In recent years, the rapid development of perovskite solar cells has garnered significant attention due to their high power conversion efficiency and low-cost fabrication potential. However, the stability of these devices, particularly under ultraviolet (UV) light exposure, remains a critical challenge for their commercialization. As a researcher in this field, I have focused on addressing the interfacial instability in n-i-p structured perovskite solar cells, where the TiO2/perovskite interface is prone to degradation under UV illumination. This article explores the underlying causes of this instability, reviews current research progress, and proposes comprehensive strategies to enhance UV stability through interfacial engineering. The goal is to establish a robust framework for improving the durability of perovskite solar cells, enabling their long-term application in real-world environments.
The instability of the TiO2/perovskite interface under UV light primarily stems from the photocatalytic properties of TiO2 and the presence of surface defects. TiO2, commonly used as an electron transport layer in perovskite solar cells, has a band-edge absorption in the UV region. When exposed to UV light, TiO2 becomes activated, leading to the catalytic degradation of adjacent organic-inorganic hybrid perovskite materials. This process involves the formation of metastable deep-level defects at the interface, where TiO2 extracts electrons from halide anions in the perovskite structure, ultimately causing decomposition. The presence of oxygen molecules accelerates this degradation, further compromising the stability of perovskite solar cells. To quantify this, the defect density at the interface can be modeled using the following equation:
$$ N_d = -\frac{2}{q \epsilon} \left( \frac{d(C/A)^{-2}}{dV} \right)^{-1} $$
where \( N_d \) is the defect density, \( q \) is the electron charge, \( \epsilon \) is the permittivity of the perovskite material, \( C \) is the capacitance, \( A \) is the area, and \( V \) is the voltage. This equation highlights how interfacial defects contribute to instability, and reducing \( N_d \) is crucial for enhancing the UV stability of perovskite solar cells.
Current research efforts to mitigate UV-induced degradation in perovskite solar cells can be broadly categorized into two approaches: incorporating down-conversion fluorescent materials and fully covering the TiO2 surface. Down-conversion materials, such as those based on rare-earth elements, convert UV light into visible light, thereby reducing the activation of TiO2. However, these materials often fail to cover the entire UV spectrum, limiting their effectiveness. For instance, the efficiency of UV conversion can be described by the following formula:
$$ \eta_{dc} = \frac{\int \Phi_{emit}(\lambda) d\lambda}{\int \Phi_{absorb}(\lambda) d\lambda} $$
where \( \eta_{dc} \) is the down-conversion efficiency, \( \Phi_{emit} \) is the emitted photon flux, and \( \Phi_{absorb} \) is the absorbed photon flux. In practice, achieving high \( \eta_{dc} \) across all UV wavelengths remains challenging, necessitating alternative strategies for perovskite solar cells.
The second approach involves replacing or fully passivating the TiO2 surface to suppress its photocatalytic activity. Various materials have been explored, including organic compounds, carbon allotropes, metal oxides, multicomponent compounds, and metal salts. Among these, metal salts such as CsX (where X = Cl, Br, I) offer advantages like resistance to aging, lack of photocatalytic activity, and high electrical conductivity. However, a major limitation is that solution-processed metal salt films often form island-like structures, resulting in incomplete coverage of the TiO2 surface. This partial coverage only locally passivates the interface, leaving areas vulnerable to UV degradation in perovskite solar cells. The table below summarizes the properties of different passivation materials for perovskite solar cells:
| Material Type | Advantages | Disadvantages | Suitability for Perovskite Solar Cells |
|---|---|---|---|
| Organic Compounds | Easy processing, flexible | Prone to degradation, limited stability | Moderate |
| Carbon Allotropes | High conductivity, stable | Can be degraded by TiO2 under UV | Moderate |
| Metal Oxides | Robust, wide bandgap | May catalyze degradation in short UV | Low |
| Multicomponent Compounds | Tailored properties | Large particle size, unsuitable for planar structures | Low |
| Metal Salts | No aging, no photocatalysis, high conductivity | Incomplete coverage, solubility issues | High (with improvements) |
The incomplete coverage of metal salts arises from two main factors: uncontrollable crystallization behavior of the metal salt precursors within the micro-liquid film during solution processing, and the solubility of the prepared metal salt films in perovskite precursor solvents like DMF, DMSO, or GBL. To address these issues, we propose a two-step strategy: first, achieving “full” coverage through ultrafast crystallization, and second, protecting this coverage by constructing a non-dissolvable surface via mercaptobenzene adsorption. This approach aims to comprehensively passivate the TiO2 surface, thereby enhancing the UV stability of perovskite solar cells.
For the first step, we focus on preparing a full-coverage CsX film on TiO2. The process begins with substrate preparation: cleaning FTO substrates, drying with high-purity N2, and treating with UV/O3 for 15 minutes. A compact TiO2 layer is deposited by immersing the FTO in a 200 mM TiCl4 aqueous solution at 70°C for 30 minutes, followed by rinsing and drying. A porous TiO2 layer is then applied using a TiO2 paste (e.g., Dyesol 18NR-T) via blade coating, drying, and annealing at 450°C for 30 minutes. The CsX film is formed by dissolving CsX in a water/isopropanol (1:1 volume ratio) mixture at a concentration of 2.5 mg/mL, spin-coating at 6000 rpm for 5 seconds, and utilizing a low-pressure chamber apparatus to induce ultrafast crystallization. This apparatus, consisting of a drying chamber, vacuum system, and exhaust system, reduces chamber pressure to as low as 1 Pa, facilitating rapid solvent evaporation and controlled crystallization. The laboratory temperature is maintained below 20°C to minimize premature solvent evaporation, ensuring the entire process is completed within 1 minute. The crystallization kinetics can be described by the following equation:
$$ \frac{dC}{dt} = -k \cdot (C – C_{sat}) $$
where \( \frac{dC}{dt} \) is the rate of concentration change, \( k \) is the crystallization rate constant, \( C \) is the current concentration, and \( C_{sat} \) is the saturation concentration. By optimizing \( k \) through pressure control, we achieve uniform CsX coverage on TiO2, which is critical for passivating perovskite solar cells.
The second step involves adsorbing mercaptobenzene to protect the CsX film from dissolution during subsequent perovskite deposition. Mercaptobenzene, composed of a thiol group and a benzene ring, chemically adsorbs onto the CsX surface via the thiol group, while the benzene ring provides insolubility in perovskite precursor solvents. To implement this, we prepare a mercaptobenzene solution in CH2Cl2 (in which CsX is insoluble), anneal the CsX film at 150°C for 30 minutes, immerse it in the solution, and then rinse and dry rapidly. The adsorption amount is characterized using Quartz Crystal Microbalance (QCM) technology, where the frequency shift \( \Delta f_Q \) relates to the adsorbed mass per unit area \( \rho_f \Delta d_f \) by:
$$ \Delta f_Q = -\frac{f_Q^2}{N \rho_Q} \rho_f \Delta d_f $$
Here, \( f_Q \) is the resonant frequency, \( N \) is the harmonic number, and \( \rho_Q \) is the density of the quartz crystal. This allows precise monitoring of mercaptobenzene coverage, ensuring optimal protection for perovskite solar cells.
To verify the non-dissolvability of the CsX surface, we employ microscopic observation and photocatalytic product detection. For microscopic analysis, the mercaptobenzene-adsorbed CsX film is immersed in DMF, and any morphological changes are examined using high-resolution scanning electron microscopy. In photocatalytic tests, we use methyl orange dye in DMF, measuring its absorbance at 465 nm before and after UV exposure. If the absorbance remains constant, it indicates that the CsX film is protected from dissolution; a decrease suggests dissolution, while degradation of mercaptobenzene alone would occur if directly adsorbed on TiO2. This multi-faceted approach ensures the integrity of the passivation layer in perovskite solar cells.
Next, we deposit the perovskite film on the protected TiO2/CsX/mercaptobenzene interface. The perovskite precursor solution is prepared by dissolving PbI2 and CH3NH3I in DMF at specific concentrations. Spin-coating combined with the low-pressure chamber is used to rapidly remove solvents, but the wettability may be reduced due to mercaptobenzene treatment. Therefore, we systematically vary spin-coating speeds and precursor concentrations to determine the optimal conditions for achieving a uniform perovskite layer. The relationship between film thickness \( d \) and processing parameters can be expressed as:
$$ d = \frac{C \cdot V}{\omega \cdot t} $$
where \( C \) is the concentration, \( V \) is the volume, \( \omega \) is the spin speed, and \( t \) is the time. By optimizing these parameters, we ensure sufficient perovskite material for efficient light absorption and charge generation in perovskite solar cells.
To evaluate the UV stability of the TiO2/CsX/perovskite interface, we expose it to 365 nm UV light at an intensity of 523 mW/cm2 from the FTO side. We then perform thermal admittance spectroscopy using an electrochemical workstation to extract the interface defect density. Additionally, Mott-Schottky analysis provides the built-in potential \( V_{bi} \), from which we calculate the majority carrier concentration \( N_d \) and depletion width \( w \) using:
$$ N_d = -\frac{2}{q \epsilon} \left( \frac{d(C/A)^{-2}}{dV} \right)^{-1} $$
$$ w = \sqrt{\frac{2 \epsilon (V_{bi} – V_{bias})}{q N_d}} $$
where \( V_{bias} \) is the applied bias voltage. These measurements directly and indirectly assess the density of metastable deep-level defects, revealing how CsX microstructure influences interfacial stability in perovskite solar cells. The table below summarizes key parameters for UV stability assessment:
| Parameter | Symbol | Measurement Technique | Impact on Perovskite Solar Cells |
|---|---|---|---|
| Defect Density | \( N_d \) | Thermal Admittance Spectroscopy | Direct indicator of interface quality |
| Built-in Potential | \( V_{bi} \) | Mott-Schottky Analysis | Affects charge separation efficiency |
| Depletion Width | \( w \) | Mott-Schottky Analysis | Influences carrier collection |
| Crystallization Rate | \( k \) | Low-Pressure Chamber Control | Determines coverage uniformity |
In conclusion, the UV instability of perovskite solar cells primarily arises from the TiO2/perovskite interface, where TiO2‘s photocatalytic activity and surface defects lead to perovskite degradation. While current research has explored various passivation materials, metal salts like CsX offer a promising solution due to their stability and conductivity. However, achieving full coverage remains a challenge due to uncontrolled crystallization and solubility issues. Our proposed two-step strategy—ultrafast crystallization for “full” coverage and mercaptobenzene adsorption for protection—provides a comprehensive approach to interfacial stabilization. By integrating these methods, we establish a framework for enhancing the UV stability of perovskite solar cells, which is essential for their practical application. Future work will focus on optimizing these strategies for large-scale production and long-term durability, ultimately contributing to the advancement of perovskite solar cell technology.
Throughout this research, we have emphasized the importance of interfacial engineering in perovskite solar cells, and we believe that the insights gained will pave the way for more stable and efficient photovoltaic devices. The continuous improvement of perovskite solar cells relies on innovative materials and precise control over interface properties, and our work contributes to this ongoing effort by addressing one of the most critical stability issues. As we move forward, we will explore additional materials and techniques to further enhance the performance and longevity of perovskite solar cells, ensuring their role in the future of renewable energy.