Solution-based fabrication of photovoltaic cell
An ink for forming CIGS photovoltaic cell active layers is disclosed along with methods for making the ink, methods for making the active layers and a solar cell made with the active layer. The ink contains a mixture of nanoparticles of elements of groups IB, IIIA and (optionally) VIA. The particles are in a desired particle size range of between about 1 nm and about 500 nm in diameter, where a majority of the mass of the particles comprises particles ranging in size from no more than about 40% above or below an average particle size or, if the average particle size is less than about 5 nanometers, from no more than about 2 nanometers above or below the average particle size. The use of such ink avoids the need to expose the material to an H 2 Se gas during the construction of a photovoltaic cell and allows more uniform melting during film annealing, more uniform intermixing of nanoparticles, and allows higher quality absorber films to be formed.
1. A method for fabricating a liquid containing intermixed elements of groups IB and IIIA, and VIA, comprising the steps of:
forming non-oxide nanoparticles containing elements from group IB; IIIA; and VIA;
intermixing the non-oxide nanoparticles; and
mixing the non-oxide nanoparticles to form a liquid that serves as an ink,
wherein forming non-oxide nanoparticles includes a reaction of the type:
(IB)(Hal)+(IIIA)(Hal)+TOP(VIA)+TOPO→IB-IIIA-VIA,
where IB refers to an element of group IB, IIIA refers to an element of group IIIA, VIA refers to an element of group VIA and Hal refers to a halogen.
2. The method of claim 1 wherein the halogen is selected from the group consisting of: iodine (I) and chlorine (Cl).
3. The method of claim 1 wherein forming non-oxide nanoparticles includes a reaction of the type:
CuCl+InCl3(+GaI3)+TOPSe(S)+TOPO→Cu(Ga,In)Se(S)2.
4. The method of claim 1 wherein the size distribution of the particles is adjusted by controlling the reaction conditions under which the nanoparticles are made.
5. The method of claim 1 wherein trioctylphosphine oxide (TOPO) is heated to about 100° C., followed by degassing and flushing with nitrogen.
6. The method of claim 1 wherein an equimolar solution of InCl 3 and CuCl in Trioctylphosphine (TOP) is injected into hot TOPO at 100° C.
7. The method of claim 1 wherein TOPO turns bright yellow after injection, and the temperature drops to 80° C. and wherein the reaction proceeds for about 30 min to one hour, after which the temperature is increased to 250° C., and then trioctylphospine selenide (TOPSe) is injected into the TOPO solution (which comprises a mixture of Se powder and TOP) is stirred at room temperature for a time sufficient to give a clear solution of TOPSe.
8. The method of claim 1 wherein there is an initial drop in temperature to 220° C., after which the reaction stabilizes at about 250° C. and proceeds for 24-36 h.
9. The method of claim 1 wherein the reaction mixture is then cooled to about 60° C. or less, followed by the addition of excess methanol to flocculate the particles.
10. The method of claim 1 wherein there is no immediate visible precipitation on the addition of methanol.
11. The method of claim 1 wherein TOPO-capped CuInSe 2 nanoparticles are mixed with water and an optional dispersant in an aqueous liquid to form a CIGS ink.
12. The method of claim 1 wherein a ratio of nanoparticles to water in weight percentage is between about 10% and about 60%.
13. The method of claim 1 wherein a ratio of nanoparticles to water in weight percentage is about 40%.
14. The method of claim 1 wherein after coating a substrate with a film of the ink, the TOPO is decomposed by heating.
15. The method of claim 14 wherein at the same time, the heating is sufficient such that CuInSe 2 nanoparticles anneal together to form a polycrystalline film.
16. The method of claim 14 further comprising heating is sufficient such that CuInSe 2 nanoparticles anneal together to form a polycrystalline film.
17. The method of claim 1 wherein the non-oxide nanoparticles are added to a solvent to form a suspension.
18. The method of claim 1 wherein the non-oxide nanoparticles are added to a solvent to form a colloidal suspension.
19. The method of claim 14 further comprising depositing the liquid ink on a substrate; and
heating the liquid ink in one or more steps to form a photovoltaic absorber layer.