Indian Journal of Engineering & Materials Sciences Vol. 13, April; 2006, pp. 140-144 Photoelectrochemical characterization of Bi 2 S 3 thin films deposited by modified chemical bath deposition R R Ahire & R P Sharma* Thin Film and Semiconductor Laboratory, Department of Physics, G.T.Patil College, Nandurbar 425 412, India Received 5 April 2005; accepted 28 December 2005 For the photoelectrochemical (PEC) solar cell, the prime requirement is that photoelectrode/ photoanode should have bandgap close to the maximum in the visible spectrum. Bismuth sulphide (Bi 2 S 3 ) is challenging material because of its midway bandgap (E g = 1.74 ev) and absorption coefficient of the order of 10 4 cm -1. In the present investigation, bismuth sulphide (Bi 2 S 3 ) thin films of thickness about 0.14 μm have been prepared by using modified chemical bath deposition method onto glass and fluorine doped tin oxide (FTO) coated glass substrate under optimized conditions. The films are annealed at 200ºC for 2 h in air. It is found that deposited films turn from amorphous to polycrystalline after annealing. The Bi 2 S 3 /NaOH-S-Na 2 S/C cell has been fabricated by using Bi 2 S 3 annealed films and their photoelectrochemical performance has been studied. It is found that Bi 2 S 3 films are photoactive. However, conversion efficiency is low (0.056%) due to low series and high shunts resistance of Bi 2 S 3 films. IPC Code: H01C17/14 For the last decades, photoelectrochemical (PEC) processes at semiconductor, electrolyte interface have found new interest because of their solar and nonsolar applications. Bi 2 S 3 thin films have received great attention since its bandgap lies close to the range of theoretically maximum attainable energy conversion efficiency 1. It can also be used in heterojunction, IR detectors, LUX meters and switching devices. Photoelectrochemical (PEC) devices based on Bi 2 S 3 thin films have much interest and activity in solar energy research. Further, such PEC cells have been applied in rechargeable electrochemical storage septum cell and redox couple storage devices. It is known that Bi 2 S 3 films can be prepared by various methods such as chemical bath in aqueous and non-aqueous solutions 2-4, spray 5, solution gas interface 6 and electrodeposition 7,8. In these, chemical bath deposition is a simple, inexpensive and convenient method for large area deposition. Both metal ions and chalcogen ions are mixed together in bath where metallic ions and chalcogen ions are released. These ions combine together to form metal chalcogenide films on the substrate a precipitate in the bulk solution. In the present work, we report on the preparation of Bi 2 S 3 films by modified chemical bath deposition * For correspondence (E-mail: ramphalsharma@yahoo.com) Present address: Department of Physics, Dr Babasaheb Ambedkar Marathwada University, Aurangabad 431 004, India method on to fluorine doped tin oxide [FTO] coated glass substrates. Using Bi 2 S 3 films as a photoelectrode and carbon as a counter electrode formed the PEC cell and its photoelectrochemical characterization was carried out. Experimental Procedure Film deposition The Bi 2 S 3 thin films were deposited onto FTO coated glass substrates of 0.075 0.0125 m 2. For the deposition of thin films aqueous 0.003 M bismuth nitrate solutions were used as cationic precursors. The ph of this solution was 9. The anionic precursor was 0.1 M of thioacetamide [CH 2 CS - NH 2 ] with ph 11. The ph of the anionic precursor was raised with the addition of hydrazine hydrate. For the deposition of Bi 2 S 3 thin films a wellcleaned glass substrate was immersed in cationic precursor solution of bismuth nitrate [Bi(NO 2 ) 3 ] for 20 s in which Bi 3+ ions are adsorbed on the surface of the substrate. The substrate was rinsed with ion exchange water for 40 s to remove unadsorbed ions. Then substrate was immersed in an anionic precursor of thioacetamide solution for 20 s in which S 2- ions are reacted with adsorbed Bi 3+ ions on glass substrate. This was followed by rinsing again in ion exchange water for 40 s to remove unreacted S 2- ions. This completes one deposition cycle for the deposition of Bi 2 S 3 thin films. By repeating such deposition cycle
AHIRE & SHARMA: PHOTOELECTROCHEMICAL CHARACTERIZATION OF Bi 2 S 3 THIN FILMS 141 for 20 times continuous Bi 2 S 3 film on glass substrate was obtained. The deposition was carried out at room temperature (27ºC). Photoelectrochemical (PEC) characterization of Bi 2 S 3 thin film The photoactivity of film was tested by forming a PEC cell using Bi 2 S 3 thin film onto FTO coated glass as working electrode and graphite as counter electrode. Polysulphide (Na 2 S-S-NaOH) was used as an electrolyte. A 200 W tungsten filament lamp was used as the light source. The current-voltage (I-V) characterization in dark and under illumination was obtained at 82 mw/cm 2 illumination intensity. The spectral response of cell was recorded with monochromator using wavelength between 350 to 1050 nm. Transient photoresponse and capacitancevoltage (C-V) characteristics of cell were used to calculate open circuit voltage, decay constant and flat band potential, respectively. Results and Discussion Bi 2 S 3 film thickness The thickness of photelectrode has a strong impact on the performance of photoeletrochemical cell 9. Thickness of Bi 2 S 3 films was measured with the help of weight difference method employing sensitive microbalance. Thickness of the film was 0.14 μm and film was adherent well to the FTO coated glass substrate. electrical conductivity of Bi 2 S 3 thin films prepared by using above deposition bath was also confirmed from thermo-emf measurement of the films 11. Current-voltage (I-V) characteristics in dark and under light illumination The current-voltage (I-V) characteristics of PEC cells in dark and under illumination were examined with photoelectrode of Bi 2 S 3. The nature of the I-V curves indicated the formation of rectifying junction (Fig. 1). The PEC cell was illuminated and the curves were shifted in the forth quadrant indicating that the cell is generator of electricity. Increase in the current under illumination indicates that the Bi 2 S 3 films are photoactive in nature 12. Information of junction ideality factor can be obtained using diode equation as ev I= I nkt 0 e 1 (2) where n is junction ideality factor, I o is the reverse saturation current, V is the forward bias voltage and I is the forward bias current. The plot of log I versus V is shown in Fig. 2 (derived from Fig. 1) using forward bias data only. The values of junction ideality factor n d for dark and n i for under illumination were determined from the graph of log I versus V and are 12.4 and 9.5, respectively. Type of conductivity The PEC with the configuration Bi 2 S 3 /(NaOH-S- Na 2 S)/C was formed. It was seen that PEC cell gives some dark voltage V d and dark current I d with Bi 2 S 3 electrode as the negative and graphite electrode as the positive polarity ends. The origin of this voltage is attributed to the difference between two half cell potentials in the PEC cell, which can be written as E = E graphite - E Bi2S (1) 3 where E graphite and EBi 2S are the half cell potentials 3 when dipped in the polysulphide electrolyte. After illumination of the cell, the magnitude of this voltage increases with negative polarity towards the Bi 2 S 3 photoelectrode. Therefore, in the present case, cathodic behaviour of photovoltage of semiconductor was observed which indicates that the Bi 2 S 3 thin films have n-type electrical conductivity 10. The n-type Fig. 1 Current-voltage characteristics for Bi 2 S 3 polysulphide PEC cell in dark and under illumination.
142 INDIAN J. ENG. MATER. SCI., APRIL 2006 Photovoltaic output characteristics Photovoltaic output characteristics of Bi 2 S 3 PEC cell under illumination intensity 82 mw/cm 2 is shown in Fig. 3. The calculations lead to the fill factor (ff) 39% and power conversion efficiency (η) 0.056. The low conversion efficiency may be due to the low series and high shunt resistance. The series resistance R S and shunt resistance R Sh were calculated from the slopes of power output characteristic using relations. di 1 = dv I = 0 R S di 1 = dv V = 0 R Sh (3) (4) The values of R S and R Sh are found to be 53 Ω and 625 Ω, respectively. Spectral response Fig. 4 shows the plot of short circuit current (I sc ) against wavelength λ. The decrease of I sc on shorter wavelength side of the peak may be due to absorption of light in electrolyte and high surface recombination of photogenerated carrier by surface states 13. The decrease in I sc on longer wavelength side may be attributed to the transition between defect levels. Using peak value band gap energy is 1.78 ev. This agrees well with the results obtained for optical absorption of Bi 2 S 3 films 14. Fig. 2 Log I versus V plot for Bi 2 S 3 polysulphide PEC cell in dark and under illumination. Transient photoresponse Photovoltaic rise and decay of PEC cell was studied. Fig. 5 shows rise and decay characteristics of Bi 2 S 3 /polysulphide PEC cells. Theoretically, open circuit voltage (V oc ) decay curve has three distinct regions 15. These three regions correspond to high, intermediate and low level injection of minority Fig. 3 Photovoltaic output characteristics for Bi 2 S 3 polysulphide PEC cell. Fig. 4 Spectral response for Bi 2 S 3 polysulphide PEC cells.
AHIRE & SHARMA: PHOTOELECTROCHEMICAL CHARACTERIZATION OF Bi 2 S 3 THIN FILMS 143 Fig. 7 Mott-Schottky plot for Bi 2 S 3 polysulphide PEC cell. Fig. 5 Rise and decay curve for Bi 2 S 3 polysulphide PEC cell. to the presence of surface states. The decay constant, b is calculated to be equal to 1.33. Capacitance voltage (C-V) characteristics in dark Fig. 7 shows Mott-Schottky plot for the PEC cell formed with Bi 2 S 3 thin film the value of flat band potential V fb is obtained using Mott-Schottky equation. 2 2 C = ( V Vfb kt/ e) qεεοn d (6) Fig. 6 Log V oc against log t (derived from Fig. 5) carriers. The open circuit voltage V oc of the PEC cell was found to persist for some time when the light is cut off. Fig. 6 shows graph of log V oc versus log t by using relation. V oc (t) = V oc (0) e -b (5) where V oc (0) and V oc (t) are the open circuit voltages at t = 0 and t s respectively and b is decay constant the linearity of log V oc versus log t plot suggest the kinetic involved in the voltage decay process is of second order. The slow decay in V oc can be ascribed where the symbols have their usual meaning. The nature of the plot is linear. The flat band potential V fb was determined by extrapolating the linear region of this plot on the voltage axis is 0.55V. The positive slope of the plot confirmed the n-type electrical conductivity of Bi 2 S 3 which is in good agreement with the earlier values reported in literature 16. Conclusions A simple and less investigated modified chemical bath deposition method was used for depositing good quality thin films of Bi 2 S 3 on FTO coated glass. The I-V characteristics of the PEC cell formed by using Bi 2 S 3 thin film with polysulphide electrolyte (Na 2 S-S- NaOH) shows rectifying nature of the junction. Fill factor and conversion efficiency of the cell are found to be small due to low series and high shunt resistance values. The decay constant of Bi 2 S 3 films, which is found to be 1.33, indicated the second order kinetics in decay process. Film exhibits n-type electrical conductivity.
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