Designer materials: Combining the strengths of hybrid halide perovskites and organic semiconductors
We report efficient charge separation and conduction in 2D hybrid perovskite nanoplatelets by incorporating an electron acceptor chromophore. The concept used combines the properties of perovskites and organic semiconductors to design functional 2D perovskites for built-in specific applications.
Two-dimensional (2D) hybrid organic-inorganic perovskites are an emerging class of semiconductor materials related to the highly promising for solar cell application 3D perovskites. 2D perovskites differ to their 3D counterpart by the presence of large organic cations that improve the stability and expand the possibilities to tune their properties for a broad range of opto-electronic devices apart from solar cells, such as light-emitting diodes, photo-detectors, spintronics, waveguides, nano-lasers and photo-catalysis.
An established view in the hybrid perovskites field is that the optoelectronic properties are mostly determined by the inorganic component, as the valence and conduction bands are formed by the electronic orbitals of the inorganic elements, while the organic component indirectly affect the properties. However, we noticed that in most 2D perovskites the large organic cation was an alkylammonium (mostly butyl) or phenyl-alkylammonium, which lack specific functionality and affect the properties only by influencing the structure rigidity of the inorganic octahedral lattice (Fig. 1a). Coming from the organic electronics field, this felt to us like an open opportunity to tune the optoelectronic properties as through organic synthesis a wide range of functionalities is accessible. Furthermore, it is known that the elements that can form the inorganic octahedral layer are limited, while a wide variety of organic compounds can be stacked in between inorganic layers. We wondered “what if we could modify the optoelectronic properties of 2D hybrid perovskites by organic molecular design in order to built-in specific applications?”.
In 2D perovskites with non-functional organic compounds, the electronic charges (electrons and holes) are limited to the inorganic octahedral layer where they strongly interact through electrostatic interaction between the positive and negative charge (~350 meV), resulting in a strongly bound charge pair at room temperature. This property is desirable for light-emitting applications but not for solar cells and photo-catalyzers were charge separation is required. Our first idea consisted of trying to induce charge separation by introducing functionally strong electron acceptor or donor organic chromophores (such as perylene diimides, naphthalene diimides, perylene, etc.) commonly used in organic solar cells (Fig. 1b). We tested this idea by performing density functional theory (DFT) calculations finding that these large organic chromophores can be introduced to form stable 2D perovskites in which the electronic band structure is also formed by orbitals from the organic component, in principle this would be the result of charge transfer.1 However, reality is not as straightforward as computer calculations. Synthesizing these materials turned out to be very challenging.2–4 We tried many chromophores and synthetically modified them to improve their solubility in the solvents used for the inorganic elements. Nevertheless, we were not able to periodically stack the organic chromophores in between the inorganic octahedral layers. With a little inspiration from the quantum dots field, we changed the approach to overcome the stacking constraint by replacing the attached organic ligands used to stabilize colloidal 2D perovskite nanoplatelets in solution with our functional organic chromophores (Fig. 1c). This way we could create an experimental model system to study if charge transfer would occur in this material.
This idea is the basis of our recent work at the Delft University of Technology, in which we attached organic peryline diimide (PDI) chromophores to 2D CsPbBr3 nanoplatelets observing striking changes in the optoelectronic properties and charge carrier transport due to charge transfer in the material. We determined by transient absorption (TA) experiments charge transfer efficiencies of ~50% from both the nanoplatelets to the PDI (electron transfer) as well as from the PDI to the nanoplatelets (hole transfer) by selectively exciting each compound (Fig. 2a). The charge transfer is unequivocally proven by the appearance of the characteristic absorption feature of the PDI chromophore when it has accepted an electron (PDI anion) at 760 nm (Fig. 2b). In addition, we demonstrated that the charges formed in this way live long enough to be useful for application in devices such as solar cells (Fig. 2c, microsecond lifetimes). Further work should aim to optimize the charge transfer efficiency by varying the energy levels using different nanoplatelets thicknesses and organic chromophores, beside designing organic chromophores that would strongly interact to couple their orbitals and conduct.
Our most important finding is that we have shown that it is possible to design specific functionality in hybrid perovskites by combining the inorganic lattice with well-designed organic chromophores. This concept is not limited to charge separation but can also be extended to, for example, the use of chiral molecules for circularly polarized light detection or singlet fission/up-conversion chromophores to enhance light absorption.
For more details, please see our article “Overcoming the exciton binding energy in two-dimensional perovskite nanoplatelets by attachment of conjugated organic chromophores” published in Nature Communications.
Gélvez-Rueda, M.C., Fridriksson, M.B., Dubey, R.K., Jager, W.F, van der Stam, W., Grozema, F.C. Overcoming the exciton binding energy in two-dimensional perovskite nanoplatelets by attachment of conjugated organic chromophores. Nature Communications (2020). https://doi.org/10.1038/s41467-020-15869-7.
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4. Gélvez-Rueda, M. C. et al. Inducing Charge Separation in Solid-State Two-Dimensional Hybrid Perovskites through the Incorporation of Organic Charge-Transfer Complexes. J. Phys. Chem. Lett. 11, 824–830 (2020).