Figure 2. PbS quantum dot solar cells with high open-circuit voltages.

Experimentally measured open-circuit voltage (V_oc) as a function of bandgap (E_g) in PbS QD solar cells under 1 sun in air (closed circle). A linear fit to measured V_oc values yields , where q is the electronic charge (e). In addition to the linear fit, previously reported literature V_oc values are also plotted for comparison. These literature values were obtained from QD solar cells based on p-type PbS QDs employing three different types of junctions; (1) metal-QD Schottky junction (open circle)⁶,¹⁹,²⁰,²¹,²²,²³; (2) p–n heterojunction using n-type wide bandgap metal oxides such as TiO₂ or ZnO¹²,²⁴,²⁵,²⁶,²⁷, or n-type organic materials (C60 and PCBM)²⁸,²⁹ (open square); and (3) a recently introduced p–n homojunction using n-doped PbS QDs (closed triangle)³⁰. The inset illustrates the energy-band diagram of the metal-p-type semiconductor junction at thermal equilibrium.