As for thin film solar cells, Perovskite solar cells (PSCs) with organo-lead halide perovskites as light harvesters is the latest development, whose certified power conversion efficiency (PCE) = (22.1) is higher than that of multicrystalline silicon solar cells (21.3%). And CH₃NH₃PbI₃ (MAPbI₃) is one of the most common light harvesters in PSCs. However, when the MAPbI₃ film is prepared, a certain amount of PbI₂ generally remains in it. Some researchers believe that the remnant PbI₂ could play a passivation role in improving the performance of PSCs, but the others have the opposite views. What is the truth?

A recent research on the passivation effect of remnant PbI₂ in PSCs gives the answer. It was performed by the research group led by Prof. FANG Xiaodong in Anhui Institute of Optics and Fine Mechanics (AIOFM) of Chinese Academy of Sciences.

In the research, Dr. WANG Shimao and his coworkers prepared MAPbI₃ films through two-step spin-coating method, and the content and location of the remnant PbI₂ were tuned through varying the heating temperatures of PbI₂ and MAPbI₃ films respectively.

The charge recombination behavior of PSCs and the passivation effect of remnant PbI₂ were investigated through electrochemical impedance spectroscopy (EIS, Fig. 1A and B), dark current, and time-resolved fluorescence emission decay (Fig. 1C) measurements.

It was found that the charge recombination in PSCs was gradually suppressed, the fluorescence emission lifetime gradually increased with the content of unreacted PbI₂ located at TiO₂/perovskite interface increasing.

Moreover, when decomposed PbI₂ remains among the MAPbI₃ particles, the charge recombination resistance (Rrec) and fluorescence emission lifetime increased significantly.

So far, the passivation effects of the unreacted and decomposed PbI₂ in PSCs have been confirmed. This research was published in Nanoscale entitled Credible evidence for the passivation effect of remnant PbI₂in CH₃NH₃PbI₃films in improving the performance of perovskite solar cells.

This work was supported by the National Natural Science Foundation of China, the China Postdoctoral Science Foundation, the National Basic Research Program of China, and the Key Laboratory of Novel Thin Film Solar Cells, Chinese Academy of Sciences.

Fig. 1.Charateristics of PSCs and PbI₂ (A) The representative Nyquist plots of PSCs prepared at different temperatures obtained under the bias voltage of 0.8 V. (B) The variation of recombination resistance (R_rec) with the preparation temperature obtained under the bias voltages of 0.7, 0.8, 0.9 and 1.0 V. (C) Time-resolved fluorescence emission decay spectra and their fits (lines) of the MAPbI₃ films prepared at different temperatures. (Image by WANG Shimao)