Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2013 Article ID 561948 5 pageshttpdxdoiorg1011552013561948
Research ArticleTo Enhance Performance of Light Soaking Process onZnSCuIn1minusxGaxSe2 Solar Cell
Yu-Jen Hsiao1 Chung-Hsin Lu2 and Te-Hua Fang3
1 National Nano Device Laboratories Tainan 741 Taiwan2Department of Chemical Engineering National Taiwan University Taipei 617 Taiwan3Department of Mechanical Engineering National Kaohsiung University of Applied Sciences Kaohsiung 807 Taiwan
Correspondence should be addressed to Yu-Jen Hsiao yujenhsiaotwgmailcom
Received 20 September 2013 Accepted 13 October 2013
Academic Editor Teen-Hang Meen
Copyright copy 2013 Yu-Jen Hsiao et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
The ZnSCuInGaSe2heterojunction solar cell fabricated onMo coated glass is studiedThe crystallinity of the CIGS absorber layer
is prepared by coevaporated method and the ZnS buffer layer with a band gap of 321 eV The MoS2phase was also found in the
CuInGaSe2Mo system form HRTEMThe light soaking effect of photoactive film for 10min results in an increase in FF from 558
to 64 but series resistivity from 74 to 38Ω The efficiency of the devices improved from 812 to 950
1 Introduction
Various photovoltaic (PV) devices have received a lot ofattention as a renewable energy source [1] These devicesare made based on inorganic or organic materials such assilicon-based thin film solar cells [2] multijunction solarcells [3] and dye-sensitized solar cells [4] However most ofthese devices suffer from either high cost or relatively lowenergy conversion efficiency CuIn
1minus119909Ga119909Se2(CIGS)-based
thin films have received considerable attention as one of themost promising materials for thin film solar cells due totheir high absorption coefficient the potential for low costmanufacturing and high conversion efficiency for inorganicsolar cell application although MoCuInGaSe
2(CIGS) thin
film solar cells have been extensively studied [5]A high efficiency CIGS is usually fabricated by a coevap-
oration method but this is a complex process to scaleup to large areas due to the problem of nonuniformitySeveral low cost and promising alternatives to coevaporationsuitable for large scale production such as sputtering [6]electrodeposition [7] and screen printing [8] have beeninvestigated In particular sputtering of CuInGa precursorsfollowed by selenization appears to be a favored processfor thin film deposition [9] In this study a three-stage
co-evaporation deposition technology was adopted to growCIGS photovoltaicelectronic films to realize efficient solarcells To avoid the Cd-induced pollution to environment then-typeZnS buffer layer for formingCIGSPVswas introduced[10]
In this paper we report ZnSCuIn1minus119909
Ga119909Se2heterojunc-
tion solar cells fabricated on Mo coated glass The crys-tallinity of the CIGS and ZnS buffer layer were studied Themicrostructure phase was investigated in the CuInGaSe
2Mo
system by HRTEM The light soaking and electrical proper-ties of the fabricated p-CIGSn-ZnS solar cells will also bediscussed
2 Experimental
The soda-lime glasses were carefully cleaned in isopropanol-acetone ultrasonic bath to remove electrostatic charges Anapproximately 07120583m thick Mo back contact was directlydeposited by RF sputtering on glass substrate CIGS absorberlayers were deposited by evaporation of elemental Cu In Gaand Se onto Mo by three-stage co-evaporation During the1st stage of the absorber growth the substrate temperaturewas kept at 400∘C while during the 2nd and 3rd stages
2 International Journal of Photoenergy
the substrate temperature increased up to 600∘C The 100ndash300 nm thick ZnS buffer layer was deposited on CIGS filmby chemical bath deposition at 80∘C The solar cells werefinished by deposition of a ZnS buffer layer RF sputtering ofZnO Al front contacts (sim400 nm thick) and electron beamevaporation of 1 120583m thick Al contact grids for better currentcollection No antireflection (AR) coating was applied
The phase identification was performed by X-ray powderdiffraction (XRD Rigaku Dmax-33)The surface microstruc-ture was examined by scanning electron microscopy (SEMHitachi S4200) The morphology and microstructure wereexamined by transmission electron microscopy (HRTEMHF-2000 Hitachi) The absorption spectra were obtainedusing an optical spectrometer (Hitachi U-4100) and current-voltage measurements (Keithley 2410 SourceMeter) wereobtained using a solar simulator (TELTEC) with an AM 15filter under an irradiation intensity of 100mWcm2
3 Results and Discussion
The CIGS quaternary alloy absorber layer coevaporatedexhibits the characteristic peaks of chalcopyrite structure inX-ray diffraction (XRD) analysis as shown in Figure 1(a)XRD spectra also indicate that the CIGS film presents astrong (112) preferred orientation at 2120579= 2668 correspondingto chalcopyrite phases The other prominent peaks corre-sponded to the (220) and (312) directions The full widthat half maximum (FWHM) of the diffraction peak is rathersmall which indicates that the film crystallinity is fairly good
XRD patterns of ZnS with various deposition thicknessesare shown in Figure 1(b) The possible chemical reactions forthe synthesis of ZnS films are as follows
[Zn(H2O)5(OH)]+ +H+ larrrarr Zn(OH)
2+ 2H+ (1)
CH3CSNH
2+H+ + 2H
2O
larrrarr H2S + CH
3COOH +NH+
4
(2)
H2Slarrrarr HSminus +H+ larrrarr S2minus +H+ (3)
Zn2+ + S2minus 997888rarr ZnS (4)
During the reaction processes sulfide ions are releasedslowly from CH
3CSNH
2and react with zinc ions It indicates
that ZnS is produced by reaction of S2minus and Zn2+ in (4) Allof the peaks were identified to be those of the cubic ZnSphase (JCPDS card number 79-0043) [11] The crystallinityof ZnS increased along with deposition thickness When thethickness was increased from 100 to 300 nm the peaks of(111) (220) and (311) were obviously shown
Figure 2(a) has shown the UV-vis absorption spectra of200 nm ZnS film on glass and estimated the band gap For adirect band gap semiconductor the absorbance in the vicinityof the onset due to the electronic transition is given by thefollowing equation
120572 =
119862(ℎ] minus 119864119892)
12
ℎ]
(5)
(112)
(211)
(220)
(312)(400)
CIGS film
20 30 40 50 60 70 80
Inte
nsity
(au
)
2120579 (deg)
Mo(110)Mo(211)
(a)
300nm
200nm
100nm
20 30 40 50 60
Inte
nsity
(au
)
2120579 (deg)
C(111)
C(220)
C(311)
(b)
Figure 1 (a) XRD spectra of CIGS film on Mo electrode (b) XRDspectra of ZnS film synthesized using a chemical bath depositionmethod at various thicknesses
where 120572 is the absorption coefficient 119862 is the constant ℎ]is the photon energy and 119864
119892is the band gap The visible
light absorption edge of 200 nm ZnS film was at 386 nmExtrapolation of the linear region gives a band gap of 321 eVTherefore the direct band gap energy obtained from ourexperiment is 321 eV As known hydrothermal process maytransform some elemental S species to sulfur dioxides It hasdefect states like S vacancies in the band gap of ZnS [12]Therefore the sample had lower band gap than ideal crystalstructure of ZnS with 368 eV It has shown actual sampleof ZnS film before and after being deposited on CIGSMosubstrate in Figure 2(b) The color of CIGSMo was grayon the surface and then we can see brown color as ZnSfilm deposited on CIGSMo substrate Therefore we candetermine ZnS film on CIGS by color variability
Figure 3(a) shows the cross-sectional bright field TEMimage of the MoCIGSZnS stacked layers In contrast tothe relatively large grains of the CIGS layer (02 to 07 120583m)
International Journal of Photoenergy 3
300 400 500 600 700
Wavelength (nm)
Abso
rptio
n120572
(cm
minus1)
321eV for ZnS
ZnS sim200nm
(a)
CIGSMo ZnSCIGSMo
(b)
Figure 2 (a) Absorption spectra are as the function of photonenergy for the 200 nm ZnS film (b) Realized sample of ZnS filmdeposited on CIGSMo substrate
the ZnS layer consists of very small grains Although theCIGS layer exhibits substantial surface roughness (sim80 nmin average) the ZnS layer grown on top of CIGS has auniform thickness (sim200 nm) that was prepared for TEMby the focused ion beam (FIB) Each one of the layersconstituting the MoCIGSZnS system was investigated inorder to know the formation of defects as well as to getinformation regarding crystalline structure and grain sizeOn the other hand two different regions are identified in theZnS and CIGSMo films as is observed in the micrograph ofFigures 3(b)-3(c) The crystalline ZnS films are identified bythe high resolution lattice images A representative HRTEMimage enlarging a round part of the structure in Figure 3(b)is given The interplanar distances of the crystal fringes areabout 031 nm
The microstructure of the MoCuInGaSe2interfaces was
investigated in order to visualize defects and the formation
(a)
(b)
(c)
Figure 3 (a) TEM cross-section images of ZnSCIGSMo (b) high-resolution TEM image of the cubic ZnS film and (c) the interface ofCIGS and Mo
Cu
Cu
Cu
Ga
Ga
Ga
Se Se
Se
In
0 2 4 6 8 10 12
(keV)
Figure 4 EDX analysis of the CIGS film
of secondary phases as a result of possible chemical reac-tions occurring during the deposition of the stacked layersFigure 3(c) shows a typical cross-sectional HRTEM imageof the MoCIGS interface The formation of a very thinlayer (10ndash40 nm) of a new compound is observed aroundthe MoCIGS interface It seems that the new compoundcorresponds to the MoS
2phase due to the similarity with
the CuInGaSe2Mo system in which an interlayer of MoSe
2is
usually formed [13] This result makes sure that the metallicMo thin layer is converted into MoS
2during the initial
minutes of CIGS deposition The MoS2layer gives rise to
a small conduction band offset with respect to the CIGSbulk material and a small Schottky barrier at the Mo backcontact [14] Both features are good for device performancebecause the conduction band offset diminishes the backsurface recombination and then arrow Schottky barrier givesno substantial resistance to holes between CIGS and themetallic back contact The EDS line profiles indicate that theCIGS film consists of Cu In Ga and Se as shown in Figure 4In addition the atomic concentrations of Cu = 23 In =21 Ga = 10 and Se = 46 are calculated from the EDSspectrum
4 International Journal of PhotoenergyJ
(mA
cm
2)
25
20
15
10
5
0
00 02 04 06
Voltage (V)
Lighting 0minLighting 5minLighting 10min
(a)
90
80
70
60
50
40
30
20
10
400 500 600 700 800 900 1000 1100
Wavelength (nm)
EQE
()
95
(b)
Figure 5 (a) J-V characteristics of ZnSCIGS heterojunction solarcell with various light soaking times and (b) the IPCE spectrum ofCIGS solar cell with efficiency of 95
Table 1 Photovoltaic performance of the AZOZnSCIGS hetero-junction solar cell with various light soaking times under AM15Gat 100mWcm2 illumination
Before light soaking the AZOZnSCIGS heterojunctionsolar cell generally suffered from poor one-diode behavioura characteristic especially marked at low temperatures Thesituation is considerably improved after light soaking withvarious times under AM15G at 100mWcm2 illumination
The solar cell parameters of the cells used in photovoltaicmeasurements in Table 1The time needed for the parametersto saturate under illumination is also shown Before lightsoaking the problem with the cells was the low fill factor andhigh series resistivity The fill factor increased significantlywith light soaking for ZnS buffer layer while 119881oc remainsstable In Figure 5(a) J-V curves obtained in the light soakedstates for the lighting 5 and 10min
The measurements reveal that lighting into the photoac-tive film results in an increase in FF from 558 to 64but series resistivity from 74 to 38Ω The 120578 value of thedevices improved from 812 to 950 The effect of FF valueis attributed to the positive conduction band offsets (CBO)between the CIGS layer and the buffer layer and it has beensuggested that this barrier is lowered by illumination due topersistent photoconductivity (PPC) in the buffer layer [15]In this work white light-induced metastable changes to theFF are only observed for cells with buffer layers having alighting time 10min In addition the quantum efficiencies aremeasured after light soaking in Figure 5(b) The EQE spectraare similar in shape consistent with the almost unchangedshort circuit current density
4 Conclusions
In summary the ZnSCuInGaSe2heterojunction solar cell
with the light soaking process has been investigated Thecrystallinity of the CIGS absorber layer is fairly good bycoevaporated method ZnS buffer layer with a band gap of321 eV was deposited on CIGSMo sample The MoS
2phase
was found in the CuInGaSe2Mo system form HRTEM The
light soaking effect of photoactive film for 10min results in anincrease in FF from 558 to 64 but series resistivity from74 to 38Ω The 120578 value of the devices improved from 812 to950
Acknowledgment
This research is supported by the National Science CouncilTaiwan under contract no NSC 102-3113-P-002-026
References
[1] N S Lewis ldquoToward cost-effective solar energy userdquo Sciencevol 315 no 5813 pp 798ndash801 2007
[2] K L Chopra P D Paulson and V Dutta ldquoThin-film solar cellsan overviewrdquo Progress in Photovoltaics vol 12 pp 69ndash92 2004
[3] M Konagai ldquoDeposition of newmicrocrystalline materials 120583c-SiC 120583c-GeC byHWCVD and solar cell applicationsrdquoThin SolidFilms vol 516 no 5 pp 490ndash495 2008
[4] L Lu R Li K Fan andT Peng ldquoEffects of annealing conditionson the photoelectrochemical properties of dye-sensitized solarcells made with ZnO nanoparticlesrdquo Solar Energy vol 84 pp844ndash853 2010
[5] I Repins M A Contreras B Egaas et al ldquo19sdot9-efficientZnOCdSCuInGaSe2 solar cell with 81sdot2 fill factorrdquo Progressin Photovoltaics vol 16 pp 235ndash239 2008
International Journal of Photoenergy 5
[6] G S Chen J C Yang Y C Chan L C Yang and W HuangldquoAnother route to fabricate single-phase chalcogenides by post-selenization of Cu-In-Ga precursors sputter deposited from asingle ternary targetrdquo Solar EnergyMaterials and Solar Cells vol93 pp 1351ndash1355 2009
[7] Y Lai F Liu Z Zhang et al ldquoCyclic voltammetry study ofelectrodeposition of Cu(InGa)Se
2thin filmsrdquo Electrochimica
Acta vol 54 no 11 pp 3004ndash3010 2009[8] S J Ahn C W Kim J H Yun et al ldquoCuInSe
2(CIS) thin
film solar cells by direct coating and selenization of solutionprecursorsrdquo Journal of Physical Chemistry C vol 114 no 17 pp8108ndash8113 2010
[9] M S Hanssen H Efstathiadis and P Haldar ldquoDevelopment ofsmooth CuInGa precursor films for CuIn
1minus119883Ga119909Se2thin film
solar cell applicationsrdquo Thin Solid Films vol 519 no 19 pp6297ndash6301 2011
[10] T Nakada and M Mizutani ldquo18 Efficiency Cd-freeCu(InGa)Se
[11] M Scocioreanu M Baibarac I Baltog I Pasuk and T VelulaldquoPhotoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnSin cubic and hexagonal phaserdquo Journal of Solid State Chemistryvol 186 pp 217ndash223 2012
[12] W Chen Z G Wang Z J Lin and L Y Lin ldquoAbsorption andluminescence of the surface states inZnSnanoparticlesrdquo Journalof Applied Physics vol 82 no 6 pp 3111ndash3115 1997
[13] X Zhu Z Zhou Y Wang L Zhang A Li and F HuangldquoDetermining factor of MoSe
2formation in Cu(InGa)Se
2solar
cellsrdquo Solar Energy Materials and Solar Cells vol 101 pp 57ndash612012
[14] D Abou-Ras G Kostorz D Bremaud et al ldquoFormation andcharacterisation of MoSe
2for Cu(InGa)Se
2based solar cellsrdquo
Thin Solid Films vol 480-481 pp 433ndash438 2005[15] I L Eisgruber J E Granata J R Sites J Hou and J Kessler
ldquoBlue-photon modification of nonstandard diode barrier inCuInSe
2solar cellsrdquo Solar Energy Materials and Solar Cells vol
the substrate temperature increased up to 600∘C The 100ndash300 nm thick ZnS buffer layer was deposited on CIGS filmby chemical bath deposition at 80∘C The solar cells werefinished by deposition of a ZnS buffer layer RF sputtering ofZnO Al front contacts (sim400 nm thick) and electron beamevaporation of 1 120583m thick Al contact grids for better currentcollection No antireflection (AR) coating was applied
The phase identification was performed by X-ray powderdiffraction (XRD Rigaku Dmax-33)The surface microstruc-ture was examined by scanning electron microscopy (SEMHitachi S4200) The morphology and microstructure wereexamined by transmission electron microscopy (HRTEMHF-2000 Hitachi) The absorption spectra were obtainedusing an optical spectrometer (Hitachi U-4100) and current-voltage measurements (Keithley 2410 SourceMeter) wereobtained using a solar simulator (TELTEC) with an AM 15filter under an irradiation intensity of 100mWcm2
3 Results and Discussion
The CIGS quaternary alloy absorber layer coevaporatedexhibits the characteristic peaks of chalcopyrite structure inX-ray diffraction (XRD) analysis as shown in Figure 1(a)XRD spectra also indicate that the CIGS film presents astrong (112) preferred orientation at 2120579= 2668 correspondingto chalcopyrite phases The other prominent peaks corre-sponded to the (220) and (312) directions The full widthat half maximum (FWHM) of the diffraction peak is rathersmall which indicates that the film crystallinity is fairly good
XRD patterns of ZnS with various deposition thicknessesare shown in Figure 1(b) The possible chemical reactions forthe synthesis of ZnS films are as follows
[Zn(H2O)5(OH)]+ +H+ larrrarr Zn(OH)
2+ 2H+ (1)
CH3CSNH
2+H+ + 2H
2O
larrrarr H2S + CH
3COOH +NH+
4
(2)
H2Slarrrarr HSminus +H+ larrrarr S2minus +H+ (3)
Zn2+ + S2minus 997888rarr ZnS (4)
During the reaction processes sulfide ions are releasedslowly from CH
3CSNH
2and react with zinc ions It indicates
that ZnS is produced by reaction of S2minus and Zn2+ in (4) Allof the peaks were identified to be those of the cubic ZnSphase (JCPDS card number 79-0043) [11] The crystallinityof ZnS increased along with deposition thickness When thethickness was increased from 100 to 300 nm the peaks of(111) (220) and (311) were obviously shown
Figure 2(a) has shown the UV-vis absorption spectra of200 nm ZnS film on glass and estimated the band gap For adirect band gap semiconductor the absorbance in the vicinityof the onset due to the electronic transition is given by thefollowing equation
120572 =
119862(ℎ] minus 119864119892)
12
ℎ]
(5)
(112)
(211)
(220)
(312)(400)
CIGS film
20 30 40 50 60 70 80
Inte
nsity
(au
)
2120579 (deg)
Mo(110)Mo(211)
(a)
300nm
200nm
100nm
20 30 40 50 60
Inte
nsity
(au
)
2120579 (deg)
C(111)
C(220)
C(311)
(b)
Figure 1 (a) XRD spectra of CIGS film on Mo electrode (b) XRDspectra of ZnS film synthesized using a chemical bath depositionmethod at various thicknesses
where 120572 is the absorption coefficient 119862 is the constant ℎ]is the photon energy and 119864
119892is the band gap The visible
light absorption edge of 200 nm ZnS film was at 386 nmExtrapolation of the linear region gives a band gap of 321 eVTherefore the direct band gap energy obtained from ourexperiment is 321 eV As known hydrothermal process maytransform some elemental S species to sulfur dioxides It hasdefect states like S vacancies in the band gap of ZnS [12]Therefore the sample had lower band gap than ideal crystalstructure of ZnS with 368 eV It has shown actual sampleof ZnS film before and after being deposited on CIGSMosubstrate in Figure 2(b) The color of CIGSMo was grayon the surface and then we can see brown color as ZnSfilm deposited on CIGSMo substrate Therefore we candetermine ZnS film on CIGS by color variability
Figure 3(a) shows the cross-sectional bright field TEMimage of the MoCIGSZnS stacked layers In contrast tothe relatively large grains of the CIGS layer (02 to 07 120583m)
International Journal of Photoenergy 3
300 400 500 600 700
Wavelength (nm)
Abso
rptio
n120572
(cm
minus1)
321eV for ZnS
ZnS sim200nm
(a)
CIGSMo ZnSCIGSMo
(b)
Figure 2 (a) Absorption spectra are as the function of photonenergy for the 200 nm ZnS film (b) Realized sample of ZnS filmdeposited on CIGSMo substrate
the ZnS layer consists of very small grains Although theCIGS layer exhibits substantial surface roughness (sim80 nmin average) the ZnS layer grown on top of CIGS has auniform thickness (sim200 nm) that was prepared for TEMby the focused ion beam (FIB) Each one of the layersconstituting the MoCIGSZnS system was investigated inorder to know the formation of defects as well as to getinformation regarding crystalline structure and grain sizeOn the other hand two different regions are identified in theZnS and CIGSMo films as is observed in the micrograph ofFigures 3(b)-3(c) The crystalline ZnS films are identified bythe high resolution lattice images A representative HRTEMimage enlarging a round part of the structure in Figure 3(b)is given The interplanar distances of the crystal fringes areabout 031 nm
The microstructure of the MoCuInGaSe2interfaces was
investigated in order to visualize defects and the formation
(a)
(b)
(c)
Figure 3 (a) TEM cross-section images of ZnSCIGSMo (b) high-resolution TEM image of the cubic ZnS film and (c) the interface ofCIGS and Mo
Cu
Cu
Cu
Ga
Ga
Ga
Se Se
Se
In
0 2 4 6 8 10 12
(keV)
Figure 4 EDX analysis of the CIGS film
of secondary phases as a result of possible chemical reac-tions occurring during the deposition of the stacked layersFigure 3(c) shows a typical cross-sectional HRTEM imageof the MoCIGS interface The formation of a very thinlayer (10ndash40 nm) of a new compound is observed aroundthe MoCIGS interface It seems that the new compoundcorresponds to the MoS
2phase due to the similarity with
the CuInGaSe2Mo system in which an interlayer of MoSe
2is
usually formed [13] This result makes sure that the metallicMo thin layer is converted into MoS
2during the initial
minutes of CIGS deposition The MoS2layer gives rise to
a small conduction band offset with respect to the CIGSbulk material and a small Schottky barrier at the Mo backcontact [14] Both features are good for device performancebecause the conduction band offset diminishes the backsurface recombination and then arrow Schottky barrier givesno substantial resistance to holes between CIGS and themetallic back contact The EDS line profiles indicate that theCIGS film consists of Cu In Ga and Se as shown in Figure 4In addition the atomic concentrations of Cu = 23 In =21 Ga = 10 and Se = 46 are calculated from the EDSspectrum
4 International Journal of PhotoenergyJ
(mA
cm
2)
25
20
15
10
5
0
00 02 04 06
Voltage (V)
Lighting 0minLighting 5minLighting 10min
(a)
90
80
70
60
50
40
30
20
10
400 500 600 700 800 900 1000 1100
Wavelength (nm)
EQE
()
95
(b)
Figure 5 (a) J-V characteristics of ZnSCIGS heterojunction solarcell with various light soaking times and (b) the IPCE spectrum ofCIGS solar cell with efficiency of 95
Table 1 Photovoltaic performance of the AZOZnSCIGS hetero-junction solar cell with various light soaking times under AM15Gat 100mWcm2 illumination
Before light soaking the AZOZnSCIGS heterojunctionsolar cell generally suffered from poor one-diode behavioura characteristic especially marked at low temperatures Thesituation is considerably improved after light soaking withvarious times under AM15G at 100mWcm2 illumination
The solar cell parameters of the cells used in photovoltaicmeasurements in Table 1The time needed for the parametersto saturate under illumination is also shown Before lightsoaking the problem with the cells was the low fill factor andhigh series resistivity The fill factor increased significantlywith light soaking for ZnS buffer layer while 119881oc remainsstable In Figure 5(a) J-V curves obtained in the light soakedstates for the lighting 5 and 10min
The measurements reveal that lighting into the photoac-tive film results in an increase in FF from 558 to 64but series resistivity from 74 to 38Ω The 120578 value of thedevices improved from 812 to 950 The effect of FF valueis attributed to the positive conduction band offsets (CBO)between the CIGS layer and the buffer layer and it has beensuggested that this barrier is lowered by illumination due topersistent photoconductivity (PPC) in the buffer layer [15]In this work white light-induced metastable changes to theFF are only observed for cells with buffer layers having alighting time 10min In addition the quantum efficiencies aremeasured after light soaking in Figure 5(b) The EQE spectraare similar in shape consistent with the almost unchangedshort circuit current density
4 Conclusions
In summary the ZnSCuInGaSe2heterojunction solar cell
with the light soaking process has been investigated Thecrystallinity of the CIGS absorber layer is fairly good bycoevaporated method ZnS buffer layer with a band gap of321 eV was deposited on CIGSMo sample The MoS
2phase
was found in the CuInGaSe2Mo system form HRTEM The
light soaking effect of photoactive film for 10min results in anincrease in FF from 558 to 64 but series resistivity from74 to 38Ω The 120578 value of the devices improved from 812 to950
Acknowledgment
This research is supported by the National Science CouncilTaiwan under contract no NSC 102-3113-P-002-026
References
[1] N S Lewis ldquoToward cost-effective solar energy userdquo Sciencevol 315 no 5813 pp 798ndash801 2007
[2] K L Chopra P D Paulson and V Dutta ldquoThin-film solar cellsan overviewrdquo Progress in Photovoltaics vol 12 pp 69ndash92 2004
[3] M Konagai ldquoDeposition of newmicrocrystalline materials 120583c-SiC 120583c-GeC byHWCVD and solar cell applicationsrdquoThin SolidFilms vol 516 no 5 pp 490ndash495 2008
[4] L Lu R Li K Fan andT Peng ldquoEffects of annealing conditionson the photoelectrochemical properties of dye-sensitized solarcells made with ZnO nanoparticlesrdquo Solar Energy vol 84 pp844ndash853 2010
[5] I Repins M A Contreras B Egaas et al ldquo19sdot9-efficientZnOCdSCuInGaSe2 solar cell with 81sdot2 fill factorrdquo Progressin Photovoltaics vol 16 pp 235ndash239 2008
International Journal of Photoenergy 5
[6] G S Chen J C Yang Y C Chan L C Yang and W HuangldquoAnother route to fabricate single-phase chalcogenides by post-selenization of Cu-In-Ga precursors sputter deposited from asingle ternary targetrdquo Solar EnergyMaterials and Solar Cells vol93 pp 1351ndash1355 2009
[7] Y Lai F Liu Z Zhang et al ldquoCyclic voltammetry study ofelectrodeposition of Cu(InGa)Se
2thin filmsrdquo Electrochimica
Acta vol 54 no 11 pp 3004ndash3010 2009[8] S J Ahn C W Kim J H Yun et al ldquoCuInSe
2(CIS) thin
film solar cells by direct coating and selenization of solutionprecursorsrdquo Journal of Physical Chemistry C vol 114 no 17 pp8108ndash8113 2010
[9] M S Hanssen H Efstathiadis and P Haldar ldquoDevelopment ofsmooth CuInGa precursor films for CuIn
1minus119883Ga119909Se2thin film
solar cell applicationsrdquo Thin Solid Films vol 519 no 19 pp6297ndash6301 2011
[10] T Nakada and M Mizutani ldquo18 Efficiency Cd-freeCu(InGa)Se
[11] M Scocioreanu M Baibarac I Baltog I Pasuk and T VelulaldquoPhotoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnSin cubic and hexagonal phaserdquo Journal of Solid State Chemistryvol 186 pp 217ndash223 2012
[12] W Chen Z G Wang Z J Lin and L Y Lin ldquoAbsorption andluminescence of the surface states inZnSnanoparticlesrdquo Journalof Applied Physics vol 82 no 6 pp 3111ndash3115 1997
[13] X Zhu Z Zhou Y Wang L Zhang A Li and F HuangldquoDetermining factor of MoSe
2formation in Cu(InGa)Se
2solar
cellsrdquo Solar Energy Materials and Solar Cells vol 101 pp 57ndash612012
[14] D Abou-Ras G Kostorz D Bremaud et al ldquoFormation andcharacterisation of MoSe
2for Cu(InGa)Se
2based solar cellsrdquo
Thin Solid Films vol 480-481 pp 433ndash438 2005[15] I L Eisgruber J E Granata J R Sites J Hou and J Kessler
ldquoBlue-photon modification of nonstandard diode barrier inCuInSe
2solar cellsrdquo Solar Energy Materials and Solar Cells vol
Figure 2 (a) Absorption spectra are as the function of photonenergy for the 200 nm ZnS film (b) Realized sample of ZnS filmdeposited on CIGSMo substrate
the ZnS layer consists of very small grains Although theCIGS layer exhibits substantial surface roughness (sim80 nmin average) the ZnS layer grown on top of CIGS has auniform thickness (sim200 nm) that was prepared for TEMby the focused ion beam (FIB) Each one of the layersconstituting the MoCIGSZnS system was investigated inorder to know the formation of defects as well as to getinformation regarding crystalline structure and grain sizeOn the other hand two different regions are identified in theZnS and CIGSMo films as is observed in the micrograph ofFigures 3(b)-3(c) The crystalline ZnS films are identified bythe high resolution lattice images A representative HRTEMimage enlarging a round part of the structure in Figure 3(b)is given The interplanar distances of the crystal fringes areabout 031 nm
The microstructure of the MoCuInGaSe2interfaces was
investigated in order to visualize defects and the formation
(a)
(b)
(c)
Figure 3 (a) TEM cross-section images of ZnSCIGSMo (b) high-resolution TEM image of the cubic ZnS film and (c) the interface ofCIGS and Mo
Cu
Cu
Cu
Ga
Ga
Ga
Se Se
Se
In
0 2 4 6 8 10 12
(keV)
Figure 4 EDX analysis of the CIGS film
of secondary phases as a result of possible chemical reac-tions occurring during the deposition of the stacked layersFigure 3(c) shows a typical cross-sectional HRTEM imageof the MoCIGS interface The formation of a very thinlayer (10ndash40 nm) of a new compound is observed aroundthe MoCIGS interface It seems that the new compoundcorresponds to the MoS
2phase due to the similarity with
the CuInGaSe2Mo system in which an interlayer of MoSe
2is
usually formed [13] This result makes sure that the metallicMo thin layer is converted into MoS
2during the initial
minutes of CIGS deposition The MoS2layer gives rise to
a small conduction band offset with respect to the CIGSbulk material and a small Schottky barrier at the Mo backcontact [14] Both features are good for device performancebecause the conduction band offset diminishes the backsurface recombination and then arrow Schottky barrier givesno substantial resistance to holes between CIGS and themetallic back contact The EDS line profiles indicate that theCIGS film consists of Cu In Ga and Se as shown in Figure 4In addition the atomic concentrations of Cu = 23 In =21 Ga = 10 and Se = 46 are calculated from the EDSspectrum
4 International Journal of PhotoenergyJ
(mA
cm
2)
25
20
15
10
5
0
00 02 04 06
Voltage (V)
Lighting 0minLighting 5minLighting 10min
(a)
90
80
70
60
50
40
30
20
10
400 500 600 700 800 900 1000 1100
Wavelength (nm)
EQE
()
95
(b)
Figure 5 (a) J-V characteristics of ZnSCIGS heterojunction solarcell with various light soaking times and (b) the IPCE spectrum ofCIGS solar cell with efficiency of 95
Table 1 Photovoltaic performance of the AZOZnSCIGS hetero-junction solar cell with various light soaking times under AM15Gat 100mWcm2 illumination
Before light soaking the AZOZnSCIGS heterojunctionsolar cell generally suffered from poor one-diode behavioura characteristic especially marked at low temperatures Thesituation is considerably improved after light soaking withvarious times under AM15G at 100mWcm2 illumination
The solar cell parameters of the cells used in photovoltaicmeasurements in Table 1The time needed for the parametersto saturate under illumination is also shown Before lightsoaking the problem with the cells was the low fill factor andhigh series resistivity The fill factor increased significantlywith light soaking for ZnS buffer layer while 119881oc remainsstable In Figure 5(a) J-V curves obtained in the light soakedstates for the lighting 5 and 10min
The measurements reveal that lighting into the photoac-tive film results in an increase in FF from 558 to 64but series resistivity from 74 to 38Ω The 120578 value of thedevices improved from 812 to 950 The effect of FF valueis attributed to the positive conduction band offsets (CBO)between the CIGS layer and the buffer layer and it has beensuggested that this barrier is lowered by illumination due topersistent photoconductivity (PPC) in the buffer layer [15]In this work white light-induced metastable changes to theFF are only observed for cells with buffer layers having alighting time 10min In addition the quantum efficiencies aremeasured after light soaking in Figure 5(b) The EQE spectraare similar in shape consistent with the almost unchangedshort circuit current density
4 Conclusions
In summary the ZnSCuInGaSe2heterojunction solar cell
with the light soaking process has been investigated Thecrystallinity of the CIGS absorber layer is fairly good bycoevaporated method ZnS buffer layer with a band gap of321 eV was deposited on CIGSMo sample The MoS
2phase
was found in the CuInGaSe2Mo system form HRTEM The
light soaking effect of photoactive film for 10min results in anincrease in FF from 558 to 64 but series resistivity from74 to 38Ω The 120578 value of the devices improved from 812 to950
Acknowledgment
This research is supported by the National Science CouncilTaiwan under contract no NSC 102-3113-P-002-026
References
[1] N S Lewis ldquoToward cost-effective solar energy userdquo Sciencevol 315 no 5813 pp 798ndash801 2007
[2] K L Chopra P D Paulson and V Dutta ldquoThin-film solar cellsan overviewrdquo Progress in Photovoltaics vol 12 pp 69ndash92 2004
[3] M Konagai ldquoDeposition of newmicrocrystalline materials 120583c-SiC 120583c-GeC byHWCVD and solar cell applicationsrdquoThin SolidFilms vol 516 no 5 pp 490ndash495 2008
[4] L Lu R Li K Fan andT Peng ldquoEffects of annealing conditionson the photoelectrochemical properties of dye-sensitized solarcells made with ZnO nanoparticlesrdquo Solar Energy vol 84 pp844ndash853 2010
[5] I Repins M A Contreras B Egaas et al ldquo19sdot9-efficientZnOCdSCuInGaSe2 solar cell with 81sdot2 fill factorrdquo Progressin Photovoltaics vol 16 pp 235ndash239 2008
International Journal of Photoenergy 5
[6] G S Chen J C Yang Y C Chan L C Yang and W HuangldquoAnother route to fabricate single-phase chalcogenides by post-selenization of Cu-In-Ga precursors sputter deposited from asingle ternary targetrdquo Solar EnergyMaterials and Solar Cells vol93 pp 1351ndash1355 2009
[7] Y Lai F Liu Z Zhang et al ldquoCyclic voltammetry study ofelectrodeposition of Cu(InGa)Se
2thin filmsrdquo Electrochimica
Acta vol 54 no 11 pp 3004ndash3010 2009[8] S J Ahn C W Kim J H Yun et al ldquoCuInSe
2(CIS) thin
film solar cells by direct coating and selenization of solutionprecursorsrdquo Journal of Physical Chemistry C vol 114 no 17 pp8108ndash8113 2010
[9] M S Hanssen H Efstathiadis and P Haldar ldquoDevelopment ofsmooth CuInGa precursor films for CuIn
1minus119883Ga119909Se2thin film
solar cell applicationsrdquo Thin Solid Films vol 519 no 19 pp6297ndash6301 2011
[10] T Nakada and M Mizutani ldquo18 Efficiency Cd-freeCu(InGa)Se
[11] M Scocioreanu M Baibarac I Baltog I Pasuk and T VelulaldquoPhotoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnSin cubic and hexagonal phaserdquo Journal of Solid State Chemistryvol 186 pp 217ndash223 2012
[12] W Chen Z G Wang Z J Lin and L Y Lin ldquoAbsorption andluminescence of the surface states inZnSnanoparticlesrdquo Journalof Applied Physics vol 82 no 6 pp 3111ndash3115 1997
[13] X Zhu Z Zhou Y Wang L Zhang A Li and F HuangldquoDetermining factor of MoSe
2formation in Cu(InGa)Se
2solar
cellsrdquo Solar Energy Materials and Solar Cells vol 101 pp 57ndash612012
[14] D Abou-Ras G Kostorz D Bremaud et al ldquoFormation andcharacterisation of MoSe
2for Cu(InGa)Se
2based solar cellsrdquo
Thin Solid Films vol 480-481 pp 433ndash438 2005[15] I L Eisgruber J E Granata J R Sites J Hou and J Kessler
ldquoBlue-photon modification of nonstandard diode barrier inCuInSe
2solar cellsrdquo Solar Energy Materials and Solar Cells vol
Figure 5 (a) J-V characteristics of ZnSCIGS heterojunction solarcell with various light soaking times and (b) the IPCE spectrum ofCIGS solar cell with efficiency of 95
Table 1 Photovoltaic performance of the AZOZnSCIGS hetero-junction solar cell with various light soaking times under AM15Gat 100mWcm2 illumination
Before light soaking the AZOZnSCIGS heterojunctionsolar cell generally suffered from poor one-diode behavioura characteristic especially marked at low temperatures Thesituation is considerably improved after light soaking withvarious times under AM15G at 100mWcm2 illumination
The solar cell parameters of the cells used in photovoltaicmeasurements in Table 1The time needed for the parametersto saturate under illumination is also shown Before lightsoaking the problem with the cells was the low fill factor andhigh series resistivity The fill factor increased significantlywith light soaking for ZnS buffer layer while 119881oc remainsstable In Figure 5(a) J-V curves obtained in the light soakedstates for the lighting 5 and 10min
The measurements reveal that lighting into the photoac-tive film results in an increase in FF from 558 to 64but series resistivity from 74 to 38Ω The 120578 value of thedevices improved from 812 to 950 The effect of FF valueis attributed to the positive conduction band offsets (CBO)between the CIGS layer and the buffer layer and it has beensuggested that this barrier is lowered by illumination due topersistent photoconductivity (PPC) in the buffer layer [15]In this work white light-induced metastable changes to theFF are only observed for cells with buffer layers having alighting time 10min In addition the quantum efficiencies aremeasured after light soaking in Figure 5(b) The EQE spectraare similar in shape consistent with the almost unchangedshort circuit current density
4 Conclusions
In summary the ZnSCuInGaSe2heterojunction solar cell
with the light soaking process has been investigated Thecrystallinity of the CIGS absorber layer is fairly good bycoevaporated method ZnS buffer layer with a band gap of321 eV was deposited on CIGSMo sample The MoS
2phase
was found in the CuInGaSe2Mo system form HRTEM The
light soaking effect of photoactive film for 10min results in anincrease in FF from 558 to 64 but series resistivity from74 to 38Ω The 120578 value of the devices improved from 812 to950
Acknowledgment
This research is supported by the National Science CouncilTaiwan under contract no NSC 102-3113-P-002-026
References
[1] N S Lewis ldquoToward cost-effective solar energy userdquo Sciencevol 315 no 5813 pp 798ndash801 2007
[2] K L Chopra P D Paulson and V Dutta ldquoThin-film solar cellsan overviewrdquo Progress in Photovoltaics vol 12 pp 69ndash92 2004
[3] M Konagai ldquoDeposition of newmicrocrystalline materials 120583c-SiC 120583c-GeC byHWCVD and solar cell applicationsrdquoThin SolidFilms vol 516 no 5 pp 490ndash495 2008
[4] L Lu R Li K Fan andT Peng ldquoEffects of annealing conditionson the photoelectrochemical properties of dye-sensitized solarcells made with ZnO nanoparticlesrdquo Solar Energy vol 84 pp844ndash853 2010
[5] I Repins M A Contreras B Egaas et al ldquo19sdot9-efficientZnOCdSCuInGaSe2 solar cell with 81sdot2 fill factorrdquo Progressin Photovoltaics vol 16 pp 235ndash239 2008
International Journal of Photoenergy 5
[6] G S Chen J C Yang Y C Chan L C Yang and W HuangldquoAnother route to fabricate single-phase chalcogenides by post-selenization of Cu-In-Ga precursors sputter deposited from asingle ternary targetrdquo Solar EnergyMaterials and Solar Cells vol93 pp 1351ndash1355 2009
[7] Y Lai F Liu Z Zhang et al ldquoCyclic voltammetry study ofelectrodeposition of Cu(InGa)Se
2thin filmsrdquo Electrochimica
Acta vol 54 no 11 pp 3004ndash3010 2009[8] S J Ahn C W Kim J H Yun et al ldquoCuInSe
2(CIS) thin
film solar cells by direct coating and selenization of solutionprecursorsrdquo Journal of Physical Chemistry C vol 114 no 17 pp8108ndash8113 2010
[9] M S Hanssen H Efstathiadis and P Haldar ldquoDevelopment ofsmooth CuInGa precursor films for CuIn
1minus119883Ga119909Se2thin film
solar cell applicationsrdquo Thin Solid Films vol 519 no 19 pp6297ndash6301 2011
[10] T Nakada and M Mizutani ldquo18 Efficiency Cd-freeCu(InGa)Se
[11] M Scocioreanu M Baibarac I Baltog I Pasuk and T VelulaldquoPhotoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnSin cubic and hexagonal phaserdquo Journal of Solid State Chemistryvol 186 pp 217ndash223 2012
[12] W Chen Z G Wang Z J Lin and L Y Lin ldquoAbsorption andluminescence of the surface states inZnSnanoparticlesrdquo Journalof Applied Physics vol 82 no 6 pp 3111ndash3115 1997
[13] X Zhu Z Zhou Y Wang L Zhang A Li and F HuangldquoDetermining factor of MoSe
2formation in Cu(InGa)Se
2solar
cellsrdquo Solar Energy Materials and Solar Cells vol 101 pp 57ndash612012
[14] D Abou-Ras G Kostorz D Bremaud et al ldquoFormation andcharacterisation of MoSe
2for Cu(InGa)Se
2based solar cellsrdquo
Thin Solid Films vol 480-481 pp 433ndash438 2005[15] I L Eisgruber J E Granata J R Sites J Hou and J Kessler
ldquoBlue-photon modification of nonstandard diode barrier inCuInSe
2solar cellsrdquo Solar Energy Materials and Solar Cells vol
[6] G S Chen J C Yang Y C Chan L C Yang and W HuangldquoAnother route to fabricate single-phase chalcogenides by post-selenization of Cu-In-Ga precursors sputter deposited from asingle ternary targetrdquo Solar EnergyMaterials and Solar Cells vol93 pp 1351ndash1355 2009
[7] Y Lai F Liu Z Zhang et al ldquoCyclic voltammetry study ofelectrodeposition of Cu(InGa)Se
2thin filmsrdquo Electrochimica
Acta vol 54 no 11 pp 3004ndash3010 2009[8] S J Ahn C W Kim J H Yun et al ldquoCuInSe
2(CIS) thin
film solar cells by direct coating and selenization of solutionprecursorsrdquo Journal of Physical Chemistry C vol 114 no 17 pp8108ndash8113 2010
[9] M S Hanssen H Efstathiadis and P Haldar ldquoDevelopment ofsmooth CuInGa precursor films for CuIn
1minus119883Ga119909Se2thin film
solar cell applicationsrdquo Thin Solid Films vol 519 no 19 pp6297ndash6301 2011
[10] T Nakada and M Mizutani ldquo18 Efficiency Cd-freeCu(InGa)Se
[11] M Scocioreanu M Baibarac I Baltog I Pasuk and T VelulaldquoPhotoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnSin cubic and hexagonal phaserdquo Journal of Solid State Chemistryvol 186 pp 217ndash223 2012
[12] W Chen Z G Wang Z J Lin and L Y Lin ldquoAbsorption andluminescence of the surface states inZnSnanoparticlesrdquo Journalof Applied Physics vol 82 no 6 pp 3111ndash3115 1997
[13] X Zhu Z Zhou Y Wang L Zhang A Li and F HuangldquoDetermining factor of MoSe
2formation in Cu(InGa)Se
2solar
cellsrdquo Solar Energy Materials and Solar Cells vol 101 pp 57ndash612012
[14] D Abou-Ras G Kostorz D Bremaud et al ldquoFormation andcharacterisation of MoSe
2for Cu(InGa)Se
2based solar cellsrdquo
Thin Solid Films vol 480-481 pp 433ndash438 2005[15] I L Eisgruber J E Granata J R Sites J Hou and J Kessler
ldquoBlue-photon modification of nonstandard diode barrier inCuInSe
2solar cellsrdquo Solar Energy Materials and Solar Cells vol
