Exciton dissociation at nanostructured donor-acceptor interfaces

Photovoltaic solar cells based on organic systems are emerging as a viable technology platform, with current devices exhibiting good performance and durability. Common to almost all of these new devices is a nanostructured interface that comprises a electron donor system, often a conjugated polymer such as poly (3-hexylthiophene), and more often than not, C ₆₀ as an electron acceptor. A unique and essential feature of these interfaces is the ability to efficiently dissociate the photo-generated excitons into free carriers and, more importantly, to very effectively inhibit the reverse, recombination process. A uniform consensus on why this happens has yet to emerge and it is therefore a topic of great interest. Although they are considered to be a viable technology, solar cell power conversion efficiencies have only just exceeded 8%, and while this value is impressive, a further factor of two, at minimum, is needed. Incremental improvements in device performance cannot achieve this goal, and it requires a more fundamental, basic research to be focused on the problem. It is this type of approach that motivates this presentation. The talk will focus on the use of time-resolved microwave conductivity, using pulsed laser excitation, as a tool for probing both the production and loss of free carriers that result from exciton dissociation. Two systems will be examined: (i) the use of single-wall carbon nanotubes (SWNTs) instead of PCBM as the electron acceptor when dispersed in a conjugated polymer film, and (ii) the dissociation of excitons generated directly in PCBM when dispersed in a range of hole-accepting polymers.