Energy levels in organic semiconductors: tuning and doping

The energy level tuning effect, observed when mixing halogenated versions of similar molecules, has recently been identified as a new way to modify the bulk ionisation energy and electron affinity of an organic semiconducting thin film, and increase the open-circuit voltage in organic solar cells. In this thesis, the energy level tuning effect is extended to organic field-effect transistors (OFETs). A method for the fabricating ambipolar devices with balanced hole and electron transport is established. This is achieved by using blends of two host molecules, zinc phthalocyanine (ZnPc) and its eight-times fluorinated derivative (F8ZnPc), with energy levels that shift based on mixing ratio. The semiconducting behaviour of the bottom-gate bottom-contact OFETs can be tuned continuously from unipolar p-type, through ambipolar, and finally to unipolar n-type. For the ambipolar devices, the optimum balance between the hole and electron mobilities is found for the blend of 1:1.5 weight ratio of ZnPc:F8ZnPc with hole and electron mobilities of (8.3 ± 0.2)×10 −7 cm2V−1s−1 and (5.5 ± 0.1)×10−7 cm2V−1s−1 , respectively. An application of the ambipolar devices in a complementary-like voltage inverter circuit, with performance comparable to an inverter based on separate ZnPc and F8ZnPc OFETs, is demonstrated. Next, a p-dopant (F6-TCNNQ) is introduced into the blend of ZnPc: F8ZnPc to investigate the impact of the energy level tuning effect on the molecular doping process. The doping efficiency is investigated using photothermal deflection spectroscopy and electron paramagnetic resonance spectroscopy, and is found to depend upon host mixing ratio. The experimentally observed trend is explained with a statistical model that includes both shifts of the host’s ionisation energies, and, importantly, the electron affinity of the dopant. The energy level shift in the model is in close agreement with the 0.86 eV shift in ionisation energy observed experimentally. As further validation, the model reproduces the measured trend at low temperature. The energy level tuning effect therefore has a crucial impact on the molecular doping process. The practice of comparing host and dopant energy levels must consider the long-range electrostatic shifts to consistently explain the doping mechanism in organic semiconductors.