Introduction

There are two methods fabricating the perovskite solar cells. In general, solution process that adds anti-solvent to spin coating solution MAPbI3 is easy to use. The process of solution shows low cost, but is difficult to control. The whole device manufacturing leads to low error tolerance. Another method of perovskite is thermal evaporation, which makes mass production possible and has high error tolerance, but it requires more time to fabricate and needs more cost.

Fabrication

Combining the advantages of thermal evaporation and solution process is the new method which has been created by our laboratory, called sandwich evaporation technique (SET).

PEDOT:PSS was spin-coated on ITO conductive glass. Then MAI/DMF solution was prepared and spin-coated on ITO substrates coated with PEDOT:PSS. Then PbI₂ was evaporated and MAI powder was evaporated in low pressure chamber to complete the active layer. Finally, PC₆₀BM was spin-coated as the electron transport layer. Then BCP was evaporated as the surface modification layer and Ag electrode respectively.

Results and discussions

Based on the above steps, we found that crystallization of perovskite improved when kept in low vacancy lower than 0.13 Pa. In order to further improve the quality, the process was applied to SET process to slow down the reaction rate.

As shown in the SEM (Scanning Electron Microscope), we compared the cross-section of devices that just finished and have been controlled at 0.13Pa for several days. The cross-section of the perovskite that has just been fabricated in Fig. (a) is not flat. The Electron-hole pairs will recombine in the crevice, which seriously affects the performance of device. The crystalline grain shown in fig. (b) after several days of preserving under low vacancy has relatively better flatness. The unreacted perovskite in the evaporation process can continue to finish completely, and the sandwich structure in MAI-PBI₂-MAI further promoted the full progress of the reaction. Finally, the highly quality perovskite solar cells were formed.

The PL (Photoluminescence Spectroscopy) of perovskite which fabricated with SET also first increased for several days and reached a maximum intensity when kept at low vacancy for several days.

CH3NH3Iaq.⇋CH3NH2aq.+HIaq.

4HIaq.+O2⇋I2s+H2Oaq.

2HIaq.⇋H2g+I2s

The intensity of X-ray we measured was increased from 1^st day to 7^th day and we got the maximum X-ray pattern and 7^th day.

The absorption of our samples had the same phenomenon. And the increasing absorption, explained that the improvement of the short current Jsc. Hence we got the highest Jsc , 23.66 mA/cm² at 7^th day.

Conclusion

The MAPbI₃ formed with MAI-PbI₂-MAI was successfully applied to the SET process. MAPbI₃ film first Further optimized the crystalline by controlling at low-pressure. The continuous and uniform perovskite layer have good performance for the 0.91V of V_oc, 23.66 mA/cm² of J_sc, 76.4% of fill factor, and 16.51% of efficiency. We believe that the SET process shows the great potential and has universal use in any of ABX₃ perovskite solar cells’ fabrication.