High-throughput printing of semiconductor precursor layer from nanoflake particles
Methods and devices are provided for transforming non-planar or planar precursor materials in an appropriate vehicle under the appropriate conditions to create dispersions of planar particles with stoichiometric ratios of elements equal to that of the feedstock or precursor materials, even after selective forces settling. In particular, planar particles disperse more easily, form much denser coatings (or form coatings with more interparticle contact area), and anneal into fused, dense films at a lower temperature and/or time than their counterparts made from spherical nanoparticles. These planar particles may be nanoflakes that have a high aspect ratio. The resulting dense films formed from nanoflakes are particularly useful in forming photovoltaic devices.
1. A method comprising:
formulating an ink of particles wherein about 50% or more of the particles are flakes each containing at least one element from group IB, IIIA and/or VIA and having a non-spherical, planar shape, wherein overall amounts of elements from group IB, IIIA and/or VIA contained in the ink are such that the ink has a desired stoichiometric ratio of the elements, wherein the flakes each have a longest dimension between about 5 microns and greater than about 500 nm;
coating a substrate with the ink to form a precursor layer; and
processing the precursor layer in one or more steps to form a dense film.
2. The method of claim 1 wherein the flakes have an aspect ratio of at least about 5 or more.
3. The method of claim 1 wherein the flakes may have an aspect ratio of at least about 10 or more.
4. The method of claim 1 wherein the flakes have an aspect ratio of at least about 15 or more.
5. The method of claim 1 wherein the planar shape of the flakes creates greater surface area contact between adjacent flakes that allows the dense film to form at a lower temperature and/or shorter time as compared to a film made from a precursor layer using an ink of spherical nanoparticles wherein the nanoparticles have a substantially similar material composition and the ink is otherwise substantially identical to the ink of claim 1 .
6. The method of claim 1 wherein the planar shape of the flakes creates greater surface area contact between adjacent flakes that allows the dense film to form at an annealing temperature at least 50 degrees C. less as compared to a film made from a precursor layer using an ink of spherical nanoparticles that is otherwise substantially identical to the ink of claim 1 .
7. The method of claim 1 wherein the planar shape of the flakes creates greater surface area contact between adjacent flakes relative to adjacent spherical nanoparticles and thus promotes increased atomic intermixing as compared to a film made from a precursor layer made from an ink of claim 1 .
8. The method of claim 1 wherein the planar shape of the flakes creates a higher packing density in the precursor layer that forms the dense film as compared to a film made from a precursor layer made from an ink of spherical nanoparticles of the same composition that is otherwise substantially identical to the ink of claim 1 .
9. The method of claim 1 wherein the planar shape of the flakes creates a packing density of at least 76% in the precursor layer.
10. The method of claim 1 wherein the planar shape of the flakes creates a packing density of at least 95% in the precursor layer.
11. The method of claim 3 wherein the planar shape of the flakes results in grain sizes of at least about 2.0 μm in at least one dimension in the semiconductor absorber of a photovoltaic device.
12. The method of claim 3 wherein the planar shape of the flakes results in grain sizes of at least about 1.0 μm in at least one dimension in the semiconductor absorber of a photovoltaic device.
13. The method of claim 1 wherein the planar shape of the flakes provides a material property to allow for a more even distribution of group IIA material throughout in the dense film as compared to a film made from a precursor layer made from an ink of spherical nanoparticles that is otherwise substantially identical to the ink of claim 5 .
14. The method of claim 1 wherein the flakes have length of less than about 2 microns and a thickness of less than 100 nm.
15. The method of claim 1 wherein the flakes have length of less than about 1 microns and a thickness of less than 50 nm.
16. The method of claim 1 wherein the flakes are comprised of a binary alloy of group IB, IIIA elements.
17. The method of claim 1 wherein the flakes are comprised of a ternary alloy of group IB, IIIA elements.
18. The method of claim 1 wherein the flakes are comprised of a quaternary alloy of group IB, IIIA, and/or VIA elements.
19. The method of claim 1 wherein the flakes have between about 5 wt % of oxygen to about 0.1 wt % oxygen.
20. The method of claim 1 wherein the film is formed from a precursor layer of the flakes and a layer of a sodium containing material in contact with the precursor layer and/or nanoflakes containing at least one of the following materials: Cu—Na, In—Na, Ga—Na, Cu—In—Na, Cu—Ga—Na, In—Ga—Na, Na—Se, Cu—Se—Na, In—Se—Na, Ga—Se—Na, Cu—In—Se—Na, Cu—Ga—Se—Na, In—Ga—Se—Na, Cu—In—Ga—Se—Na, Na—S, Cu—S—Na, In—S—Na, Ga—S—Na, Cu—In—S—Na, Cu—Ga—S—Na, In—Ga—S—Na, or Cu—In—Ga—S—Na; and/or an ink containing the nanoflakes and a sodium compound with an organic counter-ion or a sodium compound with an inorganic counter-ion.