Mixed Halide Research Articles

Hybrid Bulk‐Heterojunction of Colloidal Quantum Dots and Mixed‐Halide Perovskite Nanocrystals for High‐Performance Self‐Powered Broadband Photodetectors

AbstractSelf‐powered broadband photodetectors exhibit excellent self‐powered and wide‐band photoresponse from visible to infrared region and attract enormous attention due to their promising applications in imaging, sensing, and optical communication. PbSe colloidal quantum dots (CQDs) and halide perovskites nanocrystals (NCs) are commonly used for photodetectors due to their strong absorption capability, tunable bandgap, and high aspect ratio. However, due to suffering from low charge carrier mobility and high trap density, the performance of individual PbSe CQDs and perovskites‐based photodetectors is not satisfactory. Integration of PbSe CQDs with inorganic mixed‐halide perovskite nanomaterials can provide an opportunity to overcome these drawbacks. In this work, a hybrid nanocomposite of PbSe CQDs blended with all‐inorganic mixed halide perovskite NCs is integrated to fabricate bulk‐heterojunction‐based high‐performance photodetectors. The transportation of photogenerated carriers is enhanced by employing electrons‐ and holes‐extracting layers. As a result, the photoresponsivity of 6.16 A W−1 and a specific detectivity of 5.96 × 1013 Jones with an ON/OFF current ratio of 105 is obtained for bulk‐heterojunction photodetector ITO/ZnO/PbSe:CsPbBr1.5I1.5/P3HT/Au in the self‐powered mode. Meanwhile, the device performance of the fabricated photodetector is numerically simulated by using Technology Computer‐Aided Design software, and the physical mechanisms for photogenerated carriers’ transportation are discussed in detail.

Mixed halide head perovskites thin films: Stability and growth investigation

Herein, we investigate the mixed halide perovskite films with the formula MAPbX3 where (X3 = Br3, Cl3, I3, Br2Cl, Br2I, and I2Cl) prepared by Spin-Coating in order to study the effects of partial and total substitution of Br. For this purpose, we have carried out a series of characterization techniques, including x-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–visible spectroscopic; we find that all films content with partial Bromide exhibit two peaks located at 2θ angles of 14,94° and 30,2° matching to crystallographic planes (110) and (220) respectively correspond to perovskite structure, in the case of MAPbCl3, MAPbI3 or mixed MAPbI2Cl the characteristics peaks (110) and (220) were identified at 2θ angles of 14,16° and 28,53° respectively. The surface morphology of the different mixed lead halides films was examined by SEM, MAPbBr2I has the highest crystallinity and grain size. Furthermore, All the mixed halide films exhibited strong absorbance with bandgap values ranging between 1.5 eV and 2.5 eV. Moreover, we investigated the crystal structure, morphology, and color change of MAPbX3 after four weeks under dark with 22% humidity; the Bromide-doped samples demonstrate a higher degree of stability based on the study of their degradation mechanisms.

High Open Circuit Voltage Over 1V Achieved in Tin-Based Perovskite Solar Cells with a 2D/3D Vertical Heterojunction.

2D–3D mixed tin halide perovskites are outstanding candidate materials for lead‐free perovskite solar cells (PSCs) due to their improved stability and decreased trap density in comparison with their pure 3D counterparts. However, the mixture of multiple phases may lead to poor charge transfer across the films and limit the device efficiency. Here, a stacked quasi‐2D (down)–3D (top) double‐layered structure in perovskite films prepared via vacuum treatment is demonstrated, which can result in a planar bilayer heterojunction. In addition, it is found that the introduction of guanidinium thiocyanate (GuaSCN) additive can improve the crystallinity and carrier mobility in the 2D perovskite layer and passivate defects in the whole film, leading to a long carrier lifetime (>140 ns) in photoluminescence measurements. As a result, the PSCs show a high open circuit voltage (VOC) up to 1.01 V with a voltage loss of only 0.39 V, which represents the record values ever reported for tin‐based PSCs. The champion device exhibits a power conversion efficiency (PCE) of 13.79% with decent stability, retaining 90% of the initial PCE for 1200 h storage in N2‐filled glovebox.

Acid‐ and Base‐Stable Cs2Pt(Cl,Br)6 Vacancy‐Ordered Double Perovskites and Their Core–Shell Heterostructures for Solar Water Oxidation

The stability of the absorber materials in an aqueous medium is the key to developing successful photoelectrochemical (PEC) solar fuel devices. The halide perovskite materials provide an opportunity to tune desired optoelectronic properties and show very high photovoltaic power conversion efficiency. However, their stability is poor as they decompose instantly in an aqueous electrolyte medium. Here the most stable vacancy ordered double perovskites Cs2PtCl6 and Cs2PtBr6, which remain intact in a wide range of pH values between 1 and 13 is reported. These materials also possess excellent absorption properties covering a significant portion of the visible spectrum. Like conventional ABX3 materials, these ultrastable materials offer tunability in optical properties via mixed halide sites. Through anion exchange, the conversion of Cs2PtCl6 to Cs2PtBr6 through core–shell conversion mechanism is shown. The latter led to the formation of type‐II heterostructures. The electrochemical properties of these materials are investigated in detail and their ability to carry out solar water oxidation on an unprotected photoanode, with photocurrent density of >0.2 mA cm−2 at 1.23 V (vs. RHE) is demonstrated.

Impedance spectroscopy study of defect/ion mediated electric field and its effect on the photovoltaic performance of perovskite solar cells based on different active layers

Effect of defects mitigation on the photovoltaic (PV) performance of three different types of perovskite solar cells (PSCs), namely, single halide (methylammonium lead triiodide, MAPbI 3 ), mixed halide (MAPbI x Cl 3-x ) and phenyl-C71-butyric acid methyl ester (PC 71 BM) based bulk heterojunction (MAPbI 3 -PC 71 BM) PSCs is studied using impedance and transient spectroscopy as a function of dc voltage bias. Parameters extracted from impedance spectra and transient signals are quantitatively analysed to understand the operation mechanism of each PSC. Among the PSCs, the low frequency capacitance (C lf ) values obtained near to the open circuit voltage (V OC ), where charge carriers are restricted inside the device, are found to be minimum for MAPbI 3 based PSCs. Whereas, bulk capacitance (C hf ), high frequency resistance (R hf ) and low frequency resistance (R lf ) values are obtained to be highest for the MAPbI 3 based device which exhibited the best power conversion efficiency (PCE) of 16%. The extracted impedance parameters suggest lessened defect or ion accumulation, improved charge storage capacity/charge life-time, reduced recombination losses and ion transport for the MAPbI 3 device, favoring performance enhancement. The characteristic time constants for the high frequency and low frequency processes also specify longer charge carrier lifetime and faster interfacial charge transfer process near to V OC in MAPbI 3 based PSCs. In addition, transient photocurrent and photovoltage analysis confirms the existence and interplay of ion induced electric field, built-in and applied external field. C lf , C hf , R lf and R hf values as well as transient responses are observed to be greatly dependent on the chemical composition, grain size, grain boundaries and pinholes formed in the perovskite active layer. Based on the results from impedance measurements and transient spectroscopy, the working mechanism of each PSC is proposed in view of the interplay among ion induced electric field, built-in-field and applied electric field. • Operation of three different perovskite solar cells during J-V characterisation. • Defects/ions ion movement and its consequence on the photovoltaic parameters. • The electric field imbalance due to ionic movement in perovskite solar cells.

Enhancing the Electronic Properties and Stability of High-Efficiency Tin-Lead Mixed Halide Perovskite Solar Cells via Doping Engineering.

Overcoming Voc loss to increase the efficiency of perovskite solar cells (PSCs) has been aggressively studied. In this work, we introduce and compare rubidium iodide (RbI) and potassium iodide (KI) alkali metal halides (AMHs) as dopants in a tin-lead (SnPb)-based perovskite system to improve the performance of PSCs by enhancing their Voc. Improvement in terms of surface morphology, crystallinity, charge transfer, and carrier transport in the SnPb perovskites was observed with the addition of AMH dopants. Significant power conversion efficiency improvement has been achieved with the incorporation of either dopant, and the highest efficiency was 21.04% in SnPb mixed halide PSCs when the RbI dopant was employed. In conclusion, we can outline the enhancement strategy that yields a remarkable efficiency of >20% with a smaller Voc loss and improved storage, light, and thermal stability in SnPb PSCs via doping engineering.

Stable CsPbX3 mixed halide alloyed epitaxial films prepared by pulsed laser deposition

The pulsed laser deposition (PLD) technique has been proved to be able to grow oxide thin films with high structural quality with precisely controlled composition and thickness to achieve designed optical and electronical properties established in alloyed semiconductors and heterostructures. In this Letter, inorganic halide perovskite CsPb(IxClyBr1−x−y)3 epitaxial alloyed films on (001)-SrTiO3(STO) substrates were grown by PLD. The film crystal quality, phase stability, and the epitaxial relationship between the film and substrate were characterized with a detailed x-ray diffraction technique like high-resolution reciprocal spatial mapping and ϕ-scan. In addition, the photocarrier dynamics of the alloyed epitaxial films were investigated by photophysics spectroscopy, including steady and femtosecond transient optical absorption spectroscopy and temperature-dependent and time-resolved photoluminescence spectroscopy. The bandgap of the CsPbX3 films was tuned from 1.75 to 2.98 eV by substituting X with I/Br/Cl and their mixture of different ratios. Free exciton emissions were observed at a low temperature photoluminescence spectrum (PL, 10 K), which confirmed the high crystal and optical quality of the epitaxial perovskite alloyed films except the CsPbI3 film. The femtosecond transient absorption spectra also showed that such perovskite films are of very low concentration of exciton trap states. These results indicated that PLD is a powerful technology for growing high quality inorganic halide perovskite films with a tunable bandgap covering the full visible light range, which provided more options for CsPbX3 based panchromatic LED and other optoelectronic devices.

Stable one dimensional (1D)/three dimensional (3D) perovskite solar cell with an efficiency exceeding 23%

AbstractThe interface properties of organic–inorganic mixed halide perovskite solar cells play a significant role in their photovoltaic performance. Defect states at the interface can decrease the power conversion efficiency through defect‐assisted charge recombination and degrade the long‐term stability. Low‐dimensional perovskites exhibit excellent stabilities. They can be combined with three dimensional (3D) perovskites to utilize their advantages for fabrication of high‐performance devices. In this study, we successfully construct a robust interface for perovskite films through mixed‐dimensional engineering. After the introduction of a one dimensional (1D)‐structured capping layer on the 3D perovskite through post‐treatment, the number of defect states in the perovskite film is significantly decreased, while the lifetime of the photocarriers is increased. In addition, the 1D capping layer can serve as a diffusion barrier to reduce the ion migration and protect the vulnerable 3D perovskite against moisture. Perovskite solar cells based on stacked 1D/3D structures exhibit a power conversion efficiency of 23.3% and maintain 80% of the initial efficiency after storage for over 800 h under ambient conditions with a humidity of 50%. Their performances are superior to those of the 3D devices. This study provides a simple approach to simultaneously enhance the efficiency and stability of perovskite solar cells and may guide the development of other solar cells.image

Low‐temperature crystallization and growth of CsPbIBr 2 films through PbX 2 ‐DMSO adduct towards stable and efficient carbon‐based all‐inorganic perovskite solar cells

In the realm of solar cells, currently, the carbon-electrode-based, all-inorganic perovskite solar cells (C-IPSCs) based on cesium lead mixed halide perovskites have drawn much attention. Band-gap tunability and environmental stability of these perovskites (CsPbIBr2) have been gaining attraction as a suitable absorber material for device fabrication. Fabrication of a high-performing and stable C-IPSC based on CsPbIBr2 is a challenging process due to high-temperature fabrication, poor solution deposition of perovskite film, and high recombination at perovskite/carbon interface. In this work, a simple single-step deposition process was adopted to engineer a good quality perovskite film by replacing conventional precursors with DMSO-adducts of lead halide (PbX2). Incorporation of the adduct-based precursor enables crystallization of CsPbIBr2 at a low temperature (~150°C). This enhances the crystallinity of the film, improves the grain size and defects passivation which imparts endurance to the perovskite against phase change and thermal stress. Thereby, enhancing the light absorption and suppressing the non-radiative recombination in CsPbIBr2 film. As a result, the open-circuit voltage is enhanced up to 1.18 V and fill factor up to 57%, further improving the power conversion efficiency by 59.4% leading to a champion device efficiency of 5.9%. Adduct-based C-IPSC displayed substantial enhancement of ambient stability and the adduct-based perovskite exhibited excellent thermal stability.

Nanoscale Photoexcited Carrier Dynamics in Perovskites.

The optoelectronic properties of lead halide perovskite thin films can be tuned through compositional variations and strain, but the associated nanocrystalline structure makes it difficult to untangle the link between composition, processing conditions, and ultimately material properties and degradation. Here, we study the effect of processing conditions and degradation on the local photoconductivity dynamics in [(CsPbI3)0.05(FAPbI3)0.85(MAPbBr3)0.15] and (FA0.7Cs0.3PbI3) perovskite thin films using temporally and spectrally resolved microwave near-field microscopy with a temporal resolution as high as 5 ns and a spatial resolution better than 50 nm. For the latter FACs formulation, we find a clear effect of the process annealing temperature on film morphology, stability, and spatial photoconductivity distribution. After exposure of samples to ambient conditions and illumination, we find spectral evidence of halide segregation-induced degradation below the instrument resolution limit for the mixed halide formulation, while we find a clear spatially inhomogeneous increase in the carrier lifetime for the FACs formulation annealed at 180 °C.

Wide-Bandgap Organic-Inorganic Lead Halide Perovskite Solar Cells.

Under the groundswell of calls for the industrialization of perovskite solar cells (PSCs), wide‐bandgap (>1.7 eV) mixed halide perovskites are equally or more appealing in comparison with typical bandgap perovskites when the former's various potential applications are taken into account. In this review, the progress of wide‐bandgap organic–inorganic hybrid PSCs—concentrating on the compositional space, optimization strategies, and device performance—are summarized and the issues of phase segregation and voltage loss are assessed. Then, the diverse applications of wide‐bandgap PSCs in semitransparent devices, indoor photovoltaics, and various multijunction tandem devices are discussed and their challenges and perspectives are evaluated. Finally, the authors conclude with an outlook for the future development of wide‐bandgap PSCs.

Halide Segregation in Mixed Halide Perovskites: Visualization and Mechanisms

Photoinduced halide segregation in mixed halide perovskites is an intriguing phenomenon and simultaneously a stability issue. In-depth probing this effect and unveiling the underpinning mechanisms are of great interest and significance. This article reviews the progress in visualized investigation of halide segregation, especially light-induced, by means of spatially-resolved imaging techniques. Furthermore, the current understanding of photoinduced phase separation based on several possible mechanisms is summarized and discussed. Finally, the remained open questions and future outlook in this field are outlined.

Effect of Chlorine Addition on the Performance and Stability of Electrodeposited Mixed Perovskite Solar Cells

In this work, electrodeposition was explored as an alternative technique for elaborating large-scale and homogeneous perovskite layers for photovoltaic application. This method was used to elaborate simple MAPbI3 and mixed halide perovskites MAPbI3–xClx. PbO2 was first electrodeposited in a controlled manner. Next, the conversion into perovskite was conducted with various iodine/chlorine ratios. This produced different compositions of perovskites combining an interesting range of microstructural and functional properties. These perovskites, obtained for the first time using electrodeposition, were then tested in solar cell devices. The performances obtained with these innovative electrodeposited mixed perovskites are excellent, better than those obtained with most of the common forms of MAPbI3 published so far. In addition, the stability of these electrodeposited perovskites has been evaluated under mild aging conditions (40 °C, under vacuum or an ambient atmosphere) for 500 h. For the treatment under vacuum, a maturation process was evidenced inducing an improvement of PCE close to 40%, to reach 7%. These results are of great interest, especially considering the active area tested close to 0.2 cm2. The addition of Cl in the lattice not only limits the formation of by-products (PbI2, MAPbCl3, etc.) and enhances the efficiency of the solar cells but also largely increases the stability.

Unveiling Charge Carrier Recombination, Extraction, and Hot-Carrier Dynamics in Indium Incorporated Highly Efficient and Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs) have been propelled into the limelight over the past decade due to the rapid‐growing power conversion efficiency (PCE). However, the internal defects and the interfacial energy level mismatch are detrimental to the device performance and stability. In this study, it is demonstrated that a small amount of indium (In3+) ions in mixed cation and halide perovskites can effectively passivate the defects, improve the energy‐level alignment, and reduce the exciton binding energy. Additionally, it is confirmed that In3+ ions can significantly elevate the initial carrier temperature, slow down the hot‐carrier cooling rate, and reduce the heat loss before carrier extraction. The device with 1.5% of incorporated In3+ achieves a PCE of 22.4% with a negligible hysteresis, which is significantly higher than that of undoped PSCs (20.3%). In addition, the unencapsulated PSCs achieve long‐term stability, which retain 85% of the original PCE after 3,000 h of aging in dry air. The obtained results demonstrate and promote the development of practical, highly efficient, and stable hot‐carrier‐enhanced PSCs.

Highly stable mixed halide perovskite quantum dots synthesized in the presence of fluorous ligands

AbstractThis work probes the potential of fluorous fatty acids as stabilizing ligands in the synthesis of mixed halide perovskite quantum dots (MHP QDs). Traditionally, MHP QDs are synthesized in the presence of oleic acid (OA) and oleyl amine (OAm). Here fluorous fatty acids have been used along with OA and OAm to yield MHP QDs with higher stability. Increased fractions of fluorous molecules in reaction mixture require the process to be carried out at a higher temperature due to the distinctive temperature‐dependent mixing behavior of perfluorinated ligands. Best of fluorous acid functionalized QDs in solution showed a 9% decrease in their photoluminescence quantum yield (PLQY) over 30 days compared to 27% in those without. The composition of fluorous fatty acids during synthesis influences the size, form and optical properties of MHP QDs. Further in situ addition of non‐carboxylic acid fluorous ligands (thiols) are also shown to the give similar improvements in stability and optical properties of MHP QDs. The in situ fluorous ligand functionalization studied in this work presents a new method to improve the stability of MHP‐QD while providing a thermally controlled handle to tune the structure of nanoparticle films.

Omnibearing Interpretation of External Ions Passivated Ion Migration in Mixed Halide Perovskites.

Fundamental understanding of ion migration inside perovskites is of vital importance for commercial advancements of photovoltaics. However, the mechanism for external ions incorporation and its effect on ion migration remains elusive. Herein, taking K+ and Cs+ co-incorporated mixed halide perovskites as a model, the impact of external ions on ion migration behavior has been interpreted via multiple dimensional characterization aspects. The space-effect on phase segregation inhibition has been revealed by the photoluminescence evolution and in situ dynamic cathodoluminescence behaviors. The plane-effect on current suppression along grain boundary has been evidenced via visualized surface current mapping, local current hysteresis, and time-resolved current decay. And the point-effect on activation energy incremental for individual ions has been also probed by cryogenic electronic quantification. All these results sufficiently demonstrate the passivated ion migration results in the eventually improved phase stability of perovskite, of which the origin lies in various ion migration energy barriers.

Homogeneous Grain Boundary Passivation in Wide‐Bandgap Perovskite Films Enables Fabrication of Monolithic Perovskite/Organic Tandem Solar Cells with over 21% Efficiency

AbstractMonolithic perovskite/organic tandem solar cells have attracted increasing attention due to their potential of being highly efficient while compatible to facile solution fabrication processes. One of the limiting factors for improving the performance of perovskite/organic tandem cells is the lack of wide‐bandgap perovskites with suitable bandgap, film quality, and optoelectronic properties for front cells. In addition, the development of low‐bandgap organic bulk‐heterojunction (BHJ) rare cells with extended absorption in the infrared range is also critical for improving tandem cells. This work has carefully optimized mixed halide wide‐bandgap perovskite (MWP) films by introducing a small amount of formamidinium (FA+) cations into the basic composition of MA1.06PbI2Br(SCN)0.12, which provides an effective means to modulate the crystallization properties and phase stability of the films. At optimized conditions, the MA0.96FA0.1PbI2Br(SCN)0.12 forms high‐quality films with grain boundaries homogeneously passivated by PbI2, leading to a reduction in defect states and an enhancement in phase stability, enabling the fabrication of perovskite solar cells with a power conversion efficiency(PCE) of 17.4%. By further integrating the MWP front cell with an organic BHJ (PM6:CH1007) rare cell composed of a nonfullerene acceptor with absorption extended to 950 nm, a tandem cell with PCE over 21% is achieved.

Synthesis of Stable Lead-Free Cs3 Sb2 (Brx I1- x )9 (0 ≤ x≤ 1) Perovskite Nanoplatelets and Their Application in CO2 Photocatalytic Reduction.

Exploring photocatalysts to foster CO2 photoreduction into high value-added chemicals is of great significance. Lead halide perovskites (LHPs) have recently been extensively investigated as photocatalysts, owing to their facile fabrication and prominent optoelectronic properties. However, the toxicity of lead and instability will hinder their future large-scale applications. To address these challenges, a series of lead-free Sb-based all-inorganic mixed halide perovskite Cs3 Sb2 (Brx I1- x )9 (0 ≤ x≤ 1) nanoplatelets (NPLs) is synthesized. The perovskite NPLs are prepared using a ligand-assisted re-precipitation approach at 50 °C. The authors observe the tunability of their optical band gaps from 2.1 to 2.5eV, and they can maintain the excellent stability over 120 h under heating at 100 °C or UV light irradiation. The resultant materials are employed as efficient photocatalysts for visible-light driven CO2 reduction at the gas-solid interface. The Cs3 Sb2 (Br0.7 I0.3 )9 perovskite NPLs afford an impressive overall yield of 27.7µmol g-1 for the selective photocatalytic conversion of CO2 into CO. This study represents a significant demonstration for practical artificial photosynthesis by using LHP materials as photocatalysts.

Study on the halide effect of MA4PbX6·2H2O hybrid perovskites – From thermochromic properties to practical deployment for smart windows

Thermochromic smart windows empowered by the unique material properties and reversible thermochromism of dihydrated methylammonium lead halide hybrid perovskites (MA4PbX6·2H2O; X: halide) have risen as novel yet promising candidates for thermochromic smart windows, in which the halide plays a crucial role to the functionality and performance. In this study, the effects of halide types and mixing ratios on the thermochromic properties and practical deployment of MA4PbX6·2H2O halide hybrid perovskites are comprehensively studied by extensive fabrication and characterization processes as well as fundamental and applicational investigation tools. The potential perovskite compositions for thermochromic smart window applications are identified with respect to the desired optical properties (i.e. luminous transmittance and solar modulation ability) and transitional properties (i.e. transition temperature, hysteresis width and transition time). Moreover, thermal-regulating performance of the applicable candidate mixed halide hybrid perovskite MA4PbI5Br1·2H2O is demonstrated in a model-house field investigation. This work provides not only an inclusive database of the thermochromic properties of MA4PbX6·2H2O halide hybrid perovskites, but also an application manual about their deployment for thermochromic smart windows, facilitating the development of smart windows and sustainable buildings.

Suppressing photoinduced phase segregation in mixed halide perovskites

Suppressing photoinduced phase segregation in mixed halide perovskites