Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)
DOI: https://doi.org/10.29363/nanoge.hopv.2023.186
Publication date: 30th March 2023
Surfaces and grain boundaries (GBs) are likely the sources of energy losses and degradation in metal-halide perovskites (MHPs). Efficient perovskite solar cells (PSCs) make use of surface passivation layers to improve energy alignment, enhance stability and reduce trap states on MHP surfaces [1], while the local GB structures are more difficult to control. Still, PSCs reach excellent power conversion efficiencies of >25% despite the presence of GBs in their polycrystalline MHP absorber [2]. To date, our understanding of GBs is still far behind, and the literature remains quite controversial. On the one hand, GBs appear benign on charge transport and light absorption but, on the other hand, come along with enhanced defect densities and structural instabilities [3]. To gain control over GBs in MHPs, we must derive a deeper understanding of their structural dynamics and electronic properties under finite temperatures.
We performed ab initio molecular dynamics (AIMD) to shed light on the structural dynamics and electronic properties at the grain boundaries of MHPs using cesium lead iodide, CsPbI3, as our model system [4]. We observe halide-driven structural healing of the GB due to a facile migration of iodine ions in GBs as a response to the lattice strain in the GB region. Density functional theory (DFT) calculations reveal a substantial reduction of hole trap states upon healing of the GB, while shallow electron trap states form by strain-induced Pb–Pb dimers in the GB, likely being a source of open-circuit voltage losses by enhanced non-radiative recombination. On a longer timescale, our AIMD simulations reveal the spontaneous formation of iodine Frenkel defects near lead-iodine-rich GB regions. DFT calculations show a gradual reduction of the defect formation energy from bulk (1 eV) > GB(0.5 eV) > surface (0.1 eV). Overall, our simulations reveal a competition between moderate impact on the electronic properties by structural healing and a detrimental impact on the point defect densities, both connected to the facile migration of iodine ions in GBs. Based on these insights on an atomistic resolution, tailored passivation strategies are derived to mitigate detrimental defect formation at GBs.
The Ministero dell'Istruzione dell'Universit`a e della Ricerca (MIUR) and Universit`a degli Studi di Perugia are acknowledged for financial support through the program “Dipartimenti di Eccellenza 2018–2022” (Grant AMIS) to F. D. A. W. K, E.M and F. D. A. further acknowledge funding by the PON project “Tecnologia per celle solari bifacciali ad alta Efficienza a 4 terminali per utility scale” (BEST-4U) of the Italian Ministry MIUR (CUP B88D1900016005).
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