Research Background
1. Narrow bandgap Pb-Sn perovskite solar cells are suitable as bottom cells for series solar cells, and are expected to exceed the single junction S-Q limit.
2. The coordination between 2-AD and perovskite can inhibit the oxidation of Sn2+, thereby reducing the p-doping level and increasing the open circuit voltage
3. Sn Pb mixed PSCs modified by 2-AD achieved high PCE of 22.31% and excellent stability
1、 Narrow bandgap Pb-Sn perovskite solar cells are expected to exceed the single junction S-Q limit
The power conversion efficiency (PCE) of organic inorganic lead based perovskite solar cells (PSC) has reached 26.1%, which is equivalent to crystalline silicon solar cells. However, due to the large bandgap of lead based PSC, its current efficiency approaches the Shockley Queisser (S-Q) limit. By adjusting the Sn/Pb ratio, the band gap of Sn Pb mixed perovskite can reach the lowest value of 1.2 eV, which can achieve higher theoretical efficiency than lead based perovskite. In addition, low bandgap Sn Pb mixed perovskite is crucial for all perovskite based series solar cells and is expected to exceed the S-Q limit. However, the rapid crystallization of tin containing perovskite leads to the formation of defects in the body or interface of the perovskite film. Meanwhile, Sn2+is easily oxidized to Sn4+, leading to severe p-type doping.
2、 Achievement Introduction
Cao Jiupeng and Qin Tianshi from Nanjing University of Technology published research results on improving the performance of Sn Pb mixed perovskite solar cells in the AFM journal, introducing the introduction of imidazole derivative 2 (aminomethyl) imidazole dihydrochloride (2-AD) into Sn Pb mixed perovskite to prepare Sn Pb mixed PSCs. The lone pair electrons of nitrogen can coordinate with metal ions to regulate the crystallization process, thereby improving the uniformity of perovskite films. Simultaneously, NH2 groups can form coordination bonds with uncoordinated metal ions to passivate multiple types of defects. In addition, Cl - anions can improve the quality of perovskite films and passivate defects by forming Pb/Sn Cl bonds. In addition, the coordination between 2-AD and perovskite can inhibit the oxidation of Sn2+, thereby reducing the p-doping level and increasing the open circuit voltage. Thanks to these advantages, Sn Pb mixed PSCs modified by 2-AD achieved a high PCE of 22.31% and excellent stability.
3、 Results and Discussion
Key point 1:2-AD can effectively passivate defects
Calculate the simulated electrostatic potential (ESP) plot of 2-AD to determine the charge distribution. Nitrogen atoms with negative charge centers can coordinate with Pb2+/Sn2+, which is beneficial for improving the crystallinity of perovskite. 2-AD can form coordination bonds with perovskite to passivate crystal defects. In addition, NH2 tails can occupy FA/MA vacancies on the surface of perovskite grains. The X-ray diffraction (XRD) analysis results show that the peak intensity of the 2-AD modified perovskite film is much higher than that of the control film, indicating that the introduction of 2-AD can improve the crystallinity of the perovskite film. In addition, after doping with 2-AD, the diffraction peak slightly shifts towards a higher angle, indicating that Cl is doped into the lattice. Due to the stronger Pb Cl bond than Pb I bond, the introduction of Cl can effectively passivate defects. Meanwhile, the UV visible absorption spectrum of the perovskite film shows that the sample modified with 2-AD exhibits increased light absorption due to the enhanced film quality. Tauc shows that the band gaps of the control and 2-AD doped perovskite films are 1.24 and 1.25 eV, respectively, which can be attributed to Cl alloying into the perovskite lattice.
Figure 1 ESP, Defect Passivation Diagram, and XRD Graph of 2AD
Key point 2: 2-AD has a better effect on the crystallization of perovskite
In order to further evaluate the effect of 2-AD on the crystallization of perovskite, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were performed on perovskite films. Due to the fast crystallization rate of tin containing perovskite, the perovskite film is controlled to contain some small grains. With the introduction of 2-AD, perovskite films exhibit larger grain sizes and more compact morphology. This can be attributed to the Lewis acid-base interaction between 2-AD and perovskite, which slows down the crystal growth rate of tin based perovskite. The cross-sectional SEM of perovskite indicates that the perovskite film has a similar thickness. AFM images indicate that the addition of 2-AD can significantly reduce the surface roughness of the perovskite film, from 36.88 nm to 26.53 nm. High quality perovskite thin films can reduce defect density and facilitate interface charge transfer.
Figure 2- Effect of AD on the crystallization of perovskite
Key point 3: 2-AD will inhibit the oxidation of Sn2+
Fourier transform infrared spectroscopy (FTIR) testing was conducted to demonstrate the interaction between perovskite and 2-AD, where the N-H bond and C=H bond move towards a higher number of regions due to coordination with Sn2+ions. The chemical interaction between 2-AD and perovskite was studied by X-ray photoelectron spectroscopy (XPS) due to the electron donating properties of N-containing groups. After introducing 2-AD, the Sn3d, Pb 4f, and I3d peaks shifted towards lower binding energies, indicating that the interaction between 2-AD and perovskite passivated the uncoordinated Pb/Sn ions. In addition, the Cl peak of XPS confirms the incorporation of 2-AD into the perovskite film. The fitting curve of Sn3d indicates that the Sn4+ratio of the 2-AD modified perovskite membrane is lower than that of the control membrane, indicating that the incorporation of 2-AD inhibits the oxidation of Sn2+. The decrease in the proportion of Sn4+can be attributed to the increase in local electron density around Sn2+after the introduction of 2-AD, which makes it more difficult for Sn2+to oxidize.
Figure 3 Spectral characterization of perovskite thin films doped with 2-AD
Key point 4: The performance of 2-AD modified perovskite solar cells is good
Inverted PSC with ITO/PEDOT: PSS/perovskite/PCBM/BCP/Ag structure was prepared to investigate the effect of 2-AD on optoelectronic properties. The standard displays 19.25% PCE, with an open circuit voltage (VOC) of 0.82 V, a short circuit current (JSC) of 31.95 mA cm-2, and a fill factor (FF) of 73.47% (forward scanning direction). The 2% mol 2-AD modification device shows a significant enhancement of 22.31% PCE, with VOC of 0.89 V, JSC of 32.51 mA cm-2, and FF of 77.12%. Then, measure the long-term stability of the unpackaged device under continuous illumination in an N2 glove box and an ambient atmosphere with a relative humidity of 20%. The improved 2-AD device maintained an initial efficiency of 80% after continuous illumination in the glove box for 800 hours, while the control device degraded to 40% under the same conditions. When stored in the ambient atmosphere under continuous illumination, the control device rapidly decays to only 10% of the original efficiency after 180 hours. In contrast, the improved 2-AD device remained above 70% after 180 hours, indicating that the introduction of 2-AD can greatly improve the stability of the device.
Figure 4 Performance and stability testing of perovskite solar cells
Key point 5: Density function theory (DFT) proves the inhibitory effect of 2-AD on Sn2+
Density Function Theory (DFT) calculations are used to better understand the inhibitory effect of 2-AD on Sn2+oxidation. The adsorption energy (Eads) of O2 molecules on the surface of perovskite is -0.41eV. On the other hand, the functional groups of NH2 and CVN are -0.83 and -0.9eV, respectively, indicating that 2-AD can block the binding of O2 molecules to Sn2+. The calculated electron density distribution shows that after adsorbing O2, the electron cloud density of Sn2+is significantly transferred to the O2 molecule, resulting in Sn2+being easily oxidized by O2. When 2-AD is introduced, the electron density around Sn2+significantly increases, effectively inhibiting the oxidation of Sn2+.
Figure 5 Density Functional Calculation Results
4、 Summary
In summary, the author demonstrated the introduction of a multifunctional imidazole derivative 2-AD to aid in the crystallization of mixed Sn Pb perovskite. This - NH2 and CVN functional group can coordinate with Sn2+/Pb2+ions, slowing down crystal crystallization and increasing grain size. Meanwhile, doping Cl - into the lattice is beneficial for improving the quality of perovskite thin films. In addition, 2-AD can effectively inhibit the oxidation of Sn2+and reduce defect density. Therefore, the PCE of mixed Sn Pb PSCs increased from 19.25% to 22.31%, exhibiting excellent long-term stability. This work provides an effective solution for the preparation of high-performance mixed Sn Pb PSC.
5、 References
Liu B;Chen H; Cao J; Chen X; Xie J; Shu Y; Yan F; Huang W; Qin T.Imidazole Derivative Assisted Crystallization for High-Efficiency Mixed Sn–Pb Perovskite Solar Cells,Adv. Funct. Mater（2023）
DOI: 10.1002/adfm.202310828
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