Proceedings of the European Summer School of Photovoltaics 4 â 7 ...

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Dyes in Dye-Sensitised Solar Cells There are certain requirements that dye molecules have to meet in order to be applicable in DSSCs. Obviously, they must have strong absorption over the visible spectrum and preferably also in near IR, that carries around 47% of energy of the Sun. Moreover, the dyes have to be stable both in their ground and excited state as they have to withstand millions of redox turnovers. Furthermore, they must have suitable energy levels, both excited and ground, in relation to the semiconductor conduction band edge and redox charge mediator, respectively. The first compounds that were used as sensitisers were organometallics. They were followed later on by purely organic chromophores. The latter are preferred because they do not contain transition metals ions and thus can be potentially cheaper to mass produce. The mechanism of sensitisations differs between the two families of compounds: purely organic conjugated molecules exhibit π π* excitation whereas transition-metal/ligand complexes transfer an electron from a non-bonding lone pair d- orbital of metal to a π* orbital of ligand. There are some minor differences in the mechanism of sensitisation between the two chromophore families [12] but both of them have been experimentally proven to be useful in DSSC. For the time being organometallics bring slightly better results. In order to minimise losses inside the cell, electron transfer from the dye to the semiconductor and further to the outer circuit must be rendered much more competitive than the other “parasitic” electron pathways. One approach to this challenge is to investigate the link between the chromophore and the surface of the titania. Proper anchoring conditions have to provide an ultra fast electron path. The carboxylic (–COOH) group that is responsible for this process has been included in all the chromophores for DSSC applications up to now and it has shown to bring good results. Another solution to achieve more competitive sensitisation and electron transfer is to slow down the “parasitic” electron pathways, i.e. backtransfer to the electrolyte and to the ground state of the ionised dye molecule (Fig. 3A). The former in particular has turned out to be harmful to the operation of the cell (Fig. 3B). An approach proposed to overcome this problem includes engineering the molecules to contain additional parts responsible for blocking access of the electrolyte to the surface [14, 15]. Recently a new molecular design for a chromophore has been proposed [15, 16], according to which molecules should involve electron donating group(s), π conjugated bridge, electron withdrawing group(s) and anchoring group (D-π-A, in the given order). In the ground state the electron density is higher at the electron donating region whereas upon excitation it is moved to the acceptor group and thus towards the semiconductor surface. In this way electron approaches the surface when entering the excited state what is believed to facilitate its transfer to the semiconductor. The D-π-A design also has another advantage: slowing down the “parasitic” electron backtransfer from the conduction band of the semiconductor to the ionised chromophore. Namely, when the electron has been withdrawn from the dye, the electron density in the region of the electron donating group, that is far away from the semiconductor surface, decreases. Thus it is more probable for the chromophore to be regenerated by the electrolyte than by the electron trapped at the semiconductor surface. Experimental section The authors will investigate other possible anchoring groups, inter alia sulfonate group (‐SO ‐ ), to check whether they can render 3 electron transfer to the outer circuit more competitive, as this topic has not been studied extensively. Besides that we will focus on the structure-properties relations of the chromophore. We will investigate a few families of molecules with a relatively simple common backbone. They are highly conjugated and they involve groups donating or withdrawing electrons via resonance. Examp- D r Fig. 4. The backbones of the examples of families of the compounds to be investigated experimentally. A r = EWG r – electron withdrawing group; D r = EDG r = electron donating group; the subscript r stands for resonance; R is substituted by benzene or naphthalene derivatives les of families that have been chosen are shown in Fig. 4. Various substituents will be tried out to check their influence on the cell performance. This, in conjunction with the relative simplicity of the molecules is believed to let the relation between structure and properties of the dyes be scrutinised. Basing on the results hopefully some hints to the chromophore design will be formulated. The authors start by crystallising the chosen compounds in order to perform X-ray diffraction experiments on them, which allows a precise description of the structure of the compound. Absorption spectra of the investigated chromophores both in solution as well as anchored to the mesoporous titania surface will also be collected. Then a complete solar cell will be fabricated and characterised. All these result will allow us to see whether the chosen chromophores are suited for use in PV cells. Of course, besides the processes at the semiconductor-choromophore-electrolyte interface, there are many more factors influencing the cell performance. They involve working temperature, electrolyte conductivity, substrate conductivity, cell width and cell contacts, counter electrode, iodine concentration, mass transport conditions as well as titania layer thickness, to name the most important [11], which are beyond the scope of this work. References SO 3 - A r [1] Grätzel M.: Photovoltaic and photoelectrochemical conversion of solar energy, Phil. Trans. R. Soc. A 365, 993 (2007). [2] REN21, Renewable Energy Policy Network for the 21 st century, Renewables 2011 – Global Status Report, www.ren21.net [3] United States Energy Information Administration, Annual Energy Outlook 2011, www.eia.gov [4] Oregan, B.; Grätzel, M.: A low-cost, high-efficienty solar-cell based on dye-sensitized colloidal TiO 2 films, Nature 1991, 353, 737–740. [5] Low K. S.: PhD progress report, University of Cambridge, Cavendish Laboratory, Department of Physics, Structure and Dynamics Group (2009). [6] Desilvestro H.: Ionic Liquids – The Panacea for Dye Solar Cells?, Dyesol Ltd (2007), www.dyesol.com [7] Nogueira A.F., C. Longo, M.-A. De Paoli: Polymers in dye sensitized solar cells: overview and perspectives, Coordination Chemistry Reviews 248 (2004) 1455–1468. [8] Weiss D. S., M. Abkowitz: Advances in Organic Photoconductor Technology, Chem. Rev. 110 (2010), 479 [9] Longo C. et al.: DSSCs: A Successful Combination of Materials, J. Braz. Chem. Soc., 14, 889-901, (2003). [10] Roncali J.: Molecular Bulk Heterojunctions: An Emerging Approach to Organic Solar Cells, Acc. of Chemical Research, vol. 42, No. 11 (2009) 1719-1730. [11] Desilvestro H.: What Physical Factors Affect Current-Voltage Characteristics of Dye Solar Cells?, Dyesol Ltd (2008), www.dyesol.com [12] Prezhdo O. V., W. R. Duncan, V. V. Prezhdo: Dynamics of the Photoexcited Electron at the Chromophore–Semiconductor Interface, Acc. Chem. Res. 41, 339 (2008). [13] G. Smestad, Sol. Energy Mater. Sol. Cells 55, 157 (1998). [14] Justin Thomas K. R. et al.: 2,3-Disubstituted Thiophene-Based Organic Dyes for Solar Cells, Chem. Mater. (2008) 20, 1830–1840. [15] Mingfei Xu et al.: Energy-Level and Molecular Engineering of Organic D-π-A Sensitizers in Dye‐Sensitized Solar Cells, J. Phys. Chem. C (2008), 112, 19770–19776. [16] Masataka Katono et al.: D-π-A Dye System Containing Cyano-Benzoic Acid as Anchoring Group for Dye-Sensitized Solar Cells, Langmuir (2011) 27, 14248–14252. Dr N - O 3 S SO 3 - N R 128 Elektronika 6/2012
Azaheterocyclic materials for organic photovoltaic cells Natalia Nosidlak 1) , Monika Pokladko-Kowar 1) , Ewa Gondek 1) , Andrzej Danel 2) , Jerzy Sanetra 1 1) Institute of Physics, Cracow University of Technology, Krakow, Poland 2) Department of Chemistry, University of Agriculture, Krakow, Poland Recently we can observe growing interest in organic photovoltaic cells. Nowadays, when the development of renewable sources of energy is necessary, photovoltaic energy becomes an opportunity to reduce consumption of fossil fuels. Organic photovoltaic cells are easy built by deposition techniques like spin coating, moreover PV devices have lower weight, flexible shape and low production cost in comparison with inorganic equivalents [1]. These features have fuelled the interest of both science and industry. The conversion of solar light into electric power requires the generation of both negative and positive charges. The operation of a photovoltaic cell can be generally divided into three basic steps: light absorption, charge separation and charge collection of the appropriate carriers to anode and metallic cathode. (2) where P light – power irradiated at cell surface unit [3]. Experiment For the purpose of this article the regioregular P3OT (poly 3-octylthiophene) (Fig. 2) with HOMO level of about – 5,2 eV and LUMO level about – 2,85 eV, as polymer matrice were choosen. conversion step light absorption exciton creation exciton diffusion charge separation charge transport charge collection INCYDENT PHOTONS reflection and transmission recombination of charges limited mobility of charges recombination near electrodes barriers at electrodes SEPARATED CHARGE AT ELECTRODES loss mechanism recombination of excitons exciton transfer with subsequent recombination of excitons no charge separation and subsequent recombination of excitons Fig. 1. The specific conversion steps and loss mechanisms in an organic solar cell When photons are absorbed, only small part of incident light is absorbed because of size of band gap. Required band gap is about 1,1 eV, when most of organic semiconductors has 2.0 eV. This fact reduce possibility of absorption to 30%. Also some part of incident light is lost because of reflection and transmission. In case of exciton creation and diffusion, loss mechanism includes recombination of excitons. Length of diffusion should be equal to thickness of film, but is about 10nm when thickness of film is about 100 nm. In the next step of conversion loss mechanism includes recombination of excitons but also cases when charges are no separated. This situation have place when difference between ionization potential and electron affinity is insufficient and excitons are able to jump into the material with a lower band gap without charge separation. Charge transport efficiency is limited by recombination especially when applied material used as transporting medium is same for electrons and holes. The last step of conversion is charge collection on electrodes. Losses here are caused by recombination near electrodes and by potential barrier of electrodes [2]. Parameters describing fulfilled solar cells: V OC – open circuit voltage, I SC – short circuit current, I mpp , V mpp – voltage and current at the maximum power point, FF – fill factor determined by: (1) η – energy conversion efficiency calculated by the following formula: Fig. 2. The principal molecular chemical formula for poly(3-octylthiophene) The active layer of solar cells contain P3OT and three kind of 1H- pyrazolo[3,4-b]quinoxalines: PAQX1, PAQX2, PAQX3, PAQX4 (Fig. 3). a) b) Ph Ph N N N N N CH 3 c) d) H 3 C N C H 3 F N CH 3 M eO N C H 3 N N N Fig. 3. Investigated chromophore molecules of: a) PAQX1, b) PAQX2, c) PAQX4, d) PAQX3 Photovoltaic cells were fabricated on ITO (Indium Tin Oxide) covered glass slides (15×15 mm) which were cleaned in an ultrasonic bath using organic solvents. Next the ITO was covered with PEDOT:PSS thin film by spin-coating and left for 30 minutes in vacuum heater at 70 o C. After 30 minutes the active layer (P3OT + PQXx) was spinned over. At the end photovoltaic cells were defined by aluminum electrodes thermally evaporated in vacuum. The device has following architecture: ITO/PEDOT:PSS (poly(3,4-ethylene dioxythiophene)-poly-(styrene sulphonate)/ active layer/Al. Aluminum electrode acts as cathode (collecting N N N N N N Elektronika 6/2012 129
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Page 9 and 10: Summary The silver nanopowder paste
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