Iron Doped CdSe Films with Improved Photosensitivity and Stability for

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Fe Doped CdSe Films with Improved Photosensitivity and Stability for use in Liquid Junction Solar Cell Atanu Jana, Mukul Hazra, and Jayati Datta ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00853 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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ACS Applied Energy Materials

Fe Doped CdSe Films with Improved Photosensitivity and Stability for use in Liquid Junction Solar Cell Atanu Jana1,2, Mukul Hazra1,2, Jayati Datta1,2* 1. Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, India 2. Department of Chemistry, Renewable Energy Research Centre, Heritage Institute of Technology, Kolkata-700 107, India * Corresponding author: E-mail: [email protected]; [email protected] ABSTRACT In the present investigation, transition metal Fe has been introduced in low levels in the CdSe matrix to formulate CdFeSe films for application in photo-electrochemical solar cells. Periodic voltammetry was employed to deposit the ternary films on FTO glass. Spectral characteristics, morphology and composition of the matrices were determined by the respective physio-chemical methods. The band gap energies of CdFeSe thin films were increased with the increase of Fe content. Electrochemical impedance spectroscopy, chronoamperometry and photo-sensitivity of the film matrices were studied. In order to derive the functional parameters at the anodeelectrolyte interface, the cell configuration, FTO/CdFeSe/S2-–Sx2-/Pt, coupled with calomel reference electrode was used. Anodic striping voltammetry was employed to investigate on the inherent stability of the film matrices. The performance screening of the films ultimately indicates the best output in terms of photo-conversion efficiency (%η), fill factor (%FF) and durability, at an optimal Fe content in the matrix. KEYWORDS CdFeSe film, cyclic voltammetry, anodic stripping voltammetry, EIS, Photoelectrochemical performance

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1. Introduction Thin films semiconductors (SC) belonging to group II-IV, V, VI series have wide-range of photovoltaic applications as energy materials in different categories of optical and electronic devices.1–5 One of the direct band gap (Gr II-VI) photo-sensitive materials, CdSe (1.74 eV)6, 7 can be readily tailored to n-type conductivity in bulk as well as in thin films, and energetically tuned to the solar spectrum by modifications in terms of morphology, size of the crystallites and increased stability.8 Similarly, polycrystalline FeSe (II-VI) semiconductor with a direct band gap 1.23 eV is another attractive option for energy harvesting in solar cells.9 The FeSe nano particles (NPs) mainly exist in tetragonal and hexagonal structure and bear suitable electrical and optical properties desirable in photo-electrochemical energy conversion.10-12 In recent times, considerable attention has been devoted to formulate hybrid SC matrices by intermixing photo-sensitive materials or using donor impurities to achieve energy efficient functional properties in terms of widening intra gap impurity bands, creation of band tails and band gap renormalization.13 The presence of transition metal Fe as a dopant material is expected to produce considerable photosensitivity effect in a number of host lattices14, 15 and CdSe one of them. The CdFeSe composite band gap can be tuned by means of compositional variation, in the energy range 1.23eV (FeSe) to 1.72eV (CdSe), covering the essential part of the visible spectrum. The electro-deposition technique has gained popularity for the growth of elementary as well as multielemental thin films for energy of applications.16–24 The present study also involves eletroco-deposition of Fe, Cd chalcogenide nano-particles (NPs) using respective precursors in the electrolytic bath. S. Thanikaikarasan et. al.13 reported high temperature synthesis of CdSe:Fe films on ITO glass and for which 38% fill factor (FF) and 2.21% conversion efficiency were found using tungsten filament (150W) lamp as light source. However, XRD pattern did not show any peak for the mixed chalcogenides. S.K. Shinde et al.25 also developed CdFeSe films through galvanostatic method and reported maximum photo-conversion efficiency of 0.38% with 46% FF. There are few other reports available on optimizing dopant concentration to modify the surface 2 ACS Paragon Plus Environment

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and the physical properties of CdFeSe composite films.26-29 However in any of the above reports, co-deposition mechanism and its correlation with the matrix morphology were not discussed. Furthermore the stability study of the films towards photo-degradation has neither been attempted. In the present investigation, authors synthesize CdFeSe thin films at variable dopant concentration, by using cyclic voltammetry as an effective tool to achieve compact, uniform nanostructured SC films. The deposited films were extensively subjected to structural, morphological, compositional, optical and electrochemical performance evaluation for validation in photoelectrochemical (PEC) cell. Investigations were extended to derive stability of the matrix towards photo-dissolution. 2. Experimental Section 2.1. CdFeSe semiconductor films deposition CdFeSe thin films were deposited on FTO glass substrate (Dyesol, Australia) by periodic voltammetry from an electrolytic bath containing 0.2 M CdCl2, 1 x 10–3 M SeO2 along with FeSO4 in variable concentrations in the range 0.05 M–0.25 M, maintaining an acidic pH of 2. The cohesiveness of the film was ascertained by using TX-100 surfactant. The detailed film synthesis process has been discussed in our previous study.30 The voltammetric cycle depth was optimized at 250 scans during the film growth process, as described in our previous report.30 In the present investigation, the same depth of cycle was fixed during preparation of the composite (CdFeSe) films under controlled voltammetry.

2.2. Optical measurement and surface characterizations by SEM-EDX, XRD and XPS analysis JASCO V–530 UV–VIS Spectrophotometer, Japan was used to record the UV spectrum of the deposited films. The respective band gap energies were evaluated from the differential transmittance plot in the wavelength range 350 nm to 1000 nm. The matrix morphology was determined using scanning electron microscope (JEOL JSM-6700F FESEM) while the 3 ACS Paragon Plus Environment


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composition was obtained through energy dispersive analysis of X–ray (EDX) employing Hitachi S3400N, Japan, combined with INCA software. In order to obtain the crystallinity of the films, XRD patterns were recorded in Philips PW 1710, X–ray diffractometer. The detailed experimental setup can be obtained from our previous report.30 The elemental composition and the oxidation states of the CdFeSe composite films were investigated through X-ray photoelectron spectroscopy (XPS) (Omicron Nanotechnology Instrument (Serial No.0571)). 2.3. Polarization Study of the CdFeSe films One of the important criteria of the deposited films for PEC application, is the inherent stability in aqueous medium. In this respect, anodic polarization was studied by linear sweep voltammetry (LSV) and typical corrosion parameters like corrosion potential (Ecorr), corrosion current (Icorr), corrosion rate (Rcorr) were calculated from the corresponding Tafel plots.31,32 The detailed experimental procedure has been discussed in our previous repot.30 After polarization, the aliquots of the working electrolyte were subjected to anodic stripping voltammetry (ASV) for quantitative analysis of the dissolved elements (Cd, Fe) by using same instrumental setup maintioned in our earlier report.33 Notably, glassy carbon and large area Pt foil were used as counter electrodes during estimation of Cd and Fe respectively.

2.4. Electrochemical Characterizations of SC electrode – electrolyte interface Electrochemical impedance spectroscopic (EIS) measurements were performed to derive the charge transfer kinetics at the CdFeSe photo-anode/0.5 M S2––Sx2– electrolyte interface in a PEC cell. The experimental setup is similar as described earlier report.30 The equivalent circuit (EC) parameters were calculated from the corresponding Nyquist plots and the Mott–Schottky plots derived the nature of charge carrier and flat band potential.32 The current–voltage (J–V) measurements was recorded under 50 mW/cm2 light intensity, using xenon lamp and the performance output was derived thereof. The chronoamperometric measurements enabled the


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determination of photo-current decay in the fabricated PEC system using CdFeSe matrices as the photo-anode. 3. Results and Discussion Figure 1(i) represents the voltammetric features of CdFeSe electro-deposition on FTO substrate at variable amount of Fe2+ in the preparative bath under a fixed cycle depth of 250. The voltammograms starts with cathodic current indicating nucleation of Cd on the glass substrate by the reduction process Cd2+ + 2e → Cd at E0red = –0.63 V, while formation of elemental Se from the acidic source, H2SeO3 + 4H+ + 4e → Se + 3H2O at E0red = 0.496 V, is a spontaneous process. On the other hand, presence of FeSO4 in the preparative bath initiates reduction of Fe2+ (Fe2+ + 2e → Fe) at E0red = –0.691 V, that merges with the Cd2+ reduction almost at similar potential. All the potentials are expressed with respect to aqueous SCE. In all cases the multi-elemental codeposition procedure is followed by surface diffusion and the film growth takes place with the possible formulation of CdSe, FeSe and CdFeSe NPs as the matrix components. Large cathodic current generation at much higher negative potential indicated the H2 evolution reaction. In the anodic sweep, the robust oxide peak at ~ –0.43 V is associated with oxide formation of Cd and Fe as well as partial dissolution of the elements from the matrix.

(ii)

(i)

Figure 1. (i) Cyclic voltammograms for the electro-deposition of CdFeSe on FTO substrate at different [Fe2+] in preparative bath and inset: zoom portion of the Cd and Fe reduction region; (ii) CV of bare CdSe and CdFeSe (0.15M Fe2+) composite film showing positive shift in case of both oxidation and reduction peaks. All the films deposited at 250 periodic scans. 5 ACS Paragon Plus Environment


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Figure 1(ii) represents the voltammograms of the respective ternary CdFeSe or bare CdSe films developed with or without presence of FeSO4 in the bath. A significant positive shift observed in case of both oxidation and reduction peaks reflects the presence of Fe in the matrix forming CdFeSe or a blended structure of CdSe and FeSe.13 In fact incorporation of Fe in the CdSe matrix is confirmed from elemental compositional analysis (SEM-EDX) as well as XRD analysis discussed in later sections. Overall, the observed peak shifts with variable Fe2+ concentration (Figure1i) may be accounted for the structural and compositional changes occurring on the film surface during the growth process. (b)

(a)

(c)

Figure 2. (a) Transmittance spectra and the corresponding derivative plot (inset) of the CdFeSe films developed at different Fe2+ concentration; (b) %T of bare CdSe and CdFeSe; (c) variation of band gap with the incorporation of [Fe2+] in the preparative bath.


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Figure 2a represents the UV–visible transmittance spectra in the wave length range 350-1100 nm for the composite CdFeSe films at different Fe2+ concentrations, and the corresponding derivative plots are shown in the inset. A distinct absorption peak was observed (Figure 2b) at 674 nm, corresponding to band gap energy 1.86 eV, representing blue-shift with respect to bare CdSe at 1.72 eV; while two peaks appearing at around 918 and 548 nm confirm the presence of FeSe and Fe2O3 respectively. The variation of band gap energies is shown in Figure 2c. A sharp increase in the band energy is initially observed with Fe incorporation in the CdSe matrix, indicative of the presence of high energy oxides, Fe2O3. For the rest of the Fe2+ concentration the band gap values for CdSe remains almost the same. However, the band gap corresponding to the peak at 918 nm for FeSe, increases from 1.35 to1.39 eV with Fe content up to 0.15 M, beyond which there is a decline in the band gap, probably due to the loss of stoichiometry in the composite matrices. XRD patterns obtained for bare CdSe and composite CdFeSe films are shown in Figure 3a. For both the films the features resembles cubic phase (JCPDS 19-0191) for CdSe and the (h k l) values are indexed accordingly. The diffraction peaks of cubic CdSe films are found at 2 values 25.52O and 42.27O corresponding to lattice planes (111) and (110) respectively. Further, peaks appearing at 31.77O (101) and 54.62O (103) indicates the presence of hexagonal FeSe phase in the composite matrices [JCPDS 26-0795]. Whereas, hexagonal and rhombohedral Fe2O3 phase is also formed for high level Fe content (0.25M) CdFeSe matrix which is indicated by the peaks at 31.55O and 40.25O, respectively [JCPDS 40-1139, 33-0664]. The interesting feature in Figure 3a is that a new and unidentified diffraction peak appears at 2=22.68O for the composite films when FeSO4 concentration in bath is higher than 0.10 (M). This is further support the existence of ternary formulation of CdFeSe in the matrix. The crystallite sizes in the films are calculated using DebyeScherrer’s equation.34 The crystallite size for bare CdSe (10–12 nm) are found to decrease with increasing Fe concentration and attains narrow range between 3-5 nm. Regardless of the feeble existence of Fe in the matrix, the lattice shrinkage with increasing Fe content is possibly due to replacement of the lattice sites of Cd+2 ion (0.97 Å) by smaller size Fe2+ ion (0.74 Å) at selected 7 ACS Paragon Plus Environment


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sites in the ternary matrix as presented in Figure 3c. With the gradual inclusion of Fe in the matrix, further changes noted are peak shift to higher 2 observed for the diffraction pattern at 25.52O and decrease in peak intensities corresponding to CdSe (111) plane (Figure 3b). Z. G. Ju et.al.35 also reported similar observations when Fe is incorporated in CdSe matrix.

(a)

(b)

(c)

Figure 3. (a) XRD pattern of CdFeSe films developed at various concentration of Fe in the preparative bath (b) peak shift with increasing level of Fe and (c) schematic representation of CdFeSe lattice. [Note: C = Cubic; H = Hexagonal; Rh = Rhombohedral]

The dislocation densities () for the films (length of dislocation lines per unit volume) were calculated using the relation  = 1/D2 where D is crystaline size.36 In the present investigation 8 ACS Paragon Plus Environment

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 value of bare CdSe is found to be 6.90 x 1015 lines m-2 which increases upto 40.0 x 1015 lines m2

after incorporation of the transition metal in the matrix. This may be another reason for

increasing the band gap of CdFeSe compared to bare CdSe system (discussed in previous section). However, the intermediate films (0.15 M Fe2+) show lower  values  x 1015 lines m-2 indicating decrease in inter-planar spacing that translates to closed pack composite formulation with moderate Fe content.

(a)

(b)

(c)

(d)

Figure 4. Typical FESEM images of CdFeSe thin films developed at (a) 0.10 M (b) 0.15 M and (c) 0.20 M Fe2+; (d) Typical EDX study for the film deposited at 0.15M Fe2+ in CdFeSe film. Table 1: EDX data for the CdFeSe films. Samples (Fe2+ variation)

Atomic percentage Cd

Fe

Se

CdFeSe (0.05 M Fe2+)

47.41

0.20M Fe2+) is likely to minimize the electron-hole pair separation, possibly due to the presence of high energy Fe-oxide or hydroxide in the film matrices. The double layer feature build up across the electrode-electrolyte junction was further revealed in the Nyquist plots (Figure 9b) derived from EIS records under illumination. Inset of Figure 9b 14 ACS Paragon Plus Environment

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shows the corresponding Randles equivalent circuit (EC) model, consisting of a solution resistance (Rs), charge transfer resistances (R1 and R2) and a non-ideal capacitor, constant phase elements (CPE1 and CPE2) corresponding to the double layer capacitance for the composite films at the interfacial region of a PEC cell system. The impedance data are summarized in Table 3. The formation of FeSe phase is evident with Fe incorporation into CdSe matrix. Effectively a coupled charge transfer reaction is represented by R1 (for high energy CdSe) and R2 (for low energy FeSe) at the electrified interface. Evidently the matrices formed under 0.10–0.15 (M) Fe2+ in the bath exhibit better efficiency to overcome the charge transfer resistance (Rct) and lead to enhanced reaction kinetics at the electro-electrolyte interface (Figure 10a). The films also bear moderately high capacitance values (CPE1 & CPE2) at the same Fe level. Hence the combined effect of low Rct and high CPE may be responsible for the exuberance in the PEC performance of the film developed under 0.15 M Fe in the preparative bath. The Bode version of EIS records (log |Z| vs. log f ) is represented by Figure 10b. In high and low frequency region, the contribution of the imaginary component is quite less and the Randles cell performed mostly as a resistor. In the intermediate frequency range, impedance of the imaginary part plays major role; capacitive behavior prevails in the cell and phase angle move toward 90o. The inset of Figure 10b reflects the transition of capacitive to resistive behavior till an optimal level of Fe is reached in the matrix. The maximum phase shift (max) for CdFeSe films follow the order 0.05>0.10>0.15

Iron Doped CdSe Films with Improved Photosensitivity and Stability for

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