Temperature-dependent Degradation Modes in CdS/CdTe Devices

Abstract

AbstractA set of 24 identically made CdS/CdTe devices were subjected to accelerated lifetime testing (ALT) under open-circuit bias, 1 sun illumination, and temperatures of 60, 80, 100, and 120 °C. A total of 6 identical devices were tested for statistical purposes at each temperature. Current density-voltage (JV) measurements were made on stressed cells for up to 2000+ hours. Device performance parameters were calculated as a function of temperature and stress time using discrete element circuit models. Forward current behavior was represented by two parallel diodes to simulate recombination currents in the quasi-neutral (QNR) and space-charge (SCR) regions. Backcontact behavior was studied using a parallel combination blocking diode and shunt conductance. A systematic pattern of degradation was apparent with increased stress temperature. At 60 °C, degradation associated with the CdTe/backcontact dominates. At temperatures above 80 °C, greater losses in fill factor (FF) and open-circuit voltage (Voc) were observed. Recombination current modeling of JV data attributes this to increased space-charge recombination. Calculated diffusion lengths based upon an Arrhenius-derived activation energy of 0.63 eV in this temperature-range suggests Cu diffusion into the SCR is mechanistically responsible for the observed increased recombination, and decreased Vocand FF. At lower temperatures (60 to 80 ºC), degradation was considerably slower with a measured activation energy of 2.9 eV.