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Investigation of ZnO nanorod solar cells with layer-by-layer deposited CdTe quantum dot absorbers PDF

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Investigation of ZnO nanorod solar cells with layer-by-layer deposited CdTe quantum dot absorbers Joe Briscoe School of Applied Science Cranfield University Supervisors: Dr Steve Dunn and Prof Robert Dorey PhD Thesis August 3, 2011 (cid:13)c Cranfield University, 2011. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder. i Abstract Innovation in solar cell design is required to reduce cost and compete with tradi- tionalpowergeneration. Currentinnovativesolartechnologiesincludenanostructured dye-sensitisedsolarcellsandpolymersolarcells,whichbothcontainorganicmaterials with limited lifetime. This project aims to combine the advantages of ZnO nanorods andquantumdot(QD)absorbersinanall-inorganicsolarcell,usingthelayer-by-layer (LbL)processtoincreaselightabsorptioninthecell. The parameters that affect the aqueous chemical growth of ZnO nanorods were investigatedonAg-coatedsubstratesinordertoimprovethedensityandalignmentof the nanorods. Rods 3–6µm long and90–500nm in diameter were grown on fluorine- dopedtinoxide(FTO)-coatedglasssubstratesforuseinsolarcells. ZnO nanorods were doped with antimony (Sb) in-situ during their aqueous syn- thesis to make them p-type. Direct addition of Sb acetate to the reaction adversely affected the nanorod morphology, which was avoided by first dissolving the Sb ac- etateinethyleneglycol. Opticalandelectricalpropertiesofthenanorodswerealtered withSb-doping,butp-typebehaviourwasnotprovenconclusively. ZnOnanorodswereconformallycoatedwithCdTeQD-polymerfilmsusingaLbL process. Increasing the number of coated layers increased the level of light absorp- tionatwavelengthsof500-900nmduetoabsorptionbytheQDs. Airannealingofthe QD-polymerfilmsabove200◦Cledtooxidationofthefilm,whichdidnotoccurwhen annealing in vacuum. Annealing in vacuum up to 350◦C led to a slight reduction in quantum confinement effects attributed to increased interaction between the nanopar- ticles due to reduced separation. At 450◦C the polymer was completely removed and noquantumconfinementremainedintheCdTe. To complete the solar cells CuSCN was deposited between the LbL-coated ZnO nanorods by repeatedly spreading a solution of CuSCN in propyl sulphide on the sur- faceandallowingittodry. Thisfilmfilledbetweenthecoatednanorods,butthedrying and quantity of solution used had to be carefully controlled to avoid cracking in the film. Spin-coatingofCuSCNsolutionswasattempted,butfilmssuitableforsolarcells were not produced. Poly(styrenesulfonate)-doped poly(3,4- ethylenedioxythiophene) (PEDOT:PSS)wasdepositedbyspin-coatingasanalternativetoCuSCN,butthefilm onlypenetrated∼200nmbelowthenanorodtips. Solar cells were produced with different thicknesses of LbL films, annealed com- ponents and other variations. A model was proposed whereby carriers are extracted from the LbL film through exciton transfer between QDs. Annealing of the ZnO nanorods in air and reduction of the cracks in the CuSCN film both improved the efficiency of the solar cells. Annealing of the LbL film in vacuum improved the per- formance of the cells by increasing the efficiency of charge transfer. In devices with annealed LbL films 50-layer devices had higher efficiency than 30-layer devices and cells using CuSCN had a higher efficiency than those with PEDOT:PSS. The best cells produced used 50 layer CdTe-polymer films annealed at 350◦C in vacuum with CuSCN. These produced an energy conversion efficiency of 0.0062%, which com- paresto1–3%forsimilarcellsintheliteratureand10–20%forcommercialdevices. ii ThisthesisisdedicatedtoPaulandMonicaBriscoe. I was privileged to know them for so long and I am so glad they knew I would achievethis. iii Acknowledgements I would like to thank all those who have helped and supervised me in my PhD. Steve Dunn and Diego Gallardo who helped me to make a firm start, develop my skills as a researcher, and guided me through my PhD. Rob Dorey who stepped in to make sure I completed my work successfully and who has provided me with a mass of helpful and excellent suggestions to develop this thesis. Andy Stallard, Matt Taunt and Chris Shaw, without whom I could never have gathered all the data I needed. I would also like to thank all my colleagues in our group with whom I’ve chatted, discussedandbrain-stormedmyworkonmanyoccasionsandwhohavemademefeel verymuchathomeandcomfortableinmywork. My thanks also go out to my parents, Lyn and Bob, who have supported me through this process, and without whom I would never have made it here. My special thanks go to Jane Clubb, who has been there for me always, helped me through my troublesandprovidedmewithagreatdealofstrengthandsupport. iv Publications and Presentations Journal papers Briscoe, J., Gallardo, D. E., Lesnyak, V. and Dunn, S., Influence of annealing oncompositionandopticalpropertiesofCdTenanoparticlelayer-by-layerfilms, JournalofNanoscienceandNanotechnology. 11(6),5270-5273(2011). Briscoe, J., Gallardo, D. E., Hatch S., Lesnyak, V., Gaponik, N. and Dunn, S., Enhanced quantum dot deposition on ZnO nanorods for photovoltaics through layer-by-layer processing. Journal of Materials Chemistry. 21 (8), 2517-2523 (2011). Briscoe, J., Gallardo, D. E. and Dunn, S., In situ antimony doping of solution- grownZnOnanorods,ChemicalCommunications,2009,1273-1275. Briscoe,J.andDunn,S.,Extrathinabsorbersolarcellsbasedonnanostructured semiconductors,MaterialsScienceandTechnology. Underreview. Conference proceedings Briscoe, J., Gallardo, D. E. and Dunn, S., Layer-by-layer CdTe nanoparticle absorbers for ZnO nanorod solar cells - the influence of annealing on cell per- formance,MRSProceedings1260,T06-02(2010). Briscoe, J., Gallardo, D. E. and Dunn, S., Antimony doped ZnO nanorods - a changefromntoptype? MRSProceedings1256,N16-33(2010). Damitha, A. A., Adikaari, T., Briscoe, J., Dunn, S., Carey, J. D. and Silva, S. R. P., Effect of transparent electrode on the performance of bulk heterojunction solarcells,MRSProceedings,1270,HH14-23(2010). Oral presentations Self-assembled CdTe nanoparticle absorbers for ZnO nanorod solar cells - the influence of annealing on cell performance. Materials Research Symposium, SanFrancisco,April2010. In-situ antimony doping of zinc oxide nanorods grown in aqueous solution. ElectroceramicsXI,Manchester,September2008. Poster presentations Thermal annealing of layer-by-layer deposited nanoparticle composites. MRS, April2010. AntimonydopedZnOnanorods-achangefromntoptype? MRS,April2010. Self-assembled CdTe nanoparticle absorbers for ZnO nanorod solar cells. SET forBritain,HousesofParliament,March2010. Contents 1 Introduction 1 1.1 Background—electricitygeneration . . . . . . . . . . . . . . . . . . 1 1.1.1 Challengesinphotovoltaics . . . . . . . . . . . . . . . . . . 2 1.2 Aimsandobjectives . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Thesisstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 LiteratureReview 7 2.1 Solarcells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.1 Solarcelltheory . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.2 Solarcellmaterials . . . . . . . . . . . . . . . . . . . . . . . 10 2.2 Extra-thinabsorbersolarcells . . . . . . . . . . . . . . . . . . . . . 11 2.2.1 Dye-sensitisedsolarcells . . . . . . . . . . . . . . . . . . . . 13 2.2.2 Solid-stateholecollectors . . . . . . . . . . . . . . . . . . . 14 2.2.3 Inorganicabsorbers . . . . . . . . . . . . . . . . . . . . . . . 16 2.2.4 PorousTiO -basedetasolarcells . . . . . . . . . . . . . . . 18 2 2.2.5 Zincoxidenanorod-basedsolarcells . . . . . . . . . . . . . . 22 2.3 Zincoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.3.1 Zincoxidenanorodgrowthmethods . . . . . . . . . . . . . . 30 2.3.2 ZnOPhotoluminescence . . . . . . . . . . . . . . . . . . . . 34 2.3.3 P-typeZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.4 CdTenanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.4.1 Layer-by-layerdeposition . . . . . . . . . . . . . . . . . . . 40 2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3 Experimental 44 3.1 ZnOnanorodsynthesis . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.1.1 ZnOseedlayer . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.1.2 FTOetching . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.1.3 Aqueouschemicalnanorodgrowth . . . . . . . . . . . . . . 45 3.1.4 Sb-dopednanorods . . . . . . . . . . . . . . . . . . . . . . . 48 3.2 SynthesisofCdTenanoparticles . . . . . . . . . . . . . . . . . . . . 48 3.3 Layer-by-layerdepositionofCdTenanoparticles . . . . . . . . . . . 49 3.3.1 Annealingoflayer-by-layerfilms . . . . . . . . . . . . . . . 50 3.4 Copperthiocyanatedeposition . . . . . . . . . . . . . . . . . . . . . 50 v vi CONTENTS 3.5 PEDOT:PSSdeposition . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.6 Devicecompletion . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.7 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.7.1 Scanning-electronmicroscopy . . . . . . . . . . . . . . . . . 53 3.7.2 X-raydiffraction . . . . . . . . . . . . . . . . . . . . . . . . 53 3.7.3 X-rayphotoelectronspectroscopy . . . . . . . . . . . . . . . 53 3.7.4 Opticalabsorption . . . . . . . . . . . . . . . . . . . . . . . 53 3.7.5 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . 54 3.7.6 Electricalandphotovoltaiccharacterisation . . . . . . . . . . 54 4 ZnOnanorodsynthesis 55 4.1 Nanorodmorphologyandnucleation . . . . . . . . . . . . . . . . . . 55 4.1.1 NanorodgrowthonAg . . . . . . . . . . . . . . . . . . . . . 55 4.1.2 Seedlayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.1.3 Nanorodgrowthonseedlayer . . . . . . . . . . . . . . . . . 58 4.1.4 GrowthwithPEI . . . . . . . . . . . . . . . . . . . . . . . . 58 4.2 Opticalproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.2.1 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.2.2 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . 61 4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 5 Sb-dopedZnOnanorods 64 5.1 Morphologyandcomposition . . . . . . . . . . . . . . . . . . . . . . 64 5.2 Opticalproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.2.1 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.2.2 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . 72 5.3 Electricalproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 6 CdTenanoparticlelayer-by-layerfilms 77 6.1 Morphologyandcomposition . . . . . . . . . . . . . . . . . . . . . . 77 6.2 Opticalproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 6.2.1 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 6.2.2 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . 82 6.3 Annealedfilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 6.3.1 Compositionaleffects . . . . . . . . . . . . . . . . . . . . . 85 6.3.2 Opticalchanges . . . . . . . . . . . . . . . . . . . . . . . . . 88 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 7 Solarcells 95 7.1 ZnOnanorodsforsolarcells . . . . . . . . . . . . . . . . . . . . . . 95 7.2 LbL-coatednanorodsforsolarcells . . . . . . . . . . . . . . . . . . 97 7.3 Copperthiocyanate . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 7.3.1 Spin-coatingtrialswithethylsulphide . . . . . . . . . . . . . 99 CONTENTS vii 7.3.2 Earlytrialsusingpropylsulphide . . . . . . . . . . . . . . . 100 7.3.3 SC1andSC2devices . . . . . . . . . . . . . . . . . . . . . . 101 7.3.4 SC3devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 7.4 PEDOT:PSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 7.5 Photovoltaicproperties . . . . . . . . . . . . . . . . . . . . . . . . . 110 7.5.1 RoleofZnOinthesolarcells . . . . . . . . . . . . . . . . . 112 7.5.2 RoleofLbLCdTe-PDDAfilmsinthesolarcells . . . . . . . 116 7.5.3 AnnealingLbLfilmsinair . . . . . . . . . . . . . . . . . . . 119 7.5.4 AnnealingofZnO . . . . . . . . . . . . . . . . . . . . . . . 120 7.5.5 AnnealingLbLfilmsinvacuum . . . . . . . . . . . . . . . . 121 7.5.6 ComparisonofsolarcellscontainingCuSCNandPEDOT:PSS 124 7.5.7 ImprovementinCuSCN . . . . . . . . . . . . . . . . . . . . 126 7.5.8 IncreasinglayersincellswithannealedLbLfilms . . . . . . . 127 7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 8 ConclusionsandFutureWork 132 8.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.1.1 Backgroundandaims . . . . . . . . . . . . . . . . . . . . . . 132 8.1.2 Stagesofwork . . . . . . . . . . . . . . . . . . . . . . . . . 133 8.1.3 Completionofobjectives . . . . . . . . . . . . . . . . . . . . 135 8.2 FutureWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 References 141 List of Figures 2.1 ExampleJ-Vbehaviourofanon-idealsolarcellinthedarkandunder illumination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2 AM1.5solarspectrum. . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3 Equivalentcircuitforasolarcell. . . . . . . . . . . . . . . . . . . . . 10 2.4 Schematicofanextra-thinabsorbersolarcell. . . . . . . . . . . . . . 11 2.5 EnergybanddiagramandschematicofananostructuredDSSC. . . . 13 2.6 EnergybanddiagramthebandalignmentoftheTiO /dye/CuSCNand 2 TiO /dye/CuIsystems. . . . . . . . . . . . . . . . . . . . . . . . . . 15 2 2.7 SEMimagesofCuSCNproducedbySILARmethod. . . . . . . . . . 16 2.8 Energy band diagram showing the change in alignment between the conduction bands of TiO and PbS with and without quantum con- 2 finementeffects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.9 BandalignmentfortheTiO /Se/CuSCNsolarcell. . . . . . . . . . . 19 2 2.10 EQEandtransmissionspectraofTiO /In(OH)S/Sb S /CuSCN,TiO / 2 2 3 2 CdS/CuSCNandTiO /In(OH)S/Cu S/CuSCNsolarcells. . . . . . . . 21 2 2 2.11 Band diagram and SEM image of TiO /In(OH) S /Pb(OH) S / PE- 2 x y x y DOT:PSSsolarcellstructure. . . . . . . . . . . . . . . . . . . . . . . 21 2.12 SEM images of ZnO nanorods produced by MOCVD with secondary rodsgrowingfromthesurface. . . . . . . . . . . . . . . . . . . . . . 23 2.13 EffectofaddingPEItothechemicalsynthesisofZnOnanorods. . . . 23 2.14 ZnO nanorods grown by electrochemical deposition and coated with a-Sibychemicalvapourdeposition. . . . . . . . . . . . . . . . . . . 24 2.15 Uncoated and CdSe-coated ZnO nanorods grown by electrochemical deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.16 PredictedbandalignmentforZnO/CdTe/CuSCN&ZnO/CdSe/CuSCN. 26 2.17 SEMimagesofthecross-sectionofaZnOnanorod/In S /CuSCNso- 2 3 larcellandZnOnanorodswithIn S coating. . . . . . . . . . . . . . 27 2 3 2.18 Change in J , V and FF in the ZnO nanorod/In S /CuSCN solar sc oc 2 3 cell as local thickness of In S layer is increased and proposed band 2 3 structureoftheIn S –CuSCNinterfacebeforeandafterannealing. . . 27 2 3 2.19 Increase in quantum efficiency of the ZnO nanorod/CdSe/MEH-PPV solarcellwithdifferentannealingtimesinair/CdCl at380◦C. . . . . 28 2 2.20 ZnOnanorodsgrownbythevapour-solidmechanism. . . . . . . . . . 31 2.21 ZnOnanorodsgrownusingzincnitrateandHMT. . . . . . . . . . . . 33 viii

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Cranfield University. Supervisors: Dr Steve Dunn and Prof Robert Dorey. PhD Thesis. August 3, 2011 cс Cranfield University, 2011. All rights reserved. fuels. Within the renewable option there are a number of technologies such as wind, tidal, solar photovoltaic (PV), geothermal, biomass, etc. Many
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