"The all-inorganic CsPbIBr2 perovskite has received more and more attention due to its suitable band gap and excellent environmental stability. However, the requirements of high temperature processing limit its application in flexible devices. Walid A. City University of Hong Kong. In the article Seed-Assisted Growth for Low-Temperature-Processed All-Inorganic CsPbIBr2 Solar Cells with Efficiency over 10%, Daoud prepared high-quality CsPbIBr2 calcium titanium by introducing methyl ammonium halide (MAX, X = I, Br, Cl) to reduce the crystallization temperature Low-temperature seed-assisted growth (SAG) method of mineral film. The mechanism is attributed to the fact that MA+-based perovskite seeds act as crystal nuclei, thereby reducing the formation energy of CsPbIBr2 during annealing. The study found that it was prepared at low temperature (150℃) The perovskite treated with methyl ammonium bromide has micron-sized crystal grains and excellent charge performance, with an efficiency of 10.47%. In addition, through the SAG method, the methyl ammonium bromide treatment based on high temperature (250 ℃) treatment The efficiency of perovskite devices reached 11.1%, which is currently the highest efficiency of CsPbIBr2 solar cells. DOI: 10.1002/smll.202001535"

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Refinement of important knowledge points

1. The cubic phase band gap of halide lead iodine perovskite
CsPbI3 1.73eV
CsPbI2Br 1.91eV (add it yourself)
CsPbIBr2 2.05eV
CsPbBr3 2.3eV
   
2. A brief history of CsPbIBr2 development
In 2016, Ma et al. used the dual-source thermal evaporation method to manufacture the first device based on CsPbIBr2 with a PCE of 4.7%. Soon thereafter, they increased the PCE to 6.3% (2016) by using spray assisted deposition. Subsequently, Li et al. used the conventional one-step method, followed by gas-assisted treatment, and the PCE was 8.02% (2017). Afterwards, Zhu et al. obtained a PCE of 9.16% (2018) through a simple intermolecular exchange strategy, and further increased the PCE to 10.71% (2019) with the help of interface modification. Recently, Subhani et al. introduced an SmBr3 interface layer between TiO2 and CsPbIBr2 for effective charge transfer and obtained a PCE of 10.88% (2019). In this paper, the seed-assisted growth method is used to obtain the highest efficiency of 11.1%. (See the citation in the paper for details)

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Graphic analysis

Figure 1. a) Schematic diagram of different manufacturing methods. The surface SEM images of the CsPbIBr2 perovskite film deposited by different methods and annealed at 150 ℃ and c) 250 ℃. The ratio is 1μm.
For CPG film, the precursor is directly annealed after spin coating without any treatment. For the IEG film, 200 μL methanol solution containing 10 mg mL-1 CsI was spin-coated on CsPbIBr2 at a speed of 4000 rpm for 15 s. For SAG perovskite, 200μL of MAX (X = I, Br, Cl) solution (isopropanol solution containing 10 mg mL-1) was dropped onto CsPbIBr2 and spin-coated at 4000 rpm for 15 seconds, and then proceeded accordingly deal with. By SEM, SAG has the largest grain.
seed-assisted growth=SAG
conventional pathway growth=CPG
intermolecular exchange growth=IEG

Figure 2. (a~c) XRD pattern of CsPbIBr2 thin film; (d~f) J-V curve of CsPbIBr2 thin film device; (g~i) relationship between annealing temperature and efficiency; J) reaction pathway

XRD shows that at the same temperature, the film produced by the SAG method has a purer phase and better film quality, and can form a purer cubic phase at a lower temperature; the JV curve proves that the device produced by the SAG method has better electrical properties; annealing The relationship between temperature and efficiency shows that the SAG efficiency is the best at the same temperature, and the three methods are all temperature-dependent, and the higher the annealing temperature, the higher the efficiency. In addition, when the temperature of the SAG method is greater than 150°C, the device efficiency is less affected by the annealing temperature, indicating that a higher efficiency device can be obtained at a lower temperature, which can be used for flexible battery applications. Figure J shows the reaction pathways of the three methods. SAG reduces the activation energy required for the reaction by forming an intermediate phase, that is, the corresponding product can be obtained at a lower temperature (PS: This method is described earlier, DMSO is placed Perhaps a device with a lower annealing temperature and better efficiency can be formed).

Figure 3. CsPbIBr2 films treated with different methylammonium halides (MAX, X = I, Br, Cl) a) XRD pattern; b) UV-visible absorption spectrum; c) Steady-state photoluminescence (PL) spectrum. D) SEM image, the inset shows the grain size distribution. The ratio is 1μm. ; G) Cross-sectional SEM image. The ratio is 500 nm. (Annealing temperature 150℃)
In order to further study the possible changes in the photoelectric performance of CsPbIBr2 perovskite affected by halide anions, three different methyl halide (MAX), such as MAI, MABr and methyl ammonium chloride (MACl), were used. And the corresponding perovskites are called Pvsk-I, Pvsk-Br and Pvsk-CI respectively. From the XRD point of view, the difference is not big, indicating that they are all pure cubic phases. But the two graphs b and c show that the band gap width of Pvsk-I, Pvsk-Br and Pvsk-CI is increased. In addition, the PL peak of Pvsk-Br is the strongest, indicating that the non-radiative recombination of the device is suppressed. d~f show that the crystal grain size of Br is the largest. In addition, the uniformity of the film after CI is the worst, and serious short-circuit effects will occur, which is not conducive to the manufacture of devices. The cross-section g~i graph also shows the problem of non-uniformity.

Figure 4. Pbsk-1, Pvsk-Br and Pvsk-Cl films; (a~f) XPS characterization (g~h) UPS spectrum; i) energy level diagram
Two additional Pb0 peaks at 136.8 and 141.7 eV were observed in the Pb 4f spectrum of the film (Figure 4f). Lead metal is generally considered to be the recombination center of carriers in the film, which leads to the degradation of device performance. The Pb0 peak intensity of the Pvsk-Br film is the lowest, indicating that the metal lead content of the perovskite film is low. How to make the energy band diagram of i through g and h is described in detail in the text, (a brief introduction: the valence band top EVBM = 21.22-(Ecutoff-Eonset) can be measured by UPS, and the band gap can be measured by an ultraviolet spectrophotometer Width to calculate the bottom of the conduction band, and finally make an energy band diagram). According to the energy band diagram, the valence band top of the Pvsk-Br film is the highest, and the hole transport layer has the lowest energy (0.3 eV), which is conducive to the extraction of photo-generated holes and improves the device performance.

Figure 5. Pvsk-1, Pvsk-Br and Pvsk-Cl PSC performance characterization; a) device structure; b) cross-sectional SEM image; c) the best efficiency JV curve; d) EQE curve; e) steady-state photocurrent and PCE; f) Device efficiency

Figure 6. a) Time-resolved photoluminescence (TRPL) spectrum; b) The light intensity correlation between Jsc (inset) and Voc; c) The dark JV curve of a pure electronic device with inserted structure; (d~e) stable Sex Research

Figure 7. (a~c) Surface SEM image. d~f) J-V curve of the reverse and forward scanning of the device

Table 1 Comparison of this work with other CsPbIBr2

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to sum up
1. This article reduces the formation energy by introducing MAX (X = I, Br, CI), and proves a low-temperature SAG method for high-quality CsPbIBr2 films. Through this strategy, it is possible to obtain micron-sized crystal grains with a complete film coverage at 150°C, and obtain the best efficiency of 10.47%.
2. The PCE of the PSC obtained by the SAG method of the Pvsk-Br film at 250°C reached 11.1%, which is the highest efficiency of the inorganic CsPbIBr2 PSC reported so far.
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