Effect of thermal annealing on the structure of the small molecule (electro-donor) DRCN5T: tunneling spectroscopies analysis

Post-treatments of the organic solar cells (OSCs) active layers, such as thermal, solvent vapor and applied external electric field, can lead to very different morphologies that directly impact the power conversion efficiency (PCE) of the OSCs. These post-treatments could improve bulk morphology, which reduce charge carrier recombination and, therefore, improving the photovoltaic performance of bulk heterojunction (BHJ). Here, thin solid films of the small molecule DRCN5T and DRCN5T:[70]PCBM blend, were analyzed through scanning tunneling microscopy (STM) and spectroscopy (STS) after different temperatures treatments. DRCN5T is an electro-donor compound used in the OSCs active layer, which reach ~10% of PCE. These very thin solid films (monolayers) exhibit a worm-like pattern without thermal annealing (TA) (amorphous behavior), however after applying TA at 120°C, the small molecule crystallizes: its structural geometry becomes a well-defined organized one.