Exploring the behavior of electron-hole pairs in working organic light emitting diodes

Abstract

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.

Correlation between bias-dependent ESR signals and magnetic field effects in organic light emitting diodes

“…This spectral resemblance was also confirmed at other bias-voltages. 24 The EDMR and ELDMR signals are thus given by common ESR transitions regardless of the bias. EDMR measurements were also performed for the electron-only device (ITO/PEI/SY-PPV/PEI/Al) and the holeonly device (ITO/MoO 3 /SY-PPV/MoO 3 /Al) of SY-PPV [ Fig.…”

“…28,30 In this case, the EDMR and ELDMR spectra can be explained by the sum of spectral components from hole and electron carriers. 31,32 In reality, the observed spectra can be reproduced by the sum of two Gaussian curves with the full width half maximum (FWHM) of 3.1 G and 14.5 G. 24 Interestingly, the EDMR spectrum of the hole-only device was fitted well with a single Gaussian curve with a FWHM of 4.0 G: note that the FWHM is almost independent of the bias (see the supplementary material) and this width is relatively close to the smaller width of the OLED sample. Moreover, the feature of the smaller width in the hole carrier corresponds well with the recent reports that the MC curve of the hole-only device in polymer OLEDs is narrowed than that of the electron-only device.…”

“…Considering the ESR-induced reduction of n SP , the negative EDMR signal indicates the relation of d SP k TPE > d TP k SPE , where d SP and d TP are the dissociation rates of SP and TP into carriers, respectively, and k SPE and k TPE are the transition rates of SP to singlet exciton and TP to TE, respectively: for detail, see the supplementary file of Ref. 24.…”

“…Recently, we showed that the normalized intensities of electrically-detected and electroluminescence (EL)-detected magnetic resonance (EDMR and ELDMR, respectively) signals of OLEDs, calculated by the EDMR and ELDMR intensities divided by the dark current and the EL intensity, respectively, are reduced by increasing the operation bias of OLED. 24 The signal-reduction was attributed to the dissociation of e-h pairs by the electric field. 24 Here, we discuss the biasdependent ESR properties of OLED in detail through comparison of those with hole-only and electron-only devices, and thereby, we address the role of e-h pairs on the ESR signals and MFEs in both the current and EL properties.…”

“…24 The signal-reduction was attributed to the dissociation of e-h pairs by the electric field. 24 Here, we discuss the biasdependent ESR properties of OLED in detail through comparison of those with hole-only and electron-only devices, and thereby, we address the role of e-h pairs on the ESR signals and MFEs in both the current and EL properties. In particular, the results of the biasdependence experiments indicate that both the efficiencies of MC and MEL are reduced by increasing the forward bias due to the dissociation of e-h pair.…”