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

Abstract

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.

Chemical insight into the hydrothermal deposition of Sb2(S,Se)3 towards delicate microstructure engineering

“…2b). Because the melting points of S and Se are lower than 250 °C, 51,52 they will become volatile liquids at 350 °C, giving rise to a desired in situ sulfuration/selenization process due to the slow decomposition rate of Sb 2 (S x ,Se y ) 3 at 350 °C. The traditionally explored sulfuration and selenization processes, using S, H 2 S, Se or H 2 Se as the S/Se source, 53–57 exhibit a S/Se gradient diffusion process, that is, the concentration of S/Se vapor in the absorber layer is much lower than that of the environment.…”

“…2b). Because the melting points of S and Se are lower than 250 C, 51,52 they will become volatile liquids at 350 C, giving rise to a desired in situ sulfuration/selenization process due to the slow decomposition rate of Sb 2 (S x ,Se y ) 3 at 350 C. The traditionally explored sulfuration and selenization processes, using S, H 2 S, Se or H 2 Se as the S/Se source, [53][54][55][56][57] exhibit a S/Se gradient diffusion process, that is, the concentration of S/Se vapor in the absorber layer is much lower than that of the environment. We hypothesized that sulfuration/selenization possessing an in situ reaction, such as the annealing of metal polysulfoselenide at an appropriate temperature, could offer a locally high concentration of the S/Se steam atmosphere in the absorber layer.…”

Spiers Memorial Lecture: Next generation chalcogenide-based absorbers for thin-film solar cells

“…Despite the reduced chemical abundance, if chemical complexity presages lm processing and defect control challenges, then polycrystalline Se, an elemental ptype semiconductor with a direct bandgap of 1.8-2.0 eV (depending on processing), provides interesting opportunities for PV, particularly as the top cell in a tandem device. [54][55][56][57][58][59][60][61] With a trigonal structure comprising hexagonally packed helical chains of Se atoms (Fig. 4a), 62 crystalline selenium offers a high absorption coefficient (a > 10 5 cm À1 for wavelengths below $600 nm) 55 and facile lowtemperature processing (i.e., Se melts at $220 C and may be readily evaporated or solution processed).…”

“…Recent work has focused on trying to invert high-performance device structures from superstrate to substrate conguration (a preferred geometry for tandem use), with an appropriate n-type heterojunction partner and without introducing damage to the Se absorber during the buffer deposition. 59 A 3.9% PCE inverted p-i-n device comprising poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/Se/phenyl-C61-butyric acid methyl ester (PCBM) layers has recently been demonstrated. 60 In this structure, PCBM serves as both an ETL and a blocking layer to inhibit reaction between Se and the top Ag contact used in the device.…”

Solution Processed Fabrication of Se–Te Alloy Thin Films for Application in PV Devices

“…Although Se did not get much attention as individual photovoltaic material in past years, it has been part of some high-efficiency metal chalcogenide solar cell materials like Cu­(In,Ga)­Se 2 , Cu 2 ZnSnSe 4 , and Cd­(Se,Te) . Recently, few research groups have again started working on pure Se-based photovoltaic devices. − The reason for this can be their properties and ease of fabrication. Se has a high absorption coefficient and can be fabricated at temperatures below its melting point which is 220 °C, making it a material suitable for scalable deposition.…”