Composition Engineering in Doctor‐Blading of Perovskite Solar Cells

Organic–inorganic halide perovskite (OIHP) solar cells with efficiency over 18% power conversion efficiency (PCE) have been widely achieved with lab scale spin‐coating method which is however not scalable for the fabrication of large area solar panels. The PCEs of OIHP solar cells made by scalable deposition methods, such as doctor‐blading or slot‐die coating, have been lagging far behind than spin‐coated devices. In this study the authors report composition engineering in doctor‐bladed OIHP solar cells with p–i–n planar heterojunction structure to enhance their efficiency. Phase purer OIHP thin films are obtained by incorporating a small amount of cesium (Cs⁺) and bromine (Br^?) ions into perovskite precursor solution, which also reduces the required film formation temperature. Pinhole free OIHP thin films with micrometer‐sized grains have been obtained assisted by a secondary grain growth with added methylammonium chloride into the precursor solution. The OIHP solar cells using these bladed thin films achieved PCEs over 19.0%, with the best stabilized PCE reaching 19.3%. This represents a significant step toward scalable manufacture of OIHP solar cells.     

Inkjet‐Printed Micrometer‐Thick Perovskite Solar Cells with Large Columnar Grains

Transferring the high power conversion efficiencies (PCEs) of spin‐coated perovskite solar cells (PSCs) on the laboratory scale to large‐area photovoltaic modules requires a significant advance in scalable fabrication methods. Digital inkjet printing promises scalable, material, and cost‐efficient deposition of perovskite thin films on a wide range of substrates and in arbitrary shapes. In this work, high‐quality inkjet‐printed triple‐cation (methylammonium, formamidinium, and cesium) perovskite layers with exceptional thicknesses of >1 µm are demonstrated, enabling unprecedentedly high PCEs > 21% and stabilized power output efficiencies > 18% for inkjet‐printed PSCs. In‐depth characterization shows that the thick inkjet‐printed perovskite thin films deposited using the process developed herein exhibit a columnar crystal structure, free of horizontal grain boundaries, which extend over the entire thickness. A thin film thickness of around 1.5 µm is determined as optimal for PSC for this process. Up to this layer thickness X‐ray photoemission spectroscopy analysis confirms the expected stoichiometric perovskite composition at the surface and shows strong deviations and inhomogeneities for thicker thin films. The micrometer‐thick perovskite thin films exhibit remarkably long charge carrier lifetimes, highlighting their excellent optoelectronic characteristics. They are particularly promising for next‐generation inkjet‐printed perovskite solar cells, photodetectors, and X‐ray detectors.     

Aerosol‐Jet‐Assisted Thin‐Film Growth of CH3NH3PbI3 Perovskites—A Means to Achieve High Quality,Defect‐Free Films for Efficient Solar Cells

A high level of automation is desirable to facilitate the lab‐to‐fab process transfer of the emerging perovskite‐based solar technology. Here, an automated aerosol‐jet printing technique is introduced for precisely controlling the thin‐film perovskite growth in a planar heterojunction p–i–n solar cell device structure. The roles of some of the user defined parameters from a computer‐aided design file are studied for the reproducible fabrication of pure CH₃NH₃PbI₃ thin films under near ambient conditions. Preliminary power conversion efficiencies up to 15.4% are achieved when such films are incorporated in a poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate‐perovskite‐phenyl‐C71‐butyric acid methyl ester type device format. It is further shown that the deposition of atomized materials in the form of a gaseous mist helps to form a highly uniform and PbI₂ residue‐free CH₃NH₃PbI₃ film and offers advantages over the conventional two‐step solution approach by avoiding the detrimental solid–liquid interface induced perovskite crystallization. Ultimately, by integrating full 3D motion control, the fabrication of perovskite layers directly on a 3D curved surface becomes possible. This work suggests that 3D automation with aerosol‐jet printing, once fully optimized, could form a universal platform for the lab‐to‐fab process transfer of solution‐based perovskite photovoltaics and steer development of new design strategies for numerous embedded structural power applications.     

Combinatorial Screening of Polymer:Fullerene Blends for Organic Solar Cells by Inkjet Printing

Polymer:fullerene blends were screened in a combinatorial approach using inkjet printing thin film libraries for photovoltaic devices. The application of inkjet printing enabled a fast and simple experimental workflow from film preparation to the study of structure‐property‐relationships with a very high material efficiency. Inkjet printing requires less material for the preparation of thin film libraries in comparison to other dispensing techniques, like spin‐coating. Two polymers (PCPDTBT, PSBTBT) and two fullerene derivatives (mono‐PCBM, bis‐PCBM) were investigated in various blend ratios, concentrations, solvent ratios, and film thicknesses. Morphological and optical properties of the inkjet printed films were investigated and compared with spin‐coated films. This study shows the principle of an experimental setup from solution preparation to film characterization for the combinatorial investigation of large polymer:fullerene libraries.     

Exceptionally Enhanced Electrode Activity of (Pr,Ce)O2−δ‐Based Cathodes for Thin‐Film Solid Oxide Fuel Cells

It is shown that an electrochemically‐driven oxide overcoating substantially improves the performance of metal electrodes in high‐temperature electrochemical applications. As a case study, Pt thin films are overcoated with (Pr,Ce)O_2?δ (PCO) by means of a cathodic electrochemical deposition process that produces nanostructured oxide layers with a high specific surface area and uniform metal coverage and then the coated films are examined as an O₂‐electrode for thin‐film‐based solid oxide fuel cells. The combination of excellent conductivity, reactivity, and durability of PCO dramatically improves the oxygen reduction reaction rate while maintaining the nanoscale architecture of PCO layers and thus the performance of the PCO‐coated Pt thin‐film electrodes at high temperatures. As a result, with an oxide coating step lasting only 5 min, the electrode resistance is successfully reduced by more than 1000 times at 500 °C in air. These observations provide a new direction for the design of high‐performance electrodes for high‐temperature electrochemical cells.     

In Situ Probing the Crystallization Kinetics in Gas-Quenching-Assisted Coating of Perovskite Films

The pursuit of commercializing perovskite photovoltaics is driving the development of various scalable perovskite crystallization techniques. Among them, gas quenching is a promising crystallization approach for high-throughput deposition of perovskite films. However, the perovskite films prepared by gas-quenching assisted blade coating are susceptible to the formation of pinholes and frequently show inferior crystallinity if the interplay between film coating, film drying, and crystallization kinetics is not fully optimized. That arguably requires a thorough understanding of how single processing steps influence the crystallization kinetics of printed perovskite films. Here, in situ optical spectroscopies are integrated into a doctor-blading setup that allows to real-time monitor film formation during the gas-quenching process. It is found that the essential role of gas quenching treatment is in achieving a smooth and compact perovskite film by controlling the nucleation rate. Moreover, with the assistance of phase-field simulations, the role of excessive methylammonium iodide is revealed to increase grain size by accelerating the crystal growth rate. These results show a tailored control of crystal growth rate is critical to achieving optimal film quality, leading to fully printed solar cells with a champion power conversion efficiency of 19.50% and mini solar modules with 15.28% efficiency are achieved.     

CsI‐Antisolvent Adduct Formation in All‐Inorganic Metal Halide Perovskites

The excellent optoelectronic properties demonstrated by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high‐performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead‐halide precursor salts. These adduct‐based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all‐inorganic perovskites such as CsPbI₃. In this work, inorganic perovskite composition CsPbI₃ is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A‐site cation salts. These CsI adducts control nucleation more so than the PbI₂–dimethyl sulfoxide (DMSO) adduct and demonstrate how the A‐site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI–MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.     

Precursor Decomposition,Microstructure, and Porosity of Yttria Stabilized Zirconia Thin Films Prepared by Aerosol‐Assisted Chemical Vapor Deposition

Microstructures of yttria stabilized zirconia thin films deposited by aerosol assisted chemical vapor deposition (AA‐CVD) are correlated with the thermal decomposition behavior of the corresponding metal precursors, zirconium and yttrium 2,4‐pentanedionate. Process conditions of AA‐CVD are investigated with the aim of producing dense and compact YSZ thin films for applications as gas‐tight electrolyte. Based on systematic cross sectional scanning transmission electron microscopy (STEM) investigations and conductivity measurements, the development of percolating nanoporosity is observed in samples prepared at temperatures between 350 °C and 600 °C at standard solution throughput. Compact columnar thin films with bulk conductivity are obtained at 600 °C by reducing the metal content of the precursor solution and at 450 °C by reducing the solution throughput.     

Vacuum Deposited Triple‐Cation Mixed‐Halide Perovskite Solar Cells

Hybrid lead halide perovskites are promising materials for future photovoltaics applications. Their spectral response can be readily tuned by controlling the halide composition, while their stability is strongly dependent on the film morphology and on the type of organic cation used. Mixed cation and mixed halide systems have led to the most efficient and stable perovskite solar cells reported, so far they are prepared exclusively by solution‐processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. In this report, thermal vacuum deposition with multiple sources (3 and 4) is used to prepare for the first time, multications/anions perovskite compounds. These thin‐film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability. The importance of the control over the film morphology is highlighted, which differs substantially when these compounds are vacuum processed. Avenues to improve the morphology and hence the performance of fully vacuum processed multications/anions perovskite solar cells are proposed.     

Fused‐Ring Electron Acceptor ITIC‐Th: A Novel Stabilizer for Halide Perovskite Precursor Solution

Solution‐processed perovskite solar cells have great potential for low‐cost roll‐to‐roll fabrication. However, the degradation of aged precursor solutions will become a critical obstacle to mass production. In this report, a small molecule (ITIC‐Th) is employed to stabilize the perovskite precursor solution containing mixed cations and halides. It is found that ITIC‐Th can effectively suppress the formation of yellow δ‐phase in the films made from aged precursor solutions. Consequently, the devices fabricated from the aged precursor solution with ITIC‐Th experience much less efficiency drop with the increase of the precursor aging time—from 19.20% (fresh) to 16.55% (39 d), compared with the devices made from conventional precursor solutions dropping from 18.07% (fresh) to 1.76% (39 d). The characterizations suggest that ITIC‐Th is beneficial for CH₃NH₃⁺ cations to be incorporated into the crystal structure, facilitating the formation of perovskite phase. Furthermore, the presence of ITIC‐Th in the perovskite thin film gives rise to additional photocurrent as well as improved fill factor due to the well‐matched energy levels, the passivation of defects, and the complementary absorption spectra, suggesting a new route toward future high‐efficiency solar cells—incorporating organic non‐fullerene acceptors and halide perovskite materials into the same active layer.     

Formation of CuInSe2 and CuGaSe2 Thin‐Films Deposited by Three‐Stage Thermal Co‐Evaporation: A Real‐Time X‐Ray Diffraction and Fluorescence Study

Thin film solar cells based on co‐evaporated Cu(In,Ga)Se₂ absorber films present the highest efficiencies among current polycrystalline thin‐film technologies. Thanks to the development of a novel experimental setup for in situ growth studies, it was possible to follow the formation of the crystalline phases during such deposition processes for the first time. This synchrotron‐based energy‐dispersive X‐ray diffraction and fluorescence setup is suited for real‐time studies of thin film vapor deposition processes. Focusing on the growth of CuInSe₂ and CuGaSe₂ fabricated by three‐stage processing, we find that the phase transitions in the Cu‐In‐Se system follow the reported pseudo‐binary In₂Se₃‐Cu₂Se phase diagram. This requires a transformation of the Se sublattice during the incorporation of Cu‐Se into the In₂Se₃ precursor film from the first process stage. In the Cu‐Ga‐Se system, besides an increase in the lattice spacings, we observe no transformation of the Se sublattice. Furthermore, the structural defects of the Ga‐Se precursor film are preserved until the CuGaSe₂ stoichiometry is reached. By means of model calculations of the fluorescence signals, we confirm in both systems the segregation of Cu₂Se at the surface near a concentration of 25 at.% Cu shortly after the recrystallization of the films. The modeling also reveals that Cu₂Se penetrates into the CuInSe₂ film, whereas it remains at the surface of the CuGaSe₂ film.     

The Development of Hydrazine‐Processed Cu(In,Ga)(Se,S)2 Solar Cells

The hydrazine‐based deposition of Cu(In,Ga)(S,Se)₂ (CIGS) thin films has attracted considerable attention in recent years due to its potential for the high‐throughput production of photovoltaic devices based on this absorber material. This article provides an introduction as well as presenting a complete picture of the current status of hydrazine‐based CIGS solar‐cell fabrication, including the three major steps of this deposition process: dissolution of the precursor materials in hydrazine, deposition of a film from the resulting precursor solution, and the completion and characterization of a photovoltaic device following absorber deposition. Recent discoveries are then discussed, regarding the dissolution chemistry of the relevant precursor complexes in hydrazine, which together represent the true foundation of this processing method. Recent studies on CIGS film formation are then summarized, including the control and analysis of the crystalline phase, electronic bandgap, and film morphology. Finally, the latest progress in high‐performance device fabrication is highlighted, with a focus on optoelectronic characterization including current–voltage, junction capacitance, and minority carrier lifetime measurements. Finally, a discussion and future outlook is provided.     

Large‐Grained Perovskite Films Enabled by One‐Step Meniscus‐Assisted Solution Printing of Cross‐Aligned Conductive Nanowires for Biodegradable Flexible Solar Cells

Increasing performance demand associated with the short lifetime of consumer electronics has triggered fast growth in electronic waste, leading to serious ecological challenges worldwide. Herein, a robust strategy for judiciously constructing flexible perovskite solar cells (PSCs) that can be conveniently biodegraded is reported. The key to this strategy is to capitalize on meniscus‐assisted solution printing (MASP) as a facile means of yielding cross‐aligned silver nanowires in one‐step, which are subsequently impregnated in a biodegradable elastomeric polyester. Intriguingly, the as‐crafted hybrid biodegradable electrode greatly constrains the solvent evaporation of the perovskite precursor solution, thereby generating fewer nuclei and in turn resulting in the deposition of a large‐grained dense perovskite film that exhibits excellent optoelectronic properties with a power conversion efficiency of 17.51% in PSCs. More importantly, the hybrid biodegradable electrode‐based devices also manifest impressive robustness against mechanical deformation and can be thoroughly biodegraded after use. These results signify the great potential of MASP for controllably assembling aligned conductive nanomaterials for biodegradable electrodes. As such, it represents an important endeavor toward environmentally friendly, multifunctional and flexible electronic, optoelectronic, photonic, and sensory materials and devices.     

Carbon Nanoparticles in High‐Performance Perovskite Solar Cells

In the past few years, organic–inorganic metal halide ABX₃ perovskites (A = Rb, Cs, methylammonium, formamidinium (FA); B = Pb, Sn; X = Cl, Br, I) have rapidly emerged as promising materials for photovoltaic applications. Tuning the film morphology by various deposition techniques and additives is crucial to achieve solar cells with high performance and long‐term stability. In this work, carbon nanoparticles (CNPs) containing functional groups are added to the perovskite precursor solution for fabrication of fluorine‐doped tin oxide/TiO₂/perovskite/spiro‐OMeTAD/gold devices. With the addition of CNPs, the perovskite films are thermally more stable, contain larger grains, and become more hydrophobic. NMR experiments provide strong evidence that the functional groups of the CNPs interact with FA cations already in the precursor solution. The fabricated solar cells show a power‐conversion efficiency of 18% and negligible hysteresis.     

Electrospray‐Assisted Fabrication of Moisture‐Resistant and Highly Stable Perovskite Solar Cells at Ambient Conditions

An electrospray deposition technique to fabricate a perovskite (CH₃NH₃PbI₃) layer for highly stable and efficient perovskite solar cells at ambient humidity (30%–50% relative humidity) conditions is demonstrated. A detailed study is conducted to determine the effect of different electrospray parameters on the device performance and to provide a mechanistic explanation of the superior stability of the films. Due to the controlled reactivity that results in the formation of a smooth perovskite film, these cells exhibit stability exceeding 4000 h, in contrast to much lower stability of those fabricated by conventional spin coating methods. Furthermore, the perovskite film deposited by electrospray methods exhibits a self‐healing behavior when exposed to moisture. The authors hypothesize the formation of an intermediate metastable phase and smooth morphology of the film as the reason for this enhanced stability. Electrospray is a scalable technique that provides precise control over the amount of material required for deposition, reducing significant material loss that occurs in conventional solution‐based methods. Overall, this work shows that stability of perovskite solar cells can be improved by fabrication using a well controlled and optimized electrospray technique, without the use of any additives or cell encapsulants.     

Self‐Assembled 2D Perovskite Layers for Efficient Printable Solar Cells

2D organic–inorganic hybrid Ruddlesden–Popper perovskites have emerged recently as candidates for the light‐absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D‐perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spin‐coating step. In this work, a facile, spin‐coating‐free, directly scalable drop‐cast method is reported for depositing precursor solutions that self‐assemble into highly oriented, uniform 2D‐perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm²). This is the highest PCE to date for a solar cell with 2D‐perovskite layers fabricated by nonspin‐coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D‐perovskite layer deposition using a slot‐die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot‐die coating on a glass substrate or R2R slot‐die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.     

Enhanced Efficiency of Hot‐Cast Large‐Area Planar Perovskite Solar Cells/Modules Having Controlled Chloride Incorporation

Organic–inorganic perovskite photovoltaics are an emerging solar technology. Developing materials and processing techniques that can be implemented in large‐scale manufacturing is extremely important for realizing the potential of commercialization. Here we report a hot‐casting process with controlled Cl^? incorporation which enables high stability and high power‐conversion‐efficiencies (PCEs) of 18.2% for small area (0.09 cm²) and 15.4% for large‐area (≈1 cm²) single solar cells. The enhanced performance versus tri‐iodide perovskites can be ascribed to longer carrier diffusion lengths, improved uniformity of the perovskite film morphology, favorable perovskite crystallite orientation, a halide concentration gradient in the perovskite film, and reduced recombination by introducing Cl^?. Additionally, Cl^? improves the device stability by passivating the reaction between I^? and the silver electrode. High‐quality thin films deployed over a large‐area 5 cm × 5 cm eight‐cell module have been fabricated and exhibit an active‐area PCE of 12.0%. The feasibility of material and processing strategies in industrial large‐scale coating techniques is then shown by demonstrating a “dip‐coating” process which shows promise for large throughput production of perovskite solar modules.     

Effect of Precursor Solution Aging on the Crystallinity and Photovoltaic Performance of Perovskite Solar Cells

Perovskite materials due to their exceptional photophysical properties are beginning to dominate the field of thin‐film optoelectronic devices. However, one of the primary challenges is the processing‐dependent variability in the properties, thus making it imperative to understand the origin of such variations. Here, it is discovered that the precursor solution aging time before it is cast into a thin film, is a subtle but a very important factor that dramatically affects the overall thin‐film formation and crystallinity and therein factors such as grain growth, phase purity, surface uniformity, trap state density, and overall solar cell performance. It is shown that progressive aging of the precursor promotes efficient formation of larger seeds after the fast nucleation of a large density of small seeds. The hot‐casting method then leads to the growth of large grains in uniform thin‐films with excellent crystallinity validated using scanning microscopy images and X‐ray diffraction patterns. The high‐quality films cast from aged solution is ideal for thin‐film photovoltaic device fabrication with reduced shunt current and good charge transport. This observation is a significant step toward achieving highly crystalline thin‐films with reliability in device performance and establishes the subtle but dramatic effect of solution aging before fabricating perovskite thin‐films.     

Atmospheric‐Pressure Chemical Vapor Deposition of Iron Pyrite Thin Films

Iron pyrite (cubic FeS₂) is a promising candidate absorber material for earth‐abundant thin‐film solar cells. In this report, single‐phase, large‐grain, and uniform polycrystalline pyrite thin films are fabricated on glass and molybdenum‐coated glass substrates by atmospheric‐pressure chemical vapor deposition (AP‐CVD) using the reaction of iron(III) acetylacetonate and tert‐butyl disulfide in argon at 300 °C, followed by sulfur annealing at 500–550 °C to convert marcasite impurities to pyrite. The pyrite‐marcasite phase composition depends strongly on the concentration of sodium in the growth substrate and the sulfur partial pressure during annealing. Phase and elemental composition of the films are characterized by X‐ray diffraction, Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, Rutherford backscattering spectrometry, and X‐ray photoelectron spectroscopy. The in‐plane electrical properties are surprisingly insensitive to phase and elemental impurities, with all films showing p‐type, thermally activated transport with a small activation energy (≈30 meV), a room‐ temperature resistivity of ≈1 Ω cm, and low mobility. These ubiquitous electrical properties may result from robust surface effects. These CVD pyrite thin films are well suited to fundamental electrical studies and the fabrication of pyrite photovoltaic device stacks.     

Vacuum‐Assisted Growth of Low‐Bandgap Thin Films (FA0.8MA0.2Sn0.5Pb0.5I3) for All‐Perovskite Tandem Solar Cells

All‐perovskite multijunction photovoltaics, combining a wide‐bandgap (WBG) perovskite top solar cell (E_G ≈1.6–1.8 eV) with a low‐bandgap (LBG) perovskite bottom solar cell (E_G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum‐assisted growth control (VAGC) of solution‐processed LBG perovskite thin films based on mixed Sn–Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well‐established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge‐carrier lifetime. The improved optoelectronic characteristics enable high‐performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four‐terminal all‐perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active‐area solar cells up to 1 cm².