Team Information

First Author: Qiufeng Ye 

Corresponding Authors: Pingqi Gao, Xingwang Zhang, Jingbi You

DOI: 10.1002/adma.201905143

Highlights

• Cesium-Based Inorganic Perovskite Solar Cells: Cesium-based inorganic perovskite solar cell architectures offer enhanced device stability and hold significant potential for practical applications.
• Open-Circuit Voltage (V_oc) Loss: Due to potential charge recombination leading to substantial open-circuit voltage (V_oc) losses, the power conversion efficiency of inorganic perovskite solar cells remains lower compared to organic-inorganic hybrid perovskite solar cells.
• Insulating Shunt Blocking Layer: This study demonstrates the utilization of an insulating lithium fluoride (LiF) shunt blocking layer on a tin oxide (SnO₂) electron transport layer, enabling better energy level alignment with the CsPbI_3-xBr_x bandgap and facilitating interface defect passivation.
• EQE Differential Spectra: EQE differential spectra reveal that the incorporation of lead chloride (PbCl₂) into the CsPbI_3-xBr_x precursor solution significantly improves the crystallinity of the perovskite film and suppresses charge recombination within the perovskite. Consequently, the CsPbI_3-xBr_x perovskite solar cells exhibit outstanding performance with a bandgap optimized at 1.77 eV, achieving a remarkable V_oc of up to 1.25 V and an efficiency of 18.64%.
• Photostability: The CsPbI_3-xBr_x perovskite solar cells demonstrate high photostability under continuous 1 sun equivalent illumination for over 1000 hours, with less than 6% efficiency degradation.

Introduction

In October 2019, Advanced Materials published research by Prof. You Jingbi et al. on cesium lead inorganic perovskite solar cells achieving efficiencies exceeding 18% through reduced charge recombination. This study introduces an inorganic lithium fluoride (LiF) shunt blocking layer between SnO₂ and the CsPbI_3-xBr_x perovskite, which modifies the conduction band of the electron transport layer and suppresses surface defects.

Furthermore, the research incorporated a small amount of lead chloride (PbCl₂) into the CsPbI_3-xBr_x perovskite precursor, further mitigating recombination within the perovskite film. Consequently, a power conversion efficiency of 18.64% was achieved for inorganic CsPbI_3-xBr_x perovskite solar cells, with a maximum open-circuit voltage (V_oc) reaching 1.25 V and a V_oc loss reduced to 0.52 V. Simultaneously, the CsPbI_3-xBr_x perovskite solar cells exhibited excellent photostability under continuous 1 sun equivalent illumination, demonstrating less than 6% efficiency degradation over 1000 hours.

Background

Organic-inorganic hybrid perovskite materials, with their high absorption coefficients, large carrier mobilities, tunable bandgaps, and long carrier lifetimes, have emerged as one of the most promising absorber layers for next-generation solar cells. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has soared from 3.8% to over 24% in the past decade. Despite their high efficiency, the thermal and moisture sensitivity of the organic cations remains a major concern for device stability. Addressing long-term stability is therefore a primary focus within the perovskite solar cell community.

As an alternative, all-inorganic perovskites (CsPbX₃, X = I, Br, Cl, or their mixtures) have garnered increasing attention due to their superior thermal stability.

Despite significant progress in inorganic perovskite solar cells, the power conversion efficiency still lags behind that of hybrid perovskite solar cells, even compared to I-Br mixed perovskites with similar bandgaps (1.75 eV). Open-circuit voltage loss (V_oc loss) remains a major contributing factor to the lower PCE performance of inorganic perovskite solar cells, which is closely related to bandgap alignment and defects at the interfaces or within the perovskite bulk.

Figure Analysis

Figure 1. a) Schematic diagram of the device structure of an inorganic perovskite solar cell; LiF is used to modify the SnO₂ surface. b) Bandgap alignment of each layer in the CsPbI_3-xBr_x inorganic perovskite solar cell.

In this study, CsPbI_3–xBr_x perovskite films were fabricated via a simple one-step deposition method using cesium iodide (CsI), HPbI_3+x (x varying from 0.3 to 0.5), and lead bromide (PbBr₂) precursor solutions. SnO₂, with a conduction band of approximately 4.3 eV, serves as the electron transport layer, matching well with hybrid perovskites and acting as an efficient charge selective layer.

However, this alignment is not ideal for inorganic perovskites with shallower conduction bands. A low work function and nearly insulating LiF electron selective layer was thermally deposited onto the SnO₂ layer to modify the SnO₂ surface work function and passivate defects (Figure 1a). X-ray photoelectron spectroscopy (XPS) results confirmed the presence of LiF on the SnO₂ surface.

Following LiF modification, the SnO₂ conduction band shifted from 4.3 eV to 4.01 eV. This shift leads to better energy level alignment with the conduction band minimum of CsPbI_3-xBr_x (3.94 eV) (Figure 1b). This modification enhances the built-in potential within the device and reduces interface recombination.

Figure 2. Photographs of CsPbI_3-xBr_x perovskite precursor solutions with varying PbCl₂ concentrations: 0% PbCl₂, 5% PbCl₂, 10% PbCl₂, and 12% PbCl₂.

Different concentrations (0%, 5%, and 10% relative to the CsI content) of PbCl₂ were added to the CsPbI_3-xBr_x perovskite precursor. Scanning electron microscopy (SEM) images of the CsPbI_3-xBr_x films with varying PbCl₂ content are shown. The control perovskite film exhibits several pinholes, while upon the addition of 5% PbCl₂, the perovskite film becomes denser. With the addition of 10% PbCl₂, the CsPbI₃ crystal size increases to over 2 μm. This improvement is likely due to Cl incorporation slowing down the perovskite crystallization process. The authors attempted to add more Cl to the precursor and found that exceeding 10% PbCl₂ resulted in precipitation (Figure S5).

Figure 3. a) J-V characteristics of inorganic CsPbI_3-xBr_x perovskite solar cells with varying PbCl₂ concentrations: without PbCl₂ (dark gray), 5% PbCl₂ (red), and 10% PbCl₂ (blue) using an Enlitech solar simulator and IVS-KA6000 software. b) Efficiency histograms of inorganic CsPbI_3-xBr_x PVSCs with varying PbCl₂ concentrations: without PbCl₂ (dark gray), 5% PbCl₂ (red), and 10% PbCl₂ (blue).

After calibrating the light intensity using a standard cell in the SRC-2020 WPVS format, IV tests were conducted on the devices. The light intensity was controlled by the IVS-KA6000 software which also controlled the Enlitech solar simulator and the Keithley SMU source meter. Perovskite solar cells were measured in a nitrogen glovebox using reverse scans (1.4 V → 0 V, step size 0.02 V) and forward scans (0 V → 1.4 V, step size 0.02 V). The IVS-KA6000 allows for comprehensive parameter determination, including voltage step size, voltage scan rate, and delay time control, to account for hysteresis effects in perovskite solar cells and obtain the most accurate IV curves.

To further validate the effectiveness of the experimental results, efficiency histograms were employed to demonstrate the performance differences under various concentration conditions, providing compelling scientific evidence. However, testing a large number of perovskite solar cells can be time-consuming.

In particular, a single glass substrate may contain multiple sub-cells. The time interval between measurements of each sub-cell can lead to temperature increases due to simulator illumination, deviating from the STC condition of 25°C. For most solar cell materials, an increase in temperature results in a decrease in V_oc. This can introduce errors in the device conversion efficiency. Therefore, accelerating the measurement switching speed between sub-cells can effectively improve the test results, especially for experiments involving efficiency histograms.

Both Enlitech’s GIV-M6 manual sample switching box and the GIV-A08 automatic switching box can significantly reduce the switching time between sub-cells by a factor of ten or more. This ensures minimal temperature influence and optimal PCE performance measurement for perovskite solar cells, as shown in Figure 3b.

It is noteworthy that the PbCl₂ additive significantly improves the black phase stability of CsPbI₃-based perovskite films. The authors found that CsPbI_3-xBr_x perovskite containing 10% PbCl₂ can maintain its black phase for 120 hours in ambient air at 40% humidity. In contrast, the control CsPbI_3-xBr_x only survived for less than 24 hours. Considering the positive impact of PbCl₂ on CsPbI_3-xBr_x, solar cells based on SnO₂/LiF electron transport layers were fabricated in this study. Perovskites with the PbCl₂ additive exhibited improved device performance, attributed to the simultaneous increase in V_oc and FF (Figure 3a).

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

From the EQE differential spectra results (Figure 3c), the bandgaps of the CsPbI_3-xBr_x perovskite absorber layers are 1.75, 1.76, and 1.77 eV, exhibiting a clear blueshift. This indicates that the introduction of a certain amount of Cl into the CsPbI_3-xBr_x lattice results in a blueshift, with the magnitude of the shift increasing significantly with PbCl₂ incorporation. This is likely due to the reduction in defect states resulting from the large grain size and reduced grain boundaries. Shallow trap levels, suggested to be present on the surface or within the bulk, with fewer non-radiative losses, significantly passivate the perovskite defects.

Figure 3. d) Relationship between PCE and V_oc loss for inorganic perovskite solar cells reported in this work and literature.

This study attributes the enhanced device performance, resulting in a PCE of up to 18.64%, to the increased grain size and reduced grain boundaries leading to trap passivation. The V_oc, J_sc, and FF are 1.234 V, 18.3 mA cm^-2, and 82.58%, respectively. Remarkably, the V_oc loss is 0.52 V, which represents a significant improvement for inorganic perovskite solar cells. It can be hypothesized that the enhanced device performance stems from the growth of large grain sizes and the reduction of grain boundaries, leading to trap passivation, with a slight sacrifice in J_sc due to the small blueshift.

Figure 4. a) Relationship between J_sc and light intensity; b) Relationship between V_oc and light intensity.

Light intensity-dependent V_oc can provide valuable insights into the recombination mechanisms within PV devices (https://www.sciencedirect.com/science/article/abs/pii/S2211285518300442). At V_oc, there is no net current (J = 0 mA cm^-2) flowing through the device, so all photogenerated charge carriers should recombine within the perovskite film. The corresponding charge carrier recombination process is reflected by the ideality factor n, which is determined by the slope of V_oc versus the incident light intensity, as shown in the following equation:

where q is the elementary charge, k is the Boltzmann constant, T is the temperature, and Φ is the light intensity. The KA-Viewer software enables accurate fitting of the ideality factor n. When the ideality factor n approaches 2, Shockley-Read-Hall (SRH) type, trap-assisted recombination dominates. Conversely, for free electron and hole recombination, the ideality factor should be 1.

This study utilizes Enlitech’s solar simulator with automatically adjustable incident light intensity, coupled with the IVS-KA6000 software for automated measurement of light intensity-dependent J_sc and V_oc. A linear relationship between J_sc and light intensity was observed for devices both with and without LiF (Figure 4b). This indicates the absence of interface barriers or carrier imbalance even with the introduction of insulating LiF.

Additionally, the relationship between V_oc and light intensity (Sun-Voc) is shown in Figure 4a. The KA-Viewer software enables accurate fitting of the ideality factor n. The SnO₂-based device exhibits a slope corresponding to an ideality factor n = 1.94, while the device with LiF shows a smaller slope with n = 1.59. As previously mentioned, defect-assisted recombination within the device can cause deviations in the slope and ideality factor. Therefore, these results further confirm that the optimized SnO₂ layer effectively suppresses trap-assisted recombination in perovskite solar cells.

Figure 5. a) J-V characteristics of the best inorganic CsPbI_3-xBr_x perovskite solar cell. b) External Quantum Efficiency (EQE) of the perovskite solar cell. c) J-V hysteresis characteristics. d) Stable power output (SPO) of the CsPbI_3-xBr_x perovskite solar cell under maximum power point conditions.

The J-V curve and parameters for the best-performing device are presented in Figure 5a. The EQE curve in Figure 5b was obtained using the Enlitech QE-R system. The QE-R software includes a built-in function to calculate the J_sc(EQE) by integrating the current density over the AM1.5G spectrum. The J_sc derived from the external quantum efficiency (EQE) is 17.71 mA cm^-2, as shown in Figure 5b, which closely matches the measured J_sc (IV). The J-V hysteresis and stable power output (SPO) are shown in Figures 5c and 5d, achieving a champion SPO of 16.9%.

Device stability was also investigated in this study. The SnO₂/LiF based CsPbI_3-xBr_x perovskite solar cells demonstrated excellent photostability against phase separation. After storage in a N₂ glovebox and continuous white LED illumination for over 1000 hours, it retained more than 94% of its initial PCE.

Conclusion

This study achieved an encouraging power conversion efficiency of 18.64% from reverse scans of inorganic CsPbI_3-xBr_x perovskite solar cells; the open-circuit voltage (V_OC) can reach up to 1.25 V, and the V_OC loss can be as low as 0.52 V. Simultaneously, CsPbI_3-xBr_x perovskite solar cells demonstrated excellent photostability under continuous 1 sun equivalent illumination for over 1000 hours, with less than 6% efficiency degradation. This highly efficient SnO₂/LiF based CsPbI_3-xBr_x can promote fundamental research on all-inorganic perovskites and their potential application in photovoltaic and optoelectronic devices.

To further enhance the power conversion efficiency of inorganic perovskite solar cells beyond 20%, the authors suggest that the key lies in minimizing open-circuit voltage losses arising from charge recombination at the charge transport layer interfaces and within the inorganic perovskite layer itself.

Publication Information

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

Ye, Qiufeng; Zhao, Yang; Mu, Shaiqiang; Ma, Fei; Gao, Feng; Chu, Zema; Yin, Zhigang; Gao, Pingqi; Zhang, Xingwang; and You, Jingbi

DOI: 10.1002/adma.201905143