Daniel Trefz

This manuscript provides the first systematic characterization of the electrochemical properties of the high mobility n-type polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis (dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) and its corresponding monomer 2,6-bis(2-bromothien-5-yl)naphthalene-1,4,5,8-tetracarboxylic-N,N′-bis(2-octyldodecyl) diimide (Br-NDI2OD-T2-Br) by cyclic voltammetry and in situ spectroelectrochemistry. Both monomer and polymer reveal a 2-fold reduction to the dianion via a radical anion species. The comparison between monomeric and polymeric species allows the explanation of the electrochemical behavior of P(NDI2OD-T2) according to redox polymers with localization of charges on the naphthalene bisimide unit. Measurements with electrolyte gated transistors suggest electron hopping transport according to mixed valence conductivity. In the last section of this paper we discuss a significant first cycle effect upon electrochemical reduction which had not been reported for n-type polymers before. The effect is even more pronounced for samples with controlled morphology, that is, high amounts of aggregation in the films. In agreement with solution experiments we attribute the appearance of the signal at −1.04 V (E1/2 = −1.00 V) to the radical anion form of the solvated species.

Its inherent strong tendency to aggregate in solution is used in the following study to prepare highly anisotropic films of the n-type copolymer poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)). Solvent vapor annealing (SVA) allows to tune the size of oriented domains in spherulite-like superstructures with alignment up to several hundreds of micrometers. Blade coating (BC), on the other hand, yields square centimeter large perfectly oriented films with dichroic ratios of 18 and charge transport anisotropies up to 14. On the nanometer scale highly oriented fibers of form I are visible in the oriented areas with the fiber long axis parallel to the chain direction. We give experimental evidence that structure formation does involve liquid crystal (LC) mesophases at high solution concentrations which are frozen upon solvent removal. Temperature post-treatment of the oriented films gives, on the other hand, evidence for a classical semicrystalline nature of this polymer with spherulites consisting of crystalline and amorphous domains. These findings point to a different growth behavior than previously discussed for the well-studied p-type polymer poly(3-hexylthiophene) and suggests that the definition and distinction between liquid-crystalline and semicrystalline nature might need to be reassessed.

PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance.

We highlight the influence of processing conditions on polymorphism and structure formation on the mesoscale for the family of PCPDTBT polymers with branched alkyl side chains. Direct correlations of morphology to the chemical structure and to transistor device performance are established. We found that up to four different packing motifs could be realized depending on the polymer derivative and the processing conditions: amorphous, π-stacked, cross-hatched and dimer-containing polymorphs. While C- and F-PCPDTBT display similar packing behavior organizing in π-stacked and dimer-like structures, Si-PCPDTBT gives rise to cross-hatched structures upon simple deposition from solution. The observed differences in chain packing for C-/F-PCPDTBT versus Si-PCPDTBT are attributed to differences in backbone conformations and aggregation behavior in solution. The effect of polymorphism on charge transport is probed using field-effect transistors, in which both π-stacked and cross-hatched polymer chain arrangements yield the highest hole mobilities. Mesoscopic morphology and mobility simulations rationalize our experimental findings by relating mobility to distributions of electronic coupling elements between the chains.