Search

Abstract The electronic properties of metal halide perovskites (MHPs) are crucial for achieving the full potential of MHPs‐based optoelectronic devices, and they are extensively studied by photoelectron spectroscopy (PES). However, there are numerous complexities during PES measurements, which can result in inconsistent experimental data or even misinterpretation of results. Here, it is demonstrated that in the presence of charge selective junctions with MHPs, significant electronic energy level realignment can occur during PES measurements due to the sample excitation with high‐energy photons used in PES, amounting up to over 0.8 eV in the present case. This is caused by unbalanced charge carrier accumulation within the perovskite due to the charge‐selective interface. X‐ray photoelectron spectroscopy further reveals that photoexcitation due to bremsstrahlung, as produced in commonly employed twin‐anode lab‐sources, can readily produce sizable photoinduced shifts of core levels of MHPs films, whereas monochromatized X‐ray lab‐sources (irradiation flux reduced by >50 times by eliminating the bremsstrahlung) induce negligible shifts within the range of presently applied anode powers. The data and measurement conditions presented here are intended to enable others to obtain reliable MHP electronic property information from PES measurements.

A mismatch between quasi-Fermi level splitting and open-circuit voltage is detrimental to wide bandgap perovskite pin solar cells. Here, through theoretical and experimental approaches, the authors optimize n- and p-type interfaces to achieve open-circuit voltage of 1.29 V and T80 of 3500 h at 85 °C.

Effective transport layers are essential to suppress non-radiative recombination losses. Here, the authors introduce phenylamino-functionalized ortho-carborane as an interfacial layer, and realise inverted perovskite solar cells with efficiency of over 23% and operational stability of T97 = 400 h.

Mitigating the migration of mobile ions within perovskite solar cells is a crucial step on the way to improving their stability. In the past, transient capacitance measurements were applied to extract information about mobile ions, including their activation energy, diffusion coefficient, density, and polarity. However, in this work, we show that the interpretation of capacitance transients is more complex than originally proposed because of the intrinsic nature of the perovskite and the contributions of charge transport layers to the capacitance. Using drift-diffusion simulations and light-intensity-dependent capacitance transient measurements, we show that the direction of capacitance transients is not linked to the polarity of the migrating species. Instead, the direction of the transients is linked to the layer of the cell that dominates capacitance modulation. This work illustrates that transport layers can be crucial for the capacitance and impedance response of perovskite solar cells, and therefore, for characterizing mobile ions in perovskites.

The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in n-i-p configuration using vacuum deposited C60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from ~1.23 eV for the bare absorber, just ~90 meV below the radiative limit, to ~1.10 eV when C60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of ~30–40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure.

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

Long diffusion lengths of photoexcited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Time-resolved photoconductance measurements are often used to determine diffusion lengths in conventional semiconductors. However, effects such as polaron formation or multiple trapping can lead to time-varying mobilities and lifetimes that are not accounted for in the conventional calculation of the diffusion length. Here, a generalized analysis is presented that is valid for time-dependent mobilities and time-dependent lifetimes. The diffusion length is determined directly from the integral of a photoconductivity transient and can be applied regardless of the nature of carrier relaxation. To demonstrate our approach, photoconductivity transients are measured from 100 fs to 1 µs by the combination of time-resolved terahertz and microwave spectroscopy for BiVO_{4}, one of the most studied metal oxide photoanodes for photoelectrochemical water splitting. The temporal evolution of charge carrier displacement is monitored and converges after about 100 ns to a diffusion length of about 15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a-Si:H, c-Si, and halide perovskite, which underlines its potential to determine the diffusion length in a wide range of semiconductors, including disordered materials.

In organic solar cells, the photogeneration of free charge carriers is limited by the dissociation of interfacial charge transfer states. Here, the authors study the impact of charge carrier mobilities in operational devices and show that the slowest charge carriers limit the dissociation of charge transfer states.