Whispering Gallery Mode Enabled Efficiency Enhancement: Defect and Size Controlled CdSe Quantum Dot Sensitized...

A synergetic approach of employing smooth mesoporous TiO₂ microsphere (SμS-TiO₂)-nanoparticulate TiO₂ (np-TiO₂) composite photoanode, and size and defect controlled CdSe quantum dots (QD) to achieve high efficiency (η) in a modified Grätzel solar cell, quantum dot sensitized whisperonic solar cells (QDSWSC), is reported. SμS-TiO₂ exhibits whispering gallery modes (WGM) and assists in enhancing the light scattering. SμS-TiO₂ and np-TiO₂ provide conductive path for efficient photocurrent charge transport and sensitizer loading. The sensitizer strongly couples with the WGM and significantly enhances the photon absorption to electron conversion. The efficiency of QDSWSC is shown to strongly depend on the size and defect characteristics of CdSe QD. Detailed structural, optical, microstructural and Raman spectral studies on CdSe QD suggest that surface defects are prominent for size ~2.5 nm, while the QD with size > 4.5 nm are well crystalline with lower surface defects. QDSWSC devices exhibit an increase in η from ≈0.46% to η ≈ 2.74% with increasing CdSe QD size. The reported efficiency (2.74%) is the highest compared to other CdSe based QDSSC made using TiO₂ photoanode and I^-/I₃^- liquid electrolyte. The concept of using whispering gallery for enhanced scattering is very promising for sensitized whisperonic solar cells.