Emerging Approaches in Enhancing the Efficiency and Stability in Non‐Fullerene Organic Solar Cells

Abstract

the center, side-chains hanging out from the molecular plane, and two strong compact electron-withdrawing end groups. Independent modifications of these three parts provide diverse NFA molecular structures, thus enabling strong absorption in the visible and/or near infrared (NIR) regions, easily tunable energy levels, and finely tunable crystallinity. [28-32] In addition, low voltage losses are a significant feature of NFA OSCs compared to their fullerene counterparts, contributing to the rapidly increasing PCE. [33-36] At the same time, the operational stability of NFA OSCs has also been demonstrated to be promising, [37-41] although further improvement is required to meet the standard for practical applications. Along with the rapid development of materials and devices, the fundamental understanding of OSCs is also moving forward, though at a slower pace compared with that of material development and device engineering. One of the greatest impediments to developing a fundamental understanding of OSCs lies in the bulk heterojunction (BHJ) structure. The invention of BHJ, which is a milestone in OSC research, increases the interfacial area between the electron donor and electron acceptor materials, overcoming the issues of short exciton diffusion length, limited exciton lifetime, and charge separation that limits bilayer junctions. [42] Together with these advantages, the BHJ structure also presents challenges. The multiple phases and complex interfaces bring about sophisticated hierarchical morphologies and complicated charge dynamics, which are difficult for experimental observation and control. In NFA-based systems, some of these challenges have even been manifested. The similarity in the element constitution makes it difficult to spatially distinguish the electron donor and electron acceptor materials. Additionally, the resemblance of energy makes the spectra indistinguishable for charge-transfer (CT) states and singlet excitons. [33] Furthermore, OSC materials are updated so fast that the new materials can be distinct from previous ones, and the characteristics may be entirely different. This review covers emerging approaches for enhancing the efficiency and stability of non-fullerene OSCs. We highlight that recent advances on non-fullerene OSCs result from the coordinated development of donors and acceptors; those who are interested in a comprehensive understanding of donor materials are referred to recent review articles on this topic. [43-49] In this review, we mainly focus on the development of acceptors. We summarize new strategies for high short-circuit current (J SC) and fill factor (FF). A variety of methods, for example, extended absorption and effective morphology control, are discussed. We also introduce design rules for low energy losses, especially by The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non-fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physi...

Achieving Efficient Ternary Organic Solar Cells Using Structurally Similar Non‐Fullerene Acceptors with Varying Flanking Side Chains

“…[ 1–15 ] Recently, non‐fullerene OSCs have been developing rapidly with power conversion efficiencies (PCEs) of over 17% achieved by single‐junction devices. [ 16–22 ] These advances can be largely attributed to the emergence of high‐performance non‐fullerene acceptors (NFAs). [ 23–33 ] Since NFAs have excellent tunability of optical and electrochemical properties, it is feasible to tailor the structures and match with various donor polymers.…”

Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

“…Benefiting from the excellent tunability of the electronic structure and blend morphology for donor (D)-acceptor (A)-type electron acceptors, [1][2][3][4] organic photovoltaics (OPVs) have achieved high power conversion efficiencies (PCEs) above 17% within a short period of time, [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] indicating a bright future for commercial applications, provided that the three key issues, efficiency, cost, and stability, can be synergistically addressed. [20][21][22][23][24][25][26][27][28] Currently, most high-performance D-A acceptors contain an electron-rich core and strong electron-deficient 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) terminals (Figure 1a), 29 recognized as one of the main factors that limit the device lifetime because the exocyclic double bonds formed by the kinetically reversible Knoevenagel condensation reaction (KCR) are highly labile upon photooxidation, [30][31][32][33][34] ZnO-catalyzed photodegradation, 35,36 and base-induced [37][38][39] decomposition (Figure 1a). To increase the device stability, stabilizers, …”

Suppressing Kinetic Aggregation of Non‐Fullerene Acceptor via Versatile Alloy States Enables High‐Efficiency and Stable Ternary Polymer Solar Cells

“…[ 18–20 ] Especially, the slow evolution from the initially optimal BHJ morphologies at metastable state to thermodynamically steady case at equilibrium, the aggregation and/or crystallization of NFAs, and the D/A demixing will induce the severe degradation of exciton dissociation, subsequent charge carrier transport, and extraction and give rise to the serious non‐radiative recombination, thereby shortening the lifetime of OPV devices. [ 21–23 ] Therefore, it is imperative that the facile and effective approach should be utilized to inhibit the related morphological instability.…”