In my analysis of the rapidly evolving field of photovoltaic technology, the perovskite solar cell has emerged as one of the most promising candidates for next-generation energy conversion due to its exceptional optoelectronic properties, tunable bandgap, and low-cost solution processability. The power conversion efficiency (PCE) of perovskite solar cells has now surpassed 27%, approaching the Shockley-Queisser limit for single-junction devices. However, despite this remarkable progress, the long-term operational stability and charge transport efficiency of perovskite solar cells remain critical bottlenecks that hinder their large-scale commercialization. I have observed that the soft ionic lattice of organic-inorganic lead halide perovskites is prone to generating abundant defects during rapid crystallization, including Schottky defects and Frenkel defects, which significantly promote ion migration and degrade device performance. To address these challenges, I focus my review on a class of organometallic molecules—ferrocene (Fc) and its derivatives—that have demonstrated extraordinary potential as additives, interface modifiers, and charge transport materials in perovskite solar cells.
Ferrocene is a prototypical metallocene featuring a sandwich structure in which an iron atom is coordinated between two cyclopentadienyl anions. This unique arrangement endows ferrocene with remarkable thermal stability, excellent resistance to air and moisture, and significant electrochemical activity. The Fe–cyclopentadienyl bond dissociation energy is comparable to that of covalent C–C bonds, providing ferrocene with outstanding chemical robustness. The Fe(II) center located between the two parallel cyclopentadienyl rings enables reversible oxidation, facilitating a wide range of redox reactions. I note that the Fc/Fc\(^+\) redox couple has a well-defined reversible potential, which is central to its defect passivation and charge transfer capabilities. Because the electronic structure of ferrocene can be finely tuned through functionalization, Fc derivatives can construct more rational energy level alignments and internal electric fields at the perovskite/transport layer interface, enabling band offset optimization and barrier modulation. This promotes effective separation and directional transport of photogenerated carriers rather than merely modulating the bandgap.
In my review, I systematically cover the latest advances of ferrocene monomers and various functionalized ferrocene derivatives in bulk phase regulation, surface defect passivation, and heterointerface engineering of perovskite solar cells. I analyze their impact on key device parameters, including short-circuit current density (\(J_{SC}\)), open-circuit voltage (\(V_{OC}\)), fill factor (FF), and long-term stability. I propose that future research should focus on precise functional group regulation, in-situ characterization techniques, and low-cost scalable molecular design to promote the development of high-efficiency, stable, and commercializable perovskite solar cells based on ferrocene systems.
Fundamental Properties and Redox Chemistry of Ferrocene
The unique sandwich structure of ferrocene gives rise to its exceptional properties. I can express the redox equilibrium of ferrocene as:
$$ \text{Fc} \rightleftharpoons \text{Fc}^+ + e^- $$
where the standard redox potential \(E^\circ(\text{Fc}^+/\text{Fc})\) is approximately 0.40 V versus the saturated calomel electrode in acetonitrile, but can be tuned by substituents. The reversible nature of this redox couple allows ferrocene to act as an electron shuttle, accepting and donating electrons without irreversible decomposition. This is crucial for defect passivation in perovskite solar cells because defects such as Pb\(^0\) and I\(^0\) can be healed through cyclic electron transfer. The Fe(II) center can be oxidized to Fe(III) under suitable conditions, and the resulting ferrocenium ion (Fc\(^+\)) can participate in further reactions. The thermodynamic stability of ferrocene is reflected in its high decomposition temperature (above 400 °C) and resistance to acid, base, and UV radiation. These characteristics make ferrocene and its derivatives ideal for interface engineering in perovskite solar cells, where harsh environmental stress and photothermal conditions prevail.
I can summarize the key physical properties of ferrocene in Table 1.
| Property | Value/Description |
|---|---|
| Molecular formula | Fe(C\(_5\)H\(_5\))\(_2\) |
| Structure | Sandwich, eclipsed D\(_{5h}\) |
| Fe–Cp bond dissociation energy | ~4.0 eV (comparable to C–C) |
| Redox potential \(E^\circ\)(Fc\(^+\)/Fc) | ~0.40 V vs. SCE |
| Thermal decomposition temperature | >400 °C |
| Solubility | Nonpolar solvents; functionalized derivatives can be polar |
| Electron transfer | Reversible, outer-sphere |
When incorporated into a perovskite solar cell, ferrocene and its derivatives can function as defect passivators, interface modifiers, and stability enhancers. I will now discuss the mechanistic roles in detail.
Defect Passivation Mechanisms in Perovskite Solar Cells
The soft ionic lattice of perovskite solar cells inevitably contains various defects. Schottky defects involve equal missing of A-site cations and X-site anions, leading to excess PbI\(_2\). Frenkel defects include single-ion vacancies at A, B, and X sites. These defects create deep trap states that facilitate non-radiative recombination and ion migration. Ferrocene and its derivatives address these issues through a unique perpetual recovery cycle. I can describe the chemical reactions for defect healing using ferrocene as follows:
$$ 2\text{Fc} + \text{I}_2 \rightarrow 2\text{FcI} $$
$$ 2\text{FcI} + \text{Pb}^0 \rightarrow \text{PbI}_2 + 2\text{Fc} $$
$$ \text{FcI} + \text{PbI}_2 \rightarrow \text{FcPbI}_3 $$
$$ \text{FcPbI}_3 + \text{MA}^+ \rightarrow \text{MAPbI}_3 + \text{Fc} $$
These reactions form a two-step cycle that continuously regenerates ferrocene while converting defects into active light-absorbing sites. The one-dimensional perovskite FcPbI\(_3\) has appropriate steric structure and optimal dissociation energy, allowing the repair of A-site, B-site, and X-site defects simultaneously. I find this mechanism particularly elegant because it not only passivates defects but also recycles the passivating agent, enabling exceptional longevity. In one study, perovskite solar cells with ferrocene-based healing achieved lifetimes exceeding 10,000 hours under inert conditions. The redox potential of the Fc/Fc\(^+\) couple allows charge transfer between I\(^0\) (X-site) and Pb\(^0\) (B-site) defects, effectively oxidizing I\(^0\) back to I\(^-\) and reducing Pb\(^0\) to Pb\(^{2+}\). This dual action reduces deep trap densities and suppresses non-radiative recombination.
Furthermore, lithium ions from doped hole transport layers such as Spiro-OMeTAD can migrate into the perovskite layer, causing degradation. Ferrocene interlayers can immobilize Li\(^+\) through interaction with the cyclopentadienyl rings. I can express the adduct formation as:
$$ \text{Fc} + \text{Li}^+ \rightarrow [\text{Fc}\cdot\text{Li}]^+ $$
This interaction prevents Li\(^+\) accumulation at the perovskite/HTL interface, leading to more uniform distribution and slower migration. The ferrocene layer also acts as a barrier against oxygen and moisture, further enhancing stability.
Interface Engineering and Charge Dynamics
In perovskite solar cells, the interfaces between the perovskite absorber and charge transport layers play a decisive role in determining device performance. Ferrocene derivatives can form stable interfacial layers that reduce interface defects and optimize charge transport channels. They promote strong interactions between the perovskite layer and charge transport materials such as PEDOT:PSS, PTAA, and self-assembled monolayers. By adjusting the interfacial energy level structure, ferrocene derivatives help optimize current density and open-circuit voltage. I can express the PCE as:
$$ \text{PCE} = \frac{J_{SC} \times V_{OC} \times \text{FF}}{P_{in}} $$
where \(P_{in}\) is the incident power density (typically 100 mW·cm\(^{-2}\) under AM1.5G). Ferrocene derivatives can increase \(V_{OC}\) by reducing non-radiative recombination, increase \(J_{SC}\) by improving charge extraction, and increase FF by reducing series resistance and improving shunt resistance. The energy level alignment can be described by the Schottky barrier height:
$$ \phi_B = \phi_M – \chi_S $$
where \(\phi_M\) is the metal work function and \(\chi_S\) is the semiconductor electron affinity. Ferrocene derivatives can modify the effective work function of transport layers, reducing the barrier and facilitating ohmic contact.
I summarize representative ferrocene derivatives and their functions in perovskite solar cells in Table 2.
| Year | Fc Type | Device Structure | Function | \(J_{SC}\) (mA·cm\(^{-2}\)) | \(V_{OC}\) (V) | FF (%) | PCE (%) |
|---|---|---|---|---|---|---|---|
| 2021 | Fc | FTO/SnO\(_2\)/MAPbI\(_3\)(Fc)/Spiro-OMeTAD/Au | Defect passivation, stability enhancement | 24.11 | 1.144 | 78.01 | 21.90 |
| 2022 | FcTc\(_2\) | ITO/PTAA/Cs\(_{0.05}\)(FA\(_{0.98}\)MA\(_{0.02}\))\(_{0.95}\)Pb(I\(_{0.98}\)Br\(_{0.02}\))\(_3\)/FcTc\(_2\)/C\(_{60}\)/BCP/Ag | Enhanced charge transport, stability | 25.68 | 1.184 | 82.32 | 25.00 |
| 2023 | DBzFe | FTO/SAMs/FA\(_{0.95}\)Cs\(_{0.05}\)PbI\(_3\)(DBzFe)/C\(_{60}\)/BCP/Ag | Suppress ion migration, improve film quality | 24.95 | 1.15 | 82.00 | 24.95 |
| 2023 | Fc-16F | FTO/TiO\(_2\)/Cs\(_{0.05}\)FA\(_{0.85}\)MA\(_{0.10}\)Pb(I\(_{0.97}\)Br\(_{0.03}\))\(_3\)/Fc-16/Spiro-OMeTAD/Ag | Suppress ion migration, redox activity | 25.18 | 1.128 | 81.02 | 23.02 |
| 2023 | DmFc | ITO/PEDOT:PSS/perovskite(DmFc)/PCBM/Ag | Improve crystallinity | 21.95 | 0.99 | 77.88 | 16.92 |
| 2024 | FcA | FTO/TiO\(_2\)/Cs\(_{0.03}\)FA\(_{0.95}\)MA\(_{0.02}\)Pb(I\(_{0.98}\)Br\(_{0.02}\))\(_3\)/FcA/Spiro-OMeTAD/Au | Defect passivation, enhanced charge transport | 24.96 | 1.140 | 81.06 | 23.06 |
| 2024 | OFFcA | FTO/TiO\(_2\)/Cs\(_{0.03}\)FA\(_{0.95}\)MA\(_{0.02}\)Pb(I\(_{0.98}\)Br\(_{0.02}\))\(_3\)/OFFcA/Spiro-OMeTAD/Au | Improve film morphology | 25.12 | 1.147 | 81.97 | 23.62 |
| 2024 | FAAT | ITO/FAAT/MAPbI\(_3\)/PCBM/Bphen/Al | Enhance charge separation and transport | 23.14 | 1.101 | 74.81 | 19.05 |
| 2024 | FcTTPc | ITO/Me-4PACz/perovskite/FcTTPc/PCBM/BCP/Ag | Defect passivation, band alignment | 25.68 | 1.184 | 82.32 | 25.00 |
| 2024 | Fc\(_2\)Tc\(_2\) | ITO/PTAA/perovskite/Fc\(_2\)Tc\(_2\)/C\(_{60}\)/BCP/Ag | Surface activation, charge dynamics enhancement | 25.93 | 1.194 | 84.24 | 26.08 |
| 2024 | DPPF | FTO/NiO\(_x\)/perovskite/DPPF/PCBM/BCP/Ag | Defect passivation, interface enhancement | 23.43 | 1.06 | 83.57 | 20.82 |
| 2024 | FDA-PVC | ITO/SnO\(_2\)/MAPbI\(_3\)/FDA-PVC/Spiro-OMeTAD/AgAl | Band alignment, enhanced photovoltaic performance | 24.05 | 1.10 | 78.02 | 20.67 |
| 2024 | FcP\(_2\) | glass/FTO/SnO\(_2\)/perovskite/FcP\(_2\)/Spiro-OMeTAD/Au | Enhanced charge transport, improved morphology | 26.02 | 1.144 | 83.3 | 24.80 |
| 2024 | FcAD | ITO/PEDOT:PSS/FcAd/quasi-2D perovskite/TPBi/LiF/Al | Phase regulation, defect passivation | — | — | — | — |
| 2024 | FcPF\(_6\) | FTO/ZnTiO\(_3\)/perovskite/FcPF\(_6\)/Spiro-OMeTAD/Au | Suppress ion migration, enhance hole transport | 25.60 | 1.18 | 82.71 | 25.02 |
| 2025 | FcFk\(_2\) | ITO/PEDOT:PSS/BHJ/FcFk\(_2\)/Ag | Regulate interfacial electronic structure | 27.10 | 0.907 | 80.10 | 19.70 |
| 2025 | HEFc | FTO/HTLs/perovskite(HEFc)/PCBM/Ag | Suppress non-radiative recombination | 25.55 | 1.194 | 84.26 | 25.70 |
From this table, I observe that the introduction position and molecular structure of Fc derivatives significantly influence the performance enhancement pathway. When ferrocene molecules act as bulk additives or surface passivation layers, they mainly reduce defect state density and suppress non-radiative recombination, thereby improving \(V_{OC}\) and stability. When used as interface modifiers or transport layer regulators, they optimize energy level matching and carrier extraction efficiency, improving FF and \(J_{SC}\). High-efficiency devices (PCE > 25%) often employ Fc derivatives with multiple functional groups (e.g., FcTc\(_2\), FcTTPc, and Fc\(_2\)Tc\(_2\)) that simultaneously achieve defect passivation and interface charge dynamics optimization. This indicates that molecular structure complexity and interfacial synergistic regulation are important directions for efficiency improvement.
Single-Side Functionalized Ferrocene Derivatives
Single-side functionalization of ferrocene introduces functional groups such as carboxyl, hydroxyl, and amino groups. Common derivatives include ferrocene carboxylic acid (FcA), 3-ferrocenyl acrylic acid (FcUnA), 3-ferrocenyl propionic acid (FcSaA), ferrocene carboxaldehyde (FcCHO), (dimethylaminomethyl)ferrocene (FcAm), and ferrocenemethanol (FcCH\(_2\)OH/FcMeOH). These derivatives can be further modified by covalent attachment of amino acid residues to form polar –COOH head groups. Fluorinated ferrocenes yield derivatives such as octafluoropentyl ferrocene carboxylate (OFFcA). A molecular fluorination strategy with long perfluoroalkyl chains produces bis-ferrocene carboxylate hexadecafluorodecyl (Fc-16F). The carboxyl group coordinates with undercoordinated Pb\(^{2+}\) on the perovskite surface, while the ferrocene core provides redox activity. The fluorinated chains enhance hydrophobicity and moisture resistance.
For example, OFFcA, synthesized from FcA, acts as an interstitial conductive medium to regulate the heterointerface between the perovskite layer and the hole transport layer. The synergistic effect between the C=O group and F substituents effectively eliminates Pb defects and excess PbI\(_2\). The F substituents enhance the oxidation ability of the Fc/Fc\(^+\) couple, promoting defect passivation. Devices modified with OFFcA retained 82.7% of their initial PCE after 50 days at 60% relative humidity, compared to only 28.3% for control devices.
Double-Side Functionalized and Polymer Ferrocene Derivatives
Double-side functionalization involves symmetric modification with two functional groups. Examples include ferrocene-bisthiophene-2-carboxylate (FcTc\(_2\)), 1,1′-dimethylferrocene (DmFc), dibenzoylferrocene (DBzFe), 1,1′-bis(diphenylphosphino)ferrocene (DPPF/FcP\(_2\)), and the newly designed FcTTPc with carboxylate and thiophene groups around a central Fc core. Polyferrocene molecules such as Fc\(_2\)Tc\(_2\) and tri-ferrocene-bisthiophene-2-carboxylate Fc\(_3\)Tc\(_2\) have also been applied. These molecules enable multi-directional defect passivation and have shown remarkable improvements in perovskite solar cell performance.
FcTc\(_2\) interacts strongly with the perovskite surface through thiophene groups that coordinate with lead, reducing interface defects. Thiophene also forms π-π stacking interactions with C\(_{60}\) and PCBM, enhancing charge transport. The certified PCE of FcTc\(_2\)-treated devices reached 24.3% with over 98% initial PCE retention. FcTTPc, with carboxylate and thiophene functional groups, optimizes the perovskite/electron transport layer interface. The carboxylate and thiophene groups reduce interface defects, while thiophene forms π-π interactions with the electron transport layer, improving charge transport and energy level alignment.
DPPF, an anti-UV ferrocene derivative, interacts with uncoordinated Pb\(^{2+}\) to reduce defect density and enhance electron transport. The hydrophobic phenyl groups passivate defects and protect the active layer from moisture. Unencapsulated devices retained over 80% of initial PCE after 1,400 hours at 25 °C and 50–60% relative humidity. DmFc, introduced in a volatile mixed solvent system, enables one-step ambient air film formation. Its reversible DmFc/DmFc\(^+\) redox couple repairs intrinsic defects such as I\(^0\) and Pb\(^0\), reducing trap density and enlarging grain size.
DBzFe as an additive enables efficient and stable methylammonium-free perovskite solar cells. It passivates defects, improves film quality, and suppresses ion migration. The strong chemical interaction between DBzFe and perovskite reduces defect density and non-radiative recombination, while relieving film stress. Devices achieved 23.5% PCE with high stability.
FcP\(_2\) employs a chemical bridging strategy, anchoring and coordinating Pb atoms through strong Pb–P bonds. This passivates surface and grain boundary defects. The delocalized electronic nature of FcP\(_2\) promotes balanced interfacial charge transport. Density functional theory confirms effective interaction with the PbI\(_2\)-terminated FAPbI\(_3\) surface. The n-i-p perovskite solar cell achieved 25% PCE and retained over 95% initial PCE after 1,000 hours of continuous operation under 1 sun illumination.
FAAT (ferrocene acetylacetonate) is a novel hole transport material candidate. It possesses appropriate energy levels and high hole mobility. Devices based on FAAT achieved 19.05% PCE, demonstrating potential as a low-cost hole transport material. FAAT modification of CsPbBr\(_3\) quantum dots significantly increased hole transfer rates.
I summarize the functional groups and their roles in Table 3.
| Fc Derivative | Functional Groups | Primary Role | Key Mechanism |
|---|---|---|---|
| FcA | Carboxyl | Defect passivation | Coordination with Pb\(^{2+}\), redox cycling |
| FcUnA | Acrylic acid | Interface modification | Carboxyl binding, film quality improvement |
| FcSaA | Propionic acid | Passivation | Carboxyl coordination, stability enhancement |
| FcCHO | Aldehyde | Surface modification | Chemical interaction with defects |
| FcAm | Dimethylaminomethyl | Interface dipole | Energy level tuning |
| FcCH\(_2\)OH | Hydroxyl | Passivation | Hydrogen bonding with halides |
| OFFcA | Fluorinated carboxyl | Moisture resistance | Hydrophobic barrier, enhanced redox |
| Fc-16F | Perfluoroalkyl chain | Ion migration suppression | Hydrophobic, redox activity |
| FcTc\(_2\) | Bisthiophene carboxylate | Charge transport | Pb coordination, π-π stacking |
| DmFc | Dimethyl | Crystallinity improvement | Redox repair, grain enlargement |
| DBzFe | Dibenzoyl | Ion migration suppression | Chemical interaction, stress relief |
| DPPF | Bis(diphenylphosphino) | UV resistance | Pb\(^{2+}\) coordination, hydrophobic phenyls |
| FcP\(_2\) | Phosphine | Chemical bridging | Pb–P bonds, delocalized electrons |
| FcTTPc | Carboxylate, thiophene | Interface regulation | Pb coordination, π-π with ETL |
| Fc\(_2\)Tc\(_2\) | Polyferrocene bisthiophene | Surface activation | Charge dynamics, defect passivation |
| Fc\(_3\)Tc\(_2\) | Tri-ferrocene bisthiophene | Interface engineering | Multi-site passivation |
Stability Enhancement and Ion Migration Suppression
Ion migration in perovskite solar cells, primarily of MA\(^+\) and I\(^-\), originates from the weak ionic structure and low activation energy, exacerbated by defects such as excess PbI\(_2\), Pb, and I vacancies. Under light, electric field, or heat, I\(^-\) and MA\(^+\) can easily overcome migration barriers, leading to lattice rearrangement and oxidation-reduction reactions with oxygen and photogenerated holes. This causes local lattice collapse and hysteresis. Ferrocene derivatives effectively suppress ion migration through multiple mechanisms.
First, the redox-active Fc/Fc\(^+\) couple can repeatedly heal defects, reducing the channels for ion migration. Second, functional groups such as carboxyl, phosphine, and thiophene strongly coordinate with undercoordinated Pb\(^{2+}\), immobilizing surface ions. Third, fluorinated chains and phenyl groups create hydrophobic barriers that prevent moisture and oxygen ingress. Fourth, ferrocene interlayers can immobilize Li\(^+\) from doped hole transport layers, preventing Li\(^+\) accumulation at interfaces.
For Sn-Pb mixed perovskite solar cells, Sn\(^{2+}\) oxidation to Sn\(^{4+}\) is a critical stability issue. A ferrocene capping layer creates a reducing environment due to the electron-donating π orbitals of cyclopentadienyl, suppressing Sn\(^{2+}\) oxidation. The Fc layer also acts as a barrier against oxygen and moisture. Narrow-bandgap Sn-Pb mixed perovskite solar cells with a ferrocene capping layer achieved 21.02% PCE and significantly improved photostability and storage stability.
I can express the oxidation of Sn\(^{2+}\) and its suppression by ferrocene as:
$$ \text{Sn}^{2+} \rightarrow \text{Sn}^{4+} + 2e^- $$
$$ \text{Fc} \rightarrow \text{Fc}^+ + e^- $$
$$ \text{Sn}^{4+} + 2\text{Fc} \rightarrow \text{Sn}^{2+} + 2\text{Fc}^+ $$
The ferrocene capping layer thus acts as a sacrificial reductant, continuously protecting Sn\(^{2+}\) from oxidation. This redox buffering effect is a unique advantage of ferrocene-based materials in perovskite solar cells.
Cross-Scale Mechanistic Framework
Based on my systematic analysis, I propose a cross-scale mechanistic framework for ferrocene-engineered perovskite solar cells. At the molecular scale, the reversible Fc/Fc\(^+\) redox couple and tunable functional groups provide the structural basis for defect coordination, electron donation, and dipole modulation. At the interface scale, ferrocene molecules form chemical bonds or π-π interactions with undercoordinated Pb\(^{2+}\), halide vacancies, and transport layer materials. This leads to energy level matching, charge transport channel reconstruction, and ion migration suppression. At the device scale, these effects manifest as simultaneous improvements in open-circuit voltage, fill factor, and long-term operational stability. The overall pathway can be summarized as:
$$ \text{Molecular Design} \rightarrow \text{Interface Electronic and Defect State Regulation} \rightarrow \text{Macroscopic Photovoltaic Performance Enhancement} $$
This molecular–interface–device hierarchical coupling model provides a unified framework for understanding the role of organometallic molecules in perovskite photovoltaics. I believe this framework can guide the rational design of next-generation ferrocene derivatives for perovskite solar cells.
Comparative Analysis of Device Parameters
To further quantify the impact of ferrocene derivatives, I analyze the statistical distribution of device parameters. The PCE improvement can be decomposed into contributions from \(J_{SC}\), \(V_{OC}\), and FF. I define the relative enhancement as:
$$ \Delta \text{PCE} = \frac{\text{PCE}_{\text{modified}} – \text{PCE}_{\text{control}}}{\text{PCE}_{\text{control}}} \times 100\% $$
$$ \Delta J_{SC} = \frac{J_{SC,\text{modified}} – J_{SC,\text{control}}}{J_{SC,\text{control}}} \times 100\% $$
$$ \Delta V_{OC} = \frac{V_{OC,\text{modified}} – V_{OC,\text{control}}}{V_{OC,\text{control}}} \times 100\% $$
$$ \Delta \text{FF} = \frac{\text{FF}_{\text{modified}} – \text{FF}_{\text{control}}}{\text{FF}_{\text{control}}} \times 100\% $$
From the reported data, ferrocene derivatives typically increase \(V_{OC}\) by 30–80 mV and FF by 3–8% absolute, while \(J_{SC}\) improvements are more modest (0.5–2 mA·cm\(^{-2}\)). The most significant PCE gains arise from simultaneous \(V_{OC}\) and FF enhancement, indicating that defect passivation and interface charge transport optimization are the dominant mechanisms. For example, Fc\(_2\)Tc\(_2\) increased PCE from ~23% to 26.08%, corresponding to an 11.6% relative improvement. FcTTPc and FcTc\(_2\) achieved PCEs of 25.00%, with \(V_{OC}\) around 1.184 V and FF above 82%.
I summarize the statistical ranges in Table 4.
| Parameter | Typical Control Range | Typical Modified Range | Relative Improvement |
|---|---|---|---|
| \(J_{SC}\) (mA·cm\(^{-2}\)) | 22–25 | 23–27 | +2–8% |
| \(V_{OC}\) (V) | 1.00–1.12 | 1.05–1.20 | +3–8% |
| FF (%) | 74–80 | 78–85 | +4–7% absolute |
| PCE (%) | 17–23 | 20–26.5 | +10–20% |
Synthesis and Processing Considerations
In my assessment, the practical implementation of ferrocene derivatives in perovskite solar cells depends not only on their functional performance but also on their synthetic accessibility, solubility, and compatibility with device fabrication processes. Most ferrocene derivatives can be synthesized through well-established routes, including Friedel-Crafts acylation, lithiation followed by electrophilic quenching, and palladium-catalyzed cross-coupling reactions. For example, ferrocene carboxylic acid can be prepared by lithiation of ferrocene with n-butyllithium followed by reaction with carbon dioxide. Fluorinated derivatives such as OFFcA and Fc-16F require fluorinated alkyl halides or perfluoroalkyl carboxylic acids as reactants. The synthetic cost and complexity increase with the number and type of functional groups, but the raw materials are generally inexpensive and readily available.
Solubility is a critical parameter for solution-processed perovskite solar cells. Unsubstituted ferrocene is nonpolar and has limited solubility in polar solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), which are commonly used for perovskite precursor solutions. Functionalization with polar groups such as carboxyl, hydroxyl, and amino groups significantly enhances solubility in polar solvents. For interface modification, ferrocene derivatives are often dissolved in chlorobenzene, chloroform, or isopropanol and spin-coated onto the perovskite surface. The concentration typically ranges from 0.5 to 5 mg·mL\(^{-1}\). The choice of solvent must avoid dissolving or damaging the underlying perovskite layer. Orthogonal solvents such as chlorobenzene and toluene are preferred for top-surface modification.
Thermal stability is another important consideration. Ferrocene and many of its derivatives have decomposition temperatures above 300 °C, which is well above the typical annealing temperatures used in perovskite solar cell fabrication (100–150 °C). This ensures that the ferrocene molecules remain intact during device processing. However, some functional groups may undergo thermal degradation at elevated temperatures. For example, carboxyl groups can decarboxylate above 200 °C, and fluoroalkyl chains can decompose at high temperatures. Therefore, I recommend careful control of annealing temperature and time when incorporating ferrocene derivatives.
The deposition position of ferrocene derivatives is also crucial. They can be introduced as additives in the perovskite precursor solution, as an antisolvent additive, as a surface passivation layer, as an interface modifier between the perovskite and charge transport layers, or as a capping layer. Each position leads to different distribution profiles and interaction mechanisms. Bulk additives can passivate defects throughout the film but may disrupt crystallization if not carefully controlled. Surface passivation layers primarily heal top-surface defects and form protective barriers. Interface modifiers between the perovskite and transport layers can optimize energy level alignment and charge extraction. Capping layers provide environmental protection and redox buffering. I have summarized the advantages and disadvantages of each approach in Table 5.
| Incorporation Strategy | Advantages | Disadvantages | Typical Derivatives |
|---|---|---|---|
| Bulk additive | Passivates bulk and grain boundary defects; improves crystallinity | May disrupt perovskite crystallization; uneven distribution | DmFc, DBzFe, Fc |
| Antisolvent additive | Uniform distribution; controls nucleation and growth | Requires careful solvent engineering | HEFc |
| Surface passivation | Heals surface defects; simple processing | Limited to surface; may not affect bulk | FcA, OFFcA, FcTTPc |
| Interface modifier | Optimizes energy level alignment; enhances charge extraction | Requires orthogonal solvent; may increase series resistance | FcTc\(_2\), FcP\(_2\), DPPF |
| Capping layer | Protects against moisture and oxygen; redox buffering | May block charge transport if too thick | Fc, Fc-16F |
I also note that the molecular weight and steric bulk of ferrocene derivatives can influence film morphology. Small molecules like Fc and DmFc can easily diffuse into the perovskite grain boundaries, while larger molecules like polyferrocenes tend to remain at the surface or interface. The optimal molecular design must balance diffusion, passivation, and charge transport properties.
Comparison with Other Organometallic Passivators
To contextualize the advantages of ferrocene derivatives, I compare them with other organometallic passivators used in perovskite solar cells, such as europium (Eu\(^{3+}\)/Eu\(^{2+}\)) and gadolinium (Gd\(^{3+}\)/Gd\(^{2+}\)) complexes, as well as main-group organometallic compounds. The key difference is the reversible Fc/Fc\(^+\) redox couple, which allows continuous regeneration and catalytic defect healing. Eu and Gd complexes also have multiple oxidation states, but their redox potentials are less tunable and their solubility in organic solvents is often poor. Ferrocene derivatives offer superior molecular design flexibility because the cyclopentadienyl rings can be functionalized with a wide range of groups without disrupting the redox center.
Moreover, ferrocene is a neutral, stable, and commercially available compound with low toxicity. In contrast, many rare-earth organometallics are expensive and may have supply chain concerns. The iron center in ferrocene is earth-abundant and biocompatible. Therefore, I consider ferrocene derivatives to be more suitable for scalable and sustainable manufacturing of perovskite solar cells.
Another comparison can be made with organic passivators such as phenethylammonium iodide (PEAI), butylammonium iodide (BAI), and various ammonium salts. These organic molecules effectively passivate defects and form 2D/3D heterostructures, but they lack redox activity. They cannot heal metallic Pb\(^0\) or I\(^0\) defects through electron transfer. Ferrocene derivatives, by contrast, can chemically reduce Pb\(^0\) and oxidize I\(^0\), directly converting detrimental defects into benign or even beneficial species. This dual redox capability is unique and explains the exceptional stability improvements observed in ferrocene-engineered perovskite solar cells.
I can express the defect healing selectivity using the standard reduction potentials. The reduction of Pb\(^{2+}\) to Pb\(^0\) has a standard potential of approximately -0.13 V vs. NHE, while the oxidation of I\(^-\) to I\(^0\) has a standard potential of approximately 0.54 V vs. NHE. The Fc\(^+\)/Fc couple has a potential of about 0.40 V vs. SCE (approximately 0.64 V vs. NHE). Therefore, Fc can reduce I\(^0\) to I\(^-\) and oxidize Pb\(^0\) to Pb\(^{2+}\), both thermodynamically favorable. I can write the cell potentials:
$$ E^\circ(\text{Fc}^+/\text{Fc}) = +0.64 \text{ V vs. NHE} $$
$$ E^\circ(\text{I}^0/\text{I}^-) = +0.54 \text{ V vs. NHE} $$
$$ E^\circ(\text{Pb}^{2+}/\text{Pb}^0) = -0.13 \text{ V vs. NHE} $$
The positive potential difference for I\(^0\) reduction (\(0.64 – 0.54 = 0.10\) V) and for Pb\(^0\) oxidation (\(0.64 – (-0.13) = 0.77\) V) indicates that both reactions are spontaneous. This thermodynamic analysis confirms the unique ability of ferrocene to heal both oxidative and reductive defects simultaneously.
Advanced Characterization and In-Situ Studies
To fully understand the mechanisms of ferrocene derivatives in perovskite solar cells, advanced characterization techniques are essential. I recommend the use of in-situ grazing incidence X-ray diffraction (GIXRD) to monitor crystallization and phase evolution during ferrocene incorporation. In-situ photoluminescence (PL) and time-resolved PL can track defect passivation and non-radiative recombination in real time. X-ray photoelectron spectroscopy (XPS) depth profiling can reveal the distribution of ferrocene molecules and their chemical interactions with perovskite components. Kelvin probe force microscopy (KPFM) can map surface potential and work function changes induced by ferrocene interface layers. Conductive atomic force microscopy (c-AFM) can measure local charge transport at grain boundaries and interfaces.
I also see great value in synchrotron-based techniques such as X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) to probe the electronic structure and oxidation state of the iron center in ferrocene during device operation. In-situ electron paramagnetic resonance (EPR) can detect radical species such as Fc\(^+\) and monitor redox cycling. These advanced techniques will provide unprecedented insights into the dynamic behavior of ferrocene molecules in perovskite solar cells.
For theoretical studies, density functional theory (DFT) calculations can model the adsorption geometry, binding energy, and charge transfer between ferrocene derivatives and perovskite surfaces. Molecular dynamics simulations can reveal diffusion pathways and aggregation behavior. Machine learning interatomic potentials can extend simulations to larger length and time scales. Combining experimental and computational approaches will accelerate the rational design of ferrocene derivatives.
Scalability and Commercialization Prospects
For ferrocene-engineered perovskite solar cells to reach commercial viability, several challenges must be addressed. First, the cost of ferrocene derivatives must be low enough to justify their use in large-area modules. Ferrocene itself is inexpensive, but functionalization adds cost. I estimate that the material cost for a typical surface passivation layer (1 mg·mL\(^{-1}\), 50 μL per cm\(^{2}\)) is on the order of 0.01–0.05 USD per cm\(^{2}\), which is acceptable for high-efficiency solar cells. Second, the deposition process must be compatible with roll-to-roll manufacturing. Spin coating is not suitable for large-area production; instead, slot-die coating, blade coating, or spray coating should be developed. Ferrocene derivatives with high solubility and good film-forming properties are suitable for these scalable techniques. Third, long-term stability must be demonstrated under real-world conditions, including damp heat testing (85 °C, 85% relative humidity) and UV exposure. Ferrocene derivatives with hydrophobic functional groups and redox buffering capabilities are promising for meeting these requirements.
Fourth, environmental health and safety considerations must be evaluated. Ferrocene is generally considered to have low toxicity, but some derivatives may pose hazards. For example, fluorinated compounds can be persistent in the environment. I recommend using biodegradable or easily recoverable functional groups where possible. Life cycle assessment should be conducted to quantify the environmental impact of ferrocene production and disposal.
Fifth, integration with tandem architectures is a promising direction. Ferrocene derivatives can serve as interlayers or recombination layers in perovskite/silicon tandem solar cells. Their redox activity and energy level tunability make them ideal for optimizing the tunnel junction. I expect to see increased research on ferrocene-based interlayers for tandem devices in the coming years.
In summary, the commercialization prospects for ferrocene-engineered perovskite solar cells are promising, but require concerted efforts in molecular design, process engineering, and stability testing. I am optimistic that ferrocene derivatives will become a standard component in high-performance perovskite solar cells.
Future Directions and Molecular Engineering Strategies
In my view, future development of ferrocene molecules for perovskite solar cells will increasingly focus on precise functionalization and multi-scale synergistic molecular design. First, it is necessary to build libraries of functional groups that can target specific defect types and electronic structures. For example, multidentate carboxylic/phosphine ligands can strongly coordinate Pb\(^{2+}\), fluorinated segments can inhibit water and oxygen ingress, and thiophene or heteroaryl rings can construct high π-π stacking interfaces. This will enable fine regulation of defect state density, band bending, and carrier dynamics.
Second, by adjusting the spatial configuration and dipole moment distribution of the ferrocene backbone, it is possible to construct controllable built-in electric fields and gradient polarization environments at the molecular scale. This can further promote exciton dissociation and directional carrier transport, laying the foundation for high-efficiency, low-hysteresis devices.
Third, the design of ferrocene molecules will evolve toward scenario-specific and architecture-adaptive applications. For narrow-bandgap Sn-Pb mixed perovskite solar cells, ferrocene capping layers with both reducing and blocking properties should be prioritized to slow Sn\(^{2+}\) oxidation and suppress deep-level defects. For inverted p-i-n perovskite solar cells and tandem devices, ferrocene-based interlayers or polyferrocene networks with simultaneous energy level alignment, good film formation, and high conductivity are needed to ensure consistency and scalability from small-area devices to large-area modules.
Fourth, combining in-situ spectroscopy, in-situ electron microscopy, and multiscale theoretical calculations can establish predictive models for molecular reaction pathways, interface reconstruction, and long-term failure mechanisms. This can guide high-throughput screening and rational design of ferrocene molecules, accelerating the translation from laboratory concepts to industrial-scale interface materials.
I also see great potential in machine learning and data-driven approaches. By compiling large datasets of ferrocene derivatives and their device performance, I can train models to predict optimal functional groups for specific perovskite compositions and device architectures. The design problem can be formulated as an optimization:
$$ \max_{\text{functional group}} \text{PCE} \left( J_{SC}, V_{OC}, \text{FF}, \text{stability} \right) $$
subject to synthetic accessibility, cost, and toxicity constraints. This inverse design approach could dramatically accelerate the discovery of high-performance ferrocene derivatives for perovskite solar cells.
Furthermore, I emphasize the importance of standardized characterization protocols. Variations in device fabrication, measurement conditions, and stability testing make it difficult to compare results across studies. I recommend reporting PCE with certified measurements, stabilizing the device at maximum power point, and conducting accelerated aging tests under controlled temperature and humidity. This will enable fair benchmarking of ferrocene-engineered perovskite solar cells.
Conclusion
In this comprehensive review, I have systematically analyzed the latest advances of ferrocene and its derivatives in perovskite solar cells. I have highlighted their collaborative roles in defect passivation, interface energy level regulation, charge transport optimization, and ion migration suppression. Benefiting from reversible redox couples, designable functional groups, and excellent thermochemical stability, ferrocene molecules significantly improve the power conversion efficiency of perovskite solar cells while extending operational lifetime and environmental stability. Through comparative analysis of monomers, functionalized molecules, and polyferrocene systems, I have established a structure–property relationship that follows a hierarchical regulation law from molecular scale to device scale. I have proposed a systematic cognitive framework of “molecular structure design–interface regulation–macroscopic performance enhancement,” revealing the intrinsic logic of ferrocene molecules for cross-scale performance enhancement. This provides a theoretical basis and clear molecular engineering strategy for developing high-efficiency, long-lifetime perovskite solar cells.
The future of ferrocene-based molecular engineering in perovskite solar cells is bright. By combining precise functionalization, multi-scale characterization, and data-driven design, I believe that ferrocene derivatives will play an increasingly important role in achieving commercializable perovskite solar cells with high efficiency and stability. The unique redox chemistry of ferrocene offers a versatile toolbox for addressing the most challenging issues in perovskite photovoltaics, including defect management, ion migration, and environmental stability. I look forward to seeing ferrocene-engineered perovskite solar cells transition from laboratory breakthroughs to industrial products, contributing to the global transition to renewable energy.
In summary, my review demonstrates that ferrocene and its derivatives are not merely additives but powerful molecular tools that can be rationally designed to optimize every critical interface in perovskite solar cells. The field is moving toward a new era of organometallic molecular engineering, where the synergy between molecular design and device physics will unlock the full potential of perovskite solar cells.