Electrical, optical and structural characterization of interfaces containing poly(3-alkylthiophenes) (P3ATs) and polydiphenylamine on ITO/TiO2: interaction between P3ATs polymeric segments and TiO2

Abstract

With the aim of studying the use of conjugated polymers poly(3-methylthiophene) (P3MT), poly(3-hexylthiophene) (P3HT) and polydiphenylamine (PDPA) to make up the active layer of inverted organic solar cells forming the interface with TiO₂, and also help shed light on the optical and electronic properties applied to develop this technology, the interfaces between films containing P3MT, P3HT and PDPA on ITO electrode were electrochemically prepared, after chemically depositing a film of TiO₂. The systems under investigation were designated ITO/TiO₂/P3MT, ITO/TiO₂/PDPA/P3MT, ITO/TiO₂/PDPA, ITO/TiO₂/P3HT and ITO/TiO₂/PDPA/P3HT and characterized by Raman techniques (Spectroscopy and Microscopy), Electrochemical Impedance Spectroscopy (EIS) and Photoluminescence (PL). In this study, the aromatic, semiquinone and quinone segments in the polymer matrices of the poly(3-alkylthiophenes) (P3ATs) and PDPA at these interfaces were monitored and characterized by comparison with films of their homopolymers by means of Raman spectroscopy and EIS. Raman imaging demonstrates that the P3MT film can be incorporated into the titanium oxide crystalline lattice. The systems containing P3MT or P3HT were found to strongly interact with the TiO₂, stabilizing the P3AT radical cation segments, and the presence of PDPA destabilized this interaction. These findings were complemented by low-temperature (15 K) PL spectra, revealing a reduction in the intensity and displacement of the band associated with radical cation emission, observed in the absence of TiO₂ in the system under investigation.