Abstract
Low‐bandgap polymers are sensitive to various degradation processes, which strongly decrease their lifetime. The chemical and physical changes occurring in the low‐bandgap polymer with benzodithiophene units poly[4,8‐bis(5‐(2‐ethylhexyl)thiophene‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th) and its blend with the fullerene derivative [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) are followed during irradiation‐induced aging by combination of various characterization methods. The active layer morphology is investigated using atomic force microscopy (AFM) as well as in‐operando grazing incidence small angle X‐ray scattering (GISAXS), indicating morphological alterations and material loss due to chemical modifications. Optical spectroscopy gives insights into these chemical processes which lead to significant absorption losses under ambient conditions. Independent of the energy of the absorbed photons, but only in combination with oxygen, the excitation of the polymer leads to a fatal increase in oxidation probability. Fourier transform infrared (FTIR) data highlight the sensitivity of the conjugated polymer backbone to oxidation, a result of lost conjugation and therefore absorption capability. With combined AFM height and infrared (IR) mapping, the chemical degradation and material loss is confirmed on a nanoscale. Although the chemical structure is seriously damaged, the blend morphology is not undergoing major changes.
Highlights
Low-bandgap polymers are sensitive to various degradation processes, which conversion efficiencies (PCEs) and insuffistrongly decrease their lifetime
In low-bandgap poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b 0]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT):PC71BM solar cells, the evaporation of the processing additive led to a decrease in the domain sizes and formation of islands and traps which was attributed to be the reason for the observed dominant loss in fill factor (FF).[18]
Even though we see a slight decrease in domain size for intermediate structures (R2), this cannot be connected to a simultaneous loss in FF and occurs in structures larger than the ones discussed for PCPDTBT:PC71BM solar cells
Summary
For the in-operando GISAXS experiment, we prepare typical PTB7-Th:PC71BM bulk heterojunction solar cells in conventional device architecture on an indium tin oxide (ITO)-covered glass substrate, with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) sandwiched in between as blocking layer and without device encapsulation. Several factors can influence the scattering contrast, namely the material composition of the respective domains, the domain abundance or polydispersity.[48] A merging of large clusters R1 to even larger clusters beyond the resolution limit might account for the loss in I1 in combination with the increase in I0, yet this would not significantly change the observed OPV parameters, as the length scales far exceed the exciton diffusion length. The aged sample seems to have lost material on the surface which increases the contrast between the elevated and deeper domains This could explain the increased scattering intensity at very low qy values found in the GISAXS study due to an increased contrast between large domains as well as an increased surface roughness. To look into chemical changes in more detail, thin-film samples of PTB7-Th are investigated with respect to their optical properties using UV–vis absorption spectroscopy
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