(Nanowerk Spotlight) There is currently a tremendous amount of interest in the solution processing of inorganic materials. Low cost, large area deposition of inorganic materials could revolutionize the fabrication of solar cells, LEDs, and photodetectors. The use of inorganic nanocrystals to form these structures is an attractive route as the ligand shell that surrounds the inorganic core allows them to be manipulated and deposited using organic solvents.
The most common methods currently used for film formation are spin coating and dip coating, which provide uniform thin films but limit the geometry of the substrate used in the process. The same nanocrystal solutions used in these procedures can also be sprayed using an airbrush, enabling larger areas and multiple substrates to be covered much more rapidly.
The trade-off is the roughness and uniformity of the film, both of which can be substantially higher. Reporting their findings in a recent online edition of ACS Applied Materials & Interfaces (“Inorganic Photovoltaic Devices Fabricated Using Nanocrystal Spray Deposition”), researchers have now attempted to quantify these differences for a single-layer solar cell structure, and found the main difference to be a reduction in the open circuit voltage of the device. SEM images of the top surface of the deposited films following deposition and sintering, showing (a) CdTe spin coated and (b) CdTe spray coated. The scale bar in both images represents 200 nm. (Reprinted with permission from American Chemical Society)
“Our work was motivated by a desire to coat larger substrate areas more efficiently,” Edward Foos, a research scientists in the Materials Synthesis and Processing Section of the Chemistry Division at the Naval Research Laboratory, and first author of the paper, tells Nanowerk. “Our initial work indicated that if the layers were thick enough to cover the substrate completely and avoid pinhole formation that would lead to shorting of the device, then the increased surface roughness might be tolerable.”
He adds that this is the first time the impact of this surface roughness on the performance characteristics has been directly compared for these types of devices.
The team prepared single-layer Schottky-barrier solar cells using spray deposition of inorganic (CdTe) nanocrystals with an airbrush. The spray deposition results in a rougher film morphology that manifests itself as a 2 orders of magnitude higher saturation current density compared to spin coating. “We’re currently working to improve the spray coating process to improve the layer uniformity,” says Foos. “If the surface roughness can be reduced, then the overall device performance should increase.” The team is confident that further optimization of the spray process to reduce this surface roughness and limit the Voc suppression should be possible and eventually lead to comparable performances between the two deposition techniques. “Importantly” Foos points out, “the spray-coating process enables larger areas to be covered more efficiently, reducing waste of the active layer components, while enabling deposition on asymmetric substrates.
These advantages should be of substantial interest as inorganic nanocrystal-based solar cells become increasingly competitive as third-generation devices.” The team’s next step will be the fabrication of more complex device architectures that incorporate multiple solution processed layers. These structures will have an even smaller tolerance for variation. In addition, the deposition chemistry used must not interfere with the material applied in the previous step.
By Michael Berger. Copyright © Nanowerk