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18.64% Cs Lead Inorganic Perovskite Solar Cells

Scientific Column

Scientists Jingbi You et. al. in CAS Reported Cesium Lead Inorganic Solar Cell with 18.64%, Voc=1.25V, 1000hr Performance.

Cesium Lead Inorganic Solar Cell with Efficiency beyond 18% via Reduced Charge Recombinat

First Author: Qiufeng Ye

Corresponding Authors: Pingqi Gao, Xingwang Zhang, and Jingbi You

DOI: 10.1002/adma.201905143

Highlights
  1. The structure of cesium-based inorganic perovskite solar cells has improved stability and has good application prospects.
  2. The large open circuit voltage (Voc) might loss due to the charge recombination, and the power conversion efficiency of inorganic PSCs is lower than hybrid PSCs.
  3. This article points out that on the electron transport layer of tin dioxide, the use of an insulating shunt group of faulty lithium oxide had better align the band gap with CsPbI3xBrx and it can use for interface defect passivation.
  4. From the EQE differential spectrum, it is found that adding lead chloride to the CsPbI3xBrx precursor significantly increases the crystallinity of the perovskite film, and the charge recombination in the perovskite can be inhibited. Therefore, the CsPbI3xBrx perovskite solar cell is optimized at the band gap of 1.77eV, showing excellent performance, the best Voc is as high as 1.25V, and the efficiency is 18.64%
  5. The CsPbI3xBrx perovskite solar cell has a high degree of 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 a cesium-inorganic perovskite solar cell, which aims to reduce the charge recombination efficiency of more than 18% developed by Professor Jingbi You from Chinese Academy of Sciences. In this article, an inorganic shunt barrier layer lithium fluoride (LiF) was developed between SnO2 and CsPbI3xBrx perovskite, which promotes the conduction band of the electron transport layer and suppresses surface defects.

  In addition, the study also added a small amount of lead chloride (PbCl2) to the CsPbI3xBrx perovskite precursor to further inhibit the recombination in the perovskite film. Therefore, the power conversion efficiency of 18.64% inorganic CsPbI3xBrx perovskite solar cells is obtained, 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 CsPbI3xBrx 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 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 the 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
Schematic diagram of the device structure of an inorganic perovskite solar cell

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 CsPbI3xBrx 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 CsPbI3xBrx 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.

  Although 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 of LiF, the conduction band of SnO2 has been moved from 4.3 eV to 4.01 eV. This will result in better energy level alignment with the minimum conduction band of CsPbI3xBrx (3.94 eV) (Figure 1b). This modification can enhance the inherent potential within the device and reduce interface recombination.

CsPbI3-xBrx perovskite precursor solution with different concentrations of PbCl2: 0% PbCl2, 5% PbCl2, 10% PbCl2 and 12% PbCl2.

Figure 2. Photograph of CsPbI3xBrx 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 CsPbI3xBrx perovskite precursor. The CsPbI3xBrx film contains PbCl2 in different PbCl2 scanning electron microscopes. The control perovskite film showed several pinholes. When 5% PbCl2 was added at the same time, the perovskite film became dense, 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).

JV characteristics of inorganic CsPbI3-xBrx perovskite solar cells

Figure 3. a) Typical JV characteristics of inorganic CsPbI3xBrx 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 CsPbI3xBrx 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, the perovskite solar cell was measured with 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 measurement 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. But testing a large numbers 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 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 CsPbI3xBrx perovskite containing 10% PbCl2 can maintain the black phase for 120 hours in ambient air with 40% humidity. In contrast, the control CsPbI3xBrx can only survive for less than 24 hours. Considering the positive effect of PbCl2 on CsPbI3xBrx, 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).

EQE spectral differential graph

Figure 3 c). EQE spectral differential graph, which can be used to judge the band gap of perovskite solar cells.

  According to the result of the EQE differential spectrum shown in Figure 3 c), the band gaps of the CsPbI3xBrx perovskite absorption layer are 1.75, 1.76, and 1.77 eV, showing a clear blue shift. This result shows that when a certain amount of Cl is introduced into the crystal lattice of CsPbI3xBrx, the blue shift intensity increases significantly with the precipitation of PbCl2, which 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 be carried out on the surface or in the bulk, with less non-radiation loss, and significantly passivating perovskite defects.

PCE and Voc-loss of inorganic perovskite solar cells

Figure 3. d) The relationship between PCE and Voc-loss of inorganic perovskite solar cells reported in this working literature.

  This article believes 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.

The relationship between Jsc and light intensity

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:

the slope of Voc and the incident light intensity

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.

  In this paper, Enlitech’s solar simulator can be automatically adjusted to the same incident light intensity, with the IVS-KA6000 software to automatically measure the Jsc and Voc related to the light intensity. It is found that the devices with and without LiF show the relationship between linear Jsc and light intensity (Figure 4b). This means that even if insulating LiF is introduced, there is no interface barrier or carrier imbalance.

  In addition, the relationship between Voc and light intensity (Sun-Voc) shown in Figure 4a. Using KA-Viewer software can accurately fit the ideal factor n. It can be seen that the slope of the SnO2-based device is the ideality factor n = 1.94, while the LiF-incorporated device shows a smaller slope, the ideality factor n = 1.59. As mentioned above, the defect-assisted recombination in the device may cause the slope to deviate from the ideal factor. Therefore, these results further confirm that the optimized SnO2 layer can effectively suppress trap assisted recombination in perovskite solar cells.

J-V characteristics of the best inorganic CsPbI3-xBrx perovskite solar cell

Figure 5. a) The J-V characteristics of the best inorganic CsPbI3xBrx 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 CsPbI3xBrx 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). Figure 5 b) EQE curve is the result of testing using Enlitech QE-R system. The QE-R software comes with 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 b), which closely matches the measured Jsc (IV). J-V hysteresis and stable power output (SPO), as shown in Figures 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 CsPbI3xBrx 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

  This article obtained 18.64% power conversion efficiency from the reverse scan of the inorganic CsPbI3xBrx perovskite solar cells; the open circuit voltage (Voc) can be as high as 1.25 V, and the Voc loss can be as low as 0.52 V. At the same time, the CsPbI3xBrx perovskite solar cell shows excellent light stability under continuous 1 solar equivalent illumination for more than 1000 hours, with an efficiency drop of less than 6%. This high-efficiency SnO2/LiF-based CsPbI3xBrx can promote the basic research of all-inorganic perovskites and the potential applications of photovoltaic and optoelectronic devices.

  In order to further increase the power conversion efficiency of inorganic perovskite solar cells by more than 20%, the author believes that the key is to reduce the open circuit voltage loss caused by the charge transport layer interface and the charge recombination in the inorganic perovskite layer.

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  

DOI: 10.1002/adma.201905143

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