-step process. The first is sputter deposition of metallic (AB) or oxide (ABO3) precursors; the second is sulfurization of such precursors in H2S gas.
In this contribution, we will show that LaYS3 is a particularly attractive wide-band gap photoabsorber. Experimentally, it features a direct band gap of 2.0 eV, an intense photoluminescence signal, and a relatively small offset (0.1 eV) between its band gap energy and its photoluminescence peak.
Our first attempts at fabricating a simple single-junction LaYS3 solar cell will be presented. However, in line with the scope of this session, we will especially focus on the integration of ABS3 materials, and LaYS3 in particular, with a silicon bottom cell. This poses several challenges, among which the high-temperature process required to form high-quality LaYS3 and its negative effect on the properties of Si. Another issue is the selection of an appropriate recombination layer between the Si and the LaYS3 cell, which should also serve as a barrier to interdiffusion unless the two cells are mechanically stacked.
Finally, we will show that other novel ABS3 materials, while more difficult to synthesize, may also be promising for tandem solar cells. Some of those compounds contain more earth abundant elements than La and Y, and can be grown at lower temperatures.