Spin signatures of photogenerated radical anions

By Jared SagoffOctober 1, 2010

Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. In analogy with natural photosynthesis, the key steps in PV solar energy conversion are the generation, separation and extraction of charges.

PV systems can be classified in three groups based on the active media: inorganic, organic, and hybrid devices. In spite of the fact that organic-based PVs have not demonstrated competitively high conversion efficiency so far, they are still considered an option with many attractive features—like low-cost fabrication and tunability of electronic properties of organic materials.

By blending the conjugated polymer, as P3HT, and fullerenes, C60-derivative C60-PCBM, efficient light-induced charge separation (CS) can be achieved, thus making these materials attractive for solar energy conversion.

One of the downsides of the C60-PCBM acceptor material for photovoltaic applications is a very low absorption coefficient in the visible spectral region and, as a consequence, a relatively small contribution to the photocurrent. The substitution of the C60 by C70 fullerene considerably improves the photocurrent due to the low symmetry of the C70 molecules and thus higher absorption coefficient in the visible region. However, there is little known on the photophysics in C70 containing composites.

We carried out detailed investigation of charged polarons in thin films of polymer-fullerene composites by light induced electron paramagnetic resonance (EPR) at 9.5 GHz (X-Band) and 130 GHz (D-Band). The materials studied were poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester (C60-PCBM) and two different soluble C70-derivates: C70-PCBM and diphenylmethano[70]fullerene oligoether (C70-DPM-OE).

The first experimental identification and characterization of the negative polaron localized on the C70-cage in polymer-fullerene bulk-heterojunctions has been obtained. When recorded at conventional X-band EPR, this signal is overlapping with the signal of the positive polaron, which does not allow for its direct experimental identification. Owing to the superior spectral resolution of the high-frequency D-Band EPR, we were able to separate light-induced signals from P+ and P- in PHT-C70 bulk-heterojunctions.

Comparing signals from C70-derivatives with different side-chains, we have obtained experimental proof that the polaron is localized on the cage of the C70 molecule. The obtained g-tensor parameters are of importance as these are the characteristics of the structure, symmetry, and dynamics of the localized/delocalized unpaired spin states.

This research is a collaboration between Argonne National Laboratory, the University of Wurzburg and Bavarian Centre for Applied Energy Research, (Germany) and the University of Madrid (Spain). Argonne's portion of this work was supported by the U.S. Department of Energy.