Employing an amphiphilic interfacial modifier to enhance the performance of a poly(3-hexyl thiophene)/TiO2 hybrid solar cell

We have studied two amphiphilic interfacial modifiers: low cost Cu phthalocyanine dye containing ether side chains (Cu-ph-ether dye) and a carboxylic acid-and bromine-terminated 3-hexyl thiophene oligomer (oligo-3HT-(Br)COOH, M-w similar to 5K) to enhance the interfacial interaction between poly(3-hexyl thiophene) (P3HT) and TiO2 nanorods. A large improvement in the performance of fabricated solar cells was observed using these relatively large molecular modifiers when compared to pyridine-modified TiO2 nanorods. UV-vis spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the modifiers are adsorbed and chemically bonded to TiO2 through unshared electrons associated with the modifiers. Furthermore, the new modifiers increased the hydrophobicity of TiO2 with the order of oligo-3HT-(Br) COOH > Cu-ph-ether dye > pyridine. Synchrotron X-ray spectroscopy studies of the modified hybrid films indicate the crystallinity of P3HT is increased, following the same trend as the hydrophobicity, because the new modifiers function as plasticizers, increasing the flow characteristics of the film. Moreover, the same trend is also observed for the reduced recombination rate and increased lifetime of charge carriers in the device by transient photo-voltage measurement. Thus, the oligo-3HT-(Br)COOH outperforms the Cu-ph-ether dye and pyridine in enhancing the power conversion efficiency (PCE, eta) of the solar cell. More than a two-fold improvement is shown compared to pyridine. The results are due to the large size, conductivity, and polar characteristics of the oligo-3HT-(Br)COOH unit, which facilitates both the crystallization of P3HT and the electron transport of the TiO2 nanorods. This study provides a useful route for increasing the efficiency of hybrid solar cells via the enhancement of interfacial interactions between organic donors and inorganic acceptor materials.