Nanoscale morphology control of polymer/TiO2 nanocrystal hybrids: Photophysics, charge generation, charge transport, and photovoltaic properties

We present a simple approach by using mixed solvent to control the morphology of poly(3-hexylthiophene) (P3HT)/TiO2 nanorod hybrid bulk heterojunction solar cells without any post-treatment. The effects of the controlled morphology on the optical and electrical properties are investigated. It has been a challenge to disperse polar inorganic nanocrystals at a relative high concentration into a relative nonpolar polymer. The use of mixed solvent which consists of pyridine (a poor solvent for P3HT), dichloromethane, and chloroform (a good solvent for P3HT) modifies the nanoscale morphology of P3HT/TiO2 nanorod hybrids, resulting in highly crystalline P3HT domains with well-dispersed TiO2 nanorods within polymer matrix. Study of photophysics reveals that charge carrier could form from emissive species upon photoexcitation and such a process is more efficient in highly ordered P3HT prepared by mixed solvent method. In the P3HT/TiO2 hybrid film, the formation of a bicontinuous phase-separated morphology largely improves charge separation, transport, and recombination in the hybrid devices, which are further supported by time-resolved photoluminescence spectroscopy, carrier extraction by linearly increasing voltage mobility measurement, and transient open-circuit voltage decay measurement, respectively. A result of threefold improvement of the device performance using mixed solvent has been demonstrated compared to that using a single solvent only. This simple process does not need any further thermal post-treatment and is therefore compatible with the room temperature process developed with commonly used plastic substrates for flexible solar cell applications. Our method for morphology control could also be applied to other donor-acceptor hybrid systems as a strategy for device optimization. © 2010 American Chemical Society.