Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskites Suggesting Structural Changes under Working Conditions

Material Innovation in Advancing Organometal Halide Perovskite Functionality

Organometal halide perovskites (OMHPs) have garnered much attention recently for their unprecedented rate of increasing power conversion efficiency (PCE), positioning them as a promising basis for the next-generation photovoltaic devices. However, the gap between the rapid increasing PCE and the incomplete understanding of the structure-property-performance relationship prevents the realization of the true potential of OMHPs. This Perspective aims to provide a concise overview of the current status of OMHP research, highlighting the unique properties of OMHPs that are critical for solar applications but still not adequately explained. Stability and performance challenges of OMHP solar cells are discussed, calling upon combined experimental and theoretical efforts to address these challenges for pioneering commercialization of OMHP solar cells. Various material innovation strategies for improving the performance and stability of OMHPs are surveyed, showing that the OMHP architecture can serve as a promising and robust platform for the design and optimization of materials with desired functionalities.

Open circuit potential build-up in perovskite solar cells from dark conditions to 1 sun

The high open-circuit potential (Voc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The Voc analysis is essential to understand their working mechanisms. A large number of CH3NH3PbI3-xClx PSCs were fabricated on single large-area substrates and their Voc dependencies on illumination intensity, I0, were measured showing three distinctive regions. Similar results obtained in Al2O3 based PSCs relate the effect to the compact TiO2 rather than the mesoporous oxide. We propose that two working mechanisms control the Voc in PSCs. The rise of Voc at low I0 is determined by the employed semiconductor n-type contact (TiO2 or MgO coated TiO2). In contrast, at I0 close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated EFn (as in the dye sensitized solar cells) to an EFn, directly determined by the CH3NH3PbI3-xClx absorber is suggested.

Zero-dipole molecular organic cations in mixed organic-inorganic halide perovskites: possible chemical solution for the reported anomalous hysteresis in the current-voltage curve measurements

Starting from a brief description of the main architectures characterizing the novel solar technology of perovskite-based solar cells, we focus our attention on the anomalous hysteresis experimentally found to affect the measurement of the current-voltage curve of such devices. This detrimental effect, associated with slow dynamic reorganization processes, depends on several parameters; among them, the scan rate of the measurements, the architecture of the cell, and the perovskite deposition rate are crucial. Even if a conclusive explanation of the origin of the hysteresis has not been provided so far, several experimental findings ascribe its origin to ionic migration at an applied bias and dielectric polarization that occurs in the perovskite layer. Consistently, a dipole-moment-reduced cation such as formamidinium ion is experimentally reported to quantitatively reduce the hysteresis from perovskite-based devices. By means of a density-functional theory-based set of calculations, we have predicted and characterized guanidinium ion (GA = (+)[C(NH2)3], a zero-dipole moment cation by symmetry)-based organic-inorganic halide perovskite's structural and electronic properties, speculating that such a cation and the alloys it may form with other organic cations can represent a possible chemical solution for the puzzling issue of the hysteresis.

Probing Photocurrent Generation, Charge Transport, and Recombination Mechanisms in Mesostructured Hybrid Perovskite through Photoconductivity Measurements

Conductivity of methylammonium lead triiodide (MAPbI3) perovskite was measured on different mesoporous metal oxide scaffolds: TiO2, Al2O3, and ZrO2, as a function of incident light irradiation and temperature. It was found that MAPbI3 exhibits intrinsic charge separation, and its conductivity stems from a majority of free charge carriers. The crystal morphology of the MAPbI3 was found to significantly affect the photoconductivity, whereas in the dark the conductivity is governed by the perovskite in the pores of the mesoporous scaffold. The temperature-dependent conductivity measurements also indicate the presence of states within the band gap of the perovskite. Despite a relatively large amount of crystal defects in the measured material, the main recombination mechanism of the photogenerated charges is bimolecular (band-to-band), which suggests that the defect states are rather inactive in the recombination. This may explain the remarkable efficiencies obtained for perovskite solar cells prepared with wet-chemical methods.

Energetics and dynamics in organic-inorganic halide perovskite photovoltaics and light emitters

The rapid transcendence of organic-inorganic metal halide perovskite solar cells to above the 20% efficiency mark has captivated the broad photovoltaic community. As the efficiency race continues unabated, it is essential that fundamental studies keep pace with these developments. Further gains in device efficiencies are expected to be increasingly arduous and harder to come by. The key to driving the perovskite solar cell efficiencies towards their Shockley-Queisser limit is through a clear understanding of the interfacial energetics and dynamics between perovskites and other functional materials in nanostructured- and heterojunction-type devices. In this review, we focus on the current progress in basic characterization studies to elucidate the interfacial energetics (energy-level alignment and band bending) and dynamical processes (from the ultrafast to the ultraslow) in organic-inorganic metal halide perovskite photovoltaics and light emitters. Major findings from these studies will be distilled. Open questions and scientific challenges will also be highlighted.

Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH2)2]PbI3, at 298 K

The structure of black formamidinium lead halide, α-[HC(NH₂)₂]PbI₃, at 298 K has been refined from high resolution neutron powder diffraction data and found to adopt a cubic perovskite unit cell, a = 6.3620(8) Å. The trigonal planar [HC(NH₂)₂]⁺ cations lie in the central mirror plane of the unit cell with the formamidinium cations disordered over 12 possible sites arranged so that the C–H bond is directed into a cube face, whereas the −NH₂ groups hydrogen bond (NH···I = 2.75–3.00 Å) with the iodide atoms of the [PbI₃]⁻ framework. High atomic displacement parameters for the formamidinium cation are consistent with rapid molecular rotations at room temperature as evidenced in ab initio molecular dynamic simulations.

First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers

Hysteresis in current-voltage curves has been an important issue for conversion efficiency evaluation and development of perovskite solar cells (PSCs). In this study, we explored the ion diffusion effects in tetragonal CH3NH3PbI3 (MAPbI3) and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calculations. The calculated activation energies of the anionic and cationic vacancy migrations clearly show that I(-) anions in both MAPbI3 and FAPbI3 can easily diffuse with low barriers of ca. 0.45 eV, comparable to that observed in ion-conducting materials. More interestingly, typical MA(+) cations and larger FA(+) cations both have rather low barriers as well, indicating that the cation molecules can migrate in the perovskite sensitizers when a bias voltage is applied. These results can explain the ion displacement scenario recently proposed by experiments. With the dilute diffusion theory, we discuss that smaller vacancy concentrations (higher crystallinity) and replacement of MA(+) with larger cation molecules will be essential for suppressing hysteresis as well as preventing aging behavior of PSC photosensitizers.

Interfacial Electron Transfer Barrier at Compact TiO2 /CH3 NH3 PbI3 Heterojunction

Low-temperature solution-processed CH3 NH3 PbI3 interfaced with TiO2 has recently been demonstrated as a highly successful type-II light harvesting heterojunction with ≈20% efficiency. Therefore, an efficient ultrafast photoexcited electron transfer from CH3 NH3 PbI3 to TiO2 is expected. However, by probing the photoexcited charge carrier dynamics in CH3 NH3 PbI3 /quartz, CH3 NH3 PbI3 /TiO2 (compact), and CH3 NH3 PbI3 /PCBM in a comparative study, an electron transfer potential barrier between CH3 NH3 PbI3 and the compact TiO2 (prepared with the spray pyrolysis method) formed by surface states is uncovered. Consequently, the CH3 NH3 PbI3 photoluminescence intensity and lifetime is enhanced when interfaced to compact TiO2 . The electron accumulation within CH3 NH3 PbI3 needed to overcome this interfacial potential barrier results in the undesirable large current-voltage hysteresis observed for CH3 NH3 PbI3 /TiO2 planar heterojunctions. The findings in this study indicate that careful surface engineering to reduce this potential barrier is key to pushing perovskite solar cell efficiencies toward the theoretical limit.

Light-Induced Increase of Electron Diffusion Length in a p-n Junction Type CH3NH3PbBr3 Perovskite Solar Cell

High band gap, high open-circuit voltage solar cells with methylammonium lead tribromide (MAPbBr3) perovskite absorbers are of interest for spectral splitting and photoelectrochemical applications, because of their good performance and ease of processing. The physical origin of high performance in these and similar perovskite-based devices remains only partially understood. Using cross-sectional electron-beam-induced current (EBIC) measurements, we find an increase in carrier diffusion length in MAPbBr3(Cl)-based solar cells upon low intensity (a few percent of 1 sun intensity) blue laser illumination. Comparing dark and illuminated conditions, the minority carrier (electron) diffusion length increases about 3.5 times from Ln = 100 ± 50 nm to 360 ± 22 nm. The EBIC cross section profile indicates a p-n structure between the n-FTO/TiO2 and p-perovskite, rather than the p-i-n structure, reported for the iodide derivative. On the basis of the variation in space-charge region width with varying bias, measured by EBIC and capacitance-voltage measurements, we estimate the net-doping concentration in MAPbBr3(Cl) to be 3-6 × 10(17) cm(-3).

Navigating Organo-Lead Halide Perovskite Phase Space via Nucleation Kinetics toward a Deeper Understanding of Perovskite Phase Transformations and Structure-Property Relationships

Organo-lead halide perovskite photovoltaics have developed faster than our understanding of the material itself. Using the vast body of work on perovskite processing created in just the past few years, it is possible to create a better picture of this material's complex phase-transformation behavior. This concept paper summarizes and correlates the current understanding of structural intermediates, kinetic controls, and structure-property relationships of organo-lead iodide perovskites. To this end, a new way of graphically relating information is developed, allowing the simultaneous mapping of schematic kinetic relationships between all currently prevailing perovskite deposition and growth techniques.

Photoinduced Reversible Structural Transformations in Free-Standing CH3NH3PbI3 Perovskite Films

In the pursuit to better understand the mechanisms of perovskite solar cells we performed Raman and photoluminescence measurements of free-standing CH3NH3PbI3 films, comparing dark with working conditions. The films, grown on a glass substrate and sealed by a thin glass coverslip, were measured subsequent to dark and white-light pretreatments. The extremely slow changes we observe in both the Raman and photoluminescence cannot be regarded as electronic processes, which are much faster. Thus, the most probable explanation is of slow photoinduced structural changes. The CH3NH3PbI3 transformation between the dark and the light structures is reversible, with faster rates for the changes under illumination. The results seem to clarify several common observations associated with solar cell mechanisms, like performance improvement under light soaking. More important is the call for solar-cell-related investigation of CH3NH3PbI3 to take the photoinduced structural changes into consideration when measuring and interpreting the results.

Environmental Effects on the Photophysics of Organic-Inorganic Halide Perovskites

The photophysical properties of films of organic-inorganic lead halide perovskites under different ambient conditions are herein reported. We demonstrate that their luminescent properties are determined by the interplay between photoinduced activation and darkening processes, which strongly depend on the atmosphere surrounding the samples. We have isolated oxygen and moisture as the key elements in each process, activation and darkening, both of which involve the interaction with photogenerated carriers. These findings show that environmental factors play a key role in the performance of lead halide perovskites as efficient luminescent materials.

Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping

A need to understand the excited-state behavior of organic-inorganic hybrid perovskites, such as CH3NH3PbI3, has arisen due to the rapid development of perovskite solar cells. The photoinduced processes leading to the efficient charge separation observed in these materials remain somewhat elusive. This Perspective presents an overview of the initial attempts to characterize the excited-state and charge recombination dynamics in the prototypical material CH3NH3PbI3. While much has been accomplished in designing high-efficiency solar cells, the multifaceted nature of the CH3NH3PbI3 excited state offers ample challenges for the photovoltaic community to better comprehend. Building on this foundation may enable us to tackle the stability concerns that have shadowed the rise of perovskite solar cells. Furthermore, a better understanding of the excited-state properties can provide insight into the specific properties that have thrust this material to the forefront of photovoltaic research.

The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH₃NH₃⁺ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH₃NH₃PbI₃ crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH₃NH₃⁺ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH₃NH₃⁺ in either antiferroelectric or ferroelectric domains. Collective realignment of CH₃NH₃⁺ to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1–1 ms, faster than most observed hysteresis.

Hysteresis often exists in the characterization of methylammonium lead halide-based solar cells, but is not well understood. Here, the authors use quasielastic neutron scattering to study the dynamics of dipolar organic cations and shed light on the hysteresis behaviour.

Bright Visible-Infrared Light Emitting Diodes Based on Hybrid Halide Perovskite with Spiro-OMeTAD as a Hole-Injecting Layer

Hybrid halide perovskites that are currently intensively studied for photovoltaic applications, also present outstanding properties for light emission. Here, we report on the preparation of bright solid state light emitting diodes (LEDs) based on a solution-processed hybrid lead halide perovskite (Pe). In particular, we have utilized the perovskite generally described with the formula CH3NH3PbI(3-x)Cl(x) and exploited a configuration without electron or hole blocking layer in addition to the injecting layers. Compact TiO2 and Spiro-OMeTAD were used as electron and hole injecting layers, respectively. We have demonstrated a bright combined visible-infrared radiance of 7.1 W·sr(-1)·m(-2) at a current density of 232 mA·cm(-2), and a maximum external quantum efficiency (EQE) of 0.48%. The devices prepared surpass the EQE values achieved in previous reports, considering devices with just an injecting layer without any additional blocking layer. Significantly, the maximum EQE value of our devices is obtained at applied voltages as low as 2 V, with a turn-on voltage as low as the Pe band gap (V(turn-on) = 1.45 ± 0.06 V). This outstanding performance, despite the simplicity of the approach, highlights the enormous potentiality of Pe-LEDs. In addition, we present a stability study of unsealed Pe-LEDs, which demonstrates a dramatic influence of the measurement atmosphere on the performance of the devices. The decrease of the electroluminescence (EL) under continuous operation can be attributed to an increase of the non-radiative recombination pathways, rather than a degradation of the perovskite material itself.

Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K

The methylammonium cation in [CH3NH3]PbI3 demonstrates increasing positional disorder on heating from 100 K to 352 K. In the tetragonal phase, stable between 165 K and 327 K, the cation is disordered over four sites directed toward the faces of the distorted cubic [PbI3](-) framework and migrates towards the cavity centre with increasing temperature.

The Interface between FTO and the TiO2 Compact Layer Can Be One of the Origins to Hysteresis in Planar Heterojunction Perovskite Solar Cells

Organometal halide perovskite solar cells have shown rapid rise in power conversion efficiency, and therefore, they have gained enormous attention in the past few years. However, hysteretic photovoltaic characteristics, found in these solid-state devices, have been a major problem. Although it is being proposed that the ferroelectric property of perovskite causes hysteresis in the device, we observed hysteresis in a device made of nonferroelectric PbI2 as a light absorber. This result evidently supports the fact that ferroelectric property cannot be the sole reason for hysteresis. The present study investigates the roles of some key interfaces in a planar heterojunction perovskite (CH3NH3PbI(3-x)Cl(x)) solar cell that can potentially cause hysteresis. The results confirm that the interface between fluorine doped tin oxide (FTO) substrate and the TiO2 compact layer has a definite contribution to hysteresis. Although this interface is one of the origins to hysteresis, we think that other interfaces, especially the interface of the TiO2 compact layer with perovskite, can also play major roles. Nevertheless, the results indicate that hysteresis in such devices can be reduced/eliminated by changing the interlayer between FTO and perovskite.

A Switchable High-Sensitivity Photodetecting and Photovoltaic Device with Perovskite Absorber

Amplified photocurrent gain has been obtained by photodiodes of inorganic semiconductors such as GaAs and Si. The avalanche photodiode, developed for high-sensitivity photodetectors, requires an expensive vapor-phase epitaxy manufacture process and high driving voltage (50-150 V). Here, we show that a low-cost solution-processed device using a planar-structured ferroelectric organo-lead triiodide perovskite enables light detection in a large dynamic range of incident power (10(-7)-10(-1) W cm(-2)) by switching with small voltage (-0.9 to +0.5 V). The device achieves significantly high external quantum conversion efficiency (EQE) up to 2.4 × 10(5)% (gain value of 2400) under weak monochromatic light. On a single dual-functional device, incident small power (0.2-100 μW cm(-2)) and medium to large power (>0.1 mW cm(-2)) are captured by reverse bias and forward bias modes, respectively, with linear responsivity of current. For weak light detection, the device works with a high responsivity value up to 620 A W(-1).

Ferroelectric Polarization in CH3NH3PbI3 Perovskite

We report on ferroelectric polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepared for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external electric field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelectric polarization was also observed after removal of the electric field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Additionally, we suggest the effect of perovskite crystal size (morphology) on charge collection at the interface of the ferroelectric material even though insignificant size dependency in electric polarization was observed.

Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells

Despite spectacular advances in conversion efficiency of perovskite solar cell many aspects of its operating modes are still poorly understood. Capacitance constitutes a key parameter to explore which mechanisms control particular functioning and undesired effects as current hysteresis. Analyzing capacitive responses allows addressing not only the nature of charge distribution in the device but also the kinetics of the charging processes and how they alter the solar cell current. Two main polarization processes are identified. Dielectric properties of the microscopic dipolar units through the orthorhombic-to-tetragonal phase transition account for the measured intermediate frequency capacitance. Electrode polarization caused by interfacial effects, presumably related to kinetically slow ions piled up in the vicinity of the outer interfaces, consistently explain the reported excess capacitance values at low frequencies. In addition, current-voltage curves and capacitive responses of perovskite-based solar cells are connected. The observed hysteretic effect in the dark current originates from the slow capacitive mechanisms.

Giant photoluminescence blinking of perovskite nanocrystals reveals single-trap control of luminescence

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size ∼200 × 30 × 30 nm(3). The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light.

The expanding world of hybrid perovskites: materials properties and emerging applications

Hybrid inorganic–organic perovskites have emerged over the last 5 years as a promising class of materials for optoelectronic applications. Most notably, their solar cells have achieved power conversion efficiencies above 20% in an unprecedented timeframe; however, many fundamental questions still remain about these materials. This Prospective Article reviews the procedures used to deposit hybrid perovskites and describes the resulting crystallographic and morphological structures. It further details the electrical and optical properties of perovskites and then concludes by highlighting a number of potential applications and the materials challenges that must be overcome before they can be realized.

Emergence of Hysteresis and Transient Ferroelectric Response in Organo-Lead Halide Perovskite Solar Cells

Although there has been rapid progress in the efficiency of perovskite-based solar cells, hysteresis in the current-voltage performance is not yet completely understood. Owing to its complex structure, it is not easy to attribute the hysteretic behavior to any one of different components, such as the bulk of the perovskite or different heterojunction interfaces. Among organo-lead halide perovskites, methylammonium lead iodide perovskite (CH3NH3PbI3) is known to have a ferroelectric property. The present investigation reveals a strong correlation between transient ferroelectric polarization of CH3NH3PbI3 induced by an external bias in the dark and hysteresis enhancement in photovoltaic characteristics. Our results demonstrate that the reverse bias poling (-0.3 to -1.1 V) of CH3NH3PbI3 photovoltaic layers prior to the photocurrent-voltage measurement generates stronger hysteresis whose extent changes significantly by the cell architecture. The phenomenon is interpreted as the effect of remanent polarization in the perovskite film on the photocurrent, which is most enhanced in planar perovskite structures without mesoporous scaffolds.

The chemical origin of the p-type and n-type doping effects in the hybrid methylammonium–lead iodide (MAPbI3) perovskite solar cells

A field-induced and photoinduced self-doping chemistry of the MAPbI₃ perovskite films affecting their photovoltaic performance and stability is proposed.

Chemical decoration of CH3NH3PbI3 perovskites with graphene oxides for photodetector applications

A facile solution method is developed for in situ synthesis of free-standing CH₃NH₃PbI₃/graphene hybrids to improve significantly the photodetector performance.

Photovoltaic performance and the energy landscape of CH3NH3PbI3

The methylammonium ions in CH₃NH₃PbI₃ prefer to rotate collectively, and to be parallel to their neighbours or to be polarised by working field. The polarised charges in CH₃NH₃PbI₃ screens the hindering electric field, improves the power conversion characteristics of CH₃NH₃PbI₃ photovoltaic cells. This polarised charges are also in significant part responsible for the normal and anomalous hysteresis.

Organic–inorganic halide perovskite based solar cells – revolutionary progress in photovoltaics

This review outlines the latest progress in perovskite-based solar cells, including device achievements and underlying insights and mechanisms of the perovskite materials.

Rotational dynamics of organic cations in the CH3NH3PbI3perovskite

Rotational dynamics of organic cations in the CH₃NH₃PbI₃perovskite are revealed by elastic and quasi-elastic neutron scattering and group theoretical analysis.

Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics

A reversible photo-induced instability has been found in mixed-halide photovoltaic perovskites that limits the open circuit voltage in solar cells.

We report on reversible, light-induced transformations in (CH₃NH₃)Pb(Br_xI_1–x)₃. Photoluminescence (PL) spectra of these perovskites develop a new, red-shifted peak at 1.68 eV that grows in intensity under constant, 1-sun illumination in less than a minute. This is accompanied by an increase in sub-bandgap absorption at ∼1.7 eV, indicating the formation of luminescent trap states. Light soaking causes a splitting of X-ray diffraction (XRD) peaks, suggesting segregation into two crystalline phases. Surprisingly, these photo-induced changes are fully reversible; the XRD patterns and the PL and absorption spectra revert to their initial states after the materials are left for a few minutes in the dark. We speculate that photoexcitation may cause halide segregation into iodide-rich minority and bromide-enriched majority domains, the former acting as a recombination center trap. This instability may limit achievable voltages from some mixed-halide perovskite solar cells and could have implications for the photostability of halide perovskites used in optoelectronics.