hide
Free keywords:
-
Abstract:
Hybrid inorganic-organic systems have a potential in the optimization and development of material functionalities. In hybrid solar cells, the separation of the electron-hole pairs after photogeneration is a key aspect of the light-harvesting functionality. This requires, for instance, the use of a molecular system in which the excited states have a sufficiently long lifetime to allow charge separation. We study terrylene molecules in combination with a monolayer WS2 sample as a model of a hybrid inorganic-organic system (HIOS). Transition metal dichalcogenides (TMDCs) are known for their rich optoelectronic properties. The TMDCs offer better carrier mobility, which is complementary to the properties of organic materials.
To investigate the terrylene molecules, two types of time-resolved spectroscopy techniques are used. These are the time-correlated single photon counting (TCSPC), and the broadband transient absorption (TA) spectroscopy, using a white light continuum probe. The spectra of the steady state absorption peaked at 2.2 eV, and the emission of the monomers of terrylene in a xylene-based solution show mirrored lineshapes. Both the TCSPC and the TA consistently show a decay of 3.7 ns for the excited state S1. The thin terrylene film is characterized by a strong (> 1 eV) blue-shift, due to H-aggregation. Photoexcitation of the H-aggregate, using UV light at 4.3 eV, leads to the formation of an induced absorption band around the monomer line position, and which is identified as the signature of charge-transfer (CT) interactions in the system. A phenomenological fit model allows for the disentanglement of the participating elementary processes. The fit helps to determine the contribution of the ground state aggregate, but also the determination of a contribution from a hot aggregate, which is red-shifted by 200 meV with respect to the ground state lines. The dynamics induced in the thin film are on the order of 100 ps. Structural fluctuations play a critical role in the response of the thin terrylene film. The study of terrylene in an n-heptane solvent reveals the formation of a UV band around 3.8 eV, due also to aggregation effects. The results obtained during the measurement of terrylene underline the impact of the geometry adopted by the molecular interactions in the condensed phase. Understanding and controlling terrylene-based systems can be beneficial for applications.
The linear absorption response of the hybrid sample displays a spectrum that is marked on the UV side, by the presence of H-bands similar to those observed in the thin terrylene film. The visible side of the spectrum is marked by the presence of the bright excitons, A, B, and C, characteristic of the WS2 system. In addition to these absorption lines, corresponding to the individual materials constituting the HIOS, a hybrid state is observed at 2.6 eV. To determine the properties of the excited states, broadband transient absorption measurements are performed, by selectively exciting the A exciton at 2 eV, in the WS2 part, and the terrylene aggregate at 4.3 eV. In general, the TA features measured in WS2 are also observed in the TA response of the hybrid sample. The TA response measured with the visible laser light pump indicates, distinctly, the presence of a ground state bleach (GSB) signal associated with the hybrid state. The decay time of the hybrid state is determined to be 200 ps. The response measured with the UV pump indicates that the photoexcitation of the H-aggregate is followed by an ultrafast charge transfer process towards the WS2. The presence of the carriers induces a broadening of the lines. Understanding the charge separation processes is important for light harvesting applications. The hybrid material is useful in the exploitation of new functionalities, resulting from the interactions at the interface. The system can be investigated further to determine the performances which are achievable in a real-life device.
