Method of manufacture of sodium doped CIGS/CIGSS absorber layers for high efficiency photovoltaic devices
View Patent ↗A method for processing a thin-film absorber material with enhanced photovoltaic efficiency includes forming a barrier layer on a soda lime glass substrate followed by formation of a stack structure of precursor layers. The method further includes subjecting the soda-lime glass substrate with the stack structure to a thermal treatment process with at least H 2 Se gas species at a temperature above 400° C. to cause formation of an absorber material. By positioning the substrates close together, during the process sodium from an adjoining substrate in the furnace also is incorporated into the absorber layer.
1. A method of processing a thin film absorber material with enhanced photovoltaic efficiency comprising:
providing a soda lime glass substrate having a front surface;
forming a barrier layer over the front surface, the barrier layer preventing diffusion of sodium ions from the soda lime glass substrate;
forming an electrode which includes molybdenum over the barrier layer;
depositing a first mixture of a copper species, gallium species, and sodium species to form a first thickness of a first precursor material over the electrode;
depositing a second mixture of a copper species and a gallium species to form a second thickness of a second precursor material over the first precursor material;
depositing an indium species to form a third thickness of a third precursor material over the second precursor material, to thereby form a stack structure of the first thickness, the second thickness, and the third thickness;
subjecting the soda lime glass substrate and the stack structure to a thermal treatment in the presence of H 2 Se and nitrogen at a temperature above about 400° C. to form an absorber material from interdiffusion of the copper species, the gallium species, the indium species, and the first sodium species; and
while doing so transferring a sodium species from a back surface of a soda lime glass substrate.
2. The method of claim 1 wherein the barrier layer comprises at least one of silicon dioxide, silicon nitride, and silicon oxynitride.
3. The method of claim 1 wherein the step of transferring a sodium species comprises:
reacting a sodium species from the soda lime glass substrate using the H 2 Se species to form an intermediary species of Na 2 Se;
transferring the Na 2 Se to the absorber material; and
absorbing sodium from the Na 2 Se into the absorber material.
4. The method of claim 3 wherein the sodium has a concentration of 10 18 -10 19 atoms/cc in the absorber material.
5. The method of claim 3 wherein the back surface of a soda lime glass substrate comprises a back surface of another soda lime glass substrate being subjected to the thermal treatment.
6. The method of claim 3 wherein the intermediary species of Na 2 Se is a gas phase species.
7. The method of claim 1 wherein the step of transferring a sodium species from a back surface of a soda lime glass substrate increases conversion efficiency of a resulting photovoltaic cell using the absorber material.
8. The method of claim 7 wherein the absorber material includes from 0.7 wt % to 1.5 wt % of sodium.
9. The method of claim 7 wherein the absorber material comprises an atomic ratio of copper species to indium species added to gallium species of about 0.9.
10. The method of claim 1 wherein the step of subjecting the soda lime glass substrate and the stack structure comprises heating to a temperature from about 400° C. to about 600° C.
11. A method of incorporating sodium species into thin-film photovoltaic absorber for enhancing solar energy conversion efficiency, the method comprising:
providing a first soda lime glass substrate having a front surface region and a back surface region;
forming a bottom electrode material over the front surface region;
forming a stack of precursor materials including copper, indium, gallium, and sodium, over the electrode material at a first temperature range;
disposing the first soda lime glass substrate having the stack in a furnace containing H 2 Se gas and nitrogen gas;
disposing at least a second soda lime glass substrate next to the first soda lime glass substrate; and
heating the first soda lime glass substrate and the second soda lime glass substrate to a second temperature range to cause the precursor materials to react with selenium from the H 2 Se gas to form a photovoltaic absorber having sodium from both the precursor materials and from the adjoining second soda lime glass substrate.
12. The method of claim 11 wherein the first soda lime glass substrate comprises soda lime flat glass with about 14 wt % of Na 2 O.
13. The method of claim 11 wherein sodium from the adjoining second soda lime glass substrate comprises a gas phase intermediary species of Na 2 Se formed when sodium ions from Na 2 O diffuse out of the second soda lime glass substrate and react with selenium during the thermal process.
14. The method of claim 13 wherein the second soda lime glass substrate is one of a plurality of soda lime glass substrates, each having a stack of the precursor materials formed on the front surface of each substrate, and the substrates are disposed in a parallel arrangement in a furnace.
15. The method of claim 13 wherein the first temperature range comprises a temperature range from about 20° C. to about 100° C.
16. The method of claim 13 wherein the second temperature range comprises a temperature from about 400° C. to about 550° C.
17. The method of claim 13 wherein the photovoltaic absorber material comprises a copper-indium-gallium-selenium compound having sodium of about 0.7 to 1.5 at %.