Organometal halide perovskites have attracted widespread attention as the most favorable prospective material for photovoltaic technology because of their high photoinduced charge separation and carrier transport performance. However, the microstructural aspects within the organometal halide perovskite are still unknown, even though it belongs to a crystal system. Here direct observation of the microstructure of the thin film organometal halide perovskite using transmission electron microscopy is reported. Unlike previous reports claiming each phase of the organometal halide perovskite solely exists at a given temperature range, it is identified that the tetragonal and cubic phases coexist at room temperature, and it is confirmed that superlattices composed of a mixture of tetragonal and cubic phases are self-organized without a compositional change. The organometal halide perovskite self-adjusts the configuration of phases and automatically organizes a buffer layer at boundaries by introducing a superlattice. This report shows the fundamental crystallographic information for the organometal halide perovskite and demonstrates new possibilities as promising materials for various applications.

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan–French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

Temperature effects of CH 3 NH 3 PbI 3 perovskite solar cells having simple planar architecture were investigated on the crystal structure and photovoltaic performance. The obvious changes in the CH 3 NH 3 PbI 3 crystal structure were found by varying the temperature as a consequence to the augmentation in lattice parameters and expansion of the unit cell. The expansion of the crystal gave a serious influence on the performance of the solar cells, where the differences in the coefficients of the thermal expansion (CTEs) together with the lattice mismatch between TiO 2 and perovskite materials might cause interfacial defects responsible for the deterioration in the photovoltaic performance. Interestingly, the hysteresis in the cubic phase is very small because of the less distorted angles of the CH 3 NH 3 PbI 3 structure against the temperature fluctuation.Perovskite solar cells based on CH 3 NH 3 PbI 3 have attracted enormous attention in the last few years 1 due to their rapid improvement and high certified efficiencies over 20%.2 The architectures of the devices have been categorized to mesoscopic structure or simple planar heterojunction, 3 and the devices exhibit large hysteresis in JV characteristics especially in the planar structure. 4 The solar cells present a big mismatch in the power conversion efficiency (PCE) from forward (short circuit to open circuit) and reverse scan (open circuit to short circuit), and correct estimation of the PCE has been a controversial matter in recent past. 5 Several hypotheses have been proposed to be the origin of the hysteresis in planar architecture, such as ferroelectric proprieties of CH 3 NH 3 PbI 3 , 6 ion migration inside perovskite layer, 7 chemical and structural changes in the materials, and interfacial contact between the layers.8 However, the mechanism responsible for the anomalous hysteresis remains unclear.Another possible factor of the hysteresis is the interfacial contact and lattice mismatch between compact TiO 2 and perovskite layer.9 This interface may cause the temporal delay of the JV characteristics due to rearrangement of the interface under external bias and illumination. In fact, large decrease of hysteresis has been shown by modification of the compact TiO 2 layer using fullerene derivatives (C 60 , 60-PCBM).10 In this case, there is no lattice mismatch as the inorganic interface of the crystalline. Petrozza et al.10b also showed that allowing the TiO 2 interface to rearrange under voltage bias produces a similar injection rate to that of PCBM contact. One factor that affects the interfacial properties is ion accumulation, as shown by the large capacitance associated with electrode polarization.11 From theoretical calculations, it has been found that the migration of defects originated from the diffusion of iodide vacancies or high orientation of CH 3 NH 3 + is able to modify the interface. 12 In addition, CH 3 NH 3 PbI 3 undergoes several crystal phase transitions when the temperature of the solar cell is changed. 13 Recently, several devices...

For efficient hybrid solar cells based on organometal halide perovskites, the real origin of the IV hysteresis became a big issue and has been discussed widely. In this study, simulated IV curves of different equivalent circuit models were validated with experimental IV curves of a planar perovskite solar cell with the power conversion efficiency (PCE) of 18.0% and 8.8% on reverse scan (from open circuit to short circuit) and forward scan (from short circuit to open circuit), respectively. We found that an equivalent circuit model with a series of double diodes, capacitors, shunt resistances, and single series resistance produces simulated IV curves with large hysteresis matching with the experimentally observed curves. The electrical capacitances generated by defects due to the lattice mismatch at the TiO 2 /CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3 /spiro-OMeTAD interface are truly responsible for the hysteresis in perovskite solar cells.Perovskite solar cells based on CH 3 NH 3 PbI 3 have attracted enormous attention in the last few years due to their outstanding performance as photovoltaics. The power conversion efficiency (PCE) of the devices has dramatically improved to over 20% in a relatively short duration.1,2 Despite the unique properties and higher efficiencies, several important issues, e.g., mysterious hysteresis in IV curves and durability of stabilized performance, still remained for commercialization.3,4 It has been found the hysteresis strongly depends on the device architecture, where the planar structure and Al 2 O 3 mesoscopic perovskite cells show relatively large hysteresis than TiO 2 mesoporous structure devices. 5 The typical planar structure of SnO 2 :F(FTO)/compact TiO 2 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au 6 suffers from severe hysteresis in the IV measurement.3,4 The reverse scan (from the open circuit to the short circuit) always shows higher PCE than the forward scan (from short circuit to the open circuit). Hence, such hysteresis in the IV curves creates ambiguity about the actual performance of the device, which is being suspected to be over-estimated. 7,8 The origin of hysteresis has been discussed on the intrinsic proprieties like ferroelectric polarization 9 and/or ionic migration 10 of the perovskite to date. However, there was no direct evidence that could support the above claims. It has been reported that passivation of TiO 2 layer by C60 or use of phenyl-C61-butyric acid methyl ester (PCBM) instead of TiO 2 in inverted device structure reduces the hysteresis. 1113 The passivation could minimize the trap states and improve electron transfer through the interface of TiO 2 /CH 3 NH 3 PbI 3 , resulting in the reduction of hysteresis.11 On the other hand, PCBM in the inverted cell could extract the carriers (electrons) more efficiently than TiO 2 without accumulation at the interface, and the hysteresis was eliminated. In another standpoint, lattice mismatch of the interfaces containing organic compounds could be ignored and consequently the hysteresis was reduced. The importance of la...

A compact TiO 2 interlayer between fluorine-doped tin oxide (FTO) and perovskite in planar heterojunction perovskite solar cells is essential for effective charge extraction and minimized recombination. The electrical property of the interface of the perovskite layer with the compact TiO 2 layer is thus important for achieving high power conversion efficiency in the perovskite solar cells. The motivation for the present study was to improve cell performance by improving the property of the TiO 2 / perovskite interface. Interestingly, a remarkable enhancement in cell efficiency was achieved by treatment of the compact TiO 2 layer with TiCl 4 and UV(O 3 ).Organometal halide perovskites (CH 3 NH 3 PbI 3 ) have become a new attractive photovoltaic material due to their remarkable optical and electronic properties, and impressive power conversion efficiencies. In a very short time, perovskite-based solar cells have shown a continuous rise in cell power conversion efficiency, from less than 4% in 2009 1 to over 19% in 2014 for both meso-and planar-architectures. 2,3 In the early stages, the perovskite solar cells were fabricated based on the classic mesostructure dye-sensitized solar cells where perovskite functions as light absorber.1,4 On the other hand, due to the ambipolar charge-conductivity of this material, a simple architecture can be adopted, such as a planar pn heterojunction solar cells. 5,6 These types of solar cells, which are similar to organic polymer solar cells, are very attractive for potential commercialization because of simplicity of the device structure. The standard planar structure perovskite solar cells consist of a perovskite layer sandwiched between two charge transport layers: a thin compact TiO 2 layer and a 2,2¤,7,7¤-tetrakis(N,N-di-p-methoxyphenylamine)-9,9¤-spirobifluorene (spiro-OMeTAD) layer. In such heterojunction structures, morphologies of perovskite thin film and all interfacial connections play an important role in improving the cell performance. In addition, quality of the compact layer is another important factor that affects cell performance. Recent study 7 has shown that electron transfer is significantly improved by modifying compact TiO 2 layer with fullerene (C60-SAM) and hence, the cell performance is enhanced. This can be attributed to several aspects, better adhesion between compact TiO 2 layer and perovskite active layer improving the contact resistance and charge transfer reducing back charge recombination. 8 We have investigated here alternative methods for interface engineering of compact TiO 2 layer, TiCl 4 , and UV(O 3 ). The TiCl 4 treatment has been mostly studied in dye-sensitized solar cells (DSCs) for treatment and surface modification of the mesoporous TiO 2 layer.912 On the other hand, UV(O 3 ) treatment is a wellestablished cleaning method to remove organic contaminants from different surfaces increasing the wettability. 1319In this study, we report the influence of surface treatment of the compact TiO 2 layer with TiCl 4 and UV(O 3 ) on the cell performan...