Over 19.2% efficiency of layer-by-layer organic photovoltaics enabled by a highly crystalline material as an energy donor and nucleating agent

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.