Light harvesting and electron recombination are essential factors that influence photovoltaic performance of quantum dots sensitized solar cells (QDSSCs). ZnO hollow microspheres (HMS) as architectures in QDSSCs are beneficial in improving light scattering, facilitating the enhancement of light harvesting efficiency. However, this advantage is greatly weakened by defects located at the surface of ZnO HMS. Therefore, we prepared a composite hollow microsphere structure consisting of ZnO HMS coated by TiO₂ layer that is obtained by immersing ZnO HMS architectures in TiCl₄ aqueous solution. This TiO₂-passivated ZnO HMS architecture is designed to yield good light harvesting, reduced charge recombination, and longer electron lifetime. As a result, the power conversion efficiency (PCE) of QDSSC reaches to 3.16% with an optimal thickness of TiO₂ passivation layer, which is much higher when compared to 1.54% for QDSSC based on bare ZnO HMS.

Zn_xCd_1-xSe@ZnO hollow spheres (HS) were successfully fabricated for application in quantum dot sensitized solar cells (QDSSCs) based on ZnO HS through the ion-exchange process. The sizes of the Zn_xCd_1-xSe@ZnO HS could be tuned from ~300 nm to ~800 nm using ZnO HS pre-synthesized by different sizes of carbonaceous spheres as templates. The photovoltaic performance of QDSSCs, especially the short-circuit current density (J_sc), experienced an obvious change when different sizes of Zn_xCd_1-xSe@ZnO HS are employed. The Zn_xCd_1-xSe@ZnO HS with an average size distribution of ~500 nm presented a better performance than the QDSSCs based on other sizes of Zn_xCd_1-xSe@ZnO HS. When using the mixture of Zn_xCd_1-xSe@ZnO HS with different sizes, the power conversion efficiency can be further improved. The size effect of the hollow spheres, light scattering, and composition gradient structure Zn_xCd_1-xSe@ZnO HS are responsible for the enhancement of the photovoltaic performance.

Size controllable TiO₂ hollow microspheres (HMS) were synthesized by a carbonaceous spheres (CS) template method. Based on TiO₂ HMS, the ZnS/CdS quantum dots (QDs) were loaded to form a ZnS/CdS@TiO₂ HMS photoanode for quantum dots sensitized solar cell (QDSSC). The size effects of TiO₂ HMS on photovoltaic performance were investigated, and showed that TiO₂ HMS with sizes ~560 nm produced the best short-circuit current density (J_sc) of 8.02 mA cm⁻² and highest power conversion efficiency (PCE) of 1.83%, showing a better photovoltaic performance than any other QDSSCs based on TiO₂ HMS with size ~330 nm, ~400 nm, and ~700 nm. The improvement of photovoltaic performance based on ~560 nm TiO₂ HMS which can be ascribed to the enhanced light harvesting efficiency caused by multiple light reflection and strong light scattering of TiO₂ HMS. The ultraviolet-visible (UV-vis) spectra and incident photo to the current conversion efficiency (IPCE) test results confirmed that the size of TiO₂ HMS has an obvious effect on light harvesting efficiency. A further application of ~560 nm TiO₂ HMS in ZnS/PbS/CdS QDSSC can improve the PCE to 2.73%, showing that TiO₂ HMS has wide applicability in the design of QDSSCs.