Design and Optimization of Perovskite / CIGS Tandem Solar Cells

Tandem photovoltaic devices based on CIGS and perovskite solar cell (PSC) sub-cells offer the implementation of all-thin-film tandems with all the advantages of thin-film technology, together with the possibility to achieve high efficiencies above the thermodynamic limit of single-junction solar cells. More precisely, the CIGS/perovskite tandem solar cell technology allows for low cost, light-weight, large area roll-to-roll fabrication and lower integration and installation costs, finally promising lower energy costs compared to current technologies. To this date, record power conversion efficiencies of over 24% and 27% have been achieved for monolithic two-terminal (2T) and stacked four-terminal (4T) tandems, respectively.<br/>The crucial step for coupling CIGS and PSC devices in one tandem cell lies in the optimization of the layer stack with respect to light management and current collection, which include the growth of the transparent electrode layers. These requirements become even more stringent in monolithic 2T tandems, where cell performance requires current matching between top and bottom cells. This means that in particular for the CIGS cell we need to tailor the photoactive band gap to make optimum use of the partial illumination in the spectral region above ~750 nm wavelength, which is transmitted through the perovskite cell stack. One means to realize a beneficial trade-off between gains in the open circuit voltage and losses due to reduced photoabsorption is by the design of band gap grading, i.e. Ga in-depth distribution in the CIGS absorber film.<br/>Hence, our approach to achieve high performance monolithic interconnected perovskite/CIGS 2T tandem solar cells rests on four major building blocks. (i) The adaptation of the CI(G)S bottom cell including control over the surface roughness, (ii) growth and optimization of the PSC layer stack on top, notably with focus on the hole transport layer, (iii) the design of the tunneling/recombination junction connecting the two sub-cells, and (iv) the implementation of light management structures for improved current matching.<br/>All these tasks involve a dedicated focus on the control and analysis of the interfaces in each sub-cell and between the two cells, which will be the focus on my talk. I will lay out our methodology to assess the interface chemistry and the effect on its optoelectronic properties and functionality in the device. This point is particularly relevant for the optimized PSC and significantly affects the overall device performance and stability.