PMMA as an effective protection layer against the oxidation of P3HT and MDMO-PPV by ozone
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ans-Joachim Egelhaafa) Bavarian Center for Applied Energy Research, Solar Factory of the Future, 90429 Nuremberg, Germany
Holger Hintz and Dustin Quinones Institute of Physical and Theoretical Chemistry, University of Tübingen, 72076 Tübingen, Germany
Christoph J. Brabec Bavarian Center for Applied Energy Research, Solar Factory of the Future, 90429 Nuremberg, Germany; and iMEET, Friedrich Alexander-University, Erlangen 91058, Germany
Heiko Peisert and Thomas Chassé Institute of Physical and Theoretical Chemistry, University of Tübingen, 72076 Tübingen, Germany (Received 2 January 2018; accepted 22 March 2018)
The protective effect of poly(methylmethacrylate) (PMMA) cover layers against the degradation of p-conjugated polymers by ozone and photo-oxidation, respectively, has been investigated by UV/Vis spectroscopy. The PMMA films were cast from solution at thicknesses between 20 and 100 nm on top of films of poly(3-hexylthiophene) and poly[2-methoxy-5-(39,79-dimethyloctyloxy)-1,4-phenylenevinylene]. PMMA layers of more than 65 nm in thickness reduce the oxidation rate of the p-conjugated polymers under 15 ppm of ozone in the dark by more than three orders of magnitude, whereas photo-oxidation rates under dry and humid air remain unaffected. The PMMA cover layers are hardly affected by ambient ozone over thousands of hours. Calculations of ozone and oxygen fluxes through the PMMA films reveal that ozonation rates are limited by the diffusion of ozone, whereas photo-oxidation rates are not limited by the diffusion of oxygen, due to the much larger pressure gradient of the latter.
I. INTRODUCTION
The comparably fast oxidation of p-conjugated polymers in ambient atmosphere prevents their use in unprotected (opto-)electronic devices.1–6 Although chemical antioxidants have been tested successfully in the lab for the protection of organic electronic devices against photo-oxidation,7–9 commercial organic electronics products are protected by packaging them between barriers, which provide protection not only against oxidation of the p-conjugated components but also against the degradation of low work function electrodes or hygroscopic layers by humidity.10–14 To achieve device lifetimes of several years and at the same time preserve some of the most important advantages of organic electronics, such as light weight and flexibility, high quality flexible barriers films have to be used, which easily may represent the most expensive part of the device.15–17 In the quest of reducing the cost of packaging, a lot of mechanistic studies of the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.74
degradation processes of optoelectronic devices have been undertaken, which have revealed a comprehensive picture of the culprits for device failure. Photo-oxidation by molecular oxygen18–22 and oxidation by ozone18,23–29 have been identified as the most relevant mechanisms of oxidation, whereas thermal oxidation has been found to be negligible at temperatures below 100 °C.20,21 Wh
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