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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTSchematic of the LB apparatus used to fabricate amonolayer of encapsulated nanoparticles.Figure 4.4.2.8Si nanoparticle encapsulated with organosilanes.Figure 4.4.2.9is very attractive for organic and inorganic hybriddevice applications. Organic/inorganic heterojunctionshave generated a great interest for theirpotential in developing hybrid devices as well asto understand organic semiconductors and theirinterfacial properties. We are also investigatingthe possibility of poly-Si/organic hybrid solarcells. Figure 4.4.2.10 shows the absorption spectraof poly-Si/PEDOT:PSS/P3HT on glass substratecompared with the absorption spectra of c-Si/PEDOT:PSS/P3HT structure.Absorption spectra of poly-Si/PEDOT:PSS/P3HT on glass substratecompared with absorption spectraof c-Si/PEDOT:PSS/P3HT structure.Figure 4.4.2.10colloidal dispersion. Compression of the monolayeris monitored via measurements of surface pressureand then controlled by a feedback loop. This uniquetechnique allows transfer of this ordered monolayeronto a wide range of solid substrates such asglass or Si wafers. By controlling the interspacingbetween adjacent particles, i.e. the shell thickness,by varying the molecular weight of capping species,we can control the periodicity of the film - leadingto new optical and electrical properties.Silicon (Si) nanoparticles are being used as corematerials. In order to control the interspacingbetween the particles, the termination of Sinanopaticles is carried out using organosilanes ofvarying alkyl chain lengths, as shown in Fig. 4.4.2.9.Progress to date is reported elsewhere [4.4.2.11].Once the assembly approach is mastered, itshould be possible to build up device structuresincorporating layers of quantum dots with thedoping in each layer individually controlled.4.4.2.8 Poly-Si/Organic Hetero-Junction CellRecently organic/inorganic hetero-junction thinfilms solar cells have gained importance for theircost-effective potential applications [4.4.2.12]. Thecombination of hydrogenated amorphous (a-Si:H)and/or poly-silicon (poly-Si:H) and organic materials4.4.2.9 References4.4.2.1 G. Yu, K. Pakbaz and A. J. Heeger, Appl. Phys. Lett.64 (1994), p3422.4.4.2.2 M.A. Green, K. Emery, Y. Hishikawa and W. Warta,“Solar Cell Efficiency Tables”, Versions 26-34,Progress in Photovoltaics, 2006-2009.4.4.2.3 http://www.solarmer.com4.4.2.4 A. Holzhey, C. Uhrich, E. Brier, E. Reinhild, P. Bauerle,K. Leo and M. Hoffmann, J. Appl. Phys. 104 (2008),p064510.4.4.2.5 K. Lee, J. Y. Kim and A. J. Heeger, SPIE OrganicPhotonic Materials and Devices VIII, San Jose, CA,January 23-26, 2006.4.4.2.6 L.S. Roman, O. Inganäs, T. Granlund, T. Nyberg,M. Svensson, M.R. Andersson, J.C. Hummelen,Advanced Materials 12 (3) (2000), p189–195.4.4.2.7 M. Niggemann, M. Glatthaar, A. Gombert, A.Hinsch, V. Wittwer, Thin Solid Films 451–452 (2004),p619–623.4.4.2.8 K. Tvingstedt, V. Andersson, F. Zhang, O. Inganas,Applied Physics Letters 91 (2007) p123514.4.4.2.9 N.C. Lindquist, A. Lesuffleur, and S. Oh, AppliedPhysics Letters 91 (2007), p253105.4.4.2.10 S. Pillai, K.R. Catchpole, T. Trupke, and M.A. Green,“Surface Plasmon Enhanced Silicon Solar Cells”,Journal of Applied Physics 101 (2007), p093105.4.4.2.11 L. Treiber, C. Bumby, S. Huang, G. Conibeer, 23rdEuropean Photovoltaic Solar Energy Conference,Valencia, Spain (2008).4.4.2.12 R. A. Halton, et al., Organic Electronics 10(2009), p388.58
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTSynthetic silicon. Portion of the PeriodicTable showing pathways to engineeringsemiconductors with 4 valence electrons/atom.Figure 4.4.3.14.4.3 Earth-Abundant PhotovoltaicDevices (CZTS)University Staff• Dr Xiaojing Hao• Prof. Martin GreenPostgraduate Research StudentXiaolei Liu4.4.3.1 Strand OutlineAll successfully commercialised non-concentratingphotovoltaic technologies to date are based onsilicon or the chalcogenides (semiconductorscontaining Group VI elements, specifically Te, Seand S). As indicated by Figure 4.4.3.1, the successfulchalcogenide materials, CdTe and CuInSe 2, can beregarded as “synthetic silicon” where the balancebetween atoms in these materials provides thesame average number of valence band electrons asin silicon.Unfortunately Cd and Se are toxic “heavy metals”while Te and In are amongst the 12 most rareelements in the Earth’s crust, factors that wouldseem to limit the long-term potential of theestablished chalcogenide technologies. However,as indicated in Figure 4.4.3.1, by investigating moredeeply into the Periodic Table, the compoundCu 2ZnSnS 4is uncovered with the same number ofvalence band electrons on average but involvingearth-abundant, non-toxic elements.Although the potential of this material is relativelyunexplored for photovoltaics, initial results havebeen promising with a group at IBM reporting 9.7%efficiency for small cells based on related materials(alloy of the above sulphide and the correspondingselenide). The appearance of the cells involved(Fig. 4.4.3.2) is quite close to the appearance ofcells made using CIGS technology (CuInSe 2plusCuGaSe 2alloy).The Centre’s work in this new strand of activitytakes a different direction from most of the presentinternational work in this area. The starting point isa viable high throughput manufacturing process.Efforts are to develop technology suitable for thedesired process rather than persevering with solutiongrowth processes that have given the best laboratoryresults to date. Materials selection would beguided by stable compositions formed in naturallyoccurring minerals such as kesterite and stannites.Another key area that is to be investigated in detailis epitaxial relationships to silicon to investigate thesuitability for tandem cell stacks, including stacksinvolving silicon as the lowermost layer.4.4.3.2 Simplified fabrication of CZTSthin film cellsThough various approaches have been appliedto fabricate CIGS solar cell, CIGS with the highestefficiency (up to 20.3% so far) have been achievedby the physical vapour deposition (PVD) approach,with the first 10% efficient cell demonstrated in1980, the first 15% cell in 1993 and the first 20% cellin 2008. As a substitute for CIGS thin film solar cells,CZTS has many similarities to CIGS as mentionedabove. As such, it is logical that the full potentialof high efficiency CZTS thin film solar cells will beextracted using the PVD approach.Though a major milestone was reached at theend of 2009 with the demonstration of 9.7% CZTSefficiency (some Se also in device) by a researchteam at IBM by a “hydrazine-based solution”approach [4.4.3.1], solution growth is believedinherently incapable of giving the material controlrequired for best-possible performance. Evidencefor this arises in the similar case of CIGS where thesame IBM team holds the record of 12.2% efficiencyfor solution-deposited CIGS (again hydrazine–based) [4.4.3.2], while in contrast the PVD approachgives 20.3% efficiency.Evaporation and sputtering are the two mostimportant PVD techniques. The disadvantage ofevaporation is that it is very difficult to control theCu evaporation source; and the lack of commerciallyavailable equipment for large area thermalevaporation. In contrast, magnetron sputtering is59
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