Optical and magnetic resonance studies of organic materials used in photovoltaic applications

In this work we focused on the electronic processes in active materials used in organic photovoltaics. Films of several electron donors, acceptors, and their blends were investigated by means of steady state optical and magnetic resonance probes. The efficiency of organic photovoltaics depends on film morphology, charge mobility and light absorption. Therefore we studied common donor materials with very different morphology: RR P3HT (regioregular poly(3-hexylthiophene)) and RRa P3HT (regio-random poly(3-hexylthiophene)). The charge transport is affected by regioregularity and molecular weight. Consequently, we examined RR P3HT polymers with various molecular weights. We learned that a polaron band at low photon energy only appears in the photoinduced absorption spectrum of low molecular weight RR P3HT. We studied two main approaches for improving the efficiency of organic photovoltaics: modifying the lowest (highest) unoccupied (occupied) molecular orbital, LUMO (HOMO) of the donor (acceptor) materials; as well as synthesizing polymer donors with low optical gap. TAES-V is a low-band gap polymer composed of three co-polymers having the structure of „donor-acceptor-donor?. Its record power conversion efficiency (~7%) when blended with PC70BM is partially due to the significantly red-shifted absorption. Our results show that an intrachain charge transfer exciton (CTE) is long-lived in this polymer and that it persists in the blend with PC70BM. In addition we studied three fullerene derivatives. The LUMO of a fullerene derivative can be changed by the addition of functional side groups to the fullerene cage that improves the organic solar cells performance. The addition can also lead to hindering of aggregation in the films, which consequently decreases the charge transport in solar cells. In the study of polymer/fullerene blends we mixed RR P3HT with three different fullerene derivatives. We conclude that higher power conversion efficiency of a blend is mainly due to the higher LUMO level and improved open circuit voltage. We also compared DOO-PPV (H-polymer) with DOO-PPV enriched with deuterium (D-polymer). We show that hyperfine interaction is weaker in the D-polymer and that the spin relaxation rate is four times smaller than in H-polymer. Consequently, the longer spin diffusion length makes the D-polymer better suited for higher performing organic spin-valves.