Indian Journal of Pure & Applied Physics Vol. 40, May 2002, pp. 33 1 -336

Photoelectrochemical (PEe) studies on chemically deposited BhSe3 thin films

B R Sankapa\, H M Pathan & C D Lokhande*

Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 4 1 6 004

Received 1 5 May 200 1 ; revised 22 January 2002; accepted 1 3 February 2002

Semiconducting Bi2Se, thin films have been prepared from aqueous alkaline bath onto fluorine-doped tin oxide (FrO) glass substrates (sheet resistance - 7 a-cm2) by chemical bath deposition (CBD) at room temperature (27 °C). The films formed are of nanocrystalline, smooth, homogeneous and well covered to the substrate surface as evidenced from X -ray diffraction (XRD) and scanning electron microscopy (SEM) studies, respectively. These films are annealed at 200 °C for 4 hr in air. The annealed Bi2Se, films are used for photoelectrochemical (PEC) characterization using polysulphide as an electrolyte and graphite as a counter electrode. The characteristics such as current-voltage (I-V), photovoltaic output and capacitance etc. have been studied and results are reported.

1 Introduction

One of the most efficient solar photovoltaic devices is the thin film photoelectrochemical (PEC) solar cell with conversion efficiencies TJ as high as 1 9 % (Ref. 1 ) . Bismuth triselenide (Bi2Se,), a binary semiconducting material belonging to group (V-VI) compound family, has a great technological importance owing to potential applications in photoelectrochemical devices, solar selective coatings and optoelectronic devicesz. Bi and Se glasses are used in optical and photosensitive devices. Some reports are available for the deposition of B i2Se, thin films by chemical bath deposition (CBO) method. Pramanik et at. ' have deposited Bi2SeJ thin films by CBO using bismuth nitrate as a Bj3+ ion source and sodium selenosulphite as a Se2- ion source with triethanolamine as a complexing agent. The films formed were having random distributions of small crystals. Garcia et al. 4 deposited Bi2Se, thin fi lms using N, N dimethylselenourea instead of sodium selenosulphite. They have reported annealing effect (200 DC for 1 hr in N2 atmosphere) on crystallinity and bandgap of the material. No attempt has been made for the photoelectrochemical characterisation of B i2Se, thin films formed by CBO method.

In the present investigation, the CBO method is employed to deposit thin films from aqueous alkaline medium at room temperature (27 DC). Films formed are nanocrystall ine. These fi lms can be used

as photoelectrodes in PEC cel l , since basic requirements of a good thin film electrode for PEC cells are low resistivity and large grain size. The grain size leads to reduction of grain boundary area of thin film that has important consequence for efficient energy conversion. The nanocrystall ine Bi2Se, films so obtained were annealed at 200 DC for 4 hr in air for the improvement of grain size . These films were used in photoelectrochemical (PEC) cells as a photoanode and the effect of annealing on the properties of B i2Se/polysulphide cell was studied and results are reported.

2 Experimental Details

Optimization of preparative parameters - The preparative parameters in CBO method are concentration of reactant, bath temperature and deposition time. In the presence of B j3+ ions in the bath, B izSeJ will form if the ionic product (IP) of Bj3+ and Se2. exceeds the solubility (SP) product of BizSe,. For the optimization of the concentration of Bi'+ ions, 1 0 ml volume of 0. 1 M sodium selenosulphite solution was taken in each beaker containing glass substrates and 1 0 ml of B i(NO,), of various concentrations were added in each beaker. The reactant beakers were kept at room temperature (27 DC) for 2 hr. After this the films were taken out, washed with double distilled water and then dried. Gravimetric weight difference method was employed to measure film thickness by assuming the bulk density of Bi2Se, as 7.53 gm/cc. It is

332 INDIAN J PURE & APPL PHYS, VOL 40, MAY 2002

observed that film thickness increases l inearly up to 0.05 M concentration of B i(N03)3 and attains a maximum value. After this concentration, film thickness decreases, as the rate of film formation becomes smaller than the rate of fi lm dissolution . In the similar way, optimization was done for volume and deposition time. Thus, optimized deposition conditions for B i2Se3 thin films were (0.05M) ( 1 0 ml) Bi(NO.1)3, (O. I M) (6 ml) Na2SeS03, 2 hr deposition time and 27 °C deposition temperature.

..... j < ......

1/1 c: t.I .... c: ....

20

a} A� depo !;ited b) Ann ealed

40 60 2 B ( d eg.)

Fig. I - XRD patterns of Bi2Se3 thin films for (a) as-deposited and (b) annealed at 200 DC for 4 hr in air

Reaction mechanism - Bi2Se, films can be prepared by decomposition of sodium selenosulphite in an alkaline medium containing Bi salt and a suitable complexing agent, which allow for obtaining a soluble species, Bj3+ in this medium. The decomposition of Bi3+ occurs when ionic product of B i3+ and Se2- exceeds the solubility product of Bi2Se3' The reaction takes place in alkaline medium where Bi salt, triethanolamine and sodium selenosulphite are present.

Hydrolysis of sodium selenosulphite (Na2SeS03) in alkaline medium gives Se2- ions as :

Na2SeS03 + OR � Na2S04 + HSe­

HSe- + OR � H20 +Se2-

. . . ( 1 )

. . . (2)

and the reaction of TEA bismuth (III) complex ions which releases bismuth ions in:

. . . (3)

Ions can condense as ion-by-ion to produce thin films of Bi2Se, on the substrates following the reaction :

2[Bi(TEA)'+] + 3 [Na2SeSO,] +60R � 3Na2S04 + B i2Se, + 3H20 + TEA . . . (4)

Preparation of photoanode - B i2Se, films were prepared by the CBD method using the optimized preparative conditions onto fluorine doped tin oxide (FrO) coated glass substrates (sheet resistance 7 Q­cm2) at room temperature (27 0q. The deposition was carried out for 2 hr after which the substrates were taken out, washed with double distilled water and dried in air. These films were annealed at 200°C for 4 hr in air and photoelectrochemical characterisation was carried out for as deposited and annealed B i2Se, thin films. For this, 0. 1 M polysulphide (NaOH-Na2S-S) with pH- I 0 was used as an electrolyte to check the photoactivity with graphite as a counter electrode. The type of conductivity exhibited by these fi lms was determined by noting the polarity of emf developed by PEC cell under i l lumination. The capacitance­voltage (C-V) characteristics were obtained with respect to saturated calomel electrode (SCE).

Table I - Comparison between observed and standard d-values for Bi2Se, thin films

Nature of Plane Standard Observed Illmax sample (11k!) d-values d-values %

As-deposited 1 00 3 .03 3 .08 1 00

Annealed at 020 5.83 5 .82 1 3 . 62 200 DC for 4 hr 1 30 3 . 65 3 .64 4 1 .33

02 1 3 . 3 3 3 .39 1 00 301 2.77 2.72 88.85 24 1 2. 1 7 1 . 1 8 34.03 040 1 .92 1 .93 58.08 208 1 .60 1 .60 44.44 02l.Q 1 .5 1 1 . 55 49.33 2011 1 .47 1 .45 49. 3 3 0024 1 . 1 9 1 .20 6 1 .43

..

..

. .{

SANKAPAL et al. :Bi2Se} THIN FILMS 333

Fig. 2 - Scanning electron micrograph of Bi2Se, thin films on glass substrates at magnification 1 0,000 X

( I)) 7.B (al

N 'l.4 IE 400 . u

J. 2.0 � ...

N 0\ IE 0 u ..J I c( 0 :J... ---C GJ ... ... :) lJ o A$ deposittd

- 400 • Ann ea led

-0.4 0 0.4 Vo l t a ge (V )

Fig. 3 - ( a) Current-voltage characteristics in dark for n-Bi2Se, /polysulphic!J! PEC cells for as deposited and annealed fi l ms at 200 °C for 4 hr in air, (b) Log I versus voltage plots for n-Bi2Se, /polysulphide PEC cell s in dark for as deposited and annealed films at 200 °C for 4 hr in air (inset)

3 Results and Discussion

Structural and morphological studies - The as­grown and annealed films were characterized by XRD technique with CuKa radiation . The XRD patterns obtained for the films grown on bare microslide glass plates were studied in the 28 range of 1 0- 1 000• The XRD pattern of as grown film indicates fine grains or amorphous nature of Bi2Se, thin films [Fig. I (a)] . The annealed film shows nanocrystalline grain size [Fig. I (b) ] . Table I compares the d-values for the films calculated from XRD studies, with standard d-values5.

....... N 'E u <

800 (b)

400

� o ---C GJ ... ... :) u

-400

-800

0.4 0.8 V(Volt )

o As deposited • Annealed

-0.4 0 0.4 Vol toge ( V )

Fig. 4 - (a) Current-voltage characteristics under i l lumination for n-Bi2Se/polysulphide PEC cel ls for as-deposited and annealed fi l ms at 200 °C for 4 hr in air, (b) Log I versus voltage plots for n-Bi2Sc,/polysulphide PEC cel l s under i l lumination for as-deposited and annealed fi l ms at 200 °C for 4 hr in air (inset)

Fig. 2 shows scanning electron micrograph (SEM) of as grown B i2SeJ films onto glass substrates at magnification 1 0,000 X. The film is non-porous and well uniform and well adherent to the substrate.

334 INDIAN J PURE & APPL PHYS, VOL 40, MAY 2002

Conductivity type - The PEC cell with configuration Bi2Se/O. l M (NaOH-Na2S-S)/C was formed. It was observed that even in dark the PEC cell gave some voltage Vd with B i2SeJ thin film electrode as negative and graphite electrode as positive. The origin of this voltage is attributed to the difference between two half-cell potentials in the PEC cell . After illumination of the junction, the magnitude of this voltage increased with negative polarity towards the Bi2SeJ film. It indicates that the Bi2SeJ thin fi lm is of n-type. This result agrees well with earlier report on the conductivity of Bi2SeJ thin film 4.

Current-voltage (/- V) characteristics - The crystallographic structure of B i2SeJ films estimated by X-ray diffraction and surface morphology through SEM showed that annealed films are nanocrystal line, smooth, homogeneous and well covered to the surface. The use of nanocrystalline in place of single crystal is desired from the practical stand-point realizing a large area photoelectrode in any photoelectrochemical cel l . One of the major problems in uti lizing nanocrystalline thin film semiconductors in PEC cell is the absence of space charge layer at the electrode interface. Under these circumstances, photogenerated charge carrier can move in both the directions. The current-voltage (I­V) characteristics in the dark (Fig. 3) and under il lumination (Fig. 4) were obtained for both as deposited and annealed B i2SeJ films. The nature of /- V curve is analogous to the nature reported by Killedar et at.1> for Sb2SJ thin film. In dark, [Fig, 3 (a)] , the forward bias current increase rapidly for annealed films and it may be due to enhancement in grain size of the photoelectrode besides removal of defects levels and increase in packing density. However, the behaviour of forward bias current is different under il lumination and current onset takes place [Fig. 4(a)] at small voltage than in dark. This

may be attributed to the photoconductivity exhibited by B i2SeJ thin film7•

Photovoltaic output characteristics - The photovoltaic output characteristics under il lumination of 82 mW/cm2 were studied for as deposited and annealed B i2SeJ film photoanode as shown in Fig. 5. The photovoltaic conversion efficiency (ll %) and the fil l-factor (ff %) obtained are listed in the Table 2. It is seen that conversion efficiency and fil l factor are improved for annealed Bi2Se3 photoanode due to annealing. The dependence of conversion efficiency and fi ll factor on grain size of crystallites is significant, reflecting the fact that changing the grain size does alter the junction injection levelsR• The series (R,) and shunt resistance (R,h) were evaluated from the slope of power output characteristics using the relation :

(dlldV) I=l1 = l lR,

and

. . . (5)

. . . (6)

The calculated values of R, and R,,, are listed in Table 2. For an ideal solar cell , the value of series resistance should be zero and shunt resistance should be infinite. In the present investigation, low conversion efficiencies are attributed to high value of series resistance and low value of shunt resistance of Bi2Se3 films9.

Transient photoresponse - The photovoltage rise and decay curve reveals that the increase in photovoltage is almost instantaneous . Fig. 6 represents rise and decay curves for as-deposited and annealed B i2SeJ photoanode. Magnitude of open circuit voltage ( V"J increases for annealed Bi2SeJ fi lm photoelectrode. This may be due to decrease in defect states with increase in grain size as a consequence of annealing. Fig. 7 shows graph of log

Table 2 - Performance of photo-electrochemical cells based on Bi2SeJ film electrodes

Nature of Junction ideality factor Series Shunt Fi l l Conversion Decay constant Flat Bi2Se3 tl lm resistance resistance factor eftlciency band

potential In dark In l ight (R,) (R,h) (ft)% (11)% (b) ( VIl,) IIJ IIi versus

( SeE) As-deposited 6.70 1 2.33 2 1 4 860 40.7 0.0 1 75 0.32 -0.23 Annealed at 6.20 1 5 .25 1 20 7 1 8 4 1 . 3 0.046 0.40 -0.09 200°C for 4 hr

SANKAPAL et al. :BizSe3 THIN FILMS 335

(V"J against log t by using the relation:

V"lt) = V"cCO) rh . . . (7)

where VllC(O) and V,>c(t) are open circuit voltage at t=0 and t seconds, respectively and b the decay constant. The linearity of plots in Fig. 7 suggests that the kinetics involved in voltage decay process is of second order. The slow decay in V"c ca� be ascribed to the presence of the surface states. The decay constant b is found to be increased from 0.32 to 0.43 for annealed films.

� 200 u .{ = 1t00 ... c OJ ... ... ::J U

Voltage (V") 0.04 0.08 0.12 0.16

o As depoSIted • Annealed

Fig. 5 - Photo voltaic characteristics for n-Bi2Se/polysulphide PEe cells for as-deposited and annealed films at 200 °C for 4 hr i n air

> E u

1 ao

� \60

ON 20

o As,d tPosi t ed • Annealed

40 T i m e ( s )

Fig. 6 - Photoinduced open circuit voltage (Voc) , rise and decay curves for n-Bi2SeJ/polysulphide PEe cells for as­deposited and annealed films at 200 °e for 4 hr in air

2.3 -

� .... u 2.2

� O't o ..J 2.1

1.5

o As deposited • Annealed

1 .6 1.7 1.8 Log t ( s )

Fig. 7 - Variation o f log Voc against log t (derived from Fig. 6 for decay part) for as-deposited and annealed films at 200 °C for 4 hr i n air

Fig. 8 - Matt-Schottky plots for PEe cells formed with B i2SeJ thin fi lms for as-deposited and annealed at 200 °e for 4 hr in air

Capacitance-voltage (C-V) characteristics -The depletion layer " capacitance of the cell was measured as a function of applied voltage (versus SeE) and Mott-Schottky plots were constructed. Fig. 8 shows such plots that are linear. The flat band potentials ( V1h'S) were determined by extrapolating the linear region of these plots on the voltage axis (listed in Table 2). The change in VIh of the B i2SeJ film electrode after annealing may be due to the increase in electron density. The positive slope of plot confirmed the n-type electrical conductivity of

336 INDIAN J PURE & APPL PHYS, VOL 40, MAY 2002

Bi1Se, thin fi lm, which is in good agreement with earl ier report4.

4 Conclusion

The PEC cells of as-deposited and annealed B i2Se3 thin films were formed and their properties were studied. It is found that the photoactivity was improved after annealing mainly due to the enhancement in grain size. The low value of conversion efficiency is mainly attributed to the high series and low value shunt resistance of B i2Se3 fi lm. The Bi2Se3 film exhibit IHype electrical conductivity.

Acknowledgment

The authors are thankful to UGC, New Delhi, for the financial support under the DRS: SAP programme ( 1 999-2003) and JNCASR, Bangalore, for providing SEM facil ity.

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