Chloride (CH3NH3Cl, etc.) is usually used in perovskite to control the crystallization process and grain size, suppress defects, and stabilize the α-phase FAPbI3 to prepare high-efficiency perovskite solar cells (PSCs). However, there are few studies on the influence of residual chlorine distribution on performance, especially in post-treatments such as phenethylammonium iodide (PEAI), butylammonium iodide (BAI) and cyclohexylmethylammonium iodide (CHMAI). The effect of chlorine distribution, and the synergy with 2D perovskites are unclear.
The team of Professor Yan Keyou from South China University of Technology and the team of Professor Qiu Jianhang from Shenyang Institute of Metal Research, Chinese Academy of Sciences jointly reported the influence of the enrichment of chlorine on the surface of chlorine-containing perovskite films during the post-processing process on the device performance. Among them, after cyclohexylmethylammonium iodide (CHMAI) treatment, the Cl/I atomic ratio on the upper surface of the perovskite increased from 0.037 to 0.439; the Cl/I atomic ratio on the surface of the perovskite film treated with PEAI also increased to 0.114. The CHMAI device achieved the highest efficiency of 24.42% (certified as 23.60%), better than during PEAI treatment, and also achieved a high electroluminescent efficiency (EQEEL) of 10.84%, with the highest irradiance of 170 W sr-1 m- 2, which indicates that the non-radiative recombination process of carriers in the device is effectively suppressed.
XPS spectra of (a) Pb 4f, (b) I 3d and (c) Cl 2p. (d) Depth profile of Cl element sputtered by ToF SIMS from top to bottom (Perovskite/SnO2/ITO/Glass). (e, f) Elemental 3D-mapped trajectories of the ToF SIMS depth profile of Cl. (g, h) Planar SEM images, (i) XRD diffraction pattern of the perovskite film.
After the CHMAI-treated perovskite film is thermally annealed, the surface 2D phase composition will increase, while the chloride will decrease, and the photovoltage of the corresponding device will increase, but the fill factor will decrease. Therefore, it is speculated that the enrichment of chlorine on the surface of the perovskite film can improve the charge transport and increase the fill factor. At the same time, it was verified in the non-chlorine system that CHMAI can only increase the open circuit voltage of the device, but the fill factor is very low, which again shows that the 2D phase can passivate defects, and the chlorine-rich can promote the carrier transport.
Further DFT theoretical calculations found that the Cl element has a greater binding energy on the surface of CHMA@FAPbI3 than PEA@FAPbI3, so CHMAI can better adjust the chlorine distribution. This work provides insights into post-treatment of perovskite films and chloride immobilization on the surface of perovskite films to improve photovoltaic (PV) performance.
Figure 2. Relationship between chloride redistribution and 2D passivation
(a) J-V curve of CHMAI device and (b) XPS spectrum of Cl 2p of the corresponding perovskite film, (c) J-V curve of PEAI device and (d) XPS spectrum of Cl 2p of the corresponding film. (e) The relationship between the PCE of PSCs and the atomic ratio of Cl/I on the perovskite film surface. (f) Theoretical calculation diagram of the binding energy of Cl element on the surface of CHMA@FAPbI3 and PEA@FAPbI3 films. Among them, the black ball is Pb; the purple is I; the green is Cl. (g) J-V curves of MACl-free blank control group and CHMAI modified devices and (h) statistical distribution diagrams of device VOC and FF.
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