CsPbCl3‐Driven Low‐Trap‐Density Perovskite Grain Growth for >20% Solar Cell Efficiency

Abstract Charge recombination in grain boundaries is a significant loss mechanism for perovskite (PVK) solar cells. Here, a new strategy is demonstrated to effectively passivate trap states at the grain boundaries. By introducing a thin layer of CsPbCl3 coating before the PVK deposition, a passivating layer of PbI2 is formed at the grain boundaries. It is found that at elevated temperature, Cl− ions in the CsPbCl3 may migrate into the PVK via grain boundaries, reacting with MA+ to form volatile MACl and leaving a surface layer of PbI2 at the grain boundary. Further study confirms that there is indeed a small amount of PbI2 distributed throughout the grain boundaries, resulting in increased photoluminescence intensity, increased carrier lifetime, and decreased trap state density. It is also found that the process passivates only grain surfaces, with no observable effect on the morphology of the PVK thin film. Upon optimization, the obtained PVK‐film‐based solar cell delivers a high efficiency of 20.09% with reduced hysteresis and excellent stability.

All used solutions were filtered through a 0.4-μm pore PTFE filter and were stored in a dry nitrogen atmosphere.

Device Fabrication
Preparation of the TiO 2 blocking layer: Fluorine-doped tin oxide (FTO)-coated glass with a size of 25 × 25 mm 2 was washed sequentially with detergent, deionized water, acetone, and isopropanol with ultrasonication for 10 min each, and then were dried by N 2 and treated by an O 2 plasma. The clean substrate was immersed in a 40 mM TiCl 4 aqueous solution for 30 min at 70 °C and washed with distilled water and ethanol, followed by annealing at 200 °C for 30 min in air to form a compact n-type blocking layer of TiO 2 .
Growth of the CsPbX 3 QDs film: The QDs layer was fabricated on the above-prepared TiO 2 layer at 2500 rpm by spin-coating the prepared QDs solutions with different concentrations. Immediately after the spin-coating, the prepared film was quickly dipped sequentially into saturated Pb(OAc) 2 EA solution and neat EA solution.
Growth of the PVK film: The prepared PVK precursor solution was spin-coated onto the QDs film at a speed of 4000 rpm for 20 s. CB (100 μL) was dropped onto the spinning substrate during the spin-coating step at 10 s before the end of the procedure. The film was then heated at 150 °C for 15 min.
Assembly of the PSCs: An HTL film was prepared by spin-coating the HTL solution onto the PVK film at 4000 rpm for 30 s. Finally, a gold electrode with a thickness of ~70 nm was thermally evaporated onto the Spiro-OMeTAD-coated film to finish the device fabrication.
The bare devices without any encapsulation were stored and tested upon exposure to the ambient environment (in air at relative humidity of ~30% at 25 °C).

Characterization
The film surface morphology and cross-sections were characterized by SEM and EDS (Jeol SU-8020). The TEM images were obtained using an FEI Tecnai T20 equipped with a Gatan SC200 CCD camera and LaB6 filament operated at 200 kV. The XPS measurements were performed in a VG ESCALAB MK2 system with monochromatized Al Kα radiation. XRD patterns were performed on a DX-2700 with Cu K radiation ( = 0.15418 nm). Absorbance spectra were collected using a Shimadzu UV-3600 double beam spectrometer. PL spectra were measured using a Horiba Jobin Yvon Fluoro Log2 spectrofluorometer. TRPL spectra were acquired according to a time-correlated single photon counting method using an Edinburgh Instruments FLS920 fluorescence spectrometer. The laser diode is capable of a repetition rate of 80 MHz; however, the repetition rates were adjusted as appropriate to observe the full decay. J-V curves were measured at 25 °C under AM1.5G (100 mW/cm 2 ) illumination (scan rate: 0.5 V/s, both forward (from I SC to V OC ) and reverse (from V OC to I SC ) scan modes). Here, the bare devices without any encapsulation were stored and tested upon exposure to the ambient environment (in air at relative humidity of 25%~35% at 25 °C). A black cardboard mask with a window area of 0.09 cm 2 was clipped onto the glass side to define the active area of the cell. The spectral response was taken by an EQE measurement system (QEX10, PV Measurement), which was equipped with a monochromator, a lock-in amplifier, a Xe lamp, and a current-voltage amplifier. Prior to the use of the light, the spectral response and the light intensity were calibrated using a mono-silicon detector. The frequency-dependent capacitances (C-f) of the devices were obtained by a semiconductor device analyzer (Agilent Technologies, B1500A). An AC voltage perturbation of 20 mV and a constant bias at the open voltage was maintained. Each spectrum was measured covering the range from 1 KHz to 1 MHz.