Self-organization of liquid crystalline and crystalline-conjugated materials has been used to create, directly from solution, thin films with structures optimized for use in photodiodes. The discotic liquid crystal hexa-peri-hexabenzocoronene was used in combination with a perylene dye to produce thin films with vertically segregated perylene and hexabenzocoronene, with large interfacial surface area. When incorporated into diode structures, these films show photovoltaic response with external quantum efficiencies of more than 34 percent near 490 nanometers. These efficiencies result from efficient photoinduced charge transfer between the hexabenzocoronene and perylene, as well as from effective transport of charges through vertically segregated perylene and hexabenzocoronene pi systems. This development demonstrates that complex structures can be engineered from novel materials by means of simple solution-processing steps and may enable inexpensive, high-performance, thin-film photovoltaic technology.

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The interplay between phase separation in polyfluorene blends which show photoinduced charge transfer and photovoltaic performance in photodiodes has been investigated. Phase separation length scales have been varied from several microns to tens of nanometers by limiting the time allowed for solvent-enhanced self-organization through several different processing routes. Concurrent with the decrease in feature size, an increase in maximum photovoltaic efficiency of nearly 1 order of magnitude was observed in photodiodes incorporating the phase-separated blends as the active layer. The structure of the blend films was investigated using fluorescence microscopy, fluorescence scanning near-field optical microscopy, and atomic force microscopy. In some cases, a hierarchy of micron-and nanometer-scale phase separation was observed which may explain the unexpectedly high photoresponse in devices with up to micron-scale phase separation structure. This result along with in situ fluorescence microscopy studies of the transformation process highlights the complex, multistage nature of the conjugated polymer blend formation process which generally exhibits spinodal behavior.

The emission color of fluorene-based polymers can be facilely tuned across the whole visible spectrum by copolymerization with perylene dyes. Methods are demonstrated for incorporation of the dyes in the polymer mainchain, at the chain termini, or as side chains. Efficient energy transfer causes the emission to come solely from the dye units. Efficient LEDs have been made from the copolymers with dyes in the mainchain.

Semiconducting conjugated polymers for display applications are beginning to make the transition from laboratory to market place, while research continues into their use in applications including transistors, integrated optoelectronic devices, [1] voltage regulators, [2] solar cells, and photodiodes. [3] Research into novel materials, processing techniques, and device geometries have yielded exciting developments in stability, lifetime, and performance in all areas of application. For example, efficient photodiodes have been fabricated from mixtures of electron-and holeaccepting polymers. [4,5] These polymer composites, which are deposited by spin-coating from solution, contain a high surface area of dispersed interfaces at which photogeneration of charges is efficient. The morphology of the polymer mixture is crucial to the efficient operation of these devices. In this paper, we demonstrate directly and, to the best of our knowledge, for the first time, how the photocell performance can be controlled by manipulating the morphology of the polymer blend, which we achieve by controlling the rate of solvent evaporation during spin-coating.Substituted polyfluorene-based polymers are an important example of a range of new highly-fluorescent semiconducting polymers. Poly(9,9-dioctylfluorene) was developed as a blue-emitting material for polymer light-emitting diodes (LEDs), [6] and has since been shown to exhibit nondispersive hole transport with a relatively high mobility of 4´10 ±4 cm 2 V ±1 s ±1 . [7] The thermal stability and photostability of polyfluorenes has been shown to be considerably better than those of the phenylene±vinylene polymers. [8] Polyfluorene-based materials are expected to play an important part in the emissive displays of the future.We have demonstrated previously that efficient photovoltaic devices can be fabricated from blends of semiconducting polymers with different electron affinities and ionization potentials. [4,9] Excitons photogenerated in the blend are dissociated by charge transfer of the electron to one component of the blend and hole transfer to the other component. This process of photogeneration was established in molecular semiconductors by Tang et al. [10] The interpenetrating network formed by a blend of two such polymers provides both the high surface area of spatially distributed interfaces necessary for efficient charge generation, and the means for separately collecting electrons and holes. While the probability of charge transfer at an interface in a binary blend is determined by the relative energy levels of the constituent polymers, [11] the morphology of the polymer composite plays an important part in determining the efficiency of a polymer blend photodiode.The low entropy of mixing prevents miscibility of polymers on a molecular level, and they tend to phase separate into discrete domains. [12] In equilibrium, the lowest energy state is generally attained when the two components separate to form two bulk domains. When a thin film is prepared from a blend of poly...

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.

Two di erent 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-PhC 12) leads to segregated structures optimised for charge separation and transport in photovoltaic device structures. High external quantum e ciencies up to 34% near 490 nm have been reached. The high e ciencies result from e cient photo-induced charge transfer between the materials as well as e ective 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 e ects during spin coating of the HBC-PhC 12:perylene blend result in a spontaneous vertical segregation of the HBC and the perylene normal to the plane of the spun ÿlm. 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-PhC 12 is replaced by HBC-C * 8. One probable reason for this may be the di erent phase stability of the LC phase in the HBCs, which leads to di erent ÿlm-forming properties and ÿlm morphologies.

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.