Sangita Baniya Tuesday, May 1, 2018 at 4:00 PM (110 INSCC)
Title: Optical and magneto-optical studies of unconventional semiconductors for optoelectronic applications
This dissertation comprises three separate studies on materials that serve as active layers in organic optoelectronic devices: 1) Vibration modes and electronic excitations in π-conjugated copolymers and their blend with fullerene molecules with applications in organic photovoltaic (OPV) solar cells; 2) Magnetic field effect in organic light emitting devices (OLED) based on thermally activated delayed fluorescence (TADF) compounds 3) Optical study of 2D hybrid organic inorganic perovskites with potential use in a variety of optoelectronic devices.
We measured the spectra of resonant Raman scattering and doping induced absorption (DIA) in pristine films of π-conjugated donor-acceptor (D-A) copolymers, as well as the photoinduced absorption (PIA) spectra of their blend with fullerene PCBM molecules. We found that dominant charge carriers in these copolymers are polaron excitations. In addition, we also found that the D-A copolymer chains contain strongly coupled vibrational modes having relatively strong Raman scattering intensity. Since the lower energy induced polaron absorption band overlaps with the renormalized vibrational modes, they appear as anti-resonance lines superposed onto the induced polaron absorption band in both PIA and DIA spectra. We show that the Raman scattering, DIA and PIA spectra of PTB7 (fluorinated poly-thienothiophene-benzodithiophene) are well explained by the amplitude mode model.
We also studied magnetic field effect in donor-acceptor (D-A) type TADF compounds based organic light emitting diodes (OLED). Magneto-electroluminescence (MEL) and magneto-photoluminescence (MPL) in thin films of these compounds are enhanced thermally, and the response is interpreted as due to the Δg mechanism. TADF-based OLED doped with fluorescent emitters with various concentrations were also investigated. We found that both MEL and MPL responses are thermally activated with substantially lower activation energy compared to that in the pristine undoped D–A TADF host blend. However both MPL and MEL steeply decrease with the emitter’s concentration indicating the existence of a loss mechanism in the OLED device associated with energy transfer directly into the non-emissive triplet level of the emitter.
We also studied layered (2D) hybrid perovskite semiconductors which form natural “multiple quantum wells” which have strong spin-orbit coupling due to the heavy elements in their building blocks. This property may lead to “Rashba splitting” close to the extrema in the electron bands. Using optical spectroscopies such as electroabsorption (EA), and PIA, we studied the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D lead perovskite, namely, (C6H5C2H4NH3)2PbI4. From the EA spectrum we found that the exciton binding energy in this hybrid perovskite is 190 meV for the 1s exciton; whereas the continuum shows Franz-Keldysh oscillation that unambiguously reveals the band-edge energy. We found a strong PIA band at 0.15 eV that is due to long-lived free carrier absorption, which is caused by the Rashba-splitting in this material. We obtained a Rashba splitting energy of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å in this compound. This finding shows that 2D hybrid perovskites have great promise for potential applications in optoelectronic and spintronic applications.