Poly(naphthalenediimidequaterthiophene):Poly(hexyilthiophene) Heterojunctions. Efficient Polymer-to-Polymer Electron Transfer Interfaces

Organic thin films solar cells and plastic solar cells [1] have attracted the attention of the scientific community especially as regards the performance of new conjugated polymers including their interfaces [2-4]. In this work, poly(naphthalenediimidequaterthiophene) (PNDIT4) and poly(hexyilthiophene) (P3HT) have been employed, for the first time, for engineering planar and bulk heterojunctions by the synergetic use of two techniques: electropolymerization and layer by layer deposition. Electropolymerization has been used for obtaining PNDIT4 thin films on transparent ITO/PET electrodes, starting from the synthesized monomer. Inverse Langmuir-Schaefer technique has been employed for depositing P3HT on ITO/PET supported PNDIT4 electrodes. Both the techniques allow an accurate control of the films thickness and XPS as well as contact angle have been performed to investigate the thin film chemistry and interfacial tension. Bulk heterojunctions have been obtained by the thermal annealing at 130°C of planar heterojunctions. SEM and AFM analyses have evidenced the morphological structure of the mixed polymeric blends. Steady state fluorescence studies have been performed in order to investigate the electron transfer efficiency in these two device configurations. Planar architectures displayed a fluorescence quenching of 67% while bulk heterojunctions showed a fluorescence quenching as high as 98% suggesting an efficient electron transfer from the excited P3HT to PNDIT4 The high values of quenching percentage can be attributed to the synergic effect of the following factors: suitable domain size and film thickness of the two polymeric phases; appropriate energy levels location as confirmed by cyclic voltammetry, UV-Vis spectra and DFT calculations. The relevant performance of such polymer-polymer interface is here studied also through the development of plastic solar cells.