J. Energy Chem.: Precursor fluid engineering refreshes CsPbIBr2 perovskite battery’s highest efficiency

• Research Background

  In recent years, all-inorganic perovskites (CsPbX 3 ) have attracted widespread attention due to their excellent thermal stability. Among them, CsPbIBr 2 perovskite can take into account both appropriate band gap and stability, and is considered an ideal optoelectronic material for use in many fields including solar cells, detectors, smart photovoltaic windows, etc. Currently, the reported photoelectric conversion efficiency of CsPbIBr perovskite solar cells is only 11-12%, which is still far below its theoretical limit. One of the main reasons is that the concentration of the precursor solution is low, resulting in the thickness of the perovskite film prepared by the solution spin coating method is only 250‒280 nm, which seriously affects the further improvement of light harvesting ability and photoelectric conversion efficiency.
  

  
  

  
  

  Achievement Display

  Recently, Yu Ze and others from Dalian University of Technology adopted a three-component precursor solution (TCP) consisting of cesium bromide, lead iodide and lead bromide to replace the currently widely used two-component precursor solution based on cesium iodide and lead bromide. Distribution formula (DCP). The three-component formulation increased the precursor concentration to 1.3 M, thus obtaining a 390 nm thick CsPbIBr perovskite film. At the same time, due to the stronger interaction between lead iodide and DMSO, the crystal growth process of perovskite is effectively regulated, thereby obtaining a high-quality perovskite film with larger grain size and smooth surface. On this basis, a small molecule squaryl acid modified material (SQ‒C8) was introduced between the perovskite and the hole transport material to passivate surface defects and accelerate charge transfer, ultimately achieving a photoelectric conversion efficiency of 12.8%. This is the highest efficiency currently achieved for CsPbIBr perovskite solar cells.
  
  This research work was published in the Journal of Energy Chemistry under the title "Precursor engineering enables high-performance all-inorganic CsPbIBr 2 perovskite solar cells with a record efficiency approaching 13%". The first author is Chang Qingyan, a master's student at Dalian University of Technology. and An Yidan, a postdoctoral fellow at City University of Hong Kong. The corresponding authors are Professor Ye Xuanli of City University of Hong Kong and Professor Yu Ze of Dalian University of Technology .

  
  

  Graphic and text introduction
  

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• Figure 1. DLS spectra of CsPbIBr precursor solutions: (a) DCP and (b) TCP. In situ UV-visible absorption spectra of CsPbIBr films : (c) DCP and (d) TCP. (e) Evolution of absorption intensity at 550 nm for CsPbIBr films prepared by DCP and TCP . (f) XRD patterns of CsPbIBr 2 thin films prepared by DCP and TCP.

• Time-resolved UV-visible spectroscopy (Figures 1c and 1d) shows that the CsPbIBr 2 film prepared by TCP clearly exhibits a relatively slow crystallization process (200 ~ 320 s) compared with the DCP-based film (200 ~ 270 s) . We further selected the evolution of the peak intensity at 550 nm with time to track the changes in film crystallization (Figure 1(e)). The crystal growth rate ( n pc ) of the film prepared by TCP is 0.017, which is significantly slower than the corresponding value of the film based on DCP ( n pc = 0.024). The XRD pattern (Figure 1(f)) shows that the TCP-treated film shows higher crystallinity.
  

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• SEM surface images of CsPbIBr perovskite films: (a) DCP and (d) TCP. Cross-sectional SEM images of CsPbIBr 2 perovskite films: (b) DCP and (e) TCP. AFM images of CsPbIBr 2 perovskite films: (c) DCP and (f) TCP.
• Figures 2a and 2d show that the CsPbIBr perovskite film prepared by TCP has larger grain size and fewer grain boundaries. Due to the increase in precursor concentration, the film thickness increased from 280 nm (DCP) to 390 nm (TCP) (as shown in Figures 2b and 2e). The AFM results in Figures 2c and 2f show that compared with DCP, the surface roughness of the CsPbIBr perovskite film prepared by TCP is reduced from 24.1 nm to 21.7 nm.
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• Figure 3. Optimal solar cell performance based on DCP and TCP: (a) J–V curve measured at 100 mW cm −2 (AM 1.5G). (b) IPCE pattern and corresponding integrated current. (c) Stable power output at the maximum power point under one sunlight irradiation (AM 1.5G). (d) Efficiency distribution plot based on 20 devices.
• Figure 3a shows that the TCP-based device obtains a short-circuit current Jsc of 11.50 mA cm ‒2 , which is better than the DCP-based device (10.69 mA cm ‒2 ). The IPCE spectrum (Figure 3b) further shows that TCP-based devices have higher IPCE values ​​in the 350-600 wavelength range. The main reason why TCP-based devices obtain high short-circuit current is due to the significant increase in the thickness of the perovskite light-absorbing layer, which in turn improves the light harvesting capability. Figure 4(ad) proves that the TCP precursor liquid formula effectively improves the quality of the perovskite film, reduces its defect state density, suppresses the non-radiative recombination of carriers, thereby increasing the open circuit voltage of the device. Figure 4e shows that the introduction of the SQ‒C8 interface modification layer can not only effectively passivate surface defects of perovskite, but also accelerate the extraction of charges at the interface, further increasing the photoelectric conversion efficiency to 12.8%.
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• Figure 4. CsPbIBr perovskite film: (a) PL and (b) TRPL spectra. (c) SCLC curve of electronic device only. (d) Dependence of device V oc on light intensity. (e) J − V curve of the TCP-based device with and without SQ-C8 modification measured at 100 mW cm ‒2 (AM 1.5G) . (f) Air stability of unpackaged devices (25% ± 5% RH, 25 °C).

• Summary

  This work successfully developed a three-component precursor liquid system for preparing high-quality CsPbIBr 2 perovskite films. On the one hand, the three-component precursor liquid increases the thickness of the perovskite film, thereby significantly improving the light harvesting capability. On the other hand, the three-component formula regulates the crystallization process of perovskite and obtains high-quality CsPbIBr perovskite films with larger grain size and fewer defects . This work provides a new idea for the solution method to prepare high-quality CsPbIBr perovskite films for high-efficiency solar cells and other optoelectronic devices.

  
  

  Qingyan Chang#, Yidan An#, Huaiman Cao, Yuzhen Pan, Liangyu Zhao, Yulong Chen, Yi We, Sai-Wing Tsang, Hin-Lap Yip*, Licheng Sun and Ze Yu*,  Precursor engineering enables high-performance all-inorganic CsPbIBr2 perovskite solar cells with a record efficiency approaching 13%, Journal of Energy Chemistry

  DOI:10.1016/j.jechem.2023.10.021

  https://www.sciencedirect.com/science/article/pii/S2095495623005892