Multichannel Imaging and In Situ Process Monitoring for Vacuum-Assisted Drying of Inkjet-Printed and Blade-Coated Perovskite Thin Films

With the rapid progress of laboratory-scale high performance metal halide perovskite-based photovoltaics, light emitting diodes, photo- and x-ray detectors over the past decade, the swift development of scalable methods for high throughput and large area production is pivotal for advancing the commercialization. Real-time process monitoring systems are of particular importance to control the drying, nucleation and crystallization of solution-processed perovskite thin-films, <i>e.g.</i>, by inkjet printing or blade coating. In this work, we present an <i>in-situ</i> monitoring technique for vacuum-assisted annealing of solution-processed perovskite thin-films, which is a suitable method to achieve high quality perovskite thin-films. We use a multichannel imaging system to monitor the evolution of the drying, the nucleation and the crystal formation of perovskite thin-films on areas &gt;10 cm² with sub-second resolution. The <i>in-situ</i> imaging setup allows determining the reflectance and photoluminescence (PL) image filtered through three different spectral filters, which allows determining the peak emission wavelength. Moreover, the PL is spectrally observed on a local spot to support the peak emission wavelength imaging data. The influence of key parameters of the vacuum-assisted annealing process, as well as the perovskite precursor solution, the underlying substrate properties and subsequent hot-plate annealing is investigated. Exemplary, the system identifies (i) morphological differences like surface roughness originating, <i>e.g.</i>, from a slow evacuation rate or a short evacuation time in the reflectance channel, (ii) finds a difference in slope steepness of the initial PL peak shift for lead halide-rich or -poor precursor solutions or (iii) can estimate the film thickness from the PL on-set. The drying and perovskite formation of a wide variety of perovskite material compositions including such with low cesium content is observed, for which additionally to the vacuum a heat source is needed for a crystallization to a preferred photoactive perovskite phase. As reference system, a cesium-, formamidinium-, lead halide-based perovskite precursor ink is chosen and deposited <i>via</i> inkjet printing and blade coating. For these wet-films, correlations between the monitored space- and time-resolved PL and reflectance channels and the morphology and photovoltaic device performance are obtained. Besides obvious layer defects detected by spatial analysis, a delayed or premature PL on-set or different temporal PL intensity evolution shapes on individual spots identified by time-resolved PL analysis can differentiate between perovskite solar cell performances up to 18% power conversion efficiency and below. These analysis methods allow to prescreen the deposited thin-films for their suitability for the next step in the a production line towards a complete solar cell stack and thus increase the production yield.