End-Group Engineering of Nonfullerene Acceptors for High-Efficiency Organic Solar Cells

Abstract

Conspectus In recent years, organic solar cells (OSCs) have made significant advancements due to a deeper understanding of molecular design and device technology. One area of molecular design that has contributed to these advancements is the emergence of nonfullerene small-molecule acceptors (SMAs) and polymerized SMAs. The molecular design strategy of state-of-the-art SMAs focuses on two aspects: the electron-rich central core unit and electron-deficient end groups. Different from the manipulation of the central cores, end-group engineering is a direct and efficient means to adjust the physicochemical properties and crystallization/aggregation behavior of acceptors, leading to enhanced photovoltaic performance. On the basis of our recent research advances, herein we focus on the topic of end-group engineering of nonfullerene acceptors, aiming to provide a comprehensive understanding of the optimization of end groups for the design of high-performance acceptor materials. In this Account, first, we systematically compare the difference between thiophene-fused and benzene-fused end groups in synthetic routes and molecular energy levels. Unlike the centrosymmetric benzene, the axisymmetric thiophene-fused end groups have two different fusion modes, resulting in their different frontier orbital energy levels. Second, we offer a wrought review of SMAs with thiophene-fused or thiophene derivatives-fused end groups, emphasizing the important role of thiophene derivatives-fused end groups in enhancing molecular packing, improving exciton bonding energy, and reducing energy loss in OSCs. Additionally, we reveal the specific reason why the thiophene-fused end group with an α/β fusion site and the thiophene-fused end group with a β/γ fusion site have significantly different molecular energy levels. Third, we summarize the photovoltaic parameters and conventional physicochemical properties of polymerized SMAs based on monobromobenzene-fused end groups and fluorobromine (or chlorobromide) cosubstituted benzene-fused end groups. We demonstrate that regioregular polymerized SMAs show great prospects in realizing high-performance all-polymer solar cells by eliminating the disorder of molecular backbone structure with pure monobromobenzene-fused end groups. Furthermore, the halogenation strategy (fluorination and chlorination) is also an effective method for designing high-performance PSMAs with large electron mobility induced by the intermolecular noncovalent interactions of halogen···H, halogen···S, and halogen···halogen. Finally, we analyze the role of asymmetric end group substitution for developing high-performance SMAs. In comparison with symmetric SMAs, the asymmetric one achieves low energy loss while ensuring sufficient charge separation. As a summary and perspective, we discuss the current questions regarding end groups and propose our insights into the future development of nonfullerene acceptors with novel end groups toward low-cost and high-performance OSCs.

High‐Performance Ternary Organic Solar Cells with Enhanced Luminescence Efficiency and Miscibility Enabled by Two Compatible Acceptors

“…As well‐known, the Δ E 3 of 0.169 eV is among the best values for highly efficient OSCs with PCE over 17% in the literature. [ 21,34,44,46–62 ]…”

“…[ 1–13 ] Extensive efforts in material design and device structure have led to substantial improvements in the performance of OSCs. [ 14–31 ] In this regard, the emergence of acceptor‐donor(acceptor1)donor‐acceptor (A‐DA 1 D‐A) type small‐molecule acceptors (SMAs) has played a pivotal role, given their widespread utilization as acceptors in high‐efficiency binary OSCs. [ 25,32,33 ] To further improve power conversion efficiencies (PCEs), the rational design and incorporation of a third component into the binary host system to create a ternary device has become crucial.…”

Regulating the Electron‐Deficient Component in A‐DA1D‐A Typed Small‐Molecule Acceptors for High‐Performance Organic Solar Cells†

“…In recent years, organic solar cells (OSCs) have garnered significant attention as the third generation of solar cells. [1][2][3][4][5][6][7][8][9][10][11][12] In contrast to conventional inorganic solar cells, OSCs boast several distinct advantages, including remarkable flexibility, affordability, transparency, and straightforward fabrication processes.…”

Designing a Novel Wide Bandgap Small Molecule Guest for Enhanced Stability and Morphology Mediation in Ternary Organic Solar Cells with over 19.3% Efficiency

“…[ 7–10 ] Due to the emergence of Y‐series acceptors and novel molecules, coupled with the continuous refinement of device engineering, organic photovoltaics have made significant progress in recent years. [ 11–13 ] Although OSCs have achieved power conversion efficiency (PCE) exceeding 19%, challenges related to device stability and large‐scale fabrication persist. [ 14–21 ] Recent literature has shed light on incorporating the third component as a viable strategy to enhance both the PCE and stability, placing ternary OSCs under an intensified spotlight.…”