1. Narrow bandgap Pb-Sn perovskite solar cells are suitable as bottom cells of tandem solar cells and are expected to exceed the single-junction S-Q limit.
2. As an effective defect passivator and antioxidant, GlyCl greatly inhibits the oxidation of Sn2+ and improves the device performance of narrow bandgap mixed Pb-Sn perovskite solar cells.
3. This work paves the way for fabricating low-defect lead-tin perovskites for efficient single-junction and tandem solar cells.
1. Narrow bandgap Pb-Sn perovskite solar cells are expected to exceed the single-junction S-Q limit
Mixed lead and tin-based (Pb-Sn) perovskites have gained particular attention among all perovskite families due to their narrow band gap (1.25 eV), which is even narrower than pure Pb or pure Sn-based analogues. Specifically, narrow-bandgap Pb-Sn perovskite solar cells are suitable as bottom cells of tandem solar cells and are expected to achieve high efficiencies beyond the single-junction S-Q limit.
2. Introduction to results
Wenwen Zheng of Wuhan University of Technology and Fang Guoguo of Wuhan University published the research results of bottom-up modification to improve the performance of narrow bandgap lead-tin perovskite single junction and tandem solar cells in the EES journal. They introduced a method using glycinate A general approach to passivating multi-channel defects in mixed lead-tin perovskites using GlyCl as an additive. First, the poly(3,4-ethyldioxythiophene) polystyrene sulfonic acid (PEDOT:PSS) precursor solution is used to passivate the buried interface defects, and then it is introduced into the perovskite precursor solution to inhibit Sn2+ oxidation and passivate the body. defects, and then post-processing to passivate the top surface defects. Glycine is an effective defect passivator and antioxidant that greatly inhibits the oxidation of Sn2+ and improves the device performance of narrow bandgap mixed Sn-based perovskite solar cells. As a result, the PCE of the narrow-bandgap Pb-Sn perovskite solar cell passivated by polychloride acid was significantly improved by 22.07%, and the stability was significantly improved. In addition, a 4-terminal (4T) all-perovskite tandem solar cell with a Glycl-modified narrow-bandgap Pb-Sn bottom cell achieved a PCE of 27.07%. This work paves the way for fabricating low-defect lead-tin perovskites for efficient single-junction and tandem solar cells.
3. Results and Discussion
Point 1: The film quality after GlyCl treatment is the best
The optical properties of perovskite films without and without the addition of sodium glycinate were first studied. Four different types of samples were prepared under different conditions, namely pure FA0.7MA0.3Pb0.5Sn0.5I3 deposited on pure PEDOT:PSS (control), FA0.7MA0.3Pb0.5Sn0.5I3 deposited on 10 mg/mL GlyCl plus PEDOT:PSS (bottom), 2 mol% FA0.7MA0.3Pb0.5Sn0.5I3 deposited on 10 mg/mL GlyCl plus PEDOT:PSS (bottomlk), 2 mol% FA0.7MA0. 3Pb0.5Sn0.5I3 was deposited on 10 mg/mL GlyCl in PEDOT:PSS followed by the addition of 2 mg/mL GlyCl (bottomlk/top, i.e. target). All films show good optical properties and have strong absorption capabilities between 300 and 1000nm. The absorption results show that mixed Pb-Sn halide perovskites with multiple GlyCl modifications exhibit a similar band gap of 1.25 eV. Steady-state photoluminescence (PL) measurements were also used to characterize film quality. The film was deposited on the PEDOT:PSS coating substrate, and the PL intensity of the film gradually increased when GlyCl was incorporated at the bottom, upper surface, or intermediate. Among them, the PL intensity of the film surface after GlyCl treatment is the strongest, indicating that the film quality after GlyCl treatment is the best.
Point 2: GlyCl can passivate vacancy defects, hinder the migration of I ions, and interact with perovskite
The interaction of glycine with perovskites was studied. As shown in the figure, the peak of the carbon-oxygen double bond is obviously shifted from 1621 of the control sample to 1628 of the target sample (bottom/body/top GlyCl modification), indicating that GlyCl interacts with PbI2/SnI2. X-ray photoelectron spectroscopy (XPS) measurements confirmed the presence of Cl in the final film. As shown in the figure, an additional Cl 2p peak appears for the target sample. In addition, XPS results confirmed the interaction between Pb2+ and GlyCl. Compared with the control group, the binding energies of both metal cations and halide ions are lower, which confirms the interaction between FA0.7MA0.3Pb0.5Sn0.5I3 perovskite and the carbon-oxygen double bond in GlyCl. The characteristic peaks of the elements also change slightly, indicating that GlyCl can passivate vacancy defects and hinder the migration of I ions.
Point 3: GlyCl modified perovskite solar cells have good performance
Since the assembled Pb-Sn perovskite film has superior properties, the addition of GlyCl will significantly improve the performance of the device. Here, the authors first fabricated an inverted solar cell to evaluate the effect of glycine on device performance, as shown in the figure. And comparing the external quantum efficiency (EQE) spectra of the corresponding devices, the best performing solar cell among these devices was the target solar cell passivated using GlyCl, which produced a significant PCE (22.07%) with a JSC of 32.31 mA/cm2, VOC is 0.85 V, FF 80.26% measured under reverse voltage sweep. Under forward voltage sweep, the cell also achieved a PCE of 21.77%. The EQE measurements confirmed the reliability of the JSC values obtained from the J-V scans and were very close to those obtained from the J-V curves. The steady-state efficiency of the device was monitored and reached a high value of 21.91% at a constant 0.74 V bias for 100 s.
Point 4: Device performance improvement factors
The significant improvement in device performance mainly comes from the substantial enhancement of VOC and FF, which is related to the defect density and carrier recombination in the device. Therefore, the authors performed dark current, capacitance voltage (C-V), ideality factor (n), electrochemical impedance spectroscopy (EIS) and space charge limited current (SCLC) measurements to study the effect of GlyCl modification on defect passivation and carrier compounding effects. Dark current is recorded by measuring the J-V curve of the solar cell under dark conditions. As shown, the target device has much lower dark current at both positive and negative bias voltages compared to the control device, indicating much lower leakage current and non-radiative recombination in the target device. Capacitance-voltage (C-V) measurements are used to estimate the device's built-in potential. It is obvious that compared with the control device of 0.57 V, the built-in potential of the target device is 0.67 V, and the built-in potential is more conducive to carrier separation, thereby reducing carrier recombination in the GlyCl-modified device. Ideal factors are another advantage and can be used to estimate charge extraction and recombination processes in control and target devices. The J-V characteristics of the device under different light intensities of 0.1-1 sun were obtained. The relationship between VOC and logarithmic light intensity is shown.
Point 5: Good stability
In addition to improving efficiency, multiple modifications can also improve the stability of solar cells. Figure 5a shows the shelf storage stability test of the unencapsulated device under dark conditions in a N2-filled glovebox at room temperature. The control device retains 92% of its initial PCE after 600 hours of storage. In comparison, under the same conditions, the target device with added GlyCl still accounted for 98% of its original PCE. Modification of this process can not only improve the storage stability of the device, but also improve the operational stability of the device.
4. Summary
In summary, the authors demonstrated that the addition of GlyCl can effectively modify narrow-bandgap Pb-Sn perovskite solar cells from bottom to top. Adding GlyCl can improve the film morphology of lead-Sn perovskite films, passivate defects, promote carrier transport, and inhibit the oxidation of Sn2+. Therefore, solar cells modified with multiple GlyCl reduce carrier recombination and improve carrier transport. Finally, the PCE of the best-performing Pb-Sn alloy-based single junction and 4T all-perovskite tandem solar cells were 22.07% and 27.07%, respectively. In addition, the use of GlyCl can enhance the stability of solar cells by acting as a reducing agent and surface coordination agent to passivate surface defects. This provides an effective way to further improve device performance and provides broad development prospects for narrow-bandgap hybrid Pb-Sn perovskite solar cells and multi-junction tandem perovskite solar cells.
5. References
Zhang W; Huang L; Guan H. et al. Bottom-up modification boosts the performance of narrow-bandgap lead–tin perovskite single-junction and tandem solar cells, Energ. Environ. Sci. (2023)
DOI: 10.1039/D3EE02010J
https://pubs.rsc.org/en/content/articlelanding/2023/ee/d3ee02010j
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