Organic–inorganic hybrid perovskite solar cell (PSC) is a third-generation photovoltaic technology^{[1, 2]}, and the certified power conversion efficiency (PCE) has reached 25.5% (https://www.nrel.gov/pv/cell-efficiency.html), which can rival solar cells based on crystalline-Si and other inorganic semiconductors. The intrinsic instability of perovskite materials could impede PSC commercialization^[3]. To date, a variety of strategies such as composition engineering, additive engineering, interface engineering and encapsulation technique are employed to improve the long-term stability of PSCs^[4-9]. In particular, fullerene materials with high electron mobility, high electron affinity, small reorganization energy and adjustable energy level have been widely utilized as interfacial layers or additives in PSCs for efficiency and stability improvement^[10]. Among them, fluorinated and crosslinkable fullerene derivatives can improve the stability of PSCs effectively. The fluorinated fullerene derivatives could improve the moisture stability because of the hydrophobicity of fluorine atom. The crosslinked fullerene derivatives can protect the electron-transport layers (ETLs) against solvent erosion during perovskite solution deposition, and the as-formed organic networks can improve mechanical stability of PSCs.



Humidity can deteriorate the performance of PSCs. In-situ encapsulation over perovskite film by using fluorine-substituted fullerene derivatives acquired excellent device stability against humidity destruction. In 2016, Jen et al. reported a fluoroalkyl-substituted fullerene derivative (DF-C₆₀, Fig. 1) and used it as an additive to make inverted fullerene/CH₃NH₃PbI₃ bulk-heterojunction (BHJ) PSCs^[11]. DF-C₆₀ passivated the defects of perovskite surface and facilitated charge transfer, yielding a PCE of 18.11%. The unencapsulated device demonstrated high ambient stability, after being stored under 60 ± 5% relative humidity (RH) for one month, the device kept >83% of its initial PCE. DF-C₆₀ was further incorporated into CH₃NH₃Pb_0.5Sn_0.5I₃ to make graded heterojunction (GHJ) PSCs^[12]. The GHJ device offered an improved PCE (15.61%) with reduced photovoltage deficit. Additionally, the perfluoroalkyl chains in DF-C₆₀ can effectively retard moisture penetration into Pb–Sn perovskites and also suppress Sn²⁺ oxidation. In 2017, novel fluorinated isoxazolino[60]fullerene (IS-1 and IS-2, Fig. 1) and pyrazolino[60]fullerene derivatives (PI-1 and PI-2, Fig. 1) were synthesized by Martín et al., and they were used to construct ETL-free PSCs^[13]. The suitable LUMO energy levels contribute to the increased open-circuit voltage (V_oc) and the higher PCE of CH₃NH₃PbI₃:IS2 device, while fullerene helped to improve the photostability of the device. Two fluorinated PC₆₁BM derivatives (3F-PC₆₁BM and 5F-PC₆₁BM, Fig. 1) were used as n-type additives to make CH₃NH₃PbI₃ BHJ PSCs^[14]. The incorporation of 0.1 wt% 3F-PC₆₁BM can fill pinholes and passivate the defects to form a flat and dense BHJ perovskite film, suppressing moisture permeation under ambient condition. As a result, the device kept ~80% of its initial PCE after 550 h storage in N₂, whereas the control device decayed to 80% after 240 h storage. More recently, Yang et al. developed a double-site defect passivation strategy by using a perfluoroalkyl- and pyridine-functionalized fullerene derivative (C₆₀-PyF15, Fig. 1), which was added into CH₃NH₃PbI₃ layer to make inverted fullerene/perovskite BHJ PSCs^[15]. The device gave a PCE of 20.10%, and there was coordination between pyridine moiety and Pb²⁺ and also hydrogen bonding between fluorine atom and CH₃NH₃⁺. The improved hydrophobicity and suppressed ion migration endowed the device much better ambient and thermal stabilities. The device can retain 85% and 81% of its initial PCE after exposing for 1400 and 660 h under ambient condition (25 ± 5 °C, ~35% RH) and under heating at 85 °C in N₂ without encapsulation, respectively.






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Figure1.
Molecular structures of fluorinated and crosslinkable fullerene derivatives.




Several fluoroalkyl-substituted fullerene derivatives were combined with other fullerene materials to make a moisture-resistant mixed-fullerene interfacial layer for PSCs. In 2016, Jen et al. combined N-methyl-2-(2-perfluorooctylphenyl)-3,4-fullero-pyrrolidine (F-C₆₀, Fig. 1) with bis-adduct of N-methyl-2-(2,3,4-tris(2-(2-methoxyethoxy)ethoxy)phenyl)-3,4-fulleropyrrolidium iodide (bis-C₆₀) to form a robust and efficient cathode interlayer atop of PC₆₁BM in CH₃NH₃PbCl_3–xI_x PSCs^[16]. Benefitting from the hydrophobicity of F-C₆₀, the device demonstrated a remarkable stability, retaining ~80% of its original PCE after exposing to an ambient condition (20% RH) for two weeks without encapsulation, whereas the control device with bis-C₆₀ cathode interlayer decayed to zero under the same condition. Recently, Deng et al. reported two PC₆₁BM analogues, [6, 6]-phenyl-C₆₁-butyric acid 1H, 1H-trifluoro-1-ethyl ester (3F-PC₆₁BM) and [6, 6]-phenyl-C₆₁-butyric acid 1H, 1H-tridecafluoro-1-heptyl ester (C₆F₁₃-PC₆₁BM, Fig. 1)^[17], which were used to make hybrid ETLs with PC₆₁BM in inverted CH₃NH₃PbI₃- and (FA_0.83MA_0.17)_0.95Cs_0.05Pb(I_0.9Br_0.1)₃-based PSCs, delivering PCEs of 18.37% and 19% for 1 wt% 3F-PC₆₁BM-containing device, respectively. The upward-oriented fluorocarbon chains formed a thin moisture-resistant layer. The 1 wt% C₆F₁₃-PC₆₁BM-containing CH₃NH₃PbI₃ PSCs showed a much better ambient stability, keeping over 80% PCE after 33 days storage under 40%–60% RH in air. In 2020, Feng et al. synthesized 2-(perfluorophenyl)-5-phenyl-C₆₀-fulleropyrrolidine (FP-i) and 2,5-bis-(perfluorophenyl)-C₆₀-fulleropyrrolidine (FP-ii, Fig. 1), which were introduced as ETLs in CH₃NH₃PbCl_3–xI_x PSCs^[18]. The hydrogen bonds between the organic cation of perovskite and F atom within FP-i and FP-ii, as well as the enhanced hydrophobicity led to good passivation and humidity stability, keeping 70%–85% of its initial PCE after 70 h in ambient air.



Though fullerene-based ETLs demonstrate fascinating electron transport/extraction capability and excellent electronic contact with perovskite films, the partial erosion of fullerene films by the solvents (e.g. N, N-dimethylformamide, dimethyl sulfoxide) used for processing perovskite film may yield defective film with shunting paths. Multilayer spin-coating deposition sometimes results in poor device performance. Furthermore, mechanical stress is the primary reason for mechanical failure of PSCs^[19]. Introducing crosslinkable fullerene derivatives is a sound solution to these issues.



In 2016, Snaith et al. developed two crosslinkable fullerene derivatives called sol-gel C₆₀ and PCBCB (Fig. 1) to form robust fullerene films during CH₃NH₃PbI_3–xCl_x preparation^[20]. Sol-gel C₆₀ was crosslinked via hydrolysis-condensation reaction, and crosslinked PCBCB ETL was achieved by annealing at 200 °C via ring-opening reaction of benzocyclobutene moiety. Both solvent-resistant fullerene ETLs resulted in reduced shunting paths and improved hole-blocking properties. Another novel thermally crosslinkable styrene-functionalized fullerene derivative MPMIC₆₀ (Fig. 1) was synthesized by Dauskardt et al. to replace fragile PC₆₁BM and C₆₀ ETL in both n–i–p and p–i–n CH₃NH₃PbI₃ PSCs^[19]. A solvent-resistant film formed by MPMIC₆₀ exhibited 10-fold and 14-fold enhancement in fracture resistance over PC₆₁BM and C₆₀, respectively. The device gave higher V_oc and short-circuit current density (J_sc) than PC₆₁BM device. Later on, Hsu et al. introduced a crosslinkable [6,6]-phenyl-C₆₁-butyric styryl dendron ester (C-PCBSD, Fig. 1) into CH₃NH₃PbI_xCl_3–x via anti-solvent method to enhance the crystallization of perovskite films and passivate the defects, as well as improving electron extraction^[21]. Crosslinked network of C-PCBSD could prevent the fullerene layer being washed away by subsequent solution processing, and protect the active layer against moisture. The devices can maintain 84.6% of its initial PCE after 180 h storage in ambient (40 ± 5% RH). In 2017, Petrozza et al. used in-situ crosslinked C-PCBSD in n–i–p CH₃NH₃PbI₃ PSCs^[22], and C-PCBSD can form a robust and solvent-resistant thin film atop of TiO₂ to improve the quality of upper perovskite films, promoting charge extraction. The device exhibited an enhanced PCE close to 19% with a V_oc above 1.1 V. In 2018, Liao et al. further developed a composite ETL combining C-PCBSD with π-conjugated graphdiyne (GD), which was sandwiched between TiO₂ and perovskite films in n–i–p CH₃NH₃PbI₃ PSCs^[23]. The GD introduced into C-PCBSD formed a face-on stacked film through a strong π–π stacking interaction. Besides, the thermally annealed C-PCBSD:GD film provided a robust and adhesive network with excellent solvent resistance to overcome interfacial erosion, which contributed to efficient charge extraction of ETL, improved crystallinity of perovskite films, leading to a PCE of 20.19% and improved stability (retaining 80% PCE after 500 h storage under ambient condition with 25%–30% RH). More recently, Marder et al. mixed PCBCB with a poly(methacrylate) derivative (CL) to form an ETL for p–i–n Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.9Br_0.1)₃ PSCs, which can be thermally insolubilized^[24]. The device with PCBCB:CL ETL demonstrated improved PCE (18.5%) and remarkable thermal stability, keeping 92% of its stabilized power output after 3000 h storage in N₂ at 85 °C.



Another crosslinked fullerene derivative [6, 6]-phenyl-C₆₁-butyric oxetane dendron ester (C-PCBOD, Fig. 1) via photo-curing was developed to modify TiO₂ surface in n–i–p PSCs^[25]. C-PCBOD was safe in the following deposition of CH₃NH₃PbI₃ precursor solution. It could passivate the trap-states of TiO₂ and facilitate the charge transfer, yielding PCEs of 15.9% and 18.3% for devices based on compact TiO₂ and mesoporous TiO₂ ETL, respectively. To overcome the brittleness of perovskite films, which limits the application in flexible PSCs, in 2019, Liao et al. introduced C-PCBOD as a plasticizer into CH₃NH₃PbI₃ films to form an organic network to passivate the grain boundaries as well as the trap states within perovskite layer^[26]. The rigid and flexible devices offered PCEs of 20.4% and 18.1%, respectively. Under a stretching of Δ10.0 nm/μm, the C-PCBOD-containing device retained 62% of its initial PCE, while the control device kept 42%. The robust fullerene network could prevent device decomposition in air, retaining 72% of the initial value after 960 h storage in air (25 ± 5% RH). PC₆₁BM crosslinked by 1,6-diazidohexane (DAZH, Fig. 1) via a UV light curing was reported^[27]. The crosslinked PC₆₁BM layer demonstrated improved solvent-resistance, and the FA_0.66MA_0.34PbI_2.85Br_0.15 device gave a PCE of 18.4% for small-area and 14.9% for modules, respectively.



To check the details about above-mentioned fullerene derivatives, their application, device performance parameters, and stability, you may find Table S1 in the Supporting Information. By applying fluorinated and crosslinkable fullerene derivatives, scientists successfully improved the efficiency and stability of PSCs. More innovative fullerene materials containing fluorine and crosslinkable moieties are desired.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2017YFA0402800) and the National Natural Science Foundation of China (51925206, U1932214). L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720) for financial support.

Appendix A. Supplementary materials

Supplementary materials to this article can be found online at https://doi.org/1674-4926/42/12/120201.