Comparison of Solid-State Quantum-Dot-Sensitized Solar Cells with ex Situ and in Situ Grown PbS Quantum Dots

Differences in the solar cell performance of solid-state PbS quantum-dot-sensitized solar cells fabricated with ex situ and in situ grown PbS quantum dots were investigated. The PbS quantum dots were either anchored on mesoporous TiO₂ via l-glutathione (GSH) linker exchange or prepared in situ by the successive ionic layer adsorption/reaction (SILAR) method to create quantum-dot-sensitized solar cells. Spiro-OMeTAD was used as the organic p-type hole transporting material (HTM). The performance of the cells was evaluated with current-voltage, external quantum efficiency, and impedance spectroscopy (IS) measurements, and electron lifetimes were measured with open-circuit voltage decay (OCVD), intensity-modulated photovoltage spectroscopy (IMVS) techniques as well as with IS measurements. Analysis of the experimental data indicates that the SILAR route provides more intimate contacts at both TiO₂/PbS and PbS/HTM interfaces, which results in more efficient charge injection and separation and thus higher photocurrents. However, the results of the OCVD, IMVS, and IS measurements demonstrate that the cells sensitized with PbS quantum dots prepared ex situ have longer electron lifetimes, indicating that back-transfer of electrons to the HTM is slower, probably as a consequence of passivation of surface states by the GSH ligands.