[en] 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

[en] All-polymer thin film transistors, inverters and active-matrix backplanes have been fabricated by inkjet printing technique. Source, drain and gate electrodes were printed with an aqueous dispersion of conducting polymer, poly(ethylenedioxythiophene) (PEDOT). The semiconductor and the gate dielectric were spin-coated from solutions of conjugated polymer and insulator polymer, respectively. In order to overcome the resolution limit of inkjet printing, PEDOT aqueous dispersion has been deposited onto a pre-patterned substrate which involves wettability contrast to define a channel, and polymer transistors with a channel length of 5 μm have been achieved. These transistors were applied to inverter circuits and active-matrix backplanes. Active-matrix operation of an electrophoretic display device has been demonstrated with the printed polymer active-matrix backplane

[en] We have investigated the behaviour of inverted poly(3-hexylthiophene):[6,6]-phenyl- C₆₁-butyric acid methyl ester (P3HT:PCBM) solar cells with different active layer thickness upon changing light intensity. Using white-light bias external quantum efficiency (EQE) measurements and photocurrent transient measurements we explain the different thickness dependence of device performance of inverted (ITO/ZnO/P3HT:PCBM/WO₃/Ag) and standard (ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al) cells. Whereas for inverted devices where high EQEs of up to 68% are measured under low light intensities (∼3.5 mW cm^-2), a dramatic reduction in EQE is observed with increasing white-light bias (up to ∼141.5 mW cm^-2) accompanied by a severe distortion of the EQE spectrum. For the inverted device this spectral distortion is characterized by a dip in the EQE spectrum for wavelengths corresponding to maximum light absorption and becomes more prominent with increasing active layer thickness. For regular P3HT:PCBM devices, in contrast, a less dramatic reduction in EQE with increasing light intensity and only a mild change in EQE spectral shape are observed. The change in EQE spectral shape is also different for standard devices with a relative reduction in EQE for spectral regions where light is absorbed less strongly. This asymmetry in device behaviour is attributed to unbalanced charge transport with the lower mobility carrier having to travel further on average in the inverted device structure. Thus at high light intensities charge recombination is more pronounced at the front half of the device (close to the transparent electrode) for inverted cells where most of the light is absorbed, and more pronounced at the back half of the device for standard cells. Our results therefore indicate that bulk charge transport mobilities rather than vertical composition gradients are the dominant factor in determining the performance of standard and inverted P3HT:PCBM cells.

[en] We investigate the n-type metal oxide tin (IV) oxide (SnO₂) as an electron injection and transport layer in hybrid polymer light-emitting diodes. SnO₂ is air stable and bio-safe, with high optical transparency and electrical conductivity, and with a deep valence band energy, making it highly suitable for such applications. Results reveal that SnO₂ is effective as an electron injecting cathode material when a thin hole-blocking interlayer of Cs₂CO₃ or Ba(OH)₂ is coated on it. Devices are optimized with respect to injection-layer thickness and hole-blocking layer configuration, with high performance metrics (current efficiencies of 20 cd A⁻¹, external quantum efficiencies of 6.5%) being demonstrated in the device with Ba(OH)₂ as the inorganic interlayer in the hybrid architecture. Also, we characterize thin films of spray-pyrolysis-deposited SnO₂, as compared with the commonly used interlayer material ZnO, in terms of film morphology and interfacial photophysics. (paper)

[en] We demonstrate the optimization of single-layer polymer LED structures with active layers with thicknesses of the order of 1 micron. By using a combined approach of the addition of MoO₃, as a bottom hole-injection layer, and the incorporation of such thick active layers, exceptionally high performance metrics are achieved. In particular, brightnesses of 1000 cd m⁻² at driving voltages of only 6.8 V, corresponding to a power efficiency of 7.8 lm W⁻¹, a current efficiency of 17.2 cd A⁻¹, and external quantum efficiency of 5.6%, are reported for devices based on F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3] thiadiazol-4,8-diyl)]). A side-by-side comparison, between the standard LED structure and hybrid structures, demonstrates that with MoO₃ as bottom hole-injection layer, the electron and hole charge carriers are both giving space-charge limited current for both carriers due to the Ohmic contacts. The devices hence show improved charge carrier balance, and, most importantly, high brightness at low operational voltage. Such thick active-layer devices with high performance metrics, in addition to improved engineering and processing tolerances, are thus especially important for application to high-throughput device fabrication methods. (paper)

[en] Time-dependent density functional theory (TD-DFT) is used to examine the effect of stacking in a model semiconducting polymer hetrojunction system consisting of two co-facially stacked oligomers. We find that the excited electronic states are highly sensitive to the alignment of the monomer units of the two chains. In the system we examined, the exchange energy is nearly identical to both the and band off-set at the heterojunction and to the exciton binding energy. Our results indicate that the triplet excitonic states are nearly degenerate with the singlet exciplex states opening the possibility for the interconversion of singlet and triplet electronic states at the heterojunction interface via spin-orbit coupling localized on the heteroatoms. Using Russell-Saunders theory, we estimate this interconversion rate to be approximately 700-800 ps, roughly a 5-10-fold increase compared to isolated organic polymer chains

[en] We present a comprehensive study of the hydrostatic pressure dependence of the vibrational properties of tetracene using periodic density-functional theory (DFT) within the local density approximation (LDA). Despite the lack of van der Waals dispersion forces in LDA we find good agreement with experiment and are able to assess the suitability of this approach for simulating conjugated organic molecular crystals. Starting from the reported x-ray structure at ambient pressure and low temperature, optimized structures at ambient pressure and under 280 MPa hydrostatic pressure were obtained and the vibrational properties calculated by the linear response method. We report the complete phonon dispersion relation for tetracene crystal and the Raman and infrared spectra at the centre of the Brillouin zone. The intermolecular modes with low frequencies exhibit high sensitivity to pressure and we report mode-specific Grüneisen parameters as well as an overall Grüneisen parameter $\mathrm{\gamma <!--\; ??\; -->}=2.8$. Our results suggest that the experimentally reported improvement of the photocurrent under pressure may be ascribed to an increase in intermolecular interactions as also the dielectric tensor. (paper)

[en] We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1, 4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.

[en] The spectral evolution of an intrachain neutral singlet exciton toward a charge-transfer (CT) state in solvents of increasing polarity has been monitored by time-resolved photoluminescence and ultrafast transient-absorption spectroscopy in a model conjugated random copolymer composed of electron donor and electron acceptor units. In polar solvents, a charge-like absorption superimposes the region of stimulated emission and leads to a dramatic reduction in gain implying that CT states can be detrimental for light amplification and lasing.