Two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) have been studied since the 1980s with a perspective of combining properties of both inorganic frameworks and versatile organics towards creating functional hybrid materials. In recent years, 2D OIHPs have showed impressive stability compared to their 3D counter-parts. The efficiency of 2D OIHP solar cells have been dramatically improved via a couple of methods such as hot-casting and using additives due to presumably achieved vertical alignment of the inorganic slabs.
However, most 2D OIHP based solar cells of high efficiency have employed simple aliphatic ammoniums (e.g., butylammonium, C4H9-NH2+, BA) as spacer cations. These aliphatic ammoniums are electrically insulating and not light absorbing. On the other hand, conjugated oligomers would absorb a complementary portion of the solar spectrum to that of the inorganic framework and contribute the charge transport progress. Progress has been made with 2D OIHPs based on spacer cations with single aromatic rings, such as phenethylammonium (C6H5-CH2CH2-NH3+, PEA). Yet, chemical tuning of these spacer cations and its impact on the structure of 2D OIHPs as well as the device performance of related solar cells become an interesting yet under-explored direction.
Our research group (Wei You, University of North Carolina at Chapel Hill) has investigated the effect of chemistry tuning of spacer cations on 2D OIHP solar cell performance.
We show that selectively monofluorinating PEA at different positions of the aromatic moiety can significantly affect the photovoltaic device efficiency of these 2D OIHPs (n = 4). Monofluorination of PEA does not significantly change the molecule size or add additional optoelectronic functionalities. While we observe over 10% photovoltaic efficiency when 3-fluorophenethylammonium (mF1PEA) or 4-fluorophenethylammonium (pF1PEA) is used as the organic cation in 2D OIHP based solar cells, the efficiency of solar cells based on 2-fluorophenethylammonium (oF1PEA) is less than 1%. We find the observed difference in efficiency can be explained by considering three key properties of the 2D OIHP films: phase distribution, surface morphology and crystal orientation. To further understand how the organic cation would affect the structure of these 2D OIHPs, we analyze single crystals of 2D OIHPs (n = 1) with these fluorinated PEA cations and disclose that all crystals have a similar inorganic framework structure, yet very different organic cation packing arrangements. The different packing arrangements and orientational disorder of the spacer cations result in orientational degeneracy and different formation energies, largely explaining the difference in film properties. This work provides key missing information on how spacer cations exert influence on desirable electronic properties and device performance of two-dimensional perovskites via the weak and cooperative interactions of these cations in the crystal lattice.
The related paper has been published in Nature Communications. Please see details:
Jun Hu, Iain W. H. Oswald, Samuel J. Stuard, Masrur Morshed Nahid, Ninhao Zhou, Olivia F. Williams, Zhenkun Guo, Liang Yan, Huamin Hu, Zheng Chen, Xun Xiao, Yun Lin, Zhibin Yang, Jinsong Huang, Andrew M. Moran, Harald Ade, James R. Neilson, Wei You*. Synthetic Control over Orientational Degeneracy of Spacer Cations Enhances Solar Cell Efficiency in Two-Dimensional Perovskites. Nat. Commun. https://doi.org/10.1038/s41467-019-08980-x