Moritz Riede

Abstract:

In contrast to p-dopants, highly efficient molecular n-dopants are prone to degradation in air due to their low ionization potentials, limiting the processing conditions of doped functional organic devices. In this contribution, we investigate the air-stability of pure films of the n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W2(hpp)4) and of C60 layers doped by W2(hpp)4. We find that 1/3 of the initial conductivity of the doped C thin films can be restored by thermal annealing in vacuum after a drop by 5 orders of magnitude upon air exposure. Furthermore, we show by ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements that the Fermi level shift toward the lowest unoccupied molecular orbital (LUMO) of C remains after air exposure, clearly indicating a conservation of n-doping. We explain these findings by a down-shift of the W(hpp) energy levels upon charge-transfer to a host material with deeper lying energy-levels, facilitating a protection against oxidation in air. Consequently, the observed recovery of the conductivity can be understood in terms of a self-passivation of the molecular n-doping. Hence, an application of highly efficient n-doped thin films in functional organic devices handled even under ambient conditions during fabrication is feasible. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Enhancement of the efficiency of organic solar cell devices requires knowledge about the structure of the organic layers involved. Films of the donor material dicyanovinyl-sexithiophene bearing four ethyl side-chains at thiophenes two and five DCV6T-Et(2,2,5,5) (DCV6T-Et) are prepared by thermal evaporation in high vacuum at various thicknesses and substrate temperatures. Infrared spectroscopic ellipsometry is used for determination of the molecular orientation in the thin films grown on room temperature (RT) substrate. From simulation of the IR ellipsometric data, the film thickness and the anisotropic optical constants of the DCV6T-Et films are determined. It is found that the optical constants strongly depend on the film thickness. Different average molecular orientations are determined for a few molecules thin (4 nm) and somewhat thicker (20 nm) films. Furthermore, the evolution of the surface morphology of films deposited at elevated substrate temperatures (80°C, 100°C) is studied in comparison to the thick RT-film. Atomic force microscopy images indicate that the growth on heated substrate is accompanied by an increase in grain size and surface roughness of the films. Simultaneously, the measured optical absorption spectra display structured and increased absorption in the red spectral region for the DCV6T-Et films deposited at higher substrate temperatures. The changes in surface topography and optical response relate to improved molecular arrangement induced by the substrate heating. To demonstrate the morphological influence on solar cell performance, we finally discuss DCV6T-Et/C60 planar heterojunction solar cells composed of DCV6T-Et films deposited at different substrate temperatures. © 2012 Elsevier B.V.

Abstract:

We compare two air-stable n-dopants for the fullerene C60: AOB and DMBI-POH. Conductivity and Seebeck coefficient measurements were performed at various doping concentrations and the thermal activation of the conductivity was determined. A superlinear increase of conductivity upon doping was found for DMBI-POH doped C60 reaching a maximum conductivity of 5.3 S/cm. In contrast to this, a linear rise of conductivity and an exponential thermal activation of mobility was observed for C60 doped by AOB. This suggests a different doping mechanism for the two compounds. © 2012 Elsevier B.V. All rights reserved.

Abstract:

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C60, as already employed in organic photovoltaics. By using DFT calculations and analyzing the recombination characteristics of the excited states revealed by photoinduced absorption (PIA) spectroscopy, the absorption peaks are assigned to triplet exciton, cation, and anion transitions. We determine the temperature dependent generation and recombination behavior of triplet excitons and cations in the mixed layer. At 10 K, we observe an enhanced triplet exciton generation rate compared to the pristine donor layer due to back recombination from a charge-transfer (CT) state at the donor-acceptor interface. With increasing temperature, the triplet generation rate first increases which is ascribed to an enhanced singlet exciton migration to this interface. Above 150 K, the triplet generation rate declines due to the beginning CT exciton separation, leading to the generation of free charge carriers. This temperature activated behavior is ascribed to a temperature activated increase of charge carrier mobility, facilitating CT exciton splitting. © 2012 American Chemical Society.