26 research outputs found
Transient electrically detected magnetic resonance spectroscopy applied to organic solar cells
The influence of light-induced paramagnetic states on the photocurrent
generated by polymer:fullerene solar cells is studied using spin-sensitive
techniques in combination with laser-flash excitation. For this purpose, we
developed a setup that allows for simultaneous detection of transient electron
paramagnetic resonance as well as transient electrically detected magnetic
resonance (trEDMR) signals from fully processed and encapsulated solar cells.
Combining both techniques provides a direct link between photoinduced triplet
excitons, charge transfer states, and free charge carriers as well as their
influence on the photocurrent generated by organic photovoltaic devices. Our
results obtained from solar cells based on poly(3-hexylthiophene) as electron
donor and a fullerene-based electron acceptor show that the resonant signals
observed in low-temperature (T = 80 K) trEDMR spectra can be attributed to
positive polarons in the polymer as well as negative polarons in the fullerene
phase, indicating that both centers are involved in spin-dependent processes
that directly influence the photocurrent
Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer
We investigate the delocalization of holes in the semicrystalline conjugated
polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by
directly measuring the hyperfine coupling between photogenerated polarons and
bound nuclear spins using electron nuclear double resonance spectroscopy. An
extrapolation of the corresponding oligomer spectra reveals that charges tend
to delocalize over 4.0–4.8 nm with delocalization strongly dependent on
molecular order and crystallinity of the PBTTT polymer thin films. Density
functional theory calculations of hyperfine couplings confirm that long-range
corrected functionals appropriately describe the change in coupling strength
with increasing oligomer size and agree well with the experimentally measured
polymer limit. Our discussion presents general guidelines illustrating the
various pitfalls and opportunities when deducing polaron localization lengths
from hyperfine coupling spectra of conjugated polymers
Charge Delocalization in Oligomers of Poly(2,5-bis(3-alkylÂthiophene-2-yl)ÂthienoÂ[3,2‑<i>b</i>]thiophene) (PBTTT)
We investigate theoretically
charge delocalization in radical cations,
i.e., positive polarons, formed on oligomer chains of polyÂ(2,5-bisÂ(3-alkylÂthiophene-2-yl)ÂthienoÂ[3,2-<i>b</i>]Âthiophene) (PBTTT). We use nonempirically tuned range-separated
density functionals (TRS-DFT), including LC-ωPBE, LC-BLYP, and
ωB97XD. We consider the evolution with oligomer length of the
molecular geometric and electronic structures, optical absorption
features, and spin densities. The TRS-DFT results indicate that a
positive polaron can delocalize ideally over some 10 thiophene rings
when the backbone is nonplanar and up to 14 rings for a backbone forced
to be completely planar. Interestingly, up to six polarons can coexist
side-by-side in a hexamer (which contains 24 thiophene rings), which
is consistent with the highest degrees of doping (oxidation) experimentally
achievable in polythiophene derivatives