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.

In a monolithic perovskite/c-Si tandem device, the perovskite top cell has to be deposited onto a flat c-Si bottom cell without anti-reflective front side texture, to avoid fabrication issues. We use optical simulations to analyze the reflection losses that this induces. We then systematically minimize these losses by introducing surface textures in combination with a so-called burial layer to keep the perovskite top cell flat. Optical simulations show that, even with a flat top cell, the monolithic perovskite/c-Si tandem device can reach a matched photocurrent density as high as 19.57 mA/cm².

Copper-indium-gallium-di-selenide (CIGS) is the present record holder in lab-scale thin-film photovoltaics (TF-PV). One of the problems of this PV technology is the scarcity of indium. Multi-junction solar cells allow better spectral utilization of the light spectrum, while the required current generation per layer is much lower, allowing much thinner absorber layers of CIGS. In this contribution we demonstrate working fabricated devices of CIGS bottom cells that are monolithically integrated with a hydrogenated amorphous silicon (a-Si:H) top cell. The proposed structures are a unique fusion of two distinct fabrication methods, being co-evaporation and plasma enhanced chemical vapor deposition (PE-CVD). In addition, devices without any ZnO have been processed. In those cells a nc-SiO_x:H n-layer acted as an electron recipient and lateral insulator for the CIGS p-layer, and a highly p- and n-doped nc-SiO_x:H layer served as the tunnel recombination junction. The top TCO on the a-Si:H cell was varied with ZnO:Al (AZO) and In₂O₃/Sn₂O₃ (ITO). Efficiencies of the not yet optimized devices have reached 7.9% active area efficiency (with V_oc=1.23V, FF=64%, J_sc= 9.95 mA/cm²).