Efficient and Stable Perovskite Solar Cells based on Nitrogen‐Doped Carbon Nanodots

The role of graphitic and amorphous nitrogen‐doped carbon dots (N‐CDs) as additives for perovskite solar cells (PSCs) is investigated. A detailed study of N‐CDs: perovskite (PVSK) blends through X‐ray diffraction, nuclear magnetic resonance, field emission scanning electron microscopy, UV–vis, and photoluminescence spectroscopy reveals the existence of interactions between N‐CDs and PVSK. The amorphous or graphitic nature of these carbon nanoforms, as well as the interactions between CDs and PVSK, clearly determines the photovoltaic outcome of the PSCs. Thus, a small amount of graphitic carbon dots (g‐N‐CDs) leads to more‐stable PSCs, while maintaining and even improving the power conversion efficiency (PCE). In addition, the long‐term evaluation of the g‐N‐CDs‐containing cells shows improvement of the PCE over time, up to 109% of the initial efficiency after 40 days while the reference performance is dropped to 86%.


Methods and Materials CDs Preparation and Characterization
Pristine a-N-CDs were synthesized according to a previously reported method, 1 and g-N-CDs were synthesized according to a reported procedure. 2 l u ' c ( -dimethyl-1,3-dioxane-4,6-dione), 4-dimethylaminopyridine, Kaiser k ph x LH− w pu ch f g -Aldrich. Dialysis membrane (Float-A-Lyzer, MWCO 0.5-1 KDa) were bought from Spectrum Labs. Anhydrous N,Ndimethylformamide (DMF) was purchased at Acros organics and used without any purification.

Synthesis of a-N-CDs
The synthesis of a-N-CDs with carboxylic acid groups on the surface was accomplished by post-functionalization of pristine a-N-CDs with amino group on the surface (a-N-CDs-NH 2 ) w h l u ' c ( -dimethyl-1,3-dioxane-4,6-dione). Pristine a-N-CDs (25.0 mg, . 3 l ) w lv 5 L f hy u F h l u ' c ( .5 mg, 0.17 mmol) and 4-dimethylaminopyridine (2.5 mg, 0.020 mmol) were added. The mixture was stirred under argon at 80°C for 48 h. The volatiles were removed under high vacuum, the residue was dissolved in methanol and the purification was accomplished through size exclusion chromatography using Sephadex LH-20. 3 The residue was taken up in water and lyophilized.
Fourier-transform Infrared (FT-IR) spectra (KBr) were recorded on a Perkin Elmer 2000 spectrometer. UV-Vis spectra were recorded on a PerkinElmer Lambda 35 UV-Vis spectrophotometer and baseline corrected. Fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer.
The 1 H-NMR spectra (DMF:DMSO 4:1) were taken on a Bruker NMR spectrometer (300 MHz) at room temperature. Chemical shifts (δ) are reported in ppm. The concentration of the PVSK NMR solutions was the same as that of the perovskite solution, with a CD concentration of 1 mg ml -1 for the g-N-CDs and 10 mg ml -1 for the a-N-CDs. For the MAFA NMRs the concentration was 1M with the same relative amount of g-N-CDs.
The morphologies and structural properties of the films were analyzed with an ULTRA plus ZEISS field-emission scanning electron microscope (FESEM) and a Bruker AXS-D8 Advance X-ray diffractometer with CuKa radiation.
The perovskite films for UV-Vis and photoluminescence spectroscopy were prepared by spin-coating of the precursor solutions on well-cleaned quartz glass substrates inside a glovebox and using the same conditions for the preparation of active layer in the solar cells. The exciting wavelength used was 507 nm, with a slit width of excitation and emission of 15 nm. The measurements were made in the same sensitivity of photomultiplier and same scan speed (200 nm/min).
The J-V characteristics were measured using a 450 W xenon light source (Oriel). The light intensity was calibrated with a Si photodiode equipped with an IR-cutoff filter (KG3, Schott) and it was recorded during each measurement. Current-voltage characteristics of the cells were obtained by applying an external voltage bias while measuring the current response with a digital source meter (Keithley 2400). The voltage scan rate was 20 mV s -1 , and no device preconditioning was applied before starting the measurement, such as light soaking or forward voltage bias applied for a long time. The cells were masked with a black metal mask of 0.16 cm 2 to estimate the active area and reduce the influence of the scattered light.

Substrate and electron-transporting layer preparation:
FTO-gl u ( Ω q-1) were cleaned by ultrasonication in deionized water with soap (2% Hellmanex water solution) for 15 min. After thorough rinsing with deionized water, the substrates were further ultrasonicated with EtOH and acetone for 30 minutes each. Finally, the substrates were dried under a nitrogen flow and, in order to eliminate any organic impurity, they were further treated in an UV-ozone cleaner for 15 min. Afterwards, a TiO 2 compact layer was deposited on FTO via spin-coating for 20s at 5000 rpm from a precursor solution of titanium chloride 2M in water. After the spin-coating procedure, the substrates were left at 100 °C for 10 min. Then, the mesoporous TiO 2 layer was deposited by spin coating for 10 s at 4000 rpm with a ramp of 2000 rpm s-1, using 30 nm particle paste diluted in EtOH to achieve 150-200 nm thick layer. Subsequently, the substrates were sintered following a heating ramp up to 450 °C, at which they were left for 30 min under dry air flow. The mesoporous TiO 2 was doped by spin-coating of a solution of Li-TFSI in CH 3 CN (10 mg mL -1 ) at 3000 rpm for 10 s. Finally, the electrode with Li-doped mesoporous TiO 2 was completed with a second sintering process, the same as before. After cooling down to 150 °C, the substrates were immediately transferred to a nitrogen-filled glove box for the deposition of the perovskite films.

Perovskite precursor solution and film preparation
The perovskite films were deposited from a precursor solution containing FAI (1 M), PbI 2 (1.1 M), MABr (0.2 M) and PbBr 2 (0.22 M) in anhydrous DMF:DMSO 4:1 (v:v) solvent, following a previously reported procedure. 4 The chosen amount of g-N-CDs (0.1, 0,5, 1, 2, 3, or 7.5 mg ml -1 ) was added to the solvent before the dissolution of the perovskite components. The perovskite solution was spin coated in a nitrogen-filled glove box through a two-step program at 1000 and 5000 rpm for 10 and 20 s, respectively. During the second p 5 μL f chl z w p u h p g u 5 f h end of the program. The substrates were then annealed at 100 °C for 45 min in a nitrogen-filled glove box.

Hole transporting layer and gold contact deposition
After the perovskite annealing the substrates were cooled down for few minutes and 50 μl p -OMeTAD solution (70 mmol in chlorobenzene) were spun at 4000 rpm for 20 s. Before deposition, this solution was doped with tert-butyl pyridine (3.3 mol/mol with respect to Spiro), Bis(trifluoromethane)sulfonimide lithium salt (1.8 M in acetonitrile, 0.5 mol/mol with respect to Spiro) and FK209 (0.25 M in acetonitrile, 0.05 mol/mol with respect to Spiro). Finally, an 80 nm-thick gold electrode was thermally evaporated through an appropriate shadow mask on top of the hole transporting layer to produce a completed PSC device. Figure S1. UV-Vis spectra of pristine a-N-CDs with an amino-rich surface (green line) and a-N-CDs with carboxylic acid groups on the surface (blue line) in water at 298 K.      Figure S11: Picture of one of the studied devices. Table S1: Photovoltaic parameters corresponding to the PSCs whose curves are reported in Figure 7. Approximately 10 devices were measured for each condition.