Reduced exchange narrowing caused by gate-induced charge carriers in high-mobility donor–acceptor copolymers

Variations in exciton absorption resulting from charge accumulation in various semiconducting donor–acceptor (DA) copolymer thin films were systematically investigated by gate modulation (GM) spectroscopy by using the field-effect transistor device structure. The GM spectra obtained for high-mobility DA copolymer thin films exhibited second-derivative like line shapes due to an effect of spectral broadening of ordinary exciton absorption spectra by accumulated charges. In contrast, the GM spectra obtained for relatively low-mobility DA copolymer thin films exhibited simple bleaching of exciton absorption spectra, as well as observed for non-DA-type polymers like poly(3-hexylthiophene-2,5-diyl) (P3HT). From a systematic comparison of the GM spectra with temperature-dependent absorption spectra for the polymers in solution, we found that the spectral broadening observed in the GM spectra can be attributed to a reduced effect on the exchange narrowing where excitonic transitions of individual polymer chains are coherently coupled within highly ordered crystalline domains in the polymer thin films. We discuss that the gate-induced charge accumulation in the polymer films effects to suppress the exciton coherence length, which contributes to the reduced exchange narrowing. We also discuss that the whole feature of the GM spectra can be understood in terms of a decomposition into ordered and disordered polymers and that the GM spectra can be used as fine probes for a degree of structural ordering in semiconductor channels of polymer field-effect transistors.

Figure 1

Molecular structures, molecular weights $\left(M_{\mathrm{w}}\right)$, polydispersity indices (PDI), lengths of monomer unit, and lengths of polymer chain for seven kinds of semiconducting polymers.

Figure 2

(a) Schematic device structure of a thin-film transistor. (b) GM spectra measured for thin-film transistors with seven kinds of semiconducting polymers as the semiconductor channels (black line). The red and blue lines are the absorption spectra measured for the thin films and their second derivatives, respectively.

Figure 3

Peak intensity $(\mathrm{\Delta }\alpha d/\alpha d)$ and peak width $(W_{\mathrm{GMS}}/W_{\mathrm{abs}})$ of the GM spectra normalized by those of the ordinary absorption spectra. Both parameters are plotted as a function of carrier mobility.

Figure 4

(a) Temperature dependence of the absorption spectra measured for a 1,2,4-trichlorobenzene solution of DA1, in which the molar absorption coefficient $\left(\varepsilon \right)$ is plotted as a function of the photon energy. The absorption spectrum measured for the thin film is shown for comparison (broken line). (b) Concentration dependence of the absorption spectra measured for nitrobenzene solutions of DA1 at room temperature.

Figure 5

(a) Schematic representation of the electronic structure of an isolated polymer and a polymer aggregate, in which the ground and excited states are abbreviated as GS and ES, respectively. (b) Schematic representation of absorption spectra for an isolated polymer and a polymer aggregate. (c) Mechanisms of charge-induced changes in the absorption spectra for ordered and disordered polymer aggregates.

Figure 6

(a) Fraction of polymer aggregates in the DA1 solution, as estimated from the spectral fitting. The inset shows spectral components of the polymer aggregates and isolated polymers obtained by fitting of the solution spectrum measured at 413 K. (b) Peak position and width for spectral component of polymer aggregates, as determined by Gaussian fitting. The inset shows the spectral component of polymer aggregates at 303 K whose vibrational progressions are fitted by three Gaussians. (c) Exciton coherence length $L_{\mathrm{coh}}$ estimated from the peak width of the polymer aggregates. (d) Absorption spectra measured for a 1,2,4-trichlorobenzene solution of DA1 at 453 K and a thin film of DA1 at room temperature. The red lines are vibrational progressions fitted by Gaussians.

Figure 7

(a) Difference spectra for two different temperatures for the DA1 solution; $\mathrm{\Delta }{\varepsilon }_{\mathrm{HT}}$ between 423 and $433\mathrm{K}$ and $\mathrm{\Delta }{\varepsilon }_{\mathrm{LT}}$ between 303 and $313\mathrm{K}$. (b) A summed spectrum of $\mathrm{\Delta }{\varepsilon }_{\mathrm{HT}}$ and $\mathrm{\Delta }{\varepsilon }_{\mathrm{LT}}$ with a ratio of 2:5 (red line), and a GM spectrum of a thin film of DA1 (black line). (c) Schematic representation of a DA1 thin film consisting of high- and low-coherence domains.

Figure 8

(a) Out-of-plane and in-plane x-ray diffractions measured for the thin film of DA1. Background halo pattern due to a quartz glass substrate is subtracted for the shown data. (b) Schematic for lamellar structure of DA1.