Unravelling the Photodegradation Mechanisms of a Low Bandgap Polymer by Combining Experimental and Modeling Approaches

Abstract

Large-scale introduction of Organic Solar Cells (OSCs) onto the market is currently limited by their poor stability in light and air, factors present in normal working conditions for these devices. Thus, great efforts have to be undertaken to understand the photodegradation mechanisms of their organic materials in order to find solutions that mitigate these effects. This study reports on the elucidation of the photodegradation mechanisms occurring in a low bandgap polymer, namely, Si-PCPDTBT (poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl]). Complementary analytical techniques (AFM, HS-SPME-GC-MS, UV–vis and IR spectroscopy) have been employed to monitor the modification of the chemical structure of the polymer upon photooxidative aging and the subsequent consequences on its architecture and nanomechanical properties. Furthermore, these different characterization techniques have been combined with a theoretical approach based on quantum chemistry to elucidate the evolution of the polymer alkyl side chains and backbone throughout exposure. Si-PCPDTBT is shown to be more stable against photooxidation than the commonly studied p-type polymers P3HT and PCDTBT, while modeling demonstrated the benefits of using silicon as a bridging atom in terms of photostability.

Progress in Understanding Degradation Mechanisms and Improving Stability in Organic Photovoltaics

“…Bleaching of the film's absorption is typically observed to occur on a timescale of seconds to hours under one‐sun intensities. Chemical analysis by means of infrared spectroscopy of the degradation products of a variety of polymers, including PPV's, P3HT, PCDTBT, PTB7, PBDTTPD, Si‐PCPDTBT and others shows a loss of conjugated bonds and a growth of carbonyl, ester, and alkoxy bonds, indicating the materials react with oxygen.…”

Photodegradation of Si-PCPDTBT:PCBM active layer for organic solar cells applications: A surface and bulk investigation

“…The thickness (initially 300 nm) progressively increased by more than 40 nm until 680 h of irradiation and then decreased ( À 60 nm for the most aged samples). The evolution of the film thickness depends on the competition between fragmentation of the materials provoking the migration of low molecular weight molecules out of the film and the insertion of oxygen which may increase the volume of the film [12,16]. In the present study, the thickness increases (þ40 nm) until 680 h of exposure suggesting that the insertion of substantial amounts of oxygen (Table 1 and Fig.…”

“…(Fig. SI4) The assignment is supported by a recent study in which a detailed attribution (experimental and computational methods) of the vibrational modes of the Si-PCPDTBT polymer was carried out [16]. We have selected the essential vibrational modes of the polymer and the fullerene structures which can be monitored during the photodegradation.…”

“…[9,10,12,13,16,19] The assignment of the different vibrational modes has been done by a direct comparison with the spectra of pure PC 70 BM and Si-PCPDTBT:PC 70 BM. (Fig.…”

“…Therefore, resistant organic materials and a better understanding of the photooxidation mechanism are required. The photodegradation of various conjugated polymers or blends have been reported in the literature [9][10][11][12][13][14][15][16][17].…”