|People involved:||Torbjörn Pascher, Villy Sundström, Arkady Yartsev, Tomas Österman, Yingyot Infahsaeng|
|Former members:||Suman Pal, Dimali Vithanage, Swati De|
This project is related to the following Fields, Subjects and Techniques:
|Fields:||Ultrafast Chemistry, Physics and Biology, Photochemistry and Photophysics|
|Subjects:||Polymer Solar Cells|
|Techniques:||Pump-probe spectroscopy, Time-resolved Fluorescence, Nanosecond laser flash photolysis|
Charge formation and recombination in conjugated polymer (donor) fullerene (acceptor) blends is studied extensively in our group with the help of time-resolved spectroscopy in combination with kinetic modeling. For low bandgap polyfluorene-fullerene blends (APFO3:PCBM) it has been observed that Coulombically bound electron-hole pairs are formed by an ultrafast (~100 fs) photo induced electron transfer between conjugated polymer and fullerene. The bound charges are then separated to form free charges on the time scale of ~30 ps. Finally, the free charges geminately recombine within 30 ns at low light intensities. These processes are illustrated in the potential surface diagram of Figure. It is concluded that the ~30 ns recombination time translates to an effective recombination distance of ~30 nm, comparable to the morphological structures seen in APFO3:PCBM blends. This suggests that optimization of solar cell morphology is important for reduction of recombination as well as for improving charge transport.
The detailed steps of both charge generation and recombination are not yet fully understood. Our present work aims to study the driving force (DG) dependence of charge pair formation, the mechanism of free charge formation and stabilization, as well recombination in various organic semiconductor materials, by using steady state and time resolved spectroscopic techniques.