Impact of Nonuniformities on Thin Cu(In,Ga)Se 2 Solar Cell Performance
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1012-Y08-02
Impact of Nonuniformities on Thin Cu(In,Ga)Se2 Solar Cell Performance Ana Kanevce, and James R. Sites Physics Department, Colorado State University, Fort Collins, CO, 80523 ABSTRACT Solar-cell performance degradation due to physical nonuniformities becomes more significant as the thickness of polycrystalline absorbers is reduced. ìVoltageî nonuniformities such as those due to band-gap fluctuations, variations in the back-contact proximity, and areas where the absorber is completely depleted can have very significant impact on cell performance. Similarly local shunts can seriously degrade the efficiency. ìCurrentî nonuniformities such as optical defects have generally much less impact. The analysis presented is based on Cu(In,Ga)Se2 cells, but the qualitative results should be applicable to thin-absorber devices in general. For lateral nonuniformity studies, the solar cell is simulated by a two dimensional network of parallel diodes separated by resistors. The nonuniformities are approximated by small regions of reduced photovoltage, often referred to as ìweak diodesî, and by isolated shunt resistors. The weak-diode approach allows investigation of device performance as a function of the weak-diode voltage deficit, the ratio of weak-to strong-diode area, and the weak diodesí spatial distribution. Increased TCO resistance can isolate weak diodes, thus limiting the voltage loss due to nonuniformities, but increasing fill-factor losses. INTRODUCTION Solar cells based on Cu(In,Ga)Se2 (CIGS) are the most efficient thin-film polycrystalline cells to date, with record efficiency of 19.5% [1, 2]. In the last few years, motivated by the limited availability of the element In and by the need to decrease the production cost, several groups have produced CIGS absorbers thinner than 1 µm [3-5]. Thinning the absorbers is economically viable only if respectable conversion efficiency is maintained. To minimize the losses, it is important to analyze the impact of nonuniformities on submicron absorbers. Thinabsorber devices typically have smaller grain size. Variations in grain structure can significantly affect device parameters and are likely more pronounced for submicron cells. Smaller grains generally mean smaller carrier lifetime and may be more susceptible to band-gap fluctuations. In addition to likely being less uniform, thinner CIGS performance is likely to be more sensitive to uniformity fluctuations [6]. Nonuniformities in thin-film cells have been observed with several experimental techniques, and these are reviewed in Ref. [7]. Previous nonuniformity studies [8-11] have shown that potential fluctuations in CIGS cells are unavoidable and generally detrimental to the cell performance. They also predict that a local series resistance, like the resistance of i-ZnO layer, can reduce the nonuniformity losses. With a goal to minimize losses in submicron devices, this work separates the physical causes for potential variations and analyzes which types of nonuniformities impact thin devices more severely. It connects
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