CdTe film

Aqueous cathodic electrodeposition has been shown to offer a low-cost route for the fabrication of large surface n-CdS/p-CdTe solar cells. In a typical procedure, CdTe films, 1-2 xm thick, are electrodeposited from common acidic tellurite bath over a thin window layer of a CdS-coated substrate under potential-controlled conditions. The as-deposited CdTe films are stoichiometric, exhibit strong preferential (111) orientation, and have n-type conductivity (doping density typically... 

Peter and Wang [266] invented a channel flow cell for rapid growth of CdTe films they showed that 2 p,m Aims can be deposited in less than 20 min, as opposed to the 2-3 h normally required in the conventional stirred single batch cells. The as-deposited films were structurally more disordered than the conventional ones, but after annealing and type conversion they became suitable for fabrication of efficient solar cells. A test cell with an AMI.5 efficiency approaching 6% was fabricated using a film prepared in the channel cell. 

Takahashi M, Uosaki K, Kita H (1984) Composition and electronic properties of electro-chemically deposited CdTe films. J Appl Phys 55 3879-3881... 

Murase K, Uchida H, Hirato T, Awakura YJ (1999) Electrodeposition of CdTe films from ammoniacal alkaline aqueous solution at low cathodic overpotentials. J Electrochem Soc 146 531-536... 

Duffy NW, Peter LM, Wang RL, Lane DW, Rogers KD (2000) Electrodeposition and characterisation of CdTe films for solar ceU applications. Electrochim Acta 45 3355-3365 Duffy NW, Peter LM, Wang RL (2002) Characterisation of CdS/CdTe heterojunctions by photocurrent spectroscopy and electrolyte electroreflectance/absorbance spectroscopy (EEA/EER). J Electroanal Chem 532 207-214 (see also references therein). 

An early attempt for ordered growth of a chalcogenide simple compound has been the cathodic deposition of thin (3 p,m) CdTe films on n-type (100) GaAs single crystals from an acidic aqueous electrolyte at 95 °C, which contained Cd(II) and Te traces generated electrolytically in situ by using a pure Te anode [4]. The... 

Lincot D, Kampmann A, Mokili B, Vedel J, Cortes R, Froment M (1995) Epitaxial electrodeposition of CdTe films on InP from aqueous solutions Role of a chemically deposited CdS intermediate layer. Appl Phys Lett 67 2355-2357... 

Photovoltaic response parameters for electrodeposited (polycrystalline) CdTe thin film electrodes in sulfide-polysulfide or alkaline sodium telluride PEC have been reported, primarily with no reference to the stability of the cells [100], In view of the instability of CdTe in aqueous solutions, Bhattacharya and Rajeshwar [101] employed two methods for the characterization of their electrodeposited CdTe-based PEC. In the first one, a coating of Pb02 (-100 nm thick) was deposited on the CdTe film surface by electroless deposition, and the coated films... 

Zinc telluride, ZnTe, was deposited on quartz, silicon, InAs, and GaSb substrates using Zn[TeSi(SiMe3)3]2 at temperatures between 250 °C and 350 °C. On InAs (orientation not specified) a cubic ZnTe layer was obtained. Problems of stoichiometry are encountered at temperatures below 325 °C because decomposition of the precursor is incomplete, while at higher temperatures (above 350 °C) the deposited ZnTe decomposes into Zn (which evaporates) and involatile elemental tellurium which remains. The results with the analogous cadmium precursor (1.4 torr, 290 °C) indicate that the CdTe films may be of better stoichiometry than those of ZnTe, with XRD results indicating that on a Si substrate the hexagonal phase is predominantly... 

Insufficient rinsing can also result in some codeposition if the previous reactant is not fully removed. The main drawback is the possibility of 3-D growth, which can be hard to identify with very thin deposits. Alternatively, the rinse solution may not be important. Some high quality CdTe films were formed in this group without using a separate rinse solution. That is, the reactant solutions were exchanged by each other, under potential control, suggesting some small amount of codeposition probably did occur. 

Cadmium acetate and sodium tellurite solutions have similarly been used to grow CdTe films by SILAR. The films were polycrystalline (hexagonal), and the grain size was on the order of 22nm (film thickness 272nm). The optical band gap was 1.41 eV.85... 

FIG. 28. XRD patterns for CdTe films grown with 200 cycles as a function of the Cd deposition potential. Omega had been optimized for increased surface sensitivity in each case. 

Figure 3. Spot-type ED pattern from the mosaic CdTe film on the mica substrate.

CdTe The mechanism of CdTe electrodeposition was studied intensively [194-197]. Electrodeposition of CdTe semiconductor thin films was carried out in acidic aqueous solutions at pH 1-3 [195-197]. To prevent accumulation of tellurium in CdTe film, the concentration of Te(IV) ions in acidic electrolyte must be much lower than the concentration of Cd(II). 

The electrodeposition of CdTe films from various ammonia-alkaline solutions at low cathodic overpotential [199,200] was also studied, and the mechanism of CdTe formation was discussed [201, 202]. The properties of films were electrochemically characterized. 

CdTe, a II—VI compound semiconductor with a direct band gap of 1.44 eV at room temperature, is, from its physical properties, a promising photovoltaic material. The electrodeposition of CdTe in ionic liquid was published recently by Sun et al. [38]. They were able to show that the semiconductor can be electrodeposited at elevated temperature (above 120 °C) in the Lewis basic l-ethyl-3-methylimidazolium chloride/tetrafluoroborate ionic liquid containing CdCh and TeCU. CdTe films were obtained by the underpotential deposition (UPD) of Cd on the deposited Te. The deposit composition was independent of the deposition potential within the Cd UPD regime. The crystallinity of the deposits is improved by increasing the deposition temperature, which again demonstrates the high potential of the wide thermal windows of ionic liquids for compound electrodeposition. 

Figure 6.22 Angle dependence of the reflectance at a wavelength of 532 nm for various substrate and absorber configurations, (a) Planar and a microporous Xi02 film on SnOi/glass at vertical hght incidence the specular reflectance peak disappears in the microporous films and a cosine reflectance distribution with no specular feature is obtained (b) When a thin CdTe film is deposited on the structured Xi02, the overall...

A numerical simulation of this cell based on a one-dimensional model has been carried out by Ernst (2001), Grasso et al. (2002) and by Burgelman and Grasso (2004). In the work of Ernst and Grasso et al., the spectral response data could be simulated with reasonable accuracy using only a few adjustable parameters. These simulations confirm the electron diffusion length in the p-type CdTe films to be approximately 150 nm. The recombination centre density was found to be lO cm . These data indicate that the nanocrystalline CdTe films are of inferior quality than the material used in the conventional, planar CdTe solar cells, where diffusion lengths of 2 //m and defect densities of lO cm are typical. 

Works on chemical deposition of tellurides are very limited, despite in principle the expectation that deposition should be similar to the sulfide or selenide systems. As indicated by Bode [3], telluride precusors (such as tellurourea) are very unstable which renders the deposition very difficult to achieve. Deposition has been reported with using a different process [94, 95]. Telluride precursor is introduced as dissolved Te02 (TeO in basic solutions) in the solution containing complexed metallic ions. Upon the addition of hydrazine, the reduction to Te (-II) can be slowly achieved, leading to the formation of the metallic telluride as for PbTe [95]. More recently CdTe films have been prepared this way [94]. This process can be considered as an extension of the electroless process used for the deposition of metals [2]. It is probable that other routes similar to the selenosulfite route for selenides are possible for tellurides too, but have not yet been investigated in depth. 

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