Tetracene Ultrathin Film Growth on Hydrogen-Passivated Silicon

Abstract

Inorganic−organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), an SF molecule, on hydrogen-passivated Si( 111) ] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and Xray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost upright-standing molecules. The molecular arrangement is very similar to the Tc bulk phase, with only a slightly higher average angle between the conjugated molecular plane normal and the surface normal (α) around 77°. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H−Si(111) and a-Si:H substrates as well as for (sub)mono-to several-monolayer coverages. Annealing to room temperature, however, changes the film structure toward a smaller α of about 63°. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wave functions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

Change in Tetracene Polymorphism Facilitates Triplet Transfer in Singlet Fission-Sensitized Silicon Solar Cells

“…A recent report by Niederhausen et al that deployed near-edge X-ray absorption fine structure (NEXAFS), XPS, and density functional theory (DFT) calculations also suggests that different orientations at the interface exist and could lead to a change in transfer efficiency. 38 …”

“…A recent report by Niederhausen et al that deployed near-edge X-ray absorption fine structure (NEXAFS), XPS, and density functional theory (DFT) calculations also suggests that different orientations at the interface exist and could lead to a change in transfer efficiency. 38 To extract the triplet transfer rate and transfer efficiency, we model the PL decay data considering the singlet fission process, as depicted in Figure 5a. 29,31,39 The singlet fission process in tetracene (with a rate k SF ) competes with the radiative decay (with a rate of k Rad ) through the formation of triplet pairs, which can then dissociate (at a rate of k Diss ) to form free triplets or fuse back (at a rate of k TT ) to create an excited singlet state.…”

Investigation of Singlet Fission–Halide Perovskite Interfaces

“…This is in agreement with other examples of rodlike molecules on the surface of inorganic semiconductors. 26 , 27 , 40 Furthermore, in all cases, we find that the interaction energy is observed to be weak: for comparison, the formation energy for a tetracene film is ∼−2 eV per molecule.…”

“…It has recently become possible to study some simple interfaces with first-principles computational methods. For example, density functional theory (DFT) calculations (corroborated by X-ray diffraction experiments) have helped confirm the geometry of the interface between tetracene and silicon, 26 , 27 where tetracene thin films were found to orient with their long axis perpendicular to the semiconductor surface.…”

Interlayer exciton landscape in WS$_2$/tetracene heterostructures

“…The latter attach to the surface of the TMD via van der Waals bonds and form a molecular lattice, as has been shown in previous studies 26,27 . Tetracene molecules tend to form periodic lattices rather than distribute themselves randomly on the surface [28][29][30][31][32] . Motivated by these findings, we describe the tetracene as a quasi two-dimensional structure characterized by a homo-lumo gap.…”