Two terminal (2T) perovskite/copper-indium-gallium-selenide (CIGS) tandem solar cells combine high conversion efficiency with lightweight flexible substrates which can decrease manufacturing and installation costs. In order to improve the power conversion efficiency of these tandem solar cells, the use of advanced simulation tools is crucial to estimate the loss mechanisms. In this regard, most of the available simulation works on tandem solar cells are oriented to minimize optical losses and assuming simplifications for the electrical simulations in particular in the top and bottom cell interconnection at the so-called tunnel recombination junction (TRJ) neglecting the inner physics of the complete tandem device. Therefore, the effect of charge exchange mechanism between top and bottom soler cells on the external parameters of a tandem devices is not fully understood yet. In this work, we present an experimentally validated opto-electrical model based on the fundamental semiconductor equations for the study of loss mechanisms of a reference perovskite/CIGS solar cell. Different from other numerical works, because our simulation platform includes the fundamental working mechanisms of the layers comprising the TRJ, we can properly calculate the losses related to it. We firstly present the calibration and validation of our opto-electrical model with respect to three fabricated reference solar cells: top cell only, bottom cell only and tandem device. Then, we use the calibrated model to evaluate main loss mechanisms affecting the baseline tandem device. Finally, we use the model to propose a roadmap for the optimization of monolithic perovskite/CIGS tandem solar cells.

An interdigitated back-contacted (IBC) configuration is proposed for submicron copper indium gallium (di)selenide (CIGS). In a modelling platform, the structure was opto-electrically optimized for maximum efficiency. The results are compared with a reference front/back-contacted (FBC) solar cell with similar absorber thickness and exhibiting 11.9% efficiency. The electrical passivation at the front side is accomplished by an Al₂O₃ layer, which is endowed with negative fixed charges. The results indicate that with an optimal geometry and engineered bandgap grading, the efficiency of the new IBC structure can reach 17%. Additionally, with a reasonably low defect density in the absorber layer, efficiencies as high as 19.7% and open-circuit voltage comparable with that of the record solar cell are possible with the IBC structure.

The optical losses associated with sub-micron absorbers in CIGS solar cells can be reduced by light management techniques. 3-D optical modelling was used to optimize light in-coupling and internal rear reflectance in a 750-nm thick CIGS reference solar cell. At the front side, an effective medium approximation (EMA) approach for describing optical properties of a MgF₂-based anti-reflection coating (ARC) was applied. Taking reflectance as the cost function and sequential nonlinear programming as the optimization algorithm, an optimal porous-on-compact double-layer ARC was determined. This led to a wideband light in-coupling with a 6.8% improvement in the photo-current density (J_ph) with respect to the reference solar cell without ARC. Considering the variation of the sunlight direction due to day and seasonal changes, different light incidence angles were investigated. The results indicate that in this case, our designed double-layer ARC outperforms the standard compact MgF₂ single-layer ARC. By using the EMA approach, the amount of computational memory can be reduced by a factor of 30, shortening the simulation time from four days to one hour. At the rear side of the cell, a point-contacted MgF₂/Al₂O₃ reflector, in combination with our proposed front ARC, enhances the J_ph by 11.3% considering the same reference solar cell. Compared to a much thicker cell (1600-nm thick absorber) with no light management applied, our front-and-rear optical approaches more-than-compensate optical losses resulting from using thinner absorbers. This design is suitable for industrial uptake and practical to realize. Additionally, the approach of using EMA for double-layer ARC optimization is innovative with respect to other ARC approaches applicable to not only chalcopyrite photovoltaic technologies.

A novel back-contacted solar cell based on a submicron copper indium gallium (di)selenide (CIGS) absorber is proposed and optically investigated. First, charge carrier collection feasibility is studied by band diagram analysis. Then, two back-contacted configurations are suggested and optimized for maximum current production. The results are compared with a reference front/back-contacted CIGS solar cell with a 750-nm-thick absorber. Current density production of 38.84 mA/cm ² is predicted according to our simulations for a realistic front-side texturing. This shows more than 38% improvement in optical performance compared to the reference cell and only 7.7% deviation from the theoretical Green absorption benchmark.