Anticipated performance of Cu(In,Ga)Se₂ solar cells in the thin-film limit

Abstract

The demand for alternative sources of energy is rapidly increasing. Although thin-film polycrystalline solar cells are an excellent candidate for clean energy production with competitive prices, the knowledge of device physics is still incomplete, especially in the very low thickness limit. The goal of this work is to enhance the understanding of thin-film solar cells with submicron absorbers, which would lead to significantly lower production cost and wider commercial usage. One of the largest obstacles towards increased large scale Cu(In,Ga)Se2 (CIGS) production is the expense of the elements involved, especially indium. The usage of In and the other materials, as well as the deposition time, can be significantly decreased, if the thickness could be reduced without significant efficiency loss. The main focus of this work is on CIGS cells with thin absorbers, in particular: (1) the influence of specific parameter variations on device performance, (2) the sensitivity to nonuniformities in key parameters, (3) the possibility for illumination from both sides of the cell, and (4) the role of the buffer layer between absorber and contact. As the cells become thinner, the distance between the back contact and the electric-field region narrows, or, for very thin cells, completely disappears. One of the main differences between thick and thin cells is that in thinner cells more light reaches the back-contact and back-contact recombination becomes a significant loss mechanism. A choice of the back-contact material or increased fraction of Ga towards the back are needed to avoid losses. The Ga/In ratio needed and the effect of Ga distribution through the absorber on the performance of ultra-thin cells are analyzed, and compared with the thick cells. In addition to the issues with uniform thin absorbers, there is a concern of increased sensitivity to nonuniformities. Fluctuations in the defect density, band gap, thickness or doping affect the voltage. The low voltage area then influences the nonuniform cell performance depending on the voltage difference between the "strong" and "weak" area, the ratio between the "weak" and "strong" area and the distribution of the "weak" area. Thinner cells are more sensitive to thickness nonuniformities, and therefore deposition methods that produce smooth absorbers are desirable for thin devices. In ultra-thin cells illumination from the back side, i.e. light entering CIGS from the opposite side of the junction, can yield a significant output performance. If certain optimization steps are taken, the back side performance can approach that of the front side. In the case of a back-illuminated cell, the carrier generation occurs in the bulk, making it critical to lower back-contact recombination, as well as to provide good collection. Different parameters such as thickness, doping, minority carrier lifetime and band-gap have an impact on the back illumination performance. The main optimization steps need to be directed towards short circuit current increase. In ultra-thin limit, back and front illumination performances converge. Illumination with only low energy photons in cells with the commonly used CdS buffer layer causes distortion in the current-voltage curves, whose mechanism is explained in this work. The difference in distortion for thin and thick devices is studied. Some alternatives to CdS have a great potential. CdZnS is a higher band gap candidate for buffer layer that has achieved efficiency close to the record one and with some optimization can open the door to efficiencies higher than 20 %.