Sajid N.
Malik
ab,
Abdul Qadeer
Malik
b,
Rana Farhat
Mehmood
c,
Ghulam
Murtaza
a,
Yousef G.
Alghamdi
d and
Mohammad Azad
Malik
*a
aSchools of Chemistry and Materials, The University of Manchester, Oxford Road, Manchester, M13 9PL UK. E-mail: azad.malik@manchester.ac.uk; Fax: +44 (0)161-275-4616; Tel: +44 (0)161-275-465
bSchool of Chemical & Materials Engineering, National University of Sciences and Technology, Islamabad 44000, Pakistan
cInstitute of Chemical Sciences, Bahauddin Zakariya University, Multan 60000, Pakistan
dDepartment of Chemistry, Faculty of Science & Art – Rabigh, King Abdulaziz University, Jeddah, Saudi Arabia
First published on 12th March 2015
[In(iBu2PS2)3] and [Cu(iBu2PS2)(PPh3)2] complexes have been synthesized and used as single source precursors to deposit thin films of cubic In2S3 and Cu2−xS respectively on glass substrates by aerosol-assisted chemical vapor deposition (AACVD) at 350–500 °C. Thin films of CuInS2 have also been deposited by using 1:
1 molar ratio of [In(iBu2PS2)3] and [Cu(iBu2PS2)(PPh3)2]. The deposited thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray diffraction (EDX) techniques. Deposition of films at different temperatures showed significant variation in stoichiometry and microstructure. The CuInS2 thin films were ultrasonicated in toluene along with dodecanthiol for 3 hours to obtain a suspension of CuInS2 nanocrystallites with a diameter of ca. 18 ± 2 nm and a band gap of 1.59 eV.
Copper sulfide thin films have been utilized as photo-absorber in CdS–CuxS solar cells in 1950.3 However, renewed interest of the scientific community in copper sulfide chiefly emerged with the discovery of a range of distinct crystallographic phases and stoichiometric combinations of CuxS (where 1 < x > 2). Other important features of copper sulfide include its lower toxicity as compared with other compound semiconductors and its earth abundant nature. Copper sulfide exhibits bulk band gap energies of ≥1.2 eV and has been used as a p-type semiconductor in many optoelectronic devices like solar cells, gas sensors,4 optical filters, and electro-conductive electrodes.5 Further applications of copper sulfide thin films include microwave shielding, photocatalysis,6 cathode material in lithium batteries,7 water splitting8 and as solar control coatings.9 Furthermore, it has also been investigated as an important bio-imaging,10 photo-thermal therapeutic11 and thermoelectric material.12
Indium sulfide, a wide band gap material of III–VI semiconductor family, has a band gap energy 2–2.3 eV and 2.8 eV. Owing to the stability, optical transparency, high photosensitivity, lower toxicity and photoconductive behaviour, indium sulfide thin films find their use in important technological applications like photocatalysis,13 water splitting14 and solar photovoltaics.15 Solar cells based on β-In2S3 have already shown a power conversion efficiency of 16.4% which is more or less comparable to the CdS based solar devices. Therefore, indium sulfide is being considered as an alternative material to highly toxic CdS.16
Copper indium disulfide (CuInS2) is an important ternary chalcogenide material of I–III–VI family of compound semiconductors. It has a direct band gap value of 1.5 eV which closely matches with the solar spectrum. Furthermore, CuInS2 has an excellent photo-irradiation stability as well as a high absorption coefficient (>105). Therefore, CuInS2 was investigated as the material of choice for thin film solar photovoltaics.17 Solar cells based on its quaternary selenium analogue CuInxGa1−xSe2 have already demonstrated a sub-module power conversion efficiency of 20.4%.18 Theoretical calculations suggest power conversion efficiencies of 27–32% for CuInS2 based solar cells.19 However, the practical efficiencies are limited to only 13% due to recombination losses in the space charge region.20 CuInS2 nanostructures also find other important applications like bio-imaging,21 H2 evolution from water,22 light emitting diodes23 and as a counter electrode material for dye sensitized solar cells.24
A variety of wet and dry methods have been used for deposition of metal sulfide thin films. These methods include chemical bath deposition,25 spin coating,26 chemical spray pyrolysis,27 successive ion layer adsorption and reaction (SILAR),28 physical vapor deposition (PVD),29 atomic layer deposition (ALD),30 modulated flux deposition,31 electrodeposition32 and metal–organic chemical vapor deposition (MOCVD)33 including aerosol assisted chemical vapor deposition (AACVD).34 It has been found that the thin films deposited by different techniques have different phase structure and degree of crystallinity, thus showing variable electrical as well as optoelectronic properties. Precise control over phase structure and stoichiometry of the deposited metal sulfides is still an important research challenge.
Chemical vapor deposition (CVD) offers good control over morphology, stoichiometry and phase structure of the deposited material. Furthermore, this technique has demonstrated an excellent potential for scaling up the film deposition process. Therefore, CVD routes have attracted remarkable research attention and resultantly there has been an ever increasing interest of the scientific community in the design of CVD precursors. In contrast to conventional CVD, AACVD technique circumvents the requirements of volatility and thermal stability of the precursors. Furthermore, judicious design of precursors coupled with careful control of deposition conditions like solvent, precursor concentration, carrier gas flux and deposition temperature can provide high quality thin films for technologically demanding applications. Therefore, our research efforts focus on exploring metal–organic compounds as molecular precursors for deposition of metal chalcogenide nanostructures and thin films.
Dialkyldithiophospinate compounds have, since long, been used as insecticides, additives to lubricants and flotation agent for metals on industrial scale.35 Previously, we have synthesized diisobutyldithiophosphinato-complexes of Cd and Zn and used them as single-source precursor for deposition of CdS and ZnS thin films by LP-MOCVD.36 We have also synthesized dialkyldiselenophosphinato-complexes of In(III) and Ga(III).37 Herein, we report the deposition of thin films of copper sulfide, indium sulfide and their ternary analogue CuInS2 from Ph3P-stabilized diisobutyldithiophosphinato-copper(I) and tris(diisobutyldithiophosphinato)indium(III) complexes by AACVD technique.
A Bruker D8 AXE diffractometer was used to record the p-XRD patterns of the thin films, which were scanned from 20 to 80 degrees with the step size of 0.02. Microstructural details and morphology of the thin films were examined using a Philips XL 30 FEGSEM scanning electron microscope. Carbon coating of the thin films was carried out using an Edwards E-306A coating system before SEM imaging. EDX analyses for determining elemental composition of the thin films were performed using a DX4 instrument.
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Fig. 1 TGA curves showing decomposition behaviour of (a) [In(iBu2PS2)3] and (b) [Cu(iBu2PS2)(PPh3)2] complexes. |
The TGA curve for the copper complex (II) shows that this complex also decomposes in a single step. The decomposition process starts at slightly higher temperature of 296 °C and completes at 371 °C leaving a 22.85% residue which is fairly close to the calculated value for Cu2S (21.96%).
The microstructure of as deposited Cu2−xS thin films was examined using scanning electron microscopy. SEM images of Cu2−xS films deposited at different temperatures are given in Fig. 3. The micrographs clearly show that morphology of the materials deposited at different temperatures is distinctively different. The SEM image of the thin films deposited at 350 °C showed a thick layer of poorly adsorbed material covering a very thin layer of small globular crystallites. Randomly oriented crystallites with multiple facets were observed in the films deposited at 400 °C. Thin film deposited at 450 °C demonstrated growth of similar grains with a small fraction of loose grey material. The SEM image for thin film deposited at 500 °C shows predominant growth of large rhombohedral and triangular crystallites. The crystallites deposited at 500 °C had a size of about 1.0–1.2 μm, whereas thin films deposited at 400 and 450 °C comprised of submicron sized crystallites. Results of the EDX analysis for thin films deposited on various temperatures are summarized in Table 1. Deviation from normal stoichiometric ratio of 2:
1 for copper and sulfur was observed in EDX analyses. Furthermore, variation in stoichiometric composition of individual grains was also indicated by EDX analyses.
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Fig. 3 SEM images of as deposited thin films of Cu2−xS at (a) 350 °C (b) 400 °C and (c) 450 °C and (d) 500 °C. |
Material | Deposition temperature (°C) | EDX analyses results atomic percentage (%) | |||
---|---|---|---|---|---|
Cu | In | S | P | ||
Cu2−xS, digenite | 350 | 61.9377 | — | 38.0623 | |
00-024-0061 | 400 | 62.8425 | — | 37.1575 | |
00-047-1748 | 450 | 62.1039 | — | 37.8961 | |
500 | 63.3231 | — | 36.6769 | ||
In2S3, cubic | 350 | — | 41.3432 | 58.6568 | |
01-084-1385 | 400 | — | 42.4768 | 57.5232 | |
450 | — | 41.9436 | 58.0564 | ||
500 | — | 43.0286 | 56.9714 | ||
CuInS2, roquesite | 350 | 24.9663 | 25.6206 | 46.4954 | 2.9177 |
00-027-0159 | 400 | 26.1627 | 26.5356 | 47.3017 | |
450 | 26.2351 | 26.4152 | 47.3497 | ||
500 | 26.2051 | 26.4460 | 47.3489 |
Optical band gap of the film deposited at 450 °C was determined by extrapolating the linear portion of the (αhν)2vs. hν curve to the hν axis, where (αhν)2 = 0, and was found to be ∼1.82 eV. The reported band gap values for copper sulfide thin films vary from 1.67 eV to 1.99 eV depending upon the technique used for deposition of thin films.11
Surface morphology and microstructure of the as-deposited In2S3 thin films were studied using SEM imaging. The SEM images reveal homogenous coverage of substrates by the deposited material at all the deposition temperatures. However, remarkable differences in morphology of the deposited material were evident in the SEM images of films deposited at different temperatures (Fig. 5).
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Fig. 5 SEM images of as deposited In2S3 thin films at (a) 350 °C (b) 400 °C (c) 450 °C and (d) 500 °C. |
The In2S3 films deposited at 350 and 400 °C consist of very small grains of undefined shape with no distinguishable grain boundaries. Thin film deposited at 450 °C showed cubic and rectangular grains evolving from smaller crystallites. Highly crystalline grains of cubic structure and well defined grain boundaries were observed in the film deposited at 500 °C. Such films with smooth and uniform coverage, controlled morphology and good crystallinity might offer fairly good power conversion efficiencies in solar photovoltaic devices. EDX analysis of the as-deposited thin films showed no appreciable differences in the stoichiometric composition of the material deposited at various deposition temperatures. Furthermore, no remarkable variation in composition of the individual grains could be determined within the analytical limits of EDX technique. Band gap energy calculated for In2S3 thin film deposited at 450 °C was found to be 2.15 eV. Literature values for band gap of indium sulfide thin films are 2.0 to 2.4 eV and 2.8 eV depending on their synthesis process and on their composition.41
The crystallographic phase of the deposited material was determined by the p-XRD studies. The powder X-ray diffraction patterns of the thin films deposited at various temperatures are shown in Fig. 6. The p-XRD studies demonstrated that the tetragonal phase of CuInS2 (standard ICDD pattern 00-027-0159) had been deposited at all the deposition temperatures with the material having a preferred orientation along (112) plane. This is typical for ternary copper chalcopyrite compounds deposited by CVD. Broader peaks for the material deposited at 350 °C reflect smaller size and poor crystallinity of thin films deposited at 350 °C. However, relatively sharp peaks were observed in the p-XRD pattern of thin films deposited at higher temperatures. No additional peaks from binary copper and/or indium sulfide materials were observed.
The SEM images (Fig. 7) show that the films deposited at different temperatures have a remarkably different microstructure. As indicated by the p-XRD studies, the films deposited at 350 °C consist of very small crystallites with the surface covered by loosely bound material of undefined shape.
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Fig. 7 SEM images of as deposited thin films of CuInS2 at (a) 350 °C, (b) 400 °C, (c) 450 °C (d) 500 °C (e) EDX elemental map showing uniform distribution of Cu, In and S in CuInS2 thin film. |
Small flakes forming flower like clusters were observed in the film deposited at 400 °C. The material deposited at 450 °C comprised of randomly oriented flake like structures having a length of ∼10 μm and a diameter of ca. 0.8 μm, uniformly covering the substrate. A more pronounced growth in these flake like structures was observed in the thin film deposited at 500 °C where the flakes have a length ranging from 25–35 μm. EDX analysis (Table 1) confirmed that the deposited material had a stoichiometric ratio of 1:
1
:
2 for Cu, In and S respectively. Elemental mapping (Fig. 7e) confirmed the uniform distribution of the Cu, In and S throughout the film. Furthermore, no notable differences in the stoichiometry of the individual grains could be observed. A small contamination of phosphorus (<3%) was found on the film deposited at 350 °C. No such contamination can be observed on the films deposited at higher temperatures.
The deposition of CuInS2 thin films by CVD using molecular precursors has been reported by Nomura et al. using [Bu2In(SiPr)Cu(S2CNiPr2)] as a single source precursor.42 The p-XRD studies showed the deposition of polyphasic mixtures of either CuInS2 and CuIn5S8 or CuInS2 and In6S7 phases. Hepp and co-workers synthesized a number of complexes as single-source precursors for deposition of ternary copper chalcogenide thin films.43 O'Brien et al. have also reported the deposition of CuInS2 thin films by using asymmetrical dialkyldithiocarbamate complexes as molecular precursors.44 They have also synthesized [(Ph3P)Cu(SC{O}Ph)3In(SC{O}Ph)] complex as single source precursor for CuInS2.45 However, AACVD using this precursor resulted into deposition of β-In2S3 rather than CuInS2. Similarly, Shim and co-workers have deposited CuInS2 thin films by MOCVD using bis(ethylisobutylacetato)copper(II) and tris(N,N-ethylbutyldithiocarbamato)indium(III) precursors.46
Most of these reports either involve a precursor which is difficult to synthesise or the deposited materials were a mixture of binary and ternary phases. Furthermore, deposition of CuInS2 thin films sometimes involves an intricate two-stage deposition methodology whereby copper film is deposited first, followed by deposition of indium sulfide and subsequent reaction of binary phases to form ternary CuInS2, as in case of Shim et al.46 The current work presents a simple, straightforward, non-vacuum and scalable method for the deposition of good quality, phase pure CuInS2 thin films at moderate temperatures from easily synthesized, air stable and environmentally benign molecular complexes. Post deposition treatments like annealing may further improve the microstructure, crystallinity as well as overall quality of as deposited thin films, as reported by Kelly and co-workers.47
The nanoparticles thus obtained were washed thrice with ethanol through centrifugation to remove extra dodecanthiol and dispersed in toluene. No agglomeration was observed in the nanoparticles upon storage for 1 week. p-XRD studies revealed that the diffraction pattern (Fig. 8a) recorded for thus obtained nanoparticles corresponds to the tetragonal CuInS2 (standard ICDD pattern 00-027-0159) having I2d space group. Scherrer formula was used to calculate the mean diameter of the crystallites which was found to be 18.2 nm.
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Fig. 8 (a) p-XRD (b) TEM (c) HR-TEM and (d) SAED pattern of as prepared CuInS2 nanoink. Figure in inset shows the size distribution histogram of CuInS2 nanoparticles. |
The TEM images recorded for as obtained nano-ink are shown in Fig. 8b. TEM microscopy revealed formation of well dispersed quasi-spherical nanoparticles with very little/no agglomeration. The mean diameter of the nanoparticles calculated from TEM image using image analysis software (ImageJ) was found to be 19 ± 2.2 nm which was in fair agreement with the result obtained from Scherrer equation (ca. 18.2 nm). HR-TEM image (Fig. 8c) clearly showed the lattice fringes and inter-planer distance was calculated to be 0.32 nm which corresponds to the d-spacing of the (112) plane of the tetragonal CuInS2 material. Selected area electron diffraction (SAED) pattern (Fig. 8d) indicates that the nanoparticles are polycrystalline in nature and showed three concentric diffraction rings corresponding to (112), (204) and (312) planes of the CuInS2 lattice. Nanoparticles thus obtained showed a band gap of 1.59 eV (Fig. 9) which is slightly higher than that for the bulk CuInS2 material. This may be attributed to nano-size domain of the crystallites obtained in this work.
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Fig. 9 UV-vis-NIR absorption spectrum of CuInS2 (in toluene) suspension of nanoparticles. Figure in inset shows the band gap of 1.59 eV for CuInS2 nanoink. |
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