Abstract
Perovskite solar cells based on series of inorganic cesium lead bromide and iodide mixture, CsPbBr3-xIx, 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 (CsPbBr2.9I0.1) was used as the perovskite and spiro-OMeTAD as the hole transport material (HTM). We investigated the origin of variation in open circuit voltage, Voc 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 Voc 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 Voc 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.
Highlights
Perovskite solar cells have attracted great attention as the prime energy conversion devices, owing to its reasonably low fabrication cost without compromising its photovoltaic performances
Despite CsPbI3 perovskite solar cell fabricated under vacuum-processed has achieved high efficiency of 10.5 %, subsequently the improved efficiency of mixed halides or mixed halide-mixed Pb/Sn perovskite solar cells up to 11 % has been reported, the need of high technology equipment such as vacuum chamber and glove box during the fabrication process have restrained its outdoor applications competency owing to unsolved stability issue and high production cost[12,13,14,15]
The factors that control the Voc are the adequate match of the Lowest Unoccupied Molecular Orbital (LUMO) level of the electron transport material (ETM) and the conduction band of the perovskite layer, as well as the Highest Occupied Molecular Orbital (HOMO) level of the hole transport material (HTM) and the valence band of the perovskite layer[16]
Summary
A device with high overall photoconversion efficiency of 3.98 % was obtained when small amount of iodide (CsPbBr2.9I0.1) was used as the perovskite and spiro-OMeTAD as the hole transport material (HTM). An inverted perovskite solar cell using conducting polymers as the hole-selective contact materials exhibited direct correlation between higher work functions of HTM and lower charge recombination rate, the Voc was increased[17,19]. It is notable that the Voc is originated from two factors, which are the band gap of the perovskite material and the perfect matching of electron and hole transporting interface In this manuscript, we combine two important properties in achieving high Voc in standard solar cells, which are the tunable band gap in inorganic perovskite absorbers and the capability to extract hole charge carrier properly. The stability issues were analyzed in this work, concluding that small amount of iodide stabilize all photovoltaic parameters
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