Synthesis and characterization of novel oxygenated CdSe window layer for CdTe thin film solar cells

Significant increases in the solar conversion performance of thin film CdS/CdTe solar cells have been realized in recent years by the inclusion of wider gap materials, notably CdS:O, in place of the CdS window layer. Similarly, use of CdSe in combination with CdTe has given yet further increases in current density due to the formation of CdTe1-xSex and the resulting enhanced long wavelength collection. Here we report the synthesis and properties of oxygenated CdSe films and their influence on device performance when included into CdTe solar cells. The CdSe:O films were made by pulsed laser deposition in oxygen at pressures of 0.01 and 0.03 Pa. Optical transmission measurements indicated that oxygenation acts to increase transparency in the 400-750 nm range, and that increased oxygen content acts to increase the apparent absorption threshold slightly. The higher transmission acts to increase the spectral response of CdTe solar cells in the range 350-500 nm compared to CdSe- and CdS-containing control devices. Moreover, the already known advantage of enhanced long wavelength response (850-910 nm) over that of CdS/CdTe is retained for both CdSe and CdSe:O window layers. Hence the use of CdSe:O as a window layer is demonstrated to increase the current harvesting of these solar cells by increasing both their long and short wavelength performances. These gains in the short circuit current outweigh small losses in fill factor and open circuit voltage making CdSe:O a valuable possible means to increase the performance of CdTe solar cells.