Nanoscale structure-property relationships in low-temperature solution-processed electron transport layers for organic photovoltaics

Here we elucidate the nanostructure-property relationships in low-temperature, solution-processed ZnO based thin films employed as novel electron transport layers (ETLs) in organic photovoltaic (OPV) devices. Using a lowcost zinc precursor (zinc acetate) in a simple amine-alcohol solvent mix, high-quality ETL thin films are prepared. We show that at a processing temperature of 110 °C the films are composed of nanoparticles embedded in a continuous organic matrix consisting of ZnO precursor species and stabilizers. Using a combination of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), we study the thermally induced morphological and compositional changes in the ETLs. Transient optoelectronic probes reveal that the mixed nanocrystalline/amorphous nature of the films does not contribute to recombination losses in devices. We propose that charge transport in our low-temperature processed ETLs is facilitated by the network of ZnO nanoparticles, with the organic matrix serving to tune the work function of the ETL and to provide excellent resistance to current leakage. To demonstrate the performance of our ETLs we prepare inverted architecture OPVs utilizing Poly[[4, 8-bis[(2-ethylhexyl)oxy]benzo[1, 2-b:4, 5- b′]dithiophene-2, 6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3, 4-b]thiophenediyl]] (PTB7): [6, 6]-Phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as active layer materials. The low-temperature ETL devices showed typical power conversion efficiencies (PCEs) of >7% with the champion devices achieving a PCE > 8%.