Browsing by Subject "Rubrene"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Strongly-Bound Excitons In Transition Metal Dichalcogenides And Organic Semiconductors
    (2020-05) Schulzetenberg, Aaron
    Atomically-thin, semiconducting transition metal dichalcogenides (TMDs) and organic semiconductors such as rubrene hold exceptional promise for unique and niche electronic applications which cannot be solved with conventional semiconducting crystalline materials. In particular, the process by which excitons relax in thin TMDs controls device engineering considerations including charge carrier mobility and exciton diffusion length. The decay mechanism and time scales can critically depend on interfaces, method of sample preparation and temperature. Here, I present ultrafast transient reflectivity studies of several chemical vapor deposition (CVD) grown TMD structures, including few-layer 2H MoTe2 on SiO2, MoTe2 1T’-2H homojunctions and monolayer MoS2-WS2 lateral heterojunctions on sapphire. The transient reflectivity of CVD-grown, few-layer (5-10 layers) 2H MoTe2 carried out a both room temperature and cryogenic temperatures demonstrates a temperature and fluence dependence consistent with defect-mediated exciton decay. The optical properties of MoTe2 were additionally found to be stable over the course of 8 months air exposure. The biexponential decay dynamics of monolayer MoS2 and WS2 were shown to be consistent with previous investigations. Both studies of interfaces, including the 2H-1T’ MoTe2 homojunctions and the MoS2-WS2 heterojunction were unable to observe signatures of interfacial charge transfer due to lack of sufficient spatial resolution near the interface crossover. In addition to studies on TMDs, the low-wavenumber Raman modes of both isotopically substituted 13C Rubrene and those of a structural analog to rubrene, fm-rubrene, were measured and compared to native rubrene. The 13C rubrene demonstrated a uniform shift to lower energy intermolecular mode vibrations. The modes of fm-rubrene were characterized for the first time and compared to a predicted computational Raman spectrum showing large (~4%) deviations with theory at low vibrational energies (<200cm-1), suggesting intermolecular coupling becomes influential at this threshold.
  • Thumbnail Image
    Item
    Structural Explorations into Singlet Fission Chromophores for Solar Photochemistry with Femtosecond Stimulated Raman Spectroscopy
    (2020-10) Bera, Kajari
    Understanding physical, chemical or biological changes require monitoring the molecular vibrations aka nuclear coordinates on the timescale of their movements, which is hundreds of femtoseconds. Thus, ultrafast spectroscopic techniques capable of providing molecular conformational changes on the femtosecond timescale are desirable to provide in-depth knowledge of any photophysical process. This thesis discusses the strength of using a structurally sensitive ultrafast spectroscopic technique, femtosecond stimulated Raman spectroscopy (FSRS) to understand the photophysics in a chemical process called singlet fission. Thin films and single crystals which may undergo singlet fission are promising for solar energy conversion strategies as they can generate two charge carriers by the absorption of only one photon. A major bottleneck in the singlet fission field is the challenge of designing or discovering new molecules with enhanced photophysical properties, while not altering other parameters such as crystal packing or solubility which can hinder overall device performance. Chemical modifications to acenes may lead to singlet fission-based photovoltaics with improved solar energy conversion efficiencies, but identifying how molecular structural changes impact the rate and yield of fission is challenging to model and predict. Traditionally, spectroscopic measurements have not provided much insight for rational design of new chromophores, but rather have been used to understand why an existing system works well. To this end, I have used FSRS to examine the excited state structural dynamics in organic chromophores undergoing singlet fission and have provided a predictive model for molecular designing guidelines to obtain efficient singlet fission systems for their use in solar energy based devices. Specifically, I have used ultrafast FSRS to monitor the changes in the molecular structure in rubrene during singlet fission and observed that singlet fission in crystalline rubrene is associated with a loss of electron density in the tetracene backbone. Armed with this knowledge and a hypothesis to improve singlet fission rate by reducing electron density in the tetracene core, I screened rubrene derivatives to study their excited state dynamics using FSRS. From a series of derivatives, I screened two new rubrene derivatives with electron withdrawing substituents, FM-rubrene & F-rubrene, to prime the system for singlet fission, without impacting intermolecular interactions. Using these rationally designed rubrene derivatives I found that both the rate and yield of singlet fission are significantly improved, proving that spectroscopic insight is crucial to successfully designing new chromophores. This shows that the long-held promise of spectroscopy-informed small molecule design for organic optoelectronic materials can be realized in rubrene-based singlet fission materials. As the first graduate student to use FSRS in a microscope to look at crystals, I realized that distinguishing Raman from non-Raman features can be complicated in molecular crystals, where the narrowband transient vibronic couplings due to less heterogeneous broadening can overwhelm the excited state Raman bands. To overcome this, we redesigned the ultrafast FSRS experimental setup to help ease the data extraction and interpretation associated with this technique. By adding a pair of mirrors and a slit to the existing grating filter setup, we generated an additional Raman excitation pulse that identifies Raman peaks from a pool of non-Raman peaks. In this thesis, I present the successful implementation of FSRS to provide unprecedented structural dynamics in organic semiconducting materials with the aim to achieve better performing singlet fission based-photovoltaic devices and the experimental modifications made to support FSRS as a more user-friendly technique for its widespread adoption to understand chemical reactions.