Unraveling the rapid ion migration in perovskite solar cells by circuit-switched transient photoelectric technique.

Ion migration activated by illumination is a critical factor responsible for the performance decline and stability degradation of perovskite solar cells (PSCs). While ion migration has been widely believed to be much slower than charge transport, recent research suggests that, despite the lack of understanding of the mechanism, it may also be involved in a series of rapid photoelectric responses of PSCs. Here, we report an improved circuit-switched transient photoelectric technique with nanosecond temporal resolution, which enables quantitative characterization of ion migration dynamics in PSCs across a fairly broad time window. Specifically, ion migration occurring within microseconds after illumination (corresponding to a diffusion length of ∼10-7 cm) is unambiguously identified. In conjunction with the composition engineering protocol, we justify that it arises from the short-range migration of halide anions and organic cations around the contact/perovskite interface. The rapid ion migration kinetics revealed in this work strongly complement the well-established ion migration model, which offers new insights into the mechanism of ion-carrier interaction in PSC devices.

Simultaneously improved photoluminescence, stability, and carrier transport of perovskite nanocrystals by post-synthetic perfluorobutanesulfonic acid treatment.

We report a post-synthetic treatment method based on perfluorobutanesulfonic acid (PFBA) to ameliorate the photophysical performance of perovskite nanocrystals. By virtue of the PFBA treatment, both the photoluminescence efficiency and stability of perovskite quantum dot-based colloidal solutions and the electrical conductivity of their close-packed films are simultaneously improved.

High-Fill-Factor Perovskite Solar Cells via Pseudohalide Salt Modification of the Substrate to Mitigate Nonradiative Recombination at the Interface.

The utilization of the sol-gel method for fabricating planar SnO2 as the electron transport layer (ETL) induces numerous defects on the SnO2 layer surface and perovskite film bottom, causing considerable deterioration of the device performance. Conventional inorganic salt-doped SnO2 precursor solutions used for passivation may cause incomplete substrate coverage due to the presence of inorganic salt crystals, further degrading the device performance. Here, a substrate modification approach involving the pretreatment of a fluorine-doped SnO2 (FTO) substrate with NH4PF6 is proposed. The interaction between PF6- ions and the FTO substrate enhances SnO2 film quality; excess PF6- ions decrease the number of defects on the film surface. NH4+ ions react with an -OH stabilizing agent in the SnO2 solution and are eliminated during annealing. The combined effects suppress nonradiative recombination and ion migration at the ETL-perovskite interface. The corresponding high-quality perovskite solar cells (PSCs) exhibit a fill factor of ∼0.825; PSC efficiency increases from 19.59% to 22.32%.

Influence of Two- and Three-Dimensional Engineering on the Trap State Distribution and Photophysical Properties of Lead Halide Perovskite Polycrystals.

Constructing a two- and three-dimensional (2D/3D) heterojunction structure on the surface of a 3D perovskite film, termed 2D/3D engineering, is effective in elevating the stability of perovskite polycrystal-based photovoltaic and photoelectronic devices; however, it remains controversial whether this protocol is favorable or detrimental to the device performance. Here, we prepare a series of 2D/3D perovskite films by post-treating the perovskite polycrystalline film with different concentrations of phenethylammonium iodide (PEAI). Systematic spectroscopy and electrochemical studies illustrate that PEAI can penetrate the 3D perovskite network and eliminate the intrinsic trap states of perovskite polycrystals, while the 2D perovskite nanosheets enriched on the top of the polycrystalline film may introduce additional trap states, which manipulate the photoluminescence performance and dynamics of the as-prepared perovskite films in an opposite manner. Based on this finding, the strategy of optimizing the photophysical properties of the host 3D perovskite through 2D/3D engineering is elaborated, paving the way for fabricating high-performance and high-stability perovskite polycrystalline films.

Low-Temperature Preparation of High-Quality Perovskite Polycrystalline Films via Crystallization Kinetics Engineering.

Preparation of lead halide perovskite polycrystalline films at a low annealing temperature is highly restricted by their intrinsically large crystallization activation energy, which hinders the conversion of the precursors/intermediates to perovskites and yields as-prepared polycrystals with tiny grain sizes and terrible crystal quality. Herein, we demonstrate through in-situ, real-time spectroscopic studies that both the nucleation and crystal growth kinetics can be improved without the need for a high annealing temperature by treating the film with thiourea, as accounted for by the reduced activation energy. As a consequence, the thiourea-treated perovskite polycrystalline film exhibits larger grain sizes and greater crystallinity than the untreated one. More importantly, owing to the synergistic effect of the promoted crystallization kinetics and the passivation of surface defects, the low-temperature prepared films treated with thiourea even present more prominent photophysical properties than those fabricated by using the conventional high-temperature method. The strategy of crystallization kinetics engineering proposed in this work paves the way for fabricating high-quality perovskite polycrystalline films in a low-temperature manner.

Mesoporous TiO2 layer suppresses ion accumulation in perovskite solar cells.

Ion accumulation in perovskite solar cells can be highly suppressed by a mesoporous TiO2 layer. This is evidenced by the decrease of the ion-related electrostatic potential with increasing the thickness of the mesoporous layer, accounted for by the electron population in the shallow trap states of the TiO2 nanocrystals.

Electron transport layer assisted by nickel chloride hexahydrate for open-circuit voltage improvement in MAPbI3 perovskite solar cells.

SnO2 is a promising electron transport layer (ETL) material with important applications in planar perovskite solar cells (PSCs). However, electron-hole recombination and charge extraction between SnO2 and the perovskite layer necessitates further exploration. Nickel chloride hexahydrate (NiCl2·6H2O) was introduced into the SnO2 ETL, which significantly increased the power conversion efficiency (PCE) from 15.49 to 17.36% and the open-circuit voltage (V OC) from 1.078 to 1.104 V. The improved PCE and V OC were attributed to the reduced defect states and increased energy level of the conduction band minimum. This work provides new insights into optimizing the V OC and PCE of PSCs.

Lewis Base Plays a Double-Edged-Sword Role in Trap State Engineering of Perovskite Polycrystals.

We report the double-edged-sword effect of the thiourea (a typical Lewis base) additive for tailoring the trap state distribution of perovskite polycrystalline films. Through the thiourea treatment, the polycrystal grain size is greatly increased because of the reduced crystallization activation energy, which, together with the surface defect passivation, alters the density of the energetically "deep" and "shallow" trap states in a trade-off manner. Based on this finding and further photoelectric and spectral studies, the nonmonotonic dependence of the photoluminescence intensity on the thiourea concentration and the complicated time-resolved photoluminescence behavior are excellently clarified. As a proof of concept, the photophysical performance of perovskite polycrystals is optimized via a modified Lewis base treatment by taking the proposed double-edged-sword effect into account.

Lewis Base-Mediated Perovskite Crystallization as Revealed by In Situ, Real-Time Optical Absorption Spectroscopy.

The strategy of Lewis base modification has been shown to be rather effective in fabricating high-quality perovskite crystals; however, the underlying mechanisms remain controversial owing to the lack of any systematic characterization of the crystallization process. Herein, we report a novel non-invasive optical technique, termed vertical reflection-type in situ, real-time absorption spectroscopy, to investigate the mechanisms of Lewis base-mediated optimization of perovskite crystallinity by visualizing the entire energetic landscape of crystal growth. We show that by virtue of the urea additive, a prototypical Lewis base, the growth kinetics is accelerated prominently by decreasing the activation energy from 73.7 to 41.7 kJ/mol. In addition, the self-passivation of structural disorder during thermal annealing is identified, which is shown to be further strengthened by urea modification toward a shallower distribution of trap states.

Influence of the MACl additive on grain boundaries, trap-state properties, and charge dynamics in perovskite solar cells.

Grain boundary trap passivation in perovskite films has become one of the most effective strategies for suppressing the charge recombination and enhancing the photovoltaic performance of perovskite solar cells, whereas the relevant trap-state properties and the charge carrier dynamics need to be further clarified. In this work, the CH3NH3Cl (MACl) additive is introduced into the MAI:PbI2 precursor solution to obtain perovskite films comprising various grain sizes with distinct grain boundaries and trap-state properties. The influence of grain boundary traps passivated with the MACl additive on trap-state properties and charge carrier transport/recombination dynamics is systematically studied with time-resolved spectroscopic and transient photoelectric characterization. Specifically, the MACl amount determines the content of the PbI2 residual in the final perovskite, leading to photoluminescence quenching induced by charge transfer. The trap-state distribution result reveals that the deep-level traps at the grain boundaries as the main sources of charge recombination centers are dramatically passivated. Low-temperature photoluminescence spectroscopy distinguishes and compares the trap-state emission related to different perovskite phases. Transient photoelectric measurements including photovoltage decay and charge extraction further demonstrate that the boundary trap passivation can effectively promote charge transport and inhibit charge recombination in devices treated with the optimized MACl amount. As a result, the corresponding device possesses superior photovoltaic parameters to the control device. This work proposes a systematic understanding of the grain boundary trap passivation strategy and provides a new insight into the development of high-performance perovskite solar cells.

The influence of the electron transport layer on charge dynamics and trap-state properties in planar perovskite solar cells.

Despite the outstanding photovoltaic performance of perovskite solar cells, the correlation between the electron transport layer and the mechanism of photoelectric conversion is still not fully understood. In this paper, the relationship between photovoltaic performance and carrier dynamics is systematically studied in both TiO2- and SnO2-based planar perovskite devices. It is found that the different electron transport layers result in distinct forward scan results and charge dynamics. Based on the charge dynamics results, the influence of the electron transport layer on charge carrier transport and charge recombination is revealed. More importantly, the trap-state density is characterized, which is proven to be related to the charge carrier dynamics and the specific hysteresis behaviour in the perovskite solar cells. The present work would provide new insights into the working mechanisms of electron transport layers and their effect on hysteresis.

Modification of NiO x hole transport layer for acceleration of charge extraction in inverted perovskite solar cells.

The modification of the inorganic hole transport layer has been an efficient method for optimizing the performance of inverted perovskite solar cells. In this work, we propose a facile modification of a compact NiO x film with NiO x nanoparticles and explore the effects on the charge carrier dynamic behaviors and photovoltaic performance of inverted perovskite devices. The modification of the NiO x hole transport layer can not only enlarge the surface area and infiltration ability, but also adjust the valence band maximum to well match that of perovskite. The photoluminescence results confirm the acceleration of the charge separation and transport at the NiO x /perovskite interface. The corresponding device possesses better photovoltaic parameters than the device based on control NiO x films. Moreover, the charge carrier transport/recombination dynamics are further systematically investigated by the measurements of time-resolved photoluminescence, transient photovoltage and transient photocurrent. Consequently, the results demonstrate that proper modification of NiO x can significantly enlarge interface area and improve the hole extraction capacity, thus efficiently promoting charge separation and inhibiting charge recombination, which leads to the enhancement of the device performances.

Concentration-regulated photon upconversion and quenching in NaYF4:Yb3+,Er3+ nanocrystals: nonexponentiality revisited.

Concentration quenching of rare-earth doped upconversion nanoparticles severely limits the dopant concentration, and this greatly hinders their potential applications. Therefore, it is necessary to understand the roles of dopant concentration in photon population and luminescence quenching for materials designed with improved upconversion luminescence (UCL). Herein, the excited-state dynamics of well-accepted NaYF4:Yb3+,Er3+ nanocrystals were investigated as models based on the Kohlrausch-function. The use of the Kohlrausch-function successfully disentangled the rise and decay of dynamics data and well revealed the kinetics. Photon population and concentration quenching mechanisms depending on the sensitizer concentration are deeply revealed by the regular variations of the fitting parameters. The results indicated that high doping of sensitizers will accelerate the population of both green and red emitting energy levels, but cause significant concentration quenching in green emission and little quenching in red emission. Our work opened up new pathways of kinetics analysis, which is beneficial for further mechanism development, and established detailed photon population and concentration quenching models depending on the doping concentration of the sensitizer.

Charge carrier recombination dynamics in a bi-cationic perovskite solar cell.

The compositional engineering is of great importance to tune the electrical and optical properties of perovskite and improve the photovoltaic performance of perovskite solar cells. The exploration of the corresponding photoelectric conversion processes, especially the carrier recombination dynamics, will contribute to the optimization of the devices. In this work, perovskite with mixed methylammonium (MA) and formamidinium (FA) as organic cations, MA0.4FA0.6PbI3, is fabricated to study the influence of the bi-cation on the charge carrier recombination dynamics. X-ray diffraction analysis indicates the existence of the MAPbI3-FAPbI3 phase segregation in the bi-cationic perovskite crystal. The time-resolved photoluminescence dynamics presents a relatively fast carrier recombination process ascribed to the charge transfer from MAPbI3 to FAPbI3 in the bi-cationic perovskite film. The carrier recombination dynamics investigated by transient photovoltage measurements reveals a biphasic trap-assisted carrier recombination mechanism in the bi-cationic device, which involves carrier recombination in the MAPbI3 phase and FAPbI3 phase, respectively. The ultimate presentation of the carrier recombination process is closely related to the charge transfer between the two perovskite phases.

Naturally occurring nanotube with surface modification as biocompatible, target-specific nanocarrier for cancer phototherapy.

Phototherapy has drawn increasing attention including the use of nanocarriers with high drug loading capacity and delivery efficacy for target-specific therapy. We have made use of naturally-occurring halloysite nanotubes (HNTs) to build a biomimetic nanocarrier platform for target-specific delivery of phototherapeutic agents. The HNTs were decorated with poly(sodium-p-styrenesulfonate) (PSS) to enhance the biocompatibility, and were further functionalized by lumen loading the type-II photosensitizer indocyanine green (ICG). The HNT-PSS-ICG nanocarrier, without further tethering targeting groups, was shown to associate with the membrane of giant unilamellar vesicles (GUVs) via Pickering effects. Application of HNT-PSS-ICG nanocarrier to human breast cancer cells gave rise to a cell mortality as high as 95%. The HNT-PSS-ICG nanocarrier was further coated with MDA-MB-436 cell membranes to endow it with targeting therapy performance against breast cancer, which was confirmed by in vivo experiments using breast cancer tumors in mice. The membrane-coated and biocompatible nanocarrier preferentially concentrated in the tumor tissue, and efficiently decreased the tumor volume by a combination of photodynamic and photothermal effects upon near-infrared light exposure. Our results demonstrate that the HNT-based nanocarrier by virtue of facial preparation and high loading capacity can be a promising candidate for membrane-targeting nanocarriers.

Energy transfer mechanism dominated by the doping location of activators in rare-earth upconversion nanoparticles.

Research on the energy transfer mechanism of rare-earth-doped upconversion nanoparticles (UCNPs) has been an important area due to the increasing demand for tuning multicolor emission and enhancing the upconversion efficiency; however, because of large energy mismatch, many lanthanide activators, such as Eu3+, cannot realize highly efficient near infrared-to-visible upconversion by simple codoping of Yb3+. Therefore, introduction of other ions to assist the energy transfer process is required. Herein, we prepared core-shell nanoparticles with different doping locations to investigate the upconversion energy transfer mechanism. The upconversion luminescence (UCL) of core-shell nanoparticles was investigated by steady-state luminescence and time-resolved luminescence spectra. The UCL behaviors in these different multi-activator core-shell nanoparticles were observed. The results revealed different energy transfer channels influenced by the doping location of activators. This study may open up new avenues of structure design for fine-tuning of multicolor UCL for specific applications.

Characterization of the influences of morphology on the intrinsic properties of perovskite films by temperature-dependent and time-resolved spectroscopies.

Organic-inorganic halide perovskites have attracted enormous attention owing to their promising application in photovoltaic devices. The morphology of the perovskites is the key to driving the performance of perovskite devices, which necessitates a systematic study. In this work, two typical morphologies, i.e., flake and cube, of perovskite films are fabricated, and the temperature-dependent optical absorption and photoluminescence properties of the two types of perovskite film are systematically investigated. From the temperature-dependent spectra, both exciton and phase transition temperatures of the flake film are found to be about 10 K lower than those of the cube one. Meanwhile, the influences of the morphology on the exciton binding energy, optical phonon energy and polaron binding energy are quantitatively characterized. The exciton binding of the flake film is nearly three times smaller than that of the cube one, while the phonon coupling energy and the polaron binding energy of the former are about 5 meV and 2 meV larger than those of the latter. Furthermore, the results of photoluminescence lifetime and charge separation efficiency further reveal that the charge carrier kinetics in the two kinds of perovskite films is significantly different. The current study provides a theoretical framework to understand the fundamental physics of perovskites and to promote the design and enhancement of active materials for improved optoelectronic devices.

Integrity of Membrane Structures in Giant Unilamellar Vesicles as Assay for Antioxidants and Prooxidants.

We have attempted to evaluate, on the basis of optical microscopy for a single giant unilamellar vesicle (GUV), the potency of antioxidants in protecting GUV membranes from oxidative destruction. Photosensitized membrane budding of GUVs prepared from soybean phosphatidylcholine with chlorophyll a (Chl a) and ß-carotene (ß-Car) as photosensitizer and protector, respectively, were followed by microscopic imaging. A dimensionless entropy parameter, ΔE, as derived from the time-resolved microscopic images, was employed to describe the evolution of morphological variation of GUVs. As an indication of membrane instability, the budding process showed three successive temporal regimes as a common feature: a lag phase prior to the initiation of budding characterized by LP (in s), a budding phase when ΔE increased with a rate of kΔE (in s-1), and an ending phase with morphology stabilized at a constant ΔEend (dimensionless). We show that the phase-associated parameters can be objectively obtained by fitting the ΔE-t kinetics curves to a Boltzmann function and that all of the parameters are rather sensitive to ß-Car concentration. As for the efficacy of these parameters in quantifying the protection potency of ß-Car, kΔE is shown to be most sensitive for ß-Car in a concentration regime of biological significance of <1 × 10-7 M, whereas LP and ΔEend are more sensitive for ß-Car concentrations exceeding 1 × 10-7 M. Furthermore, based on the results of GUV imaging and fluorescence and Raman spectroscopies, we have revealed for different phases the mechanistic interplay among 1O2* diffusion, PC-OOH accumulation, Chl a and/or ß-Car consumption, and the morphological variation. The developed assay should be valuable for characterizing the potency of antioxidants or prooxidants in the protection or destruction of the membrane integrity of GUVs.

Multiple-Trapping Model for the Charge Recombination Dynamics in Mesoporous-Structured Perovskite Solar Cells.

The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO2 -dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices.

Design and Fabrication of Temperature-Sensitive Nanogels with Controlled Drug Release Properties for Enhanced Photothermal Sterilization.

For better removal of excessive free radicals and harmful bacteria from the human body, the development of synergistic antioxidant and antibacterial agents is urgently required. Herein, we designed novel temperature-sensitive, curcumin (Cur)-loaded nanogels for the application of scavenging reactive oxygen species and killing pathogenic bacteria. Photothermal sterilization, different from traditional antibiotics, is a promising and effective treatment for pathogenic bacterial infection. The nanogels were fabricated by using poly(N-isopropylacrylamide) (a temperature-sensitive hydrogel) to encapsulate poly(3,4-ethylenedioxythiophene) nanoparticles (photothermal agents) and Cur through a reformative precipitation polymerization. When triggered by near-IR light, the Cur-loaded nanogels exhibited high (56.8 %), and excellent temperature-sensitive effects. Moreover, the light-induced temperature increase can also weaken the interaction between the networks of PNIPAAm and Cur, to show excellent antioxidant and antibacterial performance (90 % cell death) of the nanogels.