Kyoto University and Japan Science and Technology Agency announced on April 27 2011 that a research group led by Associate Professor Hideo Ohkita of Kyoto University has succeeded in improving the efficiency of dye-sensitized polymer solar cells, and confirmed the principle of selective dye loading. Details are published in online version of Advanced Energy Materials*.
Although polymer solar cells are featured for mechanical flexibility and low cost possibility, conversion efficiency of the cells has been low because of the light absorption limited to the wavelength around 650 nm. Selective dye loading at the polymer/fullerene interface to broaden the spectral response has not been successful due to the clustering of the dye.
The researchers' idea was to use surfactant to segregate dye effectively at the interface by utilizing the difference in surface energy of components, poly(3-hexylthiophene) (P3HT), a fullerene derivative (PCBM), and a light-harvesting dye, silicon phthalocyanine derivative (SiPc). The photocurrent density and power conversion efficiency of the ternary blend solar cells were most improved by loading SiPc with a content of 4.8 wt%. The absorption and surface energy measurements suggested that SiPc is located at the P3HT/PCBM interface. Such interfacial segregation of dye molecules can be rationally explained in terms of the surface energy of each component similar to the efffect of surfactant in soap. Crystallization of P3HT by annealing resulted in additional enhancement of segregation.