Understanding the Effects of Fluorine Substitution in Lithium Salt on Photovoltaic Properties and Stability of Perovskite Solar Cells

Abstract

Lithium bis­(trifluoromethanesulfonyl)­imide (Li-TFSI) additive is widely employed to improve the hole mobility of the hole-transporting layer (HTL) in perovskite solar cells (PSCs). However, the hygroscopic nature of Li-TFSI is not beneficial to the long-term stability of PSCs. Herein, a new more water-resistant Li-PFSI is used to replace Li-TFSI. As a result, the best power conversion efficiency (PCE) of 22.14% is achieved for Li-PFSI-treated PSCs, exceeding that of the control cell with Li-TFSI (20.25%). Importantly, the Li-PFSI-based cell shows impressive environmental and thermal stability. Moreover, we first comparatively investigate the effect of the amount of fluorine substitution in lithium salt (2F for Li-FSI, 6F for Li-TFSI, and 10F for Li-PFSI) on the HTL’s physical properties and their photovoltaic performance in PSCs. We found that more fluorine substitution can improve the HTL charge-carrier transfer and photovoltaic performance in PSCs. Our findings provide key missing information for designing new additives toward efficient and stable PSCs.

Understanding the Influence of Anion Exchange on the Hole Transport Layer for Efficient and Humidity-Stable Perovskite Solar Cells

“…The hydrophobicity of different anions is dependent on the number fraction of water-repellent fluorine atoms over the total numbers of atoms in the anions, which follows the order of PF 6 − > TFSI − > FSI − . 20 This agrees closely with the contact angle variations for different anion-doped spiro-OMeTAD films and humidity stability results. Consequently, the humidity stability of the PSC device can be further improved by the water-repelling hydrophobic PF 6 − anion-doped HTL.…”

“…The FSI-spiro film had the lowest contact angle value of 65.07°. The hydrophobicity of different anions is dependent on the number fraction of water-repellent fluorine atoms over the total numbers of atoms in the anions, which follows the order of PF 6 – > TFSI – > FSI – . This agrees closely with the contact angle variations for different anion-doped spiro-OMeTAD films and humidity stability results.…”

“…Although previous study has investigated the impact of the fluorine atom in the anion dopant on water resistance of the HTL, its influence on p-doping capability has not been elucidated, suggesting that the understanding of dopants in terms of p-doping capability toward the HTL is necessary. 20 The highest p-doping capability was obtainable using a hexafluorophosphate (PF 6 − ) anion because a strong interaction with spiro-OMeTAD is enabled while resulting in a poor affinity with Li + . While the PF 6 − anion combined with the tert-butylammonium cation was introduced, the resulting device showed relatively inferior performance presumably due to the lack of the understanding of the relationship between the dopant and its p-doping capability.…”

“…In this manner, the influence of various dopant anions of different molecular sizes on ionic interactions was evaluated in terms of the p-doping efficiency into spiro-OMeTAD. Although previous study has investigated the impact of the fluorine atom in the anion dopant on water resistance of the HTL, its influence on p-doping capability has not been elucidated, suggesting that the understanding of dopants in terms of p-doping capability toward the HTL is necessary . The highest p-doping capability was obtainable using a hexafluorophosphate (PF 6 – ) anion because a strong interaction with spiro-OMeTAD is enabled while resulting in a poor affinity with Li + .…”

Dual Modification Engineering via Lanthanide‐Based Halide Quantum Dots and Black Phosphorus Enabled Efficient Perovskite Solar Cells with High Open‐Voltage of 1.235 V

“…The Cs 3 TbCl 6 QDs modified device exhibits higher PCE and higher V bi than the control device, which is beneficial to suppress hysteresis and then improve the performance of the device. [ 30 ] The introduction of Cs 3 TbCl 6 QDs could enhance charge extraction and decrease carrier recombination on the surface of the perovskite film. The J – V curves of the control and the Cs 3 TbCl 6 QDs modified devices by reverse scan and forward scan are displayed in Figure 4f and Table S3, Supporting Information.…”