Temperature-dependent morphology-electron mobility correlations of naphthalene diimide-indacenodithiophene copolymers prepared via direct arylation polymerization

Abstract

A series of defect-free n-type copolymers poly(naphthalene diimide-alt-indacenodithiophene) P(NDI-IDT) comprising alternating naphthalene diimide (NDI) and indacenodithiophene (IDT) units is prepared using atom-economic direct arylation polycondensation (DAP). Copolymers with varying molecular weights up to M_n,SEC = 113 kg mol⁻¹ are obtained in high yield and used to investigate the optical, thermal and electrical properties as a function of chain length. Two weak endotherms are seen in differential scanning calorimetry experiments at 68 °C and 180–220 °C, depending on molecular weight, which are ascribed to side chain (T_m,1) and main chain (T_m,2) melting, respectively. Thin-film morphologies are weakly crystalline for annealing temperatures below T_m,2, with organic field-effect (OFET) mobilities being on the order of 10⁻³ cm² V⁻¹ s⁻¹. Under these conditions very low Urbach energies (E_U) between 28 and 30 meV are found. However, thermal annealing above T_m,2 results in amorphous morphologies with hypsochromically shifted optical spectra, strongly increased E_U and complete loss of mobility. In comparison to the well-investigated bithiophene analogue PNDIT2, P(NDI-IDT) thin films are significantly less crystalline, and solutions lack the typical features of aggregation. This is ascribed to the additional bulky hexylphenyl side chains of P(NDI-IDT) that hamper main chain ordering and aggregate formation and thus, interchain charge hopping. With intrachain transport being limited as well, field-effect electron mobilities are limited to ∼10⁻³ cm² V⁻¹ s⁻¹ and mostly independent of chain length. These results demonstrate the importance of both intra- and interchain transport to electron mobility for NDI main chain copolymers.