Organic light-emitting diodes with a PIN structure of only thiophene/phenylene co-oligomer derivatives

Organic light-emitting diodes with trilayer PIN structures are fabricated using only thiophene/penylene co-oligomer derivatives: 5,5'-bis(4-biphenylyl)-2,2'-bithiophene (BP2T), 5'''-bis(4-trifluoromethylphenyl)[2,2';5',2'';5'',2''']quaterthiophene (P4T-CF3), and 5,5'-bis-(4'-cyanobiphenyl-4-yl)-2,2'-bithiophene (BP2T-CN) as the P, I, and N materials, respectively. As designed from their expected frontier orbital energies, electroluminescence (EL) is obtained from the P4T-CF3 layer. When the BP2T layer is first deposited on an indium-tin-oxide (ITO) substrate, the device (Al:Li/BP2T-CN/P4T-CF3/BP2T/ITO) shows homogeneous EL. On the other hand, the device with an opposite deposition order (Au/BP2T/P4T-CF3/BP2T-CN/ITO) shows dotted EL with higher efficiency at lower bias voltages. By decreasing the deposition rate of the P4T-CF3 layer in the latter device, its morphological change results in homogeneous EL with increased density of dotted emission.