Electronic structures of p-phenylene biphenyltetracarboximide polyimide/indium-tin oxide heterostructures grown on glass substrates for organic light-emitting devices

p-Phenylene biphenyltetracarboximide (BPDA-PDA) polyimide (PI) layers were grown on indium-tin oxide (ITO)-coated glass substrates by curing the PI precursor layer. The ionization energy, which was determined from the γ-focused ion beam (FIB) measurements and corresponds to the energy level of the highest occupied molecular orbital (HOMO), of the PI layer was 4.77 eV, and the optical energy gap obtained from the absorbance measurements was 3.51 eV. The energy levels of the HOMO and the lowest unoccupied molecular orbital of the PI layer were determined on the basis of the FIB and absorbance measurements. These results can help in understanding the electronic structure of the BPDA-PDA PI/ITO heterostructures for organic light-emitting devices.