Lead halide perovskites have emerged as a highly promising class of materials for solar cells. Nevertheless, suitable procedures to analyze the exact composition of the perovskite films are rare and are not generally applicable. Here we show a simple and fast method to determine the absolute composition of mixed metal halide perovskites through a combination of nuclear magnetic resonance and total reflection X-ray fluorescence spectroscopy. The method is validated on powder mixtures and applied to thin film samples prepared by coevaporation of formamidinium iodide (FAI), CsI, PbI2 and PbBr2, allowing correlation between film composition and solar cell performance. We show that the rates of the individual precursors do not always translate linearly into film composition and that the preparation of the hole transport layer MeO-2PACz strongly influences the incorporation of FAI. Our work highlights the importance of compositional analysis and demonstrates how the developed method provides new insights into the complex coevaporation process in perovskite solar cells.

The fullerene C60 is the prevalent electron transport material (ETM) in high-efficiency perovskite/silicon tandem solar cells. However, it introduces intrinsic limitations, including high interfacial non-radiative recombination losses, the formation of mechanically weak interfaces, and high parasitic absorption. Here, we report a non-fullerene ETM based on a meta-carborane core and 9-fluorenylidene malononitrile functional groups (mCB-FMN) that addresses these challenges while maintaining the processing advantages of C60. Thermally evaporated mCB-FMN forms uniform, conformal thin films that enable efficient electron extraction and strongly suppress interfacial non-radiative recombination losses compared to C60. The introduction of this novel ETM further reduces oxygen-induced degradation of the perovskite/ETM interface, improves the nucleation of the SnOx buffer layer grown by atomic layer deposition and enhances interfacial adhesion within the perovskite/ETM/SnOx stack. Its wide optical bandgap is another key advantage, as it minimizes parasitic absorption losses in tandem solar cells. Replacing C60 with mCB-FMN in opaque p-i-n perovskite single-junction devices improves the power conversion efficiency (PCE) by 1.5% (absolute), driven by a 110 meV increase in open-circuit voltage (VOC). Proof-of-concept perovskite/silicon tandem integration of mCB-FMN yields a PCE of 31.3%, surpassing the C60-based reference by 2.4% (absolute) through simultaneous improvements in VOC and short-circuit current density. These results establish mCB-FMN as a novel non-fullerene ETM for perovskite/silicon tandem solar cells that overcomes the performance limitations of conventional C60 and highlight carborane-based compounds as a promising new class of materials for high-efficiency perovskite photovoltaics.

Self-assembling molecules (SAMs) are investigated as an alternative hole-transport layer to PEDOT:PSS for tin-lead (Sn-Pb) narrow-band gap (NBG) perovskite solar cells. Here we report on an unintended effect of the additive lead(II)-thiocyanate (Pb(SCN)(2)) in SAM-based NBG perovskite solar cells. Upon an increase in the amount of Pb(SCN)(2), SAM-based layer stacks exhibit better film properties. Photoconversion efficiency reaches 18.5% for a concentration of 1% Pb(SCN)(2) and declines drastically for higher concentrations. Time-resolved photoluminescence and surface photovoltage measurements show that carrier extraction is impeded in Pb(SCN)(2)-containing layer stacks with >1% Pb(SCN)(2), and photoelectron spectroscopy reveals the unintended appearance of SAMs on the top surface of the perovskite layer. Relocated SAMs can be removed by solvent washing of the perovskite surface, recovering electron extraction but not the solar cell performance. This study highlights that commonly used additives must be adapted accordingly when introducing SAMs to NBG solar cells.

Despite rapid advances in all-perovskite multi-junction devices, the prevalent hole-transport layer (HTL), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), constrains stability and efficiency in narrow-band-gap tin-lead (Sn-Pb) bottom cells due to acidity and parasitic absorption. Although self-assembled monolayers (SAMs) are well established as HTLs in Pb perovskites and therefore considered promising for Sn-Pb perovskites, their implementation is lagging behind. In this study, we uncover the underlying mechanism by which SAMs limit charge extraction and induce non-uniform buried interfaces in Sn-Pb perovskite films. Guided by these insights, we demonstrate that a graphene oxide/SAM bilayer concurrently mitigates electronic and ionic losses, enabling 22.1% single-junction Sn-Pb efficiency with a 0.88 V open-circuit voltage. In all-perovskite triple-junctions, the bilayer reduces parasitic absorption losses in the near-infrared, yielding 27.3% efficiency (stabilized at 27.0%). The bilayer’s chemically benign nature improved device stability, retaining 90% of initial efficiency after 770 h of operation at 25°C.

This study investigates the influence of oxygen concentration and thermal treatment on the optical and electrical properties of tin oxide (SnO x ) thin films deposited via radio frequency (RF) magnetron sputtering. The oxygen content in the sputtering process gas is systematically varied, revealing its critical role in influencing the films’ charge carrier density, mobility, and resistivity. Optimal conductivity (resistivity as low as 3.4 mΩ cm) is achieved at an argon‐oxygen mix gas flow rate of 3.4 sccm combined with quasi‐in‐situ heating, enhancing both charge carrier density and mobility. Optical analysis revealed that transmittance and absorptance depend on oxygen flow. In the long‐wavelength range, absorption scales with the number of free carriers, while in the short‐wavelength range, discrete absorption peaks below the band gap were observed, possibly originating from a secondary SnO‐like phase.

Advancing inverted perovskite solar cells requires effective strategies to mitigate nonradiative recombination at the perovskite/C-60 interface. Here, we report a volatile material that forms a thin, dense interlayer that essentially eliminates the C-60-induced nonradiative interfacial recombination loss despite not directly passivating the perovskite surface. Ultraviolet photoelectron spectroscopy highlights that the molecule forms a positive dipole layer on the surface that aligns the perovskite and C-60 energy levels for electron conduction. Furthermore, the molecule's volatile nature allows the use of a high-concentration solution that enables a high surface coverage (likely >99%) without increasing the thickness. The combination of these two effects yields an effective approach to suppressing interface recombination. The resulting triple cation perovskite solar cells achieved a power conversion efficiency of >25% and the devices maintain >90% of their initial efficiency after 1200 h of operation. Furthermore, the molecule is broadly applicable to various perovskite compositions and bandgaps.

Vapor phase deposition methods are readily able to achieve uniform coverage of large-area substrates and are widely considered promising for industrial-scale perovskite solar cell fabrication. However, as perovskite-silicon tandem solar cells approach commercialization, practical considerations of manufacturing throughput come into play. Here, it is shown that the inherent sublimation characteristics of the organic precursor formamidinium iodide (FAI) make increasing the deposition rate of FA-based co-evaporated perovskites negatively impact replicability and lead to a substantial decrease in power conversion efficiency (PCE). These losses are linked to reduced film homogeneity and the emergence of carbon-rich regions within the perovskite layer. To mitigate these rate-induced effects, two approaches are explored: source layout optimization and material preconditioning. Utilizing dual FAI sources rather than a single FAI source reduces the relative PCE drop from approximate to 23%rel to approximate to 9%rel at a deposition rate of approximate to 18 nm min-1 (14.8% PCE @ maximum power point (MPP)) compared to the baseline rate of 5 nm min-1 (16.2% PCE @MPP). Alternatively, preconditioning a single FAI source reduces the performance losses from approximate to 31%rel to approximate to 26%rel at a deposition rate of approximate to 21 nm min-1. These findings underscore the importance of tailored source strategies to enable high-rate FA-based co-evaporated perovskites without compromising device performance.

The built-in voltage (VBI) is a key parameter for solar cell operation, yet in perovskite solar cells the distribution, magnitude, and origin of the VBI remains poorly understood. In this work, we systematically studied the VBI in pin-type perovskite solar cells based on different hole transport layers (TLs). To this end, we determine the surface photovoltage (SPV) of partial and complete device stacks layer-by-layer by measuring the work function (WF) under dark and light (equivalent AM1.5G) conditions with Kelvin probe (KP) and photoemission spectroscopy (UPS) measurements in 3 different laboratories. We demonstrate that the SPV increases upon the addition of each additional layer until it equals the open-circuit voltage (VOC) of the full device. This suggests that both the electron and hole transport layer (HTL/ETL) enlarge the SPV, by improving the separation of photogenerated carriers. Yet, the contribution of both transport layers to the total SPV of the device is small (in the range of ≈100 to 200 meV) and the largest contribution to the SPV originates from the top metal electrode (≈500 meV). The results suggest that the VBI of pin-type perovskite solar cells is largely a result of the work-function difference of the electrodes. With regard to films (or incomplete cell stacks), our simulations can reproduce the measured SPV, and measured quasi-Fermi level splitting (>VOC) in partial cell stacks without a significant internal field consistent with the experimental data. This work establishes layer-by-layer SPV measurements, which are easily accessible, as a key tool for understanding device performance and internal energetics, similar to layer-by-layer QFLS measurements.

One of the challenges of excitonic materials is the accurate determination of the exciton binding energy and bandgap. The difficulty arises from the overlap of the discrete and continuous excitonic absorption at the band edge. Many researches have modeled the shape of the absorption edge of such materials on the Elliott model and its several modifications such as non-parabolic bands, magnetic potentials and electro-hole-polaron interactions. However, exciton binding energies obtained from measured data often vary strongly depending on the chosen model. Here, we propose an alternative and rather simple approach, which has previously been successful in the determination of the optical bandgap of amorphous, direct and indirect semiconductors, based on the bands-fluctuations (BF) model. In this model, the fluctuations due to disorder, temperature or lattice vibrations give rise to the well known exponential distribution of band tail states (Urbach tails). This analysis results in an analytic equation with 5 parameters only. The binding energies and optical bandgaps of GaAs and the family of tri-halide perovskites ($\textrm{MAPbX}_{3}$), $\textrm{X=Br,I,Cl}$, over a wide range of temperatures, are obtained with this model. The results for the bandgap, linewidth and exciton binding energy are in good agreement with previous reports. Moreover, due to the polar nature of perovskites, the obtained binding energies can be compared with the ones computed with a theoretical model for polar materials via a model proposed by Kane et al. In this model, the exciton is surrounded by a cloud of virtual phonons interacting via the Fr$\ddot{\textrm{o}}$lich interaction. As a consequence, the upper bound for the binding energy of the exciton-polaron system is calculated. Coincidentally, these results are in good agreement with the optical constants obtained with the EBF model.

Identification of charge carrier separation processes in perovskite/silicon tandem solar cells and recombination at buried interfaces of charge selective contacts is crucial for photovoltaic research. Here, intensity- and wavelength- dependent transient surface photovoltage (tr-SPV) is used to investigate slot-die-coated perovskite top layers deposited on n-type Heterojunction Silicon bottom cells. We show that using an appropriate combination of photon energy and/or bottom cell polarity, one can individually probe the buried interfaces of the bottom silicon cell or the perovskite`s buried interfaces of a tandem solar cell: For excitation with higher energy photons, time delays before the onset of a strong SPV signal indicate significant hole minority drift before separation in the silicon bottom cells. Furthermore, symmetric bottom Si heterojunction solar cell stacks can serve to investigate the top perovskite stack including its junction to the bottom cell, unhampered by photovoltages from the silicon substrate. Thus, investigation of the buried interfaces in tandem devices using time-resolved surface photovoltage is found to yield valuable information on charge carrier extraction at buried interfaces and demonstrates its unique potential compared to more conventional approaches that rely on photoluminescence decay kinetics.

C-60 is the prevalent electron-transport layer (ETL) in high-efficiency p-i-n perovskite single-junction and multi-junction solar cells. Here, it is demonstrated that the exposure of the C-60 ETL to ambient O-2 results in significantly increased non-radiative recombination, influencing results from commonly applied characterization techniques such as steady-state and transient photoluminescence (PL), transient surface photovoltage, as well as current density-voltage measurements. Based on PL and He-I UV photoemission spectroscopy measurements and supported by density functional theory calculations and drift-diffusion simulations, it is proposed that O-2 rapidly intercalates into the C-60 ETL, causing the formation of deep trap states and an altered charge carrier balance at the perovskite/C-60 interface. The findings reveal that the effect is reversible but can mislead experimental interpretations if disregarded, emphasizing the importance of O-2 management during device fabrication and characterization. Furthermore, it is demonstrated that this interaction enables simple PL measurements in air to serve as a novel sensing method for evaluating the barrier layer quality of the SnOx buffer layer atop C-60. This study thereby not only highlights a critical deterioration mechanism in perovskite solar cells and provides a deeper understanding of the underlying interaction between the C-60 ETL and O-2 but also offers practical avenues for future selective contact optimizations.

Vapor phase deposition processes hold great potential for industrializing the deposition of perovskite-based absorbers, offering a pathway to commercialization.

Physical vapor deposition (PVD) processes show significant promise for industrial deposition of perovskite-based absorbers, paving the way for commercialization. Thermal vacuum sublimation is a particularly attractive candidate due to its scalability, capability to create conformal coatings, and established role in the industrial production of optoelectronic devices. Economic viability of a deposition process is influenced by the achievable production throughput, a crucial factor for cost-effectiveness. In this study, we address the gap between research focus and industrial requirements by introducing and assessing three strategies to enhance production throughput in an industrial setting: (1) Increase of source temperature to maximize deposition rates. We establish operational guidelines that minimize decomposition risks, ensuring consistent and high-throughput production. (2) Improve effectiveness of individual sources in industrial settings. We evaluate the industrial feasibility of common deposition techniques, such as co-deposition and sequential deposition. Furthermore, we analyze static deposition rate profiles of key perovskite precursors and use this information for linear sublimation source modelling. (3) Differences in horizontal upscaling behavior for co-deposition and sequential deposition. We discuss the impact of strategies (1) and (2), on configuration and arrangement of linear sources. By addressing these technical and economic challenges, our work provides practical guidelines for the transition of sublimation-based perovskite deposition from laboratory research to industrial-scale production.

Understanding the sub-band gap luminescence in Ruddlesden-Popper 2D metal halide hybrid perovskites (2D HaPs) is essential for efficient charge injection and collection in optoelectronic devices. Still, its origins are still under debate with respect to the role of self-trapped excitons or radiative recombination via defect states. In this study, we characterized charge separation, recombination, and transport in single crystals, exfoliated layers, and polycrystalline thin films of butylammonium lead iodide (BA(2)PbI(4)), one of the most prominent 2D HaPs. We combined complementary defect- and exciton-sensitive methods such as photoluminescence (PL) spectroscopy, modulated and time-resolved surface photovoltage (SPV) spectroscopy, constant final state photoelectron yield spectroscopy (CFSYS), and constant light-induced magneto transport (CLIMAT), to demonstrate striking differences between charge separation induced by dissociation of excitons and by excitation of mobile charge carriers from defect states. Our results suggest that the broad sub-band gap emission in BA(2)PbI(4) and other 2D HaPs is caused by radiative recombination via defect states (shallow as well as midgap states) rather than self-trapped excitons. Density functional theory (DFT) results show that common defects can readily occur and produce an energetic profile that agrees well with the experimental results. The DFT results suggest that the formation of iodine interstitials is the initial process leading to degradation, responsible for the emergence of midgap states, and that defect engineering will play a key role in enhancing the optoelectronic properties of 2D HaPs in the future.

An intrinsic hydrogenated amorphous silicon (a‐Si:H(i)) film and a doped silicon film are usually combined in the heterojunction contacts of silicon heterojunction (SHJ) solar cells. In this work, a post‐doping process called catalytic doping (Cat‐doping) on a‐Si:H(i) is performed on the electron selective side of SHJ solar cells, which enables a device architecture that eliminates the additional deposition of the doped silicon layer. Thus, a single phosphorus Cat‐doping layer combines the functions of two other layers by enabling excellent interface passivation and high carrier selectivity. The overall thinner layer on the window side results in higher spectral response at short wavelengths, leading to an improved short‐circuit current density of 40.31 mA cm −2 and an efficiency of 23.65% (certified). The cell efficiency is currently limited by sputter damage from the subsequent transparent conductive oxide fabrication and low carrier activation in the a‐Si:H(i) with Cat‐doping. Numerical device simulations show that the a‐Si:H(i) with Cat‐doping can provide sufficient field effect passivation even at lower active carrier concentrations compared to the as‐deposited doped layer, due to the lower defect density.

Perovskite solar cells (PSCs) are promising for high-efficiency tandem applications, but their long-term stability, particularly due to ion migration, remains a challenge. Despite progress in stabilizing PSCs, they still fall short compared to mature technologies like silicon. This study explores how different piperazinium salt treatments using iodide, chloride, tosylate, and bistriflimide anions affect the energetics, carrier dynamics, and stability of 1.68 eV bandgap PSCs. Chloride-based treatments achieved the highest power conversion efficiency (21.5%) and open-circuit voltage (1.28 V), correlating with stronger band bending and n-type character at the surface. At the same time, they showed reduced long-term stability due to increased ionic losses. Tosylate-treated devices offered the best balance, retaining 96.4% efficiency after 1000 h (ISOS-LC-1I). These findings suggest that targeted surface treatments can enhance both efficiency and stability in PSCs.