Stability of Polymer:PCBM Thin Films under Competitive Illumination and Thermal Stress

Abstract

The combined effects of illumination and thermal annealing on the morphological stability and photodimerization in polymer/fullerene thin films are examined. While illumination is known to cause fullerene dimerization and thermal stress their dedimerization, the operation of solar cells involves exposure to both. The competitive outcome of these factors with blends of phenyl-C61-butyric acid methyl ester (PCBM) and polystyrene (PS), supported on PEDOT:PSS is quantified. UV-vis spectroscopy is employed to quantify dimerization, time-resolved neutron reflectivity to resolve the vertical composition stratification, and atomic force microscopy for demixing and coarsening in thin films. At the conventional thermal stress test temperature of 85 °C (and even up to the PS glass transition), photodimerization dominates, resulting in relative morphological stability. Prior illumination is found to result in improved stability upon high temperature annealing, compatible with the need for dedimerization to occur prior to structural relaxation. Modeling of the PCBM surface segregation data suggests that only PCBM monomers are able to diffuse and that illumination provides an effective means to control dimer population, and thus immobile fullerene fraction, in the timescales probed. The results provide a framework for understanding of the stability of organic solar cells under operating conditions.

Interfacial width and phase equilibrium in polymer-fullerene thin-films

“…Mixture thermodynamics and interfacial width. PS has been extensively studied in bulk, near surfaces/interfaces and in thinfilms (e.g., the dependence of the glass transition temperature on Mw 40 and film thickness 41 ), and we have previously utilised this polymer to understand morphological development during OPV fabrication and operation [42][43][44][45] . As part of a recent study on fullerene crystallisation, we uncovered rapid mixing in PS/phenyl-C60-butyric acid methyl ester (PCBM) bilayers in the limit of high PS Mw, well before crystallisation of the fullerene 43 .…”

Dynamic PCBM:Dimer Population in Solar Cells under Light and Temperature Fluctuations

“…[17] Recently, we reported a neutron reflectivity study on the competitive effects of light and temperature on the morphological stability of a polymer:fullerene blend. [18] These results found PCBM dimers to be effectively immobile compared to PCBM monomers in the blend matrix. The impact of PCBM dimerization on OSC performance appears thus nontrivial, and quantifying the PCBM dimer population throughout the processing steps, at both short-and long-term operation conditions, is important to predict and improve OSC stability.The photochemical dimerization (and polymerization) of C 60 fullerenes was reported two decades ago by Eklund and co-workers, [19] who found crosslinking of up to 20 molecules upon photoirradiation of neat fullerene films.…”

“…Upon illumination in an inert (N 2 ) atmosphere, PCBM dimerization results in a spectral UV-vis absorbance feature, that has been previously documented. [12,18] Figure 2a presents the absorbance of a PCDTBT:PCBM 1:2 thin film during 50 h illumination at 630 W m −2 from a white LED (spectrum in Figure S1, Supporting Information). Other methods used to quantify PCBM dimerization have several disadvantages: in Raman spectroscopy (visible, UV, and even near IR), for instance, the illumination itself causes dimerization and thus is convoluted in the measurement outcome; [32] high performance liquid chromatography (HPLC) requires redissolution of the thin film (often with sonication) which can potentially cause both agglomeration and dedimerization; [13] it is also comparatively time intensive.…”

“…In a polymer:fullerene blend system, there are a number of factors that will influence this including crystallinity, [28] domain size, component segregation (normal to the film surface), [29][30][31] and polymer:fullerene blend ratio. [18] Furthermore, time and temperature can nontrivially affect blend demixing and coarsening, crystallization, and surface segregation which, in turn, can modulate the local spatial arrangement of fullerenes and thus the statistics of meeting topochemical requirements.…”

Impact of Initial Bulk‐Heterojunction Morphology on Operational Stability of Polymer:Fullerene Photovoltaic Cells

“…In fact, spinodal de‐mixing has been shown to be responsible for the complex initial burn‐in degradation of OPV devices. Other studies have also identified different origins of burn‐in degradation including polymer molecular weight,crystallinity, fullerene dimerization and degradation, and interlayer interfaces . Thus, a comprehensive study of BHJ unifying both efficiency and operational stability is of utmost importance.…”