Lei Chang

Giant photostriction in organic–inorganic lead halide perovskites

Among the many materials investigated for next-generation photovoltaic cells, organic–inorganic lead halide perovskites have demonstrated great potential thanks to their high power conversion efficiency and solution processability. Within a short period of about 5 years, the efficiency of solar cells based on these materials has increased dramatically from 3.8 to over 20%. Despite the tremendous progress in device performance, much less is known about the underlying photophysics involving charge–orbital–lattice interactions and the role of the organic molecules in this hybrid material remains poorly understood. Here, we report a giant photostrictive response, that is, light-induced lattice change, of >1,200 p.p.m. in methylammonium lead iodide, which could be the key to understand its superior optical properties. The strong photon-lattice coupling also opens up the possibility of employing these materials in wireless opto-mechanical devices.

Oxygen Vacancy Induced Room-Temperature Metal–Insulator Transition in Nickelate Films and Its Potential Application in Photovoltaics

Oxygen vacancy is intrinsically coupled with magnetic, electronic, and transport properties of transition-metal oxide materials and directly determines their multifunctionality. Here, we demonstrate reversible control of oxygen content by postannealing at temperature lower than 300 °C and realize the reversible metal-insulator transition in epitaxial NdNiO₃ films. Importantly, over 6 orders of magnitude in the resistance modulation and a large change in optical bandgap are demonstrated at room temperature without destroying the parent framework and changing the p-type conductive mechanism. Further study revealed that oxygen vacancies stabilized the insulating phase at room temperature is universal for perovskite nickelate films. Acting as electron donors, oxygen vacancies not only stabilize the insulating phase at room temperature, but also induce a large magnetization of ∼50 emu/cm³ due to the formation of strongly correlated Ni²⁺ t(2g)⁶e(g)² states. The bandgap opening is an order of magnitude larger than that of the thermally driven metal-insulator transition and continuously tunable. Potential application of the newly found insulating phase in photovoltaics has been demonstrated in the nickelate-based heterojunctions. Our discovery opens up new possibilities for strongly correlated perovskite nickelates.

Thermally Induced Reversible Double Phase Transitions in an Organic–Inorganic Hybrid Iodoplumbate C4H12NPbI3 with Symmetry Breaking

A one-dimensional (1D) organic-inorganic hybrid iodoplumbate crystal (1, C4H12NPbI3, TMAPbI3) can undergo two reversible phase transitions as the temperature decreases. Its dynamic phase-transition behaviors were carefully studied by dielectric measurements, thermal analysis, and variable-temperature crystallographic studies. These results indicate that the phase transitions possess a disorder-order feature with a noncentrosymmetrical intermediate phase structure. Due to the existence of the ordered motion and reorientation of the C4H12N(+) cation, 1 undergoes two phase transitions: the first one from space group P63/m at room temperature to Pm at 163 K with symmetry breaking, and the second one from space group Pm at 163 K to P61 at 142 K with partial symmetry restoration. Our results indicate that there is an existence of a transitional structure with a low symmetry space group during the disorder-order-type phase transitions, which can provide us valuable information to deeply understand the disorder-order phase transition in organic-inorganic hybrids.

Self-powered sensitive and stable UV-visible photodetector based on GdNiO3/Nb-doped SrTiO3 heterojunctions

The properties of perovskite nickelates are very sensitive to their oxygen content, which allows us to tune their electronic structures by varying the oxygen partial pressure during film deposition. Under the optimized condition, we have obtained GdNiO3 films that are sensitive to a wide spectrum of light. By combining the GdNiO3 film with Nb-doped SrTiO3 to form a heterojunction, we design a self-powered photodetector with high sensitivity toward light with a wavelength between 650 nm and 365 nm. Under 365 nm illumination (50 μW/cm2), the device shows a responsivity of 0.23 A/W at 0 V bias, comparable to or even better than the ultraviolet photodetectors made of semiconductor materials such as GaN or ZnO. The photo-dark ratio can be close to 103 when the power light density reaches 0.6 mW/cm2. Moreover, the device performance is very stable without any decay after 6 months.

Band gap tuning of nickelates for photovoltaic applications

Hybrid perovskites have achieved tremendous success as a light absorber in solar cells during the past few years. However, the stability issue casts shadow on their practical applications. Perovskite oxides may offer an alternative. In this study, the metal–insulator transition in perovskite neodymium nickelates (NdNiO3) is systematically tuned by adjusting the oxygen partial pressure during film growth. Room temperature insulating films with different band gaps are obtained. Testing photovoltaic cells have been prepared by combining the nickelates with Nb-doped SrTiO3, and photovoltaic performance has been optimized. Our study offers a new route for designing novel photovoltaic materials.

Band alignment and electrocatalytic activity at the p-n La0.88Sr0.12FeO3/SrTiO3(001) heterojunction

Ferrite perovskites have exhibited promising p-type conductivity and oxygen evolution reaction (OER) activity. In this work, we investigate heteroepitaxial p-n junctions formed by La0.88Sr0.12FeO3 and n-SrTiO3(001). Sr substitution for La in LaFeO3 is shown to be effective for introducing p-type conductivity, lowering the optical bandgap, and enhancing electrocatalytic OER. A staggered, type-II band alignment with a large built-in potential within the LSFO forms due to the polar interface. This electronic structure facilitates charge transfer across the p-n junction and accounts for the strongly thickness-dependent extent of OER we observe.

Localization-driven metal-insulator transition in epitaxial hole-doped Nd1-x Sr x NiO3 ultrathin films.

Advances in thin film growth technologies make it possible to obtain ultra-thin perovskite oxide films and open the window for controlling novel electronic phases for use in functional nanoscale electronics, such as switches and sensors. Here, we study the thickness-dependent transport characteristics of high-quality ultrathin Nd0.9Sr0.1NiO3 (Sr-NNO) films, which were grown on LaAlO3 (0 0 1) single-crystal substrates by using pulsed laser deposition method. Thick Sr-NNO films (25 unit cells) exhibit metallic behavior with the electrical resistivity following the T  n (n  <  2) law corresponding to a non-Fermi liquid system, while a temperature driven metal-insulator transition (MIT) is observed with films of less than 15 unit cells. The transition temperature increases with reducing film thickness, until the insulating characteristic is observed even at room temperature. The emergence of the insulator ground state can be attributed to weak localization driven MIT expected by considering Mott-Ioffe-Regel limit. Furthermore, the magneto-transport study of Sr-NNO ultrathin films also confirms that the observed MIT is due to the disorder-induced localization rather than the electron-electron interactions.

Enhancing ferroelectric photovoltaic effect by polar order engineering

Destabilizing the polar order unexpectedly boosts the ferroelectric photovoltaic performance in bismuth ferrite. Ferroelectric materials for photovoltaics have sparked great interest because of their switchable photoelectric responses and above-bandgap photovoltages that violate conventional photovoltaic theory. However, their relatively low photocurrent and power conversion efficiency limit their potential application in solar cells. To improve performance, conventional strategies focus mainly on narrowing the bandgap to better match the solar spectrum, leaving the fundamental connection between polar order and photovoltaic effect largely overlooked. We report large photovoltaic enhancement by A-site substitutions in a model ferroelectric photovoltaic material, BiFeO3. As revealed by optical measurements and supported by theoretical calculations, the enhancement is accompanied by the chemically driven rotational instability of the polarization, which, in turn, affects the charge transfer at the band edges and drives a direct-to-indirect bandgap transition, highlighting the strong coupling between polarization, lattice, and orbital order parameters in ferroelectrics. Polar order engineering thus provides an additional degree of freedom to further boost photovoltaic efficiency in ferroelectrics and related materials.

In-Plane Ferroelectricity in Thin Flakes of Van der Waals Hybrid Perovskite

Collective ferroic orders in van der Waals (vdW) crystals are receiving increasing attention in 2D materials research. The interplay between spatial quantum confinement and long-range cooperative phenomena not only broadens the horizon of fundamental physics, but also enables new device paradigms and functionalities built upon vdW heterostructures. Here, the in-plane ferroelectric properties in thin flakes of vdW hybrid perovskite bis(benzylammonium) lead tetrachloride are studied. The ordering of electric dipoles along the layer plane circumvents the depolarization field and preserves the ferroelectricity down to one unit-cell thickness or two vdW layers at room temperature. The superior performance of the electromechanical energy conversion is demonstrated by exploiting its in-plane piezoelectricity. The successful isolation of ferroelectric order in atomically thin vdW hybrid perovskite paves the way for nonvolatile flexible electronic devices with the cross-coupling between strain, charge polarization, and valley degrees of freedom.