Constructing Soft Perovskite–Substrate Interfaces for Dynamic Modulation of Perovskite Film in Inverted Solar Cells with Over 6200 Hours Photostability

Abstract High‐performance perovskite solar cells (PSCs) depend heavily on the quality of perovskite films, which is closely related to the lattice distortion, perovskite crystallization, and interfacial defects when being spin‐coated and annealed on the substrate surface. Here, a dynamic strategy to modulate the perovskite film formation by using a soft perovskite–substrate interface constructed by employing amphiphilic soft molecules (ASMs) with long alkyl chains and Lewis base groups is proposed. The hydrophobic alkyl chains of ASMs interacted with poly(triarylamine) (PTAA) greatly improve the wettability of PTAA to facilitate the nucleation and growth of perovskite crystals, while the Lewis base groups bound to perovskite lattices significantly passivate the defects in situ. More importantly, this soft perovskite–substrate interface with ASMs between PTAA and perovskite film can dynamically match the lattice distortion with reduced interfacial residual strain upon perovskite crystallization and thermal annealing owing to the soft self‐adaptive long‐chains, leading to high‐quality perovskite films. Thus, the inverted PSCs show a power conversion efficiency approaching 20% with good reproducibility and negligible hysteresis. More impressively, the unencapsulated device exhibits state‐of‐the‐art photostability, retaining 84% of its initial efficiency under continuous simulated 1‐sun illumination for more than 6200 h at elevated temperature (≈65 °C).


Device fabrication
Indium tin oxide (ITO) coated glass was cleaned in deionized water, acetone, and ethanol subsequently and respectively for 10 min in an ultrasonicator (Shumei KQ300DE), then was treated with UV-ozone (ODT UV-O3 Cleaner) for 15 min. The PTAA was dissolved in toluene with a concentration of 4 mg mL -1 and spin-coated on the ITO substrate at 4000 rpm for 30 s, and then annealed on a hotplate at 100°C for 10 min. For the amphiphilic soft molecules (ASMs), CTAB, DTAB, and DTAC were dissolved in DMF (0.5 to 5 mg/mL) and spin-coated on PTAA at 6000 rpm for 70 s.
The perovskite precursor was dissolved in a mixed solvent (DMF/DMSO= 4:1 v/v) with a chemical formula of (Cs 0.05 FA 0.81 MA 0.14 )Pb(I 0.86 Br 0.14 ) 3 . Then the precursor was spin-coated on the PTAA or ASMs at 10000 rpm for 55 s. During the spin-coating at the 40 th s, 150 µL ethyl acetate was quickly dropped on the center of the substrates. The wet perovskite films were annealed at 100°C for 10 min. For the control device, DMF solution was spin-coated to treat PTAA before perovskite precursor spin-coating. Then, PCBM dissolved in chlorobenzene (20 mg/mL) was spin-coated at 2000 rpm for 60 s on the top surface of perovskite layer. Finally, the C 60 (~10 nm), LiF (~1 nm), and Cu (~100 nm) layers were thermally deposited in a vacuum chamber (< 5×10 -4 Pa). C 60 was deposited at a rate of 0.5 Å s −1 and the deposition rates of LiF and Cu were 0.1 and 10 Å s −1 , respectively.

Perovskite solar cell characterization
The current density and voltage (J-V) curves were obtained by a source meter (Keithley 2400) under AM1.5 sunlight at 100 mW cm −2 using a solar simulator (SAN-EI, XES-50S1).
The National Renewable Energy Laboratories (NREL)-calibrated KG5 filtered silicon reference cell was applied to calibrate the AM1.5 irradiance level. The J−V curves were measured along the reverse scan direction from 1.2 V to −0.3 V and the forward scan direction from −0.3 V to 1.2 V at a scan rate of 100 mV•s -1 . All of the devices without encapsulation were tested in ambient air (25°C, ~40% relative humidity). During the J-V measurement, all PSCs were masked with a 0.08 cm 2 metal aperture. External quantum efficiency (EQE) measurements were carried out using custom-built Fourier transform photocurrent spectroscopy based on a QE-R system (Enli Tech.). The J−V curves for exciton dissociation probability were measured along with the forward scan from -3 to 1.5 V by a source meter (Keithley 2400) under the illumination of AM1.5G and in the dark.
The electrochemical impedance spectra (EIS) were measured on a CHI604 electrochemical work station (CH Instrument Inc.). A 5 mV voltage amplitude was applied at an applied bias voltage of 1.1 V with frequencies between 10 5 and 100 Hz under dark conditions. Meanwhile, EIS of the corresponding devices were further measured under 1 sun illumination (AM 1.5G) with frequencies between 106 and 100 Hz. The results were fitted using the software of Zsim. All characterizations and measurements were performed in ambient conditions. Mott-Schottky fitted capacitance-voltage plots were measured on a CHI604 electrochemical work station (CH Instrument Inc.). A 10 mV voltage amplitude was applied at different direct current voltages ranging from -0.1 to 1.2 V with a frequency of 10 3 Hz under dark conditions. The V bi could be obtained from the C-V plots on the basis of Mott- is the active area, e is the elementary charge, ε and ε 0 represent the relative dielectric constant of the perovskite and vacuum dielectric constant, N is the doping density of perovskite, and V is the applied external voltage.

Device stability tests
The unencapsulated and unsealed devices were illuminated under continuous 100 mW

Perovskite film characterization
The morphologies of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and atomic force microscope (AFM). AFM was carried out using an FM-Nanoview 1000 equipped with Scanasyst-Air peak force tapping mode AFM tips from FSM-Precision Co., Ltd. The chemical compositions and structures of the perovskite films were analyzed by X-ray diffraction (Rigaku Smartlab X). X-ray photoelectron spectra (XPS) analysis was performed on an ESCALAB 250 system equipped with a monochromatic Al Kα