"Solvent annealing" effect in polymer solar cells based on poly(3-hexylthiophene) and methanofullerenes

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 t_s (varied from 20-80 s), which controls the solvent annealing time t_a, 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 t_a is greater than 1 min (t_s-50 s). For t_s < 50 s, both the short-circuit current (J_SC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < t_s < 55 s, the J_SC 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 (V_OC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.