The effect of bandgap grading on performance on lead-free perovskite solar cells

Lead (Pb)-halide perovskite materials have been widely used in perovskite solar cells (PSCs) application but the presence of Pb in this device can harm human beings and environmental to a significant extent. Thus, the search toward Pb-free-halide perovskite and low-toxicity materials has been done to replace the conventional Pbhalide perovskite for sustainable environment. Among the possible Pb-substitution strategies, the trivalent bismuth (Bi) cation possesses the same electronic configuration as Pb and comparable optoelectronic properties to the Pb-halide perovskites. Unfortunately, Bi-halide PSCs showed a very small power conversion efficiency (PCE) compared to Pb-halide PSCs. This work demonstrated how the device architecture able to overcome the efficiency related challenge in PSCs. The graded bandgap structure has been designed in Bi-halide PSCs to improve the output current and PCE by maximizing the solar spectrum. In this work, Titanium dioxide (Ti02) was used as electron transport layer (ETL) and Spiro-OMeTAD was used as hole transport layer (HTL) due to its facile implementation and high performance in electronic device. The variation of ioidine concentration in Bi-doped iodide estabishes bandgap tuning and conductivity type of the layer. The increse of iodine concentration would reduce band gaps and induce the change of semiconductor behavior from n-type to p-type. In this strategy, the absorbance component consists of three Bi-based perovskite layer with different concentration of iodine that form n- and p- type homojuctions. This configuration produces cells with desirable perforamance that effectively absorb the photons in almost all parts of the solar spectrum. The open circuit voltage (Voc) (940 mV), short current densities (Jsc) ( N25 mAcm -2 ) and fill factors ( N58%) for the best cells have shown drastic improvement compared to single active layer device. The effects of quasi-electric fields due to the band-gap variation of the active semiconductor, upon the illumination Jsc and Voc will be discussed further.