Evaluation of PbTe and SnTe as ohmic contact layers in CdTe solar cell devices

• Published in: Applied surface science 2022-10, Vol.598, p.153656, Article 153656
• Main Authors: Swartz, C.H., LeBlanc, E.G., Perkins, C., McGott, D.L., Reese, M.O., Nichols, A., Myers, T.H.
• Format: Article
• Language: English
• Subjects: Anisotropic Band Valley; Cadmium Telluride; Lead Telluride; Magnetotransport; XPS
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2022.153656

[Display omitted] •The band offset between PbTe and CdTe was measured by X-ray photoelectron spectroscopy.•PbTe was found to be a good Ohmic contact to CdTe, only when doped heavily p-type.•Hall effect measurement of the doping level faces interference from parasitic interfacial layers.•A formula is given for variable field Hall effect in L-valley semiconductors. For solar cells based on CdTe, the choice of a suitable back contact material is limited by CdTe’s deep work function. Here, we explored p-type PbTe and SnTe as ohmic contacts to CdTe. These contact layers were grown on single crystal CdTe substrates by molecular beam epitaxy, and the valence band offset between film and substrate was measured using X-ray photoemission spectroscopy. Polycrystalline device structures were also grown by sublimation to assess performance improvements. Doping was achieved in PbTe by thallium incorporation. Only the highest Tl doped PbTe resulted in a desirable band alignment with the CdTe, forming an electron reflector and no hole barrier. Time-resolved photoluminescence measurements also revealed significant photocarrier lifetime improvements for only the highest doped PbTe. Consequentially, devices incorporating the highest doped PbTe layers showed increased power conversion efficiency, primarily from increased fill factor. Doping of the PbTe was measured via Hall effect with variable magnetic field, which was required due to the formation of an n-type parasitic interface layer. To properly interpret the variable field Hall measurements, we derived an expression for the magnetic field-dependent conductivity tensor of an L-valley semiconductor.