Solution-Processed TiO₂ Nanoparticles Functionalized with Catechol Derivatives as Electron Transporting Layer Materials for Organic Photovoltaics

For organic photovoltaics (OPVs), the electron transport layer (ETL) material is crucial for collecting and transporting the electrons from the active material toward the electrode. In this study, TiO₂ nanoparticles (TNPs) are functionalized with a series of catechol (CA) derivatives possessing different electrophilicities, i.e., CA; CA attached to an electron-withdrawing cyano group, 3,4-Dihydroxy benzonitrile (CA-CN); and CA attached to an electron-donating methoxy group, 4-Methoxybenzene-1,2-diol (CA-OMe), and the resulting solution-processed films are applied as ETLs. The calculated energy level shows that the lowest unoccupied molecular orbital (LUMO) is lowered by the cyano group and raised by the methoxy group, and it forms different cascade energy levels in between photoactive layer and electrode. As a result, the V_OC of each device utilizes TNP CA, TNP CA-CN, and TNP CA-OMe as ETLs is increased to 0.82, 0.73, and 0.65 V as compared to without ETL (0.63 V), respectively. Under the optimized conditions, a power conversion efficiency (PCE) of 10.80 % is achieved for OPVs containing TNP CA as ETLs. The PCE improvement could be mainly attributed to the low recombination rate in between active layer and electrode interface. The findings of this study lead a valuable guide for designing a ligand used for the functionalized TNPs.