Today numerous cyanine dyes that are soluble in organic solvents are available, driven by more than a century of
research and development of the photographic industry. Several properties specific to cyanine dyes suggest that
this material class can be of interest for organic solar cell applications. The main absorption wavelength can be
tuned from the ultra-violet to the near-infrared. The unparalleled high absorption coefficients allow using very
thin films for harvesting the solar photons. Furthermore, cyanines are cationic polymethine dyes, offering the
possibility to modify the materials by defining the counteranion. We here show specifically how counterions can
be utilized to tune the bulk morphology when blended with fullerenes. We compare the performance of bilayer
heterojunction and bulk heterojunction solar cells for two different dyes absorbing in the visible and the near-infrared.
Light-induced Electron Spin Resonance (LESR) was used to study the charge transfers of light induced
excitons between cyanine dyes and the archetype fullerene C60. LESR results show good correlation with the cell
performance.
KEYWORDS: Interfaces, Dewetting, Coating, Solar cells, Thin films, Ions, Atomic force microscopy, Photovoltaics, Heterojunctions, Thin film solar cells
The details of the arrangement of mixtures of semiconducting materials in thin-films have a major influence on the
performance of organic heterojunction solar cells. Here, we exploit the phenomenon of spinodal dewetting during spin
coating of blends of PCBM and a cyanine dye for the design of phase separated morphologies with increased interfacial
area. AFM snapshots of as-prepared films and after selective dissolution suggest that the solution separates into transient
bilayers, which destabilize due to long-range intermolecular interactions. We propose that film destabilization is
effectively driven by electrostatic forces that build up due to mobile ions that cross the junction and dissolve partially in
PCBM. The resulting morphology type is mainly dependent on the ratio between the layer thicknesses, whereas the
dominant wavelengths formed are determined by the absolute film thickness. Solar cells were fabricated from films with
known structure and a power conversion efficiency of η = 0.29 % was measured for a vertically segregated film
consisting of a cyanine layer covering the anode and an upper phase composed of dewetted PCBM domains. We explain
the merits of this structure in contrast to a lateral separated blend morphology where the efficiency was 3 times smaller.
Significant progress is being made in the photovoltaic energy conversion using organic semiconducting materials. One
of the focuses of attention is the nanoscale morphology of the donor-acceptor mixture, to ensure efficient charge
generation and loss-free charge transport at the same time. Using small molecule and polymer blend systems, recent
efforts highlight the problems to ensure an optimized relationship between molecular structure, morphology and device
properties. Here, we present two examples using a host/guest mixture approach for the controlled, sequential design of
bilayer organic solar cell architectures that consist of a large interface area with connecting paths to the respective
electrodes at the same time. In the first example, we employed polymer demixing during spin coating to produce a rough
interface: surface directed spinodal decomposition leads to a 2-dimensional spinodal pattern with submicrometer features
at the polymer-polymer interface. The second system consists of a solution of a blend of small molecules, where phase
separation into a bilayer during spin coating is followed by dewetting. For both cases, the guest can be removed using a
selective solvent after the phase separation process, and the rough host surface can be covered with a second active,
semiconducting component. We explain the potential merits of the resulting interdigitated bilayer films, and explore to
which extent polymer-polymer and surface interactions can be employed to create surface features in the nanometer
range.
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