Ferroelectric materials have drawn widespread attention due to their switchable spontaneous polarization and anomalous photovoltaic effect. The coupling between ferroelectricity and the piezo-phototronic effect may lead to the design of distinctive photoelectric devices with multifunctional features. Here, we report an enhancement of the photovoltaic performances in the ferroelectric p-type La-doped bismuth ferrite film (BLFO)/n-type zinc oxide (ZnO) nanowire array heterojunction by rationally coupling the strain-induced piezoelectricity in ZnO nanowires and the ferroelectricity in BLFO. Under a compressive strain of -2.3% and a 10 V upward poling of the BLFO, the open-circuit voltage (VOC) and short-circuit current density (JSC) of the device increase by 8.4% and 54.7%, respectively. Meanwhile, the rise (/decay) time is modulated from 153.7 (/108.8) to 61.28 (/74.86) ms. Systematical band diagram analysis reveals that the promotion of photogenerated carriers and boost of the photovoltaic performances of the device can be attributed to the modulated carrier transport behaviors at the BLFO/ZnO interface and the superposed driving forces arising from the adding up of the piezoelectric potential and ferroelectric polarization. In addition, COMSOL simulation results of piezopotential distribution in ZnO nanowire arrays and the energy band structure change of the heterojunction further confirm the mechanisms. This work not only presents an approach to design high-performance ferroelectric photovoltaic devices but also further broadens the research scope of piezo-phototronics.

Enhanced Photovoltaic Performances of La-Doped Bismuth Ferrite/Zinc Oxide Heterojunction by Coupling Piezo-Phototronic Effect and Ferroelectricity.

Abstract Triboelectric polymer with high charge density is the foundation to promote the wide range of applications of triboelectric nanogenerators. This work develops a method to produce triboelectric polymer based on repeated rheological forging. The fluorinated ethylene propylene film fabricated by repeated forging method not only has excellent mechanical properties and good transmittance, but also can maintain an ultrahigh tribo-charge density. Based on the film with a thickness of 30 μm, the output charge density from contact-separation nanogenerator reaches 352 μC·m −2 . Then, the same film is applied for the nanogenerator with air-breakdown mode and a charge density of 510 μC·m −2 is further achieved. The repeated forging method can effectively regulate the composition of surface functional groups, the crystallinity, and the dielectric constants of the fluorinated ethylene propylene, leading to the superior capability of triboelectrification. Finally, we summarize the key parameters for elevating the electrification performance on the basis of molecular structure and related fabrication crafts, which can guide the further development of triboelectric polymers. 

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Fabrication of triboelectric polymer films via repeated rheological forging for ultrahigh surface charge density

The piezo-phototronic effect (a coupling effect of piezoelectric, photoexcitation and semiconducting properties, coined in 2010) has been demonstrated to be an ingenious and robust strategy to manipulate optoelectronic processes by tuning the energy band structure and photoinduced carrier behavior. The piezo-phototronic effect exhibits great potential in improving the quantum yield efficiencies of optoelectronic materials and devices and thus could help increase the energy conversion efficiency, thus alleviating the energy shortage crisis. In this review, the fundamental principles and challenges of representative optoelectronic materials and devices are presented, including photocatalysts (converting solar energy into chemical energy), solar cells (generating electricity directly under light illumination), photodetectors (converting light into electrical signals) and light-emitting diodes (LEDs, converting electric current into emitted light signals). Importantly, the mechanisms of how the piezo-phototronic effect controls the optoelectronic processes and the recent progress and applications in the above-mentioned materials and devices are highlighted and summarized. Only photocatalysts, solar cells, photodetectors, and LEDs that display piezo-phototronic behavior are reviewed. Material and structural design, property characterization, theoretical simulation calculations, and mechanism analysis are then examined as strategies to further enhance the quantum yield efficiency of optoelectronic devices via the piezo-phototronic effect. This comprehensive overview will guide future fundamental and applied studies that capitalize on the piezo-phototronic effect for energy conversion and storage.

Piezo-phototronic effect on photocatalysis, solar cells, photodetectors and light-emitting diodes.

ZnO nanorods array as light absorption antenna for high-gain UV photodetectors

High photoresponse sensitivity of lithium-doped ZnO (LZO) thin films for weak ultraviolet signal photodetector

Boosting output charge density is top priority for achieving high‐performance triboelectric nanogenerators (TENGs). The charge‐excitation strategy is demonstrated to be a superior approach to acquire high output charge density. Meanwhile, the molecular charge behaviors in the dielectric under a strong electric field from high charge density bring new physics that are worth exploring. Here, a rapid self‐polarization effect of a polar dielectric material by the superhigh electric field in a charge‐excitation TENG is reported, by which the permittivity of the polar dielectric material realizes self‐increase to a saturation, and thus enhances the output charge density. Consequently, an ultrahigh charge density of 3.53 mC m−2 is obtained with 7 µm homemade lead zirconate titanate−poly(vinylidene fluoride) composite film in the atmosphere with 5% relative humidity, which is the highest charge density for TENGs with high durability currently. This work provides new guidance for dielectric material optimization under charge excitation to boost the output performance of TENGs toward practical applications.

Achieving Remarkable Charge Density via Self‐Polarization of Polar High‐k Material in a Charge‐Excitation Triboelectric Nanogenerator

ZnO Thin Film Flexible UV Photodetectors: Regulation on the ZnO/Au Interface by Piezo‐Phototronic Effect and Performance Outcomes

Triboelectric nanogenerators (TENGs) are regarded as a promising technology to convert ambient low‐frequency mechanical energy into electricity for distributed power supply. Although TENGs based on triboelectrification (TE) of metal–dielectric and air breakdown have the advantage of direct current (DC) output, the unidirectional and single‐channel output mode limits their energy utilization efficiency. Here, a bidirectional and double‐channel (BDC‐TENG) based on TE of dielectric–dielectric and local corona discharge is proposed. Different from traditional DC‐TENGs, the TE process of the BDC‐TENG occurs in two dielectrics, and the double‐channel output is generated from simultaneous discharge between each dielectric and its adjacent electrode fixed on both sides of the slider. The BDC‐TENG shows unique characteristics, in that the current direction is tunable by adjusting the slider movement direction and electronegativity difference of two dielectric materials, and the average output power density is up to 3 W m–2 which is 14 times higher than that of traditional DC‐TENGs. This work provides a feasible strategy to boost the output performance of DC‐TENGs.

A High‐Performance Bidirectional Direct Current TENG by Triboelectrification of Two Dielectrics and Local Corona Discharge

The light trapping effect of zinc oxide (ZnO) ultraviolet photodetectors (UV PDs) has been established as a promising way to optimize the performance of optoelectronic devices. In this paper, we report a light trapping fabricated metal-semiconductor-metal structure, consisting of a ZnO nanowire array as a top layer light absorber supported by a ZnO film. The ZnO film is bridged between two interdigitated metal electrodes for collecting photo-generated carriers. In this connection, high-dense ZnO nanowires can be used as a light trapping unit and to transmit the photogenerated carriers towards the ZnO film. The photogenerated carriers diffuse along the longitudinal direction of the ZnO nanowire and then to the ZnO film and are collected by the applied bias electrode. Compared to present ZnO thin film UV PDs, our device has an effective light trapping effect and the enhancement of photo-generated carriers at the top interface by a ZnO nanowire array structure are highly beneficial to UV light detection as they can provide a long optical path and more surface area. In addition, when the device was connected with nanowires, a 10 times augment of responsivity appeared accompanied by a giant photo-to-dark current ratio (1.6 × 103). This novel work not only enhanced fundamental improvement of nanowires to ZnO film UV PDs, but also provided a distinct contrast between light trapping UV PDs and ZnO film UV PDs.

https://iopscience.iop.org/article/10.1088/1361-6528/ab0ee5/pdf

Improved responsivity performance of ZnO film ultraviolet photodetectors by vertical arrays ZnO nanowires with light trapping effect.

In this work, we fabricated a ZnO thin film flexible UV photodetector with a room-temperature process and investigated it with an emphasis on the influence of different external stresses. The photodetector exhibited enhancing photocurrent, sensitivity, responsivity, and response range with a tensile strain. This may originate from the improving collection ability of photogenerated carriers with the change of Schottky barrier height, which is induced by the piezo-phototronic effect. The as-obtained thin film flexible photodetector belongs to the family of photoresponse-controlled optoelectronic devices and has applications in a myriad of useful ways.

Chuncai Shan

Papers

Achieving Remarkable Charge Density via Self‐Polarization of Polar High‐k Material in a Charge‐Excitation Triboelectric Nanogenerator

ZnO Thin Film Flexible UV Photodetectors: Regulation on the ZnO/Au Interface by Piezo‐Phototronic Effect and Performance Outcomes

A High‐Performance Bidirectional Direct Current TENG by Triboelectrification of Two Dielectrics and Local Corona Discharge

Improved responsivity performance of ZnO film ultraviolet photodetectors by vertical arrays ZnO nanowires with light trapping effect.

Piezo-phototronic effect for enhanced sensitivity and response range of ZnO thin film flexible UV photodetectors