Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells

We have systematically investigated the effects of surface modification of titania $\left({\text{TiO}}_2\right)$ in hybrid ${\text{TiO}}_2∕\text{regioregular}$ poly(3-hexylthiophene) (P3HT) photovoltaic cells. By employing a series of para-substituted benzoic acids with varying dipoles and a series of multiply substituted benzene carboxylic acids, the energy offset at the ${\text{TiO}}_2∕\text{polymer}$ interface and thus the open-circuit voltage of devices can be tuned systematically by 0.25 V. Transient photovoltage measurements showed that the recombination kinetics was dominated by charge carrier concentration in these devices and were closely associated with the dark current. The saturated photocurrent of ${\text{TiO}}_2∕\text{P}3\text{HT}$ devices exhibited more than a twofold enhancement when molecular modifiers with large electron affinity were employed. The ability of modifiers to accept charge from polymers, as revealed in photoluminescence quenching measurement with blends of polymers, was shown to be correlated with the enhancement in device photocurrent. A planar geometry photoluminescence quenching measurement showed that ${\text{TiO}}_2$ substrates modified by these same molecules that accept charge quenched more excitons in regioregular P3HT than bare ${\text{TiO}}_2$ surfaces. An exciton diffusion length in P3HT as large as $6.5-8.5\text{nm}$ was extracted. By measuring the external quantum efficiency (EQE) of working devices, it was found that all of the excitons that were quenched were accountable as extracted photocurrent. EQE was effectively increased from $5\%$ to $10\%-14\%$ with certain surface modifiers; consequently exciton harvesting was more than doubled. The use of ruthenium (II) sensitizing dyes with good exciton harvesting property coupled with suppression of the recombination kinetics improved the efficiency of optimized bilayer ${\text{TiO}}_2∕\text{P}3\text{HT}$ devices from $0.34\%$ to $0.6\%$ under AM 1.5 solar illuminations. The implication of this work is directly relevant to the design of nanostructured bulk heterojunction inorganic-organic cells, in which efficient exciton harvesting and control of the recombination kinetics are key to achieving high efficiency.