Efficient Non-Fullerene Organic Photovoltaics Printed by Electrospray via Solvent Engineering

Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1-10 cm2), medium (10-100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as "fully printed/coated" devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

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.

Flow-Enhanced Flexible Microcomb Printing of Organic Solar Cells

Scalable and roll-to-roll compatible processing methods have become pressing needs to transfer organic solar cells (OSCs) to realistic energy sources. Herein a new fabrication method of flexible microcomb printing is proposed. The microcomb is based on a PET sheet micromachined into comb teeth by a laser marker. A computational fluid mechanics simulation shows that the fluid flow around the microcomb teeth induces high shear as well as extensional strain rates, which enhance the molecular alignment and lateral mass transport. The PTQ10:Y6-BO OSCs printed by the flexible microcomb demonstrate a substantially increased degree of crystallinity and phase separation with a suitable domain size. Devices printed by the flexible microcomb in air achieve PCEs of up to 15.93%, higher than those of control devices spin-coated in the N₂ glovebox. The flexibility of the PET film makes the microcomb teeth contact directly with the substrate without a suspended liquid meniscus, thus facilitating printing on soft or curved substrates. Printing of flexible OSCs and large-area devices are demonstrated. The flexible OSCs exhibit PCEs of up to 13.62%, which is the highest for flexible OSCs made by scalable printing techniques to date. These results make flexible microcomb printing a feasible and promising strategy toward the manufacture of efficient OSCs.

Printing fabrication of large-area non-fullerene organic solar cells

Organic solar cells (OSCs) based on a bulk heterojunction structure exhibit inherent advantages, such as low cost, light weight, mechanical flexibility, and easy processing, and they are emerging as a potential renewable energy technology. However, most studies are focused on lab-scale, small-area (<1 cm²) devices. Large-area (>1 cm²) OSCs still exhibit considerable efficiency loss during upscaling from small-area to large-area, which is a big challenge. In recent years, along with the rapid development of high-performance non-fullerene acceptors, many researchers have focused on developing large-area non-fullerene-based devices and modules. There are three essential issues in upscaling OSCs from small-area to large-area: fabrication technology, equipment development, and device component processing strategy. In this review, the challenges and solutions in fabricating high-performance large-area OSCs are discussed in terms of the abovementioned three aspects. In addition, the recent progress of large-area OSCs based on non-fullerene electron acceptors is summarized.

Organic Photovoltaics Printed via Sheet Electrospray Enabled by Quadrupole Electrodes

Developing manufacturing methods that are scalable and compatible with a roll-to-roll process with low waste of material has become a pressing need to transfer organic photovoltaics (OPVs) to a viable renewable energy source. For this purpose, various spray printing methods have been proposed. Among them, electrospray (ES) is an attractive option due to its negligible material waste, tunable droplet size, and tolerance to the substrate defects and roughness. Conventional ES with a circular spray footprint often makes the droplets well separated and unlikely to merge, giving rise to "coffee rings" which cause a rough and flawed film morphology. Here, a quadrupole electrode is introduced to generate a compressing electric field that squeezes the conical ES profile into the shape of a thin sheet. The numerical simulation and experimental data of the trajectories of sprayed droplets show that the quadrupole apparatus can effectively increase the long axis to short axis ratio of the oval spray footprint and hence bring droplets closer to each other and make the merging more likely for the deposited droplets. By promoting the merging of droplets, individual coffee rings are also suppressed. Thus, the quadrupole ES offers untapped opportunities for effectively reducing voids and improving the flatness of the ES-printed active layer. The devices with a PM6:N3 active layer printed by the sheet ES exhibited the highest power conversion efficiency (PCE) of up to 15.98%, which is a noticeable improvement over that (14.85%) of counterparts fabricated by a conventional conical ES. This is the highest PCE reported for ES-printed OPVs and is one of the most efficient spray-deposited OPVs so far. In addition, the all-spray-printed devices reached a PCE of 14.55%, which is also among the most efficient all-spray-printed OPVs.

Heptacyclic S,N-Heteroacene-Based Near-Infrared Nonfullerene Acceptor Enables High-Performance Organic Solar Cells with Small Highest Occupied Molecular Orbital Offsets

The reduction of energy offsets between donors and acceptors is a direct way to improve the open-circuit voltage (V_OC) and overall performance of organic solar cells (OSCs). In this work, two nonfullerene acceptors (NFAs) (BDTBO-4F and BDTBO-4Cl) were synthesized, which were composed of a heptacyclic S,N-heteroacene core and terminal units with halogen atoms, where the latter modulates the energy level of the frontier molecular orbital. Consequently, BDTBO-4Cl exhibited a deeper highest occupied molecular orbital level (E_HOMO) and lowest unoccupied molecular orbital level (E_LUMO) than BDTBO-4F. Moreover, these two NFAs exhibited high electron mobility and strong absorption at 700-900 nm. The polymer donor PM6 was combined with BDTBO-4F and BDTBO-4Cl, and the resulting OSCs exhibited outstanding power conversion efficiencies of 14.83% for the PM6:BDTBO-4F device and 13.87% for the PM6:BDTBO-4Cl device. More encouragingly, these OSCs exhibited efficient hole transfer from NFAs to PM6, despite small ΔE_HOMO(D-A) values (<0.10 eV). These results prove that modulation of E_HOMO of acceptors to decrease ΔE_HOMO(D-A) is an efficient strategy for high-performance OSCs.