Scientific Column
Scientists Jingbi You et. al. in CAS Reported Cesium Lead Inorganic Solar Cell with 18.64%, Voc=1.25V, 1000hr Performance.
First Author: Qiufeng Ye
Corresponding Authors: Pingqi Gao, Xingwang Zhang, and Jingbi You
Highlights
- The structure of cesium-based inorganic perovskite solar cells has improved stability and shows promising application prospects.
- The large open circuit voltage (Voc) may be lost due to charge recombination, and the power conversion efficiency of inorganic PSCs is lower than that of hybrid PSCs.
- This article suggests that on the electron transport layer of tin dioxide, the use of an insulating shunt group of lithium oxide better aligns the band gap with CsPbI–Br, and it can be used for interface defect passivation.
- From the EQE differential spectrum, it is found that adding lead chloride to the CsPbI–Br precursor significantly increases the crystallinity of the perovskite film, and charge recombination in the perovskite can be inhibited. Therefore, the CsPbI–Br perovskite solar cell is optimized at the band gap of 1.77eV, showing excellent performance, with the best Voc as high as 1.25V, and an efficiency of 18.64%.
- The CsPbI–Br perovskite solar cell demonstrates high light resistance under continuous 1 sun equivalent illumination for more than 1000 hours, and the efficiency drop is less than 6%.
Summary
In October 2019, Advanced Materials magazine published an article on a cesium-inorganic perovskite solar cell, which aims to increase the charge recombination efficiency to over 18%. This cell was developed by Professor Jingbi You from the Chinese Academy of Sciences. In this article, an inorganic shunt barrier layer of lithium fluoride (LiF) was developed between SnO2 and CsPbI3–xBrx perovskite, which promotes the conduction band of the electron transport layer and suppresses surface defects.
Additionally, the study also added a small amount of lead chloride (PbCl2) to the CsPbI3–xBrx perovskite precursor to further inhibit recombination in the perovskite film. As a result, the power conversion efficiency of the inorganic CsPbI3–xBrx perovskite solar cells reached 18.64%, the maximum open circuit voltage (Voc) can reach 1.25V, and the Voc loss can be reduced by 0.52V. At the same time, the CsPbI3–xBrx perovskite solar cell exhibits excellent light stability under continuous 1 sun equivalent illumination, and the efficiency drops by less than 6% within 1000 hours.
Background
The organic-inorganic hybrid perovskite material has a high absorption coefficient, high carrier mobility, adjustable band gap, and long life, and has become one of the most promising light-absorbing layers for next-generation solar cells. The power conversion efficiency (PCE) of perovskite solar cells (PSC) has soared from 3.8% to more than 24% in the past 10 years. Despite their high efficiency, thermal and moisture-sensitive organic cations are still the main problems that reduce device stability. Therefore, solving long-term stability is the main concern in the perovskite solar cell community. As an alternative, all-inorganic perovskites (CsPbX3, X = I, Br, Cl, or their mixtures), prized for their excellent thermal stability, have received increasing attention.
Although significant progress has been made in inorganic perovskite solar cells, the power conversion efficiency is still far behind that of hybrid perovskite solar cells, even when compared with the I-Br hybrid perovskite with a similar band gap (1.75 eV). In this way, it can be found that open circuit voltage loss (Voc loss) is still the main reason for the poor PCE performance of inorganic perovskite solar cells, which is closely related to band gap matching and defects in the interface or in the bulk of perovskite.
Key Results
Figure 1. a) Schematic diagram of the device structure of an inorganic perovskite solar cell; LiF was used to modify the surface of SnO2. b) The Energy band gap arrangement of each layer in the CsPbI3–xBrx inorganic perovskite solar cell.
In this paper, cesium iodide (CsI), HPbI3+x (x varies from 0.3 to 0.5), and lead bromide (PbBr2) precursor solutions were used to prepare CsPbI3–xBrx perovskite films through a simple one-step deposition. SnO2 is used as an electron transport layer with a conduction band of approximately 4.3 eV, which is perfectly matched to the hybrid perovskite and serves as an effective charge selection layer.
However, this is not a perfect match for inorganic perovskites with shallow conduction bands. The low work function and almost insulating LiF electron selective layer is thermally deposited on the SnO2 layer to modify the surface work function of SnO2 and passivate defects (Figure 1a). The X-ray photoelectron spectroscopy (XPS) results confirmed the presence of LiF on the surface of SnO2.
After the modification with LiF, the conduction band of SnO2 has moved from 4.3 eV to 4.01 eV. This will result in better energy level alignment with the minimum conduction band of CsPbI3– xBrx (3.94 eV) (Figure 1b). This modification can enhance the inherent potential within the device and reduce interface recombination.
Figure 2. Photograph of CsPbI3–xBrx perovskite precursor solution with different concentrations of PbCl2: 0% PbCl2, 5% PbCl2, 10% PbCl2 and 12% PbCl2.
Different concentrations of PbCl2 (relative to 0%, 5%, and 10% of the CsI content) were added to the CsPbI3–xBrx perovskite precursor. The CsPbI3–xBrx film contains PbCl2 in different PbCl2 scanning electron microscopes. The control perovskite film showed several pinholes. When 5% PbCl2 was added, the perovskite film became denser, and after adding 10% PbCl2, the crystal size of CsPbI3 increased to more than 2 μm. This improvement may be due to Cl incorporation slowing down the perovskite crystallization process. The author tried to add more Cl to the precursor and found that more than 10% PbCl2 would cause precipitation (Figure S5).
Figure 3. a) Typical JV characteristics of inorganic CsPbI3–xBrx perovskite solar cells with different concentrations of PbCl2 using Enlitech solar simulator and IVS-KA6000 software: withour PbCl2 (dark gray), 5% PbCl2 (red) and 10% PbCl2 (blue). b) The efficiency histogram of inorganic CsPbI3–xBrx PVSC with different concentrations of PbCl2: without PbCl2 (dark gray), 5% PbCl2 (red) and 10% PbCl2 (blue).
In this study, IVS-KA6000 software is used to control Enlitech solar simulator and Keithley SMU source meter. After using SRC-2020 WPVS to carry out reference cell light intensity, the device is tested for IV. In a nitrogen glove box, the perovskite solar cell was measured with a reverse scan (1.4 V→0 V, step size 0.02 V) and forward scan (0 V→1.4 V, step size 0.02 V). IVS-KA6000 can perform complete parameter measurements of voltage step length, voltage sweep rate, delay time control, and so on according to the hysteresis effect of perovskite solar cells to obtain the most accurate IV curve.
In order to prove the validity of the results in this study, it is scientifically convincing to use the efficiency histogram to prove the difference between the pros and cons under different concentration conditions. However, testing a large number of perovskite solar cells will consume a lot of time!
In particular, there will be multiple sub-cells on a piece of glass sample. During the test time interval between each sub-cell, the simulator’s light will cause the temperature to rise and deviate from the STC condition of 25 degrees. For most solar cell materials, a rise in temperature will cause a drop in Voc. This will cause errors in the conversion efficiency of the device. Therefore, accelerating the switching measurement speed between the sub-cells can effectively improve the test results, especially for experiments on the efficiency histogram.
Whether it is Enlitech’s GIV-M6 manual sample switching box or GIV-A08 automatic switching box, it can greatly shorten the switching time between test cells by more than ten times to ensure that the perovskite solar cells can be measured with the least effect of temperature and the best PCE performance, as shown in Figure 3. B).
It is worth noting that the PbCl2 additive significantly improves the black phase stability of the CsPbI3-based perovskite film. The author found that the CsPbI3–xBrx perovskite containing 10% PbCl2 can maintain the black phase for 120 hours in ambient air with 40% humidity. In contrast, the control CsPbI3–xBrx can only survive for less than 24 hours. Considering the positive effect of PbCl2 on CsPbI3–xBrx, this study fabricated a solar cell based on SnO2/LiF electron transport layer. It was found that the perovskite containing PbCl2 additive showed better device performance, which was attributed to the simultaneous increase of Voc and FF (Figure 3 a).
Figure 3 c). EQE spectral differential graph, which can be used to judge the band gap of perovskite solar cells.
According to the results of the EQE differential spectrum shown in Figure 3 c), the band gaps of the CsPbI3–xBrx perovskite absorption layer are 1.75, 1.76, and 1.77 eV, showing a clear blue shift. These results indicate that when a certain amount of Cl is introduced into the crystal lattice of CsPbI3–xBrx, the blue shift intensity significantly increases with the precipitation of PbCl2. This may be due to the large grain size and the decrease of grain boundaries, which leads to the decrease of defect states and shallow trapping. It is recommended to carry out this process on the surface or in the bulk, with less non-radiative loss, and significantly passivating perovskite defects.
Figure 3. d) The relationship between PCE and Voc-loss of inorganic perovskite solar cells reported in this working literature.
This article posits that the improvement of device performance is due to the growth of large grain size and the reduction of grain boundaries, which leads to the passivation of traps and a power conversion efficiency as high as 18.64%. Then Voc, Jsc and FF are 1.234 V, 18.3 mAcm-2 and 82.58%, respectively. It is worth noting that the Voc loss is 0.52 V, which is a significant improvement for inorganic perovskite solar cells. It can be assumed that the improvement in device performance is due to the growth of large grain size and the reduction of grain boundaries, resulting in passivation of traps, and the sacrifice of Jsc due to a small amount of blue shift.
Figure 4 a) The relationship between Jsc and light intensity, b) The relationship between Voc and light intensity.
The light-intensity-dependent Voc can provide important insights into the mechanism of the recombination process in PV devices. At Voc, there is no net current (J = 0 mA cm−2) through the device; therefore, all photo-generated charge carriers should be recombined in the perovskite film. The corresponding charge carrier recombination process is reflected by the ideality factor n, which is determined by the slope of Voc and the incident light intensity, as shown in the formula:
Where q is the basic charge, k is Boltzmann’s constant, T is the temperature, and Φ is the light intensity. Using KA-Viewer software can accurately fit the ideal factor n. When the ideal factor n is close to 2, Shockley-Read-Hall (SRH) type, trap-assisted compound dominates. On the contrary, in the case of recombination of free electrons and holes, the ideality factor should be 1.
Figure 5. a) The J-V characteristics of the best inorganic CsPbI3–xBrx perovskite solar cell b) The external quantum efficiency (EQE) of the perovskite solar cell c) J-V hysteresis characteristics d) Stabilized power output (SPO) of CsPbI3– xBrx perovskite solar cell under the condition of maximum power point.
The J-V curve and parameters of the best device are shown in Figure 5 a). The EQE curve in Figure 5 b) is the result of testing using the Enlitech QE-R system. The QE-R software includes a calculation function for the integrated current density Jsc (EQE) of the AM1.5G spectrum. The Jsc obtained from the external quantum efficiency (EQE) is 17.71 mA cm-2, as shown in Figure 5 b), which closely matches the measured Jsc (IV). The J-V hysteresis and stable power output (SPO), as shown in Figures 5 c) and d), reach a champion SPO of 16.9%.The stability of the device is also tested in the article. The SnO2/LiF-based CsPbI3–xBrx perovskite solar cell shows excellent light stability against phase separation. After the initial power conversion efficiency, it can be stored in an N2 glove box, and when continuous white LED illumination exceeds 1000 hours, it can still retain more than 94%.
Conclusion
Article Information
Cesium Lead Inorganic Solar Cell with Efficiency beyond 18% via Reduced Charge Recombination
Qiufeng Ye, Yang Zhao, Shaiqiang Mu, Fei Ma, Feng Gao, Zema Chu, Zhigang Yin, Pingqi Gao, Xingwang Zhang, and Jingbi You
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