Liquid Crystals for Organic Photovoltaics
As discussed in Chaps. 2 , 3 , 5 and 6 , columnar, smectic and, more recently, nematic liquid crystals are widely recognized as very promising charge-transporting organic semiconductors due to their ability to spontaneously self-assemble into highly o
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Liquid Crystals for Organic Photovoltaics Mary O’Neill and Stephen M. Kelly
8.1 Introduction The photovoltaic effect involves the generation of a photo-voltage and often a photocurrent on absorption of light in a semiconductor. The device must contain an asymmetry so that the photogenerated electrons and holes separate and travel in different directions to their respective electrodes. In an organic material the absorption of light of energy greater than the exciton energy, given by Eex D EHOMO ELUMO EBE , transfers an electron from the highest occupied molecular orbital (HOMO) of energy EHOMO to the lowest unoccupied molecular orbital (LUMO) of energy ELUMO and creates an exciton, which is a bound electron hole pair normally localised on a single molecule. The binding energy of the exciton, EBE , is large, some tenths of an eV, so that an electric field is insufficient to dissociate it. Instead the ionisation of the exciton is mostly achieved at an interface between electron-donating and electron-accepting species. Figure 8.1 shows a bilayer photovoltaic device containing an electron accepting material (A) overlaying an electron donating medium (D), both sandwiched between two dissimilar electrodes, one of which is transparent to transmit the incident light [1]. An energy level diagram of the device is given in Fig. 8.2 to illustrate the principle of operation. The D has a low ionisation potential (and thus a highlying EHOMO ) whereas the A has a high electron affinity (a low-lying ELUMO ). The photogenerated excitons diffuse to the interface and are ionised to generate free
M. O’Neill () Department of Physics and Mathematics, University of Hull, Hull HU6 7RX, UK e-mail: [email protected] S.M. Kelly Department of Chemistry, University of Hull, Hull HU6 7RX, UK e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 8, © Springer ScienceCBusiness Media Dordrecht 2013
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Fig. 8.1 Schematic of a bilayer organic photovoltaic consisting of an electron-accepting material A overlying an electron-donating film D, sandwiched between two dissimilar electrodes. The hole transporting polymer poly (3,4-ethylenedioxythiophene)/poly(styrene sulfonate) or equivalent is often inserted between the D and the anode to assist extraction of holes. The light is incident through the substrate and lower electrode
Fig. 8.2 Energy level diagram of a bilayer organic photovoltaic device. EHOMO (D) and ELUMO (A) denote the HOMO level energy of the D and the LUMO level energy of the A layer, respectively. Photons with an average photon energy larger than the optical band gap are absorbed on either side of the heterojunction (step 1). The photogenerated electron and hole thermalise and form an exciton (step 2). Excitons diffuse to the heterojunction (step 3) where they dissociate and transfer an electron [hole] into the A [D] layer (step 4). The electrons and holes are collected at the cathode an
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