Yang Shihe, Professor, Shenzhen Graduate School, Hong Kong University of science and technology and Peking University

In recent years, perovskite solar cells have become a research hotspot in the field of photovoltaic due to its high efficiency, low cost and easy assembly The conventional perovskite solar cells are usually three-layer sandwich structure of "electron transport layer / perovskite / hole transport layer" Because the common hole transport materials such as spiro ometad, PTAA and so on are generally expensive, and easily induce the decomposition of perovskite materials, the development of perovskite solar cells is greatly limited The perovskite solar cells without hole transport layer on carbon base make use of the hole transport ability of perovskite materials, which can save the hole transport layer, and replace expensive precious metals with carbon materials as electrode materials, which can greatly reduce the cost of batteries, improve the stability of perovskite solar cells, and accelerate the pace of commercialization However, for carbon based perovskite solar cells with hole transport layer removed, the difference of quasi Fermi energy levels between electrons and holes tends to be smaller, and the open circuit voltage (VOC) is limited, so the device efficiency is not high Recently, Professor Yang Shihe of Shenzhen Graduate School of Hong Kong University of science and technology and Peking University made an article on ferroelectric materials, with the title of "an ultrathin ferroelectric perovskite oxide layer for high performance hole transport material free carbon based halide perovskitesolar cells" published research paper in adv.function Material 10.1002/adfm.201806506 ）。 The key point of this work is to introduce an ultra-thin PbTiO 3 ferroelectric material between the TiO2 electron transport layer and perovskite to form TiO2 / PbTiO 3 composite Interestingly, PbTiO 3 also has perovskite structure, except that PbTiO 3 is an oxide, and the perovskite active absorption layer in solar cells is a kind of halide The induced polarization electric field of PbTiO 3 and the built-in electric field of the active layer of halogen perovskite are superposed each other (Fig 1), which can further enhance the separation efficiency of electrons and holes in the cell, reduce the interface recombination, thus breaking the limited open circuit voltage, and finally improve the efficiency of the carbon based perovskite solar cell without hole transport layer to 16.37% This efficiency is the highest efficiency of the carbon based hole free transport layer structure reported at the time of publication of the paper At the same time, this kind of battery also shows excellent stability Fig 1 A-b) energy band diagram of battery; c) schematic diagram of device structure; d) PbTiO 3 crystal cell structure (source: adv funct Mater.) the author first characterized the structure of the material (Fig 2) The morphology of Porous TiO 2 (TiO2 / PbTiO 3-0.02) covered by ultra-thin PbTiO 3 can not be observed by SEM However, through HRTEM, it can be seen that the smooth surface of TiO 2 becomes rough, and the crystal spacing corresponds to TiO 2 (101) and PbTiO 3 (101), respectively The optimal thickness (Figure C, concentration 0.02 M) for cell devices is only about 1 nm Because the signal is too weak due to the thickness of the cover, there is no obvious peak position of TiO2 / pbtio3-0.02 under the XRD test After the author further improves the cover thickness (concentration 0.2m), there is an obvious corresponding peak of tetragonal PbTiO3 The binding energy of Ti, Pb and O decreased in XPS, mainly due to the electron absorption effect of Pb ion There is no obvious change in the UV-Vis absorption intensity of TiO 2 substrate before and after covering ultra-thin PbTiO 3, and the band gap width of the two materials is not much different, so the additional loss of photon energy caused by the introduction of PbTiO 3 is very small, which is very suitable for the interface material of TiO 2 Fig 2 A-b) morphology change of ultra-thin PbTiO 3 before and after covering porous TiO 2 surface, J) absorption change of UV visible band, C-D) HRTEM, e) XRD, F-G) XPS test results (source: adv funct Mater.) using AFM The piezoelectric response of the tested material is mainly based on the first-order and second-order resonance response of the material Under the same driving voltage, the amplitude of the first-order resonance response increases with the increase of the voltage The signal mainly comes from the piezoelectric response of the material, while the second-order resonance response is mainly caused by the electrostatic force, ion movement and other factors in the sample It can be seen from Fig 3 that the first-order response of TiO2 is much smaller than the second-order response (Fig C), so there is no obvious piezoelectric effect; while the first-order response of TiO2 covered with PbTiO3 is significantly enhanced, and larger than the second-order response (Fig a), so it shows a strong piezoelectric effect It is worth noting that the effect decreases to a certain extent with the increase of calcination temperature (Fig b), which is mainly related to The microstructure of PbTiO 3 is related to the destruction of its morphology The results of ferroelectric polarization reversal test show that the phase of PbTiO 3 changes 180 ° with the change of different AC voltage, which is mainly due to the increase of voltage, resulting in the reversal of the original domain direction The butterfly curve of amplitude shows that the ultra-thin PbTiO 3 shows strong ferroelectric performance under the optimal conditions The J-V curve of the device is analyzed The results show that the introduction of ultra-thin PbTiO 3 greatly increases the open circuit voltage and current of the battery, and the optimization effect will decrease with the decrease of ferroelectric properties and the increase of PbTiO 3 thickness Fig 3 A-c piezoelectric response analysis of TiO 2 / PbTiO 3 (calcined at 450 ℃), TiO 2 / PbTiO 3 (calcined at 550 ℃) and TiO 2 under AFM test, D-E) ferroelectric polarization reversal test under different AC voltage, F-G) J-V curve of different materials and thicknesses (source: adv funct Mater.), Mott Schottky curve of TiO 2 and PbTiO 3 (Fig 4C) ）It can be seen that the ultra-thin PbTiO 3 makes the potential of TiO 2 move to the negative direction, which indicates that the introduction of PbTiO 3 improves the Fermi energy level of electrons (FIG 4A), and the valence band and conduction band energy levels of PbTiO 3 can be calculated by ups and band gap The conduction band energy level of PbTiO 3 is higher than that of TiO 2 To some extent, it can reduce the reverse electron transport, which is helpful to reduce the interface recombination The tunneling effect plays an important role in this transport mechanism The enhancement of the built-in electric field in the cell also proves that the introduction of PbTiO 3 is helpful to increase the open circuit voltage This is confirmed by the J-V curve in Figure 3 Figure 4 A) vacuum energy level diagram; b) UPS test of TiO 2 and PbTiO 3; c) Mott Schottky curve of TiO 2 and PbTiO 3; d) difference of built-in potential with and without ultra-thin PbTiO 3 covered cells (source: adv funct Mater.) finally, the author mainly tested the difference of battery performance with and without PbTiO 3 covered devices (Figure 5) It can be seen from the J-V curve that the hysteresis of the device is very small, which is related to the integration of CNT interface and the enhancement of electron hole separation efficiency; IPCE corresponds to the increase of current; the steady-state PL test results show that the introduction of PbTiO 3 enhances the fluorescence quenching, which indicates that the electron export is more effective; the transient PL test results show that the electron export time is shorter; EIS The test is mainly carried out in the dark field and near open circuit voltage Under this condition, the semi arc is mainly the composite resistance of the device It can be seen that the composite resistance of the device with PbTiO 3 cover is far greater than that of the device without PbTiO 3 cover under different bias voltage, which proves that PbTiO 3 is helpful to reduce the device interface composite Fig 5 A) positive and negative sweep J-V curves with and without PbTiO 3 covered devices; b) IPCE curves with and without PbTiO 3 covered devices; c) steady state PL tests; d) transient PL tests; E) EIS Nyquist curves; F) variation of composite resistance with bias voltage (source: adv funct Mater.) this work introduces ferroelectric materials into carbon based perovskite solar cells for the first time The polarization electric field is used to enhance the built-in electric field of the cell, so as to improve the separation efficiency of electrons and holes, reduce the compound loss of interface electrons and holes, further improve the open circuit voltage, and increase the efficiency of carbon based hole free perovskite solar cells to 16.37%, which provides a new idea for how to further improve the efficiency of perovskite solar cells in the future Professor Li Jiangyu, Shenzhen Institute of advanced technology, Chinese Academy of Sciences, and Professor Kam sing Wong, Hong Kong University of science and technology cooperated in the research work of this paper.