Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
In recent years research on molecular engineering of materials for OPV devices has been combined with the study of optimal morphology. It is well known that the morphology of photoactive materials is critical for charge-generation, charge-transport and recombination properties, thus dramatically affecting the PV performances of the material1. Electron Paramagnetic Resonance spectroscopy is a powerful tool for the investigation of morphology on a molecular level.
In this work we applied EPR spectroscopy to the study of molecular order in films of conjugated polymers and polymer/PCBM blends that are of interest for Bulk Heterojunction Solar Cells. The photophysics of these materials involves the formation of various paramagnetic states that can be detected by EPR techniques and are characterised by anisotropic magnetic interactions depending on the orientation of the molecular system in the applied magnetic field. Such anisotropy can be exploited to determine the molecular preferential orientation with respect to the plane of the film – if present. Different authors have already used steady-state CW EPR to analyse the ordering of the polymer P3HT in spin-coated and drop-casted films2,3.
We extended the analysis to several low bandgap polymers for photovoltaic applications, and to various film deposition techniques. Besides, we considered the possibility of using not only EPR spectra from intrinsic paramagnetic defects, but also spectra of photo-induced paramagnetic species. These can belong to the trapped polymer radical cation generated by absorption of light in the polymer/PCBM blends, or to the polymer triplet state, either populated via ISC or by charge recombination. Since the magnetic dipolar interaction between two unpaired electrons is much more anisotropic than the Zeeman interaction that characterises radical species, triplet state spectra often give less ambiguous results compared to polarons. In order to be able to exploit triplet signals we carried out a preliminary study on the triplet state of the polymers by means of time-resolved EPR.
(1) Jackson, N. E.; Savoie, B. M.; Marks, T. J.; Chen, L. X.; Ratner, M. a. J. Phys. Chem. Lett. 2015, 6, 77–84.
(2) Aguirre, A.; Gast, P.; Orlinskii, S.; Akimoto, I.; Groenen, E. J. J.; El Mkami, H.; Goovaerts, E.; Van Doorslaer, S. Phys. Chem. Chem. Phys. 2008, 10 (47), 7129–7138.
(3) Konkin, a.; Roth, H. K.; Scharff, P.; Aganov, a.; Ambacher, O.; Sensfuss, S. Solid State Commun. 2009, 149 (21-22), 893–897.
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