Determination of CdIn2S4 semiconductor parameters by (photo)electrochemical technique

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Physica B 393 (2007) 249–254 www.elsevier.com/locate/physb

Determination of CdIn2S4 semiconductor parameters by (photo)electrochemical technique R.R. Sawant, K.Y. Rajpure, C.H. Bhosale Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Received 14 June 2006; received in revised form 8 December 2006; accepted 10 January 2007

Abstract Semiconducting n-CdIn2S4 thin films have been deposited on amorphous and fluorine-doped tin oxide (FTO) coated glass substrates by using a well-known spray pyrolysis technique. With the objective of finding the optimum conditions for the deposition of CdIn2S4 thin films, the influence of substrate temperature on properties of the films have been studied. Photoelectrochemical (PEC) technique has been employed to optimize substrate temperature. The films are characterized by the techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by X-rays (EDAX) and PEC studies. The SEM studies reveal the compact morphology with large number of grains. EDAX studies show that the material formed at optimized substrate temperature is nearly stoichiometric. Measured values of efficiency (Z) and fill factor (FF) for the PEC cell are 1.06% and 0.47, respectively. Various physical parameters of cadmium indium sulphide (CdIn2S4) film are estimated. Energy band diagrams for CdIn2S4 and polysulphide electrolyte, before and after making junction have been constructed. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.82.Fk; 81.15.Rs; 82.47.Jk Keywords: Spray pyrolysis; CdIn2S4 thin films; XRD; PEC; SEM; EDAX; Mott–Schottky plot

1. Introduction During last few years there is a considerable interest in the preparation and characterization of chalcogenide thin films because of their potential applications in various fields of science and technology [1–7]. The ternary chalcogenides have potential applications in solar energy conversion [8–11], in optoelectronic devices [12], and in nonlinear optics [13]. Cadmium indium sulphide (CdIn2S4) has received very little attention as a prospective material for photoelectrochemical solar cells [14]. The attempts have already been made to prepare photoactive CdIn2S4 thin films by simple and low-cost chemical spray pyrolysis technique [5]. Gartner’s model was applied to determine the quantum efficiency and hence the minority carrier diffusion length, carrier concentration and flat band potential. Corresponding author. Tel.: +91 231 2609227; fax: +91 231 2691533.

E-mail address: [email protected] (K.Y. Rajpure). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.01.009

The present investigation deals with the systematic study of preparation of CdIn2S4 thin films by spray pyrolysis technique in order to get good quality thin films in terms of photoactivity by optimizing substrate temperature by photoelectrochemical (PEC) technique. Optimization of preparative parameters of photoactive semiconducting electrode by PEC method is new, reliable and unique technique in thin film technology [15]. An appropriate redox electrolyte is chosen and short-circuit current (Isc) and open-circuit voltage (Voc) of the PEC cell formed with photoelectrode is measured with respect to one of the deposition parameters. Plots of Isc and Voc against the desired deposition parameter pass through the relatively maximum value giving the optimized value. Determination of physical properties of semiconducting thin film electrode is essential in testing their suitability for further applications. The CdIn2S4 thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by X-rays (EDAX) and PEC techniques. Observed and calculated values of contact

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potentials of CdIn2S4 and polysulphide electrolyte have been compared. 2. Experimental The depositions were carried out onto commercially available glass substrates. The A.R. grade chemicals used were cadmium chloride (CdCl2), indium trichloride (InCl3) and thiourea CS(NH2)2. The films were prepared by taking equimolar (0.05 M) solutions of CdCl2, InCl3 and CS(NH2)2. In order to find the optimized condition for deposition of CdIn2S4 thin films, the depositions were carried out by varying only substrate temperature and keeping other parameters at their fixed values. The structural characterization of the films was carried out by analysing the XRD patterns obtained using a Phillips PW 1710 X-ray diffractometer with Cu-Ka radiation. The surface morphology of the spray deposited CdIn2S4 film on glass substrate was carried out by SEM model Cambridge Stereoscan 250–MK3 assembly and model Excel–30 in series. The compositional analysis of the film was studied by the EDAX attachment to abovementioned SEM model. The PEC cell was fabricated by using CdIn2S4 thin films deposited onto the fluorine-doped tin oxide (FTO) coated glass substrates as active photoelectrode, polysulphide (1 M NaOH+1 M Na2S+1 M S) solution as an electrolyte and graphite as counterelectrode. The distance between photoelectrode and counterelectrode was kept at 0.5 cm. The cell was illuminated by 500 W tungsten filament lamp. The actual input power incident on the cell was 15 mW/cm2. The water lens was interposed between the lamp and the cell to avoid direct heating of the cell. The optimized substrate temperature was obtained by using PEC technique. The Mott–Schottky plot was studied using a LCR bridge (Aplab model 4912) at 100 Hz. 3. Results and discussion 3.1. Optimization of substrate temperature by PEC technique PEC cells formed with CdIn2S4 thin films prepared at various substrate temperatures, having configuration CdIn2S4/(1 M NaOH+1 M Na2S+ 1 M S)/C are studied. It is seen that even in dark, PEC cell gives some dark voltage Vd, that may be attributed to the difference between two half-cell potentials of CdIn2S4 thin film and a graphite electrode. After illumination of the junction, the magnitude of the open-circuit voltage increases with the negative polarity towards the CdIn2S4 electrode; indicating CdIn2S4 thin films are of n-type [16]. It is seen that the short-circuit current (Isc) and open-circuit voltage (Voc) are function of substrate temperature (as shown in Fig. 1) and increase with increase in substrate temperature, attains the maximum value at 350 1C and then decrease for further increase in substrate temperature, which may be attributed to variation in the stoichiometry with respect to substrate

temperature. EDAX results confirm the formation of nearly stoichiometric compound at 350 1C. The substrate temperature plays an important role in deciding the values of Isc and Voc. Amongs the substrate temperatures, 350 1C substrate temperature is observed to be optimized from the PEC measurements, which has been confirmed by XRD studies.

3.2. XRD studies The XRD patterns shown in Fig. 2 show that the material deposited at 325 and 350 1C is polycrystalline. A matching of the observed and the standard ‘d’ values confirms that the deposited films are of CdIn2S4 having spinel cubic structure [17]. The XRD pattern of a film prepared at 375 1C shows that the films are microcrystalline, as the intensity of X-ray reflections is very small. The resulted structure of the films at 375 1C is due to relatively lower film thickness. At optimized substrate temperature, the condensation of the vapour atom with the impinged surface increases, whereas agglomeration rate acquires its peak value results into critical thickness of the films. Increase in crystallinity with substrate temperature is due to optimum rate of supply of thermal energy for recrystallization [16].

3.3. SEM and EDAX The surface morphology of the spray deposited CdIn2S4 thin film on glass substrate at optimized substrate temperature is as shown in Fig. 3. The micrograph reveals that the substrate is well covered with large number of fine grains and film surface is uniform. The compositional analysis of CdIn2S4 thin film, deposited at optimized substrate temperature of 350 1C, is determined by EDAX technique and is tabulated in Table 1. The resulting ratio of atomic % of the Cd:In:S has been found to be 1:1.78:3.02 (approximately CdIn2S3). Though the air was used to atomize the spray, considering the possibility of oxidation of sulphur at deposition temperature (350 1C), any oxides of sulphur or other compounds was not detected. Table 1 shows that the film is sulphur deficient.

3.4. Photovoltaic output characteristics Typical photovoltaic output characteristics of nCdIn2S4/ 1 M NaOH+1 M Na2S+1 M S/C cell, under illumination is shown in Fig. 4. The efficiency Z (in %) was calculated from the relation Z¼

V oc  I sc  FF  100, Pinput

where Pinput is the input light energy.

(1)

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1000

251

400

Voc

300

Isc

200

Isc / μA

600

400

Voc / mV

800

100 200

0

0 300

315

330

345

360

375

Substrate Temperature / °C Fig. 1. Variation of photocurrent (Isc) and photovoltage (Voc) versus substrate temperature for CdIn2S4 thin films.

Calculated values of Z and FF are 1.06% and 0.47, respectively. The efficiency of above-mentioned PEC solar cell can be enhanced by getting more stoichiometric CdIn2S4 material.

300 375 °C 200 100

3.5. C–V characteristics (Mott–Schottky plots)

0

Fig. 5 shows the Mott–Schottky (M–S) plots of the nCdIn2S4/polysulphide electrolyte system in dark (pH ¼ 12.5, frequency ¼ 100 Hz). The value of the flatband potential Vfb, was obtained at 1/C2s ¼ 0 on the potential axis according to well-known Mott–Schottky relation [18,19] !     1 2 kT  , (3) ¼ V  V fb 20 2s qN D q C 2s

Intensity / A.U.

300 350 °C 200 100 0 300 325 °C

(511)

(400) (220)

100

(440)

(311)

200

0 20

40

60

80

100

2θ / deg. Fig. 2. XRD patterns for spray deposited CdIn2S4 thin films deposited at different substrate temperatures.

The fill factor (FF) was calculated from the relation FF ¼

Vm  Im , V oc  I sc

(2)

where the Im and Vm are values of maximum current and voltage that can be extracted from a PEC solar cell.

where Cs is the space–charge capacitance, Vfb, the flat band potential, Ao, the permittivity of free space, As, the static permittivity of the semiconductor; 6.6 for CdIn2S4 [20] and ND, the donor concentration. Estimated values of Vfb and ND have been found to be — 1.15 eV/ SCE and 4.26  1016 cm3, respectively. The Fermi level of semiconductor when the energy bands are flat, corresponds to the flat band potential Vfb. The density of states in conduction band of a semiconductor is given by the relation   2 NC ¼ (4) ð2pme kTÞ3=2 3 h has been found to be 1.76  1018 cm3, where me (E0.17mo) is the effective electron mass for CdIn2S4 [21]. Now the Fermi level of the semiconductor is related to the donor density (ND) and the density of states in the

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252

Fig. 3. Scanning electron micrograph of typical CdIn2S4 thin film deposited at optimized substrate temperature of 350 1C (magnification 20,000  ).

Table 1 Elemental analysis of CdIn2S4 thin film deposited at optimized substrate temperature 350 1C Wt%

At%

K-ratio

Z

A

F

SK Cd L In L Total

23.49 27.20 49.31 100.00

52.18 17.24 30.59 100.00

0.2654 0.2436 0.4431

1.2230 0.9171 0.9134

0.9109 0.9764 0.9838

1.0142 1.0000 1.0000

1/Csc2, 1012 (cm4.F-2)

Element

8

6

4

2 Vfb = -1.15 V/SCE

Voltage / mV 0

50

100

150

200

250

300

350

400

Current density / μA cm-2

0

-200

0 -1.2

-1.0

-0.8

-0.6 -0.4 -0.2 0.0 0.2 Electrode Potential (V) vs SCE

0.4

0.6

Fig. 5. Mott–Schottky plot for n-CdIn2S4/1 M NaOH+1 M Na2S+1 M S/C PEC cell (f ¼ 100 Hz).

-400

-600

-800

-1000

Fig. 4. Plot of photovoltaic output characteristics for n-CdIn2S4/1 M NaOH+1 M Na2S+1 M S/C PEC cell.

conduction band (NC) by the relation N D ¼ N C exp½ðE C  E f Þ=kT,   ND E C  E f ¼ kT ln . NC

ð5Þ

Therefore, EC is situated above Ef by an amount 0.10 eV, i.e. Ec ¼ 1.25 eV/SCE. The valence band edge must be below the conduction band edge by an amount to be equal

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a

-2

253

b

-2 0.45 eV Ec

Ec

-1

0.1 eV Ef

Ef

Ef, redox

Ef, redox

0

E/eV (SCE)

E/eV (NHE)

-1

0.55 eV

0

2.2 eV

1 1

Ev

Ev

2 2 Fig. 6. n-CdIn2S4/1 M NaOH+1 M Na2S+1 M S band diagram (a) at flat band condition and (b) band bending at interface.

Table 2 Results obtained from the Mott–Schottky plots of n-CdIn2S4/polysulphide/ C PEC cell

Based on these parameters, the width of depletion layer is calculated by the relation

Physical parameter

Value

W¼

Electrolyte used Ef,redox (V/ SCE) Vfb (V/ SCE) Donor concentration, ND (cm3) Density of states in conduction band, NC (cm3) ECEF (eV) Band bending, Vbb (eV) Barrier height, VB (eV) Depletion width, W (A1) Conduction band edge, EC (eV/ SCE) Valence band edge, EV (eV/ SCE) Carrier type

1 M polysulphide (pH ¼ 12.5) 0.70 1.15 4.26  1016 1.76  1018 0.10 0.45 0.55 87.85 1.25 0.95 n

(7)

Summary of results obtained from Mott–Schottky plots for n-CdIn2S4 film polysulphide interface is given in Table 2. Measured values of photopotential (Voc ¼ 0.365 V) have been found to be relatively less than the calculated value of built-in potential (band bending ¼ 0.45 eV/SCE) from Mott–Schottky plots.

4. Conclusions

to band gap energy of the semiconductor 2.2 eV [5]. Therefore, EV is at 0.95 eV/SCE. Now, the amount of equilibrium band bending (Vbb) at the semiconductor/electrolyte interface is obtained by the difference of the redox Fermi potential (Vf,redox) of the electrolyte, and the flat band potential (Vfb) of the semiconductor in that electrolyte, i.e. Vbb ¼ Vf,redoxVfb ¼ (0.7)(1.15) ¼ 0.45 eV/SCE. The barrier height VB is calculated using the relation V B ¼ qeV bb þ ðE C  E f Þ.

  220 2s V bb 1=2 . eN D

(6)

The band bending diagram is constructed using the calculated values of various parameters and is depicted in Fig. 6.

Nearly stoichiometric thin film formation of n-CdIn2S4 by a spray pyrolysis technique is feasible. The PEC technique can be used for optimization of the preparative parameters of spray deposited semiconducting thin films. Uniform fine-grained CdIn2S4 thin films with spinel cubic structure can be obtained at optimized substrate temperature of 350 1C. The efficiency (Z) and FF for the PEC cell are found to be 1.06% and 0.47, respectively. Energy band diagram of n-CdIn2S4/polysulphide heterojunction has been constructed.

Acknowledgment Authors are thankful to the University Grants Commission (UGC), New Delhi, for financial assistance through the UGC-DRS (SAP) — IInd phase project.

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