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Here, an investigation of three-dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post-treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (ηIQE) of the devices. To investigate the 3D BHJ morphology, novel time-of-flight secondary-ion mass spectroscopy studies are employed along with conventional methods, such as UV-vis absorption, X-ray diffraction, and high-resolution transmission electron microscopy studies. The ηIQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high-performance P3HT:PCBM solar cells using the optimized solvent-annealing method is reported, and these cells show a mean power-conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm−2.

Changes in the nanoscale morphologies of the blend films of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), for high-performance bulk-heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well-ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.

Polymer-based photovoltaic cells, with periodic sub-micrometer structures as an efficient light-trapping scheme, are investigated to improve the performance of organic solar cells based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61. A soft lithographic approach that uses photoresponsive azo polymer films as masters and poly(dimethylsiloxane) as stamps is used to form surface relief gratings (SRGs) on the active layers. The effect of periodic gratings on solar cell performance is precisely investigated according to various grating conditions such as period, depth, and dimension. The solar cells with 1D and 2D SRGs present improved incident-photon-to-current conversion efficiencies and an overall increase in power conversion efficiencies, primarily resulting from the enhancement of short-circuit current density, indicating that periodic structures induce further photon absorption in the active film.