Shallow defects and variable photoluminescence decay times up to 280 µs in triple-cation perovskites.

Quantifying recombination in halide perovskites is a crucial prerequisite to control and improve the performance of perovskite-based solar cells. While both steady-state and transient photoluminescence are frequently used to assess recombination in perovskite absorbers, quantitative analyses within a consistent model are seldom reported. We use transient photoluminescence measurements with a large dynamic range of more than ten orders of magnitude on triple-cation perovskite films showing long-lived photoluminescence transients featuring continuously changing decay times that range from tens of nanoseconds to hundreds of microseconds. We quantitatively explain both the transient and steady-state photoluminescence with the presence of a high density of shallow defects and consequent high rates of charge carrier trapping, thereby showing that deep defects do not affect the recombination dynamics. The complex carrier kinetics caused by emission and recombination processes via shallow defects imply that the reporting of only single lifetime values, as is routinely done in the literature, is meaningless for such materials. We show that the features indicative for shallow defects seen in the bare films remain dominant in finished devices and are therefore also crucial to understanding the performance of perovskite solar cells.

Understanding the Thickness and Light-Intensity Dependent Performance of Green-Solvent Processed Organic Solar Cells.

For indoor light harvesting, the adjustable band gap of molecular semiconductors is a significant advantage relative to many inorganic photovoltaic technologies. However, several challenges have to be overcome that include processability in nonhalogenated solvents, sufficiently high thicknesses (>250 nm) and high efficiencies at illuminances typically found in indoor environments. Here, we report on the development and application of new methods to quantify and identify performance losses based on thickness- and intensity-dependent current density-voltage measurements. Furthermore, we report on the fabrication of solar cells based on the blend PBDB-T:F-M processed in the nonhalogenated solvent o-xylene. In the low-intensity regime, insufficiently high shunt resistances limit the photovoltaic performance and by analyzing current density voltage-curves for solar cells with various shunt resistances we find that ∼100 kΩ cm2 are required at 200 lux. We provide a unified description of fill factor losses introducing the concept of light-intensity-dependent apparent shunts that originate from incomplete and voltage-dependent charge collection. In experiment and simulation, we show that good fill factors are associated with a photo-shunt inversely scaling with intensity. Intensity regions with photo-shunt resistances close to the dark-shunt resistance are accompanied by severe extraction losses. To better analyze recombination, we perform a careful analysis of the light intensity and thickness dependence of the open-circuit voltage and prove that trap-assisted recombination dominates the recombination losses at low light intensities.

Homogenizing out-of-plane cation composition in perovskite solar cells.

Perovskite solar cells with the formula FA1-xCsxPbI3, where FA is formamidinium, provide an attractive option for integrating high efficiency, durable stability and compatibility with scaled-up fabrication. Despite the incorporation of Cs cations, which could potentially enable a perfect perovskite lattice1,2, the compositional inhomogeneity caused by A-site cation segregation is likely to be detrimental to the photovoltaic performance of the solar cells3,4. Here we visualized the out-of-plane compositional inhomogeneity along the vertical direction across perovskite films and identified the underlying reasons for the inhomogeneity and its potential impact for devices. We devised a strategy using 1-(phenylsulfonyl)pyrrole to homogenize the distribution of cation composition in perovskite films. The resultant p-i-n devices yielded a certified steady-state photon-to-electron conversion efficiency of 25.2% and durable stability.

Quantifying Charge Extraction and Recombination Using the Rise and Decay of the Transient Photovoltage of Perovskite Solar Cells.

The extraction of photogenerated charge carriers and the generation of a photovoltage belong to the fundamental functionalities of any solar cell. These processes happen not instantaneously but rather come with finite time constants, e.g., a time constant related to the rise of the externally measured open circuit voltage following a short light pulse. Herein, a new method to analyze transient photovoltage measurements at different bias light intensities combining rise and decay times of the photovoltage. The approach uses a linearized version of a system of two coupled differential equations that are solved analytically by determining the eigenvalues of a 2 × 2 matrix. By comparison between the eigenvalues and the measured rise and decay times during a transient photovoltage measurement, the rates of carrier recombination and extraction as a function of bias voltage are determined, and establish a simple link between their ratio and the efficiency losses in the perovskite solar cell.

Toward the Integration of a Silicon/Graphite Anode-Based Lithium-Ion Battery in Photovoltaic Charging Battery Systems.

Solar photovoltaic (PV) energy generation is highly dependent on weather conditions and only applicable when the sun is shining during the daytime, leading to a mismatch between demand and supply. Merging PVs with battery storage is the straightforward route to counteract the intermittent nature of solar generation. Capacity (or energy density), overall efficiency, and stability at elevated temperatures are among key battery performance metrics for an integrated PV-battery system. The performance of high-capacity silicon (Si)/graphite (Gr) anode and LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode cells at room temperature, 45, and 60 °C working temperatures for PV modules are explored. The electrochemical performance of both half and full cells are tested using a specially formulated electrolyte, 1 M LiPF6 in ethylene carbonate: diethyl carbonate, with 5 wt % fluoroethylene carbonate, 2 wt % vinylene carbonate, and 1 wt % (2-cyanoethyl)triethoxysilane. To demonstrate solar charging, perovskite solar cells (PSCs) are coupled to the developed batteries, following the evaluation of each device. An overall efficiency of 8.74% under standard PV test conditions is obtained for the PSC charged lithium-ion battery via the direct-current-direct-current converter, showing the promising applicability of silicon/graphite-based anodes in the PV-battery integrated system.

Interpretation of Mott-Schottky plots of photoanodes for water splitting.

A large body of literature reports that both bismuth vanadate and haematite photoanodes are semiconductors with an extremely high doping density between 1018 and 1021 cm-3. Such values are obtained from Mott-Schottky plots by assuming that the measured capacitance is dominated by the capacitance of the depletion layer formed by the doping density within the photoanode. In this work, we show that such an assumption is erroneous in many cases because the injection of electrons from the collecting contact creates a ubiquitous capacitance step that is very difficult to distinguish from that of the depletion layer. Based on this reasoning, we derive an analytical resolution limit that is independent of the assumed active area and surface roughness of the photoanode, below which doping densities cannot be measured in a capacitance measurement. We find that the reported doping densities in the literature lie very close to this value and therefore conclude that there is no credible evidence from capacitance measurements that confirms that bismuth vanadate and haematite photoanodes contain high doping densities.

Tantalum Oxide as an Efficient Alternative Electron Transporting Layer for Perovskite Solar Cells.

Electron transporting layers facilitating electron extraction and suppressing hole recombination at the cathode are crucial components in any thin-film solar cell geometry, including that of metal-halide perovskite solar cells. Amorphous tantalum oxide (Ta2O5) deposited by spin coating was explored as an electron transport material for perovskite solar cells, achieving power conversion efficiency (PCE) up to ~14%. Ultraviolet photoelectron spectroscopy (UPS) measurements revealed that the extraction of photogenerated electrons is facilitated due to proper alignment of bandgap energies. Steady-state photoluminescence spectroscopy (PL) verified efficient charge transport from perovskite absorber film to thin Ta2O5 layer. Our findings suggest that tantalum oxide as an n-type semiconductor with a calculated carrier density of ~7 × 1018/cm3 in amorphous Ta2O5 films, is a potentially competitive candidate for an electron transport material in perovskite solar cells.

Quantifying Efficiency Limitations in All-Inorganic Halide Perovskite Solar Cells.

While halide perovskites have excellent optoelectronic properties, their poor stability is a major obstacle toward commercialization. There is a strong interest to move away from organic A-site cations such as methylammonium and formamidinium toward Cs with the aim of improving thermal stability of the perovskite layers. While the optoelectronic properties and the device performance of Cs-based all-inorganic lead-halide perovskites are very good, they are still trailing behind those of perovskites that use organic cations. Here, the state-of-the-art of all-inorganic perovskites for photovoltaic applications is reviewed by performing detailed meta-analyses of key performance parameters on the cell and material level. Key material properties such as carrier mobilities, external photoluminescence quantum efficiency, and photoluminescence lifetime are discussed and what is known about defect tolerance in all-inorganic is compared relative to hybrid (organic-inorganic) perovskites. Subsequently, a unified approach is adopted for analyzing performance losses in perovskite solar cells based on breaking down the losses into several figures of merit representing recombination losses, resistive losses, and optical losses. Based on this detailed loss analysis, guidelines are eventually developed for future performance improvement of all-inorganic perovskite solar cells.

Comment on "Resolving spatial and energetic distributions of trap states in metal halide perovskite solar cells".

Ni et al (Research Articles, 20 March 2020, p. 1352) report bulk trap densities of 1011 cm-3 and an increase in interfacial trap densities by one to four orders of magnitude from drive-level capacitance profiling of lead halide perovskites. From electrostatic arguments, we show that the results are not trap densities but are a consequence of the geometrical capacitance and charge injection into the perovskite layer.

How To Quantify the Efficiency Potential of Neat Perovskite Films: Perovskite Semiconductors with an Implied Efficiency Exceeding 28.

Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.

Tail state limited photocurrent collection of thick photoactive layers in organic solar cells.

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Fermi-level pinning in methylammonium lead iodide perovskites.

Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for the design of high efficiency devices since the optoelectronic properties can vary on the nanometre scale. Here we show via scanning tunnelling microscopy and spectroscopy that different grains and grain facets exhibit variations in the local density of states. Modeling of the tunneling spectroscopy curves allows us to quantify the density and fluctuations of surface states and estimate the variations in workfunction on the nanometre scale. The simulations corroborate that the high number of surface states leads to Fermi-level pinning of the methylammonium lead iodide surfaces. We do not observe a variation of the local density of states at the grain boundaries compared to the grain interior. These results are in contrast to other reported SPM measurements in literature. Our results show that most of the fluctuations of the electrical properties in these polycrystalline materials arise due to grain to grain variations and not due to distinct electronic properties of the grain boundaries. The measured workfunction changes at the different grains result in local variations of the band alignment with the carrier selective top contact and the varying number of surface states influence the recombination activity in the devices.

Charge-Carrier Recombination in Halide Perovskites.

The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.

High open-circuit voltages in lead-halide perovskite solar cells: experiment, theory and open questions.

One of the most significant features of lead-halide perovskites is their ability to have comparably slow recombination despite the fact that these materials are mostly processed from solution at room temperature. The slow recombination allows achieving high open-circuit voltages when the lead-halide perovskite layers are used in solar cells. This perspective discusses the state of the art of our understanding and of experimental data with regard to recombination and open-circuit voltages in lead-halide perovskites. A special focus is put onto open questions that the community has to tackle to design future photovoltaic and optoelectronic devices based on lead-halide perovskites and other semiconductors with similar properties. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Simulating nanocrystal-based solar cells: A lead sulfide case study.

Nanocrystal-based solar cells are promising candidates for next generation photovoltaic applications; however, the most recent improvements to the device chemistry and architecture have been mostly trial-and-error based advancements. Due to complex interdependencies among parameters, determining factors that limit overall solar cell efficiency are not trivial. Furthermore, many of the underlying chemical and physical parameters of nanocrystal-based solar cells have only recently been understood and quantified. Here, we show that this new understanding of interfaces, transport, and origin of trap states in nanocrystal-based semiconductors can be integrated into simulation tools, based on 1D drift-diffusion models. Using input parameters measured in independent experiments, we find excellent agreement between experimentally measured and simulated PbS nanocrystal solar cell behavior without having to fit any parameters. We then use this simulation to understand the impact of interfaces, charge carrier mobility, and trap-assisted recombination on nanocrystal performance. We find that careful engineering of the interface between the nanocrystals and the current collector is crucial for an optimal open-circuit voltage. We also show that in the regime of trap-state densities found in PbS nanocrystal solar cells (∼1017 cm-3), device performance exhibits strong dependence on the trap state density, explaining the sensitivity of power conversion efficiency to small changes in nanocrystal synthesis and nanocrystal thin-film deposition that has been reported in the literature. Based on these findings, we propose a systematic approach to nanocrystal solar cell optimization. Our method for incorporating parameters into simulations presented and validated here can be adopted to speed up the understanding and development of all types of nanocrystal-based solar cells.

Spin-coated planar Sb2S3 hybrid solar cells approaching 5% efficiency.

Antimony sulfide solar cells have demonstrated an efficiency exceeding 7% when assembled in an extremely thin absorber configuration deposited via chemical bath deposition. More recently, less complex, planar geometries were obtained from simple spin-coating approaches, but the device efficiency still lags behind. We compare two processing routes based on different precursors reported in the literature. By studying the film morphology, sub-bandgap absorption and solar cell performance, improved annealing procedures are found and the crystallization temperature is shown to be critical. In order to determine the optimized processing conditions, the role of the polymeric hole transport material is discussed. The efficiency of our best solar cells exceeds previous reports for each processing route, and our champion device displays one of the highest efficiencies reported for planar antimony sulfide solar cells.

Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination.

Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm-2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells.

What Remains Unexplained about the Properties of Halide Perovskites?

The notion that halide perovskite crystals (ABX3 , where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.