July 23, 2019.
Hybrid inorganic-organic (MAPbI3) or pure inorganic (CsSnI3 or CsPbI3) are intensively used for fabricating the perovskite solar cell because of its high efficiency and simple fabrication process. In this work, we are interested on using inorganic CsSnxPb1-xI3 films as the p-type transporting layer in the solid-state dye-sensitized solar cells (S-DSSCs). This is because the inorganic CsSnxPb1-xI3 perovskite film is the non-volatile and has the simple preparing process. Interesting, the efficiency of CsSnI3 based S-DSSCs has been reported to be very high of ~10.2% . The reason of the superior performance is due to the high hole mobility in CsSnI3 films (585 cm2V-1s-1 at room temperature). CsPbI3 film also shows the p-type behavior, and it was successfully used as the light absorber with the good cell efficiency of 2.9%  and 4.13% . The physical and optical properties of inorganic CsSnxPb1-xI3 films are depended on the ratio of Sn/Pb. In this work, CsSnxPb1-xI3 films were prepared by simply dropping the mixed CsI, SnI2 and PbI2 solutions at five different Sn/Pb ratios (x = 1, 0.8, 0.5, 0.2 and 0) on the substrate surface. The color of CsSnxPb1-xI3 films was varied with the Sn/Pb ratio. The Sn/Pb ratio of 0.5, CsSn0.5Pb0.5I3, dictates the darkest color, whereas the other Sn/Pb ratios display dark-brown or yellow-like colors. The film morphology and crystallite will be analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Solid-state dye-sensitized solar cells (S-DSSC) were assembled by directly dropping the mixed CsI, SnI2 and PbI2 solution on the TiO2-coated-dye electrodes, and drying under the room temperature. CsSn0.5Pb0.5I3 based S-DSSC generates the highest efficiency of 3.47% amount all five conditions (CsSnxPb1-xI3, x = 1, 0.8, 0.5, 0.2 and 0). The cell impedances will be tested by Electrochemical Impedance Spectroscopy (EIS) for explaining the cell behaviors.
In this research, tetragonal CH3NH3PbI3 perovskite thin films were successfully fabricated under atmospheric air using a one-step hot-casting technique. Dendritic patterns in film morphology were observed for low casting temperature films. Full surface coverage films were achieved at high casting temperature of 180 °C. X-ray diffraction (XRD) showed that as low as 0.08° of the full width at half maximum of the (110) peak can be achieved with Ts180C sample suggesting large crystallite size in the film. Lattice parameters extracted from the XRD patterns through Nelson-Riley plots were different from those found in the literature. The energy gaps of the hot-casted films change with changing casting temperature from 1.48 -1.55 eV due to the variation of Urbach energy. The relating perovskite solar cells showed that at low casting temperatures the short circuit current density (Jsc) was higher than at higher casting temperatures, this is attributed to their lower recombination activities. The hot-casted perovskite thin films had superior structural stability compared with the two-step method films. In Figure 1 as shown below, the photoluminescence (PL) spectra showed that the hot-casted perovskite films had additional emission features at 766, 794, and 823 nm relating to trap states present in the thin films. The origins of these trap states were believed to be attributed to the presence of iodine vacancies (VI), iodine interstitial (Ii) and methylammonium ion vacancies (VMA) in the prepared films.
The tetragonal MAPbI3 perovskite thin films have been successfully fabricated by hot-casting method under atmospheric air. However, the dendritic pattern in the film morphology is induced in low casting temperature due to the lack of supersaturation and fast crystallization. Full surface coverage was established with high casting temperature of 180 °C where fast saturation and crystallization took place. The hot-casted perovskite thin films exhibited casting temperature dependence in the energy gap as the Urbach energy changes. The stability was significantly improved for hot-casted film compared to that derived from the two-step deposition method. However, the question for improved stability is still open. Furthermore, the hot-casted films suffer from several recombination channels relating to trapped holes originated from the interstitial sites and vacancies induced by the fast formation of the prepared films. The energy level of the trapped holes would lie between the work function of gold and highest occupied molecular orbital (HOMO) of poly(triaryl amine (PTAA). These recombination centers are more sensitive to the resultant PCEs than the degree of coverage of the perovskite photoactive layer. The number of trap states is relatively significant, the improvement has to be taken into account for developing MAPbI3 perovskite prepared under full air for further applications. The best cell has shown the efficiency of 5.91%, as with 13.42 mA/cm2 (Jsc), 0.89 V (Voc) and 0.50 (FF) in condition Ts at 90 oC.
Fig. 1 Model represents some possible radiative transition channels.
 In Chung, et. al., “All-solid-state dye-sensitized solar cells with high efficiency”, Nature, 2012, 485, 486-489.
 Giles E. Eperon et. al., “Inorganic caesium lead iodide perovskite solar cells”, Mater. Chem. A, 2015, 3, 19688.
 Paifeng Luo et. al., “Solvent engineering for ambient-air-processed, phase-stable CsPbI3 in perovskite solar cells”, J. Phys. Chem. Lett. 2016, 7, 3603−3608.
Assoc. Prof. Dr. Vittaya Amornkitbamrung
Solid State Physics Laboratory
Dept. of Physics, Fac. of Science, Khon Kaen University, Khon Kaen – 40002, Thailand