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-alkyl­thiophene-2-yl)­thieno­[3,2‑<i>b</i>]thiophene) (PBTTT)

We investigate theoretically charge delocalization in radical cations, i.e., positive polarons, formed on oligomer chains of poly­(2,5-bis­(3-alkyl­thiophene-2-yl)­thieno­[3,2-<i>b</i>]­thiophene) (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

Spatial Orientation and Order of Structure-Defining Subunits in Thin Films of a High Mobility n‑Type Copolymer

Orientation and order of distinct molecular subunits in solid layers of the high mobility n-type copolymer poly­[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-1,4,5,8-naphthalene­diimide-2,6-diyl]-5,5′-(2,2′-bithiophene) P­(NDI2OD-T2) are investigated by means of infrared transition moment orientational analysis. This novel spectroscopic technique based on concurrent absorbance measurements of structure-specific bands in dependence on inclination <i>and</i> polarization of the incoming light enables to determine the complete tensor of absorption independently for each transition moment. As a result, for nanometer thin films pronounced in-plane anisotropy arising from self-aggregated order is detected, which, however, is no longer discernible for micrometer thick samples. In contrast, the out-of-plane orientation (inclination of molecular subunits) is retained irrespective of the widely varying layer thicknesses (150 nm vs 1.4 μm). Thus, the conception of the sample morphology occurs as stratification of slightly misaligned layers of oriented polymers; with increasing film thickness the macroscopic in-plane order diminishes, whereas the out-of-plane orientation is preserved