Halide Perovskite Films Research Articles

Achieving high-performance in situ fabricated FAPbBr3 and electroluminescence.

Currently, metal halide perovskite films still encounter the issues of inferior film quality and interfacial electrical properties when they were constructed electroluminescence devices. Herein, efficient and pinhole-free perovskite emissive film was obtained on the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer modified by an ultrathin LiF layer. Owing to the synergetic effect of the LiF interlayer, including better regulation of the perovskite film and a more balanced charge injection capability, an efficient green light-emitting diode based on the perovskite film was achieved with a maximum current efficiency of 25.6 cd/A, which is 58% higher than that of the control device with a plasma-treated PEDOT:PSS layer. Our results not only provide a facile strategy for acquiring efficient perovskite films but also circumvent the expensive and time-consuming plasma treatment process commonly used to improve the wetting properties of the underlying films.

Mixed Halide Perovskite Films by Vapor Anion Exchange for Spectrally Stable Blue Stimulated Emission.

Solution-processed all-inorganic CsPbX3 perovskites exhibit outstanding optoelectronic properties and are being considered as a promising optical gain medium, with impressive performance in the green and red region. However, the development of CsPbX3 for blue emission is still lagging far behind, owing to difficulties in thin films synthesis and spectral instability subject to light irradiation. Here, a facile vapor anion exchange (VAE) method that enables preparation of blue-emitting perovskite films with both excellent surface morphology and good photo-stability is reported. The mixed-Br/Cl quasi-2D perovskite films show spectrally stable pure blue emission (471nm) under continuous-wave laser irradiation with power density as high as 81Wcm-2 . Furthermore, optically pumped blue amplified spontaneous emission (ASE) is realized based on the mixed-Br/Cl perovskite films. By changing the duration of VAE treatment, the ASE peak can be tuned from 537nm down to 475nm. This work not only presents a facile method to prepare high quality mixed halide Cs-based perovskite films, but also pave the way for further exploration of stable blue perovskite lasing.

Crystal-Size-Induced Band Gap Tuning in Perovskite Films.

A comprehensive picture explaining the effect of the crystal size in metal halide perovskite films on their opto‐electronic characteristics is currently lacking. We report that perovskite nanocrystallites exhibit a wider band gap due to concurrent quantum confinement and size dependent structural effects, with the latter being remarkably distinct and attributed to the perturbation from the surface of the nanocrystallites affecting the structure of their core. This phenomenon might assist in the photo‐induced charge separation within the perovskite in devices employing mesoporous layers as they restrict the size of nanocrystallites present in them. We demonstrate that the crystal size effect is widely applicable as it is ubiquitous in different compositions and deposition methods employed in the fabrication of state‐of‐the‐art perovskite solar cells. This effect is a convenient and effective way to tune the band gap of perovskites.

Crystal‐Size‐Induced Band Gap Tuning in Perovskite Films

AbstractA comprehensive picture explaining the effect of the crystal size in metal halide perovskite films on their opto‐electronic characteristics is currently lacking. We report that perovskite nanocrystallites exhibit a wider band gap due to concurrent quantum confinement and size dependent structural effects, with the latter being remarkably distinct and attributed to the perturbation from the surface of the nanocrystallites affecting the structure of their core. This phenomenon might assist in the photo‐induced charge separation within the perovskite in devices employing mesoporous layers as they restrict the size of nanocrystallites present in them. We demonstrate that the crystal size effect is widely applicable as it is ubiquitous in different compositions and deposition methods employed in the fabrication of state‐of‐the‐art perovskite solar cells. This effect is a convenient and effective way to tune the band gap of perovskites.

Color‐Stable Deep‐Blue Perovskite Light‐Emitting Diodes Based on Organotrichlorosilane Post‐Treatment

AbstractRecent studies of sky‐blue perovskite light‐emitting diodes (PeLEDs) have extensively promoted optimal device design to achieve an external quantum efficiency (EQE) above 12%. However, the development of thin‐film deep‐blue PeLEDs lags dramatically behind, especially with regards to meeting the latest Rec. 2020 standard. A trichloro(3,3,3‐trifluoropropyl) silane post‐treatment that drives the emission of perovskite into the deep‐blue region, ranging from 440 to 460 nm, which meets the Rec. 2020 standard, is proposed. The chlorine ions released from the organotrichlorosilane molecules during their polycondensation reaction provide an addition halide source to fine tune the composition of the mixed halide perovskite films, leading to increase of bandgap and deep‐blue emission. In addition, hydrogen bonds between the hydroxy groups of silane molecules and halide anions in perovskite can suppress ion migration for improving emission stability. As a result, an optimal PeLED is developed with deep‐blue emission at 458 nm and excellent color stability, which yields an EQE and luminance of 1.1% and 130 cd m−2, respectively, representing a state‐of‐the‐art result for thin‐film PeLEDs in this emission region. This work paves the way to achieve high‐performance deep‐blue PeLEDs with stable emissions to meet the demand for potential applications such as full‐color display.

Halide Perovskites: Tailoring of Ligand‐Off Nanoparticles Inks for Thin p‐Type Oxide Overlayers Formation with Maintaining Intact Halide Perovskite (Adv. Funct. Mater. 31/2021)

In article number 2100863, Hyun Suk Jung, Jun Hong Noh, and co-workers present a highly dispersive ligand-off oxide colloidal solution based on a non-polar solvent, which enables the formation of an oxide overlayer on the halide perovskite film. The p-type oxide overlayer offers the possibility of overcoming the thermal instability of conventional organic hole transport materials in perovskite solar cells.

Photoluminescence and Photoconductivity of Lead Halide Perovskite Films Modified with Mixed Cellulose Esters

Photoluminescence and Photoconductivity of Lead Halide Perovskite Films Modified with Mixed Cellulose Esters

Tin Halide Perovskite Polycrystalline Films Formed with Nonhazardous 2-Propanol Solvent

The use of safe and environmentally friendly materials is an important issue for industrial mass production processes. In this study, we demonstrate the deposition of tin halide perovskite films using environmentally benign 2-propanol as a solvent. Sequential deposition of 2-propanol solutions containing SnBr2 and CH3NH3I led to the formation of CH3NH3SnBr3−xIx, which exhibited good crystallinity and visible-light absorption.

Alleviating halide perovskite surface defects

Alleviating halide perovskite surface defects

Reducing Defects in Organic-Lead Halide Perovskite Film by Delayed Thermal Annealing Combined with KI/I2 for Efficient Perovskite Solar Cells.

This study improved quality of CH3NH3PbI3 (MAPbI3) perovskite films by delaying thermal annealing in the spin coating process and introducing KI and I2 to prepare MAPbI3 films that were low in defects for high-efficiency perovskite solar cells. The influences of delayed thermal annealing time after coating the MAPbI3 perovskite layer on the crystallized perovskite, the morphology control of MAPbI3 films, and the photoelectric conversion efficiency of solar cells were investigated. The optimal delayed thermal annealing time was found to be 60 min at room temperature. The effect of KI/I2 additives on the growth of MAPbI3 films and the corresponding optimal delayed thermal annealing time were further investigated. The addition of KI/I2 can improve perovskite crystallinity, and the conductivity and carrier mobility of MAPbI3 films. Under optimized conditions, the photoelectric conversion efficiency of MAPbI3 perovskite solar cells can reach 19.36% under standard AM1.5G solar illumination of 100 mW/cm2.

Observation of Net Stimulated Emission in CsPbBr3 Thin Films Prepared by Pulsed Laser Deposition

AbstractInorganic halid perovskites, such as CsPbX3 (X = Br, Cl, or I), have attracted a great deal of attention for next generation on‐chip optoelectronic devices for which high quality thin films are essential. So far, most utilized fabrication techniques for halide perovskite films have been based on solution synthesized methods, taking advantage of cost effectiveness and flexibility. Conventional solution processed thin films suffer from pinholes, poor morphology and/or low mole fraction when embedded in glass or polymer matrixes, unavoidably limiting their application‐level. Herein, we report the fabrication of high crystalline and coverage CsPbBr3 thin films by using a single‐step physical technique – pulsed laser deposition (PLD). As‐fabricated thin films demonstrate stimulated emission with low pump threshold at room temperature. Systematic investigations suggest that the mechanism of stimulated emission in our CsPbBr3 thin films originates from exciton−exciton inelastic scattering. These results not only shed light on high performance all‐inorganic CsPbBr3 perovskite films for various optoelectronic devices, but also enrich the comprehension of stimulated emission mechanism of all‐inorganic CsPbBr3 perovskite materials.

Asymmetric Bipolar Resistive Switching of Halide Perovskite Film in Contact with TiO2 Layer.

Halide perovskite materials such as methylammonium lead iodide (CH3NH3PbI3) have attracted considerable interest for the resistive random-access memory applications, which exploit a dramatic change in the resistance by an external electric bias. In many semiconductor films, the drift, accumulation, and chain formation of defects explain the change in the resistance by an external bias. This study demonstrates that the interface of CH3NH3PbI3 with TiO2 has a significant impact on the formation and rupture of defect chains and causes the asymmetric bipolar resistive switching in the Au/CH3NH3PbI3/TiO2/FTO device (FTO = fluorine-doped tin oxide). When a negative bias is applied to the Au electrode, iodine interstitials with the lowest migration activation energy move toward TiO2 in the CH3NH3PbI3 layer and pile up at the CH3NH3PbI3-TiO2 interface. Under the same condition, oxygen vacancies in the TiO2 layer also travel to the CH3NH3PbI3-TiO2 interface and strongly attract iodine interstitials. As a result, a Schottky barrier appears at the CH3NH3PbI3-TiO2 interface, and the resistance of Au/CH3NH3PbI3/TiO2/FTO becomes much larger than that of Au/CH3NH3PbI3/FTO in the high resistance state. The frequency dependence of the capacitance confirms the asymmetric appearance of a large space charge polarization at the CH3NH3PbI3-TiO2 interface, which causes the unique bipolar resistive switching behavior with the on/off ratio (103) and retention time (>104 seconds) at -0.85 V in Au/CH3NH3PbI3/TiO2/FTO film.

Halide Ion Mobility of Mixed Halide Perovskites in Light and Dark

The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the perovskite film during the operation of solar cells. Of particular interest is the halide ion mobility in metal halide perovskites, which plays an important role in determining the performance of perovskite solar cells. Photoinduced phase segregation seen in mixed halide perovskite films under steady state irradiation offers a convenient way to visualize halide ion segregation. Interestingly, upon storage in dark, the process is reversed and the original mixed halide composition gets restored. Whereas entropy of mixing explain the thermally activated mixing of halide ions to yield mixed halide perovskite, the opposite trend observed during photoirradiation remains an intriguing phenomenon. The threshold energy of incident light to observe halide segregation increases with increasing temperature. The diffusion of these halide species, which is tracked through changes in the absorption spectra at different temperatures, offers a direct measurement of thermally activated halide diffusion in perovskite films. The thermally activated halide exchange shows the challenges of stabilizing metal halide perovskites with varying bandgap in tandem solar cells.

Crystallization Behavior and Luminescence of Inkjet Printing CH3NH3PbBr3

AbstractLead halide perovskites display remarkable optoelectronic properties, like large absorption coefficients, high photoluminescence quantum efficiencies, and long lifetime and diffusion length of photocarriers. This system is easily fabricated using solution processes and inkjet printing is an effective way to prepare halide perovskite films and complex patterns. In this work, the crystallization behaviors of inkjet printing CH3NH3PbBr3 crystals are systematically investigated with varied I− doping, printing times, and solvents. Using N,N‐dimethylformamide (DMF) solvent, CH3NH3PbBr3−xIx (x = 0, 0.14, 0.29, 0.45, 0.59) are printed on the glass and the crystalline grains are developed from the (001) oriented tetragonal in side length of 10–50 µm to dendrite with increasing I− concentrations. The crystalline grains are kept tetragonal, while the average crystal size changes from 22 to 89 µm by increasing the number of printing from 10 to 1000 times. DMF and dimethyl sulfoxide (DMSO) are used as solvents for printing CH3NH3PbBr3, and more regular grains are obtained from DMF solvent. Several patterns are printed on glass and papers, and fluorescent two‐dimensional (2D) patterns are observed under the 480 nm excitation. The as‐printed patterns show excellent homogeneity and high reproducibility, indicating that the inkjet printing shows broad application prospects in flexible electronics.

Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells.

Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.

Graphene quantum dot-embedded perovskite photodetectors with fast response and enhanced sensitivity through bulk defect passivation

Graphene quantum dots (GQDs) synthesized by single phase solution chemistry are embedded in metal halide perovskite (MHP) films because they can act as passivating defects that are present throughout the MHP film, not only at the interface but also throughout the bulk of the film. The perovskite photodetectors composed of indium-doped tin oxide/SnO2/MHP with embedded GQDs/poly triarylamine/Au have the reduced dark current, increased detectivity, and fasted response speed due to lower trap density throughout the bulk MHP layer.

Characterization of Thin MAPb(I1–xBrx)3 Alloy Halide Perovskite Films Prepared by Sequential Physical Vapor Deposition

Lead iodide (PbI2)-rich methylammonium lead bromide-iodide (MAPb(I1–xBrx)3) thin-films were prepared by sequential physical vapor deposition of methylammonium lead tri-bromide (MAPbBr3) on methylammonium lead tri-iodide (MAPbI3) bottom layer. The structural, optical, morphological, and electrical properties of the thin-films were studied as the thickness of methylammonium bromide (MABr) was increased from 300 to 500 nm. X-ray diffractograms confirmed transformation of tetragonal MAPbI3(x is 0.0) to the cubic-like structure of MAPbBr3 (x is 1.0) as MAPb(I1–xBrx)3 (x = 0.89–0.95) and PbI2 were formed. The bromine mole ratio x decreased as MABr thickness increased. UV-Vis absorption spectra showed that the bandgap of the thin alloy film decreased from 2.21 to 2.14 eV as x decreased. Scanning electron micrographs depicted densely packed grains that entirely covered the substrate and contained very few pinholes. The average grain size increased from 150 to 320 nm as x decreased. Electrical properties showed high charge carrier mobility that increased linearly with MABr thickness. FTO/MAPb(I1–xBrx)3/Au devices using fluorine-doped tin oxide (FTO) as substrate and gold (Au) as contacts were fabricated and current-voltage characteristics were determined. Space-charge-limited current theory was applied to charge carrier mobility and trap density of MAPb(I1–xBrx)3 thin-films. The charge carrier mobility increased as x decreased. The power conversion efficiency (PCE) of FTO/MAPbBr3/Au, FTO/MAPb(I0.11Br0.89)3/Au and FTO/MAPbI3/Au solar cells were 0.56, 0.62, and 1.15%. Devices including titanium dioxide compact layer (c-TiO2) and titanium dioxide mesoporous (m-TiO2) layer as electron transport layers were also fabricated for the application of Mott-Shottky (M-S) theory. Analyses of dark current-voltage and capacitance-voltage curves of FTO/c-TiO2/m-TiO2/MAPb(I0.11Br0.89)3 solar cells revealed a sizeable built-in voltage (Vbi) of 1.6 V and an accumulation of charge at interfaces for voltages greater than 0.2 V, respectively. Similar analyses for FTO/TiO2/MAPbI3/Au showed a small Vbi of 0.7 V and no charge carrier at interfaces. The work paves a way for reproducible growth of MAPb(I1–xBrx)3 for solar cells and sheds more light on the degree of ion migration in mixed halide and pure halide perovskites.

Laser fabricated carbon quantum dots in anti-solvent for highly efficient carbon-based perovskite solar cells

Additive passivation can be an effective strategy to regulate and control the properties of organic–inorganic halide perovskite film. In this article, carbon quantum dots (CQDs), fabricated by non-focused laser irradiation of carbon nanomaterial diluted in anti-solvent ethyl acetate, denoted as EACQDs, were adopted for perovskite film defect passivation and modification of carbon-based CH3NH3PbI3 perovskite solar cells (PSCs). The size of EACQDs can be tuned by manipulating the laser fluence. The morphology of perovskite film was uncovered through scanning electron microscopy and atomic force microscopy. After embedding of EACQDs, the defect in perovskite crystal was reduced, resulting in the decreased carrier recombination and accelerated carrier transportation, which were demonstrated by electrochemical impedance spectroscopy, photoluminescence and time-resolved photoluminescence. As a consequence, with the optimization of 0.01 mg/mL EACQDs (1064 nm–300 mJ·pulse−1·cm−2–10 min), the power conversion efficiency (PCE) of carbon-based PSCs achieved a maximum value of 16.43%, which improved 23.81% when compared with the pristine PSCs of 13.27%. Furthermore, the EACQDs optimized PSCs also exhibited an excellent stability and still retained 86% of its initial PCE after 50-day storage at the room atmosphere with a humidity of 30–50%.

Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Summary Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H…I and O…H) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs.

Characterization on Highly Efficient Perovskite Solar Cells Made from One‐Step and Two‐Step Solution Processes

One‐step and two‐step methods are regarded as the main solution processes for preparing organic metal halide perovskite (PVSK) films. Both of them are also reported to produce high‐quality PVSK films and high‐performance devices. Herein, two highly efficient perovskite solar cells (PSCs) made by one‐step (19.91%) and two‐step (20.63%) methods are analyzed in detail. Particularly, the nucleation mechanism and structural characteristics of the PVSK films made from different methods are systematically discussed. Characterizations from field‐emission scanning electron microscopy (FE‐SEM), atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) show that the ion distribution in the PVSK film made by the one‐step method is less uniform horizontally, while the PVSK film made by the two‐step method is less uniform vertically. Both films contain a considerable amount of unreacted species, including PbI2, cations, and halides, which are relics from their parental fabricating process. Despite these significant differences, both films are capable to deliver nearly 20% conversion efficiency. This study depicts a clear picture of the PVSK film properties made by one‐step and two‐step methods and provides plentiful information for further improvement in the future.