Optical spacers based on metal oxide layers have been intensively studied in poly(3‐hexylthiophene) (P3HT) based polymer solar cells for optimizing light distribution inside the device, but to date, the potential of such a metal oxide spacer to improve the electronic performance of the polymer solar cells simultaneously has not yet be investigated. Here, a detailed study of performance improvement in high efficient polymer solar cells by insertion of solution‐processed ZnO optical spacer using ethanolamine surface modification is reported. Insertion of the modified ZnO optical spacer strongly improves the performance of polymer solar cells even in the absence of an increase in light absorption. The electric improvements of the device are related to improved electron extraction, reduced contact barrier, and reduced recombination at the cathode. Importantly, it is shown for the first time that the morphology of optical spacer layer is a crucial parameter to obtain highly efficient solar cells in normal device structures. By optimizing optical spacer effects, contact resistance, and morphology of ZnO optical spacers, poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6diyl] [3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl] thieno[3,4‐b]thiophenediyl]] (PTB7):[6,6]‐phenyl‐C71‐butyric acid (PC70BM) bulk heterojunction solar cells with conversion efficiency of 7.6% are obtained in normal device structures with all‐solution‐processed interlayers.

A comparative study of the optical properties of two well-known copolymers for photovoltaic applications, the PTB7 and the PTB7-Th, is presented. This study contributes to the understanding of the photophysics and permits correlation of the morphological change to the optical properties. For this purpose, Variable Angle Spectroscopic Ellipsometry (VASE) measurements are performed to determine the optical transitions as well as the band gap of PTB7 and PTB7-Th thin films from the imaginary parts of the dielectric functions. Then, the molecular aggregations in chlorobenzene solutions are investigated by means of absorbance and photoluminescence measurements as a function of concentration and temperature. These solution measurements are compared to film measurements in order to isolate interchain and intrachain optical transitions.