  The ongoing depletion of fossil fuels has led to an intensive search for
additional renewable energy sources. Solar-based technologies could provide
sufficient energy to satisfy the global economic demands in the near future.
Photovoltaic (PV) cells are the most promising man-made devices for direct
solar energy utilization. Understanding the charge separation and charge
transport in PV materials at a molecular level is crucial for improving the
efficiency of the solar cells. Here, we use light-induced EPR spectroscopy
combined with DFT calculations to study the electronic structure of charge
separated states in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerene
derivatives (C60-PCBM and C70-PCBM). Solar cells made with the same composites
as active layers show power conversion efficiencies of 3.3% (P3HT), 6.1%
(PCDTBT), and 7.3% (PTB7), respectively. Under illumination of these
composites, two paramagnetic species are formed due to photo-induced electron
transfer between the conjugated polymer and the fullerene. They are the
positive, P+, and negative, P-, polarons on the polymer backbone and fullerene
cage, respectively, and correspond to radical cations and radical anions. Using
the high spectral resolution of high-frequency EPR (130 GHz), the EPR spectra
of these species were resolved and principal components of the g-tensors were
assigned. Light-induced pulsed ENDOR spectroscopy allowed the determination of
1H hyperfine coupling constants of photogenerated positive and negative
polarons. The experimental results obtained for the different polymer-fullerene
composites have been compared with DFT calculations, revealing that in all
three systems the positive polaron is distributed over distances of 40-60 A on
the polymer chain. This corresponds to about 15 thiophene units for P3HT,
approximately three units PCDTBT, and about three to four units for PTB7. No
spin density...
