Multi junction CIGS cells
Currently, the maximum efficiency of compound thin film photovoltaic devices is close to 23%, already approaching the technological limit. In order to further increase this value, a combination of solar cells with different spectral sensitivities, called multi-junction device, is necessary. These can convert a higher fraction of the energy of the solar spectrum into electric energy because each individual cell is optimized for a specific part of the solar spectrum.
However, these multi-junction devices require new technological solutions like high-efficiency, wide band gap absorbers, transparent materials for charge transport between individual cells, low temperature deposition processes for top cells and absorber materials with matching spectral response.
Current research topics at PVcomB
Hole-selective transparent oxides for thin film tandem devices
At PVcomB we currently focus on p-type, transparent conducting materials to combine a low band-gap CIGS bottom cell with large band gap methyl ammonium lead halides (commonly known as “perovskites” . Figure 2 shows the schematic cross section of a possible perovskite/CIGS tandem cell. Prerequisites for this hole-conducting material are
- A formation temperature of below 200°C, because it would be deposited on an existing CIGS device, which degrades at higher temperature
- A band gap exceeding 1.7 eV to make it transparent for long wave-length light
- Sufficient carrier density to form a well conducting layer between top and bottom cell and formation of a good contact with the n-doped ZnO from the bottom cell.
- Chemical compatibility with both perovskite and ZnO
- Long-term stability under the conditions of outdoor use of the PV device
Promising material we are utilizing are cuprous oxide (Cu2O) and copper gallium oxide (CuGaO2), which have already been used in single-junction perovskite devices , . New materials are sought in collaboration with partners within the speedCIGS project. There, material properties are predicted based on theoretical calculations using the density functional theory (DFT). We collaborate with the Young investigator groups of Eva Unger and Steve Albrecht that work on different topics around perovskite PV.
 W. B. Yan, S. Y. Ye, Y. L. Li, W. H. Sun, H. X. Rao, Z. W. Liu, Z. Q. Bian and C. Huang, Adv. Energy Mater 6 (17) (2016).
 H. Zhang, H. Wang, W. Chen, and A. K.-Y. Jen, Adv. Mater. 2017, 29, 1604984
 "Cu2O as a potential intermediate transparent conducting oxide layer for monolithic perovskite-CIGSe tandem solar cells", Yajie Wang, Alexander Steigert, Guanchao Yin, Vladimir Parvan, Reiner Klenk, Rutger Schlatmann, and Iver Lauermann, subm. to pss 2017.