For improvement of the electron transporting and charge separation in the interface of TiO2 and perovskite, the smooth and compact Nb-doping TiO2 electron transporting layer was successfully prepared by a hydrolysis–pyrolysis method using an acidic titanium isopropoxide solution in isopropanol and niobium chloride (NbCl5) as the niobium source. The planar perovskite solar cells with the undoped TiO2 electron transporting layer gave a photoelectric conversion efficiency (PCE) of 14.56%, while the planar perovskite solar cells with the 2.5% Nb-doping TiO2 electron transporting layer obtained a PCE of 15.97%. When the TiO2 nanorod array was introduced between the 2.5% Nb-doping TiO2 electron transporting layer and the perovskite thin film, the corresponding perovskite solar cells achieved a PCE of 18.88% under illumination of simulated AM 1.5 sunlight (100 mA cm–2).

In this paper, a two-layer structured PbI2 thin film was constructed by the spin-coating procedure using a 0.80 M PbI2 solution in DMF and subsequent close-spaced vacuum thermal evaporation using PbI2 powder as a source. The bottom PbI2 thin film was compact with a sheet-like appearance, parallel to the FTO substrate, and can be easily converted to a compact perovskite thin film to suppress the charge recombination of the electrons of the TiO2 conduction band and the holes of the spiro-OMeTAD valence band. The top PbI2 thin film was porous with nano-sheet arrays, perpendicular to the FTO substrate, and can be easily converted to a porous perovskite thin film to improve the hole migration from the perovskite to spiro-OMeTAD and the charge separation at the perovskite/spiro-OMeTAD interface. The planar perovskite solar cells based on the two-layer structured PbI2 thin film exhibited a photoelectric conversion efficiency of 11.64%, along with an open-circuit voltage of 0.90 V, a short-circuit photocurrent density of 19.29 mA cm(-2) and a fill factor of 0.67.

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Solid-state gradient-band-gap PbS QDSCs achieved a PCE of 6.29% under 1 sun and 7.21% under 0.5 sun.