Research Article An Investigation of Nanocrystalline and...

Hindawi Publishing CorporationJournal of SpectroscopyVolume 2013 Article ID 321483 9 pageshttpdxdoiorg1011552013321483

Research ArticleAn Investigation of Nanocrystalline andElectrochemically Grown Cu2ZnSnS4 Thin Film UsingRedox Couples of Different Band Offset

Prashant K Sarswat1 Michael L Free1 and Gagan Kumar2

1 Department of Metallurgical engineering University of UT Salt Lake City Utah 84112 USA2 Institute for Research in Electronics and Applied Physics University of Maryland College Park MD 20742 USA

Correspondence should be addressed to Prashant K Sarswat saraswatpgmailcom

Received 31 May 2013 Accepted 12 August 2013

Academic Editor Yogendra Mishra

Copyright copy 2013 Prashant K Sarswat et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Alternative electrolytes were examined to evaluate photoelectrochemical response of Cu2ZnSnS

4films at different biasing potential

Selections of the electrolytes were made on the basis of relative Fermi level position and standard reduction potential Our searchwas focused on some cost-effective electrolytes which can produce good photocurrent during illuminationThin films were grownon FTO substrate using ink of nanocrystalline Cu

2ZnSnS

4particles as well as electrodeposition-elevated temperature sulfurization

approach Our investigations suggest that photoelectrochemical response is mostly due to conduction band-mediated processSurface topography and phase purity were investigated after each electrochemical test in order to evaluate filmquality and reactivityof electrolytes Raman examination of film and nanocrystals was conducted for comparison The difference in photocurrentresponse was explained due to various parameters such as change in charge transfer rate constant presence of dangling bonddifference in concentration of adsorbed species in electrode

1 Introduction

In the last two decades there has been significant interest indeveloping copper zinc tin sulfide Cu

2ZnSnS

4(CZTS) based

solar cell devices CZTS offers favorable optical and electronicproperties suitable for solar cell applications A tremendousimprovement has been reported in the power conversion effi-ciency of CZTS device in the last few years [1] Recently 111efficiency of photovoltaic device with standard configuration(glassMoCZTSSeCdSZnOITONi-Al) reported by IBMindeed proves that CZTS will be a leading candidate for thinfilm environmentally friendly solar cell [2] One of the greatadvantages of this material is utilization of abundant and costeffective constituent elements Despite the fact of improve-ment in efficiency many other shortcomings are also neededto be overcome such as low fill factor reduced short circuitcurrent and high recombination due to secondary phases Inall cases it is essential to examine device performance prior tofabrication of complete photovoltaic cell In most of the time

photovoltaic properties of CZTS absorber were examinedusing a ldquosemiconductor-electrolyte interfacerdquo as a testinganalog [3]This facilitates easy formation of Schottky barriersusing heterojunctions [3] A nondestructive way of photo-voltaic property evaluation is an additional benefit It can beseen that inmany cases Eu3+Eu2+ was utilized as a redox cou-ple because of various associated advantages [3ndash5] Howeverit is important to note that europium nitrate is a costly rareearth metal containing salt and strong oxidizer [6] Hencethere is an urgent need to find alternative redox systems withproperties of fast charge transfer and exhibiting photoelectro-chemical behavior of CZTS absorber material In this paperwe examine alternative electrolytes aiming to explore theenhanced photocurrent response in their respective appliedpotential range We fabricate most of the CZTS thin filmsusing electrodeposition-elevated temperature sulfurizationon transparent conducting substrate Some of the CZTS filmsare prepared using separately synthesized CZTS nanocrys-tals The CZTS film qualities are finally evaluated for each

2 Journal of Spectroscopy

CZTS conduction band

CZTS valance band

H2H2O[Fe(CN)6]3minus4minus

Eu2+3+

Fe2+Fe3+

H2OO2

Other closelylocated redox

couple

Cr3+Cr2+ Cd2+CdFe2+Fe

HOOCCOOH(aq)HCHO(aq)

minus15

minus1

minus05

0

05

1

15

2

25E NHE

0

minus1

minus2

minus3

minus4

minus5

minus6

minus7

Vaccum

Fermi equivalent level

Figure 1 A schematic diagram showing CZTS band alignment with different redox electrolytes

Table 1 Standard reduction potential for selected reactions

No Electrode reaction Standard reduction potential1 HCHO(aq) + 2H2O + 2eminus 999445999468 CH3OH + 2OHminus minus059V2 2CO2(g) + 2H+ + 2eminus 999445999468HOOCCOOH(aq) minus049V3 Cr3+ + eminus 999445999468 Cr2+ minus042V4 2SO3

2minus + 3H2O + 4eminus 999445999468 S2O32minus+ 6OHminus minus058V

5 NO2minus + H2O + eminus 999445999468 NO + 2OHminus minus046V

electrolyte system It is important to mention that the fastdeveloping area of plasmonics has great potential in enhanc-ing the performance of the solar cells [7] Plasmonics dealswith the excitation of the surface plasmons which basicallyconverts the light energy into electricity in the photovoltaicdevices [8 9] Optical field enhancement in metallic nanos-tructures is generally utilized to achieve strong enhancementof optical absorption Metallic nanoparticles or other dielec-tric nanocrystals support the energy collection of incidentphotons in surface plasmon oscillations This energy can besubsequently transferred to the semiconductor substrate in amore efficient manner in comparison with the conventionalphotovoltaic CZTS absorber These experiments and simula-tion are currently in progress The present paper is organizedas follow In Section 2 we discuss the theory of the redoxcouple Cu

2ZnSnS

4thin film band alignment In Section 3 we

discuss synthesis and characterization followed by results anddiscussion The results are summarized in Section 5

2 Theory

The important criterion for redox couple to be used for pho-toelectrochemical characterization of photovoltaic absorberlayer is that after reduction of one of the ionic species it

should not be deposited on the working electrode [3 4]Hence many of the cost-effective redox couple cannot beused for this purpose due to electrodeposition A schematicenergy band alignment diagram (Figure 1) for CZTS hasbeen drawn based on information from the literature [10 11]A Gaussian distribution of energy levels was assumed foroxidized and reduced species It can be seen that conductionband offset for many redox species such as Fe(CN)

6

3minus4minusFe2+Fe3+ and H

2OO2is very high Hence they will not be

a good choice to record photoelectrochemical response Inthe contrary conduction band offset for Eu3+Eu2+ couple isrelatively low Moreover that redox species is selected whoseredox potential is in a range where no degradation of CZTSfilm occurs during electrochemical characterization [3 4]The standard electrode potential for Eu3+Eu2+ is simminus035V(versus SHE) and it fulfills most of these requirements[3ndash5] The schematic diagram shows only ldquoestimated bandposition of CZTSrdquo because exact values are not knowndue to ongoing research for this material However thespecies whose standard reduction potential is close to thatof Eu3+Eu2+ can be well chosen The standard electrodepotential values for selected redox reactions are shown inTable 1 [12 13] the Fe2+Fe and Cd2+Cd also have standardreduction potential value close to Eu3+Eu2+ with very small

Journal of Spectroscopy 3

band offset but toxicity and deposition issue will createcomplications LiClO

4electrolyte in propylene carbonate was

also utilized for photoelectrochemical study of CZTS films[14] However perchlorates form energetic mixtures in manyorganic compounds [15] It is always recommended to chooseredox electrolyte in which charge transfer process should befast as well as reversible [16] However there are some reportswhere irreversible electrolytes were utilized as charge carrierscavenger [16] In addition CZTS is not only extensively usedin solid photovoltaic cell but also it is being utilized as anelectrochemical cell (EPC) [16 17]The ldquophotosensitized elec-trolytic oxidation effectrdquo a key phenomenon to understandsolar water splitting is now being utilized for CZTS materialalso Hence this paper will mainly focus on nonenergeticcost-effective electrolytes

3 Experimental

31 Material Copper sulfate pentahydrate zinc sulfate hex-ahydrate tin chloride chromium(III) chloride sodium sul-fite formaldehyde hydrated europium nitrate potassiumferricyanide potassium ferrocyanide and oxalic acid werepurchased from Ms Sigma Aldrich FTO coated glasssubstrate was purchased fromMs Pilkington USA

32 Synthesis and Characterizations of CZTS Films CZTSthin films were grown on fluorinated tin oxide coatedglass substrate using solution-based methods Two methodswere utilized for CZTS thin film synthesis In method 1CZTS nanocrystals were produced separately and furthercoated on FTO substrate whereas for method 2 we utilizedelectrochemical techniques For method 1 copper(II) acetate(99999) zinc(II) acetate (9999) tin(II) acetate anhy-drous (ge97) and thiourea (gt99) were grinded homo-geneously The molar composition was chosen in such amanner so that stoichiometric ratio of Cu Zn Sn in mixturewas sim2 1 1 This mixture was sintered in the presenceof evaporated sulfur environment for 2 hours at 550∘CThe annealing tube (alumina) was purged with argon for25min to displace air prior to sulfurization Elemental sulfur(999) placed on quartz boat was held at a temperature of150∘C as the sulfur source In another experiment sinteringwas carried out without sulfur After 2 hours natural coolingwas allowed and material was examined for phase detectionA sponge-type soft material was obtained which easily turnsinto powder when grinded This powder was washed inKCN (10 by weight solution in water) solution followedby washing in 50 aqueous solution of isopropyl alcohol toremove residual sulfide phases CZTS powder was dissolvedin isopropanol and stirred for 3 hours For better adhesion3 cellulose was also mixed after stirring Few drops ofterpineol were also added in the solutionThese nanocrystalswere coated on FTO substrate by immobilizing using 12-ethanedithiol linkers Nanocrystals coated film was furtherannealed (temperature sim300∘C) in an argon environment forduration of 1 hour For method 2 an electrolyte containingions of interest (Cu2+ Zn2+ and Sn2+) was used for growth ofmetallic layer [15 18] Coelectrodeposition was done by using

3-electrode cell connected to Gamry Instruments Reference600 potentiostat operated by virtual front panel softwareElectrodepositions were carried out at potentialsimminus16V Suchfilms were annealed in evaporated sulfur environment toobtain CZTS thin film A controlled sulfurization was carriedout in a tube furnace to minimize film sheet resistanceincrement Most of the sulfurized CZTS films were sim1120583mthick An inert atmosphere was maintained using continuoussupply of argon CZTS films were characterized by X-raydiffraction Raman and inductively coupled plasma opticalemission spectroscopy to verify phase and purity Moredetails about CZTS film synthesis characterizations (such asmorphology roughness and optical properties) and diodeparameters of CZTS film on various back contact can befound in earlier reports [15 18 19]

Due to fact that XRD pattern of CZTS is very similarto that of other possible sulfides phase examination wascarried out using Raman spectroscopy An R 3000 QERaman spectrometer (made by Raman Systems) was usedfor Raman spectroscopy A laser excitation of wavelength sim785 nm was used for Raman measurements The laser powerwas sim100mW Scanning electron microscopy was performedusing an FEI Quanta 600TM scanning electron microscopeBand gap measurements of CZTS thin films were done usingtransmittance data obtained from an Ocean Optics spec-trophotometer equipped with OOI Base 32 software [15 18]J-V curveswere recordedduring LED light illumination (inci-dent illuminance on substrate sim32 times 104 lux at wavelengthof sim550 nm)) and in dark by using a Gamry InstrumentsReference 600 potentiostat regulated by virtual front panelsoftware An equivalent concentration of all electrolytes solu-tionswassim007molel In a typical experiment aCZTS coatedthin film on FTO substrate was immersed in an aqueoussolution of redox electrolyte togetherwith a saturated calomelreference electrode (SCE) and a platinum counter electrodefor measurements The details of photoelectrochemical char-acterization experiments can be found in earlier report [3]3D topographical imageswere captured by using anAmScopeeyepiece camera equipped with ToupView 32 software

A flat band potential measurement was also carried outin order to estimate contribution from space charge currentThree-electrode electrochemical cell was used for flat bandmeasurementsMott-Schottky analysiswas carried out for flatband potential evaluation A substrate containing CZTS thinfilm grown on FTO coated glass was immersed in variouselectrolytes The following equation was used for flat bandpotential calculation [3]

1

1198622

119904119888

=

2

1198901205761199041205760119873

[(119881 minus 119881119891119887) minus

119870119861119879

119890

] (1)

where 119862119904119888is capacitance of the depletion region and 119870

119861is

Boltzmannrsquos constant The flat band potential was evaluatedby extrapolation to 11198622 = 0 in the Mott-Schottky plot ldquo119873rdquois the doping density ldquo119890rdquo represents electronic charge ldquo119881rdquo isapplied potential and ldquo119879rdquo represents temperature 120576

119904is the

dielectric constant of the semiconductor 1205760is the permittivity

of free space


250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)

287 cmminus1

336 cmminus1

368 cmminus1

Figure 2 Raman spectrum of CZTS nanopowder

4 Results and Discussion

Figure 2 shows the Raman spectrum of the CZTS powderover the range from 200 to 400 cmminus1 In the Raman spectruman obvious major peak located at 336 cmminus1 and two minorpeaks at 368 cmminus1 and 287 cmminus1 can clearly be seen Thisspectral data is in good agreement with the reported Ramanspectra of CZTS [5] SEM images of CZTS powder (onconducting substrate) and film surface are also shown inFigure 3 It can be seen that nanoparticles size is variable(Figure 3(a)) Few of them are less than 50 nm while someof the particles are greater than 100 nm Grains of CZTS filmaresim1120583mbig althoughwe obtained some small-size particles(see Figure 3(b))

The formation of conformal contact between CZTS andeuropium nitrate electrolyte promotes the minimizationof minority carrier diffusion towards electrolyte [4] Forredox couples enlisted in Table 1 a similar phenomenoncan be expected Detailed discussion about photocurrentgeneration phenomenon during illumination can be found inearlier reports [3 4] For evaluation of photoelectrochemicalperformance linear scan voltammograms were recordedby the potentiostat using virtual front panel software dur-ing flashing light illumination The cathodic photocurrentresponse confirms p-type photoactivity of the annealedCZTSfilms We examined both of our films (from nanocrystalsuspension and electrochemically grown) for photoelectro-chemical test and we found that photocurrent for filmgrown by electrodeposition-sulfur annealing is relativelyhigh Small photocurrent response for nanocrystalline filmcan be attributed to high recombination and short lifetime ofcarrier Hence we conducted most of our experiments usingfilm grown bymethod 2We have evaluated change in currentduring illumination at different potential for different redoxcouples and average change in current during illuminationis plotted in the same graph for comparison (see Figure 4)It can be seen that for Cr3+Cr2+ redox couple change inphotocurrent (Δ119868) is maximum for small potential range(0V to minus05 V) However we were not able to characterizeour film beyond this potential range due to its degradation

On the other hand all other proposed electrolytes showedphotocurrent response up to potential 08 V Specificallyboth Eu3+Eu2+ redox couple and oxalic acid show significantchange in photocurrent up to a potential of sim1 volt But itis interesting to note that maximum change in photocurrentusing oxalic acid as an electrolyte was twice as high com-pare to europium nitrate electrolyte solution Formaldehydesulfite and nitrite couples also showed photoelectrochemicalresponse but change in photocurrent is approximately halfcompared to that with europium nitrate It is essential tounderstand that enhanced cathodic photocurrent duringillumination indicates an occurrence of reduction reactionFor oxalic acid case it is possible due to presence of CO

2

alongwithH+ in water Although effects and actual content ofdissolved CO

2in context of CZTS are not well studied some

reports discussed aspects of dissolved CO2for other kind

of wafers [20] For this study we have neglected dissolvedCO2contribution Hence it is very likely that cathodic

photocurrent generation in this case is due to reduction ofH+into hydrogen gas A separate comparative test was conductedusing solution of sulfuric acid and oxalic acid with similarpH It can be seen that photocurrent (Figure 5) is still highcompare to that from sulfuric acid case It illustrates that thereare other additional operative mechanisms in case of oxalicacid aqueous solution which causes enhanced photovoltaicperformance This observation is encouraging and can beutilized as selective conversion of CO

2to oxalate in water

In order to confirm whether alternative electrolytes cancause any phase change or degradation in CZTS absorberdeposited on FTO Raman spectroscopy was carried outFigure 6 shows Raman spectra of a film (before and afterelectrochemical test) over the range from 250 to 400 cmminus1In all Raman spectra the two peaks are located at 3365and 3675 cmminus1 and it can be clearly seen that peak atsim3365 cmminus1 is stronger peak The Raman peak positions andrelative intensities are in good agreement with the previouslyreported Raman spectra of CZTS thin film [5 15] Nosignificant change in Raman spectra of film was observedafter electrochemical test in sodium sulfite formaldehyde orsodium nitrite solution After Lorentzian function fitting toboth spectra it was observed that both Raman linewidth andpeak intensity did not change significantly which indicatesthat CZTS filmrsquos phase integrity is not disrupted after photo-electrochemical examination in oxalic acid aqueous solution

A comparison of film surface topography is shown inFigure 7 To evaluate larger film area we used lower magnifi-cation images for analysis A change in surface topographycan be seen when the film is exposed to europium nitrateelectrolyte solution In an encircled area one can observesome changes in film roughness Thorough analysis suggeststhat less than sim20 of film area was changed However therewas more change in surface features and topography (gt45of film area) when electrochemical tests were run using oxalicacid electrolyte solution For Cr3+Cr2+ redox couple weobserved huge change in surface topography (see Figure 8)Such films were not suitable for topography comparison afteran electrochemical test However an optical image showingdamaged portion of film is shown for comparison Almost


200 nm

(a)

1 120583m

(b)

Figure 3 Scanning electronmicrograph of (a) CZTS nanocrystals on copper substrate (nanoparticles of sizesim50 nm are shownwithin circle)(b) CZTS thin film grown using nanocrystals

Light on

Response from thefilm made fromnanocrystals coating(europium nitrate)

Potential (SCE) (V)

Europium nitrateChromium chlorideOxalic acid

02

016

012

008

004

00minus05minus1minus15

Curr

ent

Potential

Chan

ge in

curr

ent (

mA

cm2)

(a)

0

0005

001

0015

002

0025

minus1 minus08 minus06 minus04 minus02 0Potential (SCE) (V)

FormaldehydeSulfiteNitrite

Chan

ge in

curr

ent (

mA

cm2)

(b)

Figure 4 Average photoelectrochemical response of CZTS thin film in different redox electrolytes change in current during illumination isshown in inset

no change in film topography was observed when photo-electrochemical tests were run using formaldehyde sodiumsulfite and sodium nitrite solutions An estimation of ionicspecies concentration pH and ionic strength was carriedout using Visual MINTEQ software in order to evaluateeffects of these parameters It was observed that oxalic acid astrong acid has lower pH value compared to that of europiumnitrate aqueous solution Hence we can expect more changein surface topography in case of oxalic acid Chromium chlo-ride solution is also slightly acidic but acidity is less compareto oxalic acid solution For sulfite and nitrite photoelectro-chemical reaction causes creation of OHminus ion still we did notobserve any significant change in surface topography A pho-tocatalytic oxidation of SO

3

2minus into S2O6

2minus is reported [18]

along with good amount of hydrogen production in the pres-ence of mercury lamp The kinetics of reported reaction is amultistage photolytic process and depends on several factorssuch as pH concentration temperature and reaction path-ways [21] It is very likely that suchmechanisms are operativehence we conducted a control test using a different copperworking electrodeWe did not observe any reportable changein photoactivity in this case Hence it can be said that most ofthe photoactivity is due to reduction of sulfite ion via CZTS-electrolyte charge transfer mechanism

It can be seen that electrode reactions 1 and 4 (sulfiteand formaldehyde) have shown higher photoelectrochemicalresponse whereas reaction 5 (nitrite) has relatively lowphotocurrent Nitrite has good absorptivity in water a broad


minus075 minus055 minus035 minus015Cu

rren

t (au

)

Potential (SCE) (V)

Sulfuric acidOxalic acid

Light on

Figure 5 A comparison of current potential behaviorphotoelec-trochemical response of CZTS thin film in oxalic acid and sulfuricacid solution

peak centered at 355 nm corresponds to 119899 rarr 120587lowast transition[22] At shorter wavelength it shows absorption due to120587 rarr 120587

lowast transition Specifically for nitrite photolysis processand species consumptionproduction involve a number ofreactions hence it is difficult to measure individual rate ofreaction However strong absorptivity in water can causeeffective reduction of photons incident to CZTS filmWe car-ried out a similar control test what we conducted for sodiumsulfite and did not find any measurable contribution to totalphotoelectrochemical current Despite the low photocurrentit can be advantageous to use sodium nitrite because of itsgood corrosion inhibition properties [23]

The situation when defect states locate within centerof the band gap can cause exchange of charge eitherfrom valence band or conduction band There is a pos-sibility that valance bandmediated charge transfer mech-anism is operative and contributing to effective photo-electrochemical response A separate experiment was con-ducted to investigate this phenomenon where photoelec-trochemical response of CZTS thin film was measuredusing Fe (CN)

6

3minus4minus redox couple From Figure 1 it canbe seen that Fe(CN)

6

3minus4minus redox couple is good choicebecause its equivalent Fermi level lies very close to midgap Figure 9 shows the current potential behavior ofCZTS working electrode immersed in Fe(CN)

6

3minus4minus dur-ing illumination and dark It can be seen that thereis not any good photocurrent response indicating thatmost of the photoelectrochemical response in case of CZTSis due to conduction band-mediated charge transfer mecha-nism

The total current is an aggregate of dark current pho-tocurrent and space charge recombination current [3] Thephotocurrent is a direct function of number of free electron

250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)

After testBefore test

3675 cmminus13365 cmminus1

Figure 6 (a) Raman spectra of CZTS film before and after testin europium nitrate solution (b) Raman spectra of CZTS filmbefore and after test in oxalic acid aqueous solution No significantchange in Raman spectra was found after electrochemical test informaldehyde sodium sulfite or sodium nitrite solution

in field free region (119899) The value of ldquo119899rdquo depends on manyfactors such as recombination rate heterogeneous chargetransfer rate diffusion rate of electron and intensity lossof photons during travel inside the solution For similarsubstrate incident power and absorption coefficient offilm photocurrent contribution from each electrolyte wasassumed to be same (light absorption in electrolyte was alsoassumed to be similar except for Cr3+Cr2+) Contributionfrom space charge current (119869SCR) depends mainly on relativedifference between flat band potential and applied voltage [3]

119869SCR = 119861radic10038161003816100381610038161003816(119881119891119887minus 119881)

100381610038161003816100381610038161198901199021198812119870119861119879 (2)

where ldquo119861rdquo is a recombination constant The value of ldquo119861rdquodepends on various factors including carrier thermal velocitydensity of traps and capture cross-section For a similarvalue of ldquo119861rdquo the flat band potential will govern the changein 119869SCR It was observed that flat band potential for oxalicacid electrolyte issim120mVmore than europium redox couple(see Figure 10) The lowest flat band potential was observedfor sodium sulfite electrolyte Altogether it can be said thatthe square root term in (2) and subsequent space chargecurrent will be different for these electrolytes It was observedthat such contribution or change is more effective towardsanodic direction however most of our photoelectrochemicalcharacterization was done at negative potential

For large negative electrode potential cathodic reactionat CZTS will be mainly due to transport control and therewill be significant loss of interfacial charge transfer control


(a) (b) (c)

(d) (e) (f)

Figure 7 3D topography of CZTS film (area sim50 120583times 50 120583) (a) Before electrochemical test in europium nitrate aqueous solution (b) Afterelectrochemical test in europium nitrate solution (c) After electrochemical test in oxalic acid aqueous solution (d) Before electrochemicaltest in formaldehyde aqueous solution (e) After electrochemical test in formaldehyde aqueous solution (f) After electrochemical test insodium sulfite aqueous solution

50120583m

Figure 8 An optical micrograph of CZTS film after electrochemicaltest in chromiumchloride solution damaged film can be seenwithinencircled area

[24] The adsorbed ldquointermediate radicalsrdquo are also veryimportant to explain photoelectrochemical kinetics [24] Inour case these radicals are different because of differentelectrolytes (and their solution parameters) and hence thereis possibility of different surface states The charge transferrate constant is related to drift velocity of outgoing electronand density of acceptor states which can be different fordifferent electrolytes [3 24] It is believed that acidity of oxalicacid causes production of dangling bond Density of suchdefect states changes exponentially due to change in potential[18] For most of the redox reaction the contribution ofsurface states was assumed to be minimal and rate (current)determining step is due to Schottky barrier type transport

Potential (SCE) (V)

Curr

ent (

A)

minus15 minus1 minus05 0 05 1 15

00008

00006

00004

00002

0

minus00002

minus00004

minus00006

No change inphotocurrent

duringillumination

Figure 9 Current potential behavior of CZTS thin film grown onFTO coated glass and immersed in redox electrolytes containingFe(CN)

6

3minus4minus redox couple during flashing light illumination

Although majority of change in dark current beyond thestandard reduction potential is due to reduction of one ofthe ionic species in solution [4] adsorption of species inelectrode also causes change in rate kinetics [23] It shouldbe noted that most of the adsorbed species are differentThe surface recombination rate constant is an important andhighly sensitive parameter and it causes significant change inphotocurrent when its value is changed twice of the originalvalue [3] Hence it will be interesting to explore the effects ofall these parameters as well as plasmonic effects [25] in detailthese experiments are in progress


minus05 100500Potential (V)

Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)

Figure 10 (a) Mott-Schottky plot for CZTS film grown on FTO coated glass substrate and (a) immersed in europium nitrate solution (b)immersed in oxalic acid aqueous solution (c) immersed in sodium nitrite aqueous solution (d) immersed in sodium sulfite aqueous solutionand (e) immersed in formaldehyde aqueous solution

5 Conclusions

In summary alternative electrolytes were examined to eval-uate photoelectrochemical performance of CZTS absorbermaterials Our primary results suggest that these elec-trolytesredox couples can be beneficial due to equivalent or

enhanced photocurrent response in their respective appliedpotential range However more details such as irreversibil-ity and charge transfer process for these electrolytes arestill needed to explore In case of sodium sulfite andformaldehyde photoelectrochemical response was satisfac-tory whereas for sodium nitrite photocurrent was less It was


observed that oxalic acid aqueous solution acts very well overthe potential range of simminus05 V to minus10 V Film quality andtopography were evaluated by comparing 3D topographicalimage and it was observed that oxalic acid redox couplescause less damage in surface features while for Cr3+Cr2+redox couple film damage was high Almost no damage wasobserved in CZTS thin film when sodium sulfite formalde-hyde and sodium nitrite were used as redox scavengerA difference in photocurrent for different electrolytes wasexplained on the basis of different defect states adsorptionof species in photoelectrode and different surface recombi-nation rate constant

References

[1] T K Todorov K B Reuter and D B Mitzi ldquoHigh-efficiencysolar cell with earth-abundant liquid-processed absorberrdquoAdvanced Materials vol 22 no 20 pp E156ndashE159

[2] T K Todorov J Tang S Bag et al ldquoBeyond 11 efficiencycharacteristics of state-of-the-art Cu

2ZnSn(SSe)

4solar cellsrdquo

Advanced Energy Materials vol 3 no 1 pp 34ndash38 2012[3] P K Sarswat and M L Free ldquoAn evaluation of depletion layer

photoactivity in Cu2ZnSnS

4thin filmrdquo Thin Solid Films vol

520 no 13 pp 4422ndash4426 2012[4] S C Riha S J Fredrick J B Sambur Y Liu A L Prieto and B

A Parkinson ldquoPhotoelectrochemical characterization of nano-crystalline thin-film Cu

2ZnSnS

4photocathodesrdquo ACS Applied

Materials and Interfaces vol 3 no 1 pp 58ndash66 2011[5] P K Sarswat and M L Free ldquoDemonstration of a sol-gel

synthesized bifacial CZTS photoelectrochemical cellrdquo PhysicaStatus Solidi (A) vol 208 no 12 pp 2861ndash2864 2011

[6] httpwwwsigmaaldrichcom[7] V E Ferry M A Verschuuren H B T Li et al ldquoLight trapping

in ultrathin plasmonic solar cellsrdquoOptics Express vol 18 no 13pp A237ndashA245 2010

[8] C S Liu G Kumar and V K Tripathi ldquoLaser mode conversioninto a surface plasma wave in a metal coated optical fiberrdquoJournal of Applied Physics vol 100 no 1 Article ID 013304 6pages 2006

[9] G Kumar and V K Tripathi ldquoExcitation of a surface plasmawave over a plasma cylinder by a relativistic electron beamrdquoPhysics of Plasmas vol 15 no 7 Article ID 073504 5 pages2008

[10] S Chen JH Yang XGGongAWalsh and SHWei ldquoIntrin-sic point defects and complexes in the quaternary kesteritesemiconductor Cu

2ZnSnS

4rdquo Physical Review B vol 81 no 24

Article ID 245204 10 pages 2010[11] M Gratzel ldquoPhotoelectrochemical cellsrdquo Nature vol 414 pp

338ndash344 2001[12] httpwwwedgovnlcaeduk12evaluationchem3202

standardreductionpotentialspdf[13] N A Lange Lange Handbook of Chemistry Mcgraw-Hill New

York NY USA 9th edition 1956[14] N M Shinde D P Dubal D S Dhawale C D Lokhande J H

Kim and J H Moon ldquoRoom temperature novel chemical syn-thesis of Cu

2ZnSnS

4(CZTS) absorbing layer for photovoltaic

applicationrdquoMaterials Research Bulletin vol 47 no 2 pp 302ndash307 2012

[15] H Vogt J Balej J E Bennett P Wintzer S A Sheikh andP Gallone ldquoChlorine oxides and chlorine oxygen acidsrdquo in

UllmannrsquoS Encyclopedia of InduStrial ChemiStry John Wiley ampSons New York NY USA 2000

[16] P K Sarswat and M L Free ldquoA Comparative study of Co-electrodeposited Cu

2ZnSnS

4absorber material on fuorinated

tin oxide and molybdenum substratesrdquo Journal of ElectronicMaterials vol 41 no 8 pp 2210ndash2215 2012

[17] R C Alkire D M Kolb J Lipkowski and P N Ross Photo-electrochemicalMaterials and Energy Conversion ProcessWiley-VCH amp Co KGaA Weinheim Germany 2010

[18] H Cherfouh T Chari A Guermoune M Siaj and B MarsanldquoPreparation and characterization of a new CuInS

2Graphene

composite electrode for application in electrochemical solarcellsrdquo in Proceedings of the 219th Electrochemical Society (ECS)Meeting abstract 1870 2011

[19] P K Sarswat M L Free and A Tiwari ldquoA factorial design ofexperiments approach to synthesize CZTS absorber materialfrom aqueous mediardquo in Novel Fabrication Methods for Elec-tronic Devices L Schmidt-Mende H J Snaith G L Whitingand D S Ginger Eds abstract G618 2010 Proceedings of theMaterials Research Society Symposium Boston Mass USAvol 1288 2011

[20] P K Sarswat and M L Free ldquoan assessment of contact engi-neering for the Cu

2ZnSnS

4-alternative back contactrdquoMaterials

Focus vol 2 no 4 pp 244ndash250 2013[21] S Kumari M Keswani S Singh et al ldquoEffect of dissolved

CO2in de-ionized water in reducing wafer damage during

megasonic cleaning in megpierdquo ECS Transactions vol 41 no5 pp 93ndash99 2011

[22] C Huang C A Linkous O Adebiyi and A T-Raissi ldquoHydro-gen production via photolytic oxidation of aqueous sodiumsulfite solutionsrdquoEnvironmental Science andTechnology vol 44no 13 pp 5283ndash5288 2010

[23] M Fischer and P Warneck ldquoPhotodecomposition of nitriteand undissociated nitrous acid in aqueous solutionrdquo Journal ofPhysical Chemistry vol 100 no 48 pp 18749ndash18756 1996

[24] S I Krakhmalev V A Vorotnikova N V Ten and N VTaranova ldquoDetermination of sodiumnitrite in complex sodiumgreaserdquo Chemistry and Technology of Fuels and Oils vol 20 no12 pp 612ndash613 1984

[25] Y K Mishra R Adelung G Kumar et al ldquoFormation of self-organized silver nanocup type structures and their plasmonicabsorptionrdquo Plasmonics vol 8 pp 811ndash815 2013

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


CZTS conduction band

CZTS valance band

H2H2O[Fe(CN)6]3minus4minus

Eu2+3+

Fe2+Fe3+

H2OO2

Other closelylocated redox

couple

Cr3+Cr2+ Cd2+CdFe2+Fe

HOOCCOOH(aq)HCHO(aq)

minus15

minus1

minus05

0

05

1

15

2

25E NHE

0

minus1

minus2

minus3

minus4

minus5

minus6

minus7

Vaccum

Fermi equivalent level

Figure 1 A schematic diagram showing CZTS band alignment with different redox electrolytes

Table 1 Standard reduction potential for selected reactions

No Electrode reaction Standard reduction potential1 HCHO(aq) + 2H2O + 2eminus 999445999468 CH3OH + 2OHminus minus059V2 2CO2(g) + 2H+ + 2eminus 999445999468HOOCCOOH(aq) minus049V3 Cr3+ + eminus 999445999468 Cr2+ minus042V4 2SO3

2minus + 3H2O + 4eminus 999445999468 S2O32minus+ 6OHminus minus058V

5 NO2minus + H2O + eminus 999445999468 NO + 2OHminus minus046V

electrolyte system It is important to mention that the fastdeveloping area of plasmonics has great potential in enhanc-ing the performance of the solar cells [7] Plasmonics dealswith the excitation of the surface plasmons which basicallyconverts the light energy into electricity in the photovoltaicdevices [8 9] Optical field enhancement in metallic nanos-tructures is generally utilized to achieve strong enhancementof optical absorption Metallic nanoparticles or other dielec-tric nanocrystals support the energy collection of incidentphotons in surface plasmon oscillations This energy can besubsequently transferred to the semiconductor substrate in amore efficient manner in comparison with the conventionalphotovoltaic CZTS absorber These experiments and simula-tion are currently in progress The present paper is organizedas follow In Section 2 we discuss the theory of the redoxcouple Cu

2ZnSnS

4thin film band alignment In Section 3 we

discuss synthesis and characterization followed by results anddiscussion The results are summarized in Section 5

2 Theory

The important criterion for redox couple to be used for pho-toelectrochemical characterization of photovoltaic absorberlayer is that after reduction of one of the ionic species it

should not be deposited on the working electrode [3 4]Hence many of the cost-effective redox couple cannot beused for this purpose due to electrodeposition A schematicenergy band alignment diagram (Figure 1) for CZTS hasbeen drawn based on information from the literature [10 11]A Gaussian distribution of energy levels was assumed foroxidized and reduced species It can be seen that conductionband offset for many redox species such as Fe(CN)

6

3minus4minusFe2+Fe3+ and H

2OO2is very high Hence they will not be

a good choice to record photoelectrochemical response Inthe contrary conduction band offset for Eu3+Eu2+ couple isrelatively low Moreover that redox species is selected whoseredox potential is in a range where no degradation of CZTSfilm occurs during electrochemical characterization [3 4]The standard electrode potential for Eu3+Eu2+ is simminus035V(versus SHE) and it fulfills most of these requirements[3ndash5] The schematic diagram shows only ldquoestimated bandposition of CZTSrdquo because exact values are not knowndue to ongoing research for this material However thespecies whose standard reduction potential is close to thatof Eu3+Eu2+ can be well chosen The standard electrodepotential values for selected redox reactions are shown inTable 1 [12 13] the Fe2+Fe and Cd2+Cd also have standardreduction potential value close to Eu3+Eu2+ with very small





3 Experimental






1

1198622

119904119888

=

2

1198901205761199041205760119873

[(119881 minus 119881119891119887) minus

119870119861119879

119890

] (1)


119861is


119904is the


of free space


250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)

287 cmminus1

336 cmminus1

368 cmminus1






2










200 nm

(a)

1 120583m

(b)


Light on


Potential (SCE) (V)


02

016

012

008

004


Curr

ent

Potential

Chan

ge in

curr

ent (

mA

cm2)

(a)

0

0005

001

0015

002

0025



Chan

ge in

curr

ent (

mA

cm2)

(b)



3

2minus into S2O6






rren

t (au

)

Potential (SCE) (V)


Light on





6


6


6



250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)





119869SCR = 119861radic10038161003816100381610038161003816(119881119891119887minus 119881)

100381610038161003816100381610038161198901199021198812119870119861119879 (2)




(a) (b) (c)

(d) (e) (f)


50120583m



Potential (SCE) (V)

Curr

ent (

A)


00008

00006

00004

00002

0

minus00002

minus00004

minus00006


duringillumination


6





Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





3 Experimental






1

1198622

119904119888

=

2

1198901205761199041205760119873

[(119881 minus 119881119891119887) minus

119870119861119879

119890

] (1)


119861is


119904is the


of free space


250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)

287 cmminus1

336 cmminus1

368 cmminus1






2










200 nm

(a)

1 120583m

(b)


Light on


Potential (SCE) (V)


02

016

012

008

004


Curr

ent

Potential

Chan

ge in

curr

ent (

mA

cm2)

(a)

0

0005

001

0015

002

0025



Chan

ge in

curr

ent (

mA

cm2)

(b)



3

2minus into S2O6






rren

t (au

)

Potential (SCE) (V)


Light on





6


6


6



250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)





119869SCR = 119861radic10038161003816100381610038161003816(119881119891119887minus 119881)

100381610038161003816100381610038161198901199021198812119870119861119879 (2)




(a) (b) (c)

(d) (e) (f)


50120583m



Potential (SCE) (V)

Curr

ent (

A)


00008

00006

00004

00002

0

minus00002

minus00004

minus00006


duringillumination


6





Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)

287 cmminus1

336 cmminus1

368 cmminus1






2










200 nm

(a)

1 120583m

(b)


Light on


Potential (SCE) (V)


02

016

012

008

004


Curr

ent

Potential

Chan

ge in

curr

ent (

mA

cm2)

(a)

0

0005

001

0015

002

0025



Chan

ge in

curr

ent (

mA

cm2)

(b)



3

2minus into S2O6






rren

t (au

)

Potential (SCE) (V)


Light on





6


6


6



250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)





119869SCR = 119861radic10038161003816100381610038161003816(119881119891119887minus 119881)

100381610038161003816100381610038161198901199021198812119870119861119879 (2)




(a) (b) (c)

(d) (e) (f)


50120583m



Potential (SCE) (V)

Curr

ent (

A)


00008

00006

00004

00002

0

minus00002

minus00004

minus00006


duringillumination


6





Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


200 nm

(a)

1 120583m

(b)


Light on


Potential (SCE) (V)


02

016

012

008

004


Curr

ent

Potential

Chan

ge in

curr

ent (

mA

cm2)

(a)

0

0005

001

0015

002

0025



Chan

ge in

curr

ent (

mA

cm2)

(b)



3

2minus into S2O6






rren

t (au

)

Potential (SCE) (V)


Light on





6


6


6



250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)





119869SCR = 119861radic10038161003816100381610038161003816(119881119891119887minus 119881)

100381610038161003816100381610038161198901199021198812119870119861119879 (2)




(a) (b) (c)

(d) (e) (f)


50120583m



Potential (SCE) (V)

Curr

ent (

A)


00008

00006

00004

00002

0

minus00002

minus00004

minus00006


duringillumination


6





Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



rren

t (au

)

Potential (SCE) (V)


Light on





6


6


6



250 300 350 400

Cou

nts (

au)

Wavenumber (1cm)





119869SCR = 119861radic10038161003816100381610038161003816(119881119891119887minus 119881)

100381610038161003816100381610038161198901199021198812119870119861119879 (2)




(a) (b) (c)

(d) (e) (f)


50120583m



Potential (SCE) (V)

Curr

ent (

A)


00008

00006

00004

00002

0

minus00002

minus00004

minus00006


duringillumination


6





Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


(a) (b) (c)

(d) (e) (f)


50120583m



Potential (SCE) (V)

Curr

ent (

A)


00008

00006

00004

00002

0

minus00002

minus00004

minus00006


duringillumination


6





Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



Cminus2

(Fminus2

cm2)

(a)


Cminus2

(Fminus2

cm2)

(b)


Cminus2

(Fminus2

cm2)

(c)


Cminus2

(Fminus2

cm2)

(d)


Cminus2

(Fminus2

cm2)

(e)


5 Conclusions





References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



References



2ZnSn(SSe)

4solar cellsrdquo






2ZnSnS









2ZnSnS







2ZnSnS






2ZnSnS





2Graphene




2ZnSnS


















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Research Article An Investigation of Nanocrystalline and...

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Transcript of Research Article An Investigation of Nanocrystalline and...