PbS and CdS Quantum Dot-Sensitized Solid-State Solar Cells: 'Old Concepts, New Results'

Lead sulfide (PbS) and cadmium sulfide (CdS) quantum dots (QDs) are prepared over mesoporous TiO2 films by a successive ionic layer adsorption and reaction (SILAR) process. These QDs are exploited as a sensitizer in solid-state solar cells with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as a hole conductor. High-resolution transmission electron microscopy (TEM) images reveal that PbS QDs of around 3 nm in size are distributed homogeneously over the TiO2 surface and are well separated from each other if prepared under common SILAR deposition conditions. The pore size of the TiO2 films and the deposition medium are found to be very critical in determining the overall performance of the solid-state QD cells. By incorporating promising inorganic QDs (PbS) and an organic hole conductor spiro-OMeTAD into the solid-state cells, it is possible to attain an efficiency of over 1% for PbS-sensitized solid-state cells after some optimizations. The optimized deposition cycle of the SILAR process for PbS QDs has also been confirmed by transient spectroscopic studies on the hole generation of spiro-OMeTAD. In addition, it is established that the PbS QD layer plays a role in mediating the interfacial recombination between the spiro-OMeTAD+ cation and the TiO2 conduction band electron, and that the lifetime of these species can change by around 2 orders of magnitude by varying the number of SILAR cycles used. When a near infrared (NIR)-absorbing zinc carboxyphthalocyanine dye (TT1) is added on top of the PbS-sensitized electrode to obtain a panchromatic response, two signals from each component are observed, which results in an improved efficiency. In particular, when a CdS-sensitized electrode is first prepared, and then co-sensitized with a squarine dye (SQ1), the resulting color change is clearly an addition of each component and the overall efficiencies are also added in a more synergistic way than those in PbS/TT1-modified cells because of favorable charge-transfer energetics.