Synthesis and investigation of low band gap energy donor-acceptor polyaniline

Solar cells has gained a great attention as a green, renewable and cheap energy resources. To overcome the challenging technical problems and improve their competitiveness with silicone solar cells, the design, synthesis and development of new materials with engineered band gap energies has found an undeniable importance.

Herein, the synthesis of a polymer with donor-acceptor structure based on polyaniline grafted to ZnO nanoparticles at one end and naphthalene moiety at the other end of chains, and investigation of their chemical structure, composition, morphology, optical and electrochemical properties is reported. The chemical structure of the materials were analyzed by FT-IR and 1H NMR spectroscopy. The organic and inorganic contents of materials were determined by thermal gravimetric analysis (TGA) and atomic absorption spectroscopy (AAS) techniques. The morphology and size of nanoparticles were observed by scanning electron microscopy (SEM). The optical and electrical band gap energy of the samples were measured by ultraviolet visible-diffuse reflectance (UV-Vis-DRS) spectroscopy and cyclic voltammetry (CV) diagrams.

The chemical structure of designed materials has been successfully confirmed by the results of FT-IR and 1H NMR spectra. TGA and AAS analysis have indicated that the synthesized final material has contained about 10% of ZnO and 90% of organic parts including toluene-2,4-diisocyanate, 2,4-diaminotoluene, polyaniline and naphthalene groups. An almost highly uniform spherical nanoparticles with sizes about 70 nm has been observed by SEM images. UV-Vis-DRS spectroscopy and CV diagrams have revealed that by grafting ZnO nanoparticles and naphthalene moiety to the polyaniline chain ends, the optical and electrical band gap energy of the sample were lowered to 1.19 and 0.95 eV, respectively. It was concluded that the grafted groups to chain ends has increased the length of conjugated system, lowering the energy level of lowest unoccupied molecular orbital (LUMO) and increasing the energy level of highest occupied molecular orbital (HOMO). Detailed analysis of CV diagrams has indicated that the effect in lowering of LUMO has been a bit more pronounced than the increasing of HOMO energy level.