Chiang Mai J. Sci. 2019; 46(5) : 1009-1014http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Investigation of Cu Doped Cadmium Sulphide Photoconductive CellsSuchittra Inthong [a], Pratthana Intawin [a], Arnon Kraipok [a], Jaruwan Kanthachan [a],Sukum Eitssayeam [a], Uraiwan Inthata [b], Manlika Kamnoy [a], Denis Sweatman [a] and Tawee Tunkasiri*[a][a] Department of Physics and Materials, Faculty of Science, Chiang Mai University, Chiang Mai 50200,

Thailand.[b] School of Science, Mae Fah Luang University, Chiang Rai 57000, Thailand.

*Author for correspondence; e-mail: tawee.tun@cmu.ac.th

Received: 14 March 2019Revised: 10 May 2019

Accepted: 16 May 2019

ABSTRACT Thinfilmcadmiumsulphidephotoconductivecellswerepreparedoncleanglassslides

byChemicalBathDeposition(CBD).Copper(Cu)ionswereusedfordoping.CupricChloride(CuCl2) and Cadmium Chloride (CdCl2)weremixedwiththiourea(CH4N2S)solution.Differentamountsof CuCl2(0.1,0.2,0.3and0.4%(molar))wereemployed.Theannealingtemperatureswere300°C,400°Cand500°C.TheXRDanalysisrevealedthattheas-depositedfilmshowedthecubicCdSphasebutthehexagonalphaseappearedat300°Candathighertemperatures.Themicrostructurestudyshowedthatthegrainsizeof 500°Cannealed0.4%CudopedCdSwasbiggest.Goodagreementwasfoundbetweencrystallitesizeandphotosensitivity.Withfurtherdevelopmentbyusingpairof dopants,thistechniquecouldproducebetterphotosensitivityof the CdS cell.

Keywords:conductivecells,photoconductivecells,Cudopedcadmiumsulphide,photosensitivity

1. INTRODUCTION Forseveraldecades,semiconductorshave

been studied to be employed as solar cells. They canbeclassifiedinto3generations.Thefirstistraditionalorwaferbasedcells.Theyaremadeof crystallinesiliconsuchasmonocrystallinesilicon and including poly-silicon. Second generationcellsarethinfilmsolarcellssuchasamorphous silicon, binary compounds such as cadmium telluride (CdTe), cadmium sulphide (CdS), gallium arsenide (GaAs) etc. and ternary compounds such as gallium arsenide phosphide

(GaAsP), Copper indium diselenide (CuInSe2) andthosehavingchalcopyritestructure.Thethirdgenerationof thesolarcell includesanumberof thinfilmtechnologieswhichhavenot yet been commercially applied and are still in research.

The most popular generation 2 com-pounds are the compounds in groups III-V andII-VIforexamplegalliumarsenide(GaAs),cadmiumsulphide(CdS),zincsulphide(ZnS)etc.Thesecompoundswereoriginallyusedas

Chiang Mai J. Sci. 2019; 46(5)1010

thesubstratesandalsoimplantedwithotherions such as neodenium (Nd) and thallium (Tl).Later,mostof theworkwasconcentratedonGaAsasitwasthecompoundcommonlyusedintechnologiessuchasfortheproductionof Gunndiodesandlasers.GaAsisaveryimportant compound semiconductor. It has manyadvantagesoversilicon,suchashighermobility,andhasthepossibilityof formingsomedevices,forexample,Gunndiodeswhichcannotberealizedwithsilicon.

Group III-V compounds can also be usedas lightemittingdiodes (LED)forexample,GaAs-PemitsredlightwhilstGaAsemitinfraredlight.Photoreflectancestudyof strainedGaAsN/GaAsT-junctionquantumwireswascarriedoutbyKlangtakaietal.[1].Structuralandmechanicalpropertiesof GaAsunderpressureupto200GPawerestudiedbyPluergphonetal.[2].Ternarycompoundssuch as CuInSe2wereintensivelystudiedbySa-yakanitetal. [3] inorderto investigatetheirelectronicstructure,forpossibleuseashighlyefficientsolarcells.However,therearestillsomecomplicationsfortheproductionof both GaAs and CuInSe2 solar cells.

In II-VI semiconductor compounds, therearequiteafewcompoundssuchasCdS,CdSe,ZnOetc.,havingawidebandgapwhichareverygoodforemployingaslightemittingdiodesandlaserdiodesforblueandultravioletapplications.Duetoproblemswithconductivity,theapplicationof thesecompoundsisstillquestionable.ZnOisthebestexample.Itshowsexcellentopticalcharacteristics,thoughit remains problematic to create high charge carrierdensitiesviadopinginthecompound[4].

Dopingof otherionsintoII-VIcompoundssuch as CdS and CdSe can increase the conduction of thematerialsandtheycanbecomephoto-cells.Bothcompoundsarebasicallyvariableresistancedeviceswhoseresistancedependsuponthesensitivitylevelof theincidentlight,theresistancefallingastheilluminationincreases.

InthisarticlewestudythelightsensitivitysomeII-VI photocells. Some II-VI materials such asCdSwereemployed.Cuionswereusedfordopinginthecompoundviaachemicalreaction.Thephotocellswerepreparedviachemicalbathdeposition(CBD).Silverpaintwasappliedtomaketheohmiccontact.Photosensitivityaswellasmicrostructureandcrystallinityof thesamplesafterannealingwerestudied.

2. MATERIALS AND METHODSCBDisatechniqueforlargeareathin

filmdepositionviaachemicalreaction.Thetechniquecanbefoundpublishedelsewhere[5].Fromourpreliminaryexperiment(notrecordedhere)wefoundthatif weemployed0.05Mof cadmium chloride (CdCl2)solutionmixedwith0.05Mof thiourea(CH4N2S) solution it can produceagoodfilmof CdS,asinvestigatedusing a scanning electron microscope (FE-SEM, JSM6335F).Therefore,inthiswork,copperdopedcadmiumsulphide(CdS)waspreparedbymixingcupricchloride inthecadmiumchloride solution (about 0.05 M, 150 cm3). Themixedsolutionwaspouredintoacleanbeakerwithamagneticstirreratthebottom.Then0.05Mthiourea(CH4N2S) solution (150 cm3)wasslowlypouredintothebeaker,whichwasheatedupto80ºC,withthemagneticbarcontinuouslystirring.Theamountsof cupricchloride (CuCl2)were0.1%,0.2%,0.3%and0.4%(molar).Eachconcentrationwasaddedintothesolutionseparately.ThepHof thesolutionwascontrolledupto9,byslowlyaddingasolutionof 25%of ammoniumhydroxide(NH4OH).Atthisstage,acleanglassslidewasimmersedvertically inthebeakerforabout1hr.Athinfilmof CudopedCdSwasthendeposited on the immersed glass slide. The filmsweredriedfor24hrsandthengraduallyannealedat300°C,400°Cand500°C.Thestructureandthesurfaceof thethinfilmswereexaminedusinganX-raydiffractometer(XRD,JDX-8030)andascanningelectronmicroscope

Chiang Mai J. Sci. 2019; 46(5) 1011

(FE-SEM,JSM6335F),respectively.Silverpaintwaspastedontomaketheohmic

contactonthefilmsurface.Themeasurementof thecurrent-voltage(I-V)characteristicwascarriedoutontheundopedanddopedCdSfilms.Filmsof anotherII-VIcompoundsuchasCdSewerealsopreparedforcomparison,employingthesamepreparationprocedure.Copperwasusedfordopingatthesameamountasthatof the Cu-doped CdS. The circuit employed to measurethecurrent-voltage(I-V)curveswasasimplecircuit,i.e.adcvoltagesupplyconnectedinparalleltothefilmandanammeterinseries.TheI-Vcurveof eachfilmwasmeasuredinthedarkandunderirradiationwithwhitelight.Aforty-wattPhillipsbulbwasemployed.Thegraphsareshowninthisreport.

3. RESULT AND DISCUSSIONSThestructureof theCudopedCdSwas

investigatedbyXRD.Thediffractogramsof differentamountsof Cu-dopedCdSfilmsshowedthattheconcentrationof dopantdidnotchangethediffractogramssignificantly.Thediffractogramsof Cu-dopedCdSat0.1%,0.2%,0.3% and 0.4% concentrations are presented

inFigure1a,togetherwiththediffractogramsof 0.4%Cu-dopedfilmannealedat300°C,400°Cand500°C.Thestandardpeaksof JCPDSfilesarealsopresented.ItappearsthattheXRDpeaksof theunannealedCdSfilmshowedcubicunitcellCdS(JCPDS,41-1049).Howeverfrom300°Candabove,thestructureof thehexagonalunitcell(JCPDS,75-1546)appeared.Thepeaksof cubicCdSstill appeared, indicating that the cubic and hexagonalCdSweremixedtogether,thoughthehexagonalphasewasmorepronounced.Inourexperiment,theas-depositedCdSfilmshowedcubicphase(f.c.c)andthisresultisinagreementwiththatobtainedbyOlivaetal.[6],whoalsofoundcubicunitcellof theas-depositedCdSfilm,usingthesamepreparationprocedure.

Afterannealingat300°CthehexagonalCdS phase started to appear. The occurrence of hexagonalCdSphaseisinaccordancewiththeresultobtainedbyGiletal.[7],whofoundthehexagonalCdSphaseafterheatingabove250°C.Uponfurtherannealingat400°Cand500°C,thehexagonalphasewasmorepronounced.Thetransformationof cubicphasetohexagonalphasecanbeexplainedas

Figure 1.(a)TheX-raydiffractogramsof theasdepositedfilms,Cu-dopedCdSat0.1%,0.2%,0.3%and0.4%,includingtheannealed0.4%Cudopedfilm,at300ºC,400ºCand500ºC.(•)=anunidentifiedpeak.(b)TheI-Vcurvesof the0.4%CudopedCdSandCdSesamples.

Chiang Mai J. Sci. 2019; 46(5)1012

follows.Theannealingtemperaturecanincreasethe lattice parameter and interplanar distance. Thelatticedeformsslowlyfromf.c.cphasetothetransitionstate.Uponslowlycoolingdownto room temperature, the atoms then rearrange andslipintothehexagonalclosepack(hcp)structure.Presumably,theyhavelessenergythanforpackinginf.c.cstructure.However,inourworkcubicpeak(111)stillappeared(Figure1a)butthepeakheightdecreasedafterannealingat400°Cand500°C.The(200)peakof cubicCdSwasusedtoevaluatethedecreasingamountof cubicphase.Themaximumpeak(111)of cubicphasecannotbeusedsinceitoverlappedwiththehexagonalpeak(002).Thequantitativemeasurementwasperformedusingtheareaunderthe(200)peakinarbitraryunits.Itwasfoundthatthecubicphasedecreaseddownto40%(at500°C)fromthatof thedepositedfilm.

InFigure1a.theunidentifiedpeak(•)intheXRDpatternappearedthroughoutfrompreparation and annealing processes. The unidentifiedpeakmayoccurfromimpurityinthechemicals.ACdOpeak(JCPDS78-0653)appearedduringannealingfrom300°Cto500°C.Thisisduetotheinfluenceof oxygenfromtheaironCdSandsotheformationof CdOthenoccurred[8].CdOisalsoann-typesemiconductorhavingabandgapof 2.18eV.Underanappliedvoltage,itcanalsocontributetothephotosensitivityalongsidethatof Cu-dopedCdS.Theexistenceof CdOmayaffectthedecreaseinresistanceof thesamplestogetherwiththeeffectof crystallitesize.

OtherII-VIcompoundssuchascadmiumselenide (CdSe) and cadmium telluride (CdTe) are in the same group and can be employed as solarcells.Thisisduetotheirphotosensitivityinthevisiblespectrum.WechoseCdSeforcomparingwithCdSfilmsas itsbandgap(1.74 eV) is closer to CdS (2.42 eV) than that of CdTe(1.49eV)atroomtemperature.CdSisanimportantn-typesemiconductor.Under

anappliedvoltage(evenwithoutexposingtothe light) the electrons can conduct through the sample.

Copper is achemicalelementwithveryhighelectricalconductivity.It isoftenemployedfordopinginsemiconductors.Petreetal.[9]studiedtheinfluenceof Cudopingonopto-electronicpropertiesof chemicallydepositedCdS.Zhuetal.[10]studiedtheeffectsof differentdopingratiosof CudopedCdSonQDSCsperformance.HabbasandAhmad[11]studiedtheeffectof dopingonsomephysicalpropertiesof chemicalsprayedCdSfilms.AfterdopingwithCuions,thechargemobilitywasthenincreasing.

TheI-Vcurvesof theunannealedandannealedat300°C400°Cand500°Cof 0.4%CudopedCdSfilmswereexposedtowhitelight.Thecurvesof thefilms(Figure1b)showedstraightlines,indicatingthatcurrent(I)variesasthevoltage(V).Moreover,atthesameappliedvoltage,thehighertemperatureannealedsampleshowedahighercurrent.Thesametrendoccurredforthe0.4%CudopedCdSeI-Vcurve.However,thephotosensitivityof theCdSefilmwasalittlebithigherthanforthatof the500°CannealedCdSsample.Thedifferencesweresmall.

Thestudyof themicrostructureof thesamplesannealedat300°C,400°Cand500°C(of the0.4%Cudopedfilm)arepresentedinFigure2.Itcanbeseenthatthegrainsizeof thesamplesdevelopedfromnanometer(annealedat300°C)tomicrometerat500°C.Thismaybeduetothedegreeof crystallinityof sampleswhichincreasedwiththehighertemperature anneal.

The resistancesof the samples inFigure1(b)werecalculatedandaretabulatedintable1togetherwiththeircrystallitesizeandmicrostrain.Thecrystallitesizes(D)of thesampleswerecalculatedfromXRDdatausingDebye-Scherrer’sequationasfollow.

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D=0.9λ/βcosθ (1)

Whereβ=full-widthathalf maximum(FWHM) λ=X-Raywavelength(1.54Å) θ=Bragg’sdiffractionangle

Themicrostrain(ε)of thesamplewascalculatedfromtheequation(2)

β=4εtanθ (2)

Equation(2)wasderivedfromBragg’sequation.

Theresistanceof theunannealedsamples(0.4%CudopedCdS)withoutexposingtothelightwasabout2×106Ω.Theresultsforthecrystallitesizeof theunannealedsamplewascomparabletothatobtainedbyAbbasandAhmad[11]whopreparedtheirsamplesbychemicalspraytechniques.Thepeakusedtocalculatethecrystallitesizeandstrain(hcpCdS)was(101),duetoitsmaximumintensity.Someotherpeakswereeitherof lowintensityoroverlappingwiththoseof cubicpeaks.From Table 1, it can be concluded that the

Figure 2.TheSEMmicrostructureof the0.4%Cu-dopedsample,annealedat(a)300ºC,(b)400ºCand(c)500ºC.

Table 1.Crystallitesizes,microstrainsandresistancesof 0.4%Cu(molar)dopedCdSfilms.

Temperature Crystallite size Microstrain Resistance(°C) D (nm) ε (×10-3) (×106 Ω)

unannealed 30 1.45 2

300°C 35 1.3 0.8

400°C 50 1.2 0.6

500°C 300 0.5 0.5

crystallitesizesof theunannealed,300°Cand400°Cannealedsamplesslowlyincreasedbutthenrapidly increasedat500°C.This is inaccordancewiththegrainsizeof thesamplesasinFigure2.Theresistanceof eachfilmslowlydroppedfrom2×106Ωdownto0.5×106Ω,asshowninTable1.Theresistanceof CdSedroppedfrom7×106Ωdownto~0.3×106Ω.

Itwas thought that thegrainsizeorthecrystallitesizemayplaythekeyroleforphotosensitivity.Asthecrystallitesizeof CdSgrewbigger,itthendecreasedthenumberof thecrystalliteboundaries.Thereforethepotentialbarriersbetweenthecrystalboundarieswerelessthanthatof smallergrains.Therefore,thechargecarriersof thecrystalconductthrough

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thegrainsmoreeasily.Inourexperimenthowever,withhightemperature(>500°C)annealedcannotgivebiggergrainsize.Instead,itproducedmoreCdOasaresult,theCdScrystalsdecrease.OtherionsdopedCdS,whichproduceasimilarmagnitudeof grainsize,couldalsobeemployed.Eitssayeametal.[11]carriedoutworkonNidopedCdSthinfilmsusingCBDtechniques.Thegrainsizeof theirsampleswasintherangeof 200-400nmwhichisinthesameorderasthatof ours(300nm).Itisalsowouldbeinterestingtoemploypairsof dopantssuchasCuandNiwhichcouldimprovethemorphologyof thethinCdSfilms.Furtherdevelopmentusingvariouspairsof dopantscouldbringbetterphotosensitivityof thephotoconductiveCdScells.

4. CONCLUSIONTheexperimentundertookthepreparation

of copperdopedCadmiumsulphidethinfilmphotoconductivecells.Thechemicalbathdeposition(CBD)techniquewasemployed.Glassslideswereusedassubstrates.Differentamountsof CuCl2 rangingfrom0.1%to0.4%wereusedfordoping.Annealingwascarriedoutat300°C,400°C,and500°C.TheSEMmicrographsof theCudopedsamplesshowedthattheCdSfilmprecipitatedontheglassslideshadgrainsizesinnanometerscaleat300°C,andgraduallygrewbiggerintothemicrometerscaleat500°C.Resultsagreedwellwithcorrelationbetweentheirphotosensitivityandtheircrystallitesizes.Photosensitivitywasbetterwiththehigherannealingtemperature.Furtherdevelopmentusingpairsof dopantscouldimprovetheCdSphotoconductivecellwhichcouldbedevelopedintolargeascaleindustry.

ACKNOWLEDGMENTTheauthorswould like to thankthe

Departmentof PhysicsandMaterials,Facultyof Science,ChiangMaiUniversity,Thailand

fortheirfinancialsupport.

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