Modeling Recombination Junctions for Tandem Solar Cells

Thin film solar cells of the 2nd and 3th generation with CIGS, CdTe, CZTS and perovskite based absorber layers are very promising solar cell technologies. One of the main advantages comparing to silicon technologies are the limited materials consumption and the flexibility to deposit them on a variety of surfaces including flexible substrates. Both silicon, CIGS and perovskite solar cells have nowadays already a descent efficiency. To further improve the efficiency of solar cells scientists are often investigating their potential in tandem structures. This can be tandem structures with a thin film solar cell on top of silicon but also a combination of thin film technologies.<br/>For monolithic solar cells not only the optical quality of the junction between both cells but also electrical losses need to be minimized. Traditionally in tandems based on III-V semiconductors this is done with tunnel junctions. However for the materials used as carrier selective contact in thin film solar cells creating heavily doped layers to enhance the tunnel probability is still questionable. Therefore recombination junctions are good candidates to fulfill the task of forming a good optical and electrical contact between the two solar cells. Nonetheless traditionally in modeling potential thin film tandem solar cells the cells are modeled individually and combined as a monolithic cell by connecting both IV curves in series and adjusting the irradiation spectrum of the bottom cell. This often results in an overestimate of the performance by not including the electrical losses in the connection in between. Many TCAD simulation packages are not including the possibility to add an interface defect that allows a direct coupling between the electron and hole currents from both sides via this recombination.<br/>In this work we developed a theoretical model for the current voltage characteristics (IV) of a recombination junction in between solar cells. Based on this IV curve the voltage loss of a tandem solar cell with known IV curves for the individual solar cells is evaluated.<br/>We implemented the equations for such a recombination defect in DeVSim TCAD software, which allows python scripting. These TCAD simulations allow to solve the continuity equations for holes and electrons for the whole tandem structure. This software package is used to calculate the efficiency of a 1D - Perovskite CIGS solar cell. These calculations show a good correspondence with our theoretical model. Based on these TCAD simulations the impact of the recombination junction and its properties on the performance of the a CIGS – Perovskite tandem solar cell is evaluated.