Carrier Recombination Kinetics Research Articles

Enhancing Photoelectrochemical Water Oxidation Using Ferromagnetic Materials and Magnetic Fields.

Photoelectrochemical (PEC) water splitting provides a promising strategy for H2 production. However, its performance is limited by severe carrier recombination and sluggish water oxidation kinetics. While numerous strategies, namely, elemental doping, morphology engineering, heterojunction formation, and catalyst modification, have been extensively explored to enhance the PEC performance, the application of external magnetic fields (MFs) to affect the catalysis or charge carrier dynamics remains yet to be exploited. Herein, BiVO4 is first selected as a representative photoanode, demonstrating that an ultrathin ferromagnetic coating based on Fe2TiO5, when combined with an external MF, boosts its solar water oxidation performance. The combined analyses of the charge transfer and separation efficiency together with ultraviolet photoelectron spectroscopy and transient absorption spectroscopy data revealed that the MF positively affects the band alignment across the BiVO4/Fe2TiO5 interface, improving the charge separation, while the oxygen evolution at the Fe2TiO5/electrolyte interface was promoted. Finally, we expand this concept to other metal oxide photoanodes, such as TiO2, WO3, and Fe2O3, demonstrating the universality of such an approach. Overall, this work pioneers a novel route to harvest external MFs and improve the PEC response of common nonmagnetic semiconductor photoelectrodes in photoelectrocatalytic conversion.

Construction of a 2D/2D Vs-ZnIn2S4/Zn-TCPP heterojunction for effective charge transfer to enhance photocatalytic hydrogen evolution

The integration of inorganic and organic semiconductor components in composites has tremendous potential in photocatalysis, owing to the ultrafast photogenerated exciton dissociation and slow carrier recombination kinetics at their interfaces, which can result in enhanced photocatalytic performances. By exploiting the synergy among morphology modulation, sulfur vacancy (Vs) engineering, and close interface control, a novel inorganic–organic Vs-ZnIn2S4/ tetrakis (4-carboxyphenyl) zinc porphyrin (Zn-TCPP) composite for photocatalytic hydrogen evolution (PHE) was successfully synthesized via microwave-assisted solvothermal and self-assembly methods. The cocatalyst-free PHE rate of Vs-ZnIn2S4/Zn-TCPP (ZZT-20 %) reaches 8.997 mmol h−1 g−1 under visible light irradiation, which is 10.9, 2.2, and 67.1 times that of ZnIn2S4, Vs-ZnIn2S4, and Zn-TCPP, respectively. Moreover, the apparent quantum yield of ZZT-20 % under 420 nm monochromatic light (1.11 %) clearly exceeds that of Vs-ZnIn2S4. Testing experiments and DFT calculations revealed that the significantly enhanced PHE activity can be mainly attributed to the Z-scheme charge transfer mechanism, which greatly improves the visible light response range and redox potentials. Remarkably, Vs sites act as electron traps, inducing the vertical transport of electrons within the bulk phase and their accumulation at the surface Zn–S bonds. Simultaneously, van der Waals forces and weak hydrogen bonds between Vs-ZnIn2S4 and the Zn-TCPP interface cause the rapid migration of photogenerated electrons to Zn-TCPP. This work not only reveals new insights into the structure–property relationships of inorganic–organic Vs-ZnIn2S4 and Zn-TCPP composites, but also provides an appropriate strategy for cleaner water resourcelization and sustainable solar energy conversion.

(Invited) Probing Spin Selective Surface Chemistry with Circularly Polarized XUV Light

Controlling spin states of surface reaction intermediates is an important but often overlooked parameter to designing semiconductor photocatalysts to mimic nature’s best photosynthetic complexes. Extreme Ultraviolet Reflection-Absorption (XUV-RA) spectroscopy enables direct observation of surface electron dynamics by combining element, oxidation, and spin state resolution, with surface sensitivity and ultrafast time resolution. While absorption of linearly polarized XUV light can probe the local oxidation and spin states of individual elements, it is insensitive to absolute spin alignment. To enable the study of spin-selective electron dynamics at interfaces, we have recently implemented ultrafast XUV Magnetic Circular Dichroism (XUV-MCD). Employing XUV-MCD in a reflection geometry, we are now studying the ultrafast surface electron dynamics that give rise to spin polarized photocurrents in magnetic and chiral semiconductors. As an example, yttrium iron garnet (Y3Fe5O12, YIG) is a ferrimagnetic oxide with a visible band gap, consisting of two sub-lattices based on octahedrally and tetrahedrally coordinated Fe(III) centers. Ultrafast XUV measurements show that electron-phonon coupling and polaron formation rates differ significantly between octahedral and tetrahedral centers leading to very different charge carrier recombination kinetics. Circular measurements show that this difference in recombination rate gives rise to long-lived, spin-polarized charge accumulation at the YIG surface that drives spin selective water oxidation. This new ability to observe real-time spin polarization at surfaces promises to provide key design parameters for semiconductor photocatalysts capable of spin selective surface chemistry.

Deep eutectic solvent as an effective tool for enhanced photocatalytic activity of Cu2S@MoS2 nano-assemblies: A green approach

The scientific world has witnessed the emergence of plethora of green strategies to ameliorate the efficiency of nano-photocatalytic materials for waste water treatment applications. This work, for the first time, highlights the effect of ZnCl2 and urea-based DES on the morphology, electronic band structure and photocatalytic efficiency of Cu2S @MoS2 NPs compared to solvents like water, EDA and DETA. XRD and FT-IR analysis confirmed the formation of NPs and quite significant changes in the pattern of M-S bonds of Cu2S@MoS2 were observed upon variation of solvents. The FE-SEM analysis of NPs prepared in 1:1 composition of DES and water (Cu2S@MoS2 NPs - 1:1) showed entirely distinct nanoflower morphology compared to spherical arrangement shown by NPs prepared using other solvents. As a result, Cu2S@MoS2 NPs - 1:1 showed superior textural properties with a surface area of 89 m2/g in contrast to 7.4, 6.2 and 18 m2/g observed for NPs synthesized in water, EDA and DETA, respectively. Besides, wide band gap of 2.75 eV for Cu2S@MoS2 NPs - 1:1 induced favorable charge carrier recombination kinetics, which played an instrumental role in the degradation of tetracycline hydrochloride (TC) and rhodamine B (RB). Cu2S@MoS2 NPs - 1:1 showed best photocatalytic activity as 97% and 96% of TC and RB was removed after 90 min of visible light irradiation with no significant decline in removal efficiency till 5 cycles. Scavenger studies hinted towards the crucial role of h+ and ⋅ OH in degradation process and a mechanism was proposed accordingly. The superior photocatalytic activity of NPs prepared using DES compared to water and organic solvents showed that the inclusion of DES in synthesis schemes of NPs is an excellent strategy for improving their efficiency towards waste water treatment applications.

Unveiling the Influence of Sulfur Doping on Photoelectrochemical Performance in BiVO4/FeOOH Heterostructures.

BiVO4 is a promising photoanode for photoelectrochemical (PEC) water splitting but suffers from high charge carrier recombination and sluggish surface water oxidation kinetics that limit its efficiency. In this work, a model of sulfur-incorporated FeOOH cocatalyst-loaded BiVO4 was constructed. The composite photoanode (BiVO4/S-FeOOH) demonstrates an enhanced photocurrent density of 3.58 mA cm-2, which is 3.7 times higher than that of the pristine BiVO4 photoanode. However, the current explanations for the generation of enhanced photocurrent signals through the incorporation of elements and cocatalyst loading remain unclear and require further in-depth research. In this work, the hole transfer kinetics were investigated by using a scanning photoelectrochemical microscope (SPECM). The results suggest that the incorporation of sulfur can effectively improve the charge transfer capacity of FeOOH. Moreover, the oxygen evolution reaction model provides evidence that S-doping can induce a "fast" surface catalytic reaction at the cocatalyst/solution interface. The work not only presents a promising approach for designing a highly efficient photoanode but also offers valuable insights into the role of element doping in the PEC water-splitting system.

Revealing Charge-Transfer Dynamics at Buried Charge-Selective Heterointerface in Highly Effective Perovskite Solar Cells.

The suboptimal carrier dynamics at the heterointerface between the perovskite and charge transport layer severely limit further performance enhancement of the state-of-the-art perovskite solar cells (PSCs). Herein, we completely map charge carrier extraction and recombination kinetics over a broad time range at buried electron-selective heterointerfaces via ultrafast transient technologies. It is revealed that the heterointerfaces carefully contain the electronic processes of free charge generation in perovskite within ∼2.8 ps, relaxation process of trap-state induced electron capturing less than ∼10.0 ps, electron extraction from perovskite to SnO2 within ∼194 ps, trap-assisted recombination within ∼2047 ps, and recombination between back-injected electrons and remaining holes within ∼8.4 ns. Moreover, we further demonstrate that the inserted poly(vinyl alcohol) (PVA) thin layer can effectively enhance the electron extraction from perovskite to SnO2, block the undesired electron back injection, and significantly suppress the nonradiative recombination, contributing to the improved device parameters of photovoltage and fill factor. This work sheds light on charge-transfer limitations at the perovskite buried heterointerface and provides an effective guide of ideal heterointerface design for promoting charge transfer and improving PSC performance.

Microstructural, electrical, and optoelectronic properties of BaSi2 epitaxial films grown on Si substrates by close-spaced evaporation

BaSi2, an earth-abundant photovoltaic material with a limiting efficiency of 32 %, has the interesting property that grain boundaries do not significantly degrade the film properties. In this study, we investigated the microstructure and its effect on the electrical and optoelectronic properties of the BaSi2 films grown by close-spaced evaporation. Microstructural analysis confirmed the (100)-oriented epitaxial growth on both Si(100) and Si(111) substrates. In addition to conventional epitaxial relationships, lattice matching with the BaSi2(01¯3) plane parallel to Si(110) was observed on Si(100) substrates. The density of the epitaxial domain boundary was found to decrease with increasing film thickness, increasing growth temperature, and using Si(100) substrates instead of Si(111). Hall effect measurements revealed the conductivity type transition at a temperature between 900 and 1000 ∘C and a decreasing tendency of carrier density with film thickness. There was no obvious correlation between mobility and grain boundary density, consistent with previous reports. Carrier recombination kinetics were evaluated by the microwave-detected photoconductivity decay method and the photoconductivity measurement under constant optical irradiation. Both analyses revealed weak negative correlations between excess carrier lifetime and grain boundary density, which clarified the superiority of large-grained BaSi2 films for photovoltaic applications.

Promoting the BiVO4 Photoanode Solar Water Oxidation Performance by Coating Hole-Blocking and Oxygen Evolution Layers

Bismuth vanadate (BiVO4)-based photoelectrochemical (PEC) water-splitting is a propitious technique for green fuel production, while the high surface charge carrier recombination and sluggish oxygen production kinetics limits its practical applications. Herein, we discuss the favorable impacts of grafting a hole-blocking SnO2 layer as an inner layer onto the fluorine-doped tin oxide and an oxygen evolution N-FeCoOx layer on BiVO4 photoanodes. Interestingly, the optimized SnO2@BiVO4/N-FeCoOx photoanode demonstrates an exceptional photocurrent density of 4.6 mA cm–2 at 1.23 V versus reversible hydrogen electrode under AM 1.5 G illumination, accompanied by the long-term stability up to 10 h. More specifically, the synergistic effect of both hole-blocking as well as oxygen evolution layers promotes the interfacial charge separation, transport, and PEC stability. Altogether, this work offers a simple and effective approach for constructing highly efficient photoanodes for PEC water splitting.

Critical Evaluation of the Photovoltaic Performance of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI3 have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI)x(BiI3)y thin films with stoichiometric ratios (x/y) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 °C) spin coating process from AgI/BiI3 solutions in dimethyl sulfoxide. Several critical observations on the basis of ultrafast broadband UV–vis transient absorption spectroscopy and photocurrent spectroscopy were made for these Ag–Bi–I materials, which will have a considerable impact on their photovoltaic performance: Their carrier recombination kinetics were independent of the initial carrier number density over the range of 1.3–6.6 × 1017 cm–3 and well approximated by a monoexponential decay with a rate constant krec in the range of 2.0–4.3 × 108 s–1, which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI)x(BiI3)y compounds, thereby suggesting strong electron–phonon coupling, which will favor charge-carrier localization and nonradiative carrier recombination, in agreement with the virtually absent photoluminescence of these materials. In addition, Fourier-transform step-scan photocurrent spectroscopy (FTPS) on AgBi2I7 provided an Urbach energy of 70 meV and found indications for deep defects (0.6 eV below the band gap), which was also consistent with a trap-mediated recombination mechanism. A combination of all of these effects is likely responsible for the still quite low light-harvesting performance of this class of materials, which has been reported to show photovoltaic conversion efficiencies (PCEs) below 5%. Finally, for “AgI-rich” compounds (3:1, 2:1), we found a substantial separate contribution of carrier loss processes through ultrafast relaxation in AgI domains with time constants of 0.73 and 2.4 ps.

Synergy of Ultrathin CoOx Overlayer and Nickel Single Atoms on Hematite Nanorods for Efficient Photo-Electrochemical Water Splitting.

To solve surface carrier recombination and sluggish water oxidation kinetics of hematite (α-Fe2 O3 ) photoanodes, herein, an attractive surface modification strategy is developed to successively deposit ultrathin CoOx overlayer and Ni single atoms on titanium (Ti)-doped α-Fe2 O3 (Ti:Fe2 O3 ) nanorods through a two-step atomic layer deposition (ALD) and photodeposition process. The collaborative decoration of ultrathin CoOx overlayer and Ni single atoms can trigger a big boost in photo-electrochemical (PEC) performance for water splitting over the obtained Ti:Fe2 O3 /CoOx /Ni photoanode, with the photocurrent density reaching 1.05mA cm-2 at 1.23V vs. reversible hydrogen electrode (RHE), more than three times that of Ti:Fe2 O3 (0.326 mA cm-2 ). Electrochemical and electronic investigations reveal that the surface passivation effect of ultrathin CoOx overlayer can reduce surface carrier recombination, while the catalysis effect of Ni single atoms can accelerate water oxidation kinetics. Moreover, theoretical calculations evidence that the synergy of ultrathin CoOx overlayer and Ni single atoms can lower the adsorption free energy of OH* intermediates and relieve the potential-determining step (PDS) for oxygen evolution reaction (OER). This work provides an exemplary modification through rational engineering of surface electrochemical and electronic properties for the improved PEC performances, which can be applied in other metal oxide semiconductors as well.

Trap-mediated bipolar charge transport in NiO/Ga2O3 p+-n heterojunction power diodes

The construction of p-NiO/n-Ga2O3 heterojunction becomes a popular alternative to overcome the technological bottleneck of p-type Ga2O3 for developing bipolar power devices for practical applications, whereas the identification of performance-limiting traps and the bipolar transport dynamics are still not exploited yet. To this end, the fundamental correlation of carrier transport, trapping and recombination kinetics in NiO/β-Ga2O3 p+-n heterojunction power diodes has been investigated. The quantitative modeling of the temperature-dependent current-voltage characteristics indicates that the modified Shockley-Read-Hall recombination mediated by majority carrier trap states with an activation energy of 0.64 eV dominates the trap-assisted tunneling process in the forward subthreshold conduction regime, while the minority carrier diffusion with near-unity ideality factors is overwhelming at the bias over the turn-on voltage. The leakage mechanism at high reverse biases is governed by the Poole-Frenkel emissions through the β-Ga2O3 bulk traps with a barrier height of 0.75 eV, which is supported by the identification of majority bulk traps with the energy level of EC − 0.75 eV through the isothermal capacitance transient spectroscopic analysis. These findings bridge the knowledge gap between bipolar charge transport and deep-level trap behaviors in Ga2O3, which is crucial to understand the reliability of Ga2O3 bipolar power rectifiers.

ZrO2-Based Photocatalysts for Wastewater Treatment: From Novel Modification Strategies to Mechanistic Insights

Zirconium dioxide (ZrO2) has garnered substantial research interest in the field of photocatalytic water treatment due to its appealing properties, such as thermal stability, considerable physical strength, and strong chemical resistance. However, the wide bandgap energy endorses less photoabsorption and rapid charge carrier recombination kinetics, thus restricting the photoactivity of ZrO2. Previously, vast research efforts have been made to improve the photoefficacy of ZrO2, and hence it is worth exploring the potential strategic modifications responsible for incremented photocatalytic efficiency. In this regard, the present review article emphasizes the optical, structural, and electronic features of ZrO2, which makes it an interesting photocatalytic material. The exceptional modification strategies that help to modulate the crystal structure, morphology, bandgap energy, and charge carrier kinetics are primarily discussed. The potential synthetic routes involving bottom-up and top-down methods are also outlined for understanding the rationale for incorporating these techniques. Moreover, the photocatalytic performance evaluation was done by investigating the photodegradation kinetics of various organic and inorganic pollutants degradation by ZrO2. Conclusively, in light of research advances involving ZrO2 photocatalyst, this review article may expedite further investigation for enhancing the large-scale photocatalytic applications for environmental and energy concerns.

Influence of the Electron Selective Contact on the Interfacial Recombination in Fresh and Aged Perovskite Solar Cells

In this work, we have used TiO2 and SnO2 layers as electron selective contact (ESC) in n-i-p perovskite solar cells configuration. To study and compare the ion migration kinetics of these ESC, CsFAMAPbIBr and MAPbI3-based devices were fabricated and characterised in fresh (1 day) and aged (28 days) conditions. Depending on the ESC and perovskite composition, devices reveal a different progression over time in terms of hysteresis and performance. Using transient photovoltage (TPV) and transient photocurrent (TPC) techniques, we studied the kinetics of carrier extraction and recombination, which showed that aged devices present slower recombination kinetics compared to their fresh counterparts, revealing a positive effect of the aging process. Finally, transient of the transient, derived from the TPV technique, discloses that TiO2 accumulates more charges in the ESC/perovskite interface compared to SnO2 and that the ion migration kinetics are directly related to the perovskite composition.

Effect of post-annealing on thermally evaporated reduced-dimensional perovskite LEDs

Reduced-dimensional perovskites (RDPs) with self-assembled multi-quantum well structures have emerged as promising candidates for light-emitting diodes (LEDs) due to their high color purity, high photoluminescence quantum yield, and decent stability. Compared to the traditional RDP film preparation methods reported in the previous literature, thermal evaporation is an appealing option for RDP film fabrication with uniform crystallization, high repeatability, and precise control. Here, based on the vacuum deposition method, we adopted a combined strategy, including annealing treatment and device structure optimization. Meanwhile, we investigated the effects of post-annealing on charge carrier recombination kinetics, exciton energy transfer, and phase distribution of vacuum-deposited RDP films. As a result, we achieved an external quantum efficiency of 6.5% for the device, which is one of the best performances among prevailing research on vacuum-processed RDP-based LEDs.

Photoinjection and carrier recombination kinetics in photoanode based on (TM)FeO3 adsorbed TiO2 quantum dots

The work being reported was carried out with motivation to address the issues being faced by DSSC by designing organic dye attached to (TM)FeO3/TiO2 structures (where TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) using first principles methods. The structural and electronic properties of (TiO2)54 quantum dot (QD), clusters (TM)FeO3, adsorbed (TM)FeO3/(TiO2)54 and dye attached (TiO2)54/(TM)FeO3 structures are described in detail. The structural stability of the clusters exhibited consistent trend in such a way that the cluster CuFeO3 is most stable whereas ZnFeO3 is least stable structure. The band gap of the QD is reduced after adsorption of the clusters to make harvesting of solar light in visible region except (TiO2)54/Fe2O3 and (TiO2)54/ZnFeO3 which are found active in infrared region of electromagnetic spectrum. The value of recombination rate is decreased whereas the electron injection rate is increased after adsorption of (TM)FeO3 on the QD.

Photoelectrochemistry of metalloporphyrin-modified GaP semiconductors.

Photoelectrosynthetic materials provide a bioinspired approach for using the power of the sun to produce fuels and other value-added chemical products. However, there remains an incomplete understanding of the operating principles governing their performance and thereby effective methods for their assembly. Herein we report the application of metalloporphyrins, several of which are known to catalyze the hydrogen evolution reaction, in forming surface coatings to assemble hybrid photoelectrosynthetic materials featuring an underlying gallium phosphide (GaP) semiconductor as a light capture and conversion component. The metalloporphyrin reagents used in this work contain a 4-vinylphenyl surface-attachment group at the β-position of the porphyrin ring and a first-row transition metal ion (Fe, Co, Ni, Cu, or Zn) coordinated at the core of the macrocycle. In addition to describing the synthesis, optical, and electrochemical properties of the homogeneous porphyrin complexes, we also report on the photoelectrochemistry of the heterogeneous metalloporphyrin-modified GaP semiconductor electrodes. These hybrid, heterogeneous-homogeneous electrodes are prepared via UV-induced grafting of the homogeneous metalloporphyrin reagents onto the heterogeneous gallium phosphide surfaces. Three-electrode voltammetry measurements performed under controlled lighting conditions enable determination of the open-circuit photovoltages, fill factors, and overall current-voltage responses associated with these composite materials, setting the stage for better understanding charge-transfer and carrier-recombination kinetics at semiconductor|catalyst|liquid interfaces.

Promoting Active Electronic States in LaFeO3 Thin-Films Photocathodes via Alkaline-Earth Metal Substitution.

The effects of alkaline-earth metal cation (AMC; Mg2+, Ca2+, Sr2+, and Ba2+) substitution on the photoelectrochemical properties of phase-pure LaFeO3 (LFO) thin-films are elucidated by X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance, and electrochemical impedance spectroscopy (EIS). XRD confirms the formation of single-phase cubic LFO thin films with a rather complex dependence on the nature of the AMC and extent of substitution. Interestingly, subtle trends in lattice constant variations observed in XRD are closely correlated with shifts in the binding energies of Fe 2p3/2 and O 1s orbitals associated with the perovskite lattice. We establish a scaling factor between these two photoemission peaks, unveiling key correlation between Fe oxidation state and Fe-O covalency. Diffuse reflectance shows that optical transitions are little affected by AMC substitution below 10%, which are dominated by a direct bandgap transition close to 2.72 eV. Differential capacitance data obtained from EIS confirm the p-type characteristic of pristine LFO thin-films, revealing the presence of sub-bandgap electronic state (A-states) close to the valence band edge. The density of A-states is decreased upon AMC substitution, while the overall capacitance increases (increase in dopant level) and the apparent flat-band potential shifts toward more positive potentials. This behavior is consistent with the change in the valence band photoemission edge. In addition, capacitance data of cation-substituted films show the emergence of deeper states centered around 0.6 eV above the valence band edge (B-states). Photoelectrochemical responses toward the hydrogen evolution and oxygen reduction reactions in alkaline solutions show a complex dependence on alkaline-earth metal incorporation, reaching incident-photon-to-current conversion efficiency close to 20% in oxygen saturated solutions. We rationalize the photoresponses of the LFO films in terms of the effect sub-bandgap states on majority carrier mobility, charge transfer, and recombination kinetics.

Scaling Laws of Exciton Recombination Kinetics in Low Dimensional Halide Perovskite Nanostructures

Carrier recombination is a crucial process governing the optical properties of a semiconductor. Although various theoretical approaches have been utilized to describe carrier behaviors, a quantitative understanding of the impact of defects and interfaces in low dimensional semiconductor systems is still elusive. Here we develop a model system consisting of chemically tunable, highly luminescent halide perovskite nanocrystals to illustrate the role of carrier diffusion and material dimensionality on the carrier recombination kinetics and luminescence efficiency. Our advanced synthetic methods provide a well-controlled colloidal system consisting of nanocrystals with different aspect ratios, halide compositions, and surface conditions. Using this system, we reveal the scaling laws of photoluminescence quantum yield and radiative lifetime with respect to the aspect ratio of nanocrystals. The scaling laws derived herein are not only a phenomenological observation but proved a powerful tool disentangling the carrier dynamics of microscopic systems in a quantitative and interpretable manner. The investigation of our model system and theoretical formulation bring to light the dimensionality as a hidden constraint on carrier dynamics and identify the diffusion length as an important parameter that distinguishes nanoscale and macroscale carrier behaviors. The conceptual distinction in carrier dynamics in different dimensionality regimes informs new design rules for optical devices where complex microstructures are involved.

Binary synergetic ions reduce defect density in ambient air processed perovskite solar cells

At present, significant research efforts are being concentrated on enhancing the performance and stability of perovskite solar cells (PSC) by lowering defect traps. In this study, NH4+ and SCN- binary ions as additive were incorporated into perovskite precursor to control the crystal growth. Our best performance based on the devices fabricated under fully open air condition was improved from 15.67 to 18.75%, boosting by 20%. The stability results display that devices containing additive maintained ~90% of the initial efficiency for 400 h in ambient air with a humidity of 30%. We first study the fundamentals of defect properties and carrier recombination kinetics behind the multifaceted role of mixed NH4+ and SCN- ions. Compared to using a single active specie as additive, mixed-ions devices exhibit effective bifacial trap passivation as well as lowered defect density in not only perovskite bulk material but also interfaces significantly, leading to facilitated electron transport. Our work can manifest a simple ambient air based approach by the mixed binary ions as additives in order to potentially promote the commercial prospects of PSCs.

Minimization of Carrier Losses for Efficient Perovskite Solar Cells through Structural Modification of Triphenylamine Derivatives.

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and successfully employed in triple-cation mixed-halide PSCs, reaching efficiencies of 19.4 %. The efficiencies, comparable to those obtained using spiro-OMeTAD, point them out as promising candidates for easily attainable and cost-effective alternatives for PSCs, given their facile synthesis from commercially available materials. Interestingly, although all these HTMs show similar chemical and physical properties, they provide different carrier recombination kinetics. Our results demonstrate that is feasible through the molecular design of the HTM to minimize carrier losses and, thus, increase the solar cell efficiencies.