Accelerated stress testing and diagnostic analysis of degradation in CdTe solar cells

Abstract

Solar cell module reliability is inextricably linked to cell-level reliability. This is particularly so with thin-film technologies. In CdTe, reliability issues historically associate with back contact stability and the use of Cu as an extrinsic dopant. Using a simple approach by which identical cells are heated under open-circuit bias and 1-sun illumination, degradation activation energies of 0.63 and 2.94 eV in laboratory-scale CdS/CdTe devices were identified in the accelerated stress temperature range of 60 to 120 °C. At lower stress temperatures, cell performance changes were linearly correlated with changes in both fill factor (FF) and short-circuit current (J sc ). At higher stress temperatures, changes in efficiency were correlated with changes in FF and open-circuit voltage (V oc ). The measured activation energy of 0.63 is associated with Cu-diffusion. During the early stage of stress testing, which may provide additional back contact annealing, improvements in FF were due to Cu-diffusion. Decreased performance observed at longer stress times (decreased FF and V oc ), according to a two-diode Pspice model, were due to both increased space-charge recombination (near the junction) and decreased recombination in the bulk. Kirkendall void formation (S-outdiffusion) at the CdS/CdTe interface is given as responsible for the 2.9 eV degradation mechanism.

Correlations of capacitance-voltage hysteresis with thin-film CdTe Solar cell performance during accelerated lifetime testing

“…The results shown in Fig. 3 use a model-fitted value of 3 ohms*cm 2 for R s which is a reasonable upper value observed during stress testing of these cells [9]. As seen in this figure, recombination occurs mostly in the space-charge except near V oc where recombination in the quasi-neutral region, i.e., between the depletion width and back contact begins to dominate.…”

“…A schematic of this basic design is shown in Figure 1. In order to ascertain the long term reliability of modules based upon CdS/CdTe, cells of this basic design were exposed to 1-sun illumination under open-circuit, V oc , bias and acceleration temperatures of 60 -120 ºC for times exceeding 1000 hours [9]. Under field-use conditions, series-connected cells nominally see voltages somewhat less than V oc , thus, open-circuit conditions represent an additional form of acceleration.…”

Towards understanding junction degradation in cadmium telluride solar cells

“…[3][4][5] In this work, numerical modeling is employed to investigate a mechanism of defect generation caused by excess charge carrier concentrations. This type of mechanism has been widely investigated for other types of semiconductors, but remains relatively unexplored in CdTe.…”

The use of 2^nd and 3^rd level correlation analysis for studying degradation in polycrystalline thin-film solar cells

“…5. This calculation used a fit-determined value of 3 Ω⋅cm 2 for Rs which is a reasonable upper value observed during stress testing of these cells [9]. As seen in this figure, recombination occurs mostly in the space charge near Voc where recombination in the quasi neutral region, i.e., between the depletion width and back contact, begins to dominate.…”

“…To ascertain durability, cells were fabricated and then exposed to 1-sun illumination under open-circuit, Voc, bias and acceleration temperatures of 60 -120 ºC for times exceeding 1000 hours [9]. Two dominant degradation mechanisms were identified in the temperature range studied.…”