Perovskite solar cells have emerged as a leading technology in the third-generation photovoltaic landscape due to their exceptional power conversion efficiencies, which have skyrocketed from initial values of around 3.8% to over 27% in recent years. This remarkable progress is largely attributed to the outstanding optoelectronic properties of perovskite materials, including high absorption coefficients, excellent charge carrier mobility, and tunable bandgaps. Furthermore, the solution processability of perovskite solar cells enables low-cost fabrication through techniques like spin-coating and blade-coating, making them highly attractive for large-scale industrial applications. Despite these advantages, the commercialization of perovskite solar cells faces significant hurdles, primarily concerning long-term stability and environmental toxicity. Issues such as ion migration, phase segregation, and sensitivity to moisture, oxygen, and heat lead to rapid degradation, while the use of toxic lead and hazardous organic solvents like DMF and DMSO poses ecological and health risks. To address these challenges, ionic liquids have gained considerable attention as versatile “green solvents” and functional materials in the fabrication of perovskite solar cells.
Ionic liquids are room-temperature molten salts composed of organic cations and organic or inorganic anions. Their unique properties, such as low volatility, high thermal stability, strong solvating power, and tunable chemical structures, make them ideal candidates for enhancing the performance and durability of perovskite solar cells. In this review, we explore the multifaceted roles of ionic liquids in perovskite solar cells, focusing on their applications as solvents, additives, modifiers, and charge transport layers. We also discuss the mechanisms by which ionic liquids improve crystallinity, passivate defects, optimize interfaces, and boost stability in perovskite solar cells. Through a detailed analysis of recent advancements, we aim to provide a comprehensive perspective on the future potential of ionic liquids in advancing perovskite solar cell technology.
Fundamental Properties of Ionic Liquids
Ionic liquids exhibit a range of physicochemical properties that are highly beneficial for perovskite solar cell applications. These properties stem from their unique ionic structures and can be tailored through careful selection of cations and anions. Below, we summarize the key characteristics of ionic liquids relevant to perovskite solar cells.
| Property | Description | Impact on Perovskite Solar Cells |
|---|---|---|
| Wide Liquid Range | Ionic liquids remain liquid over a broad temperature range (e.g., -90°C to 300°C) due to asymmetric ions and charge delocalization. | Enables processing under varied conditions and enhances thermal stability of perovskite layers. |
| Solvation Capacity | Strong electrostatic interactions and hydrogen bonding allow dissolution of diverse organic and inorganic compounds. | Facilitates the preparation of high-quality perovskite precursor solutions and controls crystallization kinetics. |
| Designability | Chemical structures of cations and anions can be modified to adjust polarity, hydrophobicity, conductivity, and melting point. | Allows customization for specific roles, such as defect passivation or interface engineering in perovskite solar cells. |
| Low Toxicity | Low volatility and high thermal stability reduce environmental and health risks compared to conventional solvents. | Promotes greener manufacturing processes for perovskite solar cells. |
| Electrochemical Properties | High ionic conductivity and wide electrochemical windows enable efficient charge transport. | Improves performance when used as charge transport layers or modifiers in perovskite solar cells. |
The solvation ability of ionic liquids can be described by the coordination between ions and perovskite precursors. For example, the interaction energy between a cation (e.g., imidazolium) and lead iodide (PbI₂) can be expressed using the following equation, which accounts for electrostatic and van der Waals forces:
$$ E_{\text{interaction}} = \frac{k \cdot q_1 q_2}{r} + \frac{A}{r^{12}} – \frac{B}{r^6} $$
where \( E_{\text{interaction}} \) is the total interaction energy, \( k \) is Coulomb’s constant, \( q_1 \) and \( q_2 \) are charges of ions, \( r \) is the distance between them, and \( A \) and \( B \) are constants for van der Waals interactions. This strong interaction facilitates the dissolution of perovskite precursors and influences crystal growth, ultimately leading to improved film quality in perovskite solar cells.
Classification of Ionic Liquids
Ionic liquids can be categorized based on their cationic structures, each offering distinct advantages for perovskite solar cells. The table below outlines common types of ionic liquids and their typical applications in perovskite solar cells.
| Type | Common Cations | Common Anions | Key Applications in Perovskite Solar Cells |
|---|---|---|---|
| Imidazolium-based | 1-alkyl-3-methylimidazolium | BF₄⁻, PF₆⁻, I⁻, Br⁻ | Defect passivation, enhanced conductivity, and stability improvement. |
| Pyridinium-based | N-alkylpyridinium | BF₄⁻, Cl⁻, TFSI⁻ | Grain growth optimization and hydrophobicity for moisture resistance. |
| Ammonium-based | Tetraalkylammonium | Halides, HSO₄⁻, RCOO⁻ | Crystallization control and interface modification. |
| Phosphonium-based | Tetraalkylphosphonium | TFSI⁻, Br⁻, BF₄⁻ | Thermal stability and charge transport enhancement. |
| Pyrrolidinium-based | N-alkyl-N-methylpyrrolidinium | Br⁻, BF₄⁻, TFSI⁻ | Chemical stability and defect passivation. |
| Piperidinium-based | N-alkyl-N-methylpiperidinium | Br⁻, BF₄⁻, TFSI⁻ | Similar to pyrrolidinium, with improved structural integrity. |
| Functionalized ILs | Custom cations/anions with specific groups (e.g., -CN, -NH₂) | Varied (e.g., BF₄⁻, TFSI⁻) | Targeted defect passivation and crystallization tuning. |
| Polymeric ILs (PILs) | Polymerized ionic liquid monomers | Varied (e.g., TFSI⁻) | Mechanical robustness, defect suppression, and strain relief. |
The diversity in ionic liquid structures allows for precise control over perovskite solar cell properties. For instance, the Hammett acidity constant (\( \sigma \)) can be used to predict the effectiveness of functionalized ionic liquids in passivating defects:
$$ \sigma = \log \left( \frac{K_a}{K_a^0} \right) $$
where \( K_a \) and \( K_a^0 \) are acid dissociation constants of substituted and unsubstituted ions, respectively. This helps in designing ionic liquids with optimal electron-donating or withdrawing groups for interacting with perovskite components.
Applications in the Perovskite Active Layer
Ionic liquids play a crucial role in optimizing the perovskite active layer, which is the core component responsible for light absorption and charge generation in perovskite solar cells. Their applications include serving as solvents, additives, and defect passivators, all of which contribute to enhanced performance and stability of perovskite solar cells.
Ionic Liquids as Solvents for Perovskite Precursors
Traditional solvents like DMF and DMSO are toxic and volatile, posing environmental and health risks. Ionic liquids offer a greener alternative due to their low vapor pressure and high solvation power. When used as single solvents or co-solvents, ionic liquids can dissolve perovskite precursors such as PbI₂ and methylammonium iodide (MAI), forming intermediate complexes that guide crystallization. For example, methylammonium formate (MAFa) interacts strongly with PbI₂ via formate anions (HCOO⁻), leading to vertically aligned perovskite films with reduced pinholes and improved coverage. The dissolution process can be modeled using the coordination number (\( N \)) between Pb²⁺ and solvent molecules:
$$ N = \frac{4\pi \rho}{3} \int_0^{r_{\text{min}}} r^2 g(r) \, dr $$
where \( \rho \) is the number density, \( r_{\text{min}} \) is the distance to the first minimum in the radial distribution function \( g(r) \), and \( r \) is the interatomic distance. This equation highlights how ionic liquids enhance solubility and control nucleation in perovskite solar cell fabrication.
In co-solvent systems, ionic liquids like methylammonium acetate (MAAc) mixed with DMSO slow down crystallization kinetics, resulting in larger grains and fewer defects. The viscosity (\( \eta \)) of such mixtures influences the film formation process:
$$ \eta = \eta_0 \exp\left( \frac{E_a}{RT} \right) $$
where \( \eta_0 \) is the pre-exponential factor, \( E_a \) is the activation energy for flow, \( R \) is the gas constant, and \( T \) is temperature. Higher viscosity in ionic liquid-based solutions delays solvent evaporation, promoting uniform perovskite layers in perovskite solar cells.
Ionic Liquids as Additives for Crystallization Control
As additives, ionic liquids modulate perovskite crystallization by forming intermediate phases with precursors. For instance, imidazolium-based ionic liquids with functional groups (e.g., -C≡N) coordinate with Pb²⁺ to create chain-like complexes that retard crystal growth. This leads to dense, pinhole-free films with enhanced optoelectronic properties. The crystallization rate (\( k \)) can be expressed using the Avrami equation:
$$ X(t) = 1 – \exp(-k t^n) $$
where \( X(t) \) is the fraction crystallized at time \( t \), and \( n \) is the Avrami exponent. Ionic liquids increase \( k \) and \( n \), indicating faster and more ordered crystallization, which is beneficial for perovskite solar cell efficiency.
Moreover, ionic liquids like 1-ethyl-3-methylimidazolium sulfate (EMIMHSO₄) release lattice strain and reduce trap densities in all-inorganic perovskite solar cells. The strain energy (\( U \)) per unit volume can be calculated as:
$$ U = \frac{1}{2} \sigma \epsilon $$
where \( \sigma \) is stress and \( \epsilon \) is strain. By minimizing \( U \), ionic liquids improve the mechanical stability of perovskite films, contributing to the longevity of perovskite solar cells.
Defect Passivation by Ionic Liquids
Defects in perovskite layers, such as iodine vacancies (\( V_I \)) and uncoordinated Pb²⁺ ions, act as non-radiative recombination centers, reducing the performance of perovskite solar cells. Ionic liquids passivate these defects through chemical interactions. For example, anions like Br⁻ in 1-ethyl-3-methylimidazolium bromide ([EMIM]Br) fill \( V_I \) sites, while cations with lone pairs (e.g., imidazolium) coordinate with Pb²⁺. The passivation efficiency can be quantified by the trap density (\( N_t \)) reduction:
$$ N_t = N_{t0} \exp\left( -\frac{E_b}{kT} \right) $$
where \( N_{t0} \) is the initial trap density, \( E_b \) is the binding energy between the ionic liquid and defect, \( k \) is Boltzmann’s constant, and \( T \) is temperature. Higher \( E_b \) values indicate stronger passivation, leading to better open-circuit voltages and fill factors in perovskite solar cells.
Functionalized ionic liquids with multiple groups (e.g., ammonium and lactate in guanidinium lactate) simultaneously passivate cation and anion defects. The overall defect passivation energy (\( \Delta G_{\text{pass}} \)) can be estimated using:
$$ \Delta G_{\text{pass}} = -RT \ln(K_{\text{eq}}) $$
where \( K_{\text{eq}} \) is the equilibrium constant for the passivation reaction. Negative \( \Delta G_{\text{pass}} \) values spontaneous reactions, enhancing the stability and efficiency of perovskite solar cells.
Applications in Interface Functional Layers
Interfaces between the perovskite layer and charge transport layers (CTLs) are critical for efficient charge extraction and minimizing recombination in perovskite solar cells. Ionic liquids serve as interface modifiers and standalone CTLs, optimizing energy level alignment and reducing losses.
Ionic Liquids as Interface Modification Materials
When applied at interfaces, ionic liquids form dipole layers that adjust work functions and passivate surface defects. For instance, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) on TiO₂ electron transport layers (ETLs) lowers the work function from 4.26 eV to 4.01 eV, improving electron injection into perovskite solar cells. The work function change (\( \Delta \Phi \)) can be related to the dipole moment (\( \mu \)):
$$ \Delta \Phi = \frac{e \mu \cos \theta}{\epsilon_0 A} $$
where \( e \) is electron charge, \( \theta \) is the tilt angle, \( \epsilon_0 \) is vacuum permittivity, and \( A \) is area. This alignment reduces energy barriers, boosting the performance of perovskite solar cells.
Similarly, ionic liquids like BMIMPF₆ on SnO₂ ETLs enhance hydrophobicity and uniformity, leading to better perovskite growth. The interfacial tension (\( \gamma \)) between layers can be modeled using:
$$ \gamma = \gamma_0 – k C $$
where \( \gamma_0 \) is the initial tension, \( k \) is a constant, and \( C \) is ionic liquid concentration. Lower \( \gamma \) values promote smoother interfaces, reducing recombination in perovskite solar cells.
In hole transport layers (HTLs), ionic liquids such as H-TFSI dope Spiro-OMeTAD, increasing conductivity and stability. The doping efficiency (\( \eta_d \)) can be expressed as:
$$ \eta_d = \frac{\sigma_{\text{doped}} – \sigma_{\text{undoped}}}{\sigma_{\text{undoped}}} \times 100\% $$
where \( \sigma \) is conductivity. Higher \( \eta_d \) values correlate with improved fill factors and reduced hysteresis in perovskite solar cells.
Ionic Liquids as Independent Charge Transport Layers
Ionic liquids can function as standalone ETLs or HTLs, eliminating the need for high-temperature processed metal oxides. For example, solid-state ionic liquids like 1-benzyl-3-methylimidazolium chloride (ss-IL) exhibit high electron mobility (\( \mu_e \)):
$$ \mu_e = \frac{\sigma}{n e} $$
where \( n \) is charge carrier density and \( e \) is electron charge. Values up to \( 10^{-3} \, \text{cm}^2 \, \text{V}^{-1} \, \text{s}^{-1} \) have been reported, comparable to conventional materials, enabling flexible and efficient perovskite solar cells.
Moreover, ionic liquids like MAAc on ITO electrodes create dipole layers that facilitate electron extraction without additional ETLs. The power conversion efficiency (PCE) of such devices can exceed 21%, demonstrating the potential of ionic liquids in simplifying perovskite solar cell architectures.
Stability Enhancement through Ionic Liquids
Stability is a paramount concern for perovskite solar cells, and ionic liquids address this through multiple mechanisms, including hydrophobicity, phase stabilization, and interface protection. The table below summarizes key studies where ionic liquids improved the stability of perovskite solar cells.
| Ionic Liquid | Device Structure | Strategy | PCE (%) | Stability Improvement |
|---|---|---|---|---|
| MA+TFA⁻ | FTO/TiO₂/MAPbI₃/Spiro-OMeTAD/Au | Hydrophobicity enhancement | 20.1 | 336 h at 40% RH, 80% PCE retention |
| FIm | FTO/c-TiO₂/m-TiO₂/Perovskite/Spiro-OMeTAD | Moisture resistance | 16.32 | 33 days at 55-60% RH, 90% PCE retention |
| HMII | FTO/SnO₂/FAPbI₃/C₆₀/BCP/Ag | Grain boundary engineering | 24.09 | 300 h at 50% RH, 90.1% PCE retention |
| MA+DFA⁻ | ITO/CPTA/Perovskite/Spiro-OMeTAD/Ag | Phase stabilization | 21.46 | 180 days in N₂, 85% PCE retention |
| BMIBr | FTO/TiO₂/MAPbI₃/Spiro-OMeTAD/Au | Thermal stability | 19.3 | 50 min at 80°C, 85% PCE retention |
| DMIMPF₆ | FTO/TiO₂/Cs₀.₀₈FA₀.₉₂PbI₃/Spiro-OMeTAD/Au | Interface passivation | 23.25 | 480 h at 45% RH, 89% PCE retention |
The degradation rate of perovskite solar cells can be modeled using the Arrhenius equation for thermal stability:
$$ k_{\text{deg}} = A \exp\left( -\frac{E_a}{RT} \right) $$
where \( k_{\text{deg}} \) is the degradation rate constant, \( A \) is the pre-exponential factor, and \( E_a \) is the activation energy for degradation. Ionic liquids increase \( E_a \), slowing down degradation and extending the lifespan of perovskite solar cells.
For humidity stability, the water contact angle (\( \theta_c \)) is a key indicator:
$$ \cos \theta_c = \frac{\gamma_{sv} – \gamma_{sl}}{\gamma_{lv}} $$
where \( \gamma_{sv} \), \( \gamma_{sl} \), and \( \gamma_{lv} \) are solid-vapor, solid-liquid, and liquid-vapor surface tensions, respectively. Ionic liquids with hydrophobic groups increase \( \theta_c \), reducing water penetration and enhancing the durability of perovskite solar cells.
Conclusion and Future Perspectives
Ionic liquids have proven to be versatile and effective materials for addressing the challenges in perovskite solar cells, including efficiency, stability, and environmental impact. Their unique properties enable precise control over crystallization, defect passivation, interface engineering, and charge transport, leading to significant improvements in the performance of perovskite solar cells. As solvents, additives, and modifiers, ionic liquids facilitate the fabrication of high-quality perovskite layers with reduced toxicity. Moreover, their application in interface functional layers enhances energy level alignment and reduces recombination losses.
Looking ahead, several research directions hold promise for further advancing the role of ionic liquids in perovskite solar cells. First, a deeper understanding of the interaction mechanisms between ionic liquids and perovskite components is needed, potentially through in-situ characterization and computational modeling. Second, the development of novel ionic liquids with tailored functional groups could optimize defect passivation and stability. Third, scaling up ionic liquid-based processes for large-area perovskite solar cell manufacturing requires attention to uniformity and cost-effectiveness. Finally, exploring ionic liquids in tandem perovskite solar cells and other emerging photovoltaic architectures could unlock new efficiencies.
In conclusion, ionic liquids represent a powerful tool for realizing high-performance, stable, and green perovskite solar cells. With continued innovation, they are poised to play a pivotal role in the commercialization and widespread adoption of perovskite solar cell technology.