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Physics Seminar

Dynamics of charge formation in organic solar cells: correlating structural and photophysical length scales

Speaker: Dr Stoichko Dimitrov (Queen Mary University of London)
Date: Wednesday 9 December 2020
Time: 15:00
Venue: Zoom

Photocurrent generation in printed organic solar cells is known to depend on the morphology of the photoactive layer, in which molecularly-mixed and pure phases with different densities and sizes of the p- and n-type semiconductors co-exist. The impact of these structures on the charge separation dynamics is not yet well understood. In this work, we address this issue by studying blends of several amorphous co-polymers and the electron acceptor PC_{70}BM. By changing their blend composition, we modulate the size and concentration of the pure and intermixed domains on the nanometre lengthscale. Then using laser spectroscopy we show that the changes in morphology correlate quantitatively with the changes in charge separation and recombination dynamics, and with changes in device photovoltaic performance. We determine the radius of geminately recombining electron-hole pairs to be 3-5 nm, which is bigger than the size of the intermixed domains (1-3 nm) in the films with excess fullerene, where pure fullerene phases of <4 nm are formed. Whilst the formation of such fullerene domains results in some (~ 20%) reduction in fullerene exciton dissociation yields, this negative impact is outweighed by the complete suppression of geminate recombination by making fullerene domains accessible for efficient electron escape.