In spite of the unprecedented advance of organohalide lead perovskites in the photovoltaics scenario, many of the characteristics of this class of materials, including their slow photoconductivity response, solar cell hysteresis, and switchable photocurrent, remain poorly understood. Many experimental hints point to defect migration as a plausible mechanism underlying these anomalous properties. By means of state-of-the-art first-principles computational analyses carried out on the tetragonal MAPbI3 (MA = methylammonium) perovskite and on its interface with TiO2, we demonstrate that iodine vacancies and interstitials may easily diffuse across the perovskite crystal, with migration activation energies as low as ∼0.1 eV. Under working conditions, iodine-related defects are predicted to migrate at the electrodes on very short time scales (<1 μs). MA and Pb vacancies, with calculated activation barriers of ∼0.5 and 0.8 eV, respectively, could be responsible for the slow response inherent to perovskites, with typical calculated migration times of the order of tens of ms to minutes. By investigating realistic models of the perovskite/TiO2 interface we show that negatively charged defects, e.g. MA vacancies, close to the electron transport layer (TiO2 in our case) modify the perovskite electronic state landscape, hampering charge extraction at selective contacts, thus possibly contributing to the observed solar cell hysteresis. We further demonstrate the role of the electron transport layer in affecting the initial concentration of defects close to the selective contacts, highlighting how charge separation at the perovskite/TiO2 interface may further change the defect distribution. We believe that this work, identifying the mobile species in perovskite solar cells, their migration across the perovskite material, and their effect on the operational mechanism of the device, may pave the way for the development of new materials and solar cell architectures with improved and stabilized efficiencies.

Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation

Organic and inorganic hybrid perovskites (e.g., CH(3)NH(3)PbI(3)), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic-inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications

Solution-processed hybrid organic–inorganic perovskites (HOIPs) exhibit long electronic carrier diffusion lengths, high optical absorption coefficients and impressive photovoltaic device performance. Recent results allow us to compare and contrast HOIP charge-transport characteristics to those of III–V semiconductors — benchmarks of photovoltaic (and light-emitting and laser diode) performance. In this Review, we summarize what is known and unknown about charge transport in HOIPs, with particular emphasis on their advantages as photovoltaic materials. Experimental and theoretical findings are integrated into one narrative, in which we highlight the fundamental questions that need to be addressed regarding the charge-transport properties of these materials and suggest future research directions. The charge transport properties of hybrid organic—inorganic perovskites, which can explain their excellent photovoltaic performance, are reviewed through an integrated summary of experimental and theoretical findings. The potential origins of these properties are discussed and future research directions are indicated.

Hybrid organic—inorganic perovskites: low-cost semiconductors with intriguing charge-transport properties

An insight into the analogies, state-of-the-art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic-organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high-efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo-, pyro-, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion.

In recent years, there has been an unprecedented rise in the performance of metal halide perovskite solar cells. They are now in a position to compete on performance with traditional crystalline solar cells, and as such the most pressing questions concern the long term operational stability of this class of solar cell. Here, recent developments in understanding and overcoming stability concerns of metal halide perovskite solar cells are highlighted. An overview of possible instability issues due to electrical, atmospheric, heat, and light stresses is provided and the different implications to the most commonly used device architectures are discussed.

Stability of Metal Halide Perovskite Solar Cells

Abstract Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter=24.26 mm, thickness=1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Non-linear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

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A Review on Piezoelectric Energy Harvesting: Materials, Methods, and Circuits

Stray magnetic field considered as harmful noise for the human body can be a ubiquitous energy source. We are surrounded with 50/60 Hz parasitic magnetic noise arising from power delivery infrastructure, but it cannot be readily utilized by traditional electromagnetic harvesters. Here, we introduce a novel magneto-mechano-electric (MME) generator with a colossal power density that can turn on 35 LEDs and drive a wireless sensor network under a weak magnetic field of 5–7 × 10−4 T at a low frequency of 60 Hz. The MME generator is a cantilever structured magnetoelectric (ME) laminate composite in which the 〈011〉 oriented anisotropic single crystal fiber composite (SFC) is bonded to Ni plate and Nd permanent magnet proof mass. The ME laminate composite has a strong ME coupling (αME ∼ 160 V cm−1 Oe−1) even without magnetic bias due to the intrinsic property of Ni. The MME generator is also found to exhibit a colossal output power density of 46 mW cm−3 Oe−2 under a weak magnetic field of 1.6 × 10−4 T at 60 Hz. This MME generator can be a ubiquitous power source for wireless sensor networks, low power electric devices, and wireless charging systems by harvesting tiny amounts of parasitic magnetic energy from our living environment.

Ubiquitous magneto-mechano-electric generator

In this paper, we report the synthesis of [001]pc/[012]Rh (pc: pseudo cubic, Rh: rhombohedral) grain oriented lead-free piezoelectric 0.93(Na0.5Bi0.5TiO3)–0.07BaTiO3 (NBT–BT) ceramic with Na0.5Bi0.5TiO3 (NBT) as the seed template. The difference in surface energy along with the chemical potential gradient between the stable NBT seeds and the metastable liquid phase was the driving force for the growth of textured grain. Interfaces in the microstructure were found to be coherent at the atomic scale facilitating the domain wall motion with an applied electric field. The strong texturing in [001]pc/[012]Rh was found to result in extra structural distortions manifested by a decreasing lattice parameter and increasing rhombohedral angle (α). The textured specimen exhibited rather ordered domains with smaller size as compared to its randomly oriented counterpart. The piezoelectric response was found to increase monotonously with the increase in the degree of texturing and the optimized microstructure was found to provide 200% enhancement in the magnitude of piezoelectric coefficient (d33 ∼ 322 pC N−1) as compared to its randomly oriented form (d33 ∼ 160 pC N−1).

Synthesis mechanism of grain-oriented lead-free piezoelectric Na0.5Bi0.5TiO3–BaTiO3 ceramics with giant piezoelectric response

Ferroelectric solar cells based on ferroelectric oxides have attracted significant attention owing to many unique advantages, such as the switchable photocurrent and photovoltage, and the above bandgap open circuit voltages. However, the small photocurrent densities of the typical ferroelectric solar cells greatly limit their photovoltaic performance. In this report, we experimentally revealed the polarization switching properties of inorganic–organic hybrid perovskites and developed ferroelectric solar cells based on the hybrid perovskites. Hybrid perovskite methylammonium lead trihalide (MAPbX3) thin films exhibited 180° domain phase switching and polarization hysteresis loops. Ferroelectric solar cells based on the mixed halide MAPbI3−xClx thin film demonstrate a power conversion efficiency of 6.7% and the ferroelectric solar cells display switchable photovoltaic effects. This work provides an alternative but exhilarating solution for high-performance ferroelectric solar cells beyond inorganic ferroelectric oxides.

Yuan Zhou

Papers

A Review on Piezoelectric Energy Harvesting: Materials, Methods, and Circuits

Ubiquitous magneto-mechano-electric generator

Synthesis mechanism of grain-oriented lead-free piezoelectric Na0.5Bi0.5TiO3–BaTiO3 ceramics with giant piezoelectric response

Ferroelectric solar cells based on inorganic-organic hybrid perovskites

Interface band structure engineering by ferroelectric polarization in perovskite solar cells