In fact, large conjugated oligomers (e.g., tetrathiophene, acenes) have been successfully incorporated into 2D OIHPs, yet only with the n = 1 phase 8, 13, 16. This improved light absorption could potentially increase the current of 2D OIHP based solar cells and further boost their efficiency. Ideally, one would like to replace such aliphatic ammoniums with functional organics, for example, conjugated oligomers that would absorb a complementary portion of the solar spectrum to that of the inorganic framework. However, these aliphatic ammoniums are electrically insulating and not light absorbing. Perhaps due to the easy accessibility of simple aliphatic ammoniums (e.g., butylammonium, C 4H 9-NH 3 +, BA), most 2D OIHP based solar cells of high efficiency have employed them as spacer cations 25, 28, 31. However, 2D OIHP solar cells have recently been demonstrated with significant high efficiency via a couple of methods such as hot-casting 28 and using additives 25, 31, 33, 34, 35, due to presumably achieved vertical alignment of the inorganic slabs. This would significantly hinder the charge transport in the vertical direction and result in a lower efficiency 9, 28, 32. However, these 2D layered perovskites, having the insulating organic cations separating these inorganic slabs, tend to adopt an orientation where these inorganic slabs would be aligned in parallel to the substrate. The lead halide-based 2D OIHPs, which have a general formula of (RNH 3) 2MA n–1Pb nX 3 n+1, have attracted much attention as alternative photovoltaic materials because of their improved stability 24, 25, 26, 27, 28, 29, 30, 31, 32. Given the vast design space for both organic and inorganic frameworks, more “exotic” functions have also been envisioned with 2D OIHPs, including singlet fission, up conversion, among others 8, 13, 18, 22, 23. Depending upon the energy levels and band gaps, these conjugated organics can contribute to light absorption and emission, and/or facilitate charge transfer between the organic and the inorganic framework 13, 14, 15, 16, 18, 19, 20, 21. As to the organics, a variety of conjugated molecule based ammoniums have been incorporated into 2D OIHPs, including oligothiophenes 13, acenes 14, 15, 16, and fullerenes 17. The inorganic framework can empower the 2D OIHPs with desirable properties such as high charge carrier mobility along the sheet-like inorganic framework 4, and the layered structure of such 2D OIHPs enables appreciable tunability in quantum confined properties (e.g., band gap, exciton binding energy) by varying the thickness of the inorganic layer 10, 11, 12. Two-dimensional (2D) organic–inorganic hybrid perovskites (OIHPs), with a perspective of combining properties of both inorganic frameworks and versatile organics towards creating functional hybrid materials, have been studied since the 1980s 1, 2, 3, 4, 5, 6, 7, 8, 9. 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. Here we demonstrate that the selection of spacer cations (i.e., selective fluorination of phenethylammonium) affects the film properties of two-dimensional perovskites, leading to different device performance of two-dimensional perovskite solar cells (average n = 4). Structural analysis reveals that 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. However, how the chemical nature of the organic cations affects the properties of two-dimensional perovskites and devices is rarely reported. Chemical tuning of spacer organic cations has attracted great interest due to their additional functionalities. Two-dimensional perovskites have emerged as more intrinsically stable materials for solar cells.
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