By using the one-step method, the ternary blends combining D18 with newly synthesized acceptors like BTP-Cy-4F^[44], QX-α^[45], BTP-TCl^[46], CH8F^[47] and AQx-18^[48] (or using donors like PJ-1^[49], D18-Cl^[50]) demonstrated potentials for over 19% efficiency. Peng et al. developed novel non-fullerene acceptor BTP-Cy-4F, featuring outer branched chains and inner cyclohexane side chains^[44]. This design reduced intermolecular aggregation, leading to wider bandgaps and elevated lowest unoccupied molecular orbital (LUMO) energy levels. Blending BTP-Cy-4F with D18 and 15 wt% BTP-eC9 in a ternary blend, a PCE of 19.36% was achieved. Besides, Liu et al. made PM6:D18:L8-BO OSCs with a double-fibril network morphology^[23]. Owing to long exciton diffusion length of D18 and reduced recombination, a PCE of 19.6% was obtained.

Facing the solubility issue of high molecular weight D18 (HW-D18), Bo et al. devised a high vapor pressure method (HP-method)^[22]. They heated HW-D18 in chloroform in a sealed vial at 100 °C, enhancing the solubility by increasing boiling point. Thus, HW-D18:L8BO OSCs gave a record PCE of 19.65% for binary OSCs (Fig. 1). Very recently, two new acceptors were developed, i.e., core fluorination (AQx-2F) and fluorination at the tail of long inner alkyl chains (DT-C8Cl)^{[41, 43]}. D18:AQx-2F and D18:DT-C8Cl binary OSCs gave PCEs of 19.7% and 19.4%, respectively.

The deposition technique for the active layer in OSCs, from a methodological standpoint, encompasses both one-step and two-step approaches, and the two-step process is a layer-by-layer (LbL) deposition or sequential deposition. The one-step method involves creating a blend of donor and acceptor materials that are simultaneously deposited, allowing for the formation of a bulk heterojunction (BHJ) that leads to efficient charge separation. On the other hand, the two-step method involves the sequential deposition of donor and acceptor materials, which can precisely control over the layer thickness and composition, leading to optimized charge-transport pathways^[38−40]. It is quite significant that D18 devices made with one-step or two-step methods offer over 19% PCEs^{[41, 42]}. These achievements underscore the versatility and potential of D18 in advancing the performance of OSCs, marking a great milestone.

The two-step process involves sequentially depositing donor and acceptor solutions, enabling independent optimization of the microstructure in each layer^[51]. By using two-step method, Huang et al., Bo et al., and Liu et al. independently reported PCEs of 19.05%, 19.05%, and 19.10%, respectively, in binary cells with D18 and L8-BO^[52−54]. Similarly, Gao et al. achieved a PCE of 19.30% from OSCs with D18-Cl and L8-BO made by a LbL method^[55]. Additionally, Bo et al. achieved a PCE of 19.21% from HW-D18/L8-BO OSCs by using HP-method combined with a LbL process ^[22].

In 2020−2021, D18 and its derivatives, distinguished as a significant group of polymer donors, were first reported by Ding et al.^{[8, 9, 21]}. Blending with Y6, D18 demonstrated >18% PCE, marking a notable advance in the field of organic photovoltaics. Since its initial demonstration, D18 and its derivatives have exhibited exceptional performance across a variety of applications, including highly efficient single-junction devices^{[22, 23]}, stretchable electronics^[24], thick-film devices^[25], efficient all-polymer systems^{[26, 27]}, indoor photovoltaics^{[28, 29]}, devices with enhanced stability^[30−32], green solvent-processed devices^{[33, 34]}, and tandem devices^[35−37]. This article will present the strategies employed with D18 and its derivatives in single-junction OSCs for achieving PCEs above 19% (Fig. 1).

By using one-step or two-step approach, spin-coated single-junction OSCs with D18 or its derivatives give over 19% PCE. However, shifting spin-coating technique to large-area printing technique for producing efficient solar modules is still challenging, especially when we will not use low-boiling-point solvents. More efforts are required in materials development.

Jen et al. has reported non-fullerene acceptors such as BS3TSe-4F^[56], T9TBO-F^[57], and 3-ClTh^[58] with PCEs over 19% in ternary LbL OSCs (Fig. 1). Moreover, Zhang et al. incorporated D18 into the bottom donor layer to build a ternary PM1:D18/L8-BO device^[59]. This configuration offered a PCE of 19.13%. Interestingly, Peng et al. made OSCs with a double-BHJ active layer, via sequentially depositing D18-Cl:BTP-eC9 and PM6:L8-BO^[42] layers. This design enhances the light absorption and maintains the optimal morphology, bypassing the complex optimization required for quaternary blends. A PCE of 19.61% was achieved.

In recent years, organic solar cells (OSCs) have garnered significant attention due to their distinctive attributes, such as flexibility, lightweight, and solution processing, which position them as alternatives for next-generation solar technologies^[1−5]. Thanks to breakthroughs in materials development, the power conversion efficiency (PCE) for single-junction OSCs has already surpassed 19%^[6−13]. The development of photoactive materials is pivotal in enhancing the PCEs, and several reviews have provided insights into materials design^[14−18]. Herein, we highlight single-junction OSCs based on D18 and its derivatives^{[19, 20]}.