Perovskite solar cells have emerged as a promising technology in the field of photovoltaics due to their high power conversion efficiencies, low-cost fabrication, and tunable optoelectronic properties. However, the presence of defects in perovskite films, such as iodine vacancies and undercoordinated lead ions, often leads to non-radiative recombination and reduced stability, limiting their commercial viability. In this study, we introduce a novel amidine-based small molecule, 2-amidinoethylenediamine dihydrobromide (2AD), as an interfacial modifier to address these challenges. By applying 2AD to mixed-cation mixed-halide perovskite films with a bandgap of 1.55 eV, we systematically investigate its impact on film morphology, crystallinity, photophysical properties, and device performance. Our approach focuses on defect passivation, grain boundary engineering, and enhanced charge carrier dynamics, ultimately leading to improved efficiency and stability in inverted perovskite solar cells. Through comprehensive characterization techniques, including scanning electron microscopy, X-ray diffraction, and time-resolved photoluminescence, we demonstrate that 2AD serves as a multifunctional agent that not only suppresses defect states but also promotes larger grain sizes and better film uniformity. The integration of 2AD into the perovskite solar cell structure results in a significant boost in power conversion efficiency, reduced hysteresis, and superior environmental stability, underscoring its potential for advancing perovskite-based photovoltaics. This work provides a detailed exploration of the underlying mechanisms and practical implications of amidine small-molecule modifications, contributing to the ongoing development of high-performance and durable perovskite solar cells.
The fabrication of perovskite solar cells involves several critical steps, each influencing the final device performance. For this study, we employed an inverted planar p-i-n structure with the configuration glass/FTO/NiOx/Perovskite/PCBM/BCP/Ag. The perovskite active layer consisted of a mixed-cation mixed-halide composition, specifically (FA0.90MA0.05Cs0.05)Pb(I0.96Br0.04)3, which was deposited using a one-step spin-coating method with chlorobenzene as the anti-solvent. To ensure reproducibility, all procedures were carried out in a controlled environment, and materials were sourced commercially without further purification unless specified. The 2AD modifier was dissolved in isopropanol at a concentration of 0.5 mg/mL and applied via spin-coating onto the perovskite surface prior to the deposition of electron transport layers. This treatment aimed to passivate surface defects and optimize the interface between the perovskite and charge transport layers. We compared the properties of control devices (without 2AD) and 2AD-treated devices to evaluate the effectiveness of the modification strategy. Key parameters, such as film thickness, solvent ratios, and annealing conditions, were meticulously optimized to achieve high-quality films. The use of 2AD aligns with broader efforts in the perovskite solar cell community to employ organic molecules for defect mitigation, as their functional groups can interact with perovskite components through hydrogen bonding or ionic interactions, thereby reducing trap states and enhancing optoelectronic properties.
To assess the morphological changes induced by 2AD treatment, we performed scanning electron microscopy (SEM) and atomic force microscopy (AFM) on the perovskite films. The control samples exhibited grain sizes ranging from 200 to 450 nm, with an average of 304 nm, while the 2AD-treated films showed a significant increase in grain size, averaging 321 nm with a distribution of 150 to 500 nm. This enlargement is attributed to the passivation effect of 2AD, which reduces nucleation sites and promotes Ostwald ripening during crystallization. Additionally, AFM measurements revealed a decrease in root-mean-square roughness from 16.6 nm for control films to 15.8 nm for 2AD-modified films, indicating a smoother surface that minimizes charge recombination at interfaces. The enhanced hydrophobicity was confirmed by contact angle tests, where the water contact angle increased from 70.1° to 74.3° after 2AD treatment, suggesting improved resistance to moisture ingress—a critical factor for the long-term stability of perovskite solar cells. These morphological improvements are summarized in Table 1, highlighting the role of 2AD in optimizing film quality for efficient charge transport and reduced defect density in perovskite solar cells.
| Sample | Average Grain Size (nm) | RMS Roughness (nm) | Contact Angle (°) |
|---|---|---|---|
| Control | 304 | 16.6 | 70.1 |
| 2AD-Treated | 321 | 15.8 | 74.3 |
X-ray diffraction (XRD) analysis provided insights into the crystallographic changes upon 2AD modification. The control films displayed characteristic peaks corresponding to the perovskite phase, with the (100) plane showing a full width at half maximum (FWHM) of 0.3949°. In contrast, the 2AD-treated films exhibited a narrower FWHM of 0.2466° for the same plane, indicating enhanced crystallinity and reduced microstrain. This is consistent with the SEM results, as larger grains typically correlate with better crystalline quality. The intensity of the (100) peak also increased after 2AD treatment, suggesting a more oriented growth and fewer defects such as unreacted PbI2, which was absent in the modified films. The XRD patterns can be described by the Bragg equation: $$ n\lambda = 2d\sin\theta $$ where \( \lambda \) is the X-ray wavelength, \( d \) is the interplanar spacing, and \( \theta \) is the diffraction angle. The reduction in FWHM implies a decrease in dislocation density, which is beneficial for minimizing non-radiative recombination pathways in perovskite solar cells. Furthermore, the stability of the crystalline structure under ambient conditions was improved, as evidenced by the retained peak intensities after aging, underscoring the role of 2AD in reinforcing the perovskite lattice against degradation.
The optical properties of the perovskite films were evaluated using ultraviolet-visible (UV-Vis) absorption spectroscopy and photoluminescence (PL) measurements. The absorption spectra showed that both control and 2AD-treated films had similar bandgaps of approximately 1.55 eV, calculated from Tauc plots using the formula: $$ (\alpha h\nu)^2 = A(h\nu – E_g) $$ where \( \alpha \) is the absorption coefficient, \( h\nu \) is the photon energy, \( A \) is a constant, and \( E_g \) is the bandgap energy. This indicates that 2AD modification does not alter the fundamental optoelectronic characteristics of the perovskite material. However, the 2AD-treated films exhibited higher absorption intensity, particularly in the visible range, which we attribute to the improved film quality and reduced light scattering from smoother surfaces. Steady-state PL spectra revealed a blue shift in the emission peak from 792 nm for control films to 791 nm for 2AD-treated films, along with a significant increase in PL intensity. This enhancement suggests a reduction in non-radiative recombination centers, as the amidine groups in 2AD effectively passivate surface defects. Time-resolved photoluminescence (TRPL) decays were fitted with a bi-exponential model: $$ I(t) = A_1 \exp(-t/\tau_1) + A_2 \exp(-t/\tau_2) + B $$ where \( \tau_1 \) and \( \tau_2 \) represent the fast and slow decay components, respectively, and \( A_1 \), \( A_2 \), and \( B \) are constants. The average carrier lifetime \( \tau_{\text{avg}} \) was calculated as: $$ \tau_{\text{avg}} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2} $$ For control films, \( \tau_{\text{avg}} \) was 0.857 μs, whereas for 2AD-treated films, it increased to 1.023 μs, indicating prolonged carrier lifetimes and suppressed charge recombination. These findings are crucial for understanding how 2AD enhances the performance of perovskite solar cells by facilitating more efficient charge extraction and collection.
| Sample | \( \tau_1 \) (μs) | \( \tau_2 \) (μs) | \( A_1 \) | \( A_2 \) | \( \tau_{\text{avg}} \) (μs) |
|---|---|---|---|---|---|
| Control | 0.312 | 1.205 | 0.45 | 0.55 | 0.857 |
| 2AD-Treated | 0.298 | 1.410 | 0.42 | 0.58 | 1.023 |
X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical interactions between 2AD and the perovskite layer. The Pb 4f spectra showed a shift to lower binding energies after 2AD treatment, with Pb 4f7/2 moving from 138.96 eV to 138.38 eV and Pb 4f5/2 from 143.82 eV to 143.22 eV. Similarly, the I 3d peaks shifted from 619.70 eV (I 3d5/2) and 631.16 eV (I 3d7/2) to 619.19 eV and 630.65 eV, respectively. The N 1s peak also exhibited a shift from 400.97 eV to 400.51 eV. These shifts suggest strong chemical bonding, likely through the formation of hydrogen bonds between the amidine groups of 2AD and iodide ions in the perovskite, which passivate iodine vacancies and reduce defect densities. The overall XPS data support the hypothesis that 2AD acts as a multifunctional passivator, improving the electronic structure at the perovskite interface and contributing to the enhanced performance of perovskite solar cells. The reduction in binding energies indicates a more stable surface, which is essential for minimizing ion migration and degradation under operational conditions.
The device performance of the fabricated perovskite solar cells was evaluated through current density-voltage (J-V) measurements under standard illumination conditions. The control devices achieved a power conversion efficiency (PCE) of 21.32%, with an open-circuit voltage (VOC) of 1.119 V, short-circuit current density (JSC) of 25.44 mA/cm², and fill factor (FF) of 74.85%. In contrast, the 2AD-treated devices exhibited a remarkable improvement, with a PCE of 23.49%, VOC of 1.118 V, JSC of 25.64 mA/cm², and FF of 81.98%. The hysteresis index (HI) was calculated using the formula: $$ \text{HI} = \frac{\text{PCE}_{\text{reverse}} – \text{PCE}_{\text{forward}}}{\text{PCE}_{\text{reverse}}} $$ For control devices, HI was 6.8%, which decreased to 5.3% after 2AD treatment, indicating reduced ionic migration and better interface quality. The enhanced FF and reduced hysteresis are directly linked to the improved film morphology and defect passivation, which minimize charge trapping and recombination losses. Statistical analysis of multiple devices confirmed the reproducibility of these results, with 2AD-treated perovskite solar cells consistently outperforming the control group. The external quantum efficiency (EQE) spectra also showed a broader response in the visible region for modified devices, correlating with the higher JSC values. These performance metrics are summarized in Table 3, demonstrating the efficacy of 2AD in optimizing the photovoltaic parameters of perovskite solar cells.
| Parameter | Control | 2AD-Treated |
|---|---|---|
| PCE (%) | 21.32 | 23.49 |
| VOC (V) | 1.119 | 1.118 |
| JSC (mA/cm²) | 25.44 | 25.64 |
| FF (%) | 74.85 | 81.98 |
| Hysteresis Index (%) | 6.8 | 5.3 |
Long-term stability tests were conducted on unencapsulated devices stored in ambient conditions (25°C and 35% relative humidity) for over 30 days. The control devices retained less than 80% of their initial PCE after this period, primarily due to moisture-induced degradation and defect proliferation. In contrast, the 2AD-treated perovskite solar cells maintained over 90% of their original efficiency, highlighting the role of 2AD in enhancing environmental robustness. This improvement can be attributed to the increased hydrophobicity, reduced defect density, and better interfacial integrity, which collectively mitigate the ingress of water and oxygen molecules. The stability data align with the morphological and chemical analyses, reinforcing the notion that 2AD modification not only boosts efficiency but also addresses one of the key challenges in perovskite solar cell technology—durability. We further investigated the thermal stability by subjecting devices to 85°C for 500 hours, and the 2AD-treated samples showed negligible degradation, whereas control devices experienced a 20% drop in PCE. These findings underscore the potential of amidine-based small molecules like 2AD in developing commercially viable perovskite solar cells with extended lifetimes.
In conclusion, our study demonstrates that the amidine small molecule 2AD serves as an effective interfacial modifier for enhancing the performance and stability of perovskite solar cells. Through comprehensive characterization, we have shown that 2AD treatment leads to larger grain sizes, smoother surfaces, improved crystallinity, and prolonged charge carrier lifetimes. These morphological and optoelectronic improvements translate into higher power conversion efficiencies, reduced hysteresis, and superior environmental stability in inverted perovskite solar cells. The chemical interactions between 2AD and the perovskite layer, as evidenced by XPS shifts, facilitate defect passivation and reduce non-radiative recombination. This work not only provides a practical strategy for optimizing perovskite solar cells but also contributes to the fundamental understanding of interface engineering in photovoltaics. Future research could explore the application of similar amidine derivatives in large-area modules and tandem configurations to further advance the commercialization of perovskite solar cells. Overall, the integration of multifunctional small molecules like 2AD represents a promising avenue for achieving high-efficiency, stable, and scalable perovskite-based energy solutions.