Perovskite silicon tandem solar cells have the potential to overcome the theoretical efficiency limit of conventional silicon solar cells, but require effective light management and well-adjusted interlayer properties in the complex stack. Transparent conductive oxide (TCO) layers allow light to pass through to the absorbers while allowing lateral charge transport to the contacts at the front side of the solar cells. Thus, optimization of TCO layers to achieve highest transparency and conductivity while having good interfacial properties with neighboring layers is important for achieving high efficiencies in tandem solar cells.

This work presents an optimization study on serial co-sputtered indium-zinc oxide (IZO) films used as front TCO layer for perovskite silicon tandem solar cells. First, the effect of oxygen partial pressure and Zn target power during the sputtering process of IZO films on the single film properties are investigated. Three groups of films with the following Zn target power (W) and oxygen partial pressure (sccm) are produced: 140 W/34 sccm, 100 W/34 sccm, 100 W/30 sccm. This process parameter range was chosen by running a series of experiments to achieve optimum electrical and optical properties of IZO layers. The optical and electrical characterization of the single films show that similar resistivity and transparency can be achieved for three different processes. However, the 100 W/34 sccm films show slightly lower average mobility (39 cm²/Vs) compared to the other processes (48 cm²/Vs and 47 cm²/Vs).

The IZO films are successfully implemented into perovskite silicon tandem solar cell structures. In our 2-terminal monolithic p-i-n device structure the perovskite top solar cell is electrically connected to the silicon bottom solar cell with an ITO layer. The perovskite solar cell is composed of PTAA and PFN hole transport bilayer, perovskite (1.64 eV bandgap), C60 and SnO₂ electron transport bilayer, IZO, Ag metallization and MgF₂ anti-reflective coating [1]. Higher FF values are obtained for lower Zn target power groups (100 W). Despite the slightly lower mobilities of the single layers, highest efficiencies were obtained with the 100 W/34 sccm sputtering parameters mainly due to higher fill factor (FF) values with an average of 77% and 76% (forward and reverse scan respectively) with highest stabilized power conversion efficiency of 24.7%. The mismatch between the single layer results and the solar cell results with the implemented IZO layers might be an indicator of varying interlayer properties in the complex tandem structure.