No abstract

Additives are widely adopted for efficient perovskite solar cells (PSCs), and proper additive design contributes a lot to PSCs' various breakthroughs. Herein, a novel additive of N,1-fluoroformamidinium iodide (F-FAI), whose cation replaces one amino group in guanidinium (GA + ) with electron-withdrawing fluorine group, is synthesized and applied as the additive for PSCs. The electron-withdrawing effect of fluorine promotes the molecular polarity of N,1-fluoroformamidine (F-FA), enhancing the interaction of N,1-fluoroformamidinium (F-FA + ) with MAPbI 3 . Compared with the nonpolar GA + , F-FA + improves the crystallinity, passivates the defect, and downshifts the Fermi level of MAPbI 3 more significantly. The charge transfer and built-in field in printable triple mesoscopic PSCs are therefore enhanced. Moreover, charge transport in MAPbI 3 is also promoted by F-FAI. With these benefits, a power conversion efficiency of 17.01% for printable triple mesoscopic PSCs with improved open-circuit voltage and fill factor is obtained with the addition of F-FAI, superior to the efficiency of 15.24% for those devices with guanidinium iodide additives.

Halide perovskite solar cells (PSCs) have drawn worldwide attention due to their great potential to be promising candidates for highly efficient and cost-effective photovoltaic technologies. [1] Benefiting from the excellent optoelectronic properties of halide perovskites together with previous abundant experience in other solar cells, especially in dye-sensitized solar cells and organic solar cells, the power conversion efficiency (PCE) of conventional PSCs has been boosted to 25.5%, [2] making PSCs one of the most efficient solar cell type. Conventional efficient PSCs in lab usually rely on costly materials such as gold or silver electrode and organic hole transport materials (HTM). [3] To further reduce the material and fabrication cost of PSCs toward commercialization, carbon electrode-based HTM-free perovskite solar cells (C-PSCs) are developed. [4] In C-PSCs, carbon electrodes (CEs) are chosen to replace metal electrodes due to their excellent adjustable electronic properties, chemical stability and low cost. [5] However, the efficiency of C-PSCs has not exceeded 20%. [6] Therefore, developing carbon materials for C-PSCs to promote their efficiency toward the comparable level as conventional PSCs is in great demand. [4] Since no additional HTM is applied in C-PSCs, adjusting the work function (WF) of CEs to realize more efficient hole extraction and more suitable energy level alignment is an effective strategy to enhance the efficiency of C-PSCs. [7,8] To realize this, Jiang et al. introduced p-type metal oxides into the CE to improve its WF for extracting holes. [9] However, the introduction of those metal oxides led to the sheet resistance increase of CEs, which resulted in series resistance increase of the C-PSCs and restricted the efficiency improvement. Doping graphite has been demonstrated as another effective method to adjust the CEs WF for improving C-PSCs performance. Duan et al. improved the efficiency of C-PSCs from 12.4% to 13.6% by applying boron-doped carbon as the electrode. [5] Yang's group improved the WF of CE and the efficiency of C-PSCs by doping graphite with boron. [10] In addition, it is found that introducing oxygen-containing functional groups into the CE can also increase the WF. Tian et al. synthesized a carbon black with much higher oxygen content and a much higher WF than common carbon black. [11] When applying it in the CE for C-PSCs, the device efficiency was enhanced from 13.6% to 15.7%. This work demonstrated that introducing oxygen into CEs is of great potential to improve C-PSCs efficiency. However, the restriction is that the related process for introducing oxygen is carried out at high temperature of about 1600 K. Therefore, developing facile methods to synthesize oxygen-rich carbon materials sustainably for CEs is worth exploring.Biomass materials, which hold the "Sustainable and green" characteristics, have been applied for different energy conversion devices. [12][13][14] Zhu et al. synthesized a ZrO 2 @cellulose acetatereinforced nanofibrous membrane for sodium-ion b...