Dual Sensitization Strategy for High-Performance Core/Shell/<i>Quasi-shell</i> Quantum Dot Solar Cells

The potential of quantum-dot sensitized solar cells (QDSCs), a promising candidate for third-generation photovoltaics, has not been fully realized with the corresponding power conversion efficiencies (PCE) still hovering below 9%. In this context, we demonstrate an optimized dual sensitization strategy that combines the linker-assisted self-assembly of QDs and successive ionic layer adsorption and reaction (SILAR) approach to assemble high-efficiency QDSCs. CdTe/CdS core/shell QDSC is chosen as the model system whose PCE, so far, has been reported at ∼3.8%. Our dual sensitization strategy comprises self-assembly of Type-II CdTe/CdS core/shell QDs on porous TiO₂ followed by deposition of an additional CdS quasi-shell through SILAR. The highest QD surface coverage was optimized by systematic pH variation, whereby PCE improved from 2.04(1)% (pH 11) to 3.696(5)% (pH 13). It was observed that while the epitaxial shell passivates the core surface traps, the nonepitaxial quasi-shell passivates the TiO₂ surface states. Thus, for core/shell, core/<i>quasi-shell</i> and core/shell/<i>quasi-shell</i> sensitized devices, PCE increased as 1.5(1)%, 3.6(4)%, and 5.69(2)%, respectively. The thickness of CdS shell and quasi-shell were optimized to achieve the PCE of CdTe/CdS/<i>CdS</i> core/shell/<i>quasi-shell</i> QDSCs as high as 6.32(9)% (6.41% for the champion cell), which notably is the highest for any aqueous processed QDSC