Design of a vapor transport deposition process for thin film materials

Abstract

A vapor transport process for continuous deposition of elemental and compound thin film materials is presented. The process saturates a carrier gas with a vapor from a subliming source. The saturated mixture is directed over a substrate at lower temperature, resulting in a supersaturation condition and subsequent film growth. The process geometry, comprising the dimensions of the saturation and deposition zones, carrier gas pressure and flow rate, and saturation zone temperature are determined by calculating worst-case characteristic times and simply insuring that the residence time of the carrier gas sufficiently exceeds these times. A model was used to design a system, which is currently being used to deposit 1–10μm thick CdTe films on a 10×10cm2 translating substrate. The process produces film thickness uniformity to within ±5% in the translation direction and across the deposition zone, with a material utilization of 50%. Linear translation speed of 12.5cm∕min has been demonstrated in depositing a 4.5μm CdTe film. The vapor transport process has also been used to deposit CdxZn1−xTe alloy films over a wide range of compositions by addition of ZnTe to the source. Photovoltaic conversion efficiencies of >13% for CdTe and >12% for CdxZn1−xTe have been achieved by devices fabricated from vapor transport deposited films deposited on to moving CdS coated substrates. Refinements are suggested for commercial-scale deposition.

Material utilisation when depositing CdTe layers by inline AP-MOCVD

“…During production of the absorber layer in CdS/CdTe thin film solar cells, the estimated material utilisation for VTD processes and sputtering/evaporation ranges from 35% to 70% [3] and 50% to 75% [4], respectively; however the requirement for ultrahigh purity (exceeding 5 N purity) source materials tends to keep the materials costs high [2,5]. Commercially available CdTe modules from First Solar (i.e.…”

How the Chlorine Treatment and the Stoichiometry Influences the Grain Boundary Passivation in Polycrystalline CdTe Thin Films

“…The partial vapor pressure of the liquid hydrocarbon depends on the temperature of the vaporizer since it is determined by the liquid-vapor equilibrium. Moreover, a gas of the FLHY family, with partial pressure between 1ˆ10 4 Pa and 4ˆ10 4 Pa, can be mixed with a chlorinated one. An inert gas such as Ar can be added to this mixture of hydrocarbons with partial pressure ranging from 1ˆ10 4 Pa to 4ˆ10 4 Pa in order to reach a maximum total pressure of 5ˆ10 4 Pa. After the CdTe treatment, the gases that exit the quartz ampoule are composed of unreacted species and decomposition fragments.…”

“…Moreover, a gas of the FLHY family, with partial pressure between 1ˆ10 4 Pa and 4ˆ10 4 Pa, can be mixed with a chlorinated one. An inert gas such as Ar can be added to this mixture of hydrocarbons with partial pressure ranging from 1ˆ10 4 Pa to 4ˆ10 4 Pa in order to reach a maximum total pressure of 5ˆ10 4 Pa. After the CdTe treatment, the gases that exit the quartz ampoule are composed of unreacted species and decomposition fragments. Since these products cannot be released directly into the environment, they must be treated with an abatement process, which releases only the inert transporting gas that can then be recycled.…”

Thin‐Film Solar Cells and Modules

“…For the VT process, the source and substrate are separated and the Cd and Te 2 species are entrained into a carrier gas and delivered to the substrate. The VT process can be operated over a broad range of pressures, but as the system pressure is increased, the carrier gas fl ow rate must be increased to obtain high CdTe fi lm growth rates since the concentration of Cd and Te 2 species entrained in the delivery gas decreases as the pressure increases [38] .…”