Boosting Long-Term Stability of Pure Formamidinium Perovskite Solar Cells by Ambient Air Additive Assisted Fabrication

Due to the high industrial interest for perovskite-based photovoltaic devices, there is an urgent need to fabricate them under ambient atmosphere, not limited to low relative humidity (RH) conditions. The formamidinium lead iodide (FAPI) perovskite α-black phase is not stable at room temperature and is challenging to stabilize in an ambient environment. In this work, we show that pure FAPI perovskite solar cells (PSCs) have a dramatic increase of device long-term stability when prepared under ambient air compared to FAPI PSCs made under nitrogen, both fabricated with N-methylpyrrolidone (NMP). The T80 parameter, the time in which the efficiency drops to 80% of the initial value, increases from 21 (in N2) to 112 days (in ambient) to 145 days if PbS quantum dots (QDs) are introduced as additives in air-prepared FAPI PSCs. Furthermore, by adding methylammonium chloride (MACl) the power conversion efficiency (PCE) reaches 19.4% and devices maintain 100% of the original performance for at least 53 days. The presence of Pb–O bonds only in the FAPI films prepared in ambient conditions blocks the propagation of α- to δ-FAPI phase conversion. Thus, these results open the way to a new strategy for the stabilization in ambient air toward perovskite solar cells commercialization.

QDs were dispersed in 400 µL of FAPI solution to get a 25 mg/mL of stock solution. The stock solution of FAPI-PbS QDs was further diluted for the preparation of 0.5, 1, 2.5, and 5 mg/mL solutions. Before the perovskite deposition, SnO2 ETL substrates were treated with UV-Ozone for 20 minutes. The perovskite solution (with and without FAPI-PbS QDs) was spin-coated at 5000 rpm for 17 seconds and 400 µL of diethyl ether solution was dropped on the spinning substrate at 9 th second. The substrate was then annealed at 100°C for 1 minute and 165°C for 10 minutes. 2,3 Subsequently, the Spiro-MeOTAD solution was deposited by spin coating at 4000 rpm for 35 s. The Spiro-MeOTAD solutions were prepared by dissolving (72.3 mg/mL in 1 mL chlorobenzene) and followed by the addition of tert-butyl pyridine (t-BP, 28.8 µL), Lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI, 17.5 µL). Finally, 80 nm of the gold counter electrode was deposited using a thermal evaporator. Note that, the perovskite thin film and Spiro-MeOTAD deposition were carried out at ambient atmosphere conditions (with a relative humidity of ~40-60%).
Structural characterization X-ray diffraction (XRD): The crystallographic information of the films were analyzed by an X-ray diffractometer (D8 Advance, Bruker AXS) (Cu Kα, the wavelength of l = 1.5406 Å) within the range of 10-55° with a step size of 0.02°. The X-ray diffraction patterns were recorded from films deposited on top of the glass substrate.

Scanning Electron Microscope (SEM):
The topographical and cross-sectional images were recorded by field emission scanning electron microscope (FEG-SEM) JEOL 3100F) operated at 5 kV. The SEM was recorded from films deposited on top of the ITO substrate.
Optoelectronic characterization UV-Vis Spectra: The absorption spectra were obtained using a UV/Vis absorption spectrophotometer (Varian, Cary 300). The UV-Vis spectra were recorded from films deposited on top of the glass substrate.

Photoluminescence Spectra (Visible and infrared), Time-Resolved Photoluminescence (TRPL) Decay profile
The emission measurement (PL spectra) and time-resolved PL (TRPL) decay were collected by Horiba Fluorolog. The photoluminescence spectra (visible and infrared) of perovskite thin films were recorded by exciting the samples at 532 nm. The PL (visible) and PL decay were recorded from films deposited on top of the glass substrate. The PL (infrared) was measured for the PbS QDs and FAPI-PbS QDs from the solution.

Incident-photon-to current conversion efficiency (IPCE): Measured using a QEPVSI-b
Oriel measurement system and measured in DC mode.
Electrochemical Impedance Measurements (EIS) were performed by applying 20 mV of a voltage perturbation at different frequencies from 1MHz to 0.1Hz in a PGSTAT-30 from Autolab and under 1 sun illumination at different applied potential, from 0 to 1.2 volts. The recombination resistance was calculated by using the equivalent circuit previously reported. 4

Solar cell characterization:
The current-voltage (J-V) curves are measured using a Keithley 2612 source meter under AM 1.5 G (1000 Wm -2 ) provided by a Solar Simulator Abet, Xenon short-arc lamp Ushio 150 watts, in the air at a temperature around 25°C and without encapsulation. Each curve is generated using 123 data points from a starting potential of 1.2 V to a final potential of -0.02 V (reverse scan; vice versa for the forward scan) using a scan rate of 10 mVs -1 . The active area of the cell is 0.121 cm 2 . The photovoltaic performance of the aged devices is measured without any encapsulation, at ambient atmosphere, and under 1 sun condition.

Raman spectroscopy:
The micro-Raman spectra were measured by WiTec apyron equipment at 532 nm excitation wavelength with an EMCCD detector. The laser power intensity used was 0.1 mW and 5 mW. The Raman experiments were conducted in ambient air conditions with a relative humidity of 55%.

Atomic Force Microscopy (AFM):
The morphology and surface roughness of the perovskite film were measured by using AFM (Concept Scientific Instrument) in resiscope mode. The scanning area was 10x10 µm 2 .
Photoelectrons were excited with the Al-Ka line (1486.7 eV) of a monochromatic x-ray source μ-FOCUS 500 (SPECS GmbH). Measurements were taken at room temperature with a passenergy of 20 eV. Figure S1. The steady-state photoluminescence (PL) spectra of FAPI (air) and FAPI (N2) perovskite thin films; a) from the front and backside, b) measurements at ambient air and N2 purging cycle.

PbS QDs additive optimization
We have shown that the addition of PbS QDs causes an increase in the long-term stability of FAPI 5 and FACsPbI 6 solar cells prepared in an inert atmosphere. Here, the effect of PbS QDs under ambient conditions has been analyzed. PbS QDs have been added in the fabrication for the analysis of their effect, hereafter named as FAPI-PbS QDs samples. In this case, prior to mixing the PbS QDs with FAPI precursor solution for the fabrication of perovskite thin films, we have performed the ligand exchange procedures as reported earlier. 5 As shown in Figure S3a Figure   S3b). Figure S4a Table S1, shows the (100) and (110) XRD peak area comparison of the reference FAPI (air) and various amounts of FAPI-PbS QDs perovskite thin films. Higher intensity peak ratio is observed for the 0.5 and 1 mg/mL (air) films than the reference FAPI (air) film. This is symbolic of relatively more favored perovskite grain growth in the (100) plane induced by the PbS QDs. 6 Once the concentration overcomes 1 mg/ml the more nucleation seeds lead to smaller grains, more grain boundaries, in turn affecting the crystallinity. 5 All the perovskite films showed a similar absorbance profile throughout the wavelength region and band edge region, Figure S6d. The bandgap of the reference FAPI (air) and various amount of FAPI-PbS QDs perovskite films are derived from Tauc plots ( Figure S6f and Table   S2), and each film shows a bandgap value of ~1.5 eV similar to the α-FAPI, confirming that the bandgap is not substantially affected despite the FAPI-PbS QDs inclusion. As shown in Figure S6e, the presence of FAPI-PbS QDs did not appreciably change the emission wavelength, as expected from the similar bandgap observed.   Figure S24; e) Steady-state Photoluminescence (PL) spectra; f) The Tauc plots. The samples for XRD, UV-Vis absorption, and PL measurements are fabricated on glass substrates.   Table S3. Figure S8. The X-ray diffraction (XRD) patterns of the FAPI (N2) and 1 mg/mL (N2) perovskite thin film prepared and annealed under N2 environment.    Figure S11. The cross-sectional scanning electron microscopy images of perovskite solar cell prepared with a) reference FAPI (air); b) 1 mg/mL (air). The device layer consists of Indium tin oxide (ITO)/SnO2/Perovskite (FAPI (air) or 1 mg/mL (air)/2,2',7,7'-tetrakis-(N,N-di-4methoxyphenylamino)-9,9'-spirobifluorene (Spiro-MeOTAD)/Au. Table S5. The photovoltaic parameters of perovskite solar cells based on reference FAPI (N2 and air) and 1 mg/mL (air) under 1 sun illumination (AM 1.5 G, 1000 Wm −2 ). All the devices are recorded at forward and reverse bias scan direction (-0.02 V to 1.2 V and vice versa) with a scan rate of 10 mV/s. The champion cell efficiency is given in the brackets and the average statistical distribution is based on 18 solar cells.  Figure S13. The IPCE spectra and integrated current density of reference FAPI (N2 and air) and 1 mg/mL (air) device. The lower IPCE at longer wavelengths (750-800 nm) for FAPI (air) film is due to the lower absorption in this region, see Figure 1a. Figure S14. The stabilized photocurrent density at maximum power point vs time for FAPI (air, N2) and 1 mg/mL (air) devices. The photocurrent at maximum power point for the reference FAPI (air and N2) and 1 mg/mL (air) devices at maximum power point photovoltage for a short-term at ambient air (relative humidity of ~60%), which shows that the FAPI-PbS QDs has an important role in the device performance stability. 5, 6 Moreover, it is observed that the device 1 mg/mL (air) reaches the maximum power point (with a stabilized PCE of 16.87%) in few seconds and that for reference FAPI (air and N2) device takes more time. Figure S15. The plot of recombination resistance (Rrec) versus applied voltage under 1 sun illumination was obtained from the fitting of EIS using the equivalent circuit previously reported. 4 Recombination resistance is similar in both devices, presenting slightly higher values for 1 mg/mL (air). where ntrap is the trap density, εr is the dielectric constants of perovskite, ε0 is the permittivity of vacuum, L is the thickness of the perovskite film, e is the elementary charge and VTFL is the trap-filled limit voltage acquired by fitting the dark I−V data. The VTFL values are found to be 0.72 V and 0.45 V for the reference FAPI (air) and 1 mg/mL (air) films, respectively. The reference FAPI (air) device shows a higher trap density of 3.8 × 10 16 cm −3 , than the trap density of 2.1 × 10 16 cm −3 of the 1 mg/mL (air). <τavg>* is the average lifetime; <τavg> = (A1τ1 2 + A2τ2 2 )/ (A1τ1+A2τ2), 12 whereas (A1+A2) = 1. Figure S17. The long-term stability (145 days) comparison of the unencapsulated reference FAPI (air) and various amount of FAPI-PbS QDs devices (the data obtained from the average of 5 different devices) the normalized photovoltaic parameters like a) open-circuit voltage (VOC); b) Short-circuit current density (JSC); c) fill factor (FF), and d) power conversion efficiency (PCE), with a relative humidity of 23% and 25°C, at dark condition. Figure S18. The photostability test of unencapsulated perovskite devices at 25°C and RH 60% (10 hours each); a) the stabilized photocurrent density and b) PCE at maximum power point vs time for the reference FAPI (air) and 1 mg/mL (air) devices. Measurements were carried out at 25ºC in the air atmosphere with RH 60%. In detail, the unencapsulated FAPI (air) based device efficiency drops to 42% from its initial value, while the device with 1 mg/mL (air) is found to be stable and a relatively lower amount of degradation is observed, retained >83% of the device efficiency. It is also observed that the device with FAPI-PbS QDs (air) reaches the maximum power point in few seconds and that for reference FAPI (air) device takes more time; this also signifies the better quality of perovskite layer and mitigated hysteresis due to the FAPI-PbS QDs inclusion. 13 Figure S19. a) The XRD patterns of the aged perovskite solar cells with gold contact (devices were stored at ambient conditions of 25 °C and RH of 23%, at the dark condition). Note that the diffraction peak denoted as (♦) is from the ITO substrate and (Δ) is from the gold contact layer. The inset shows the figure of FAPI (N2) device after one month; b) Photographic images of FAPI (N2) devices with time (the devices were stored in ambient air without encapsulation, 25 °C, RH of 23%, at the dark condition).

Stability measurements with PbS QDs
In order to investigate the influence of FAPI-PbS QDs towards the tolerance of moisture, we evaluated the stability of perovskite thin films of reference FAPI (air) and FAPI-PbS QDs in the FAPI matrix (on glass substrate without any encapsulation) by monitoring the optical and structural features under ambient conditions with RH of 40±10 % and 25 °C, at the dark. Initially, we observed the changes of XRD patterns of the fresh and aged films of reference FAPI (air) and with the inclusion of FAPI-PbS QDs films. As shown in Figure S20, the degradation of reference FAPI (air) film started with the formation of the PbI2 phase (12.70°) and converted to the δ-FAPI phase (11.80°) in a week. 2,3 Subsequently, the FAPI (air) film completely transformed into a yellow phase in two weeks with the signature of δ-FAPI peaks of 11.80° (010) and 26.30° (021). However, the degradation pathway of the perovskite samples with FAPI-PbS QDs is different than the reference FAPI (air) films. As shown in Figure S20, all the films (0.5, 1, 2.5, and 5 mg/mL (air)) started to degrade in PbI2 (12.70°) and a negligible amount of δ-FAPI (11.80°). As shown in Figure S20-S22, the comparative analysis between the SEM images, thin-film photographs, and the XRD patterns clearly show the formation of small PbI2 crystallites in all the perovskite films and for the aged perovskite film the appearance of the δ-FAPI phase (yellow phase) for the reference FAPI (air) films. While, only the reduction of the α-phase with the formation of PbI2 as degradation products observed for the films of FAPI-PbS QDs, without the complete conversion to the delta phase, Figure S20, and S21. This corroborates with the SEM measurements of the representative films after the aging process, see Figure S22.
The stability of perovskite films is furthermore verified with the absorbance spectroscopy. As shown in Figure S25 the UV-Vis absorbance spectra of reference FAPI (air) film is remarkably reduced after aging for 10 days with the appearance of the photoinactive yellow perovskite material in 14 days. On the contrary, the absorption spectra of FAPI-PbS QDs films are showed only a slight variation in absorbance after aging for 10 days, shown in Figure S25. Moreover, the samples with a higher amount of FAPI-PbS QDs (eg, 2.5 and 5 mg/mL (air)) films showed better absorbance and stability than the films with a lower amount of FAPI-PbS QDs counterparts (as shown in Figure S25). More evidently, the FAPI-PbS QDs induces a strain which constitutes the compression of the unit cell (see Figure S6b, c) and consecutive α phase stabilization through chemical interactions. 5, 6, 11     Figure S24. The UV-Vis absorbance spectra of aged perovskite thin films of FAPI (air) and FAPI (N2) fabricated on glass substrates (the samples are stored under an ambient atmosphere with a relative humidity of 40±10 % and 25°C, at dark). Note that two shoulders at 570 nm and 620 nm are observed for FAPI (air) and significantly lower absorbance is measured for FAPI (air) than for FAPI (N2) at the longer wavelength range (750-800 nm). In as deposited (Day-0) the FAPI (air) films shows a higher content of δ-FAPI and PbI2 phases, responsible of the shoulders, than FAPI (N2), see Figure 1b. The presence of these phases also is the responsible of the relatively lower of absorbance (in the longer wavelength range of 750-800 nm) observed for the FAPI (air) film when compared to the FAPI (N2) film in Day-0. However, note that these effect, shoulders and lower absorption at 750-800 nm range is also observed for FAPI (N2) Day-3 when the presence of δ-FAPI and PbI2 phases has increased.     High energy resolution X-ray photoelectron spectra of the O 1s core level measured in FAPI (air) -red curves-and FAPI (N2) -blue curves-films with the different aging degrees, which ranges between fresh and 7 days aged films.  Figure S30. a) The current-voltage (J-V) curves of reference MACl (air) device recorded at forward and reverse bias scan direction (-0.02 V to 1.2 V and vice versa) with a scan rate of 10 mV/s. The devices were fabricated at a lower relative humidity of ~30%. Figure S31. a) The current-voltage (J-V) curves of reference MACl (air) and MACl+1 mg/mL (air) device recorded at forward and reverse bias scan direction (-0.02 V to 1.2 V and vice versa) with a scan rate of 10 mV/s. b) IPCE spectra and integrated current density of reference MACl (air) and MACl+1 mg/mL (air) devices.