J. D. MacKenzie

The origin of the open-circuit voltage in polyfluorene-based photovoltaic devices

The influence of device structure on the open-circuit voltage of polyfluorene-based photovoltaic devices has been investigated. Bilayers of hole- and electron-accepting polyfluorenes have been fabricated using an aqueous “float-off” lamination technique and subsequently incorporated into organic photovoltaic devices with a range of cathodes and anodes. A scaling of the open-circuit voltage with electrode work function difference has been observed with an additional intensity- dependent contribution from the active layer within the device. This additional contribution is attributed to photoinduced generation of carriers, whereby accumulation of charge at the polymer–polymer heterojunction results in a dipole across the interface and gives rise to a diffusion current that must be counterbalanced by a drift current at open circuit.

Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing

Surface treatment and solvent evaporation control are used to promote vertical segregation in polyfluorene-blend thin films. This surface-mediated control of the compositional structure in the direction normal to the plane of the film has important implications for optimizing charge transport in solution-processed conjugated polymer-blend optoelectronics. Here, the surface energy of the hole-collector electrode of photovoltaic devices is modified by deposition of self-assembled monolayers to favor segregation of the hole-accepting component of the blend to the substrate. Devices fabricated with intentionally vertically segregated blends showed external quantum efficiencies of up to 14%, which is ten times higher than that of devices fabricated without surface modification.

Increased efficiency in vertically segregated thin-film conjugated polymer blends for light-emitting diodes

The effect of film morphology on the performance of conjugated polymer-blend light-emitting diodes (LEDs) has been investigated. Vertically segregated structures have been fabricated by varying the spinning conditions and solvents used so that phase separation normal to the substrate is induced. External quantum efficiencies show that LEDs made with vertically segregated structures are between 25% and 100% more efficient than laterally segregated LEDs, depending on the scale of lateral phase separation.

Correlation Between Molecular Structure, Microscopic Morphology, and Optical Propertiesof Poly(tetraalkylindenofluorene)s

The emission properties of polyindenofluorenes with various proportions of straight- and branched-chain alkyl substituents have been compared. The polymer with straight octyl substituents shows green emission due to formation of aggregates, while the polymer with branched 2-ethylhexyl substituents shows blue emission. Studies of the film morphology show the presence of ordered structures due to π-stacking of the polymer chains for the octyl-substituted polymer, whereas the polymer with branched side-chains shows no such order. Copolymers show intermediate behavior. A clear correlation is established between the degree of straight-chain alkyl substitution, the formation of ordered structures in the films, and the amount of long wavelength emission in the solid-state spectra.

Efficient organic photovoltaics from soluble discotic liquid crystalline materials

Two different types of soluble discotic liquid crystalline materials and a crystalline perylene dye have been used to create, directly from solution, photovoltaic devices which are compared in this work. Self-organisation of the soluble electron-accepting perylene derivative and the soluble liquid crystalline (LC) discotic material which is stable in a LC phase at room temperature (HBC-PhC12) leads to segregated structures optimised for charge separation and transport in photovoltaic device structures. High external quantum efficiencies up to 34% near 490nm have been reached. The high efficiencies result from efficient photo-induced charge transfer between the materials as well as effective transport of electrons and holes to the cathode and anode through segregated perylene and the discotic peri-hexabenzocoronene p-system. Atomic force microscopy and device characteristics suggest that the driving force for phase separation and surface energy effects during spin coating of the HBC-PhC12:perylene blend result in a spontaneous vertical segregation of the HBC and the perylene normal to the plane of the spun film. This represents a nearly ideal, self-organised structure in which vertical segregation of charge transport layers coexist with a high interfacial area between the two charge transfer components. This vertical segregation has not been observed in the spin-coated blends where the HBC-PhC12 is replaced by HBC-C8∗. One probable reason for this may be the different phase stability of the LC phase in the HBCs, which leads to different film-forming properties and film morphologies.

Raman microscopy determination of phase composition in polyfluorene composites

Confocal Raman spectroscopy with a spatial resolution of ⩽1u200aμm is used to determine the composition of binary polyfluorene composites with micro- and mesoscale phase separation. The phases are found to contain significant proportions of both constituents, implying that exciton dynamics such as charge and energy transfer may occur within a particular phase. The results presented here provide an insight into thin-film phase separation of conjugated polymer blends of interest for optoelectronic device applications. In particular, in this letter the high degree of intraphase mixing is discussed in relation to the relatively high efficiency of photovoltaic devices fabricated from these blends.

Fluorescence scanning near‐field optical microscopy of polyfluorene composites

Fluorescence scanning near‐field optical microscopy (SNOM) is used to investigate binary polyfluorene‐based composites of varying composition. The samples investigated contain blends of the polymer poly(9,9′‐dioctylfluorene‐cobenzothiadiazole), F8BT, with similar polyfluorenes of wider band gap. Images acquired from a film containing 50% by weight F8BT exhibit a high degree of correlation between the topography and fluorescence, with an F8BT‐rich phase which protrudes from the surface of the film forming isolated regions with sizes from hundreds of nanometres to several micrometres. A film containing 10% by weight F8BT also has micrometre‐size F8BT‐rich regions, but also present are small and locally varying proportions of F8BT in the other polyfluorene component phase, indicating a hierarchy of phases within this sample. The fluorescence and topographic images of a third sample studied, containing 90% by weight F8BT, display no correlation, demonstrating that it is not always appropriate to use topographic information to determine the phase structure within polymer blends. The fluorescence SNOM images acquired from these samples are able to assist our understanding of the photovoltaic efficiency of devices fabricated from these films, which are governed by the extent of the interfacial area between these two constituent polymers.

ESEM imaging of polyfluorene blend cross-sections for organic devices

Abstract We report the use of environmental scanning electron microscopy (ESEM) to determine the phase separation in the cross-section of a 200 nm thick polyfluorene blend film, of the type used in polymer photovoltaic devices and LEDs. The micron and sub-micron surface phases are found to penetrate through the film to the underlying substrate, whilst smaller surface features do not necessarily propagate through the film. The observed cross-sectional structure helps to explain the optoelectronic response of these blends and shows that ESEM is an effective tool in the characterisation of polymer blend cross-sections.

Excited-state absorption in luminescent conjugated polymer thin films: ultrafast studies of processable polyindenofluorene derivatives

Abstract Femtosecond transient absorption experiments on thin films of poly-2,8-indenofluorene, a processable step-ladder analogue of poly- p -phenylene, are presented. Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped singlet excited-state and charged excitation absorption bands. Morphological effects on the dynamics are investigated by modification of the aliphatic side chains in the monomer unit. Steric effects, controlled by the nature of the alkyl side groups, result in a group-dependent three-dimensional structure. The transient absorption dynamics are similar in all samples, in spite of distinct morphologies, indicating limited importance in the excited-state analysis.

Highly-efficient broadband waveguide outcoupling in light-emitting diodes with self-organized polymer blends

Self-organized, two-dimensional micron-scale photonic structures have been fabricated within the emissive layer of polymer blend light-emitting diodes (LEDs). The relief-and-phase grating is achieved by phase separation of two semiconducting polymers directed by a surface chemical pattern. Short-wavelength oscillations are found pinned (and are thus phase-locked) to the domain boundaries. These high-frequency harmonics mimic short-period gratings in providing efficient waveguide outcoupling but without spectral dispersion. This provides a general way to harness the waveguide modes trapped in polymer LEDs, doubling their external quantum and power efficiencies while maintaining spectral integrity with viewing angle.