A reduced electron-extraction barrier at an interface between a polymer poly(3-hexylthiophene) layer and an indium tin oxide layer

Abstract

Roles of the buried interface between polymer poly(3-hexylthiophene) (P3HT) layer and indium tin oxide (ITO) on the glass substrate have been characterized by transient photovoltage (TPV). Since P3HT is the hole-transporting material, from intuitiveness, ITO/P3HT contact (IPcontact) tends to be hole extracting. However, in this letter, the negative TPV of ITO/P3HT/Al demonstrates that IPcontact dominates the reversed built in electric field, namely pointing from ITO to Al, and is confirmed to be electron extracting. Meanwhile, an interesting biphasic feature of TPV is demonstrated in a device of ITO/P3HT:[6,6]-phenyl-C61-butyric acid methyl ester/Al. The negative component in biphasic TPV shows that IPcontact is one reason resulting in the leakage current for P3HT based solar cells in normal structures. The theoretical study is conducted, and reveals that the interaction between P3HT and ITO reduces electron barrier by 0.5eV for IPcontact. Band bending and dipole formation are two possible reasons to reduce the electron barrier. By taking advantage of the electron extraction, IPcontact is employed as a composite cathode in an inverted solar cell by pre-coating a pristine P3HT buffer layer between a blended layer and ITO. The study paves a way to characterize the buried interface in solution processable optoelectronics by observing polarity change of TPV, and to fabricate the simplified inverted organic solar cell employing IPcontact to extract electrons.