In-situ analysis of the drying process in blade-coated perovskite absorber layers for efficient solar cells
Simon Ternes a b c, Tobias Börnhorst c, Jonas A. Schwenzer a, Ihteaz M. Hossain a b, Waldemar Mehlmann c, Philip Scharfer c, Wilhelm Schabel c, Uli Lemmer a b, Bryce S. Richards a b, Ulrich W. Paetzold a b
a Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131 Karlsruhe, Germany
b Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
c Institute of Thermal Process Engineering – Thin Film Technology (TFT), Karlsruhe Institute of Technology, Engelbert Arnold Str. 4, 76131 Karlsruhe, Germany
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Simon Ternes, 137
Publication date: 11th February 2019

As a new class of solution-processable semiconductors with excellent optoelectronic characteristics, organic-inorganic perovskites are intensively investigated by a rapidly growing research community. In particular, the discovery of these perovskites has tremendous impact on the field of emerging photovoltaic technologies. The record power conversion efficiencies of over 23% have however only been demonstrated in perovskite solar cells spin-coated on active areas less than 1 cm². It is in no way trivial to fabricate equally efficient larger scale thin-film solar modules.

The key challenge of scalable solution-based deposition techniques such as blade coating arises from inhomogeneous crystal growth dynamics of the perovskite. As a consequence, rough, inhomogeneous and pinhole-rich perovskite films are typically obtained. In this study, we demonstrate precise drying control and in-situ-analysis of the drying of blade coated perovskite films. While the drying control is obtained by using a temperature-stabilized channel with a laminar flow, in-situ monitoring of the wet film shrinkage is recorded by laser reflectometry. This combined setup allows for understanding the role of key drying parameters i.e. the air- and substrate temperatures, the air velocity and the precursor composition. Eventually, we discriminate between different stages of the drying process and determine their individual impact on the crystal formation.

As compared to a film of pure DMF, the drying dynamics of the perovskite precursor film are strongly decelerated in a drying stage just before the onset of crystallization. The drying rate in this late stage determines the length of the available time period for crystal-growth. The shorter this time period i.e. the sharper the crystallization onset, the more regular are the obtained crystalline thin-films. However, drying at high air speeds is not equivalent to using high temperatures, because higher temperatures accelerate the crystal-growth. Consequently, the product of available crystallization time and crystallization growth rate determines the final layer morphology. Taking advantage of these findings, we are able to blade coat homogeneous, pinhole-free perovskite active layers at all applied environment temperatures (25°C-85°C). By this means, we are able to blade-coat perovskite active layers, irrespective of the needed drying times, yielding high power conversion efficiencies of 19.5% (17.5% stabilized). Notably, these power conversion efficiencies are comparable with the ones obtained by spin-coating the same precursor and fabricating the same solar cell stack, which indicates successful upscaling. Furthermore, we contribute to the state-of-the-art understanding of perovskite thin-film formation by developing a complete model of the perovskite drying process.

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