Project J – Solution Processed Ferroelectrics in Photovoltaic Devices
Organic-inorganic metal-halide perovskites have revolutionized the field of solution processed photovoltaics within the last few years, whereas ferroelectric titanate-based perovskites are the most widely used piezoelectric materials. Piezoelectric response, however, was also observed from solution processed metal-halides, and recently the effect was optimized in a novel compound organic- inorganic perovskite with the composition trimethylchloromethyl ammonium trichloromanganese(II) [TMCM-MnCl3], which exhibits a piezoelectric coefficient of 185 pC/N that outperforms some lead-free titanates (BaTiO3 with  poling for instance exhibits a value of 105 pC/N). Merging ferroelectrics with photovoltaics has opened interesting and useful aspects: While in conventional semiconductor photovoltaic devices, photoexcited electrons and holes are separated by built-in electric fields from p-n junctions or heterojunctions, in ferroelectric materials internal electric fields due to ferromagnetic domain walls can drive the photoexited carriers. As a consequence, in a traditional semiconductor the maximum open circuit voltage is given by the band gap of the semiconductor, whereas in ferroelectric photovoltaic devices the possibility to achieve above-bandgap voltages have been discussed. A severe disadvantage of the oxide based ferroelectric materials for photovoltaics has been that their band gap energies are too high to efficiently harvest the sun’s spectrum, whereas specially designed low band gap oxide perovskites exhibited relatively good performance only in a multilayer stacked architecture.
In this project photovoltaic devices will be developed from solution processed ferroelectric semiconductors with a perovskite-like crystal structure. The effect of the ferroelectric field on the photovoltaic performance will be optimized to combine high power conversion efficiency and high open circuit voltages. Novel molecular ferroelectric materials will be developed for these purposes in the form of thin films and single crystals based on lead free metal halides with different organic counter ions, to tune the ferroelectric response as well as the materials band gap energy.
|Prof. Dr. Wolfgang Heiß
Institute of Materials for Electronics and Energy Technology
Department of Materials Science and Engineering