ALD Buffer Layer Growth and Interface Formation on Cu(In,Ga)Se2 Solar Cell Absorbers

Jan Sterner
Abstract & Cover

Cu(ln,Ga)Se, (CIGS) thin film solar cells contain a thin layer of CdS. To avoid toxic heavy-metal-containing waste in the module production the development of a cadmium-free buffer layer is desirable. This thesis considers alternative Cd-free buffer materials deposited by Atomic Layer Deposition (ALD). Conditions of the CIGS surface necesvury for ALD growth are investigated and the heterojunction interface is characterized by band alignment studies of ZnO/CIGS and In,S3/CIGS interfaces. The thesis also includes investigations on the surface modification of the CIGS absorber by sulfurization. According to ALD theory the growth process is limited by surface saturated reactions. The ALD growth on CIGS substrates shows nucleation failure and generally suffers from surface contaminations of the CIGS layer. The grade of growth disturbance varies for different ALD precursors. The presence of surface contaminants is related to the substrate age and sodium content. Improved growth belavior is demonstrated by different pretreatment procedures. The alignment of the energy bands in the buffer/absorber interface is an important parameter for minimization of the losses in a solar cell. The valence band and conduction band offsets was determined by in situ X-ray and UV photoelectron spectroscopy during layer by layer formation of buffer material. The conduction band offset (AEA) should be small but positive for optimal solar cell electrical performance according to theory. The conduction band offset was determined for the ALD ZnO/CIGS interface (AE = -0.2 eV) and the ALD In,SICIGS interface (AEA = -0.25 eV). A high temperature process for bandgap grading and a low temperature process for surface passivation by post deposition sulfurization in H,S were investigated. It is concluded that the high temperature sulfurization of Culno.spazSe, leads to phase separation when x>0. The low temperature process did not result in enhanced device performance. 

Source of Information
Mats Boman
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics
(Uppsala, Sweden)
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