Perovskite solar cells have emerged as one of the most promising photovoltaic technologies due to their high power conversion efficiency, low-cost fabrication, and tunable optoelectronic properties. However, the performance of these devices is often limited by non-radiative recombination losses, particularly at the surface of the perovskite films. The surface defect density in polycrystalline perovskite films is typically two orders of magnitude higher than that of bulk defects, leading to significant open-circuit voltage (VOC) losses. In this study, I investigate the use of 1-naphthylmethylammonium bromide (NMABr) as a passivating agent to address these issues. By introducing NMABr, I aim to reduce surface defect density, form interface dipoles, and establish a 2D/3D perovskite heterojunction, thereby enhancing charge carrier transport and device performance. This approach provides a novel strategy for selecting surface passivators based on dipole layer formation, which could pave the way for more efficient and stable perovskite solar cells.
The fundamental challenge in perovskite solar cells lies in the solution-processed fabrication of polycrystalline films, which inherently contain a high density of surface defects. These defects, such as uncoordinated lead ions and halide vacancies, act as non-radiative recombination centers, reducing the VOC and overall efficiency. Various passivation strategies have been explored, including the use of organic ammonium salts, which can form 2D perovskite layers or interface dipoles to mitigate recombination. In this work, I focus on NMABr, a bulky ammonium salt that not only passivates surface defects but also induces an interfacial dipole moment and a 2D/3D heterostructure. This multi-functional approach enhances the built-in electric field, facilitates hole extraction, and improves energy level alignment at the interface, leading to superior device performance.
To understand the impact of NMABr on perovskite solar cells, I conducted a series of experiments, including morphological, electronic, and optoelectronic characterizations. The results demonstrate that NMABr treatment significantly reduces surface defects, increases carrier lifetime, and boosts VOC, while maintaining high stability under ambient conditions. In the following sections, I will detail the experimental methods, present and discuss the results, and conclude with the implications of this work for the future development of perovskite solar cells. Throughout this article, I will emphasize the role of perovskite solar cells in advancing photovoltaic technology, and the keyword ‘perovskite solar cell’ will be frequently referenced to highlight its importance.
Experimental Methods
The fabrication of perovskite solar cells involves several critical steps to ensure high-quality films and optimal device performance. All materials were used as received, and the procedures were carried out in controlled environments to minimize contamination. The perovskite composition used in this study was (Cs0.05FA0.80MA0.15)Pb(I0.95Br0.05)3, which is known for its excellent stability and efficiency. The substrates were indium tin oxide (ITO)-coated glass, which were cleaned sequentially in deionized water, isopropyl alcohol (IPA), and anhydrous ethanol using ultrasonic treatment for 15 minutes each. After drying with nitrogen gas, the substrates underwent UV-ozone treatment for 20 minutes to improve surface wettability.
The electron transport layer (ETL) was prepared by diluting tin oxide (SnO2) colloidal solution with deionized water in a 1:3 volume ratio. This dispersion was spin-coated onto the ITO substrates at 4000 rpm for 20 seconds, followed by annealing at 180°C for 20 minutes in air. The perovskite layer was fabricated using a two-step spin-coating method with a stoichiometric excess of lead iodide (PbI2). Specifically, a mixture of PbI2 (1.3 mol/L) with 2.5 mol% PbCl2 and 2.5 mol% CsI in a DMF/DMSO (9:1, v/v) solvent was spin-coated at 1500 rpm for 30 seconds onto the SnO2 layer. The film was then annealed at 70°C for 10 seconds in a nitrogen-filled glovebox. Subsequently, an organic salt solution containing FAI, MACl, and MABr (60:6:6 mg/mL in IPA) was spin-coated at 1700 rpm for 30 seconds, followed by annealing at 150°C for 15 minutes in air with a relative humidity of 30–40%.
For passivation, NMABr was dissolved in IPA at concentrations ranging from 1 mg/mL to 9 mg/mL. The NMABr solution (80 μL) was spin-coated onto the perovskite film at 5000 rpm for 30 seconds, and the film was annealed at 100°C for 5 minutes in a nitrogen glovebox. The hole transport layer (HTL) was prepared by dissolving 72.3 mg of Spiro-OMeTAD, 28.8 μL of 4-tert-butylpyridine (TBP), and 17.5 μL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg in 1 mL acetonitrile) in 1 mL of chlorobenzene. This solution was spin-coated at 3000 rpm for 30 seconds onto the perovskite layer. Finally, a 100 nm thick silver electrode was thermally evaporated to complete the device structure.
Characterization techniques included time-resolved photoluminescence (TRPL) using a fluorescence lifetime spectrometer, atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) to assess surface morphology and potential. Optical properties were evaluated using UV-Vis spectroscopy, and structural analysis was performed with X-ray diffraction (XRD). Device performance was measured under standard AM 1.5G illumination (100 mW/cm2) using a solar simulator, and external quantum efficiency (EQE) was recorded to validate the photocurrent. Stability tests were conducted by storing unencapsulated devices in ambient air (50 ± 10% relative humidity) and monitoring their performance over time.
The following table summarizes the key materials and their roles in the fabrication process:
| Material | Role | Concentration/Specifications |
|---|---|---|
| PbI2 | Perovskite precursor | 1.3 mol/L in DMF/DMSO |
| NMABr | Passivating agent | 1–9 mg/mL in IPA |
| Spiro-OMeTAD | Hole transport material | 72.3 mg/mL in chlorobenzene |
| SnO2 | Electron transport layer | Colloidal solution diluted 1:3 with water |
Results and Discussion
The morphological changes induced by NMABr passivation were first investigated using scanning electron microscopy (SEM). The SEM images revealed that the NMABr-treated perovskite films exhibited a reduced presence of surface PbI2 and the formation of a new layer, indicative of a 2D perovskite structure. This 2D layer likely results from the reaction between NMABr and residual PbI2, which promotes better interfacial charge transport. The reduced surface roughness and improved coverage suggest that NMABr effectively passivates surface defects and enhances the homogeneity of the perovskite film. This is crucial for minimizing non-radiative recombination in perovskite solar cells, as defects at the surface or grain boundaries can act as traps for charge carriers.
To further elucidate the electronic properties, I performed Kelvin probe force microscopy (KPFM) measurements. The surface potential of the NMABr-passivated films showed a significant increase compared to the control films, indicating a reduction in work function due to the formation of an interfacial dipole layer. This dipole moment arises from the interaction between the NMA+ cations and the perovskite surface, which alters the electron cloud density and creates a favorable energy level alignment. The gradient in energy levels facilitates hole collection at the interface while impeding electron back-transfer, resulting in a field-effect passivation that reduces VOC losses. The KPFM data corroborate the hypothesis that NMABr not only passivates defects but also modifies the interfacial energetics, which is beneficial for perovskite solar cell performance.
The charge carrier dynamics were analyzed using time-resolved photoluminescence (TRPL) spectroscopy. The TRPL decay curves were fitted with a bi-exponential function to extract the fast and slow decay components, which correspond to defect-assisted recombination and bimolecular recombination, respectively. The bi-exponential decay function is given by:
$$ F(t) = A_1 e^{-\frac{t}{\tau_1}} + A_2 e^{-\frac{t}{\tau_2}} $$
where \( A_1 \) and \( A_2 \) are the decay amplitudes, and \( \tau_1 \) and \( \tau_2 \) are the fast and slow decay lifetimes, respectively. The average carrier lifetime \( \tau_{ave} \) was calculated using the formula:
$$ \tau_{ave} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2} $$
The results, summarized in the table below, show that the NMABr-passivated films had a significantly longer average lifetime (100.628 ns) compared to the control films (37.741 ns). This increase in carrier lifetime indicates a reduction in trap-assisted non-radiative recombination, which is consistent with the effective passivation of surface defects. The enhanced lifetime also suggests improved charge extraction at the interface with the hole transport layer, contributing to higher performance in perovskite solar cells.
| Sample | \( \tau_{ave} \) (ns) | \( A_1 \) (%) | \( A_2 \) (%) | \( \tau_1 \) (ns) | \( \tau_2 \) (ns) |
|---|---|---|---|---|---|
| Control | 37.741 | 22.6 | 77.4 | 19.1 | 222.6 |
| NMABr-passivated | 100.628 | 20.81 | 79.19 | 27.60 | 228.21 |
The photovoltaic performance of the devices was evaluated by measuring current density-voltage (J-V) curves under standard illumination conditions. The NMABr-passivated perovskite solar cells exhibited a notable improvement in VOC, from an average of 1.10 V for the control devices to 1.18 V for the optimized devices. This increase in VOC is attributed to the reduced non-radiative recombination and the formation of the dipole layer, which enhances the built-in potential. The fill factor (FF) also improved, indicating better charge collection and reduced series resistance. The short-circuit current density (JSC) showed a slight increase, which can be linked to the improved charge transport in the 2D/3D heterostructure. The overall power conversion efficiency (PCE) increased by approximately 15% for the NMABr-passivated devices compared to the control. The following table summarizes the key photovoltaic parameters for devices with different NMABr concentrations:
| NMABr Concentration (mg/mL) | VOC (V) | JSC (mA/cm2) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 0 (Control) | 1.10 | 22.5 | 75.2 | 18.6 |
| 1 | 1.12 | 22.7 | 76.8 | 19.5 |
| 5 | 1.18 | 23.0 | 78.5 | 21.3 |
| 9 | 1.16 | 22.8 | 77.0 | 20.4 |
The stability of the perovskite solar cells is a critical factor for their commercial application. I conducted long-term stability tests by storing unencapsulated devices in ambient air (50 ± 10% relative humidity) and monitoring their PCE over time. The NMABr-passivated devices retained over 85% of their initial PCE after 504 hours, while the control devices degraded to less than 55% of their initial efficiency. This enhanced stability is likely due to the hydrophobic nature of the NMA+ cations, which form a protective layer that inhibits the ingress of moisture and oxygen. The 2D perovskite structure also contributes to the improved stability by providing a more robust interface. These results underscore the potential of NMABr passivation for developing durable perovskite solar cells that can withstand environmental stressors.
To further analyze the electronic structure, I considered the energy level alignment at the interface. The dipole moment introduced by NMABr can be described by the change in surface potential \( \Delta \phi \), which relates to the work function change. The dipole moment \( \mu \) can be approximated using the Helmholtz equation:
$$ \Delta \phi = \frac{\mu \cos \theta}{\varepsilon \varepsilon_0 A} $$
where \( \varepsilon \) is the dielectric constant, \( \varepsilon_0 \) is the vacuum permittivity, \( \theta \) is the tilt angle of the dipole, and \( A \) is the area per molecule. This dipole layer shifts the energy levels, facilitating better hole extraction and reducing recombination. The improved energy level alignment is a key factor in the enhanced VOC and overall performance of the perovskite solar cells.
Conclusion
In this study, I demonstrated that NMABr passivation effectively reduces surface defects in perovskite solar cells by forming an interfacial dipole layer and a 2D/3D heterojunction. The passivation mechanism involves the coordination of NMA+ cations with uncoordinated Pb2+ ions, which suppresses non-radiative recombination and enhances carrier lifetime. The dipole moment alters the surface potential, leading to better energy level alignment and improved charge transport. As a result, the NMABr-passivated devices exhibit higher VOC, PCE, and stability compared to the control devices. This work highlights the importance of dipole-based passivation strategies for advancing perovskite solar cell technology and provides a new direction for selecting surface passivators. Future research could explore other bulky ammonium salts or multifunctional molecules to further optimize the interface and push the efficiency of perovskite solar cells closer to their theoretical limits.
The success of this approach underscores the potential of interface engineering in overcoming the limitations of perovskite solar cells. By combining defect passivation with dipole formation, it is possible to achieve high performance and stability simultaneously. I believe that this strategy will inspire further innovations in the design of passivating agents for next-generation photovoltaic devices. The continued development of perovskite solar cells is essential for realizing low-cost, high-efficiency solar energy conversion, and I am confident that interface modulation will play a central role in this journey.