Recent Advances of CdTe Photovoltaics

In recent years, CdTe solar cell research has transitioned to p-type doing using group-V elements such as As or P. Compared to traditional Cu-doping, group-V doping can result in two orders of magnitude higher carrier concentrations, implying an open-circuit voltage gain of more than 100 mV. In addition, the long-term degradation rate, and temperature coefficient of CdTe solar cells are further reduced, suggesting higher energy yield at the same name plate efficiency/wattage.<br/>In 2023, efficiency measured at standard test condition of group-V doped devices surpassed Cu-doped CdTe solar cells for the first time. To date, we have certified &gt;22.6% efficiency at NREL, and demonstrated &gt;23% efficiency in-house using NREL measurement protocol. 917 mV was achieved with absorber band gap of 1.39 eV. The gain is largely due to improvement of Voc and FF compared to Cu-doped devices. Engineering the interface between CdTe and ZnTe leads to reduced interface recombination. An optimized selenium profile in part contributed to lower voltage deficit. Other improvements include a wide bandgap metal oxide buffer layer in front of the absorber, and inserting an optical reflector after the back contact to increase near band-edge QE. In addition, we also demonstrated reduced microscopic non-uniformity via lower As dosage or phosphor incorporation. Nevertheless the benefit on device Voc is yet to be materialized. Pathways for further improvement include enhanced passivation at back contact interface, and reduction of non-radiative recombination in absorber via better activation of group V dopants, reduced potential fluctuations, and identification and mitigation of defects in the absorber.