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 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. © 2012 American Chemical Society.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-C1-015-21
Acknowledgements: We thank Profs. Stephen Forrest (University of Michigan) and Peter Djurovich (University of Southern California), as well as Drs. Cody Schlenker and Zhiwei Liu for helpful discussions. We thank Francisco Navarro for the help in spectral resolved photoluminescence quenching measurements. We acknowledge financial support from the Global Photonic Energy Corporation (GPEC), the King Abdullah University of Science and Technology (KAUST, KUS-C1-015-21), and the National Science Foundation (NSF) Solar Energy Initiative (SOLAR, CHE-0934098). The NSF is also acknowledged for the funds used to acquire the X-ray diffractometer used to determine the structure of ZnTPP.tz, through an NSF CRIF Grant 1048807. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.