Organic photovoltaics (OPV) is a promising and exciting new technology for the renewable energy sector. It has the potential to deliver low-cost, customizability, light weight, semi-transparency, flexibility and scalable rapid deployment to name just a few possibilities. Critical for the efficient operation of an organic solar cell is a complete understanding of material and device properties such as light absorption, charge generation and transport, recombination and charge extraction processes in the disordered organic semiconducting junction.
In this thesis we examine in detail the charge transport and recombination processes that dominate the charge extraction efficiency in organic solar cells. In this context we use advanced methodologies such as Photoinduced Charge Extraction in Linearly Increasing Voltage (PhotoCELIV), Time-of-Flight (TOF) and Transient Photovoltage (TPV) to understand and model the fundamental factors that control these processes. In particular we study a number of electron donor and acceptor materials. Specifically we investigate a new solution processable small-molecule electron acceptor material (K12), in neat form as well as in operating OPV devices. We find that in the neat material, the dominant transport is n-type (electrons). Furthermore, the electron transport in K12 is strongly dependent on its morphology. Under optimized process conditions that lead to a microcrystalline morphology, the electron mobility was found to reside between 10^-4 - 10^-3 cm^2V^-1s^-1. We further investigate the effect of temperature on the electron transport in this novel n-type material and find that it behaves in a similar manner as in conventional low-conductivity p-type materials like P3HT. We also examine the carrier recombination dynamics and observe that the transport is limited by Langevin-type bimolecular recombination with the product of carrier lifetime and mobility being constant and temperature independent. Additionally we measure a carrier mobility enhancement effect at high carrier concentrations.
K12 as electron acceptor in devices in combination with P3HT shows successful photo current generation. However, we observe that the carrier extraction is limited by Langevin-type bimolecular recombination. In comparison, the two high performing prototypical donor-acceptor systems P3HT:PC61BM and PCDTBT:PC71BM show a reduced bimolecular recombination. We also find that the charge transport is significantly slower in the K12:P3HT device than in the other two systems. We suggest that trapped carriers in the K12 phase are a significant detrimental effect on the device performance, in conjunction with impeded electron transport due to non-optimal K12 morphology.
Finally we compare the carrier dynamics of the two high performing systems in operating devices. We find that although the PCDTBT:PC71BM systems exhibits a less reduced bimolecular carrier recombination than P3HT:PC61BM it still outperforms the latter significantly. Considering the fundamental structural difference in morphology of the two systems these results provide more evidence pointing to the conclusion that the reduction in bimolecular recombination is indeed strongly determined by the scale of separation of the donor and acceptor phase, reducing the probability of a free electron and hole encounter. A strongly reduced bimolecular recombination is beneficial for an organic solar cell but is not necessarily mandatory for high device performance, as the PCDTBT:PC71BM system demonstrates.
The here presented work hopefully deepens our understanding of transport and recombination processes occurring in organic semiconducting systems. In particular, the results on the small-molecule n-type electron acceptor K12 provides new insights for alternatives to the common fullerene based acceptors, in the neat form and in devices. The bipolar transport in complex networks formed by the donor and acceptor phases in bulk-heterojunction solar cells is one of the most critical aspects for efficiency in OPV and a deeper fundamental understanding points the way forwards to significant improvements in device performance and resulting commercial viability.