Advanced Functional Materials and Devices (AFMD) Group

Abstract:

The maximum open-circuit voltage V of bulk-heterojunction solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photoactive absorber blend. We investigate blend layers comprising zinc-phthalocyanine (ZnPc) and the buckminster fullerene C with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:C from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(C) gap by 0.25 eV. The trend in this gap correlates with the change in the charge transfer energy measured by Fourier-transform photocurrent spectroscopy as well as with the observed open-circuit voltage of solar cells containing ZnPc:C as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:C blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the C phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested. © 2013 American Physical Society.

Abstract:

By combining computer simulations, grazing incidence, and powder X-ray-diffraction measurements we reconstruct the crystal structure of a thin film of terminally dicyanovinyl-substituted quaterthiophene (DCV4T). The crystal structure differs from the known single crystal structure of the same compound, but resembles the molecular packing of a methylated DCV4T. Charge transport simulations on the molecular level show that the 2 dimensional thin-film charge-transport network is well suited for hole transport in solar cells. © The Royal Society of Chemistry 2013.

Abstract:

High-efficiency white OLEDs fabricated on silver nanowire-based composite transparent electrodes show almost perfectly Lambertian emission and superior angular color stability, imparted by electrode light scattering. The OLED efficiencies are comparable to those fabricated using indium tin oxide. The transparent electrodes are fully solution-processable, thin-film compatible, and have a figure of merit suitable for large-area devices.

Abstract:

The role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S-shaped current-voltage ( J-V ) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric fi eld and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the ? s to ms regime. In combination with drift-diffusion simulation data, we demonstrate a pile-up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S-kinks in the J-V curve and can be distinguished from each other and from other reasons for S-kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S-shaped J-V curve close to open circuit to evaluate whether their electrodes are optimized. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.