Di(2-pyridyl)ketone stabilized titanium dioxide nanoparticles for the room temperature processed electron transporting layer in organic photovoltaics

Abstract

Monodisperse TiO2 nanoparticles are obtained through hydrolysis of titanium butoxide in the presence of di(2-pyridyl)ketone (DPK) and para-toluenesulfonic acid. The DPK ligand stabilized the surface of the TiO2 nanoparticle through complexation with TiO2. DPK stabilized TiO2 (TNP-DPK) can be dispersed without aggregation in alcoholic solutions at concentrations higher than 1 M. The synthesized particles have 4-5 nm size and spherical shape at amorphous phase. The atomic force microscope image reveals that only 0.1 wt% of TNP-DPK solution can form a conformal and dense film on top of the active layer. The TNP-DPK solution is used for an electron transporting layer (ETL) in organic photovoltaics (OPV) that utilizing poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) as a photoactive layer. The power conversion efficiency of the OPV is significantly improved from 7.18% to 9.08% by inserting the TNP-DPK layer between photoactive layer and Al electrode. Based on the internal quantum efficiency and transient photo voltage experiments, the TNP-DPK layer is found to improve the charge collection efficiency and reduce the charge recombination at the active layer/electrode interface. These results clearly show that the solution processed TNP-DPK layer can play a role as an efficient ETL in OPVs. (C) 2015 Elsevier B.V. All rights reserved.