The Synthesis And Characterization Of Bipolar Solution-Processible And Electrochemical Polymerization Small Molecules

To obtain high efficient red-emission material our team has recently synthesized solution-processible material on the basis of green-emission material. Narrow gap naphtho[2,3-c][1,2,5]thiadiazole (NTZ) unit with fluorenes connected to the both sides functions as rigid luminous plane. We connect flexible alkyls chains to the 9th carbon of fluorene to endow the material glass-like properties essential to solution-processible materials. As excellent hole-transporting (IIT) and electro active unit carbazole improves HT ability and enables it to be prepared into device by electrochemical polymerization (EP). By cyclic voltammetry curves we could conclude that EP process can not break up luminescence of main chain because oxidation potential of carbazole is lower than luminous main chain; NTZ unit as electron-transporting (ET) unit enhances electron injection and transporting capacity. With both HT units and ET units together in one molecule it is in possession of bipolar qualities. Through NMR, MS and element analysis we have characterized the structure of the material and prove that we get high purity material. We investigate thermal quality by TGA and DSC. TGA curve shows that thermal decomposition (Td) temperature is 429℃which is high enough for application of devices. DSC curve with only glass transition temperature (Tg) and no any other phase transition indicates that it is a good amorphous material. High solid quantum efficiency 0.62 is obtained which shows strong red light-emitting. We also survey electrochemical property by cyclic voltammetry (CV). It proves that oxidation potential of main chain is higher than carbazole. A single-layer device was fabricated by spin-coating with the structure ITO/PEDOT:PSS/TCNzC (spin-coating)/Ba (10 nm)/Al (100 nm). AFM image shows that film surface is very smooth with the root mean square roughness (RMS) of only 0.32 nm. The devices shows the maximum brightness of 2210 cd m-2, a luminous efficiency of 0.52 cd A-1 and a power efficiency of 0.32 lm W-1. To improve performance of device we also fabricated double layer device with TPBi as hole-blocking layer and the device shows a luminous efficiency of 7.18 cd A-1 and a power efficiency of 2.25 lm W-1. Otherwise we fabricated single-layer and double-layer device by electrochemical polymerization with the same structure as spin-coating device, the device shows a rather high luminous efficiency of 1.5 cd A-and a power efficiency of 5.8 lm W-1 which is comparable to spin-coating devicBased on molecule structure of tetracarbazole (TC) materials, we designed red, green and blue (RGB) octcarbazole (OC) structure. The luminous core whose gap gets lower gradually to generate RGB emission was reserved but fluorene unit next to the core with flexible side alkyls chains connected to the 9th C position was doubled. So OC-materials will still keep the advantage of TC-materials but get enhanced in other aspects:bigger molecule weight results in better thermal stability; one molecule with more flexible chains and more electro active units will favor improving the efficiency of spin-coating device and stability of electrochemical polymerization films. We have investigated the qualities of OC-materials and the result proves our assumption:the Td and Tg of OC-materials have been enhanced compared to corresponding TC-materials. That demonstrates the improvement of thermal stability. The maximum peak of emission has a red shift relative to TC-materials because conjugate plane is extended. From cyclic voltammetry curve we could notice that the oxidation potential of main chain is higher than carbazole and therefore OC-materials still could be prepared into device without affecting light-emitting of main chain. Only single-layer devices of OC-materials by spin-coating were fabricated. We have compared efficiency of single-layer device of OC-materials and TC-materials. It turned out that the efficiency has been improved after updating molecule structure. The enhancement of efficiency should be attributed that we increase flexible chains in one molecule so as to improve solution-processible quality. The single-layer devices of RGB materials show luminous efficiency of 1.5 cd A-1,6.4 cd A-1 and 1.3 cd A-1 respectively.