In 2015, a nonfullerene acceptor (NFA) ITIC was reported by Zhan et al., bringing OSC to a new era. Many efficient NFAs have been developed, and the PCEs of solar cells have soared to ~19%^{[1, 2]}. NFAs always exhibit much low optical bandgap with strong absorption in 600–900 nm, so the development of wide-bandgap (WBG) polymer donors with good light-harvesting ability in 400–700 nm is desirable^[8-12]. The pairing of WBG polymer donors and low-bandgap (LBG) NFAs presents reduced voltage loss (V_loss), and the highest occupied molecular orbital (HOMO) offset between donor and acceptor can be very small, even close to zero^{[2, 3]}.

Currently, most efficient polymer donors are synthesized through multi-step reactions, exhibiting high cost. Cheap and high-performance polymer donors well matching those LBG NFAs are needed^[25-30]. We are expecting single-junction OSCs with >20% PCE.

Organic solar cells (OSCs) as a promising photovoltaic technology have attracted great attention due to its unique advantages, such as solution processing, low cost, lightweight and excellent mechanical flexibility^[1-5]. Conventional OSCs always employ fullerene derivatives, e.g. PC₆₁BM, PC₇₁BM and IC₇₀BA, as electron acceptors. Fullerene derivatives present weak absorption in the visible region, while polymer donors show excellent light-harvesting ability in the visible and even near-infrared (NIR) regions. Many medium- or low-bandgap polymer donors have been developed for complementary absorption, and the power conversion efficiencies (PCEs) for fullerene-based OSCs reach ~12%^{[6, 7]}.

The polymer donors mentioned above are based on BDTT donor unit, and they need complex syntheses. Li et al. reported a low-cost polymer donor PTQ10 with thiophene and 6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline units, which was synthesized from commercial materials via a two-step synthesis (yield 87.4%). PTQ10:BTP-FTh:IDIC cells demonstrated a PCE of 19.05%^[2].

BDTT is one of the best building blocks in constructing D-A conjugated polymers^[13]. The two-dimensional conjugated structure and weak electron-donating ability endow its copolymers with good hole mobilities and low HOMO energy levels. In 2015, Hou et al. reported PM6 (PBDB-TF), and its fullerene solar cells gave a 9.2% PCE (Fig. 1). PM6 offered over 18% PCE when blending with Y-series NFAs^[13]. PM6 works very well with most NFAs, and has become one of the best commercial polymer donors. In addition, the chlorinated derivative PM7 (PBDB-TCl) achieved over 17% PCE in PM7:Y6 cells^[14]. What's more, some donor or acceptor units as the third component were introduced into PM6. Efficient terpolymer donors were obtained by using random D-A copolymerization to tune the energy levels and absorption. Li et al. introduced an electron-withdrawing unit 2,5-bis(4-(2-ethylhexyl)thiophen-2-yl)pyrazine into PM6 backbone to get a D-A₁-D-A₂ type terpolymer PMZ-10. PMZ-10:Y6 solar cells gave a PCE of 18.23%^[15].

In 2020, Ding et al. reported a milestone WBG polymer donor D18 based on DTBT unit with large molecular plane and strong electron-withdrawing capability^{[1, 16]}. D18:Y6 cells offered a PCE of 18.22%, with an open-circuit voltage (V_oc) of 0.859 V, a short-circuit current density (J_sc) of 27.70 mA/cm² and a FF of 76.6%, which was the first report on single-junction OSCs with over 18% efficiency^[1]. Then, the chlorinated analogue D18-Cl was reported. D18-Cl:N3 cells and D18-Cl:N3:PC₆₁BM cells delivered PCEs of 18.13% and 18.69%, respectively^{[17, 18]}. Later, D18-B and D18-Cl-B were also developed via side-chain engineering. D18-B:N3:PC₆₁BM and D18-Cl-B:N3:PC₆₁BM cells offered PCEs of 18.53% and 18.74%, respectively^[19]. D18 derivatives have been developed and present good performance^{[20, 21]}.

Hou et al. reported two dithieno[3,2-f:2′,3′-h]quinoxaline (DTQx)-based polymer donors PBQx-TF and PBQx-TCl with fluorinated or chlorinated BDTT as the donor units^{[22, 23]}. 19.0% and 18.0% PCEs were achieved for PBQx-TF:F-BTA3:eC9-2Cl and PBQx-TCl:BTA3:BTP-eC9 cells, respectively. Very recently, Hou et al. developed a WBG polymer donor PB2F containing fluorinated BDTT and 1,3,4-thiadiazole units with a very deep HOMO level of –5.64 eV. PB2F:PM6:BTP-eC9 cells gave a PCE of 18.6%^[24].

J. Cao thanks the National Natural Science Foundation of China (21604021), Hunan Provincial Natural Science Foundation (2018JJ3141), and the Innovation Team of Huxiang High-level Talent Gathering Engineering (2021RC5028). L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600), the National Natural Science Foundation of China (51922032 and 21961160720), and the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02) for financial support.