High-Performance CsPbI2Br Perovskite Solar Cells with Zinc and Manganese Doping
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Photovoltaic performances of CsPbI2Br solar cells are still lower than those of hybrid inorganic–organic perovskite solar cells, and researchers are exploring ways to improve their efficiencies. Due to its higher thermal stability in comparison with the generally studied hybrid inorganic–organic perovskites, all-inorganic CsPbI2Br has recently attracted great attention. By utilizing the combination of MnCl2 and ZnCl2 particles doping to modulate film growth, it is found that MnCl2 and ZnCl2 particles infiltrate into the holes of the CsPbI2Br lattice through the growth procedure, leading to suppressed nucleation and reduced growth rate. The combination assists to achieve higher CsPbI2Br crystalline grains for increased Jsc as high as 15.66 mA cm−2 and FF as large as 73.37%. It is indicated that a specific combination of ZnCl2-MnCl2 doping can fundamentally improve the film surface morphology, reduce trap density, and suppress the recombination of carriers. Consequently, power conversion efficiency (PCE) is significantly improved from 13.47 to 14.15% compared with the reference device without doping.
KeywordsPerovskite solar cell Defect density Stability CsPbI2Br ZnCl2-MnCl2 doping
External quantum efficiency
Scanning electron microscope
X-ray photoelectron spectroscopy
Hybrid organic–inorganic perovskites have aroused great concerns because of their excellent electronic and optical properties [1, 2, 3, 4, 5, 6, 7] such as high mobility of the charge carriers and tunable band gap [8, 9, 10, 11]. Notably, the power conversion efficiency (PCE) of perovskite-based organic–inorganic hybrid solar cells has improved from 3.8 to 23.3% through the cation exchange [12, 13, 14, 15, 16, 17]. There are still challenges to overcome all environmental degradation . Up to now, the cesium lead halide perovskite solar cells have been researched by many groups [19, 20, 21, 22]. The large band gap of CsPbBr3 is about 2.3 eV, which is too large to absorb long-wavelength lights [23, 24]. The CsPbI3 has a low band gap of 1.73 eV, but it degrades rapidly from black phase to yellow phase at ambient temperature [25, 26]. CsPbI2Br perovskite shows a desirable band gap of 1.91 eV and is stable in the black phase in ambient air [19, 20]. It is demonstrated that the size of microcrystalline grain is a key factor for increasing the efficiency of solar cell. [27, 28, 29, 30]. It appears that the grain boundaries in the surface of perovskite film suppress the recombination of the charges in their trap states . Meanwhile, grain boundaries can provoke external states near the edge of the valence band which will impede the spread of the hole . Therefore, it is desirable that the CsPbI2Br has a huge particle size and a low trap charge density . For this purpose, the doping of impurities was explored extensively by incorporating several ions into the host lattice to modulate the performance of the film . For example, by incorporating potassium into CsPbI2Br, these large CsPbI2Br crystallites could be obtained to improve the formation of charge carriers and the better charge transport increases PCE . Chu et al. used KCl as an additive material to obtain uniform and dense MAPbI3 perovskite films with large grain-size nanocrystals . Liu et al. reported that the addition of Mn2+ with a certain amount could significantly improve the crystalline grain size and achieve superior solar cell performance . All-inorganic CsPbI2Br has recently attracted great attention due to its higher thermal stability in comparison with the generally studied hybrid inorganic–organic perovskites. In the paper, it is indicated that a specific combination of ZnCl2-MnCl2 doping can fundamentally improve the film surface morphology, reduce trap density, and suppress the recombination of carriers. Consequently, PCE is significantly improved from 13.47 to 14.15% compared with the reference device without doping. To the best of our knowledge, the PCE of 14.15% is among the best performance of CsPbI2Br perovskite solar cells.
Results and Discussion
We prepared 1.0 M using solution CsBr together with equal stoichiometric PbI2 in mixed solvents of DMF and DMSO as the precursor solution. Through a one-step spin-coating method, a 350-nm film (measured by profilometer) was obtained after being annealed at 150 °C. To study the effect of additive on the film morphology and the device performance, we incorporated different contents of ZnCl2-MnCl2 (0%, 0.25%, and 0.50%) molar ratio, marked by CsPbI2Br-0%, CsPbI2Br-0.25%, and CsPbI2Br-0.50%, respectively, into the CsPbI2Br precursor solution.
Comparison of the device parameters of the perovskite solar cells based on different CsPbI2Br-ZnCl2-MnCl2 films
Jsc [mA cm−2]
Materials and Methods
The SnO2 were bought from Alfa Aesar. CsBr, ZnCl2, MnCl2, (DMSO), and (DMF) were bought from Sigma-Aldrich. spiro-OMeTAD and PbI2 were bought from Xi’an Polymer Light Technology Corp.
Initially, the ITO glasses were successively cleaned by applying detergent, isopropyl alcohol, acetone solvents about 20 min, and deionized water. The process is also followed by removing the substances remain in the substrates through oxygen plasma processing approximately for 10 min. The SnO2 were diluted in ultrapure water at a volume ratio of 1:6. Firstly, glass substrates were spin coated by SnO2 layer at 3000 rpm for 40 s, and then were annealed at 150 °C for 30 min. To prepare a perovskite precursor, CsBr, PbI2, ZnCl2, and MnCl2 were stoichiometrically dissolved in a mixed solvent of DMSO and DMF with a volume ratio of 1.4:1 to form a 1.0 M solution. The solution was filtered through a 0.22-μm pore PTFE filter, and then stirred at 70 °C for 2 h. The precursor solution was then spin coated on the SnO2/ITO substrate firstly at 1000 rpm with accelerating rate of 1000 rpm for 12 s, after that at 5000 rpm with accelerating rate of 3000 rpm not more than 30 s. Then, 100 μL of chlorobenzene (CB) were distilled onto the rotating substrate during the second step spin-coating with the time of 10 s before the end of the process. Afterwards, the film was first annealed at 50 °C for 1 min and then at 150 °C for 5 min. An HTL film was prepared by spin-coating spiro-OMeTAD solution onto the formed CsPbI2Br film at 4000 rpm with accelerating rate of 3000 rpm for 30 s. The spiro-OMeTAD solution consisted of 72.3 mg Spiro-OMeTAD, 17.5 μL bis (trifluoro methane) sulfonamide lithium salt (Li-TFSI) stock solution (520 mg Li-TFSI in 1 mL acetonitrile), 28.8 μL 4-tertbutylpyridine, and 1 mL chlorobenzene. At the end, the Au film with a thickness of 80 nm was deposited through thermal evaporation.
The Rigaku-2500 X-ray diffraction meter was used to measure the X-ray diffraction patterns. The top-view SEM images were attained using a scanning electron microscope (SEM, HITACH2100). Keithley 2420 was used to measure the solar cell J–V characteristics under AM 1.5 sunlight at an irradiance of 100 mW cm−2 provided by a solar simulator (Newport, Oriel Sol3A Class AAA, 94043A). The intensity of light was measured by monocrystalline silicon reference cell with a KG5 window (Newport, Oriel 91150). Impedance spectroscopy was measured by Zennium (Zahner). EQE was recorded using a Newport Oriel IQE-200 by a power source (Newport 300 W xenon lamp, 66920) with a monochromatic instrument (Newport Cornerstone 260). The device area is 0.044 cm2.
In summary, we got inorganic CsPbI2Br solar cells by incorporating ZnCl2-MnCl2 into the CsPbI2Br precursor solution. When the ZnCl2-MnCl2 content achieves 0.25%, the device shows a champion PCE of 14.15%, with FF of 73.37%, Jsc of 15.66 mA cm−2, and Voc of 1.23 eV. The enhanced photovoltaic performance is associated to improved surface morphology, reduced trap density, and suppressed charge recombination. This work could guide fundamental researches in the Cesium lead halide perovskites and promote their potential applications for solar cell.
The Table of Contents Entry
A simple compositional engineering technique is used to improve the film quality and device performance. By incorporating MnCl2+ZnCl2 into the CsPbI2Br film, the CsPbI2Br perovskite solar cell attains an outstanding efficiency of 14.15% and good long-term stability. In addition, the fabrication process is highly reproducible and inexpensive.
Special thanks to Professor Dr. Jizheng Wang for his valuable suggestions from, Institute of Chemistry, Chinese Academy of Sciences,
This research work were supported by the (NSFC) National Natural Science Foundation of China (Grant numbers No. 61675024, and No. 61874009).
Availability of Data and Materials
The results of this article are incorporated within the article datasets.
UK fabricated the devices and performed the experiments. UK and AAK analyzed the data. YZ, AZ, and NU revised the manuscript. Finally, all authors approved the final manuscript to be submitted.
The authors declare that they have no competing interests.
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