Preparation of Cu(In,Ga)Se2 thin films by pulse electrodeposition

doi:10.1016/j.jallcom.2010.12.031

Journal of Alloys and Compounds

Volume 509, Issue 8, 24 February 2011, Pages L129-L133

https://doi.org/10.1016/j.jallcom.2010.12.031 Get rights and content

Abstract

Cu(In,Ga)Se₂ thin films were prepared from aqueous solution by pulse electrodeposition. It was found that the co-deposition of the species occurred under a 3D growth with instantaneous nucleation. The morphology of the pulse-electrodeposited film can be improved by adjusting the duty cycle. The significant loss of indium and reduction of In–Se compound(s) accordingly were observed with decrease of duty cycle. Chalcopyrite structure Cu(In,Ga)Se₂ films with p-type behavior and enhancement in crystallinity were obtained after annealing treatment in Ar atmosphere.

Introduction

Cu(In, Ga)Se₂ (CIGS) is a very promising semiconductor material for solar cell [1], [2] and photoelectrochemical hydrogen production application [3]. Thin film CIGS solar cell devices, fabricated from a multistep physical vapor deposition (PVD) process, have demonstrated a record conversion efficiency of 20.3% [4]. The PVD technology is excellent for good quality film growth, but difficult to scale up. There is thus great interest in developing low cost non-vacuum-based techniques such as electrodeposition [5]. Several studies on electrodeposition of CIGS thin film have been reported [6], [7], [8], and these studies have mainly focused on the potentiostatic electrodeposition technique using direct current (DC) mode. There are very few research that attempts to achieve pulse electrochemical growth of CIGS films [9] but fails to incorporate gallium into the films during electrodeposition, even if ternary CuInSe₂ thin film prepared by pulse electrodeposition have been reported [10], [11], [12]. Pulsed electrodeposition is an advanced form of electrodeposition, which offers better control over deposit properties by controlling the interfacial electrochemical reaction for the formation of CIGS. In contrast to the DC electrodeposition where only potential or current can be controlled, in pulse electrodeposition, a number of variables like pulse waveform, cathodic/anodic pulses, on/off pulse time or duty cycle θ, and applied potential, etc. [13] can be independently varied and offer effective ways to control properties such as microstructure, adhesion, composition, crystallinity, optical and electrical properties. On the other hand, the study of the mechanism of CIGS electrodeposition is very insufficient [14], especially the nucleation and growth mechanism of CIGS during deposition has not been reported to our knowledge, although an understanding of CIGS nucleation and growth mechanism will help in producing CIGS deposits with desirable properties.

In this work, Cu(In,Ga)Se₂ thin films were pulse electrodeposited from acidic solution. The preliminary results of investigation of the electrochemical nucleation and growth mechanism by cyclic voltammetry and chronoamperometry, as well as film composition, morphology and structure properties were presented.

Section snippets

Experimental

The electrochemical analyses, including cyclic voltammetric (CV) and chronoamperometric (CA) measurements, and electrodeposition of Cu–In–Ga–Se films were performed in a typical three-electrode cell where the working electrode was Mo/glass(Mo film thickness ≈0.7 μm and active area = 1cm × 1 cm), the counter electrode was a platinum plate, and the reference electrode was a saturated calomel electrode (SCE). All potentials are reported with respect to this reference. The electrolyte bath contained 3

Results and discussion

Fig. 1(a) illustrates the typical cyclic voltammogram for co-deposition of quaternary Cu–In–Ga–Se film. As seen in Fig. 1(a), the curve displays two well-defined cathodic peaks, two anodic peaks and a double crossing between the cathodic and anodic branches. The first cathodic peak between −0.30 V and −0.43 V corresponds to the formation of Cu–Se binary compound(s) and the second cathodic peak at about −0.6 V is attributed to the co-deposition of Cu, In, Ga and Se according to the composition of

Conclusions

Cu(In,Ga)Se₂ thin films were pulse-electrodeposited from aqueous solution. The cyclic voltammetric studies show the potential range for Cu, In, Ga and Se co-electrodeposition and the chronoamperometric investigation reveals the instantaneous nucleation and 3D diffusion controlled growth mode for co-electrodeposition. The pulse-electrodeposited films show the best morphology for duty cycle of 67%. With the decrease of duty cycle, significant loss of indium, and reduction of In–Se compound(s)

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Effective control over the relative content of In is achieved during pulse electrodeposition of CIS films previously (Mandati et al., 2013b). Liu et al reported a significant effect on In and In-Se compounds in pulse electrodeposition of CIGS films while concluding a diffusion-controlled growth with instantaneous nucleation mechanism (Liu et al., 2011). Fu et al have demonstrated the use of pulse and pulse-reverse plating for CIGS films by successfully eliminating the Cu2−xSe secondary phase (Fu et al., 2009).
Fabrication of Cu(In,Ga)Se₂ (CIGS) absorber layers containing two-dimensional nano-flake structures using a single stage pulse electrodeposition technique is reported for the first time, wherein CuCl₂, InCl₃, GaCl₃ and H₂SeO₃ are used as precursors in a pH 3 buffer. The method employs tri-sodium citrate as complexing agent. The phenomenon of intrinsic electrochemical dissolution associated with pulse electrodeposition technique is efficiently utilized to obtain CIGS nano-flakes. The presence of tri-sodium citrate and the relaxation time during pulse electrodeposition play crucial role in achieving control over composition and morphology of CIGS films thereby aiding in the formation of nano-flakes. Evolution of nano-flake structures is systematically investigated with the increase in deposition time during pulse electrodeposition. Elemental analysis reveals the stoichiometric composition of nano-flake films while the formation of chalcopyrite phase-pure CIGS is confirmed by XRD and Raman analyses. The bandgap of CIGS nano-flakes is inferred to be about 1.21 eV from Tauc’s plot. Mott-Schottky studies unveil the p-type conductivity of the CIGS with a flat-band potential and carrier density values of −0.15 V and 5.2 × 10¹⁶ cm⁻³, respectively. Photoelectrochemical characterization of CIGS films affirms their photoactivity and the photoresponse is almost 20 times compared to the traditional planar CIGS films. Nanostructured CIGS films fabricated by low-cost pulse electrodeposition method reduce materials consumption while promising excellent photoresponse and are suitable for photovoltaic and photoelectrochemical applications.
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This, in turn, results in greater deposit homogeneity as well as precise control over stoichiometry and deposition rate. During the deposition of thin films of ternary/quaternary systems like CIS/CIGS, such control over the composition of individual elements is particularly crucial to obtain single-phase by controlling the formation of secondary phases like Cu-Se, In-Se, Ga-Se, etc., thereby making PC electrodeposition an attractive technique [26,27]. Furthermore, by varying the duty cycle, reduction in porosity can be achieved by avoiding entrapment of hydrogen during deposition and results in a highly dense and compact CIGS films with enhanced photoelectrochemical performance [18,19].
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LetterPreparation of Cu(In,Ga)Se2 thin films by pulse electrodeposition

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

Sol. Energy Mater. Sol. Cells

Thin Solid Films

Sol. Energy Mater. Sol. Cells

J. Alloys Compd.

Electrochem. Commun.

Sol. Energy Mater. Sol. Cells

Electrochim. Acta

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

Thin Solid Films

Letter
Preparation of Cu(In,Ga)Se₂ thin films by pulse electrodeposition