Samantha Kaczaral1,Samuel Schreiber2,Daniel Morales Jr2,Keith White2,Michael Toney1,2,David Moore3,Michael McGehee1,2,3
University of Colorado, Boulder1,University of Colorado Boulder2,National Renewable Energy Laboratory3
Samantha Kaczaral1,Samuel Schreiber2,Daniel Morales Jr2,Keith White2,Michael Toney1,2,David Moore3,Michael McGehee1,2,3
University of Colorado, Boulder1,University of Colorado Boulder2,National Renewable Energy Laboratory3
An automated gas quenching process improves the reproducibility of perovskite solar cells by eliminating processing variables that change over time or from facility to facility. Broad human error can be eliminated by automating the process. Gas quenching, in comparison to antisolvent quenching, prevents the buildup of solvents in the spinning atmosphere. However, a further understanding of how gas quenching parameters can affect perovskite film formation is necessary to develop a robust methodology to create high performing devices. The developed methodology minimizes performance differences between batches of devices and allows for systematic film formation studies.<br/><br/>Gas quenching film formation has previously been studied with respect to the complexing solvent, NMP compared to DMSO. However, our experiments show that film formation changes based on the nitrogen quenching pressure/flowrate. The experiments were done on a 1.73 eV bandgap perovskite with a composition of DMA<sub>0.1</sub>Cs<sub>0.3</sub>FA<sub>0.6</sub>Pb(I<sub>0.8</sub>Br<sub>0.2</sub>)<sub>3</sub>. As the nitrogen quench pressure increases, the homogeneity of the film improves based on XRD analysis. As the gas quench pressure increases from 30 psi to 80 psi, the ratio of peak area for PVK(100):PbI<sub>2</sub>(100) increases from 0.77 to 5.54 respectively. Based on in-situ photoluminescence spectra, PL transients also change as a function of gas quench pressure during the fabrication process. At pressures less than 50 psi, a dual peak is observed in the first 10 seconds of the quench which then homogenizes into a single peak. However, at pressures over 50 psi, only a single peak is observed during the entire duration of film formation.<br/><br/>One explanation of increasing heterogeneity at lower pressures is that certain salts (e.g. PbBr<sub>2</sub>, CsI) nucleate first during the spin process when they cannot complex strongly with DMF/NMP. Excess PbI<sub>2</sub> that remains complexed with NMP increases the final film heterogeneity. As pressure increases, the solvents are removed at a faster rate which decreases the PbI<sub>2</sub> remaining in the solvate phase. In-situ GIWAXS data will be shown to describe the structural evolution of the film as a function of quenching pressure.<br/><br/>Film formation using the gas quenching method is less sensitive to the processing environment, compared to the antisolvent method. Gas quenched films show almost identical PL transients during film formation in a solvent free environment and a DMF saturated environment. Dual PL peaks were present at the same location and duration for both environments and the films ended at the same bandgap post quench. For the antisolvent method in a solvent free environment, dual PL peaks were present throughout the entire duration of the spin. However, in a DMF saturated environment, only a single PL peak was present for the duration of fabrication and the post-spin coat bandgap did not match that of the clean environment. Resiliency against solvent buildup during fabrication further improves the reproducibility of our automated gas quench system.<br/><br/>Upon setting up an automated gas quench system at both CU Boulder and NREL, our first observation is that pressure at the nitrogen source outlet is not a sufficient metric for describing our fabrication parameters. The optimum pressure for gas quenching at NREL is 100 psi; however, that caused the perovskite solution to be blasted off the substrate at CU. Based on this observation, it is necessary to report the flowrate of the nitrogen quench instead of the pressure, since tube diameter, curvature, and length can all result in different flowrates for a given input pressure. Using a systematic procedure, multiple researchers at NREL can obtain over 18% devices across multiple batches. The developed methodology can be applied to other facilities to achieve over 18% efficiency across multiple batches. The automated gas quenching process reproducibly resulted in high performing devices and allowed for systematic film formation studies.