Behavior of photoexcited electrons in hole-transport material-free perovskite solar cells

Hole-transport material-free perovskite solar cells were prepared by utilizing a spin-coating method. To optimize the dropping conditions of the toluene as an antisolvent in the nitrogen atmosphere, the CH3NH3PbI3/TiO2/fluorine-doped tin oxide (FTO)/glass specimen consisted of a high-density tissue, and crystal faults such as voids and cracks were not observed on its surface. By controlling the thickness of the mesoporous TiO2 layer with the rotation speed (x) of a spin coater, it was speculated that the thicker mesoporous TiO2 layer enables not only an efficient electron extraction from the CH3NH3PbI3 perovskite layer but also a smooth transition of electrons to the FTO electrode. Moreover, the precursor solution for CH3NH3PbI3 perovskite crystals was optimized for its molar concentration (y). The energy conversion efficiency (η) gradually increased from η = 5.8% to 9.6% with an increase in y to 2.6 M above which it decreased. The reason to obtain a superior energy conversion efficiency is so that the larger interface between the mesoporous TiO2 and perovskite layers is able to extract photoexcited electrons effectively. The above facts show for that the perovskite solar cells that have a larger area are synthesized with good reproducibility.