The Effect of Electrolyte Composition on the Electrodeposition of CdTe from Ammonia-alkaline Solutions.

Tanaka

et al. 2005

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The behavior of CdTe in deposition and dissolution in a basic ammoniacal aqueous media ͑pH 10.7͒ containing Cd͑II͒ and Te͑IV͒ species was investigated in situ using an electrochemical quartz crystal microbalance ͑QCM͒. The instantaneous current efficiency of CdTe deposition during potentiostatic cathodic electrolysis, under dark conditions, increased up to 80-90% at the beginning of the electrolysis, but later gradually decreased to ca. 60%, resulting in an overall efficiency of around 70%. In contrast, the overall current efficiency under illumination with visible light increased steeply to 100%. Two anodic waves appeared during the positive-going scan of a cyclic voltammogram in the CdTe deposition bath, and were assigned to ͑i͒ a two-electron decomposition of CdTe into Cd͑II͒ ions and elemental Te followed by ͑ii͒ a four-electron dissolution of the resulting Te into Te͑IV͒ ions. These observed phenomena are discussed using a potential-pH diagram devised for the Cd-Te-NH 3 -H 2 O system.Since the pioneering work on CdTe electrodeposition by Kröger's group in the late 1970s, 1 cathodic electrodeposition has become one of the key techniques for the preparation of thin-layered compound semiconductors 2,3 ͑e.g., CdTe͒ for photovoltaic applications, and the cell made up of an n-CdS/p-CdTe heterojunction has been put into production on an industrial scale. 4 Although aqueous acidic sulfate solutions 1 have historically been employed as the bath for CdTe electrodeposition, we have proposed that aqueous basic, or alkaline, solutions containing ammonia 5-10 or amine ligands 11,12 are also suitable for the electrodeposition of a uniform CdTe layer, since these basic solutions have a relatively high solubility for Te͑lV͒ species as TeO 3 2− ions. In the cases of both acidic and basic baths, the deposition of stoichiometric CdTe takes place at potentials positive to the Nernst potential for elemental Cd deposition. 1,2,7,13 Regarding the acidic baths, for example, Kampmann et al. found that a deposition potential of only ϩ5 mV vs. the elemental Cd deposition led to the formation of CdTe with relatively high crystallinity. 2 Although the basic baths gave CdTe layers having a lower crystallinity relative to the layers from the acidic baths, 14 the nearer the deposition potential approaches the potential for elemental Cd, the higher the CdTe deposition current becomes. 7,10 At these potentials, Cd atoms can electrodeposit accompanying the formation of the CdTe compound only when Te atoms exist at the growing surface or, in other words, the Cd atoms forming CdTe compound cannot deposit unless Te atoms exist at the surface. The mechanism of CdTe deposition is, therefore, considered to be ͑i͒ a cathodic deposition of surface tellurium atoms, Te͑IV͒ + 4e → Te ͑ads͒ , followed by ͑ii͒ an adsorption, and ͑iii͒ a kind of underpotential reduction of the Cd͑II͒ ions to form CdTe: Cd͑II͒ + Te ͑ads͒ + 2e → CdTe.In order to discuss the thermodynamics and mechanism of CdTe deposition, we devised a potential-pH diagram for the Cd-Te-NH 3 -...

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confidence: 99%

Electrochemical QCM Studies of CdTe Formation and Dissolution in Ammoniacal Basic Aqueous Electrolytes

Tanaka

et al. 2005

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“…Recently, we have reported that a crystalline CdTe film can be electrodeposited from ammoniacal alkaline baths with pH 10.7. [8][9][10] In the present study, the effects of (i) cathode potential, i.e., cathode overpotential, and (ii) bath composition of Cd(II) and Te(IV) species were investigated in terms of the morphology and stoichiometry of the resulting films, as well as the deposition current density. Experimental Ammoniacal aqueous electrolytes containing CdSO 4 , TeO 2 , NH 3 , and (NH 4 ) 2 SO 4 , where the NH 3 /NH 4 ϩ molar ratio was four, were employed for the CdTe electrodeposition; the composition of the electrolytes used in this study is summarized in Table I.…”

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confidence: 99%

Electrodeposition of CdTe Films from Ammoniacal Alkaline Aqueous Solution at Low Cathodic Overpotentials

Uchida

et al. 1999

Cathodic electrodeposition of CdTe films was studied using aqueous ammonia-alkaline electrolytic baths (pH 10.7; temperature 343 K) in which Cd(II) and Te(IV) species were dissolved to form Cd(NH 3 ) 4 2ϩ and TeO 3 2Ϫ ions, respectively. From the solution, 60 mM Cd(II)Ϫ10 mM Te(IV)Ϫ4.0 M NH 3 Ϫ1.0 M NH 4 ϩ (M ϭ mol dm Ϫ3 ), a flat and smooth polycrystalline CdTe film (thickness, ca. 1 m) with nearly stoichiometric composition was deposited at a constant cathode potential, ranging from Ϫ0.70 to Ϫ0.30 V vs. SHE, whereas dendrite CdTe accompanying elemental cadmium was obtained at Ϫ0.80 V. The deposition behavior was fully explained by an underpotential deposition mechanism taking the calculated redox potentials of Te 0 /Te IV O 3 2Ϫ and Cd 0 /Cd II (NH 3 ) 4 2ϩ pairs into consideration. During electrodeposition of nearly stoichiometric crystalline CdTe, the current density was decreasing monotonously.

“…Although aqueous sulfate electrolytes with pH 1-2 have historically been studied for CdTe electrodeposition, we have proposed that ammoniacal basic aqueous electrolytes are also suitable, because basic solutions have a relatively high solubility of Te͑IV͒ species. [4][5][6][7][8][9] From the ammoniacal basic electrolytes, we successfully obtained smooth and flat polycrystalline CdTe deposits with a nearly stoichiometric composition at potentials positive to that of bulk-Cd deposition. 6 Furthermore, it turned out that the deposition rate was considerably increased by photoirradiation of the cathode surface during the electrodeposition.…”

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confidence: 99%

Electrical Properties of CdTe Layers Electrodeposited from Ammoniacal Basic Electrolytes

Miyake

et al. 2003

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The electrical properties of polycrystalline CdTe layers electrodeposited from ammoniacal basic electrolytes were examined by resistivity and Hall effect measurements. As-prepared CdTe layers electrodeposited in the dark and under illumination were both found to be p-type with resistivity of 10 7 -10 9 ⍀ cm, carrier density of 10 9 -10 11 cm Ϫ3 , and mobility ca. 1 cm 2 V Ϫ1 s Ϫ1 . By decreasing the Cd͑II͒ concentration of the electrolytes, the Cd content of the CdTe layer slightly decreased and the carrier density increased correspondingly, whereas deposition potential did not affect the electrical properties. The temperature dependence of the resistivity revealed that the activation energy of conductivity was 0.67 eV, which suggested that the small carrier density was attributed to a compensation effect of semiconducting CdTe compound.