Characterization of Charge-Carrier Transport in Semicrystalline Polymers: Electronic Couplings, Site Energies, and Charge-Carrier Dynamics in Poly(bithiophene-alt-thienothiophene) [PBTTT]

We establish a link between the microscopic ordering and the charge-transport parameters for a highly crystalline polymeric organic semiconductor, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). We find that the nematic and dynamic order parameters of the conjugated backbones, as well as their separation, evolve linearly with temperature, while the side-chain dynamic order parameter and backbone paracrystallinity change abruptly upon the (also experimentally observed) melting of the side chains around 400 K. The distribution of site energies follows the behavior of the backbone paracrystallinity and can be treated as static on the time scale of a single-charge transfer reaction. On the contrary, the electronic couplings between adjacent backbones are insensitive to side-chain melting and vary on a much faster time scale. The hole mobility, calculated after time-averaging of the electronic couplings, reproduces well the value measured in a short-channel thin-film transistor. The results underline that to secure efficient charge transport in lamellar arrangements of conjugated polymers: (i) the electronic couplings should present high average values and fast dynamics, and (ii) the energetic disorder (paracrystallinity) should be small.