Abstract
The research fields in this thesis involve organic light-emitting diodes and organic photovoltaics. In chapter two, we synthesize four iridium complexes (DBQ)2Ir(pyi), (DBQ)2Ir(3mpyi), (DBQ)2Ir(4mpyi) and (DBQ)2Ir(5mpyi) bearing the high rigidity ligand, dibenzo[f,h]quinoline, to enhance the thermal stability of complexes. Therefore, these four complexes show high thermal decomposition temperatures (Td) around 445~489 °C. We use 2-(1H-imidazol-2-yl)pyridine (pyi) as an ancillary ligand and perform methylation at 3-, 4-, and 5- positions of pyridine rings to yield 2-(1H-imidazol-2-yl)-3-methylpyridine (3mpyi), 2-(1H-imidazol-2-yl)-4-methylpyridine (4mpyi) and 2-(1H-imidazol-2-yl)-5-methylpyridine (5mpyi), respectively, to fine tune their emission color. Interestingly, the absorption, photoluminance, and electrochemical properties of these four complexes show no significant differences; however, the emission quantum yield and emission lifetime are dramatically changed. The complexes with methylation on the pyridine ring show higher emission quantum yields relative to the one without methylation. Notably, through TD-DFT calculation, we find that the methylation of pyridine ring can change the energy level of triplet excited state and affect the emission quantum yield. One of the device using (DBQ)2Ir(4mpyi) as a dopant and CzSiPO as a host exhibit high EQE of 27%, current efficiency of 103.6 cd/A and power efficiency of 108.6 lm/W with driving voltage at 2.5 V. In chapter three, we develop five new electron transport materials, BIpPPy, BImPPy, BImSiPPy, BIdmPPy and BIdpPPy by using benzimidazole and pyridine moieties because benzimidazole-based materials show high thermal stability in literature while pyridine ring can offer good electron mobility and suitable energy level. This new generation of electron transporting materials exhibit triplet energy gap of 2.54~2.72 eV, thermal decomposition temperature (Td) of 310~419 °C and glass transition temperature (Tg) of 62~116 °C. In order to assess the transporting properties, we make electron-only devices with these materials and compare them with commercially available TPBI, TAZ, BCP/Alq3. As expected, the new generation electron transporting materials in this chapter show better transporting capacity than those of commercially available materials. In red PHOLED, the best device using BIQS as a host and BIdmPPy as a ETM shows a maximum current efficiency of 34.7 cd/A and a maximum power efficiency of 28.9 lm/W with CIE coordinates of (0.67, 0.33). In green PHOLED, the best device using BCPO as a host and BIdpPPy as an ETM exhibits a maximum EQE of 26.7%, a maximum current efficiency of 102.1 cd/A and a maximum power efficiency of 106.9 lm/W with CIE coordinates of (0.28, 0.64). In blue PHOLED, the best device using BCPO as a host and BIdmPPy as an ETM exhibits a maximum EQE of 21.5%, a maximum current efficiency of 47.0 cd/A and a maximum power efficiency of 44.9 lm/W with CIE coordinates of (0.14, 0.34). Research in chapter five can be separate into two parts. In the first part, we synthesize two organometallic alternating conjugated polymers, TPT-TPyPt and TPT-TIqPt, consisting of indacenodithiophene (thiophene/phenylene/thiophene; TPT) and cyclometalated platinum(II) moieties and compare their optical, electrochemical, photovoltaic properties, space charge limited currents and their structures (via density functional theory) with those of related platinum-free organic polymers, TPT-TPy and TPT-TIq. The absorption bands of the metal-based polymers are significantly red-shifted relative to those of the metal-free polymers. We fabricate OPVs incorporating blends of these polymers and [6,6]-phenyl-C71-butyric acid methyl ester at various weight ratios. One of these devices exhibit a power conversion efficiency (PCE) of 2.9%, with a short-circuit current density of 7.7 mA/cm2, an open-circuit voltage of 0.78 V and a fill factor of 0.48 under AM 1.5G (100 mW/cm2) illumination. The second part is mainly based on side chain improvement of TPT-TPyPt. As we know, polymer side chains have influence on thin film morphology and carrier mobility, thus we investigate the effects of different side chains through atomic force microscopy (AFM), powder X-ray diffraction (XRD) and space charge limit current (SCLC) measurements. One of the best OPV device using 26TPT-TPyPt/PCBM as an active layer shows high PCE of 3.2%, a maximum short-circuit current of 7.51 mA/cm2 and an open- circuit voltage of 0.77 V with fill factor of 0.551. The introduction of 2-ethylhexyl into these new generation polymer can improve the morphology of the active layer and minimize the domain size of the active layer relative to TPT-TPyPt-based device. To the best of our knowledge, this PCE is the highest reported for a polymer solar cell based on a cyclometalated platinum-type polymer. Parts of this research are accepted in the journal “Solar Energy Materials & Solar Cells”.