Introduction The all-inorganic wide-band gap CsPbI2Br perovskite (1.92 eV) trades off stability and light absorption performance, showing a great application potential in tandem solar cells and flexible solar cells. However, its power conversion efficiency (PCE) and stability still need to be further improved. Usually, the CsPbI2Br perovskite films derived from solution process will inevitably generate various defects at interfaces or grain boundaries as non-radiative recombination centers for photogenerated carriers, and they are sensitive to water molecules to degrade, thus reducing the photovoltaic performance and stability of the devices. It was reported that the interface modification of functional cations (i.e., alkali metal ions, organic amine ions, etc.) and anions (i.e., halogen ions, acetic acid ions, etc.) had a positive effect on improving the efficiency and stability of perovskite solar cells (PSCs). In this paper, a modifier tetrabutylammonium hexafluorophosphate (TBAPF6) with both defect passivation and hydrophobic functions was used to modify the interface of CsPbI2Br/Carbon for the improvement of the PCE and stability of hole-free carbon-based CsPbI2Br PSCs with an architecture of indium tin oxide (ITO)/SnO2/CsPbI2Br/TBAPF6/Carbon. TBAPF6 as an ionic liquid with the Lewis base properties was used as a dopant or an interfacial modifier of perovskite films, which was conducive to passivating the defects of devices. The surface modification of CsPbI2Br perovskite films was carried out at different concentrations of TBAPF6, and its effect on the film quality of CsPbI2Br perovskite and the photovoltaic performance and environmental the stability (including wet and thermal stability) of CsPbI2Br PSCs devices was investigated. Methods Lead iodide (PbI2, 99.999%, in mass fraction, the same below, Advanced Election Technology Co., Ltd.), lead bromide (PbBr2, 99.999%, Advanced Election Technology Co., Ltd.), cesium iodide (CsI, 99.999%, Advanced Election Technology Co., Ltd.), tetrabutylammonium hexafluorophosphate (TBAPF6, 98%, Aladdin Co.), dimethyl sulfoxide (DMSO, analytical reagent, Sigma-Aldrich Co.), tin (IV) oxide colloid (SnO2, Alfa Co.), isopropanol (99.5%, Aladdin Co.), ITO-etched glass (square resistance 7-9 -/square, Advanced Election Technology Co., Ltd.), and conductive carbon paste (Huamin New Material Technology Co., Ltd.). 1.0 mol/L CsPbI2Br perovskite precursor was prepared via dissolving CsI, PbBr2, and PbI2 at a molar ratio of 1.0:0.5:0.5 in DMSO. The ITO-etched glass substrates were cleaned with ethanol, acetone, isopropanol, and ethanol under ultrasonication for 15 min and dried in an oven, and then treated with UV ozone for 25 min. The SnO2 colloid precursor was spin-coated onto ITO glass substrates at 3 000 r/min for 30 s and then put on a hot plate in ambient air at 150 ℃ for 30 min. After 10 min for UV ozone, 44 μL perovskite precursor solution was deposited on the SnO2 layer through a spin-coating process at 1 000 r/min for 10 s and then 3 000 r/min for 30 s in anitrogen-filled glove box. After the spin-coating process, the obtained films were placed on a hot plate at 40 ℃ for 3 min and then annealed at 150 ℃ for 5 min. Afterwards, 45 μL TBAPF6 solution in isopropanol (IPA) with different concentrations (i.e., 0, 0.5, 1.0 and 2.0 mg/mL) was spin-coated on the CsPbI2Br films at 4 000 r/min for 30 s and then annealed at 120 ℃ for 10 min, respectively. Finally, the carbon electrode was deposited on top of the device by a doctor-blading method and then the prepared device was completed after annealing at 120 ℃ for 20 min. The morphology of the absorber layers was observed by a model JSEM-5610LV field-emission scanning electron microscope (SEM, JEOL Co., Japan). The X-ray diffraction (XRD) pattern was recorded by a model AXS X-ray diffractometer (Bruker Co., Germany) with Cu Kα radiation (λ=1.54 -). The current density-voltage (J-V) curves of devices were obtained by a model Keithley 2400 source meter under an illumination of AM 1.5G (100 mW/cm2) with cell area controlled at 0.09 cm2 by a black metal mask. The ultraviolet photoelectron spectra (UPS) were determined by a model ESCALAB 250XI ultraviolet photoelectron spectroscope (Thermo Co., USA) and a model PHI 5 000 Versa Probe III (ULVAC-PHI. Inc., Japan), and the test light source energy was 21.22 eV. The UV-Vis absorption spectra were recorded by a model UV-3600plus spectrophotometer (Shimadzu Co., Japan) in a wavelength range from 300 to 800 nm at room temperature. The steady-state photoluminescence (PL) was obtained by a model FLS1000 UltraFast fluorescence spectrometer (Edinburgh Co., UK) with an excitation wavelength of 450 nm at room temperature. The time-resolved photolumine-scene (TRPL) spectra were obtained on a model Delta Flex fluorescence spectrometer (HORIBA Scientific Co., Japan) using a time-dependent single photon counting method. The electrochemical impedance spectroswpy (EIS) were carried out with a model PP211&CHI1030B electrochemical workstation under illumination with a bias at 0 V in a frequency range of 1 Hz-2 MHz. Results and discussion Based on density functional theory, the Gauss View and Gaussian were used to conduct molecular modeling and electrostatic potential simulation analysis for TBAPF6. The cation is a quaternary ammonium ion (TBA+) containing 4 butyl groups, and the anion is 6 fluorophosphate ion (PF6-). TBAPF6 has the Lewis base properties, in which nitrogen has some uncoordinated lone pair electrons, which can effectively passivate the uncoordinated Pb2+ through electrostatic action. TBA+ can interact with negatively charged defects such as Cs+ vacancies through ionic and hydrogen bonds. Since anion PF6- is negatively charged, positively charged defects (such as halogen vacancies) are passivated by electrostatic action, and fluorine atoms on the anion also play a hydrophobic role to a certain extent. The effect of TBAPF6 interface modifier on the surface morphology of CsPbI2Br perovskite films was analyzed by SEM. The surface morphology of the pristine sample is uneven and rugged. Clearly, there are many pinholes on the surface of the film and cannelures between the grains possibly due to the corrosion of water molecules on the grain boundaries. These characteristics are the main causes of leakage current. Compared with the pristine film, the surface morphology of TBAPF6-modified film is more smooth, uniform and dense. There are few pinholes on the surface and no cannelures between grains. Therefore, TBAPF6 modification can effectively improve the surface morphology of CsPbI2Br perovskite films to prevent the erosion of water molecules in air, and the dense microstructure is conducive to improving the photovoltaic performance and stability of the devices. The phase composition of CsPbI2Br perovskite films treated at different concentrations of TBAPF6 was analyzed by XRD. Clearly, all the samples have characteristic diffraction peaks with 2θ of 14.5° and 29.6°, corresponding to the planes (100) and (200) of perovskite phase. The characteristic peak intensity increases significantly with the increase of TBAPF6 concentration. When the concentration increases upto 1.0 mg/mL, the characteristic diffraction peak intensity reaches the maximum value. However, the characteristic peak intensity decreases when the concentration of TBAPF6 further increases. The pristine device has a PCE of 9.93%, with an open circuit voltage (Voc) of 1.16 V, a short circuit current density (Jsc) of 14.02 mA/cm2, and a fill factor (FF) of 60%. As TBAPF6 concentration increases from 0 to 1.0 mg/mL, the device with an architecture of ITO/SnO2/CsPbI2Br/TBAPF6/carbon exhibits the maximum PCE of 12.04%, with Voc of 1.24 V, Jsc of 14.60 mA/cm2, and FF of 67%. To test the repeatability, 30 devices based on different concentrations of TBAPF6 were set up. The overall PCE distribution of the modified device is significantly greater than that of the pristine device. The PCE distribution of the modified device at 0.5 mg/mL is mainly in a range of 11.0%-11.5%, and some can reach more than 11.5%. The PCE of 1.0 mg/mL modified device increases, mainly distributes 12%, which is the optimum overall distribution of PCE (i.e., 12.04%). The PCE of 2.0 mg/mL modified device is mainly distributed in a range of 11.0%-11.8%, which decreases slightly. Compared with the optimized device, the PCE of the pristine device is mainly distributed in a range of 7%-10%. Therefore, 1.0 mg/mL TBAPF6 is an optimal concentration of the interface modifier. The optical band gap (Eg) of the pristine and 1.0 mg/mL TBAPF6-modified CsPbI2Br perovskite films is 1.92 eV and 1.91 eV, respectively. The Fermi level (Ef) of the pristine and modified samples can be calculated as -5.23 eV and -5.25 eV, respectively, which drops from -5.23 eV to -5.25 eV before and after modification, and is closer to the valency band, indicating that CsPbI2Br perovskite film changes from N-type to P-type semiconductor. The valence maximum (Ev) moves up by 0.05 eV, which reduces the energy level shift of the CsPbI2Br/carbon electrode interface, thus effectively reducing Voc loss and promoting the hole transfer between perovskite and carbon electrode. At last, in the humidity environment of 35%-40%, 80% of the initial efficiency of the optimal 1.0 mg/mL TBAPF6 modified device is maintained after 16 h, and in air at 85 ℃ after 72 h, 70.2% of the initial efficiency is maintained, indicating that the moisture and thermal stability are greatly improved, compared with the pristine device. Conclusions The results showed that TBAPF6 interface modification agent could effectively passivate the surface defects of CsPbI2Br perovskite films, reduce the non-radiative recombination of carriers, improve the energy level arrangement of CsPbI2Br/carbon electrode interface, and promote the carrier transport. It played a hydrophobic role. Thus, the PCE and stability of all inorganic hole-free carbon-based CsPbI2Br PSCs were improved. The microstructure of TBAPF6 modified CsPbI2Br films was more smooth and dense with a higher crystallinity. The Eg decreased slightly, and the light absorption capacity increased. The Ef was closer to the valence band, indicating the transition from N-type to P-type semiconductor. The upward movement of Ev was conducive to the hole transfer at the CsPbI2Br/carbon electrode interface. The steady-state photoluminescence intensity was more intense, indicating a longer average carrier lifetime. The decrease of dark current and the increase of recombination resistance indicated that the defects of the modified device were effectively passivated, and the probability of carrier non-radiative recombination was reduced. The molecular modeling and electrostatic potential simulation showed that PF6- of TBAPF6 passivated the positively charged halogen vacancies through electrostatic action, while TBA+ passivated the negatively charged Cs+ vacancies through ionic and hydrogen bonding. Some functional groups with long alkyl chains and fluorine atoms effectively prevented the intrusion of water molecules in air, thus improving the wet stability of the devices. The optimal 1.0 mg/mL TBAPF6 modified device exhibited a champion PCE of 12.04%, with Voc of 1.24 V, Jsc of 14.60 mA/cm2, and FF of 67%, demonstrating the superior moisture and thermal stability, compared with the pristine device.