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2011 Bahas Solar Simulator Characterising dye-sensitised solar cells.pdf

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Optik 122 (2011) 1225–1230 Contents lists available at ScienceDirect Optik j our nal homepage: www. el sevi er . de/ i j l eo Characterising dye-sensitised solar cells Laura L. Tobin a,b,c , Thomas O’Reilly a,d , Dominic Zerulla a,d , John T. Sheridan a,b,c,∗ a SFI-Strategic Research Cluster in Solar Energy Conversion, Belfield, Dublin 4, Ireland b UCD Communications and Optoelectronic Research Centre, Belfield, Dublin 4, Ireland c School of Electrical, Electronic and Mechanical Engineering,
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  Optik 122 (2011) 1225–1230 Contents lists available at ScienceDirect Optik  journal homepage: www.elsevier.de/ijleo Characterising dye-sensitised solar cells Laura L. Tobin a , b , c , Thomas O’Reilly a , d , Dominic Zerulla a , d , John T. Sheridan a , b , c , ∗ a SFI-Strategic Research Cluster in Solar Energy Conversion, Belfield, Dublin 4, Ireland b UCD Communications and Optoelectronic Research Centre, Belfield, Dublin 4, Ireland c School of Electrical, Electronic and Mechanical Engineering, College of Engineering, Mathematics and Physical Sciences, University College Dublin,National University of Ireland, Belfield, Dublin 4, Ireland d School of Physics, College of Engineering, Mathematics and Physical Sciences, University College Dublin,National University of Ireland, Belfield, Dublin 4, Ireland a r t i c l e i n f o  Article history: Received 25 January 2010Accepted 28 July 2010 Keywords: PhotovoltaicGrätzel cellsSolar energyDye-sensitisedBiomimetic a b s t r a c t In today’s society there is a vast and in many cases not fully appreciated dependence on electrical powerfor everyday life. Furthermore, with growing energy and environmental concerns arising due to fossilfueldepletionandclimatechange/globalwarming,everincreasingattentionisbeinggiventoalternativeand/or renewable sources of energy such as biomass, hydropower, geothermal, wind and solar energy.Devices such as photovoltaic cells are therefore of enormous importance. The more widely used andcommerciallyavailablesilicon(semiconductor)basedcellscurrentlyhavethegreatestreportedefficien-cies and have received considerable attention. However the manufacturing of these cells is complex andexpensiveduetothecostanddifficultyofproducingandprocessingpuresilicon.Onealternativetechnol-ogybeingexploredisthedevelopmentofdye-sensitisedsolarcells(DSSCs)orGrätzelcells.Inthispaperwe report on our current work to develop simple test equipment and optoelectronic models describingthe performance and behaviours of DSSCs. We describe some of the background to our work and alsosomeofourinitialexperimentalresults.Basedontheseresultsweaimtocharacterisetheopto-electricalproperties and bulk characteristics of simple dye-sensitised solar cells and then to proceed to test newcell compositions. © 2010 Elsevier GmbH. All rights reserved. 1. Introduction Conventionalsiliconandmoreexoticquantumbasedsolarcellscontinue to drive the solar energy production and dominate thecommercialmarket.Howeveroverthelastdecademomentumhasgrowninsupportoforganicsolarcelltechnology.Siliconsolarcellsflourished commercially over the years in large part due to priordevelopments within the semiconductor industry, i.e., because of the technical expertise developed and the substantial history of financialinvestmentinfabricationplantintheelectronicsarea[1].PV cells with efficiencies of up to ∼ 40% have been reported [2,3].Thedisadvantageofsiliconsolarcellsisthattheassociatedman-ufacturingprocessremainsrelativelycomplexandcostly[4].While it would seem that there is a plentiful supply of the required rawmaterial, since after oxygen, silicon is the second most abundantelementandcomprisesof25.7%oftheEarth’scrust.Themajordis- ∗ Corresponding author at: School of Electrical, Electronic and Mechanical Engi-neering, College of Engineering, Mathematics and Physical Sciences, UniversityCollege Dublin, National University of Ireland, Belfield, Dublin 4,Ireland. Tel.: +353 1 716 1927. E-mail address:  John.Sheridan@ucd.ie (J.T. Sheridan). advantage of working with silicon is that it is not found free innaturebutisfoundasoxides(e.g.,sand,quartz,flint)andassilicates(e.g., granite, asbestos, clay). Energy intensive material process-ing and the use of expensive clean room fabrication condition aretherefore essential when working with this material [5].Oneeconomicallyviableandflexiblealternativetothe  p – n  junc-tionphotovoltaic(PV)devicesisthinfilmorganic-materialdevices,specificallydye-sensitisedsolarcells(DSSCs).Insiliconsystemsthesemiconductor takes on the dual role of charge carrier transportandlightabsorption,whereasintheDSSCthesetwooperationsareseparated [6].The Science Foundation Ireland Strategic Research Cluster forAdvanced Biomimetic Materials for Solar Energy Conversion is anew research cluster based in Ireland, formed with the expressedintention of bringing together industry and academia to producerenewable energy solutions. Our specific area of research is inbiomimetic DSSCs and their electrical properties.An attractive feature of DSSCs is that the concept derives fromthe area of biomimetics, in other words DSSC devices are made soas to imitate or mimic nature in some way. In this case there areanalogies to the process of photosynthesis. In both cases incominglight is absorbed by an organic dye and electrons are produced,resultingintheproductionofpositiveandnegativechargecarriers. 0030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2010.07.028  1226  L.L. Tobin et al. / Optik 122 (2011) 1225–1230 Fig. 1.  Schematic of the energy flow in a dye-sensitised solar cell. A schematic of the cross-section of a typical DSSC can be seeninFig.1.Amonolayerofanorganicdyeisattachedtoamesoscopic film of a wide bandgap oxide semiconductor. The dye absorbsincident light and produces electrons, which are injected into theconduction band of the semiconductor oxide. The electrons aretransportedacrossthenanoparticle/nanocrystallinTiO 2 layertothecurrentcollector(anode).Fromheretheelectronspassthroughtheexternal circuit and subsequently to the counter electrode (cath-ode). The sensitiser is regenerated by the organic hole conductorwhich transports the positive charges to the counter electrodewhere recombination occurs. Under solar exposure there is no netchemicalchangeandlightabsorbedisinpartconvertedtoelectricalpower.Both natural photosynthesis and DSSCs use organic dyes suchas anthcyanins. In DSSCs a mesoscopic film of titanium dioxidereplaces nicotinamide adenine dinucleotide phosphate (NADP + ),and carbon dioxide replaces the electron acceptor. Iodide and tri-ode(I − ,I 3 − )replacethewaterandoxygeninphotosynthesisastheelectron donor and oxidation product [7].Operational devices do not require-clean room conditions forfabrication and can be made reasonably robust to environmentalconditions, unlike silicon solar cells.This paper is organised as follows: in Section 2 a brief overview of photoelectric material and their use in solar cells is discussed.Section3describesaverysimplemethodologyforcreatingaDSSC. Initialexperimentalresultsandsomebasicmodellingarereportedin Section 4. Work on a testing rig for measuring multiple DSSCs is given in Section 5. Section 6 contains a brief discussion and a conclusion. 2. Overview  Edmund Becquerel is credited as being the first to report thephotoelectric effect in 1839. He noted that a photocurrent wasproduced when silver coated platinum electrodes were immersedin electrolyte [8,9]. The next significant development came from Willoughby Smith’s experiments in 1873 on electrical currentspassing though a bar of crystalline selenium, which found that itsresistance was reduced when the bar of crystalline selenium wasexposed to light. Following on from this in 1897 William Adamsand Richard Day reported that a current was produced when lightfell upon selenium, which had two heated platinum contacts andno external power supply [10].In 1894 Charles Fritts constructed the first large area solar cellusing plates made from two different metals with molten sele-nium compressed between them. Fritts was able to apply a thinsemitransparent layer of gold leaf onto a thin film of selenium toproduce the first thin film photovoltaic. The results of this experi-ment paved the way for the study of solar cells in the 20th century[11,12]. Heinrich Hertz experimentally observed the photoelectriceffect in 1887 with a spark gap generator where a spark was pro-duced upon the detection of electromagnetic waves. Hertz notedthat a charged object will more willingly lose its charge when illu-minatedbyultravioletlight[1,13,14].InthesameyearJamesMoser first reported on the dye-sensitised photoelectrochemical cell onanilluminatedsemiconductor[15].FollowingHertz’sexperiments AleksandrStoletovreportedin1888–1891,thattheelectriccurrentwas proportional to the intensity of the incoming electromagneticfield [16].Philipp von Lenard in 1902 also extended the research carriedout by Hertz on the photoelectric effect. He showed the variationin electron energy with light frequency by shining ultraviolet lighton a metal in a vacuum. In the presence of an electric field theseelectrons can be accelerated or retarded and in the presence of amagnetic field, their paths can be curved. Von Lenard showed thatthe calculated maximum electron kinetic energy is determined bythe frequency of the incident light. These experiments were com-plex in that they required freshly cut metal in order to use puremetal. However the metal oxidised quickly. At the time of theseexperiments the existence of photons was a matter of debate, andvonLenardsobservationswerequalitativeratherthanquantitative[17,18]. These results seemed to conflict with Maxwell’s electro-magnetic theory of light which predicted that the electron energywas proportional to the intensity of the radiation [19]. The quanti- tativesolutioncamefromEinsteinin1905when,followingPlancksproposal, he described light as being composed of discrete quantaor photons as opposed to continuous waves [18,20]. This was veri- fiedbyMillikanin1916[20,21]andEinsteinreceivedanobleprize for this work on the photoelectric effect.The photoelectric effect was also detected in copper-cuprousoxide thin film structures, in lead sulphide and thallium sulphide.In these cells a layer of semitransparent metal when depositedonto the semiconductor provided the asymmetric electronic junc-tion[22].In1904Hallwachsconstructedasemiconductorjunction solar cell using copper and copper oxide which was a prototypeof thin film Schottky devices [23]. Goldman and Brodsky in 1914 proposed the existence of a (potential) barrier to current flow atone of the interfaces of the semiconductor metal, i.e., a rectify-ingaction.Throughoutthe1930sMottandSchottkypioneeredthedevelopment of a theory of metal semiconductor barrier layers.In 1918 Czochralski developed a method to measure crystalli-sation rates of metal which was then adapted by Bell Labs in the1950s to grow single crystals of silicon [23,24]. The photovoltaic effectwasdiscoveredincadmiumselenide(CdSe)byAudobertandStora in 1932 which opened up the way to II–VI solar cells [23]. It wasTealandLittlein1948whoadaptedtheCzochralskimethodof crystal growth to fabricate single-crystalline germanium and soonthereafter silicon [25].The development of silicon electronics followed the discoveryofatechniquetomanufacture  p – n  junctionsinsiliconinthe1950s.The  p – n  junction structure architecture produced superior rectify-ing action and photovoltaic performance [1].Prior to 1953 the most efficient photovoltaic devices were sele-nium photocells with a maximum efficiency of 0.8%. This changeddramatically in 1954 when Chapin, Fuller and Pearson combinedtheirresearchtoproducethefirstsiliconsolarcellwithanefficiencyof 6%. Other materials such as gallium arsenide, indium phosphideandcadmiumtelluridewerestudiedforusein  p – n  junctionphoto-voltaicdevices,butsiliconhasremainedtheprincipalphotovoltaicmaterial. Silicon has benefitted from the advances in silicon tech-nology driven by the microelectronics industry [26,27].In the 1970s alternative energy sources research was spurredon by the energy crisis in the oil dependent western world. Therewas a growth in funding for research and development of photo-  L.L. Tobin et al. / Optik 122 (2011) 1225–1230 1227 voltaics and a variety of methods were developed which reducedmanufacturingandmaterialcostswhilesimultaneouslyimprovingdevice efficiency. Photochemical junctions were explored with aviewtolowercosts,andalternativematerialsincludedamorphoussilicon, polycrystalline silicon and organic conductors. To improveefficiencies, tandem and other multiple band gap structures werealso developed [1].With the discovery of the hole in the Ozone layer in 1985 therewasrenewedimpetustodevelopcleanerenvironmentallyfriendlytechnologies.In1991BrianO’ReganandMichaelGrätzelreportedanew solar cell concept based on biomimetics, specifically mimick-ing the photosynthesis process in plants [6]. This type of solar cell is known as a dye-sensitised solar cell. A very significant advan-tage of this technology is that it is less sensitive to the directionof the incident illumination than traditional PV. Thus, in the dif-fuse natural lighting prevalent in more northerly regions it couldhave significant advantages. Producers aim to mass produce suchcellsasflexiblethinplasticsheetssuchsheetsmayalsoofferssomedistinct advantages over heavier more rigid PV substrates. To dateDSSC efficiencies are not competitive with mass produced siliconsolar cells, with efficiencies of 5–10% being quoted, however it isexpected that one day these DSSCs will improve significantly andbecome commercially viable.Photovoltaic production has expanded at a rate of 15–25% perannum since the latter half of the 1990s and this has allowed asignificant reduction in manufacturing costs [1]. Photovoltaics are finally starting to become competitive energy suppliers as con-ventional electricity supplies have become more expensive. Oneexample of growth is in commercial solar panels for residentialhouses. 3. Methodology  We have implemented rudimentary DSSCs and performedmeasurements to characterise the current–voltage ( I  – V  ) valuesobtained. These DSSCs consist of two glass microscope slides(4cm × 2cm), which have a layer of transparent conducting coat-ing on one face of each slide as shown in Fig. 1. Fluorine doped SnO 2  was our material of choice for the transparent conductingcoating.OtheroxidessuchasZnOandSnO 2  canandhavebeenpre-viously investigated [16]. By measuring the electrical conductivity tofindtheresistanceoftheglassslides,thetransparentconductingcoatingwasimmediatelydistinguishablefromthenon-conductingface.One basic technique which can be used to produce a very thinuniform layer (monolayer) of TiO 2  is to mask three edges on oneside of the conductive face of the glass slide with Scotch tape.This forms a mould into which the TiO 2  solution can flow or bedrop cast. Another reason for localising the layer by masking theconductive glass is to allow for the simple introduction of elec-trical contacts. Using a pipette, several drops of the commercialcolloidal TiO 2  were transferred onto the slide and spread over theunmasked area to produce an even layer of approximately 10  mdepth.Once the TiO 2  containing solution dries the mask is removedwith care. Sintering is required to ensure that the layer of TiO 2 adheresontotheglassslide.Usingafurnacepre-heatedtoapproxi-mately450 ◦ Cthesinteringtimeisroughly5–10min.TheTiO 2 layeris transformed from a white colour into a brownish colour, whichisduetothenaturalorganicmatterintheTiO 2  layerrevertingbackto its former white colour once it has been successfully sintered tothe substrate. The glass slide must be cooled slowly after the sin-tering process to avoid excessive thermal stress which can lead tocracking or detachment (flaking) of the layer. This is the negativeelectrode (the anode) of the device. Fig. 2.  Circuit schematic for measuring the current–voltage characteristics of the(DSSC) solar cell. P: 10k   potentiometer; V: voltmeter; I: ammeter. One option is to dye the negative electrode. In the experimentreported here dried Hibiscus flowers were soaked in a petri-dishfilled with boiling water, i.e., at 100 ◦ C. The slides were immersedface up in the solution for approximately 10min to ensure that thedye had completely penetrated the TiO 2  layer. This can be deter-minedvisuallybythepurple-redstainingoftheTiO 2  layer.Thedyehas been absorbed by the TiO 2 . Tweezers were used to remove theslides from the petri-dish and distilled water was used to gentlyrinse off any excess dye. The slides were then left face up to dry.The counter electrode (the cathode) was formed by coating thetransparent conducting layer on the second glass slide with a lightcarbon layer. This coating can be introduced in several ways, mostsimply by using a HB pencil to directly apply a graphite coating tothe transparent conducting surface. Any loose or excess graphiteparticles can easily be eliminated by brushing or blowing themaway. This layer acts as the catalyst for the triiodide–iodide (I 3 − ,I − ) regeneration reaction.Thepositiveandnegativeelectrodeswereplacedtogetherwiththe catalyst-coating electrode on top of the TiO 2  layer. The twoglass slides were offset with respect to one another. This displace-ment while still ensuring overlap of the counter electrode will, atthe two ends of the slides, provides two exposed conducting innercell surface areas (at either end of the cell) which are exposedto the air. Mechanical clips were used to hold the glass slides inplace. To activate the cell several drops of the electrolyte solution(triiodide–iodide) were placed at the edge of the glass slides andcapillary action drew the solution into the gap between the anodeand cathode. The inners surfaces, at the exposed ends of the twoslides, provide the cell electrode contact points [7]. 4. Results ForthissolarcellHibiscusdyeextractedfromcelldriedHibiscusflowers was used for the dye and a carbon coating of graphite wasusedforthecounterelectrode.Othercyanindyescommonlyfoundin biological systems (plants) were explored such as raspberry andblackberry juice. However the observed current values were farlowerthanthosemeasuredusingtheHibiscusdyeemployedinourcell. Using a 20W 12V halogen lamp as a light source, positioned20cm from the solar cell and using the electronic circuit describedin Fig. 2, the current and voltage were measured and an  I  – V   curvewas obtained as in Fig. 3.In standard semiconductor diode theory [28,29] the forward biased diode current,  i D , is commonly approximated as a functionof the voltage  V   across the diode using the following expression i D  = I  s  exp  qV kT   − 1   (1) I  S   is commonly referred as the  saturation  or  scale current  . q =1.6 × 10 − 19 C is the charge on the electron,  k =1.38 × 10 − 23  J/Kis Boltzmann’s constant and  T   is the temperature in Kelvin(0K= − 273.15 ◦ C). Often these terms are lumped together into a  1228  L.L. Tobin et al. / Optik 122 (2011) 1225–1230 Fig.3.  Typicalcurrent–voltage( I  – V  )curveforasolarcellstainedwithHibiscusdye. single variable  V  T   called the  thermal voltage V  T   = kT q ,  (2)which at room temperature has the value  V  T  ∼ 25mV.In our case we propose to model our DSSC using the schematicrepresentation shown in Fig. 4. We assume that the current source output,  i S  , is linearly proportional to the illuminating solar lightintensity (power)  P  S  ,(3) i S  = ˛ P  s .ApplyingKirchhoff’slaw(currentconservationatanode)intheforward biased case (i.e.,  V  >0), it can be shown that I  ( V  ) = i S − i D  = ˛P  s − i D .  (4)Assuming our DSSC exhibits a diode like behaviour, then usingEqs. (1) and (2) gives I  ( V  ) = ˛P  s − I  s  exp   V V  T   − 1  .  (5)Clearlytherelationshipbetweenoutputvoltageandoutputcur-rent is nonlinear,  I  =  f  ( V  ), and Ohms Law will at best be obeyed ina piece-wise fashion. We note that we do not require either  i S   or V  T   to be governed by the same relationship (arise due to the samephysical effects) as the variables in Eq. (1), however we hope the analogy may prove to be of some value.An important point on the  I  – V   curve (i.e., the plot of the out-put current as a function of the output voltage), will be the pointat which maximum power is supplied. This is sometimes referredto as the ‘knee of the curve’. An example of an experimental curveis given in Fig. 5. Three points of significance are labelled on this graph. Point (i)  I  = I  SC  the short circuit current when  V  =0; Point(ii) the knee point  { I  MAX ,  V  MAX }  which is the point of maximumpower,  P  MAX = I  MAX V  MAX , output, and Point (iii)  V  = V  OC  the opencircuit voltage when  I  =0.We wish to use these values at these points in our experimen-tal curves in order to reduce the number of independent variables Fig. 4.  Schematic diagram of illustrating our proposed model of the operation of aDSSC. Fig. 5.  Theoretically predicted and experimentally obtained  I  – V   curves for a DSSC. appearing in our proposed model. In this way the appropriatenessof the model can be tested.For Point (i) it is clear that I   = I  SC  = i S  = ˛P  s  (6)For Point (iii) Eq. (5) can be re-written as 0 = i S − I  s  exp  V  oc  V  T   − 1  .  (7)This equation can re-written as I  s  exp  V  oc  V  T   − 1  = i S .  (8)InordertoexaminePoint(ii)wemustnowdefineanexpressionfor our output power  P  P  ( V  ) = I  ( V  ) V   =  ˛P  s − I  s  exp   V V  T   − 1  V.  (9)Eq. (9) is plotted in Fig. 6. Inordertoidentifyourmaximumoutputpowerpointwerecallthat at the extremum value,  P  MAX , for which  dP  / dV  =0. Taking thederivative of Eq. (9) with respect to  V   gives ˛P  s − I  s  exp   V V  T   − 1  + V   − I  s  exp   V V  T   − 1   1 V  T   .  (10)We set this equal to zero in order to find the value of   V  = V  MAX at which  P  = P  MAX . ˛P  s − I  s  exp  V  MAX V  T   − 1  1 +  V  MAX V  T   ≡ 0 ,  (11) ⇒ ˛P  s − I  s  exp  V  MAX V  T   + V  MAX V  T  exp  V  MAX V  T   − 1 − V  MAX V  T   ≡ 0 .  (12) Fig. 6.  Output power,  P  , plotted as a function of DSSC output voltage,  V  .
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