Cu2ZnSnS4 Films Grown on Flexible Substrates by Dip Coating Using ...

Journal of ELECTRONIC MATERIALS, Vol. 44, No. 12, 2015

DOI: 10.1007/s11664-015-4103-z 2015 The Minerals, Metals & Materials Society

Cu2ZnSnS4 Films Grown on Flexible Substrates by Dip Coating Using a Methanol-Based Solution: Electronic Properties and Devices A.E. RAKHSHANI1,2,3 and S. THOMAS1 1.—Physics Department, Faculty of Science, Kuwait University, P.O. Box 5969, 13060 Safat, Kuwait. 2.—e-mail: [email protected]. 3.—e-mail: [email protected]

The deposition of device quality Cu2ZnSnS4 (CZTS) films on flexible substrates by simple and cost-effective techniques is of great interest for solar cell applications. In this work, CZTS films were deposited on lightweight flexible substrates by successive dip coating using a nontoxic, methanol-based precursor solution. The films were characterized by x-ray diffraction, energy dispersive x-ray analysis, scanning electron microscopy, atomic force microscopy, optical transmission spectroscopy, photocurrent spectroscopy and admittance spectroscopy. The films prepared by this technique have direct band gaps of 1.5–1.6 eV, a p-type resistivity of 1 X cm, an acceptor concentration of 1017 cm3 and structural and morphological properties that are suitable for device applications. Four defect levels with activation energies of 5.4 meV, 18.8 meV, 70 meV, and 221 meV were detected in the films. All but the shallowest defect level were attributed to the native VCu, CuZn, and VSn acceptor-type defects. For further assessment of the films, Schottky barrier and heterojunction diodes were fabricated and characterized. The results signified that the device quality CZTS films can be synthesized by the dipcoating method used in this study. Key words: Cu2ZnSnS4, dip coating, photocurrent, defect level, Schottky barrier, heterojunction

INTRODUCTION Cu2ZnSnS4 (CZTS) is a quaternary compound semiconductor that is used as an absorber layer in the fabrication of thin film solar cells. CZTS is composed of earth-abundant, nontoxic and lowcost elements, in contrast to the CdTe and CuInxGa1xSe2 currently used in solar cells. CZTS is an intrinsic p-type semiconductor with crystal structures of stannite and kesterite, with the latter phase being more thermodynamically stable. CZTS has a direct band gap energy of 1.4–1.6 eV (the optimum value for a single-junction cell) and a high optical absorption coefficient of over 104 cm1 in the visible wavelength region.1,2 Owing to these optimal properties, the conversion efficiency of CZTS-based (Received July 22, 2015; accepted September 28, 2015; published online October 15, 2015)

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cells can theoretically reach 32% (Shockley–Queisser limit).2 By partial replacement of S with Se, the band gap of CZTS(Se) can be tuned in the range of 0.9–1.6 eV, which renders the material even more attractive for the fabrication of tandem solar cells. The synthesis, properties and photovoltaic application of CZTS films have been reviewed.1,2 CZTS(Se) films have been prepared by a variety of techniques, such as evaporation,3 sputtering,4 using a scalpel,5 spray pyrolysis,6 sol–gel processing,7 nanocrystal synthesis,8 electrodeposition,9 spin coating,10 and dip coating11–15 and by other vacuum-based and solution-based methods. The solution-based techniques are attractive because they are simple, fast, and scalable and can be performed at low temperatures and in non-vacuum environments. The best recorded efficiency of CZTS(Se)-based solar cells is 12.6% (2013);16 in this case, the absorber layer was prepared by spin coating from a hydrazine-based

Cu2ZnSnS4 Films Grown on Flexible Substrates by Dip Coating Using a Methanol-Based Solution: Electronic Properties and Devices

solution. Special care was needed for handling hydrazine because of its toxic and flammable properties. Other non-hazardous solution-based techniques, such as spray pyrolysis,6 electrodeposition,9 and spin coating,10 have yielded solar cells with conversion efficiencies of 8–9%. Dip coating is another solution-based route, which is even simpler than spin coating and does not require expensive equipment. CZTS films can be fabricated from dipcoated Cu2SnS3 and ZnS stack layers11 as well as from dip-coated stacks of SnSx and ZnS.12 Absorber films can also be processed by dipping the substrate in a solution of ammonia containing Cu, Zn, Sn and Na2S2O3, followed by annealing the coated film in air.13 In another approach, CZTS films have been prepared via successive dipping of the substrate in a cationic solution containing Cu, Zn and Sn and an anionic solution of thioacetamide.14 CZTS films were also synthesized by Chaudhuri and Tiwari from a single and non-toxic solution of methanol containing Cu, Zn, Sn and thiourea, as described below.15 The glass substrate was dipped into the precursor solution, followed by decomposition of the deposit into CZTS by air annealing the wet substrate at 200C. The deposition cycle was repeated several times to obtain the desired thickness. This simple and low-cost technique produced photoconductive films, while reports on the photoconductivity of CZTS are sparse. The suitable temperature range for the decomposition process was found to be 190–270C. Films grown on glass were found to be p-type with a resistivity of approximately 1 Xcm. The concentration and mobility of the holes were estimated from the thermoelectric power (+86 lV K1) to be 3.4 9 1019 cm3 and 0.1 cm2 v1 s1, respectively.15 To the best of our knowledge, this simple deposition technique has not been utilized to fabricate device-quality films on flexible conducting substrates, and no attempt has been made to study the electronic properties of these films. The aim of this study was to prepare CZTS films on flexible conducting substrates using the successive dip-coating method with a modified precursor solution to achieve higher quality films suitable for device applications. We used a higher decomposition temperature and a higher zinc concentration in the precursor solution than those reported originally;15 the goal was to obtain films with a Zn-rich/Sn-poor composition that would limit the formation of certain undesirable phases. Here, we report new results for the electronic properties of the films and for the fabrication and characteristics of Schottky-barrier (SB) and heterojunction diodes on these films.

EXPERIMENTAL PROCEDURE For the preparation of the precursor solution, 100 mM of copper sulfate (CuSO4Æ5H2O, Aldrich 99.99%), 100 mM of zinc accetate (Zn(O2CCH3)2, Aldrich 99.99%), 50 mM of tin chloride (SnCl2Æ

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2H2O, Aldrich 99.99%) and 500 mM of thiourea (SC(NH2)2, Alfa Aesar 99%) were dissolved in methanol (CH3OH, Sima-Aldrich, 99.8%). Then, 40 mL of the precursor solution was stirred to transparency while 4 mL 50% HCl was slowly added. The concentration of zinc in the precursor solution was chosen to be twice that reported by Chaudhuri et al.15 for preparing CZTS films with a Zn-rich/Snpoor composition. Films were deposited on different substrates, including glass, flexible molybdenum (Mo) and stainless steel (SS). A pre-cleaned substrate was dipped into the precursor solution for approximately 30 s. After the removal from the solution, the substrate was dried in hot air and then heat treated in air at 250C for 10 min. This yielded a uniform and adherent film of CZTS with a shiny black appearance and a thickness of 120 nm, which was measured by a surface profiler. To increase the film thickness, the deposition cycle was repeated 10–15 times. A post-deposition annealing, which enhanced the film’s properties, was performed mainly at 350C in air (30–60 min); in some cases, the film was annealed at 400C in an Ar (90%)/N2 mixture at atmospheric pressure. A SB diode was fabricated on the chemically untreated surface of the CZTS film by vacuum deposition of circular Al contacts, with thicknesses of 50–100 nm and diameters of 1.5–2.0 mm. A heterojunction diode was fabricated by chemical bath deposition of a CdS buffer layer (50–100 nm) on the chemically untreated surface of the CZTS films, followed by brief heat treatment in air (100C, 10 min) prior to the deposition of Al contacts. The chemical bath used was an alkaline solution containing CdCl2 (1.9 mM), SC(NH2)2 (3.8 mM), NH4Cl (15.7 mM) and NH4OH (0.85 mM). The chemicals used had a minimum purity of 99%. Films were characterized using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) (Jeol JSM 7001F), x-ray diffraction (XRD) (Siemens D500, CuKa: 0.154056 nm), atomic force microscopy (AFM) (Agilent 5420) and UV–Vis– NIR optical spectrometry (Shimadzu, SolidSpec3700). The films’ lateral photoconductivity was measured across two parallel conducting strips (1– 2 mm separation) deposited on the film surface under a suitable potential difference. The photocurrent, normalized by the incident photon flux, was determined at different incident wavelengths. The measurements were performed using a setup consisting of a grating monochromator (Sciencetech 9050), lock-in amplifier (Stanford Research SR 530), current amplifier (Keithley 428), mechanical chopper and light source (tungsten-halogen). For measurements of photoconductivity at different temperatures, samples were placed in an optical cryostat (Oxford DN 1704). The resistivity of films grown on glass was determined by the fourprobe technique. The hot-probe technique verified the conductivity of the films as p-type. The current–voltage (IV) and capacitance–voltage (CV)

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Rakhshani and Thomas

RESULTS AND DISCUSSION Composition and Structure The atomic ratios of elements in the films grown on glass were Zn/Cu = 0.84 ± 0.08, Sn/Cu = 0.40 ± 0.03, and (Cu + Zn + Sn)/S = 1.04 ± 0.07. After post-deposition annealing at 400C, these ratios changed to Zn/Cu = 0.71 ± 0.06, Sn/Cu = 0.40 ± 0.03, and (Cu + Zn + Sn)/S = 1.16 ± 0.07. No significant changes were detected in the film composition when post-deposition annealing was performed at 350C. These results indicate that the films were Zn-rich and slightly Sn-poor with respect to the copper content. Within measurement accuracy, the atomic ratio of metals to sulfur was unity before and after the annealing process. XRD patterns for the as-grown and annealed films, which are depicted in Fig. 1, showed only the (112), (200), (220) and (312) diffraction peaks of CZTS; these appeared at 2h = 28.5, 33.0, 47.3 and 56.2, respectively, and corresponded to the lattice constants a = b = 0.5427 nm and c = 1.0848 nm (JCPDS 26-0575). No other XRD lines were observed, suggesting that a pure CZTS phase was synthesized. ZnS and Cu2SnS3 impurity phases that produce overlapping lines were discarded based on the fact that their optical band gap transitions were not detected. The average size of the crystallites D in the films was estimated from the full width at half maximum b of the (112) peak using the Scherrer equation D = 0.94 k/(b cosh), where k (= 0.15406 nm) is the wavelength of the incident x-ray.11 D increased slightly from 5.4 nm in the as-grown film to 6.8 nm after the annealing process at 400C. The micro-strain S and the density of dislocations q in the films were estimated using S = Dcosh/4 and q =1/D2.11 For the as-grown films and the films annealed at 350C, S = 6.7 9 103 and q = 3.4 9 1014 m2 Annealing at 400C improved the structural properties of the films and yielded S = 3.4 9 103 and q = 1.4 9 1014 m2. The surface morphology of the SS substrate and a film deposited on SS before and after being annealed at 350C is shown in Fig. 2. Annealing at 400C was avoided for films grown on SS, because otherwise these films developed micro-cracks in the top layers. No pores, cracks or voids were detectable in the SEM micrographs of Fig. 2, suggesting good structural quality of the film. It is also evident from Fig. 2 that the annealed film had a smoother surface; the AFM-determined surface roughness of the films, averaged over six measurements from different areas, yielded 9.0 nm for the as-grown and

25

112 20

Intensity (arb. u.)

characteristics of the diodes were measured by a source/measure unit (Keithley 236) and a CV analyser (Keithley 590,100 kHz), respectively. A Device Analyzer (Keysight B1500A) was used for admittance spectroscopy measurements in the frequency range of 1 kHz–5 MHz.

200 220 312

(c)

15

(b)

10

(a)

5 0 0

20

40

60

80

2θ (deg) Fig. 1. XRD patterns for a film on glass: (a) as-grown, (b) annealed at 350C, and (c) annealed at 400C.

5.7 nm for the annealed film. The smoother surface of the annealed film could be attributed to the removal of the defect clusters, which may exist in the as-grown films. This removal had a pronounced effect on the optical transmittance of the films, which will be discussed. Optical Transmittance and Band Gap Energy Figure 3a shows the optical transmittance (Tr) spectra for the as-grown and annealed films. Annealing increased the film transmittance considerably, probably as a result of the removal of scattering centres such as defect clusters. To determine the optical band gap of the films, the convenient method of plotting (aE)m against E was used, where a is the film optical absorption coefficient at the incident photon energy E.17 The plot is expected to yield a straight line whose intercept on the axis for E yields a direct band gap for m = 2 or an indirect band gap for m = 1/2. In this work, ln(1/Tr), which varies in proportion to a, was used for the evaluation of band gap. A more elaborate approach for the calculation of a from the transmittance and reflectance data18 was also examined using the reflectance data for a single crystal of CZTS.19 Both methods produced the same results, verifying the accuracy of the simpler approach. From the plots shown in Fig. 3b, the direct band gap of the as-grown and annealed films was measured as 1.50 eV. The scattered data points seen in Fig. 3b were due to the relatively large thickness (1.8 lm) of the film used, where the intensity of the transmitted light was too low and noisy, especially for the as-grown film. Photocurrent Analysis Photocurrent spectroscopy was also used as a complementary method for the evaluation of band


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200 nm

Transmittance (%)

(a) 30

(a)

25 Annealed

20 15 10 5

(b)

As-grown

0 500

1000

1500

Wavelength (nm) (b)

Annealed (α E) (arb. u.)

200 nm

As-grown

2

(c)

1.50 eV

200 nm

Fig. 2. SEM plan view of a film on SS (a) before and (b) after being annealed at 350C. (c) SEM plan view of the SS substrate.

gap energy. This technique is especially useful when the measurement of optical transmittance is difficult owing to large film thickness. The lateral photocurrent of a film with thickness d, which is normalized by the incident photon flux R, should vary in proportion to a over a suitable range of photon energies where ad is less than unity.20 Therefore, a plot of (RE)m against E is expected to yield a straight line whose horizontal intercept measures the band gap energy. Figure 4a shows such a plot for a film grown on glass and annealed at 350C. The as-grown films did not exhibit a strong photoconductivity suitable for this analysis. The photoconductivity of films, similar to their optical transmittance, was enhanced considerably after post-deposition annealing. The horizontal intercept of the plot in Fig. 4a denotes a direct band gap of 1.56 eV, which is in agreement with 1.50 eV obtained from the transmittance data. The calculated band gaps for CZTS are in the range of 1.49–1.64 eV

1

2

3

4

E (eV) Fig. 3. (a) Optical transmittance of a film deposited on glass before and after being annealed at 400C. (b) Plots of (aE)2 versus photon energy E for the same film (a is the optical absorption coefficient).

for the kesterite phase and 1.30–1.42 eV for the stannite phase.21 Figure 4b shows the temperature dependence of photocurrent for the sample in Fig. 4a using a He/ Ne laser as the excitation source. The three distinct regions of the plot (A, B, C) can be explained by a theory proposed by Simmons and Taylor.22,23 The theory predicts that at very low temperatures (T), the photocurrent density Jph should be independent of T. In the intermediate region A, Jph is expected to increase with increasing T and to attain a peak value. In the high temperature region B, Jph is expected to decrease with increasing T. The temperature dependence of Jph in the regions A and B are given by Simmons and Taylor as Region A : JPh ¼ qleðGNV =vth rN Þ1=2 expðEa =2kT Þ; (1)

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2

(R.E) (arb. u.)

(a)

1.56 eV

1.5

1.6

1.7

1.8

E (eV)

(b) -1.2

Schottky Barrier and Heterojunction Diodes C

-1.4 -1.6 ln Iph (arb. u.)

3.7 9 1017 cm3, using p = Nvexp(f/kT), NV = 1.6 9 1019 cm3 (Ref. 24) and f = 98 meV. This result is in good agreement with the density of acceptors determined from the CV analysis of SB and heterojunction devices, as will be discussed. The increase of Jph with T in region C can be attributed to a second defect level whose activation energy was measured from Eq. 1 as 221 meV. The 70-meV activation energy, determined from Fig. 4b, is in close agreement with that measured in Na-doped CZTS (72–94 meV).24 This defect level can be attributed to (0/–)CuZn, which has a theoretical activation energy of 100 meV.25 The activation energy of the deeper defect level (221 meV) matches closely with the calculated values for Vsn and CuSn defects.26 We assign the activation energy of 221 meV to the Vsn acceptor-type defect because the sample was Sn-poor in composition.

-1.8

B

-2

A

-2.2 -2.4 -2.6 -2.8 3

4

5

6

7

8

9

10

11

-1

1000/T (K ) Fig. 4. (a) Variation in the normalized photocurrent R versus photon energy E. (b) The temperature dependence of photocurrent Iph.

Region B : JPh ¼ qleðG=vth rN Þexp½ðn Ea Þ=kT ; (2) where q is electron charge, k is Boltzmann’s constant, l is the holes mobility, e is the applied electric field, G is the excess carrier photogeneration rate (proportional to the intensity of incident light), NV is the valence band density of states and vth is the thermal velocity of holes. r and N denote the capture cross section and the density of a defect trapping level, respectively. Ea is the activation energy of a defect trapping level and f is the Fermi energy, both with respect to the edge of the valence band. The temperature dependence of the pre-exponential factors in Eqs. 1 and 2 is very weak. Figure 4b shows the variation of Jph with T in regions A and B, as predicted by the theory. From the linear slopes fitted to the data points in those regions, we obtained Ea = 70 meV and f = 98 meV. The concentration of free holes in the valence band was p =

To the best of our knowledge, the device application of CZTS films prepared by the method used in this study has not been demonstrated. SB devices were fabricated on films deposited on SS and annealed at 350C. The annealed films had a p-type resistivity of 1 X cm. Gold and aluminium circular contacts were thermally evaporated on the chemically untreated films. A preliminary chemical treatment with some reported etchants, including KCN, did not yield superior devices. Gold was found to produce ohmic contact with the films, as concluded from the linear IV plots of the SS-CZTS-Au devices. The electrical contact at the SS-CZTS interface was apparently ohmic, probably as a result of the presence of interface defects. Aluminium produced a rectifying contact with the films. As a result, diode-type IV characteristics were exhibited with a forward current corresponding to the negative bias of Al. Figure 5a shows a typical IV plot and the device configuration (inset). Ignoring the effects of leakage current, the IV behaviour for ideal SB diodes is27 I ¼ I0 ½expðqVj =nkTÞ 1;

(3)

where q is the electron charge, n is the ideality factor, k is Boltzmann’s constant, T is the absolute temperature, Vj is the voltage drop across the junction (Vj = V IRs), and Rs is the diode series resistance. When thermionic emission plays a dominant role, the reverse biased saturation current I0 is given by I0 ¼ AA T 2 expðU=kTÞ;

(4)

where A is the device area, A* is the effective Richardson constant (A* = 63.6 A cm2 K2 using an effective mass m* = 0.53 m for CZTS) and U is the zero-field barrier height at the metal-film interface. For the SB device in Fig. 5a, the saturation current is I0 = 200 nA, and its rectification


(a) 0.4 Al CZTS SS

I (mA)

0.2

0.0

-1.5

-1

-0.5

0

0.5

1

1.5

V (V)

(b) 10-2 2.5 16 -2

C (10 F )

-2

I (A)

10

-4

10-6

0.0 -2.5

-1.5

-0.5

0.5

V (V)

10-8 -0.5

0.5

1.5

2.5

3.5

2 (η.E) (arb. u.)

(c)

1.50 eV 1.2

1.3

1.4

1.5

factor (ratio of the forward to reverse current) r = 9.4 (±1.0 V) is relatively low. The barrier height and the ideality factor were obtained as U = 0.71 eV and n = 4.5 by fitting the forward IV data to Eqs. 3 and 4. The device capacitance measured at a test frequency of 100 kHz was 5.8 nF and independent of the bias voltage. This suggests that the measured capacitance was a combination of the junction capacitance and a dominant bias-independent capacitance. The latter may be related to the effect of interface states or the presence of an insulating layer in the device structure. A brief annealing step after the deposition of Al contacts improved the device characteristics appreciably. Figure 5b and its inset show the forward IV plot and the CV plot for a typical device with the improved characteristics. The rectification factor increased to r = 24 (±1.0 V), the saturation current reduced to I0 = 30 nA, the barrier height increased to U = 0.76 eV and the ideality factor reduced to n = 3.4. The large n values exceeding unity indicate the deviation of these devices from ideal behaviour. Generally, the polycrystalline nature of the films, large series resistances, contributions from space charge-limited conduction and parasitic rectifying junctions within the device28 and the presence of interface states in equilibrium with the film are the plausible reasons for yielding large n values.29 The junction built-in potential V0 and the concentration of uncompensated acceptors Na in the film were determined to be V0 = 0.53 V and Na = 7.2 9 1017 cm3 from fitting the experimental data, in the inset of Fig. 5b, to the relationship27 C2 ¼ ð2=qer e0 Na A2 ÞðV0 kT=q VÞ:

V (V)

1.6

1.7

1.8

E (eV) Fig. 5. (a) IV plot and device configuration (inset) of a SB device. (b) Forward IV plot and CV plot (inset) of a heat-treated device. (c) Plot of (gE)2 versus photon energy E, where g is the external quantum efficiency of a zero-biased SB diode.

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(5)

In the above expression, e0 is the permittivity of free space and er is the static dielectric constant, taken to be 7.0.30 Deviation from the nonlinear behaviour, observable in the inset plot of Fig. 5b, suggests a non-uniform distribution of Na along the depth of the film. Na increased from 7.2 9 1017 cm3 in the vicinity of junction to 1.091019 cm3 in the bulk of the film. The latter which was obtained from the slope of the C2–V plot in the bias range of 1.5 V to 2.0 V agrees with the estimated hole concentration of 3.4 9 1019 cm3 for similar films.15 Reports on Na values in films prepared by different techniques are scarce. Nevertheless, Dhakal et al. report Na = 5.8 9 1016 cm3 and p (Na) = 5.7 9 1016 cm3 for sputter-deposited films.31 Xinkun et al. report p = 1016–1018 cm3 for films prepared by thermal and electron-beam evaporation.32 Using Na = 7.291017 cm3 in the junction vicinity and taking NV = 1.691019 cm3, the position of the Fermi level from the valence band f was determined from f = (kT/ q)ln(NV/Na); this in turn yielded another value for the barrier height, where U = q(V0 + f). The results

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(a)

(a) 2.0 1.5

160 K

0 -0.50

0.50

V (V)

10-7

120 K

C (nF)

-1.50

I (A)

10

-5

240 K 210 K 180 K

17 -2

10

C-2 (10 F )

5 -3

N* = 1.5x1017 cm-3 V0 = 0.45 V

90 K

0.5

n = 1.9

Temperature increase

Io = 0.1 nA r = 2.1x105 (±1V) 10

0.0 3 10

-9

-1.5

-1

-0.5

0

0.5

1

4

6

10

10 Frequency (Hz)

1.5

V (V)

(b) 0.2 CZTS

CdS

EC

1.50 eV

C=

0.05 eV

2.43 eV EV

++++++++

2 -1 -2 ln(Fo/T ) (s K )

(b)

18.8 meV

0

-0.2 5.4 meV -0.4

V = 0.98 eV

-0.6 3 Fig. 6. (a) A typical IV and CV plot (inset) for the heterojunction diodes. (b) Schematic band diagram of a CZTS/CdS heterojunction.

4

5

6

7

8

9

10 11 12

-1

1000/T (K ) Fig. 7. (a) Frequency dependence of the capacitance of a zero-biased heterojunction diode at different temperatures. (b) Temperature dependence of the inflection frequency F0.

obtained were f = 80.3 meV and U = 0.61 eV. The latter value is smaller than the 0.76-eV barrier height measured from the device IV characteristics. Such a discrepancy could be due to various effects, including the non-uniform distribution of Na. The short-circuit photocurrent of the device normalized by the incident photon flux (external quantum efficiency at zero bias) g was measured as a function of the incident photon energy E and was also used for the evaluation of the film band gap. Figure 5c shows a plot of (gE)2 against E, which yielded a direct band gap of 1.50 eV, in excellent agreement with those obtained from the optical transmittance and lateral photoconductivity measurements. Heterojunction diodes, which were fabricated on Mo foil substrates, presented superior device parameters to those of the SB diodes. For the heterojunction diodes with a thin buffer layer of CdS inserted between CZTS and Al, the junction was at the CZTS-CdS interface and the role of Al was to provide ohmic contact to CdS. The forward current corresponded to the negative voltage bias

applied to Al. In heterojunction diodes, the actual IV relationship is complex; nevertheless, the general form of the IV equation is still similar to that of a SB diode and is generally dominated by thermoionic emission of one type of carrier over an effective barrier height U’, which replaces U in Eq. 4.27 Figure 6a shows the IV plot for a heterojunction diode, whose device parameters were measured to be n = 1.9, r = 2.19105 (±1 V), I0 = 0.1 nA, and U¢ = 0.91 eV. A higher value of U¢ = 0.97 eV was measured for another device with parameters n = 2.0, r = 8.49105, and I0 = 10 pA. The measured values of U¢ are very close to 0.98 eV, the theoretical valence band offset energy at the CdS/CZTS interface (Fig. 6b),33 which apparently acts as a barrier height for the transport of holes across the junction. The value of the ideality factor measured, n = 2.0– 2.1, indicates the dominance of the Shockley-ReadHall recombination process through a single trap in the depletion region. Recombination through the


interface states normally yields n ‡ 2, where both n and I0 increase with the increase of the density of interface states.34 The CV plot in the inset of Fig. 6a can be expressed by Eq. 5 if the dielectric constants of CdS and CZTS are taken to be approximately the same (7.0) and Na is replaced by N* = NaNd/(Na + Nd), where Nd is the concentration of uncompensated donors in the CdS film. The intercept of the linear fit in the inset plot of Fig. 6a yields the device built-in potential V0 = 0.45 V, and the slope of the line gives N* = 1.5 9 1017 cm3. Using Na = 7.2 9 1017 cm3, measured from the CV plot of the SB diode and the N* value, the concentration of donors in the CdS film is calculated to be Nd = 1.9 9 1017 cm3. Admittance Spectroscopy For the characterization of defect levels by admittance spectroscopy, we used a heterojunction diode. The zero-biased junction capacitance was measured as a function of frequency from 1 kHz to 5 MHz at different temperatures, using an AC test voltage of 30 mV. Figure 7a shows the result. The inflection observable in the plots signifies a defect level. The observed increasing inflection frequency F0 (=x0/2p) with increasing temperature is predicted by theory through35,36 x0 ¼ 2et ¼ 2vth rNV expðEa =kT Þ:

(6)

In the above equation, et is the emission rate of the defect level at temperature T, and k, Ea, NV, vth, and r are the same parameters as in Eqs. 1 and 2. Accounting for the temperature dependence of vth and NV, Eq. 6 can then be written as F0 = bT2exp(Ea/kT), where b is a temperature-independent proportionality constant. Therefore, an Arrhenius plot of F0/T2 against 1/T will yield the value of Ea. This is demonstrated in Fig. 7b, where the plot gives two slopes corresponding to two traps with activation energies of 18.8 meV and 5.4 meV. The former activation energy is attributed to the acceptor-type (0/)VCu native defect, whose calculated activation energy is 20 meV.25 The formation energy of VCu is higher than that for the acceptortype CuZn defect; therefore, the p-type conductivity in CZTS is governed by the CuZn defect, which has a calculated activation energy of 100–120 meV.25,26 Using current-mode deep level transient spectroscopy, Das and co-workers recently measured the activation energies of the VCu and CuZn defects as 30 ± 10 meV and 120 ± 40 meV, respectively. The concentrations and capture cross sections of these defects have also been determined.37 Photoluminescence measurements have revealed the presence of two shallow acceptor levels in CZTS with activation energies of 30 ± 5 meV and 10 ± 5 meV, which are in close agreement with our results.38 The origin of the shallowest level detected in our film (5.4 meV) and of the reported one (10 ± 5 meV) have not been identified.

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CONCLUSIONS In conclusion, we have successfully deposited CZTS films with a zinc-rich/Sn-poor composition on flexible substrates via a simple successive dip-coating method from a single and nontoxic precursor solution. Post-deposition annealing enhanced the optical transmittance and photoconductivity of the films. Films with good structural and morphological properties and with band gap energies of 1.5 eV1.6 eV were grown on various substrates. Photocurrent spectroscopy and admittance spectroscopy techniques were used to measure the activation energies of native defects VCu, CuZn and VSn. Heterojunction and SB diodes were fabricated on the films, and the electronic properties of the films were evaluated. These devices, which showed excellent characteristics, demonstrated that the films possess the essential properties for further development as an absorber layer. ACKNOWLEDGEMENTS Support from the Research Sector of Kuwait University under Research Project RS01/13 and the general facility Projects GS03/01, GE01/07 and GE01/08 is gratefully acknowledged. REFERENCES 1. H. Wang, Int. J. Photoenergy 2011, 801292 (2011). 2. M.P. Suryawanshi, G.L. Agawane, S.W. Shin, P.S. Patil, J.H. Kim, and A.V. Moholkar, Mater. Technol. 28, 98 (2013). 3. I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann, W.C. Hsu, A. Goodrich, and R. Noufi, Sol. Energy Mater. Sol. Cells 101, 154 (2012). 4. D.H. Kuo, J.T. Hsu, and A.D. Saragih, Mater. Sci. Eng., B 186, 94 (2014). 5. Q. Guo, G.M. Ford, W.C. Yang, C.J. Hages, H.W. Hillhouse, and R. Agrawal, Sol. Energy Mater. Sol. Cells 105, 132 (2012). 6. G. Larramona, S. Bourdais, A. Jacob, C. Chone, T. Muto, Y. Cuccaro, B. Delatouche, C. Moisan, D. Pere, and G. Dennler, J. Phys. Chem. Lett. 5, 3763 (2014). 7. Q. Tian, G. Wang, W. Zhao, Y. Chen, Y. Yang, L. Huang, and D. Pan, Chem. Mater. 26, 3098 (2014). 8. H. Zhou, T.B. Song, W.C. Hsu, S. Luo, S. Ye, H.S. Duan, C.J. Hsu, W. Yang, and Y. Yang, J. Am. Chem. Soc. 135, 15998 (2013). 9. D. Colombara, A. Crossay, L. Vauche, S. Jaime, M. Arasimowicz, P.P. Grand, and P.J. Dale, Phys. Stat. Solidi A 212, 88 (2015). 10. M. Werner, C.M. Sutter-Fella, Y.E. Romanyuk, and A.N. Tiwari, Thin Solid Films 582, 308 (2015). 11. S. Kahraman, S. C ¸ etinkaya, H.A. C ¸ etinkara, and H.S. Gu¨der, Thin Solid Films 550, 36 (2014). 12. A. Wangperawong, J.S. King, S.M. Herron, B.P. Tran, K. Pangan-Okimoto, and S.F. Bent, Thin Solid Films 519, 2488 (2011). 13. N.M. Shinde, C.D. Lokhande, J.H. Kim, and J.H. Moon, J. Photochem. Photobiol. A 235, 14 (2012). 14. N.M. Shinde, D.P. Dubal, D.S. Dhawale, C.D. Lokhande, J.H. Kim, and J.H. Moon, Mater. Res. Bull. 47, 302 (2012). 15. T.K. Chaudhuri and D. Tiwari, Sol. Energy Mater. Sol. Cells 10, 46 (2012). 16. W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, and D.B. Mitzi, Adv. Energy Mater. 4, 1301465 (2013). 17. J. Tauck, Mater. Res. Bull. 5, 721 (1970). 18. A.E. Rakhshani, J. Appl. Phys. 81, 7988 (1997).

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