XPS analysis of the chemical degradation of PTB7 polymers for organic photovoltaics

Abstract

The chemical degradation of the Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] or ' PTB7' has been studied using X-ray Photoelectron Spectroscopy (XPS). This material system appears to be intrinsically unstable especially when illuminated in air and XPS studies confirm the rapid photo-degradation is related to changes in chemical structure of the polymer. In particular, XPS spectra show an initial reduction in relative CC intensity, suggests loss of the alkoxy side chains. This is followed by a dramatic increase in the level of oxygen-bonded species, especially CO at ~286.5 eV and C(=O)O at 289.2 eV, indicative of COOH and OH group formation, and oxidation of S. The XPS results support the view that using processing additives reduces the chemical stability of the polymer and provides insight into strategies to improve molecular design to ensure higher chemical stability.

Waterborne polyurethane as a carbon coating for micrometre-sized silicon-based lithium-ion battery anode material

“…The two peaks at about 501 and 940 cm −1 for the Si@CNO sample indicate the crystalline nature of Si [ 33 ]. The two peaks at 1349 cm −1 (D band) and 1580 cm −1 (G band) are characteristic of the carbon material in Si@CNO [ 34 ]. The one wide peak at 470 cm −1 of Si indicates that the Si sample is amorphous.…”

An insight into the air stability of the benchmark polymer:fullerene photovoltaic films and devices: A comparative study

“…A notable reduction in carbon species (sp 2 and sp 3 carbons, peaks intensity has significantly reduced) suggests the loss of alkoxy side chains. At the same time, the oxygen bonded species have been significantly increased that indicates the formation of COOH and OH group formation and probably the oxidation of 'S' too [19]. Oxidative species in the BHJ layer may generally function as the charge-carrier trapping sites [44,45].…”

“…are considerably increased [19,35]. These oxidized species can potentially cause the degradation of the BHJ materials, leading to an inability of the polymer:fullerene blend to transfer the charge [29,36,37], while the peak intensities for both the sp 2 and sp 3 carbon components (C=C and C-C) have been significantly reduced.…”

“…For example, amorphous copolymer based on poly (2,7- thieno [3,4-b]thiophenediyl]] (PTB7) demonstrated very promising results with the blend PTB7:PC 71 BM as photoactive layer compared with P3HT and PCDTBT [16,17]. However, a number of studies showed that PTB7 is a relatively unstable material [18,19].…”

Use of gas cluster ion source depth profiling to study the oxidation of fullerene thin films by XPS

“…A problem in elucidating the degradation mechanisms of PC 61 BM has been its relative stability compared to other materials present in the device. In many cases relatively rapid degradation of other materials limits the device lifetime [7][8][9]. Due to this clear degradation, less attention has been paid to the influence of the PC 61 BM itself, which has often been assumed to be stable.…”