Quantifying Crystallinity in High Molar Mass Poly(3-hexylthiophene)

Abstract

We demonstrate a method to determine absolute crystallinity in high molar mass poly(3-hexylthiophene) (P3HT), as used in commercially relevant organic photovoltaic devices, using enthalpy of fusion and melting temperature values derived from differential scanning calorimetry (DSC) and 13C CPMAS NMR. By studying P3HT with molar masses ranging from 3.6 to 49 kg/mol and using recent work on oligomeric 3-hexylthiophene, we demonstrate several critical items. First, that proper extrapolation to infinite chain lengths, i.e., crystal size, yields values for the equilibrium melting temperature Tm0 of 272 ± 6 °C and the enthalpy of fusion per crystalline repeat unit ΔHu of 49 ± 2 J/g for Form I crystals of P3HT. Second, that a simple correction for crystal size using Tm0 is critical for determining an accurate degree of crystallinity from enthalpy measurements because of finite crystal size effects. Furthermore, our results demonstrate that the ordered fraction of P3HT measured from 13C NMR is indistinguishable from the DSC-determined crystalline fraction, once crystal size corrections are properly implemented. The connection between crystal size and melting temperature is affirmed by successive self-nucleation and annealing (SSA) measurements, which, when performed as a function of molar mass, allowed us to identify the molar mass at which chain folding occurs in P3HT in the melt, ≈11.5 kg/mol.