Copper indium gallium selenide (CIGS) thin film solar cells are now in production for energy conversion devices. This cell provides a high absorption coefficient, high temperature stability, and is low-cost compared to other collection materials. A CIGS solar cell consists of several layers of materials, with Cu(In1Ga)Se2 as the energy absorber layer sandwiched between a substrate, electrical contacts, and a buffer layer. The substrate may be made of glass or made of a polymer material that provides flexibility.
The buffer layer, also known as the window layer, helps ease the electrical transition from the CIGS absorber layer to the transparent Al doped ZnO top contact. The buffer is made of a material that provides a band gap layer between the absorber and the electrical transmitter. In the world of semiconductors, “band gap” is the difference in energy between the valence band and the conduction band of a solid material.
CIGS solar cells have traditionally used cadmium sulfide (CdS) as the buffer layer. However, this material is highly toxic, especially if inhaled, and cadmium compounds are classified as carcinogenic. Owing to this problem, solar cell developers are exploring alternative materials.
Oxide-based wide-band gap materials are attractive for an extensive range of applications, such as solar collector cells, functional coatings, (opto)electronic devices, or sensors. Zinc oxide can be doped with certain elements to enhance the intrinsic n-type conductivity of the semiconductor, or with divalent elements such as Mg, which increase or decrease the band gap due to differences in the crystalline structures such as zincite- or periclase-type structures, or nanorods.
In a recent technical poster presentation, our scientists demonstrated how layers of Zn1-XMgX can be applied as a potential substitute for CdS in CIGS solar cells. Although there are several techniques for depositing this buffer material, studies have shown that spray pyrolysis is a relatively safe and effective method of layering Zn1-XMgX films.
Spray pyrolysis is a versatile solution-based growth technique that yields high-quality ZnO films. This method is easy to upscale, making it suitable for large area depositions. In spray pyrolysis, a precursor solution containing volatile metal salts forms a mist of droplets that are directed onto a heated substrate.
Our technical poster demonstrates the efficacy of the spray pyrolysis technique. It shows how X-ray diffraction using a grazing-incidence sampling accessory (GIXRD) for thin film characterizations, and the complementary EDXRF technique, are capable of characterizing layers within a CIGS solar cell stack. We show how EDXRF will determine layer thicknesses in the CIGS stack and how GIXRD measures Zn1-XMgX layers with varying Mg concentrations in order to characterize the crystalline growth of the material as the quantity of Mg varies. The poster additionally looks at the growth of films influenced by varying concentrations of actinium compounds (ZnAc and HAc) in the spray pyrolysis application. These studies will enable CIGS film and other solar collection device developers to find safer, cheaper and more efficient materials for emerging green technologies.
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