Electrodeposition of CdSe photoabsorber thin films in the presence of selected organic additives

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Electrodeposition of CdSe photoabsorber thin films in the presence of selected organic additives S. Hamilakis n, D. Balgis, K. Milonakou-Koufoudaki, C. Mitzithra, C. Kollia, Z. Loizos School of Chemical Engineering, National Technical University of Athens, 9, Iroon Polytechniou Street, Zografou Campus, Athens 15 780, Greece

art ic l e i nf o

a b s t r a c t

Article history: Received 28 November 2014 Accepted 14 January 2015

Photoabsorbing CdSe semiconductive thin films were prepared by cathodic electrodeposition onto titanium substrates from an acidic aqueous electrolytic bath, containing some selected, commercially available organic salts as additives, specifically monosodium L-glutamate and choline chloride. The products obtained were fully characterized with XRD and SEM–EDAX techniques and their photoelectrochemical behavior was studied using a photoelectrochemical cell (PEC). It is observed that the use of both additives leads to more uniform and in many cases to better crystallized deposits. All films, taken in the presence of the additives, exhibit some differences in their semiconductive behavior, in comparison to the pure CdSe ones. However, the addition of monosodium L-glutamate salt into the bath brought about a clear improvement in photoresponse of the deposits, whereas the use of the choline chloride salt clearly led to a deterioration of their photoconductivity. It is considered that the organic ions of the salts (L-glutamate anion and choline cation) are potentially adsorbed on the CdSe deposits, thus introducing crystal defects, which modify the electric properties of the final products. & 2015 Published by Elsevier B.V.

Keywords: Solar energy materials Cadmium selenide Hybrid semiconductors Organic additives Choline chloride Monosodium L-glutamate

1. Introduction Cadmium chalcogenides, such as CdSe, CdTe and Cd(Se,Te) alloys, are well-known semiconductive compounds presenting a particular interest as they have found applications in the field of photocatalysis and conversion of solar energy [1–5]. They belong to the compounds formed between elements of 12th (zinc group) and 16th (chalcogens: oxygen group) of the periodic table, e.g. CdSe and CdTe. These example compounds possess direct energy gaps (1.7 and 1.5 eV, respectively), which are more efficient to the absorption of electromagnetic radiation. Moreover, using these compounds, the exploitation of a large part of the photons present in the solar spectrum can be attained. Cathodic electrodeposition of Cd chalcogenides is extensively investigated in [6,7], where the concept of potential preparation of compact, polycrystalline, semiconductive compound films by underpotential deposition (upd) of Cd, in a potentiostatic manner, was described. The lattice structure of CdSe can be found in the forms of zinc blende (cubic) and wurtzite (hexagonal). The former is a metastable phase, constituting the almost exclusive product of an electrochemical formation process, while the latter is the thermodynamically stable structure obtained either by annealing the cubic phase or directly by various, electroless deposition techniques [2–4].

n

Corresponding author. Tel: þ 30 210 772 3258; fax: þ30 210 772 3188. E-mail address: [email protected] (S. Hamilakis).

Photoelectrochemical research has a far-reaching interest in cadmium chalcogenide semiconductors since they can be effectively used as active electrodes in relatively stable photoelectrochemical cells (PEC) for solar energy conversion. Moreover, polycrystalline anodes, particularly of CdSe, have signified the potential advantages of the liquid–solid junction compared to solid state ones [1–5,8,9]. In our previous work [10] we have investigated the role played by some slightly water soluble fullerene derivatives, introduced in the electrolytic bath during the electrodeposition of cadmium chalcogenides. It was found that these chemical species can be codeposited with the inorganic ones, giving hybrid systems possessing improved semiconductive behavior such as their photoresponse in PEC. In the present work we attempted to continue and extend our research using in the place of the fullerene salts some low cost and commercially available organic compounds, such as monosodium L-glutamate and choline chloride. These salts, readily soluble in the water, provide in their aqueous solutions (working pH¼2.2) organic species, specifically L-glutamate cations (glutamic acid isoelectric point: pH¼3.2) and choline cations, respectively. Thus, their behavior may differ during the electrodeposition process. Moreover, it is already known that glutamic salts and their derivatives are used as surfactants or additives in metal electroplating baths, modifying the grain size of the deposits [11,12]. Organic additives often tend to favor the development of most crystallites to some dominant textures, mostly inhibiting crystal growth towards other crystallographic axes [13], so

http://dx.doi.org/10.1016/j.matlet.2015.01.052 0167-577X/& 2015 Published by Elsevier B.V.

Please cite this article as: Hamilakis S, et al. Electrodeposition of CdSe photoabsorber thin films in the presence of selected organic additives. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.01.052i

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influencing the properties of the deposits. We may expect that the above chemical species should function as additives, introducing crystal defects, modifying thus the electric properties of the deposits.

2. Experimental CdSe thin films were developed potentiostatically, using a potentio-scan system with a conventional three electrode setup. The cathode was a rotating Ti disc electrode (∅ 12 mm; cathode's rotation rate: 500 rpm). The counter electrode was a large platinum plated grid. The potential of the working electrode was

monitored against an Hg/HgSO4 saturated sulfate reference electrode (SSE). The electrolytic bath for CdSe plating was an aqueous solution containing typically 0.2 mol/L CdSO4 and 2  10  3 mol/L SeO2 being kept constant at 857 1 1C. The concentration of the additives (monosodium L-glutamate or choline chloride) was set to 2  10  3 mol/L. The bath pH was adjusted to 2.2. All deposits were examined by X-Ray Diffraction (XRD; Siemens D5000 using a Cu Kα X-ray source) and Scanning Electron Microscopy (SEM; FΕΙ-Quanta 200) techniques. Compositional data were obtained by Energy Dispersive X-ray (EDAX) analysis. Photoresponse studies were performed in a photoelectrochemical cell (PEC) with a three electrode configuration comprising platinum wire rods as

Fig. 1. XRD diagram of CdSe thin films prepared by electrodeposition at  0.9,  1.0 and  1.1 V vs. SSE in the presence of monosodium L-glutamate (a) and choline chloride (b) additives in comparison with the diagrams of pure CdSe.

Fig. 2. EDAX diagrams and SEM micrographs of CdSe thin films prepared by electrodeposition at  1.0 V vs. SSE in the presence of monosodium L-glutamate (before and after surface etching) and choline chloride additives in comparison those of pure CdSe.

Please cite this article as: Hamilakis S, et al. Electrodeposition of CdSe photoabsorber thin films in the presence of selected organic additives. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.01.052i

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counter and reference electrodes. An alkaline sulfide–polysulfide solution (S2x  1 M NaOH, 1 M Na2S, 1 M S solution) was used as the working redox electrolyte. The PEC measurements were conducted under a white illumination generated by a halogen lamp and focused in front of the quartz window of the cell. Illumination intensity was 1000 W/m2.

3. Results and discussion Fig. 1 summarizes the XRD diagrams of the new semiconductive thin films prepared in the presence of monosodium L-glutamate and choline chloride additives in comparison with the diagrams of pure CdSe. It is found that all specimens exhibit a well-developed cubic zinc blende structure with a predominating crystalline orientation towards the [111] crystallographic axis, like the electrodeposited pure CdSe [1–3]. An EDAX carbon peak (see Fig. 2) appears in all specimens, prepared in the presence of the additives, as resulted from the SEM–EDAX investigations, probably suggesting the development of a hybrid system or, at least, the introduction of crystal defects, which modify the electric properties of the final products. Fig. 2 indicatively presents the EDAX diagrams and SEM micrographs of CdSe thin films electrodeposited at  1.0 V vs. SSE in

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the presence of glutamate (before and after surface etching) and choline additives in comparison to those of pure CdSe. All the asprepared semiconductive thin films have good crystallized structure with nano-scaled grain sizes. Chemical species derived from the additives are incorporated not only superficially but even in the bulk of the deposit; indeed, EDAX carbon peaks still exists after the surface etching, caused by the contact of the layer with the corrosive sulfide–polysulfide solution during the PEC measurements. Table 1 summarizes the four parameters of the photoconversion curves (short circuit current, jsc, open circuit potential, VOC, fill factor, FF, and photoelectrochemical efficiency, η) for the CdSe thin films prepared at  0.9,  1.0 and  1.1 V mV vs. SSE, in the presence of monosodium L-glutamate and choline chloride additives, used directly as absorbed electrodes in a conventional PEC. Fig. 3 illustrates the current-potential photoresponses for the films electrodeposited in the presence of the glutamate salt, which present the best solar energy conversion efficiencies. For comparison, the corresponding data of the pure CdSe, taken at the same conditions, are provided, too. All photocurrents are anodic, that is characteristic of an n-type behavior due to the variations of stoichiometry. It is also observed that all deposits with the choline chloride additive have clearly inferior properties such as short circuit current and photoelectrochemical efficiencies when compared with the reference specimens.

Table 1 Photoelectrochemical parameters of CdSe thin films prepared by electrodeposition in the presence of monosodium L-glutamate and choline chloride additives. Deposition potential (V/SSE)

JSC (μΑ) VOC (mV) FF η (%)

CdSe/monosodium L-glutamate

Pure CdSe

CdSe/choline chloride

 0.9

 1.0

 1.1

 0.9

 1.0

 1.1

 0.9

 1.0

 1.1

2128  412 0.312 0.274

2160  412 0.356 0.317

2701  367 0.299 0.296

3738  426 0.412 0.656

4207  393 0.306 0.507

4903  291 0.345 0.492

489  184 0.338 0.030

1018  272 0.295 0.082

296  277 0.265 0.022

Fig. 3. Current density vs. electrochemical potential given by CdSe thin films prepared by electrodeposition at  0.9,  1.0 and  1.1 V mV vs. SSE (curves a, b and c, respectively) in the presence of Na L-glutamate additive (curves II) in comparison with the diagrams of pure CdSe (curves I), used directly as absorbed electrodes in a conventional PEC in the dark and under illumination of 1000 W/m2.

Please cite this article as: Hamilakis S, et al. Electrodeposition of CdSe photoabsorber thin films in the presence of selected organic additives. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.01.052i

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On the contrary, all specimens deposited with the glutamate additive have improved properties compared to the reference ones, displaying approximately twice as many short circuits current and photoconversion efficiencies. 4. Conclusions Two low cost and commercially available organic compounds, specifically monosodium L-glutamate and choline chloride, have been selected as additives during the electrodeposition of photoabsorbing CdSe semiconductive thin films. All the as-prepared thin films have a uniform and well-crystallized structure with nano-scaled grain sizes, providing anodic type currents, a typical behavior of n-type semiconductors. Chemical species derived from the additives are incorporated in the inorganic compound probably in the frame of an electro-codeposition process, leading to a kind of a hybrid semiconductive system. The thin films deposited with the choline chloride additive have inferior semiconductive properties, as derived from the short circuit current and photoelectrochemical efficiency values, when compared with the reference specimens. On the other hand, all the monosodium L-glutamate specimens had improved and superior properties compared not only to the reference ones but also with the products, obtained with other electro-codeposition techniques [10], exhibiting the highest solar energy conversion efficiencies. It is considered that organic chemical species from the additives can create crystal defects on the CdSe deposits, impacting positively or negatively (in the cases of the glutamate and the choline chloride, respectively) the photoconductivity of the final products.

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Please cite this article as: Hamilakis S, et al. Electrodeposition of CdSe photoabsorber thin films in the presence of selected organic additives. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.01.052i

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