Here we report a highly active electrochemical water splitting electrode fabricated from colloidal IrOx nanoparticles and nanoporous Si stabilized by conformal TiO2. The colloidal amorphous IrOx nanoparticles are highly active for oxygen evolution reaction. However, their application for water splitting has a dilemma that the traditional annealing process could lead to low activities but the nanoparticles based electrode without annealing usually exhibited low stability. This nanostructured water splitting electrode exhibited both the high activities as the colloidal IrOx nanoparticles and comparable stability as a traditional thermal annealing fabricated electrode. The impedance study revealed that conformal TiO2 significantly inhibits the interface oxidation and maintains the high activity of colloidal IrOx nanoparticles catalysts. The conformal TiO2 interface engineering combined with nanocatalysts would be a promising strategy to achieve balanced activities and stability for water splitting.Keywords: Conformal TiO2; IrOx nanoparticles; Nanoporous silicon; Water splitting;

Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3) have emerged as a revolutionary class of light-absorbing semiconductors that has demonstrated a rapid increase in efficiency within a few years of active research. Controlling perovskite morphology and composition has been found critical to developing high-performance perovskite solar cells. The recent development of solution chemistry engineering has led to fabrication of greater than 15–17%-efficiency solar cells by multiple groups, with the highest certified 17.9% efficiency that has significantly surpassed the best-reported perovskite solar cell by vapor-phase growth. In this Perspective, we review recent progress on solution chemistry engineering processes and various control parameters that are critical to the success of solution growth of high-quality perovskite films. We discuss the importance of understanding the impact of solution-processing parameters and perovskite film architectures on the fundamental charge carrier dynamics in perovskite solar cells. The cost and stability issues of perovskite solar cells will also be discussed.

A novel α-FeOOH/mesoporous carbon (α-FeOOH/MesoC) composite prepared by in situ crystallization of adsorbed ferric ions within carboxyl functionalized mesoporous carbon was developed as a novel visible light assisted heterogeneous Fenton-like catalyst. The visible light active α-FeOOH nanocrystals were encapsulated in the mesoporous frameworks accompanying with surface attached large α-FeOOH microcrystals via C–O–Fe bonding. Assisting with visible light irradiation on α-FeOOH/MesoC, the mineralization efficiency increased owing to the photocatalytic promoted catalyzing H2O2 beyond the photothermal effect. The synergistic effect between α-FeOOH and MesoC in α-FeOOH/MesoC composite improved the mineralization efficiency than the mixture catalyst of α-FeOOH and MesoC. The iron leaching is greatly suppressed on the α-FeOOH/MesoC composite. Interestingly, the reused α-FeOOH/MesoC composites showed much higher phenol oxidation and mineralization efficiencies than the fresh catalyst and homogeneous Fenton system (FeSO4/H2O2). The XPS, XRD, FTIR, and textural property results reveal that the great enhancement comes from the interfacial emerged oxygen containing groups between α-FeOOH and MesoC after the first heterogeneous Fenton-like reaction. In summary, visible light induced photocatalysis assisted heterogeneous Fenton-like process in the α-FeOOH/MesoC composite system improved the HO• production efficiency and Fe(III)/Fe(II) cycle and further activated the interfacial catalytic sites, which finally realize an extraordinary higher degradation and mineralization efficiency.

•The sulfurated [NiFe]-LDH (FeNiS) nanoparticles were fabricated via the sulfurizing treatment of [NiFe]-based LDH.•The FeNiS nanoparticles enhance catalytic activity of CdTe/CdS for hydrogen evolution.•The FeNiS nanoparticles exhibited enhanced activities than [NiFe]-LDH.•The low cost and sulfur tolerant FeNiS nanoparticles are promising co-catalysts candidate for photocatalysis.Binary transition metals layered double hydroxides (LDH) such as [NiFe] have been developed as the promising low cost and high performance electrocatalysts for hydrogen evolution. However their applications as co-catalysts in photocatalysis like CdTe/CdS quantum dots (QDs) for hydrogen evolution are limited by their ineffective contact with QDs nanoparticles. We report on the synthesis of the sulfurated [NiFe]-LDH (FeNiS) nanoparticles via the sulfurizing treatment of [NiFe]-based LDH. The sulfurizing treatment can successfully break the 2D [NiFe]-LDH into FeNiS nanoparticles by replacing the OH group via the S group. FeNiS nanoparticles provide more bonding sites for CdTe/CdS QDs due to the sulfur species formed on the FeNiS nanoparticles. The junction between sulfur species and Cd2+ of CdTe/CdS QDs could facilitate electrons transfer between CdTe/CdS QDs and FeNiS nanoparticles and then significantly enhance their photocatalytic hydrogen production. The photocatalytic activity of FeNiS-CdTe/CdS is much better than [NiFe]-CdTe/CdS QDs and Pt-CdTe/CdS QDs. In all, this novel FeNiS nanoparticles would be a promising low cost co-catalyst for energy and environmental photocatalysis.Download high-res image (120KB)Download full-size image

•Uniform mesoporous TiO2 films were deposited on a hierarchical carbon foam.•Carbon doping enhance it visible light photocatalytic activity.•3D mesoTiO2/hydro-CF foams significantly improve the photodecomposition of VOCs.Carbon foams (CFs) were prepared by using waste polyurethane foams (PUFs) as hard templates and phenolic resin as a carbon source. The obtained CFs were treated to be hydrophilic with plenty of carboxyle group by a wet oxidization method. Mesoporous TiO2 films were then facilely deposited on hydrophilic carbon foams (hydro-CFs) because the hydrophilic carbon surface facilitates the formation of uniform coating layer. The macroporous hydro-CFs act as not only the supports of TiO2 films but also the adsorbent for enriching the VOCs at the interface of hydro-CF and mesoTiO2 films. The UV–vis and visible light irradiation photocatalytic oxidation of acetone and toluene was evaluated on mesoTiO2/hydro-CF, which is higher than that of pure mesoTiO2 and mesoTiO2/CF. The mesoTiO2/hydro-CF even showed visible light activity for acetone degradation due to the plausible carbon doping due to the strong interaction between the TiO2 precursor and the hydro-CFs.Download high-res image (165KB)Download full-size image

•The difficult-to-make phase pure α-FAPbI3 perovskite with large grain was facilely fabricated.•A flash crystallization help inhibited the unwanted formation of δ-FAPbI3 at lower temperature.•The facile PMMA coating help realization of a flash crystallization of α-FAPbI3 at appropriate temperature.The α-HC(NH2)2PbI3 (FAPbI3) perovskite with narrower band gap and higher thermal stability is a promising candidate for high performance perovskite optoelectronic applications. However, it is difficult to achieve fast growth of a phase pure α-FAPbI3 perovskite because the unwanted δ-FAPbI3 usually first forms at lower temperature then converted into the wanted α-FAPbI3 at higher temperature. It takes long time annealing to complete the conversion of δ-FAPbI3 to α-FAPbI3, which could also lead to uncontrollable degradation of α-FAPbI3. Here we report a facile flash deposition of large grain phase pure α-FAPbI3 without any δ-FAPbI3 impurity by inhibiting the δ phase crystallization. In this method, a simple PMMA sealing to synchronize a flash α-crystallization process at appropriate temperature with the unwanted δ phase crystallization. Consequently, a high quality phase pure α-FAPbI3 with large grain size was fabricated into high performance perovskite solar cells. The strategy of flash crystallization would be a promising method for deposition of phase pure perovskite in future.Download high-res image (163KB)Download full-size image

Organic and inorganic hybrid perovskites have emerged as revolutionary optoelectronic semiconductors, which are promising for various applications especially in photovoltaics and light-emitting diodes. Perovskite solar cells have demonstrated unprecedented progress on PCE within a very short time in the history of photovoltaics. Perovskite solar cells have more than 20% PCE and advantages such as easy-to-process and tunable bandgaps have made them favorable for commercialization. Extending the absorption to a longer wavelength and addressing the challenge of stability in metal halide perovskites has become the most important research focus. However, composition engineering could be a potential solution by further tuning the band gap for the metal halide perovskites and, thus, enhance their stability. The mixed cation perovskite is one of the most practical and successful strategies for composition engineering. Here, recent progress on mixed cation metal halide perovskites is reviewed and this includes the development of binary, ternary and two-dimensional/three-dimensional mixed cation perovskites in chemical synthesis and device performance and stability progress. The prospects and challenges for the mixed cation hybrid perovskites are also considered.

Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic–inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

We demonstrate a facile non-CH3NH3I one-step fabrication of high quality CH3NH3PbI3 perovskite via direct gas/solid deposition of spin coating HI + PbI2 precursor solution in low concentration CH3NH2 atmospheres. This HI + PbI2 precursor solution was prepared by direct addition of stoichiometric aqueous HI acid into PbI2 DMF solution without any other chemicals. This novel one-step approach can be extended to fabricate different lead halide perovskites such as CH3NH3PbI2Br and C2H5NH2PbI3 by spin coating HBr + PbI2 precursor solution in CH3NH2 atmosphere or HI + PbI2 in C2H5NH2 atmosphere, respectively. In all, our novel one-step approach is a promising candidate for fabricating high performance perovskites by one-step reaction of gaseous CH3NH2 with HX and PbX2 (X = I, Br).

Perovskite films were prepared using single step solution deposition at different annealing temperatures and annealing times. The crystal structure, phases and grain size were investigated with XRD, XPS and SEM/EDX. The prepared films show a typical orientation of tetragonal perovskite phase and a gradual transition at room temperature from the yellow intermediate phase to the black perovskite phase. Films with high purity were obtained by sintering at 100 °C. In addition, the chemical composition and crystal struture of intermediate phase were investigated in detail. FTIR, UV-vis and NMR spectra revealed the occurance of DMF complexes. Interestingly, the intermediate phase could be transformed to the black perovskite phase upon X-ray irradiation. In addition, the recovery of the aged perovskite films from a yellow intermediate phase back to the black perovskite was shown to be viable via heating and X-ray irradiation.

Organolead halide perovskites exhibit superior photoelectric properties, which have given rise to the perovskite-based solar cells whose power conversion efficiency has rapidly reached above 20% in the past few years. However, perovskite-based solar cells have also encountered problems such as current–voltage hysteresis and degradation under practical working conditions. Yet investigations into the intrinsic chemical nature of the perovskite material and its role on the performance of the solar cells are relatively rare. In this work, Raman spectroscopy is employed together with CASTEP calculations to investigate the organic–inorganic interactions in CH3NH3PbI3 and CH3NH3PbBr3−xClx perovskite single crystals with comparison to those having ammonic acid as the cations. For Raman measurements of CH3NH3PbI3, a low energy line of 1030 nm is used to avoid excitation of strong photoluminescence of CH3NH3PbI3. Raman spectra covering a wide range of wavenumbers are obtained, and the restricted rotation modes of CH3–NH3+ embedded in CH3NH3PbBr3 (325 cm−1) are overwhelmingly stronger over the other vibrational bands of the cations. However, the band intensity diminishes dramatically in CH3NH3PbBr3−xClx and most of the bands shift towards high frequency, indicating the interaction with the halides. The details of such an interaction are further revealed by inspecting the band shift of the restricted rotation mode as well as the C–N, NH3+ and CH3 stretching of the CH3NH3+ as a function of Cl composition and length of the cationic ammonic acids. The results show that the CH3NH3+ interacts with the PbX3− octahedral framework via the NH3+ end through N+–H⋯X hydrogen bonding whose strength can be tuned by the composition of halides but is insensitive to the size of the organic cations. Moreover, an increase of the Cl content strengthens the hydrogen bonding and thus blueshifts the C–N stretching bands. This is due to the fact that Cl is more electronegative than Br and an increase of the Cl content decreases the lattice constant of the perovskite. The findings of the present work are valuable in understanding the role of cations and halides in the performance of MAPbX3-based perovskite solar cells.

Here we report a high performance nanoporous silicon photoelectrode deposited with earth abundant [Mo3S13]2− nanoclusters for hydrogen generation. The earth abundant [Mo3S13]2− nanoclusters with controllable loading can be facilely deposited onto nanoporous Si via drop coating of the monodispersed [Mo3S13]2− suspension followed by drying. The nanoporous Si photoelectrode exhibited a ∼200 mV lower onset potential for photocurrent by reducing the interface resistance for hydrogen generation. The deposition of [Mo3S13]2− also passivates the nanoporous Si with a more stable photocurrent for hydrogen generation than b-Si in a 20000 s test.

CdTe quantum dots (QDs) have an extended absorbance region compared to that of CdSe or CdS QDs for solar utilization; however, their low activities, especially the chemical stability, limit their applications in photocatalytic hydrogen evolution. We report on enhanced visible-light-driven hydrogen evolution based on CdTe QDs via forming CdTe/CdS core/shell and using sulfur tolerant catalysts of the [Mo3S13]2– nanocluster. The aqueous synthesized CdTe/CdS QDs exhibit much better photocorrosion resistance than regular CdTe QDs for photocatalytic hydrogen generation. The sulfur compound covered CdTe/CdS QDs are facilely decorated with the low cost sulfur tolerant [Mo3S13]2– nanoclusters to exhibit enhanced visible-light photocatalytic H2 generation than the CdTe QDs catalyzed with classical cocatalysts of Pt. In all, the combination of sulfur tolerant [Mo3S13]2– nanoclusters and CdTe/CdS core/shell structure significantly enhance the activity and stability of CdTe QDs for visible-light photocatalytic hydrogen evolution.Keywords: Core/shell; Hydrogen evolution; Quantum dots; [Mo3S13]2− nanoclusters;

•The grain boundary PbI2 passivated CH3NH3PbI3 and CH3NH3PbI2Br perovskites were facilely fabricated.•The beneficial and side effect of PbI2 can be controlled by using hydrohalide deficient precursor of PbI2·xHX (X=I, Br).•PbI2 passivated CH3NH3PbI2Br perovskites exhibited 14.5% efficiency with 86% IPCE.We demonstrate a controllable formation of grain boundary PbI2nanoplates passivated CH3NH3PbI3 and CH3NH3PbI2Br perovskites for high performance solar cells with up to 17.8% and 14.4% efficiencies, which are higher than the corresponding phase pure perovskite solar cells. The PbI2 passivated planar perovskite films were facilely prepared via direct gas/solid reaction of hydrohalide deficient PbI2·xHI/Br precursor with CH3NH2 gas. The amount of PbI2 impurities can be controlled by adjusting the hydrohalide deficiency in the precursors. The crystal growth investigation suggested that the PbI2 is highly like to form during the annealing crystallization process instead of existing in either the PbI2·xHI/Br precursor films or as grown perovskite films. The PbI2 with controllable amount locating at grain boundary could effectively passivate the perovskites with a longer PL lifetime and enhanced Voc.

We demonstrate a facile morphology-controllable sequential deposition of planar CH3NH3PbI3 (MAPbI3) film by using a novel volume-expansion-adjustable PbI2·xMAI (x: 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·xMAI 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%.

We demonstrate high humidity tolerant one-step and sequential deposition methods to fabricate high quality planar CH3NH3PbI3 perovskite films with the assistance of HCl. The addition of stoichiometric HCl into PbI2 precursor solution from 33 wt% hydrochloric acid leads to the formation of a novel HCl·PbI2 precursor film, which can be easily thermally decomposed back into PbI2. This novel intermediate planar HCl·PbI2 precursor film can be completely converted into a compact planar MAPbI3 film within only 10 s at room temperature via sequential deposition. In another novel one step method the precursor solution of PbI2 + MAI + HCl obtained by adding stoichiometric HCl into regular PbI2 + MAI precursor solution was used to fabricate a very smooth planar MAPbI3 film by just spin coating. Both the one step and sequential deposition methods can be used to fabricate high quality planar perovskite films in a hood under ambient conditions with up to 60% humidity level for high efficiency planar perovskite solar cells.

We demonstrate a three-step sequential solution process to prepare PbI2-free CH3NH3PbI3 perovskite films. In this three-step method, a thermally unstable stoichiometric PbI2·CH3NH3Cl precursor film is first deposited on the mesoporous TiO2 substrate, followed by thermal decomposition to form PbI2, which is finally converted into CH3NH3PbI3 by dipping in a regular isopropanol solution of CH3NH3I at room temperature. In comparison to the two-step approach using similar processing conditions, the three-step method enables the formation of the PbI2 film through the thermal decomposition of the PbI2·CH3NH3Cl precursor film. This facilitates a rapid conversion of PbI2 to CH3NH3PbI3 without any traceable residue PbI2 in the final conversion step, leading to an improved device performance.

CH3NH3PbI3 perovskite-based optoelectronics have attracted intense research interests recently because of their easy fabrication process and high power conversion efficiency. Herein, we report a novel photodetector based on unique CH3NH3PbI3 perovskite films with island-structured morphology. The light-induced electronic properties of the photodetectors were investigated and compared to those devices based on conventional compact CH3NH3PbI3 films. The island-structured CH3NH3PbI3 photodetectors exhibited a rapid response speed (<50 ms), good stability at a temperature of up to 100 °C, a large photocurrent to dark current ratio (Ilight/Idark > 1 × 104 under an incident light of ∼6.59 mW/cm2, and Ilight/Idark > 1 × 102 under low incident light ∼0.018 mW/cm2), and excellent reproducibility. Especially, the performance of the island-structured devices markedly exceed that of the conventional compact CH3NH3PbI3 thin-film devices. These excellent performances render the island-structured device to be potentially applicable for a wide range of optoelectronics.Keywords: CH3NH3PbI3 perovskites; high sensitivity; island-structured thin film, sensor; photodetector

We demonstrate a facile synthetic approach for preparing mixed halide perovskite (CH3NH3)Pb(Br1−xClx)3 single crystals by the solvothermal growth of stoichiometric PbBr2 and [(1 − y)CH3NH3Br + yCH3NH3Cl] DMF precursor solutions. The band gap of (CH3NH3)Pb(Br1−xClx)3 single crystals increased and the unit cell dimensions decreased with an increase in Cl content x, consistent with previous theoretical predictions. Interestingly, the Cl/Br ratio in the (CH3NH3)Pb(Br1−xClx)3 single crystals is larger than that of the precursor solution, suggesting an unusual crystal growth mechanism.

•Our nanocomposite IrO2/Ti anode exhibits a high OER activity.•Highly active IrO2 nanoparticles was synthesized by solution chemistry.•Anodized Ti nanotube substrate was deposited with IrO2 nanoparticles.•The shorter Ti nanotube is more effective for IrO2/Ti anode fabrication.•The IrO2 in our nanocomposite IrO2/Ti anode is still be nanoparticles.IrO2 is one of the most active water oxidant catalysis for the oxygen evolution anode owing to its chemical stability and outstanding catalytic activities, however iridium (Ir) is also one of the rarest metals in the world. In this report, a novel highly active nanostructured IrO2/Ti anode for oxygen evolution reaction (OER) was fabricated by depositing IrO2 nanoparticles onto Ti nanotube substrates. This nanostructured IrO2/Ti anode exhibits comparable oxygen evolution catalytic activities as the traditional IrO2/Ti dimensional stable anode with only 1% iridium consumption of the traditional method. This nanostructured IrO2/Ti anode would be a promising candidate method for fabricating high effective IrO2/Ti anode with low Ir consumption.

Recently, the photocatalysts have attracted lots of attention and efforts due to their great potential for environmental remediation application. Toxic ions in water are an increasing environmental pollutant with the fast development. Numerous researches have been made to develop photocatalysts to treat ionic pollutants under the illumination of ultraviolet light and visible light. Here, photocatalytic remediation of toxic ionic pollutants has been reviewed. This review summarized and discussed various photocatalysts including TiO2, modified TiO2, metal oxides, metalsulfides, and nitrides and their recent progress in removing ionic pollutants such as heavy metal ion. The latest achievements and their future prospects of photocatalytic remediation of ion pollutant have also been reviewed.光催化除了广泛应用于有机废水的降解外,目前也逐步被用于水中离子型污染物的处理。本文对能够降解离子型污染物的光催化剂进行了归类总结和讨论。光催化可以有效地降解离子型污染物,为了提高其对紫外光和可见光的利用效率,开发和应用了各种新型光催化剂。这些催化剂主要包括:二氧化钛、改性二氧化钛、金属氧化物、金属硫化物和氮化物以及它们的复合物。最后,总结和展望了近期光催化还原离子型污染物的研究成果以及光催化材料在降解离子型污染物应用方面的前景。

CH3NH3PbI3 perovskite layered films deposited on substrates with and without a titania support structure have been prepared and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450–800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. In addition, for the first time we could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red-shifted at ∼780 nm. We show that an additional bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to estimate relative amounts of PbI2 in the samples. Kinetic analysis reveals that perovskite films with less PbI2 show faster relaxation rates than those containing more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples containing PbI2 are due to a passivation effect, in line with other recently reported work.

Hybrid organometallic halide perovskite CH3NH3PbI2Br (or MAPbI2Br) nanosheets with a 1.8 eV band gap were prepared via a thermal decomposition process from a precursor containing PbI2, MABr, and MACl. The planar solar cell based on the compact layer of MAPbI2Br nanosheets exhibited 10% efficiency and a single-wavelength conversion efficiency of up to 86%. The crystal phase, optical absorption, film morphology, and thermogravimetric analysis studies indicate that the thermal decomposition process strongly depends on the composition of precursors. We find that MACl functions as a glue or soft template to control the initial formation of a solid solution with the main MAPbI2Br precursor components (i.e., PbI2 and MABr). The subsequent thermal decomposition process controls the morphology/surface coverage of perovskite films on the planar substrate and strongly affects the device characteristics.

We demonstrate a three-step sequential solution process to prepare PbI2-free CH3NH3PbI3 perovskite films. In this three-step method, a thermally unstable stoichiometric PbI2·CH3NH3Cl precursor film is first deposited on the mesoporous TiO2 substrate, followed by thermal decomposition to form PbI2, which is finally converted into CH3NH3PbI3 by dipping in a regular isopropanol solution of CH3NH3I at room temperature. In comparison to the two-step approach using similar processing conditions, the three-step method enables the formation of the PbI2 film through the thermal decomposition of the PbI2·CH3NH3Cl precursor film. This facilitates a rapid conversion of PbI2 to CH3NH3PbI3 without any traceable residue PbI2 in the final conversion step, leading to an improved device performance.

Organolead halide perovskites exhibit superior photoelectric properties, which have given rise to the perovskite-based solar cells whose power conversion efficiency has rapidly reached above 20% in the past few years. However, perovskite-based solar cells have also encountered problems such as current–voltage hysteresis and degradation under practical working conditions. Yet investigations into the intrinsic chemical nature of the perovskite material and its role on the performance of the solar cells are relatively rare. In this work, Raman spectroscopy is employed together with CASTEP calculations to investigate the organic–inorganic interactions in CH3NH3PbI3 and CH3NH3PbBr3−xClx perovskite single crystals with comparison to those having ammonic acid as the cations. For Raman measurements of CH3NH3PbI3, a low energy line of 1030 nm is used to avoid excitation of strong photoluminescence of CH3NH3PbI3. Raman spectra covering a wide range of wavenumbers are obtained, and the restricted rotation modes of CH3–NH3+ embedded in CH3NH3PbBr3 (325 cm−1) are overwhelmingly stronger over the other vibrational bands of the cations. However, the band intensity diminishes dramatically in CH3NH3PbBr3−xClx and most of the bands shift towards high frequency, indicating the interaction with the halides. The details of such an interaction are further revealed by inspecting the band shift of the restricted rotation mode as well as the C–N, NH3+ and CH3 stretching of the CH3NH3+ as a function of Cl composition and length of the cationic ammonic acids. The results show that the CH3NH3+ interacts with the PbX3− octahedral framework via the NH3+ end through N+–H⋯X hydrogen bonding whose strength can be tuned by the composition of halides but is insensitive to the size of the organic cations. Moreover, an increase of the Cl content strengthens the hydrogen bonding and thus blueshifts the C–N stretching bands. This is due to the fact that Cl is more electronegative than Br and an increase of the Cl content decreases the lattice constant of the perovskite. The findings of the present work are valuable in understanding the role of cations and halides in the performance of MAPbX3-based perovskite solar cells.

Perovskite films were prepared using single step solution deposition at different annealing temperatures and annealing times. The crystal structure, phases and grain size were investigated with XRD, XPS and SEM/EDX. The prepared films show a typical orientation of tetragonal perovskite phase and a gradual transition at room temperature from the yellow intermediate phase to the black perovskite phase. Films with high purity were obtained by sintering at 100 °C. In addition, the chemical composition and crystal struture of intermediate phase were investigated in detail. FTIR, UV-vis and NMR spectra revealed the occurance of DMF complexes. Interestingly, the intermediate phase could be transformed to the black perovskite phase upon X-ray irradiation. In addition, the recovery of the aged perovskite films from a yellow intermediate phase back to the black perovskite was shown to be viable via heating and X-ray irradiation.

Organic and inorganic hybrid perovskites have emerged as revolutionary optoelectronic semiconductors, which are promising for various applications especially in photovoltaics and light-emitting diodes. Perovskite solar cells have demonstrated unprecedented progress on PCE within a very short time in the history of photovoltaics. Perovskite solar cells have more than 20% PCE and advantages such as easy-to-process and tunable bandgaps have made them favorable for commercialization. Extending the absorption to a longer wavelength and addressing the challenge of stability in metal halide perovskites has become the most important research focus. However, composition engineering could be a potential solution by further tuning the band gap for the metal halide perovskites and, thus, enhance their stability. The mixed cation perovskite is one of the most practical and successful strategies for composition engineering. Here, recent progress on mixed cation metal halide perovskites is reviewed and this includes the development of binary, ternary and two-dimensional/three-dimensional mixed cation perovskites in chemical synthesis and device performance and stability progress. The prospects and challenges for the mixed cation hybrid perovskites are also considered.

Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic–inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

We demonstrate high humidity tolerant one-step and sequential deposition methods to fabricate high quality planar CH3NH3PbI3 perovskite films with the assistance of HCl. The addition of stoichiometric HCl into PbI2 precursor solution from 33 wt% hydrochloric acid leads to the formation of a novel HCl·PbI2 precursor film, which can be easily thermally decomposed back into PbI2. This novel intermediate planar HCl·PbI2 precursor film can be completely converted into a compact planar MAPbI3 film within only 10 s at room temperature via sequential deposition. In another novel one step method the precursor solution of PbI2 + MAI + HCl obtained by adding stoichiometric HCl into regular PbI2 + MAI precursor solution was used to fabricate a very smooth planar MAPbI3 film by just spin coating. Both the one step and sequential deposition methods can be used to fabricate high quality planar perovskite films in a hood under ambient conditions with up to 60% humidity level for high efficiency planar perovskite solar cells.

We demonstrate a facile non-CH3NH3I one-step fabrication of high quality CH3NH3PbI3 perovskite via direct gas/solid deposition of spin coating HI + PbI2 precursor solution in low concentration CH3NH2 atmospheres. This HI + PbI2 precursor solution was prepared by direct addition of stoichiometric aqueous HI acid into PbI2 DMF solution without any other chemicals. This novel one-step approach can be extended to fabricate different lead halide perovskites such as CH3NH3PbI2Br and C2H5NH2PbI3 by spin coating HBr + PbI2 precursor solution in CH3NH2 atmosphere or HI + PbI2 in C2H5NH2 atmosphere, respectively. In all, our novel one-step approach is a promising candidate for fabricating high performance perovskites by one-step reaction of gaseous CH3NH2 with HX and PbX2 (X = I, Br).

We demonstrate a facile synthetic approach for preparing mixed halide perovskite (CH3NH3)Pb(Br1−xClx)3 single crystals by the solvothermal growth of stoichiometric PbBr2 and [(1 − y)CH3NH3Br + yCH3NH3Cl] DMF precursor solutions. The band gap of (CH3NH3)Pb(Br1−xClx)3 single crystals increased and the unit cell dimensions decreased with an increase in Cl content x, consistent with previous theoretical predictions. Interestingly, the Cl/Br ratio in the (CH3NH3)Pb(Br1−xClx)3 single crystals is larger than that of the precursor solution, suggesting an unusual crystal growth mechanism.

We demonstrate a new strategy for the in situ formation of highly luminescent CH3NH3PbBr3 perovskite planar film via the reaction between PbBr2 and methylamine gas. The obtained CH3NH3PbBr3 perovskite planar film exhibited similar quantum confinement to solution chemistry synthesized colloidal CH3NH3PbBr3 quantum dots. Such quantum confinement was realized by a PbOx/Pb(OH)2 framework, which is a by-product formed in situ from the reaction of PbBr2 and methylamine gas under ambient conditions.