Simulation of Polycrystalline Cu(In,Ga)Se2 Solar Cells in Two Dimensions

The extent to which grain boundaries (GBs) in polycrystalline materials may be detrimental, benign, or even beneficial is explored with numerical simulations in two dimensions. We focus on the effects of GB recombination in Cu(In,Ga)Se2 (CIGS) solar cells and its effects on solar-cell performance. The simulations predict that (1) for device efficiency exceeding 17%, the effective GB recombination velocity must be less than 104 cm/s; (2) grain boundaries within the space-charge region (SCR) lower the open-circuit voltage, whereas the short-circuit current is reduced by grain boundaries in the bulk material; and (3) horizontal GBs are relatively benign unless they are located in the SCR. Modifications to the electronic structure near grain boundaries show that charge-induced band-bending at grain boundaries will most likely have a negative effect on device performance, whereas a down-shift in the valence-band energy at the grain surface can effectively passivate the GBs and reduce the effective recombination velocity. For the models considered, GBs generally have a deleterious effect on efficiency, and GBs alone can not explain the apparent superiority of polycrystalline over single-crystalline CIGS materials. INTRODUCTION Conversion efficiencies of Cu(In,Ga)Se2 solar cells have nearly reached 20% [1]. It is often argued that these improvements have been achieved despite, or because of, the presence of grain boundaries (GBs). A frequently used argument for the latter is that the much less investigated, crystalline counterparts have achieved substantially lower efficiencies, around 10%–12% [2]. This work uses numerical simulations to predict the changes to CIGS solar-cell efficiencies that are introduced by the presence of GBs. The effects of uncharged horizontal and columnar GBs are investigated and quantified. Detailed analysis shows how different parts of the GB affect the carrier collection and forward recombination. For moderate values of GB recombination velocities (∼105 cm/s), the effects of electrostatic charging of GBs and valence-band offsets are investigated. Charges establish an electrostatic potential, which has been reported in the literature for CIGS solar cells [3–5]. A valence-band offset was calculated from first principles [6], and Cu-depletion at GBs [7] was experimentally observed; the latter is also predicted to cause a down-shift of the valence-band energy [8]. Similar Cu depletion and band-gap widening toward the valence band were observed at the CIS surface [9]. Our findings suggest that a plausible reason behind highly efficient thin-film CIGS solar cells (η > 17%) is an inherent valence-band offset due to surface Cu-depletion that passivates GBs. No configuration considered predicted a beneficial effect of GBs on solar efficiency and this suggests that the superiority of polycrystalline over single-crystalline material has to originate from poor interface or bulk properties of the single-crystalline material and not from the absence of GBs. A comprehensive study of the GB problem, including a detailed description of the model, a consideration of a broader parameter space, detailed analysis including analytical treatment, and F14.1.1 Mater. Res. Soc. Symp. Proc. Vol. 865 © 2005 Materials Research Society