Lead Iodide Films Research Articles

High‐Performance Directly Patterned Nanograting Perovskite Photodetector with Interdigitated Electrodes

AbstractSolution‐processed organic–inorganic metal halide perovskites have recently attracted tremendous attention in the photodetector community due to their excellent optoelectronic properties and facile fabrication. The main challenge in perovskite photodetectors (PSPDs) is to achieve high responsivity and fast speed simultaneously. In this work, this challenge is overcome by employing a directly patterned nanograting methylammonium lead iodide (MAPbI3) film in metal‐semiconductor‐metal (MSM) PSPD on interdigitated indium tin oxide (ITO) electrodes. Because of the improved perovskite morphology after directly patterning by nanoimprint lithography, as well as the enhanced electric field intensity by the perovskite nanograting and interdigitated electrodes, the PSPDs have responsivity of 441 A W−1, detectivity of 8.32 × 1012 Jones, response time of 10.7 µs, all of which are among the best performances in MSM PSPDs. Moreover, the PSPDs maintain excellent photocurrent performance after 20 days of air exposure. The approach opens a path to manufacturing‐friendly, high‐performance, and reliable PSPDs and paves the way toward perovskite‐based optoelectronic circuits.

Strong Induced Circular Dichroism in a Hybrid Lead‐Halide Semiconductor Using Chiral Amino Acids for Crystallite Surface Functionalization

AbstractChirality is a desired property in functional semiconductors for optoelectronic, catalytic, and spintronic applications. Here, introducing enantiomerically‐pure 3‐aminobutyric acid (3‐ABA) into thin films of the 1D semiconductor dimethylammonium lead iodide (DMAPbI3) is found to result in strong circular dichroism (CD) in the optical absorption. X‐ray diffraction and grazing incidence small angle X‐ray scattering (GISAXS) are applied to gain molecular‐scale insights into the chirality transfer mechanism, which is attributed to a chiral surface modification of DMAPbI3 crystallites. This study demonstrates that the CD signal strength can be controlled by the amino‐acid content relative to the crystallite surface area. The CD intensity is tuned by the composition of the precursor solution and the spin‐coating time, thereby achieving anisotropy factors (gabs) as high as 1.75 × 10–2. Grazing incidence wide angle scattering reveals strong preferential ordering that can be suppressed via tailored synthesis conditions. Different contributions to the chiroptical properties are resolved by a detailed analysis of the CD signal utilizing an approach based on the Mueller matrix model. This report of a novel class of chiral hybrid semiconductors with precise control over their optical activity presents a promising approach for the design of circularly polarized light detectors and emitters.

A facile healing of two-step deposited MAPbI3 perovskite on TiO2 nanorod through dynamic methylamine treatment

The highly crystalline TiO2 nanorod as an underlying electron transport layer in perovskite solar cell has gained much attention due to its directional charge transport, better carrier extraction and enhanced light scattering. However, the morphology of perovskite film is critical to the photovoltaic performance, which makes it challenging to realize a high-quality perovskite film readily on an uneven nanorod-shaped underlying layer. In this regard, we develop a facile strategy to modify the two-step deposited methylammonium lead iodide (MAPbI3) film, deposited on hydrothermally synthesized TiO2 nanorod, by rapidly introducing methylamine vapor during the rotation, which enables another control to film planarization via a shear force induced by air flow, to investigate the influence of methylamine (MA) healing on such perovskite solar devices. The morphology and porosity of PbI2 on nanorod-based TiO2 are systemically studied by controlling the mixed solvent effect of dimethylformamide and dimethyl sulfoxide to satisfy a preferred PbI2 for the two-step deposited perovskite using TiO2 nanorod. The performance of such two-step deposited perovskite film after the developed MA treatment can be further improved, which suggests the occurrence of liquified MAPbI3 during the rotation to provide the positive healing effect on the film smoothness, pore infiltration, and unreacted PbI2 residues. The perovskite film after a long MA treatment is found to suffer from the formation of more defects or disorders than the defect tolerance in MAPbI3, which outweighs the healing advantages. Such a trend highlights the importance of delicately optimizing the developed MA treatment process, which can possibly achieve a high-performance perovskite solar cell of using two-step deposited MAPbI3 on TiO2 NR.

Carrier lifetime measurement of perovskite films by differential microwave photoconductivity decay

We measure the minority carrier lifetime of perovskite films by differential microwave photoconductivity decay (μ-PCD). Clear decay curves can be detected from bare and laminated methylammonium lead iodide (MAPbI3) films by the differential μ-PCD. The degradation of the bare and laminated MAPbI3 films under air exposure at room temperature is clearly observed as the continuous change of the decay curves. The differential μ-PCD can thus be a quick and non-destructive method for the characterization of the electrical quality of perovskite films and modules.

Ambient air processed highly oriented perovskite solar cells with efficiency exceeding 23% via amorphous intermediate

The preparation of perovskite solar cells (PSCs) in ambient air can effectively reduce the production cost and is more conducive to commercial production. However, ambient air fabrication of perovskite film always causes rough surface and nonuniform coverage which increase non-radiative recombination in PSCs. In this work, we adopted an amorphous intermediate strategy in lead iodide (PbI2) film to prepare the oriented perovskite film under ambient air condition. This amorphous intermediate formed by the interaction force between cesium iodide (CsI) and PbI2, which promotes perovskite phase inversion in perovskite precursor film and finally the perovskite grains grow perpendicular to the substrate in the ambient air. As a result, this ambient air processed-PSCs with the perovskite film passivated with butylammonium iodide (BAI) obtained a power conversion efficiency (PCE) of 23.46%, which is equivalent to that of two-step preparation in glovebox. Furthermore, the PSC maintains 85% of its initial efficiency under more than 1000 h of continuous illumination and processes a recoverable PCE at temperatures below 65 °C under photothermal cycling stress.

Metal Halide Perovskites Demonstrate Radiation Hardness and Defect Healing in Vacuum.

Herein, we subject formamidinium lead iodide films to oxygen-containing gases (flowing O2 or free diffusion of lab atmosphere), inert gases (flowing He, Ar, or N2), and vacuum. Our films are irradiated by Cu Kα X-rays and held at 75 °C while X-ray diffraction is recorded. Under all gas conditions, we observe a reproducible 1.1 ± 0.5 Å3 perovskite lattice contraction from an initial unit cell volume of 256.5 ± 0.8 Å3 concurrent with continuous perovskite loss and lead iodide growth. Oxygen-containing gases increase the reaction rates without materially altering perovskite structural changes. Under the same temperature and irradiation conditions in vacuo, a self-healing reaction is observed, exhibited by a reproducible (0.9 ± 0.3 Å3) lattice expansion and stabilization of the perovskite. Interactions between the perovskite, defects, and minority phases are simulated by generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) density functional theory. Lattice contraction indicates an increase in the concentration of Schottky defects─pairs of formamidinium and iodine vacancies. Under irradiation in every atmospheric condition, a solid solution of Schottky defects with a concentration of several percent diffuses and precipitates forming lead iodide and consuming the defects. In the presence of ionized gases, this framework is modified to include the continual loss of formamidinium and iodine ions from the perovskite forming Schottky defects.

A Modified Sequential Deposition Route for High-Performance Carbon-Based Perovskite Solar Cells under Atmosphere Condition.

Carbon-based hole transport material (HTM)-free perovskite solar cells have exhibited a promising commercialization prospect, attributed to their outstanding stability and low manufacturing cost. However, the serious charge recombination at the interface of the carbon counter electrode and titanium dioxide (TiO2) suppresses the improvement in the carbon-based perovskite solar cells’ performance. Here, we propose a modified sequential deposition process in air, which introduces a mixed solvent to improve the morphology of lead iodide (PbI2) film. Combined with ethanol treatment, the preferred crystallization orientation of the PbI2 film is generated. This new deposition strategy can prepare a thick and compact methylammonium lead halide (MAPbI3) film under high-humidity conditions, which acts as a natural active layer that separates the carbon counter electrode and TiO2. Meanwhile, the modified sequential deposition method provides a simple way to facilitate the conversion of the ultrathick PbI2 capping layer to MAPbI3, as the light absorption layer. By adjusting the thickness of the MAPbI3 capping layer, we achieved a power conversation efficiency (PCE) of 12.5% for the carbon-based perovskite solar cells.

Carbon monoxide induced self-doping in methylammonium lead iodide films and associated long-term degradation effects

Carbon monoxide interacts strongly with the MAPbI3film surface and can displace the adsorbed O2leading to loss of the organic moiety, accompanied by lowering of the work function and softening of the perovskite film due to formation of PbI2.

Efficient approach for optical and morphological characterization of hybrid perovskite films based on reflectance and transmittance measurements

Organo-inorganic perovskites (OIPs) have been intensively studied due to their potential application in low-cost, high-efficiency energy conversion in solar cells. Despite the great improvement in the quality of OIP films, wide dispersion in the same batch of perovskite-based devices remains an obstacle to obtaining highly reproducible results. For that reason, new and efficient strategies for testing deposition results is essential. Here we present a simple and efficient procedure for characterizing optical and morphological properties based on simultaneous reflectance and transmittance measurements under normal incidence over a methylammonium lead iodide film. The proposed method provides qualitative and quantitative morphological information associated with the film roughness as well as information about the position of the optical gap and possible contributions to optical dispersion in the structure that can be used as a simple diagnostic tool to optimize film deposition. Results are compared and validated with electronic and atomic force microscopy, as well as first-principles calculations.

Effect of Pristine Graphene on Methylammonium Lead Iodide Films and Implications on Solar Cell Performance

The relatively low stability of solar cells based on hybrid halide perovskites is the main issue to be solved for the implementation in real life of these extraordinary materials. Degradation is accelerated by temperature, moisture, oxygen, and light and mediated by halide easy hopping. The approach here is to incorporate pristine graphene, which is hydrophobic and impermeable to gases and likely limits ionic diffusion while maintaining adequate electronic conductivity. Low concentrations of few-layer graphene platelets (up to 24 × 10–3 wt %) were incorporated to MAPbI3 films for a detailed structural, optical, and transport study whose results are then used to fabricate solar cells with graphene-doped active layers. The lowest graphene content delays the degradation of films with time and light irradiation and leads to enhanced photovoltaic performance and stability of the solar cells, with relative improvement over devices without graphene of 15% in the power conversion efficiency, PCE. A higher graphene content further stabilizes the perovskite films but is detrimental for in-operation devices. A trade-off between the possible sealing effect of the perovskite grains by graphene, that limits ionic diffusion, and the reduction of the crystalline domain size that reduces electronic transport, and, especially, the detected increase of film porosity, that facilitates the access to atmospheric gases, is proposed to be at the origin of the observed trends. This work demonstrated how the synergy between these materials can help to develop cost-effective routes to overcome the stability barrier of metal halide perovskites, introducing active layer design strategies that allow commercialization to take off.

Surface electrical properties modulation by multimode polarizations inside hybrid perovskite films investigated through contact electrification effect

Surface electrical properties is of great significance for developing high-performance organic-inorganic hybrid perovskite based electronic devices. The photovoltage-induced ions transport and redistribution at the surface region have been well studied, but their contributions to the surface electrical properties are still lack of experimental evidences. In this article, a self-powered polymer-based contact electrification probe (PCE-probe) is proposed to investigate the photovoltage- or applied bias-induced ion transport polarization (PI) and ferroelectric polarization (PF) inside the methylammonium lead iodide films (MAPI). Results show that both the PI- and PF-induced ion transport and redistribution create a similar local ion-doping region near the surface. Positive or negative ion-doping produces a n-type or p-type layer, which enhances the interficial junction inside MAPI-based solar cells and benefits the seperation and transfer of photogenerated carriers or excitons. The PI and PF effects can be added up or subtracted from each other depending on the polarization configuration. A qualitative relationship between the PCE-probe output and CE-effect, PI- and PF-induced transferred surface charges is investigated. These results can help to understand the polarization nature inside and the high power conversion efficiency of MAPI-based photovoltaic devices, and provide a feasible analysis method of PCE-probe for detecting surface electrical properties.

Anchoring CsPbI3 crystalline for stable and efficient perovskite solar cells with perfluorocarbon-assisted shield

The inorganic perovskite solar cells (PSCs) are now playing an increasingly essential role in the realm of clean energy. However, the application of inorganic perovskite solar cells is usually limited as the spontaneous phase transform. Here, we applied the 1H, 1H-Perfluorooctylammonium iodide (PFOAI) upon the surface of the cesium lead iodide (CsPbI3) films by taking advantage of its excellent defect passivation ability and hydrophobicity. Decreased recombination upon the surface and postponed degradation of the device were achieved for the observation of lower trap density and longer carrier lifetime and better stability based on the PFOAI-CsPbI3 films. The champion PFOAI-CsPbI3 based device showed an increased power conversion efficiency (PCE) of 18.24% with the excellent fill factor (FF) of 81.77% and Jsc of 20.51 mA cm−2 and remain ∼85% of its initial PCE for 1000 h (25 °C) in the N2 glovebox. This work adds slightly weight on the balance of balancing the stability and high efficiency of the inorganic PSCs.

Robust, High-Performing Maize-Perovskite-Based Solar Cells with Improved Stability.

Herein, we focus on improving the long-term chemical and thermomechanical stability of perovskite solar cells (PSCs), two major challenges currently limiting their commercial deployment. Our strategy incorporates a long-chain starch polymer into the perovskite precursor. The starch polymer confers multiple beneficial effects by forming hydrogen bonds with the methylammonium iodide precursor, templating perovskite growth that results in a compact and homogeneous film deposited in a simple one-step coating (antisolvent-free). The inclusion of starch in the methylammonium lead iodide films strongly improves their thermomechanical and environmental stability while maintaining a high photovoltaic performance. The fracture energy (Gc) of the film is increased to above 5 J/m2 by creating a nanocomposite that provides intrinsic reinforcement at grain boundaries. Additionally, improved optoelectronic properties achieved with the starch polymer enable good photostability of the active layer and enhanced resistance to thermal cycling.

Out-of-equilibrium processes in crystallization of organic-inorganic perovskites during spin coating

Complex phenomena are prevalent during the formation of materials, which affect their processing-structure-function relationships. Thin films of methylammonium lead iodide (CH3NH3PbI3, MAPI) are processed by spin coating, antisolvent drop, and annealing of colloidal precursors. The structure and properties of transient and stable phases formed during the process are reported, and the mechanistic insights of the underlying transitions are revealed by combining in situ data from grazing-incidence wide-angle X-ray scattering and photoluminescence spectroscopy. Here, we report the detailed insights on the embryonic stages of organic-inorganic perovskite formation. The physicochemical evolution during the conversion proceeds in four steps: i) An instant nucleation of polydisperse MAPI nanocrystals on antisolvent drop, ii) the instantaneous partial conversion of metastable nanocrystals into orthorhombic solvent-complex by cluster coalescence, iii) the thermal decomposition (dissolution) of the stable solvent-complex into plumboiodide fragments upon evaporation of solvent from the complex and iv) the formation (recrystallization) of cubic MAPI crystals in thin film.

MAPbI3 Deposition by LV-PSE on TiO2 for Photovoltaic Application

Hybrid perovskites are one of the most popular materials nowadays due to their very exclusive properties. To mitigate costs, complexity, and environmental impact, in this work, we have prepared methylammonium lead iodide (MAPbI3) films by a two-step Low-Vacuum Proximity-Space-Effusion (LV-PSE). The LV-PSE method exploits the low vacuum and the short diffusion path from the precursor source to have high thermal energy and partial pressure of the sublimated species close to the substrate. To this aim, the substrate is located at a medium distance (∼2 cm) from the melting pots in a low-vacuum chamber at ∼4 × 10−2 mbar. In the first step, a PbI2 film is deposited on a substrate; in the second step, the conversion into MAPbI3 occurs via an adsorption-incorporation-migration mechanism through the evaporation of methylammonium iodide (MAI) reagents. To exploit the potential of the conversion reaction, 190 nm MAPbI3 layers are deposited on TiO2 substrates. The layers were characterized in terms of crystal structure by X-ray diffraction (XRD) analyses, which showed the exclusive presence of MAPbI3 confirming the complete conversion of the PbI2 film. Scanning Electron Microscopy (SEM) analyses revealed a flat uniform pinhole-free coverage of the substrates and good conformational coverage of the TiO2 underlayer. Transmission Electron Microscopy (TEM) analyses addressed the formation of the tetragonal phase and the absence of the amorphous phase in the film. Spectroscopic ellipsometry (SE) analyses were used to explore the optical properties and the stability of the MAPbI3 layer at different temperatures and ambient conditions. As proof of concept, solar cell architectures were prepared using TiO2 as Electron Transporting Layer (ETL), Spiro-OMeTAD as Hole Transporting Layer (HTL), and Au as a contact to exploit the new up-scalable and clean deposition method. Using just ∼190 nm thick layers, the best efficiency reached with this architecture was 6.30%.

Retrieval of Optical Constants of Undoped Flash-Evaporated Lead Iodide Films from an Analysis of Their Normal Incidence Transmission Spectra Using Swanepoel’s Transmission Envelope Theory of Non-Uniform Films

Abstract: Normal-incidence transmission-wavelength (T_exp (λ)-λ) spectra of 1 and 1.2 m thick flash-evaporated lead iodide (PbI2) films on 1.1 mm thick glass slides held at 200 ℃ display well-spaced several interference-fringe maxima and minima in the λ-range 520-900 nm, without exhibiting a transparent region and with the maxima lying well below the substrate transmission. Below 520 nm, these T_exp (λ)-λ curves drop steeply to zero (at λ ≼ 505 nm), signifying crystalline-like PbI2 film absorption. As corrections of measured (PbI2 film/substrate) transmittance data for substrate absorption and spectrometer slit-width effect were marginal over the studied λ-range, the observed low transmittance of (PbI2 film/substrate) system was related to PbI2 film thickness non-uniformity (∆d), which causes shrinkage of both maxima and minima and leads to significant film optical absorption that reduces both maxima and minima. The McClain ENVELOPE algorithm was utilized, with a minor modification, to construct maxima T_M (λ_max/λ_min) and minima T_m (λ_min/λ_max) envelope curves, which were analyzed by Swanepoel’s envelope method of non-uniform films using an approach that takes account of dispersive substrate refractive index n_s (λ) and circumvents the non-availability of a high-λ transparency region. In such analytical approach, ∆d was varied till a re-generated T_gen (λ)-λ curve matches the T_exp (λ)-λ curve. An average thickness d ̅ of the film, besides its refractive index n(λ) and absorption coefficient α(λ) in the weak, medium and strong absorption regions, were then obtained. The energy-dependence of α(λ) is discussed in view of interband electronic transition models. The obtained results are consistent with other literature studies on similar flash-evaporated PbI2 films. Keywords: PbI2, Optical constants and bandgap, Swanepoel's transmission envelope method, Non-uniform films.

Uniaxially Oriented Monolithically Grained Perovskite Films for Enhanced Performance of Solar Cells

High-quality metal-halide perovskite films with large grains, high crystallinity, and low defect density are vital to fabricate high-performance perovskite solar cells (PSCs). Here, we show that a combined method involving the methylammonium chloride (MACl) additive strategy and inverted annealing process can produce highly crystalline and (110) oriented methylammonium lead-iodide (MAPbI3) films with micron-sized grains of up to 4 μm and low trap density. The MACl additive plays critical roles on tuning the crystallization process, enhancing crystallinity, promoting grain growth, and inducing preferred crystal orientation of the perovskite film under an adequate thermal activation. Meanwhile, the inverted annealing approach enables the vulnerable MAPbI3 layer to withstand the applied thermal stress, thus effectively avoiding its decomposition. The optoelectronic properties of films are significantly improved with repressed nonradiative recombination of carriers, as evaluated by photoluminescence (PL) analysis, enabling the corresponding PSCs to achieve power conversion efficiencies (PCEs) of up to 19.86% under a reverse scan, as compared to PCEs below 17% for the control photovoltaics. More importantly, we have decoupled the functions of MA+ and Cl– and demonstrated a significant synergistic effect between the two constituents, enabling the MACl additive to show superior effects on modifying nucleation and crystal growth dynamics in contrast to its NH4Cl and MAI counterparts.

1,10-Phenanthroline as an Efficient Bifunctional Passivating Agent for MAPbI3 Perovskite Solar Cells.

Passivation is one of the most promising concepts to heal defects created at the surface and grain boundaries of polycrystalline perovskite thin films, which significantly deteriorate the photovoltaic performance and stability of corresponding devices. Here, 1,10-phenanthroline, known as a bidentate chelating ligand, is implemented between the methylammonium lead iodide (MAPbI3) film and the hole-transport layer for both passivating the lead-based surface defects (undercoordinated lead ions) and converting the excess/unreacted lead iodide (PbI2) buried at interfaces, which is problematic for the long-term stability, into "neutralized" and beneficial species (PbI2(1,10-phen)x, x = 1, 2) for efficient hole transfer at the modified interface. The defect healing ability of 1,10-phenanthroline is verified with a set of complementary techniques including photoluminescence (steady-state and time-resolved), space-charge-limited current (SCLC) measurements, light intensity dependent JV measurements, and Fourier-transform photocurrent spectroscopy (FTPS). In addition to these analytical methods, we employ advanced X-ray scattering techniques, nano-Fourier transform infrared (nano-FTIR) spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to further analyze the structure and chemical composition at the perovskite surface after treatment at nanoscale spatial resolution. On the basis of our experimental results, we conclude that 1,10-phenanthroline treatment induces the formation of different morphologies with distinct chemical compositions on the surface of the perovskite film such that surface defects are effectively passivated, and excess/unreacted PbI2 is converted into beneficial complex species at the modified interface. As a result, an improved power conversion efficiency (20.16%) and significantly more stable unencapsulated perovskite solar cells are obtained with the 1,10-phenanthroline treatment compared to the MAPbI3 reference device (18.03%).

Creation and investigation of electronic defects on methylammonium lead iodide (CH_3NH_3PbI_3) films depending on atmospheric conditions

Methylammonium lead iodide (MAPbI_3) (CH_3NH_3PbI_3) is popular material for edge technology application, but it still includes many uncertainties. Particularly, molecular and electronic degradation (electronic defect distribution) and mobility–lifetime product still hold many mysteries. Stemming from the atmospheric or light-induced degradation, mobility–lifetime product changes are still unknown and haven’t been studied up to now. In this study, mobility–lifetime product change was investigated depending on degradation source such as atmospheric and light soaked. MAPbI_3 films were deposited by thermal chemical vapor deposition (thermal CVD). Structural analysis was done by X-ray diffraction (XRD), respectively. Deposited MAPbI_3 films were exposed to laboratory ambient, vacuum atmosphere, deionized water vapor (DIWV) atmosphere and UV light soaking at constant temperature (300K) to define changes on mobility–lifetime product.

“Visible” Phase Separation of MAPbI3/δ‐FAPbI3 Films for High‐Performance and Stable Photodetectors

AbstractSimultaneous achievement of elevated photocurrent by increasing grain size of perovskite films and reduced dark current by introducing passivation phase as well as improved stability is significant and tough challenge for perovskite‐based photodetector devices. Herein, in‐situ formation of “visible” formamidinium lead iodide (δ‐FAPbI3) phase within a high‐quality methylamine lead iodide (MAPbI3) film is reported by a pressure‐induced phase separation strategy. The MAPbI3 single‐crystal grains are adequately grown and parts of them are over 20 µm in lateral dimension. More importantly, the incursion of δ‐FAPbI3 with high resistance behaves like an organic scaffold to passivate the trap state, limit cation diffusion, and increase intrinsic resistance of the MAPbI3 film. Accordingly, the MAPbI3/δ‐FAPbI3 photodetector devices exhibit excellent photoelectrical performance, including high detectivity, responsivity, and on/off ratio due to long carrier lifetime and low defect density. Furthermore, the unsealed MAPbI3/δ‐FAPbI3 film device retains over 94% of its initial photocurrent after 15 days in ambient conditions, exhibiting significantly improved stability. The introduction of “useless” δ‐FAPbI3 phase in high‐quality perovskite films opens up a new way toward improved photoelectrical performance and stability.