the interface between an electrode and a photoactive layer, an interfacial layer that is capable of selectively transporting electrons or holes is required. [6][7][8][9][10][11][12] Generally, transition metal oxides such as zinc oxide (ZnO), [13] titanium dioxide (TiO 2 or TiO x ) [14] and tin oxide (SnO 2 or SnO x ) [15] are used as interlayer materials because of their optical transparency, high electron mobilities, and chemical stability. [16] Usually, these interlayer materials are deposited using vacuum deposition techniques such as atomic layer deposition, electronbeam deposition, and sputtering. [17][18][19][20][21] However, these vacuum deposition techniques can damage the organic materials in the photoactive layers.To overcome this limitation, solution processing of interfacial layers has been introduced because of its advantages over the vacuum processes, such as less destructive fabrication, large-area manufacturing, low costs, and a high output production. [22][23][24][25][26] However, solution processes mostly involve sintering for hydrolyzing the metal oxide precursors, and sintering requires high temperatures that affect the morphological and chemical stability of the organic photoactive layers. [27,28] In addition, the sintering temperatures are usually higher than the glass temperature of plastic substrates, resulting in a limited substrate selection. Therefore, it is necessary to develop a solution processable transition metal oxide nanoparticle that does not require a high-temperature hydrolysis process. [22,[29][30][31][32][33][34][35] One method is to synthesize highly dispersive metal nanoparticles in an organic solvent, and then coating these nanoparticles directly onto the photoactive layer. [36][37][38] Recently, we reported the preparation of highly dispersive titanium oxide (TiO 2 ) nanoparticles (TNPs) by bounding TiO 2 with organic ligands. [31,39] We found that combining the TNPs with an organic ligand containing a phenyl group stabilized the surface of the TNPs and formed a robust electron transporting layer (ETL), because of the strong π-π interactions between the particles. These TNPs were well dispersed in organic solvents, easily spin-coated onto a photoactive layer at room temperature and utilized as efficient ETLs in OPVs. 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, TiO 2 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 ...