Progression of metalorganic chemical vapour-deposited CdTe thin-film PV devices towards modules

This paper reports important developments achieved with CdTe thin-film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high-transparency window layer (Cd0.3Zn0.7S) and a device-activation anneal. These increased the fill factor and open-circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd0.3Zn0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm2 in order to assess its impact on the device series resistance and fill factor. Finally, micro-module devices utilising series-connected 2-cm2 sub-cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module-to-cell efficiency ratio of 0.9 was demonstrated for a six-cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on.