Abstract
Incorporation of CdSe layers into CdTe thin film solar cells has recently emerged as a route to improve cell performance. It has been suggested that the formation of lower band gap CdTe(1-x)Se(x) phases following Se diffusion induces bandgap grading which may increase the carrier lifetime and thereby open circuit voltage. In this study we investigate the impact of CdSe incorporation on CdTe solar cell performance. We demonstrate that the standard CdS/CdTe device architecture is incompatible with Se incorporation, owing to large optical losses. An alternative cell structure with an oxide partner layer replacing the CdS with SnO2/CdSe/CdTe is developed, leading to cell efficiencies of > 13.5%. The differences in processing required for effective selenium incorporation are investigated with performance improvements resulting from additional post-growth annealing. Finally, other oxides such as TiO2, ZnO and FTO are demonstrated to be unsuitable partner layers but highlight that the choice of partner layer is key to further improving the performance.
Highlights
CdTe has established itself as the most competitive of the thin-film photovoltaics (PV) technologies currently on the market, demonstrating high performance (> 22%), long-term stability and one of the lowest costs per kWh (~0.0387 $ kWh) [1,2].CdS was for a long time considered to be essential in achieving high performance
Through the cell led work in this paper we have identified a number of key factors related to the incorporation of CdSe layers into CdTe solar cell structures
We established that the CdS/CdSe/CdTe device structure may be fundamentally limiting due to enhanced optical losses
Summary
CdTe has established itself as the most competitive of the thin-film photovoltaics (PV) technologies currently on the market, demonstrating high performance (> 22%), long-term stability and one of the lowest costs per kWh (~0.0387 $ kWh) [1,2]. It has been suggested that there is a bandgap grading within the CdTe(1-x)Sex layer resulting in a subsequent increase in carrier lifetime [9] This change in the nature of the device junction via the incorporation of CdSe may make it possible to partner CdTe(1-x)Sex directly with simple oxide layers without the need for CdS. We demonstrate that use of a SnO2 in a direct junction with CdTe(1-x)Sex, and optimisation of the intermixing, allows the performance of CdS/CdTe devices to be matched. This increases the JSC, concomitant VOC losses remain problematic
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