Efficient Poly(3-hexylthiophene)-Based Bulk Heterojunction Solar Cells Fabricated by an Annealing-Free Approach

A simple method for preparing spin-coated photoactive films with high crystalline poly(3-hexylthiophene) (P3HT) without annealing was developed. It involves the incorporation of a small amount of the P3HT monomer, 3-hexylthiophene (3HT), into the P3HT/[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) solution from which the films were prepared. The UV-visible spectrum of the thus-prepared film includes pronounced crystallization peaks with higher absorbance over the whole range of wavelengths, 450-650 nm, than that of the film prepared from a coating solution without 3HT. X-ray diffraction (XRD) data also reveal a smaller interlayer distance between conjugated backbones in the 3HT-processed film than the conventional film. The TEM image of the as-cast 3HT-processed film demonstrates a nanoscaled phase separation and interpenetrating network morphology like those of a thermally annealed conventional film. Polymer solar cells based on such 3HT-processed bulk heterojunction films exhibit an enhanced photocurrent and fill factor without any pre- or postannealing treatment and a similar power conversion efficiency and incident photo-to-electron conversion efficiency to those of conventional annealed solar cells, indicating the feasibility of this annealing-free approach for fabricating high-efficiency polymer solar cells.