Selenium-Substituted Non-Fullerene Acceptors: A Route to Superior Operational Stability for Organic Bulk Heterojunction Solar Cells

Abstract

Non-fullerene acceptors (NFAs) for organic solar cells (OSCs) have significantly developed over the past five years with continuous improvements in efficiency now over 18%. However, a key challenge still remains in order to fully realize their commercialization potential: the need to extend device lifetime and to control degradation mechanisms. Herein, we investigate the effect of two different molecular engineering routes on the widely utilized ITIC NFA, to tune its optoelectronic properties and interactions with the donor polymer in photoactive blends. Heavier selenium (Se) atoms substitute sulfur (S) atoms in the NFA core in either outer or inner positions, and methyl chains are attached to the end groups. By investigating the effects of these structural modifications on the long-term operational stability of bulk-heterojunction OSC devices, we identify outer selenation as a powerful strategy to significantly increase device lifetime compared to ITIC. Combining outer selenation and methylation results in an impressive 95% of the initial OSC efficiency being retained after 450 h under operating conditions, with an exceptionally long projected half-lifetime of 5600 h compared to 400 h for ITIC. We find that the heavier and larger Se atoms at outer-core positions rigidify the molecular structure to form highly crystalline films with low conformational energetic disorder. It further enhances charge delocalization over the molecule, promoting strong intermolecular interactions among acceptor molecules. Upon methylation, this strong intermolecular interaction stabilizes acceptor domains in blends to be resilient to light-induced morphological changes, thereby leading to superior device stability. Our results highlight the crucial role of NFA molecular structure for OSC operational stability and provide important NFA design rules via heteroatom position and end-group control.

High‐Performance Ladder‐Type Heteroheptacene‐Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions

“…In fact, similar phenomenon of enhanced noncovalent interaction with reduced core‐end‐group dihedrals and shorter interatomic distances for Se⋅⋅⋅O was observed by Labanti et al. in a series of indacenodithieno[3,2‐b]thiophene‐based nonfullerene acceptors [31] …”

Efficient organic solar cells with superior stability based on PM6:BTP-eC9 blend and AZO/Al cathode

“…By which, morphology change, chemical reaction, and energy level mismatch could happen at all function layers or/and their contact, thus degrading the device performance. And in the real application, the performance decay is usually a result of a complex interplay of these multiple processes ( Labanti et al., 2021 ).…”

“…But relatively less has been dedicated to efficient NFA-based systems about this topic ( Du et al., 2020 ). To promote the stability of NFA-based OSCs, different strategies including tailoring of NFA ( Du et al., 2020 ; Hu et al., 2020 ; Labanti et al., 2021 ) and donor ( Zhang et al., 2021 ), designing new active materials ( Bin et al., 2020 ), ternary-blend method ( Yang et al., 2020 ), utilization of stabilizer/dopant ( Linglong Ye et al., 2020 ; Zhang et al., 2019 ), and interface engineering ( Lin et al., 2020 ; Yao et al., 2020 ), have been adopted. Focusing on photo-stability, Brabec ( Du et al., 2020 ) and coworkers systematically studied the PM6/derivatives: IT-4F blend systems.…”

“…With a similar inverted structure, Kim et al. ( Labanti et al., 2021 ), optimized the interaction between ITIC and donor PBDB-T by selenium-substitution on ITIC to stabilized the blend, thus promoting the device photo-stability.…”

“…T80 (80% of the initial PCE) lifetime of $1,300 h can be obtained for optimal inverted devices (ITO/ZnO/blend/MoO 3 /Al) under illumination and open circuit condition. With a similar inverted structure, Kim et al (Labanti et al, 2021), optimized the interaction between ITIC and donor PBDB-T by selenium-substitution on ITIC to stabilized the blend, thus promoting the device photo-stability.…”