Improving the electron mobility of TiO2nanorods for enhanced efficiency of a polymer–nanoparticle solar cell

Abstract

The poly(3-hexyl thiophene):TiO₂ nanorod (P3HT:TiO₂) solar cell has a better thermal stability than the P3HT:PCBM solar cell; however, the former has a lower power conversion efficiency (PCE) than the latter. We would like to enhance the PCE of P3HT:TiO₂ solar cell by improving the electron mobility of anatase TiO₂ nanorods. Two novel approaches: (1) ripening and (2) boron doping for TiO₂ nanorods were explored. TiO₂ nanorods were synthesized first by sol–gel process in the presence of an oleic acid surfactant at 98 °C for 10 h. The size of the TiO₂ nanocrystal is about 35 nm in length and 5 nm in diameter. The insulating oleic acid on the TiO₂ nanorods was replaced by pyridine (as-synthesized TiO₂) for good compatibility and charge transport between P3HT and TiO₂ in the application of hybrid P3HT:TiO₂ nanorod solar cells. The crystallinity of the as-synthesized TiO₂ nanorods was increased through ripening (120 °C, 24 h) by using an autoclave reactor while the size of the nanocrystals was not significantly changed. Boron doped TiO₂ nanorods (B-doped TiO₂) were synthesized using the same sol–gel process of as-synthesized TiO₂ nanorods but by replacing 0.7 at.% Ti with B using boron n-butoxide instead of titanium tetraisopropoxide. The UV-Vis spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses indicate the B is present in TiO₂ nanorods as substitutional defects which can be either Ti–O–B or O–Ti–B bonding, with a B 1 s binding energy of 192.1 eV. The ripening process is more effective at increasing the crystallinity of TiO₂ nanorods than boron doping, as shown by XRD and Raman spectroscopy. The electron mobility of the TiO₂ nanorods is improved from 6.21×10⁻⁵ to 2.33×10⁻⁴ (cm² V⁻¹ s) and 5.27×10⁻⁴ (cm² V⁻¹ s) for ripened TiO₂ and B-doped TiO₂, respectively, as compared with as-synthesized TiO₂. The PCE of P3HT:TiO₂ solar cells was increased by 1.31 times and 1.79 times under A. M. 1.5 illumination for ripened and B-doped TiO₂, respectively, as compared with as-synthesized TiO₂. The B-doped TiO₂ has the highest mobility and PCE, mainly due to the presence of partially reduced Ti⁴⁺ by boron atom with delocalized electrons.