CBD processes

The CBD CdS on CIGS or CIS devices produce superior solar cells relative to solar cells made by dry chemical vapor deposition (CVD) or physical vapor deposition (PVD) of CdS. Several groups15-20 put forward plausible reasons for the superior performance, including selective etching or removal of native oxides by ammonia and also that the CBD process does not cause any physical damage, which could occur during CVD or PVD processes. 

The current state-of-the-art CBD process for CdS consists of cadmium ions, thiourea, and ammonia. The deposition of a uniform CBD CdS thin film occurs because of a chemical reaction of controlled sulfur and cadmium ions in the alkaline solution. The slow release of sulfur from thiourea via decomposition and slow release of Cd2+ ions from CdfNI I3)2 allow the following chemical reactions ... 

In general, the CBD process starts by a quick mixing of stable solutions and proceeds under the influence of temperature control and stirring. The reaction advances as a function of time. The degree of reaction a (the ratio between the amount of substance that has reacted and the total amount able to react) can be expressed by the following equation24 ... 

Most CBD processes start slowly at a specific bath temperature, then accelerate, and eventually slow down again. Nucleation sites appear instantly on the substrates, the moment the solution contacts the substrate. In most cases, it is better to initiate the nucleation sites by inserting the substrate in a metalion solution, instead of a solution mixture containing both cations and anions. When cations and anions are mixed together, the resultant compounds begin to precipitate as soon as the ionic product, also called the solubility product,... 

It is also reported that anions can be preferentially adsorbed in the CBD process, which leads to different levels of impurities. Hashimoto et al.28 compared the iodide and chloride impurities in the CBD CdS thin films when the films were deposited from an aqueous solution of iodides and chlorides, respectively. The CdS films deposited using iodide contained 3% iodine, whereas the CdS grown from chlorides contained less than 1% chlorine. 

In general, CBD processes have been investigated by a sizable group of scientists worldwide. The CBD technique has been used to prepare a wide range of sulfides, selenides, and oxides. The following paragraphs only cite selected papers of interest to the author, primarily relating to the field of... 

Figure 10.2 shows typical distillate composition profiles for close boiling mixtures using a solvent in a CBD column. The CBD process becomes a conventional BED process with the addition of the solvent. Due to the addition of solvent, the components can be separated at high purity using a small column with low reflux ratio. See Safrit and Westerberg (1997) and Low and Sorensen (2002) for unconventional BED processes. 

ZnO, and Zn(OH)2) for the CBD process [261]. Optimal film compositions must be identified before large-scale preparation of these films for solar cell applications because CBD of ZnS is more complex and difficult than that of CdS [262]. Although significant efforts have been undertaken to make CIGS-based cells free ofCd, until now, such cells usually have lower efficiency and less reproducible behavior than their Cd-containing counterparts. The continued investigation of the deposition conditions leading to superior film properties for the CBD of ZnS in basic aqueous ammonia solutions would be worthwhile. 

The chemical bath deposition (CBD) process uses a controlled chemical reaction to achieve the deposition of a thin film by precipitation. CBD has been used to prepare ZnS films (by route C). In a typical experiment, substrates are immersed in an alkaline solution containing the chalcogenide source, the metal ion and added base. A chelating agent is also added, in order to control the hydrolysis of the metal ioa The process rehes on the slow release of ions into an alkaline solution in which the free metal ion is buffered at a low concentration. 

Fig. 3.16 Reaction schemes of different CBD mechanisms for compound semiconductors (a) atom-by-atom process (b) aggregation of colloids and (c) mixed process. (Reprinted from [247], Copyright 2009, with permission from Elsevier)...

Figure 17.27 The EPL process involves a fusion protein containing an intein tag plus a CBD. The fusion protein is captured on an immobilized chitin resin and after removal of contaminating proteins, it is eluted using thiophenol, which cleaves at the thioester bond between the intein and the desired expressed protein. This releases a phenylth-ioester-activated protein that can be used in the native chemical ligation reaction with another peptide containing an N-terminal cysteine residue. Conjugation results in a native amide (peptide) bond formed between them.

Compared with the CBD technique, electrodeposition requires some additional capital equipment (i.e., suitable power supplies and electrodes). Major advantages of the electrodeposition process include the insignificant amount of waste generation. The electrodeposition bath can be reused for an unlimited number of cycle times when salts are replenished in the bath. The major drawback for electrodeposition is that it requires conductive substrates, which limits the application of this process in several key technologies. 

However, a single mixture (binary or multicomponent) can be separated into several products (single separation duty) and multiple mixtures (binary or multicomponent) can be processed, each producing a number of products multiple separation duties) using only one CBD column (Logsdon et al., 1990 Mujtaba and Macchietto, 1996 Sharif et al., 1998). 

Unlike continuous distillation, batch distillation is inherently an unsteady state process. Dynamics in continuous distillation are usually in the form of relatively small upsets from steady state operation, whereas in batch distillation individual species can completely disappear from the column, first from the reboiler (in the case of CBD columns) and then from the entire column. Therefore the model describing a batch column is always dynamic in nature and results in a system of Ordinary Differential Equations (ODEs) or a coupled system of Differential and Algebraic Equations (DAEs) (model types III, IV and V). 

Low and Sorensen (2002) and Low (2003) considered optimal operation of BED processes in CBD and MVC columns using azeotropic mixtures. They used profit as the measure to compare the performances of such columns. For a given separation task, an MVC column was found to be superior to a CBD column. However, for maximum profitability most of the initial feed needed to be charged in... 

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