3BO.9.1 CIGS Devices with Increased Bandgap Energy: Results of the EFFCIS Project
Tuesday, 7th September 2021
15:15 - 16:45
W. Witte, D. Hariskos, S. Paetel, M. Maiberg, S. Zahedi-Azad, P. Pistor, H. Kempa, R. Scheer, D. Hauschild, V. van Maris, M. Blankenship, L. Weinhardt, C. Heske, J. Keutgen, O. Cojocaru-Mirédin, E. Ghorbani, K. Albe, X. Jin, R. Schneider, D. Gerthsen, A. Nikolaeva, J.A. Marquez-Prieto, M. Krause, S. Schäfer, D. Abou-Ras, T. Unold, R. Mainz, J. Seeger, F. Wilhelmi, M. Hetterich, M. Schweiger, B. Dimmler, M. Powalla
One of the goals of the German joint research project EFFCIS is to gain a better physical and chemical understanding of devices with Cu(In,Ga)Se2 (CIGS) absorbers with increased bandgap energy Eg > 1.3 eV. Especially the reduction of the open-circuit voltage (VOC) deficit, the influence of alkali metal post-deposition treatments (PDT) on performance and properties of high-Eg CIGS solar cells/modules, and the nature of the CIGS/buffer interface are in the focus of the project. PDT with Rb or Cs significantly reduces the VOC deficit for high-Eg CIGS solar cells, compared with KF- or un-treated absorbers. In a solar cell with Eg = 1.48 eV and 14.2% efficiency, the VOC deficit is reduced to 0.64 V; for a cell with Eg = 1.38 eV and 17.2% efficiency, values as low as 0.53 V can be achieved. The order of alkali supply and the interplay between Na and Rb (or Cs) is important in terms of solar cell parameters. Electrostatic potential fluctuations extracted from electron beam induced current (EBIC) measurements are in the same range for samples with Ga/(Ga+In) ratios (GGIs) of 0.30 and 0.66, indicating also similar VOC losses. For high-Eg CIGS, atom probe tomography revealed the distribution of alkali elements on the nanometer scale, and the crystal structure was imaged by highresolution transmission electron microscopy. Cross-sectional Kelvin probe force microscopy measurements showed lower local work functions with alkali PDT at grain boundaries exposed by the cleaving process (for both GGI = 0.3 and 0.8). The external photoluminescence quantum yield reduces around 3 orders of magnitude from Eg = 1.2 to 1.6 eV, indicating enhanced non-radiative recombination in the absorber layer for increasing Eg. With in-situ energy-dispersive X-ray diffraction, the time-dependent formation of crystal phases during CIGS growth processes with high Eg was assessed. The combination of X-ray and ultraviolet photoelectron (XPS and UPS) as well as inverse photoemission spectroscopies (IPES) indicates that, with increasing GGI, the surface bandgap increases and the conduction band shifts away from the Fermi energy. In addition, a pronounced clifflike conduction band offset (CBO) at the solution-grown CdS/CIGS (GGI = 0.95) interface is found. Ab-initio density functional theory calculations for CBOs between high-Eg CIGS absorbers and various buffer materials could help to find suitable alternative candidates for a replacement of the CdS buffer.