Extended carrier lifetimes and diffusion in hybrid perovskites revealed by Hall effect and photoconductivity measurements

Impressive performance of hybrid perovskite solar cells reported in recent years still awaits a comprehensive understanding of its microscopic origins. In this work, the intrinsic Hall mobility and photocarrier recombination coefficient are directly measured in these materials in steady-state transport studies. The results show that electron-hole recombination and carrier trapping rates in hybrid perovskites are very low. The bimolecular recombination coefficient (10⁻¹¹ to 10⁻¹⁰ cm³ s⁻¹) is found to be on par with that in the best direct-band inorganic semiconductors, even though the intrinsic Hall mobility in hybrid perovskites is considerably lower (up to 60 cm² V⁻¹ s⁻¹). Measured here, steady-state carrier lifetimes (of up to 3 ms) and diffusion lengths (as long as 650 μm) are significantly longer than those in high-purity crystalline inorganic semiconductors. We suggest that these experimental findings are consistent with the polaronic nature of charge carriers, resulting from an interaction of charges with methylammonium dipoles.

Hybrid perovskites exhibit long carrier diffusion lengths and lifetimes. Here, Chen et al. show experimentally that carrier recombination in perovskites is far from Langevin and closer to the best direct-bandgap semiconductors, which can be explained by the dipolar polaronic nature of charge carriers.

Intrinsic Charge Transport across Phase Transitions in Hybrid Organo-Inorganic Perovskites

Hall effect measurements in CH3 NH3 PbBr3 single crystals reveal that the charge-carrier mobility follows an inverse-temperature power-law dependence, μ ∝ T(-) (γ) , with the power exponent γ = 1.4 ± 0.1 in the cubic phase, indicating an acoustic-phonon-dominated carrier scattering, and γ = 0.5 ± 0.1 in the tetragonal phase, suggesting another dominant mechanism, such as a piezoelectric or space-charge scattering.

Perovskite Materials for Light-Emitting Diodes and Lasers

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Thin Insulating Tunneling Contacts for Efficient and Water-Resistant Perovskite Solar Cells

Tunneling contacts made of any insulating polymers, a champion technology in silicon solar cells, are shown to increase the stabilized efficiency of perovskite solar cells (PSCs) to 20.3%. The tunneling layers spatially separate photo-generated electrons and holes at the perovskite-cathode interface and reduce charge recombination. The tunneling layers made of hydrophobic polymers also significantly enhance the resistance of PSCs to water-caused damage.

Photoluminescence Blinking of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods Induced by Surface Traps

Photoluminescence (PL) of organometal halide perovskite materials reflects the charge dynamics inside of the material and thus contains important information for understanding the electro-optical properties of the material. Interpretation of PL blinking of methylammonium lead iodide (MAPbI₃) nanostructures observed on polycrystalline samples remains puzzling owing to their intrinsic disordered nature. Here, we report a novel method for the synthesis of high-quality single-crystal MAPbI₃ nanorods and demonstrate a single-crystal study on MAPbI₃ PL blinking. At low excitation power densities, two-state blinking was found on individual nanorods with dimensions of several hundred nanometers. A super-resolution localization study on the blinking of individual nanorods showed that single crystals of several hundred nanometers emit and blink as a whole, without showing changes in the localization center over the crystal. Moreover, both the blinking ON and OFF times showed power-law distributions, indicating trapping-detrapping processes. This is further supported by the PL decay times of the individual nanorods, which were found to correlate with the ON/OFF states. Furthermore, a strong environmental dependence of the nanorod PL blinking was revealed by comparing the measurements in vacuum, nitrogen, and air, implying that traps locate close to crystal surfaces. We explain our observations by proposing surface charge traps that are likely related to under-coordinated lead ions and methylammonium vacancies to result in the PL blinking observed here.

Engineering of CH3 NH3 PbI3 Perovskite Crystals by Alloying Large Organic Cations for Enhanced Thermal Stability and Transport Properties

The number of studies on organic-inorganic hybrid perovskites has soared in recent years. However, the majority of hybrid perovskites under investigation are based on a limited number of organic cations of suitable sizes, such as methylammonium and formamidinium. These small cations easily fit into the perovskite's three-dimensional (3D) lead halide framework to produce semiconductors with excellent charge transport properties. Until now, larger cations, such as ethylammonium, have been found to form 2D crystals with lead halide. Here we show for the first time that ethylammonium can in fact be incorporated coordinately with methylammonium in the lattice of a 3D perovskite thanks to a balance of opposite lattice distortion strains. This inclusion results in higher crystal symmetry, improved material stability, and markedly enhanced charge carrier lifetime. This crystal engineering strategy of balancing opposite lattice distortion effects vastly increases the number of potential choices of organic cations for 3D perovskites, opening up new degrees of freedom to tailor their optoelectronic and environmental properties.

Strategy to Boost the Efficiency of Mixed-Ion Perovskite Solar Cells: Changing Geometry of the Hole Transporting Material

The hole transporting material (HTM) is an essential component in perovskite solar cells (PSCs) for efficient extraction and collection of the photoinduced charges. Triphenylamine- and carbazole-based derivatives have extensively been explored as alternative and economical HTMs for PSCs. However, the improvement of their power conversion efficiency (PCE), as well as further investigation of the relationship between the chemical structure of the HTMs and the photovoltaic performance, is imperatively needed. In this respect, a simple carbazole-based HTM X25 was designed on the basis of a reference HTM, triphenylamine-based X2, by simply linking two neighboring phenyl groups in a triphenylamine unit through a carbon-carbon single bond. It was found that a lowered highest occupied molecular orbital (HOMO) energy level was obtained for X25 compared to that of X2. Besides, the carbazole moiety in X25 improved the molecular planarity as well as conductivity property in comparison with the triphenylamine unit in X2. Utilizing the HTM X25 in a solar cell with mixed-ion perovskite [HC(NH2)2]0.85(CH3NH3)0.15Pb(I0.85Br0.15)3, a highest reported PCE of 17.4% at 1 sun (18.9% under 0.46 sun) for carbazole-based HTM in PSCs was achieved, in comparison of a PCE of 14.7% for triphenylamine-based HTM X2. From the steady-state photoluminescence and transient photocurrent/photovoltage measurements, we conclude that (1) the lowered HOMO level for X25 compared to X2 favored a higher open-circuit voltage (Voc) in PSCs; (2) a more uniform formation of X25 capping layer than X2 on the surface of perovskite resulted in more efficient hole transport and charge extraction in the devices. In addition, the long-term stability of PSCs with X25 is significantly enhanced compared to X2 due to its good uniformity of HTM layer and thus complete coverage on the perovskite. The results provide important information to further develop simple and efficient small molecular HTMs applied in solar cells.

Electronic and Ionic Transport Dynamics in Organolead Halide Perovskites

Ion migration has been postulated as the underlying mechanism responsible for the hysteresis in organolead halide perovskite devices. However, the electronic and ionic transport dynamics and how they impact each other in organolead halide perovskites remain elusive to date. Here we report a systematic investigation of the electronic and ionic transport dynamics in organolead halide perovskite microplate crystals and thin films using temperature-dependent transient response measurements. Our study reveals that thermally activated ionic and electronic conduction coexist in perovskite devices. The extracted activation energies suggest that the electronic transport is easier, but ions migrate harder in microplates than in thin films, demonstrating that the crystalline quality and grain boundaries can fundamentally modify electronic and ionic transport in perovskites. These findings offer valuable insight on the electronic and ionic transport dynamics in organolead halide perovskites, which is critical for optimizing perovskite devices with reduced hysteresis and improved stability and efficiency.

Color-Pure Violet-Light-Emitting Diodes Based on Layered Lead Halide Perovskite Nanoplates

Violet electroluminescence is rare in both inorganic and organic light-emitting diodes (LEDs). Low-cost and room-temperature solution-processed lead halide perovskites with high-efficiency and color-tunable photoluminescence are promising for LEDs. Here, we report room-temperature color-pure violet LEDs based on a two-dimensional lead halide perovskite material, namely, 2-phenylethylammonium (C6H5CH2CH2NH3(+), PEA) lead bromide [(PEA)2PbBr4]. The natural quantum confinement of two-dimensional layered perovskite (PEA)2PbBr4 allows for photoluminescence of shorter wavelength (410 nm) than its three-dimensional counterpart. By converting as-deposited polycrystalline thin films to micrometer-sized (PEA)2PbBr4 nanoplates using solvent vapor annealing, we successfully integrated this layered perovskite material into LEDs and achieved efficient room-temperature violet electroluminescence at 410 nm with a narrow bandwidth. This conversion to nanoplates significantly enhanced the crystallinity and photophysical properties of the (PEA)2PbBr4 samples and the external quantum efficiency of the violet LED. The solvent vapor annealing method reported herein can be generally applied to other perovskite materials to increase their grain size and, ultimately, improve the performance of optoelectronic devices based on perovskite materials.

Tunable Near-Infrared Luminescence in Tin Halide Perovskite Devices

Infrared emitters are reasonably rare in solution-processed materials. Recently, research into hybrid organo-lead halide perovskite, originally popular in photovoltaics,1-3 has gained traction in light-emitting diodes (LED) due to their low-cost solution processing and good performance.4-9 The lead-based electroluminescent materials show strong colorful emission in the visible region, but lack emissive variants further in the infrared. The concerns with the toxicity of lead may, additionally, limit their wide-scale applications. Here, we demonstrate tunable near-infrared electroluminescence from a lead-free organo-tin halide perovskite, using an ITO/PEDOT:PSS/CH3NH3Sn(Br1-xIx)3/F8/Ca/Ag device architecture. In our tin iodide (CH3NH3SnI3) LEDs, we achieved a 945 nm near-infrared emission with a radiance of 3.4 W sr(-1) m(-2) and a maximum external quantum efficiency of 0.72%, comparable with earlier lead-based devices. Increasing the bromide content in these tin perovskite devices widens the semiconductor bandgap and leads to shorter wavelength emissions, tunable down to 667 nm. These near-infrared LEDs could find useful applications in a range of optical communication, sensing and medical device applications.

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

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.

Atomic Layer Deposition of TiO2 for a High-Efficiency Hole-Blocking Layer in Hole-Conductor-Free Perovskite Solar Cells Processed in Ambient Air

In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO2 as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. First, uniform, pinhole-free TiO2 films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO2 films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode were fabricated in ambient air, and the effect of thickness of TiO2 compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The best-performance devices based on optimized TiO2 compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCEs) of as high as 7.82%. Furthermore, they can maintain over 96% of their initial PCE after 651 h (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability.

High-performance perovskite CH3NH3PbI3 thin films for solar cells prepared by single-source physical vapour deposition

In this work, an alternative route to fabricating high-quality CH3NH3PbI3 thin films is proposed. Single-source physical vapour deposition (SSPVD) without a post-heat-treating process was used to prepare CH3NH3PbI3 thin films at room temperature. This new process enabled complete surface coverage and moisture stability in a non-vacuum solution. Moreover, the challenges of simultaneously controlling evaporation processes of the organic and inorganic sources via dual-source vapour evaporation and the heating process required to obtain high crystallization were avoided. Excellent composition with stoichiometry transferred from the powder material, a high level of tetragonal phase-purity, full surface coverage, well-defined grain structure, high crystallization and reproducibility were obtained. A PCE of approximately 10.90% was obtained with a device based on SSPVD CH3NH3PbI3. These initial results suggest that SSPVD is a promising method to significantly optimize perovskite CH3NH3PbI3 solar cell efficiency.

Free Carriers versus Excitons in CH3NH3PbI3 Perovskite Thin Films at Low Temperatures: Charge Transfer from the Orthorhombic Phase to the Tetragonal Phase

We have investigated the dynamic optical properties of CH3NH3PbI3 (MAPbI3) perovskite thin films at low temperatures using time-resolved photoluminescence, optical transient absorption (TA), and THz TA spectroscopy. Optical spectroscopic results indicate that the high-temperature tetragonal phase still remains in the MAPbI3 thin films at low temperatures in addition to the major orthorhombic phase. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase. Consequently, the near-band-edge optical responses of the photocarriers in both the tetragonal and orthorhombic phases of the MAPbI3 thin films are more accurately described by a free-carrier model, rather than an excitonic model even at low temperatures.

Simultaneous Observation of an Intraband Transition and Distinct Transient Species in the Infrared Region for Perovskite Solar Cells

Solar cells based on organometal-halide perovskites such as CH3NH3PbI3 have emerged as a promising next-generation photovoltaic system, but the underlying photophysics and photochemistry remain to be established because of the limited availability of methods to implement the simultaneous and direct measurement of various charge carriers and ions that play a crucial role in the operating device. We used nanosecond time-resolved infrared (IR) spectroscopy to investigate, with high molecular specificity, distinct transient species that are formed in perovskite solar cells after photoexcitation. In CH3NH3PbI3 planar-heterojuction solar cells, we simultaneously observed infrared spectral signatures that are associated with an intraband transition of conduction-band electrons, Fano resonance, and the spiro-OMeTAD cation having an exceptionally short lifetime of 1.0 μs (at ∼1485 cm(-1)). The present results show that the time-resolved IR method offers a unique capability to elucidate these important transients in perovskite solar cells and their dynamic interplay in a comprehensive manner.

Pressure-Induced Structural and Optical Properties of Organometal Halide Perovskite-Based Formamidinium Lead Bromide

Organometal halide perovskites (OMHPs) are attracting an ever-growing scientific interest as photovoltaic materials with moderate cost and compelling properties. In this Letter, pressure-induced optical and structural changes of OMHP-based formamidinium lead bromide (FAPbBr3) were systematically investigated. We studied the pressure dependence of optical absorption and photoluminescence, both of which showed piezochromism. Synchrotron X-ray diffraction indicated that FAPbBr3 underwent two phase transitions and subsequent amorphization, leading directly to the bandgap evolution with redshift followed by blueshift during compression. Raman experiments illustrated the high pressure behavior of organic cation and the surrounding inorganic octahedra. Additionally, the effect of cation size and the different intermolecular interactions between organic cation and inorganic octahedra result in the fact that FAPbBr3 is less compressible than the reported methylammonium lead bromide (MAPbBr3). High pressure studies of the structural evolution and optical properties of OMHPs provide important clues in optimizing photovoltaic performance and help to design novel OMHPs with higher stimuli-resistant ability.

Ultrahigh sensitivity of methylammonium lead tribromide perovskite single crystals to environmental gases

One of the limiting factors to high device performance in photovoltaics is the presence of surface traps. Hence, the understanding and control of carrier recombination at the surface of organic-inorganic hybrid perovskite is critical for the design and optimization of devices with this material as the active layer. We demonstrate that the surface recombination rate (or surface trap state density) in methylammonium lead tribromide (MAPbBr₃) single crystals can be fully and reversibly controlled by the physisorption of oxygen and water molecules, leading to a modulation of the photoluminescence intensity by over two orders of magnitude. We report an unusually low surface recombination velocity of 4 cm/s (corresponding to a surface trap state density of 10⁸ cm^-2) in this material, which is the lowest value ever reported for hybrid perovskites. In addition, a consistent modulation of the transport properties in single crystal devices is evidenced. Our findings highlight the importance of environmental conditions on the investigation and fabrication of high-quality, perovskite-based devices and offer a new potential application of these materials to detect oxygen and water vapor.

Advances in Perovskite Solar Cells

Organolead halide perovskite materials possess a combination of remarkable optoelectronic properties, such as steep optical absorption edge and high absorption coefficients, long charge carrier diffusion lengths and lifetimes. Taken together with the ability for low temperature preparation, also from solution, perovskite-based devices, especially photovoltaic (PV) cells have been studied intensively, with remarkable progress in performance, over the past few years. The combination of high efficiency, low cost and additional (non-PV) applications provides great potential for commercialization. Performance and applications of perovskite solar cells often correlate with their device structures. Many innovative device structures were developed, aiming at large-scale fabrication, reducing fabrication cost, enhancing the power conversion efficiency and thus broadening potential future applications. This review summarizes typical structures of perovskite solar cells and comments on novel device structures. The applications of perovskite solar cells are discussed.

Halide Perovskites: Poor Man's High-Performance Semiconductors

Halide perovskites are a rapidly developing class of medium-bandgap semiconductors which, to date, have been popularized on account of their remarkable success in solid-state heterojunction solar cells raising the photovoltaic efficiency to 20% within the last 5 years. As the physical properties of the materials are being explored, it is becoming apparent that the photovoltaic performance of the halide perovskites is just but one aspect of the wealth of opportunities that these compounds offer as high-performance semiconductors. From unique optical and electrical properties stemming from their characteristic electronic structure to highly efficient real-life technological applications, halide perovskites constitute a brand new class of materials with exotic properties awaiting discovery. The nature of halide perovskites from the materials' viewpoint is discussed here, enlisting the most important classes of the compounds and describing their most exciting properties. The topics covered focus on the optical and electrical properties highlighting some of the milestone achievements reported to date but also addressing controversies in the vastly expanding halide perovskite literature.

Hysteresis-Suppressed High-Efficiency Flexible Perovskite Solar Cells Using Solid-State Ionic-Liquids for Effective Electron Transport

An efficiency of flexible perovskite solar cells (Pvs-SCs) of 16.09% is achieved, the highest value reported for flexible Pvs-SCs to date. The outstanding performance is attributed to the superior features of alternative electron-transport materials, such as antireflection, a suitable work function, high electron mobility, and a reduced trap-state density of the perovskite material.

Overcoming the Interface Losses in Planar Heterojunction Perovskite-Based Solar Cells

A scalable, hysteresis-free and planar architecture perovskite solar cell is presented, employing a flame spray synthesized low-temperature processed NiO (LT-NiO) as hole-transporting layer yielding efficiencies close to 18%. Importantly, it is found that LT-NiO boosts the limits of open-circuit voltages toward an impressive non-radiative voltage loss of 0.226 V only, whereas

PEDOT

PSS suffers from significant large non-radiative recombination losses.

Controlled Substitution of Chlorine for Iodine in Single-Crystal Nanofibers of Mixed Perovskite MAPbI3- x Clx

Longer carrier diffusion length and improved power conversion efficiency have been reported for thin-film solar cell of organolead mixed-halide perovskite MAPbI3- x Cl x in comparison with MAPbI3 . Instead of substituting I in the MAPbI3 lattice, Cl-incorporation has been shown to mainly improve the film morphology of perovskite absorber. Well-defined crystal structure, adjustable composition (x), and regular morphology, remains a formidable task. Herein, a facile solution-assembly method is reported for synthesizing single-crystalline nanofibers (NFs) of tetragonal-lattice MAPbI3- x Cl x with the Cl-content adjustable between 0 ≤ x ≤ 0.75, leading to a gradual blueshift of the absorption and photoluminescence maxima from x = 0 to 0.75. The photoresponsivity (R) of MAPbI3 NFs keeps almost unchanging at a value independent of the white-light illumination intensity (P). In contrast, R of MAPbI3- x Cl x NFs decreases rapidly with increasing both the x and P values, indicating Cl-substitution increases the recombination traps of photogenerated free electrons and holes. This study provides a model system to examine the role of extrinsic Cl ions in both perovskite crystallography and optoelectronic properties.

Hydrogen-like Wannier-Mott Excitons in Single Crystal of Methylammonium Lead Bromide Perovskite

A thorough investigation of exciton properties in bulk CH3NH3PbBr3 perovskite single crystals was carried out by recording the reflectance, steady-state and transient photoluminescence spectra of submicron volumes across the crystal. The study included an examination of the spectra profiles at various temperatures and laser excitation fluencies. The results resolved the first and second hydrogen-like Wannier-Mott exciton transitions at low temperatures, from which the ground-state exciton's binding energy of 15.33 meV and Bohr radius of ∼4.38 nm were derived. Furthermore, the photoluminescence temperature dependence suggested dominance of delayed exciton emission at elevated temperatures, originating from detrapping of carriers from shallow traps or/and from retrapping of electron-hole pairs into exciton states. The study revealed knowledge about several currently controversial issues that have an impact on functionality of perovskite materials in optoelectronic devices.

Hybrid Perovskite/Perovskite Heterojunction Solar Cells

Recently developed organic-inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus toward new device architectures holds great potential to induce the next leap in device performance. Here, we demonstrate a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide-angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices.

Effective-mass model and magneto-optical properties in hybrid perovskites

Hybrid inorganic-organic perovskites have proven to be a revolutionary material for low-cost photovoltaic applications. They also exhibit many other interesting properties, including giant Rashba splitting, large-radius Wannier excitons, and novel magneto-optical effects. Understanding these properties as well as the detailed mechanism of photovoltaics requires a reliable and accessible electronic structure, on which models of transport, excitonic, and magneto-optical properties can be efficiently developed. Here we construct an effective-mass model for the hybrid perovskites based on the group theory, experiment, and first-principles calculations. Using this model, we relate the Rashba splitting with the inversion-asymmetry parameter in the tetragonal perovskites, evaluate anisotropic g-factors for both conduction and valence bands, and elucidate the magnetic-field effect on photoluminescence and its dependence on the intensity of photoexcitation. The diamagnetic effect of exciton is calculated for an arbitrarily strong magnetic field. The pronounced excitonic peak emerged at intermediate magnetic fields in cyclotron resonance is assigned to the 3D_±2 states, whose splitting can be used to estimate the difference in the effective masses of electron and hole.

Perovskite Photodetectors based on CH3 NH3 PbI3 Single Crystals

As very important light-harvesting materials, organic-inorganic perovskites have showed an alluring prospect in optoelectronic devices. However, most of the perovskite devices, such as photodetectors reported recently, are based on the polycrystalline films of perovskites, and the large grain boundaries and amounts of defects existing in films are unfavorable for the further improvement of device performance. Herein, high-quality single crystals of CH₃ NH₃ PbI₃ were successfully prepared through a simple solution immersing method. The obtained CH₃ NH₃ PbI₃ crystals showed the morphologies of nanowires and nanoplates, which were confirmed to belong to the same tetragonal phase. Both nanowire- and nanoplate-based photodetectors exhibited improved performance over CH₃ NH₃ PbI₃ polycrystalline thin films, with an on/off ratio approaching 10³ , which is one of the highest values reported so far for perovskite photodetectors.

Large Grained Perovskite Solar Cells Derived from Single-Crystal Perovskite Powders with Enhanced Ambient Stability

In this study, we demonstrate the large grained perovskite solar cells prepared from precursor solution comprising single-crystal perovskite powders for the first time. The resultant large grained perovskite thin film possesses a negligible physical (structural) gap between each large grain and is highly crystalline as evidenced by its fan-shaped birefringence observed under polarized light, which is very different from the thin film prepared from the typical precursor route (MAI + PbI2).

Effect of Electron Transporting Layer on Bismuth-Based Lead-Free Perovskite (CH3NH3)3 Bi2I9 for Photovoltaic Applications

Methylammonium iodo bismuthate ((CH3NH3)3Bi2I9) (MBI) perovskite is a promising alternative to rapidly progressing hybrid organic-inorganic lead perovskites because of its better stability and low toxicity compared to lead-based perovskites. Solution-processed perovskite fabricated by single-step spin-coating and subsequent heating produced polycrystalline films of hybrid perovskite (CH3NH3)3Bi2I9), whose morphology was influenced drastically by the nature of substrates. The optical measurements showed a strong absorption band around 500 nm. The devices made on anatase TiO2 mesoporous layer showed good performance with current density over 0.8 mA cm(-2) while the devices on brookite TiO2 layer and planar (free of porous layer) was inefficient. However, all the MBI devices were stable to ambient conditions for more than 10 weeks.

The Role of Trap-assisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots

Hybrid metal halide perovskites have been paid enormous attentions in photophysics research, whose excellent performances were attributed to their intriguing charge carriers proprieties. However, it still remains far from satisfaction in the comprehensive understanding of perovskite charge-transport properities, especially about trap-assisted recombination process. In this Letter, through time-resolved transient absorption (TA) and photoluminescence (PL) measurements, we provided a relative comprehensive investigation on the charge carriers recombination dynamics of CH₃NH₃PbBr₃ (MAPbBr₃) perovskite films and quantum dots (QDs), especially about trap-assisted recombination. It was found that the integral recombination mode of MAPbBr₃ films was highly sensitive to the density distribution of generated charge carriers and trap states. Additional, Trap effects would be gradually weakened with elevated carrier densities. Furthermore, the trap-assisted recombination can be removed from MAPbBr₃ QDs through its own surface passivation mechanism and this specialty may render the QDs as a new material in illuminating research. This work provides deeper physical insights into the dynamics processes of MAPbBr₃ materials and paves a way toward more light-harvesting applications in future.

High-Efficiency Flexible Solar Cells Based on Organometal Halide Perovskites

Flexible and light-weight solar cells are important because they not only supply power to wearable and portable devices, but also reduce the transportation and installation cost of solar panels. High-efficiency organometal halide perovskite solar cells can be fabricated by a low-temperature solution process, and hence are promising for flexible-solar-cell applications. Here, the development of perovskite solar cells is briefly discussed, followed by the merits of organometal halide perovskites as promising candidates as high-efficiency, flexible, and light-weight photovoltaic materials. Afterward, recent developments of flexible solar cells based on perovskites are reviewed.

Flexible Hybrid Organic-Inorganic Perovskite Memory

Active research has been done on hybrid organic-inorganic perovskite materials for application to solar cells with high power conversion efficiency. However, this material often shows hysteresis, which is undesirable, shift in the current-voltage curve. The hysteresis may come from formation of defects and their movement in perovskite materials. Here, we utilize the defects in perovskite materials to be used in memory operations. We demonstrate flexible nonvolatile memory devices based on hybrid organic-inorganic perovskite as the resistive switching layer on a plastic substrate. A uniform perovskite layer is formed on a transparent electrode-coated plastic substrate by solvent engineering. Flexible nonvolatile memory based on the perovskite layer shows reproducible and reliable memory characteristics in terms of program/erase operations, data retention, and endurance properties. The memory devices also show good mechanical flexibility. It is suggested that resistive switching is done by migration of vacancy defects and formation of conducting filaments under the electric field in the perovskite layer. It is believed that organic-inorganic perovskite materials have great potential to be used in high-performance, flexible memory devices.

Photo-induced halide redistribution in organic-inorganic perovskite films

Organic–inorganic perovskites such as CH₃NH₃PbI₃ are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH₃NH₃PbI₃ films under illumination. We demonstrate that the photo-induced ‘brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance.

Visual evidence for photo-induced ionic migration in perovskite films without contacts is lacking. Here, the authors use a unique combination of confocal photoluminescence microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH₃NH₃PbI₃ films under illumination.

Optical characterization of voltage-accelerated degradation in CH3NH3PbI3 perovskite solar cells

We investigate the performance degradation mechanism of CH₃NH₃PbI₃ perovskite solar cells under bias voltage in air and nitrogen atmospheres using photoluminescence and electroluminescence techniques. When applying forward bias, the power conversion efficiency of the solar cells decreased significantly in air, but showed no degradation in nitrogen atmosphere. Time-resolved photoluminescence measurements on these devices revealed that the application of forward bias in air accelerates the generation of non-radiative recombination centers in the perovskite layer buried in the device. We found a negative correlation between the electroluminescence intensity and the injected current intensity in air. The irreversible change of the perovskite grain surface in air initiates the degradation of the perovskite solar cells.

Halide-Substituted Electronic Properties of Organometal Halide Perovskite Films: Direct and Inverse Photoemission Studies

Solution-processed perovskite solar cells are attracting increasing interest due to their potential in next-generation hybrid photovoltaic devices. Despite the morphological control over the perovskite films, quantitative information on electronic structures and interface energetics is of paramount importance to the optimal photovoltaic performance. Here, direct and inverse photoemission spectroscopies are used to determine the electronic structures and chemical compositions of various methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I), revealing the strong influence of halide substitution on the electronic properties of perovskite films. Precise control over halide compositions in MAPbX3 films causes the manipulation of the electronic properties, with a qualitatively blue shift along the I → Br → Cl series and showing the increase in ionization potentials from 5.96 to 7.04 eV and the change of transport band gaps in the range from 1.70 to 3.09 eV. The resulting light absorption of MAPbX3 films can cover the entire visible region from 420 to 800 nm. The results presented here provide a quantitative guide for the analysis of perovskite-based solar cell performance and the selection of optimal carrier-extraction materials for photogenerated electrons and holes.

Sliding three-phase contact line of printed droplets for single-crystal arrays

Controlling the behaviours of printed droplets is an essential requirement for inkjet printing of delicate three-dimensional (3D) structures or high-resolution patterns. In this work, molecular deposition and crystallization are regulated by manipulating the three-phase contact line (TCL) behaviour of the printed droplets. The results show that oriented single-crystal arrays are fabricated based on the continuously sliding TCL. Owing to the sliding of the TCL on the substrate, the outward capillary flow within the evaporating droplet is suppressed and the molecules are brought to the centre of the droplet, resulting in the formation of a single crystal. This work provides a facile strategy for controlling the structures of printed units by manipulating the TCL of printed droplets, which is significant for realizing high-resolution patterns and delicate 3D structures.

Solution-Grown Monocrystalline Hybrid Perovskite Films for Hole-Transporter-Free Solar Cells

High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3 NH3 PbBr3 /Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3 NH3 PbBr3 solar cells to date.

Heterostructured WS2 /CH3 NH3 PbI3 Photoconductors with Suppressed Dark Current and Enhanced Photodetectivity

Heterostructured photoconductors based on hybrid perovskites and 2D transition-metal dichalcogenides are fabricated and characterized. Due to the superior properties of CH3 NH3 PbI3 and WS2 , as well as the efficient interfacial charge transfer, such photoconductors show high performance with on/off ratio of ≈10(5) and responsivity of ≈17 A W(-1) . Furthermore, the response times of the heterostructured photoconductors are four orders of magnitude faster compared to the counterpart of a perovskite single layer.

The In-Gap Electronic State Spectrum of Methylammonium Lead Iodide Single-Crystal Perovskites

The density of trap states within the bandgap of methylammonium lead iodide single crystals is investigated. Defect states close to both the conduction and valence bands are probed. Additionally, a comprehensive electronic characterization of crystals is carried out, including measurements of the electron and hole mobility, and the energy landscape (band diagram) at the surface.

Topological nature of nonlinear optical effects in solids

There are a variety of nonlinear optical effects including higher harmonic generations, photovoltaic effects, and nonlinear Kerr rotations. They are realized by strong light irradiation to materials that results in nonlinear polarizations in the electric field. These are of great importance in studying the physics of excited states of the system as well as for applications to optical devices and solar cells. Nonlinear properties of materials are usually described by nonlinear susceptibilities, which have complex expressions including many matrix elements and energy denominators. On the other hand, a nonequilibrium steady state under an electric field periodic in time has a concise description in terms of the Floquet bands of electrons dressed by photons. We show theoretically, using the Floquet formalism, that various nonlinear optical effects, such as the shift current in noncentrosymmetric materials, photovoltaic Hall response, and photo-induced change of order parameters under the continuous irradiation of monochromatic light, can be described in a unified fashion by topological quantities involving the Berry connection and Berry curvature. We found that vector fields defined with the Berry connections in the space of momentum and/or parameters govern the nonlinear responses. This topological view offers a route to designing nonlinear optical materials.

Solution-processed photodetectors based on organic-inorganic hybrid perovskite and nanocrystalline graphite

We present here solution-processed photodetectors based on a methyl ammonium lead iodide perovskite (MAPbI3) and nanocrystalline graphite (NCG) hybrid composite. The highest responsivity of the best MAPbI3/NCG photodetector was 795 mA W(-1) at 500 nm visible light, which is almost twice as high as that of the NCG-free MAPbI3 photodetector (408 mA W(-1)). The enhanced performance of the MAPbI3/NCG photodetector arises from the improved charge extraction at the MAPbI3/NCG interface. The dependence of photodetector performance on the mass percentage of NCG (the ratio of NCG to MAPbI3) in the hybrid materials is also reported here, and is correlated to the fabrication process. Moreover, by comparing the responsivity of the devices with different channel lengths, we show that the performance of hybrid photodetectors can be further tuned by tailoring the channel length.

MAPbI(3-x)Br(x) mixed halide perovskites for fully printable mesoscopic solar cells with enhanced efficiency and less hysteresis

Hybrid lead-halide perovskite solar cells (PSCs) are promising alternatives to silicon-based cells due to their high photovoltaic performance and low cost. We report herein fully printable perovskite solar cells with a mesoporous TiO2/Al2O3/C architecture in combination with mixed-halide MAPbI(3-x)Br(x) perovskites. A maximum conversion efficiency of 13.49% can be achieved with an increased open circuit voltage of 1.01 V, which is higher than the MAPbI3-based devices. A reduced anomalous hysteresis in the J-V curve measurement has been demonstrated in perovskite solar cells based on MAPbI2.4Br0.6 perovskite, which is directly linked to the characteristically slow kinetics measured through electrochemical impedance spectroscopy.

Highly Efficient Reproducible Perovskite Solar Cells Prepared by Low-Temperature Processing

In this work, we describe the role of the different layers in perovskite solar cells to achieve reproducible, ~16% efficient perovskite solar cells. We used a planar device architecture with PEDOT:PSS on the bottom, followed by the perovskite layer and an evaporated C₆₀ layer before deposition of the top electrode. No high temperature annealing step is needed, which also allows processing on flexible plastic substrates. Only the optimization of all of these layers leads to highly efficient and reproducible results. In this work, we describe the effects of different processing conditions, especially the influence of the C₆₀ top layer on the device performance.