Lingping Kong

Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic-inorganic trihalide perovskites

Significance The emergence of organic–inorganic hybrid lead triiodide perovskite materials promises a low-cost and high-efficiency photovoltaic technology. Although the high-power conversion efficiency of this technology has been successfully demonstrated, further improvement appears to be limited without further narrowing the band gap while also retaining or even synergistically prolonging the carrier lifetime. We report a synergistic enhancement in both band gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) of organic–inorganic hybrid lead triiodide perovskite materials under mild pressures below ∼0.3 GPa. This work could open new territory in materials science, and new materials could be invented using the experimental and theoretical guidelines we have established herein. The organic–inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley–Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon–electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.

Electron-Rotor Interaction in Organic-Inorganic Lead Iodide Perovskites Discovered by Isotope Effects.

We report on the carrier-rotor coupling effect in perovskite organic-inorganic hybrid lead iodide (CH3NH3PbI3) compounds discovered by isotope effects. Deuterated organic-inorganic perovskite compounds including CH3ND3PbI3, CD3NH3PbI3, and CD3ND3PbI3 were synthesized. Devices made from regular CH3NH3PbI3 and deuterated CH3ND3PbI3 exhibit comparable performance in band gap, current-voltage, carrier mobility, and power conversion efficiency. However, a time-resolved photoluminescence (TRPL) study reveals that CH3NH3PbI3 exhibits notably longer carrier lifetime than that of CH3ND3PbI3, in both thin-film and single-crystal formats. Furthermore, the comparison in carrier lifetime between CD3NH3PbI3 and CH3ND3PbI3 single crystals suggests that vibrational modes in methylammonium (MA(+)) have little impact on carrier lifetime. In contrast, the fully deuterated compound CD3ND3PbI3 reconfirmed the trend of decreasing carrier lifetime upon the increasing moment of inertia of cationic MA(+). Polaron model elucidates the electron-rotor interaction.

The role of tetragonal side morphotropic phase boundary in modified relaxor-PbTiO3 crystals for high power transducer applications

Morphotropic phase boundary (MPB) in ferroelectric materials leads to improved properties due to the structural instability. The manganese modified Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals with MPB composition were investigated, the structure/property relationship was established. The tetragonal side MPB (coexistence of 91% tetragonal and 9% monoclinic phases) was confirmed by X-ray synchrotron data, while relaxor behavior was detected by Raman characterization and dielectric measurement. Crystals with such MPB composition possess high “figure of merit” (d33·Q33 ∼ 106 pC/N), being one order higher when compared with their pure rhombohedral counterparts. Together with high Curie temperature (∼229 °C) and temperature stability of properties, demonstrating a promising candidate for high power transducer applications.

In situ structure characterization of Pb(Yb1/2Nb1/2)O3-PbTiO3 crystals under high pressure-temperature

The stability field of the piezoelectric/ferroelectric phase of solid solution 0.47Pb(Yb1/2Nb1/2)O3-0.53PbTiO3 (PYN-PT) has been studied using in situ x-ray diffraction (XRD) and Raman spectroscopy techniques under high pressure and high temperature conditions to observe the evolution of features. PYN-PT remains in the piezoelectric tetragonal phase up to approximately 6.5 GPa at room temperature then transforms to a paraelectric cubic phase, which exhibits local disorder. The cubic phase is stable up to 50 GPa. Based on the high pressure and high temperature XRD results, we present a pressure-temperature phase diagram of PYN-PT which constraints the stability region of the ferroelectric phase.

Growth of magnesium aluminate nanocrystallites

Nanocrystalline magnesium aluminate was synthesized with the coprecipitation method. Its growing behaviors as a function of temperature were studied with synchrotron X-ray diffraction (XRD) and Raman spectroscopy. It is found that the particle growth was greatly inhibited at temperatures below 1000 °C due to the hydroxide precursor reactants. Above 1000 °C, magnesium aluminate nanoparticles start to grow fast. After two hours annealing at 1200 °C, the grain size changes by multiple folds, suggesting that oriented attachment may occur. Above 1200 °C, the grain size changes in various directions are much smaller than the average grain size, indicating the oriented attachment mechanisms become inactive in the growth of MgAl2O4 nanoparticles with sizes larger than 42 nm.

Origin of the enhanced piezoelectric thermal stability in BiScO3-PbTiO3 single crystals

BiScO3-PbTiO3 single crystals were reported to possess high piezoelectric coefficient of 1200 pC/N and Curie temperature of >400 °C, exhibiting excellent thermal stability of properties up to 350 °C. However, the origin of the thermal stability is yet unclear. In this research, high resolution synchrotron-based technique was used to study the temperature driven structural evolution in BiScO3-PbTiO3 system, where two competing symmetries and local distortion were observed, accounting for the high piezoelectric activity. A strong correlation between thermal stability of structure and temperature-dependent properties was established, which will benefit the design of ferroelectric materials with broad temperature usage range.

An insight into the origin of low-symmetry bridging phase and enhanced functionality in systems containing competing phases

High piezoelectric activity of ferroelectrics with morphotropic phase boundary (MPB) compositions has been the focus of numerous recent investigations. The concept of a bridging low-symmetry phase between competing phase structures of the MPB composition remains controversial due to the compositional inhomogeneity near the MPB and the lack of appropriate experimental techniques to delineate the complex crystal structures. We have studied a simple ferroelectric BaTiO3 by employing a high resolution synchrotron-based technique, in which the formation of different symmetry regions due to chemical inhomogeneity can be ruled out. We observed two types of thermotropic phase boundaries, revealing the importance of interphase-strain in the formation of a bridging phase between competing phases and the enhancement of functionality.

Isothermal pressure-derived metastable states in 2D hybrid perovskites showing enduring bandgap narrowing

Significance Metastable materials often exhibit unexpected striking properties that are not available in stable state. While metastable states are generally achieved by rapid cooling of materials from high temperature, it is imperative to explore other nonthermal routes to access metastable states, especially for heat-vulnerable materials. Here, we report that work by pressure, namely, a compression−decompression cycle under ambient temperature, can drive thermosusceptible organic−inorganic hybrid perovskites to their metastable state, in which the perovskites show enduring bandgap narrowing for significantly broadened solar absorption. This pressure-derived route provides a fundamental path to obtain metastable materials with unprecedented performance. Materials in metastable states, such as amorphous ice and supercooled condensed matter, often exhibit exotic phenomena. To date, achieving metastability is usually accomplished by rapid quenching through a thermodynamic path function, namely, heating−cooling cycles. However, heat can be detrimental to organic-containing materials because it can induce degradation. Alternatively, the application of pressure can be used to achieve metastable states that are inaccessible via heating−cooling cycles. Here we report metastable states of 2D organic−inorganic hybrid perovskites reached through structural amorphization under compression followed by recrystallization via decompression. Remarkably, such pressure-derived metastable states in 2D hybrid perovskites exhibit enduring bandgap narrowing by as much as 8.2% with stability under ambient conditions. The achieved metastable states in 2D hybrid perovskites via compression−decompression cycles offer an alternative pathway toward manipulating the properties of these “soft” materials.