Models of light absorption enhancement in perovskite solar cells by plasmonic nanoparticles.

Numerous experiments have demonstrated improvements on the efficiency of perovskite solar cells by introducing plasmonic nanoparticles, however, the underlying mechanisms are still not clear: the particles may enhance light absorption and scattering, as well as charge separation and transfer, or the perovskite's crystalline quality. Eventually, it can still be debated whether unambiguous plasmonic increase of light absorption has indeed been achieved. Here, various optical models are employed to provide a physical understanding of the relevant parameters in plasmonic perovskite cells and the conditions under which light absorption may be enhanced by plasmonic mechanisms. By applying the recent generalized Mie theory to gold nanospheres in perovskite, it is shown that their plasmon resonance is conveniently located in the 650-800 nm wavelength range, where absorption enhancement is most needed. It is evaluated for which active layer thickness and nanoparticle concentration a significant enhancement can be expected. Finally, the experimental literature on plasmonic perovskite solar cells is analyzed in light of this theoretical description. It is estimated that only a tiny portion of these reports can be associated with light absorption and point out the importance of reporting the perovskite thickness and nanoparticle concentration in order to assess the presence of plasmonic effects.

Unveiling of a puzzling dual ionic migration in lead- and iodide-deficient halide perovskites (d-HPs) and its impact on solar cell J-V curve hysteresis.

Halide perovskite solar cells are characterized by a hysteresis between current-voltage (J-V) curves recorded on the reverse and on the forward scan directions, and the suppression of this phenomenon has focused great attention. In the present work, it is shown that a special family of 3D perovskites, that are rendered lead -and iodide- deficient (d-HPs) by incorporating large organic cations, are characterized by a large hysteresis. The strategy of passivating defects by K+, which has been successful in reducing the hysteresis of 3D perovskite perovskite solar cells, is inefficient with the d-HPs. By glow discharge optical emission spectroscopy (GD-OES), the existence of the classic iodide migration in these layers is unveiled, which is efficiently blocked by potassium cation insertion. However, it is also shown that it co-exists with the migration of the large organic cations characteristics of d-HPs which are not blocked by the alkali metal ion. The migration of those large cations is intrinsically linked to the special structure of the d-HP. It is suggested that it takes place through channels, present throughout the whole perovskite layer after the substitution of PbI+ units by the large cations, making this phenomenon intrinsic to the original structure of d-HPs.

New Dication-Based Lead-Deficient 3D MAPbI3 and FAPbI3 "d-HPs" Perovskites with Enhanced Stability.

Toxicity induced by the presence of lead and the rather poor stability of halide perovskite semiconductors represent the major issues for their large-scale application. We previously reported a new family of lead- and iodide-deficient MAPbI3 and FAPbI3 perovskites called d-HPs (for lead- and iodide-deficient halide perovskites) based on two organic cations: hydroxyethylammonium HO-(CH2)2-NH3+ (HEA+) and thioethylammonium HS-(CH2)2-NH3+ (TEA+). In this article, we report the use of an organic dication, 2-hydroxypropane-1,3-diaminium (2-propanol 1,3 diammonium), named PDA2+, to create new 3D d-HPs based on the MAPbI3 and FAPbI3 network with general formulations of (PDA)0,88x(MA)1-0,76x[Pb1-xI3-x] and (PDA)1,11x(FA)1-1,22x[Pb1-xI3-x], respectively. These d-HPs have been successfully synthesized as crystals, powders, and thin films and exhibit improved air stability compared to their reference MAPbI3 and FAPbI3 perovskite counterparts. PDA2+-based deficient MAPbI3 was also tested in operational perovskite solar cells and exhibited an efficiency of 13.0% with enhanced stability.

Two-in-One Sensor Based on PV4D4-Coated TiO2 Films for Food Spoilage Detection and as a Breath Marker for Several Diseases.

Certain molecules act as biomarkers in exhaled breath or outgassing vapors of biological systems. Specifically, ammonia (NH3) can serve as a tracer for food spoilage as well as a breath marker for several diseases. H2 gas in the exhaled breath can be associated with gastric disorders. This initiates an increasing demand for small and reliable devices with high sensitivity capable of detecting such molecules. Metal-oxide gas sensors present an excellent tradeoff, e.g., compared to expensive and large gas chromatographs for this purpose. However, selective identification of NH3 at the parts-per-million (ppm) level as well as detection of multiple gases in gas mixtures with one sensor remain a challenge. In this work, a new two-in-one sensor for NH3 and H2 detection is presented, which provides stable, precise, and very selective properties for the tracking of these vapors at low concentrations. The fabricated 15 nm TiO2 gas sensors, which were annealed at 610 °C, formed two crystal phases, namely anatase and rutile, and afterwards were covered with a thin 25 nm PV4D4 polymer nanolayer via initiated chemical vapor deposition (iCVD) and showed precise NH3 response at room temperature and exclusive H2 detection at elevated operating temperatures. This enables new possibilities in application fields such as biomedical diagnosis, biosensors, and the development of non-invasive technology.

Additive Engineering for Stable and Efficient Dion-Jacobson Phase Perovskite Solar Cells.

Because of their better chemical stability and fascinating anisotropic characteristics, Dion-Jacobson (DJ)-layered halide perovskites, which owe crystallographic two-dimensional structures, have fascinated growing attention for solar devices. DJ-layered halide perovskites have special structural and photoelectronic features that allow the van der Waals gap to be eliminated or reduced. DJ-layered halide perovskites have improved photophysical characteristics, resulting in improved photovoltaic performance. Nevertheless, owing to the nature of the solution procedure and the fast crystal development of DJ perovskite thin layers, the precursor compositions and processing circumstances can cause a variety of defects to occur. The application of additives can impact DJ perovskite crystallization and film generation, trap passivation in the bulk and/or at the surface, interface structure, and energetic tuning. This study discusses recent developments in additive engineering for DJ multilayer halide perovskite film production. Several additive-assisted bulk and interface optimization methodologies are summarized. Lastly, an overview of research developments in additive engineering in the production of DJ-layered halide perovskite solar cells is offered.

Exploring Solar Cells Based on Lead- and Iodide-Deficient Halide Perovskite (d-HP) Thin Films.

Perovskite solar cells have become more and more attractive and competitive. However, their toxicity induced by the presence of lead and their rather low stability hinders their potential and future commercialization. Reducing lead content while improving stability then appears as a major axis of development. In the last years, we have reported a new family of perovskite presenting PbI+ unit vacancies inside the lattice caused by the insertion of big organic cations that do not respect the Goldschmidt tolerance factor: hydroxyethylammonium HO-(CH2)2-NH3+ (HEA+) and thioethylammonium HS-(CH2)2-NH3+ (TEA+). These perovskites, named d-HPs for lead and halide-deficient perovskites, present a 3D perovskite corner-shared Pb1-xI3-x network that can be assimilated to a lead-iodide-deficient MAPbI3 or FAPbI3 network. Here, we propose the chemical engineering of both systems for solar cell optimization. For d-MAPbI3-HEA, the power conversion efficiency (PCE) reached 11.47% while displaying enhanced stability and reduced lead content of 13% compared to MAPbI3. On the other hand, d-FAPbI3-TEA delivered a PCE of 8.33% with astounding perovskite film stability compared to classic α-FAPI. The presence of TEA+ within the lattice impedes α-FAPI degradation into yellow δ-FAPbI3 by direct degradation into inactive Pb(OH)I, thus dramatically slowing the aging of d-FAPbI3-TEA perovskite.

Managing Interfacial Defects and Carriers by Synergistic Modulation of Functional Groups and Spatial Conformation for High-Performance Perovskite Photovoltaics Based on Vacuum Flash Method.

Interfacial nonradiative recombination loss is a huge barrier to advance the photovoltaic performance. Here, one effective interfacial defect and carrier dynamics management strategy by synergistic modulation of functional groups and spatial conformation of ammonium salt molecules is proposed. The surface treatment with 3-ammonium propionic acid iodide (3-APAI) does not form 2D perovskite passivation layer while the propylammonium ions and 5-aminopentanoic acid hydroiodide post-treatment lead to the formation of 2D perovskite passivation layers. Due to appropriate alkyl chain length, theoretical and experimental results manifest that COOH and NH3 + groups in 3-APAI molecules can form coordination bonding with undercoordinated Pb2+ and ionic bonding and hydrogen bonding with octahedron PbI6 4- , respectively, which makes both groups be simultaneously firmly anchored on the surface of perovskite films. This will strengthen defect passivation effect and improve interfacial carrier transport and transfer. The synergistic effect of functional groups and spatial conformation confers 3-APAI better defect passivation effect than 2D perovskite layers. The 3-APAI-modified device based on vacuum flash technology achieves an alluring peak efficiency of 24.72% (certified 23.68%), which is among highly efficient devices fabricated without antisolvents. Furthermore, the encapsulated 3-APAI-modified device degrades by less than 4% after 1400 h of continuous one sun illumination.

Control of perovskite film crystallization and growth direction to target homogeneous monolithic structures.

Getting performant organo-metal halide perovskite films for various application remains challenging. Here, we show the behavior of solvent and perovskite elements for four different perovskites families and nine different initial precursor solution systems in the case of the most popular preparation process which includes an anti-solvent dripping-assisted spin coating of a precursor solution and a subsequent thermal annealing. We show how the initial solution composition affects, first, the film formed by spin coating and anti-solvent dripping and, second, the processes occurring upon thermal annealing, including crystal domain evolution and the grain growth mechanism. We propose a universal typology which distinguishes three types for the growth direction of perovskite crystals: downward (Type I), upward (Type II) and lateral (Type III). The latter results in large, monolithic grains and we show that this mode must be targeted for the preparation of efficient perovskite light absorber thin films of solar cells.

What Can Glow Discharge Optical Emission Spectroscopy (GD-OES) Technique Tell Us about Perovskite Solar Cells?

The emerging broad range of applications of the glow discharge optical emission spectroscopy (GD-OES) technique in the field of perovskite solar cells (PSCs) research is reviewed. It can provide a large palette of information by easily and quickly tracking the depth distribution of light to heavy elements. After a discussion of the advantages and the limitations of the technique and a comparison with other analytical techniques, how GD-OES is employed to give structural information on perovskite solar cells is shown. GD-OES has allowed the full perovskite film formation process investigation, from the initial precursor layers containing soaking and complexed solvent to the final crystallized 3D perovskite layers. The A-site elemental cations distribution is followed-up during the film formation. In addition, this technique gives a deep insight into the action mechanism of additives and their effects on the film formation. It provides fruitful information on optimized light absorbing layers and on the selective contact layers which ensure the charge transport in PSCs. It allows to directly visualize halide ions migration and their blocking by ad-hoc chemical engineering and to study the films and PSCs ageing. GD-OES opens new perspectives to explain the final performances of the devices.

Heterojunction In Situ Constructed by a Novel Amino Acid-Based Organic Spacer for Efficient and Stable Perovskite Solar Cells.

The optical properties and stability of metal halide perovskites can be improved by reducing their dimensionality. Because defects at the perovskite film grain body and boundaries cause significant energetic losses by nonradiative recombination, perovskite films with manageable crystal size and macroscopic grains are essential to improve the photovoltaic properties. Through theoretical calculation models and experiments, we show that the carboxyl group of 4-ammonium butyric acid-based cation (4-ABA+) can interact with the three-dimensional (3D) perovskite to produce in situ a secondary grain growth by post-treatment. It passivates the trap defects and broadens the light absorption. 4-ABA+ could induce a 2D capping layer on top of 3D mixed cation-based perovskite to construct a 2D/3D heterojunction. The 4-ABA+-modified perovskite film consists of large-sized grains with extremely low trap state densities and possesses a longer charge carrier lifetime and good stability, resulting in efficient perovskite solar cells with a champion efficiency of 23.16% and a VOC of 1.20 V. We show that the 4-ABA+-treated devices outperform the 3-ammonium propionic acid (3-APA+)- and 5-ammonium valeric acid (5-AVA+)-treated ones. Moreover, the devices exhibit high stability under high humidity and continuous light soaking conditions. This work gives a hint that our approach based on 4-ABA+ treatment is key to achieving better electrical properties, a controlled crystal growth, and highly stable perovskite solar cells.

Nanosensors Based on a Single ZnO:Eu Nanowire for Hydrogen Gas Sensing.

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 µM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H2 gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H2 gas. A dual-mode nanosensor is shown to detect UV/H2 gas simultaneously for selective detection of H2 during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

Stabilizing Perovskite Precursor by Synergy of Functional Groups for NiOx -Based Inverted Solar Cells with 23.5 % Efficiency.

Perovskite solar cells suffer from poor reproducibility due to the degradation of perovskite precursor solution. Herein, we report an effective precursor stabilization strategy via incorporating 3-hydrazinobenzoic acid (3-HBA) containing carboxyl (-COOH) and hydrazine (-NHNH2 ) functional groups as stabilizer. The oxidation of I- , deprotonation of organic cations and amine-cation reaction are the main causes of the degradation of mixed organic cation perovskite precursor solution. The -NHNH2 can reduce I2 defects back to I- and thus suppress the oxidation of I- , while the H+ generated by -COOH can inhibit the deprotonation of organic cations and subsequent amine-cation reaction. The above degradation reactions are simultaneously inhibited by the synergy of functional groups. The inverted device achieves an efficiency of 23.5 % (certified efficiency of 23.3 %) with an excellent operational stability, retaining 94 % of the initial efficiency after maximum power point tracking for 601 hours.

TiO2/Cu2O/CuO Multi-Nanolayers as Sensors for H2 and Volatile Organic Compounds: An Experimental and Theoretical Investigation.

TiO2/Cu2O/CuO multi-nanolayers highly sensitive toward volatile organic compounds (VOCs) and H2 have been grown in various thicknesses by a cost-effective and reproducible combined spray-sputtering-annealing approach. The ultrathin TiO2 films were deposited by spray pyrolysis on top of sputtered-annealed Cu2O/CuO nanolayers to enhance their gas sensing performance and improve their protection against corrosion at high operating temperatures. The prepared heterostructures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet visible (UV-vis) and micro-Raman spectroscopy. The gas sensing properties were measured at several operating temperatures, where the nanolayered sensors with oxide thicknesses between 20 and 30 nm (Cu2O/CuO nanolayers) exhibited a high response and an excellent selectivity to ethanol vapor after thermal annealing the samples at 420 °C. The results obtained at an operating temperature of 350 °C demonstrate that the CuO/Cu2O nanolayers with thicknesses between 20 and 30 nm are sensitive mainly to ethanol vapor, with a response of ∼150. The response changes from ethanol vapors to hydrogen gas as the thickness of the CuO/Cu2O nanolayers changes from 50 to 20 nm. Density functional theory-based calculations were carried out for the geometries of the CuO(1̅11)/Cu2O(111) and TiO2(111)/CuO(1̅11)/Cu2O(111) heterostructures and their sensing mechanism toward alcohols of different chain lengths and molecular hydrogen. The reconstructed hexagonal Cu2O(111) surface and the reconstructed monoclinic CuO(1̅11) and TiO2(111) facets, all of which terminate in an O layer, lead to the lowest surface energies for each isolated material. We studied the formation of the binary and ternary heteroepitaxial interfaces for the surface planes with the best-matching lattices. Despite the impact of the Cu2O(111) substrate in lowering the atomic charges of the CuO(1̅11) adlayer in the binary sensor, we found that it is the different surface structures of the CuO(1̅11)/Cu2O(111) and TiO2(111)/CuO(1̅11)/Cu2O(111) devices that are fundamental in driving the change in the sensitivity response observed experimentally. The experimental data, supported by the computational results, are important in understanding the use of the multi-nanolayered films tested in this work as reliable, accurate, and selective sensor structures for the tracking of gases at low concentrations.

Chlorides, other Halides, and Pseudo-Halides as Additives for the Fabrication of Efficient and Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs) are attracting a tremendous attention from the scientific community due to their excellent power conversion efficiency, low cost, and great promise for the future of solar energy. The best PSCs have already achieved a certified power conversion efficiency (PCE) of 25.5 % after an unprecedented rapid performance rise. However, high requirements with respect to large area, high-efficiency devices, and stability are still the challenges. Major efforts, especially for achieving a high degree of chemical control, have been made to reach these targets. The use of halide additives has played a critical role in improving the efficiency and stability. The present paper reviews the important breakthroughs in PSC technologies made by using halide additives, especially chloride, and pseudo-halide additives for the preparation of the perovskite layers, other layers, and interfaces of the devices. These additives help perovskite (PVK) crystallization and layer morphology control, grain boundary reduction, bulk and interface defects passivation, and so on. Normally, these halide additives play different roles depending on their categories and their location. Herein, recent progresses made due to additives employment in every possible layer of PSCs are reviewed, with focus on chloride, other halides, and pseudo-halides as additives in PVK films, halide additives in carrier transport layers, and at PVK-contact interfaces. Finally, an outlook of engineering of these additives in PSC progress is given.

Comparison of Thermal Annealing versus Hydrothermal Treatment Effects on the Detection Performances of ZnO Nanowires.

A comparative investigation of the post-electroplating treatment influence on the gas detecting performances of single ZnO nanorod/nanowire (NR/NW), as grown by electrochemical deposition (ECD) and integrated into nanosensor devices, is presented. In this work, hydrothermal treatment (HT) in a H2O steam and conventional thermal annealing (CTA) in a furnace at 150 °C in ambient were used as post-growth treatments to improve the material properties. Herein, the morphological, optical, chemical, structural, vibrational, and gas sensing performances of the as-electrodeposited and treated specimens are investigated and presented in detail. By varying the growth temperature and type of post-growth treatment, the morphology is maintained, whereas the optical and structural properties show increased sample crystallization. It is shown that HT in H2O vapors affects the optical and vibrational properties of the material. After investigation of nanodevices based on single ZnO NR/NWs, it was observed that higher temperature during the synthesis results in a higher gas response to H2 gas within the investigated operating temperature range from 25 to 150 °C. CTA and HT or autoclave treatment showed the capability of a further increase in gas response of the prepared sensors by a factor of ∼8. Density functional theory calculations reveal structural and electronic band changes in ZnO surfaces as a result of strong interaction with H2 gas molecules. Our results demonstrate that high-performance devices can be obtained with high-crystallinity NWs/NRs after HT. The obtained devices could be the key element for flexible nanoelectronics and wearable electronics and have attracted great interest due to their unique specifications.

Effects of 5-Ammonium Valeric Acid Iodide as Additive on Methyl Ammonium Lead Iodide Perovskite Solar Cells.

During the past decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has risen rapidly, and it now approaches the record for single crystal silicon solar cells. However, these devices still suffer from a problem of stability. To improve PSC stability, two approaches have been notably developed: the use of additives and/or post-treatments that can strengthen perovskite structures and the use of a nontypical architecture where three mesoporous layers, including a porous carbon backcontact without hole transporting layer, are employed. This paper focuses on 5-ammonium valeric acid iodide (5-AVAI or AVA) as an additive in methylammonium lead iodide (MAPI). By combining scanning electron microscopy (SEM), X-ray diffraction (XRD), time-resolved photoluminescence (TRPL), current-voltage measurements, ideality factor determination, and in-depth electrical impedance spectroscopy (EIS) investigations on various layers stacks structures, we discriminated the effects of a mesoscopic scaffold and an AVA additive. The AVA additive was found to decrease the bulk defects in perovskite (PVK) and boost the PVK resistance to moisture. The triple mesoporous structure was detrimental for the defects, but it improved the stability against humidity. On standard architecture, the PCE is 16.9% with the AVA additive instead of 18.1% for the control. A high stability of TiO2/ZrO2/carbon/perovskite cells was found due to both AVA and the protection by the all-inorganic scaffold. These cells achieved a PCE of 14.4% in the present work.

PEAI-Based Interfacial Layer for High-Efficiency and Stable Solar Cells Based on a MACl-Mediated Grown FA0.94MA0.06PbI3 Perovskite.

Among the three-dimensional (3D) organic-inorganic hybrid perovskites (OIHPs), mixed formamidinium and methylammonium cation lead iodide is one of the most promising for solar cell application. After optimizing the use of a methylammonium chloride (MACl) additive for the preparation of compact, high-quality, and large crystal grain layers made of a pure α-phase perovskite with the FA0.94MA0.06PbI3 composition, the treatment of the perovskite surface by a 2-phenylethylammonium iodide (PEAI) solution has been performed. This treatment, without any thermal annealing, leads notably to the spontaneous formation of a crystallized (PEA)2PbI4 two-dimensional (2D) perovskite nanolayer at the film surface due to partial organic cation dissolution. This buffer layer is shown to favor a fast transfer of the holes toward the hole transporting layer (HTL) and to reduce the recombinations at and near the perovskite/HTL interface in perovskite solar cells (PSCs). It is shown to boost their maximum power conversion efficiency (PCE) from 20.37 to 22.18%, while the hysteresis becomes negligible. A comprehensive study of the electrical response of the device has been performed. The electrical impedance spectroscopy (EIS) measurements have been fitted with ad hoc equivalent electrical circuits. The electrical responses due to interface stabilization, the intrinsic dielectric relaxation of the perovskite, and the charge depletion and charge recombinations have been distinguished. The low-frequency capacitance is analyzed as a charge recombination capacitance. The perovskite surface buffer layer is notably shown to suppress charge recombinations from the boosting of the high- and low-frequency recombination resistances as well as from the marked decrease of the low-frequency recombination capacitance. The prepared devices are proven to be especially resistant to electrical stresses, light irradiation, and moisture.

Improving the heterointerface in hybrid organic-inorganic perovskite solar cells by surface engineering: Insights from periodic hybrid density functional theory calculations.

A periodic hybrid density functional theory computational strategy is presented to model the heterointerface between the methylammonium lead iodide (MAPI) perovskite and titanium dioxide (TiO2 ), as found in perovskite solar cells (PSC), where the 4-chlorobenzoic acid (CBA) ligand is used to improve the stability and the band alignment at the interface. The CBA ligand acts as a bifunctional linker to efficiently connect the perovskite and the oxide moieties, ensuring the stability of the interface through Ti-O and Pb-Cl interactions. The computed density of states reveals that the perovskite contributes to the top of the valence band while the oxide contributes to the bottom of the conduction band with a direct bandgap of 2.16 eV, indicating a possible electron transfer from MAPI to TiO2 . Dipole moment analysis additionally reveals that the CBA ligand can induce a favorable effect to improve band alignment and thus electron transfer from MAPI to TiO2 . This latter has been quantified by calculation of the spin density of the reduced MAPI/CBA/TiO2 system and indicates an almost quantitative (99.94%) electron transfer from MAPI to TiO2 for the surface engineered system, together with an ultrafast electron injection time in the femtosecond timescale. Overall, the proposed DFT-based computational protocol therefore indicates that surface engineering and the use of a bifunctional linker can lead to a better stability, together with improved band alignment and electron injection in PSC systems.

Pd-Functionalized ZnO:Eu Columnar Films for Room-Temperature Hydrogen Gas Sensing: A Combined Experimental and Computational Approach.

Reducing the operating temperature to room temperature is a serious obstacle on long-life sensitivity with long-term stability performances of gas sensors based on semiconducting oxides, and this should be overcome by new nanotechnological approaches. In this work, we report the structural, morphological, chemical, optical, and gas detection characteristics of Eu-doped ZnO (ZnO:Eu) columnar films as a function of Eu content. The scanning electron microscopy (SEM) investigations showed that columnar films, grown via synthesis from a chemical solutions (SCS) approach, are composed of densely packed columnar type grains. The sample sets with contents of ∼0.05, 0.1, 0.15, and 0.2 at% Eu in ZnO:Eu columnar films were studied. Surface functionalization was achieved using PdCl2 aqueous solution with additional thermal annealing in air at 650 °C. The temperature-dependent gas-detection characteristics of Pd-functionalized ZnO:Eu columnar films were measured in detail, showing a good selectivity toward H2 gas at operating OPT temperatures of 200-300 °C among several test gases and volatile organic compound vapors, such as methane, ammonia, acetone, ethanol, n-butanol, and 2-propanol. At an operating temperature OPT of 250 °C, a high gas response Igas/Iair of ∼115 for 100 ppm H2 was obtained. Experimental results indicate that Eu doping with an optimal content of about 0.05-0.1 at% along with Pd functionalization of ZnO columns leads to a reduction of the operating temperature of the H2 gas sensor. DFT-based computations provide mechanistic insights into the gas-sensing mechanism by investigating interactions between the Pd-functionalized ZnO:Eu surface and H2 gas molecules supporting the experimentally observed results. The proposed columnar materials and gas sensor structures would provide a special advantage in the fields of fundamental research, applied physics studies, and ecological and industrial applications.

Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayers for High-Performance Perovskite Solar Cells.

The family of organic-inorganic hybrid perovskite (OIHPs) materials is one of the most promising for very high-efficiency photovoltaic solar cell application. In the present work, the effect of a series of self-assembled monolayers placed at the TiO2-perovskite junction, on the functioning of triple cation perovskite solar cells has been investigated. We show that employing 4-chlorobenzoic acid leads to the marked boosting of the solar cell performances. The starting pristine cell had a power conversion efficiency (PCE) of 20.3% and the chemical engineering permitted to reach a PCE up to 21.35%. Our experimental study completed by density functional theory calculations and modeling show that this progress is due to the reduction of interfacial states, to the improvement of the quality of the OIHP material and to the structural continuity between TiO2 and the OIHP. Especially, we demonstrate that the interfacial chemical interactions are important to consider in the design of highly efficient devices.