Chemical Annealing of
Zinc Tetraphenylporphyrin Films:
Effects on Film Morphology and Organic Photovoltaic Performance
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Abstract
We present a chemical annealing process for organic thin
films.
In this process, a thin film of a molecular material, such as zinc
tetraphenylporphyrin (ZnTPP), is exposed to a vapor of nitrogen-based
ligand (e.g., pyrazine, pz, and triazine, tz), forming a film composed
of the metal–ligand complex. Fast and quantitative formation
of the complex leads to marked changes in the morphology and optical
properties of the film. X-ray diffraction studies show that the chemical
annealing process converts amorphous ZnTPP films to crystalline ZnTPP·ligand
films, whose porphryin planes lie nearly parallel to the substrate
(average deviation is 8° for the ZnTPP·pz film). Organic
solar cells were prepared with ZnTPP donor and C<sub>60</sub> acceptor
layers. Devices were prepared with and without chemical annealing
of the ZnTPP layer with a pyrazine ligand. The devices with chemically
annealed ZnTPP donor layer show an increase in short-circuit current
(<i>J</i><sub>SC</sub>) and fill factor (<i>FF</i>) relative to analogous unannealed devices, presumably because of
enhanced exciton diffusion length and improved charge conductivity.
The open circuit voltages (<i>V</i><sub>OC</sub>) of the
chemically annealed devices are lower than their unannealed counterpart
because of enhanced polaron pair recombination at the donor/acceptor
heterojunction. A net improvement of 5–20% in efficiency has
been achieved, after chemical annealing of ZnTPP films with pyrazine