Device physics of copper(indium,gallium)selenide(2) thin-film solar cells.

摘要

Thin-film solar cells have the potential to be an important contributor to the global energy demand by the mid-21st-century. Cu(In,Ga)Se 2 (CIGS) solar cells, which have achieved laboratory efficiencies close to 20%, are highly attractive, because their band gap is near the optimal value, their polycrystallinity is not significantly detrimental to their performance, and the broad choice of heterojunction partners available allows additional degrees of freedom for optimizing their performance. Although steady progress has been made for CIGS solar cells, this progress has largely been driven by empirical optimization rather than by in-depth understanding of appropriate physical models. This thesis is intended to fill some of the gaps that exist between state-of-the-art experimental solar cells and their device physics. The level of complexity involved is largely prohibitive to analytical treatment and, hence, numerical approaches are primarily utilized.; The dominant topics for CIGS solar cells covered in this dissertation are (1) variation in the Ga/(Ga+In) stoichiometry ("grading"), (2) the formation of "good" heterojunctions, (3) photoconductive effects in window or buffer materials, (4) the apparently benign or even beneficial presence of grain-boundaries (GBs), including a discussion of charged GBs and the effects of Cu-depletion near GBs. This work establishes a baseline model for CIGS solar cells and, from this starting point, the device physics relating to these questions is discussed and principles are identified that apply to a broad range of devices.; CIGS grading is shown to have only small potential to improve device performance. This conclusion conflicts with earlier studies, and it is shown that the difference arises in the evaluation of the grading benefit, in particular, the proper choice of the reference performance. Band-gap increases toward the front of the device are most likely detrimental, while band-gap increases toward the back can be modestly beneficial. The popular "double grading" approach achieves only very small additional gains over the simple back grading approach. Very thin-absorber cells can benefit substantially from back grading, because in this case, grading can mitigate detrimental back-contact surface recombination.; The window/absorber interface is studied and, in good agreement with experiments, a limitation of the open-circuit voltage is observed for wide-band-gap CdS/CIGS solar cells. (Abstract shortened by UMI.)