Graduation Year

2005

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Christos Ferekides, Ph.D.

Committee Member

Don Morel, Ph.D.

Committee Member

Yun-Leei Chiou, Ph.D.

Committee Member

A.N.V. Rao, Ph.D.

Committee Member

Richard Gilbert, Ph.D.

Keywords

Ii-vi devices, Illumination, Thermal, Lightsoak, Stress, Degradation, Solar

Abstract

CdTe/CdS solar cells have shown great potential for terrestrial solar power applications. To be commercially viable they need to operate efficiently for about 30 years. CdS/CdTe solar cells fabricated at USF have shown record efficiencies upto 16.5% [46]. This research involves the study of thermal stress (TS) and light soaking (LS) on the stability of high efficiency (>10%) solar cells. The change in key electrical parameters Voc, FF, Jsc, A and Jo are quantified for more than 2000 hours of stressing.

The device degradation was found to increase with stress temperature for TS. Below 100oC, the changes were due to collection and recombination losses. Above 100oC, "shunting" mechanisms were found to start affecting the device performance. A fast drop in performance within the first 500 hours was observed. It is believed to be due to an increase in deep-level Cu-related defects that increase with stress temperature. Diffusion of Cui+ ions from the back contact along CdTe grain boundaries had been previously reported [16]. An increase in light/dark J-V crossover and bulk Rs with stress time and temperature was observed. A slow degradation component attributed to Cu-related substitutional defect [23] formation/diffusion to the junction and CdS is proposed. This should compensate the CdS over time and increase its photoconductivity/resistivity.

An improvement in the current collection and FF within 100 hours of LS was observed. This is possibly due to the enhancement of Cui+ diffusion into the junction and CdS during LS as previously reported [16]. A reduction in light/dark J-V crossover was observed, possibly due to an increase in CdS doping and reduction in the CdS/SnO2 front contact barrier. However, a fast decrease in Voc and increase in recombination current was also observed in the first 1000 hours of LS. This is possibly due to the existence of higher concentration of Cu-related deep level defects at the junction. A larger decrease in Voc was found for LS than TS at the same operating temperature.

A continuous drop in performance over time is observed for both TS and LS. The existence of a slow degradation component involving the formation/diffusion of Cu- related substitutional defects at the junction and CdS is proposed. The concentration of this defect is probably not high enough in CdS for LS samples to affect their photoconductivity and cause light/dark J-V crossover in 2000 hours.

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