Relating chemical structure and molecular packing to charge transport in conjugated polymers
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Abstract
Polymer solar cells are a promising, renewable source of electricity, which currently exhibit low efficiencies. To improve solar cell performance, one needs to understand how microscopic processes are linked to the material's chemistry, local ordering, and macroscopic composition. In the following a combination of first-principles calculations, molecular dynamics simulations, perturbative energy calculations and, Marcus charge transfer theory are used for multiscale studies of two polymer systems, Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b'] dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and Poly[2,5-bis (3-tetradecylthiophen-2-yl) thieno [3,2-b] thiophene] (PBTTT).
In PCPDTBT and its four derivatives, a number of possible crystalline morphologies are simulated in a bottom-up scheme, starting from the chemical structure. Four polymorphs, including two π-stacked configurations, are found and verified against experimental data. Also, the key degrees of freedom, responsible for crystal formation, are identified. In π-stacked structures, transfer integrals, site energies, and mobilities are calculated. The calculated energetic disorder is high and comparable to the one in amorphous materials. It originates from electrostatics of individual donor and acceptor units in the backbone and from the disordered structure of the side-chains. Chemical substitution increases the disorder, while the push-pull architecture has no effect on it. Resulting mobilities are hindered by the large energetic disorder.
For the second polymer, PBTTT, the available NMR data is combined with molecular dynamics simulations and an analytical model to assess its dynamics and macroscopic composition. The dynamics is addressed via a generalized order parameter. Elevated temperature MD simulations are used to extrapolate side-chain dynamics to the μs time-scale. The macroscopic composition is resolved via an analytical model which combines microscopic parameters obtained from simulations and macroscopic averages extracted from NMR measurements. The original NMR data suggests a two-mesophase composition with 1:1 crystalline to amorphous mesophase ratio. In combination with the analytical model and simulations, a three-mesophase model is proposed in which crystalline, intermediate and amorphous mesophases have 8%, 42% and 50% volume fractions. A formula is derived, which can be used to interpret the crystalline composition of other semi-crystalline polymers.