Harnessing solar energy as an alternative resource continues to stimulate organic photovoltaic (OPV) research for potential in low cost fabrication on mechanically flexible substrates.  Significant efforts have been devoted towards improving OPV efficiency above 10%; however, challenges still remain to understand, control, and optimize the photon-to-current conversion within the photoactive layer. OPV architectures using small molecular weight organic semiconductors to create molecular heterojunctions through non-covalent interactions or self-organization have gained increasing interest for creating high-performance devices with minimal processing.  Our focus has been designing and testing p-type semiconductors that have shape complementarily to the standard n-type C60 acceptors, and despite the progress in utilizing these materials in OPVs, many questions still remain.  The photophysics of contorted hexabenzocoronene (HBC) and second generation dibenzotetrathiphenocoronene (6-DBTTC) p-type semiconducting materials were studied in toluene and as solution-processed, blended films with [6,6]-phenyl-C61-butyric acid methyl ester PCBM  by femtosecond transient absorption spectroscopy. Photoinitiated charge generation and recombination spectra and kinetics were measured to probe the electron-hole pair dynamics created upon exciton dissociation in blended films in the visible and near-infrared region. These studies also examine charge recombination dynamics as a function of photoactive film thickness by probing the PCBM radical anion at 1030 nm.  Based on the kinetic analysis, approximately 60-65% of the charges recombine within the first 43-50 ps indicating a large Coulombic attraction between the electron and hole that is not overcome despite the favorable driving force for charge separation.  Interestingly, the recombination rates measured for 6-DBTTC are independent of photoactive film thickness and charge carrier yields double in comparison to HBC, indicating greater charge delocalization in 6-DBTTC.  Despite the similar molecular structure and electronic properties of HBC and 6-DBTTC, their film morphology and microstructure differs significantly when blended with PCBM and directly correlates with OPV efficiency.

Ultrafast dynamics of photoinduced charge generation and recombination in hexabenzocoronene molecular semiconductors

244th National Meeting (2012)

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