Here, we investigated the separate and combined effects of guanidinium (Gua⁺) and/or thiocyanate (SCN⁻) ions on the opto-electronic properties of mixed Sn–Pb perovskites, which are used as absorber layers in photovoltaics. Therefore, we spin-coated Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ thin films with GuaI, Pb(SCN)₂ or GuaSCN, in the absence or presence of SnF₂. By comparing the (micro)structural and opto-electronic properties of the perovskite films, we elucidated the functions of both ions. We found that SCN⁻ suppresses tin oxidation and doping, reduces crystal defects and improves the carrier transport properties, regardless of SnF₂ addition. We demonstrate that this is due to coordination with Sn²⁺ and scavenging of Sn⁴⁺ in the spin-coating solution, resulting in a pile-up of SnO_x at the film surface. Gua⁺ is incorporated to a limited extent into the 3D cubic perovskite structure. Gua⁺ cannot suppress tin oxidation and doping, making this additive useless without SnF₂. Conversely, when combined with SnF₂, Gua⁺ enhances the carrier transport properties. Combining Gua⁺ and SCN⁻ until a maximum addition of 4 mol% and SnF₂ results in large grains and pinhole-free films with superior charge carrier transport properties, leading to a substantial increase in the pseudo-open circuit voltage of 50 mV. Addition of >4 mol% GuaSCN leads to the formation of Gua-based 2D perovskites, including Gua_xFA_2−xSn_yPb_1−yI₄ and Gua_xFA_3−xSn_yPb_2−yI₇, which do not improve the carrier dynamics. In short, we observe a synergistic effect on addition of Gua⁺ and SCN⁻ ions, which leads to improved structural and opto-electronic properties, which is promising for implementation in solar cells.

Hybrid organic-inorganic perovskites (PVKs) offer exceptional optoelectronic performance, yet reproducible and scalable co-evaporation remains challenging. This study examines the interplay of factors affecting compositional control during three-source PVK deposition. We identify chamber pressure, precursor cross-contamination, and flux instability – especially from organic salts such as formamidinium iodide (FAI) – as major sources of variability. A critical influence is the occurrence of cross-reading, where omnidirectional evaporation of FAI contributes to the reading on the quartz crystal microbalance (QCM) sensors monitoring the inorganic precursors like caesium bromide (CsBr) and lead iodide (PbI₂) even though shielding is present. This effect, strongly dependent on FAI load, deposition rate, and QCM sensor position, erroneously inflates measured fluxes, leading to inaccurate rate control and unintentional compositional drift. Maintaining A-, B- and X-site stoichiometry therefore requires dynamic adjustment of precursor rates, particularly at higher deposition speeds where mean free path limitations come into play. We demonstrate the successful deposition of perovskite layers at a deposition speed of 27.8 nm min⁻¹ as the practical ceiling for the investigated Cs_xFA_1-xPb(I_1-xBr_x)₃ composition within our experimental framework. These findings highlight the delicate balance between deposition speed, precursor stability, and film quality, underscoring the need for improved delivery systems - such as continuous precursor feedthrough, multiple organic sources, alternative vapor transport or flash evaporation methods – to achieve reproducible, fast and large-scale fabrication of high-performance PVK films.

The potential of the vapor-phase deposition of metal halide perovskites (MHPs) for solar cells remains largely untapped, particularly in achieving rapid deposition rates. In this study, we employ in situ photoluminescence (PL) to monitor the growth dynamics of MHPs deposited via pulsed laser deposition (PLD), with rates ranging from 6 to 80 nm/min. Remarkably, the PL intensity evolution remains consistent across both low- and high-deposition rates, indicating that increased deposition rates do not significantly alter the fundamental mechanisms driving MHP formation via PLD. However, microstructural analysis and time-resolved microwave conductivity (TRMC) measurements reveal that increasing deposition rates lead to randomly oriented films on contact layers and reduced charge mobility compared with films grown at lower deposition rates. These findings emphasize the critical role of controlling initial nucleation and the value of in situ PL monitoring in optimizing the vapor-phase deposition of MHPs for enhanced photovoltaic performance at high deposition rates.

Thanks to their direct band-gap, high absorption coefficient, low manufacturing cost, and relative abundance of component materials, perovskite materials are strong candidates for the next generation of photovoltaic devices. However, their complex photochemistry and photophysics are hindering their development. This is due, in part, to the complex charge carrier recombination pathways in these materials, as well as their instability during measurements. Here, a new characterization methodology is detailed that allows the measurement, with high certainty, of the intrinsic parameters of a single perovskite sample, such as the trap state concentration and carrier mobilities. This methodology is based on a combination of time-resolved microwave photoconductivity (TRMC) and time-resolved photoluminescence (TRPL) spectroscopy. Compared to TRPL only, this methodology is faster, does not lead to significant changes in the perovskite properties over time, and increases the certainty of the parameters retrieved. Using this methodology, green solvent systems are studied to replace the traditional harmful solvents usually used when spin–coating perovskites. Although devices made using the greener solvents presented lower efficiencies, TRMC and TRPL measurements highlighted that the perovskites made with these solvents can achieve the same performance compared to the traditional solvent system.

The efficiency of quantum dot (QD) light-emitting diodes is limited by inefficient hole injection into the valence levels of the QDs. Electrochemical doping, where mobile ions form electrical double layers (EDLs) at electrodes, offers a route to removing injection barriers. While QD light-emitting electrochemical cells (QLECs) have shown promise, prior studies relied on additional charge injection layers, complicating the study of charge injection into QDs. In this work, devices with a simple ITO/QD active layer/Al structure were fabricated using highly photoluminescent ligand-exchanged CdSe/CdS/ZnS QDs, poly(ethylene oxide), and lithium trifluoromethanesulfonate as electrolyte. We show that the dense QD films in these QLECs can be electrochemically doped, transport charges, and exhibit electroluminescence. Symmetrical cyclic voltammograms and operando photoluminescence measurements prove that these devices function as electrochemically doped LECs. Spectroelectrochemical experiments on separately n- and p-doped QD films indicate that hole injection remains the primary limitation in QLEC performance. These findings demonstrate that using EDLs to facilitate charge injection in QD light-emitting devices is promising, but significant challenges remain to be solved before electron and hole injections are balanced.

Sequential thermal evaporation is an emerging technique for obtaining perovskite (PVK) photoactive materials for solar cell applications. Advantages include solvent-free processing, accurate stoichiometry control, and scalable processing. Nevertheless, the power conversion efficiency (PCE) of PVK solar cells (PSCs) fabricated by evaporation still lags behind that of solution-processed PSCs. Here, based on multi-cycle sequential thermal evaporation, we systematically investigate the effects of the post-deposition annealing temperature on the PVK properties in terms of surface morphology, opto-electronic properties, and device performance. We find that the average grain size increases to almost 1 μm and charge carrier mobilities exceed 50 cm2 V−1 s−1 when the annealing temperature is increased to 170 °C. We introduce a trace of PbCl2 to the multi-cycle sequential deposition to improve the absorber crystallinity at a lower annealing temperature of 150 °C, as evidenced by the XRD and PL analyses. The resulting PSC in a p–i–n structure yields a PCE of 18.5% with a cell area of 0.09 cm2. With the same deposition parameters, the cell area is scaled up to 0.36 cm2, achieving champion PCEs of 17.06%. This indicates the great potential of this technology for the commercialization of PSCs in the future.

Last year’s mixed Sn–Pb perovskites have been applied as low-bandgap absorbers in efficient solar cells. However, the performance is still limited by tin oxidation, resulting in doping and defects. Here we perform a quantitative analysis on how tin oxidation affects the optoelectronic properties of spin-coated Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃with varying SnF₂additions ranging from 0 to 20 mol %. First, optical spectroscopy is used to determine the fraction of Sn⁴⁺in the spin-coating solution, which varies depending on the purity of the starting SnI₂precursor. By applying steady-state microwave conductance, a large decrease in the dark conductivity from ∼100 to <∼1 S m^–1in the spin-coated films on going from 0 to 2 mol % SnF₂is observed. We conclude that, without SnF₂, ∼12% of the Sn⁴⁺in solution leads to mobile carriers in the form of free holes, p₀, in the perovskite layer. Upon SnF₂addition, p₀decreases to <1 × 10¹⁶cm^–3. We infer that a ∼70 times excess of SnF₂over the initial concentration of Sn⁴⁺in solution is required to scavenge the Sn⁴⁺and obtain layers with reduced doping. Although the reduction of p₀and defects results in increased carrier lifetimes, higher SnF₂additions are also required to decrease the surface defects, leading to even longer lifetimes close to 200 ns. The reduced doping of these perovskite films with SnF₂makes them ideal candidates for efficient solar cells; however, SnF₂also induces compositional heterogeneity and accumulation of SnO_xat the surface.

Lead halide perovskites have attracted significant attention for their wide-ranging applications in optoelectronic devices. A ubiquitous element in these applications is that charging of the perovskite is involved, which can trigger electrochemical degradation reactions. Understanding the underlying factors governing these degradation processes is crucial for improving the stability of perovskite-based devices. For bulk semiconductors, the electrochemical decomposition potentials depend on the stabilization of atoms in the lattice-a parameter linked to the material’s solubility. For perovskite nanocrystals (NCs), electrochemical surface reactions are strongly influenced by the binding equilibrium of passivating ligands. Here, we report a spectro-electrochemical study on CsPbBr₃ NCs and bulk thin films in contact with various electrolytes, aimed at understanding the factors that control cathodic degradation. These measurements reveal that the cathodic decomposition of NCs is primarily determined by the solubility of surface ligands, with diminished cathodic degradation for NCs in high-polarity electrolyte solvents where ligand solubilities are lower. However, the solubility of the surface ligands and bulk lattice of NCs are orthogonal, such that no electrolyte could be identified where both the surface and bulk are stabilized against cathodic decomposition. This poses inherent challenges for electrochemical applications: (i) The electrochemical stability window of CsPbBr₃ NCs is constrained by the reduction potential of dissolved Pb²⁺ complexes, and (ii) cathodic decomposition occurs well before the conduction band can be populated with electrons. Our findings provide insights to enhance the electrochemical stability of perovskite thin films and NCs, emphasizing the importance of a combined selection of surface passivation and electrolyte.

To increase the open-circuit voltage in solar cells based on triple cation, mixed halide perovskites, reducing recombination processes at the interfaces with transport layers (TLs) is key. Here, we investigated the charge carrier dynamics in bilayers and trilayers of Cs_0.05MA_0.10FA_0.85Pb(I_0.97Br_0.03)₃ (CsMAFA) combined with TLs using time-resolved microwave conductance (TRMC) measurements without and with bias illumination (BI). In the bilayers, we find balanced mobilities for electrons and holes in CsMAFA and nearly quantitative carrier extraction. The small, rapidly decaying TRMC signals for n-i-p- and p-i-n triple layers indicate both carriers are extracted. Applying BI leads to the charging of the TLs and the corresponding electric field prevents additional charge extraction, which demonstrates long-lived charge separation over the CsMAFA/TLs. Most importantly, for all bilayer combinations showing long-lived charge separation, an increase of the quasi-Fermi level splitting with respect to that of the CsMAFA layer is found.

The presence of defects at the interface between the perovskite film and the carrier transport layer poses significant challenges to the performance and stability of perovskite solar cells (PSCs). Addressing this issue, we introduce a dual host-guest (DHG) complexation strategy to modulate both the bulk and interfacial properties of FAPbI₃-rich PSCs. Through NMR spectroscopy, a synergistic effect of the dual treatment is observed. Additionally, electro-optical characterizations demonstrate that the DHG strategy not only passivates defects but also enhances carrier extraction and transport. Remarkably, employing the DHG strategy yields PSCs with power conversion efficiencies (PCE) of 25.89% (certified at 25.53%). Furthermore, these DHG-modified PSCs exhibit enhanced operational stability, retaining over 96.6% of their initial PCE of 25.55% after 1050 hours of continuous operation under one-sun illumination, which was the highest initial value in the recently reported articles. This work establishes a promising pathway for stabilizing high-efficiency perovskite photovoltaics through supramolecular engineering, marking a significant advancement in the field.

To boost the efficiency of perovskite solar cells beyond the limit of a single-junction cell, tandem cells are employed, requiring low bandgap materials. This is realized by partially substituting lead(II) (Pb2+) with tin(II) (Sn2+) in the perovskite structure. In this work, a scalable method is presented to produce formamidinium lead tin iodide (FAPb0.5Sn0.5I3) films by sequential thermal evaporation (sTE) of PbSnI4, which is an alloy of SnI2 and PbI2, and FAI, in vacuum. Annealing at 200 °C yields a highly oriented and crystalline layer comprising grains over 1 µm on average. Photoconductance measurements reveal carrier lifetimes exceeding 2 µs and mobilities ≈100 cm2/(Vs). Structural analysis confirms that, while interdiffusion is abundant even at room temperature, the complete conversion requires high temperatures. Although the incorporation of Cs+ into the A-site of the perovskite increases the grain size, charge carrier dynamics are reduced. A comparison between the sTE films and spin-coated samples of the same composition demonstrates the superior photoconductance of the sTE films, without the need for any additives. Overall, this study showcases the potential of sTE for producing high-quality low band gap (LBG) perovskite materials.

Additives are commonly used to increase the performance of metal-halide perovskite solar cells, but detailed information on the origin of the beneficial outcome is often lacking. Herein, the effect of glycine hydrochloride is investigated when used as an additive during solution processing of narrow-bandgap mixed Pb–Sn perovskites. By combining the characterization of the photovoltaic performance and stability under illumination, with determining the quasi-Fermi level splitting, time-resolved microwave conductivity (TRMC), and morphological and elemental analysis a comprehensive insight is obtained. Glycine hydrochloride is able to retard the oxidation of Sn2+ in the precursor solution, and at low concentrations (1–2 mol%) it improves the grain size distribution and crystallization of the perovskite, causing a smoother and more compact layer, reducing non-radiative recombination, and enhancing the lifetime of photogenerated charges. These improve the photovoltaic performance and have a positive effect on stability. By determining the quasi-Fermi level splitting on perovskite layers without and with charge transport layers it is found that glycine hydrochloride primarily improves the bulk of the perovskite layer and does not contribute significantly to passivation of the interfaces of the perovskite with either the hole or electron transport layer (ETL).

Mixed Sn-Pb halide perovskites are promising absorber materials for solar cells due to the possibility of tuning the bandgap energy down to 1.2-1.3 eV. However, tin-containing perovskites are susceptible to oxidation affecting the optoelectronic properties. In this work, we investigated qualitatively and quantitatively metastable oxygen-induced doping in isolated ASn_xPb_1-xI₃ (where A is methylammonium or a mixture of formamidinium and cesium) perovskite thin films by means of microwave conductivity, structural and optical characterization techniques. We observe that longer oxygen exposure times lead to progressively higher dark conductivities, which slowly decay back to their original levels over days. Here oxygen acts as an electron acceptor, leading to tin oxidation from Sn²⁺ to Sn⁴⁺ and creation of free holes. The metastable oxygen-induced doping is enhanced by exposing the perovskite simultaneously to oxygen and light. Next, we show that doping not only leads to the reduction in the photoconductivity signal but also induces long-term effects even after loss of doping, which is thought to derive from consecutive oxidation reactions leading to the formation of defect states. On prolonged exposure to oxygen and light, optical and structural changes can be observed and related to the formation of SnO_x and loss of iodide near the surface. Our work highlights that even a short-term exposure to oxygen immediately impairs the charge carrier dynamics of the perovskite, while structural perovskite degradation is only noticeable upon long-term exposure and accumulation of oxidation products. Hence, for efficient solar cells, exposure of mixed Sn-Pb perovskites to oxygen during production and operation should be rigorously blocked.

The technique of alloying FA+ with Cs+ is often used to promote structural stabilization of the desirable α-FAPbI3 phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in CsxFA1−xPbI3 perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs+ tuning (Cs0.15FA0.85PbI3) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains’/domains’ carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs0.15FA0.85PbI3 devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

A better understanding of the materials' fundamental physical processes is necessary to push hybrid perovskite photovoltaic devices towards their theoretical limits. The role of the perovskite grain boundaries is essential to optimise the system thoroughly. The influence of the perovskite grain size and crystal orientation on physical properties and their resulting photovoltaic performance is examined. We develop a novel, straightforward synthesis approach that yields crystals of a similar size but allows the tuning of their orientation to either the (200) or (002) facet alignment parallel to the substrate by manipulating dimethyl sulfoxide (DMSO) and tetrahydrothiophene-1-oxide (THTO) ratios. This decouples crystal orientation from grain size, allowing the study of charge carrier mobility, found to be improved with larger grain sizes, highlighting the importance of minimising crystal disorder to achieve efficient devices. However, devices incorporating crystals with the (200) facet exhibit an s-shape in the current density-voltage curve when standard scan rates are used, which typically signals an energetic interfacial barrier. Using the drift-diffusion simulations, we attribute this to slower-moving ions (mobility of 0.37 × 10^-10 cm² V^-1 s^-1) in combination with a lower density of mobile ions. This counterintuitive result highlights that reducing ion migration does not necessarily minimise hysteresis.

State-of-the-art triple cation, mixed halide perovskites are extensively studied in perovskite solar cells, showing very promising performance and stability. However, an in-depth fundamental understanding of how the phase behavior in Cs_0.05FA_0.85MA_0.10Pb(I_0.97Br_0.03)₃ (CsMAFA) affects the optoelectronic properties is still lacking. The refined unit cell parameters a and c in combination with the thermal expansion coefficients derived from X-ray diffraction patterns reveal that CsMAFA undergoes an α–β phase transition at ≈280 K and another transition to the γ-phase at ≈180 K. From the analyses of the electrodeless microwave photoconductivity measurements it is shown that shallow traps only in the γ-phase negatively affect the charge carrier dynamics. Most importantly, CsMAFA exhibits the lowest amount of microstrain in the β-phase at around 240 K, corresponding to the lowest amount of trap density, which translates into the longest charge carrier diffusion length for electrons and holes. Below 200 K a considerable increase in deep trap states is found most likely related to the temperature-induced compressive microstrain leading to a huge imbalance in charge carrier diffusion lengths between electrons and holes. This work provides valuable insight into how temperature-dependent changes in structure affect the charge carrier dynamics in FA-rich perovskites.

Perovskite-based solar cells have been rapidly developed, with record power conversion efficiencies now exceeding 25%. In order to rationally improve the efficiency of these devices, it is important to understand and quantify the dynamics of the excess charge carriers.

At ambient conditions, rare-earth oxyhydride thin films show reversible photochromism and photoconductivity, while their mechanism and relation are still unclear. In this work, this question is explored with in situ time-resolved measurements of both optical and transport properties of Gd-based oxyhydride thin films. It is found that p-type large polaron conduction is the initial mechanism of charge transport; however, upon photo-darkening, a 10⁴-fold increase of conductivity occurs, and n-type carriers become dominant. Further, photochromism and photoconductivity are shown to originate from a single process, as indicated by the fact that the photoconductivity is exponentially proportional to the increase of optical absorption. This exponential relation, notably, cannot stem from any of the optically absorbing species thought responsible for photochromism and, therefore, suggests that their formation is accompanied by a concerted increase of negative charge carriers in the Gd oxyhydride films.

Metal-halide perovskites deposited by wet-chemical deposition have demonstrated great potential for various electronic applications, including solar cells. A remaining question is how light-induced excess charges become distributed over such polycrystalline material. Here, we examine the local conductive properties of MAPbI₃ and CsFAPbI₃ by using scanning microwave microscopy (sMIM) in the dark and light. sMIM is an atomic force microscopy (AFM)-based technique measuring variations of the in-phase and out-of-phase signals due to changes in the tip-sample interaction, yielding MIM-Re and MIM-Im images, respectively. Combining this information leads to a picture for CsFAPbI₃ in which excess charges are distributed evenly over the grains, but due to local defect-rich areas, possibly related to different crystal facets, local perturbations in carrier concentration exist. For solar cells, this distribution in carrier concentration under illumination leads to variation in the local Fermi level splitting, which should be suppressed to reduce the voltage deficit.

Multiple-source thermal evaporation is emerging as an excellent technique to obtain perovskite (PVK) materials for solar cell applications due to its solvent-free processing, accurate control of stoichiometric ratio, and potential for scalability. Nevertheless, the currently reported layer-by-layer deposition approach is afflicted by long processing times caused by the multiple repetitions of thin films, which hinder industrial uptake. On the other hand, the coevaporation entails higher complexity due to the challenges of controlling the sublimation of multiple sources simultaneously. In this work, we propose a simplified approach consisting of a single-cycle deposition (SCD) of three thick precursor layers to obtain high-quality Cs0.15FA0.85PbI2.85Br0.15 (CsFAPbIBr) films. After annealing, the optimized PVK film exhibits comparable properties to the one deposited by multicycle deposition in terms of crystal structure, in-depth uniformity, and optoelectrical properties. Also, the formation and evolution of SCD PVK during annealing are investigated. We found that, in the competitive processes of precursor diffusion and reaction, the presence of cesium bromide can assist precursor mixing driven by the annealing treatment, demonstrating a reaction-limited process in the PVK conversion. With this simplified SCD approach, a PVK film is obtained with expected optical and opto-electronic properties, providing an appealing way for future thermally evaporated PVK device preparation.