In collaboration with researchers at the University of Tokyo and at the University of California in Santa Barbara, a group around Daisuke KITAZAWA at Toray has synthesized a new semiconductor polymer PBDTTT-OFT” that is heat resistent and shows high energy conversion efficiency. PBDTTT – OFT has a skeleton similar to “PBDTTT – EFT (or PTB 7 – Th)” which has been widely used as a material for organic solar cells, but differs by a linear side chain and can form a film with high crystallinity. As a consequence,reduction in conductivity due to heating is smaller than that of conventional PBDTTT-EFT. In a sealed film structure (double sealing film) composed of two layers, a polymer excellent in liquid repellency and in gas barrier property with greatly improved atmospheric stability was realized. Further, when the change in energy conversion efficiency after 5 minutes of placing the polymer on a heated hot plate was measured, it had hardly changed even after heating at 100 ° C.