Charge transport in amorphous low bandgap conjugated polymer/fullerene films

Abstract

The structural and charge transport properties of a low bandgap copolymer poly(3-hexylthiophene-alt-6,7-dimethyl-4,9-bis-(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (P(3HT-MeTDQ)) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are investigated. Thermal analysis, X-ray scattering diffraction (XRD), atomic force microscopy and transmission electron microscopy (TEM) of P(3HT-MeTDQ) reveal that the polymer is amorphous in solid state. As the hole mobility of P(3HT-MeTDQ) was measured by the time-of-flight photoconductivity method, the mobility was 3.35 × 10−4 cm2/V s, which is very comparable to that of semicrystalline poly(3-hexyl thiophene). When the mobility of amorphous P(3HT-MeTDQ) was analyzed according to the Gaussian disorder model, the polymer has the energetic and positional disorders with the values of σ = 62 meV and Σ = 1.7, respectively, indicating that the polymer has a relatively narrow Gaussian distribution of transport states. Interestingly, when P(3HT-MeTDQ) is blended with PCBM, the amorphous P(3HT-MeTDQ) becomes partially ordered, as evidenced by observation of two discernible XRD peaks at 2θ = 5° (d = 17.7 Å) and 25.5° (d = 3.5 Å) corresponding to the interchain distance and π-stacking distance, respectively. The bicontinuous network morphology was identified at the blend with 60 wt. % PCBM by TEM, at which the charge carrier transport changes from hole-only to ambipolar.

Structure and Gas Barrier Properties of Polyimide Containing a Rigid Planar Fluorene Moiety and an Amide Group: Insights from Molecular Simulations

“…Comparatively, Kapton with a relatively lower intensity at 2θ = 18.53° was observed. The interchain distance ( d -spacing) can be determined from the peak in the WAXD curve . The calculated d -spacing for Kapton was 4.78 Å, which was decreased to 4.24 Å for FAPPI (Table ).…”

Phase Behavior of Amorphous/Semicrystalline Conjugated Polymer Blends

“…As shown in Figure 3 a, P(NDI2OD-T2) was a semicrystalline polymer with T m = 324 °C (1st heating) or 318 °C (2nd heating) and T c = 294 °C (1st cooling). On the other hand, PCPDTBT did not show any melting point, indicating it was a typical amorphous polymer [ 42 ]; see Figure 3 b–d. About the glass transition temperature ( T g ), P(NDI2OD-T2) showed it at −44 °C in the DSC thermogram, which agreed with Gu et al’s report ( T g = −40 °C) within experimental uncertainties [ 35 ].…”

“…Importantly, if we recall the DSC results in Figure 3 , when the crystallite size (related with π-stacking) is larger than 2 nm, we may observe the melting transition in DSC. It is notable also that, in polymer science, the typical unit cell, chain-folded lamella and spherulite have dimensions of ~0.2–2 nm, ~10–50 nm thick and several microns wide, and ~100–1000 μm, respectively [ 42 ]. In the case of conjugated polymer, thin-film samples for OPV, we usually observe up to a lamella scale because a film thickness is ~100–200 nm.…”

“…Here, the crystallite size (t) could be estimated based on Scherrer's equation, = 0.9 ( θ ⁄ [43,44], where λ is the X-ray wavelength (= 0.154 nm) and B is the full width at half maximum (FWHM) at the diffraction angle θ. The results are summarized in Table 1, indicating that the crystallite size decreases from 2.5 nm at 100% P(NDI2OD-T2) to 2.0 nm at cell, chain-folded lamella and spherulite have dimensions of ~0.2-2 nm, ~10-50 nm thick and several microns wide, and ~100-1000 μm, respectively [42]. In the case of conjugated polymer, thin-film samples for OPV, we usually observe up to a lamella scale because a film thickness is ~100-200 nm.…”

Phase Diagrams of Ternary π-Conjugated Polymer Solutions for Organic Photovoltaics

“…(2) Then through the quenching process, the L-L demixing will precede any crystallization. Then, based on these assumptions, a semicrystalline polymer can be treated as an amorphous chain molecule [ 61 ], which could be phase-separated through the L-L phase transition [ 3 , 16 ]. For example, the ternary P3HT solutions were studied as a function of temperature, molecular weight, solvent species [CB, chloroform (CF), toluene (TOL)], processing additive (DIO, and ODT), and electron acceptor (PC 61 BM, PC 71 BM, and ITIC).…”