Both bulk heterojunction and quantum funnel strategies provide methods to effectively drive carriers toward the junction interface where electron-hole separation. Proper band alignment at the junction is crucial for charge separation and hence device performance. As shown in Fig. 5, when the conduction band of PbS CQDs is lower than TiO
2,
Jsc is less than optimal because electron needs to overcome the energy barrier to inject into TiO
2 layer, but the
Voc is maximized due to the large separation between the quasi-Fermi level of electrons in TiO
2 and quasi-Fermi level of holes in PbS CQDs film. When the conduction band of PbS CQDs is higher than TiO
2, electron injection into TiO
2 layer is facilitated but at the price of poor
Voc. Only when the band alignment between PbS CQDs and TiO
2 layer is perfectly match, both
Jsc and
Voc would be optimized leading to best device efficiency. Guided by this understanding, Liu et al. at University of Toronto studied doping of TiO
2 electrode to tune its conduction band in the hope of optimally aligning the band between TiO
2 and PbS CQDs film [
24]. Many dopants were tried to be included into TiO
2, and Zr doping showed promising improvements in CQDs photovoltaic performance: device employed Zr doped TiO
2 electrodes achieved 5.6% efficiency under simulated AM1.5 illumination with a
Jsc of 19 mA/cm
2, while the device employed undoped TiO
2 electrode had an efficiency of 3.8% with a
Jsc 16.1 mA/cm
2. Field effect transistor (FET) measurements eliminated the difference in electron mobility and concentration between doped TiO
2 electrodes, and cyclic voltammetry study revealed that Zr doping yield TiO
2 electrode had the conduction band at -3.95 eV, best matching the 950 nm PbS CQDs employed in the device fabrication. Optimal band alignment resulted in balanced
Voc and
Jsc, providing the best performing device.