Supporting Information

Transcription

1 Solution-processed Nickel Oxide Hole Injection/Transport Layers for Efficient Solution-processed Organic Light- Emitting Diodes Supporting Information 1. C 1s high resolution X-ray Photoemission Spectroscopy (XPS) signals of s-nio x annealed at 275 C and 500 C The C 1s signals of s-nio x annealed at 275 C and 500 C are shown in Figure SI-1. The main peaks in both C 1s signals are adventitious carbon peaks which are calibrated to a binding energy of ev. The shoulder peak in C 1s signal of 275 C annealed s- NiO x has a binding energy of ev, which is corresponding to the acetate component in the nickel ink precursor. 1,2 This result revealed that the nickel ink precursor did not fully decompose when annealed at 275 C.

2 275 C C 1s Intensity 500 C Intensity Binding Energy (ev) Figure SI-1. The XPS spectra of C 1s signals. The spectra on top is acquired from 275 C annealed s- NiO x and the one on bottom is acquired from 500 C annealed s-nio x. The take-off angel is Dark-injection measurement configuration The dark-injection (DI) space-charge-limited (SCL) transient measurement was carried out to calculate the s-nio x injection efficiency. Figure SI-2 shows the long voltage step applied to a device and the transient current density (J-t) characteristics in a DI transient measurement. At time t = t DI, the current density reaches to its maximum value of J DI. For t > t DI, the current density begins to decay until it reaches a steady-state value of J SS. The ratio of J DI to J SS is expressed as: = [ ( +3) ( +2) ] ( )

3 For an ideal Ohmic injection with field-independent condition, n equals zero and J DI /J SS equals 1.21, deviation from this ideal value of 1.21 indicates that the charge injection contact deviates from an Ohmic injection. To calculate the hole mobility and injection efficiency, the transient time t tr can be expressed as: The hole mobility is calculated from: = = Where V is the applied voltage and d is the film thickness. The space-charge-limited current density (J SCL ) can be calculated as: = 9 8 exp (0.89 ) Where V is the applied voltage, d is the film thickness, µ 0 and γ are temperature-dependent parameters derived from: = exp ( ) The hole injection efficiency can be calculated from: = Figure SI-2. An ideal dark injection space-charge-limited current transient with circuit RC decay.

4 3. Field-effect mobility measurement configuration The field-effect mobility of s-nio x was measured via a p-type thin film transistor (TFT). A 100 nm thick s-nio x film is spin-coated onto the Si/SiO 2 substrates and annealed at either 275 C or 500 C. The gold is used as the source and drain electrodes. The channel width 1 mm and the channel length is 40 µm. The capacitance of the SiO 2 (300 nm) is 10 nf cm -2. The field-effect mobility is calculated from: =( 2 )( ) Where I DS is the source-drain saturation current, is the applied gate voltage, W is the channel width, and C i is the capacitance of the SiO 2. For the s-nio x annealed at 500 C, the slope extracted from the I DS 1/2 - curve is , corresponding to a field-effect mobility of cm 2 V -1 s -1. For the s-nio x annealed at 275 C, the source drain current is low without any modulation effect. No saturation region is found in the source drain current, as shown in Figure SI-3. I DS (µa) =-150V =-100 V =-50 V = 0 V V DS (V) Figure SI-3. The I DS -V DS output curves of the s-nio x annealed at 275 C. 4. Electro-luminescence (EL) measurement The EL spectra were measured with an Ocean Optics HR4000 spectrometer, the devices were driven at a constant current density of 2.5 ma cm -2. Figure SI-3 shows the EL spectra of both solution-processed and thermal-evaporated green phosphorescent OLEDs.

5 The different shoulder-to-peak ratios are probably due to the difference of refractive index or micro-cavity effect. (a) Normalized Intensity (a.u.) (b) Normalized Intensity (a.u.) PEDOT:PSS As-prepared NiOx UV-ozone treate NiOx Wavelength (nm) PEDOT:PSS HIL/NPB HTL UV-ozone treated NiOx HIL/HTL Wavelength (nm) Figure SI-4. The EL spectrum of (a) solution-processed and (b) thermal-evaporated green phosphorescent OLEDs. 5. Device Performance with 275 C annealed s-nio x HIL/HTL Figure SI-5 shows the device performance with the s-nio x HIL/HTL annealed at a lower temperature of 275 C. The device shows a larger turn-on voltage of 4.5 V and a reduced current efficiency and power efficiency compared to the s-nio x annealed at 500 C.

6 (a) Current Density (ma cm -2 ) E-3 1E-4 1E-5 I-V curve L-V curve 1E Voltage (V) Luminescence (Cd m -2 ) (b) Current Efficiency (Cd A -1 ) Current Efficiency Power Efficiency Luminescence (Cd m -2 ) Power Efficiency (lm W -1 ) Figure SI-5. (a) I-V-L curve and (b) efficiency curve of the green phosphorescent OLED with s- NiO x annealed at 275 C. 6. Shelf stability measurement To evaluate the stability of the devices, devices with a PEDOT:PSS HIL and s-nio x HIL/HTL were both driven at a constant current level of 1 ma/cm 2. Pictures of the active areas of these devices are shown in Figure SI-6. Dark spots appear in the PEDOT: PSS device within two weeks and grew with time. On the other hand, s-nio x devices showed no dark spots after 6 weeks of storage in air.

7 Figure SI-6. The EL pictures of the PEDOT:PSS and NiO x devices taken in ambient atmosphere. References: [1] Chakroune, N.; Viau, G.; Ammar, S.; Poul, L.; Veautier, D.; Chehimi, M. M.; Mangeney, C.; Villain, F.; Fiévet, F. Langmuir, 2005, 21, [2] Payne, B. P. Doctor of Philosophy Thesis, the University of Western Ontario, August, 2011.