Project J – Solution Processed Ferroelectrics in Photovoltaic Devices
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 [001] 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.
