Aggregation of indoline dyes as sensitizers for ZnO solar cells

Properties of indoline dye, D149, as a sensitizer for electrodeposited porous ZnO has been studied in detail. Control of adsorption time and use of co-adsorbing cholic acid (CA) were essential to achieve high efficiencies, because of aggregation of D149 to worsen the fill factor of solar cells. Adsorption isotherm measured for D149 on ZnO powder indicated 1.5 layer adsorption at saturation. Use of CA significantly improved the cell performance as it prevents aggregation and decreases the amount of D149 adsorbed. Impedance analysis of the cells with different levels of dye loading showed that dye aggregates form deep traps to promote recombination even at low cell voltages, thus resulting in an I–V curve with rounded shoulder with poor fill factor. A new indoline dye DN-7 turned out to behave much better in this regard, allowing foolproof reproduction of stably high efficiencies as its aggregation is much more relaxed than D149 and worsening of the fill factor can be avoided.