Surface heterojunction based on n-type low-dimensional perovskite film for highly efficient perovskite tandem solar cells

ABSTRACT Enhancing the quality of junctions is crucial for optimizing carrier extraction and suppressing recombination in semiconductor devices. In recent years, metal halide perovskite has emerged as the most promising next-generation material for optoelectronic devices. However, the construction of high-quality perovskite junctions, as well as characterization and understanding of their carrier polarity and density, remains a challenge. In this study, using combined electrical and spectroscopic characterization techniques, we investigate the doping characteristics of perovskite films by remote molecules, which is corroborated by our theoretical simulations indicating Schottky defects consisting of double ions as effective charge dopants. Through a post-treatment process involving a combination of biammonium and monoammonium molecules, we create a surface layer of n-type low-dimensional perovskite. This surface layer forms a heterojunction with the underlying 3D perovskite film, resulting in a favorable doping profile that enhances carrier extraction. The fabricated device exhibits an outstanding open-circuit voltage (VOC) up to 1.34 V and achieves a certified efficiency of 19.31% for single-junction wide-bandgap (1.77 eV) perovskite solar cells, together with significantly enhanced operational stability, thanks to the improved separation of carriers. Furthermore, we demonstrate the potential of this wide-bandgap device by achieving a certified efficiency of 27.04% and a VOC of 2.12 V in a perovskite/perovskite tandem solar cell configuration.

Single junction wide-bandgap perovskite solar cells fabrication.NiO x nanocrystal (10 mg mL −1 in H 2 O and IPA mixed solvent with volume ratio of 3:1) layers were first spin-coated on ITO substrates at 3,000 rpm for 25 s in air without annealing, Subsequently the substrates were immediately transferred to the glovebox.2PACz solution (0.5 mg mL −1 ) in ethanol was spin-coated on the NiO x film at 3,000 rpm for 25 s and then annealed at 100 o C for 10 min.For the perovskite film fabrication, the substrate was spun at 4,000 rpm for 35 s with an acceleration of 1,000 rpm, 150 μL Anisole was dropped onto the substrate during the last 5 s of the spinning.The substrates were then transferred onto a hotplate and heated at 100 o C for 10 min.The organic salts surface treatment solutions were prepared by dissolving FPA, or FPA and EDA, or EDA in IPA with different concentrations.The optimal concentration of EDA used in devices was 1 mg mL -1 .The surface treatment was finished by depositing 100 µL organic salts solution onto the perovskite film surface at a spin rate of 3,000 rpm for 25 seconds with a 1,000 rpm s -1 acceleration.The film was then annealed at 100 o C for 10 min.After cooling down to room temperature, the substrates were transferred to the evaporation system and a 23-nm-thick C 60 film was deposited on top by thermal evaporation at a rate of 0.2 A s -1 , followed by evaporation of 8 nm BCP at a rate of 0.2 A s -1 , then finished by thermal evaporation of 100-nm-thick Ag electrode.
Monolithic 2-T all-perovskite tandem solar cell fabrication.The wide-bandgap perovskite solar cell fabrication was completed as described above until the deposition of C 60 .
Then followed by atomic layer deposition of SnO 2 , 20 nm SnO 2 was deposited at 90 º C using precursors of tetrakis(dimethylamino) tin (iv) (99.9999%) and deionized water as the precursors.And 1 nm layer of Au was deposited by thermal evaporation.
Next, PEDOT: PSS (diluted at a 1:1 volume ratio in IPA) was spin-coated onto the wide-bandgap subcell at 4,000 rpm for 30 s and annealed at 120 °C for 10 min.After cooling, the substrates were immediately transferred to a glovebox for the deposition of perovskite films.The perovskite films were deposited with a two-step spin-coating procedure: (1) 1,000 rpm for 10 s with an acceleration of 200 rpm s -1 , (2) 4,000 rpm for 45s with an acceleration of 1,000 rpm s -1 , 250 µL anisole was dropped onto the substrate during the second spin-coating step at 5 s before the end of the procedure.
The substrates were then treated on hotplate at 60 o C for 30 s, then 100 o C for 7 min.
Post-treatment with EDA was carried out by spin-coating a solution of 0.8 mg mL -1   EDA in IPA at 4,000 rpm for 25 s, followed by annealing at 100 °C for 5 min.After cooling, the substrates were transferred to thermal evaporation system, 23 nm C 60 , 8 nm BCP and 100 nm Ag were sequentially deposited on top of the perovskite layer by thermal evaporation.
Normal bandgap (1.5 eV) perovskite solar cell fabrication.The processes of NiO x and 2PACz layers are same as described above.The precursor was spin coated on the substrate at 1,500 and 6,000 rpm for 15 s and 25 s, respectively.During the second stage, 150 μL anisole was dropped onto the surface at the final 10 s, and the substrate was annealed at 100 o C for 10 min.The surface treatment was finished by depositing 100 µL organic salts solution (FEDA, 1mg mL -1 ) onto the perovskite film surface at a spin rate of 3,000 rpm for 25 s with a 1,000 rpm s -1 acceleration.After cooling, the substrates were transferred to thermal evaporation system, 23 nm C 60 , 8 nm BCP and 100 nm Ag were sequentially deposited on top of the perovskite layer by thermal evaporation.

PL.
To avoid environmentally induced degradation, perovskite films were encapsulated by quartz glass and UV-glue.PL spectra were detected using a spectrofluorometer (Fluorolog; HORIBA FL-3) with an exciting wavelength of 400 nm.

GIWAXS. GIWAXS measurement was performed by employing a beam energy of 10 keV and a PILATUS detector at the BL17B1 beamline of Shanghai Synchrotron
Radiation Facility (SSRF), Shanghai, China.GIWAXS patterns were acquired using incident angle of 0.4°, and the sample located about 330 mm away from the detector (Pilatus).Using the GIXGUI Matlab toolbox to correct and reshape raw patterns [3]., where σ is the source-drain conductivity, e is the elementary charge (1.6×10 −19 C),  m is the transconductance,  is the channel length (100 μm),  ox is the oxide capacitance per unit area (11.5 nF/cm 2 ),  is the effective channel width (1 mm), and  ds is the bias voltage (60 V).

SEM, UV-vis, and ToF-SIMS measurements
The surface and cross-sectional microscope images were taken by means of a scanning electron microscope (SEM, JSM-7800, JEOL).The ultraviolet-visible spectra (UV-vis) of the perovskite films deposited on NiOx/2PACz were measured using an UV-vis spectrophotometer (Agilent cary5000).ToF-SIMS analysis was carried out with a ToF-SIMS 5-100 instrument (ION-TOF GmbH, Germany).

Solar cell characterizations.
The calibration of light was enabled by a KG-2 Si diode with a solar simulator (Enli Tech, Taiwan, China).A Keithley 2400 source unit was employed to obtain J-V curves under simulated AM1.5G solar illumination at 100 mW cm -2 (1 sun).For the measurement of the J-V curve, the solar cell was masked applying an aperture mask with an area of 0.042 cm 2 .The applied bias ranges from 0 V to 1.36 V for wide-bandgap perovskite solar cell, from 0 V to 0.86 V for narrow-bandgap solar cell, from 0 V to 2.15 V for tandem solar cell at room temperature in a glovebox.No preconditioning was used before J-V measurements.A DFT.All the DFT-based first-principles calculations are performed by using the Vienna Ab-initio Simulation Package [4,5] (VASP) with the projector augmented wave [6] (PAW) method.Generalized-gradient approximation [7] (GGA) formulated by Perdew, Burke, and Ernzerhof (PBE) is used as the exchange-correlation functional.And the plane-wave cutoff energy is set to 400 eV.The Gamma-centered k-point mesh with a grid spacing of 2π×0.03Å -1 is used for electronic Brillouin-zone integration.DFT-D2 46 method of Grimme was implemented to take into account the long-range van der Waals (vdWs) interaction.For the defect formation energy calculation, the thickness of vacuum for 3D-slab structure is set to be 15 Å and the initial 2D structure is based on reported structure.[8] The defect formation energy of                For our TSC device, the hysteresis factor of 0.042 cm 2 device is 0.016, and the hysteresis factor of 0.113 cm 2 is 0.015.

FET characterization. ( 1 )
photolithography on a p++ Si/SiO 2 (300 nm) substrate, which was rendered hydrophilic in advance by oxygen plasma.Subsequently, the photolithographic patterns were converted into metal electrodes by electron-beam evaporation and lift-off of a 5 nm Cr/30 nm Au layer.The perovskite precursor solution was spin-coated onto the substrate with pre-fabricated electrodes and annealed for follow-up device measurement.(2) Electrical measurements.The gate voltage was applied under a sweep rate of 0.6 V/s, using 10-ms-wide pulses generated by the Keysight B2912B source meter.The measurement was performed at room temperature in a vacuumed probe station with pressure of ~0.5 Pa.Carrier concentration (n) was calculated by  =  ox  ⅆs ⅇ m Thermo Fisher ESCALAB 250XI.All perovskite films fabricated on ITO/NiOx/2PACz substrate and were transferred from glovebox to ESCALAB 250XI chamber using a portable gas-tight capsule.A sample bias of -10 V was applied for UPS acquisition.IPES: Inverse photoemission spectroscopy (IPES) measurement was performed using a customized ULVAC-PHI LEIPS instrument with Bremsstrahlung isochromatic mode.KPFM and AFM.The amplitude-modulation KPFM was operated combined with a Cypher S atomic force microscopy (AFM; Asylum Research, Oxford Instruments) and a HF2LI Lock-in amplifier (Zurich Instruments) in N 2 -filled glovebox.The resonance frequency ω0 and spring constant of AFM conducting tips are ~127 kHz and 5.0 Nm −1 , respectively.TPV and TPC.A 640 nm diode laser was used to modulate the V OC on top of a constant light bias.The pulse duration was set to 1 μs and the repetition rate to 50 Hz by the function generator of the oscillator.The digital oscilloscope recorded the data induced by the light perturbation, using 1 MΩ input impedance for the TPV measurement and 50 Ω impedance for TPC measurement.EL.The current density-luminance-luminescence (J-V-L) characteristics and EQE values were acquired by a Keithley 2612B source meter and a fiber integrating sphere (FOIS-1) couple with a QE Pro650 spectrometer (SpectrumTEQ-EL system, Ocean Optics).The EQE of LED can be defined as EQE= emitted photons out LED second ⁄ injected electrons second ⁄ .The counts of photons per second are collected by an integrating sphere and a fiber spectrometer and the counts of electrons are collected with Keithley 2612B source meter.The LED devices were tested on top of the integrating sphere, and only forward light emission could be collected, which is consistent with the standard OLED characterization method.All the device test processes were performed in the N 2 -filled glovebox.
commercial system (Solar cell scan 100, Beijing Zolix Instruments Co., Ltd) was used to measure the EQE spectra.And the calibration of light intensity was carried out by a standard photodetector (QE-B3/S1337-1010BQ, Zolix).The light beam was chopped at 180 Hz and the response of the cell was acquired by a Stanford Research SR830 lock-in amplifier.For all-perovskite tandem solar cell, the bias illumination from highly-bright LEDs with emission FPAks of 850 and 460 nm was employed to measure the spectral response of the top and bottom subcells, respectively, respectively.EQE measurements were performed in ambient air, and no bias voltage was applied during the EQE measurements.The solar cell certification measurements were carried out by Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences.
Figure S1.Perovskites film morphology.a,SEM and b,AFM images of control, FPA, FEDA, and EDA films.The scale bar in SEM is 100 nm.c,KPFM images of the surface potential of perovskite films.

Figure S2 .
Figure S2.High-resolution N 1s spectra and C 1s spectra of pristine and treated perovskite films obtained by XPS characterization.

Figure S5 .
Figure S5.UPS data for the pristine and different EDA solvents post-treated films.

Figure S6 .
Figure S6.UPS data for the pristine and post-treated films.PDA is propane-1,3-diammonium iodide and HAD is hexane-1,6-diammonium iodide, and all post-treatment precursor concentration is 1 mg mL -1 .The corresponding WF and E VBM are summarized in the inset.

Figure S7 .
Figure S7.EQE measurement of FEDA solar cell, showing a device bandgap of 1.77 eV.

Figure S8 .
Figure S8.Certification of photovoltaic performance of the wide-bandgap FEDA device measured by Shanghai Institute of Microsystem and Information Technology (SIMIT).

Figure S9 .
Figure S9.J-V curves and statistic parameters of the normal bandgap (1.5 eV) perovskite devices.

Figure S10 .
Figure S10.J-V curve of the tandem solar cell based on control device.

Figure S11 .
Figure S11.Certification of photovoltaic performance of the tandem perovskite device measured by SIMIT.The FEDA tandem device has an independently certified PCE of 26.59% (27.04%) under reverse (forward) scan.

Figure S12 .
Figure S12.EQE spectra of tandem solar cells and the integrated J SC values are 15.40 mA cm −2 and 15.27 mA cm −2 for top and bottom subcells, respectively.

Figure S13 .
Figure S13.The statistic of FEDA film based tandem solar cells.

Figure
Figure S14 J-V curve of the best-performing TSC device (0.113 cm 2 )

Table S1 . Representative works about wide-bandgap (1.77 eV) solar cells.
Parameters of the certified cell are shown in brackets.