Prof Henry Snaith FRS

Abstract:

We present a microscopic and electronic investigation of the polymeric anode poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) used as an electrode in photovoltaic and single carrier diodes. PEDOT:PSS is processed from aqueous solution as a colloidal dispersion with excess PSS present. We modify the PEDOT:PSS solution by the addition of a high boiling point alcohol, glycerol, which is known to increase the conductivity of the spin-coated film. Atomic force microscopy indicates swelling and greater aggregation of the PEDOT-rich colloidal particles found in this system. Current-voltage characteristics of 'hole-transporting' diodes, formed with gold contacts, suggest less surface enrichment of PSS in the glycerol modified electrode. Through Kelvin probe microscopy, we find the surface potential of glycerol modified PEDOT:PSS decreases by approximately 0.12 eV, which we assign to a reduction in surface enrichment by PSS. Photovoltaic diodes, using a PFB:F8BT polymer blend as the photo-active layer, and glycerol modified PEDOT:PSS anodes are significantly improved as compared to those with unmodified PEDOT:PSS anodes. This is likely to be due to improved hole-injection from the active polymer film into the PEDOT:PSS anode. This emphasises the electronic consequences of the morphological reorientation of the PEDOT and PSS. © 2005 Published by Elsevier Ltd.

Abstract:

Solution-processed photovoltaic devices based on blends of conjugated polymers and inorganic semiconductor tetrapods show high efficiencies due to the good electron transport perpendicular to the plane of the film. Here, we show that by using a high-boiling-point solvent, 1,2,4-trichlorobenzene, instead of chloroform for spin-coating, we can typically obtain a threefold increase in solar power conversion efficiency in devices based on CdSe tetrapods and the poly (p -phenylenvinylene) derivative O C1 C10 -PPV. The optimized devices show AM1.5 solar power conversion efficiencies of typically 2.1% with some devices as high as 2.8%. The results can be explained by the occurrence of vertical phase separation which leads to an optimal structure for charge collection. Evidence for this structure is obtained by environmental scanning electron microscopy, photocurrent action spectra measurements, time-resolved photoluminescence, and spectroscopic measurements of exciton dissociation and charge-carrier recombination. © 2005 American Institute of Physics.

Abstract:

We report microscopic and device based analysis of thin films of polyfluorene nanoparticles. We use an electroplating technique to form a complete monolayer of polymer nanoparticles on conductive and polymer-coated substrates. We find the electroplated film to be insoluble in organic solvents, and thus are able to build up multilayer structures of electroactive polymers which are originally soluble in common solvents. By spin-coating an F8BT layer from an organic solvent on top of a PFB:F8BT nanoparticle film, we form a multilayer structure. Capping with an aluminium cathode produces a photovoltaic device with substantial performance characteristics. (C) 2004 Elsevier B.V. All rights reserved.

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.