Investigation of Chalcogenide Absorber Materials for Photovoltaic Applications

Abstract

The synthesis of morphologically good thin chalcogenide films via the two-stage route is a chemical challenge. The reactivity towards the chalcogen-bearing reactants of the different metals within the precursor film is a trade-off between thermodynamic driving force and kinetics of binary sulfide formation. In this work, CuSbS2 and Cu3BiS3 thin films were produced by conversion of stacked and co-electroplated metal precursor layers in the presence of elemental sulfur vapour. Ex-situ XRD and SEM/EDS analyses of RTP treated samples were employed to study the reaction sequence and create ‘‘Time-Temperature-Reaction’’ diagrams for the description of the reaction kinetics. Modified Pilling–Bedworth coefficients were introduced for the interpretation of the experimental results. The chalcogenizing conditions have a strong influence on the following aspects: (1) Extent of intermediate phase segregation and/or crystallite size (2) Thermodynamic (de)stabilization at low temperature (3) Thermodynamic (de)stabilization at high temperature The design of a successful synthetic route needs consideration of all these points, so that tailored choices of precursor film configuration and profiles of temperature and reactant partial pressure are made. The synthesis of single crystals of the system Cu-Zn-Sn-S via the Chemical Vapour Transport (CVT) with iodine was investigated. Current knowledge of CVT in multinary systems is limited. A computation of the thermochemistry of the system was performed with the intent to estimate the risk for compositionally incongruent mass transport. Experimental studies reveal no meaningful effect of the iodine pressure employed on the composition of the CZTS products. However, samples obtained under different I2 pressure showed different morphology and had slightly different unit cell sizes. Longitudinally isothermal treatments were carried out with the intent to form large size crystals. Under the investigated conditions, the experiments resulted in the formation of crystals with 2D predominance. A possible explanation for this phenomenon is proposed, based on considerations of the vessel’s thermal conductivity and decomposition/crystallization rate at the steady-state equilibrium.

Date of Award	31 Jul 2012
Original language	English
Awarding Institution	University of Bath
Supervisor	Laurie Peter (Supervisor) & Frank Marken (Supervisor)