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Although the conversion reactions of electron-hole (e-h) pairs into emissive excitons are essential in the operation process of organic light emitting diodes (OLEDs), experimental research on behaviors of e-h pairs in operating OLEDs has not been developed so far. Here, three types of magnetic-resonance techniques that detect changes in current, photoluminescence intensity, and electroluminescence (EL) intensity induced by electron spin-resonance transitions have been applied to a working OLED as a function of operation bias. Combined use of these techniques reveals that e-h pairs exist in the OLED with completely different bias dependence from carriers and excitons. It is shown that the EL process is classified into three regions depending on the bias: pair-accumulation, pair-dissociation/recombination, and non-pair-formation regions.

The bias dependent behaviors in magnetic field effects (MFEs) of the current and the electroluminescence (EL) intensity in organic light emitting diodes (OLEDs) have been investigated from electrically-detected and EL-detected magnetic resonance (EDMR and ELDMR) techniques. An EDMR signal was not detected from the electron-only device, and the hole-only device gave only a much smaller EDMR signal than the OLED device. Both the EDMR and ELDMR signals observed from the OLED are concluded to primarily arise from the spin-dependent reaction of electron-hole (e-h) pairs. Both the normalized EDMR and ELDMR signal intensities decrease by increasing the operation bias of OLED, because the increased bias enhances the dissociation and recombination of e-h pairs beyond the increase in the pair-density by the bias. The bias-dependence curves of magneto-conductances and magneto-EL intensities are demonstrated to be very similar to those of the normalized EDMR and ELDMR, respectively. This similarity gives direct evidence that e-h pairs determine the MFEs of the present OLEDs at room temperature and that the MFEs are reduced by bias-dependent dissociation and recombination of e-h pairs. The bias-dependent EDMR and ELDMR experiments are thus effective as probing methods to examine the magnetic field properties via e-h pairs of OLEDs.

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.

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