Issue 11, 2015

Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers

Abstract

Donor–acceptor copolymers are an important class of conjugated polymer on account of their chemically tunable energy levels and ambipolar charge transport properties. These materials typically exhibit two strong absorption bands in the UV-visible range, whose natures have previously been explored using theoretical analyses. In this work, we experimentally elucidate the electronic origins of these transitions and consider their effects on photostability using resonant Raman spectroscopy and transient absorption spectroscopy. In particular, we identify two dominant electronic transitions for a material comprising diketopyrrolopyrrole (DPP) acceptor and selenophene donor units: a strong transition at low energy (520–1150 nm) that is localised within the DPP unit, and a much weaker transition at higher energy (320–520 nm) that is delocalised along the conjugated backbone of both the DPP and selenophene units. The polymer is found to have good photostability under prolonged excitation in the low energy absorption band, but is much less stable to higher energy excitation. In this latter case, the selenophene ring is identified as the photodegradation site. These effects are correlated with ultrafast transient absorption spectroscopy results, which associate the photodegradation with a higher yield of polaron pairs arising from excitation of the higher energy transition. Our findings provide insight into the design of photostable low energy gap conjugated copolymers for application in organic photovoltaic devices and demonstrate a specific vulnerability of the selenophene donor unit.

Graphical abstract: Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers

Supplementary files

Article information

Article type
Paper
Submitted
26 jun 2015
Accepted
03 ago 2015
First published
03 ago 2015

Energy Environ. Sci., 2015,8, 3222-3232

Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers

S. Wood, J. Wade, M. Shahid, E. Collado-Fregoso, D. D. C. Bradley, J. R. Durrant, M. Heeney and J. Kim, Energy Environ. Sci., 2015, 8, 3222 DOI: 10.1039/C5EE01974E

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