In recent years, perovskite solar cells have emerged as a focal point in photovoltaic research due to their exceptional optoelectronic properties, low processing costs, and scalability for large-area fabrication, showcasing immense application potential. These devices can be categorized into conventional (n-i-p) and inverted (p-i-n) structures. With advancements in self-assembled monolayer hole-transport materials and optimized perovskite crystallization, inverted perovskite solar cells have surpassed their conventional counterparts in power conversion efficiency (PCE). Moreover, inverted structures address issues like instability and poor reproducibility associated with doped hole-transport layers in conventional devices, making them the current mainstream in perovskite solar cell research.
Fullerene C60 and its derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are widely employed as electron transport layer (ETL) materials in inverted perovskite solar cells owing to their high electron affinity, suitable energy level alignment with perovskites, and efficient electron transport capabilities. However, C60 suffers from poor solubility, necessitating high-temperature evaporation for device fabrication, and exhibits insufficient passivation of perovskite defects. PCBM, while more soluble, is prone to severe self-aggregation, leading to non-uniform coverage on perovskite surfaces. Additionally, disordered molecular stacking in PCBM layers results in suboptimal charge transport properties. These issues contribute to significant charge accumulation and recombination at interfaces, impeding charge extraction and transport, and ultimately limiting the PCE of perovskite solar cells.
In this study, we designed and synthesized a novel alkylfullerene derivative, FC10, via a one-step Prato reaction and incorporated it into PCBM to form an FC10-PCBM blend as the ETL for perovskite solar cells. The introduction of FC10 effectively suppressed PCBM self-aggregation, improved film morphology, reduced pinholes, enhanced interfacial contact with the perovskite layer, and increased electron mobility. Consequently, the perovskite solar cells achieved a superior PCE of 19.65% compared to 17.97% for reference devices. This work underscores the potential of developing novel fullerene-based ETL materials to optimize interfacial morphology and charge transport, thereby advancing perovskite solar cell technology.
The synthesis of FC10 was carried out through a classic Prato reaction, as illustrated in the referenced content. Fullerene C60, decanal, and sarcosine were dissolved in toluene and refluxed at 110°C for 9 hours. After cooling, the solvent was removed under reduced pressure, and the crude product was purified via column chromatography using carbon disulfide and dichloromethane mixtures as eluents. The final product, FC10, was obtained as a yellowish solid with a yield of 48.6%. Structural confirmation was achieved using nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS), with NMR peaks corresponding to the expected hydrogen environments and HRMS confirming the molecular mass.
For device fabrication, indium tin oxide (ITO) substrates were cleaned and treated with UV ozone. A nickel oxide (NiOx) layer was spin-coated and annealed, followed by the deposition of a self-assembled monolayer (SAM) using [4-(3,6-dimethoxy-9H-carbazol-9-yl)butyl]phosphonic acid (MeO-4PACz). The perovskite layer with a composition of Rb0.05Cs0.05MA0.05FA0.85Pb(I0.95Br0.05)3 was deposited via spin-coating with an antisolvent treatment. Phenethylammonium chloride was applied for surface passivation. The ETL was formed by spin-coating either PCBM or an FC10-PCBM blend (10:1 mass ratio) in chlorobenzene. Finally, a bathocuproine (BCP) buffer layer and silver electrodes were deposited. All processes except NiOx deposition were conducted in a nitrogen-filled glovebox.
Material characterization included X-ray diffraction (XRD) to assess perovskite crystal structure, scanning electron microscopy (SEM) for surface morphology, atomic force microscopy (AFM) for roughness analysis, and current-density-voltage (J-V) measurements for photovoltaic performance. Charge transport and recombination were evaluated using space-charge-limited current (SCLC) methods and light intensity-dependent measurements.
XRD patterns of pristine perovskite films and those coated with PCBM or FC10-PCBM showed no significant shifts in peak positions or intensities, indicating that the incorporation of FC10 did not alter the perovskite crystal structure. This suggests that the performance enhancements stem from morphological and charge transport improvements in the ETL.
SEM images revealed distinct differences in surface morphology. PCBM-coated perovskite films exhibited numerous pinholes due to uneven coverage, which could facilitate moisture ingress and ion migration, degrading device performance and stability. In contrast, FC10-PCBM films showed a more uniform and pinhole-free surface, promoting better interfacial contact and charge transfer. AFM analysis further confirmed these observations, with root-mean-square roughness values decreasing from 17.3 nm for PCBM to 6.81 nm for FC10-PCBM, indicating smoother surfaces that enhance charge extraction.
The photovoltaic performance of devices with PCBM and FC10-PCBM ETLs was evaluated under AM1.5G illumination. J-V curves demonstrated that FC10-PCBM-based devices achieved higher open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF), leading to a PCE improvement from 17.97% to 19.65%. External quantum efficiency (EQE) measurements corroborated the JSC values, with integrated current densities of 22.56 mA/cm² for PCBM and 23.17 mA/cm² for FC10-PCBM, consistent with J-V results. The enhanced performance is attributed to improved charge transport and reduced recombination.
| Electron Transport Layer | JSC (mA/cm²) | VOC (V) | FF (%) | PCE (%) |
|---|---|---|---|---|
| PCBM | 21.95 | 1.075 | 76.11 | 17.97 |
| FC10-PCBM | 23.40 | 1.088 | 77.15 | 19.65 |
To investigate charge transport properties, electron-only devices with ITO/ZnO/ETL/Ca/Al structures were fabricated, and electron mobility was calculated using the SCLC method. The Mott-Gurney equation was applied:
$$J = \frac{9 \varepsilon_0 \varepsilon_r \mu V^2}{8 L^3}$$
where \( J \) is the current density, \( \varepsilon_0 \) is the vacuum permittivity, \( \varepsilon_r \) is the relative permittivity, \( \mu \) is the electron mobility, \( V \) is the applied voltage, and \( L \) is the film thickness. The electron mobility for FC10-PCBM was determined to be \( 3.3 \times 10^{-3} \, \text{cm}^2 \cdot \text{V}^{-1} \cdot \text{s}^{-1} \), significantly higher than \( 3.7 \times 10^{-4} \, \text{cm}^2 \cdot \text{V}^{-1} \cdot \text{s}^{-1} \) for PCBM. This enhancement facilitates efficient charge transport and reduces recombination losses.
Charge recombination dynamics were analyzed through light intensity-dependent JSC and VOC measurements. The relationship \( J_{\text{SC}} \propto P_{\text{light}}^\alpha \) was used to assess bimolecular recombination, where \( \alpha \) close to 1 indicates minimal recombination. For FC10-PCBM devices, \( \alpha = 0.989 \), compared to 0.957 for PCBM, suggesting suppressed bimolecular recombination. Similarly, the slope of VOC versus ln(Plight) was lower for FC10-PCBM (1.22 kT/q) than for PCBM (1.72 kT/q), indicating reduced trap-assisted monomolecular recombination. These findings align with the higher electron mobility and improved film morphology.
The enhanced performance of perovskite solar cells with FC10-PCBM ETLs can be attributed to several factors. First, FC10 mitigates PCBM self-aggregation, leading to a more uniform and pinhole-free film that improves interfacial contact and reduces defect states. Second, the higher electron mobility promotes efficient charge extraction and transport, minimizing recombination. Third, the smooth surface morphology facilitates better charge collection at the electrodes. These synergistic effects result in higher JSC, VOC, and FF, ultimately boosting PCE.
Further analysis of the charge transport mechanisms can be modeled using the drift-diffusion equation for electron current density in perovskite solar cells:
$$J_n = q \mu_n n E + q D_n \frac{dn}{dx}$$
where \( J_n \) is the electron current density, \( q \) is the elementary charge, \( \mu_n \) is the electron mobility, \( n \) is the electron concentration, \( E \) is the electric field, and \( D_n \) is the electron diffusion coefficient. The enhancement in \( \mu_n \) for FC10-PCBM directly contributes to higher \( J_n \), reducing losses and improving overall device efficiency.
In addition, the interface between the perovskite and ETL plays a critical role in charge extraction. The energy level alignment can be described using the Schottky-Mott rule:
$$\Phi_B = \Phi_{\text{ETL}} – \chi_{\text{perovskite}}$$
where \( \Phi_B \) is the energy barrier, \( \Phi_{\text{ETL}} \) is the work function of the ETL, and \( \chi_{\text{perovskite}} \) is the electron affinity of the perovskite. FC10-PCBM likely provides better energy level matching, facilitating electron injection and reducing interface recombination.
To quantify the recombination losses, the ideality factor \( n \) can be extracted from the dark J-V characteristics using the diode equation:
$$J = J_0 \left( \exp\left(\frac{qV}{nkT}\right) – 1 \right)$$
where \( J_0 \) is the reverse saturation current density. A lower ideality factor for FC10-PCBM-based devices would indicate dominant bimolecular recombination, consistent with the light intensity measurements.
The stability of perovskite solar cells is another crucial aspect influenced by the ETL. The improved coverage and reduced pinholes in FC10-PCBM films likely enhance device stability by preventing moisture and oxygen penetration. Long-term stability tests under ambient conditions could further validate this hypothesis, but initial results are promising.
In summary, the incorporation of alkylfullerene derivative FC10 into PCBM-based ETLs effectively addresses key limitations such as self-aggregation, poor morphology, and low charge transport. This strategy leads to significant improvements in the performance of perovskite solar cells, highlighting the importance of molecular engineering in developing advanced ETL materials. Future work could explore other fullerene derivatives with tailored functional groups to further optimize interface properties and charge dynamics in perovskite solar cells.
The development of efficient and stable perovskite solar cells remains a vibrant research area, with ongoing efforts focused on interface engineering, material design, and scalability. The findings presented here contribute to this broader context by demonstrating a practical approach to enhancing ETL functionality. As perovskite solar cell technology progresses toward commercialization, innovations in charge transport layers will play a pivotal role in achieving high efficiency and durability.
In conclusion, we have successfully synthesized and applied a novel alkylfullerene derivative, FC10, to modulate the interfacial morphology and charge transport properties of PCBM-based ETLs in inverted perovskite solar cells. The FC10-PCBM blend exhibits suppressed self-aggregation, higher electron mobility, and more uniform perovskite coverage, resulting in reduced recombination and improved photovoltaic parameters. This work provides valuable insights into the design of fullerene-based ETLs for high-performance perovskite solar cells, paving the way for future advancements in photovoltaic technology.