Electron–Hole Pairs Generated in the Crystalline Phase of Polymer Diodes Studied by Electrically Detected Magnetic Resonance Techniques

Abstract

The properties of electron−hole (e−h) pairs generated in a working poly(3-hexylthiophene) (P3HT) diode are investigated by electrically detected magnetic resonance (EDMR) techniques. The EDMR intensity is shown to increase with increasing density of injected electrons for a given hole density, demonstrating that the EDMR signal arises from an e−h pair. The EDMR spectrum consists of two Gaussian curves, one of which gives a g-value very similar to that of hole carriers, suggesting that the EDMR spectrum is given by the sum of electron spin resonance (ESR) spectra from hole and electron carriers forming e−h pairs. Bias-dependent correlation between carriers and e−h pairs under diode operation is examined directly from independent measurements of near-infrared (NIR) spectroscopy for carriers and EDMR for e−h pairs. When the bias is increased to above a threshold, the EDMR signal is strongly reduced despite a gradual increase in the NIR signals, providing evidence that the e−h pairs are dissociated by an electric field in the bias region. Because of this property of the pair, the correlation between the carriers and e−h pairs inside the diode changes depending on the bias magnitude: the carriers and e−h pairs coexist or only the carriers exist. The e−h pair is presumed to be formed across different lamellae because of the crystalline phase of the P3HT film. Although the intensity of the EDMR signal from the interlamellar e−h pair is small, the intensity increases substantially when the sample is exposed to air, suggesting the formation of a magnetoreactive species that incorporates oxygen.

Recombination of Free Carriers and Space Charges in Poly(3-hexylthiophene), as Revealed by Electrically and Capacitively Detected Magnetic Resonances

“…The resonance field corresponds to g = 2.00, and the negative polarity indicates a reduction of the photocurrent by ESR. The spectrum consists of narrow ( C n ) and broad ( C b ) components that substantially overlap each other, in agreement with the optically detected magnetic resonance spectrum of the P3HT film and the EDMR spectrum of a P3HT-based diode in the dark . The simulation using two Gaussian functions (Figure a) clarifies that the half-widths at half-maxima ( B 1/2 ) for the C n and C b components were 0.11 and 0.50 mT, respectively.…”

“…The spectrum consists of narrow (C n ) and broad (C b ) components that substantially overlap each other, in agreement with the optically detected magnetic resonance spectrum of the P3HT film 39 and the EDMR spectrum of a P3HT-based diode in the dark. 11 The simulation using two Gaussian functions (Figure 3a) clarifies that the half-widths at half-maxima (B 1/2 ) for the C n and C b components were 0.11 and 0.50 mT, respectively. The photo-EDMR spectrum of the ITO|P3HT|ITO device shows similar characteristics (Figure 3b).…”

“…Electrically detected magnetic resonance (EDMR), which records the conductivity change due to electron spin resonance (ESR) of paramagnetic species, is a powerful experimental technique to investigate the encounter pairs with spin-dependent dynamics in organic devices, including e–h pairs, − bipolarons, − and triplet-charge pairs . A related technique, capacitively detected magnetic resonance (CDMR), which is a combination of capacitance spectroscopy and ESR, is useful for investigating spin-dependent reactions of encounter pairs at the space-charge region. ,− Therefore, in the present article, to clarify the recombination dynamics of the free carriers and the space charges, we conducted both EDMR and CDMR for two types of devices under illumination: one with Schottky contact of regioregular poly­(3-hexylthiophene-2,5-diyl) (P3HT) and one with Ohmic contact of regioregular P3HT.…”

Photogenerated carrier dynamics of TIPS-pentacene films as studied by photocurrent and electrically detected magnetic resonance

“…In the dynamics shown in Scheme 1, the equal amounts of electrons and holes are expected to generate by photoexcitation followed by the charge separation, which are as observed in the EDMR of OLED. 22 However, in the present system, the intensity of the two components in the EDMR signal differed significantly and the area-integrated intensity of Lorentzian component expected with higher mobility was much smaller than that of the Gaussian component with relatively low mobility. Therefore, the possibility of the former mechanism can be rule out.…”

Probing an Isolated Conjugated Polymer Molecule with Radiation-Generated Spin-Correlated Polaron Pairs

“…The isotropic value of the g -factor, ( g xx + g yy + g zz )/3, for the P3HT-rra negative polarons is approximately 2.0035. This value is lower than the value of 2.0045 observed for these particles in solution but falls within the range of values of 2.0031–2.0044 obtained by EPR spectroscopy of the solid phase in steady-state experiments. − The scatter between the observed average g -values may be attributed to differences between the conformations of P3HT molecules that lead to a different partial averaging of the g -tensor anisotropy during migration of a negative polaron along the polymer chain. This result implies that in this regard, the polaron migration along a P3HT-rra molecule isolated in the PVC matrix does not differ substantially from the migration in pristine P3HT-rra.…”