Electron Spin Resonance of Thin Films of N,N′-Di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) Doped by Iodine Vapor

Abstract

We have successfully observed the ESR signals of radical cations in the thin films of N,N¤-di(1-naphthyl)-N,N¤-diphenylbenzidine (NPB), typical hole-transport material, for the first time to our best knowledge. In order to characterize the radical cation state, ESR spectra obtained upon chemical doping with iodine are analyzed combined with density functional theory (DFT) calculation. The ESR line width is inversely correlated with doping concentration.Organic light-emitting diodes (OLEDs) have been actively studied because of their excellent characteristics such as high efficiency, low cost productivity, and flexibility.1,2 Aromatic amines are widely used as hole-transport materials in the devices. Among them, N,N¤-di(1-naphthyl)-N,N¤-diphenylbenzidine (NPB) (Scheme 1) is a typical one with high thermal stability.3 Blue-color luminescence was also reported in an OLED containing NPB layer.4 In order to understand the holetransport properties, it is quite important to elucidate the nature of cationic state. Since cations are usually accompanied by radical spins, electron spin resonance (ESR) spectroscopy is a useful technique for this purpose. Although the thin films of NPB have been characterized by various methods like photoemission, 5 time-of-flight, 6 infrared, 7 and Raman spectroscopies, 8 the ESR study of NPB film has not been reported to our best knowledge. We have utilized this technique for other organic materials in thin films 9 and in organic devices. 1012 In the present letter, we carried out chemical doping by exposing the NPB films to iodine vapor and successfully observed ESR signals arising from the induced radical cations. We also performed density functional theory (DFT) calculations to obtain insights to the radical cation state.By thermal vacuum evaporation under the pressure of 4 © 10 ¹4 Pa, NPB was deposited on a quartz substrate cleaned with isopropyl alcohol and acetone. The NPB film is 80nm in thickness and 0.84 cm 2 in area. Subsequently, the film was doped using iodine vapor under vacuum of 1 © 10 ¹1 Pa for 1 h, then it was sealed in an ESR sample tube. The doping concentration was reduced by exhausting the sample tube using a diffusion pump and was controlled by the length of the exhaustion time. ESR measurements for the sample were performed with a JEOL JES-FA200 ESR spectrometer.13 DFT calculations of the B3LYP/6-31G(d) level were carried out for an isolated NPB cation.14 The molecular geometry was fully optimized. The principal values of g tensor were computed using the gauge-independent atomic orbital method. Figure 1 shows observed ESR spectra: (a) spectrum observed right after the iodine doping and (b) that measured after an exhausting time of five hours. As seen in this figure, clear ESR signals were observed in both cases. In Figure 1a, g value, peak-to-peak ESR line width ¦H pp , and spin concentration of the ESR spectrum are 2.0040, 0.676 mT, and 2.65 © 10 20 cm ¹3, respectively. Meanwhile, in Figure 1b, they are 2.0036, 1.06 mT, and 2.12 © 10 19 cm ¹3, respectively. The spi...

Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium

“…In order to further confirm the localization of charge carriers, temperature dependence of the ΔH pp was measured with the sample in the ratio of 1:1 in the temperature range between 4.2 and 300 K. Generally, in case of highly mobile charge carriers, ΔH pp is expected to be narrowed as increasing the temperature because of motional narrowing. Our previous study on NPB thin films doped by iodine vapor showed that the ΔH pp is considerably narrowed as increasing the temperature due to the motional narrowing [9]. On the other hand, in this study, only small temperature dependence was observed, indicating almost no motional narrowing behavior for charges in Mg-doped Alq 3 .…”

“…We have applied this spectroscopy not only to organic semiconducting materials [4] but also to organic semiconductor devices under operation such as organic field-effect transistors (OFETs) [5][6][7] and organic solar cells (OSCs) [8]. We also performed ESR study to investigate charge-carrier states of radical cations in N,N'-Di(1-naphthyl)-N,N'-diphenylbenzidine (NPB), a typical hole transporting material in OLEDs [9]. Pawlik et al demonstrated that ESR study is also effective for OLEDs [10].…”

Direct observation of spins at bathocuproine (BCP) interfaces: An electron spin resonance study on BCP/metal (Al or Au) thin films

“…If electron spins are immobile, the observed ESR linewidth does not depend on temperature because the hyperfine interactions does not alter [31,32]. However, if electron spins are thermally activated and become mobile in amorphous solids, the hyperfine interactions become averaged and effectively small, which results in the narrowing of the ESR linewidth [30]. From the observation of the motional narrowing of the ESR linewidth, we conclude that a majority of the spins at the interfaces between BCP and Al are mobile.…”

“…Actually, we have clarified spin states of charge carriers and their spin concentration not only in organic semiconducting materials [24] but also in organic semiconductor devices such as organic field-effect transistors (OFETs) [25][26][27] and OPVCs [28,29]. Our ESR investigations are also involved in the charge-carrier states of radical cations in a typical hole-transporting material [30] and radical anions in a typical organic light-emitting material tris(8-hydroxyquinoline) aluminum (Alq 3 ) [31,32]. We note that there is yet no ESR study of BCP on this issue.…”

Direct Observation of Radical States and the Correlation with Performance Degradation in Organic Light‐Emitting Diodes During Device Operation

“…To further exclude the possibility of the radical formation due to NPB‐related molecules from the origin of the signal B, we have examined the contribution of halogens remained in NPB molecules. It has been reported that the g factor of NPB molecules becomes larger by iodine doping to NPB molecules . In addition, halogens are generally used for the NPB synthesis and may possibly remain in NPB molecules.…”