Introducing Lead-Free Hybrid Organic-Inorganic Halobismuthates as High Performance Piezoelectrics

The piezoelectric effect is used in applications as diverse and useful as inkjet printers and atomic force microscopes. Organic-inorganic hybrid (OIH) ferro- and piezo-electric materials are attracting interest over traditional piezo-ceramics due to their more facile fabrication, mechanical flexibility and substantially reduced toxicity compared to traditional ceramics. Moreover, molecular ferro- and piezo-electrics often possess semiconductor bandgaps within the optical range, and have therefore been studied for the anomalous photovoltaic properties arising from their intrinsic non-centrosymmetry. Recent reports have shown that incorporating ferroelectric OIH perovskites into conventional halide perovskite solar cells and applying electrical poling, significantly improved photovoltaic performance can be achieved via suppression of interfacial recombination due to ferroelectricity-induced modification of the built-in field. Indeed, by this method open-circuit voltages surpassing the halide perovskite bandgap have even been reported.
Here, we introduce a family of novel, stable, bismuth-based OIH halide piezoelectrics, which crystallise in a polar space group at room temperature and exhibit bandgaps within the optical range (>2 eV), in close agreement with those calculated from first-principles methods. To achieve a detailed understanding of how the structure of these halobismuthate materials leads to their piezoelectric and optoelectronic activity, and how composition-tuning within the family can alter these properties, we present a combined variable-temperature solid-state nuclear magnetic resonance (ssNMR), single-crystal X-ray diffraction (scXRD) and thermal analysis study. This powerful combination of characterisation techniques allows us to understand the atomic-level structure and dynamic motion of both the organic and inorganic components of these materials as their non-centrosymmetric structures evolve through complimentary phase transitions at elevated temperature, and thus determine the structural origin of their piezoelectric activity.
Finally, a combination of piezoresponse force microscopy, polarization hysteresis loops and pyroelectric measurements on both single crystals and thin films of the best performing member of this halobismuthate family demonstrate the highest piezoelectric coefficient yet reported for any halobismuthate piezoelectric.