2.1. Fabrication of perovskite solar cells based on MAPbI3 micro-needles

Fluorine-doped tin-oxide (FTO) glass substrates were properly washed in an ultrasonic bath containing acetone followed by rinsing with ethanol and water for 20 min. The precursor solution was made by mixing 0.05 M zinc acetate dihydrate in ethanol : water (30 : 70 v/v) solution. Primarily, a condensed layer of ZnO was formed onto the FTO glass via a spin coating route at 2500 rpm for 15 s and heated at 125 °C for 5 min. After that, a mesoporous layer of ZnO was deposited using an electrospray method under optimum film deposition conditions at a precursor spraying rate of 0.004 mL min⁻¹, along a substrate to nozzle gap of 4.5 cm by employing a constant high voltage of 6.5 kV.²¹ The temperature of the hot plate was set at 150 °C to effectively evaporate the solvent during the entire process. PbI₂ micro-needles were synthesized using the hydrothermal synthesis at a low temperature of 100 °C.²⁰ To synthesize PbI₂ micro-needles, one solution was prepared by dissolving 0.95 g of lead acetate (Pb(CH₃CO₂)₂) in 10 mL of acetic acid and 0.04, 0.07 and 0.1 g of CTAB (to check the effect of the surfactant amount on the morphology) were also added to the same solution. Another solution was formed by dissolving 0.4193 g of potassium iodide (KI) in 5 mL of distilled water. After that, the solution of KI was poured drop-wise into the (Pb(CH₃CO₂)₂) solution in acetic acid, using continuous magnetic stirring to produce a yellow-colored solution. By effective stirring for 10 min, 15 mL of deionized water (DI) were poured into the final solution and subsequently transferred into an autoclave and heated at 100 °C for 8 h to grow the PbI₂ micro-needles. After cooling down the final product of PbI₂ micro-needles, they were rinsed with distilled water to remove the residual salts and eventually dried at 60 °C for 4 h for further processing. Hydrothermally grown PbI₂ micro-needles were further deposited as a layer by dissolving them in 1.5 mL of N,N-dimethylformamide (DMF) at 70 °C. After that, 20 μL of PbI₂ micro-needle solution was poured onto the substrate surface and it was held for 10 s and then spin-coated at 3000 and 6000 rpm for 5 s, respectively. MAPbI₃ micro-needles were formed by pouring a 200 μL MAI solution in isopropanol (35 mg : 5 mL) with the addition of 50 μL of DMF and boarded on the PbI₂ micro-needle layer for 40 s and spin-coated at 4000 rpm for 20 s followed by drying at 130 °C for 10 min. The HTL was deposited by spin coating 30 μL of spiro-OMeTAD (80 mg of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene, 8.4 μL of 4-tert-butylpyridine, and 51.6 μL of bis(trifluoromethane) sulfonamide lithium salt (Li-TFSI) solution (154 mg mL⁻¹ in acetonitrile) in 1 mL chlorobenzene) solution for 10 s at 2000 rpm. Finally, a 90 nm thick top layer of Au was formed by thermal evaporation. For comparison, a perovskite absorber with a nanocubic morphology was obtained via a two-step sequential deposition method as described previously by us.²²