Search

Increasing the stability of perovskites is essential for their integration in commercial photovoltaic devices. Halide mixing is suggested as a powerful strategy toward stable perovskite materials. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate. Here we successfully determine the exact location of the I and Cl anions in the CH3NH3PbBr3–yIy and CH3NH3PbBr3–zClz mixed halide perovskite lattices and correlate it with the enhanced stability we find for the latter. By combining scanning tunneling microscopy and density functional theory, we predict that, for low ratios, iodine and chlorine incorporation have different effects on the electronic properties and stability of the CH3NH3PbBr3 perovskite material. In addition, we determine the optimal Cl incorporation ratio for stability increase without detrimental band gap modification, providing an important direction for the fabrication of stable perovskite devices. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a half-cell device architecture. Our findings provide an answer to the current debate on halide incorporation and demonstrate their direct influence on device stability.

An ideal anti-counterfeiting label not only needs to be unclonable and accurate but also must consider cost and efficiency. But the traditional physical unclonable function (PUF) recognition technology must match all the images in a database one by one. The matching time increases with the number of samples. Here, a new kind of PUF anti-counterfeiting label is introduced with high modifiability, low reagent cost (2.1 × 10−4 USD), simple and fast authentication (overall time 12.17 s), high encoding capacity (2.1 × 10623), and its identification software. All inorganic perovskite nanocrystalline films with clonable micro-profile and unclonable micro-texture are prepared by laser engraving for lyophilic patterning, liquid strip sliding for high throughput droplet generation, and evaporative self-assembling for thin film deposition. A variety of crystal film profile shapes can be used as “specificator” for image recognition, and the verification time of recognition technology based on this divide-and-conquer strategy can be decreased by more than 20 times. © 2021 Wiley-VCH GmbH

An ideal anti-counterfeiting label not only needs to be unclonable and accurate but also must consider cost and efficiency. But the traditional physical unclonable function (PUF) recognition technology must match all the images in a database one by one. The matching time increases with the number of samples. Here, a new kind of PUF anti-counterfeiting label is introduced with high modifiability, low reagent cost (2.1 × 10−4 USD), simple and fast authentication (overall time 12.17 s), high encoding capacity (2.1 × 10623), and its identification software. All inorganic perovskite nanocrystalline films with clonable micro-profile and unclonable micro-texture are prepared by laser engraving for lyophilic patterning, liquid strip sliding for high throughput droplet generation, and evaporative self-assembling for thin film deposition. A variety of crystal film profile shapes can be used as “specificator” for image recognition, and the verification time of recognition technology based on this divide-and-conquer strategy can be decreased by more than 20 times. © 2021 Wiley-VCH GmbH

An ideal anti-counterfeiting label not only needs to be unclonable and accurate but also must consider cost and efficiency. But the traditional physical unclonable function (PUF) recognition technology must match all the images in a database one by one. The matching time increases with the number of samples. Here, a new kind of PUF anti-counterfeiting label is introduced with high modifiability, low reagent cost (2.1 × 10−4 USD), simple and fast authentication (overall time 12.17 s), high encoding capacity (2.1 × 10623), and its identification software. All inorganic perovskite nanocrystalline films with clonable micro-profile and unclonable micro-texture are prepared by laser engraving for lyophilic patterning, liquid strip sliding for high throughput droplet generation, and evaporative self-assembling for thin film deposition. A variety of crystal film profile shapes can be used as “specificator” for image recognition, and the verification time of recognition technology based on this divide-and-conquer strategy can be decreased by more than 20 times.

Atomistic characterization of surface termination and the corresponding mechanical properties of single-crystal methylammonium lead tribromide (MAPbBr(3)) are performed using combined atomic force microscopy (AFM) measurements and density functional theory (DFT) calculations. A clean MAPbBr(3) surface is obtained by in situ cleavage in ultrahigh vacuum at room temperature, and the subsequent AFM measurements of the as-cleaved MAPbBr(3) exhibit the coexistence of two different surface terrace types with step height differences corresponding to about half the thickness of a PbI6 octahedron layer. Concurrent friction force microscopy measurements show that the two surfaces result in two distinct friction values. Based on DFT calculations, we attribute the higher-friction and lower-friction surfaces to MABr-terminated flat and PbBr2-terminated vacant surface terminations, respectively. The calculated electronic band structures of the various MABr- and PbBr2-terminated surfaces show that the midgap states are absent, revealing the defect-tolerant nature of the ideal single-crystal MAPbBr(3) surfaces.

Electron transport layer (ETL) is a functional layer of great significance for boosting the power conversion efficiency (PCE) of perovskite solar cells (PSCs). To date, it is still a challenge to simultaneously reduce the surface defects and improve the crystallinity in ETLs during their low-temperature processing. Here, a novel strategy for the mediation of in situ regrowth of SnO2 nanocrystal ETLs is reported: introduction of controlled trace amounts of surface absorbed water on the fluorinated tin oxide (FTO) or indium–tin oxide (ITO) surfaces of the substrates using ultraviolet ozone (UVO) pretreatment. The optimum amount of adsorbed water plays a key role in balancing the hydrolysis–condensation reactions during the structural evolution of SnO2 thin films. This new approach results in a full-coverage SnO2 ETL with a desirable morphology and crystallinity for superior optical and electrical properties, as compared to the control SnO2 ETL without the UVO pretreatment. Finally, the rigid and flexible PSC devices based on the new SnO2 ETLs yield high PCEs of up to 20.5% and 17.5%, respectively. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

An upscalable perovskite film deposition method combining raster ultrasonic spray coating and chemical vapor deposition is reported. This method overcomes the coating size limitation of the existing stationary spray, single-pass spray, and spin-coating methods. In contrast with the spin-coating method (>90% Pb waste), negligible Pb waste during PbI2 deposition makes this method more environmentally friendly. Outstanding film uniformity across the entire area of 5 cm x 5 cm is confirmed by both large-area compatible characterization methods (electroluminescence and scattered light imaging) and local characterization methods (atomic force microscopy, scanning electron microscopy, photoluminescence mapping, UV-vis, and X-ray diffraction measurements on multiple sample locations), resulting in low solar cell performance decrease upon increasing device area. With the FAPb(I0.85Br0.15)(3) (FA = formamidinium) perovskite layer deposited by this method, champion solar modules show a power conversion efficiency of 14.7% on an active area of 12.0 cm(2) and an outstanding shelf stability (only 3.6% relative power conversion efficiency decay after 3600 h aging). Under continuous operation (1 sun light illumination, maximum power point condition, dry N-2 atmosphere with <5% relative humidity, no encapsulation), the devices show high light-soaking stability corresponding to an average T-80 lifetime of 535 h on the small-area solar cells and 388 h on the solar module.

Perovskite solar cells (PSCs) have attracted great attention in the past few years due to their rapid increase in efficiency and low-cost fabrication. How-ever, instability against thermal stress and humidity is a big issue hindering their commercialization and practical applications. Here, by combining thermally stable formamidinium-cesium-based perovskite and a moisture-resistant carbon electrode, successful fabrication of stable PSCs is reported, which maintain on average 77% of the initial value after being aged for 192 h under conditions of 85 degrees C and 85% relative humidity (the "double 85" aging condition) without encapsulation. However, the mismatch of energy levels at the interface between the perovskite and the carbon electrode limits charge collection and leads to poor device performance. To address this issue, a thin-layer of poly(ethylene oxide) (PEO) is introduced to achieve improved interfacial energy level alignment, which is verified by ultraviolet photoemission spectroscopy measurements. Indeed as a result, power conversion efficiency increases from 12.2% to 14.9% after suitable energy level modification by intentionally introducing a thin layer of PEO at the perovskite/carbon interface.

While organic-inorganic hybrid perovskite solar cells are emerging as promising candidates for next-generation solar cells with fascinating power conversion efficiency, the instability of perovskites remains a significant bottleneck for their commercialization. An atomic scale understanding of the degradation of hybrid perovskites, however, is only in its beginning stages because of the difficulty in preparing well-defined surface conditions for characterization. Using atomic force microscopy at ultra-high vacuum and room temperature, we report the first direct observation of the degradation process of a cleaved methylammonium lead bromide, MAPbBr(3) (MA: CH3NH3+), single crystal. Upon in situ cleavage, atomic force microscopy images show large flat terraces with monolayer height steps, which correspond to the surface of cubic MAPbBr(3) with methylammonium ligand termination. While this surface can be prepared via the cleavage process and is energetically stable, we observe that after several weeks under dark and vacuum conditions it degrades and produces clusters surrounded by pits. Guided by density functional theory calculations, we propose a degradation pathway that initiates even at low humidity levels and leads to the formation of surface PbBr2 species. We finally identify the electronic structure of the MA-bromine-terminated flat surface and find that it is correlated with a strong field-induced degradation of the MAPbBr(3) only at positive sample bias voltages.

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn(0.5)Ge(0.5)l(3)) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N-2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost-effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO2 are systematically studied. Ar and O-2 are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO2 films. With the optimized structure, SnO2 films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO2 electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T-80 operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO2 film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm(2) fabricated on 5 x 5 cm(2) substrates (geometry fill factor = 91%), and a T-80 operational lifetime of 515 h.

In recent years, metal halide perovskite opens the new era of the photovoltaic field. The highest power conversion efficiency has exceeded 23%. Although most of reported perovskite solar cells (PSCs) were fabricated by solution methods, the vacuum thermal evaporation method has its unique advantages, such as easy controlling of film thickness, compatibility with tandem cell designs, flexibility to tune interfaces, free of solvent, etc. In this paper, we review the development and progress of the vacuum thermal evaporation method for fabricating PSCs. © 2018 Elsevier B.V.

The rapid progress of organic-inorganic metal halide perovskite solar cells (PSCs) has attracted broad interest in photovoltaic community. A typical PSC consists of anode/cathode, a perovskite layer as absorber, and carrier transport layer(s) (electron/hole transport layer(s)), which are stacked together, resulting in multi-interfaces between these layers. Charge extraction and transport in these solar cell devices are strongly influenced by the interfaces and in particular the energy level alignment (ELA). It is the synergy of multiple interfaces and bulk films embedded in the cell architecture that has led to the extraordinary success of PSCs. Here, the authors review the progress of the studies on energy level alignment in PSCs, including several sections: methods for deriving ELA, semiconductor type of perovskite, bottom layer-dependent energy level shift of perovskite, density of states-governed ELA, ELA for specific interfaces, instability-induced ELA variation, and defects and ion migration-induced ELA variation. Perspective and outlook for precisely determining ELA, designing the device architecture, and fabricating high performance PSCs are discussed.

The rapid progress of organic-inorganic metal halide perovskite solar cells (PSCs) has attracted broad interest in photovoltaic community. A typical PSC consists of anode/cathode, a perovskite layer as absorber, and carrier transport layer(s) (electron/hole transport layer(s)), which are stacked together, resulting in multi-interfaces between these layers. Charge extraction and transport in these solar cell devices are strongly influenced by the interfaces and in particular the energy level alignment (ELA). It is the synergy of multiple interfaces and bulk films embedded in the cell architecture that has led to the extraordinary success of PSCs. Here, the authors review the progress of the studies on energy level alignment in PSCs, including several sections: methods for deriving ELA, semiconductor type of perovskite, bottom layer-dependent energy level shift of perovskite, density of states-governed ELA, ELA for specific interfaces, instability-induced ELA variation, and defects and ion migration-induced ELA variation. Perspective and outlook for precisely determining ELA, designing the device architecture, and fabricating high performance PSCs are discussed.

In recent years, metal halide perovskite opens the new era of the photovoltaic field. The highest power conversion efficiency has exceeded 23%. Although most of reported perovskite solar cells (PSCs) were fabricated by solution methods, the vacuum thermal evaporation method has its unique advantages, such as easy controlling of film thickness, compatibility with tandem cell designs, flexibility to tune interfaces, free of solvent, etc. In this paper, we review the development and progress of the vacuum thermal evaporation method for fabricating PSCs. © 2018 Elsevier B.V.

The rapid progress of organic-inorganic metal halide perovskite solar cells (PSCs) has attracted broad interest in photovoltaic community. A typical PSC consists of anode/cathode, a perovskite layer as absorber, and carrier transport layer(s) (electron/hole transport layer(s)), which are stacked together, resulting in multi-interfaces between these layers. Charge extraction and transport in these solar cell devices are strongly influenced by the interfaces and in particular the energy level alignment (ELA). It is the synergy of multiple interfaces and bulk films embedded in the cell architecture that has led to the extraordinary success of PSCs. Here, the authors review the progress of the studies on energy level alignment in PSCs, including several sections: methods for deriving ELA, semiconductor type of perovskite, bottom layer-dependent energy level shift of perovskite, density of states-governed ELA, ELA for specific interfaces, instability-induced ELA variation, and defects and ion migration-induced ELA variation. Perspective and outlook for precisely determining ELA, designing the device architecture, and fabricating high performance PSCs are discussed.

In this work all-inorganic perovskite CsPbIBr2 are doped with Mn to compensate their shortcomings in band structure for the application of perovskite solar cells (PSCs). The novel Mn-doped all-inorganic perovskites, CsPb1-xMnxI1+2xBr2-2x, are prepared in ambient atmosphere. As the concentration of Mn2+ ions increases, the bandgaps of CsPb1-xMnxI1+2xBr2-2x decrease from 1.89 to 1.75 eV. Additionally, when the concentration of Mn dopants is appropriate, this novel Mn-doped all-inorganic perovskite film shows better crystallinity and morphology than its undoped counterpart. These advantages alleviate the energy loss in hole transfer and facilitate the charge-transfer in perovskites, therefore, PSCs based on these novel CsPb1-xMnxI1+2xBr2-2x perovskite films display better photovoltaic performance than the undoped CsPbIBr2 perovskite films. The reference CsPbIBr2 cell reaches a power conversion efficiency (PCE) of 6.14%, comparable with the previous reports. The CsPb1-xMnxI1+2xBr2-2x cells reach the highest PCE of 7.36% (when x= 0.005), an increase of 19.9% in PCE. Furthermore, the encapsulated CsPb0.995Mn0.005I1.01Br1.99 cells exhibit good stability in ambient atmosphere. The storage stability measurements on the encapsulated PSCs reveal that PCE is dropped by only 8% of the initial value after >300 h in ambient. Such improved efficiency and stability are achieved using low-cost carbon electrodes (without expensive hole transport materials and Au electrodes).

2,2 ',7,7 '-tetrakis(N,N-di-p-methoxyphenylamine)-9,9 '-spirobifluorene (spiro-MeOTAD) has been widely employed as a hole transport layer (HTL) in perovskite-based solar cells. Despite high efficiencies, issues have been reported regarding solution processed spiro-MeOTAD HTL such as pinholes and the strong dependence of electrical properties upon air exposure, which poses challenges for solar cell stability and reproducibility. In this work, we perform a systematic study to unravel the fundamental mechanisms for the generation of pinholes in solution-processed spiro-MeOTAD films. The formation of pinholes is closely related to the presence of small amounts of secondary solvents (e.g. H2O, 2-methyl-2-butene or amylene employed as a stabilizer, absorbed moisture from ambient, etc), which have low miscibility in the primary solvent generally used to dissolve spiro-MeOTAD (e.g. chlorobenzene). The above findings are not only applicable for spiro-MeOTAD (a small organic molecule), but also applicable to polystyrene (a polymer). The influence of secondary solvents in the primary solvents is the main cause for the generation of pinholes on film morphology. Our findings are of direct relevance for the reproducibility and stability in perovskite solar cells and can be extended to many other spin-coated or drop-casted thin films.

Hybrid lead halide perovskites have emerged as promising active materials for photovoltaic cells. Although superb efficiencies have been achieved, it is widely recognized that long-term stability is a key challenge intimately determining the future development of perovskite-based photovoltaic technology. Herein, we present reversible and irreversible photodecomposition reactions of methylammonium lead iodide (MAPbI(3)). Simulated sunlight irradiation and temperature (40-80 degrees C) corresponding to solar cell working conditions lead to three degradation pathways: (1) CH3NH2 + HI (identified as the reversible path), (2) NH3 + CH3I (the irreversible or detrimental path), and (3) a reversible Pb(0) + I-2(g) photodecomposition reaction. If only the reversible reactions (1) and (3) take place and reaction (2) can be avoided, encapsulated MAPbI(3) can be regenerated during the off-illumination timeframe. Therefore, to further improve operational stability in hybrid perovskite solar cells, detailed understanding of how to mitigate photodegradation and thermal degradation processes is necessary. First, encapsulation of the device is necessary not only to avoid contact of the perovskite with ambient air, but also to prevent leakage of volatile products released from the perovskite. Second, careful selection of the organic cations in the compositional formula of the perovskite is necessary to avoid irreversible reactions. Third, selective contacts must be as chemically inert as possible toward the volatile released products. Finally, hybrid halide perovskite materials are speculated to undergo a dynamic formation and decomposition process; this can gradually decrease the crystalline grain size of the perovskite with time; therefore, efforts to deposit highly crystalline perovskites with large crystal sizes may fail to increase the long-term stability of photovoltaic devices.

Besides high efficiency, the stability and reproducibility of perovskite solar cells (PSCs) are also key for their commercialization. Herein, we report a simple perovskite formation method to fabricate perovskite films with thickness over 1 mu m in ambient condition on the basis of the fast gas-solid reaction of chlorine-incorporated hydrogen lead triiodide and methylamine gas. The resultant thick and smooth chlorine-incorporated perovskite films exhibit full coverage, improved crystallinity, low surface roughness and low thickness variation. The resultant PSCs achieve an average power conversion efficiency of 19.1 +/- 0.4% with good reproducibility. Meanwhile, this method enables an active area efficiency of 15.3% for 5 cmx 5 cm solar modules. The un-encapsulated PSCs exhibit an excellent T-80 lifetime exceeding 1600 h under continuous operation conditions in dry nitrogen environment.

2,2,7,7-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene (spiro-MeOTAD) hole transport material (HTM) is a milestone in the history of perovskite solar cells (PSCs). Proper choice of HTMs is key factor for efficient charge extraction and stability in solar cells. Spiro-MeOTAD is proven to be the most suitable HTM for testing PSCs due to its facile implementation and high performance. Similarly, spiro-MeOTAD is receiving attention in other applications other than in solar cells due to its desirable properties. However, spiro-MeOTAD is under debate regarding the topics of cost-performance, long-term stability, degradation issues (induced by temperature, additives, film quality, and environmental conditions), coating technologies compatibility, reliance on additives, and hysteresis. In this review, the advent of spiro-MeOTAD, and related aforementioned issues about spiro-MeOTAD are discussed. In addition, spiro-MeOTAD properties, alternative and new additives, other applications, and new HTMs that is comparable or outperforms spiro-MeOTAD in PSCs are summarized. In the outlook, the future research directions based on reported results that warrant further investigations are outlined.

For the first time, we intentionally deposit an ultrathin layer of excess methylammonium iodide (MAI) on top of a methylammonium lead iodide (MAPI) perovskite film. Using photoelectron spectroscopy, we investigate the role of excess MAI at the interface between perovskite and spiro-MeOTAD hole-transport layer in standard structure perovskite solar cells (PSCs). We found that interfacial, favorable, energy-level tuning of the MAPI film can be achieved by controlling the amount of excess MAI on top of the MAPI film. Our XPS results reveal that MAI dissociates at low thicknesses (<16 nm) when deposited on MAPbI(3). It is not the MAI layer but the dissociated species that leads to the interfacial energy-level tuning. Optimized interface energetics were verified by solar cell device testing, leading to both an increase of 19% in average steady-state power conversion efficiency (PCE) and significantly improved reproducibility, which is represented by a much lower PCE standard deviation (from 15 +/- 2% to 17.2 +/- 0.4%).

The electrical and friction properties of omega-(trans-4-stilbene)alkylthiol self-assembled monolayers (SAMs) on Au(111) were investigated using atomic force microscopy (AFM) and near edge x-ray absorption fine structure spectroscopy (NEXAFS). The sample surface was uniformly covered with a molecular film consisting of very small grains. Well-ordered and flat monolayer islands were formed after the sample was heated in nitrogen at 120 oC for 1 h. While lattice resolved AFM images revealed a crystalline phase in the islands, the area between islands showed no order. The islands exhibit substantial reduction (50percent) in friction, supporting the existence of good ordering. NEXAFS measurements revealed an average upright molecular orientation in the film, both before and after heating, with a narrower tilt-angle distribution for the heated fim. Conductance-AFM measurements revealed a two orders of magnitude higher conductivity on the ordered islands than on the disordered phase. We propose that the conductance enhancement is a result of a better pi-pi stacking between the trans-stilbene molecular units as a result of improved ordering in islands.

The electrical and friction properties of omega-(trans-4-stilbene)alkylthiol self-assembled monolayers (SAMs) on Au(111) were investigated using atomic force microscopy (AFM) and near edge x-ray absorption fine structure spectroscopy (NEXAFS). The sample surface was uniformly covered with a molecular film consisting of very small grains. Well-ordered and flat monolayer islands were formed after the sample was heated in nitrogen at 120 oC for 1 h. While lattice resolved AFM images revealed a crystalline phase in the islands, the area between islands showed no order. The islands exhibit substantial reduction (50percent) in friction, supporting the existence of good ordering. NEXAFS measurements revealed an average upright molecular orientation in the film, both before and after heating, with a narrower tilt-angle distribution for the heated fim. Conductance-AFM measurements revealed a two orders of magnitude higher conductivity on the ordered islands than on the disordered phase. We propose that the conductance enhancement is a result of a better pi-pi stacking between the trans-stilbene molecular units as a result of improved ordering in islands.

Thermal gravimetric and differential thermal analysis (TG-DTA) coupled with quadrupole mass spectrometry (MS) and first principles calculations were employed to elucidate the chemical nature of released gases during the thermal decomposition of CH3NH3PbI3. In contrast to the common wisdom that CH3NH3PbI3 is decomposed into CH3NH2 and HI, the major gases were methyliodide (CH3I) and ammonia (NH3). We anticipate that our findings will provide new insights into further formulations of the perovskite active material and device design that can prevent methylammonium decomposition and thus increase the long-term stability of perovskite-based opto-electronic devices.

The use of a solid hole transport layer (HTL) was transformational for the recent perovskite solar cell (PSC) revolution in solar energy technology. Often high efficiency PSC devices employ heavily doped hole transport materials such as spiro-MeOTAD. Independent of HTL chemistry, lithium-bis-trifluoromethanesulfonyl-imide (LiTFSI) and tert-butylpyridine (TBP) are commonly used as additives in HTL formulations for PSCs. LiTFSI and TBP were originally optimized for dye sensitized solar cells, where their roles have been extensively studied. However, in the case of PSCs, the function of TBP is not clearly understood. In this study, properties of the HTL composite deposited on flat silicon substrates were systematically measured at several length scales, e.g., macroscopically (profilometry, 4-point probe conductivity, and thermogravimetrydifferential thermal analysis), microscopically, and at the nanoscale to investigate film morphology, conductivity, and dopant distribution. Microscopic distributions of spiro-MeOTAD, LiTFSI, and TBP were determined using 2D Fourier transform infrared (FTIR) microscopy and electrostatic atomic force microscopy (EFM). Our findings reveal that the main role of TBP is to prevent phase segregation of LiTFSI and Spiro-MeOTAD, resulting in a homogeneous hole transport layer. These properties are critical for charge transport in the HTL bulk film as well as at the perovskite/HTL and HTL/electrode interfaces and for efficient solar cell performance.

Doping properties of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) hole transport layer are investigated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy under air exposure. XPS results reveal that 3 h exposure of Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI) doped spiro-MeOTAD to air results in the migration of LiTFSI from the bottom to the top across the spiro-MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of similar to 135 nm and a density of similar to 3.72 holes/mu m(2). In addition, cross-sectional scanning electron microscope images reveal that these pinholes form channels across the doped spiro-MeOTAD film. Optical microscopy and Fourier transform infrared microscopy images confirm the presence of large pinholes with diameters in the range of 1-20 mu m and a density of similar to 289 holes/mm(2) as well. The presence of pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air., Correction to article published here April 2022: https://doi.org/10.1021/cm504022q

Doping properties of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) hole transport layer are investigated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy under air exposure. XPS results reveal that 3 h exposure of Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI) doped spiro-MeOTAD to air results in the migration of LiTFSI from the bottom to the top across the spiro-MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of similar to 135 nm and a density of similar to 3.72 holes/mu m(2). In addition, cross-sectional scanning electron microscope images reveal that these pinholes form channels across the doped spiro-MeOTAD film. Optical microscopy and Fourier transform infrared microscopy images confirm the presence of large pinholes with diameters in the range of 1-20 mu m and a density of similar to 289 holes/mm(2) as well. The presence of pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air., Correction to article published here April 2022: https://doi.org/10.1021/cm504022q

Doping properties of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) hole transport layer are investigated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy under air exposure. XPS results reveal that 3 h exposure of Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI) doped spiro-MeOTAD to air results in the migration of LiTFSI from the bottom to the top across the spiro-MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of similar to 135 nm and a density of similar to 3.72 holes/mu m(2). In addition, cross-sectional scanning electron microscope images reveal that these pinholes form channels across the doped spiro-MeOTAD film. Optical microscopy and Fourier transform infrared microscopy images confirm the presence of large pinholes with diameters in the range of 1-20 mu m and a density of similar to 289 holes/mm(2) as well. The presence of pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air., Correction to article published here April 2022: https://doi.org/10.1021/cm504022q

Doping properties of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) hole transport layer are investigated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy under air exposure. XPS results reveal that 3 h exposure of Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI) doped spiro-MeOTAD to air results in the migration of LiTFSI from the bottom to the top across the spiro-MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of similar to 135 nm and a density of similar to 3.72 holes/mu m(2). In addition, cross-sectional scanning electron microscope images reveal that these pinholes form channels across the doped spiro-MeOTAD film. Optical microscopy and Fourier transform infrared microscopy images confirm the presence of large pinholes with diameters in the range of 1-20 mu m and a density of similar to 289 holes/mm(2) as well. The presence of pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air., Correction to article published here April 2022: https://doi.org/10.1021/cm504022q

Doping properties of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) hole transport layer are investigated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy under air exposure. XPS results reveal that 3 h exposure of Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI) doped spiro-MeOTAD to air results in the migration of LiTFSI from the bottom to the top across the spiro-MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of similar to 135 nm and a density of similar to 3.72 holes/mu m(2). In addition, cross-sectional scanning electron microscope images reveal that these pinholes form channels across the doped spiro-MeOTAD film. Optical microscopy and Fourier transform infrared microscopy images confirm the presence of large pinholes with diameters in the range of 1-20 mu m and a density of similar to 289 holes/mm(2) as well. The presence of pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air., Correction to article published here April 2022: https://doi.org/10.1021/cm504022q

We show that the interaction between magnetic microspheres, fabricated by coating glass microspheres with a layer of nickel, can be controlled by varying the frequency of the applied magnetic field. By floating two such microspheres on the meniscus of glycerin and applying an ac magnetic field, it is shown that the spheres achieve an equilibrium separation owing to the balance between the repulsive dipole-dipole interaction and the "attractive" force due to the weight of the particles. A monotonic decrease of the magnetorheological effect with frequency increasing is observed. Good agreement between theory and experiment is observed. © 2006 American Institute of Physics.

We show that the interaction between magnetic microspheres, fabricated by coating glass microspheres with a layer of nickel, can be controlled by varying the frequency of the applied magnetic field. By floating two such microspheres on the meniscus of glycerin and applying an ac magnetic field, it is shown that the spheres achieve an equilibrium separation owing to the balance between the repulsive dipole-dipole interaction and the "attractive" force due to the weight of the particles. A monotonic decrease of the magnetorheological effect with frequency increasing is observed. Good agreement between theory and experiment is observed. © 2006 American Institute of Physics.

We show that the interaction between magnetic microspheres, fabricated by coating glass microspheres with a layer of nickel, can be controlled by varying the frequency of the applied magnetic field. By floating two such microspheres on the meniscus of glycerin and applying an ac magnetic field, it is shown that the spheres achieve an equilibrium separation owing to the balance between the repulsive dipole-dipole interaction and the "attractive" force due to the weight of the particles. A monotonic decrease of the magnetorheological effect with frequency increasing is observed. Good agreement between theory and experiment is observed. © 2006 American Institute of Physics.

The electromagnetic band gap properties through a two-dimensional plate constructed with centimetre-sized spherical foam balls surrounded by aluminium rings are studied in the microwave range of 0.7-18 GHz. We observe from both experiments and finite-difference time-domain simulations that multiple band gaps appear and correspond to different resonant modes localized in the rings. The principal stop band, the lowest mode, appears at low frequencies and is robust to any lattice structure, even disorder. However, the remaining auxiliary stop bands, caused by high-order resonances, are usually located at higher frequencies and are more sensitive to the configuration of the structure formed by the ring resonators. We note that the stop bands would be broadened and strongly attenuated if more layers of the same plates were packed together. In addition, even or odd modes can be excited under different orientations of the rings with respect to the polarization of the illuminating radiation.

The electromagnetic band gap properties through a two-dimensional plate constructed with centimetre-sized spherical foam balls surrounded by aluminium rings are studied in the microwave range of 0.7-18 GHz. We observe from both experiments and finite-difference time-domain simulations that multiple band gaps appear and correspond to different resonant modes localized in the rings. The principal stop band, the lowest mode, appears at low frequencies and is robust to any lattice structure, even disorder. However, the remaining auxiliary stop bands, caused by high-order resonances, are usually located at higher frequencies and are more sensitive to the configuration of the structure formed by the ring resonators. We note that the stop bands would be broadened and strongly attenuated if more layers of the same plates were packed together. In addition, even or odd modes can be excited under different orientations of the rings with respect to the polarization of the illuminating radiation.

The electromagnetic band gap properties through a two-dimensional plate constructed with centimetre-sized spherical foam balls surrounded by aluminium rings are studied in the microwave range of 0.7-18 GHz. We observe from both experiments and finite-difference time-domain simulations that multiple band gaps appear and correspond to different resonant modes localized in the rings. The principal stop band, the lowest mode, appears at low frequencies and is robust to any lattice structure, even disorder. However, the remaining auxiliary stop bands, caused by high-order resonances, are usually located at higher frequencies and are more sensitive to the configuration of the structure formed by the ring resonators. We note that the stop bands would be broadened and strongly attenuated if more layers of the same plates were packed together. In addition, even or odd modes can be excited under different orientations of the rings with respect to the polarization of the illuminating radiation.

Magnetic anisotropy of particulate magnetic materials is investigated with respect to the particle shape and compacting geometry. Uniform sized micro-rods and microspheres, coated with nickel, were used. Our results reveal that uniformly aligned rods show the largest magnetic anisotropy, while the field-oriented suspension of coated micro-rods shows somewhat weaker magnetic anisotropy, which is in excellent quantitative agreement with the theoretical calculation. The magnetic anisotropy drops significantly when the micro-rods were replaced by microspheres, even when they were aligned in chain-like formations.

Magnetic anisotropy of particulate magnetic materials is investigated with respect to the particle shape and compacting geometry. Uniform sized micro-rods and microspheres, coated with nickel, were used. Our results reveal that uniformly aligned rods show the largest magnetic anisotropy, while the field-oriented suspension of coated micro-rods shows somewhat weaker magnetic anisotropy, which is in excellent quantitative agreement with the theoretical calculation. The magnetic anisotropy drops significantly when the micro-rods were replaced by microspheres, even when they were aligned in chain-like formations.

Electrorheological (ER) and Magnetorheological (MR) fluids are a class of colloidal suspensions, typically consisting of polarizable solid particles dispersed in a relative non-polarizable liquid. Drastic enhancement in viscosity and shear stress results from the field-induced structures, when ER (or MR) fluids are subject to an external electric (or magnetic) field. This property offers ER and MR fluids many potential applications in various areas. In the thesis I report the experimental results on magnetic response of particulate materials with different compacting process and particle shapes. It is found the magnetic anisotropy largely depends on the compacting process and component particle shape: uniformly aligned rods show the largest magnetic anisotropy; while the field-oriented suspension of coated microrods obtained through the MR effect shows somewhat weaker magnetic anisotropy. The magnetic anisotropy drops significantly when the microrods were replaced by microspheres, even when they were aligned in chain-like formations. A simple model is employed to interpret the role of shape anisotropy of the constituting particles. In the thesis experimental results are also reported for rheological property of ER fluids with different particle shapes and the evolution of a single magnetic micro-rod in an external dc field.

Electrorheological (ER) fluids with microspheres and micro-rods as the component solid phase were examined experimentally to investigate the influences of particle geometry on ER performance. It is observed that for both dried and water-activated ER fluids, the shape of the constituent particles plays a significant role, i.e. for particles with same diameters, the microsphere-based ER fluids show better ER performance than micro-rod-based ER fluids. We found from experiments that the aspect ratio of particles is an important parameter for the ER activities. The tendency is that the ER effect decreases with the increase of the aspect ratio, while this phenomenon becomes much weaker in the case when dried particles were substituted for the ones with moisture.

Electrorheological (ER) fluids with microspheres and micro-rods as the component solid phase were examined experimentally to investigate the influences of particle geometry on ER performance. It is observed that for both dried and water-activated ER fluids, the shape of the constituent particles plays a significant role, i.e. for particles with same diameters, the microsphere-based ER fluids show better ER performance than micro-rod-based ER fluids. We found from experiments that the aspect ratio of particles is an important parameter for the ER activities. The tendency is that the ER effect decreases with the increase of the aspect ratio, while this phenomenon becomes much weaker in the case when dried particles were substituted for the ones with moisture.

Electrorheological (ER) and Magnetorheological (MR) fluids are a class of colloidal suspensions, typically consisting of polarizable solid particles dispersed in a relative non-polarizable liquid. Drastic enhancement in viscosity and shear stress results from the field-induced structures, when ER (or MR) fluids are subject to an external electric (or magnetic) field. This property offers ER and MR fluids many potential applications in various areas. In the thesis I report the experimental results on magnetic response of particulate materials with different compacting process and particle shapes. It is found the magnetic anisotropy largely depends on the compacting process and component particle shape: uniformly aligned rods show the largest magnetic anisotropy; while the field-oriented suspension of coated microrods obtained through the MR effect shows somewhat weaker magnetic anisotropy. The magnetic anisotropy drops significantly when the microrods were replaced by microspheres, even when they were aligned in chain-like formations. A simple model is employed to interpret the role of shape anisotropy of the constituting particles. In the thesis experimental results are also reported for rheological property of ER fluids with different particle shapes and the evolution of a single magnetic micro-rod in an external dc field.

Electrorheological (ER) and Magnetorheological (MR) fluids are a class of colloidal suspensions, typically consisting of polarizable solid particles dispersed in a relative non-polarizable liquid. Drastic enhancement in viscosity and shear stress results from the field-induced structures, when ER (or MR) fluids are subject to an external electric (or magnetic) field. This property offers ER and MR fluids many potential applications in various areas. In the thesis I report the experimental results on magnetic response of particulate materials with different compacting process and particle shapes. It is found the magnetic anisotropy largely depends on the compacting process and component particle shape: uniformly aligned rods show the largest magnetic anisotropy; while the field-oriented suspension of coated microrods obtained through the MR effect shows somewhat weaker magnetic anisotropy. The magnetic anisotropy drops significantly when the microrods were replaced by microspheres, even when they were aligned in chain-like formations. A simple model is employed to interpret the role of shape anisotropy of the constituting particles. In the thesis experimental results are also reported for rheological property of ER fluids with different particle shapes and the evolution of a single magnetic micro-rod in an external dc field.