  Recent demonstrations of inverted thermal activation of charge mobility in
polymer field-effect transistors have excited the interest in transport regimes
not limited by thermal barriers. However, rationalization of the limiting
factors to access such regimes is still lacking. An improved understanding in
this area is critical for development of new materials, establishing processing
guidelines, and broadening of the range of applications. Here we show that
precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron
transporting copolymer results in single crystal-like and voltage-independent
mobility with vanishing activation energy above 280 K. Key factors are uniaxial
molecular alignment and thermal annealing at temperatures within the melting
endotherm of films. Experimental and computational evidence converge toward a
picture of electrons being delocalized within crystalline domains of increased
size. Residual energy barriers introduced by disordered regions are bypassed in
the direction of molecular alignment by a more efficient interconnection of the
ordered domains following the annealing process.
