Researchers from Washington University recently described a novel technique to fabricate perovskite solar cells, using an aerosol-based technique called electrospray deposition.
In the first step, PbI2 is deposited onto a TiO2-coated, fluorine-doped tin oxide glass substrate by spin coating to form a nearly uniform, yellow coating. Next, a solution of methyl ammonium iodide (MAI) is electrosprayed by pumping it through a capillary needle at a high voltage and generating monodispersed charged droplets in Taylor cone-jet mode. The charged droplets travel in the electric field toward the grounded substrate and the solvent from these droplets evaporates before reaching the substrate. The dry MAI nanoparticles then react with the PbI2 layer to form the dark brown colored perovskite. The perovskite formation can be achieved in 40 minutes with optimized MAI concentration, flow rate, and substrate-to-needle distance.
The scientists found that controlling the perovskite formation reaction using electrospray deposition can provide a uniform, smooth perovskite surface morphology, which prevents moisture absorption under ambient conditions, greatly enhancing the stability of the device. A rough surface, which arises if a spin-coating method is used, is more hydrophilic because the contact angle with the water droplet is lower.
To test the stability, the perovskite film was intentionally sprayed with water and heated to 100 °C; this caused it to degrade to PbI2. However, the crystalline structure of the perovskite film reformed, changing from yellow back to dark brown. The XRD spectra of the film before and after exposure to water are identical, suggesting that a self-healing mechanism takes place.
Electrospray is a scalable technique that provides precise control over the amount of material required for deposition, reducing significant material loss that occurs in conventional solution-based methods.
The team stated that this work shows that stability of perovskite solar cells can be improved by fabrication using a well controlled and optimized electrospray technique, without the use of any additives or cell encapsulants.