Growth and characterization of orthorhombic cesium lead tri-iodide perovskite thin-films by sequential physical vapor deposition for solar cells

• Sequential physical vapor deposition of CsPbI 3 has been demonstrated. • Films exhibited large average crystallites and uniform surface coverage with polycrystalline grains. • Devices delivered PCEs correlating to the density of trap states. This study used the sequential physical vapor deposition (SPVD) technique to grow poly-crystalline yellow phase cesium lead triiodide (γ-CsPbI 3 ). The effect of CsI thickness on structural, optical, morphological, and electrical properties of γ-CsPbI 3 was investigated. Crystallographic parameters of γ-CsPbI 3 as-deposited and 100 °C annealed pure-phase were determined using X-ray diffraction (XRD). Computed lattice constants were a = 4.88, b = 9.96, and c = 16.5 Å, with an average crystallite size increasing from 170 − 243 nm, and micro-strain decreasing with an increase of cesium iodide (CsI) thickness from 200 to 500 nm. Field-emission scanning electron microscopy (FE-SEM) images showed uniform surface coverage with polycrystalline grains. Average grain size increased from 168 to 235 nm with increasing CsI thickness, resulting in large, pinhole-free, and tightly packed grains. Furthermore, atomic force microscopy (AFM) surface analysis demonstrated a reduction in surface roughness from 44.0 to 38.3 nm as CsI thickness increased from 200 to 500 nm. The ultraviolet–visible (UV–Vis) spectra showed an increase in bandgap from 2.24 to 2.38 eV for as-deposited and a decrease from 2.37 to 2.05 eV for annealed films as the thickness of CsI increased. Current–voltage (J-V) measurements revealed a correlation between the trap-filled limit voltage (V TFL ) and defect density. For electron-only devices with film thicknesses of 300, 400, 500, and 600 nm, the trap state densities were found to be 1.40 × 10 15 , 1.95 × 10 15 , 2.12 × 10 15 , and 2.39 × 10 15 cm −3 , respectively. The corresponding electron mobilities were 2.34 × 10 1 , 6.30 × 10 1 , 4.72 × 10 2 , and 1.07 × 10 2 cm 2 /V·s. Additionally, perovskite solar cells (PSCs) fabricated under ambient air conditions revealed an improvement in power conversion efficiency (PCE) from 2.0 to 4.93 % for the FTO/c-TiO 2 /CsPbI 3 /Au architecture. This work suggests a way of improving precursor’s reaction using SPVD by controlling the film thickness, reducing defect density through SPVD, and therefore highlighting film thickness optimization in perovskites.