Interfacial Passivation Engineering of Perovskite Solar Cells with Fill Factor over 82% and Outstanding Operational Stability on n-i-p Architecture

Tremendous efforts have been dedicated toward minimizing the open-circuit voltage deficits on perovskite solar cells (PSCs), and the fill factors are still relatively low. This hinders their further application in large scalable modules. Herein, we employ a newly designed ammonium salt, cyclohexylethylammonium iodide (CEAI), for interfacial engineering between the perovskite and hole-transporting layer (HTL), which enhanced the fill factor to 82.6% and consequent PCE of 23.57% on the target device. This can be associated with a reduction of the trap-assisted recombination rate at the 3D perovskite surface, via formation of a 2D perovskite interlayer. Remarkably, the property of the 2D perovskite interlayer along with the cyclohexylethyl group introduced by CEAI treatment also determines a pronounced enhancement in the surface hydrophobicity, leading to an outstanding stability of over 96% remaining efficiency of the passivated devices under maximum power point tracking with one sun illumination under N2 atmosphere at room temperature after 1500 h.


Experimental Section
Synthesis of CEAI 2-Cyclohexylethylamine (0.63 g, 5.0 mmol) (TCI) was dissolved in 20 mL MeOH at 0 o C, hydroiodic acid solution (1 mL) (Sigma-Aldrich, 57% in water) was added dropwise. After the mixture was stirred for 1 h, the solution was removed to room temperature and stirred for another 2 h. After solvent evaporation, the resulted solid residue was washed with diethyl ether three times, affording white solid product (1.10 g, 87 % yield). 1  Substrates were heated at 450 o C and kept at this temperature for 15 min before and 30 min after the spray of the precursor solution, then left to cool down to room temperature.
Mesoporous TiO 2 layer was spin-coated at 4000 rpm for 20 s, with the acceleration rate of 2000 rpm/s, using a 30 nm TiO 2 paste (Dyesol 30 NR-D) diluted in ethanol with 1:6 volume ratio.
After the spin-coating, the substrates were dried at 80 ℃ for 10 min and then sintered at 450 o C for 30 min under dry air flow.

Perovskite layer
The perovskite precursor solution was prepared by dissolving a mixture of cesium iodide (0.07 mmol, TCI Co. Ltd.), methylammonium bromide (0.14 mmol, Dyenamo), formamidinium iodide (1.19 mmol, Dyenamo), lead iodide (1.45 mmol, Alfa Co. Ltd.) in 1 mL mixture of DMF and DMSO (DMF:DMSO=4:1 v/v, Acros). The perovskite solution was spin-coated through two-step program (1000 rpm for 10 s and 6000 rpm for 20 s) with pouring chlorobenzene as an anti-solvent 5s before the end of the second step. Then the substrates were annealed at 100 o C for 1 h in dry air. The CEAI was dissolved in IPA and the solution was spin-coated at 4000 rpm for 20s on the as-prepared perovskite films and dried on a hot plate at 100 o C for 10 min.

Hole transporting layer and Au top contact
The substrates were cooled down to room temperature after annealing the perovskite. A spirofluorene-linked methoxy triphenylamines (spiro-MeOTAD, Merck) solution was deposited by spin coating at 4000 rpm for 20 s, as hole-transporting material. 90 mg spiro-MeOTAD was dissolved in 1 ml chlorobenzene, doped by 20.6 μL bis(trifluoromethylsulfonyl)imide lithium salt solution (520 mg/mL LiTFSI in acetonitrile), and 35.5 μL 4-tert-butylpyridine (tBP, Sigma-Aldrich). Finally, 80 nm of Au top electrode was deposited through thermal evaporator under high vacuum with an active area of 0.16 cm 2 .

Characterization
The solar cell devices were measured using a 300 W Xenon light source (Oriel). The spectralmismatch between AM 1.5 G and the solar simulator was calibrated by a Schott K113 where J ph is the photogeneration current (approximated by J SC ) and J 0 is the dark saturation current.
Plotting the subtracted J SC at each light intensity from the J SC at one sun illumination (i.e. plotting pJ) against V OC gives pseudo J-V curve, which is essentially a light J-V curve without series resistance. However, the non-radiative recombination processes are still present in the cell, even though no current is drawn from it. Thus, pFF obtained from the pJV curve represents a case where non-radiative recombination losses of the FF are still present, but transport losses (e.g. transport of carriers through perovskite bulk, their extraction and transport through the charge-selective layers, losses at interfaces) are not. This gives a unique opportunity to disentangle between the contributions of transport and non-radiative recombination losses in a cell.

Supplementary Note 2
To get the FF value in the absence of transport and non-radiative losses -FF max , Shockley-Queisser approximation can be used, which considers that the only loss of the photo-excited charge carriers happens due to radiative recombination process: and J 0 depend on the semiconductor absorption spectrum, its bandgap energy must ℎ be known to calculate FF max . Based on the PL spectra shown in Figure S3, the band gap of the studied perovskite was determined to be 1.56 eV. Thus, a J-V curve in the case of Shockley-Queisser limit has been calculated via publicly available python-based script (https://github.com/marcus-cmc/Shockley-Queisser-limit) and plotted in Figure 4d (yellow J-V curve), having a FF max of 90.3% for a perovskite absorber investigated in this study.