2, Indian Institute of Information Technology, Design and Manufacturing, Jabalpur, , India
There is great interest in Pb-free alternatives to hybrid lead perovskites. Replacing of Pb2+ with Sn2+, to form MASnI3, not only removes Pb but also reduces the bandgap to 1.3 eV, leading to a higher Shockley–Queisser limit. Unfortunately, the photovoltaic performance of MASnI3 solar cells is limited to just ~7% because a) the diffusion lengths in Sn perovskite films are much lower than Pb perovskite films, and b) MASnI3 films degrade way too quickly in air due to oxidation of Sn2+ to Sn4+. Both diffusion length and stability are the function of film morphology in terms of crystallite grain sizes. Grain-boundary defects lead to carrier recombination and degradation initiates at the grain boundaries due to permeation of oxygen and moisture through it.
In this work we demonstrate a post-deposition vapor anneal treatment that improves the grain size of MASnI3 films from 100-200 nm to 1-2 microns, one of the highest numbers ever reported for this material. The increase in the film crystallinity was confirmed using XRD, where the FWHM of the films reduced from 0.35° in as-deposited film to 0.32° in annealed film. The improvement in the morphology of the MASnI3 layer is the key to the realization of stable and higher efficient lead-free perovskite solar cells.
Perovskite films were spincoated followed by annealing at 100°C. The as-deposited films showed the tetragonal MASnI3 with primary peak position at 24.7° corresponding to (111) crystallographic plane. SEM images show continuous films with grain-size of just 100-200 nm. For improving the morphology, the MASnI3 films were exposed to Methylamine (MA) vapors for 8-10 seconds. The MA vapors react with MASnI3, forming an optically bleached stable Lewis pair complex of SnI2.xMA at room temperature. The left over methylammonium iodide (MAI) also crystalizes out as confirmed by XRD, leading to dusky white film. To recover the perovskite, the SnI2.xMA complex needs to breakdown. This is accomplished by an annealing step in which temperature is progressively increased from 20°C to 150°C, followed by 1 hour annealing at 150°C. During annealing at 150°C, the complex breaks off to SnI2 and volatile MA. The SnI2 reacts with the crystallized MAI already in the film, forming MASnI3 film which is black in color. XRD confirms that the annealed film is phase-pure tetragonal MASnI3, with lower FWHM than as-deposited films with primary peak at 14.1° corresponding to (001) plane. SEM confirms that the annealed films have larger grains of 1-2 um. Annealing at lower temperatures resulted in partial conversion of SnI2.xMA complex, whereas annealing at higher temperatures lead to the degradation of the perovskite film into SnI2. The slow ramping of the temperature is also important because it allows precise control on the nucleation and grain-growth of the MASnI3 films. The work demonstrates that careful grain-growth can enable large grain size even in Sn-perovskites, which hitherto has been a challenge.