Characterization of organic materials for three-dimensional heterojunction solar cell applications by electron paramagnetic resonance.

摘要

The pressure imposed to the environment by the use of fossil energy sources is too high and new alternatives are needed. Plastic (organic) photovoltaics is catching the attention of scientists because it offers interesting perspectives for large area/low cost/environmentally friendly/energy production. The widely used 3-D bulk heterojunction structure in organic solar cells comprises donor/acceptor blends, in which a soluble fullerene derivative (PCBM) acting as acceptor is embedded in a polymer matrix acting as an electron donor. An incident photon can create an exciton in the donor which can be separated into electron and hole at the donor acceptor interface. The electron is allowed in the acceptor while de hole stays in the donor. The light induced charge transfer gives rise to positive radicals on the polymer chains and negative radicals on the fullerene molecules. Electron Paramagnetic Resonance (EPR) is a powerful tool to monitor the charge transfer processes in these blends, and also allowed an in-depth investigation of the radical states in doped P3HT and MDMO-PPV, the standard polymers presently in use in state-of-the-art devices.;Newly synthesized materials aimed to improve the available organic solar cells technologies, namely: materials with a better solubility (in environmentally more friendly solvents), acceptors (as alternative to PCBM) and materials with a lower band-gap (for better sunlight capture). Continuous wave EPR on composites of these conjugated polymers with PCBM allow the detection of efficient charge transfer from donor to acceptor and the discrimination of the g-tensor components of the polaron in the different polymers.;Theoretical studies in PPV and thiophene positive radical states (P +) motivated the study of the polarons formed in MDMO-PPV and P3HT which can contribute to the development of more efficient photovoltaic devices. Extensive pulsed EPR (ESEEM, HSYCORE and pulsed ENDOR) experiments in a range of frequencies revealed hyperfine couplings with nuclei in the vicinity of the radicals formed and allowed the study of the orientation dependent interactions at the molecular level. These EPR results provide valuable information regarding the orientation of these polymers on the spin coating substrates and the extension of the polaron.