Surface Texture-Induced Enhancement of Optical and

Surface Texture-Induced Enhancement of Optical andPhotoelectrochemical Activity of Cu2ZnSnS4 Photocathodes

PRASHANT K. SARSWAT,1,3 NIPON DEKA,2 S. JAGAN MOHAN RAO,2

MICHAEL L. FREE,1 and GAGAN KUMAR2

1.—Department of Metallurgical Engineering, University of Utah, 135 S, 1460 E, Room 412,Salt Lake City, UT 84112, USA. 2.—Department of Physics, Indian Institute of TechnologyGuwahati, Guwahati 781039, India. 3.—e-mail: [email protected]

The objective of this work is to understand and improve the photocatalyticactivity of Cu2ZnSnS4 (CZTS) through postgrowth modification techniques tocreate surface textures. This objective can be achieved using a combination ofsolvents, etching agents, and anodization techniques. One of the most effectivesurface treatments for enhancing the surface properties of photovoltaicmaterials is formation of nanoscale flakes, although other surface modifica-tions were also evaluated. The superior performance of textured films can beattributed to enhanced surface area of absorber material exposed to elec-trolyte, ZnS deficiency, and high catalytic activity due to reduced charge-transfer resistance. Fine-tuning of ion flux and electrolyte stoichiometry canbe used to create a controlled growth algorithm for CZTS thin films. Theresulting information can be utilized to optimize film properties. The utility ofnanostructured or engineered surfaces was evaluated using photoelectro-chemical measurements. Finite-difference time-domain (FDTD)-assistedsimulations were conducted for selected texturing, revealing enhanced surfacearea of absorbing medium that ultimately resulted in greater power loss oflight in the medium.

Key words: CZTS, surface texture, photoelectrochemical properties

INTRODUCTION

Among renewable energy sources, photovoltaic(PV) solar energy and hydrogen production fromphotoelectrochemical (PEC) water splitting drivenby solar power are two clean technologies that canbe used for environmentally friendly electricalpower production.1 In the case of solar hydrogen, astandard PEC cell is needed with one electrode thatcomprises a photoactive material to simultaneouslyallow solar light harvesting and oxidation (photoan-ode) and another for reduction (on the photocath-ode).1 Similarly, in the case of a PV cell, an efficientabsorber layer is needed to produce sufficient elec-tron–hole pairs when illuminated with photons.2

These research needs can be achieved if theabsorber layer (photoelectrode) performance can be

enhanced while not using costly methods or mate-rials for device production. Cu2ZnSnS4 (CZTS) isamong the top candidates with potential to meet the�US $1/W production cost target. Hence, significantresearch into solar hydrogen and photovoltaicenergy production using this material has beencarried out.

The relative position of conduction bands of CZTSand its derivatives are appropriate for producinghydrogen by reduction of H+ ions.3 Close examina-tion of the CZTS band diagram suggests that theoxygen evolution reaction is not possible at thiselectrode. The conduction band of CZTS is estimatedto lie at �0.7 V below the hydrogen evolutionpotential, whereas the valence band is located closeto the standard electrode potential of Fe2+/Fe3+.4,5

CZTS, a p-type photovoltaic absorber material, hasreceived significant attention due to its attractivefeatures such as utilization of abundant metals andhigh absorption coefficient.6,7 CZTS can be

(Received October 19, 2016; accepted April 13, 2017;published online May 8, 2017)

Journal of ELECTRONIC MATERIALS, Vol. 46, No. 8, 2017

DOI: 10.1007/s11664-017-5531-8� 2017 The Minerals, Metals & Materials Society

5308

http://crossmark.crossref.org/dialog/?doi=10.1007/s11664-017-5531-8&domain=pdf

http://crossmark.crossref.org/dialog/?doi=10.1007/s11664-017-5531-8&domain=pdf

synthesized using a variety of vacuum- or solution-based methods.8–10 Although CZTS has achievedgreat popularity due to its Earth-abundant con-stituent elements and low cost, it suffers from issuessuch as reduced open-circuit potential when used ina PV cell, high recombination rate, and low solar-to-hydrogen conversion efficiency.

Alternative Route to Modify ElectronicProperties of CZTS

The two primary ways to change physical prop-erties of CZTS are to alter the chemical compositionor change the domain size to nanoscale. Nanostruc-tures with narrow size distribution, straight pas-sage, and self-ordered arrangement have attractedremarkable attention due to their enhanced electri-cal, electronic, and optical properties.11 It is knownthat a sharp reduction due to electron–hole recom-bination occurs in bulk. In contrast, rapid separa-tion of electron–hole pairs occurs on the surface forbetter charge-carrier trapping and faster interfacialcharge-carrier transfer for photocatalysts with por-ous and nanoflake-type structure.12 Another greatadvantage of nanoflake and nonporous structures istheir superior cycling stability.13 Three-dimensional(3D) flower-type structures form due to assembly ofprimary nanosheets, in many cases being con-structed from two-dimensional (2D) heterostruc-tures.12 Growth of these structures often follows alateral oriented attachment (LOA) mechanism witha subsequent phase transformation rather than thecommonly observed oriented-attachment (OA)mechanism.12 CZTS nanostructured films withthese properties have great potential for use inphotocatalysis, sensing, solar energy conversion,and nanostructure templates. Periodic or multilay-ered hollow nanostructures are among the mostattractive forms of CZTS, because they offer advan-tages over other forms such as straight nanochan-nels with uniform longitudinal morphology.Electrons confined in ordered/periodic CZTS nanos-tructures may exhibit discrete energy levels ratherthan bands. Fabrication of such nanostructures hasbeen accomplished by conventional microfabricationtechniques derived from the microelectronics indus-try, such as e-beam lithography and photolithogra-phy. Most of these techniques deliver the requiredstructural accuracy, but achieving nanosize fea-tures involves complex and expensive tools, andthey are restricted to small sample size. Suchexpensive approaches will be cost prohibitive forsolar hydrogen production or electricity generationfrom solar photovoltaics.

It has been reported that transition metals ortheir alloys (e.g., Zr, Sn, Nb, Ta, Hf, W, and Fe) canalso form nanoporous structures when electrochem-ical techniques are used.14 It is believed that poreformation is induced by competition between chem-ical reactions at the semiconductor–electrolyteinterface. Postgrowth modification techniques have

also been applied to create WO3 nanoflakes withrough surface.15 Photoactivity enhancement of H2-annealed WO3 was observed due to enhanced donordensity15 when introducing a moderate amount ofoxygen vacancies to form substoichiometric WO3�x.In the case of WO3 also, chemical etching was foundto be successful to increase the surface area, leadingto faster charge transfer and lower overpotential foroxygen and hydrogen evolution. A dual etching–reduction method applied to pregrown WO3 nano-flakes in weakly acidic condition has also beeninvestigated. This method allows for controlledintroduction of oxygen vacancies under mild solu-tion conditions, resulting in enhanced PEC perfor-mance. In general, semiconductor surfaces can bemodified using a variety of methods such as photo-electrochemical texturing, maskless electrochemicaltexturing, reactive plasma etching, and etching of amasked surface. Surface texturing using etchingsolution is very popular for Si substrates, wheredifferent electrolytes have been utilized to createparticular surface patterns.16 Isotropic texturing ispossible for Si wafers, where anodic polarizationtechniques have been applied while using stronglyalkaline solution. For Si, it was observed that thereflection value was almost half that measured on achemically preetched surface.17 Enhanced photolu-minescence performance was reported18 for porousn-GaN prepared by etching the GaN substrate usingH2SO4 and H2O2.

However, few reports are available on the effectsof shape, architecture, and solution treatment onthe optical and electronic properties of CZTS; Forexample, hierarchical structure of CZTS crystalscan provide a high roughness factor that improvestheir electrocatalytic performance.19 Single-crys-talline CZTS nanosheet assemblies have been fab-ricated on fluorinated tin oxide (FTO)-coated glasssubstrates.3 These nanosheet-coated electrodesshowed better charge transportation activity,improved light absorption, and reduced charge-carrier recombination.3 Tree-like nanowireheterostructures were utilized to facilitate unas-sisted solar water splitting.20 Tree-like structuresprovide large surface area as well as shorter trans-port distance that carriers must cover before arriv-ing at the semiconductor–electrolyte surface.Improved CZTS-based solar cell efficiency wasreported when absorber layers were treated withmild hydrochloric acid solution.21 Literature regard-ing electrochemical texturing mainly focuses onthree materials: Si, titania nanotubes (TNTs), andanodic aluminum oxide (AAO). Earlier selectiveetching studies were performed on semiconductorssuch as GaAs and InP substrates.22 It was observedthat, by changing the volume ratio of acid tohydrogen peroxide, the selectivity and etch ratecould be varied. Etchants based on H2O2 and H2SO4

have been utilized for selective etching of Si andInGaAs. In most of these cases, acid and H2O2

combinations are more effective than ammonium

Surface Texture-Induced Enhancement of Optical and Photoelectrochemical Activity ofCu2ZnSnS4 Photocathodes

5309

hydroxide–hydrogen peroxide solutions in terms ofetching selectivity, smoothness of the etched sur-face, and abruptness of the etch-defined edge.23

Hence, it is interesting to investigate whether thesemethods can also be applied to CZTS films. Wepresent herein electrochemical methods to engineerthe grains of bulk CZTS for improved opticalproperties.

EXPERIMENTAL PROCEDURES

For growth of CZTS films, electrolyte containingthe ions of interest (Cu2+, Zn2+, and Sn2+) was usedfor growth of a metallic layer. Coelectrodepositionwas carried out at potential of ��1.8 V using athree-electrode system. Coelectrodeposited filmswith metallic layers were sulfurized in an evapo-rated sulfur environment to obtain CZTS thin films.Most of the sulfurized CZTS films were �1 lmthick. Inert atmosphere was maintained usingcontinuous supply of argon. CZTS films were char-acterized by x-ray diffraction analysis, Raman spec-troscopy, and inductively coupled plasma opticalemission spectroscopy (ICPOES) to verify phase andpurity. More details about CZTS film synthesis,characterization (such as morphology, roughness,and optical properties), and diode parameters ofCZTS film on various back-contacts can be found inearlier reports.6,24

These films were used as parent (untreated) filmsfor creating different nanostructures over them. Tocreate nanoflakes over pristine film, a solutionmixture comprising 3 mL 0.05 M CZTS nanocrys-talline solution, 0.02 g oxalic acid, 0.01 g urea,0.01 g thiourea, and 0.5 mL 6 M HCl was addedinto 12.5 mL acetonitrile, and the resultant solutionwas transferred to a Teflon-lined stainless-steelautoclave. CZTS nanocrystalline solution was pre-pared using CZTS powder made using a methoddescribed elsewhere.5 The FTO-CZTS substrate wasplaced at an angle against the wall of the Teflonliner with the conducting side (with the coated seedlayer) facing down. Solvothermal synthesis wasconducted at 180�C for 2 h. After synthesis, theautoclave was allowed to cool down to room tem-perature. The substrate was taken out andannealed in air at 500�C for 1 h, being denotedFTO-CZTS-NF. In a separate set of experiments,three different chloride salts of Cu, Zn, and Sn wereused to replace CZTS nanocrystalline solution.Another separate set of experiments was performedin which baseline FTO-CZTS substrate was treatedwith solution containing 0.01 M H2O2 and 0.01 MH2SO4. Anodization was carried out at 10 V influorinated ethylene glycol solution (0.3 wt.%NH4F + 3 wt.% H2O) under constant stirring at60 rpm for 10 min using Pt foil as counterelectrode.Most of the area except CZTS film was precoveredby organic protective coating (nitrocellulose-butylacetate) that was later removed using solvent(acetone or ethyl alcohol). In the case of anodization,

bulk CZTS films grown on FTO substrates wereutilized as positive electrodes.

We also conducted finite-difference time-domain(FDTD) simulations to validate some of the resultsby conducting numerical investigation of theabsorption of incident light by submicron-scalesurface textures in a CZTS layer.25 The surfacetexturing was assumed to be random with differentgrain shapes and sizes. For this numerical study, weconsidered trapezoidal and spherical texturing andcompared our results with those for a CZTS layerwithout texturing.

Characterization

X-ray diffraction (XRD) analysis of thin films wasconducted using a Philips X’Pert XRD diffractome-ter with Cu Ka radiation in the 2h range from 20� to80� with step size of �0.005�. To examine thebandgap of the CZTS film, ultraviolet–visible (UV–Vis) optical transmission spectroscopy (OceanOptics DH 2000 Bal) was utilized. The energybandgap of CZTS thin film was determined fromTauc plots showing the relationship between inci-dent photon energy (hm) on the material (abscissa)and (ahm)1/n (where n = 1/2 for direct allowed tran-sitions) on the ordinate axis, where a is the absorp-tion coefficient of the material. The morphology ofthe various synthesized bulk and nanostructuredCZTS films and nanoflake arrays was characterizedby field-emission scanning electron microscopy (Hi-tachi S-4800 SEM) in ultrahigh vacuum(�10�9 kPa). An intense electron beam from atungsten filament at an accelerating voltage of3 kV and emission current of 15 lA was used forimaging at very high magnification. Transmissionelectron microscopy was conducted using an FEITitan� microscope with the capability to image at0.8-A resolution in TEM mode and 1.4-A resolutionin scanning TEM (STEM) mode. The instrumentwas equipped with an energy-dispersive analysis ofx-rays (EDAX) detector for x-ray analysis andelemental mapping. Raman measurements wereconducted using a R 3000 QE Raman spectrometerat excitation wavelength of �785 nm and laserpower of �95 mW. The wavelength stability of theRaman spectrometer was better than 1 cm�1 driftover a 12-h period. Raman peak analysis wascarried out using RSI scan software.

RESULTS AND DISCUSSION

Phase (Structural) and Optical Properties

Figure 1a shows a characteristic x-ray diffrac-togram for CZTS film (after sulfurization), matchingperfectly with the kesterite structure of CZTS inJoint Committee on Powder Diffraction Standards(JCPDS) card no. 26-0575 with peaks correspondingto (112), (103), (200), (105), (220), (312), (224), (314),(008), and (332) planes. Most of these peaks matchwith earlier reported XRD peaks for CZTS film.6

Sarswat, Deka, Jagan Mohan Rao, Free, and Kumar5310

Additional phase examination was conducted usingRaman spectroscopy, discussed in the ‘‘Raman Exam-ination’’ section. Figure 1b shows a Tauc plot (a2

versus incident photon energy). The bandgap of thefilm was determined by extrapolating the a2 versusphoton energy curve to zero (x-axis). Using thismethod, a bandgap value of �1.38 eV was obtained,which is close to reported literature values.26

Growth of CZTS Film and Postgrowth Modifi-cation

Prior to surface texture modification, CZTS filmswere examined by transmission electron micro-scopy. Cross-sectional images were acquired afterproper sample preparation by focused ion beammilling. We also analyzed the film stoichiometryacross the film thickness (Fig. 2). It was observedthat the film growth was uniform over the FTOsubstrate with very little discontinuity. However, atsome places, clear separation between grains wasvisible (Fig. 2, inset). Energy-dispersive spec-troscopy (EDS) elemental analysis (Fig. 2, inset)suggested presence of all required elements (Cu, Zn,Sn, and S). Another important step was surfaceexamination of the CZTS films, revealing dis-cernible variation in film morphology between the

CZTS bulk and nanoflake films (Fig. 3). The nano-flake films grew perpendicular to the substrate,with thickness of the nanoflake film ranging from30 nm to 100 nm with height of �1 lm. Thesenanoflakes are different from the large, coppersulfide (CuxS)-type hexagonal structures (Fig. 4).These large hexagonal flakes often grow along withCZTS grains due to excess copper or longer sulfu-rization time. CZTS film treated with H2O2 andH2SO4 showed a flower-like pattern throughout.The grain diameter was smaller in this case(�400 nm to 500 nm) while the thickness of thesesmall grains was �40 nm.

Photoelectrochemical Performance

To measure the suitability of CZTS electrodes foruse as hydrogen evolution electrodes or as anabsorber layer for thin films, one can use anothercounterelectrode (or photoanode) and appropriateredox scavengers. Some commonly used solutionsare aqueous Eu3+ redox electrolyte, I�/I3

� redoxcouple, and phosphate buffer. Formation of confor-mal contact between CZTS and redox species (suchas europium nitrate electrolyte in this case) pro-motes minimization of minority-carrier diffusiontowards the electrolyte. For evaluation of

Fig. 1. (a) X-ray diffractogram of CZTS film on FTO-coated glass substrate, with actual peak positions (determined using JCPDS file andsoftware suite) of kesterite CZTS phase (pink lines). (b) Squared absorption coefficient (a2) versus photon energy hm (eV) curve for CZTS film.

Fig. 2. (a) High-resolution transmission electron micrograph of CZTS film coated on FTO-coated glass substrate. Inset shows clear separationbetween grains. Another inset presents point EDS elemental analysis of CZTS film. (b) Surface EDS elemental analysis of CZTS film.


5311

photoelectrochemical performance, linear scanvoltammograms were recorded by a potentiostatusing virtual front-panel software during flashinglight illumination. The active area of the photocath-odes was �0.50 cm2. The cathodic photocurrentresponse confirmed the p-type photoactivity of theannealed CZTS films. We examined our pristinefilms as well as films after postmodification byphotoelectrochemical testing, finding that the pho-tocurrent for film containing nanosized flakes wasenhanced (Fig. 5). The change in current was�0.22 mA, almost twice the current response ofthe parent film. However, films treated with H2O2/H2SO4 did not exhibit high photocurrent response,with a change in photocurrent of only 0.08 mA.These photoresponse curves were recorded at biaspotential of �0.70 V. The low photocurrent response

of these films can be attributed to high recombina-tion and short lifetime of carriers. Prior to examin-ing other postmodified samples, we also performedKCN etching of selected baseline samples. Therewas not much change in the PEC response afterKCN treatment compared with baseline. This resultindicates that there were not many inclusions orCuS traces in the baseline solution.21 This observa-tion is consistent with earlier reports.21 A separateset of experiments was used to compare the perfor-mance of fresh samples using I–V scans on selectedsamples. It was observed that film with nanoflakeswas greatly superior throughout the potential scanrange. Although NH4F-treated films exhibited someimprovement, they were not superior to films withnanoflakes (Fig. 6a).

Fig. 3. High-resolution scanning electron micrographs (recolored) of: (a) CZTS baseline sample, (b) CZTS film with nanoflakes (postmodificationusing solvothermal treatment), (c) CZTS film after H2O2/H2SO4 treatment, (d) another area of CZTS film after H2O2/H2SO4 treatment, (e) CZTSfilm after NH4F treatment, and (f) CZTS film after NH4F treatment (another view).


Electrochemical Impedance Spectroscopy(EIS) and Mott–Schottky Analysis

The effects of etching and shape of CZTS grainswere examined using electrochemical impedancespectroscopy measurements. The Mott–Schottkyplots for different electrodes exhibited negativeslope, characteristic of p-type semiconductors.5

The series resistance of the photoelectrochemicalcell is associated with the charge-transfer resistance

(RCT) between the photoelectrode and electrolyteand can be expressed as

Rs ¼ RFTO þ RCT þ RDiff ; ð1Þ

where RFTO is the resistance of the substrate andRDiff represents the diffusion impedance associatedwith the electrolyte. We performed Nyquist analysisfor different bias voltages, focusing in particular onthe regions of potential where the photocurrentshowed a significant change during chopped illumi-nation. The values of Rs and RCT were obtainedusing an appropriate fitting model in the EISspectrum analyzer. The magnitude of Rs was smallin most cases for electrodes made from CZTSnanoflake film, as seen in Fig. 6b (electrolyte andother conditions were kept similar). The relativelysmall value of RCT indicates faster charge-transferactivity with less recombination. To measure RCT

for different photocathodes, the real part of theimpedance (Z¢) for high-frequency semicircles wasmeasured. The RCT value decreased significantlywhen CZTS nanoflake-type electrodes were used. Ithas been reported that RCT values are different forCZTS kesterite and wurtzite.27 Hence, it will beinteresting to explore whether changes in thephotobehavior of the electrodes are due to phasetransformation or structural differences.

Raman Spectroscopy

To confirm whether CZTS films with electrochem-ical texturing exhibited any phase change or degra-dation, Raman spectroscopy (Fig. 7a, b, and c) was

Fig. 4. High-resolution scanning electron micrograph (recolored) ofCZTS film showing long flakes, rich in copper sulfide (CuxS) phase.These hexagonal flakes are several times higher and thicker thanthose shown in Fig. 1b.

Fig. 5. (a–d) Photoelectrochemical response (I–t curves) of different CZTS films in 0.025 M europium nitrate electrolyte solution. A three-electrode system and chopped AM1.5 illumination were used to measure the performance of the CZTS electrode.


5313

carried out. Figure 7a shows the Raman spectra of afilm (before and after electrochemical etching) in therange from 250 cm�1 to 400 cm�1. In all Ramanspectra, the three peaks are located at �337 cm�1,288 cm�1, and 368 cm�1, and it can be clearly seenthat the peak at �337 cm�1 is stronger. The Ramanpeak positions and relative intensities are in goodagreement with previously reported Raman spectrafor CZTS thin film. No significant change in theRaman spectra of the film was observed afterelectrochemical testing in the cases of H2O2-H2SO4

and solvothermal treatment (Fig. 7c). After Lorent-zian function fitting to most of the spectra, it wasobserved that neither the Raman peak position norshape changed significantly, indicating that thephase integrity of the CZTS film was not disruptedafter nanostructure formation. However, it can beseen that NH4F-treated samples showed a shift of�3 cm�1 in the Raman ‘A’ mode of vibration (Fig.7b). This indicates removal of the top layer rich inkesterite phase and possible exposure of disorderedkesterite (or wurtzite) and/or copper tin sulfide(CTS) phases. The reduction in Raman peak inten-sity is consistent with loss of materials due toetching.

Proposed Mechanism

Different solvent combinations have a uniqueeffect on the morphology and properties of thetarget substrate; For example, oxalic acid results inmultiple nanopore connections and hierarchicallybranched nanopores, whereas the combination ofoxalic acid and phosphoric acid results in formationof serrated channels due to generation of oxygen gasbubbles. Similarly, six-step anodization using oxalicacid results in formation of microflower-like struc-tures. Aqueous HF results in formation of well-tuned resonant structures or microcavities,28

whereas the HF–KMnO4 combination results inmeso–macroporous multilayer formation. In case ofhydrogen peroxide-based reaction, it is believed thatan oxidation reaction takes place, resulting information of elemental sulfur and/or sulfates

depending on the solution pH.29 Note that, withdecreasing pH, the tendency for oxidation increases,but at pH above 8, both sulfur and sulfide speciesare unstable and tend to form SxOy

n� species.30 Thesuperior performance of nanoflake-based films canbe attributed to enhanced surface area of absorbermaterial exposed to electrolyte, ZnS deficiency, andhigh catalytic activity due to reduced charge-trans-fer resistance. One important observation here isthat use of an optimum HCl concentration isessential, since this helps to form two-dimensionalflakes or hexagonal structures, and acts as aselective etchant for ZnS. Note that a plausiblemechanism for the formation of Cu2SnS3 proceedsvia the reaction between Cu+, Sn4+, and S2�. Thepossible mechanism of CZTS flake formation can bewritten as31,32

HCl ! Hþ þ Cl�

NH2CSNH2 þ 2H2O ! 2NH3 þ H2S þ CO2

SnCl2 þ H2S ! SnS þ 2HCl

SnS ! Sn2þ þ S2�

2Sn2þ þ 4Hþ þ O2 ! 2Sn4þ þ 2H2O

Sn4þ þ 2S2� ! SnS2

Similarly, there will be formation of ZnS and Cu2S.These sulfides (Cu, Zn, and Sn) will react33 and formCZTS flakes in presence of a sulfur source8:

Cu2S sð Þ þ ZnS sð Þ þ SnS gð Þ þ 1=2S2 gð Þ ! Cu2ZnSnS4

It has been reported that Cu–Sn–S intermediatecompounds in the CZTS phase evolution contributeto formation of kesterite grains synthesized byelemental sulfur.34 However, there is a lack ofsulfur role in phase-selective growth. Some reportssuggest that Cu–Zn–S intermediates exist in thewurtzite phase evolution process.34 Hence, anyvariation in precursor composition or ratio will notonly affect the phase purity but also the type ofphase, viz. kesterite or wurtzite. Often, the type ofphase (wurtzite or kesterite) also governs the

Fig. 6. (a) I–V curves of different CZTS films in 0.025 M europium nitrate electrolyte solution. A three-electrode system and chopped AM1.5illumination were used to measure the performance of the CZTS electrode. (b) Nyquist plots for baseline CZTS electrode and after solvothermaltreatment.


electrode performance, as seen in the case of highelectrocatalytic activity for wurtzite crystals.27 Fur-ther improvement of photoactivity can be achievedusing techniques such as secondary-phase removalby KCN, metal loading, and induction of secondarypore formation on nanoflakes. It has been reportedand verified that loading of metals that act ascocatalysts on the photocatalyst surface, such as Pt,Au, Ru, Pd, Ni, or Rh, is essential for enhancing thephotocurrent performance of some electrodes.35 TheFermi levels of most of these metals are lower thanthe conduction band (CB) of oxide photoelectrodes,thus photoexcited electrons can be transferred fromthe conduction band of the photoelectrode to metalparticles deposited on its surface, while photogen-erated holes in the valence band (VB) remain on thephotocatalyst. Accumulated electrons on metal par-ticles can then be used to carry out a reductionreaction, while holes on the photocatalyst can beused to carry out the oxidation reaction. Hence,suitable, cost-effective, and stable metals or theiralloys can be identified for further photoactivityenhancement. A significant increase in efficiency isexpected after such photocatalyst modification withmetal particles. For the Cu(InGa)Se2 (CIGS)-basedwater splitting system, it was observed that modi-fication with Pt or an n-type layer was veryeffective. Photocurrent increase of �16 times wasobserved after cathodic modification with CdS/ZnO.36 Without any surface modification or

texturing of the CZTS electrode, a photocurrentchange of �1 mA was observed at pH 7.37 Hence, itis anticipated that surface modification and forma-tion of a cascade band structure will enhance thephotocurrent performance.

Numerical Study of Randomly Textured CZTSStructure

Electromagnetic analysis of the absorber mediumin a solar cell is often essential to determine theabsorption for different geometrical patterns.38,39

Surface texturing of solar cells increases the com-plexity of the structures, making it difficult toobtain analytical solutions for their performance.However, finite-element method (FEM) and finite-difference time-domain (FDTD) approaches havebeen used to analyze optical wave propagation insolar cells.40–43 Such simulations lead to improvedunderstanding of light propagation and absorptionin CZTS solar cells.

For our simulations, we considered three differentgeometries for which we calculated the absorbance(Fig. 8): a plain CZTS layer without texturing, arandom trapezoidally textured CZTS layer, and arandom spherically textured CZTS layer. TheseCZTS surface textures were considered as theyresemble the experimentally fabricated samples,enabling us to understand the findings of ourexperimental results qualitatively.

Fig. 7. (a) Raman spectra of as-prepared and postgrowth-modified CZTS films. It can be seen that all films exhibited three main Raman peaks.(b) Magnified portion of Raman spectra showing wavenumber shift for ‘A’ mode when films were treated with NH4F. (c) Raman spectra of as-prepared and H2O2-treated CZTS films.


5315

Figure 8 shows the absorbance of incident lightversus wavelength for the cases where the surfacehas spherical and trapezoidal texturing (red andgreen traces, respectively), as well as no texturingfor comparison (blue traces). It is clear from thisgraph that, when one introduces texturing into theCZTS layer, the absorbance increases. This is due tothe fact that texturing results in enhanced surfacearea of absorbing medium, ultimately resulting ingreater power loss of light in the medium. Note thatspherical texturing results in greater absorbancethan trapezoidal texturing. We believe that this is

due to the greater surface area of the sphericallytextured surface in our case.

To investigate the power loss of incident light inthe solar cell medium, we further calculated thepower loss profiles in various layers of the medium.The results for different surface texturing as well aswithout texturing of the CZTS layer are shown inFig. 9. To calculate these power loss profiles, weused the finite-element time-domain solver of thecommercially available CST Microwave Studio soft-ware. As seen in Fig. 9, our simulation geometryconsisted of three layers, i.e., glass substrate,conducting tin oxide film, and CZTS absorbingmedium, as was the case experimentally. In thesimulations, we assumed glass substrate thicknessof 1 lm, while the conducting tin oxide and CZTSlayers were assumed to be 200 nm and 700 nmthick, respectively. A waveguide port at distance of�2 lm from the top of the front surface (CZTS-coated substrate) was used as a plane-wave sourcewith amplitude of �1 V/m. The time-averagedpower loss Q(x, y) was calculated as44

Qðx; yÞ ¼ 1

2ce0na � Eðx; yÞj j2; ð2Þ

where c is the speed of light in free space, e0 is thepermittivity of free space, a is the absorptioncoefficient, n is the real part of the complex refrac-tive index, and E is the electric field.

The average power loss profiles within the CZTSlayer without texturing and with random trape-zoidal or spherical texturing for incident wave-length of �510 nm are shown as 3D views inFig. 9a, b, and c, respectively.

Fig. 8. Absorbance of light versus wavelength in CZTS layer withtrapezoidal or spherical surface texture, as well as without texturingfor comparison. Surface texturing results in enhanced absorbance.

Fig. 9. Power loss profiles at incident wavelength of 510 nm for CZTS layer in two different perspectives: (a–c) 3D view and (d–f) in x–y plane,for CZTS layer without texturing and with random trapezoidal or spherical texturing, respectively.


For comparison, we also calculated the averagepower loss density in the CZTS layer in all cases. Inthe case of the plain CZTS layer, this was�0.907 9 1018 W/m3, whereas in the case of therandom trapezoidal and the spherical geometries, itwas found to be �2.6 9 1018 W/m3 and�2.642 9 1018 W/m3, respectively, i.e., an increaseof 181% and 191%. To provide a comprehensivepicture of the power loss profiles, we also presentthe power loss profiles in a different perspective (inthe y–z plane) in Fig. 9d, e, and f for the three casesdiscussed above.

We understand that the low absorption or highreflectivity of incident light is due to the abruptdiscontinuity in dielectric constant at the air–CZTSinterface.45 It is believed that a graded transition indielectric constant will reduce the reflectivity.46

Such a graded surface can be produced usingvarious surface texturing techniques,47,48 but iseffective for antireflection when the characteristicdimension of the texture is much smaller than thesmallest wavelength of the incident light. Informa-tion derived from the simulation results suggestthat nanotexturing of CZTS layers significantlyenhanced the power absorption. This prediction isvalidated by the high photoelectrochemical perfor-mance in the case of textured surface.

CONCLUSIONS

Different solution-based postgrowth modificationmethods were proposed and used to texture thesurface of CZTS. The nature of the surface texture orpattern, selectivity, and etch rate strongly dependedon the solvent combination and etching agentapplied. It was observed that the electrochemicalperformance of CZTS film with nanoflakes wasenhanced compared with other textured films. Wesuggest a mechanism in which postgrowth of CZTSnanoflakes occurs first via formation of sulfides of Cu,Zn, and Sn and subsequent reaction with sulfur.FDTD simulations were performed using CST Micro-wave Studio, revealing that texturing of the CZTSlayer resulted in enhanced absorbance, mainly due toenhanced surface area of absorbing medium, ulti-mately resulting in greater power loss of light in themedium. The average power loss density of the CZTSlayers was calculated in three different cases. For aplain CZTS layer, it was �0.907 9 1018 W/m3,whereas for random trapezoidal and spherical tex-turing, it was found to be �2.6 9 1018 W/m3 and�2.642 9 1018 W/m3, respectively.

CONFLICT OF INTEREST

The authors declare that they have no conflicts ofinterest.

REFERENCES

1. Z. Li, W. Luo, M. Zhang, J. Feng, and Z. Zou, EnergyEnviron. Sci. 6, 347 (2013).

2. M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher,Q. Mi, E.A. Santori, and N.S. Lewis, Chem. Rev. 110, 6446(2010).

3. B.-J. Li, P.-F. Yin, Y.-Z. Zhou, Z.-M. Gao, T. Ling, and X.-W.Du, RSC Adv. 5, 2543 (2015).

4. M.P. Suryawanshi, S.W. Shin, U.V. Ghorpade, K.V. Gurav,C.W. Hong, G.L. Agawane, S.A. Vanalakar, J.H. Moon,J.H. Yun, P.S. Patil, J.H. Kim, and A.V. Moholkar, Elec-trochim. Acta 150, 136 (2014).

5. P.K. Sarswat, M.L. Free, and G. Kumar, J. Spectrosc. 2013,9 (2013).

6. P.K. Sarswat, M. Snure, M.L. Free, and A. Tiwari, ThinSolid Films 520, 1694 (2012).

7. P.K. Sarswat and M.L. Free, Thin Solid Films 520, 4422(2012).

8. S. Chen, H. Tao, Y. Shen, L. Zhu, X. Zeng, J. Tao, and T.Wang, RSC Adv. 5, 6682 (2015).

9. P.K. Sarswat and M.L. Free, Physica Status Solidi (a) 208,2861 (2011).

10. N. Kamoun, H. Bouzouita, and B. Rezig, Thin Solid Films515, 5949 (2007).

11. G. Carraro, C. Maccato, A. Gasparotto, T. Montini, S.Turner, O.I. Lebedev, V. Gombac, G. Adami, G. Van Ten-deloo, D. Barreca, and P. Fornasiero, Adv. Funct. Mater.24, 372 (2014).

12. Y. Yu, G. Chen, Q. Wang, and Y. Li, Energy Environ. Sci. 4,3652 (2011).

13. D. Yan, H. Zhang, S. Li, G. Zhu, Z. Wang, H. Xu, and A. Yu,J. Alloys Compd. 607, 245 (2014).

14. K. Wang, G. Liu, N. Hoivik, E. Johannessen, and H.Jakobsen, Chem. Soc. Rev. 43, 1476 (2014).

15. W. Li, P. Da, Y. Zhang, Y. Wang, X. Lin, X. Gong, and G.Zheng, ACS Nano 8, 11770 (2014).

16. A.R. Silva, J. Miyoshi, J.A. Diniz, I. Doi, and J. Godoy,Energy Procedia 44, 132 (2014).

17. M. Abburi, T. Bostrom, and I. Olefjord, Mater. Chem. Phys.139, 756 (2013).

18. K. Al-Heuseen, M.R. Hashim, and N.K. Ali, J. Electrochem.Soc. 158, D240 (2011).

19. J. Xu, X. Yang, Q.-D. Yang, T.-L. Wong, and C.-S. Lee, J.Phys. Chem. C 116, 19718 (2012).

20. C. Liu, J. Tang, H.M. Chen, B. Liu, and P. Yang, Nano Lett.13, 2989 (2013).

21. A. Fairbrother, E. Garcıa-Hemme, V. Izquierdo-Roca, X.Fontane, F.A. Pulgarın-Agudelo, O. Vigil-Galan, A. Perez-Rodrıguez, and E. Saucedo, J. Am. Chem. Soc. 134, 8018(2012).

22. G.C. DeSalvo, W.F. Tseng, and J. Comas, J. Electrochem.Soc. 139, 831 (1992).

23. C. Juang, K.J. Kuhn, and R.B. Darling, J. Vac. Sci. Tech-nol. B 8, 1122 (1990).

24. P. Sarswat and M. Free, J. Electron. Mater. 41, 2210(2012).

25. C. Persson, J. Appl. Phys. 107, 053710 (2010).26. P.M.P. Salome, P.A. Fernandes, J.P. Leitao, M.G. Sousa,

J.P. Teixeira, and A.F. da Cunha, J. Mater. Sci. 49, 7425(2014).

27. J. Kong, Z.-J. Zhou, M. Li, W.-H. Zhou, S.-J. Yuan, R.-Y.Yao, Y. Zhao, and S.-X. Wu, Nanoscale Res. Lett. 8, 1(2013).

28. P.N. Patel, M. Vivekanand, and A.K. Panchal, Adv. Nat.Sci. Nanosci. Nanotechnol. 3, 035016 (2012).

29. M. Nusheh, H.G. Ahuett, and A. Arrambide, Recent Re-searches in Metallurgical Engineering: From Extraction toForming (Rijeka: InTech, 2012).

30. P. Somasundaran, Encyclopedia of Surface and ColloidScience (London: Taylor & Francis, 2006).

31. J. Feng, J. Chen, B. Geng, H. Feng, H. Li, D. Yan, R. Zhuo,S. Cheng, Z. Wu, and P. Yan, Appl. Phys. A 103, 413 (2011).

32. J.J. Scragg, Copper Zinc Tin Sulfide Thin Films for Pho-tovoltaics: Synthesis and Characterisation by Electro-chemical Methods (Berlin: Springer, 2011).

33. M.C. Johnson, C. Wrasman, X. Zhang, M. Manno, C.Leighton, and E.S. Aydil, Chem. Mater. 27, 2507 (2015).


5317

34. B. Zhou, D. Xia, and Y. Wang, RSC Adv. 5, 70117(2015).

35. Y.-J. Yuan, Z.-J. Ye, H.-W. Lu, B. Hu, Y.-H. Li, D.-Q. Chen, J.-S. Zhong, Z.-T. Yu, and Z.-G. Zou, ACS Catal. 6, 532 (2016).

36. M.G. Mali, H. Yoon, B.N. Joshi, H. Park, S.S. Al-Deyab,D.C. Lim, S. Ahn, C. Nervi, and S.S. Yoon, ACS Appl.Mater. Interfaces 7, 21619 (2015).

37. L. Rovelli, S.D. Tilley, and K. Sivula, ACS Appl. Mater.Interfaces 5, 8018 (2013).

38. C. Haase and H. Stiebig, Appl. Phys. Lett. 91, 061116(2007).

39. C. Haase and H. Stiebig, Prog. Photovolt. Res. Appl. 14, 629(2006).

40. J.P. Mailoa, Y.S. Lee, T. Buonassisi, and I. Kozinsky, J.Phys. D Appl. Phys. 47, 085105 (2014).

41. R. Dewan, V. Jovanov, S. Hamraz, and D. Knipp, Sci. Rep.4, 6029 (2014).

42. B. Ananthoju, S.M. Mopurisetty, H. Tyagi, D. Bahadur,N. Medhekar, S. Ganguly, and M. Aslam, in PhotovoltaicSpecialist Conference (PVSC), 2015 IEEE 42nd, p. 1(2015).

43. N. Bednar, N. Severino, and N. Adamovic, Appl. Sci. 5,1735 (2015).

44. R. Dewan and D. Knipp, J. Appl. Phys. 106, 074901 (2009).45. R.B. Stephens and G.D. Cody, Thin Solid Films 45, 19

(1977).46. H.-C. Yuan, V.E. Yost, M.R. Page, P. Stradins, D.L.

Meier, and H.M. Branz, Appl. Phys. Lett. 95, 123501(2009).

47. D. Macdonald, A. Cuevas, M.J. Kerr, C. Samundsett, D.Ruby, S. Winderbaum, and A. Leo, Sol. Energy 76, 277(2004).

48. M. Suryawanshi, G. Agawane, S. Bhosale, S. Shin, P. Patil,J. Kim, and A. Moholkar, Mater. Technol. 28, 98 (2013).


Documents

Surface Texture-Induced Enhancement of Optical and