Date: 04.12.09, Time: 15:30 h
Charge Separation from Excitons in Organic Semiconductors
Professor Sir Richard Friend
University of Cambridge
Current designs for organic photovoltaic diodes depend on the ionisation of photogenerated excitons at the heterointerface between electron-accepting and hole-accepting semiconductors. It is very well established that photoinduced electron transfer across the heterointerface can be very rapid and very efficient. Nevertheless, this step is not sufficient to achieve full separation of electron from hole, and we have found several examples of systems where the electron and hole form a coulombically bound state (charge transfer exciton) that is not readily dissociated. I will review recent research in Cambridge that reveals the effects that these states have on charge generation in PV diodes:
For the case of the polymer donor acceptor blend PFB:F8BT the charge transfer exciton is weakly fluorescent and is sufficiently long-lived (40 ns) that the dominant decay pathway is through intersystem crossing to form a triplet exciton on the F8BT that lies lower in energy than the charge-transfer state .
We have recently been able to study the dynamics of photogenerated charges in P3HT:PCBM blends through measurements of transient optical absorption carried out on a working device under DC bias . With no net photocurrent, recombination occurs at time scales of order 10 - 100ns, but under reverse bias, recombination is suppressed and replaced by sweep-out of carriers at > 100ns. This provides evidence that recombination is geminate.
 S Westenhoff et al, JACS 130 13653 (2008)
 R A Marsh, J Hodgkiss and R H Friend (to be published)