Low Temperature Aggregation Transitions in N3 and Y6 Acceptors Enable Double‐Annealing Method That Yields Hierarchical Morphology and Superior Efficiency in Nonfullerene Organic Solar Cells

Abstract

Thermal transition of organic solar cells (OSCs) constituent materials are often insufficiently researched, resulting in trial-and-error rather than rational approaches to annealing strategies to improve domain purity to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the modern highperformance acceptors Y6 and N3. Here, by using an optical method, it is discovered that the acceptor N3 has a clear solid-state aggregation transition at 82 °C. This unusually low transition not only explains prior optimization protocols, but the transition informs and enables a double-annealing method that can fine-tune aggregation and the device morphology. Compared with 16.6% efficiency for PM6:N3:PC 71 BM control devices, higher efficiency of 17.6% is obtained through the improved protocol. Morphology characterization with x-ray scattering methods reveals the formation of a multilength scale morphology. Moreover, the double-annealing method is illustrated and easily transferred and validated with Y6-based devices, using the transition of Y6 at 102 °C. As a result, the PCE improved from 16.0% to 16.8%. Design of highperformance acceptors with yet lower aggregation transitions might be required for OSCs to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low.

The Role of Demixing and Crystallization Kinetics on the Stability of Non‐Fullerene Organic Solar Cells

“…The best efficiencies of 9.5% and 9.6% are achieved for FTAZ:ITIC‐2F and FTAZ:ITIC‐2Cl, respectively, with the devices annealed at 160 °C (close to the T g of the NFAs, a common optimization condition). [ 12 ] It is noted that the relatively low absorption at wavelength below 400 nm of non‐fullerene OSC can deliver an EQE about 30%, which has been observed in other reports and is likely due to the efficient exciton dissociation and charge collection and absorption of 40–50% of the incident photons in actual devices. [ 54 ] The short‐circuit current density ( J SC ) estimated from EQE is 16.7 and 17.9 mA cm −2 for 160 °C annealed FTAZ:ITIC‐2F and FTAZ:ITIC‐2Cl devices, respectively, which are in a good agreement with the obtained J sc from J–V curves.…”

“…[ 4–7 ] Owing to the rapid development of high‐performance non‐fullerene small molecular acceptors (SMAs), PCE of over 15–17% has been achieved in various systems. [ 8–12 ] These promising results make non‐fullerene OSCs competitive with other types of next‐generation solar technology. Among start‐of‐the‐art non‐fullerene SMAs, the fused‐ring electron acceptor named ITIC, comprising a indacenodithieno[3,2‐b]‐thiophene core and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile end‐groups, represents the most extensively studied SMA that achieved high performance with various donor polymers.…”

Modulation of Morphological, Mechanical, and Photovoltaic Properties of Ternary Organic Photovoltaic Blends for Optimum Operation

“…Most recently, the PCE of this ternary system was increased to >17.5% and the coherence length of the molecular packing was increased by applying a double annealing protocol by Qin et al. [ 36 ] We surmise that applying this processing protocol could be a possible solution to further boost the mechanical performance of the PM6:N3:PCBM ternary film. Although the device efficiency and crack‐onset strain maximized at the same blend composition in our case, we would like to emphasize that the best efficiency and COS values may not be necessarily obtained in the same blend composition for other material systems.…”

“…They realized a certified PCE of 16.42% by combining PC 71 BM with PM6:N3, which outperforms its binary counterpart PM6:N3. More recently, a double annealing method improved the performance of PM6:N3:PC 71 BM to 17.6%; [ 36 ] the ternary blend system is thus among the best‐performing OPVs. Despite these advances, the mechanical properties of the active layer of these ternary blend OPVs remain poorly understood.…”

Thermodynamic Properties and Molecular Packing Explain Performance and Processing Procedures of Three D18:NFA Organic Solar Cells

“…[ 5–9 ] With Y6 [ 10 ] and its derivatives as acceptors, power conversion efficiencies (PCEs) of over 16% have now been reported by polymer solar cells for PM6‐ and PTQ10‐based devices. [ 11–17 ] Very recently Ding et al. have designed a new polymer donor, namely D18, that achieved a research PCE of 18.2% (institution certified 17.6%) when blended with Y6.…”