Koen Vandewal

Varying polymer crystallinity in nanofiber poly(3-alkylthiophene): PCBM solar cells: Influence on charge-transfer state energy and open-circuit voltage

The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (ECT) and on the open-circuit voltage (Voc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between ∼0.1 and ∼0.9 by temperature control. ECT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as ECT=ECT0−0.2fu2002eV. Alkyl side-chain length only influences ECT0. Voc relates to ECT as Voc=ECT/q−0.6u2002V.

Modeling the temperature induced degradation kinetics of the short circuit current in organic bulk heterojunction solar cells

We acknowledge the institute for the promotion of science and technology in Flanders (IWT Vlaanderen) for funding via the IWT-SBO project Polyspec, the Fund for Scientific Research, Flanders (Belgium) (F.W.O.) in the framework of project G.0252.04 and the Special Research Fund (B.O.F.).

Nanoscale electrical characterization of organic photovoltaic blends by conductive atomic force microscopy

Conductive atomic force microscopy (CAFM) is introduced to perform electrical characterization of organic photovoltaic blends with high spatial resolution. Reference blends used in organic bulk heterojunction solar cells are investigated. The ability of CAFM to electrically evidence phase separated donor and acceptor regions is demonstrated. Furthermore, local spectroscopy is performed to analyze charge transport mechanisms in the blends. Significant modifications of the electrical properties of the semiconducting polymers are shown to occur after blending with fullerene derivatives. Finally, the sensitivity of CAFM to photoelectrical phenomena is revealed. Current variations of few picoamperes are locally observed under illumination of P3HT:PCBM.

Ground-state charge-transfer complex formation in hybrid poly(3-hexyl thiophene):titanium dioxide solar cells

The existence of a ground-state charge-transfer (CT) complex in a conjugated polymer:metal oxide nanoparticle bulk heterojunction photovoltaic cell is demonstrated by Fourier-transform photocurrent spectroscopy (FTPS). The CT complex between poly(3-hexylthiophene) (P3HT) and titanium dioxide (TiO2) is characterized by a weak additional photocurrent band (onset 1eV) in the FTPS spectra, situated below the conjugated polymer bandgap of 2eV. The presence of CT interaction between P3HT and TiO2 in relation to frontier orbital alignment is discussed, as well as the contribution of a sub-bandgap interfacial CT state to the electron transfer process in P3HT:TiO2 solar cells.

Bipolar Charge Transport in Fullerene Molecules in a Bilayer and Blend of Polyfluorene Copolymer and Fullerene

Efficient polymer solar cells typically contain the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which promotes dissociation of excited states and enhances charge transpo ...