Fill Factor Loss in a Recombination Junction for Monolithic Tandem Solar Cells

One of the challenges for monolithic tandems is a transparent region that combines the carrier-selective contacts of both solar cells. In this article a theoretical calculation for the voltage loss at the maximum power point of a tandem solar cell is presented, induced by such an interface where direct tunneling is very unfavorable. The continuity equations for electrons and holes at the interface with interface defects introducing a single level that can interact with carriers at both sides have been implemented in the Technology Computer Aided Design (TCAD) package DevSim. The analytic model is compared with the TCAD simulations for the specific case of a CIGS/perovskite solar cell. The recombination junction is formed between the window layer of the CIGS and the hole transport layer (HTL) of the perovskite. The window layer of a CIGS solar cell consists of a part which is nearly undoped (i-ZnO) and a part which is heavily doped (ZnO:Al). First a recombination junction between i-ZnO and the HTL is discussed. It is concluded that for this structure a sufficiently small interface band gap energy (Δ E g < 0.6 eV) and a surface recombination velocity higher than 1.4 × 10 6 cm/s can have limited losses induced by the recombination junction. Second, the impact of the ZnO:Al layer on the voltage loss in the recombination junction is calculated. This increased doping in the ZnO increases the built-in voltage of the recombination junction and is therefore detrimental for the fill factor of the tandem solar cell.