PbS and CdS Quantum Dot‐Sensitized Solid‐State Solar Cells: “Old Concepts, New Results”

Abstract

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.

Advancing colloidal quantum dot photovoltaic technology

“…Alternative loading methods have included using chemical bath deposition (CBD) [166][167][168], successive ionic layer adsorption and reaction (SILAR) [161,169,170] and ion layer gas adsorption and reaction to form CQDs in situ [171]. A recent study has shown that although the SILAR loading method provides more intimate interfacial contact, resulting in efficient charge injection and higher photocurrents, sensitization using ex situ synthesized CQDs leads to longer electron lifetimes [172].…”

The role of photonics in energy

“…[85][86][87][88] Chalcogenide nanoparticles with quantum confinement properties have also been considered and used as sensitizers in DSSCs. 89,90 These solar cells offer a series of advantages over the more traditional BHJ OPVs based on the polymer/fullerene systems. Some of the expected advantages are (1) a contribution to light absorption by an inorganic acceptor can lead to the generation of more photocarriers, due to their larger linear absorption coefficients compared to those of fullerene derivatives; (2) the absorption of nanoparticles can be tuned to cover a broad solar spectral range, as a result of modification of their size and shape, complementary to that of the organic electron donor/hole transporter; (3) the physical dimensions of some inorganic semiconductors can be tailored to produce 1-D nanostructures, to allow efficient exciton dissociation, i.e., charge separation and electron transporting pathways simultaneously; (4) ultrafast and efficient photoinduced charge carrier transfer between the electron acceptor (inorganic nanoparticles) and the electron donor (the organic semiconductor); (5) the acceptors have relatively high electron mobility; and (6) good photo-and chemical stability.…”

The Application of Inorganic Nanomaterials in Dye-Sensitized Solar Cells

“…The in situ preparation method, where the QDs are directly generated on the surface of metal oxide film electrode, mainly includes chemical bath deposition (CBD) (Diguna et al, 2007) and successive ionic layer adsorption and reaction (SILAR) (Lee et al, 2009a). There are many of advantages of the in situ deposition approaches.…”