Fluoroaromatic Cation‐Assisted Planar Junction Perovskite Solar Cells with Improved V_OC and Stability: The Role of Fluorination Position

Abstract

The notoriously poor stability of organic–inorganic hybrid perovskite solar cells is a crucial issue restricting the commercial application of such burgeoning technology. Passivation of bulk perovskite absorber by fluorinated aromatic ammonium salt via low‐dimensional perovskites has been proved to be an effective way of improving stability and efficiency. Herein, the influence of fluorination position (ortho‐, meso‐, and para‐) on the aromatic moiety is studied in terms of their dipole moments and the ability to reduce defect density, extend carrier lifetimes, and assist charge transfer. In addition to the improved power conversion efficiency (PCE) from 19.17% to above 20%, the device treated with 2‐(o‐fluorophenyl)ethylamine iodide exhibits a remarkable open‐circuit voltage (VOC) of 1.21 V. While the 2‐(p‐fluorophenyl)ethylamine iodide‐treated device shows only 1% loss of its initial value under ambient atmosphere (with RH of 10–30%) without encapsulation for 1440 h storage. The molecular structure of fluorinated aromatic cations plays multiple roles in passivating the interface of the perovskite device.

Review of Interface Passivation of Perovskite Layer

“…The introduction of hydrophobic fluorinated aromatic amine large-volume cation strategy on the surface of the 3D perovskite films forms a 3D-2Dstacked perovskites, which combines the stability of 2D perovskite and the high performance of 3D perovskite to improve the efficiency and stability of the devices [ 87 ]. Zhou et al systematically compared the effects of 3D–2D stacked perovskites on the performance and stability of a device by treating the surface of the 3D perovskite with aromatic cations placed at different positions [ 88 ]. The hydrophobic structure not only blocks the invasion of water molecules, but also can passivate the surface defects of the perovskite and effectively promote the transport of holes to the HTM layer.…”

Dually‐Passivated Perovskite Solar Cells with Reduced Voltage Loss and Increased Super Oxide Resistance

“…Thee ncouraging results above could be attributed to the benign effect of FPD in improving the film quality and effective passivation of the bulk defects in the perovskite film. Despite the improvement in PCEs,the devices incorporating FPD still suffer from ahigh V OC deficit of 0.46 V. This voltage loss is strongly related to the recombination at defects on the surface of the perovskite films.I nt his context, we expect to further reduce surface defects on the 3D perovskite when using the FPD additive as the inside passivator.According to our previous report, [37] the introduction of oFPEAI isopropanol (IPA) solution onto the surface of the 3D perovskite could in situ form a2Dperovskite passivating layer. Figure 2a shows XRD patterns for the 3D perovskite (C), the 3D perovskite incorporating FPD (C-FPD), the 3D perovskite only treated with the oFPEAI (C/oFPEAI), and the 3D perovskite incorporating FPD before treating with the oFPEAI (C-FPD/oFPEAI).…”

“…Moreover,the pristine 3D perovskite (C) started to bleach after 132 ha nd turned completely yellow after 180 h. TheC-FPD perovskite shows superior thermal stability compared to the control, suggesting that the FPD significantly retarded the degradation of perovskites.A ccording to the previous discussion, the FPD additive can enhance the thermal stability of perovskite [14,50] since strong hydrogen bonding (F•••H À N) between fluorine atoms and MA + can immobilize MA + . [14] Besides,the long-term moisture stability of the passivated perovskite films was evaluated in contrast to the pristine 3D perovskite film (C) when all devices were stored in acabinet with an RH of 30 AE 5%.T he moisture resistivity of C/ oFPEAI devices has been thoroughly studied in our previous works, [36,37] where the hydrophobic fluorophenyl groups acted as the moisture barrier to prevent the invasion of H 2 Of rom ambiance in high humidity environments.I nt his case,a fter 71 days,t he C-FPD/oFPEAI devices retained over 97 %o f their initial efficiency,w hich was similar to the C/oFPEAI device (96 %), while the PCE of the PSC based on pristine perovskite (C) dropped to 85 %. (Figure 5d)…”

“…[13][14][15]35] Secondly,ahydrophobic 2D perovskite layer is formed in situ on the 3D perovskite layer by successively depositing 2-(2-Fluorophenyl)ethylamine iodide after the antisolvent-treatment to passivate the defects on the surface.A sar esult, the novel DP strategy prolongs carrier lifetime through defect passivation and, hence,lifts the V OC from 1.10 Vt o1 .18 V, corresponding to a V OC deficit of 0.39 V. In the complete devices,w ea chieved ac hampion stabilized PCE as high as 23.80 %. It is found that besides the excellent thermal and moisture resistant properties [9,14,18,33,36,37] provided by the DP strategy,t he effective passivation of site-vacancy-type defects (e.g., iodine vacancy) successfully suppressed the generation of O 2 À species.E specially,t his is the first time to observe the decrease of destructive O 2 À species by forming a2 Dp erovskite layer. Therefore,the DP strategy-based PSCs also showed remarkable long-term stability,r etaining over 85 %o ft he initial efficiency after heating on ahot plate at 100 8 8Cfor 30 hunder relative humidity (RH) of 70 %a nd over 97 %o ft he initial PCE after 71 days under a3 0% RH environment conditions without encapsulation.…”

Dimensionality and Defect Engineering Using Fluoroaromatic Cations for Efficiency and Stability Enhancement in 3D/2D Perovskite Photovoltaics

“…In comparison with the other aliphatic alkylammonium derivatives, the short‐chained aromatic cation such as FPEA + have many inherent advantages for the role of organic spacer cation, such as additional delocalization of positive charge on the p ‐fluorophenyl ring, reduced interlayer spacing, high dielectric constant, and low exciton binding energy. [ 24,25 ] In the molecular structure of FPEAI, the substitution of the hydrogen atom by the highly electronegative fluorine atom (fluorination) can induce strong polarity and large dipole moment, thereby enhancing the polarization in its crystal lattice. [ 26 ] The enhanced polarization in the FPEAI are highly beneficial for the charge separation and transport in the PSC device.…”