Interfacial Nanostructuring of ZnO Nanoparticles by Fullerene Surface Functionalization for “Annealing-Free” Hybrid Bulk Heterojunction Solar Cells

The effect of the fullerene carboxylic acid (PCBA) on the electronic properties of the ZnO nanoparticles (nc-ZnO) has been investigated. The resulting surfaces are characterized by Fourier transform infrared, X-ray photoelectron spectroscopy, and transmission electron microscopy. Fundamental to this work are steady state absorption and photoluminescence measurements, which confirm the presence of appreciable electronic couplings between ZnO nanoparticles and fullerene. Meanwhile, PL quenching and time-resolved absorption measurements of the PCBA–ZnO particles demonstrate efficient charge transfer from PCBA to ZnO. Hybrid bulk heterojunction solar cells based on nanocrystalline ZnO (nc-ZnO) capped with PCBA and regioregular poly­(3-hexylthiophene) are processed from solution and characterized to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active-layer properties and device parameters. Interface modification of a ZnO nanoparticle's surface is conducted to yield an increasing device performance from 0.59% to 1.20% with a short circuit current density (<i>J</i>_sc) of 5.39 mA/cm² by enhancing charge separation and improving compatibility. In addition, the influence of the annealing treatment on the device with surface modification is found to be of no consequence because of the formation of ordered nanoscale aggregates directly as-cast. It also provides an “annealing-free” approach to achieve optimized hybrid nanoscale morphology