Air-knife-assisted spray coating of organic solar cells

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have risen dramatically since the introduction of the “Y-series” of non-fullerene acceptors. However, the demonstration of rapid scalable deposition techniques to deposit such systems is rare. Here, for the first time, we demonstrate the deposition of a Y-series-based system using ultrasonic spray coating─a technique with the potential for significantly faster deposition speeds than most traditional meniscus-based methods. Through the use of an air-knife to rapidly remove the casting solvent, we can overcome film reticulation, allowing the drying dynamics to be controlled without the use of solvent additives, heating the substrate, or heating the casting solution. The air-knife also facilitates the use of a non-halogenated, low-toxicity solvent, resulting in industrially relevant, spray-coated PM6:DTY6 devices with PCEs of up to 14.1%. We also highlight the obstacles for scalable coating of Y-series-based solar cells, in particular the influence of slower drying times on blend morphology and crystallinity. This work demonstrates the compatibility of ultrasonic spray coating, and use of an air-knife, with high-speed, roll-to-roll OSC manufacturing techniques

Alumina nanoparticles enable optimal spray-coated perovskite thin film growth on self-assembled monolayers for efficient and reproducible photovoltaics

The power conversion efficiencies of metal halide perovskite photovoltaics have increased rapidly over the past decade attracting significant academic and industrial interest. The ease with which high performance perovskite photovoltaics can be fabricated through solution processing routes has opened up significant possibilities for fabrication through existing, industrially mature high-throughput solution coating techniques such as spray-coating. The power conversion efficiencies of spray-coated metal halide perovskite photovoltaics are limited by non-radiative recombination at the interfaces with charge transport layers necessitating the implementation of new charge transport layers. The self-assembled monolayer (SAM) charge transport layers have resulted in record perovskite photovoltaic device performances, due to reduced non-radiative recombination. However, poor wettability associated with some SAMs significantly limits their applicability, this is exaggerated for droplet-based scalable technologies like spray-coating. Here we report an optimised aluminium oxide nanoparticle interlayer which enables spray-coating of triple cation metal halide perovskite thin films and devices onto Me-4PACz (([4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid)). Our interlayer results in improved structural and optoelectronic properties of spray-coated perovskite thin films, compared to those fabricated through spin-coating. These improved properties enable the fabrication of p–i–n photovoltaic devices with efficiencies over 20% – some of the highest reported for both spray-coated devices in p–i–n architecture, and devices having a spray-coated “triple cation” perovskite active layer

Perovskites on ice : an additive-free approach to increase the shelf-life of triple-cation perovskite precursor solutions

The development of stable perovskite precursor solutions is critical if solution-processable perovskite solar cells (PSCs) are to be practically manufacturable. Ideally, such precursors should combine high solution stability without using chemical additives that might compromise PSC performance. Here, we show that the shelf-life of high-performing perovskite precursors can be greatly improved by storing solutions at low-temperature without the need to alter chemical composition. We find that devices fabricated from solutions stored for 31-days at 4°C achieve a champion PCE of 18.6% (97% of original PCE). The choice of precursor solvent also impacts solution shelf-life, with DMSO-based solutions having enhanced solution stability compared to those including DMF. We explore the compositions of aged precursors using NMR spectroscopy and analyse films made from these solutions using X-ray diffraction. We conclude that the improvement in precursor solution stability is directly linked to the suppression of an addition-elimination reaction and the preservation of higher amounts of methylammonium within solution

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

Polariton condensation in an organic microcavity utilising a hybrid metal-DBR mirror

We have developed a simplified approach to fabricate high-reflectivity mirrors suitable for applications in a strongly-coupled organic-semiconductor microcavity. Such mirrors are based on a small number of quarter-wave dielectric pairs deposited on top of a thick silver film that combine high reflectivity and broad reflectivity bandwidth. Using this approach, we construct a microcavity containing the molecular dye BODIPY-Br in which the bottom cavity mirror is composed of a silver layer coated by a SiO2 and a Nb2O5 film, and show that this cavity undergoes polariton condensation at a similar threshold to that of a control cavity whose bottom mirror consists of ten quarter-wave dielectric pairs. We observe, however, that the roughness of the hybrid mirror—caused by limited adhesion between the silver and the dielectric pair—apparently prevents complete collapse of the population to the ground polariton state above the condensation threshold

Binary solvent system used to fabricate fully annealing‐free perovskite solar cells

High temperature post-deposition annealing of hybrid lead halide perovskite thin films—typically lasting at least 10 min—dramatically limits the maximum roll-to-roll coating speed, which determines solar module manufacturing costs. While several approaches for “annealing-free” perovskite solar cells (PSCs) have been demonstrated, many are of limited feasibility for scalable fabrication. Here, this work has solvent-engineered a high vapor pressure solvent mixture of 2-methoxy ethanol and tetrahydrofuran to deposit highly crystalline perovskite thin-films at room temperature using gas-quenching to remove the volatile solvents. Using this approach, this work demonstrates p-i-n devices with an annealing-free MAPbI3 perovskite layer achieving stabilized power conversion efficiencies (PCEs) of up to 18.0%, compared to 18.4% for devices containing an annealed perovskite layer. This work then explores the deposition of self-assembled molecules as the hole-transporting layer without annealing. This work finally combines the methods to create fully annealing-free devices having stabilized PCEs of up to 17.1%. This represents the state-of-the-art for annealing-free fabrication of PSCs with a process fully compatible with roll-to-roll manufacture

Direct integration of perovskite solar cells with carbon fibre substrates

Integrating photovoltaic devices onto the surface of carbon fibre-reinforced polymer substrates should create materials with high mechanical strength that are also able to generate electrical power. Such devices are anticipated to find ready applications as structural, energy-harvesting systems in both the automotive and aeronautical sectors. Here, we demonstrate the fabrication of triple-cation perovskite n-i-p solar cells onto the surface of planarised carbon fibre-reinforced polymer substrates, with devices utilising a transparent top ITO contact. These devices also contain a "wrinkled" SiO2 interlayer placed between the device and substrate which alleviates thermally-induced cracking of the bottom ITO layer. Our devices were found to have a stabilised power conversion efficiency of 14.5% and a specific power (power per weight) of 21.4 W g-1 (without encapsulation), making them highly suitable for mobile power applications. This article is protected by copyright. All rights reserved

Gas‐Assisted Spray Coating of Perovskite Solar Cells Incorporating Sprayed Self‐Assembled Monolayers (Adv. Sci. 14/2022)

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