In recent years, perovskite solar cells have emerged as a promising technology for next-generation photovoltaics due to their high power conversion efficiency, low-cost fabrication, and tunable optoelectronic properties. However, the performance and stability of these devices, particularly in the inverted (p-i-n) structure, are often limited by interfacial defects and residual lead iodide (PbI2) in the perovskite layer. These issues lead to non-radiative recombination, ion migration, and degradation under environmental stressors. In this study, we address these challenges by introducing phenformin hydrochloride (PEFCl) as a passivating agent for the perovskite interface. Our findings demonstrate that PEFCl effectively reduces PbI2 residues, enhances crystallinity, and pass defects at grain boundaries and surfaces, resulting in improved efficiency and stability of inverted perovskite solar cells.
The inverted perovskite solar cell structure, which consists of a stack of NiOx/2PACz/perovskite/PCBM/BCP/Ag, was fabricated using a two-step spin-coating method. This approach is advantageous for low-temperature processing and compatibility with tandem and flexible devices. However, the two-step method often leaves excess PbI2 at the surface and buried interfaces, which can act as recombination centers and facilitate ion migration. To mitigate this, we incorporated PEFCl dissolved in 2,2,2-trifluoroethanol (TFEA) as a post-treatment on the perovskite film. The passivation mechanism involves the interaction of PEFCl molecules with undercoordinated lead atoms and halide vacancies, thereby reducing defect density and improving charge carrier dynamics.
To evaluate the impact of PEFCl on the perovskite film properties, we performed X-ray diffraction (XRD) analysis. The XRD patterns revealed a significant reduction in the PbI2 peak at 12.76° for the PEFCl-treated samples, indicating a decrease in residual PbI2 content. Concurrently, the intensity of the perovskite peaks at 14.1°, 24.46°, and 28.3° increased, suggesting enhanced crystallinity and improved phase purity. This is attributed to the ability of PEFCl to promote grain growth and reduce nucleation sites for PbI2 formation. The enhanced crystallinity is crucial for minimizing defect states and improving the overall performance of perovskite solar cells.
Surface morphology and roughness were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM images showed that the control films exhibited small-sized perovskite crystals with清晰晶界 and numerous PbI2 clusters. In contrast, the PEFCl-treated films displayed larger, more compact grains with reduced PbI2 residues. The AFM measurements confirmed a decrease in surface roughness from 22 nm for the control to 19.1 nm for the PEFCl-treated films. This reduction in roughness facilitates the deposition of subsequent layers, such as the electron transport layer (ETL), and minimizes interfacial recombination in perovskite solar cells. The improved morphology is linked to the passivation of surface defects and the promotion of homogeneous crystallization.
Optical characterization through UV-visible absorption spectroscopy showed no significant difference between the control and PEFCl-treated films, indicating that PEFCl does not incorporate into the perovskite lattice but acts solely as a surface modifier. However, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements revealed substantial improvements in the optoelectronic properties. The PL intensity increased by approximately 50% after PEFCl treatment, signifying reduced non-radiative recombination. The TRPL decay curves were fitted using a bi-exponential function:
$$Y = A_1 \exp\left(-\frac{t}{\tau_1}\right) + A_2 \exp\left(-\frac{t}{\tau_2}\right) + \gamma_0$$
where \(A_1\) and \(A_2\) are amplitudes, \(\tau_1\) and \(\tau_2\) are decay lifetimes, and \(\gamma_0\) is an offset. The average carrier lifetime \(\tau_{ave}\) was calculated as:
$$\tau_{ave} = \frac{A_1 \tau_1^2 + A_2 \tau_2^2}{A_1 \tau_1 + A_2 \tau_2}$$
For the control films, \(\tau_1 = 150.01\) ns and \(\tau_2 = 651.87\) ns, yielding \(\tau_{ave} = 561.07\) ns. In contrast, the PEFCl-treated films exhibited \(\tau_1 = 320.73\) ns and \(\tau_2 = 1276.25\) ns, with \(\tau_{ave} = 1177.03\) ns. This near-doubling of the carrier lifetime underscores the effectiveness of PEFCl in passivating defects and suppressing charge recombination, which is critical for enhancing the open-circuit voltage (\(V_{OC}\)) and overall efficiency of perovskite solar cells.
The photovoltaic performance of the devices was evaluated through current density-voltage (J-V) measurements and external quantum efficiency (EQE) analysis. The table below summarizes the key parameters for both control and PEFCl-treated perovskite solar cells:
| Sample | \(V_{OC}\) (V) | \(J_{SC}\) (mA/cm2) | FF | PCE (%) |
|---|---|---|---|---|
| Control (Forward) | 1.10 | 24.50 | 0.80 | 21.53 |
| Control (Reverse) | 1.10 | 24.45 | 0.79 | 21.39 |
| PEFCl (Forward) | 1.14 | 24.75 | 0.82 | 22.47 |
| PEFCl (Reverse) | 1.14 | 24.79 | 0.81 | 22.30 |
The PEFCl-treated devices achieved a notable increase in \(V_{OC}\) from 1.10 V to 1.14 V and a boost in PCE from 21.53% to 22.47%. The improvement in \(J_{SC}\) and fill factor (FF) is attributed to the reduced recombination and enhanced charge extraction. The EQE spectra showed integrated current densities of 23.59 mA/cm2 and 24.19 mA/cm2 for the control and PEFCl-treated devices, respectively, consistent with the J-V results. These findings highlight the role of interfacial passivation in optimizing the performance of perovskite solar cells.
Stability testing was conducted by storing unencapsulated devices in ambient air with a relative humidity of 50±5% at room temperature. The normalized PCE as a function of time is presented in the table below:
| Time (h) | Control PCE (%) | PEFCl PCE (%) |
|---|---|---|
| 0 | 100 | 100 |
| 100 | 85 | 95 |
| 200 | 70 | 90 |
| 300 | 60 | 85 |
| 400 | 55 | 80 |
| 500 | 50 | 70 |
After 500 hours, the control devices retained only 50% of their initial PCE, whereas the PEFCl-treated devices maintained 70% of their original efficiency. This enhanced stability is associated with the hydrophobic nature of PEFCl, which repels moisture, and the reduction in PbI2-mediated degradation pathways. Water contact angle measurements confirmed an increase from 60° for the control to 66° for the PEFCl-treated films, further supporting the improved moisture resistance.
In conclusion, our study demonstrates that PEFCl serves as an effective passivating agent for inverted perovskite solar cells. By reducing PbI2 residues, enhancing crystallinity, and passivating interfacial defects, PEFCl treatment leads to significant improvements in \(V_{OC}\), PCE, and long-term stability. The carrier lifetime doubling and the suppression of non-radiative recombination underscore the potential of molecular passivation strategies for advancing perovskite solar cell technology. Future work will focus on optimizing the concentration and application methods of PEFCl to further push the efficiency and stability boundaries of perovskite solar cells, paving the way for their commercial adoption.
The general formula for the power conversion efficiency of a solar cell is given by:
$$\text{PCE} = \frac{V_{OC} \times J_{SC} \times \text{FF}}{P_{\text{in}}}$$
where \(P_{\text{in}}\) is the incident light power density. For our PEFCl-treated perovskite solar cells, the enhanced \(V_{OC}\) and FF contribute directly to the higher PCE. Additionally, the defect passivation can be modeled using the Shockley-Read-Hall recombination theory, where the recombination rate \(R\) is expressed as:
$$R = \frac{n p – n_i^2}{\tau_p (n + n_1) + \tau_n (p + p_1)}$$
Here, \(n\) and \(p\) are the electron and hole concentrations, \(n_i\) is the intrinsic carrier density, and \(\tau_n\) and \(\tau_p\) are the carrier lifetimes. By reducing the defect density, PEFCl increases \(\tau_n\) and \(\tau_p\), thereby lowering \(R\) and improving \(V_{OC}\). This mechanistic insight reinforces the importance of interfacial engineering in perovskite solar cells.
Overall, the integration of PEFCl into the fabrication process of inverted perovskite solar cells offers a straightforward and effective approach to address key challenges in efficiency and stability. As research in perovskite photovoltaics continues to evolve, such passivation strategies will be crucial for achieving high-performance and durable devices, ultimately contributing to the widespread deployment of perovskite solar cells in renewable energy applications.