Graded bandgap device architecture of perovskite solar cells

This study purposes graded bandgap design for lead-free perovskite solar cells which aim to maximize the solar spectrum with good output current and and better power conversion efficiency (PCE) by improving the solar cell archtecture. Titanium dioxide (Ti02) was used as electron transport layer (ETL) and SpiroOMeTAD was used as hole transport layer (HTL) due to its facile implementation and high performance in electronic device. Lead-free bismuth oxyiodide (BiOI) was chosen to replace conventional lead-halide perovskite absorber layer. BiOI has iso-electronic properties to lead-halide perovskite with high efficient light absorption, high thermal stability andphotocatalic activity, excellent photo-generated charge carrier. The variation ofioidine concentration in BiOI establishes bandgap tuning and conductivity type of the layer BiOI films. The increase of iodine concentration would reduce band gaps and induce the change of semiconductor behaviour from n-type to p-type. In this strategy, the absorbance component consists of three BiOI perovskite layer with different concentration of iodine that form n- and p- type homojuctions. BiOI with half concentration of iodine (BIOI 0.5) is first perovskite layer, then the second perovskite layer is BiOI with same concentration of iodine (BIOI 1.0) and the third perovskite layer is BiOI with double concentration of iodine (BIOI 2.0). This configuration produces cells with desirable performance that effectively absorb the photons in almost all parts of the solar spectrum. Both open circuit voltage (Voc) (940 mV') and fill factors (—58%) for the best cells have shown drastic improvement over single active layer device and the short current densities (Jsc) measured are in the range (20-30) mAcm-2 The effects of quasielectric fields, caused by the band-gap variation of the active semiconductor, upon the illumination current density and open-circuit voltage of a solar cell will be discussed.