REVERSIBLE, METASTABLE, ULTRAFAST PHOTOINDUCED ELECTRON TRANSFER FROM SEMICONDUCTING POLYMERS TO BUCKMINSTERFULLERENE AND IN THE CORRESPONDING DONOR/ACCEPTOR HETEROJUNCTIONS
The results of comprehensive studies of photoinduced electron transfer from semiconducting (conjugated) polymers to buckminsterfullerene are reviewed. Steady state and femtosecond time-resolved photoinduced absorption (photoexcitation spectroscopy), steady state and picosecond time-resolved photoluminescence, steady state and picosecond photoconductivity, and steady state light-induced electron spin resonance measurements are summarized as experimental evidence which demonstrates ultrafast, long lived photoinduced electron transfer. Comparative studies with different semiconducting polymers as donors demonstrate that in degenerate ground state polymers, soliton excitations form before the electron transfer can occur; thereby inhibiting charge transfer and charge separation. In non-degenerate ground state systems, photoinduced electron transfer occurs in less than 10−12 s , quenching the photoluminescence as well as the intersystem crossing into the triplet manifold. The importance of electron–phonon coupling and structural relaxation following photoexcitation in these quasi-one-dimensional semiconducting polymers is proposed as a principal contribution to the stabilization of the charge separated state. Utilizing thin films of the semiconducting polymer (donor) and buckminsterfullerene (acceptor) to form a heterojunction interface, we have fabricated bilayers which functioned as photodiodes and as photovoltaic cells. The results are discussed in terms of opportunities for solar energy conversion, for photodiode detector devices, and for a variety of other applications which use photoinduced charge separation.