|About the Book|
Current developments in organic solar cells based on donor-acceptor blends require understanding and control of photoinduced charge transfer and electronic state dynamics. In this work the ultrafast dynamics of photoexcitations in pi-conjugatedMoreCurrent developments in organic solar cells based on donor-acceptor blends require understanding and control of photoinduced charge transfer and electronic state dynamics. In this work the ultrafast dynamics of photoexcitations in pi-conjugated organic semiconductors were studied using a low-intensity, high-repetition rate laser system in the broad mid- to near-infrared (IR) spectral range from 0.25 to 1.1 eV, and a high-intensity, low-repetition rate laser system in the spectral range from 1.2 to 2.5 eV, in the time domain up to 1 ns with 150 fs resolution. We also applied CW photomodulation spectroscopy along with excitation spectrum, modulation frequency sweeps, photoluminescence and electroabsorption to study the excited states of pi-conjugated polymers and acceptor-donor blends.-One current drawback to organic solar cell efficiency is negligible absorption in the near infrared spectral range of the solar spectrum. We provide and compare evidence that poly(2-methoxy-5(2-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) [electron donor] blended with 2,4,7-trinitrofluorenone (TNF) [strong electron acceptor] form a below-gap charge transfer complex (CTC) state that can extend absorption into the near infrared. The transient PA measurements also show that significant charge species are initially photogenerated, a majority of which geminately recombine within 8-10 ps, but the few that escape geminate recombination are subsequently captured in long-lived traps. In addition polarons could be also photogenerated with high efficiency at near-IR excitation, with similar fate. This demonstrates that a CTC state exists below the MEH-PPV polymer optical gap, but with low dissociation efficiency. We compare our results to those in blends of MEH-PPV/C60 where apparently a charge transport pathway to the electrodes is formed and the obtained CTC state has higher dissociation efficiency.-The most efficient all-organic photovoltaic (OPV) cells to date (∼6% power conversion efficiency) are based on bulk heterojunctions of polythiophene/fullerene derivative composites. Using primarily ultrafast spectroscopy, we study the charge generation steps in these materials. From our data, we foremost conclude that the photogenerated excitons in the polymer phase disappear within about 30 ps without a simultaneous increase in polarons within reach of our spectral range. Judging from the quantum efficiency of charge photo generation in the blend, this indicates that the singlet exciton undergoes a transformation, probably due to charge separation or energy migration into the fullerene network at the polymer/fullerene interface.-We also studied the effect of high hydrostatic pressure, P on the photophysics of a MEH-PPV film in a diamond anvil cell up to 53 kbar. We found that the cw photoluminescence band weakens, loses its zero-phonon satellite, and red-shifts by ∼2 meV/kbar- similar to that of the triplet photoinduced absorption (PA) band. The transient singlet exciton PA band slightly blue-shifts with P, and changes its decay dynamics- whereas the transient polaron PA band increases fourfold up to 70% of the exciton PA, and shows a superimposed vibrational Fano-type antiresonance. These pressure-induced effects confirm that charge generation is indeed an interchain phenomenon, and indicates enhanced aggregate formation caused by the increased interchain interaction with pressure.