Journal
CHEMISTRY OF MATERIALS
Volume 24, Issue 13, Pages 2583-2591Publisher
AMER CHEMICAL SOC
DOI: 10.1021/cm3012777
Keywords
organic solar cell; chemical annealing porphyrin; structural reorganization; crystalline organic film
Funding
- Global Photonic Energy Corporation (GPEC)
- King Abdullah University of Science and Technology (KAUST) [KUS-C1-015-21]
- National Science Foundation (NSF) Solar Energy Initiative (SOLAR) [CHE-0934098]
- NSF CRIF [1048807]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [0934098, 1048807] Funding Source: National Science Foundation
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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 degrees for the ZnTPP.pz film). Organic solar cells were prepared with ZnTPP donor and C-60 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 (J(SC)) and fill factor (FF) relative to analogous unannealed devices, presumably because of enhanced exciton diffusion length and improved charge conductivity. The open circuit voltages (V-OC) 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.
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