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HL: Fachverband Halbleiterphysik

HL 27: Focus Session: Perspectives in Cu(In,Ga)Se 1

HL 27.1: Hauptvortrag

Donnerstag, 8. September 2022, 09:30–10:00, H33

What limits state-of-the-art chalcopyrite solar cells? — •Susanne Siebentritt — Laboratory for Photovoltaics, Department of Physics and Materials Science, University of Luxembourg

Chalcopyrite solar cells have reached 23.4% efficiency, less than Si solar cells. Why are chalcopyrite solar cells not better? State of the art chalcopyrite solar cells are based on an absorber with a band gap gradient in depth, to keep electrons from the back contact and to reduce non-radiative recombination at the back contact. However, this graded band gap profile can decrease the short circuit current because of a rather low absorptance near the absorption edge. Additionally, the gradual increase of the absorptance leads to radiative loss in the open circuit voltage (VOC). Additional fluctuations and disorder lead to exponential band tails and to radiative and non-radiative VOC losses. These Urbach tails are larger in polycrystalline films than in epitaxial films, indicating a contribution of grain boundaries, however, the difference is only a few meV in Urbach energy, indicating a common source independent of grain boudaries. The dependence of the Urbach energy on the net doping level hints to electrostatic fluctuations as a main source of tail states. In addition to these limitations of the short circuit current and the open circuit voltage, the diode factor of most chalcopyrite solar cells is high, implying a low fill factor. It became only recently clear, that metastable defects contribute massively to the increased diode factor of these solar cells.