Identifying kinetic rate limiters in As doping of pX CdTe using sX CdTe

Cadmium Telluride (CdTe) Photovoltaic (PV) technology has traditionally relied on Copper (Cu) doping to reach device efficiency of over 20%. However, Cu doped devices degrade faster and the p-type dopant concentration remains stalled at 10¹⁵ cm^-3. This limits us from reaching the detailed balance limit of 1.14 V Voc target for CdTe with a bandgap of 1.45 eV. Using group V dopants such as Phosphorus (P), Arsenic (As), etc., this limit can be overcome by producing more stable devices and pushing the active dopant concentration to over 10¹⁶ cm^-3. This feat has been achieved in single-crystal (sX) CdTe technology, but still remains a challenge in polycrystalline (pX) CdTe. Activating As in pX CdTe is challenging particularly due to its high reactivity with Oxygen (O), defect complex formation with O, Chlorine (Cl) and Cd, segregation of As around grain boundaries and the amount of As incorporated in the film: flux ratio compared to CdTe. Using our co-evaporator with incorporated Arsenic effusion cell, we have succesfully fabricated pX and sX CdTe, in-situ doped with As, both reaching >10¹⁶ cm^-3 active dopant concentration and with <8% efficient device in pX CdTe. Since sX-CdTe:As does not require Cl for activating the dopant in CdTe absorber and is devoid of any grain boundary, our work uses sX-CdTe:As as a pedestal to introduce one change at a time and study its role in As activation and device performance.