Impact of UV‐Visible Light on the Morphological and Photochemical Behavior of a Low‐Bandgap Poly(2,7‐Carbazole) Derivative for Use in High‐Performance Solar Cells

Abstract

This paper reports on the photochemical behavior upon exposure to UV-visible light of a poly(2,7-carbazole) derivative for use in high-performance solar cells. Poly[ N -9 ′ -hepta-decanyl-2,7-carbazole-alt -5,5-(4 ′ ,7 ′ -di-2-thienyl-2 ′ ,1 ′ ,3 ′benzothiadiazole)] (PCDTBT) is one of a relatively large class of push-pull carbazole-based copolymers that have been synthesized to better harvest the solar spectrum. The 2,7-carbazole building block of PCDTBT is also used with different electron-accepting units in a large variety of low-band-gap polymers. The photochemical and morphological behavior of PCDTBT thin fi lms is investigated from the molecular scale to the nanomechanical properties. The photo-oxidation mechanism is shown to be governed by chain-scission and cross-linking reactions. It results in dramatic evolution of the morphology, roughness and stiffness of thin PCDTBT fi lms. Based on the identifi cation of several photoproducts formed along the macromolecular chains or released into the gas phase, the main pathways of PCDTBT photochemical evolution are discussed. These processes fi rst involve the scission of the C-N bond between the carbazole group and the tertiary carbon atom bearing the alkyl side-chain. Modifi cations of the chemical structure of PCDTBT, the evolution of its UV-visible absorbance, and its nanomechanical properties initiated by light irradiation are shown to be closely related. 479

Long-Term Stable Organic Photodetectors with Ultra Low Dark Currents for High Detectivity Applications

“…A reorganization of PCDTBT films due to the modification of the 2,7-carbazole building block under illumination has also been reported to explain the burn-in45. The responsivity drops by 9% during the burn-in period.…”

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.…”

“…[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].…”

Characterization of the degradation process of Si-PCPDTBT:PC70BM(1:2) blend layers deposited on ITO/glass substrate

“…The sputtering time till arriving at the ITO interface becomes significantly reduced with increasing photooxidation which may be related to modification of sputtering rates with oxidation and/or loss of material due to the desorption of volatile oxidation products. Indeed, previous studies revealed that photooxidation of organic materials can be manifested by a formation of low weight photoproducts in gas phase due to the presence of holes and erosion [18]. In order to compare the surface and bulk oxidization properties, the evolution of the O À ion fragment intensity was extracted from all depth profiles.…”