Can Grain Boundaries Improve the Performance of Cu(In,Ga)Se 2 Solar Cells?
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1012-Y09-01
Can Grain Boundaries Improve the Performance of Cu(In,Ga)Se2 Solar Cells? Kurt Taretto1, and Uwe Rau2 1 Universidad Nacional del Comahue, Buenos Aires 1400, NeuquÈn, 8300, Argentina 2 IEF-5, Photovoltaics, Forschungszentrum J¸lich, J¸lich, 52425, Germany
ABSTRACT Two-dimensional numerical device simulations investigate the influence of grain boundaries on the performance of Cu(In,Ga)Se2 solar cells focussing on the question whether or not grain boundaries can improve the efficiency of those devices. The results unveil the following statements: (i) The mere introduction of a grain boundary by adding localized defects into a device that has a high performance from the beginning is not beneficial. (ii) Polycrystalline solar cells can outperform monocrystalline ones, if the total number of defects is equal in both devices i.e. a given number of recombination centers is better dealt with if these defects are concentrated at the grain boundary rather than homogeneously distributed in the bulk. (iii) A significant improvement of carrier collection via the grain boundaries is found if the bulk of the devices is assumed as relatively poor. In this situation, addition of defects that are not much recombination active but provide a large charge density at the grain boundaries can improve the device performance. (iv) Almost perfect passivation of grain boundaries by an internal band offset in the valence band requires an internal barrier of at least to 300 meV. INTRODUCTION Solar cells made from Cu(In,Ga)Se2 (CIGS) provide the highest power conversion efficiency of all thin-film photovoltaic devices [1]. The grain size g of these polycrystalline absorber films hardly exceeds the film thickness d of typically d = 1.5 - 2 µm. The electronic activity of grain boundaries (GBs) in such a situation is much more critical than in multi-crystalline Si solar cells with g being of the order of g = 5-10 mm. Interestingly, solar cells made from polycrystalline CIGS absorbers markedly outperform their monocrystalline counterparts, a situation that is very similar to that of polycrystalline CdTe solar cells but very different to that of multicrystalline /monocrystalline Si solar cells. During the last few years, considerable research effort was directed to understand why the polycrystalline materials CdTe [2,3] and CIGS [4-15] yield such excellent photovoltaic absorbers. Especially, the question was discussed whether, and if yes, how grain boundaries (GBs) can be beneficial for CIGS and CdTe solar cells whereas they are detrimental for most other photovoltaic materials. The present work uses two-dimensional simulations of polycrystalline CIGS solar cells to investigate the influence of GBs on their electrical performance. Some of the present results have an overlap to earlier work [16,17]. Here, we concentrate on the question whether and under which circumstances polycrystalline material can outperform a single crystalline semiconductor material in terms of photovoltaic performance. We find that such a situation cannot be constr
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