Synthesis, band gap tuning, characterization and photosensitization of TiO 2 nanoparticles for application in hybrid solar cells

Abstract

In this work an endeavor was made to fabricate hybrid bulk heterojunctions solar cells based on doped and un-doped TiO 2 nanoparticles blended with the well-known organic polymer Poly(3-hexyl thiophene) and co-grafted with porphyrin and carminic acid. Charge transfer complex formation between the donor and acceptor dyes helped in increasing the photo induced generated current. To achieve this end sol-gel method was employed to synthesize anatase titania, which was characterized by electronic absorption spectroscopy, X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The first part of the research deals with the band gap modulation of TiO 2 by doping it with selected transition metals (2-8% Cu, Ni and Cr). This was aimed to study the effect of tuning the band gap of TiO 2 on the efficiency of the fabricated solar cells by extending the absorption spectrum of titania to the visible region. The doped materials were also characterized using optical and morphological techniques to investigate their properties. Direct band gap of TiO 2 (3.9 eV) was found to be significantly reduced to 2.94, 3.40 and 3.60 eV for 2% Cu, 4% Ni and 2% Cr-doped materials respectively. Higher dopant concentrations induced the widening of the band gap according to the Burstein-Moss phenomenon. XPS results confirmed the substitution of Ti 4+ ions by the doped transition metal ions. Bulk heterojunctions solar cells were fabricated using un- doped and doped titania in combination with P3HT in order to investigate the effect of doping on the performance of the device. An enhancement of the photo-generated current was observed by using doped titania. This increase could be ascribed to the tuning of the band gap of titania to absorb effectively in the visible region. Maximum photocurrent was obtained by employing Cu-doped TiO 2 . However the value of FF was reduced owing to the low V oc values. The second portion of this research is dedicated to investigate the effect of photosensitization of TiO 2 and M-TiO 2 (M= Cu, Ni, Cr) on the efficiency of solar cells. A metallated phthalocyanine (Ni-Pc) was used to functionalize TiO 2 nano- particles in an attempt to extend the absorption spectrum of titania to visible region. The grafted materials were optically analyzed using electronic absorption, fluorescence IVemission and FT-IR spectroscopy to study the successful chemisorption of the dye on TiO 2 surface. These dye sensitized TiO 2 and M-TiO 2 were employed to fabricate solid state solar cells using P3HT. I-V measurements were performed to see the effect of dye concentration on the performance of solar cells. The plots showed that maximum I sc was achieved using 15 μM of Ni-Pc and the efficiency of the device was enhanced 3 times as compared to pristine titania blended with P3HT. The doped grafted titania using 15 μM of Ni-Pc showed higher I sc compared to un-doped grafted materials but the FF was reduced. This lowered the overall efficiency. The effect of co-grafting was also investigated by preparing photo-active nano-hybrid material consisting of titania nanoparticles, carminic acid and sulphonic acid functionalized porphyrin. Adsorption of free base porphyrin on TiO 2 resulted in its metallation which was evidenced by the disappearance of two out of four Q-bands in the UV-visible spectra of porphyrin. The adsorption of carminic acid resulted in the formation of charge transfer complex with titania nanoparticles. This was confirmed by the electronic absorption and fluorescence emission spectroscopies. Energy level diagram showed that the interaction among the constituents of the nano hybrid assembly permitted the flow of electron in a cascade manner from carminic acid to TiO 2 .This also allowed direct flow of electrons either from carminic acid or porphyrin towards titania. The material was used as an active blend in hybrid bulk hetero-junction solar cells. Co- functionalized (co-grafted) TiO 2 based devices were found three times more efficient than the reference device but morphology of the device proved a major setback.