The power conversion efficiency (PCE) of organic photovoltaics (OPVs) has exceeded 18% with narrow bandgap, non-fullerene materials Y6 or its derivatives when used as an electron acceptor. The PCE improvement of OPVs is due to strong photon harvesting in near-infrared light range and low energy loss. Meanwhile, ternary strategy is commonly recognized as a convenient and efficient means to improve the PCE of OPVs. In this review article, typical donor and acceptor materials in prepared efficient OPVs are summarized. From the device engineering perspective, the typical research work on ternary strategy and tandem structure is introduced for understanding the device design and materials selection for preparing efficient OPVs.

Wide bandgap polymer donor PM6 and narrow bandgap polymer acceptor PY-IT were selected to construct all-polymer solar cells (all-PSCs) with a layer-by-layer (LbL) or bulk heterojunction (BHJ) structure. The solvent additive 1-chloronaphthalene (CN) plays a vital role in improving the performance of all-PSCs. The power conversion efficiency (PCE) of LbL all-PSCs is improved to 15.81% from 13.67% by incorporating 1 vol% CN in PY-IT solution, benefiting from the simultaneously enhanced short circuit current density (J_SC) of 22.61 mA cm⁻² and fill factor (FF) of 73.62%. A similar phenomenon is also observed in BHJ all-PSCs whose PCE increased from 13.29% to 15.29% by incorporating 1 vol% CN in PM6:PY-IT blend solution, originating from the optimized phase separation for better exciton dissociation and charge transport in the BHJ active layers. Over 15% PCE improvement can be obtained in the BHJ and LbL all-PSCs by incorporating appropriate additive CN as a morphology regulator. It should be noticed that the PCEs of LbL all-PSCs with CN are higher than those of other all-PSCs, indicating that the LbL processing method combined with the additive should be a prospective strategy to fabricate highly efficient all-PSCs.

A series of layer-by-layer (LbL) ternary organic photovoltaics (OPVs) were prepared with polymer PM1 as a donor, small molecule L8-BO as an acceptor, and a highly crystalline small molecule D18A as the third component. The power conversion efficiency (PCE) of LbL OPVs can be increased from 18.31% to 19.25% by incorporating 30 wt% D18A into the PM1 layer, resulting from the synergistically increased short circuit current density (J_SC) of 27.02 mA cm⁻², open circuit voltage (V_OC) of 0.909 V, and fill factor (FF) of 78.38%. The main contributions of the appropriate D18A on performance improvement can be summarized as follows: (i) serving as the energy donor to transfer its energy to L8-BO for improving exciton utilization, leading to J_SC improvement of ternary LbL OPVs; (ii) acting as a nucleating agent to induce a more ordered molecular arrangement of PM1 for more effective charge transport, resulting in an FF increment of ternary LbL OPVs; (iii) reducing the non-radiative recombination loss for the increased V_OC of ternary LbL OPVs. This work indicates that introducing highly crystalline small molecule material should be an effective method to synergistically optimize the donor and acceptor layers for achieving highly efficient LbL OPVs.