(Invited) Interface Architecture Between TiO2/Perovskite, Perovskite/Hole Transport Layer, and Perovskite Grain Boundary for MAPbI3, MASnPbI3 and CsPbI3 Solar Cells
Authors: Hayase Shuzi 
Published: February 2020

The relationship between photovoltaic performances and interface structures on MAPbI3, CsPbI3, and MAPbI3 are reported. In addition, role of Cl anions are at the interfaces are discussed. In order to examine the interface structure, quartz crystal microbalance sensor (QCM) was used. The amount of PbI2 adsorption on TiO2 surface increased as the adsorption time and leveled off at a certain time. PbI2 still remained even after the solvent only (DMF) was passed on the TiO2 layer on QCM (namely rinsing with DMF), suggesting that the PbI2 was tightly bonded on the TiO2 surface. The structure was found to be Ti-O-Pb linkage by XPS analysis. We concluded that the Ti-OH on the surface of TiO2 reacts with I-Pb-I to form Ti-O-Pb-I. The surface trap density was measured by thermally stimulated current (TSC) method. Before the PbI2 passivation, the trap density of TiO2 was 1019 cm3. The trap density decreased to 1016/cm3 after the PbI2 passivation, suggesting that the TiO2 surface trap was passivated with I-Pb-I.  It was found that Jsc increased with an increase in the Ti-O-Pb density. We concluded that the interface between TiO2 and perovskite layer has passivation structure consisting of Ti-O-Pb-I which decreases the trap density of the interfaces and suppresses charge recombination. The effect of Cl anion on high efficiency is still controversial when perovskite layer is prepared by one step method from the mixture of MAI and PbCl2. It was found that adsorption density of PbCl2 on TiO2 surface was much higher than that of PbI2 from the experiment using QCM sensor. After the surface was washed with DMF, Cl and Pb were detected. These results suggest that the TiO2 surface was much more passivated by PbCl2 than by PbI2. This may explain partially the high efficiency when the perovskite layer was fabricated by one step process consisting of MAI and PbCl2 solution. We also observed that the crystal size increased with an increase in the amount of Cl anion which of course one of the explanation of the high efficiency. The interface of hole transport layer/perovskite layer, and between perovskite layer /perovskite layer (grain boundary) was passivated with organic amines in the same way. The passivation was also effective for increasing Voc and Jsc (Efficiency: 17.5%). This was explained by the results of transient absorption spectroscopy that the charge recombination time between hole transport payer/perovskite layer increased from 0.3 to 60 μsec. 

It was found that the interface between TiO2 and perovskite has the structure of Ti-O-Pb-I. The bonding becomes the anchoring moieties for perovskite layers on TiO2 and passivates the surface traps of TiO2, which suppresses the charge recombination at the interface. It was proved that the perovskite solar cell efficiency increased with an increase in the Ti-O-Pb-I density. Cl ions which have been still controversial also increased the density of Ti-O-Pb-Cl groups between TiO2 and perovskite lauers, which help enhance the efficiency. Passivation of perovskite/hole transport layer was also tried and it was proved to be effective for increasing the efficiency (17.5%).