Gradient-band-gap strategy for efficient solid-state PbS quantum-dot sensitized solar cells.

To improve charge separation and enhance open-circuit voltage (Voc) in solid-state quantum-dot sensitized solar cells (QDSCs), gradient-band-gap PbS quantum-dots were first and easily constructed by two-step spin-coating the Pb(NO3)2 solution and the mixed solution of Na2S and 1,2-ethanedithiol via successive ionic layer absorption and reaction (SILAR). The fabricated solid-state gradient-band-gap PbS QDSCs exhibited a Voc of 0.70 V, a short-circuit photocurrent density (Jsc) of 9.65 mA·cm-2, a fill factor (FF) of 0.60, and a photoelectric conversion efficiency (PCE) of 4.08%, while the inverse gradient-band-gap PbS QDSCs showed a Voc of 0.59 V, a Jsc of 5.86 mA·cm-2, an FF of 0.49 and a PCE of 1.69%. By optimization, the best solid-state gradient-band-gap PbS QDSCs achieved a Voc of 0.65 V and a PCE of 6.29% under 1 sun, and a Voc of 0.60 V and a PCE of 7.21% under 0.5 sun. The Voc of 0.65 V was relatively high, and the PCE of 6.29% was the highest value among solid-state QDSCs constructed using SILAR.