Annealing effect on CdS films: transition from glass to ITO

Structural and optical properties of CdS films deposited on glass and ITO substrates and annealed in H2, N2 or H2+N2 atmospheres at 250 °C and 400 °C were compared. The optical thickness 405±10 nm of the CdS films was found not to be influenced by any annealing conditions or substrate. The as deposited films on both substrates had similar diffraction patterns with one narrow peak corresponding to the (111) cubic plane at 2θ 26.70° and 26.75°, respectively. Higher density of nucleation centres on the ITO surface generated denser CdS films. In the annealing process of CdS/ITO a slower shift of both (111) peak and lattice constant was observed in the direction of pure zinc blende modification. Similar values of band gap and transmittance were determined for as-deposited CdS while annealing at 400 °C decreased transmittance by 5% and 16% for CdS on glass and ITO, respectively. Transmittance dropped because of the destruction of hydroxide group in the CdS lattice, the formation of cadmium excess and the reduction of SnO2 to black SnO in the ITO structure.


Introduction
CdTe-and CIGS-based photovoltaic solar cells in a superstrate configuration are made on glass substrates coated by transparent conductive oxide (TCO) like tin oxide (SnO 2 ) or indium tin oxide (ITO) over which CdS layer is deposited. Combined TCO-CdS window layers [1][2][3][4] and the effect of these bilayers on the final photovoltaic response [5] had been studied by many authors. Usually TCOcoated glass substrates with suitable photovoltaic quality are commercially available, while the optimization of CdS thin films remains in progress at laboratory scale.
As an optimization route for CdS thin film, annealing in different atmospheres at certain timepressure-temperature conditions may be used. In [6] we studied the properties of CdS/glass thin films after H 2 annealing. A relationship between annealing conditions and desirable properties of CdS on glass was obtained: temperatures up to 400 o C had a weak influence on the morphological and optical properties; 1h annealing was enough to reach stable electrical properties; CdS films annealed at 250 -400 o C had resistivity in the region of 2.3·10 -2 -6.1 Ω·cm, respectively. It is interesting to verify if the knowledge gained about the properties of annealed CdS/glass may be applied for the annealed CdS/ITO/glass structures. Similar studies have shown the influence of annealing in air [7] or nitrogen [8] on CdS properties when deposited on different substrates such as glass, ITO, SnO 2 or other materials. However, information about CdS/ITO/glass bilayers annealed in H 2 atmosphere is scarce. This paper compares the optical, structural and electrical properties of CdS layers deposited on glass and ITO substrates and annealed in H 2 , N 2 and H 2 +N 2 gases.

Experiment
Polycrystalline CdS films were deposited by chemical bath deposition (CBD) on both uncoated sodalime glass and commercial ITO coated glass substrates. The plates were accurately cleaned and immersed in the deposition solution. The temperature and agitation speed of the solution were 85 o C and 500 rpm, respectively. One deposition lasted for 30 minutes, but for thicker CdS films the process was repeated three times. After deposition the films were vacuum dried at 120 o C to remove most of the secondary phases of water, hydroxides and organic impurities. This drying was the last stage of preparation for the so-called as-deposited CdS layers. We choose the thermal treatment conditions based on our investigation on H 2 annealing of CdS/glass: normal pressure, 1 h duration at 250 o C and 400 o C. For comparison, annealing in N 2 and H 2 +N 2 (H 2 concentration of was 5%) was also applied.
Crystallographic investigations were performed using the X-ray diffraction (XRD) technique in the Bragg-Brentano (θ-2θ) geometry by a Rigaku Ultima IV diffractometer with Cu-Kα radiation. Crystallite size, lattice constant and interplanar distance were computed by the PDXL software (Version 1.4.0.3) on the Rigaku system. The optical characteristics were evaluated at room temperature in the range of 200-2500 nm on the Jasco V-670 UV-VIS spectrophotometer equipped with an integrating sphere. Total optical transmission and reflection spectra were used to determine the band gap and optical thickness of CdS.

Results and discussions
Optical properties were analyzed on the basis of transmittance spectra and E g values shown in figure  1a and table 1, respectively.  Almost similar changes of band gap were found for CdS on glass and ITO substrates, independent of the annealing atmosphere. When the annealing temperature increased, the band gap of CdS/glass gradually decreased for all three atmospheres from 2.45 eV (as-deposited CdS) to 2.35 eV (annealed at 400 o C). With ITO the E g of CdS reached a constant value (2.375 eV) for N 2 -containing ambients at both annealing temperatures (table 1).
We estimated the integral transmittance in the region of 500-1000 nm as the area under the T plot (figure 1b). With a glass substrate a similar trend of CdS transmittance was observed for all annealing gases -a slow decrease at 250 o C followed by an abrupt decrease at 400 o C (~5 %). However, the lowest transmittance was emphasized for a CdS sample annealed in N 2 . The decreased transmitance of CdS could be explained by hydroxide group incorporated into the CdS lattice the destruction of which starts at 300 o C [6]. Independent of the annealing atmosphere, the hydroxide group incorporated into the CdS lattice leads to the formation of CdS 1-n n(OH) S solid alloy. When temperature is applied, H 2 O evaporates and Cd precipitate [9] is formed (1, 2).
When CdS/glass was annealed, the Cd precipitate (2) decreased the transmittance of CdS (figure 1b). However, in the presence of H 2 the CdO decomposes, as oxygen is eliminated by the release of H 2 O, and CdS band gap increases [6]. Therefore, the CdS transmittance was higher after the H 2 annealing than that of N 2 one. This reveals an advantage of H 2 annealing for CdS films.
For a CdS film on ITO, the transmittance decreased faster up to 250 o C than with the glass case. Thus, the contribution of Cd precipitate (2) to the decomposition of SnO 2 should be taken into account (3,4). Both products of reaction (3) resulted in the reduced transmittance of the CdS/ITO bilayer.
SnO Sn SnO   (4) Still, when CdS/ITO was annealed at 400 o C in H 2 , the transmittance decreased by 16% compared to as-deposited CdS. A new reaction (5) may compete with (3) due to higher activity of the H 2 compared to the metallic Cd from CdS. These processes resulted in the blackening of the ITO substrate and an abrupt drop of CdS transmittance.
The transmittance behavior of CdS on both glass and ITO substrates suggested that H 2 +N 2 mixture instead of pure H 2 or N 2 gases is needed for annealing.
The structural properties of CdS were analyzed based on XRD measurement results. Similar XRD patterns with one narrow (111) diffraction peak of the cubic plane at 26.70° and 26.75° were found for CdS films on glass and ITO substrates, respectively. Figure 2 shows that both peaks are in the close proximity of the (002) hexagonal plane at 26.83°. Twice higher (111) peak of CdS/ITO than that of CdS/glass suggests a more oriented CdS film. This is attributed to the strong influence of the oriented crystalline surface of ITO. The high density of nucleation centers on ITO generates a denser CdS film, which might also explain the decreased transmittance of CdS on ITO (figure1b).   Interestingly for both substrates, the main (111) peak shifted in the direction of the zinc blende CdS plane at 26.46° in all three ambients when the annealing temperature increased ( figure 3). However, a slower shift of the main cubic peak and lattice constant (table 1) in the direction of zinc blende modification of CdS was found in the case of the ITO substrate during annealing. The shift of this main peak infers that the stressed lattice of as-deposited CdS relaxed during annealing at 250 -400 o C, after which a more pure cubic CdS phase was obtained. The comparison between the three annealing atmospheres in the case of glass substrate shows that the H 2 ambient contributed to a more cubic structure of CdS, while the H 2 +N 2 ambient had a weak influence on the CdS lattice. In the case of the ITO substrate no significant differences were found when CdS was annealed at higher temperatures.

Conclusions
An abrupt reduction of transmittance was found for CdS/ITO annealed in H 2 at 400 o C. This might limit the applicability of CBD CdS for superstrate solar cells. We minimized the transmittance by annealing CdS/ITO in a mixture of H 2 + N 2 . The oriented ITO contributes to denser CdS films. The typical shift of the main (111) peak is characteristic for CdS on both glass and ITO. Generally, the structural and optical properties of CdS films on glass and ITO are quite similar.