Stability of CdTe solar cell prepared with rapid-thermal processed ZnTe:Cu back contacts and different metallizations

Abstract

This research examines the stability of CdTe cells prepared with rapid-thermal processed ZnTe:Cu back contacts. Two degradation sets were performed in this work. In the first set, CdTe cells with different window layers; CdS (standard device) and alloy CdS1−yTey (alloy device) were dark stressed at 85⁰C. The alloy devices has an additional high resistive transparent (HRT) layer between the transparent conducting oxide (TCO) and the alloy. The second set studies the degradation of standard CdTe devices with different back contact metallization (Au and Ti) under thermal stressing at 75 and 85⁰C with and without illumination. Dark and light current-voltage (JV), dark capacitance-voltage (CV), temperature dependent current-voltage (JVT), and biased admittance spectroscopy (AS) are used to study the degradation in CdTe cells. In the first set, a significant crossover between dark and light JV for standard and alloy devices is observed and this suggests a photoconductivity in the front layer. The crossover is less pronounced in the alloy device and can be attributed to its different front layer structure. The barrier heights, measured by temperature current voltage (JVT), are 0.44 eV and 0.37 eV for standard and alloy devices respectively. The net acceptor density deduced from CV is high for alloy device (6 × 1014cm−3) compared to the standard device (7 × 1013cm−3). Also, the standard device shows almost fully depleted device at zero bias, and thus no defect signature is seen in AS. However, performing AS at forward bias yields defect signature at 0.32 eV. Alloy device is not fully depleted at zero bias and AS reveals two defect signatures with activation energies 0.12 eV and 0.42 eV. The degradation of standard device under thermal stress at 85C is attributed to the decrease in fill factor (FF), while both FF and VOC degrade in the alloy device. In the second degradation set, the change in performance parameters were quantified for 165 hours of stressing. Under our test conditions the devices show moderate degradation that is different for light and dark stress. Efficiency degradation of dark stressed devices is mainly attributed to the decrease in FF. Ti contacts show more degradation than Au under similar conditions, but this is attributed to an increase in series resistance due to contact oxidation. JV curves for dark stressed devices show a partial roll-over at a forward bias slightly higher than VOC. The JV curve starts to bend after VOC similar to the roll-over behavior and then bows toward higher current. This is seen also for the standard device in the first set. After light stressing, JV curves show high forward current. The devices exhibited significant degradation at 85⁰C. The decrease in efficiency is attributed to both FF and VOC, but the later degraded slight more. CV results show a higher drop in net apparent acceptor density (NA) in the case of high temperature ALT and this can be correlated to the decrease in VOC. JVT results yields a barrier height of 0.37 eV for gold and Ti contact. A defect signature at 0.37 eV is seen for Ti device using forward bias AS and it is expected to be the back barrier since it agrees with JVT results. Devices with gold back contact is thinner than Ti and fully depleted and no defect is observed using forward bias AS. Using the JV and CV results we suggest a model to explain the degradation mechanisms of devices under different stressing conditions based on a reduction in hole density in the absorber. The proposed model is consistent with the electromigration of Cu from the back contact, with no significant changes at the contact itself. SCAPS-1D simulation is used to test this model and reproduce and explain most of the important results. For instance, the partial roll-over seen for dark stressed devices is attributed to an increase in back contact electron recombination at a bias greater than VOC.