Bismuth vanadate (BiVO4) is one of the most efficient light absorbing metal oxides for solar water splitting.

Perovskite solar cells (PSCs) exhibit a series of distinctive features in their optoelectronic response which have a crucial influence on the performance, particularly for long‐time response. Here, a survey of recent advances both in device simulation and optoelectronic and photovoltaic responses is provided, with the aim of comprehensively covering recent advances. Device simulations are included with clarifying discussions about the implications of classical drift–diffusion modeling and the inclusion of ionic charged layers near the outer carrier selective contacts. The outcomes of several transient techniques are summarized, along with the discussion of impedance and capacitive responses upon variation of bias voltage and irradiance level. In relation to the capacitive response, a discussion on the J–V curve hysteresis is also included. Although alternative models and explanations are included in the discussion, the review relies upon a key mechanism able to yield most of the rich experimental responses. Particularly for state‐of‐the‐art solar cells exhibiting efficiencies around or exceeding 20%, outer interfaces play a determining role on the PSC's performance. The ionic and electronic kinetics in the vicinity of the interfaces, coupled to surface recombination and carrier extraction mechanisms, should be carefully explored to progress further in performance enhancement.

Perovskite solar cells (PSCs) usually suffer an anomalous hysteresis in current-voltage measurements that leads to an inaccurate estimation of the device efficiency. Although ion migration, charge trapping/detrapping and accumulation have been proposed as a basis for the hysteresis, the origin of hysteresis has not been apparently unraveled. Herein we reported a tunable hysteresis effect based uniquely on open-circuit voltage variations in printable mesoscopic PSCs with a simplified triple-layer TiO2/ZrO2/Carbon architecture. The electrons are collected by the compact TiO2/mesoporous TiO2 (cTiO2/mp-TiO2) bilayer, and the holes are collected by the carbon layer. By adjusting the spray deposition cycles for the cTiO2 layer, we achieved hysteresis-normal, hysteresis-free, and hysteresis-inverted PSCs. Such unique trends of tunable hysteresis are analysed by considering the polarization of the TiO2/perovskite interface, which can accumulate positive charges reversibly. Successfully tuning the hysteresis effect clarifies the critical importance of the c-TiO2/perovskite interface in controlling the hysteretic trends observed, providing important insights towards the understanding of this rapidly developing photovoltaic technology.

The dynamic hysteresis of perovskite solar cells consists of the occurrence of significant deviations of the current density-voltage curve shapes depending on the specific conditions of measurement such as starting voltage, waiting time, scan rate, and other factors. Dynamic hysteresis is a serious impediment to stabilized and reliable measurement and operation of the perovskite solar cells. In this Letter, we formulate a model for the dynamic hysteresis based on the idea that the cell accumulates a huge quantity of surface electronic charge at forward bias that is released on voltage sweeping, causing extra current over the normal response. The charge shows a retarded dynamics due to the slow relaxation of the accompanying ionic charge, that produces variable shapes depending on scan rate or poling value and time. We show that the quantitative model provides a consistent description of experimental results and allows us to determine significant parameters of the perovskite solar cell for both the transient and steady-state performance.

Perovskite materials are becoming a major player for the future energy scenario. In only a few years, they have demonstrated extraordinary capabilities for optoelectronic applications, promising the highest efficiency at the lowest cost. However, despite the numerous studies reported in the literature, the photophysical behavior and device physics for this new technology remain unclear. Here we reveal fundamental insights into the operation mechanism of the state-of-the-art perovskite solar cells, shedding light on the origins of the opencircuit potential and the hysteretic behavior. HIGHLIGHTS The interplay between the chargetransporting layers and opencircuit potentialThe nature of the contacts: a critical factor for the interfacial charge accumulationThe quasi-Fermi level splitting and recombination inside the perovskite rules the V OC The electric field is not the dominant extraction mechanism for the photocarriers Ravishankar et al., SUMMARYPerovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential (V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

The analysis of the impedance spectroscopy (IS) data of perovskite solar cells (PSCs) has been challenging so far, with the low-frequency phenomena in particular yielding ambivalent results and interpretations. We tackle this problem by carrying out intensity-modulated photocurrent spectroscopy (IMPS) measurements at open-circuit (OC) conditions on CH3NH3PbBr3 cells prepared by the flash infrared annealing method with different electron-selective contacts. We identify the existence of a capacitance of the order 10–4 F·cm–2 that is not discernible from IS measurements, which is attributed to the accumulation of anions at the perovskite/spiro-OMeTAD interface, which also likely includes an electronic component. This interface is a dominant recombination pathway at lower voltages and can account for the large disparity in fill factors observed in PSCs. By developing detailed models for the IMPS response at both OC and short-circuit conditions, we also confirm that the arcs observed in the upper quadrant of the IMPS spectra are not related to transport times, as is commonly interpreted, but time constants results from the combination of the series resistance and capacitors within the circuit. By combining the insights from IMPS and IS measurements, we develop a more complete equivalent circuit for PSCs that can be used as a basis for further research with different perovskite materials and contact layers.

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.

Frequency domain techniques are useful tools to characterize processes occurring on different time scales in solar cells and solar fuel devices. Intensity-modulated photocurrent spectroscopy (IMPS) is one such technique that links the electrical and optical responses of the device. In this review, a summary of the fundamental application of IMPS to semiconductor photoelectrodes and nanostructured solar cells is presented, with a final goal of understanding the IMPS response of the perovskite solar cell (PSC) to shed light on its complex physical mechanisms of operation. The historical application of IMPS that connects its transfer function to the charge transfer efficiency of the semiconductor electrode and subsequently the considerations of diffusive transport for the dye-sensitized solar cell is summarized. These models prioritize the association of spectral features with time constants, which has led to a neglect of other absolute aspects of the spectra by the photovoltaic community. We clarify these aspects by developing the fundamental connection between the absolute value of the IMPS transfer function and the external quantum efficiency (EQEPV) of a photovoltaic cell. Basic models for the solar cell are developed using kinetic equations and equivalent circuits (EC), stressing their equivalence and the advantage of the EC representation to adequately account for different capacitances in the system. A critique of the current interpretations of the PSC IMPS spectra is performed, where time constants and their evolution are associated with characteristic transport processes of either electronic or ionic carriers within the PSC. These are clarified using the EC representation to identify that the generated characteristic processes are only related to coupling between different elements of the EC and are not reflective of transport phenomena in general. Furthermore, a general model is developed that identifies charge accumulation at the interfaces as a general feature for both low- and high-efficiency PSCs, whose charging/discharging resistances are the main factor in controlling the electrical response of the device. This model shows a separation of the photovoltage within the PSC that causes a reduction in its EQEPV at low frequencies. Further development of the PSC will involve gaining control over the low-frequency charge kinetics in the device to overcome these limitations.