Charge carrier dynamics at organic interfaces for polymer photovoltaics

The increased energy demand and the importance to find renewable energy resources have been addressed by a scientific effort to improve the power conversion efficiency of organic solar cells. In particular, polymer-based solar cells have experienced a great increase in power conversion efficiency, which recently reached a value over 10%. The possibility to further improve the performance of polymer-based solar cells has encouraged the research to have a close look at the key parameters, which play an important role in the mechanism of charge generation. Specifically, the photo-physical processes and the time scales involved in converting excitons to charges are still poorly understood. Also, the microscopic origin and the nature of the driving force allowing interfacial electron-hole pairs to become free charges and the role played by the sample microstructure in a bulk heterojunction have to be clarified. We have used transient absorption spectroscopy, which provides information about the excited states dynamics with <100 fs time resolution, and THz time domain spectroscopy, which is additionally able to distinguish between bound and free charges and to bring insight to local mobility with a < 300 fs time resolution. We have excited our samples at different wavelengths in order to be able to selectively excite mainly the polymer or the fullerene. In Chapter 3, we present transient absorption measurements for a polymer known for good packing, long-range ordering and high mobility: pBTTT. We have investigated neat pBTTT and pBTTT blended with PCBM, either in 1:1 weight ratio (one-phase microstructure) presenting an intermixed co-crystal phase between polymer and PCBM, or in 1:4 weight ratio (two-phase microstructure) having the intermixed phase and neat fullerene regions. We found that charges are promptly generated in the 1:1 blends, but mostly recombine geminately, while the presence of neat fullerene domains (1:4 blends) is a driving force for free charge generation. In Chapter 4, we present the study of additional pBTTT:PCBM blends, obtained using additives of different lengths and in different quantity. In contrast to the previous 1:1 and 1:4 microstructures, there is now a neat polymer and fullerene phase, and an intermixed phase (three-phase microstructure), or the blend is predominately phase separated (two-phase microstructure). We show that prompt charge generation occurs mainly in the intermixed phase, while the presence of neat polymer domains leads to delayed exciton dissociation and reduces the yield of charge generation. Neat polymer and fullerene domains contribute nevertheless to the spatial separation of electron-hole pairs. Finally, in Chapter 5, we present the transient absorption and THz investigation of a donor-acceptor co-polymer (PBDTTPD) in the neat phase and blended with PCBM in 1:2 weight ratio. The blend has a three-phase microstructure, consisting of neat amorphous polymer aggregates, amorphous intermixed regions, and neat fullerene domains. Differently from the pBTTT:PCBM systems, the presence of large (8 nm) polymer domains in PBDTTPD:PCBM is not an obstacle to the generation of charges, since the delocalized excitons efficiently dissociate with a 1 ps time constant. The delayed charge separation was also observed by THz-time domain spectroscopy at very low fluence.

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