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.

Multi-junction solar cells are considered for various applications, as they tackle various loss mechanisms for single junction solar cells. These losses include thermalization and non-absorption below the band gap. In this work, a tandem configuration comprising copper-indium-gallium-di-selenide (CIGS) and hydrogenated amorphous silicon (a-Si:H) absorber layers is studied. Two main challenges are addressed in this work. Firstly, the natural roughness of CIGS is unfavorable for monolithically growing a high quality a-Si:H top cell. Some sharp textures in the CIGS induce shunts in the a-Si:H top junction, limiting the electrical performance of such a configuration. To smoothen this interface, the possibility of mechanically polishing the intermediate i-ZnO layer has been explored. The second challenge that is addressed, is the significant current mismatch in these tandem architectures. To enhance absorption in the current-limiting top cell, the ZnO:Al front electrode was textured by means of wet-etching the entire tandem stack. We demonstrated that one can manipulate the morphology of the random textures by varying the growth conditions of the ZnO:Al, leading to better light management in these devices.