2D PEA2SnI4 Inkjet-Printed Halide Perovskite LEDs on Rigid and Flexible Substrates

Lead-free PEA2SnI4-based perovskite LEDs are successfully inkjet-printed on rigid and flexible substrates. Red-emitting devices (λmax = 633 nm) exhibit, under ambient conditions, a maximum external quantum efficiency (EQEmax) of 1% with a related brightness of 30 cd/m2 at 10 mA/cm2.


Ink preparation
0.16 M precursor solution was prepared by dissolving stoichiometric PEAI and SnI 2 in a solvent mixture of DMF:DMSO (4:1) and stirred at room temperature. For the inks with additives, SnF 2 (0.016 M) and w NaBH 4 (1 mM) NaBH 4 , respectively, were added to the precursor solutions.

Inkjet printing and thermal treatments of PEA 2 SnI 4 layer
The details of the printing procedure of the PEA 2 SnI 4 are indicated as follows: the ink formulation was based on mixing DMF/DMSO solvents (4:1). An ink ejection frequency of 2.0 kHz and a resolution of 560 drops per inch (DPI) was used. During the PEA 2 SnI 4 layer printing, the platen substrate temperature was kept constant at 20 °C in order to promote a proper spreading and adhesion among the different sublayers piled up to complete the thick full layer. After the inkjetprinting process, the layers were submitted to the curing process based on vacuum annealing at 110 °C for 15 minutes.

Device Fabrication
The PEDOT:PSS solution was filtered with 0.45 m PVDF filter and spin coated on top of ITO at 5,000 rpm (2,000 rpm/s of acceleration) for 40 s and annealed at 130 °C for 20 min in ambient conditions. After the hole transporting layer (HTL) deposition the substrates were introduced in a N 2 -filled glovebox, for the PEA 2 SnI 4 inkjet printing layer deposition. The filtered solution of perovskite ink, with a viscosity around ~3 cP, was printed with a fixed drop-volume of 10 pL through 21-μm diameter nozzle by using cartridge for Dimatix printers (Fujifilm Dimatix Inc.) on top of PEDOT:PSS-coated layers. The preparation of the inks and the printing processes were both carried out in a glove box.
Finally, the substrates were transferred to a vacuum chamber, to evaporate 40 nm of PO-T2T, 1 nm of LiF and 100 nm of Al. The area of the flexible device was defined by the evaporation mask, which was 0.08 cm 2 .

Inkjet-printed film/device characterization
The morphology and structure of Inkjet-printed perovskite thin films were studied by field emission scanning electron microscope (FEGSEM -JEOL 3100F) operated at 15 kV.
The crystalline microstructure, orientation and size of the NCs were determined by XRD using a Japan Rigaku D/Max-IIA X-ray diffractometer using Cu Kα radiation, l = 1.5406 Å, operating at 40 keV and 40 mA.
The optical properties were evaluated by measuring the transmittance and reflectance spectra, by using an integrating sphere (Bentham PV300 EQE system), using monochromated light from a Xe and quartz halogen dual lamp source through the 300-1100 nm range, and collecting the transmitted (or reflected) light with an InGaAs photodetector. Finally, the emission properties were determined by acquiring photoluminescence (PL) spectra of the films grown onto Si substrates, exciting the samples with the 325-nm line of a He-Cd laser, with a power density of 8 × 10 3 W cm -2 , and analyzing the emitted light with a single-grating monochromator coupled to a GaAs photomultiplier.
External quantal efficiency (EQE) measurements were performed with a QEPVSI-b Oriel system. Stability measurements were performed fxing the voltage Figure S1 : XRD pattern of inkjet-printed PEA 2 SnI 4 after vacuum annealing; detail of highresolution XRD spectra (in the range from 4.5° to 6.5°) of PEA 2 SnI 4 films (inset). In order to achieve the bet device structure, we selected PO-T2T as the electron-transporting acceptor due to its flat band energy level (−3.5 eV) 1,2 which matches well with the conduction band of PEA 2 SnI 4 (−3.3 eV) 3 . Beside reduced losses at the ETL/perovskite interface, PO-T2T has a higher electron mobility (≈10 −3 cm 2 V −1 s −1 ), low refractive index (∼1.72). 2,4 In the same way the PEDOT:PSS 4083 was selected for is high hole conductivity (10 −2 to 10 −3 S cm −1 ). 5 Figure S3: Energy level diagram.     Figure S6: EL spectral characteristics of inkjet-printed PEA 2 SnI 4 devices is red emitting with a peak centered at 630 nm with FWHM of 25 nm at different operation voltage. Biblioraphy: