A hybrid inorganic–organic light-emitting diode using Ti-doped ZrO2 as an electron-injection layer

We have fabricated stable efficient iridium(iii)-bis-5-(1-(naphthalene-1-yl)-1H-phenanthro[9,10-d]imidazole-2-yl) benzene-1,2,3-triol (acetylacetonate) [Ir(NPIBT)2 (acac)] doped inverted bottom-emissive green organic light-emitting diodes using Ti-doped ZrO2 nanomaterials as the electron injection layer. The current density (J) and luminance (L) of the fabricated devices with Ti-doped ZrO2 deposited between an indium tin oxide cathode and an Ir(NPIBT)2 (acac) emissive layer increased significantly at a low driving voltage (V) compared with control devices without Ti-doped ZrO2. The Ti-doped ZrO2 layer can facilitate the electron injection effectively and enhances the current efficiency (ηc) of 2.84 cd A−1 and power efficiency (ηp) of 1.32 lm W−1


Introduction
Great research effort has been directed to the area of organic light emitting diodes (OLEDs) to enhance device stability and efficiency for future display applications. [1][2][3][4] However, environmental stability and efficiency are the major issues for the development of device structures. [5][6][7][8] The low work function metals, barium or calcium as electron injecting layers (EILs) may easily degrade in the presence of oxygen and moisture. 9,10 Due to low-cost, visible-light transparency, environmental stability, carrier transport properties and tuning lm morphology to micrometer scales of metal oxides they are both attractive candidates in OLEDs. [11][12][13][14][15] They are used as hole (HIL)/ electron (EIL) injection layers in hybrid organic-inorganic lightemitting diodes (HyLEDs). 11,12 Titanium dioxide 11,12,16 /zinc oxide 13,[17][18][19] lms are employed as EIL where as molybdenum trioxide 11,13 is used as HIL in HyLEDs. HyLEDs based on poly (9,9-dioctyluorene-altenzothiadiazole) (F8BT) as an electroluminescent layer combined with ZnO and MoO 3 as EIL and HIL, respectively, exhibit maximum luminance (6500 cd m À2 ). 13 Efficiencies of HyLEDs were tuned by both hole/electron injection from metal oxide EIL into E LUMO of emissive layer. [20][21][22][23][24][25][26][27][28] Bolink et al., fabricated ITO/TiO 2 /F8BT/MoO 3 /Au HyLEDs results poor efficiences due to (i) higher energy barrier for injection from ITO to TiO 2 and (ii) poor hole blocking functionality of titania. 29 Therefore, it is urgent need to nd alternate EILs which enable effective electron injection into emissive layer E LUMO . 30 Herein, we present HyLEDs based on newly synthesized electron injection layer Ti-doped ZrO 2 and Ir(NPIBT) 2 (acac) as emitting layer. The efficiencies of HyLEDs imply that Ti-doped ZrO 2 is an potential carrier injection material with reduced wt% improves the efficiencies. The improved HyLEDs performances compared to previous ndings show that our device structure can be used to harvest efficient electroluminescence.

Materials and instrumental techniques
The structure of emissive materials was conrmed with 1 H/ 13 C NMR and mass spectra, recorded with Bruker 400 MHz spectrometer and Agilent (LCMS VL SD), respectively. Oxidation potentials were measured from potentiostat CHI 630A electrochemical analyzer. The Lambda 35 and Lambda 35 spectrophotometer with integrated sphere (RSA-PE-20) instrument (PerkinElmer) was employed to measure absorbance in both solution and lm states. Emissive properties (PL) were analyzed with PerkinElmer uorescence spectrometer (LS55) measurement. Thermal characteristics such as decomposition (T d ) and glass transition (T g ) temperatures was analyzed with Perki-nElmer thermal analysis system (10 C min À1 ; N 2 ow rate of 100 mL min À1 ) and NETZSCH-DSC-204 (10 C min À1 under N 2 atmosphere), respectively. The PL QY (quantum yield) was measured using quinine sulphate (0.54) as reference and MoO 3 are used as emissive layer and barrier-reducing HIL and electron-blocking layer, respectively. All the layers were deposited on ITO plate by thermal evaporation unit with glove box under optimized evaporation rates. The thicknesses have been monitored using quartz crystal digital thickness monitor. The current density-voltage and light intensity of HyLEDs were measured using Keithley 2400 source measuring unit. The EL spectra of the devices were carried out in ambient atmosphere without further encapsulations.

Results and discussion
The smooth surface of SEM images of Ti-doped ZrO 2 nanomaterial is due to the use of PVP K-30 templating agent (Fig. 1). The energy dispersive X-ray spectra (EDS) of Ti-doped ZrO 2 nanomaterials conrm the respective constituent elements and absence of other elements reveal the purity of nanomaterials. Doping percentage of titanium in Ti-doped ZrO 2 is 28.0 ( Fig. 1). The powder X-ray diffraction (XRD) pattern of Ti-doped ZrO 2 along with JCPDS of tetragonal ZrO 2 is displayed in Fig. 2 and the observed diffraction pattern matches with that of tetragonal ZrO 2 (card no. 81-1546). The average crystal size has been deduced using Scherrer equation as 18 nm and the surface area is 58.4 m 2 g À1 , respectively. The TEM images of Ti-doped ZrO 2 reveal the spherical shape nanoparticulate character of Ti-doped ZrO 2 which is important for display applications. Spherical particles will increase the lm brightness and resolution of images because of lower light scattering of emitted light and possess higher packing density compared to irregular shaped particles. The distance between lattice fringes was estimated as 0.289 nm corresponds to 101 plane of tetragonal ZrO 2 (Fig. 2). Composition of Ti-doped ZrO 2 was analysed by XPS and the spectrum shows presence of titanium, oxygen and zirconium (Fig. 3). Binding energy peaks of Zr 3d 5/2 and 3d 3/2 were observed at 183.5 and 186.1 eV, respectively and are attributed to Zr 4+ . 38 The doublet observed at 458.8 and 463.0 eV corresponding to Ti 2p 3/2 and 2p 1/2 core levels conrm Ti 4+ in Tidoped ZrO 2 . 39 The O1s peak at 530.8 eV is due to bulk oxygen in ZrO 2 and 531.5 eV may be due to the oxygen of Zr-OH. 40 DRS spectra of Ti-doped ZrO 2 and pure ZrO 2 is compared in Fig. 4: bare ZrO 2 shows typical absorption at 248 nm due to transition of electron from valence band to conduction band while titanium doped ZrO 2 show absorption at 252 nm and broad emission at 473 nm which is due to electric transition from conductive band to recombination band (Fig. 4). The broad bandwidth indicates the existence of different recombination sites which was observed in transition metal oxide semiconductors commonly. [41][42][43] FT-IR spectra of Ti-doped ZrO 2 and pure ZrO 2 show a strong absorption peak at 793 and 815 cm À1 corresponds to Zr-O vibrational modes of ZrO 2 phase (Fig. 2) (Fig. 5). The peak with dominant intensity stemmed from n 0 ¼ 0 to n ¼ 0 transition of 3 MLCT/ 3 p-p* to S 0 whereas a shoulder peak with lower intensity derived from n 0 ¼ 0 to n ¼ 1 electronic transition. [54][55][56] The radiative lifetime of Ir(NPIBT) 2 (acac) is 1.62 ms and PL quantum yield (F) is 0.52. The radiative (k r ) and nonradiative (k nr ) decay rate constants have been calculated from the formulae, F ¼ F ISC {k r /(k r + k nr )}, k r ¼ F/s, k nr ¼ (1/s) À (F/s) and s ¼ (k r + k nr ) À1 [Fquantum yield; slifetime; F ISCintersystem-crossing yield]. [57][58][59][60][61] Rate constants reveal that the radiative emission (3.2 Â 10 8 s À1 ) in Ir(NPIBT) 2 (acac) is slightly predominant over non-radiative transition (3.0 Â 10 8 s À1 ). From DFT [DFT/B3LYP/6-31G (d,p)] analysis, it was shown that the highest occupied molecular orbital (HOMO) is dominantly distributed over d(Ir) and p(C^N) whereas the lowest unoccupied molecular orbital (LUMO) is localized on C^N ligand of the iridium complex (Fig. 7). Ir(NPIBT) 2 (acac) complex exhibit a distorted octahedral geometry around the iridium atom with two cyclometalated NPIBT ligand and one ancillary acetylacetonate (acac) ligand. The NPIBT ligand adopt eclipsed conguration and two nitrogen atoms N(5) and N(7) reside at trans-N,N chelate disposition and the Ir-N distance lie between 2.06 and 2.10 A. The cyclometalated carbon atoms C(12) and C(21) are mutually cis around the iridium atom and Ir-C distance lie between 2.00 and 2.04 A. Due to stronger Ir-C bonding  interaction of the NPIBT ligand which weakens the Ir-C bonds at their trans disposition. Electron rich phenyl fragments of Ir(NPIBT) 2 (acac) shows trans effect, thus trans-C,C geometry is thermodynamically higher energy and kinetically more labile, called transphobia and it is conrmed by Ir-C bond length Ir-C av ¼ 2.02 A is shorter than Ir-N bond length, Ir-N av ¼ 2.08 A. 49,51 The electrochemical behaviour of Ir(NPIBT) 2 (acac) exhibit reversible one-electron oxidation wave at E ox 1/2 ¼ 0.42 V vs. Fc/Fc + , which supports the electrochemical stability of the complex (Fig. 6 The thermal characterization (T d5 and T g ) of Ir(NPIBT) 2 (acac) have been analyzed by DSC and TGA measurements to test its  Paper suitability for lm formation. The TGA of Ir(NPIBT) 2 (acac) exhibits high decomposition temperature (T d5 ) of 404 C, high glass transition temperature (T g ) of 158 C and the melting point (T m ) is 372 C (Fig. 6). The green emissive material Ir(NPIBT) 2 (acac) exhibits excellent thermal property and could be subjected to vacuum-evaporation without decomposition. 63-65

Electroluminescent performances
For an efficient HyLEDs device the solid lm of the Ti-doped ZrO 2 nanomaterials should uniformly deposit over the ITO plate and the surface morphology of coated ITO substrates with increasing concentrations of Ti-doped ZrO 2 (1.0, 2.0 and 3.0%) was analysed through atomic force microscopy (Fig. 8). The thickness and root mean square (RMS) roughness of Ti-doped ZrO 2 layer from 1.0, 2.0 and 3.0% solutions are 20, 22 and 28 nm and 2.91, 2.74 and 2.42 nm, respectively. The RMS roughness of Ti-doped ZrO 2 lms was much smoother than that of ITO (4.47 nm) and it was slightly decreased as the concentration of Ti-doped ZrO 2 increased. The absorption edge of Tidoped ZrO 2 lm was observed at 252 nm (4.92 eV, Fig. 4) which is not band gap of semiconductor and is difference between conduction band edge of Ti-doped ZrO 2 and LUMO of Ir(NPIBT) 2 (acac) that determined the injection barrier. The average lifetime (s) (Fig. 6) obtained from decay curves are summarized in Table 1. As thickness of Ti-doped ZrO 2 layer increases the lifetime also increases [1.41 ns (0%); 1.94 ns (1.0%); 2.01 ns (2.0%) and 2.29 ns (3.0%)]. The 3.0% wt Tidoped ZrO 2 blocks exciton quenching efficiently by surface quenching or nonradiative energy transfer quenching mechanisms. The current density-voltage and luminance-voltage variation of the devices with various Ti-doped ZrO 2 concentrations as well as control device are shown in Fig. 8. The current density and luminance increases signicantly by nano Ti-doped ZrO 2 layer (I-III) compared to control devices IV (0% Ti-ZrO 2 ) and V(3% ZrO 2 ). The 3.0% Ti-doped ZrO 2 nanoparticles exhibit maximum luminance of 26 432 cd m À2 at driving voltage 7.0 V whereas luminance of 1469 cd m À2 at 12.0 V was harvested from control devices IV and V. The device with 3.0% Ti-doped ZrO 2 layer also shows higher current efficiency h c (2.04 cd A À1 ) and power efficiency h p (1.15 lm W À1 ) than those of control device (IV). These higher efficiencies reveal that the Ti-doped ZrO 2 layer in combination with Ir(NPIBT) 2 (acac) makes electron injection more efficiently through the improved energy level matching at the interface between ITO and Ir(NPIBT) 2 (acac). The current density and luminance increases as the concentration of Ti-doped ZrO 2 increases because the total thickness of the device increases. As the thickness of the Ti-ZrO 2 layer increases up to 4.0%, the current density and luminance slightly increases. This may be due to the fact that the injection and transport of electrons become better as the surface coverage of Ti-ZrO 2 on top of ITO becomes better with increasing Ti-ZrO 2 layer thickness and the Ti-ZrO 2 layer has relatively higher electron mobility than organic materials. [66][67][68] The systematic study for the device optimization by controlling the thickness and morphology of Ti-ZrO 2 are under way. Therefore use of Ti-doped ZrO 2 layer in an optoelectronic device is of current interest owing to advantages of process ability at low temperature, surface roughness and photostability. The band gap energy of TiO 2 is much lower (3.2 eV) than that of ZrO 2 (5.0 eV) and this may probably be the reason for effective performances of Ti 4+ doped zirconia used as electron injection material. The lowering of band gap results in lowering of the CB level of semiconductor nano oxide Ti-ZrO 2 . The reduced potential drive required for promotion of electron from ITO to CB edge of the synthesised nanomaterial reects the enhanced performances of devices. The Ti-doped ZrO 2 layer inject electron efficiently due to improved energy level matching at ITO/Ti-doped ZrO 2 / emitting layer interface. Comparing the performances of the HyLEDs with different Ti-doped ZrO 2 layer thicknesses, a higher efficiency was obtained in the device with thicker Ti-doped ZrO 2 lm because the ability of electron injection becomes improved and the Ti-doped ZrO 2 layer has relatively higher electron mobility than organic materials. As more electrons injected, the electron-hole balance is enhanced results higher efficiencies than that of control device. Holes are injected from Au anode coated with MoO 3 HIL into highest occupied molecular orbital (HOMO) of Ir(NPIBT) 2 (acac) and electrons are injected from Tidoped ZrO 2 -EIL-coated ITO cathode into LUMO of Ir(NPIBT) 2 (acac). As conduction band of Ti-doped ZrO 2 is situated higher than LUMO of emissive material Ir(NPIBT) 2 this leads to activationless electron injection from metal-oxide into emissive material. The deeper valence band of Ti-doped ZrO 2 should results efficient hole blocking functionality results improved efficiencies. 30 The performances of devices along with thickness of Ti-doped ZrO 2 layer are summarized in Table 1. The normalized EL spectra of HyLEDs (Fig. 8) shows emission around 516 nm measured at the current density of 5.1 mA cm À2 and all the EL spectra are in the same shape irrespective of thickness of Ti-doped ZrO 2 layer since the layer is highly transparent. The use of Ti-doped ZrO 2 as EIL may due to an lower energy barrier between the conduction level of Ti-doped ZrO 2 and LUMO of Ir(NPIBT) 2 (acac) and better hole-blocking ability of Ti-doped ZrO 2 . The efficiencies of newly synthesised electron injection layer of Ti-ZrO 2 are compared with those of various recently reported electron injection layer ( Table   Table 1 Device performances of HyLEDs/Ti-ZrO 2 (I-IV)/ZrO 2 (V)/without Ti-ZrO 2 (VI)/Ir(NPIBT) 2   S1 †). 19,30,69 It can be seen that the performances of electron injection layer of Ti-ZrO 2 based devices are among the best in terms of power and current efficiencies and we believe that adopting Ti-ZrO 2 nanoparticles as EIL is meaningful, a lot in terms of good potential candidate for future displays as well as the device performances.

Conclusion
In conclusion, the efficient HyLEDs have been fabricated using ITO/Ti-doped ZrO 2 nanomaterials as a transparent cathode. The device with 3.0% nano Ti-doped ZrO 2 layer shows higher efficiencies of h c (2.04 cd A À1 ) and h p (1.15 lm W À1 ) at lower driving voltage compared to control device due to improved energy level alignment. As more electrons are injected to the emitting layer from 3.0% Ti-doped ZrO 2 , electron-hole balance becomes to be improved and hence the higher efficiencies. Ti-doped ZrO 2 is a good potential candidate for use as EIL in hybrid organicinorganic light-emitting diodes due to better hole-blocking ability of Ti-doped ZrO 2 .

Conflicts of interest
There are no conicts of interest.