Polaron spin dynamics in high-mobility polymeric semiconductors
S. Schott, U. Chopra, V. Lemaur, A. Melnyk, Y. Olivier, R. DiPietro, R. Carey, X. Jiao, C. Jellett, M. Little, A. Marks, C. McNeill, I. McCulloch, E. McNellis, D. Andrienko, D. Beljonne, J. Sinova, H. Sirringhaus
Polymeric semiconductors exhibit exceptionally long spin lifetimes and recently observed micrometer spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin-orbit and hyperfine interactions lie at the origin of their long spin lifetimes but the coupling mechanism of a spin to its environment remains elusive. Here we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges' hopping motion at low temperatures while an Elliott-Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime charge spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains.
Generic model for lamellar self-assembly in conjugated polymers: linking mesoscopic morphology and charge transport in P3HT
C. Greco, A. Melnyk, K. Kremer, D. Andrienko, K. Daoulas
We develop a generic coarse-grained model of soluble conjugated polymers capable of describing their self-assembly into a lamellar mesophase. Polymer chains are described by a hindered-rotation model where interaction centers represent entire repeat units including side chains. We introduce soft anisotropic nonbonded interactions to mimic the potential of mean force between atomistic repeat units. The functional form of this potential reflects the symmetry of the molecular order in a lamellar mesophase. The model can generate both nematic and lamellar (sanidic smectic) molecular arrangements. We parametrize this model for a soluble conjugated polymer poly(3-hexylthiophene) (P3HT) and demonstrate that the simulated lamellar mesophase matches morphologies of low molecular weight P3HT experimentally observed at elevated temperatures. A qualitative charge-transport model allows us to link local chain conformations and mesoscale order to charge transport. In particular it shows how coarsening of lamellar domains and chain extension increase the charge carrier mobility. By modeling large systems and long chains we can capture transport between lamellar layers which is due to rare but thermodynamically allowed backbone bridges between neighboring layers.
The PCPDTBT Family: Correlations between Chemical Structure Polymorphism and Device Performance
F. Fischer, G. Schulz, D. Trefz, A. Melnyk, M. Brinkmann, D. Andrienko, S. Ludwigs
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.
Macroscopic Structural Compositions of π-Conjugated Polymers: Combined Insights from Solid-State NMR and Molecular Dynamics Simulations
A. Melnyk, M. Junk, M. McGehee, B. Chmelka, M. R. Hansen, D. Andrienko
J. Phys. Chem. Lett.,
Molecular dynamics simulations are combined with solid-state NMR measurements to gain insight into the macroscopic structural composition of the π-conjugated polymer poly(25-bis(3-tetradecyl-thiophen-2-yl)thieno[32-b]thiophene) (PBTTT). The structural and dynamical properties as established by the NMR analyses were used to test the local structure of three constitutient mesophases with: (i) crystalline backbones and side chains (ii) lamellar backbones and disordered side chains or (iii) amorphous backbones and side chains. The relative compositions of these mesophases were then determined from the deconvolution of the 1H and 13C solid-state NMR spectra and dynamic order parameters. Surprisingly based on molecular dynamics simulations the powder composition consisted of only 28% of the completely crystalline mesophase while 23% was lamellar with disordered sidechains and 49% amorphous. The protocol presented in this work is a general approach and can be used for elucidating the relative compositions of mesophases in π-conjugated polymers.
Sub-ns Triplet State Formation by Non-Geminate Recombination in PSBTBT:PC70BM and PCPDTBT:PC60BM Organic Solar Cells
F. Etzold, I. A. Howard, N. Forler, A. Melnyk, D. Andrienko, M. R. Hansen, F. Laquai
Energy and Environmental Science,
The solid-state morphology and photo-generated charge carrier dynamics in low-bandgap polymer:fullerene bulk heterojunction photovoltaic blends using the donorÃÂ¢ÃÂÃÂacceptor type copolymers PCPDTBT or its silicon-substituted analogue PSBTBT as donors are compared by two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) and femto-to microsecond broadband Vis-NIR transient absorption (TA) pumpÃÂ¢ÃÂÃÂprobe spectroscopy. The 2D solid-state NMR experiments demonstrate that the film morphology of PCPDTBT:PC60BM blends processed with additives such as octanedithiol (ODT) are similar to those of PSBTBT:PC60BM blends in terms of crystallinity phase segregation and interfacial contacts. The TA experiments and analysis of the TA data by multivariate curve resolution (MCR) reveal that after exciton dissociation and free charge formation fast sub-nanosecond non-geminate recombination occurs which leads to a substantial population of the polymer's triplet state. The extent to which triplet states are formed depends on the initial concentration of free charges which itself is controlled by the microstructure of the blend especially in case of PCPDTBT:PC60BM. Interestingly PSBTBT:PC70BM blends show a higher charge generation efficiency but less triplet state formation at similar free charge carrier concentrations. This indicates that the solid-state morphology and interfacial structures of PSBTBT:PC70BM blends reduces non-geminate recombination leading to superior device performance compared to optimized PCPDTBT:PC60BM blends.