In recent years, perovskite solar cells have emerged as a promising photovoltaic technology due to their high power conversion efficiency, low-cost processing potential, and compatibility with flexible substrates. However, the fabrication of high-performance perovskite solar cells typically requires controlled environments, such as gloveboxes, to avoid degradation from moisture and oxygen. Scaling up production to industrial levels necessitates developing methods that enable ambient air processing without compromising device performance. One critical challenge is controlling the crystallization of perovskite films, as the quality of the light-absorbing layer directly impacts the efficiency and stability of perovskite solar cells. In this study, we explore a solvent regulation strategy to enhance the crystallization of perovskite films under ambient conditions, focusing on the substitution of traditional solvents with alternatives that offer stronger coordination with perovskite precursors.
The crystallization process of perovskite films is highly dependent on the solvent system used in the precursor solution. Common solvents like N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) can form unstable intermediate complexes with perovskite components, leading to defects and poor film quality. For instance, these solvents often facilitate the formation of δ-FAPbI3, a non-perovskite phase that reduces the performance of perovskite solar cells. To address this, we investigated the use of N-ethyl-2-pyrrolidone (NEP) as a co-solvent, leveraging its stronger coordination ability with PbI2 compared to conventional solvents like N-methyl-2-pyrrolidone (NMP). Our approach involves theoretical calculations and experimental validations to demonstrate how solvent regulation can suppress undesirable intermediate formation and promote the growth of high-quality α-FAPbI3 crystals. This strategy not only improves the efficiency of perovskite solar cells but also enhances their stability, making it suitable for ambient fabrication techniques like blade-coating.
We began by performing extended Hückel theoretical calculations to analyze the electronic properties of NMP and NEP solvents. These calculations revealed that the carbonyl oxygen in NEP has a higher electronegativity than that in NMP, resulting in a stronger interaction with Pb2+ ions. The charge density distribution showed that the oxygen atom in NEP carries a charge of approximately -0.7000 e, while in NMP, it is -0.6979 e. This slight difference enhances the coordination stability of NEP with PbI2, as described by the following equation for the coordination energy:
$$ E_{\text{coord}} = k \frac{q_1 q_2}{r} $$
where \( E_{\text{coord}} \) is the coordination energy, \( k \) is a constant, \( q_1 \) and \( q_2 \) are the charges on the oxygen and Pb2+ ions, respectively, and \( r \) is the distance between them. The higher charge magnitude in NEP leads to a more stable complex, reducing the formation of metastable intermediates. Additionally, Fourier-transform infrared (FTIR) spectroscopy confirmed the stronger interaction, as the C=O stretching vibration in NEP shifted to lower wavenumbers when coordinated with PbI2, indicating a stronger bond formation.
To fabricate perovskite solar cells under ambient conditions (10-20% relative humidity and room temperature), we employed a blade-coating technique combined with vacuum-assisted solvent extraction. This method accelerates solvent evaporation, promoting rapid crystallization and dense film formation. The precursor solutions were prepared by dissolving PbI2, FAI, and MACl in mixed solvents of DMF with varying volumes of NEP or NMP. For example, the DMF-NEP-7.5% solution contained 7.5% NEP by volume in DMF, while the control used DMF-NMP-7.5%. The solutions were blade-coated onto SnO2-coated ITO substrates, followed by vacuum drying and annealing at 150°C for 10 minutes. The resulting perovskite films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible spectroscopy, and photoluminescence (PL) measurements.
The structural and morphological analysis of the perovskite films revealed significant improvements with NEP-based solvents. XRD patterns showed sharper and more intense peaks for the α-FAPbI3 phase in DMF-NEP-7.5% films compared to DMF-NMP-7.5%, indicating higher crystallinity and reduced defects. The full width at half maximum (FWHM) of the (100) peak decreased from 0.80° to 0.68°, which can be quantified by the Scherrer equation:
$$ \tau = \frac{K \lambda}{\beta \cos \theta} $$
where \( \tau \) is the crystallite size, \( K \) is the shape factor, \( \lambda \) is the X-ray wavelength, \( \beta \) is the FWHM in radians, and \( \theta \) is the Bragg angle. This reduction in FWHM correlates with larger crystal sizes, as confirmed by SEM images, where DMF-NEP-7.5% films exhibited grain sizes of approximately 260 nm, compared to 220 nm for DMF-NMP-7.5%. The enhanced morphology contributes to better charge transport and reduced recombination in perovskite solar cells.
Optical properties further supported the benefits of solvent regulation. Steady-state PL measurements demonstrated higher peak intensities for DMF-NEP-7.5% films, suggesting lower non-radiative recombination losses. The PL intensity ratio between samples can be expressed as:
$$ \frac{I_{\text{NEP}}}{I_{\text{NMP}}} = \exp\left(-\frac{\Delta E}{kT}\right) $$
where \( I \) is the PL intensity, \( \Delta E \) is the energy difference related to defect states, \( k \) is Boltzmann’s constant, and \( T \) is temperature. The increased intensity for NEP-based films indicates a reduction in trap states, which is crucial for high-performance perovskite solar cells. UV-visible absorption spectra also showed superior light absorption in the visible range for DMF-NEP-7.5% and DMF-NEP-10% films, enhancing the short-circuit current density in devices.
We fabricated complete perovskite solar cells with a structure of ITO/SnO2/Perovskite/PEAI/Spiro-OMeTAD/Ag and evaluated their photovoltaic performance. The current-density-voltage (J-V) curves were measured under AM 1.5G illumination, and the key parameters are summarized in Table 1. The DMF-NEP-7.5% devices achieved a champion power conversion efficiency (PCE) of 21.05%, with an open-circuit voltage (Voc) of 1.08 V, short-circuit current density (Jsc) of 24.30 mA/cm², and fill factor (FF) of 80.36%. In contrast, the DMF-NMP-7.5% control devices had a maximum PCE of 20.25%, with Voc = 1.07 V, Jsc = 23.48 mA/cm², and FF = 80.57%. The improvement in PCE is primarily attributed to the enhanced crystallinity and reduced defect density in NEP-based perovskite solar cells.
| Solvent System | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) | Average PCE (%) |
|---|---|---|---|---|---|
| DMF-NMP-7.5% | 1.07 | 23.48 | 80.57 | 20.25 | 18.43 |
| DMF-NEP-5% | 1.06 | 23.20 | 79.50 | 19.50 | 17.80 |
| DMF-NEP-7.5% | 1.08 | 24.30 | 80.36 | 21.05 | 19.50 |
| DMF-NEP-10% | 1.07 | 24.10 | 80.00 | 20.80 | 19.20 |
| DMF-NEP-12.5% | 1.06 | 23.50 | 79.80 | 19.90 | 18.10 |
The statistical analysis of PCE values from 20 devices for each solvent system (Figure 1) showed that DMF-NEP-7.5% had a tighter distribution around higher efficiencies, confirming the reproducibility of the solvent regulation strategy. Electrochemical impedance spectroscopy (EIS) under dark conditions revealed larger semicircles for DMF-NEP-7.5% devices, indicating higher recombination resistance. The Nyquist plots were fitted using an equivalent circuit model consisting of series resistance (Rs) and charge transfer resistance (Rct), with the recombination resistance Rrec given by:
$$ R_{\text{rec}} = R_{\text{ct}} \left(1 + \frac{\omega}{\omega_0}\right) $$
where \( \omega \) is the angular frequency and \( \omega_0 \) is characteristic frequency. The increased Rrec for NEP-based perovskite solar cells suggests suppressed non-radiative recombination, leading to longer carrier lifetimes and improved Voc.
Stability testing of unencapsulated perovskite solar cells under ambient conditions (25±5°C, 10-20% RH) demonstrated that DMF-NEP-7.5% devices retained 82.1% of their initial PCE after 1200 hours, compared to 78.2% for DMF-NMP-7.5%. The enhanced stability can be modeled using a first-order degradation kinetics equation:
$$ PCE(t) = PCE_0 \exp(-kt) $$
where \( PCE_0 \) is the initial efficiency, \( k \) is the degradation rate constant, and \( t \) is time. The lower \( k \) value for NEP-based devices indicates slower degradation, attributable to the higher chemical stability of the perovskite films. This makes the solvent regulation approach particularly valuable for ambient fabrication of perovskite solar cells.
To assess the universality of our solvent regulation strategy, we applied it to mixed-cation perovskite compositions, such as FA0.95Cs0.05PbI3. The devices fabricated with DMF-NEP-7.5% achieved a maximum PCE of 19.83% and an average PCE of 19.30%, outperforming the DMF-NMP-7.5% counterparts (19.38% and 18.42%, respectively). This consistency across different perovskite formulations highlights the broad applicability of NEP-based solvent systems in improving the performance of perovskite solar cells.
In conclusion, our solvent regulation strategy using NEP as a co-solvent effectively enhances the crystallization quality of perovskite films under ambient conditions, leading to higher efficiency and stability in perovskite solar cells. The stronger coordination between NEP and PbI2 suppresses unstable intermediate formation, promoting the growth of phase-pure α-FAPbI3 with larger grains and fewer defects. The champion devices achieved a PCE of 21.05%, with excellent environmental stability. This approach is also successful in mixed-cation systems, demonstrating its potential for scalable, ambient fabrication of perovskite solar cells. Future work will focus on optimizing solvent mixtures for large-area modules and integrating with other printing techniques to advance the commercialization of perovskite photovoltaics.
The development of solvent-regulated perovskite solar cells represents a significant step toward overcoming the limitations of traditional fabrication methods. By enabling high-performance device production in air, this strategy reduces costs and complexity, paving the way for widespread adoption of perovskite solar cells in renewable energy applications. Further investigations into solvent-perovskite interactions and crystallization kinetics will continue to drive innovations in this field, ultimately contributing to the global transition to sustainable energy sources.