Improving the electron mobility of TiO2 nanorods for enhanced efficiency of a polymer–nanoparticle solar cell

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.