Enhancement of electrochemically deposited pristine CdTe film electrode photoelectrochemical characteristics by annealing temperature and cooling rate

Optik - International Journal for Light and Electron Optics 197 (2019) 163220

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Original research article

Enhancement of electrochemically deposited pristine CdTe film electrode photoelectrochemical characteristics by annealing temperature and cooling rate

T

Ahed H. Zyouda, Doa' H. Abdelhadia, Mohamed H.S. Helala, Samer H. Zyoudb, Heba Bsharata, Sohaib M. Abu-Alroba, Nordin Sablic,d, Naser Qamhiehe, ⁎ Abdul Razack Hajamohideene, Hikmat S. Hilala, a

SSERL, Chemistry Department, An-Najah National University, Nablus, Palestine College of Humanities and Science, Department of Mathematics and Basic Science, Ajman University, Ajman, United Arab Emirates c Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia d Institute of Advance Technology (ITMA), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia e Department of Physics, UAE University, Al-Ain, United Arab Emirates b

A R T IC LE I N F O

ABS TRA CT

Keywords: CdTe Film electrodes PEC performance Annealing Cooling rate

Photoelectrochemical (PEC) characteristics of CdTe film electrodes, known to have low conversion efficiency when used in their pristine form, can be significantly enhanced by carefully controlling their annealing temperature and cooling rate. Pristine CdTe films were electrodeposited onto FTO/Glass substrates which were used as anodes. To reach films with optimal characteristics, different applied preparation potentials were intentionally examined, namely 1.0, 1.1 and 1.2 V, vs. Ag/AgCl reference electrode (or 1.2, 1.3 and 1.4 V NHE, respectively) where the 1.1 V applied potential showed best PEC characteristics, and was thus followed unless otherwise stated. To study effect of annealing temperature, three temperatures (150, 200 and 250 ºC) were attempted to enhance PEC characteristics of the deposited films. Effect of cooling rate, on PEC performance of pre-annealed films, was also studied using quenching and slow cooling. Films quenched from annealing at all temperatures showed lower PEC performance compared to non-annealed electrode. Film electrodes slowly cooled from 150 or 200 ºC show enhanced PEC performance compared to quenched or non-annealed films. Film slowly cooled from 250 ºC exhibited lower PEC performance than the quenched counterpart. Annealing at 250 ºC lowered PEC for both quenching and slow cooling. As a low band gap semiconductor film electrode, it is recommended to slowly cool CdTe from low annealing temperatures, and to quickly cool them from relatively higher annealing temperature. The annealing temperature and cooling rate effects on CdTe film PEC performance are attributed to their effects on other physical characteristics, namely crystallinity, morphology and chemical composition. The optimal conversion efficiency (6.9%) was observed for film deposited at 1.1 V applied potential when annealed at 200 ºC and slowly cooled to room temperature.

⁎

Corresponding author. E-mail address: [email protected] (H.S. Hilal).

https://doi.org/10.1016/j.ijleo.2019.163220 Received 14 May 2019; Accepted 13 August 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.

Optik - International Journal for Light and Electron Optics 197 (2019) 163220

A.H. Zyoud, et al.

1. Introduction Due to their special features, cadmium telluride (CdTe) thin film electrodes are commonly used in various electrical and optical applications [1], such as nano-devices, sensors, solar cells [2]. CdTe is a II–VI a narrow band (˜1.5 eV) semiconductor with very high absorption coefficient (more than 105 cm−1) which is higher than that for silicon [3,4]. Its band gap value is suitable for visible solar light spectrum [5]. CdTe accounts for more than half thin film industry [6,7]. Despite the wide utilizations of CdTe thin films, in their pristine form they exhibit low conversion efficiency in photoelectrochemical (PEC) conversion processes. CdTe films electrodeposited onto Ni substrates showed low conversion efficiencies in the range 0.61–5% depending on conduction type and redox couples used [8]. Pristine CdTe film electrodes also showed low PEC performance when deposited onto FTO [9] or ITO [10] substrates. Spray pyrolysis deposited CdTe films showed conversion efficiency of 3.4% [11] while chemical bath deposited films showed lower value of 2.5%. Higher conversion efficiency values are reported for CdTe by treatment with CdCl2 [12–14]. Efficiency value of 17.5% or higher for monolithic CdTe is reported, albeit under special conditions [6,15] but such value has not been reported by others. To enhance their low PEC performance, CdTe films are commonly used in tandem with other systems such as CdS films [9]. Literature [16] shows that CdTe/CdS films deposited onto ITO substrates exhibit 3.5% conversion efficiency, which can be increased to 9% if Ag nanowires are deposited onto the films. Optimized performance effeciency of 9.82% has been reported for CdTe/CdS film electrodes by controlling annealing temperature of the CdS layer [17]. Conversion efficiencies in the range 5–13% were also reported for multi-junction cells based on CdTe/CdS combinations [18]. Multi-junction CdS/CdTe/ZnTe/ZnTe:Cu cells showed high conversion efficiency of 13.38% [19]. High conversion efficiency has also been reported for Cu doped CdS/CdTe film electrodes [20]. Efficiency of up to 14.1% was reported for CdS/CdTe:Cu/CNT system [21]. Treatment with CdCl2 also enhanced PEC performance for CdS/CdTe film electrodes [13,14]. With their relatively narrow band gap values, CdTe electrodes are expected to be unstable under PEC experimental conditions [22]. Yun et al. also highlighted the tendency of CdTe solar cells to photodegrade [23]. CdTe films may thus produce Cd2+ and Te2− ions, both of which are hazardous and environmentally unfriendly. In our efforts to enhance PEC characteristics of different pristine metal chalcogenide film electrodes by simple methods, this work is devoted to enhance CdTe film electrode. The strategy is to improve physical characteristics of the film electrode itself first. Further enhancement can be then achieved by modification with different coatings as described above in literature. In addition to enhancing PEC characteristics of pristine CdTe film electrodes, it is also imperative to stabilize them under PEC conditions. We earlier reported on effect of annealing temperature and cooling rate on PEC stability and conversion efficiency of different types of semiconductor electrodes [24–28]. Results showed that for medium to narrow band gap semiconductors, it is advisable to use lower annealing temperatures [24,27,29]. Using higher annealing temperatures for prolonged times has negative impact on semiconductor stability and conversion efficiency. Therefore, fast cooling is advisable for semiconductors annealed at relatively higher temperatures. For stable semiconductors with wider band gaps, such as ZnO, Al2O3 or TiO2, it is common to use higher annealing temperatures without concern about cooling rates [30–34]. As CdTe film electrodes are unstable with low PEC conversion efficiency, as stated above, this work will show how such materials can be modified by controlling annealing temperature, in their pristine form. Effect of cooling rate on such film electrodes will be assessed here for the first time, to our knowledge. Electrodeposited film electrodes are intentionally used here as they are reported to have lower PEC performance than other counterparts, which adds value to the present study. This work includes 5 Sections. The Abstract summarizes objectives and findings. The Introduction (Section 1) surveys relevant literature, highlights the necessity to do similar works and shows how the present work is novel. The Experimental Section (2) describes details of preparations, characterizations and measurement. Results and Discussion (Section 3) describes detailed results together with their discussions. The Conclusion (Section 4) summarizes main findings and recommendations. 2. Experimental 2.1. Starting materials Starting materials (CdSO4. 8/3 H2O, TeO2) and other common solvents were purchased from Aldrich. Redox couple materials (FeCl2, FeCl3, Na2S and sulfur, were purchased from Riedel. NaOH and KI were purchased from Riedel or Frutarom. All chemicals were obtained in pure form and were used without further purification. Highly conducting (7 Ω−2) and transparent (> 80% for longer than 300 nm wavelengths) FTO/glass substrates (2 × 4 cm2) were purchased from Aldrich. 2.2. Equipment A Shimadzu UV-1601 spectrometer was used to measure solid state electronic absorption spectra for CdTe films using FTO/Glass substrates as background. X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert PRO X-ray diffractometer (XRD), with CuKa (λ = 1.5418 Å), in the laboratories of the United Arab Emirates University (UAEU) Al-Ain, UAE. Field Emission Scanning Micrographs (SEM) and Energy Dispersion X-ray Spectra (EDS) were measured on A Jeol-EO Scanning Electron Microscope at the UAEU Al-Ain, UAE. Annealing was conducted using a thermostated horizontal tube furnace. CdTe samples were inserted inside a 30 cm long Pyrex glass tube under inert atmosphere. The tube was then placed inside the tube furnace, after reaching the desired temperature, for 60 min. For fast cooling, the Pyrex tube was taken from the furnace and allowed to cool in less than 5 min. Slow cooling was achieved 2

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by leaving the tube inside the furnace and lowering temperature to reach room temperature at a cooling rate of 50 °C per 30 min. 2.3. CdTe film preparation CdTe films were electrodeposited at room temperature onto glass/FTO substrates in a three electrode cell. The glass/FTO substrate (2 × 4 cm2) was used as a cathode working electrode, a platinum sheet as anode counter electrode and Ag/AgCl as reference electrode. The preparation was repeated for three different times for reproducibility purposes. In literature, CdSO4 has been used as a source for Cd2+ and TeO2 as a source for Te2− ions, with 1.5 : 2 mole ratio, respectively [35] while higher Cd2+ ions to Te2− ions are also reported [36]. Other Cd2+ ion sources, such as Cd(NO3)2 [13] or CdCl2 [14] have been described. In this work, pre-prepared aqueous solutions of CdSO4 (50 mL, 0.02 M) and TeO2 (50 mL, 0.01 M) at pH ˜2 were mixed together. The Cd2+ ions were used in excess (with 2:1 Cd to Te ratio, respectively) here intentionally to see if the preparation ratio may increase Cd/Te atom ratio in the final product. FTO/Glass substrates were vertically dipped inside the solution and other electrodes added. Electrodeposition was then started under inert atmosphere with gentle stirring for 5 min using (1.0, 1.1 or 1.2 V vs. Ag/AgCl reference). The potentials are slightly lower than described in earlier reports intentionally. Literature suggests that with higher applied potential the physical properties and PEC characteristics for CdTe were highly enhanced and the conductivity switched from p-type to n-type [13,14]. Others used even lower potentials of ˜0.3 V [36] and ˜0.7 V [10] in CdTe film electrode preparations. The resulting electrodeposited film electrode was carefully cut into 8 smaller electrodes. Each electrode was annealed at a controlled temperature and cooled either quickly or slowly, as described above. 2.4. PEC measurement Photoelectrochemical (PEC) experiments were performed inside a 3-electrode cell equipped with a thermostated water bath (˜25 °C) to avoid temperature change. Photocurrent density vs. potential (J-V) plots were measured on a computerized PAR 263A Potentiostat-Galvanostat. Experiments were repeated three times for reproducibility purposes. The glass/FTO/CdTe films were used as working electrodes vs. a platinum sheet counter electrode. The reference electrode was connected to the working electrode. The potentiostat internal cell reference, which was pre-calibrated and resembled NHE reference, was therefore used as a reference for potential. The redox couple/electrolyte system was placed inside the PEC cell, and the electrodes were dipped inside. The solution was stirred with high purity N2 (99.999%) gas bubbling for 5 min. Stirring was then stopped, and the nitrogen inlet rose to leave steady stream of nitrogen above the solution. Light illumination was made using a 50 W solar simulator lamp, with 400–800 nm radiation range. The light intensity was measured at the CdTe electrode surface and was 0.00084 W/cm2. The low intensity was intentionally used to avoid any possible electrode overheating. Photo J-V plots were measured under such low illumination intensity to void any temperature change. Other PEC characteristics (open circuit potential VOC, short circuit current density JSC, fill factor FF and external conversion efficiency η%) were measured from the J-V plots. Contrary to some earlier reports, where Na2S2O3 was used as electrolyte with no regenerative redox couples [13,14], this work followed a different method. Three different aqueous redox couples were examined here namely I−(0.1 M)/I3−(0.1 M), S-2 (0.10 M)/ S (0.10 M)/NaOH (0.10 M) or FeCl2 (0.10 M)/FeCl3 (0.10 M). The Fe2+/Fe3+ showed best PEC characteristics and was used hereinafter unless otherwise stated. Electrode stability under PEC conditions was examined by measuring JSC vs. time under relatively low illumination intensity (0.00041 W/cm2) to avoid overheating. A polarographic analyzer (Pol 150) with a polarographic stand (MDE 150) and Ag/AgCl reference was used at 0.00 V potential. All measurements were made under nitrogen atmosphere at constant temperature using FeCl2 (0.10 M)/FeCl3 (0.10 M) redox couple. 3. Results and discussion Electrochemically deposited CdTe film electrodes prepared at different potentials (1.0, 1.1 and 1.2 V). Effect of deposition potential on materials band gap and on PEC performance was first studied. The best deposition potential was then chosen for preparing electrodes for further study. 3.1. Effect of deposition potential Characteristics of CdTe electrodes are sensitive to deposition potential. Three deposition potentials were examined here. Fig. 1 shows the Tauc plots, constructed from electronic absorption spectra measured for different CdTe films deposited at 1.0, 1.1 and 1.2 V (vs. Ag/AgCl). Values of (αhν)2 vs. hν were plotted assuming direct band gap, where α is absorptivity, h is Planck's constant and ν is frequency for direct band systems as reported earlier [37–39]. Film prepared at 1.0 V shows poor linearity in the range 1.5–2.5 eV. Other films show clearer linear relationships with band gap value of ˜1.5 eV in congruence with earlier values [3,4,36]. The electronic absorption spectra confirm that the deposited films involved CdTe. Preliminary PEC study was performed on films deposited at different potentials. Fig. 2 shows that the film prepared at 1.1 V exhibits best photo J-V plot, in terms of JSC and VOC values, among the series. Table 1 summarizes values of PEC characteristics for 3

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Fig. 1. Tauc plots measured for CdTe films electrodeposited at different potentials (a) 1.0, (b) 1.1 and (c) 1.2 V.

Fig. 2. Effect of deposition potential of photo J-V plots CdTe thin films. Films were deposited at a) 1.0 V, b) 1.1 V and c) 1.2 V. Table 1 Effect of deposition potential on PEC characteristics of non-annealed CdTe thin film electrodes. Entry

Deposition potential (V)

VOC (V)

JSC (A/cm2)

FF %

η%

1 2 3

1.0 1.1 1.2

−0.221 −0.249 −0.208

7.58 × 10−4 9.77 × 10−4 8.18 × 10−4

21.08 20.79 21.69

4.2 6.02 4.39

different prepared film electrodes, and shows that the film deposited at 1.1 V has highest performance. Therefore, unless otherwise stated, deposition at 1.1 V has been followed throughout this work. Films deposited at 1.1 V have been the subject of further investigation here.

3.2. Effect of annealing temperature and cooling rate As stated above, films deposited at 1.1 V were only considered in heat treatment here. Films were annealed at different temperatures (room temperature, 150, 200 and 250 °C) and cooled back to room temperature by either quenching or slow cooling. To rationalize effect of treatment on PEC characteristics of different CdTe film electrodes, the influence of heat treatment on other physical characteristics has been studied. Effect on optical properties, namely band gap value, was studied. Effect on RXD patterns, including particle size and imperfection density, was also studied. SEM was used to study effect of treatment on film morphology and agglomerate size. Composition was studied using EDS measurement. Other parameters, such Hall effect and magnetism are not included and need further study. Due to its relatively narrow band gap, CdTe film electrodes are expected to be relatively unstable [22]. For this reason, films were annealed at relatively low temperatures, namely 150, 200 and 250 °C. Higher temperatures were avoided based on earlier studies 4

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Table 2 Average values of thickness and electrical conductivity measured for different CdTe film electrodes. (Average values were calculated from three measurements). Annealing temperature, ºC

Cooling rate

Film thickness, nm ( ± 10%)

Electrical conductivity, (Ω.cm)−1 ( ± 15%)

Non-annealed 150 150 200 200 250 250

– Fast Slow Fast Slow Fast Slow

˜340 ˜333 ˜330 ˜331 ˜327 ˜325 ˜320

1.27 × 10−7 2.15 × 10−7 4.42 × 10−7 3.85 × 10−7 5.65 × 10−7 2.35 × 10−7 2.55 × 10−7

made on other narrow to medium band gap film electrodes such as those of CdS [40], CdSe [24] and others [41]. Values of different film thicknesses were estimated gravimetrically [42,43]. Calculations were made based on re-dissolved Cd2+ and Te2− ion concentrations determined by polargraphic stripping analysis, while taking into consideration molar mass, density and percentage for different existing components (CdTe, TeO2 and Te). Film conductivity was directly measured by a digital multi-meter. Table 2 summarizes film thickness and sheet conductivity values for different films. Electronic absorption spectra for CdTe film electrodes deposited at 1.1 V and annealed at different temperatures (room temperature, 150, 200 and 250 °C), with different cooling rates, were measured, and the Tauc plots for different film electrodes are shown in Fig. 3. The band gap values for the films are 1.50, 1.40, 1.40 and 1.30 eV, respectively, as summarized in Table 3. The values are comparable to reported values [36] and slightly smaller than other ones for films prepared by electrochemical deposition [14]

Fig. 3. Tauc plots measured for CdTe films annealed at different temperatures. 5

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Table 3 Effect of annealing temperature and cooling rate on band gap value for CdTe films. All films were deposited at 1.1 V. Annealing temperature (ºC)

Cooling rate

Band gap value (eV)

Non-annealed 150 150 200 200 250 250

== Quenched Slowly cooled Quenched Slowly cooled Quenched Slowly Cooled

1.50 1.40 1.40 1.40 1.40 1.30 1.30

and chemical deposition [44]. The lowering in band gap values with annealing temperature is due to sintering of smaller particles together [45,46]. Table 3 shows that with increased annealing temperature, the band gap value decreases for both slowly cooled and quenched films. With annealing, small particles are sintered together forming larger particles. In the nano-scale, band gap red shift is known for larger particles. This is due to smaller relative surface area for larger particles and smaller relative number of surface atoms [41]. XRD patterns were measured for different films, as shown in Fig. 4. All films showed reflections characteristic for CdTe, as compared to literature [47]. In addition to the CdTe reflections, other reflections for TeO2 and elemental Te are observed [48,49]. FTO reflections [50] are observed in Fig. 4, and the higher reflection at 78° has been used as reference for relative reflection height ratio calculation, as described below. Relative reflection heights vary for different films depending on annealing temperature and on cooling rate. The film quenched from 150 °C shows higher CdTe reflection height than the non-annealed counterpart, indicating less imperfections in the former. The film slowly cooled from 200 °C also shows similar behavior. The film quenched from 250 °C shows not much enhancement in CdTe reflection height. The film slowly cooled from 200 °C exhibits the highest CdTe reflection among different films. The results indicate that slow cooling from 200 °C yields CdTe films with less imperfections than other counterparts. The presence of TeO2 can be observed in all films including the one slowly cooled from 200 °C. Based on Scherrer equation [51,52], average CdTe particle sizes were calculated based on the 38° reflection, for different films. Table 4 shows average calculated values for different films. The Table shows that annealing increases average particle size, and the slowly cooled film from 200 °C having largest particle size. Values of (CdTe reflection height at 38°/FTO reflection height at 78°) ratios are calculated from Fig. 4 and summarized in Table 4. The Table shows that annealing at 150 or 200 °C enhances crystallinity of the CdTe films in term of relative reflection height, in cases of slow and fast cooling. Lowering in relative reflection height can be observed for films annealed at the higher temperature (250 °C) for both cases of slow and fast cooling.

Fig. 4. Effects of annealing temperature and cooling rate on CdTe film crystallinity. XRD patterns measured for CdTe films annealed at different temperatures, followed by quenching or slow cooling. 6

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Table 4 Effect of annealing and cooling rate on CdTe XRD patterns. Temperature ºC

Particle size (nm)

XRD major peak ratio CdTe/FTO (at 38° and 78° respectively)

Non-annealed 150 Fast 150 Slow 200 Fast 200 Slow 250 Fast 250 Slow

45 47 52 47 52 45 47

0.50 1.80 0.86 0.75 1.30 0.44 0.69

SEM micrographs, measured for different films are shown in Fig. 5. The SEM shows that the films involve agglomerates. The agglomerates involve smaller particles, as confirmed by XRD above. Fig. 5 shows that the agglomerate sizes for slowly cooled films are larger than their quenched counterparts. No systematic size variations can be observed for films annealed at different

Fig. 5. SEM micrographs measured for different prepared CdTe films. 7

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Table 5 Effect of annealing temperature and cooling rate on values of atom%. Values were measured from EDS for different Glass/FTO/CdTe films deposited at 1.1 V applied potential. Sample description

C

O

Si

Cd

Sn

Te

Non-annealed 150 Fast 150 Slow 200 Fast 200 Slow 250 Fast 250 Slow

– 10.02 13.73 10.85 14.77 14.88 11.10

53.99 49.94 40.86 50.78 45.60 47.91 48.13

7.00 5.03 4.46 5.35 9.10 5.71 5.05

2.59 2.42 4.37 2.16 4.65 2.14 2.44

30.94 26.84 25.87 25.94 15.06 25.14 28.16

5.49 5.75 10.71 4.92 10.82 4.22 5.11

temperatures. Chemical compositions for different films were calculated from EDS. The EDS results and elemental analysis data for different films are shown in the Supplementary Figure (S1). Atom% results are summarized in Table 5 below. Despite the fact that all films were electro-deposited from solutions with higher Cu/Te ratio than 2/1, all films involved Te to Cd atom ratios higher than 1. In earlier reports [10,12–14,18,51], varying applied potential (two electrode system used) affected Te/Cd mole ratios, where n-type conductivity appeared at higher deposition potentials (1.368 V using two electrode cells) due to occurrence of higher Cd elemental ratio. In another report, n-type conductivity was reported for CdTe film electrodes electro-deposited at lower potentials of ˜0.70 V using three electrode systems [10]. In this work, the applied potential (1.10 V vs Ag/AgCl, 1.30 V vs NHE) yields higher Te/Cd atom ratios. Different elements observed belong to their corresponding layers in the Glass/FTO/CdTe, including Si (from glass), Sn (from FTO), Cd (from CdTe), Te (from CdTe and TeO2) and O (from different layers). As oxygen exists in glass and FTO, extra oxygen is attributed to TeO2. Table 5 shows that among different films, the ones slowly cooled from 150 and 200 °C exhibit higher Cd and Te abundance, based on Cd and Te atom% values. While CdTe, Te and TeO2 phases are evidenced by XRD patterns, the extra tellurium in Table 5 exists in the unreacted TeO2 and Te elemental forms. Annealing at higher temperature 250 °C shows lowering in both Cd and Te, due to the relative instability of CdTe films. The results are consistent with earlier reports for other low band gap semiconductor films [41]. Slow cooling from ambient temperature is expected to enhance film composition, for low band gap films [41], and CdTe is no exception here. Table 5 results will be revisited while discussing PEC performance below. Plots of photo-current density vs. applied potential (J-V) are shown in Fig. 6. The plots show that the film electrodes exhibit n-type semiconduction. This is observed from the positive short circuit photo-current density values. In literature, CdTe normally exhibit ptype conduction when Te/Cd ratio is higher than 1:1, and n-type conductivity when the ratio is less than 1 [10,13,14], depending on used deposition potential. Therefore, CdTe conductivity type is determined by excessive materials, being either Cd or Te [53]. Table 5

Fig. 6. Effect of annealing temperature and cooling rate on CdTe film electrode PEC performance. Photocurrent J-V plots were measured for films deposited at 1.1 V using FCl2/FeCl3 redox couple. 8

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Table 6 Effect of annealing CdTe film electrode on its PEC characteristics. All films were deposited at 1.1 V (Vs. Ag/AgCl ref.). Redox couple aqueous FeCl2/ FeCl3. Entry 1 2 3 4 5 6 7

Annealing Temperature (ºC) None 150 150 200 200 250 250

Cooling rate

VOC (V)

– Fast Slow Fast Slow Fast Slow

−0.221 −0.214 −.287 −0.173 −0.228 −0.186 −0.197

JSC (A) −4

7.58 × 10 5.66 × 10−4 9.21 × 10−4 4.77 × 10−4 1.22 × 10−3 4.24 × 10−4 4.68 × 10−4

FF

η%

21.08 21.82 21.63 21.90 20.81 21.81 20.73

4.20 3.14 6.80 2.15 6.92 2.04 2.27

shows that the Te/Cd ratio is higher than 1, but Fig. 6 still confirms n-type for the film electrodes deposited at 1.1 V potential. The EDS results show that the excess Te is in the TeO2 phase. TeO2 itself is reported to have n-type conductivity [54]. Treatment of CdTe may also convert its surface into p-type as reported earlier [55,56]. Electronic conductivity in single crystalline TeO2 was reported and p- to n-type switching by temperature was studied [57]. Oxide materials normally have n-type conductivity due to oxygen loss vacancies [58,59]. Therefore, the excess TeO2 phase is presumably responsible for observed n-type behavior of the film electrodes here. Fig. 6 shows that different film electrodes exhibit different PEC characteristics. Values of open circuit potential (VOC), short circuit current (JSC), fill factor (FF) and conversion efficiency (ɳ) are extracted from Fig. 6 and summarized in Table 6. PEC characteristics for different films are summarized in Table 6. The Table shows that annealing followed by quenching lowers PEC performance in all electrodes. Table 6 (entries 1, 2, 4 & 6), shows that the non-annealed film exhibits higher conversion efficiency (4.2%) than films quenched after annealing. Annealing at 150 °C lowered the efficiency to 3.14%, while annealing at 200 or 250 °C lowered the efficiency to 2.15% or 2.04%, respectively. The Table suggests that annealing has negative effect on CdTe film electrodes electrochemically deposited at 1.1 V. Based on earlier results with different metal chalcogenide film electrodes [24] this behavior is not unexpected. Electrodeposited films normally have high uniformity and adherence to the FTO surface. This is a favorable characteristic in PEC applications. When heated, the films may undergo distortions with increased imperfections as earlier witnessed in electrochemically deposited CdS film electrodes [24,60]. Such effects lower the PEC performance of electrodeposited film electrodes. The efficiency lowering for the quenched electrodes here cannot be explained based on crystallinity lowering or imperfection increase. Table 4 shows that films quenched from 150 or 200 °C have higher crystallinity, than the non-annealed film, but yet have lower PEC performance. The PEC performance lowering is thus due to film composition. Table 5 shows lowering in Cd and Te atom% values for quenched films. In some cases, annealing was reported to enhance PEC performance for film electrodes, by affecting interconnectivity, particle sintering and lowering sheet resistance [41]. The trend observed in Table 6 shows that annealing, followed by quenching, lowers PEC performance. As stated in Section 1 above, low band gap film electrodes may undergo efficiency lowering on annealing. In-depth analysis of quenching effect on metal chalcogenide film electrode PEC performance has been recently reviewed [41]. Table 6 (entries 1, 3 & 5) shows that slow cooling of pre-annealed CdTe film electrodes (from 150 or 200 °C) enhances its PEC performance. As reported earlier, annealing the film electrode creates more imperfections and unstable components in the heated film. Such unstable components may not be able to go back to stable positions on quenching. Slow cooling allows return of unstable components to stable positions, which results in enhanced PEC performance. Such rationale is justified by Tables 4 and 5. Table 4 shows larger particle size and higher relative reflection ratios for films slowly cooled from 150 and 200 °C, indicating higher crystallinity in both films. Table 5 also shows higher atom% values for Cd and Te in both films. Therefore, the enhanced PEC performance for films slowly cooled from 150 and 200 °C is justified by enhanced film crystallinity and composition. As earlier expected [41] annealing CdTe film electrodes, with low band gap value, at higher temperatures has negative effect on the film PEC. Table 6 (entries 6 &7) shows that the 250 °C annealing temperature is not suitable for CdTe film electrodes, at both quenching and slow cooling. Heating a low band gap semiconductor at relatively elevated temperature causes crystallinity lowering as shown in Table 4. Moreover, high temperature annealing affects film composition as shown in Table 5. The results are thus consistent with assumptions stated above in Section 1 for low band gap semiconducting electrodes. Effect of annealing temperature and cooling rate on CdTe film electrode stability under PEC conditions has been studied, as shown in Fig. 7. The Figure shows that the films slowly cooled from 150 and 200 °C sustained highest short circuit photocurrent density (JSC) values for relatively longer time than other counterparts. Films quenched from 150 and 200 °C showed lowered stability. The film slowly cooled from higher temperature (250 °C) exhibited JSC value decrease with time, showing lower stability than the one quenched from same temperature. The stability study results confirm the above PEC results, in the sense that low band gap film electrodes annealed at lower temperatures should be slowly cooled. Annealing at relatively higher temperatures should be followed with fast cooling so as to minimize time of exposure to heat, as discussed above. Collectively, the results show that annealing CdTe film electrodes at ambient temperature, followed by slow cooling, affects film crystallinity and film composition. Such effects are the main reasons for film electrode PEC performance and stability. The maximum conversion efficiency value (6.9%) can be further enhanced by further treatment. Work is under way here to further modify the present enhanced CdTe films, as described in earlier literature, aiming at reaching much higher conversion efficiencies.

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Fig. 7. Stability study for different CdTe film electrodes. Plots of JSC vs. time measured for films (a) Non-annealed, (b) quenched from 150 ○C, (c) Slowly cooled from 150 ○C, (d) Quenched from 200 ○C, (e) Slowly cooled from 200 ○C, Quenched from (f) 250 ○C, and (g) Slowly cooled from 250 ○ C. All measurements taken at room temperature, using FCl2/FeCl3 redox couple under light intensity 0.00041 W/cm2.

4. Conclusion Photoelectrochemical performance of CdTe film electrodes, electrodeposited onto Glass/FTO substrates, can be enhanced by controlling annealing temperature and cooling rate. As a narrow band gap semiconductor, CdTe film electrodes should not be heated at higher temperatures. When annealed at moderate temperature, slow cooling is recommended and yields higher PEC performance. Annealing and cooling rate affect CdTe film electrode PEC performance by affecting other properties such as crystallinity and chemical composition. Optimal PEC performance is achieved by annealing the film electrodes at 200 °C, followed by slow cooling. The study shows the possibility to enhance as-deposited CdTe film electrode PEC performance, by controlling annealing temperature and cooling rate, as a starting step prior to further modification with other materials. Declaration of Competing Interest The authors declare that there are no conflicts of interest in this work. Acknowledgements Parts of the results are based on DHA Thesis supervised by HSH and AZ. Other experimental results are based on SAR supervised by HSH and AZ. AZ contributed with new experiments. NS contributed with band gap calculation. NQ, ARH and SZ contributed with XRD, SEM & EDS measurements. MHSH and HB contributed with literature search and ideas. Financial support (Grant # ANNU-1819Sc013) to this work from Deanship of Scientific Research, ANU, is acknowledged. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.ijleo.2019. 163220. References [1] A. Arce-Plaza, F. Sánchez-Rodriguez, M. Courel-Piedrahita, O.V. Galán, V. Hernandez-Calderon, S. Ramirez-Velasco, M.O. López, CdTe thin films: deposition techniques and applications, Coatings and Thin-Film Technologies, IntechOpen, 2018. [2] Z. Gutierrez, P.G. Zayas-Bazán, O. de Melo, F. de Moure-Flores, J. Andraca-Adame, L. Moreno-Ruiz, H. Martínez-Gutiérrez, S. Gallardo, J. Sastré-Hernández, G. Contreras-Puente, CdS/CdTe heterostructures for applications in ultra-thin solar cells, Materials 11 (2018) 1788. [3] G.L. Maxwell, Characterization and Modeling of CdCl2 Treated CdTe/CdS Thin-Film Solar Cells, Colorado State University. Libraries, 2007. [4] D. Kwon, Studies of Sputtered Cadmium Telluride and Cadmium Selenide Solar Cells, The University of Toledo, 2012. [5] H. Kuo-Jui, Electron-Reflector Strategy for CdTe Thin-film Solar Cells, in, PhD Thesis, Colorado State University, Department of Physics, USA, 2010. [6] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 46), Prog. Photovolt. Res. Appl. 23 (2015) 805–812. [7] M. Bouroushian, Electrochemistry of the chalcogens, Electrochemistry of Metal Chalcogenides, Springer, 2010, pp. 57–75. [8] Y. Guo, X. Deng, Electrodeposition of CdTe thin films and their photoelectrochemical behaviour, Sol. Energy Mater. Sol. Cells 29 (1993) 115–122. [9] J. Su, T. Minegishi, M. Katayama, K. Domen, Photoelectrochemical hydrogen evolution from water on a surface modified CdTe thin film electrode under simulated sunlight, J. Mater. Chem. A 5 (2017) 4486–4492. [10] O. Echendu, K. Okeoma, C. Oriaku, I. Dharmadasa, Electrochemical deposition of CdTe semiconductor thin films for solar cell application using two-electrode and three-electrode configurations: a comparative study, Adv. Mater. Sci. Eng. 2016 (2016). [11] V. Nikale, S. Shinde, C. Bhosale, K. Rajpure, Physical properties of spray deposited CdTe thin films: PEC performance, J. Semicond. 32 (2011) 033001. [12] N. Abdul-Manaf, H. Salim, M. Madugu, O. Olusola, I. Dharmadasa, Electro-plating and characterisation of CdTe thin films using CdCl2 as the cadmium source, Energies 8 (2015) 10883–10903. [13] H. Salim, V. Patel, A. Abbas, J. Walls, I. Dharmadasa, Electrodeposition of CdTe thin films using nitrate precursor for applications in solar cells, J. Mater. Sci. Mater. Electron. 26 (2015) 3119–3128. [14] A. Ojo, I. Dharmadasa, Analysis of electrodeposited CdTe thin films grown using cadmium chloride precursor for applications in solar cells, J. Mater. Sci. Mater. Electron. 28 (2017) 14110–14120. [15] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (Version 45), Prog. Photovolt. Res. Appl. 23 (2015) 1–9.

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