Kengo Hamada

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, 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.

Highly Efficient 17.6% Tin–Lead Mixed Perovskite Solar Cells Realized through Spike Structure

Frequently observed high Voc loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

Enhancement of efficiency for mixed metal Sn/Pb perovskite solar cells from the view point of hetero-interface traps (Conference Presentation)

Absorption edge of perovskite (PVK) solar cells consisting of MAPbI3 is 800nm. According to our simulation, light harvesting in the area of near IR is also necessary for enhancing the efficiency more. We have already reported that mixed metal perovskite (MAPbSnI3) shows photo-conversion in IR region (1-6). The short circuit current (Jsc) was high, reaching to 30 mA/cm2 because of the wide range of light harvesting. However, the open-circuit voltage (Voc) was lower than 0.3 V and the estimated voltage loss was 0.6-0.7 V, which was much larger than that of MAPbI3 (0.4 V), suggesting the presence of high density charge recombination center. We found that Ti-O-Sn bonds formed at the interface between Tiania and MAPbSnI3 layer, create trap states, resulting in increasing charge recombination at the interfaces. The surface trap density and the trap depth was quantitatively monitored by thermally stimulated current methods. When the Ti-O-Sn linkage was formed at the interface between TiO2 and PVK, the trap density increased by one order of magnitude. The interface was analyzed by XPS and confirmed that Ti-O-Sn linkage was formed. After removing the Ti-O-Sn bond at the interface between TiO2 and MAPbSnI3, the efficiency drastically increased from 4.0 % to 13.8 % and the stability was improved. It was proved that interface architecture is quite important for enhancing the MAPbSnI3 solar cells.nReferencesn1. S. Nakabayashi, et al., J. Photonics for Energy; 2015, 5, 057410. 2. Y. Ogomi, et al., J. Phys. Chem. Lett. 2014, 5, 1004-1011.