The improved photovoltaic performance under reduced sun
intensities is attributed to the reduction in carrier recombination
in the solar cell. Carrier recombination is an important process in
QDSCs e a large carrier recombination can lead to significant
degradation in the performance of a solar cell. In a QDSC,
electron-hole recombination can take place at the interfaces
between the TiO2/PbeSbeS/electrolyte. Using a reduced light
intensity produces a smaller number of electrons, which leads to
a reduction in carrier recombination and hence, an improved
photovoltaic response. In other words, when the PCE is measured
under a reduced light intensity, this represents the best efficiency
that the cell could achieve in this work. The decreasing VOC under
reduced light intensities is also related to the reduction in the
number of carriers. The theoretical upper limit for VOC is
EF Eredox, where EF is the quasi Fermi level of TiO2, and Eredox is
the electrolyte redox potential. EF shifts down as the carrier
density is reduced, which results in a smaller VOC. It is noteworthy
that the cell has a large normalized JSC ¼ 20 mA cm2
(10% sun). For a solar cell with Eg ¼ 1.5e1.6 eV, the theoretical
maximal JSC would be ~25 mA/cm2 [32]. Here, the normalized JSC
reached approximately 80% of the maximal JSC, an impressive
result for a relatively new material