Electrodeposition of novel poly(naphthalenediimide-quaterthiophene) thin films and applications in plastic optoelectronics devices

A novel symmetric naphthalenediimide-quaterthiophene derivative (NDIT4d) has been polymerized on different substrates including glassy carbon and ITO/PET electrodes by means of electrochemical methods. XPS and UV-VIS spectroscopy as well as cyclic voltammetry have been employed for characterizing the thin film chemical features, the band gap and the HOMO and LUMO levels. DFT computational studies were in close agreement with the experimental observables also showing intriguing geometrical effects on the band gap energy values. The comparison of the energy levels locations of the electrodeposited poly(naphthalenediimide-quaterthiophene) derivative (e-PNDIT4) and P3HT thin films transferred by the Inverse Langmuir-Schaefer (ILS) technique, demonstrates that e-PNDIT4 behaves as acceptor at the interface with P3HT. Steady state fluorescence studies have been performed in order to study the electron transfer in both planar (double layer) and bulk heterojunctions made of e-PNDIT4 and P3HT. The bulk heterojunction - obtained by thermal annealing of the double layer system - has shown a fluorescence quenching of about 97%, indicating an efficient electron transfer from the excited P3HT to e-PNDIT4. Furthermore, electrochromic properties of e-PNDIT4 thin films have been studied by means of spectroelectrochemical measurements. Transmittance versus time plots, recorded under an applied square wave potential, showed an optical contrast of about 20%. These studies open new possibilities in terms of all-polymer devices including the employment of a new class of symmetric push-pull derivatives and the development of fabrication processes exploiting the synergetic use of electrochemical and layer-by-layer deposition methods.