Tunable Open Circuit Voltage by Engineering Inorganic Cesium Lead Bromide/Iodide Perovskite Solar Cells

Perovskite solar cells based on series of inorganic cesium lead bromide and iodide mixture, CsPbBr_3-xI _x , where x varies between 0, 0.1, 0.2, and 0.3 molar ratio were synthesized by two step-sequential deposition at ambient condition to design the variations of wide band gap light absorbers. A device with high overall photoconversion efficiency of 3.98 % was obtained when small amount of iodide (CsPbBr_2.9I_0.1) was used as the perovskite and spiro-OMeTAD as the hole transport material (HTM). We investigated the origin of variation in open circuit voltage, V_oc which was shown to be mainly dependent on two factors, which are the band gap of the perovskite and the work function of the HTM. An increment in V_oc was observed for the device with larger perovskite band gap, while keeping the electron and hole extraction contacts the same. Besides, the usage of bilayer P3HT/MoO₃ with deeper HOMO level as HTM instead of spiro-OMeTAD, thus increased the V_oc from 1.16 V to 1.3 V for CsPbBr₃ solar cell, although the photocurrent is lowered due to charge extraction issues. The stability studies confirmed that the addition of small amount of iodide into the CsPbBr₃ is necessarily to stabilize the cell performance over time.