Materials Processing.
All materials were purchased from Sigma Aldrich. First, two different molar concentrations (0.15 M in 1-butanol and 0.3 M in 1-butanol) of titanium diisopropoxide bis (acetylacetone) were prepared and sonicated for 30 min. A solution of mesoporous titanium oxide paste was prepared by dissolving a titania paste in ethanol (20% in ethanol) before sonicating continuously for 2 hours. A solution of lead (II) iodide (PbI2) (> 98.9% purity, Sigma Aldrich) was also prepared from a mixture of 599.3 mg in 1 ml of DMF:DMSO (19:1, volume ratio). This was stirred continuously on a magnetic stirrer at 500 rpm for 2 hours at a constant temperature of 60 oC. The solution was then filtered through a 0.20 µm mesh filter.
Subsequently, a solution of formamidinium (FA)-rich mixed-organic halides was prepared by dissolving 60 mg of formamidinium iodide (FAI) (Sigma Aldrich), 6 mg of methylammonium bromide (MABr) (Sigma Aldrich) and 6 mg of methylammonium chloride (Sigma Aldrich) in 1 ml of isopropyl alcohol (Sigma Aldrich). The mixture was sonicated for 1 hour in an ultrasonic bath. Finally, a solution of 2,2',7,7'-tetrakis (N,N-di-p-methoxxyphenylamine)-9,9' -spirobifluorene (Spiro-OMeTAD) (> 99% purity, Sigma Aldrich) was prepared by mixing 72 mg of Spiro-OMeTAD, 17.5 µl of lithium bis (trifluoromethylsulphony) imide (Li-FTSI) (Sigma Aldrich) (500 mg in 1 ml of acetonitrile), and 28.2 µl of 4-tert-butylpyridine (tBP) (Sigma Aldrich) in 1 ml of chlorobenzene.
Fabrication of Layered PSCs.
FTO-coated glass and bare glass substrates were cut into dimensions 25 mm x 25 mm and sonicated successively (each for 15 min) in decon-90 detergent, deionized water, acetone (Sigma Aldrich), and IPA (Sigma Aldrich). The cleaned substrates were then blow-dried in nitrogen gas, prior to UV-Ozone cleaning (Novascan, Main Street, Ames, IA, USA) for 20 minutes to remove organic residuals. An electron transport layer (ETL) (that comprises compact and mesoporous layers of titanium oxide) was deposited onto FTO-coated glass, following a previous protocol22.
First, a compact titanium oxide (c-TiO2) was spin-coated onto the cleaned FTO-coated glass from the 0.15 M solution of titanium diisopropoxide bis (acetylacetone) at 2000 rpm for 30 s. This was annealed at 150 oC for 5 min before spin coating the 0.3 M solution of titanium diisopropoxide bis (acetylacetone) at 2000 rpm for 30 s. The deposited c-TiO2 was then sintered in a furnace (Lindberg Blue M, Thermo Fisher Scientific) at 500 oC for 30 minutes. The sample was allowed to cool down to room-temperature (~ 22 oC) overnight. Subsequently, a mesoporous titanium oxide (mp-TiO2) layer was spin coated from the solution of titanium oxide paste at 5000 rpm for 30 s before sintering at 500 oC for 30 min in a furnace (Lindberg Blue M, Thermo Fisher Scientific).
A perovskite layer was deposited onto mp-TiO2/c-TiO2/FTO-coated and a bare glass in two steps. The solution of PbI2 was spin-coated onto the mp-TiO2 layer at 1500 rpm for 30 s. This was annealed at 70 oC for 1 min. The solution of FA-rich mixed-organic halides was then spin-coated onto the PbI2 layer at 1300 rpm for 30 s. This was followed by annealing at 130 oC for 15 min before spin-coating the solution of Spiro-OMeTAD onto the perovskite layer at 5000 rpm for 30 s. It is important to note here that all layers (from c-TiO2 to Spiro-OMeTAD) were deposited under ambient conditions (room temperature was 22–25% and relative humidity was 20–60%). Finally a 80.0 nm thick gold (Au) layer was thermally evaporated onto the Spiro-OMeTAD layer using an Edward E306A evaporation system (Edward E306A, Easton PA, USA). The evaporation was carried out under a vacuum pressure of ~ 1.0 × 10− 6 Torr at a deposition rate of 0.15 nm s− 1.
Pressure Experiments.
To improve interfacial surface contacts and to heal grain-boundary defects, a range of pressures (0–10 MPa) was applied to the perovskite multilayers and the fabricated perovskite solar cells. A MicroTester Instron machine (MicroTester, Instron 5848, Norwood, MA, USA) was used with a PDMS anvil that protects the device. First, the PDMS anvil was fabricated from a mixture of Sylgard 184 silicone elastomer base and Sylgard 184 silicone elastomer curing agent (Dow Corning Corporation, Midland, MI) in a ratio 10:1 by weight. The mixture was degassed and cured (at 65oC for 2 hours) in a mold with polished silicon base.
The PDMS anvil was then cut out into the dimensions of the device‘s glass substrate and attached to the crosshead of the Instron machine with the surface that was cured against the polished silicon facing down. A schematic of the set-up before the ramping of the head of Instron is shown in Fig. 1a. The Instron was set to ramp in compression at a displacement rate of 1.0 mm min− 1 to press and hold down the device (Fig. 1b) at 3 MPa for 5 minutes. Unloading was then carried out at a displacement rate of ~ 1.0 mm min− 1. This was then repeated for different peak pressures (from 3 MPa to 10 MPa) on the PSCs and perovskite layers. The pressure – time curves that show the regions of loading, holding, and unloading are presented in Fig. 1c.
Characterization of Layered PSCs.
The optical absorbances of the multilayer films were measured using an Avantes UV-Vis spectrophotometer (AvaSpec-2048, Avantes, BV, USA). The PL and TRPL measurements were obtained by exciting the films with a 405 nm picosecond laser source (Aura Technology PIXEA) using 270 nW incident power, 2 kHz repetition rate and 50% of tuning. The excitation signal was sent to a Horiba MicOS microscope optical spectrometer system that consists of a Horiba iHR550 spectrometer, a luminescence microscope with a 50X Edmund Optics Plan Apo NIR Mitutoyo objective, and a Horiba Synapse EM CCD camera. The PL spectra and TRPL measurements were then obtained using a single photon counter module (SPD-OEM-VIS, Aurea Technology) and an acquisition software interface.
The microstructures of perovskite films and the cross section of PSCs were obtained using a field-emission scanning electron microscope (SEM) (JEOL JSM-700F, Hollingsworth & Vose, MA, USA) with an SEM working distance of 10–11 mm at a low accelerating voltage of 5 kV. Current density against voltage (J-V) were measured (before and after the pressure treatment) using a Keithley source meter unit (SMU2400) (Keithley, Tektronix, Newark, NJ, USA) that was connected to an Oriel solar simulator (Oriel, Newport Corporation, Irvine, CA, USA) under AM1.5G illumination of 90 mW cm− 2. The SMU was operated using a KickStart instrument control software. The solar simulator was calibrated using a 918D high performance calibrated photodiode sensor (Newport). The device was masked to expose an area of 0.13 cm2 to illumination.