Organic solar cells are a premier candidate for cost-effective, industrial-scale, renewable energy production. However, while it is necessary for the power conversion efficiencies of such devices to improve in order for this vision to be fully realized, many of the crucial physical processes occurring within them are not yet completely understood. Focusing on the newest generations of materials, we investigated the charge generation, transport, and recombination dynamics in optimized, thin film bulk heterojunction (BHJ) solar cells through time-resolved transient photocurrent (TPC) measurements. This technique is capable of yielding detailed insights into the carrier mobility, different recombination mechanisms, the carrier density, and the density of states in ways that are largely inaccessable via most steady state approaches.

The design and construction of a high-speed, high-resolution transient photocurrent system is presented. Subsequent TPC measurements on optimized polymer and small molecule solar cells revealed that the charge carrier generation profile in efficient devices is largely independent of the internal electric field, which allowed for accurate quantification of the competition between charge carrier sweep-out and recombination in such cells. In addition, geminate recombination via charge transfer excitons was found to be generally absent as a photocurrent loss mechanism, which implies that the charge transfer state intermediary in the carrier generation process does not have a binding energy significantly greater than thermal energy.

Charge carrier transport through BHJ solar cells is fundamentally dispersive due to the disordered structure and complex film morphology within the photoactive layer. The resulting distribution of carrier mobilities is dependent on the density of states, and has a profound impact on device performance. Unfortunately, currently available measurement techniques yield only a single number for cross-cell carrier mobility, which is insufficient when viewed in the context of understanding transport through dispersive materials. Here we present a novel application of transient photocurrent and short circuit variable time-delayed collection field measurements. This unique combination allowed us to reconstruct the complete mobility distribution for the photogenerated carriers in optimized BHJ solar cells, enabling a much richer visualization of the transport dynamics than was previously possible. We demonstrate the versatility of these techniques on optimized devices comprising polymer and small molecule donors, as well as conventional and inverted architectures.