Snaith group

Abstract:

We report the first successful application of an ordered bicontinuous gyroid semiconducting network in a hybrid bulk heterojunction solar cell. The freestanding gyroid network is fabricated by electrochemical deposition into the 10 nm wide voided channels of a self-assembled, selectively degradable block copolymer film. The highly ordered pore structure is ideal for uniform infiltration of an organic hole transporting material, and solid-state dye-sensitized solar cells only 400 nm thick exhibit up to 1.7% power conversion efficiency. This patterning technique can be readily extended to other promising heterojunction systems and is a major step toward realizing the full potential of self-assembly in the next generation of device technologies.

Abstract:

We integrate mesostructured titania arrays into dye-sensitized solar cells by replicating ordered, oriented one-dimensional (1D) columnar and three-dimensional (3D) bicontinuous gyroid block copolymer phases. The solar cell performance, charge transport, and recombination are investigated. We observe faster charge transport in 1D "wires" than through 3D gyroid arrays. However, owing to their structural instability, the surface area of the wire arrays is low, inhibiting the solar cell performance. The gyroid morphology, on the other hand, outperforms the current state-of-the-art mesoporous nanoparticle films.

Abstract:

Here, the use of metal oxide layers both for charge transport and injection into an emissive semiconducting polymer and also for the control of the in-plane waveguided optical modes in light-emitting diodes (LEDs) is reported. The high refractive index of zinc oxide is used to confine these modes away from the absorbing electrodes, and include a nano-imprinted grating in the polymer layer to introduce distributed feedback and enhance optical out-coupling. These structures show a large increase in the luminescence efficiency over conventional devices, with photoluminescence efficiency increased by up to 45%. Furthermore, optically-pumped lasing in hybrid oxide polymer LEDs is demonstrated. A tuneable lasing emission is also obtained in a single device structure by employing a graduated thickness of a zinc oxide inter-layer. This demonstrates the scope for using such architectures to improve the external efficiency of organic semiconductor LEDs, and opens new possibilities for the realization of polymer injection lasers. ©2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

An investigation of the function of an indolene-based organic dye, termed D149, incorporated in to solid-state dye-sensitized solar cells using 2,20,7,70- tetrakis(N,N-di-p-methoxypheny-amine)-9,90-spirobifluorene (spiro- OMeTAD) as the hole transport material is reported. Solar cell performance characteristics are unprecedented under low light levels, with the solar cells delivering up to 70% incident photon-to-current efficiency (IPCE) and over 6% power conversion efficiency, as measured under simulated air mass (AM) 1.5 sun light at 1 and 10mW cm-2. However, a considerable nonlinearity in the photocurrent as intensities approach "full sun" conditions is observed and the devices deliver up to 4.2% power conversion efficiency under simulated sun light of 100mW cm-2. The influence of dye-loading upon solar cell operation is investigated and the thin films are probed via photoinduced absorption (PIA) spectroscopy, time-correlated single-photon counting (TCSPC), and photoluminescence quantum efficiency (PLQE) measurements in order to deduce the cause for the non ideal solar cell performance. The data suggest that electron transfer from the photoexcited sensitizer into the TiO2 is only between 10 to 50% efficient and that ionization of the photo excited dye via hole transfer directly to spiro-OMeTAD dominates the charge generation process. A persistent dye bleaching signal is also observed, and assigned to a remarkably high density of electrons "trapped" within the dye phase, equivalent to 1.8×1017 cm-3 under full sun illumination. it is believed that this localized space charge build-up upon the sensitizer is responsible for the non-linearity of photocurrent with intensity and nonoptimum solar cell performance under full sun conditions.2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

The morphology of TiO2 plays an important role in the operation of solid-state dye-sensitized solar cells. By using polyisoprene-block- ethyleneoxide (PI-b-PEO) copolymers as structure directing agents for a sol-gel based synthesis of mesoporous TiO2, we demonstrate a strategy for the detailed control of the semiconductor morphology on the 10 nm length scale. The careful adjustment of polymer molecular weight and titania precursor content is used to systematically vary the material structure and its influence upon solar cell performance is investigated. Furthermore, the use of a partially sp 2 hybridized structure directing polymer enables the crystallization of porous TiO2 networks at high temperatures without pore collapse, improving its performance in solid-state dye-sensitized solar cells. © 2009 The Royal Society of Chemistry.