Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

Abstract

Two-dimensional numerical device simulations investigate the influence of grain boundaries ͑GBs͒ on the performance of Cu͑In, Ga͒Se 2 solar cells. We find that the electronic activity of grain boundaries can reduce the efficiency of Cu͑In, Ga͒Se 2 solar cells from 20% to below 12% making proper passivation of GBs a primary requirement for high efficiency. Cell efficiencies larger than 19% require GB defect densities below 10 11 cm −2 . Also, an internal band offset in the valence band due to a Cu-poor region adjacent to the GBs could effectively passivate grain boundaries that are otherwise very recombination active. It is shown that such a barrier must be more than 300 meV high and at least 3 nm wide to virtually suppress all grain boundary recombination. Contrariwise, such a barrier represents an obstacle for hole transport reducing carrier collection across grain boundaries that are not perpendicular to the cell surface. We further find that inverted grain boundaries lead to an accumulation of the short circuit current along the grain boundary, which in certain situations enhances the total short circuit current. However, we do not find any beneficial effect of any type of grain boundaries on the overall cell efficiency.

Optical spintronics in organic-inorganic perovskite photovoltaics

“…This echoes similar scenarios put forth to explain the high photovoltaic conversion efficiency of other polycrystalline materials such as CdTe and copper indium gallium selenide (CIGS) [57–62]. Although the overall role of grain boundaries in these materials remains an open question [55, 63, 64], it’s generally accepted that charged grain boundaries which induce band bending sufficient to cause type inversion at the grain boundary core act as an efficient charge collectors rather than as recombination centers [64] (under short circuit conditions). In this case, the electrostatic field near a grain boundary separates electrons and holes, and the carrier which is attracted to the defective grain boundary core avoids recombination because of the type inversion which occurs there.…”

Direct Insight into Grain Boundary Reconstruction in PolycrystallineCu(In,Ga)Se2with Atomic Resolution

“…The width of this suggested barrier is larger than the extensions of the regions at TBs with compositional changes, as measured in the present work. Nevertheless, tunneling of charge carriers to the TB is very probable for widths below 3 nm, as calculated by Taretto et al [7].…”

“…In particular, for large band bending such that the GBs become type-inverted, i.e., that the electrons become majority carriers, the recombination at GBs may be significantly reduced. Two-dimensional device modeling [5][6][7] showed, however, that such a bend bending at the GBs leads to a reduction in the open-circuit voltage and is thus not beneficial to the overall device efficiency.…”

A straightforward method to prepare chalcopyrite CIGS films by one-step sputtering process without extra Se supply

“…When the band bending is taken into consideration, the grain boundaries which are perpendicular to the film surface could suppress the recombination and improve the carrier collection. By contrast, the grain boundaries parallel to the film surface could become the obstacle for the hole transport and block the current collection [13]. Consequently, the columnar granular structure with the perpendicular grain boundaries might be advantageous for current collection.…”