Organic devices based on nickel nanowires transparent electrode

Herein, we demonstrate a facile approach to synthesize long nickel nanowires and discuss its suitability to replace our commonly used transparent electrode, indium-tin-oxide (ITO), by a hydrazine hydrate reduction method where nickel ions are reduced to nickel atoms in an alkaline solution. The highly purified nickel nanowires show high transparency within the visible region, although the sheet resistance is slightly larger compared to that of our frequently used transparent electrode, ITO. A comparison study on organic light emitting diodes and organic solar cells, using commercially available ITO, silver nanowires, and nickel nanowires, are also discussed.

solution. The as-prepared NWs exhibits excellent transparency with good crystalline and surprisingly comparable OLEDs performance compared with our commercially available ITO. Figure 2 shows the morphology of the Ni NWs by scanning electron microscopy (SEM) with different magnification in which the diameters of Ni NWs ranged from 80 to 100 nm. Our Ni NWs were very long, with lengths up to approximately ~60 μ m. Figure S1 illustrates the distributions of Ni NW diameters and lengths. Observably, the presence of short Ni NWs is from deteriorated long Ni NWs during the purification process. The XRD pattern of Ni NWs is shown in Fig. 3. There are three peaks of Ni that correspond to different crystallization directions of (111), (200), and (220) located at 44.4°, 51.7°, and 76° which indicated that Ni NWs has a cubic crystal structure. Figure 4 collects the important features of Ni NWs deposit of glass substrates. Figure 4a shows the transmittance at 550 nm vs the sheet resistance (R S ). We obtained a good relation between the conductivity and transparency, specifically for the transparency below 90%. For the transparencies of 80%, 85%, 90% and 95%, our prepared Ni NWs demonstrate a R S of 12, 14, 19, and 48 Ω /sq., respectively. We suspect that the percolation threshold of Ni NWs was exceeded which makes the electrodes less conducive for a transparency over 95%. Figure S2 illustrates the transparency vs Ni NWs, where the 90% transparency is roughly 0.035 Ni NW g.m −2 . It is also worth noting   that our approach does not require a high annealing temperature, which gives excellent compatibility with solution process devices, which are relatively sensitive to temperature. It is worth noting that the best quality of Ag NWs deposited on glass substrate had the lowest R S of about 10.2 Ω /sq. along with 89.9% transparency 37 .

Results and Discussion
Meanwhile, Fig. 4b depicts the haze factor at 500 nm transmittance. Haze factor is a relatively important property for various optoelectronic devices [38][39][40] . Haze factor depends on Ni NWs network density in which an 11% haze value corresponds to approximately 85% transparency, whereas only 4% were obtained at a 97% transparency. This trend was expected since the high density of Ni NWs caused more light scattering. One should also note that there is no optimum haze factor value since it varies with applications. Figure 4c illustrates an absorption spectrum of Ni NWs thin film. In order to determine the chemical state of Ni NWs, X-Ray photoelectron spectroscopy (XPS) measurements were conducted (Fig. 4d). The binding energy of the most intense Ni 2p 3/2 component of the Ni 2p doublet located at 855.5 eV corresponds to nickel oxide (Ni 2+ ). In agreement with this is the observation of a satellite line, at about 8 eV higher binding energy of each Ni 2p component, which is the fingerprint of Ni 2+ ions. It is likely that Ni NWs exposed to ambient atmosphere quickly become oxidized. Figure 4e shows the average diameter and average grain diameter as a function of Ni 2+ concentrations. As one sees, the average diameter increases with concentration. It is worth noting that the Ni NWs prepared in this work are relatively smooth and uniform. The uniformity of NWs changed completely when the concentration was doubled (from 15 to 30 mM) and detailed regarding uniformity changes will be published elsewhere. On the other hand, the average grain diameter increases linearly and becomes saturated above 35 mM. We conclude that the prepared Ni NWs is efficient and provides an easy path to achieve high performance Ni NWs based TCEs. As we already have an excellent TCE, the next possible step is to implement the prepared transparent Ni NWs electrodes in OLEDs, as shown in Fig. 5.
Please note that besides fabricating organic devices based on Ni NWs and ITO based TCEs, we also prepared organic devices employing Ag NWs as the TCE. The basic architecture of the OLED consists of seven layers: Glass/ Ni NWs/PEDOT:PSS (30 nm)/TAPC (20-nm)/10 ± 1 wt% Ir(2-phq) 3 :MTXSFCz (35-nm)/TmPyPB (55-nm)/ LiF(0.9-nm)/Al (100-nm), where poly(3,4-ethylenedioxythiophene) (PEDOT) was used as the hole-injection layer (HIL), 1,1-bis[4-[N,N'-di(p-tolyl)amino]phenyl] cyclohexane (TAPC) was used as the hole-transporting layer (HTL), and 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB) was used as the electron-transporting layer (ETL) and hole-blocking layer (HBL). Figure 5a,b show the current density− voltage− luminance characteristics of the OLEDs prepared, using Ni NWs as the bottom electrode. The results were compared with the properties of a reference OLED, prepared with our commercially available Ag NWs and ITO. The OLEDs with an ITO electrode demonstrated a slightly higher operating voltage (Fig. 5c,d), marginally lower current density, and a lower luminous efficiency (14.16 cd/A and 12.20 lm/W) than the device prepared with Ni NWs (15.23 cd/A and 12.91 lm/W). The large electron injection energy barrier, from the ITO (with a work function of ~4.8 eV) to the PEDOT:PSS (with a lowest unoccupied molecular orbital, LUMO, energy level of ~2.2 eV), impeded electron injection and reduced the current density. The improved performance of the device prepared with Ni NWs (conduction band of ~3.87 eV), convincingly illustrated that Ni NWs TCE could effectively reduce the electron injection barrier from the anode to the electron injection layer, and thus increase the electron injection into the emitting layer. Moreover, we have also fabricated OLED utilizing Ag NWs as the bottom electrode. The operating voltage for Ag NWs is 3.4 V. The current, and power efficiencies are 13.28 cd/A, and 11.12 lm/W, respectively. The probably explanation for a higher operating due to the mismatch of the energy alignment between Ag NWs and PEDOT:PSS. In addition, our approach offers an effective strategy for preparing air-stable, efficient, and flexible display and lighting devices to replace the more brittle devices based on ITO electrodes. Based on our literature review, this is the first OLED realized with Ni NWs TCE.
Another possible demonstration has been realized by the fabrication of organic solar cells (OSCs). In this final demonstration, we employed poly [4,8- Figure 6 illustrates the typical current density-voltage and external quantum efficiency characteristics measured at 100 mW/cm 2 . Our proposed Ni NWs based OSC shows an efficiency of about 6.21% compared to the efficiency of ITO based OSC of 8.17% (Fig. 6a). Our proposed transparent electrode demonstrates relatively low J sc and FF compared to the ITO transparent electrode due to low shunt and high series resistance. We believe that good morphology of the active layer is the key parameter for obtaining high FF. In general the series resistance indicates the integral conductivity of the device directly related to its internal carrier mobility while shunt resistance associated to the loss of current density via carrier recombination within the device, particularly at the interfaces of each layer. Shunt resistance is basically due to the manufacturing defects, rather than poor device design. High series resistance implies that the interphase contact is less desirable and the conductivity of every layer of the device is low. A low shunt resistance suggests that the power loss in the device through an alternate current path is very large, resulting in small FF. It is worth noting that the V oc obtained in all devices are similar since it is solely determined by the HOMO and LUMO of the active materials used. The control devices fabricated employing Ag NWs show a slightly less performance to Ni NWs devices. The Ag NWs demonstrated a J sc = 11.35 mA/cm 2 , a V oc = 0.76 V, a FF = 60.70%, and a PCE = 5.24%. We attribute this undesirable device performance of Ag NWs to a higher RMS roughness of Ag NWs compared to that of Ni NWs (Ag NWs = > 30 nm vs. Ni NWs = <13 nm) and high R S (possible causing short-circuiting issue) (See Supporting Information for more details of Ag NWs). Figure 6b shows the EQE of all devices, and we can see that the EQE of Ni NWs based OSC exceeded 60% in the visible. On the other hand, the EQE of Ag NWs is very much less compared to that of Ni NWs with an average of 50.98% in the visible region. Although, the performance of Ni NWs based OSC is slightly lower compared to that of the ITO based OSC, it is appealing for future application and could be used to replace our commonly used transparent electrode, ITO.

Conclusions
In conclusion, we developed a novel approach to fabricating high performance transparent electrodes based on nickel nanowires. The nanowires are synthesized according to a facile direct template-free method. The sheet resistance of 19 Ω /sq. at 90% is obtained without any acidic treatment. The electrode fabrication process is carried out at room temperature from nanowires in solution. There is no need for post-treatments such as thermal annealing of forming gas. Moreover, we also demonstrate the fabrication of a highly efficient organic light emitting diode, comparing our proposed transparent electrode with a commercially available ITO electrode, in which we demonstrated that the performance of our proposed transparent electrode has outperformed the organic light emitting diode based on the ITO electrode.