UC Santa Barbara

his thesis details a morphological study of organic semiconductors (OSCs) that is dopedthrough the use of small molecule dopants and a spectroscopic study of photon upconversion of organic molecules. As OSCs suffer lower intrinsic conductivities than their inorganic counterparts, comprehensive understandings of successful doping strategies are of great importance. Impacts of added dopants on the intermolecular arrangement of the OSC are crucial considerations as charge transport relies upon the close arrangement of conjugated portions of OSC polymers or small molecules. The study discussed herein utilizes solid state nuclear magnetic resonance (ssNMR) and electron paramagnetic resonance (EPR) techniques to compare interactions between two dopants F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane) and BCF (tris[pentafluorophenyl] borane), when blended with the polymer PCPDTBT (P4) (poly[2,6-(4,4-bis(2-hexadecyl)-4H-cyclopenta[2,1-b;3,4- b′]dithiophene)-alt-4,7(2,1,3‑benzothiadiazole). The congregation of ssNMR and EPR results provides details explaining the previously observed lower P4 doping efficiency exhibited by F4TCNQ in comparison to BCF. Comparison of 1H and 13C ssNMR spectra from P4, P4:BCF, and P4:F4TCNQ blends reveal dopants alter the local arrangement of P4, both broadening and decreasing the intensity of the signal produced in the NMR spectra. Hyperfine interactions resolved via two-dimensional pulsed EPR measurements provided evidence that F4TCNQ intercalates between the conjugated moieties of the P4 polymer backbone. In contrast, BCF was found to be closely associated, but not inserted between the P4 backbone units. Both spectroscopic methods reveal a more severe disruption of the local arrangement of P4 when F4TCNQ serves as the dopant in comparison to BCF. Examination of the 19F ssNMR spectra illustrate an additional environment in which F4TCNQ molecules form aggregates outside of the P4 polymer, in agreement with previous studies suggesting F4TCNQ forms clusters due to its low solubility in organic solvents popularly used for OSC processing. Advancing the frontier of research in the field of photon upconversion involves seeking improvements in the parameters for efficiency (upconversion quantum yield) and performance (anti-Stokes shift) as well as reducing material costs. Herein a novel ternary upconversion system is designed which exhibits an over two-fold improvement in upconversion efficiency compared to the corresponding binary scheme. In both systems, visible light is first absorbed by the sensitizer 4CzIPN, a molecule which displays thermally activated delayed fluorescence (TADF) without relying on costly or toxic heavy metals, and ultimately emitted from p-terphenyl in order to preserve the anti-Stokes shift of 0.83 eV. 4CzIPN still demonstrated considerable delayed fluorescence in the presence of p-terphenyl, as a result of inefficient energy harvesting. Comparing the delayed fluorescence from 4CzIPN in the presence of various acceptors in combination with DFT simulations of excited state energy levels allowed the identification of 1- methylnaphthalene as the ideal intermediate acceptor to add to the binary system, producing a ternary blend with an enhanced upconversion efficiency of 7.6 %.