“Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3‐hexylthiophene) and Methanofullerenes

Abstract

The self‐organization of the polymer in solar cells based on regioregular poly(3‐hexylthiophene) (RR‐P3HT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) is studied systematically as a function of the spin‐coating time ts (varied from 20–80 s), which controls the solvent annealing time ta, the time taken by the solvent to dry after the spin‐coating process. These blend films are characterized by photoluminescence spectroscopy, UV‐vis absorption spectroscopy, atomic force microscopy, and grazing incidence X‐ray diffraction (GIXRD) measurements. The results indicate that the π‐conjugated structure of RR‐P3HT in the films is optimally developed when ta is greater than 1 min (ts ∼ 50 s). For ts < 50 s, both the short‐circuit current (JSC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < ts < 55 s, the JSC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered π‐conjugated structure of RR‐P3HT to 1.2 % for a less‐ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon‐to‐electron conversion efficiency spectrum of the less‐ordered solar cell shows a clear loss in both the overall magnitude and the long‐wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open‐circuit voltage (VOC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.

On the morphology of polymer‐based photovoltaics

“…48 Li et al used a slow drying process to control the P3HT crystallization and phase separation. 64,65 They found that P3HT was better ordered, as evidenced by the enhanced absorption. By using time-of-flight mobility measurements, a more balanced charge transport was also obtained for the slow-dried samples.…”

Carbon nanotubes for organic/inorganic hybrid solar cells

“…[20]. The photoactive layers were "solvent annealed" overnight and dried at 110 °C for 10 min according to [97]. Thickness of the P3HT/PCBM layer was ~120 nm.…”

Plasmon-loss imaging of polymer-methanofullerene bulk heterojunction solar cells

“…Following exciton dissociation the free carriers must have contiguous percolation pathways to drift/diffuse towards the collection electrodes, a process which is favoured by larger sized domains. Processing methods such as thermal, 2 solvent, 3 annealing, and solvent additives 4 improve device efficiency by modifying the active layer morphology. Transmission electron microscopy (TEM) is an ideal technique for probing morphology at nanometre length scales, but in the case of BHJ solar cells imaging is complicated by the low contrast between the carbon-rich donor and acceptor phases.…”