{'fraction_non_alphanumeric': 0.026759474091260633, 'fraction_numerical': 0.01531322505800464, 'mean_word_length': 4.512361466325661, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Direct measurement of the magnetic field effects on carrier mobilities and recombination in tri-(8-hydroxyquinoline)-aluminum based light-emitting diodes

The magnetic field effects on the carrier mobilities and recombination in tri-(8-hydroxyquinoline)-aluminum (Alq3) based light-emitting diodes have been measured by the method of transient electroluminescence. It is confirmed that the magnetic field has no effect on the electron and hole mobilities in Alq3 layers and can decrease the electron-hole recombination coefficient. The results imply that the dominant mechanism for the magnetic field effects in Alq3 based light-emitting diodes is the interconversion between singlet e-h pairs and triplet e-h pairs modulated by the magnetic field when the driving voltage is larger than the onset voltage of the electroluminescence.

The effects of magnetic fields (MFEs) on the emission and current in organic light emitting diodes (OLEDs) with nonmagnetic electrodes have been observed in 2003. 1 Since then, many groups have found that the MFEs are general phenomena which can be observed in a range of both low-molecular weight molecules and high-molecular weight polymers.  Despite the large progress in the research of MFEs, its underlying mechanism still deserves further investigations. Up to date, two competing basic models have been proposed, one is the excitonic model, [8][9][10][11] and the other is the bipolaron model. 12 The exciton model bases on that the magnetic field can either modulate the interconversion between singlet e-h pairs (e-h 1 ) and triplet e-h pairs (e-h 3 ) or modulate the reaction between e-h 3 and free carriers. The e-h pair is referred to the electron and hole that locate on different molecules and have Coulombic attraction to each other. Prigodin et al. 8 and Bergeson et al. 9 have suggested that organic magnetoresistance (OMAR) is due to the change in the e-h recombination coefficient caused by the change in the singlet-triplet mixing. Desai et al. 10 find that OMAR occurs only when there is light emission from the devices, which suggests that the OMAR is related to exciton formation. They and Hu et al. 11 suggest that the effect may be ascribed to the reaction between e-h 3 and free carriers. The bipolaron mode bases on the formation of doubly charged bipolarons during the hopping transport through the organic film, which was proposed by Bobbert et al. 12 The bipolaron model considers the inter-charge spin interaction and suggests that the mobilities are a function of inter-charge spin interaction. Therefore, it becomes crucial to experimentally test the MFEs on carrier mobilities and recombination to understand the underlying mechanism of MFEs in organic semiconductors.
In this work, we have measured the MFEs on carrier mobilities and e-h recombination process in tri-(8-hydroxyquinoline)-aluminum (Alq 3 ) based OLEDs by using transient electroluminescence (EL) method [26][27][28][29] . Compared to the method using constant voltage or current source, the transient EL method has the following merits  Figure 1 (a). The NPB is the hole-transport layer while the Alq 3 functions as both emissive and electron-transport layer. Figure 1 (a) shows the full EL response of device A at the driving voltage from 2 V to 4 V with and without the magnetic field. As can be seen from figure 1 (b), the rising edges of the EL pulses with (red line) and without (blue line) the magnetic field overlap perfectly, thus the magnetic field has no effect on the delay times of t d . In generally, the t d is thought to be the transit time for the injected electrons from the cathode to the recombination zone in Alq 3 layer of device A, because the hole mobility in NPB layer is at least one order of magnitude larger than the electron mobility in Alq 3 layer, the transit time for hole in the NPB layer can be neglected. The mobility of electron in Alq 3 layer can be calculated by  e =L/(t d E), approximately, where L is the thickness of Alq 3 layer and E is the electric field in Alq 3 layer (the E field in the NPB layer is neglected). Figure 3 shows the calculated mobility of electron in Alq 3 at the driving voltage from 2 V to 4 V, which is consistent with the results reported by S. Barth et al. 28 The magnetic field has no effect on the electron mobility in Alq 3 layer. This experimental result indicates that the effect of magnetic fields on the electron transport process is not the dominant mechanism for the MFEs in this Alq 3 -based device when the driving voltage is larger that the onset voltage of the EL. Desai et al.
suggest the magnetic fields may influence the carrier mobility caused by the scatter effect of the triplet on the carriers 10 . Since the t d is only related to the electron transport process in Alq 3 layer and is obtained before the creation of the excitons in our experiment, the scatter effect of the triplet on the mobility of electrons derived from t d can be ignored. Thus our result that the magnetic field has no effect on the electron mobility in Alq 3 layer has no contradiction with the suggestion of Desai et al.
Where Ф EL is the EL intensity. Then the bimolecular recombination coefficient r can be expressed as: 33 Where e is the electron charge, d is the thickness of the device and j is the current density. The parameters of A and S in equation (2) 34 As can be seen from figure 2 (b), the rising edges of the EL pulses with (red line) and without (blue line) the magnetic field overlap perfectly, which verifies the magnetic field has no effect on the hole mobility in Alq 3 layer. The EL decay curves for the device with and without the magnetic field are separated from each other in the slow component as shown in figure 2 (d). Therefore, it confirms than the magnetic field affects the bimolecular recombination process again. The similar results can be obtained for the driving voltage from 6 V to 12 V. provide the evidences that the magnetic field has no effect on the electron and hole mobilities in Alq 3 layer. Therefore, it can be suggested that the effect of magnetic fields on the carrier transport process is not the dominant mechanism when the driving voltage is larger than the onset voltage of the EL, and the magnetic field actually can decrease the e-h recombination coefficient.
We are grateful for financial support from National Natural Science Foundation of China (grant numbers 60878013 and 60706016) and "111" Program (B06009).