Self‐Organized Semiconducting Polymer‐Incorporated Mesostructured Titania for Photovoltaic Applications

Abstract

The quest for alternative energy sources has generated a world-wide effort to prepare materials and devices designed to harvest clean abundant energy resources such as solar radiation. Considerable interest has focused on hybrid photovoltaic systems combining an organic chromophore, conjugated polymer or dye, and an n-type inorganic semiconductor. [1][2][3] A suitable energy band offset between the organic and inorganic components ensures that photo-excited electronhole pairs formed near the organic/inorganic interface will dissociate into free carriers, with the electrons preferably on one component and the holes on the other. A promising hybrid donor-acceptor pair for photovoltaics are poly(p-phenylene vinylene) (PPV)-type conjugated polymers and titanium dioxide, [4,5] owing to the high optical absorption of the polymer in the visible range and the band offset at the titania/PPV interface, which induces electron transfer from the photo-excited polymer to titanium dioxide. To provide efficient exciton dissociation into charges prior to recombination, the phase separation between the organic and inorganic species must be on the same scale as the polymer exciton diffusion length, that is, 8-20 nm. [6,7] Furthermore, carrier transport to the electrodes requires that the organic and inorganic phases form continuous networks through the entire film.[8] The challenge is, therefore, to direct a donor-acceptor organic-inorganic phase separation on a sub-20 nm length scale, and at the same time obtain continuity and orientation of each individual phase on a much longer length scale. To achieve this goal we developed a new and general synthetic approach that employs the co-assembly of a titania precursor species, a conjugated polymer, and an amphiphilic structure-directing agent to form conjugated-polymerincorporated mesoscopically ordered titania films. The use of non-aqueous synthetic conditions enabled, for the first time, the incorporation of the highly hydrophobic conjugated polymers within the mesostructured titania host matrix during its formation. Judicious selection of the surface-active agent type and concentration directed the deposition of a cubically ordered through-film interpenetrating titania and semiconducting polymer networks with ca. 15 nm organic-inorganic phase separation. Incorporation of the conjugated polymer into the 3D titania matrix enhances its photostability, and integration of the novel hybrid material into a photovoltaic device results in improved device performances. This study demonstrates the ability to direct the self-organization of functional components into hierarchically ordered materials with improved characteristics for electronic and opto-electronic applications.Several approaches have been previously applied to prepare nanoscale interpenetrating conjugated-polymer/titania networks, but they typically lack at least one of the important characteristics, that is, the sub-20 nm organic-inorganic phase separation or the through-film connectivity. Mixing the semiconducting polymers wi...

In situ syntheses of semiconducting nanoparticles in conjugated polymer matrices and their application in photovoltaics.

“…In the presence of an amphiphilic block copolymer as templating agent, a cubic interpenetrating bi-continuous network of the donor and acceptor phase was formed. However, only efficiencies of 0.034% have been obtained [67]. Possible reasons might be the remaining structuring agent in the layer or again the low crystallinity of the nanostructures.…”

Harnessing self-assembly strategies for the rational design of conjugated polymer based materials

“…It enhances the potential for technologytransfer from the laboratory to the consumer, e.g. for solid-state sensors and displays, whilst simultaneously providing an alternative route to exert control over polymer conformation and orientation, 11,77,78 as well as improving environmental stability. 78,79 Moreover, weak physical interactions can also be exploited and coordinated to yield desirable morphologies at the organic-inorganic interface.…”

“…A number of methods have been proposed to address these difficulties, which include (i) the use of water-soluble CPEs; 79 (ii) introducing a zwitterionic mediator; 80 (iii) the use of polar aprotic solvents; 78,81,82 and (iv) co-templating with non-ionic surfactants. 77,78,[81][82][83] We will now consider each approach in turn.…”

“…The addition of a conjugated polymer into the precursor solution promotes co-assembly of the hydrophobic CP, the amphiphilic SDA and the hydrophilic inorganic oxide, circumventing macrophase separation. 77,78,[81][82][83] By modifying parameters such as the surfactant type, concentration, solvent and temperature, it is possible to fabricate lms exhibiting a variety of morphologies, including hexagonal closed-packed, cubic and lamellar structural motifs. Frey and coworkers have shown that the chemical composition and structural organisation at the organic-inorganic interface can have a signicant inuence on the photovoltaic properties of these materials.…”

“…Frey and coworkers have shown that the chemical composition and structural organisation at the organic-inorganic interface can have a signicant inuence on the photovoltaic properties of these materials. 77,92 In a comprehensive study they used a series of ethylene oxide (EO)-propylene oxide (PO) block-co-polymer surfactants as structure-directing agents to template the formation of MEH-PPV-titania cubic mesostructured lms. 92 The length of the hydrophilic EO segments and the type of hydrophobic segment were shown to critically inuence the extent of interfacial interaction, such that SDAs containing long hydrophilic segments and moderately hydrophobic PO blocks (e.g.…”