The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and organic photovoltaics. High-power conversion efficiency can be realized in both mesoporous and thin-film device architectures. We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As we show, hybrid perovskites exhibit spontaneous electric polarization; we also suggest ways in which this can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and reduction of recombination through segregation...

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Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells

Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, solution-processed and highly efficient solar cells. Here, we show that, in OTP-based photovoltaic devices with vertical and lateral cell configurations, the photocurrent direction can be switched repeatedly by applying a small electric field of <1 V μm(-1). The switchable photocurrent, generally observed in devices based on ferroelectric materials, reached 20.1 mA cm(-2) under one sun illumination in OTP devices with a vertical architecture, which is four orders of magnitude larger than that measured in other ferroelectric photovoltaic devices. This field-switchable photovoltaic effect can be explained by the formation of reversible p-i-n structures induced by ion drift in the perovskite layer. The demonstration of switchable OTP photovoltaics and electric-field-manipulated doping paves the way for innovative solar cell designs and for the exploitation of OTP materials in electrically and optically readable memristors and circuits.

Giant switchable photovoltaic effect in organometal trihalide perovskite devices

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

Halide perovskites have recently emerged as promising materials for low-cost, high-efficiency solar cells. The efficiency of perovskite-based solar cells has increased rapidly, from 3.8% in 2009 to 19.3% in 2014, by using the all-solid-state thin-film architecture and engineering cell structures with mixed-halide perovskites. The emergence of perovskite solar cells revolutionized the field not only because of their rapidly increased efficiency, but also flexibility in material growth and architecture. The superior performance of the perovskite solar cells suggested that perovskite materials possess intrinsically unique properties. In this review, we summarize recent theoretical investigations into the structural, electrical, and optical properties of halide perovskite materials in relation to their applications in solar cells. We also discuss some current challenges of using perovskites in solar cells, along with possible theoretical solutions.

Halide perovskite materials for solar cells: a theoretical review

Most known ferroelectric photovoltaic materials have very wide electronic bandgaps (that is, they absorb only high-energy photons) but here a family of perovskite oxides is described that have tunable bandgaps, allowing their use across the whole visible-light spectrum. The spontaneous electrical polarization that characterizes a ferroelectric material is attractive for solar-cell applications as the positive and negative charges generated by light absorption have a natural tendency to separate, making them easier to harvest efficiently. Unfortunately most known ferroelectrics have wide electronic bandgaps — that is they absorb only higher energy photons that make up a small fraction of the solar spectrum. Ilya Grinberg and colleagues now show that a classic ferroelectric can be chemically engineered to tune the bandgap over a broad range, achieving strong absorption and photocurrent generation across the solar spectrum. Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties1,2,3,4,5,6,7. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p–n junction solar cell2,6,8,9,10. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol–gel thin-film deposition and sputtering3,5. Recent work3,5,11 has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric–electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10−4 to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7–4 electronvolts) of ferroelectric oxides, which allow the use of only 8–20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 − x[BaNi1/2Nb1/2O3 − δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1–3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

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Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials

Solid solutions in the (x)PbTiO3–(1−x)Bi(Zn3∕4W1∕4)O3 system have been examined by x-ray diffraction, dielectric measurements, and thermal analysis. Bi(Zn3∕4W1∕4)O3 increases the tetragonality and Curie temperature of PbTiO3 which reach a value of 1.08 and 530°C, respectively, at the limit of the single-phase perovskite forming region (x∼0.8). The observation of a sustained increase in the tetragonality in this system is similar to the behavior of the (x)PbTiO3–(1−x)Bi(Zn1∕2Ti1∕2)O3 system and highlights the unique properties of Bi-based systems when the B sites contain high concentrations of highly polarizable cations.

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Enhanced tetragonality in (x)PbTiO3–(1−x)Bi(B′B″)O3 systems: Bi(Zn3∕4W1∕4)O3

The ternary perovskite system PbTiO3-Bi(Zn1/2Ti1/2)O3-Bi(Mg1/2Ti1/2)O3 was investigated in an attempt to capture the enhanced tetragonality of the PbTiO3-Bi(Zn1/2Ti1/2)O3 binary system at a morphotropic phase boundary (MPB) to improve piezoelectric performance. Results from the (1−x)[0.75 PbTiO3-0.25Bi(Zn1/2Ti1/2)O3]-(x)Bi(Mg1/2Ti1/2)O3 pseudo-binary solution indicate the formation of an MPB-type phase boundary with a high TC (585 °C at x = 0.55). Multiple dielectric transitions were observed in a region of the ternary diagram characterized by high occupancy of the A-site by Bi3+ (>40%) and the B-site by ferroelectrically active cations (>70%). Temperature-dependent diffraction studies revealed the persistence of a tetragonal structure in at least a subset of the crystal to the higher temperature dielectric transition.

Multiple dielectric transitions in the PbTiO3-Bi(Zn1/2Ti1/2)O3-Bi(Mg1/2Ti1/2)O3 system

The ternary perovskite systems PbTiO3-BiFeO3-Bi(Mg1/2Ti1/2)O3 and PbTiO3-BiFeO3-BiScO3 were investigated to clarify the relationship between the observance of multiple dielectric transitions and B-site chemistry. Multiple dielectric transitions were observed for a range of compositions in the Bi(Mg1/2Ti1/2)O3-containing system. A temperature-dependent diffraction study confirmed the existence of a cubic and tetragonal co-existence region at temperatures between the two transitions. No similar dielectric behavior was observed in the PbTiO3-BiFeO3-BiScO3 system.

David M. Stein

Papers

Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials

Enhanced tetragonality in (x)PbTiO3–(1−x)Bi(B′B″)O3 systems: Bi(Zn3∕4W1∕4)O3

Multiple dielectric transitions in the PbTiO3-Bi(Zn1/2Ti1/2)O3-Bi(Mg1/2Ti1/2)O3 system

Effects of B-site chemistry on BiFeO3-containing enhanced tetragonality systems