Abstract
1.Spirally Configured cis-Stilbene/Fluorene Hybrids as Ambiplor Templates for Organic Light Emitting Diode Applications Organic light emitting diodes (OLEDs) have been intensively investigated in the recent years for their potential applications in next generation full-color at panel displays and solid state lighting. However, the recombination efficiency of holes and electrons with unbalanced-charge carriers is one of the key factors for making bad device efficiencies. Therefore, we developed a new class of cis-stilbene/fluorene spiro hybrid systems with hole-transporting, electron-transporting and ambipolar organic fluorescent materials for optoelectronic applications. These types of materials exhibited a stable amorphous glassy state (Tg:120-167 oC) and stable decomposition temperatures (Td: >400 oC). One of the fluorescent materials, N-STIF-P(O)Ph2, had ambipolar charge transport feature with balanced hole and electron mobilities (μh: 6.510-6 cm2/Vs; μe: 5.110-6 cm2/Vs @7.3 105 V/cm). This feature allowed us to utilize N-STIF-P(O)Ph2 successfully in a single-layer device (i.e., ITO/PEDT:PSS/ N-STIF-P(O)Ph2/LiF/Al) with excellent performance. The single layer device emitted bluish green light and showed a turn-on voltage of 2.5 V, a maximum brightness of 73,359 cd/m2 at 5,611 mA/cm2 (8.5 V), operational current efficiency of 3.29 cd/A, power efficiency of 2.84 lm/W and EQE of 1.28% at 20 mA/cm2 with CIE color coordinates of (0.21, 0.47). Next, we demonstrated red-emitting PhOLED using the P(O)Ph2-STIF-P(O)Ph2 as the electron-transporting type host material and [Os(bpftz)2(PPhMe2)2, OS1] as red dopant (i.e., ITO/PEDT:PSS/NPB /TCTA/ P(O)Ph2-STIF-P(O)Ph2: 10 wt% OS1/3TPYMB/LiF/Al). This device with highly efficient performance was successfully achieved, with maximum current efficiency of 22.2 cd/A, power efficiency of 23.3 lm/W, EQE of 16%, and a maximum brightness of 29,602 cd/m2 at 1,759 mA/cm2 (10 V) with CIE color coordinates of (0.63, 0.36). And then, we fabricated fluorescent white OLEDs based on 0.4 wt% rubrene-doped Cbz-STIF-P(O)Ph2 (i.e., ITO/PEDT:PSS/NPB /TCTA/ Cbz-STIF-P(O)Ph2: 0.4 wt% Rubrene/TPBI/LiF/Al). This device showed a turn-on voltage of 3.1 V, a maximum brightness of 87,800 cd/m2 at 1,719 mA/cm2 (14.5 V), operational current efficiency of 11.3 cd/A, power efficiency of 4.23 lm/W and EQE of 3.62% at 1,000 cd/m2 with CIE color coordinates of (0.45, 0.48). Compared with the CIE color coordinates of the ideal white light (CIE: 0.33, 0.33), there were 36-45% gaps between our device and ideal WOLED. However, our device was very close to the white light by the naked eye. And this device had already achieved the best performance and the most stable state. And Next, we tried to fabricate blue OLED based on thermally activated delayed fluorescence (TADF) device configuration developed by Prof. Adachi. For this blue OLED, we used the DPEPO as host material and Cbz-STIF-cbz as TADF material (i.e., ITO/PEDOT:PSS/NPB/TCTA/CzSi/DPEPO: 10 wt% Cbz-STIF-cbz/DPPS/BmPyPB/LiF/Al). Although there was no TADF observed in Cbz-STIF-cbz, this was still good device performance for blue OLED. And this device showed a turn-on voltage of 4.1 V, a maximum brightness of 2,778 cd/m2 at 538 mA/cm2 (13.5 V), maximum current efficiency of 2.5 cd/A, power efficiency of 2.19 lm/W and EQE of 3.81% with CIE color coordinates of (0.15, 0.07). Finally, we used a highly efficient and ambipolar-type material, N-STIF-CN, as electron transporting material for green, yellow and red PHOLEDs. Compared with those devices which used common electron transporting materials (i.e., Alq3, TPBI, BmPyPB, etc.), our devices had further improved the device efficiencies and lifetime. For example, we demonstrated green PhOLED using the industry’s device configuration (i.e., ITO/HAT-CN /HT-01: 3 wt% F4-TCNQ /NPB/ TPBI: 5 wt% Ir(ppy)3/ ETL/ LiF/Al), without any hole blocking layer, N-STIF-CN could further supplant ET-01 which was often used in the industry as superior ET material with improved power efficiency by 29%, current efficiency by 29%, and EQE by 28% at 1,000 cd/m2. And for device lifetime tests, the half-life of N-STIF-CN was 210 hours and it was very close to the half-life of ET-01 which was 270 hours under the initial brightness of 1,000 cd/m2. Therefore, N-STIF-CN was very promising material based electron transporting layer for PHOLEDs. 2.Spirally Configured cis-Stilbene/Fluorene Hybrids as Ambiplor Templates for Dye-Sensitized Solar Cell Applications A new class of cis-stilbene/fluorene spiro hybrid systems with di-p-tolylamine donor and combined benzothiadiazole (BTD) and thiophene (T) acceptor units at C-3 and C-7, respectively, were synthesized as two novel D-π-π-A-A-featured dye N-STIF-T-BTD-CA and D-π-A-π-A-featured dye N-STIF-BTD-T-CA for dye-sensitized solar cell applications. These two dyes whose maximum absorption wavelength were observed at 470 nm and 523 nm, respectively, and the molar absorption coefficient were observed at 20,801 M-1 cm-1 and 21,690 M-1 cm-1, respectively. The best device performance was D-π-A-π-A-featured dye N-STIF-BTD-T-CA, and it showed a conversion efficiency (η) of up to 4.26% (Voc = 643 mV, JSC = 8.81 mA/cm2, FF = 0.74) under AM 1.5 G conditions. And the best IPCE values achieved 49.3% within the 400–550 nm absorption range.