2.3. Fabrication of Perovskite Solar Cells

All the starting materials for solar cell preparation were purchased from Sigma–Aldrich (St. Louis, MO, USA) unless otherwise specifically stated, and used as received. The hole transporting material 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine) 9,9′-spirobifluorene (spiro-OMeTAD) was purchased from Merck KGaA, Darmstadt, Germany.

For CH₃NH₃PbI₃ (MAPI) film deposition, 1 M PbI₂ and methylammonium iodide were dissolved in dimethylformamide and stirred for at least 2 h at room temperature. To facilitate the substrate wetting properties of sputter prepared AZO layer, 60 μL of phenyl-C61-butyricacid (PCBA 0.1 mg/mL) in 1,2-dichlorobenzene solution was spin cast on AZO layer at 2000 rpm for 50 s. Once the MAPI precursor materials were completely dissolved, 80 μL of this solution was spin cast on LT-TiO₂ and AZO substrates at 3000 rpm for 50 s. While the substrates started spinning, right after 8 s, 200 μL of toluene (orthogonal solvent) was continuously dripped onto the substrates. Furthermore, the substrates were annealed at 100 °C for 25 min on a hot plate inside a N₂ filled glovebox, resulting in an approximately 400 nm thick uniform MAPI layer. Successively, a hole transporting layer was deposited onto the perovskite layer by spin casting a chlorobenzene solution of spiro-OMeTAD (72.3 mg/mL), 43.2 μL of 4-tert-butyl-pyridine (TBP), and 26.3 μL of LiTFSI (520 mg/mL in acetonitrile) at 2000 rpm for 50 s in the glovebox. These substrates were stored overnight in a dry box (relative humidity < 10%) to facilitate the air doping of spiro-OMeTAD layer. Finally, approximately 70 nm thick gold back contact was deposited in a PVD chamber, completing the solar cells. The active area of the fabricated devices is either 0.09 or 0.16 cm² defined by the cross-section area of the bottom and top electrodes.

Characterization of the solar cells was carried out by using an Oriel solar simulator under AM 1.5, 1000 W m⁻² conditions and a Keithley 2400 source meter. The current–voltage (I–V) curves were measured from +1.5 V to 0.5 V and back at the scan rate of approximately 130 mV s⁻¹ under N₂ atmosphere. The photovoltaic parameters such as J_sc, V_oc, fill factor (FF) and PCE were determined from the corresponding I–V curves using a home-built software.