Japan Science and Technology Agency (JST) and the University of Tokyo announced on May 16, 2011 that Associate Professor Keisuke Tajima of the University of Tokyo and his colleagues succeeded in improving output voltage of organic thin-film photovoltaic cells by tailoring bilayer interfaces under the support of JST. Details are published online in Nature Materials on May 15, 2011*.
Although thin-film organic photovoltaic cells are featured for the possibilities of flexibility, low cost, and mass production, its challenge is the improvement of conversion efficiency. Interfacial layer between two materials composing the cells is crucial to determine the performance, because light-generated excitons at the interface are separated to electrons and holes to generate electricity.
Researchers connected two films of organic semiconductor P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) by film-transfer method without disturbing their flat surface. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment.
Bilayer organic photovoltaic devices were fabricated with interfacial dipole moments that were selected to align upward or downward at the junction between P3HT and PCBM. The open-circuit voltage of the P3HT/PCBM devices was varied from 0.3 to 0.95 V with the direction of dipole moments. The best result far more exceeded the open-circuit voltage of 0.55-0.6V ordinarily obtained in P3HT/PCBM cells without interfacial modification. Improved open-circuit voltage is realized by tailoring bilayer interfaces.
