Abstract
This study discloses the introduction of charge transport moieties onto the conjugated polymer structures and their application on light-emitting diodes and solar cells. PPP derivatives are promising candidate deep blue materials for commercialization due to their ease to be synthesized. Since the modification of PPP derivatives through incorporating charge transport moieties are scarce, this study demonstrates for the first time to clarify the structure/property relationship, photoluminescence and electroluminescence of the charge transport moiety modified PPP (EHB-PPP-Cz). Furthermore, we utilize the other charge transport moiety modified PPP (Cz1000-PPP) to elucidate the blocking effects of triplet back energy transfer from triplet state of phosphorescent dopants to that of the polymer and their chemical compatibility. Finally, we show that the charge transport moiety modified polymer derivatives and their application on solar cell to discuss their influences on exciton dissociation rate. This study discloses the design and synthesis of poly(para-phenylene) derivatives (EHB-PPP and EHB-PPP-Cz), and reports their structure-property relationships. The incorporation of alkoxy-phenyl substitute can adjust the energy level and cause a blue-shifted PL and, therefore promote the fluorescent quantum efficiency and acquire a deeper blue emission. The present copolymer (EHB-PPP-Cz) exhibits the shortest wavelength among the electroluminescent materials (λmax= 403 nm). From the measurement results of single carrier devices, the introduction of carbazole unit can solve the drawback of charge imbalance of the material. Therefore, EHB-PPP-Cz shows better hole injection capability and balances its charge transport capabilities. The device (ITO/PEDOT/EHB-PPP-Cz/TPBI/CsF/Ca/Al) based on this material exhibits stable blue emission (spectrum remains unchanged upon successive operation) with luminous efficiency of 1.12 (cd/A), external quantum efficiency of 2.32% and CIE (Commission Internationale de I’Eclairage) coordinate (0.16, 0.05). Here, DRO-PPP is utilized as the host material for electro-phosphorescence PLED application due to its large spectra overlap and excellent HOMO LUMO energy level match with guest materials. Even though the triplet energy of DRO-PPP is lower than phosphorescent material, the long distance of its side chain length can suppress back triplet energy transfer from guest to the host. As a result, the electrophosphorescence PLED based on DRO-PPP exhibits a luminous efficiency of 15 cd/A, external quantum efficiency of 3.72%. Furthermore, we incorporate the carbazole to the end of the flexible side chain of polymer (Cz100-PPP) due to its excellent chemical compatibility with guest material. From the results of Stern-Volmer and AFM measurements, the modified polymer not only exhibits the suppression of back triplet energy transfer from guest to the host, but also excellent chemical compatibility between host-guest materials. While doping with 8 wt% of Ir-G complex, the device shows performance with the highest external quantum efficiency of 8.25% and luminous efficiency of 30 cd/A (195 cd/m2, 0.668 mA/cm2), the highest luminance of 6127 cd/m2 (15 cd/A, 40 mA/cm2) and turn on electric field of 1.4 MV/cm and EL λmax at 520 nm. To the best of our knowledge, the present device efficiency is the highest one among electro-phosphorescent PLED with green emission to date. While doping with 8 wt% Ir-R complex, the device exhibits the best performance for red emission with the highest external quantum efficiency of 4.34% and efficiency of 4.5 cd/A (323 cd/m2, 7.122 mA/cm2) the highest luminance of 829 cd/m2 (3.7 cd/A, 22 mA/cm2), turn on electric field of 1.4 MV/cm. Also, when doping with 0.5 wt% Ir-G and 0.5 wt% Ir-R complexes, the device exhibits best performance for white emission with the highest external quantum efficiency of 8.65% and efficiency of 16.8 cd/A (84 cd/m2, 0.5 mA/cm2) the highest luminance of 344 cd/m2 (5.9 cd/A, 5.8 mA/cm2) and turn on electric field of 1.2 MV/cm. To improve the efficiency of polymer based solar cell, we introduce high electron deficient moiety to the end of side chain of PPV and PF polymers. The devices based on MEH-PPV with OXD moiety (POPD-MEH-PPV (37/63)), BTAZ moiety (T60) and PF with OXD moiety (OXD50PFO) exhibit significant enhanced efficiency than the unmodified analogues. Moreover, we utilize luminescence decay and time of flight measurement (TOF) measurements to clarify the function of BTAZ and OXD moieties. The experimental results reveal that the OXD and BTAZ moieties can increase exciton dissociation rate and promote electron transport. Hence, the efficiencies of the devices based on OXD and BTAZ modified polymers (POPD-MEH-PPV (37/63), T60 and OXD50PFO) are higher than the unmodified analogues by a factor of 2.1, 1.8 and 29, respectively.