Prof Henry Snaith FRS

Abstract:

The transport properties of the blended polyfluorene layers were studied using layered structures containing both pristine and the blended layers. It was shown that for a polymer blend device where there is a fine scale of phase separation, only charges generated close to either electrode can be collected. The open-circuit voltage of a polymer blend device was found to be lower than that of a bi-layer device as a result of direct paths from cathode to anode of each component of the blend. The results show that a charge injection barrier from the electrodes into the polymer film helps to retain a high open-circuit voltage in the blend.

Abstract:

We present a compositional analysis of the phase separation, nano-structure and electrical performance of blended hole-accepting and electron-accepting polyfluorene derivatives, in films and in photovoltaic devices. We use varying molecular weights to vary the thin film morphology, without altering the blend composition. We show that photoluminescence quenching is insensitive to variations in the blend morphology but the photovoltaic quantum yield is strongly dependent on morphology. This indicates that charge transport, and not charge generation, is the factor that limits device performance. We develop a model for the charge transport within a meso-scale phase separated film and estimate the distance which charges can travel within the minor component of each phase. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

Polymer blends allow control of microstructure in donor-acceptor photovoltaic devices. Here we present measurements of devices containing polyfluorene blend layers of different thicknesses, and we are able to extract characteristic transport lengths for electrons and holes. We also present analytical and numerical modeling of single-layer and bilayer photovoltaic devices, which demonstrates the importance of bound polaron pairs formed after the initial electron transfer from donor to acceptor. Field-assisted dissociation of these polaron pairs is a critical process in determining device performance.