Spatial Atmospheric Pressure Atomic Layer Deposition of Tin Oxide as an Impermeable Electron Extraction Layer for Perovskite Solar Cells with Enhanced Thermal Stability

Zinc Aluminum Oxide Encapsulation Layers for Perovskite Solar Cells Deposited Using Spatial Atomic Layer Deposition

An atmospheric-pressure spatial atomic layer deposition system is used to rapidly deposit 60 nm zinc-aluminum oxide (Zn-AlOx ) thin-film-encapsulation layers directly on perovskite solar cells at 130 °C without damaging the temperature-sensitive perovskite and organic materials. Varying the Zn/Al ratio has a significant impact on the structural properties of the films and their moisture barrier performance. The Zn-AlOx films have higher refractive indexes, lower concentrations of OH─ groups, and lower water-vapor transmission rates (WVTR) than AlOx films without zinc. However, as the Zn/Al ratio increases beyond 0.21, excess Zn atoms segregate, leading to an increase in the number of available hydroxyl groups on the surface of the deposited film and a slight increase in the WVTR. The stability of the p-i-n formamidinium methylammonium lead iodide solar cells under standard ISOS-D-3 testing conditions (65 °C and 85% relative humidity) is significantly enhanced by the thin encapsulation layers. The layers with a Zn/Al ratio of 0.21 result in a seven-fold increase the time required for the cells to degrade to 80% of their original efficiency.

Atmospheric-pressure atomic layer deposition: recent applications and new emerging applications in high-porosity/3D materials

Atomic layer deposition (ALD) is a widely recognized technique for depositing ultrathin conformal films with excellent thickness control at Ångström or (sub)monolayer level. Atmospheric-pressure ALD is an upcoming ALD process with a potentially lower ownership cost of the reactor. In this review, we provide a comprehensive overview of the recent applications and development of ALD approaches emphasizing those based on operation at atmospheric pressure. Each application determines its own specific reactor design. Spatial ALD (s-ALD) has been recently introduced for the commercial production of large-area 2D displays, the surface passivation and encapsulation of solar cells and organic light-emitting diode (OLED) displays. Atmospheric temporal ALD (t-ALD) has opened up new emerging applications such as high-porosity particle coatings, functionalization of capillary columns for gas chromatography, and membrane modification in water treatment and gas purification. The challenges and opportunities for highly conformal coating on porous substrates by atmospheric ALD have been identified. We discuss in particular the pros and cons of both s-ALD and t-ALD in combination with their reactor designs in relation to the coating of 3D and high-porosity materials.

Atmospheric atomic layer deposition of SnO2 thin films with tin(II) acetylacetonate and water

Due to its unique optical, electrical, and chemical properties, tin dioxide (SnO₂) thin films attract enormous attention as a potential material for gas sensors, catalysis, low-emissivity coatings for smart windows, transparent electrodes for low-cost solar cells, etc. However, the low-cost and high-throughput fabrication of SnO₂ thin films without producing corrosive or toxic by-products remains challenging. One appealing deposition technique, particularly well-adapted to films presenting nanometric thickness is atomic layer deposition (ALD). In this work, several metalorganic tin-based complexes, namely, tin(IV) tert-butoxide, bis[bis(trimethylsilyl)amino] tin(II), dibutyltin diacetate, tin(II) acetylacetonate, tetrakis(dimethylamino) tin(IV), and dibutyltin bis(acetylacetonate), were explored thanks to DFT calculations. Our theoretical calculations suggest that the three last precursors are very appealing for ALD of SnO₂ thin films. The potential use of these precursors for atmospheric-pressure spatial atomic layer deposition (AP-SALD) is also discussed. For the first time, we experimentally demonstrate the AP-SALD growth of SnO₂ thin films using tin(II) acetylacetonate (Sn(acac)₂) and water. We observe that Sn(acac)₂ exhibits efficient ALD activity with a relatively large ALD temperature window (140-200 °C), resulting in a growth rate of 0.85 ± 0.03 Å per cyc. XPS analyses show a single Sn 3d_5/2 characteristic peak for Sn⁴⁺ at 486.8 ± 0.3 eV, indicating that a pure SnO₂ phase is obtained within the ALD temperature window. The as-deposited SnO₂ thin films are in all cases amorphous, and film conductivity increases with the deposition temperature. Hall effect measurements confirm the n-type nature of SnO₂ with a free electron density of about 8 × 10¹⁹ cm^-3, electron mobility up to 11.2 cm² V^-1 s^-1, and resistivity of 7 × 10^-3 Ω cm for samples deposited at 270 °C.

Spatial atmospheric pressure molecular layer deposition of alucone films using dimethylaluminum isopropoxide as the precursor

Trimethylaluminum is the most used aluminum precursor in atomic and molecular layer deposition (ALD/MLD). It provides high growth-per-cycle (GPC), is highly reactive and is relatively low cost. However, in the deposition of hybrid alucone films, TMA tends to infiltrate into the films requiring very long purge steps and thereby limiting the deposition rate (nm s^-1) of the process. From our previous studies, we know that dimethylaluminum isopropoxide (DMAI) could be a potential candidate to substitute TMA in alucone depositions as it does not seem to infiltrate into the films. In this study, we perform a more detailed investigation of MLD of alucone on an atmospheric pressure spatial MLD system using DMAI as the aluminum precursor. The effect of deposition temperature and reactant purge times on the overall GPC has been investigated and a decreasing GPC with increasing deposition temperature and increasing EG purge time has been observed. Furthermore, the DMAI alucone films have been compared for their chemical environment and degradation with the films prepared using TMA and EG, showing striking similarities between the two. The results demonstrate that DMAI can be used as an alternative precursor to TMA for MLD of alucone films and this work can be used as a guide for designing efficient MLD processes in the future.

Perovskite-organic tandem solar cells with indium oxide interconnect

Multijunction solar cells can overcome the fundamental efficiency limits of single-junction devices. The bandgap tunability of metal halide perovskite solar cells renders them attractive for multijunction architectures¹. Combinations with silicon and copper indium gallium selenide (CIGS), as well as all-perovskite tandem cells, have been reported^2-5. Meanwhile, narrow-gap non-fullerene acceptors have unlocked skyrocketing efficiencies for organic solar cells^6,7. Organic and perovskite semiconductors are an attractive combination, sharing similar processing technologies. Currently, perovskite-organic tandems show subpar efficiencies and are limited by the low open-circuit voltage (V_oc) of wide-gap perovskite cells⁸ and losses introduced by the interconnect between the subcells^9,10. Here we demonstrate perovskite-organic tandem cells with an efficiency of 24.0 per cent (certified 23.1 per cent) and a high V_oc of 2.15 volts. Optimized charge extraction layers afford perovskite subcells with an outstanding combination of high V_oc and fill factor. The organic subcells provide a high external quantum efficiency in the near-infrared and, in contrast to paradigmatic concerns about limited photostability of non-fullerene cells¹¹, show an outstanding operational stability if excitons are predominantly generated on the non-fullerene acceptor, which is the case in our tandems. The subcells are connected by an ultrathin (approximately 1.5 nanometres) metal-like indium oxide layer with unprecedented low optical/electrical losses. This work sets a milestone for perovskite-organic tandems, which outperform the best p-i-n perovskite single junctions¹² and are on a par with perovskite-CIGS and all-perovskite multijunctions¹³.

Tuning the band gap and carrier concentration of titania films grown by spatial atomic layer deposition: a precursor comparison

Spatial atomic layer deposition retains the advantages of conventional atomic layer deposition: conformal, pinhole-free films and excellent control over thickness. Additionally, it allows higher deposition rates and is well-adapted to depositing metal oxide nanofilms for photovoltaic cells and other devices. This study compares the morphological, electrical and optical properties of titania thin films deposited by spatial atomic layer deposition from titanium isopropoxide (TTIP) and titanium tetrachloride (TiCl₄) over the temperature range 100-300 °C, using the oxidant H₂O. Amorphous films were deposited at temperatures as low as 100 °C from both precursors: the approach is suitable for applying films to temperature-sensitive devices. An amorphous-to-crystalline transition temperature was observed for both precursors resulting in surface roughening, and agglomerates for TiCl₄. Both precursors formed conformal anatase films at 300 °C, with growth rates of 0.233 and 0.153 nm s^-1 for TiCl₄ and TTIP. A drawback of TiCl₄ use is the HCl by-product, which was blamed for agglomeration in the films. Cl contamination was the likely cause of band gap narrowing and higher defect densities compared to TTIP-grown films. The carrier concentration of the nanofilms was found to increase with deposition temperature. The films were tested in hybrid bilayer solar cells to demonstrate their appropriateness for photovoltaic devices.

Liquid atomic layer deposition as emergent technology for the fabrication of thin films

Atomic layer deposition (ALD) is widely recognized as a unique chemical vapor deposition technique for the fabrication of thin films with high conformality and precise thickness control down to the Ångstrom level, thereby allowing surface and interface nanoengineering. However, several challenges such as the availability of chemical precursors for ALD and the use of vacuum conditions have hampered its widespread adoption and scalability for mass production. In recent years, the liquid phase homolog of ALD, liquid atomic layer deposition (LALD), has emerged as a much simpler and versatile strategy to overcome some of the current constraints of ALD. This perspective describes the different strategies that have been explored to achieve conformality and sub-nanometer thickness control with LALD, as well as the current challenges it faces to become a part of the thin-film community toolbox, in particular its automation and compatibility with different types of substrates. In this regard, the important role of LALD as complementary technology to ALD is emphasized by comparing the different pathways to deposit the same material and the precursors used to do so.

Gentle Materials Need Gentle Fabrication: Encapsulation of Perovskites by Gas-Phase Alumina Deposition

Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites' surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites' degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite-metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Enhancing the Efficiency and Stability of Triple-Cation Perovskite Solar Cells by Eliminating Excess PbI2 from the Perovskite/Hole Transport Layer Interface

Metal halide perovskites are promising contenders for next-generation photovoltaic applications due to their remarkable photovoltaic efficiency and their compatibility with solution-processed fabrication. Among the various strategies to control the crystallinity and the morphology of the perovskite active layer and its interfaces with the transport layers, fabrication of perovskite solar cells from precursor solutions with a slight excess of PbI₂ has become very common. Despite this, the role of such excess PbI₂ is still rather controversial, lacking consensus on its effect on the bulk and interface properties of the perovskite layer. In this work, we investigate the effect of removing the excess PbI₂ from the surface of a triple-cation mixed-halide Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite layer by four different organic salts on their photovoltaic performance and stability. We show that treatments with iodide salts such as methylammonium iodide (MAI) and formamidinium iodide (FAI) can lead to the strongest beneficial effects on solar cell efficiency, charge recombination suppression, and stability while non-iodide salts such as methylammonium bromide (MABr) and methylammonium chloride (MACl) can also provide improvement in terms of charge recombination suppression and stability to a moderate extent in comparison to the untreated sample. Under optimized conditions and continuous solar illumination, the MAI- and FAI-treated devices maintained 81 and 86% of their initial power conversion efficiency (PCEs), respectively, after 100 h of continuous illumination (versus 64% for the untreated solar cell with excess PbI₂). Our study demonstrates that eliminating excess PbI₂ at the perovskite/hole transport layer (HTL) interface by treating the perovskite surface with organic salts is a simple and efficient route to enhance the efficiency, and in particular the stability of perovskite solar cells.

In-situ spatial and temporal electrical characterization of ZnO thin films deposited by atmospheric pressure chemical vapour deposition on flexible polymer substrates

A technique is presented for collecting data on both the spatial and temporal variations in the electrical properties of a film as it is deposited on a flexible substrate. A flexible printed circuit board substrate with parallel electrodes distributed across its surface was designed. Zinc oxide films were then deposited on the flexible substrate at different temperatures via atmospheric pressure chemical vapour deposition (AP-CVD) using a spatial atomic layer deposition system. AP-CVD is a promising high-throughput thin film deposition technique with applications in flexible electronics. Collecting data on the film properties in-situ allows us to directly observe how deposition conditions affect the evolution of those properties in real-time. The spatial uniformity of the growing film was monitored, and the various stages of film nucleation and growth on the polymer substrate were observed. The measured resistance of the films was observed to be very high until a critical amount of material has been deposited, consistent with Volmer-Weber growth. Furthermore, monitoring the film resistance during post-deposition cooling enabled immediate identification of metallic or semiconducting behaviour within the conductive ZnO films. This technique allows for a more complete understanding of metal chalcogen film growth and properties, and the high volume of data generated will be useful for future implementations of machine-learning directed materials science.

16.8% Monolithic all-perovskite triple-junction solar cells via a universal two-step solution process

Perovskite semiconductors hold a unique promise in developing multijunction solar cells with high-efficiency and low-cost. Besides design constraints to reduce optical and electrical losses, integrating several very different perovskite absorber layers in a multijunction cell imposes a great processing challenge. Here, we report a versatile two-step solution process for high-quality 1.73 eV wide-, 1.57 eV mid-, and 1.23 eV narrow-bandgap perovskite films. Based on the development of robust and low-resistivity interconnecting layers, we achieve power conversion efficiencies of above 19% for monolithic all-perovskite tandem solar cells with limited loss of potential energy and fill factor. In a combination of 1.73 eV, 1.57 eV, and 1.23 eV perovskite sub-cells, we further demonstrate a power conversion efficiency of 16.8% for monolithic all-perovskite triple-junction solar cells.

Integrating several different perovskite absorber layers in a multi-junction solar cell imposes a great processing challenge. Here, the authors demonstrate a versatile two-step solution process for fabricating monolithic all-perovskite triple-junction solar cells.

Rapid Scalable Processing of Tin Oxide Transport Layers for Perovskite Solar Cells

The development of scalable deposition methods for perovskite solar cell materials is critical to enable the commercialization of this nascent technology. Herein, we investigate the use and processing of nanoparticle SnO₂ films as electron transport layers in perovskite solar cells and develop deposition methods for ultrasonic spray coating and slot-die coating, leading to photovoltaic device efficiencies over 19%. The effects of postprocessing treatments (thermal annealing, UV ozone, and O₂ plasma) are then probed using structural and spectroscopic techniques to characterize the nature of the np-SnO₂/perovskite interface. We show that a brief "hot air flow" method can be used to replace extended thermal annealing, confirming that this approach is compatible with high-throughput processing. Our results highlight the importance of interface management to minimize nonradiative losses and provide a deeper understanding of the processing requirements for large-area deposition of nanoparticle metal oxides.

Composite Encapsulation Enabled Superior Comprehensive Stability of Perovskite Solar Cells

Solar cells based on organometal hybrid perovskites have exhibited promising commercialization potential owing to their high efficiency and low-cost manufacturing. However, the poor outdoor operational stability of perovskite solar cells restricted their practical application, and moisture permeation and organic compounds volatilization are realized as the main factors accelerating performance degradation. Herein, we developed a composite encapsulation, by sequentially depositing a compact Al₂O₃ layer and a hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane layer on the completed device, to efficiently circumvent vapor permeability. Thus, the stability of the encapsulated perovskite solar cells was systematically investigated under simulated operational conditions. It was found that the MAPbI₃ perovskite was prone to decay into solid PbI₂ and organic vapor at high temperature or upon light illumination, and the decomposition was reversible in a well-encapsulated environment, resulting in reversible performance degradation and recovery. The enhanced thermal stability was ascribed to the competition between the perovskite decomposition and reverse synthesis. The as-prepared high-quality, multilayered encapsulation scheme demonstrated superior sealing property, and no obvious performance decline was observed when the device was stored under ambient air, continuous light illumination, double 85 condition (85 °C, 85% humidity), or even water immersion. Therefore, this work paves the way for a scalable and robust encapsulation strategy feasible to hybrid perovskite optoelectronics in a reproducible manner.

In-situ observation of nucleation and property evolution in films grown with an atmospheric pressure spatial atomic layer deposition system

Atmospheric pressure—spatial atomic layer deposition (AP-SALD) is a promising open-air deposition technique for high-throughput manufacturing of nanoscale films, yet the nucleation and property evolution in these films has not been studied in detail. In this work, in situ reflectance spectroscopy was implemented in an AP-SALD system to measure the properties of Zinc oxide (ZnO) and Aluminum oxide (Al2O3) films during their deposition. For the first time, this revealed a substrate nucleation period for this technique, where the length of the nucleation time was sensitive to the deposition parameters. The in situ characterization of thickness showed that varying the deposition parameters can achieve a wide range of growth rates (0.1–3 nm/cycle), and the evolution of optical properties throughout film growth was observed. For ZnO, the initial bandgap increased when deposited at lower temperatures and subsequently decreased as the film thickness increased. Similarly, for Al2O3 the refractive index was lower for films deposited at a lower temperature and subsequently increased as the film thickness increased. Notably, where other implementations of reflectance spectroscopy require previous knowledge of the film’s optical properties to fit the spectra to optical dispersion models, the approach developed here utilizes a large range of initial guesses that are inputted into a Levenberg-Marquardt fitting algorithm in parallel to accurately determine both the film thickness and complex refractive index.

Extremely Robust Gas-Quenching Deposition of Halide Perovskites on Top of Hydrophobic Hole Transport Materials for Inverted (p-i-n) Solar Cells by Targeting the Precursor Wetting Issue

Lead halide perovskite solar cells afford high power conversion efficiencies, even though the photoactive layer is formed in a solution process. At the same time, solution processing may impose some severe dewetting issues, especially if organic, hydrophobic charge transport layers are considered. Ultimately, very narrow processing windows with a relatively large spread in device performance and a considerable lab-to-lab variation result. Here, we unambiguously identify dimethylsulfoxide (DMSO), which is commonly used as a co-solvent and complexing agent, to be the main reason for dewetting of the precursor solution on hydrophobic hole transport layers, such as polytriarylamine, in a gas-quenching-assisted deposition process. In striking contrast, we will show that N-methyl-2-pyrrolidon (NMP), which has a lower hydrophilic-lipophilic-balance, can be favorably used instead of DMSO to strongly mitigate these dewetting issues. The resulting high-quality perovskite layers are extremely tolerant with respect to the mixing ratio (NMP/dimethylformamide) and other process parameters. Thus, our findings afford an outstandingly robust, easy to use, and fail-safe deposition technique, yielding single (MAPbI₃) and double (FA_0.94Cs_0.06PbI₃) cation perovskite solar cells with high efficiencies (∼18.5%). Most notably, the statistical variation of the devices is significantly reduced, even if the deposition process is performed by different persons. We foresee that our results will further the reliable preparation of perovskite thin films and mitigate process-to-process variations that still hinder the prospects of upscaling perovskite solar technology.

New development of atomic layer deposition: processes, methods and applications

Atomic layer deposition (ALD) is an ultra-thin film deposition technique that has found many applications owing to its distinct abilities. They include uniform deposition of conformal films with controllable thickness, even on complex three-dimensional surfaces, and can improve the efficiency of electronic devices. This technology has attracted significant interest both for fundamental understanding how the new functional materials can be synthesized by ALD and for numerous practical applications, particularly in advanced nanopatterning for microelectronics, energy storage systems, desalinations, catalysis and medical fields. This review introduces the progress made in ALD, both for computational and experimental methodologies, and provides an outlook of this emerging technology in comparison with other film deposition methods. It discusses experimental approaches and factors that affect the deposition and presents simulation methods, such as molecular dynamics and computational fluid dynamics, which help determine and predict effective ways to optimize ALD processes, hence enabling the reduction in cost, energy waste and adverse environmental impacts. Specific examples are chosen to illustrate the progress in ALD processes and applications that showed a considerable impact on other technologies.

Efficient light harvesting with a nanostructured organic electron-transporting layer in perovskite solar cells

Nanostructures have proved to be advantageous in light harvesting, improving the power conversion efficiency (PCE) of photovoltaic devices. However, the reported light-harvesting nanostructures all require extra processing beyond that for device fabrication, with multiple steps for nano-patterned structures or plasmonic nano-particles. Here we synthesized a conjugated polymer PFPDI which could simply form a nanostructured film on perovskite by spin coating. PFPDI possesses a perylene diimide-based backbone and phosphite ester pendants, which makes it a robust electron-transporting material (ETM) in perovskite solar cells. Furthermore, the perovskite solar cells with PFPDI as the electron-transporting layer (ETL) exhibited a higher light-harvesting efficiency compared to the standard phenyl-C61-butyric acid methyl ester (PCBM) devices. The JSC of the PFPDI device was enhanced from 19.71 mA cm-2 to 23.43 mA cm-2. SEM images and reflectance spectra confirmed that the PFPDI formed ditch-like nanostructures on perovskite film and induced a better light-harvesting efficiency. Further research indicated that the interaction of P[double bond, length as m-dash]O with Pb was essential to the formation of the nanostructures of PFPDI on perovskite. Therefore, our work not only offers an efficient organic ETM, but also opens up new horizons for simply constructing nanostructures with light-harvesting ability in photovoltaic devices.

Atomic layer deposition of a SnO2 electron-transporting layer for planar perovskite solar cells with a power conversion efficiency of 18.3

High-efficiency planar type perovskite solar cells were fabricated by atomic layer deposition (ALD) of SnO2 and subsequent annealing at 180 °C. As-dep. SnO2 layers prepared by post-annealing at 180 and 300 °C, respectively, were used as electron transporting layers (ETLs). ALD-TiO2 layers were also prepared by post annealing at 400 °C, and the thicknesses of all ETLs were around 12 nm. PL quenching, optical band gap measurement, UPS, and conductive AFM results show that SnO2 can more appropriately be used as an ETL compared to TiO2.

Atomic layer deposition for efficient and stable perovskite solar cells

Extended understandings of perovskite solar cells by recent ALD application studies as well as challenges toward enhancing the efficiency and stability will be addressed.

In Situ Atmospheric Deposition of Ultrasmooth Nickel Oxide for Efficient Perovskite Solar Cells

Organic-inorganic perovskite solar cells have attracted significant attention due to their remarkable performance. The use of alternative metal-oxide charge-transport layers is a strategy to improving device reliability for large-scale fabrication and long-term applications. Here, we report solution-processed perovskite solar cells employing nickel oxide hole-extraction layers produced in situ using an atmospheric pressure spatial atomic-layer deposition system, which is compatible with high-throughput processing of electronic devices from solution. Our sub-nanometer smooth (average roughness of ≤0.6 nm) oxide films enable the efficient collection of holes and the formation of perovskite absorbers with high electronic quality. Initial solar-cell experiments show a power-conversion efficiency of 17.1%, near-unity ideality factors, and a fill factor of >80% with negligible hysteresis. Transient measurements reveal that a key contributor to this performance is the reduced luminescence quenching trap density in the perovskite/nickel oxide structure.

Low-Temperature Plasma-Assisted Atomic-Layer-Deposited SnO2 as an Electron Transport Layer in Planar Perovskite Solar Cells

In this work, we present an extensive characterization of plasma-assisted atomic-layer-deposited SnO₂ layers, with the aim of identifying key material properties of SnO₂ to serve as an efficient electron transport layer in perovskite solar cells (PSCs). Electrically resistive SnO₂ films are fabricated at 50 °C, while a SnO₂ film with a low electrical resistivity of 1.8 × 10^-3 Ω cm, a carrier density of 9.6 × 10¹⁹ cm^-3, and a high mobility of 36.0 cm²/V s is deposited at 200 °C. Ultraviolet photoelectron spectroscopy indicates a conduction band offset of ∼0.69 eV at the 50 °C SnO₂/Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_2.7Br_0.3) interface. In contrast, a negligible conduction band offset is found between the 200 °C SnO₂ and the perovskite. Surprisingly, comparable initial power conversion efficiencies (PCEs) of 17.5 and 17.8% are demonstrated for the champion cells using 15 nm thick SnO₂ deposited at 50 and 200 °C, respectively. The latter gains in fill factor but loses in open-circuit voltage. Markedly, PSCs using the 200 °C compact SnO₂ retain their initial performance at the maximum power point over 16 h under continuous one-sun illumination in inert atmosphere. Instead, the cell with the 50 °C SnO₂ shows a decrease in PCE of approximately 50%.

Low-Temperature Atomic Layer Deposition of Metal Oxide Layers for Perovskite Solar Cells with High Efficiency and Stability under Harsh Environmental Conditions

Rapid progress achieved on perovskite solar cells raises the expectation for their further development toward practical applications. Moisture sensitivity of perovskite materials is one of the major obstacles which limits the long-term durability of the perovskite solar cells, especially in outdoor operation where rainfall and water accumulation on the solar panels often occur. Micro/nanopinholes within the functional layers of the devices usually lead to water vapor penetration, thus subsequent decomposition of perovskites, and finally poor device performance and shortened operational lifetime. In this work, low-temperature atomic layer deposition (ALD) technique was utilized to incorporate pinhole-free metal oxide layers (TiO₂ and Al₂O₃) into an inverted perovskite solar cell consisting of indium tin oxide/NiO/perovskite/PC₆₁BM/TiO₂/Ag. The interface properties between the inserted TiO₂ layer and the perovskite layer were investigated by X-ray photoelectron spectroscopy. The results showed that TiO₂ ALD fabrication process had made negligible degradation to the perovskite layer. The TiO₂ layer can significantly reduce interfacial charge recombination loss, improve interfacial contact, and enhance water resistance. A maximum power conversion efficiency (PCE) of 18.3% was achieved for devices with TiO₂ interface layers. A stacked Al₂O₃ encapsulation layer was designed and deposited on top of the devices to further improve device stability under harsh environmental conditions. The encapsulated devices with the best performance retained 97% of the initial PCE after being stored in ambient condition for a thousand hours. They also showed great water resistance, and no significant degradation in terms of PCE and photocurrent of the devices was observed after they were immersed in deionized water for as long as 2 h. Our approach offers a promising way of developing highly efficient and stable perovskite solar cells under real-world operational conditions.