Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces

NiOx Passivation in Perovskite Solar Cells: From Surface Reactivity to Device Performance

Nonstoichiometric nickel oxide (NiOx) is one of the very few metal oxides successfully used as hole extraction layer in p-i-n type perovskite solar cells (PSCs). Its favorable optoelectronic properties and facile large-scale preparation methods are potentially relevant for future commercialization of PSCs, though currently low operational stability of PSCs is reported when a NiOx hole extraction layer is used in direct contact with the perovskite absorber. Poorly understood degradation reactions at this interface are seen as cause for the inferior stability, and a variety of interface passivation approaches have been shown to be effective in improving the overall solar cell performance. To gain a better understanding of the processes happening at this interface, we systematically passivated specific defects on NiOx with three different categories of organic/inorganic compounds. The effects on NiOx and the perovskite (MAPbI3) deposited on top were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Here, we find that the perovskite's structural stability and film formation can be significantly affected by the passivation treatment of the NiOx surface. In combination with density functional theory (DFT) calculations, a likely origin of NiOx-perovskite degradation interactions is proposed. The surface passivated NiOx layers were incorporated into MAPbI3-based PSCs, and the influence on device performance and operational stability was investigated by current-voltage (J-V) characterization, impedance spectroscopy (IS), and open circuit voltage decay (OCVD) measurements. Interestingly, we find that a superior structural stability due to interface passivation must not relate to high operational stability. The discrepancy comes from the formation of excess ions at the interface, which negatively impacts all solar cell parameters.

Sub-bandgap Photocurrent Spectra of p-i-n Perovskite Solar Cells with n-Doped Fullerene Electron Transport Layers and Bias Illumination

In p-i-n perovskite solar cells optical excitation of defect states at the interface between the perovskite and fullerene electron transport layer (ETL) creates a photocurrent responsible for a distinct sub-bandgap external quantum efficiency (EQE). The precise nature of these signals and their impact on cell performance are largely unknown. Here, the effect of n-doping the fullerene on the EQE spectra is studied. The n-doped fullerene is either deposited from solution or by coevaporation. The latter method is used to create undoped-doped fullerene bilayers and investigate the effect of the proximity of the doped region on the EQE spectra. The intensity of the sub-bandgap EQE increases when the ETL is n-doped and also when the device is biased with green light. Using these results, the sub-bandgap EQE signal is attributed to originate from electron trap states in the perovskite with an energy below the conduction band that are filled by excitation with low-energy photons. The trapped electrons give rise to photocurrent when they are collected at a nearby electrode. The enhanced sub-bandgap EQE observed when the ETL is n-doped or bias light is applied, is related to a higher probability to extract trapped electrons under these conditions.

Improving FAPbBr3 Perovskite Crystal Quality via Additive Engineering for High Voltage Solar Cell over 1.5 V

Lead bromide-based perovskites are promising materials as the top cells of tandem solar cells and for application in various fields requiring high voltages owing to their wide band gaps and excellent environmental resistances. However, several factors, such as the formation of bulk and surface defects, impede the performances of corresponding devices, thereby limiting the efficiencies of these devices as single-junction devices. To reduce the number of defect sites, urea is added to the formamidinium lead bromide (FAPbBr3) perovskite material to increase its grain size. Nevertheless, urea undesirably reacts with lead(II) bromide (PbBr2) in the perovskite structure, creating unfavorable impurities in the device. To solve this problem, herein, in addition to urea, we introduced formamidinium chloride (FACl) into FAPbBr3. Owing to the synergistic effect of urea and FACl, the FAPbBr3 film quality effectively improved due to suppression of the generation of impurities and stabilization of film crystallinity. Consequently, the FAPbBr3 single-junction solar cell constructed using FACl and urea as additives demonstrated a power conversion efficiency of 9.6% and an open-circuit voltage of 1.516 V with negligible hysteresis. This study provides new insights into the use of additive engineering for overcoming the energy losses caused by defects in perovskite films.

Understanding and Mitigating Atomic Oxygen-Induced Degradation of Perovskite Solar Cells for Near-Earth Space Applications

Combining high efficiency with good radiation tolerance, perovskite solar cells (PSCs) are promising candidates to upend expanding space photovoltaic (PV) technologies. Successful employment in a Near-Earth space environment, however, requires high resistance against atomic oxygen (AtOx). This work unravels AtOx-induced degradation mechanisms of PSCs with and without phenethylammonium iodide (PEAI) based 2D-passivation and investigates the applicability of ultrathin silicon oxide (SiO) encapsulation as AtOx barrier. AtOx exposure for 2 h degraded the average power conversion efficiency (PCE) of devices without barrier encapsulation by 40% and 43% (w/o and with 2D-PEAI-passivation) of their initial PCE. In contrast, devices with a SiO-barrier retained over 97% of initial PCE. To understand why 2D-PEAI passivated devices degrade faster than less efficient non-passivated devices, various opto-electrical and structural characterications are conducted. Together, these allowed to decouple different damage mechanisms. Notably, pseudo-J-V curves reveal unchanged high implied fill factors (pFF) of 86.4% and 86.2% in non-passivated and passivated devices, suggesting that degradation of the perovskite absorber itself is not dominating. Instead, inefficient charge extraction and mobile ions, due to a swiftly degrading PEAI interlayer are the primary causes of AtOx-induced device performance degradation in passivated devices, whereas a large ionic FF loss limits non-passivated devices.

Impact of Dipole Effect on Perovskite Solar Cells

Interface modification and bulk doping are two major strategies to improve the photovoltaic performance of perovskite solar cells (PSCs). Dipolar molecules are highly favored due to their unique dipolarity. This review discusses the basic concepts and characteristics of dipoles. In addition, the role of dipoles in PSCs and the corresponding conventional characterization methods for dipoles are introduced. Then, we systematically summarize the latest progress in achieving efficient and stable PSCs in dipole materials at several key interfaces. Finally, we look forward to the future application directions of dipole molecules in PSCs, aiming at providing deep insight and inspiration for developing efficient and stable PSCs.

Charge Transfer Kinetics in Halide Perovskites: On the Constraints of Time-Resolved Spectroscopy Measurements

Understanding photophysical processes in lead halide perovskites is an important aspect of optimizing the performance of optoelectronic devices. The determination of exact charge carrier extraction rate constants remains elusive, as there is a large and persistent discrepancy in the reported absolute values. In this review, we concentrate on experimental procedures adopted in the literature to obtain kinetic estimates of charge transfer processes and limitations imposed by the spectroscopy technique employed. Time-resolved techniques (e.g., transient absorption-reflection and time-resolved photoluminescence spectroscopy) are commonly employed to probe charge transfer at perovskite/transport layer interfaces. The variation in sample preparation and measurement conditions can produce a wide dispersion of the measured kinetic parameters. The selected time window and the kinetic fitting model employed introduce additional uncertainty. We discuss here evaluation strategies that rely on multiexponential fitting protocols (regular or stretched) and show how the dispersion in the reported values for carrier transfer rate constants can be resolved.

Interface Passivation of a Pyridine-Based Bifunctional Molecule for Inverted Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have recently been demonstrated to be promising renewable harvesters because of their prominent photovoltaic power conversion efficiency (PCE), although their stability and efficiency still have not reached commercial criteria. Trouble-oriented analyses showcase that defect reduction among the grain boundaries and interfaces in the prepared perovskite polycrystalline films is a practical strategy, which has prompted researchers to develop functional molecules for interface passivation. Herein, the pyridine-based bifunctional molecule dimethylpyridine-3,5-dicarboxylate (DPDC) was employed as the interface between the electron-transport layer and perovskite layer, which achieved a champion PCE of 21.37% for an inverted MAPbI3-based PSC, which was greater than 18.64% for the control device. The mechanistic studies indicated that the significantly improved performance was mainly attributed to the remarkably enhanced fill factor with a value greater than 83%, which was primarily due to the nonradiative recombination suppression offered by the passivation effect of DPDC. Moreover, the promoted carrier mobility together with the enlarged crystal size contributed to a higher short-circuit current density. In addition, an increase in the open-circuit voltage was also observed in the DPDC-treated PSC, which benefited from the improved work function for reducing the energy loss during carrier transport. Furthermore, the DPDC-treated PSC showed substantially enhanced stability, with an over 80% retention rate of its initial PCE value over 300 h even at a 60% relative humidity level, which was attributed to the hydrophobic nature of the DPDC molecule and effective defect passivation. This work is expected not only to serve as an effective strategy for using a pyridine-based bifunctional molecule to passivate perovskite interfaces to enhance photovoltaic performance but also to shed light on the interface passivation mechanism.

Unveiling the Potential of Two-Terminal Perovskite/Organic Tandem Solar Cells: Mechanisms, Status, and Challenges

Perovskite/organic tandem solar cells (PO-TSCs) demonstrate exceptional suitability for emerging applications such as building-integrated photovoltaics, wearable devices, and greenhouse farming. By leveraging the distinctive attributes of perovskite and organic materials, which encompass expanded solar spectrum utilization, chemically benign solubility, and soft nature, PO-TSCs position themselves as ideal candidates for high-performance semi-transparent photovoltaics (ST-PVs). Despite these advantages, their development significantly lags behind other perovskite-based counterparts, such as perovskite/perovskite, perovskite/silicon, and perovskite/Cu(In, Ga)Se2. To address existing challenges and unlock the full potential of PO-TSCs, an exploration of the fundamental mechanisms governing tandem photovoltaic devices is embarked. Delving into critical aspects such as charge generation/separation, energy level alignment, and material choices becomes pivotal for optimizing PO-TSC performance. The investigation of monolithic two-terminal PO-TSCs offers insights into achievements and barriers, recognizing the competitive landscape with other TSC counterparts. Further scrutiny of perovskite absorbers and organic absorbers in TSCs reveals strategies aimed at enhancing stability and efficiency. The discussion extends to interconnection layers, elucidating their role in optimizing light transmission and balancing carrier recombination. In conclusion, a compelling outlook on the dynamic landscape of PO-TSCs is presented, highlighting the remarkable efficiency progression and signaling their potential to revolutionize solar energy harvesting technologies.

Cyano-Coordinated Tin Halide Perovskites for Wearable Health Monitoring and Weak Light Imaging

Low-toxicity tin halide perovskites with excellent optoelectronic properties are promising candidates for photodetection. However, tin halide perovskite photodetectors have suffered from high dark current owing to uncontrollable Sn2+ oxidation. Here, 2-cyanoethan-1-aminium iodide (CNI) is introduced in CH(NH2)2SnI3 (FASnI3) perovskite films to inhibit Sn2+ oxidation by the strong coordination interaction between the cyano group (C≡N) and Sn2+. Consequently, FASnI3-CNI films exhibit reduced nonradiative recombination and lower trap density. The self-powered photodetector based on FASnI3-CNI exhibits low dark current (1.04 × 10-9 A cm-2), high detectivity (2.2 × 1013 Jones at 785 nm), fast response speed (2.62 µs), and good stability. Mechanism studies show the increase in the activation energy required for thermal emission and generated carriers, leading to a lower dark current in the FASnI3-CNI photodetector. In addition, flexible photodetectors based on FASnI3-CNI, exhibiting high detectivity and fast response speed, are employed in wearable electronics to monitor the human heart rate under weak light and zero bias conditions. Finally, the FASnI3-CNI perovskite photodetectors are integrated with a 32 × 32 thin-film transistor backplane, capable of ultraweak light (170 nW cm-2) real-time imaging with high contrast, and zero power consumption, demonstrating the great potential for image sensor applications.

Surface heterojunction based on n-type low-dimensional perovskite film for highly efficient perovskite tandem solar cells

Enhancing the quality of junctions is crucial for optimizing carrier extraction and suppressing recombination in semiconductor devices. In recent years, metal halide perovskite has emerged as the most promising next-generation material for optoelectronic devices. However, the construction of high-quality perovskite junctions, as well as characterization and understanding of their carrier polarity and density, remains a challenge. In this study, using combined electrical and spectroscopic characterization techniques, we investigate the doping characteristics of perovskite films by remote molecules, which is corroborated by our theoretical simulations indicating Schottky defects consisting of double ions as effective charge dopants. Through a post-treatment process involving a combination of biammonium and monoammonium molecules, we create a surface layer of n-type low-dimensional perovskite. This surface layer forms a heterojunction with the underlying 3D perovskite film, resulting in a favorable doping profile that enhances carrier extraction. The fabricated device exhibits an outstanding open-circuit voltage (VOC) up to 1.34 V and achieves a certified efficiency of 19.31% for single-junction wide-bandgap (1.77 eV) perovskite solar cells, together with significantly enhanced operational stability, thanks to the improved separation of carriers. Furthermore, we demonstrate the potential of this wide-bandgap device by achieving a certified efficiency of 27.04% and a VOC of 2.12 V in a perovskite/perovskite tandem solar cell configuration.

Structurally Flexible 2D Spacer for Suppressing the Electron-Phonon Coupling Induced Non-Radiative Decay in Perovskite Solar Cells

This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron-phonon coupling of perovskite in perovskite solar cells (PSCs). Via A-site cation engineering, a weaker electron-phonon coupling in perovskite has been achieved by introducing the structurally soft cyclohexane methylamine (CMA+) cation, which could serve as a damper to alleviate the mechanical stress caused by lattice oscillations, compared to the rigid phenethyl methylamine (PEA+) analog. It demonstrates a significantly lower non-radiative recombination rate, even though the two types of bulky cations have similar chemical passivation effects on perovskite, which might be explained by the suppressed carrier capture process and improved lattice geometry relaxation. The resulting PSCs achieve an exceptional power conversion efficiency (PCE) of 25.5% with a record-high open-circuit voltage (VOC) of 1.20 V for narrow bandgap perovskite (FAPbI3). The established correlations between electron-phonon coupling and non-radiative decay provide design and screening criteria for more effective passivators for highly efficient PSCs approaching the Shockley-Queisser limit.

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.

Ammonium Additive Engineering in Antisolvents for Improving Perovskite/Charge-Transport-Layer Interfaces toward Efficient Lead-Tin Alloyed Perovskite Solar Cells

Although significant advancements have been achieved in lead-tin (Pb-Sn) alloyed perovskite solar cells (PSCs), their power conversion efficiency (PCE) remains inferior to that of their Pb-based counterparts, primarily due to higher open-circuit voltage (Voc) losses and lower fill factors (FFs). Herein, we report both perovskite top and bottom interfacial improvements by incorporating a facile fluorophenylethylammonium iodide (p-FPEAI)/ethyl acetate (EA) solution during the film crystal growth. Based on the analysis of perovskite crystallization, film growth, and strain relaxation, the mechanisms behind these interfacial improvements have been well understood. Furthermore, p-FPEAI could reduce the defect density and nonradiative recombination losses, thus attributing to the improved Voc and FF. Finally, the treated device achieved a PCE of 20.14% with a Voc of up to 0.84 V, which is among the highest reported values so far for Pb-Sn alloyed PSCs without additional precursor additives. In addition, the unencapsulated p-FPEAI-treated device maintained its initial efficiency of approximately 92% after being kept in a nitrogen atmosphere for 1 month, in contrast to the control device which retained only 30% of its initial value. Our findings provide a comprehension for understanding the effect of bulky cations as antisolvents on fabricating highly efficient Pb-Sn alloyed perovskite solar cells.

High-Efficiency and Ultra-Stable Cesium-Bismuth-Based Lead-free Perovskite Solar Cells without Modification

Perovskite solar cells (PSCs) have become a new photovoltaic technology with great commercial potential because of their excellent photovoltaic performance. However, the toxicity and poor environmental stability of Pb in Pb-based perovskites limit its large-scale application. Exploring alternatives to Pb is an available approach to develop environmentally friendly PSCs. As an adjacent element of Pb, Bi shows many similar physical and chemical properties; therefore, it is commonly applied for B site substitution in Pb-based PSCs. CsBiSCl2, a new Pb-free perovskite system, was synthesized for the first time as a light absorber. By preparing DMABiS2 as an intermediate, Cs-Bi-based CsBiSCl2 perovskite films with a band gap over 2.012 eV were prepared by introducing CsCl, and the optimal annealing temperature, time, and stoichiometric ratio of the film were explored in this work. The conventional structure of CsBiSCl2 PSCs achieved a power conversion efficiency (PCE) of 10.38%, and the efficiency declined by only 3% after aging in air for 150 days, showing excellent stability, which is one of the most stable devices in inorganic PSCs. This work opens up a new road for the future development of environmentally friendly and commercially stable lead-free PSCs.

Unveiling dominant recombination loss in perovskite solar cells with a XGBoost-based machine learning approach

Remarkable and intelligent perovskite solar cells (PSCs) have attracted substantial attention from researchers and are undergoing rapid advancements in photovoltaic technology. These developments aim to create highly efficient energy devices with fewer dominant recombination losses within the realm of third-generation solar cells. Diverse machine learning (ML) algorithms implemented, addressing dominant losses due to recombination in PSCs, focusing on grain boundaries (GBs), interfaces, and band-to-band recombination. The extreme gradient boosting (XGBoost) classifier effectively predicts the recombination losses. Our model trained with 7-fold cross-validation to ensure generalizability and robustness. Leveraging Optuna and shapley additive explanations (SHAP) for hyperparameter optimization and investigate the influence of features on target variables, achieved 85% accuracy on over 2 million simulated data, respectively. Because of the input parameters (light intensity and open-circuit voltage), the performance evaluation measures for the dominant losses caused by the recombination predicted by proposed model were superior to those of state-of-the-art models.

Rational molecular design of multifunctional self-assembled monolayers for efficient hole selection and buried interface passivation in inverted perovskite solar cells

Self-assembled monolayers (SAMs) have been widely employed as the bottom-contact hole-selective layer (HSL) in inverted perovskite solar cells (PSCs). Besides manipulating the electrical properties, molecularly engineering the SAM provides an opportunity to modulate the perovskite buried interface. Here, we successfully introduced Lewis-basic oxygen and sulfur heteroatoms through rational molecular design of asymmetric SAMs to obtain two novel multifunctional SAMs, CbzBF and CbzBT. Detailed characterization of single-crystal structures and device interfaces shows that enhanced packing, more effective ITO work function adjustment, and buried interface passivation were successfully achieved. Consequently, the champion PSC employing CbzBT showed an excellent power conversion efficiency (PCE) of 24.0% with a high fill factor of 84.41% and improved stability. This work demonstrates the feasibility of introducing defect-passivating heterocyclic groups into SAM molecules to help passivate the interfacial defects in PSCs. The insights gained from this molecular design strategy will accelerate the development of new multifunctional SAM HSLs for efficient PSCs.

Effect of sub-bandgap defects on radiative and non-radiative open-circuit voltage losses in perovskite solar cells

The efficiency of perovskite solar cells is affected by open-circuit voltage losses due to radiative and non-radiative charge recombination. When estimated using sensitive photocurrent measurements that cover the above- and sub-bandgap regions, the radiative open-circuit voltage is often unphysically low. Here we report sensitive photocurrent and electroluminescence spectroscopy to probe radiative recombination at sub-bandgap defects in wide-bandgap mixed-halide lead perovskite solar cells. The radiative ideality factor associated with the optical transitions increases from 1, above and near the bandgap edge, to ~2 at mid-bandgap. Such photon energy-dependent ideality factor corresponds to a many-diode model. The radiative open-circuit voltage limit derived from this many-diode model enables differentiating between radiative and non-radiative voltage losses. The latter are deconvoluted into contributions from the bulk and interfaces via determining the quasi-Fermi level splitting. The experiments show that while sub-bandgap defects do not contribute to radiative voltage loss, they do affect non-radiative voltage losses.

In Situ Dual-Interface Passivation Strategy Enables The Efficiency of Formamidinium Perovskite Solar Cells Over 25

Perovskite solar cells (PSCs) are promising candidates for next-generation photovoltaics owing to their unparalleled power conversion efficiencies (PCEs). Currently, approaches to further improve device efficiencies tend to focus on the passivation of interfacial defects. Although various strategies have been developed to mitigate these defects, many involve complex and time-consuming post-treatment processes, thereby hindering their widespread adoption in commercial applications. In this work, a concise but efficient in situ dual-interface passivation strategy is developed wherein 1-butyl-3-methylimidazolium methanesulfonate (MS) is employed as a precursor additive. During perovskite crystallization, MS can either be enriched downward through precipitation with SnO2 , or can be aggregated upward through lattice extrusion. These self-assembled MS species play a significant role in passivating the defect interfaces, thereby reducing nonradiative recombination losses, and promoting more efficient charge extraction. As a result, a PCE >25% (certified PCE of 24.84%) is achieved with substantially improved long-term storage and photothermal stabilities. This strategy provides valuable insights into interfacial passivation and holds promise for the industrialization of PSCs.

Unraveling radiation damage and healing mechanisms in halide perovskites using energy-tuned dual irradiation dosing

Perovskite photovoltaics have been shown to recover, or heal, after radiation damage. Here, we deconvolve the effects of radiation based on different energy loss mechanisms from incident protons which induce defects or can promote efficiency recovery. We design a dual dose experiment first exposing devices to low-energy protons efficient in creating atomic displacements. Devices are then irradiated with high-energy protons that interact differently. Correlated with modeling, high-energy protons (with increased ionizing energy loss component) effectively anneal the initial radiation damage, and recover the device efficiency, thus directly detailing the different interactions of irradiation. We relate these differences to the energy loss (ionization or non-ionization) using simulation. Dual dose experiments provide insight into understanding the radiation response of perovskite solar cells and highlight that radiation-matter interactions in soft lattice materials are distinct from conventional semiconductors. These results present electronic ionization as a unique handle to remedying defects and trap states in perovskites.

Scalable Solution-Processed Hybrid Electron Transport Layers for Efficient All-Perovskite Tandem Solar Modules

All-perovskite tandem solar cells offer the potential to surpass the Shockley-Queisser (SQ) limit efficiency of single-junction solar cells while maintaining the advantages of low-cost and high-productivity solution processing. However, scalable solution processing of electron transport layer (ETL) in p-i-n structured perovskite solar subcells remains challenging due to the rough perovskite film surface and energy level mismatch between ETL and perovskites. Here, scalable solution processing of hybrid fullerenes (HF) with blade-coating on both wide-bandgap (≈1.80 eV) and narrow-bandgap (≈1.25 eV) perovskite films in all-perovskite tandem solar modules is developed. The HF, comprising a mixture of fullerene (C60 ), phenyl C61 butyric acid methyl ester, and indene-C60 bisadduct, exhibits improved conductivity, superior energy level alignment with both wide- and narrow-bandgap perovskites, and reduced interfacial nonradiative recombination when compared to the conventional thermal-evaporated C60 . With scalable solution-processed HF as the ETLs, the all-perovskite tandem solar modules achieve a champion power conversion efficiency of 23.3% (aperture area = 20.25 cm2 ). This study paves the way to all-solution processing of low-cost and high-efficiency all-perovskite tandem solar modules in the future.

Influence of the carbazole moiety in self-assembling molecules as selective contacts in perovskite solar cells: interfacial charge transfer kinetics and solar-to-energy efficiency effects

The use of self-assembled molecules (SAMs) as hole transport materials (HTMs) in p-i-n perovskite solar cells (iPSCs) has triggered widespread research due to their relatively easy synthetic methods, suitable energy level alignment with the perovskite material and the suppression of chemical defects. Herein, three new SAMs have been designed and synthesised based on a carbazole core moiety and modified functional groups through an efficient synthetic protocol. The SAMs have been used to understand the SAM/perovskite interface interactions and establish the relationship between the SAM molecular structure and the resulting performance of the perovskite-based devices. The best devices show efficiencies ranging from 18.9% to 17.5% under standard illumination conditions, which are very close to that of our benchmark EADR03, which has been recently commercialised. Our work aims to provide knowledge on the structure of the molecules versus device function relationship.

Compact Hole-Selective Self-Assembled Monolayers Enabled by Disassembling Micelles in Solution for Efficient Perovskite Solar Cells

Self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). However, most SAM molecules are amphiphilic in nature and tend to form micelles in the commonly used alcoholic processing solvents. This introduces an extra energetic barrier to disassemble the micelles during the binding of SAM molecules on the substrate surface, limiting the formation of a compact SAM. To alleviate this problem for achieving optimal SAM growth, a co-solvent strategy to disassemble the micelles of carbazole-based SAM molecules in the processing solution is developed. This effectively increases the critical micelle concentration to be above the processing concentration and enhances the reactivity of the phosphonic acid anchoring group to allow densely packed SAMs to be formed on indium tin oxide. Consequently, the PSCs derived from using MeO-2PACz, 2PACz, and CbzNaph SAM HSLs show universally improved performance, with the CbzNaph SAM-derived device achieving a champion efficiency of 24.98% and improved stability.

Surface Engineering of Methylammonium Lead Bromide Perovskite Crystals for Enhanced X-ray Detection

The surface quality of lead halide perovskite crystals can extremely influence their optoelectronic properties and device performance. Here, we report a surface engineering crystallization technique in which we in situ grow a polycrystalline methylammonium lead tribromide (MAPbBr3) film on top of bulk mm-sized single crystals. Such MAPbBr3 crystals with a MAPbBr3 passivating film display intense green emission under UV light. X-ray photoelectron spectroscopy demonstrates that these crystals with emissive surfaces are compositionally different from typical MAPbBr3 crystals that show no emission under UV light. Time-resolved photoluminescence and electrical measurements indicate that the MAPbBr3 film/MAPbBr3 crystals possess less surface defects compared to the bare MAPbBr3 crystals. Therefore, X-ray detectors fabricated using the surface-engineered MAPbBr3 crystals provide an almost 5 times improved sensitivity to X-rays and a more stable baseline drift with respect to the typical MAPbBr3 crystals.

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

Interface passivation for 31.25%-efficient perovskite/silicon tandem solar cells

Silicon solar cells are approaching their theoretical efficiency limit of 29%. This limitation can be exceeded with advanced device architectures, where two or more solar cells are stacked to improve the harvesting of solar energy. In this work, we devise a tandem device with a perovskite layer conformally coated on a silicon bottom cell featuring micrometric pyramids-the industry standard-to improve its photocurrent. Using an additive in the processing sequence, we regulate the perovskite crystallization process and alleviate recombination losses occurring at the perovskite top surface interfacing the electron-selective contact [buckminsterfullerene (C₆₀)]. We demonstrate a device with an active area of 1.17 square centimeters, reaching a certified power conversion efficiency of 31.25%.

Interface engineering for high-performance, triple-halide perovskite-silicon tandem solar cells

Improved stability and efficiency of two-terminal monolithic perovskite-silicon tandem solar cells will require reductions in recombination losses. By combining a triple-halide perovskite (1.68 electron volt bandgap) with a piperazinium iodide interfacial modification, we improved the band alignment, reduced nonradiative recombination losses, and enhanced charge extraction at the electron-selective contact. Solar cells showed open-circuit voltages of up to 1.28 volts in p-i-n single junctions and 2.00 volts in perovskite-silicon tandem solar cells. The tandem cells achieve certified power conversion efficiencies of up to 32.5%.

Enhancing charge extraction in inverted perovskite solar cells contacts via ultrathin graphene:fullerene composite interlayers

Improving the perovskite/electron-transporting layer (ETL) interface is a crucial task to boost the performance of perovskite solar cells (PSCs). This is utterly fundamental in an inverted (p-i-n) configuration using fullerene-based ETLs. Here, we propose a scalable strategy to improve fullerene-based ETLs by incorporating high-quality few-layer graphene flakes (GFs), industrially produced through wet-jet milling exfoliation of graphite, into phenyl-C61-butyric acid methyl ester (PCBM). Our new composite ETL (GF:PCBM) can be processed into an ultrathin (∼10 nm), pinhole-free film atop the perovskite. We find that the presence of GFs in the PCBM matrix reduces defect-mediated recombination, while creating preferential paths for the extraction of electrons towards the current collector. The use of our GF-based composite ETL resulted in a significant enhancement in the open circuit voltage and fill factor of triple cation-based inverted PSCs, boosting the power conversion efficiency from ∼19% up to 20.8% upon the incorporation of GFs into the ETL.

Effects of Chemical Valences of Sulfur on the Performance of CsFAMA Perovskite Solar Cells

The low electrical conductivity and the high surface defect density of the TiO₂ electron transport layer (ETL) limit the quality of the following perovskite (PVK) layers and the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Sulfur was reported as an effective element to passivate the TiO₂ layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO₂/PVK interfaces, CsFAMA PVK layers, and solar cells using TiO₂ ETL layers treated with Na₂S, Na₂S₂O₃, and Na₂SO₄, respectively. Experimental results show that the Na₂S and Na₂S₂O₃ interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO₂/PVK interface, and improve the device efficiency and stability. Meanwhile, the Na₂SO₄ interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO₂/PVK interface and device performance. These results indicate that S^2- can obviously improve the quality of TiO₂ and PVK layers and TiO₂/PVK interfaces, while SO₄^2- has little effects, even negative effects, on PSCs. This work can deepen the understanding of the interaction between sulfur and the PVK layer and may inspire further progress in the surface passivation field.

Photovoltaic Performance of FAPbI3 Perovskite Is Hampered by Intrinsic Quantum Confinement

Formamidinium lead trioiodide (FAPbI₃) is a promising perovskite for single-junction solar cells. However, FAPbI₃ is metastable at room temperature and can cause intrinsic quantum confinement effects apparent through a series of above-bandgap absorption peaks. Here, we explore three common solution-based film-fabrication methods, neat N,N-dimethylformamide (DMF)-dimethyl sulfoxide (DMSO) solvent, DMF-DMSO with methylammonium chloride, and a sequential deposition approach. The latter two offer enhanced nucleation and crystallization control and suppress such quantum confinement effects. We show that elimination of these absorption features yields increased power conversion efficiencies (PCEs) and short-circuit currents, suggesting that quantum confinement hinders charge extraction. A meta-analysis of literature reports, covering 244 articles and 825 photovoltaic devices incorporating FAPbI₃ films corroborates our findings, indicating that PCEs rarely exceed a 20% threshold when such absorption features are present. Accordingly, ensuring the absence of these absorption features should be the first assessment when designing fabrication approaches for high-efficiency FAPbI₃ solar cells.

Precise Control of Crystallization and Phase-Transition with Green Anti-Solvent in Wide-Bandgap Perovskite Solar Cells with Open-Circuit Voltage Exceeding 1.25 V

Wide-bandgap perovskite solar cells (PSCs) have attracted a lot of attention due to their application in tandem solar cells. However, the open-circuit voltage (V_OC ) of wide-bandgap PSCs is dramatically limited by high defect density existing at the interface and bulk of the perovskite film. Here, an anti-solvent optimized adduct to control perovskite crystallization strategy that reduces nonradiative recombination and minimizes V_OC deficit is proposed. Specifically, an organic solvent with similar dipole moment, isopropanol (IPA) is added into ethyl acetate (EA) anti-solvent, which is beneficial to form PbI₂ adducts with better crystalline orientation and direct formation of α-phase perovskite. As a result, EA-IPA (7-1) based 1.67 eV PSCs deliver a power conversion efficiency of 20.06% and a V_OC of 1.255 V, which is one of the remarkable values for wide-bandgap around 1.67 eV. The findings provide an effective strategy for controlling crystallization to reduce defect density in PSCs.

Opportunities and challenges of hole transport materials for high-performance inverted hybrid-perovskite solar cells

Inverted perovskite solar cells (inverted-PSCs) have exhibited advantages of longer stability, less hysteresis, and lower fabrication temperature when compared to their regular counterparts, which are important for industry commercialization. Because of the great efforts that have been conducted in the past several years, the obtained efficiency of inverted-PSCs has almost caught up with that of the regular ones, 25.0% versus 25.7%. In this perspective, the recent studies on the design of high-performance inverted-PSCs based on diverse hole transport materials, as well as device fabrication and characterization are first reviewed. After that, the authors moved on to the interface and additive engineering that were exploited to suppress the nonradiative recombination. Finally, the challenges and possible research pathways for facilitating the industrialization of inverted-PSCs were envisaged.

Unveiling and Modulating the Interfacial Reaction at the Metal-Hole Conductor Heterojunction toward Reliable Perovskite Solar Cells

Interfaces between functional layers in perovskite solar cells (PSCs) are of paramount importance in determining their efficiency and stability, but the interaction and stability of metal-hole conductor (HC) interfaces have received less attention. Here, we discover an intriguing transient behavior in devices which induces a profound efficiency fluctuation from 9 to 20% during the initial performance testing. Air exposure (e.g., oxygen and moisture) can significantly accelerate this nonequilibrium process and simultaneously enhance the device maximal efficiency. Structural analysis reveals that the chemical reaction between Ag and HC occurred during the metal deposition by thermal evaporation, leading to the formation of an insulating barrier layer at their interfaces, which results in a high charge-transport barrier and poor device performance. Accordingly, we propose a metal diffusion-associated barrier evolution mechanism to understand the metal/HC interfaces. To mitigate these detrimental effects, we strategically develop an interlayer strategy by introducing an ultrathin layer of molybdenum oxide (MoO₃) between Ag and HC, which is found to effectively suppress the interfacial reaction, yielding highly reliable PSCs with instant high efficiency. This work provides new insights into understanding the metal-organic interfaces, and the developed interlayer strategy can be generally applicable to engineer other interfaces in realizing efficient and stable contacts.

Stabilization of FAPbI3 with Multifunctional Alkali-Functionalized Polymer

The defects located at the interfaces and grain boundaries (GBs) of perovskite films are detrimental to the photovoltaic performance and stability of perovskite solar cells. Manipulating the perovskite crystallization process and tailoring the interfaces with molecular passivators are the main effective strategies to mitigate performance loss and instability. Herein, a new strategy is reported to manipulate the crystallization process of FAPbI₃ -rich perovskite by incorporating a small amount of alkali-functionalized polymers into the antisolvent solution. The synergic effects of the alkali cations and poly(acrylic acid) anion effectively passivate the defects on the surface and GBs of perovskite films. As a result, the rubidium (Rb)-functionalized poly(acrylic acid) significantly improves the power conversion efficiency of FAPbI₃ perovskite solar cells to approaching 25% and reduces the risk of lead ion (Pb²⁺ ) leakage continuously via the strong interaction between CO bonds and Pb²⁺ . In addition, the unencapsulated device shows enhanced operational stability, retaining 80% of its initial efficiency after 500 h operation at maximum power point under one-sun illumination.

Myth behind Metastable and Stable n-Hexylammonium Bromide-Based Low-Dimensional Perovskites

In perovskite solar cells, passivating the surface or interface that contains a high concentration of defects, specifically deep-level defects, is one of the most important topics to substantially enhance the power conversion efficiency and stability of the devices. Long-chain alkylammonium bromides have been widely and commonly adapted for passivation treatment. However, the mechanism behind is still not well explored as the formation route and the exact structure of these alkylammonium bromide-based low-dimensional perovskites are unclear. Herein, we investigate the physical and chemical properties of an n-hexylammonium bromide (HABr)-based low-dimensional perovskite including both thin films and single crystals. First of all, the HA₂PbBr₄ perovskite film and aged single crystal demonstrate different X-ray diffraction patterns from those of the fresh as-prepared single crystal. We found that the fresh HA₂PbBr₄ single crystal exhibits a metastable phase as its structure changes with aging due to the relaxation of crystal lattice strains, whereas the HA₂PbBr₄ perovskite film is pretty stable as the aged single crystal. Upon reacting with FAPbI₃, HABr can be intercalated into the FAPbI₃ lattice to form a mixed-cation perovskite of HAFAPbI₃Br, which is in a dynamic equilibrium of decomposition and formation. In contrast, the reaction of HABr with excess PbI₂ forms a stable HA₂PbI₂Br₂ perovskite. Based on such findings, we rationally develop a HA₂PbI₂Br₂-passivated FACs-based perovskite by reacting HABr with excess PbI₂, the photovoltaics based on which are more stable and efficient than those passivated by the HAFAPbI₃Br perovskite. Our discovery paves way for a more in-depth study of bromide-containing low-dimensional perovskites and their optoelectronic applications.

Distinguishing Electron Diffusion and Extraction in Methylammonium Lead Iodide

Charge diffusion and extraction are crucial steps in the operation of solar cells. Here we show that time-resolved photoluminescence can be used to study electron diffusion in hybrid perovskite films and subsequent transfer to the adjacent electron extraction layer. As diffusion and transfer to the extraction layer are consecutive processes, they can be hard to distinguish, but by exciting from each side of the sample we can separate them and identify which process limits charge extraction. We find that the introduction of a fullerene monolayer between the methylammonium lead iodide (MAPbI3) and the electron-transporting SnO2 layers greatly increases the electron transfer velocity between them to the extent that electron diffusion limits the rate of electron extraction. Our results suggest that increasing the electron diffusion coefficient in MAPbI3 would further enhance the electron extraction rate, which could result in more efficient n-i-p type solar cells.

Recent Advances in Wide-Bandgap Organic-Inorganic Halide Perovskite Solar Cells and Tandem Application

Perovskite-based tandem solar cells have attracted increasing interest because of its great potential to surpass the Shockley-Queisser limit set for single-junction solar cells. In the tandem architectures, the wide-bandgap (WBG) perovskites act as the front absorber to offer higher open-circuit voltage (VOC) for reduced thermalization losses. Taking advantage of tunable bandgap of the perovskite materials, the WBG perovskites can be easily obtained by substituting halide iodine with bromine, and substituting organic ions FA and MA with Cs. To date, the most concerned issues for the WBG perovskite solar cells (PSCs) are huge VOC deficit and severe photo-induced phase separation. Reducing VOC loss and improving photostability of the WBG PSCs are crucial for further efficiency breakthrough. Recently, scientists have made great efforts to overcome these key issues with tremendous progresses. In this review, we first summarize the recent progress of WBG perovskites from the aspects of compositions, additives, charge transport layers, interfaces and preparation methods. The key factors affecting efficiency and stability are then carefully discussed, which would provide decent guidance to develop highly efficient and stable WBG PSCs for tandem application.

MDACl2-Modified SnO2 Film for Efficient Planar Perovskite Solar Cells

The electron transport layer (ETL) with excellent charge extraction and transport ability is one of the key components of high-performance perovskite solar cells (PSCs). SnO2 has been considered as a more promising ETL for the future commercialization of PSCs due to its excellent photoelectric properties and easy processing. Herein, we propose a facile and effective ETL modification strategy based on the incorporation of methylenediammonium dichloride (MDACl2) into the SnO2 precursor colloidal solution. The effects of MDACl2 incorporation on charge transport, defect passivation, perovskite crystallization, and PSC performance are systematically investigated. First, the surface defects of the SnO2 film are effectively passivated, resulting in the increased conductivity of the SnO2 film, which is conducive to electron extraction and transport. Second, the MDACl2 modification contributes to the formation of high-quality perovskite films with improved crystallinity and reduced defect density. Furthermore, a more suitable energy level alignment is achieved at the ETL/perovskite interface, which facilitates the charge transport due to the lower energy barrier. Consequently, the MDACl2-modified PSCs exhibit a champion efficiency of 22.30% compared with 19.62% of the control device, and the device stability is also significantly improved.

A Low-Cost and Lithium-Free Hole Transport Layer for Efficient and Stable Normal Perovskite Solar Cells

The most widely used material as a hole-transport layer (HTL) for effective normal perovskite solar cells (PSCs) is still 2,2',7,7'-Tetrakis[N, N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD), which requires heavy doping with the hydroscopic Lithium bis(trifluoromethanesulfonyl)imide (Li-ΤFSI). However, the long-term stability and performance of PCSs are frequently hampered by the residual insoluble dopants in the HTL, Li⁺ diffusion throughout the device, dopant by-products, and the hygroscopic nature of Li-TFSI. Due to the high cost of Spiro-OMeTAD, alternative efficient low-cost HTLs, such as octakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine) (X60), have attracted attention. However, they require doping with Li-TFSI, and the devices develop the same Li-TFSI-derived problems. Here, we propose Li-free 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI) as an efficient p-type dopant of X60, resulting in a high-quality HTL with enhanced conductivity and deeper energy levels The optimized X60:EMIM-TFSI-enabled devices exhibit a higher efficiency of 21.85% and improved stability, compared to the Li-TFSI-doped X60 devices. The stability of the optimized EMIM-TFSI-doped PSCs is greatly improved, and after 1200 hr of storage under ambient conditions, the resulting PSCs maintain 85% of the initial PCE. These findings offer a fresh method for doping the cost effective X60 as the HTL with a Li-free alternative dopant for efficient, cheaper, and reliable planar PSCs.

Ba3.5Cu7.55In1.15Se9: A Wide-Bandgap Copper Indium Selenide Reveals Strong Luminescence and Third-Harmonic Generation

A new copper indium selenide, Ba_3.5Cu_7.55In_1.15Se₉, was synthesized by the KBr flux reaction at 800 °C. The compound crystallizes with orthorhombic Pnma, a = 46.1700(12) Å, b = 4.26710(10) Å, c = 19.8125(5) Å, and Z = 8. The structural framework mainly consists of four sites of cubane-type defective M₄Se₃ (M = Cu, Cu/In) units with disordered Cu⁺/In³⁺ ions present at the part corner of each unit. The single crystal emits intense photoluminescence at 657 nm with a relative quantum yield (RQY) 0.2 times that of rhodamine 6G powder. The compound belongs to a direct band gap at 1.91 eV, analyzed by Tauc's plot, and the energy is close to the PL position. The Hall effect measurement on a pressed pellet reveals an n-type conductivity with a carrier concentration of 3.358 × 10¹⁷ cm^-3 and a mobility of 24.331 cm² V^-1 s^-1. Furthermore, the compound produces a strong nonlinear third-harmonic generation (THG), with an χ_S⁽³⁾ value of 1.3 × 10⁵ pm²/V² comparable to 1.6 × 10⁵ pm²/V² for AgGaSe₂ measured at 800 nm.

Efficient and Moisture-Stable Inverted Perovskite Solar Cells via n-Type Small-Molecule-Assisted Surface Treatment

Defect states at the surface and grain boundaries of perovskite films have been known to be major determinants impairing the optoelectrical properties of perovskite films and the stability of perovskite solar cells (PeSCs). Herein, an n-type conjugated small-molecule additive based on fused-unit dithienothiophen[3,2-b]-pyrrolobenzothiadiazole-core (JY16) is developed for efficient and stable PeSCs, where JY16 possesses the same backbone as the widely used Y6 but with long-linear n-hexadecyl side chains rather than branched side chains. Upon introducing JY16 into the perovskite films, the electron-donating functional groups of JY16 passivate defect states in perovskite films and increase the grain size of perovskite films through Lewis acid-base interactions. Compared to Y6, JY16 exhibits superior charge mobility owing to its molecular packing ability and prevents decomposition of perovskite films under moisture conditions owing to their hydrophobic characteristics, improving the charge extraction ability and moisture stability of PeSCs. Consequently, the PeSC with JY16 shows a high power conversion efficiency of 21.35%, which is higher than those of the PeSC with Y6 (20.12%) and without any additive (18.12%), and outstanding moisture stability under 25% relative humidity, without encapsulation. The proposed organic semiconducting additive will prove to be crucial for achieving highly efficient and moisture stable PeSCs.

π-Expanded Carbazoles as Hole-Selective Self-Assembled Monolayers for High-Performance Perovskite Solar Cells

Carbazole-derived self-assembled monolayers (SAMs) are promising hole-selective materials for inverted perovskite solar cells (PSCs). However, they often possess small dipoles which prohibit them from effectively modulating the workfunction of ITO substrate, limiting the PSC photovoltage. Moreover, their properties can be drastically affected by even subtle structural modifications, undermining the final PSC performance. Here, we designed two carbazole-derived SAMs, CbzPh and CbzNaph through asymmetric or helical π-expansion for improved molecular dipole moment and strengthened π-π interaction. The helical π-expanded CbzNaph has the largest dipole, forming densely packed and ordered monolayer, facilitated by the highly ordered assembly observed in its π-scaffold's single crystal. These synergistically modulate the perovskite crystallization atop and tune the ITO workfunction. Consequently, the champion PSC employing CbzNaph showed an excellent 24.1 % efficiency and improved stability.

Cooperative Adsorption of Metal-organic Complexes on CsPbI2 Br Perovskite Surface for Photovoltaic Efficiency Exceeding 17

Inorganic perovskite solar cells have attracted wide attention due to their excellent thermodynamic stability and suitable bandgap as the top absorber materials for tandem solar cells. However, the power conversion efficiencies (PCEs) of the perovskite cells can be considerably limited by the non-radiative energy loss caused by grain boundaries and surfaces. Here, the synergistic functionalization of CsPbI₂ Br perovskites was demonstrated by using a metal-organic complex. Experimental and theoretical studies revealed that the adsorption energy of the passivator could be a good descriptor to evaluate the surface passivation effect. The cooperative adsorption could eliminate the unsaturated surface sites, reduce the surface energy, and thus benefit device performance. The CsPbI₂ Br solar cells passivated by zinc diethyldithiocarbamate showed a champion power conversion efficiency of 17.15 % and retained 94 % of their initial efficiency after working under 1 sun illumination for 720 h in N₂ atmosphere.

Overcoming C60-induced interfacial recombination in inverted perovskite solar cells by electron-transporting carborane

Inverted perovskite solar cells still suffer from significant non-radiative recombination losses at the perovskite surface and across the perovskite/C₆₀ interface, limiting the future development of perovskite-based single- and multi-junction photovoltaics. Therefore, more effective inter- or transport layers are urgently required. To tackle these recombination losses, we introduce ortho-carborane as an interlayer material that has a spherical molecular structure and a three-dimensional aromaticity. Based on a variety of experimental techniques, we show that ortho-carborane decorated with phenylamino groups effectively passivates the perovskite surface and essentially eliminates the non-radiative recombination loss across the perovskite/C₆₀ interface with high thermal stability. We further demonstrate the potential of carborane as an electron transport material, facilitating electron extraction while blocking holes from the interface. The resulting inverted perovskite solar cells deliver a power conversion efficiency of over 23% with a low non-radiative voltage loss of 110 mV, and retain >97% of the initial efficiency after 400 h of maximum power point tracking. Overall, the designed carborane based interlayer simultaneously enables passivation, electron-transport and hole-blocking and paves the way toward more efficient and stable perovskite solar cells.

Metal Halide Perovskite/Electrode Contacts in Charge-Transporting-Layer-Free Devices

Metal halide perovskites have drawn substantial interest in optoelectronic devices in the past decade. Perovskite/electrode contacts are crucial for constructing high-performance charge-transporting-layer-free perovskite devices, such as solar cells, field-effect transistors, artificial synapses, memories, etc. Many studies have evidenced that the perovskite layer can directly contact the electrodes, showing abundant physicochemical, electronic, and photoelectric properties in charge-transporting-layer-free perovskite devices. Meanwhile, for perovskite/metal contacts, some critical interfacial physical and chemical processes are reported, including band bending, interface dipoles, metal halogenation, and perovskite decomposition induced by metal electrodes. Thus, a systematic summary of the role of metal halide perovskite/electrode contacts on device performance is essential. This review summarizes and discusses charge carrier dynamics, electronic band engineering, electrode corrosion, electrochemical metallization and dissolution, perovskite decomposition, and interface engineering in perovskite/electrode contacts-based electronic devices for a comprehensive understanding of the contacts. The physicochemical, electronic, and morphological properties of various perovskite/electrode contacts, as well as relevant engineering techniques, are presented. Finally, the current challenges are analyzed, and appropriate recommendations are put forward. It can be expected that further research will lead to significant breakthroughs in their application and promote reforms and innovations in future solid-state physics and materials science.

Sn-Based Perovskite Halides for Electronic Devices

Because of its less toxicity and electronic structure analogous to that of lead, tin halide perovskite (THP) is currently one of the most favorable candidates as an active layer for optoelectronic and electric devices such as solar cells, photodiodes, and field-effect transistors (FETs). Promising photovoltaics and FETs performances have been recently demonstrated because of their desirable electrical and optical properties. Nevertheless, THP's easy oxidation from Sn²⁺ to Sn⁴⁺ , easy formation of tin vacancy, uncontrollable film morphology and crystallinity, and interface instability severely impede its widespread application. This review paper aims to provide a basic understanding of THP as a semiconductor by highlighting the physical structure, energy band structure, electrical properties, and doping mechanisms. Additionally, the key chemical instability issues of THPs are discussed, which are identified as the potential bottleneck for further device development. Based on the understanding of the THPs properties, the key recent progress of THP-based solar cells and FETs is briefly discussed. To conclude, current challenges and perspective opportunities are highlighted.

Efficient and Stable Perovskite Solar Cells with a High Open-Circuit Voltage Over 1.2 V Achieved by a Dual-Side Passivation Layer

Suppressing nonradiative recombination at the interface between the organometal halide perovskite (PVK) and the charge-transport layer (CTL) is crucial for improving the efficiency and stability of PVK-based solar cells (PSCs). Here, a new bathocuproine (BCP)-based nonconjugated polyelectrolyte (poly-BCP) is synthesized and this is introduced as a "dual-side passivation layer" between the tin oxide (SnO₂ ) CTL and the PVK absorber. Poly-BCP significantly suppresses both bulk and interfacial nonradiative recombination by passivating oxygen-vacancy defects from the SnO₂ side and simultaneously scavenges ionic defects from the other (PVK) side. Therefore, PSCs with poly-BCP exhibits a high power conversion efficiency (PCE) of 24.4% and a high open-circuit voltage of 1.21 V with a reduced voltage loss (PVK bandgap of 1.56 eV). The non-encapsulated PSCs also show excellent long-term stability by retaining 93% of the initial PCE after 700 h under continuous 1-sun irradiation in nitrogen atmosphere conditions.

Organic Solar Cell With Efficiency Over 20% and VOC Exceeding 2.1 V Enabled by Tandem With All-Inorganic Perovskite and Thermal Annealing-Free Process

Organic solar cells (OSCs) based on polymer donor and non-fullerene acceptor achieve power conversion efficiency (PCE) more than 19% but their poor absorption below 550 nm restricts the harvesting of high-energy photons. In contrast, wide bandgap all-inorganic perovskites limit the absorption of low-energy photons and cause serious below bandgap loss. Therefore, a 2-terminal (2T) monolithic perovskite/organic tandem solar cell (TSC) incorporating wide bandgap CsPbI₂ Br is demonstrated as front cell absorber and organic PM6:Y6 blend as rear cell absorber, to extend the absorption of OSCs into high-energy photon region. The perovskite sub-cell, featuring a sol-gel prepared ZnO/SnO₂ bilayer electron transporting layer, renders a high open-circuit voltage (V_OC ). The V_OC is further enhanced by employing thermal annealing (TA)-free process in the fabrication of rear sub-cell, demonstrating a record high V_OC of 2.116 V. The TA-free Ag/PFN-Br interface in organic sub-cell facilitates charge transport and restrains nonradiative recombination. Consequently, a remarkable PCE of 20.6% is achieved in monolithic 2T-TSCs configuration, which is higher than that of both reported single junction and tandem OSCs, demonstrating that tandem with wide bandgap all-inorganic perovskite is a promising strategy to improve the efficiency of OSCs.

Quantifying Charge Carrier Recombination Losses in MAPbI3/C60 and MAPbI3/Spiro-OMeTAD with and without Bias Illumination

To increase the open-circuit voltage in perovskite-based solar cells, recombination processes at the interface with transport layers (TLs) should be identified and reduced. We investigated the charge carrier dynamics in bilayers of methylammonium lead iodide (MAPbI₃) with C60 or Spiro-OMeTAD using time-resolved microwave conductance (TRMC) measurements with and without bias illumination (BI). By modeling the results, we quantified recombination losses in bare MAPbI₃ and extraction into the TLs. Only under BI did we find that the density of deep traps increases in bare MAPbI₃, substantially enhancing trap-mediated losses. This reversible process is prevented in a bilayer with C60 but not with Spiro-OMeTAD. While under BI extraction rates reduce significantly in both bilayers, only in MAPbI₃/Spiro-OMeTAD does interfacial recombination also increases, substantially reducing the quasi Fermi level splitting. This work demonstrates the impact of BI on charge dynamics and shows that adjusting the Fermi level of TLs is imperative to reduce interfacial recombination losses.

Interpretation of the Recombination Lifetime in Halide Perovskite Devices by Correlated Techniques

The recombination lifetime is a central quantity of optoelectronic devices, as it controls properties such as the open-circuit voltage and light emission rates. Recently, the lifetime properties of halide perovskite devices have been measured over a wide range of the photovoltage, using techniques associated with a steady state by small perturbation methods. It has been remarked that observation of the lifetime is affected by different additional properties of the device, such as multiple trapping effects and capacitive charging. We discuss the meaning of delay factors in the observations of recombination lifetime in halide perovskites. We formulate a general equivalent circuit model that is a basis for the interpretation of all the small perturbation techniques. We discuss the connection of the recombination model to the previous reports of impedance spectroscopy of halide perovskites. Finally, we comment on the correlation properties of the different light-modulated techniques.