The Impact of Driving Force and Temperature on the Electron Transfer in Donor–Acceptor Blend Systems

We discuss whether electron transfer from a photoexcited polymer donor to a fullerene acceptor in an organic solar cell is tractable in terms of Marcus theory, and whether the driving force Δ<i>G</i>₀ is crucial in this process. Considering that Marcus rates are presumed to be thermally activated, we measured the appearance time of the polaron (i.e., the radical-cation) signal between 12 and 295 K for the representative donor polymers PTB7, PCPDTBT, and Me-LPPP in a blend with PCBM as acceptor. In all cases, the dissociation process was completed within the temporal resolution of our experimental setup (220–400 fs), suggesting that the charge transfer is independent of Δ<i>G</i>₀. We find that for the PCPDTBT:PCBM (Δ<i>G</i>₀ ≈ −0.2 eV) and PTB7:PCBM (Δ<i>G</i>₀ ≈ −0.3 eV) the data is mathematically consistent with Marcus theory, yet the condition of thermal equilibrium is not satisfied. For MeLPPP:PCBM, for which electron transfer occurs in the inverted regime (Δ<i>G</i>₀ ≈ −1.1 eV), the dissociation rate is inconsistent with Marcus theory but formally tractable using the Marcus–Levich–Jortner tunneling formalism which also requires thermal equilibrium. This is inconsistent with the short transfer times we observed and implies that coherent effects need to be considered. Our results imply that any dependence of the total yield of the photogeneration process must be ascribed to the secondary escape of the initially generated charge transfer state from its Coulomb potential

Facile Method for the Investigation of Temperature-Dependent C₆₀ Diffusion in Conjugated Polymers

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C₆₀ molecules from a C₆₀ layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C₆₀ molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C₆₀ can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C₆₀ diffusion is thermally activated and not driven by WFL-type collective motion above <i>T</i>_g but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials