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Addressing absorber quality in CD1-xMGxTE wide bandgap solar cells for tandem applications


Tandem photovoltaic (PV) solar cells, which use multiple absorbing layers to convert light into electricity, have the potential to surpass the conversion efficiency limits of PV which uses a single absorber. This has been proven using epitaxially grown III-V semiconductors, but these are expensive and are only commonly used for extra-terrestrial applications. To realize terrestrial, cost effective tandem PV, low cost production of these highly efficient cells is required. Using absorbers which are similar to cost effective, mass produced PV such as CdTe, Si, or CIGS, this is possible. Si and CIGS have appropriate properties for the IR absorbing layer in a tandem cell, but there is no common PV material with the ideal properties for the UV/Visible light absorbing layer, although CdTe is quite close. Even better, CdTe's properties can be altered to those of ideal with the addition of ternary elements such as Mg, Zn, and Mn. Issues still remain however as the quality of solar cells produced using ternary alloys of CdTe is much lower than that of the base material. These quality issues seem to stem from the CdTe bulk passivation process, which involves a thermal treatment in the presence of Cl (commonly CdCl2 is used as a source) to passivate the grain boundaries and catalyze the recrystallization and grain growth process which annihilates detrimental planar crystalline defects in the absorbing material. The work presented in this thesis addresses issues with the absorber quality of solar cells using Cd1-xMgxTe by using concurrent Cl sources with CdCl2, diffusion barriers during CdCl2, and tweaking the absorber material with the addition of quaternary elements or novel layers in the device stack. This work culminated in the production of a 10.6% efficient device, a record for devices using CdMgTe as an absorber, and concludes with paths for future improvements in device performance.


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II-VI semiconductors


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