Discotic Liquid Crystal Dye Sensitized Solar Cells DLC-DSSCs

Ammar A. Khan^a, Timothy D. Wilkinson^a, Malik M. Qasim^a, Muhammad A. Kamarudin^a

^aUniversity of Cambridge - UK, The Old Schools, Trinity Ln, Cambridge CB2 1TN, UK, Cambridge, United Kingdom

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Ammar A. Khan, 265
Publication date: 5th February 2015

Dye sensitized solar cells have been an area of intense research for more than two decades now, and efficiencies exceeding 12 % have been achieved using Ruthenium and organic dyes in liquid electrolyte based systems [1]. More recently, the inclusion of organo-metallic halide perovskite material into the DSSC architecture as a sensitizer has propelled DSSCs to remarkable efficiencies, and the latest research cite almost 19.3 % efficient solar cell devices [2]. However, iodide based redox electrolytes in liquid DSSCs present leakage/stability problems, in addition to high loss in potentials that limit open circuit voltages V_oc[3]. Solid state hole transporters like spiro-OMeTAD have had significant success in solid state DSSCs, but complex doping mechanisms, poor interfacial contact and low fill fractions are challenges that researchers continue to study [4]. Liquid crystals are thermally stable mesophases, and possess orientation/positional order that exceeds that of liquids, but is less than that of a crystal. More specifically, Discotic LCs (DLCs) represent a particularly interesting class of materials due to their semiconducting nature [5]. DLC molecules are composed of highly conjugated poly-aromatic cores, surrounded by several flexible insulating alkyl chains, that give them liquid crystalline properties. In addition, the molecules tend to minimize their free energy by pi-stacking into columns (face-on), essentially forming conducting wires surrounded by insulating chains. This gives these mesophases high carrier mobility, alongside the advantages of ordered packing and low-temperature solution processing, potentially making them ideal for hole transporting solutions in DSSCs. The preliminary results on the incorporation of hexa-alkyloxytriphenylene HAT 6 in the liquid DSSC architecture are presented in Figure 1 (a), (b). The DLC films are capillary filled to form 20 µm films in contact with the mesoporous TiO₂ in a sandwich cell arrangement. In addition, initial results on the effects of doping the bulk HAT 6 DLC with the strong electron acceptor molecule F4-TCNQ (Figure 1 (a)) are also presented, and shows improvement in the photovoltaic (PV) characteristics. The preliminary results, while promising, highlight the need for significant improvements before high efficiencies can be realized. We discuss possible constrains, that include interfacial contact, alignment of the DLC films, device thickness and different electrode materials to have a more ohmic contact between the HTL and the electrodes. Finally, updated device architecture is proposed.
Figure 1 (a) I-V measurements recorded under AM 1.5 standard illumination for triphenylene HAT 6 HTLs, both with and without 0.1 wt % F4TCNQ doping. (b) Schematic of the liquid crystalline DSSC device. N 719 was utilized as a sensitizer, and a commercial Titania and platinum pastes were used (dyesol).
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