Lead Iodide Thin Films Research Articles

Ferroelectric Poling of Methylammonium Lead Iodide Thin Films

AbstractSeemingly contradictory reports on polar domains and their origin have surrounded the controversial discussion about the ferroelectricity of the methyl ammonium lead iodide (MAPbI3) thin films that are commonly employed in perovskite solar cells. In this work, microscopic modulations of the polar domain patterns upon application of an electric poling field are correlated with macroscopic changes to the currents through the MAPbI3 layer. Piezoresponse force microscopy is used to monitor the widening, narrowing, generation or extinction of polar domains, as well as shifts of the domain walls at room temperature under an in‐plane electric poling field that is applied between two laterally organized electrodes. This poling leads to a net polarization of individual grains and the thin film itself. Macroscopically, this net polarization results in a persistent shift of the diode characteristics that is measured across the channel between the electrodes. Both the modulation of the polar domains upon electric poling and the concurrent persistent shift of the electric currents through the device are the unambiguous hallmarks of ferroelectricity, which demonstrate that MAPbI3 is a ferroelectric semiconductor.

Precursor vapor deposited perovskite solar cells with smooth absorber layer

Preparation of a smooth methylammoniumleadiodide (CH3NH3PbI3) perovskite absorber film is one of the most critical processes in fabricating a perovskite photovoltaic cell with high efficiency. In this study, we report an original coating method to prepare CH3NH3PbI3 films by using the solvent vapors of precursor methylamine (MA) and hydroiodic acid (HI) solutions on lead (II) iodide (PbI2) thin films. In this strategy, the vapors of MA and HI solutions were briefly treated to the PbI2 coated substrate and then the resultant film was annealed to obtain smooth perovskite thin layer. After the topographic analyses of the thin films, the surface roughnesses of the perovskite films were found to be limited by the surface roughnesses of the PbI2 films before precursor vapor exposure. Another important aspect of our approach is that there is no need to synthesize methylammonium iodide (CH3NH3I) salt for active layer preparation. Furthermore, this method enables large-area fabrication of high quality films by a very simple and brief process. After the characterization of preliminary devices with this method, we obtained a short circuit current density of 14.67 mA/cm2, an open circuit voltage of 0.815 V, a fill-factor of 0.59, and a power conversion efficiency of 7.08%.

Ambipolar Transport in Methylammonium Lead Iodide Thin Film Transistors

We report clear room temperature ambipolar transport in ambient-air processed methylammonium lead iodide (MAPbI3) thin-film transistors (TFTs) with aluminum oxide gate-insulators and indium-zinc-oxide source/drain electrodes. The high ionicity of the MAPbI3 leads to p-type and n-type self-doping, and depending on the applied bias we show that simultaneous or selective transport of electrons and/or holes is possible in a single MAPbI3 TFT. The electron transport (n-type), however, is slightly more pronounced than the hole transport (p-type), and the respective channel resistances range from 5–11 and 44–55 MΩ/μm. Both p-type and n-type TFTs show good on-state characteristics for low driving voltages. It is also shown here that the on-state current of the n-type and p-type TFTs is highest in the slightly PbI2-rich and MAI-rich films, respectively, suggesting controllable n-type or p-type transport by varying precursor ratio.

A modified high-temperature vapour deposition technique for fabricating $$\hbox {CH}_{3}\hbox {NH}_{3}\hbox {PbI}_{3}$$ thin films under an ambient atmosphere

A modified two-step deposition technique was performed by vapour depositing methylammonium iodide (MAI) on lead iodide $$(\hbox {PbI}_{2})$$ films to fabricate good quality methylammonium lead iodide (MAPI) thin films under an ambient atmosphere, and their properties were compared with conventional two-step solution processed MAPI thin films. Scanning electron microscopy results depicted that the MAI vapour-processed films have a uniform coverage and planar surface compared with the non-uniformly distributed granule-like morphology of the solution-processed MAPI film. X-ray diffraction results confirm that the vapour-processed films have better crystallinity compared to the conventional solution-processed MAPI thin films. An enhancement in the optical absorption and emission was observed for the vapour-processed films. The higher processing temperature of the modified-vapour deposition eliminates the effect of moisture during the ambient atmosphere processing of the MAPI films.

Light‐Induced Defect Generation in CH3NH3PbI3 Thin Films and Single Crystals

In a period of only a few years, the power conversion efficiency of organic–inorganic perovskite solar cells has surpassed a value of 24.2%. However, a major drawback is the lack of long‐term stability, which is partially related to the dissociation of organic cations under prolonged illumination. This degradation mechanism is not limited to ambient temperatures. At low temperatures (T = 5 K), illumination of methyl ammonium lead iodide (CH3NH3PbI3) thin films with a photon energy of Eph = 3.4 eV results in the formation of localized trap states located about 100 meV within the bandgap. These light‐induced defects are metastable, and annealing at T ≥ 15 K removes the localized states. Defect creation is not limited to polycrystalline perovskites but is also observed in single‐crystal CH3NH3PbI3. The experimental data are discussed in terms of a two‐level model where the metastable state is separated from the annealed state by an energy barrier.

Perovskite phase formation in formamidinium–methylammonium lead iodide bromide (FAPbI3)1-x(MAPbBr3)x materials and their morphological, optical and photovoltaic properties

This manuscript describes the synthesis of formamidinium lead iodide (FAPbI3) thin films annealed at different temperatures to investigate the perovskite phase formation temperature and check its effect on morphology and optical properties. By using these materials, inverted perovskite solar cells devices were also fabricated for the photovoltaic properties. Devices with FAPbI3 annealed at 130 °C and higher temperature have shown power conversion efficiency ~ 11%. Second, formamidinium–methylammonium lead iodide bromide (FAPbI3)1-x (MAPbBr3)x (x = 0, 0.05, 0.1, 0.15, 0.2 and 1) was used as the absorbing layer in perovskite solar cells and the best PCE of 14.99% was observed with x = 0.1. Films were characterized by XRD, UV–vis absorption spectroscopy, photoluminescence, SEM and AFM.

Characterization of Methylammonium Lead Iodide Thin Films Fabricated by Exposure of Lead Iodide Layers to Methylammonium Iodide Vapor in a Closed Crucible Transformation Process

In this work, the characterization of methylammonium lead iodide (MAPI) layers fabricated with a modified two‐step deposition technique under high vacuum is presented. Thereby, PbI2, deposited in an open sublimation process, is exposed to methylammonium iodide (MAI) vapor in a closed crucible. The fabricated layers are examined with scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV/VIS absorption spectroscopy, and photoelectron spectroscopy (PES). In addition, planar solar cells incorporating the MAPI layers are produced. The obtained XRD data show that MAPI with high perovskite phase purity can be fabricated in a broad substrate temperature range between 75 °C and 150 °C. The SEM measurements show that with increasing substrate temperature the morphology of the MAPI layers undergoes significant changes which can be separated into three distinct processes, taking place simultaneously: formation of the perovskite by incorporation of MAI into the PbI2 grains, recrystallization of the perovskite grains, and an Ostwald ripening like growth of the recrystallized grains. By the combination of the UV/VIS and in vacuo PES data, band diagrams for PbI2, MAI, and MAPI can be drawn which appear to be independent on the substrate temperature.

Probing the Microstructure of Methylammonium Lead Iodide Perovskite Solar Cells

The microstructure of absorber layers is pivotally important for all thin‐film solar technologies. Using electron backscattered diffraction (EBSD), the crystal orientation in methylammonium lead iodide thin films with submicrometer resolution is reported. For the vast majority of (110) oriented grains, the c‐axis of the perovskite unit cell is oriented in‐plane. Although some adjacent grains exhibit the same in‐plane horizontal orientation of the c‐axis, no universal horizontal orientation of the c‐axis within the sample plane exists. The (110) crystal orientation correlates with an in‐plane orientation of the ferroelectric polarization as investigated by vertical and lateral piezoresponse force microscopy (PFM). The individual grains with different crystal orientations that exhibit different ferroelectric patterns and surface potentials are identified. The strong correlation between crystal orientation and ferroelectric polarization allows conclusions to be drawn about the microstructure from PFM measurements and, likewise, the ferroelectric polarization to be derived from crystallographic observations by EBSD.

Fabrication and characterization of La doped PbI2 nanostructured thin films for opto-electronic applications

Herein, we report a facile, cost-effective, solution processed fabrication method for pure and lanthanum (La3+) doped lead iodide (PbI2) thin films and investigated structural, optical, dielectric and electrical properties. Synthesis of PbI2 was confirmed by X-ray diffraction (XRD) and FT-Raman spectroscopy analyses. The fabricated films were grown along (001) plane, with crystallite size in the range of 12–14 nm and observed that crystallinity increases and defects decreases with the increase of La doping concentration. The morphology of fabricated films was observed to be spherical nanoparticles assembled nanosheets, distributed uniformly throughout the films. Moreover, homogeneous La doping in PbI2 was seen via EDX elemental mapping image. Optical band gap was calculated from Ultra violet–visible–near infrared (UV–Vis–NIR) spectra. The band gap of pure PbI2 film was noticed ∼2.44 eV which increase to 2.63 eV for 3% La3+ doped film. Absorption index, refractive index and dielectric constant values were calculated from optical data and found to be strongly dependent on incident photon energy and all parameters show higher values for La doped PbI2 films. I-V characteristics of pure and La-doped PbI2 thin films were carried out in the device configuration viz. FTO/PbI2 (or La-doped PbI2)/Au at room temperature. I-V curve for the pure PbI2 films shows the ohmic behavior while La doped PbI2 films shows non ohmic behavior attributed to space charge limited current. The electrical resistivity values for pure PbI2 film is found to be very high about 1.8 × 1011 Ω-cm.

Scalable fabrication of high-quality crystalline and stable FAPbI3 thin films by combining doctor-blade coating and the cation exchange reaction.

Formamidinium lead iodide (FAPbI3) is one of the most extensively studied perovskite materials due to its narrow band gap and high absorption coefficient, which makes it highly suitable for optoelectronic applications. Deposition of a solution containing lead iodide (PbI2) and formamidinium iodide (FAI) or sequential deposition of PbI2 and FAI usually leads to the formation of films with a poor morphology and an unstable crystal structure that readily crystallize into two different polymorphs: the photoinactive yellow phase and the photoactive black phase. In this work, 2D 2-phenylethylammonium lead iodide (PEA2PbI4) thin films are deposited by a scalable doctor-blade coating technique and used as a growth template for the high-quality 3D FAPbI3 perovskite thin films which are obtained by organic cation exchange. We report the structural, morphological and optical properties of these converted 3D FAPbI3 perovskite films which we compare to the directly deposited 3D FAPbI3 films. The converted FAPbI3 thin films are compact, smooth, and highly oriented and exhibit better structural stability in comparison with the directly deposited 3D films. These results not only underscore the importance of the employed deposition techniques in fabricating highly crystalline and stable perovskite thin films but also provide a strategy to easily obtain very compact perovskite layers using doctor-blade coating.

Scalable Graphene‐on‐Organometal Halide Perovskite Heterostructure Fabricated by Dry Transfer

AbstractGraphene, a single layer conductor, can be combined with other functional materials for building efficient optoelectronic devices. However, transferring large‐area graphene onto another material often involves dipping the material into water and other solvents. This process is incompatible with water‐sensitive materials such as organometal halide perovskites. Here, a dry method is used and succeeded, for the first time, in stacking centimeter‐sized graphene directly onto methylammonium lead iodide thin films without exposing the perovskite film to any liquid. Photoemission spectroscopy and nanosecond time‐resolved photoelectrical measurement show that the graphene/perovskite interface does not contain significant amount of contaminants and sustain efficient interfacial electron transfer. The use of this method in fabricating graphene‐on‐perovskite photodetectors is further demonstrated. Besides a better photoresponsivity compared to detectors fabricated by the conventional perovskite‐on‐graphene structure, this dry transfer method provides a scalable pathway to incorporate graphene in multilayer devices based on water‐sensitive materials.

Cation exchange assisted dimensional down-conversion of perovskite thin films using vapor annealing: An interplay of tolerance factor

This work demonstrates a method to form 1-D ethylammonium lead iodide (EAPbI3) thin-films by dimensional down-conversion of 3-D methylammonium lead iodide (MAPbI3) perovskite. The process leverages cation-exchange that occurs during exposure to ethylamine vapor and subsequent annealing. Based on the extensive materials characterization, a detailed mechanism of the transformation is proposed. The transformation also improves the morphology of the films, yielding compact EAPbI3 films with roughness of 50-100 nm, low-pin-hole density, and grain-size of 1-3 µm. The process can be used to fabricate various EAPbI3 optoelectronic devices. Since lower dimensional perovskites are more stable against moisture than MAPbI3, the process can also be used to fabricate 3D-1D graded perovskite interfaces for materials study and to stabilize MAPbI3 solar cells with a protective outer layer of EAPbI3.

Temperature-dependent photoluminescence spectra and decay dynamics of MAPbBr3 and MAPbI3 thin films

The steady-state spectra and fluorescence lifetimes are investigated under vacuum for methylammonium lead bromide and iodide (CH3NH3PbBr3 or MAPbBr3, and CH3NH3PbI3 or MAPbI3) thin films by stably controlling the sample temperature in the range of 78 K to 320 K. The transformation of spectrum features and lifetime components are proved to be quite sensitive to the temperatures in accordance with the phase transition of structures. Our work demonstrates that the halide anions I- and Br- lead to remarkable differences on optical characteristics. Due to the distinct behaviors of excitons, electron-hole pairs and free carriers in decay channels, MAPbI3 has much longer lifetime and higher low-temperature fluorescence efficiency than those of MAPbBr3. The findings provide possible choices for certain perovskites under various ambient temperature conditions to display photovoltaic or luminescent advantage.

On the structure and physical properties of methyl ammonium lead iodide perovskite thin films by the two-step deposition method

In the present work, we synthesized methyl ammonium lead iodide perovskite (CH3NH3PbI3) thin films using the two-step deposition technique. The method involves conversion of spin coated lead iodide (PbI2) thin films by dipping in a solution of CH3NH3I (MAI) followed by annealing. In this work, the samples were grown by varying the time of dipping and the concentration of MAI solution. The thin films formed were analyzed to determine their crystalline structure, morphology, elemental composition, chemical states and physical properties. The films showed tetragonal crystal structure with compact morphology. The elemental and the respective oxidation state implied the formation of CH3NH3PbI3 thin films. Also, these thin films exhibited direct optical absorption, and the band gap values were in the range of 1.5–1.6 eV. The dark conductivity value of the films varied from 10−6 (Ω cm)−1 to 10−4 (Ω cm)−1 and all the thin films were photoconductive.

On the importance of ferroelectric domains for the performance of perovskite solar cells

This work analyzes in detail the effect of ferroelectric polarization patterns in methylammonium lead iodide (MAPbI3) thin-films on the J-V characteristics of the corresponding solar cells. The simulations are based on a finite-element discretization of the drift-diffusion equations and take into account the polarization pattern experimentally derived from piezoresponse force micrographs. Based on the knowledge of the crystalline structure, symmetry considerations and electrical simulations, we discuss models for the polarization orientation pattern and magnitude of the ferroelectric domains. We conclude that the in-plane polarization vectors have 45° orientation towards the domain walls and form herring-bone structures. The presence of ordered ferroelectric domains, even with a weak characteristic polarization magnitude enhances the power conversion efficiencies and are mandatory to reproduce the experimental J-V characteristics.

Vis-Near-Infrared Photodetectors Based on Methyl Ammonium Lead Iodide Thin Films by Pulsed Laser Deposition

Organic–inorganic hybrid perovskite materials are considered as promising candidates for emerging thin-film photodetectors. In this work, we discuss the application of the CH3NH3PbI3 thin films by pulsed laser deposition for photodetection applications. With this method, we obtained good perovskite film coverage on fluorine-doped tin oxide-coated substrates and observed wel- developed grains. The films showed no sign of degradation over several months of testing. We investigated the surface morphology and surface roughness of the films by field emission scanning electron microscopy and atomic force microscopy. The optical response of the films was studied using ultraviolet–visible and photoluminescence spectroscopy. We carried out a study on the solar and infrared photodetection of CH3NH3PbI3 thin films. The values of the responsivity, sensitivity, external quantum efficiency and specific detectivity under 1 sun illumination and 0.7 V bias were 105.4 A/W, 1.9, 2.38 × 104% and 1.5 × 1012 Jones, respectively.

Electrical Transport Mechanisms and Photoconduction in Undoped Crystalline Flash-Evaporated Lead Iodide Thin Films

The flash-evaporation technique was utilized to fabricate undoped 1.35-μm and 1.2-μm thick lead iodide films at substrate temperatures \( T_{\rm{s}} = 150 \)°C and 200°C, respectively. The films were deposited onto a coplanar comb-like copper (Cu-) electrode pattern, previously coated on glass substrates to form lateral metal–semiconductor–metal (MSM-) structures. The as-measured constant-temperature direct-current (dc)-voltage (\( I\left( {V;T} \right) - V \)) curves of the obtained lateral coplanar Cu-PbI2-Cu samples (film plus electrode) displayed remarkable ohmic behavior at all temperatures (\( T = 18 - 90\,^\circ {\hbox{C}} \)). Their dc electrical resistance \( R_{\rm{dc}} (T \)) revealed a single thermally-activated conduction mechanism over the temperature range with activation energy \( E_{\rm{act}} \approx 0.90 - 0.98 \,{\hbox{eV}} \), slightly less than half of room-temperature bandgap energy \( E_{\rm{g}} \) (\( \approx \,2.3\, {\hbox{eV}} \)) of undoped 2H-polytype PbI2 single crystals. The undoped flash-evaporated \( {\hbox{PbI}}_{\rm{x}} \) thin films were homogeneous and almost stoichiometric (\( x \approx 1.87 \)), in contrast to findings on lead iodide films prepared by other methods, and were highly crystalline hexagonal 2H-polytypic structure with c-axis perpendicular to the surface of substrates maintained at \( T_{\rm{s}} { \gtrsim }150^\circ {\hbox{C}} \). Photoconductivity measurements made on these lateral Cu-PbI2-Cu-structures under on–off visible-light illumination reveal a feeble photoresponse for long wavelengths (\( \lambda > 570\,{\hbox{nm}} \)), but a strong response to blue light of photon energy \( E_{\rm{ph}} \) \( \approx \,2.73 \, {\hbox{eV}} \) (\( > E_{\rm{g}} \)), due to photogenerated electron–hole (e–h) pairs via direct band-to-band electronic transitions. The constant-temperature/dc voltage current–time \( I\left( {T,V} \right) - t \) curves of the studied lateral PbI2 MSM-structures at low ambient temperatures (\( T < 50^\circ {\hbox{C}} \)), after cutting off the blue-light illumination, exhibit two trapping mechanisms with different relaxation times. These strongly depend on \( V \) and \( T \), with thermally generated charge carriers in the PbI2 mask photogenerated (e–h) pairs at higher temperatures.

Broadband, Ultrafast, Self‐Driven Photodetector Based on Cs‐Doped FAPbI3 Perovskite Thin Film

AbstractIn this study, a high‐performance photodetector comprised of formamidinium cesium lead iodide (FA1−xCsxPbI3) thin film is developed. The Cs‐doped FAPbI3 perovskite material is synthesized through a simple spin‐coating method, via which FA1−xCsxPbI3 with different Cs doping levels (x = 0.1, 0.15, 0.2, and 0.3) can be obtained. Further optoelectronic characterization reveals that the FA0.85Cs0.15PbI3 photodetector exhibits reproducible sensitivity to irradiation with wavelengths in the range from 240 to 750 nm, whereas it is weakly sensitive to wavelengths longer than 750 nm. The responsivity and specific detectivity are estimated to be around 5.7 A W−1 and 2.7 × 1013 cm Hz1/2 W–1, respectively. It is also worth noting that the present perovskite photodetector demonstrates an ultrafast response speed (tr/tf: 45 ns/91 ns) at zero bias voltage, which is probably related to the ultrafast lifetime and high quality of thin film according to the Hall effect study. Finally, this device shows a weak degradation in sensitivity to white light after storage at ambient condition for 45 days. The totality of the broadband sensitivity, high specific detectivity, ultrafast response speed, and self‐driven property renders the FA1−xCsxPbI3 an idea material for high‐performance photodetectors application.

Determination of the True Lateral Grain Size in Organic-Inorganic Halide Perovskite Thin Films.

In this letter, methylammonium lead iodide (MAPbI3) thin films were examined via piezoresponse force microscopy (PFM) and nanoindentation (NI) to determine if long-range atomic order existed across the full width and depth of the apparent grains. From the PFM, the piezoelectric response of the films was strongly correlated with low-index planes of the crystal structure and ferroelastic domains in macroscale solution-grown MAPbI3 crystals, which implied long-range order near the top surface. From the NI, it was found that the induced cracks were straight and extended across the full width of the apparent grains, which indicated that the long-range order was not limited to the near-surface region, but extended through the film thickness. Interestingly, the two MAPbI3 processes examined resulted in subtle differences in the extracted electro-mechanical and fracture properties, but exhibited similar power conversion efficiencies of >17% in completed devices.

Slow Electron–Hole Recombination in Lead Iodide Perovskites Does Not Require a Molecular Dipole

Hybrid organic/inorganic lead iodide perovskites of the formula APbI3, where A is a molecular cation such as methylammonium, exhibit remarkably slow photoinduced charge carrier recombination rates, for reasons that remain uncertain. Prevalent hypotheses credit this behavior to the unique dipolar nature of the molecular cation. Herein, transient terahertz spectroscopy is applied to solution-processed, all-inorganic, perovskite-phase cesium lead iodide (CsPbI3) thin films, which lack such a dipole. The recombination kinetics are studied as a function of the initial photoinduced carrier concentration and the wavelength of excitation. A kinetic model combining diffusion and recombination is fit to the data, from which the rate constants are determined, revealing a bimolecular recombination rate of 10–10 cm3 s–1, comparable to high-quality, single-crystal, direct-gap semiconductors. This rate, as well as a charge carrier mobility > 30 cm2 V–1 s–1 measured herein for CsPbI3, are similar to values reported for t...