Dr Pascal Kaienburg

Abstract:

We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ). Electron paramagnetic resonance (EPR) spectroscopy discloses the presence of two main paramagnetic species distinguished by two different g-tensors, which are assigned based on density functional theory calculations to the formation of a positive polaron on the host and a radical anion on the dopant. Close inspection of the EPR spectra shows that radical anions on the dopants couple in an antiferromagnetic manner at device-relevant doping concentrations, thereby suggesting the presence of dopant clustering, and that positive polarons on the molecular host move by polaron hopping with an activation energy of 5 meV. This activation energy is substantially smaller than that inferred from electrical conductivity measurements (∼233 meV), as the latter also includes a (major) contribution from charge-transfer state dissociation. It emerges from this study that probing the electron spin can provide rich information on the nature and dynamics of charge carriers generated upon doping molecular semiconductors, which could serve as a basis for the design of the next generation of dopant and host materials.

Abstract:

The real-time morphological evolution of vacuum-deposited α-sexithiophene (α-6T) on a weakly interacting (glass) substrate at ambient temperature is reported. In situ grazing-incidence small-angle X-ray scattering (GISAXS) enabled the observation of nanoscale aggregates, while in situ grazing-incidence wide-angle scattering (GIWAXS) allowed the study of the molecular-scale morphology. The in situ GISAXS measurements revealed that the α-6T growth proceeds via a Stranski-Krastanov mode, whereby 2-4 complete monolayers are deposited, followed by subsequent layers formed via island growth. In situ GIWAXS also showed the evolution of the polymorph composition during the thin-film growth. Initially, the disordered β-phase and the low-temperature (LT)-phase are deposited in nearly equal proportion until a thickness of 8 nm, whereby the LT-phase begins to dominate until a final α-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the β-phase. The change in the polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain relief as the thin film moves from surface to bulk-mediated growth. The GISAXS findings were confirmed through direct imaging using ex situ atomic force microscopy (AFM) at various thicknesses, revealing the existence of both initial the initial and intermediate monolayers and final island morphologies. The findings reveal the real-time morphological evolution of α-6T across both the molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of the thin-film microstructure in device performance, it is expected that these results will aid in the development of structure-property relationships necessary to realize the full potential of organic electronics.

Abstract:

The interplay of light absorption, charge-carrier transport, and charge-carrier recombination determines the performance of a photovoltaic absorber material. Here we analyze the influence on the solar-cell efficiency of the absorber material properties absorption coefficient α, charge-carrier mobility μ, and charge-carrier lifetime τ, for different scenarios. We combine analytical calculations with numerical drift-diffusion simulations to understand the relative importance of these three quantities. Whenever charge collection is a limiting factor, the αμτ product is a good figure of merit (FOM) to predict solar-cell efficiency, while for sufficiently high mobilities, the relevant FOM is reduced to the ατ product. We find no fundamental difference between simulations based on monomolecular or bimolecular recombination, but strong surface-recombination affects the maximum efficiency in the high-mobility limit. In the limiting case of high μ and high surface-recombination velocity S, the α/S ratio is the relevant FOM. Subsequently, we apply our findings to organic solar cells which tend to suffer from inefficient charge-carrier collection and whose absorptivity is influenced by interference effects. We estimate that a modest increase in absorption strength by a factor of 1.5 leads to a relative efficiency increase of more than 10% for state-of-the-art organic solar cells.

Abstract:

Simultaneous control over both the energy levels and Fermi level, a key breakthrough for inorganic electronics, has yet to be shown for organic semiconductors. Here, energy level tuning and molecular doping are combined to demonstrate controlled shifts in ionisation potential and Fermi level of an organic thin film. This is achieved by p-doping a blend of two host molecules, zinc phthalocyanine and its eight-times fluorinated derivative, with tunable energy levels based on mixing ratio. The doping efficiency is found to depend on host mixing ratio, which is explained using a statistical model that includes both shifts of the host's ionisation potentials and, importantly, the electron affinity of the dopant. Therefore, the energy level tuning effect 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.