Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis

Facile synthesis of CoOx@C/Ti-Fe2O3 photoanodes for efficient photoelectrochemical water oxidation

The development of photoelectrochemical (PEC) water splitting is hindered by the slow kinetics of four-electron processes for the oxygen evolution reaction (OER) and severe charge recombination. Amorphous carbon was chosen as a carrier for the active sites due to its exceptional conductivity and strong loading capacity. In addition, this enhanced performance was attributed to the loading of oxides of cobalt. Here, amorphous carbon-covered cobalt oxides chosen as a co-catalyst loaded on α-Fe2O3 (noted as CoOx@C/Ti-Fe2O3) have been synthesized, and they show a high current density (2.86 mA cm-2 under 1.23 V vs. RHE), and a low onset potential (0.611 V vs. RHE). Experimental analysis demonstrates that the charge transfer and separation leading to accelerated OER dynamics and improved PEC performance are enhanced by CoOx@C effectively. This study provides new ideas for designing high-performance photoelectrochemical electrodes based on amorphous carbon co-catalysts.

Inhibition of Ion Migration for Highly Efficient and Stable Perovskite Solar Cells

In recent years, organic-inorganic halide perovskites are now emerging as the most attractive alternatives for next-generation photovoltaic devices, due to their excellent optoelectronic characteristics and low manufacturing cost. However, the resultant perovskite solar cells (PVSCs) are intrinsically unstable owing to ion migration, which severely impedes performance enhancement, even with device encapsulation. There is no doubt that the investigation of ion migration and the summarization of recent advances in inhibition strategies are necessary to develop "state-of-the-art" PVSCs with high intrinsic stability for accelerated commercialization. This review systematically elaborates on the generation and fundamental mechanisms of ion migration in PVSCs, the impact of ion migration on hysteresis, phase segregation, and operational stability, and the characterizations for ion migration in PVSCs. Then, many related works on the strategies for inhibiting ion migration toward highly efficient and stable PVSCs are summarized. Finally, the perspectives on the current obstacles and prospective strategies for inhibition of ion migration in PVSCs to boost operational stability and meet all of the requirements for commercialization success are summarized.

Role of Dipolar Organic Cations on Light-triggered Charge Transfer at TiO2 /CH3 NH3 PbI3 Interfaces

The TiO2 /MAPbI3 (MA=CH3 NH3 ) interfaces have manifested correlation with current-voltage hysteresis in perovskite solar cells (PSCs) under light illumination conditions, but the relations between the photo-induced charge transfer and the collective polarization response of the dipolar MA cations are largely unexplored. In this work, we adopt density functional theory (DFT) and time-dependent DFT approach to study the light-triggered charge transfer across the TiO2 /MAPbI3 interfaces with MAI- and PbI-exposed terminations. It is found that regardless of the surface exposure of the MAPbI3 , the photo-induced charge transfer varies when going from the ground-state geometries to the excited-state configurations. Besides, thanks to the electrostatic interactions between the ends of MA cations and the photogenerated electrons, the photo-induced charge transfer across the interfaces is enhanced (weakened) by the negatively (positively) charged CH3 (NH3 ) moieties of the MA species. Resultantly, the positively charged iodine vacancies at the TiO2 /MAPbI3 interfaces tend to inhibit the charge transfer induced by light. Combining with the energy level alignment which is significantly modulated by the orientation of the MA species at the interfaces, the dipolar MA cations might be a double-edge sword for the hysteresis in PSCs with the TiO2 /MAPbI3 interfaces.

Novel Spiro-Core Dopant-Free Hole Transporting Material for Planar Inverted Perovskite Solar Cells

Hole-transporting materials (HTMs) have demonstrated their crucial role in promoting charge extraction, interface recombination, and device stability in perovskite solar cells (PSCs). Herein, we present the synthesis of a novel dopant-free spiro-type fluorine core-based HTM with four ethoxytriisopropylsilane groups (Syl-SC) for inverted planar perovskite solar cells (iPSCs). The thickness of the Syl-SC influences the performance of iPSCs. The best-performing iPSC is achieved with a 0.8 mg/mL Syl-SC solution (ca. 15 nm thick) and exhibits a power conversion efficiency (PCE) of 15.77%, with J_sc = 20.00 mA/cm², V_oc = 1.006 V, and FF = 80.10%. As compared to devices based on PEDOT:PSS, the iPSCs based on Syl-SC exhibit a higher V_oc, leading to a higher PCE. Additionally, it has been found that Syl-SC can more effectively suppress charge interfacial recombination in comparison to PEDOT:PSS, which results in an improvement in fill factor. Therefore, Syl-SC, a facilely processed and efficient hole-transporting material, presents a promising cost-effective alternative for inverted perovskite solar cells.

Ionic and electronic energy diagrams for hybrid perovskite solar cells

The development of photoelectrochemical devices based on mixed ionic-electronic conductors requires knowledge of transport, generation and reaction of electronic and ionic charge carriers. Thermodynamic representations can significantly help the understanding of these processes. They should be simple and reflect the necessity of dealing with ions and electrons. In this work, we discuss the extension of energy diagrams commonly used to describe electronic properties of semiconductors to the defect chemical treatment of electronic and ionic charge carriers in mixed conducting materials as introduced in the context of nanoionics. We focus on hybrid perovskites in relation to their use as the active layer material of solar cells. Owing to the presence of at least two ion types, a variety of native ionic disorder processes have to be dealt with in addition to the single fundamental electronic disorder process as well as potential frozen-in defects. Various situations are discussed that show how such generalized level diagrams can be usefully applied and appropriately simplified in the determination of the equilibrium behavior of bulk and interfaces in solar cell devices. This approach can serve as a basis for investigating the behavior of perovskite solar cells, but also other mixed-conducting devices operating under bias.

A DFT Study of Alkaline Earth Metal-Doped FAPbI3 (111) and (100) Surfaces

Density functional theory calculations have been performed to study the effect of replacing lead by alkaline earth metals on the stability, electronic and optical properties of the formamidinium lead triiodide (FAPbI₃) (111) and (100) surfaces with different terminations in the form of FAPb_1-xAE_xI₃ structures, where AE is Be, Mg or Ca. It is revealed that the (111) surface is more stable, indicating metallic characteristics. The (100) surfaces exhibit a suitable bandgap of around 1.309 and 1.623 eV for PbI₅ and PbI₆ terminations, respectively. Increases in the bandgaps as a result of Mg- and Ca-doping of the (100) surface were particularly noted in FAPb_0.96Ca_0.04I₃ and FAPb_0.8Ca_0.2I₃ with bandgaps of 1.459 and 1.468 eV, respectively. In the presence of Be, the band gap reduces critically by about 0.315 eV in the FAPb_0.95Be_0.05I₃ structure, while increasing by 0.096 eV in FAPb_0.96Be_0.04I₃. Optimal absorption, high extinction coefficient and light harvesting efficiency were achieved for plain and doped (100) surfaces in the visible and near UV regions. In order to improve the optical properties of the (111)-PbI₃ surface in initial visible areas, we suggest calcium-doping in this surface to produce FAPb_0.96Ca_0.04I₃, FAPb_0.92Ca_0.08I₃, and FAPb_0.88Ca_0.12I₃ structures.

Light-Controlled Reconfigurable Optical Synapse Based on Carbon Nanotubes/2D Perovskite Heterostructure for Image Recognition

Two-dimensional (2D) halide perovskite material is characterized by a mixed conducting behavior that possesses both electronic and ionic conductivity. The study on the influence of the light on ion migration in the 2D perovskite is helpful to improve the performance of perovskite-based optoelectronic devices. Here, we constructed an exfoliated 2D perovskite/carbon nanotubes (CNTs) heterostructure optical synapse, in which CNTs can be used as nanoprobes to qualitatively observe the ion aggregation or dissipation process in 2D perovskite, and found that light significantly changes the memory curve of the reconfigurable optical synapses. Through the molecular dynamic simulation, the dynamic process of ion migration in the heterostructure was simulated and the electrostatic interaction effect of nonequilibrium charge distribution of CNTs on iodide ion was demonstrated. Finally, an effective light-controlled process was realized through the synapses, which in situ regulated the performance of the weight-value discretized BP (WD-BP) neural network. This work lays a foundation for the future development of intelligent nano-optoelectronic devices.

Carbon dots with different energy levels regulate the activity of metal-free catalyst for hydrogen peroxide photoproduction

Artificial photoproduction of hydrogen peroxide (H₂O₂) from H₂O and O₂ by metal-free catalysts (e.g., graphitic carbon nitride) is regarded as an ultra-clean approach. Metal-free catalysts are often hindered by unpropitious rapid charge recombination and unfavorable selectivity. Herein, three carbon dots (CDs1 to CDs3) decorated modified-carbon nitride (CDs1-NCN, CDs2-NCN and CDs3-NCN) were designed and fabricated, which show diverse activity of H₂O₂ photoproduction. Among them, CDs1-NCN, as a two-channel photocatalyst, achieves H₂O₂ production with high efficiency (1938 μmol h^-1 g^-1). This process is at normal pressure and without sacrificial agent under visible region (λ≥420nm), which is 27.5- times higher than that of pristine C₃N₄. The apparent quantum efficiency can be calculated to 7.03 % (λ=365nm). In this system, CDs with different energy levels dominate the activity of metal-free catalyst for hydrogen peroxide photoproduction. Combining with photoelectrochemical test and transient photovoltage analysis, the active site and the catalytic mechanism of these composite catalysts are also clarified. Our work provides a clearly insight for understanding of the regulation of interfacial electron transport in metal-free photocatalysts.

Self-Enhancement of Efficiency and Self-Attenuation of Hysteretic Behavior of Perovskite Solar Cells with Aging

Spontaneous enhancement of the photovoltaic performance of perovskite solar cells (PSCs) after aging has been reported, but the underlying reasons for such behavior are still under debate. Herein, we demonstrate that this spontaneous improvement effect accompanied by self-attenuation of hysteresis in the current-voltage characteristics is time- and temperature-dependent. Moreover, it is universal to PSCs based on a range of mixed-ion perovskites and coupled to different hole- and electron-transporting materials. Time-resolved confocal fluorescence microscopy and other characterization techniques suggest that the "self-healing" phenomenon is accompanied by the homogenization and enhancement of the charge extraction efficiency as well as suppressed recombination throughout cm²-scale perovskite layers. These dynamic effects need to be accounted for when assessing the initial and stabilized performance of PSCs.

Emission Spectroscopy Investigation of the Enhancement of Carrier Collection Efficiency in AgBiS2-Nanocrystal/ZnO-Nanowire Heterojunction Solar Cells

Eco-friendly solar cells were fabricated using interdigitated layers comprising ZnO nanowires (NWs) and infrared absorbing AgBiS₂ nanocrystals (ITO/ZnO NWs/AgBiS₂/P3HT/Au). The quality of ZnO NWs was studied using photoluminescence and Raman spectroscopy to identify the defects in ZnO NWs influencing solar cell performance. Oxygen vacancies and Zn interstitial sites, among various recombination sites, were observed to be the main sites for carrier recombination, which hinders the carrier collection in the solar cells. Accordingly, the power conversion efficiency of AgBiS₂ solar cells exhibited a good correlation with the number of oxygen vacancies. The structural order and electron-phonon interaction in ZnO NWs were also investigated via Raman scattering spectroscopy. A lower concentration of oxygen vacancies and zinc interstitials (Zn_i) resulted in a higher structural order as well as a weaker electron-phonon interaction in ZnO NWs. When ZnO NWs were treated at 500 °C in oxygen with the lowest oxygen vacancy concentration, the solar cells (500-O₂ solar cell (SC)) demonstrated an external quantum efficiency of approximately 70% in the visible region and a corresponding internal quantum efficiency of more than 80%. The 500-O₂ SC exhibited a power conversion efficiency of 5.41% (J_SC = 22.21 mA/cm², V_OC = 0.41 V, and FF = 60%) under quasi one-sun illumination. New methods that can efficiently reduce oxygen vacancies and Zn_i without affecting the structural order of ZnO NWs would further enhance the carrier collection efficiency. Moreover, since ZnO is a key electron transport material for constructing not only colloidal quantum dot solar cells but also other emerging solar cells, such as organic thin-film solar cells, the present findings provide significant information for improving their performance.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

The charge carrier dynamics in organic solar cells and organic-inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

Surface band bending and carrier dynamics in colloidal quantum dot solids

Band bending in colloidal quantum dot (CQD) solids has become important in driving charge carriers through devices. This is typically a result of band alignments at junctions in the device. Whether band bending is intrinsic to CQD solids, i.e. is band bending present at the surface-vacuum interface, has previously been unanswered. Here we use photoemission surface photovoltage measurements to show that depletion regions are present at the surface of n and p-type CQD solids with various ligand treatments (EDT, MPA, PbI₂, MAI/PbI₂). Using laser-pump photoemission-probe time-resolved measurements, we show that the timescale of carrier dynamics in the surface of CQD solids can vary over at least 6 orders of magnitude, with the fastest dynamics on the order of microseconds in PbS-MAI/PbI₂ solids and on the order of seconds for PbS-MPA and PbS-PbI₂. By investigating the surface chemistry of the solids, we find a correlation between the carrier dynamics timescales and the presence of oxygen contaminants, which we suggest are responsible for the slower dynamics due to deep trap formation.

Electric-Field-Induced Ion Migration Behavior in Methylammonium Lead Iodide Perovskite

Ionic movement inside organometal halide perovskites (OMHP) materials has been widely reported to be linked with stability issues in the perovskite-based optoelectronic devices. However, the dynamic processes of the ionic movement and how they influence the devices are still not well-understood. In this work, we applied an external electric field to the CH₃NH₃PbI₃ crystal and simultaneously monitored the PL behaviors. Two successive PL responses were observed in the same location of the crystal. First, an irreversible PL quenching was observed caused by the photo-annealing effect under an electric field accompanied by a permanent morphology change. The annealed area also showed reversible PL variation, which was attributed to the activation-deactivation of the radiative recombination centers induced by the migration of the iodine ions. Such results can help us gain a deep insight into how the ionic movements in OMHPs influence the performance of the perovskite-based optoelectronic devices under working conditions.

Theory of Hysteresis in Halide Perovskites by Integration of the Equivalent Circuit

Perovskite solar cells show a number of internal electronic-ionic effects that produce hysteresis in the current-voltage curves and a dependence of the temporal response on the conditions of the previous stimulus applied to the sample. There are many models and explanations in the literature, but predictive methods that may lead to an assessment of the solar cell behavior based on independent measurements are needed. Here, we develop a method to predict time domain response starting from the frequency domain response measured by impedance spectroscopy over a collection of steady states. The rationale of the method is to convert the impedance response into a set of differential equations, in which the internal state variables emerge naturally and need not be predefined in terms of a physical (drift/diffusion/interfaces) model. Then, one solves (integrates) the evolution for a required external perturbation such as voltage sweep at a constant rate (cyclic voltammetry). Using this method, we solve two elementary but relevant equivalent circuit models for perovskite solar cells and memristors, and we show the emergence of hysteresis in terms of the relevant time and energy constants that can be fully obtained from impedance spectroscopy. We demonstrate quantitatively a central insight in agreement with many observations: regular hysteresis is capacitive, and inverted hysteresis is inductive. Analysis of several types of perovskite solar cells shows excellent correlation of the type of equivalent circuit and the observed hysteresis. A new phenomenon of transformation from capacitive to inductive hysteresis in the course of the current-voltage curve is reported.

Importance of methylammonium iodide partial pressure and evaporation onset for the growth of co-evaporated methylammonium lead iodide absorbers

Vacuum-based co-evaporation promises to bring perovskite solar cells to larger scales, but details of the film formation from the physical vapor phase are still underexplored. In this work, we investigate the growth of methylammonium lead iodide (MAPbI[Formula: see text]) absorbers prepared by co-evaporation of methylammonium iodide (MAI) and lead iodide (PbI[Formula: see text]) using an in situ X-ray diffraction setup. This setup allows us to characterize crystallization and phase evolution of the growing thin film. The total chamber pressure strongly increases during MAI evaporation. We therefore assume the total chamber pressure to be mainly built up by an MAI atmosphere during deposition and use it to control the MAI evaporation. At first, we optimize the MAI to PbI[Formula: see text] impingement ratios by varying the MAI pressure at a constant PbI[Formula: see text] flux rate. We find a strong dependence of the solar cell device performance on the chamber pressure achieving efficiencies > 14[Formula: see text] in a simple n-i-p structure. On the road to further optimizing the processing conditions we vary the onset time of the PbI[Formula: see text] and MAI deposition by delaying the start of the MAI evaporation by t = 0/8/16 min. This way, PbI[Formula: see text] nucleates as a seed layer with a thickness of up to approximately 20 nm during this initial stage. Device performance benefits from these PbI[Formula: see text] seed layers, which also induce strong preferential thin film orientation as evidenced by grazing incidence wide angle X-ray scattering (GIWAXS) measurements. Our insights into the growth of MAPbI[Formula: see text] thin films from the physical vapor phase help to understand the film formation mechanisms and contribute to the further development of MAPbI[Formula: see text] and related perovskite absorbers.

Asymmetric Bipolar Resistive Switching of Halide Perovskite Film in Contact with TiO2 Layer

Halide perovskite materials such as methylammonium lead iodide (CH₃NH₃PbI₃) have attracted considerable interest for the resistive random-access memory applications, which exploit a dramatic change in the resistance by an external electric bias. In many semiconductor films, the drift, accumulation, and chain formation of defects explain the change in the resistance by an external bias. This study demonstrates that the interface of CH₃NH₃PbI₃ with TiO₂ has a significant impact on the formation and rupture of defect chains and causes the asymmetric bipolar resistive switching in the Au/CH₃NH₃PbI₃/TiO₂/FTO device (FTO = fluorine-doped tin oxide). When a negative bias is applied to the Au electrode, iodine interstitials with the lowest migration activation energy move toward TiO₂ in the CH₃NH₃PbI₃ layer and pile up at the CH₃NH₃PbI₃-TiO₂ interface. Under the same condition, oxygen vacancies in the TiO₂ layer also travel to the CH₃NH₃PbI₃-TiO₂ interface and strongly attract iodine interstitials. As a result, a Schottky barrier appears at the CH₃NH₃PbI₃-TiO₂ interface, and the resistance of Au/CH₃NH₃PbI₃/TiO₂/FTO becomes much larger than that of Au/CH₃NH₃PbI₃/FTO in the high resistance state. The frequency dependence of the capacitance confirms the asymmetric appearance of a large space charge polarization at the CH₃NH₃PbI₃-TiO₂ interface, which causes the unique bipolar resistive switching behavior with the on/off ratio (10³) and retention time (>10⁴ seconds) at -0.85 V in Au/CH₃NH₃PbI₃/TiO₂/FTO film.

Elucidating the Trajectory of the Charge Transfer Mechanism and Recombination Process of Hybrid Perovskite Solar Cells

Perovskite-based solar cells (PSCs) have attracted attraction in the photovoltaic community since their inception in 2009. To optimize the performance of hybrid perovskite cells, a primary and crucial strategy is to unravel the dominant charge transport mechanisms and interfacial properties of the contact materials. This study focused on the charge transfer process and interfacial recombination within the n-i-p architecture of solar cell devices. The motivation for this paper was to investigate the impacts of recombination mechanisms that exist within the interface in order to quantify their effects on the cell performance and stability. To achieve our objectives, we firstly provided a rationale for the photoluminescence and UV-Vis measurements on perovskite thin film to allow for disentangling of different recombination pathways. Secondly, we used the ideality factor and impedance spectroscopy measurements to investigate the recombination mechanisms in the device. Our findings suggest that charge loss in PSCs is dependent mainly on the configuration of the cells and layer morphology, and hardly on the material preparation of the perovskite itself. This was deduced from individual analyses of the perovskite film and device, which suggest that major recombination most likely occur at the interface.

Effects of ion migration and improvement strategies for the operational stability of perovskite solar cells

The fundamental factor affecting the stability of perovskite solar cells, ion migration, has been reviewed, which is found to be closely related to the degradation of perovskite solar cells. Characterization methods like impedance spectroscopy and galvanostatic measurement to identify ion migration in perovskite films have been reviewed. The influence of light on ion migration was further discussed, which could largely explain the photo-stability decay in most perovskite solar cells. Finally, several solutions to inhibit ion migration for better operational stability of perovskite solar cells were summarized, including bulk passivation, interface passivation and grain boundary passivation. Several strategies have also been proposed to further improve the stablity of perovskite solar cells.

Hysteretic Ion Migration and Remanent Field in Metal Halide Perovskites

The gap in understanding how underlying chemical dynamics impact the functionality of metal halide perovskites (MHPs) leads to the controversy about the origin of many phenomena associated with ion migration in MHPs. In particular, the debate regarding the impact of ion migration on current-voltage (I-V) hysteresis of MHPs devices has lasted for many years, where the difficulty lies in directly uncovering the chemical dynamics, as well as identifying and separating the impact of specific ions. In this work, using a newly developed time-resolved time-of-flight secondary ion mass spectrometry CH3NH3 + and I- migrations in CH3NH3PbI3 are directly observed, revealing hysteretic CH3NH3 + and I- migrations. Additionally, hysteretic CH3NH3 + migration is illumination-dependent. Correlating these results with the I-V characterization, this work uncovers that CH3NH3 + redistribution can induce a remanent field leading to a spontaneous current in the device. It unveils that the CH3NH3 + migration is responsible for the illumination-associated I-V hysteresis in MHPs. Hysteretic ion migration has not been uncovered and the contribution of any ions (e.g., CH3NH3 +) has not been specified before. Such insightful and detailed information has up to now been missing, which is critical to improving MHPs photovoltaic performance and developing MHPs-based memristors and synaptic devices.

Improving efficiency and stability of colorful perovskite solar cells with two-dimensional photonic crystals

Colorful solar cells have been much sought after because they can generate electricity and concurrently satisfy ornamentation purposes. Owing to their outstanding power conversion efficiency and flexibility in processing, perovskite solar cells (PSCs) have the great potential to become both efficient and aesthetically appealing. Here, we specially devise and fabricate two novel electron transport layers (ETLs) for PSCs with two-dimensional (2D) photonic crystal structures, namely the 2D inverse opal (IO) structured SnO₂ (IOS) and SnO₂-TiO₂ composite (IOST), using the template-assisted spin-coating method. The synergistic structure and material modifications to the ETLs lead to a number of unique features, including the remarkable electron transfer ability, vivid colors and good protection to the infiltrated perovskite films. Furthermore, the IOS and IOST ETLs are effectively incorporated into the CH₃NH₃PbI₃-based PSC devices that deliver the best efficiency of 16.8% with structural colors.

Improved Carrier Collection and Hot Electron Extraction Across Perovskite, C60, and TiO2 Interfaces

The use of C₆₀ as an interfacial layer between TiO₂ and methylammonium lead iodide perovskite is probed to reduce the current-voltage hysteresis in perovskite solar cells (PSCs) and, in turn, to impact the interfacial carrier injection and recombination processes that limit solar cell efficiencies. Detailed kinetic analyses across different time scales, that is, from the femtoseconds to the seconds, reveal that the charge carrier lifetimes as well as the charge injection and charge recombination dynamics depend largely on the presence or absence of C₆₀. In addition, we corroborate that C₆₀ is applicable in hot carrier PSCs as it is capable of extracting hot carriers generated throughout the early time scales following photoexcitation.

Potassium doping-induced variations in the structures and photoelectric properties of a MAPbI3 perovskite and a MAPbI3/TiO2 junction

The intrinsic and K⁺-doping-induced properties of a MAPbI₃ material and a MAPbI₃/TiO₂ junction have been elucidated.

Light-Enhanced Ion Migration in Two-Dimensional Perovskite Single Crystals Revealed in Carbon Nanotubes/Two-Dimensional Perovskite Heterostructure and Its Photomemory Application

Two-dimensional (2D) hybrid perovskite sandwiched between two long-chain organic layers is an emerging class of low-cost semiconductor materials with unique optical properties and improved moisture stability. Unlike conventional semiconductors, ion migration in perovskite is a unique phenomenon possibly responsible for long carrier lifetime, current-voltage hysteresis, and low-frequency giant dielectric response. While there are many studies of ion migration in bulk hybrid perovskite, not much is known for its 2D counterparts, especially for ion migration induced by light excitation. Here, we construct an exfoliated 2D perovskite/carbon nanotube (CNT) heterostructure field effect transistor (FET), not only to demonstrate its potential in photomemory applications, but also to study the light induced ion migration mechanisms. We show that the FET I-V characteristic curve can be regulated by light and shows two opposite trends under different CNT oxygen doping conditions. Our temperature-dependent study indicates that the change in the I-V curve is probably caused by ion redistribution in the 2D hybrid perovskite. The first principle calculation shows the reduction of the migration barrier of I vacancy under light excitation. The device simulation shows that the increase of 2D hybrid perovskite dielectric constant (enabled by the increased ion migration) can change the I-V curve in the trends observed experimentally. Finally, the so synthesized FET shows the multilevel photomemory function. Our work shows that not only we could understand the unique ion migration behavior in 2D hybrid perovskite, it might also be used for many future memory function related applications not realizable in traditional semiconductors.

Interfacial recombination kinetics in aged perovskite solar cells measured using transient photovoltage techniques

The reduction of interfacial charge recombination kinetics in perovskite solar cells is key to increase device photovoltaic efficiencies. Thus, it is necessary to fully understand which are the major carrier losses and, thereafter, how they can be minimized. Transient Photovoltage (TPV) has been widely used to study carrier recombination in solar cells under operando conditions. Interestingly, a novel negative transient deflection appears in perovskite solar cells when carrying out TPV measurements and it has been related to the ionic accumulation at the perovskite interfaces, which is a process that requires great attention to fully understand how perovskite solar cells work. Herein, we moved one step further and continuously monitored the evolution of the negative trace with the aging of the perovskite solar cell. Importantly, we demonstrated that the negative signal changes with aging of the solar cells and such a change can be directly related to the enhancement of the open circuit voltage and fill factor of the devices and thus the solar cell efficiency. We postulate that this increase in efficiency is due to better/faster ion redistribution within the perovskite material.

Electrical-Field-Driven Tunable Spectral Responses in a Broadband-Absorbing Perovskite Photodiode

Controllably manipulating the spectral response of broadband-absorbing semiconductors is crucial for developing wavelength-selective optoelectronic devices. In this article, we report for the first time, the bias-dependent spectral responses for a metal-halide perovskite photodiode. Tunable external quantum efficiencies in the short- and long-wavelength regimes, and the full spectral range (ca. 300-800 nm) are observed when the device is operated under short-circuit, and forward and reverse bias conditions, respectively. This observation is understood by the interplay of wavelength-dependent penetration depth and barrier formation within the photodiode device stack. The general applicability of this concept is confirmed by a systematic study on a series of mixed-halide perovskite devices. These results suggest that the proposed concept allows as a promising platform and should inspire further exploration of multispectral responsive optoelectronic devices.

Nanostructured Perovskite Solar Cells

Over the past decade, lead halide perovskites have emerged as one of the leading photovoltaic materials due to their long carrier lifetimes, high absorption coefficients, high tolerance to defects, and facile processing methods. With a bandgap of ~1.6 eV, lead halide perovskite solar cells have achieved power conversion efficiencies in excess of 25%. Despite this, poor material stability along with lead contamination remains a significant barrier to commercialization. Recently, low-dimensional perovskites, where at least one of the structural dimensions is measured on the nanoscale, have demonstrated significantly higher stabilities, and although their power conversion efficiencies are slightly lower, these materials also open up the possibility of quantum-confinement effects such as carrier multiplication. Furthermore, both bulk perovskites and low-dimensional perovskites have been demonstrated to form hybrids with silicon nanocrystals, where numerous device architectures can be exploited to improve efficiency. In this review, we provide an overview of perovskite solar cells, and report the current progress in nanoscale perovskites, such as low-dimensional perovskites, perovskite quantum dots, and perovskite-nanocrystal hybrid solar cells.

Energetic disorder in perovskite/polymer solar cells and its relationship with the interfacial carrier losses

Previous reports have observed a direct relationship between the polymer poly(3-hexylthiophene) molecular weight (MW) and the perovskite solar cell (PSC) efficiency. Herein, we analyse how the differences in MW and the differences in energetic disorder influence the interfacial carrier losses in the PSCs under operation conditions and explain the observed differences. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Investigation on the Overshoot of Transient Open-Circuit Voltage in Methylammonium Lead Iodide Perovskite Solar Cells

Although the performance of hybrid organic-inorganic perovskite solar cells (PSCs) is encouraging, the detailed working principles and mechanisms of PSCs remain to be further studied. In this work, an overshoot phenomenon of open-circuit voltage (V_oc) was observed when the illumination light pulse was switched off. The evolution of the V_oc overshoot was systematically investigated along with the intensity and the width of the light pulse, the background illumination, and pretreatment by different bias. Based on the experimental results, we could conclude that the V_oc overshoot originated from carrier motion against carrier collection direction, which happened at the ionic-accumulation-induced band bending areas near the interfaces between the perovskite active layer and the two carrier transport layers. The investigation on the V_oc overshoot can help us to better understand ionic migration, carrier accumulation, and recombination of PSCs under open-circuit conditions.

The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells

Charge-selective contact layers in perovskite solar cells influence the current density-voltage hysteresis, an effect related to ion migration in the perovskite. As such, fullerene-based electron transport layers (ETLs) suppress hysteresis by reducing the mobile ion concentration. However, the impact of different ETLs on the electronic properties of other constituent device layers remains unclear. In this Kelvin probe force microscopy study, we compared potential distributions of methylammonium lead iodide-based solar cells with two ETLs (planar TiO₂ and C60-functionalized self-assembled monolayer) with different hysteretic behavior. We found significant changes in the potential distribution of the organic hole transport layer spiroMeOTAD, suggesting the formation of a neutral spiroMeOTAD/iodide interface due to a reaction between iodide with p-doped spiroMeOTAD in the TiO₂ cell. Our results show that the ETL affects not only the mobile ion concentration and the recombination at the perovskite/ETL interface but also the resistance and capacitance of the spiroMeOTAD.

Photocarrier dynamics in perovskite-based solar cells revealed by intensity-modulated photovoltage spectroscopy

We studied perovskite photovoltaic devices with intensity-modulated photovoltage spectroscopy. Two coexisting relaxation times are found in accordance with the results of previous impedance spectroscopy (IS) measurements. The slower time constant is independent of the light power while the faster one is inversely proportional to the light power. We employed the surface polarization picture used in the IS analysis augmented by a plausible assumption that the surface polarization is proportional to the light intensity to explain the inverse power dependence of the fast time constant. Because the surface polarization results from the surface accumulated charges, its lateral (parallel to the electrode) distribution and dynamics should be known. We present evidence that the surface accumulated charges indeed form a two-dimensional layer, and have a finite binding energy and a diffusion length.

Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells

Perovskite solar cells are known to show very long response time scales, on the order of milliseconds to seconds. This generates considerable doubt over the validity of the measured external quantum efficiency (EQE) and consequently the estimation of the short-circuit current density. We observe a variation as high as 10% in the values of the EQE of perovskite solar cells for different optical chopper frequencies between 10 and 500 Hz, indicating a need to establish well-defined protocols of EQE measurement. We also corroborate these values and obtain new insights regarding the working mechanisms of perovskite solar cells from intensity-modulated photocurrent spectroscopy measurements, identifying the evolution of the EQE over a range of frequencies, displaying a singular reduction at very low frequencies. This reduction in EQE is ascribed to additional resistive contributions hindering charge extraction in the perovskite solar cell at short-circuit conditions, which are delayed because of the concomitant large low-frequency capacitance.

Interstitial Occupancy by Extrinsic Alkali Cations in Perovskites and Its Impact on Ion Migration

Recent success in achieving highly stable Rb-containing organolead halide perovskites has indicated the possibility of incorporating small monovalent cations, which cannot fit in the lead-halide cage with an appropriate tolerance factor, into the perovskite lattice while maintaining a pure stable "black" phase. In this study, through a combined experimental and theoretical investigation by density functional theory (DFT) calculations on the incorporation of extrinsic alkali cations (Rb⁺ , K⁺ , Na⁺ , and Li⁺ ) in perovskite materials, the size-dependent interstitial occupancy of these cations in the perovskite lattice is unambiguously revealed. Interestingly, DFT calculations predict the increased ion migration barriers in the lattice after the interstitial occupancy. To verify this prediction, ion migration behavior is characterized through hysteresis analysis of solar cells, electrical poling, temperature-dependent conductivity, and time-dependent photoluminescence measurements. The results collectively point to the suppression of ion migration after lattice interstitial occupancy by extrinsic alkali cations. The findings of this study provide new material design principles to manipulate the structural and ionic properties of multication perovskite materials.

Degradation Kinetics of Inverted Perovskite Solar Cells

We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)_0.83Cs_0.17Pb(I_0.8Br_0.2)₃, CH₃NH₃PbI₃, and CH₃NH₃PbI_3-xCl_x perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH₃NH₃PbI_3-xCl_x solar cells due to oxygen exposure occurs over shorter timescales than FA_0.83Cs_0.17Pb(I_0.8Br_0.2)₃ mixed-cation devices. We attribute these oxygen-induced losses in the power conversion efficiencies to the formation of electron traps within the perovskite photoactive layer. Our results highlight that the formamidinium-caesium mixed-cation perovskites are much less sensitive to oxygen-induced degradation than the methylammonium-based perovskite cells, and that further improvements in perovskite solar cell stability should focus on the mitigation of trap generation during ageing.

Effect of Fullerene Passivation on the Charging and Discharging Behavior of Perovskite Solar Cells: Reduction of Bound Charges and Ion Accumulation

Ion accumulation of organometal halide perovskites (OHPs) induced by electrode polarization of perovskite solar cells (PSCs) under illumination has been intensely studied and associated with a widely observed current-voltage hysteresis behavior. This work is dedicated to the investigation of the behavior of charged species at the compact TiO₂/OHP interface with respect to electrode polarization in PSC devices. By providing a comprehensive discussion of open-circuit voltage ( V_OC) buildup and V_OC decay under illumination and in the dark for the PSCs modified with [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) at the TiO₂/OHP interface and their corresponding electrochemical impedance spectroscopies (EISs), a justified mechanism is proposed attempting to elucidate the dynamics of interfacial species with respect to the time and frequency domains. Our results demonstrate that the retarded V_OC buildup and decay observed in PSC devices are related to the formation of bound charges in TiO₂, which is essential to neutralize the oppositely charged ions accumulating at the OHP side. Besides, inserting a thicker PCBM at the TiO₂/OHP interface as a passivation layer can alleviate the electrode polarization more efficiently as verified by the low dielectric constant measured from EIS. Moreover, photoluminescence measurements indicate that PCBM at the TiO₂/OHP interface is capable of passivating a trap state and improving charge transfer. However, with respect to the time scale investigated in this work, the reduction of the hysteresis behavior on a millisecond scale is more likely due to less bound charge formation at the interface rather than shallow trap-state passivation by PCBM. After all, this work comprehensively demonstrates the interfacial properties of PSCs associated with PCBM passivation and helps to further understand its impact on charging/discharging as well as device performance.

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode

Solar cells based on methylammonium lead triiodide (MAPbI3) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long-term stability of MAPbI3-based solar cells has yet to be achieved. Besides the well-known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current-voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI3. In addition to Au, many other metals have been used as electrodes in MAPbI3 solar cells. However, how the external metal impurities introduced by electrodes affect the long-term stability of MAPbI3 solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI3 based on first-principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI3 and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI3 while having low resistivities and suitable work functions for carrier extraction.

Modulation of Charge Recombination in CsPbBr3 Perovskite Films with Electrochemical Bias

The charging of a mesoscopic TiO2 layer in a metal halide perovskite solar cell can influence the overall power conversion efficiency. By employing CsPbBr3 films deposited on a mesoscopic TiO2 film, we have succeeded in probing the influence of electrochemical bias on the charge carrier recombination process. The transient absorption spectroscopy experiments conducted at different applied potentials indicate a decrease in the charge carrier lifetimes of CsPbBr3 as we increase the potential from -0.6 to +0.6 V vs Ag/AgCl. The charge carrier lifetime increased upon reversing the applied bias, thus indicating the reversibility of the photoresponse to charging effects. The ultrafast spectroelectrochemical experiments described here offer a convenient approach to probe the charging effects in perovskite solar cells.

Opto-electronic characterization of third-generation solar cells

We present an overview of opto-electronic characterization techniques for solar cells including light-induced charge extraction by linearly increasing voltage, impedance spectroscopy, transient photovoltage, charge extraction and more. Guidelines for the interpretation of experimental results are derived based on charge drift-diffusion simulations of solar cells with common performance limitations. It is investigated how nonidealities like charge injection barriers, traps and low mobilities among others manifest themselves in each of the studied cell characterization techniques. Moreover, comprehensive parameter extraction for an organic bulk-heterojunction solar cell comprising PCDTBT:PC₇₀BM is demonstrated. The simulations reproduce measured results of 9 different experimental techniques. Parameter correlation is minimized due to the combination of various techniques. Thereby a route to comprehensive and accurate parameter extraction is identified.

Effect of Low Temperature on Charge Transport in Operational Planar and Mesoporous Perovskite Solar Cells

Low-temperature optoelectrical studies of perovskite solar cells using MAPbI₃ and mixed-perovskite absorbers implemented into planar and mesoporous architectures reveal fundamental charge transporting properties in fully assembled devices operating under light bias. Both types of devices exhibit inverse correlation of charge carrier lifetime as a function of temperature, extending carrier lifetimes upon temperature reduction, especially after exposure to high optical biases. Contribution of bimolecular channels to the overall recombination process should not be overlooked because the density of generated charge surpasses trap-filling concentration requirements. Bimolecular charge recombination coefficient in both device types is smaller than Langevin theory prediction, and its mean value is independent of the applied illumination intensity. In planar devices, charge extraction declines upon MAPbI₃ transition from a tetragonal to an orthorhombic phase, indicating a connection between the trapping/detrapping mechanism and temperature. Studies on charge extraction by linearly increasing voltage further support this assertion, as charge carrier mobility dependence on temperature follows multiple-trapping predictions for both device structures. The monotonously increasing trend following the rise in temperature opposes the behavior observed in neat perovskite films and indicates the importance of transporting layers and the effect they have on charge transport in fully assembled solar cells. Low-temperature phase transition shows no pattern of influence on thermally activated electron/hole transport.

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third-generation photovoltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research "Molecular Ultrafast Science and Technology," a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mechanisms of photoinduced charge separation in dye-sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here.

Atomic Layer Deposition of Electron Selective SnOx and ZnO Films on Mixed Halide Perovskite: Compatibility and Performance

The compatibility of atomic layer deposition directly onto the mixed halide perovskite formamidinium lead iodide:methylammonium lead bromide (CH(NH₂)₂, CH₃NH₃)Pb(I,Br)₃ (FAPbI₃:MAPbBr₃) perovskite films is investigated by exposing the perovskite films to the full or partial atomic layer deposition processes for the electron selective layer candidates ZnO and SnO_x. Exposing the samples to the heat, the vacuum, and even the counter reactant of H₂O of the atomic layer deposition processes does not appear to alter the perovskite films in terms of crystallinity, but the choice of metal precursor is found to be critical. The Zn precursor Zn(C₂H₅)₂ either by itself or in combination with H₂O during the ZnO atomic layer deposition (ALD) process is found to enhance the decomposition of the bulk of the perovskite film into PbI₂ without even forming ZnO. In contrast, the Sn precursor Sn(N(CH₃)₂)₄ does not seem to degrade the bulk of the perovskite film, and conformal SnO_x films can successfully be grown on top of it using atomic layer deposition. Using this SnO_x film as the electron selective layer in inverted perovskite solar cells results in a lower power conversion efficiency of 3.4% than the 8.4% for the reference devices using phenyl-C₇₀-butyric acid methyl ester. However, the devices with SnO_x show strong hysteresis and can be pushed to an efficiency of 7.8% after biasing treatments. Still, these cells lacks both open circuit voltage and fill factor compared to the references, especially when thicker SnO_x films are used. Upon further investigation, a possible cause of these losses could be that the perovskite/SnO_x interface is not ideal and more specifically found to be rich in Sn, O, and halides, which is probably a result of the nucleation during the SnO_x growth and which might introduce barriers or alter the band alignment for the transport of charge carriers.

Charge Injection, Carriers Recombination and HOMO Energy Level Relationship in Perovskite Solar Cells

We present a comparative study between a series of well-known semiconductor polymers, used in efficient organic solar cells as hole transport materials (HTM), and the state-of-the art material used as hole transport material in perovskite solar cells: the spiro-OMeTAD. The observed differences in solar cell efficiencies are studied in depth using advanced photoinduced spectroscopic techniques under working illumination conditions. We have observed that there is no correlation between the highest occupied molecular orbital (HOMO) energy levels of the organic semiconductors and the measured open-circuit voltage (V_OC). For instance, spiro-OMeTAD and P3HT have a comparable HOMO level of ~5.2 eV vs vacuum even though a difference in V_OC of around 200 mV is recorded. This difference is in good agreement with the shift observed for the charge vs voltage measurements. Moreover, hole transfer from the perovskite to the HTM, estimated qualitatively from fluorescence quenching and emission lifetime, seems less efficient for the polymeric HTMs. Finally, the recombination currents from all devices were estimated by using the measured charge (calculated using photoinduced differential charging) and the carriers’ lifetime and their value resulted in accordance with the registered short-circuit currents (J_SC) at 1 sun.

Memristive property's effects on the I-V characteristics of perovskite solar cells

The unfavorable I–V characteristics of perovskite solar cells (PSCs), such as the I–V hysteresis phenomena, have been one major obstacle for their future practical application. However, corresponding analysis based on traditional theories have shown non-negligible flaws and failed for satisfactory explanation. To present a novel mechanism, here we utilize for the first time the memristive property of the perovskite material to analyze the I–V characteristics of PSCs. The obtained joint physical model and the deduced equation may help solving the long-existent mysteries of the I–V characteristics of PSCs. On the basis of our analysis and memristor theory, we also propose an original device optimization strategy for PSCs, which may help further increase their performance to the limit.

Fully Solution-Processed n-i-p-Like Perovskite Solar Cells with Planar Junction: How the Charge Extracting Layer Determines the Open-Circuit Voltage

Fully solution-processed direct perovskite solar cells with a planar junction are realized by incorporating a cross-linked [6,6]-phenyl-C61-butyric styryl dendron ester layer as an electron extracting layer. Power conversion efficiencies close to 19% and an open-circuit voltage exceeding 1.1 V with negligible hysteresis are delivered. A perovskite film with superb optoelectronic qualities is grown, which reduces carrier recombination losses and hence increases V _oc .