Contact and On-Distance Effects in Organic Semiconductor–Insulator Junctions: Implications for Conductivity

Interfacial phenomena play a crucial role in the performance of organic electronic devices. In the research described herein, we have studied the junction between regioregular poly(3-hexylthiophene-2,5-diyl) (R-P3HT), a model organic semiconductor, and the insulator poly(4-vinylpyridine) (P4VP). Pyridine rings in the chemical structure of P4VP possess a large dipole moment and can easily form complexes with d-block metals. We have composed R-P3HT and P4VP systems in two main mutual arrangements: (i) layer-by-layer systems and (ii) laterally spread domains resulting from phase separation. In the bilayer arrangement, we added an additional layer of Al2O3 that was thick enough to eliminate contact interactions between the materials. We have demonstrated that the presence of P4VP has a positive influence on the semiconductor in-plane conductance, even without direct molecular contact. The effect can be further enhanced by the formation of Br2Co(py)2 complexes on the P4VP surface. This is direct evidence of the importance of molecular dipoles and the influence of their electric fields on the conductivity of R-P3HT in a field-transistor-like insulator–semiconductor junction. The second system allows us to study the physicochemical properties of direct interactions between the two materials. Due to the secondary phase separation, we were able to investigate two different ranges of molecular contact: (i) at the interface between P4VP-rich and R-P3HT-rich regions and (ii) within the P4VP-rich domain where R-P3HT is intermixed. Detailed analysis with a combination of conductive atomic force microcopy, Raman spectroscopy, and other techniques shows no evidence of doping of the R-P3HT through the contact with P4VP, even though the intermixing of R-P3HT into the P4VP matrix results in high conductance within the domain, which is most a likely consequence of altered molecular ordering. Our results point to the primacy of the dipole-derived interactions dominating the conductance increase in this system.