THIN FILM MICROSTRUCTURE OF SOLUTION PROCESSABLE PYRENE-BASED SMALL MOLECULES FOR ELECTRONIC APPLICATIONS
Leah A. Lucas
Dean M. DeLongchamp, Mentor
Polymers Division, MSEL
B206 Bldg 224
No – member
No – mentor member
The intelligent design and synthesis of organic semiconductors requires an understanding of the complex relationships between primary chemical structure and processing, thin film microstructure, and final device performance. We study the effect of primary chemical structure in semiconducting 6,7,15,16-tetrakis(alkylthio)quinoxalino[2',3':9,10]-phenanthro[4,5-abc]phenazine, TQPP-[SR]4 by altering R to be C12H25, C10H21, C8H17, and C6H13. Members of this TQPP-[SR]4 synthetic series are solution-processable charge transport materials for organic electronics applications1 such as organic field effect transistors (OFETs). We characterize TQPP-[SR]4 thin film microstructure as it varies with side chain length using a combination of polarized photon absorption spectroscopies (X-ray, vis, and infrared), X-ray diffraction, and scanning probe techniques. This effective characterization strategy allows the complete determination of crystal order and molecular orientation within thin films of this complex molecule.
We find that the TQPP core orientation changes dramatically depending on the length of its side chains. Longer side chains enforce a core orientation with greater potential for pi-overlap in the source-drain plane of OFETs. Moreover, TQPPs with longer side chains exhibit an additional phase transition above room temperature; heating films above that phase transition and then cooling increases the lateral size of crystalline TQPP domains. Surprisingly, TQPP-SC8H17 exhibits room temperature polymorphism, where the core packing motif changes after thermal treatment. The thin film microstructure and core packing styles appear to directly correlate with variations in OFET performance; packing styles with improved pi overlap and larger lateral domain size exhibit greater field effect carrier mobility.