Photoinduced Degradation Research Articles

Profiling photo-induced degradation for operationally perovskite solar cells in space environment

Perovskite solar cells (PSCs) have excellent specific power and are potential external power sources for spacecraft. However, the low air pressure in the space environment hinders the operational stability of PSCs, and there is still a lack of understanding of their degradation mechanisms. Here, from the perspective of device performance, morphology and ion migration, PSCs are found to undergo severe degradation under space light. Further studies have shown that vacuum can significantly accelerate the degradation rate compared to atmospheric pressure. Photoinduced ion migration and volatilization of organic matter are the main causes of irreversible loss of PSCs. The results of this research provide important insights into the potential applications of PSCs in space. • Photo-induced degradation for perovskite solar cells in space is investigated. • Vacuum accelerates the device degradation rate compared to normal pressure. • Ion migration and perovskite decomposition are contributors to device efficiency loss.

Daylight LED promotes photochemical ring contraction of 2-amine-4H-pyran-3-carbonitriles with consequent loss of their antifungal activity.

The initial objective of our work was to synthesize a series of 2-amino-4H-pyran-3-carbonitriles to be tested for their antifungal activities against economically relevant phytopathogenic fungi. Fourteen compounds were prepared in up to 94% yield and shown percentages of Botrytis cinerea inhibition above 70%. Despite the promising biological results, we observed that stock solutions prepared for biological tests showed color changing when kept for a few days on the laboratory bench, under room conditions, illuminated by common LED daylight tubes (4500-6000k). This prompted us to investigate the possible photo-induced degradation of our compounds. FT-IR ATR experiments evidenced variations in the expected bands for functional of -amino-4H-pyran-3-carbonitriles stored under LED daylight. Following, HPLC-UV analysis showed reductions in the intensity of chromatographic peaks of 2-amino-4H-pyran-3-carbonitriles, and but not for solutions kept in the dark. A solution of (E)-2-amino-8-(4-nitrobenzylidene)-4-(4-nitrophenyl)-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile underwent 84.4% of conversion after 72h of exposure to continuous LED daylight in a BOD chamber, and the reaction product was isolated in 36% yield and characterized as (E)-7-cyano-5-(4-nitrobenzylidene)-8-(4-nitrophenyl)bicyclo[4.2.0]oct-1(6)-ene-7-carboxamide (7*). Despite freshly prepared solutions of 2-amino-4H-pyran-3-carbonitriles produced antifungal activities, these solutions lost biological activity when left on the bench for a week. Besides, compound 7* formed from photo-induced degradation of 7 also showed no antifungal activity. With this, we hope to bring two contributions: (1) production of cyclobutenes through photochemical reactions of 2-amino-4H-pyran-3-carbonitriles can be carried out through exposure to simple white LED daylight; (2) biological applications of such 2-amino-4H-pyran-3-carbonitriles may be impaired by their poor photostability.

Coherent Acoustic Interferometry during the Photodriven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate (Ti-OH*).

The oxygen evolution reaction (OER) from water requires the formation of metastable, reactive oxygen intermediates to enable oxygen-oxygen bond formation. Conversely, such reactive intermediates could also structurally modify the catalyst. A descriptor for the overall catalytic activity, the first electron and proton transfer OER intermediate from water, (M-OH*), has been associated with significant distortions of the metal-oxygen bonds upon charge-trapping. Time-resolved spectroscopy of in situ, photodriven OER on transition metal oxide surfaces has characterized M-OH* for the charge trapping and the symmetry of the lattice distortions by optical and vibrational transitions, respectively, but had yet to detect an interfacial strain field arising from a surface coverage M-OH*. Here, we utilize picosecond, coherent acoustic interferometry to detect the uniaxial strain normal to the SrTiO3/aqueous interface directly caused by Ti-OH*. The spectral analysis applies a fairly general methodology for detecting a combination of the spatial extent, magnitude, and generation time of the interfacial strain through the coherent oscillations' phase. For lightly n-doped SrTiO3, we identify the strain generation time (1.31 ps), which occurs simultaneously with Ti-OH* formation, and a tensile strain of 0.06% (upper limit 0.6%). In addition to fully characterizing this intermediate across visible, mid-infrared, and now GHz-THz probes on SrTiO3, we show that strain fields occur with the creation of some M-OH*, which modifies design strategies for tuning catalytic activity and provides insight into photo-induced degradation so prevalent for OER. To that end, the work put forth here provides a unique methodology to characterize intermediate-induced interfacial strain across OER catalysts.

An attempt to the nondestructive investigation of photo-induced potato postharvest quality degradation – Preliminary results

AbstractAmong improper harvest and/or postharvest storage conditions, the effect of direct sunlight plays an important role in quality degradation of potato resulting in the development of green surface color based on chlorophyll formation associated with the formation of poisonous chemicals – glycoalcaloids – known as α-chaconine and α-solanine. Yellow skinned and fleshed potatoes with or without visible initial marks of green surface color were stored at normal room temperature under direct natural (sun)light conditions for almost two months. The aim of this study was the preliminary investigation of the sunlight induced formation of chlorophyll related compounds in potato indirectly by the detection of chlorophyll development. This attempt was based on nondestructive determination of chlorophyll related spectral and fluorescence indices for both sunlight exposed and unexposed potato sides. For both potato groups the chlorophyll content related DA-index® and chlorophyll fluorescence characteristics (F0, Fm, Fv and Fv/Fm) increased during the storage period representing chlorophyll formation. In the case of Fm, Fv and Fv/Fm values, the yellow samples reached the values of the initial spotted green samples by the 7th–9th days. From this time, the chlorophyll fluorescence values changed only minimally. After storage day 34, in the case of both at day 0 yellow and green spotted potatoes, the sunny side's F0 value was lower than that of shaded side. Close relationship was found between the results of Walz monitoring-PAM (Pulse Amplitude-Modulated) chlorophyll fluorometer and the PSI (Photon Systems Instruments) chlorophyll fluorescence imaging device (e.g. Fv R2 = 0.7226). According to our preliminary results, the Vis/NIR DA-meter®, the monitoring-PAM and the chlorophyll fluorescence imaging fluorometers were found to be suitable nondestructive devices for further investigations concerning the postharvest chlorophyll formation based greening phenomena, which is associated with solanine development in potato.

The Role of Additives in Suppressing the Degradation of Liquid-Exfoliated WS2 Monolayers.

Group VI transition metal dichalcogenides (TMDs) are considered to be chemically widely inert, but recent reports point toward an oxidation of monolayered sheets in ambient conditions, due to defects. To date, the degradation of monolayered TMDs is only studied on individual, substrate-supported nanosheets with varying defect type and concentration, strain, and in an inhomogeneous environment. Here, degradation kinetics of WS2 nanosheet ensembles in the liquid phase are investigated through photoluminescence measurements, which selectively probe the monolayers. Monolayer-enriched WS2 dispersions are produced with varying lateral sizes in the two common surfactant stabilizers sodium cholate (SC) and sodium dodecyl sulfate (SDS). Well-defined degradation kinetics are observed, which enable the determination of activation energies of the degradation and decouple photoinduced and thermal degradation. The thermal degradation is slower than the photoinduced degradation and requires higher activation energy. Using SC as surfactant, it is sufficiently suppressed. The photoinduced degradation can be widely prevented through chemical passivation achieved through the addition of cysteine which, on the one hand, coordinates to defects on the nanosheets and, on the other hand, stabilizes oxides on the surface, which shield the nanosheets from further degradation.

Photo-induced degradation of PFASs: Excited-state mechanisms from real-time time-dependent density functional theory

Per- and polyfluoroalkyl substances (PFASs) are hazardous, carcinogenic, and bioaccumulative contaminants found in drinking water sources. To mitigate and remove these persistent pollutants, recent experimental efforts have focused on photo-induced processes to accelerate their degradation; however, the mechanistic details of these promising degradation processes remain unclear. To shed crucial insight on these electronic-excited state processes, we present the first study of photo-induced degradation of explicitly-solvated PFASs using excited-state, real-time time-dependent density functional theory (RT-TDDFT) calculations. Furthermore, our large-scale RT-TDDFT calculations show that these photo-induced excitations can be highly selective by enabling a charge-transfer process that only dissociates the CF bond while keeping the surrounding water molecules intact. Collectively, the RT-TDDFT techniques used in this work (1) enable a new capability for probing photo-induced mechanisms that cannot be gleaned from conventional ground-state DFT calculations and (2) provide a rationale for understanding ongoing experiments that are actively exploring photo-induced degradation of PFASs and other environmental contaminants.

Enhanced malachite green photolysis at the colloidal-aqueous interface

Colloids, such as natural particulate matter and microplastics, can play a significant role in the fate and transport of organic contaminants. Specifically, these small nano-to micron-sized particles provide large surface area; thus, particle-aqueous interfacial chemistry becomes significant. In this work, we present an experimental investigation of interfacial photokinetics of malachite green cation (MG+) adsorbed at the surface of polystyrene carboxyl (PSC) microspheres suspended in aqueous solution. Second harmonic generation (SHG), an interfacial selective laser spectroscopic tool, has been used to probe the buried interface. It is revealed that relative to the bulk, photoinduced degradation of MG+ is accelerated by approximately 10-fold at this noncatalytic particle surface. By measuring the SHG-based surface electronic spectra, we have also demonstrated that N-demethylated intermediates of MG+ remain at the interface until they are further decomposed. MG+ exhibits a bathochromic shift at the interface. Together with strong binding affinity and faster initial rate of photodegradation of MG+ at the interface, this work highlights that adsorption and surface photolysis are important pathways by which organic compounds can be transformed within the aquatic environment. Moreover, this research also stimulates further questions on the enrichment of reactive species at the colloidal-aqueous interface and their influence on facilitating decompositions of organic pollutants.

Strategies and Applications for Improving the Stability of Black Phosphorus in Physical Environment

As a new type of 2D material, black phosphorus (BP) has been gradually attracting attentions since it was rediscovered in 2014 because of its many excellent properties, such as strong anisotropic properties, optical‐response properties, layer‐dependent bandgap, and so on. 2D BP is becoming an excellent candidate in the fields of electronics, optics, and so on. However, the applications of 2D BP are greatly limited and challenged by many adverse factors, such as the low yield, poor stability, and high price of BP nanosheets (NSs). Among them, the poor stability of BP is an important factor, which is an urgent problem to be solved. The main reason for its poor stability is the instability of its environment. The degradation factors of 2D BP are studied, including photoinduced degradation and water‐catalyzed degradation. At present, some feasible methods are explored to improve the degradation, which include passivation, surface functionalization, and doping metal or nonmetal ions. This review not only clarifies the structure, properties, degradation mechanism, and methods to improve the BP stability, but also shows more strategic means for the design and application of stable BP‐based nanocomposite materials in the fields of electronics, optics, biomedicine, and fire safety.

Elucidation of the photoinduced transformations of Aliskiren in river water using liquid chromatography high-resolution mass spectrometry

Aliskiren was selected as a compound of potential concern among a suspect screening list of more than 40,000 substances on a basis of high occurrence, potential risk and the absence of information about its environmental fate.This study investigated the photoinduced degradation of aliskiren in river water samples spiked at trace levels exposed to simulated sunlight. A half-life time of 24 h was observed with both direct and indirect photolysis playing a role on pollutant degradation. Its photo-induced transformation involved the formation of six transformation products (TPs), elucidated by LC-HRMS - resulted from the drug hydroxylation, oxidation and moieties loss with subsequent cyclization structurally. The retrospective suspected analysis performed on a total of 754 environmental matrices evidenced the environmental occurrence of aliskiren and two TPs in surface waters (river and seawater), fresh water, sediments and biota. In silico bioassays suggested that aliskiren degradation undergoes thought the formation of TPs with distinct toxicity comparing with the parent compound.

Cyanine Dyes for Photo-Thermal Therapy: A Comparison of Synthetic Liposomes and Natural Erythrocyte-Based Carriers

Cyanine fluorescent dyes are attractive diagnostic or therapeutic agents due to their excellent optical properties. However, in free form, their use in biological applications is limited due to the short circulation time, instability, and toxicity. Therefore, their encapsulation into nano-carriers might help overcome the above-mentioned issues. In addition to indocyanine green (ICG), which is clinically approved and therefore the most widely used fluorescent dye, we tested the structurally similar and cheaper alternative called IR-820. Both dyes were encapsulated into liposomes. However, due to the synthetic origin of liposomes, they can induce an immunogenic response. To address this challenge, we proposed to use erythrocyte membrane vesicles (EMVs) as “new era” nano-carriers for cyanine dyes. The optical properties of both dyes were investigated in different biological relevant media. Then, the temperature stability and photo-stability of dyes in free form and encapsulated into liposomes and EMVs were evaluated. Nano-carriers efficiently protected dyes from thermal degradation, as well as from photo-induced degradation. Finally, a hemotoxicity study revealed that EMVs seem less hemotoxic dye carriers than clinically approved liposomes. Herein, we showed that EMVs exhibit great potential as nano-carriers for dyes with improved stability and hemocompatibility without losing excellent optical properties.

The Use of Green‐Solvent Processable Molecules with Large Dipole Moments in the Electron Extraction Layer of Inverted Organic Solar Cells as a Universal Route for Enhancing Stability

AbstractEmploying a large dipole interlayer has recently been one of the most captivating interfacial engineering approaches in organic solar cells (OSCs). In this work, the effect of using green‐solvent processable molecules with large dipole moments as electron extraction layers (DM‐EELs) on OSCs’ stability is investigated. The inverted OSCs are based on two donor–acceptor systems, with histidine and sarcosine as representative DM‐EELs and with ZnO as a control EEL. Stability results illustrate that while the dominant degradation mechanism depends on the donor–acceptor system (photoinduced degradation vs temporal degradation), using DM‐EELs substantially suppresses degradation and enhances stability in both cases. The voltage‐dependent ideality factor characteristics show that the DM‐EEL OSCs exhibit minimal recombination mechanism changes even after UV exposure. By contrast, ZnO cells are limited by significant shunt formation and UV‐induced surface recombination. Low‐temperature measurements verify that unlike the indium tin oxide (ITO)/ZnO contact, the ITO/DM‐EEL contact is not affected by trap states or work function variations. As a result, the ITO/DM‐EEL contact maintains its electron‐selectivity and resists UV‐induced surface recombination observed in the ZnO cells. The results show that using green‐solvent processable materials with large dipole moments as EELs can provide a universal route for enhancing the stability of inverted OSCs.

Bicarbazole-based oxime esters as novel efficient photoinitiators for photopolymerization under UV-Vis LEDs

Five novel oxime ester type photoinitiators based on the bicarbazole structure were designed and synthesized to extend the absorption wavelength range of photoinitiators to the near-ultraviolet–visible (UV–vis) region and ensure the high efficiency of photoinduced polymerization. Introducing alkyl chains with different lengths and/or cyclizations and further α-carbonyl groups influenced absorption and photolysis properties. The rationality of the designed molecules was proven by using UV–vis spectra, and liquid chromatography–mass spectrometry (LCMS) and in situ 1H NMR experiments were also performed to explain the mechanism of photoinduced degradation. Importantly, the photopolymerization and storage stabilities of the optimized photoinitiators in photocuring formulations were also tested and were found to be better than those of commercial photoinitiators, e.g., OXE 02.

Photo-induced degradation and toxicity change of decabromobiphenyl ethers (BDE-209) in water: Effects of dissolved organic matter and halide ions

BDE-209 is a widely used brominated flame retardant that is ubiquitous in the aquatic environment, especially in marine water. However, photodegradation of BDE-209 in seawater is still not fully understood. In this work, the photodegradation kinetics of BDE-209 in water was studied and the effects of seawater dissolved organic matter (S-DOM) and halide ions (Cl−, Br−) were evaluated. S-DOM inhibited the degradation of BDE-209 through dynamic quenching and light shielding effect. However, with the coexistence of S-DOM, Cl− and Br−, the photodegradation of BDE-209 was significantly promoted. The promotional effect is attributed to the generation of excited triplet state S-DOM, singlet oxygen, and reactive halogen radicals. The results of density functional theory calculation showed that •Cl addition reaction on C-Br sites of BDE-209 is the main reaction pathway of BDE-209 with chlorine radical, which leads to the generation of mixed Cl/Br substituted intermediates. The acute toxicity and estrogenic effects of BDE-209 solution were enhanced during simulated sunlight irradiation. These results indicate that the environmental factors in seawater play important roles in the photodegradation of BDE-209, and contribute to the potential ecological risks of PBDEs in the marine environment.

Photocatalytic performance of ferric vanadate (FeVO4) nanoparticles synthesized by hydrothermal method

In this study, Ferric vanadate nanoparticles (FeVO4-NPs) were synthesized by a facile solution growth single-pot autoclave hydrothermal method. The as-synthesized powder was extensively characterized by XRD, SEM, EDS, XPS, DRS, FT-IR, BET and PL spectroscopy for physicochemical properties. The average particles size of nanoparticles was found to be 80 nm to 0.12 μm. The photocatalytic response of the FeVO4 catalyst was examined for the decomposition of Methylene Blue (MB) dye solution under visible light. The photoluminescence spectrum study revealed the potential of FeVO4-NPs in the red region of visible light. FeVO4-NPs efficiently catalyze reaction initiator (H2O2 ~ 0.05 ml dose) to generate strong OH• radical with augmented photocatalytic degradation performance i.e. 98.74% in 180 min. Various factors such as Hydrogen peroxide (H2O2) loading amount as an initiator (special case), dye absorption, stability and reusability were discussed critically. The photo-induced degradation activity was greatly enhanced for low initiator concentrations. This work is helpful for rapid and low-cost preparation of FeVO4-NPs for Fenton-like photocatalytic degradation of the water pollutants.

Bandgap Engineering in a Staggered-Type Oxide Perovskite Heterojunction for Efficient Visible Light-Driven Photocatalytic Dye Degradation.

Oxide perovskite materials with ABO3 structure have been widely employed for photocatalytic applications. However, owing to the disadvantageous electron-hole recombination process and wide bandgap of some materials, the photocatalytic performance is seemingly restricted. Coupling two catalysts together through the formation of a heterojunction ensures effective charge carrier separation. The intimate interaction between the materials is propitiously useful for charge transfer, thereby increasing the efficacy. In this study, the photocatalytic activity of a KxNa(1-x)NbO3-BaBiO3 (KNN-BBO) heterojunction material for the degradation of Rhodamine 6G organic dye was investigated. The materials were extensively characterized by X-ray diffraction, UV-Vis diffused reflectance spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and N2 adsorption isotherms. The degradation efficiency of the organic contaminant under 1 sun simulated sunlight is monitored by spectral analysis from UV-Vis absorption spectroscopy. The resistance to charge transfer was also observed by electrochemical impedance spectroscopy. The effect of the sintering temperature on the photoinduced degradation activity was also included in our study. An unsintered KNN-BBO (UKB) composite material is found to be the most efficient catalyst with 84% removal efficiency as compared to the sintered one (SKB). This is attributed to the reduced bandgap with staggered-type band alignment, increased surface area, and surface oxygen vacancy states. Together with the crucial findings of this work, a probable mechanism for enhanced photocatalytic activity has been proposed here.

Photochemically induced cyclic morphological dynamics via degradation of autonomously produced, self-assembled polymer vesicles

Autonomous and out-of-equilibrium vesicles synthesised from small molecules in a homogeneous aqueous medium are an emerging class of dynamically self-assembled systems with considerable potential for engineering natural life mimics. Here we report on the physico-chemical mechanism behind a dynamic morphological evolution process through which self-assembled polymeric structures autonomously booted from a homogeneous mixture, evolve from micelles to giant vesicles accompanied by periodic growth and implosion cycles when exposed to oxygen under light irradiation. The system however formed nano-objects or gelation under poor oxygen conditions or when heated. We determined the cause to be photoinduced chemical degradation within hydrated polymer cores inducing osmotic water influx and the subsequent morphological dynamics. The process also led to an increase in the population of polymeric objects through system self-replication. This study offers a new path toward the design of chemically self-assembled systems and their potential application in autonomous material artificial simulation of living systems.

Bifunctional TiO2/CeO2 reactive adsorbent/photocatalyst for degradation of bis-p-nitrophenyl phosphate and CWAs

TiO2/CeO2 nanocomposites were prepared by simple refluxing of peroxo titanium and cerium complexes in water at ambient air and they were used for degradation of several organophosphorus compounds (OPC). The combination of high surface reactivity of ceria with photoactivity of titania led to i) rapid reactive adsorption of bis-p-nitrophenyl phosphate (BNPP) forming p-nitrophenyl phosphate (NP) and ii) subsequent photo-induced degradation of both BNPP and NP. The composites were also three and four times more efficient for rapid degradation of chemical warfare agents (CWAs) soman and VX in nonane compared to bare ceria and titania, respectively. The strong interaction of Ti with Ce within the composites led to the formation of Ce3+ and Ti<4+ states, reduction of titania crystallite size, change of acid-base and surface properties and synergetic effects that are all responsible for highly improved degradation efficiency of these bifunctional nanocomposites with considerable potential of application for abatement of various toxic compounds.

Bio-directed synthesis of gold nanoparticles using Ananas comosus aqueous leaf extract and their photocatalytic activity for LDPE degradation

A bio-directed synthesis of gold nanoparticles (Au NPs) was developed via the reduction of hydrogen tetrachloroaurate (III) (HAuCl4·3H2O) solution by the aqueous leaf extract of Ananas comosus. The polyphenol stabilized Au NPs were characterized by UV–visible, Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD)/selected area electron diffraction (SAED), high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) analyses. The HRTEM images revealed that Au NPs were well dispersed with spherical structures. The size ranges from 7.39 to 32.09 nm with average particle size of 18.85 ± 6.74 nm. The peaks of XRD analysis at (2θ) 37.96°, 44.06°, 64.54°, 77.50° and 81.73° were respectively assigned to (111), (200), (220), (311) and (222) planes of the face-centered cubic (fcc) lattice of gold. The photocatalytic potential of Au NPs was studied through the solid-phase degradation of low-density polyethylene (LDPE) film. The photoinduced degradation of LDPE@Au nanocomposite film was higher than that of the pure LDPE film. The weight loss of LDPE@Au (1.0 wt%) nanocomposite film steadily increased and reached 51.4 ± 4.8% in 240 h under solar light irradiation, compared to the photo-induced LDPE with only 8.6 ± 0.7%. However, LDPE film with 1.0% Au NPs gave a weight loss value of 4.72 ± 0.71 under the dark condition at the end of 240 h. Thus, LDPE film with 1.0% Au NPs showed a degradation efficiency of 90.8% under solar irradiation after 240 h. The sustainability of the nanoparticles was confirmed through reusability in the photocatalytic degradation reaction up to five consecutive cycles without substantial loss in its catalytic performance.

High-Performance Electron-Transport-Layer-Free Quantum Junction Solar Cells with Improved Efficiency Exceeding 10%

Colloidal quantum dot solar cells (CQDSCs) are good candidates for low-cost power generators, due to their wide light-response range, high theoretical efficiency, and solution processability. Nevertheless, the generally used metal oxide electron transport layer (MOETL) induced various problems, blocking the performance enhancement of CQDSCs, such as band alignment mismatch, high-energy photon loss, and photoinduced interfacial degradation. In the work described herein, we constructed high-efficiency and air-stable MOETL-free solar cells based on quantum junction device structure, through manipulating the semiconductor and surface-trap properties of PbS CQDs by ligand engineering. A record power conversion efficiency of 10.5% was successfully achieved in our MOETL-free quantum junction solar cells (QJSCs). Increased photogenerated current density was obtained in MOETL-free QJSCs because of the ultraviolet and near-infrared photoresponse enhancement. These unencapsulated MOETL-free QJSCs show long-term air-storage stability (>4000 h). Our work successfully demonstrates the MOETL-free quantum junction as a high-efficiency and stable structure for solar cells, paving a way for application of CQDSCs in the full-spectrum, scalable-production, portable, and flexible photovoltaic technology.

Influence of grain size on the photo-stability of perovskite solar cells

Organometallic perovskite solar cells have recently attracted huge attention as one of the most promising photovoltaic technologies. Despite higher efficiency achieved in recent years, the instability of perovskite solar cells remains as the biggest problem to be solved. Recently, researchers have developed several encapsulation techniques to enhance the device stability against moisture, but the stability of the device under light soaking did not receive that much attention. Previously, we had studied the device physics behind photo-induced degradation of the perovskite solar cells, and we found that the performance degradation during light exposure is attributed to the generation and migration of mobile ions. In this paper, we investigated the effect of the grain size on the photo-stability and mobile ion generation in perovskite solar cells. We found that with larger perovskite film grain size, the ion generation rate decreased significantly under light exposure, which leads to less performance degradation. This result was further confirmed by our ion density measurement using transient ionic current.