A simple strategy passivating ZnO electron transport layers for realizing high efficiency in organic solar cells

Sol-gel processed ZnO films are widely used as electron transport layers (ETLs) in organic solar cells (OSCs). However, defects caused by -COOH, -OH, and vacancies of oxygen in ZnO ETLs are harmful for electron extraction and transport processes in OSCs. Here, a simple and effective strategy is reported to passivate free -OH via hydrogen bond of HO...NH2. A small molecule of biuret is introduced into ZnO precursor as an additive to form ZnO films. The hydrogen bond of HO...NH2 is confirmed by oS and FT-IR in this study, which passivates traps originated from free -OH. The passivated ZnO ETLs exhibit better morphology and higher electron mobility than pristine ZnO ETLs. In OSCs with fullerene and non-fullerene light absorbers, the cells with biuret-doped ZnO ETLs obtain improved power conversion efficiency (PCE), which prove the generality of biuret doped ZnO ETLs. Eventually, a maximal PCE of 18.19 % is obtained when PM6:L8-BO is selected as light absorber. Furthermore, the contributions of biuret are discussed in this work.