Abstract
In this work, we perform an extensive campaign of three-dimensional numerical simulations of CIGS solar cell structures to investigate the effect of a surface-passivated CIGS with point contacts openings on the cell performance parameters (Jsc, Voc, FF and η). Detailed analysis of the combination of passivation thickness, point contact size and pitch is performed under the hypothesis of highly defective CIGS front surface and ideal chemical passivation: efficiencies close to the case of ideal (i.e., defect-free) CdS/CIGS interface can be achieved by optimized nanometer-scale point contact arrays. To account for field-effect passivation due to positive residual charge density, Qf, within the passivation layer, we vary Qf in the range 1010–1013cm−2 under the two extreme scenarios of ideal or ineffective chemical passivation. Several examples of CIGS cells with different buffer layers (CdS, ZnO, ZnMgO, In2S3, Zn(O, S)) are also analyzed. We find that a positive Qf in the interval 1012– 5·1012cm−2 can help completely recover the ideal cell efficiency, irrespective of the chemical passivation effect and even in the presence of unfavorable conduction band alignment at the buffer/CIGS heterojunction. This may help devising solutions with buffer materials alternative to CdS, boosting the performance of otherwise surface-limited cells. The effect of grain boundary defect density and position with respect to point contacts is also addressed, with a grain dimension of 750nm.
Highlights
Improvement in conversion efficiency of Cu(In1−xGax)Se2 (CIGS) solar cells has been steady and remarkable, with laboratory-scale CIGS cells reaching efficiencies above 20% on a polyimide substrate [1] and beyond 22% [2,3] on soda-lime glass, exceeding those of other thin-film technologies
All traps are energetically located at the CIGS mid-gap: our simulations showed that the effect of both interface acceptor-like and donor-like defects is largely independent of the trap energy level as long as the latter is within ± 0.3 eV from the intrinsic Fermi level
This work details the application of 3D numerical simulation to the optimization of CIGS solar cells with passivated buffer/absorber interface and front-side point contacts
Summary
Improvement in conversion efficiency of Cu(In1−xGax)Se2 (CIGS) solar cells has been steady and remarkable, with laboratory-scale CIGS cells reaching efficiencies above 20% on a polyimide substrate [1] and beyond 22% [2,3] on soda-lime glass, exceeding those of other thin-film technologies. Major electrical losses in the cell are non-radiative bulk and interface recombination, which are among the first and foremost performance limiters to take care of. The assumption that interface recombination does not play a major role in CIGS solar cells with CdS buffer layer is supported by several publications discussing the transport mechanism in CIGS solar cells (e.g., [6]). Applying this interpretation to the measured values of the parameters in the one diode model will result in the conclusion that interface recombination is not dominant. The improvements obtained with the recently introduced alkaline post deposition treatment, which affects the interface between CIGS and buffer layer, strongly support the assumption that interface recombination is still limiting the device performance [8]
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