In recent years, perovskite solar cells have emerged as a promising technology in the photovoltaic field due to their remarkable progress in power conversion efficiency. However, the instability of organic-inorganic hybrid perovskites under environmental stressors such as moisture, oxygen, and heat has limited their practical applications. To address this, all-inorganic perovskites, particularly CsPbBr3, have gained attention for their superior stability and carrier transport properties. Despite these advantages, all-inorganic CsPbBr3 perovskite solar cells suffer from significant non-radiative recombination at interfaces, which hampers their efficiency. In this study, we explore the use of SnCl2 as an interface modifier on the TiO2 electron transport layer to enhance the crystallinity and morphology of CsPbBr3 films, thereby reducing carrier recombination and improving device performance.
The fabrication of efficient perovskite solar cells requires precise control over interface properties to minimize energy losses. The TiO2 electron transport layer commonly used in these devices often exhibits low electron mobility and high defect densities, leading to increased recombination. By modifying the TiO2 surface with SnCl2, we aim to passivate defects, optimize energy level alignment, and facilitate better charge extraction. Our approach involves spin-coating SnCl2 solutions of varying concentrations onto TiO2 layers before depositing the CsPbBr3 perovskite layer. We systematically investigate the effects of this modification on film quality, crystal structure, and photovoltaic parameters.
To understand the underlying mechanisms, we employ various characterization techniques, including scanning electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, and transient photocurrent measurements. The results demonstrate that SnCl2 modification significantly improves the perovskite film’s coverage and reduces phase impurities, leading to enhanced open-circuit voltage, short-circuit current density, and fill factor. The optimal SnCl2 concentration of 1.5 mg·mL−1 yields a power conversion efficiency of 9.92%, compared to 8.01% for unmodified devices. This improvement is attributed to reduced trap-assisted recombination and improved charge transport at the TiO2/CsPbBr3 interface.
The following sections detail the experimental procedures, results, and discussions, supported by tables and mathematical formulations to elucidate the key findings. We begin by describing the materials and methods used in device fabrication, followed by an analysis of the structural and morphological changes induced by SnCl2 modification. Subsequently, we present the photovoltaic performance and stability data, concluding with insights into the role of SnCl2 in enhancing the efficiency of all-inorganic perovskite solar cells.
Experimental Section
We used fluorine-doped tin oxide (FTO) glass substrates with a sheet resistance of 15 Ω·cm−2. The materials included cesium bromide (CsBr, 99.999%), lead bromide (PbBr2, 99.999%), and SnCl2·2H2O (99.99%). Solvents such as N,N-dimethylformamide (DMF), ethanol, and isopropanol were employed without further purification. The TiO2 electron transport layer was deposited using a TiCl4 precursor solution, and the CsPbBr3 perovskite layer was formed via a two-step spin-coating process.
For device fabrication, we first cleaned the FTO substrates sequentially with detergent, deionized water, acetone, and ethanol. The TiO2 layer was deposited by immersing the substrates in a 0.2 mmol·mL−1 TiCl4 solution at 70°C for 35–40 minutes, followed by annealing at 500°C for 60 minutes. SnCl2 solutions with concentrations of 0, 1, 1.5, and 2 mg·mL−1 were spin-coated onto the TiO2 layer and annealed at 180°C for 30 minutes. The CsPbBr3 perovskite was then deposited by sequentially spin-coating PbBr2 in DMF (1 mol·L−1) and CsBr in a water/butanol mixture (1.17 mol·L−1), with annealing steps at 100°C and 250°C. Finally, a carbon electrode was applied using screen printing, and the devices were annealed at 120°C for 15 minutes.
Characterization involved current density-voltage (J-V) measurements under AM 1.5G illumination, atomic force microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy, space charge limited current measurements, and transient photovoltage decay were used to analyze charge transport and recombination dynamics.
Results and Discussion
Morphological and Structural Properties
The introduction of SnCl2 significantly influenced the morphology of the CsPbBr3 perovskite films. Atomic force microscopy images revealed that unmodified films had a root mean square roughness of 62.2 nm, indicating a relatively uneven surface. In contrast, films modified with 1.5 mg·mL−1 SnCl2 exhibited a reduced roughness of 51.9 nm, suggesting improved surface smoothness. This enhancement is crucial for minimizing interfacial defects and promoting better contact with the carbon electrode, which can boost the performance of perovskite solar cells.
Scanning electron microscopy further confirmed these findings. Unmodified CsPbBr3 films displayed numerous pinholes and irregular grain sizes, whereas SnCl2-modified films showed a dense, uniform coverage with minimal voids. This improved morphology reduces non-radiative recombination sites, facilitating more efficient charge extraction in perovskite solar cells.
X-ray diffraction patterns indicated that SnCl2 modification enhanced the crystallinity of the CsPbBr3 phase. The diffraction peaks at 21.6°, 34.5°, and 37.9°, corresponding to the (110), (210), and (211) planes of CsPbBr3, intensified after modification. Concurrently, peaks associated with CsPb2Br5 impurities at 27.8°, 29.8°, and 31.6° weakened. This reduction in secondary phases contributes to a more ordered crystal structure, which is beneficial for charge carrier transport in perovskite solar cells. The contact angle measurements showed that SnCl2 treatment increased the wettability of the TiO2 surface, promoting better spreading of precursor solutions and leading to higher-quality perovskite films.
X-ray photoelectron spectroscopy analysis of the Sn3d core level confirmed the presence of Sn2+ on the TiO2 surface after SnCl2 modification. The intensity of the Sn2+ peak increased relative to Sn4+, indicating that SnCl2 effectively passivates the interface and plays a key role in modifying the electron transport layer.
Photovoltaic Performance
We evaluated the photovoltaic performance of devices with different SnCl2 concentrations. The J-V curves under standard illumination conditions showed that all parameters, including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and power conversion efficiency (PCE), improved with SnCl2 modification up to an optimal concentration of 1.5 mg·mL−1. The results are summarized in Table 1.
| SnCl2 Concentration (mg·mL−1) | VOC (V) | JSC (mA·cm−2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 0 | 1.51 | 7.23 | 73.41 | 8.01 |
| 1 | 1.53 | 7.40 | 77.98 | 8.83 |
| 1.5 | 1.59 | 7.62 | 81.35 | 9.92 |
| 2 | 1.53 | 7.49 | 78.35 | 8.98 |
The optimal device achieved a VOC of 1.59 V, JSC of 7.62 mA·cm−2, FF of 81.35%, and PCE of 9.92%. This represents a significant improvement over the unmodified device, highlighting the effectiveness of SnCl2 interface modification in enhancing the performance of perovskite solar cells. The steady-state power output measured at maximum power point conditions confirmed the stability of the modified devices, with a PCE of 9.04% maintained over 400 seconds of continuous illumination.
Statistical analysis of 15 devices for each condition demonstrated that SnCl2 modification not only improved the average performance but also enhanced the reproducibility of perovskite solar cells. The reduced variability in photovoltaic parameters indicates a more uniform interface and film quality.
Charge Transport and Recombination Analysis
Electrochemical impedance spectroscopy was conducted to investigate the charge transport and recombination processes. In dark conditions, the recombination resistance (Rrec) increased from 4,808.3 Ω for unmodified devices to 9,616.6 Ω for SnCl2-modified devices, indicating suppressed carrier recombination. Under illumination, the charge transfer resistance (Rct) decreased from 434.5 Ω to 368.1 Ω, suggesting enhanced electron extraction at the TiO2/CsPbBr3 interface. These results align with the improved FF and JSC observed in SnCl2-modified perovskite solar cells.
Space charge limited current measurements were used to estimate the trap density in the perovskite films. The trap-filled limit voltage (VTFL) decreased from 1.357 V to 1.173 V after SnCl2 modification. The trap density (ntrap) was calculated using the formula:
$$n_{trap} = \frac{2 \epsilon_0 \epsilon_r V_{TFL}}{e d^2}$$
where ε0 is the vacuum permittivity, εr is the relative permittivity of CsPbBr3, e is the elementary charge, and d is the film thickness. The values of ntrap were 1.9076 × 1022 cm−3 for unmodified films and 8.7541 × 1021 cm−3 for modified films, confirming that SnCl2 reduces defect states in perovskite solar cells.
Transient photocurrent and photovoltage decay measurements provided further insights into carrier dynamics. The carrier extraction lifetime decreased from 10.20 μs to 6.58 μs with SnCl2 modification, indicating faster charge separation. The recombination lifetime increased from 1.35 ms to 1.64 ms, reflecting reduced non-radiative recombination. These dynamics contribute to the higher VOC and PCE in SnCl2-modified perovskite solar cells.
The relationship between short-circuit current density and light intensity (JSC-I) can be described by:
$$J_{SC} \propto I^\alpha$$
where α is an exponential factor. For unmodified devices, α was 0.912, which increased to 0.943 after SnCl2 modification. This shift toward unity suggests reduced bimolecular recombination and improved charge transport in perovskite solar cells. Similarly, the open-circuit voltage versus light intensity (VOC-I) follows:
$$V_{OC} = \frac{n k_B T}{e} \ln I + B$$
where n is the ideality factor. The value of n decreased from 2.33 to 2.22 with SnCl2 modification, indicating suppressed trap-assisted recombination.
Stability Assessment
We evaluated the long-term stability of unmodified and SnCl2-modified perovskite solar cells under ambient conditions. Over 30 days, the PCE of modified devices retained 80.55% of its initial value, compared to 60.32% for unmodified devices. This enhanced stability is attributed to the improved film morphology and reduced defect density, which mitigate degradation pathways in perovskite solar cells.
Conclusion
In this study, we demonstrated that SnCl2 interface modification on TiO2 electron transport layers significantly enhances the performance of all-inorganic CsPbBr3 perovskite solar cells. By optimizing the SnCl2 concentration to 1.5 mg·mL−1, we achieved a power conversion efficiency of 9.92%, with improvements in open-circuit voltage, short-circuit current density, and fill factor. The modification promotes better perovskite crystallinity, reduces phase impurities, and minimizes non-radiative recombination. Electrochemical and transient analyses confirm enhanced charge extraction and reduced trap densities. Furthermore, SnCl2-modified devices exhibit superior stability, making this approach a viable strategy for advancing perovskite solar cell technology. Future work will focus on scaling up the fabrication process and exploring other interface modifiers to further push the efficiency boundaries of perovskite solar cells.