Chemical Bath Deposition, also called Chemical Solution Deposition and CBD, is a method of thin-film deposition (solids forming from a solution or gas), using an aqueous precursor solution. Chemical Bath Deposition typically forms films using heterogeneous nucleation (deposition or adsorption of aqueous ions onto a solid substrate), to form homogeneous thin films of metal chalcogenides (mostly oxides, sulfides, and selenides) and many less common ionic compounds. Chemical Bath Deposition produces films reliably, using a simple process with little infrastructure, at low temperature (<100˚C), and at low cost. Furthermore, Chemical Bath Deposition can be employed for large-area batch processing or continuous deposition. Films produced by CBD are often used in semiconductors, photovoltaic cells, and supercapacitors, and there is increasing interest in using Chemical Bath Deposition to create nanomaterials.
Uses
Chemical Bath Deposition is useful in industrial applications because it is extremely cheap, simple, and reliable compared to other methods of thin-film deposition, requiring only aqueous solution at (relatively) low temperatures and minimal infrastructure. The Chemical Bath Deposition process can easily be scaled up to large-area batch processing or continuous deposition.
Chemical Bath Deposition forms small crystals, which are less useful for semiconductors than the larger crystals created by other methods of thin-film deposition but are more useful for nano materials. However, films formed by Chemical Bath Deposition often have better photovoltaic properties (band electron gap) than films of the same substance formed by other methods.
Historical Uses
Chemical Bath Deposition has a long history but until recently was an uncommon method of thin-film deposition.
In 1865, Justus Liebig published an article describing the use of Chemical Bath Deposition to silver mirrors (to affix a reflective layer of silver to the back of glass to form a mirror), though in the modern day electroplating and vacuum deposition are more common.
Around WWII, lead sulfide (PbS) and lead selenide (PbSe) CBD films are thought to have been used in infrared detectors. These films are photoconductive when formed by Chemical Bath Deposition.
Chemical Bath Deposition has a long history in forming thin films used in semiconductors as well. However the small size of deposited crystals is not ideal for semiconductors and Chemical Bath Deposition is rarely used to manufacture semiconductors in the modern day.
Photovoltaics
Photovoltaic cells are the most common use of films deposited by Chemical Bath Deposition because many films have better photovoltaic properties when deposited via CBD than when deposited by other methods. This is because thin films formed by Chemical Bath Deposition exhibit greater size quantization, and therefore smaller crystals and a greater optical band gap, than thin films formed by other methods. These improved photovoltaic properties are why Cadmium Sulfide (CdS), a thin film common in photovoltaic cells, is the substance most commonly deposited by CBD and the substance most commonly investigated in CBD research papers.
Chemical Bath Deposition is also used to deposit buffer layers in photovoltaic cells because CBD does not damage the substrate.
Optics
Chemical Bath Deposition films can be made to absorb certain wavelengths and reflect or transmit others as desired. This is because films formed by Chemical Bath Deposition have an electronic bandgap which can be precisely controlled. This selective transmission can be used for anti-reflection and anti-dazzling coatings, solar thermal applications, optical filters, polarizers, total reflectors, etc. The films deposited by Chemical Bath Deposition have possible applications in anti-reflection, anti-dazzling, thermal control widow coatings, optical filters, total reflectors, poultry protection and warming coatings, light emitting diodes, solar cell fabrication and varistors.