No abstract

No abstract

We theoretically investigated the features of hot carrier solar cells, from which photogenerated carriers are extracted before they are completely thermalized. There are three channels of energy dissipation from photogenerated carriers that lowers the conversion efficiency: thermalization in the absorber, emission from the absorber, and thermodynamically unavoidable heat flux to the ambient. The emission increases with increasing carrier density in the absorber, whereas the heat flux decreases. Previous calculations of the conversion efficiency have been carried out under the supposition of no thermalization of carriers. In this case, the dominant process of energy dissipation is the emission, like conventional solar cells represented by the Shockley and Queisser formula. In practice, the carriers should be extracted to external circuits immediately after photogeneration because they are partially thermalized. This restriction leads to a much smaller carrier density and consequently more significant energy dissipation by heat flux, whereas the influence of the emission is negligible. As a result, the conversion efficiency is considerably lower than the values under the supposition of no thermalization. To suppress the heat flux to improve conversion efficiency, a smaller effective electron mass and a higher carrier temperature are required, as well as more intense irradiation. When the effective electron mass is much smaller than that of holes, the thermalization of holes has little influence on lowering the conversion efficiency.

We have fabricated Cu2Sn1-xGexS3 thin-film solar cells by cosputtering deposition of Cu and Sn followed by sulfurization in S and GeS2 vapors. The conversion efficiency was significantly improved to be as high as 6.0% compared with the values of Cu2SnS3 solar cells similarly fabricated. Scanning electron microscopy observation revealed that alloying with Ge accelerated the grain growth during the sulfurization process, contributing to the improvement in the conversion efficiency. The bandgap energy of Cu2Sn0.83Ge0.17S3 was about 1.0 eV, which is suitable for bottom cells used in double-junction solar cells.

No abstract