Thinning ferroelectric films for high-efficiency photovoltaics based on the Schottky barrier effect

Achieving high power conversion efficiencies (PCEs) in ferroelectric photovoltaics (PVs) is a longstanding challenge. Although recently ferroelectric thick films, composite films, and bulk crystals have all been demonstrated to exhibit PCEs >1%, these systems still suffer from severe recombination because of the fundamentally low conductivities of ferroelectrics. Further improvement of PCEs may therefore rely on thickness reduction if the reduced recombination could overcompensate for the loss in light absorption. Here, a PCE of up to 2.49% (under 365-nm ultraviolet illumination) was demonstrated in a 12-nm Pb(Zr0.2Ti0.8)O3 (PZT) ultrathin film. The strategy to realize such a high PCE consists of reducing the film thickness to be comparable with the depletion width, which can simultaneously suppress recombination and lower the series resistance. The basis of our strategy lies in the fact that the PV effect originates from the interfacial Schottky barriers, which is revealed by measuring and modeling the thickness-dependent PV characteristics. In addition, the Schottky barrier parameters (particularly the depletion width) are evaluated by investigating the thickness-dependent ferroelectric, dielectric and conduction properties. Our study therefore provides an effective strategy to obtain high-efficiency ferroelectric PVs and demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. An approach to boost the power conversion efficiencies (PCEs) of ferroelectric photovoltaics (PVs) is proposed based on the Schottky barrier effect. This approach leverages the thinning of a ferroelectric film to somewhere close to the depletion width, which can simultaneously suppress the recombination and lower the series resistance. Using this approach, we achieve a PCE up to 2.49% (under 365-nm ultraviolet illumination) in the 12-nm Pb(Zr0.2Ti0.8)O3 ultrathin films. Our study provides insightful guidance on how to design and tailor the ferroelectric films to achieve high PCEs, and also demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. Eliminating stray electrical effects in ultra-thin films can help optimize an unconventional solar energy technology. Ferroelectric materials have internal dipoles that spontaneously move photogenerated charges toward external circuits, producing higher power outputs than predicted by theory. Zhen Fan from South China Normal University in Guangzhou and colleagues now report that the dimensions of ferroelectric thin films distinctly affect how efficiently they convert sunlight into electricity. Measurements of solar cells containing lead-zirconium-titanate ferroelectrics with different thicknesses revealed a jump in conversion efficiencies when the film reached a thickness of 12 nanometers. Further analysis showed that this thickness correlates with the solar cell’s ‘depletion width’, a zone formed when metal electrodes contact the film. The electric field in the depletion zone complements the pushing actions of the ferroelectric dipoles, lowering electrical losses compared to thicker ferroelectric films.