High-performance quantum dot-sensitized solar cells based on sensitization with CuInS ₂ quantum dots/CdS heterostructure

A high-performance quantum dot-sensitized solar cell (QDSSC) is reported, which consists of a TiO ₂/CuInS ₂-QDs/CdS/ZnS photoanode, a polysulfide electrolyte, and a CuS counter electrode. The sensitization process involves attaching presynthesized CuInS ₂ QDs (3.5 nm) to a TiO ₂ substrate with a bifunctional linker, followed by coating CdS with successive ionic layer adsorption and reaction (SILAR) and ZnS as the last SILAR layer for passivation. This process constructs a sensitizing layer that comprises CdS nanocrystals, closely packed around the earlier-linked CuInS ₂ QDs, which serve as the pillars of the layer. The CuS counter electrode, prepared via successive ionic solution coating and reaction, has a small charge transfer resistance in the polysulfide electrolyte. The QDSSC exhibits a short-circuit photocurrent (J _sc) of 16.9 mA cm ^-2, an open-circuit photovoltage (V _oc) of 0.56 V, a fill factor of 0.45, and a conversion efficiency of 4.2% under one-sun illumination. The heterojunction between the CuInS ₂ QDs and CdS extends both the optical absorption and incident photon conversion efficiency (IPCE) spectra of the cell to a longer wavelength of approximately 800 nm, and provides an IPCE of nearly 80% at 510 nm. The high TiO ₂ surface coverage of the sensitizers suppresses recombination of the photogenerated electrons. This results in a longer lifetime for the electrons, and therefore, the high V _oc value. The notably high J _sc and V _oc values demonstrate that this sensitization strategy, which exploits the quantum confinement reduction and other synergistic effects of the CuInS ₂-QDs/CdS/ZnS heterostructure, can potentially outperform those of other QDSSCs.