Controllable Sequential Deposition of Planar CH₃NH₃PbI₃ Perovskite Films via Adjustable Volume Expansion

We demonstrate a facile morphology-controllable sequential deposition of planar CH₃NH₃PbI₃ (MAPbI₃) film by using a novel volume-expansion-adjustable PbI₂·<i>x</i>MAI (<i>x</i>: 0.1–0.3) precursor film to replace pure PbI₂. The use of additive MAI during the first step of deposition leads to the reduced crystallinity of PbI₂ and the pre-expansion of PbI₂ into PbI₂·<i>x</i>MAI with adjustable morphology, which result in about 10-fold faster formation of planar MAPbI₃ film (without PbI₂ residue) and thus minimize the negative impact of the solvent isopropanol on perovskites during the MAI intercalation/conversion step. The best efficiency obtained for a planar perovskite solar cell based on PbI₂·0.15MAI is 17.22% (under one sun illumination), which is consistent with the stabilized maximum power output at an efficiency of 16.9%

Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement

Here we demonstrate a radically different chemical route for the creation of HC­(NH₂)₂PbI₃ (FAPbI₃) perovskite thin films. This approach entails a simple exposure of as-synthesized CH₃NH₃PbI₃ (MAPbI₃) perovskite thin films to HC­(NH)­NH₂ (formamidine or FA) gas at 150 °C, which leads to rapid displacement of the MA⁺ cations by FA⁺ cations in the perovskite structure. The resultant FAPbI₃ perovskite thin films preserve the microstructural morphology of the original MAPbI₃ thin films exceptionally well. Importantly, the myriad processing innovations that have led to the creation of high-quality MAPbI₃ perovskite thin films are directly adaptable to FAPbI₃ through this simple, rapid chemical-conversion route. Accordingly, we show that efficiencies of perovskite solar cells fabricated with FAPbI₃ thin films created using this route can reach ∼18%

Comparison of Recombination Dynamics in CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ Perovskite Films: Influence of Exciton Binding Energy

Understanding carrier recombination in semiconductors is a critical component when developing practical applications. Here we measure and compare the monomolecular, bimolecular, and trimolecular (Auger) recombination rate constants of CH₃NH₃PbBr₃ and CH₃NH₃PbI₃. The monomolecular and bimolecular recombination rate constants for both samples are limited by trap-assisted recombination. The bimolecular recombination rate constant for CH₃NH₃PbBr₃ is ∼3.3 times larger than that for CH₃NH₃PbI₃ and both are in line with that found for radiative recombination in other direct-gap semiconductors. The Auger recombination rate constant is 4 times larger in lead-bromide-based perovskite compared with lead-iodide-based perovskite and does not follow the reduced Auger rate when the bandgap increases. The increased Auger recombination rate, which is enhanced by Coulomb interactions, can be ascribed to the larger exciton binding energy, ∼40 meV, in CH₃NH₃PbBr₃ compared with ∼13 meV in CH₃NH₃PbI₃

Multiple Step Growth of Single Crystalline Rutile Nanorods with the Assistance of Self-Assembled Monolayer for Dye Sensitized Solar Cells

A novel multiple step growth (MSG) process has been developed to synthesize rutile nanorods (NRs) on fluorine-doped tin oxide (FTO) glass with the assistance of a self-assembled monolayer (SAM) aiming to increase the internal surface area of the 1D materials for dye sensitized solar cell (DSSC) applications. The experimental result reveals that the SAM layer can be selectively decomposed at the tip of the nanorod, namely the rutile (001) surface, due to the anisotropic photocatalytic property of the rutile. The remaining SAM layer on the side-wall of the NRs remains intact and serves as water repellent which prevents the radial growth of the NRs during the next step hydrothermal synthesis; therefore, the spacing between the NRs and the porosity of the NR array can be retained after additional growth cycles. On the other hand, introduction of a middle layer formed via TiCl₄ solution treatment before the next growth cycle is found to be an effective way to control the diameters of the newly grown NRs. The performance of DSSC made from the rutile NRs grown using the MSG technique has been examined, and it is significantly affected by the internal surfaces of the NRs. Furthermore, the MSG combined with NR etching treatment by acid at low temperature (150 °C) leads to a significant enhancement in the solar cell performance. The gigantic wettability difference of the NRs before and after the SAM treatment as well as the MSG method could be adapted to prepare superhydrophobic and superhydrophilic nanostructured patterns for other applications

Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films

In spite of the rapid rise of metal organic halide perovskites for next-generation solar cells, little quantitative information on the electronic structure of interfaces of these materials is available. The present study characterizes the electronic structure of interfaces between semiconducting single walled carbon nanotube (SWCNT) contacts and a prototypical methylammonium lead iodide (MAPbI₃) absorber layer. Using photoemission spectroscopy we provide quantitative values for the energy levels at the interface and observe the formation of an interfacial dipole between SWCNTs and perovskite. This process can be ascribed to electron donation from the MAPbI₃ to the adjacent SWCNT making the nanotube film <i>n</i>-type at the interface and inducing band bending throughout the SWCNT layer. We then use transient absorbance spectroscopy to correlate this electronic alignment with rapid and efficient photoexcited charge transfer. The results indicate that SWCNT transport and contact layers facilitate rapid charge extraction and suggest avenues for enhancing device performance

Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys

Goldschmidt tolerance factor (<i>t</i>) is an empirical index for predicting stable crystal structures of perovskite materials. A <i>t</i> value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH­(NH₂)₂PbI₃ (FAPbI₃) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δ_H-phase (yellow phase), with δ_H-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI₃ is stabilized to an orthorhombic structure (δ_O-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI₃ with CsPbI₃, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA_1–<i>x</i>Cs_<i>x</i>PbI₃ is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA_0.85Cs_0.15PbI₃ perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI₃ film for which the α-to-δ_H phase transition (associated with yellowing appearance) is accelerated by humidity environment. Due to phase stabilization, the FA_0.85Cs_0.15PbI₃ solid-state alloy showed better solar cell performance and device stability than its FAPbI₃ counterparts. Our studies suggest that tuning the tolerance factor through solid-state alloying can be a general strategy to stabilize the desired perovskite structure for solar cell applications

Crystal Morphologies of Organolead Trihalide in Mesoscopic/Planar Perovskite Solar Cells

The crystal morphology of organolead trihalide perovskite (OTP) light absorbers can have profound influence on the perovskite solar cells (PSCs) performance. Here we have used a combination of conventional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), in cross-section and plan-view, to characterize the morphologies of a solution-processed OTP (CH₃NH₃PbI₃ or MAPbI₃) within mesoporous TiO₂ scaffolds and within capping and planar layers. Studies of TEM specimens prepared with and without the use of focused ion beam (FIB) show that FIBing is a viable method for preparing TEM specimens. HRTEM studies, in conjunction with quantitative X-ray diffraction, show that MAPbI₃ perovskite within mesoporous TiO₂ scaffold has equiaxed grains of size 10–20 nm and relatively low crystallinity. In contrast, the grain size of MAPbI₃ perovskite in the capping and the planar layers can be larger than 100 nm in our PSCs, and the grains can be elongated and textured, with relatively high crystallinity. The observed differences in the performance of planar and mesoscopic-planar hybrid PSCs can be attributed in part to the striking differences in their perovskite-grain morphologies

Ultrafast Imaging of Carrier Transport across Grain Boundaries in Hybrid Perovskite Thin Films

For optoelectronic devices based on polycrystalline semiconducting thin films, carrier transport across grain boundaries is an important process in defining efficiency. Here we employ transient absorption microscopy (TAM) to directly measure carrier transport within and across the boundaries in hybrid organic–inorganic perovskite thin films for solar cell applications with 50 nm spatial precision and 300 fs temporal resolution. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation

Polarization and Dielectric Study of Methylammonium Lead Iodide Thin Film to Reveal its Nonferroelectric Nature under Solar Cell Operating Conditions

Researchers have debated whether methylammonium lead iodide (MAPbI₃), with a perovskite crystal structure, is ferroelectric and therefore contributes to the current–voltage hysteresis commonly observed in hybrid perovskite solar cells (PSCs). We thoroughly investigated temperature-dependent polarization, dielectric, and impedance spectroscopies, and we found no evidence of ferroelectric effect in a MAPbI₃ thin film at normal operating conditions. Therefore, the effect does not contribute to the hysteresis in PSCs, whereas the large component of ionic migration observed may play a critical role. Our temperature-based polarization and dielectric studies find that MAPbI₃ exhibits different electrical behaviors below and above ca. 45 °C, suggesting a phase transition around this temperature. In particular, we report the activation energies of ionic migration for the two phases and temperature-dependent permittivity of MAPbI₃. This study contributes to the understanding of the material properties and device performance of hybrid perovskites

Defect Tolerance in Methylammonium Lead Triiodide Perovskite

Photovoltaic applications of perovskite semiconductor material systems have generated considerable interest in part because of predictions that primary defect energy levels reside outside the bandgap. We present experimental evidence that this enabling material property is present in the halide-lead perovskite, CH₃NH₃PbI₃ (MAPbI₃), consistent with theoretical predictions. By performing X-ray photoemission spectroscopy, we induce and track dynamic chemical and electronic transformations in the perovskite. These data show compositional changes that begin immediately with exposure to X-ray irradiation, whereas the predominant electronic structure of the thin film on compact TiO₂ appears tolerant to the formation of compensating defect pairs of V_I and V_MA and for a large range of I/Pb ratios. Changing film composition is correlated with a shift of the valence-band maximum only as the halide–lead ratio drops below 2.5. This delay is attributed to the invariance of MAPbI₃ electronic structure to distributed defects that can significantly transform the electronic density of states only when in high concentrations