Chiang Mai J. Sci. 2019; 46(5) : 1009-1014 http://epg.science.cmu.ac.th/ejournal/ Contributed Paper Investigation of Cu Doped Cadmium Sulphide Photoconductive Cells Suchittra 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: [email protected]Received: 14 March 2019 Revised: 10 May 2019 Accepted: 16 May 2019 ABSTRACT Thin film cadmium sulphide photoconductive cells were prepared on clean glass slides by Chemical Bath Deposition (CBD). Copper (Cu) ions were used for doping. Cupric Chloride (CuCl 2 ) and Cadmium Chloride (CdCl 2 ) were mixed with thiourea (CH 4 N 2 S) solution. Different amounts of CuCl 2 (0.1, 0.2, 0.3 and 0.4 % (molar)) were employed. The annealing temperatures were 300 °C, 400 °C and 500 °C. The XRD analysis revealed that the as-deposited film showed the cubic CdS phase but the hexagonal phase appeared at 300 °C and at higher temperatures. The microstructure study showed that the grain size of 500 °C annealed 0.4 % Cu doped CdS was biggest. Good agreement was found between crystallite size and photosensitivity. With further development by using pair of dopants, this technique could produce better photosensitivity of the CdS cell. Keywords: conductive cells, photoconductive cells, Cu doped cadmium sulphide, photosensitivity 1. I NTRODUCTION For several decades, semiconductors have been studied to be employed as solar cells. They can be classified into 3 generations. The first is traditional or wafer based cells. They are made of crystalline silicon such as monocrystalline silicon and including poly-silicon. Second generation cells are thin film solar cells such as amorphous 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 (CuInSe 2 ) and those having chalcopyrite structure. The third generation of the solar cell includes a number of thin film technologies which have not yet been commercially applied and are still in research. The most popular generation 2 com- pounds are the compounds in groups III-V and II-VI for example gallium arsenide (GaAs), cadmium sulphide (CdS), zinc sulphide (ZnS) etc. These compounds were originally used as
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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.
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
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
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.
The resistancesof the samples inFigure1(b)werecalculatedandaretabulatedintable1togetherwiththeircrystallitesizeandmicrostrain.Thecrystallitesizes(D)of thesampleswerecalculatedfromXRDdatausingDebye-Scherrer’sequationasfollow.
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
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