Light energy conversion by mesoscopic PbS quantum dots/TiO ₂ heterojunction solar cells

Solid state PbS quantum dots (QDs)/TiO ₂ heterojunction solar cells were produced by depositing PbS QDs on a 500 nm thick mesoscopic TiO ₂ films using layer-by-layer deposition. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO ₂ device produces a short circuit photocurrent (J _sc) of 13.04 mA/cm ², an open circuit photovoltage (V _oc) of 0.55 V and a fill factor (FF) of 0.49, corresponding to a light to electric power conversion efficiency (η) of 3.5% under AM1.5 illumination. The electronic processes occurring in this device were investigated by transient photocurrent and photovoltage measurements as well as impedance spectroscopy in the dark and under illumination. The investigations showed a high resistivity for the QD/metal back contact, which reduces drastically under illumination. EIS also indicated a shift of the depletion layer capacitance under illumination related to the change of the dipole at this interface.