Abstract
Organic solar cells (OSCs) are attractive as an alternative to inorganic devices for their easy fabrication and solution-processability. A major and unsolved problem with bulk heterojunction devices remains the optimization of the network morphology. Here, we discuss the influence of the 1,8-diiodooctane (DIO) solvent additive on the efficiency of OSCs and show that by selectively controlling the crystallization of the organic material, the power conversion efficiency (PCE) can be increased by about 30%. For P3HT:PCBM-based devices, the power conversion efficiency (PCE) was increased from 3.7% to 4.9% for PCPDTBT:P3HT:PCBM-based devices from 3.2% to 4.1%. This improvement is due to the higher I, which is in agreement with the higher external quantum efficiency (EQE) observed on the devices fabricated with DIO. We correlate this to an increase of the surface roughness observed with atomic force microscopy (AFM) analysis. We demonstrate that the effect of the DIO additive is equivalent to a high-temperature thermal annealing.