The Multiple Roles of (Ag,Cu)(In,Ga)(S,Se) ₂ in Tandem Solar Cells

The chalcopyrite material (Ag,Cu)(In,Ga)(S,Se)₂ is investigated as both bottom cell and top cell in tandem devices, due to its bandgap range, which ranges from 1.0 for Ag_xCu_1-x)InSe₂ with x=0 to 0.25, to 1.8 eV for AgGaSe₂ or 2.4 eV for CuGaS₂. The present world record of 23.65 % efficiency made in collaboration with First Solar ETC and Uppsala University has an effective bandgap of 1.12 eV and consists of (Ag,Cu)(In,Ga)Se₂ with an Ag/(Ag+Cu) ratio of 0.1. In-depth grading of the Ga/(Ga+In) ratio is used as a means to form a charge selective back contact, which reduces the charge density at the back contact, while keeping a good ohmic contact. It is also possible to use a sulfur-based surface treatment resulting in S substituting for part of the Se at the surface as in the former 23.35 % record device by Solar Frontier.<br/>While both the present and former world record cells use back contacts based on molybdenum, new designs are needed to optimize the cells for use in tandem devices. For the bottom cell to be competitive to crystalline silicon with its 1.1 eV bandgap, very low effective bandgaps are targeted down to 1.0 eV, which means that the surface consists of (Ag,Cu)InSe₂ or CuInSe₂. Furthermore, given the present Si development, bi-faciality will also be an important asset, which requires a transparent back contact. For the top cell in a tandem, it will be necessary to significantly increase the bandgap to values above 1.5 eV and also to deposit the cells on a transparent back contact. A high bandgap means that either a high sulfur and/or high gallium concentration is needed.<br/>A low bandgap (A)CIGSe bottom cell can be combined either with a perovskite top cell, with CdTe or with a high bandgap (A)CIG(S)Se top cell. For the CIGS-perovskite tandem, both 2-terminal and 4-terminal solutions are possible and a record monolithic two-terminal device made at the Helmholz Zentrum Berlin reaches 24.2 % using a CIGS bottom cell with 1.1 eV bandgap. This device differs from many of the devices presented recently in the literature, since it is a fully functional tandem. In several publications, higher numbers are presented as four terminal results, using a perovskite cell as a filter and measuring the devices separately. For the all-CIGS tandem in the on-going project SITA, funded by the EU, we are investigating a back illumination configuration for the top cell, but with a concept using either current or voltage matching for the full module.<br/>Both for bottom cell and top cell scale-up, the properties of the transparent contacts are important. For a four-terminal solution, avoiding parasitic absorption caused by free carriers is even more crucial than for two terminal devices. Additionally, for the transparent contacts deposited on the substrate before the CIGS deposition, the demands on chemical integrity are high. The contacts need to be able to sustain high temperatures in a corrosive environment without losing either transparency nor conductivity.<br/>In this contribution, the challenges in adapting the CIGS device to obtain a suitable tandem partner, both as bottom and as a top cell are discussed and new developments aiming to increase the efficiency and the energy yield are discussed.