Selenium Thin-Film Solar Cells with Cadmium Sulfide as a Heterojunction Partner

Elemental selenium (Se) is experiencing a renaissance as a p-type direct wide bandgap (1.95 eV) photoabsorber, appropriate for integration with lower bandgap materials in tandem photovoltaic devices. However, single-junction selenium devices are typically in the superstrate configuration with the charge-separating p–n junction located very close to the substrate. For tandem devices, the p–n junction should ideally lie near the surface of the cell to maximize photocarrier collection, implying that the n-type heterojunction partner should be deposited on top of Se. Since Se is a soft, low-melting-point material, a low-damage deposition technique should then be identified for the partner material. Here, we investigate the suitability of CdS grown by chemical bath deposition as an n-type partner for Se cells. We demonstrate the first functioning CdS/Se solar cell in the standard superstrate configuration. Although it may be possible to realize a substrate configuration, we find that the bandgap and electron affinity of CdS are not optimal for efficient carrier absorption and transport in selenium devices, suggesting that CdS is a poor partner for selenium.