Abstract:
We study the charge recombination kinetics and photovoltaic performance of composites of poly (9,9-dioctylfluorene co-bithiophene) polymer with nanocrystalline TiO2. Transient optical spectroscopy confirms that photoexciation of the
polymer leads to electron transfer to the TiO
2 and indicates that charge recombination is slow with a half-life of 100µs
to 10ms. Polymer penetration into thick porous TiO
2 layers is improved by melt-processing and treatment of the TiO
2
surface. We study the photovoltaic characteristics of devices with different layer thickness and interface morphology.
Quantum efficiency (QE) of all devices is increased by reducing the TiO
2 and polymer layer thickness. Inserting a thin
porous TiO
2 layer in to a thin bi-layer device increases the QE by a factor of five. The improved device shows peak
QE and monochromatic power conversion efficiencies of over 11% and 1% at 440nm respectively. The device
produced a short-circuit current density of 300
µAcm-2, a fill factor of 0.24 and an open-circuit voltage of 0.8V under
AM1.5 illumination. The fill factor is increased from 0.24 to 0.40 by introducing an additional dip-coating layer and
overall power conversion efficiency is increased by 50 %. However, the device produced degraded current-voltage
characteristics. We investigate this using an alternative polymers and different top contact metals.
