Lead Iodide Thin Films Research Articles

Stability in Photoluminescence and Photovoltaic Properties of Formamidinium Lead Iodide Quantum Dots

Formamidinium lead iodide (FAPI) thin films in the perovskite crystalline phase have piqued interest for single-junction solar cells due to their optimal bandgap, long photocarrier lifetime, and intrinsic structural stability. However, trap states on the surface and within the lattice impede the performance and stability of FAPI bulk crystalline film cells. Strategies such as film crystallization, deposition optimization, precursor engineering, and interface property tailoring aim to overcome these limitations. One approach involves forming FAPI quantum dots (QDs). By reducing the size, we achieve better quality and stable alpha-phase FAPI on a nanometer scale. In this contribution, we report synthesizing of alpha-phase FAPI QDs with a mean size distribution of 20 nm. These QDs exhibit a narrow photoluminescence emission peak at 1.61 eV and higher absolute quantum yields ranging from 70% to 86%. Compared to bulk FAPI with a 1.54 eV gap, a blue shift is observed due to the quantum confinement effect.However, achieving degradation-free perovskite solar cells under prolonged light soaking remains a challenge. Furthermore, we investigate the stability of FAPI bulk and QDS films in a humid chamber and after exposure to AM1.5G light in a humid setting (RH ~ 30%, T ~ 28°C). Our approach allows us to form homogeneously dispersed films of quantum dots with thicknesses of several hundred nanometers. These films are studied for photoluminescence and photovoltaic properties. Prototype solar cells incorporating synthesized FAPI QDs showed a power conversion efficiency of around 8%. These findings indicate considerably more stability in photoluminescence and photovoltaic properties of FAPI QDs for enhanced light-soaking stability compared to bulk FAPI films.

Static anti-solvent treatment on lead iodide in two-step deposition of methylammonium lead iodide perovskite thin films

Static anti-solvent treatment is rarely employed for perovskite thin film fabrication, even though this method is dependent only on fewer thin film growth parameters as compared to the dynamic anti-solvent application. Thus, studies on the influence of static anti-solvent treatment on the properties of perovskite thin films is important to gain insight to the growth processes, which is very scarce as well. In this study, we investigate the static anti-solvent treatment to tailor the morphology of Lead Iodide (PbI2) thin films for the fabrication of precursor-free Methylammonium Lead Iodide (MAPI) perovskite thin films through the two-step deposition method using Chlorobenzene as an anti-solvent. Remarkable influence of the anti-solvent loading time on the film morphology of PbI2 and MAPI thin films is observed. The crystallinity of PbI2 decreases upon increasing the anti-solvent loading time. A complete conversion of the precursors to MAPI along with the largest grain size are attained even at short loading times, ∼5s. Grain size does not increase considerably at higher loading times. Linear absorption studies on MAPI thin films show an increase in the Urbach energy with an increase in the loading time, which indicates an increase in the defect density. The increase in defect density with anti-solvent loading time is attributed to the concurrent decrease in the crystallinity of the PbI2 thin film. Steady-state and time-resolved photoluminescence (PL) studies performed on the MAPI thin films show highest PL intensity and life time at an optimum anti-solvent loading time (∼5s), which arises due to the trade off between largest grain size and lowest defect density. These observations not only highlights the need for an optimum crystallinity in the PbI2 thin films for the fabrication of superior quality MAPI thin films but also infers an optimum static anti-solvent loading time as the increase in grain size and life time enhances the device performance while an increase in defect density degrades the performance.

Solution‐Processed, Highly Crystalline, and Oriented MAPbI3 Thin Films by Engineering Crystal‐Growth Kinetics

Growing oriented and monocrystalline layers of lead halide perovskites over device substrates helps to harness their outstanding optoelectronic properties. Epitaxial growth of lead halide perovskites for device fabrication is limited by the lack of lattice‐matched substrates and the requirement of compact pinhole‐free films. Most optoelectronic devices use amorphous substrates, hindering oriented epitaxial growth. Herein, highly crystalline methylammonium lead iodide (MAPbI3) thin films over amorphous substrates are demonstrated by meticulously optimizing the nucleation and growth kinetics in spin‐coating. The “epitaxial‐like” films enable large‐area crystalline layer fabrication, with larger than 100 μm spherulitic grains oriented along [200] and [224] planes. The compact, highly crystalline, and oriented films of MAPbI3 formed over indium tin oxide/SnO2 are used to fabricate perovskite solar cells (PSCs) with an area of 1 cm2. Despite the perovskite films being highly oriented and crystalline, the PSCs’ performances highlight the critical role the interfaces play in photovoltaic cells.

The Synergetic Ionic and Electronic Features of MAPbI3 Perovskite Films Revealed by Electrochemical Impedance Spectroscopy

AbstractPerovskites have emerged as a forerunner of electronics research due to their vast potential for optoelectronic applications. The numerous combinations of constituent ions and the potential for doping of perovskites lead to a high demand to track the underlying electronic properties. Solution‐based electrochemistry is particularly promising for detailed and facile assessment of perovskites. Here, electrochemical impedance spectroscopy (EIS) of methylammonium lead iodide (MAPbI3) thin films is performed and model them with an equivalent circuit that accounts for solvent, ionic, and thin film effects. A dielectric constant consistent with prior AC studies and a diffusion constant harmonious with cation motion in MAPbI3 are extracted. An electrical double layer thickness in the perovskite film of 54 nm is obtained, consistent with lithium doping in perovskite films. Comparing the EIS and equivalent circuit model of perovskite films to control ITO‐only data enabled the assignment of the ions at each interface. This comparison implied a double layer of primarily lithium ions inside the perovskite film at the solution interface with significant recombination of ions on the solution side of the interface. This demonstrates EIS as a powerful tool for studying the fundamental charge accumulation and transport processes in perovskite thin films.

Role of perovskite thickness on optoelectronic properties in lead bromide and lead iodide thin film perovskites: A DFT study

Role of perovskite thickness on optoelectronic properties in lead bromide and lead iodide thin film perovskites: A DFT study

Bright future by controlling α/δ phase junction of formamidinium lead iodide doped by imidazolium for solar cells: Insight from experimental, DFT calculations and SCAPS simulation

In this study, we investigated the potential of Formamidinium lead iodide (FAPbI3) thin films as an absorber layer in perovskite-based solar cells, due to their good optoelectronic properties. Specifically, we investigated the effects of doping FAPbI3 with imidazolium (IM) at various percentages (ranging from 0% to 8%) on the stability and phase of the material at the α/δ phase junction. To do this, we used a range of analytical techniques, including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence spectroscopy (PL), and UV-visible spectroscopy. Our findings showed that 6% IM-doped FAPbI3 led to homogeneous perovskite films with a stable α-phase perovskite, large grain size, and no holes. We confirmed these results via Density Functional Theory (DFT) studies. Additionally, we analyzed the impact of 6% IM-doped FAPbI3 in the context of solar cell applications using SCAPS simulations. We introduced a stacked layer-based perovskite structure (FTO/TiO2/FAPbI3/MAPbI3/Spiro − MeoTAD/Au) and found that the device had a high power conversion efficiency (PCE) of 26.10%, with a fill factor (FF) of 88%, open-circuit voltage (VOC) of 1.18 V, and short-circuit current density (JSC) of 25.18 mA/cm2.

Preparation of High-Crystallinity and Large-Grain Br-doped Methylammonium Lead Iodide Thin Films at the Temperature Range of 100~140°C

Preparation of High-Crystallinity and Large-Grain Br-doped Methylammonium Lead Iodide Thin Films at the Temperature Range of 100~140°C

Orientation and Grain Size in MAPbI3 Thin Films: Influence on Phase Transition, Disorder, and Defects

In recent years, record efficiencies of halide perovskite-based devices have been achieved by processing high-quality thin films, where small morphology differences seem to be relevant for optimized optoelectronic functionality. However, a detailed understanding on how small morphological changes in perovskite films affect their structural and optoelectronic properties is still missing. Here, we investigate the influence of small morphology differences (i.e., increased grain size and crystallographic orientation), which are induced by hot-pressing methylammonium lead iodide (MAPbI3) thin films, on the structural properties, phase transition behavior, energetic disorder, and defects. To this end, detailed temperature-dependent photoluminescence (PL) and absorption analyses from 300 K down to 5 K are performed. The morphology differences, confirmed by scanning electron microscopy and X-ray diffractometry analyses, result in an increased phase transition temperature for hot-pressed (HP) films, which we attribute to less strain. Moreover, fluence-dependent and transient PL measurements reveal a lower defect density in HP films. Here, besides grain size, also the degree of orientation appears to enhance the charge carrier lifetimes. The identified interdependence of strain and defect properties with film morphology suggests small differences in the perovskite’s energetic disorder. Our work thus emphasizes the importance that even small structural differences in halide perovskites have on their optoelectronic functionality, spurring their further optimization.

Efficient Narrowband Photoconductivity of the Excitonic Resonance in Two-Dimensional Ruddlesden-Popper Perovskites Due to Exciton Polarons.

Filter-less, wavelength-selective photodetectors made of perovskite usually rely on the charge collection narrowing mechanism, which intrinsically limits the response times. Using the narrow excitonic peak of, e.g., two-dimensional (2D) Ruddlesden-Popper perovskites as direct absorbers to realize color-selective photodetectivity promises faster responses. However, one major challenge in realizing such devices remains the separation and charge carrier extraction of the tightly bound excitons. Here, we report on filter-less color-selective photoconductivity in 2D perovskite butylammonium lead iodide thin film devices, exhibiting a distinct resonance in the photocurrent spectrum with a full width at half-maximum of 16.5 nm that correlates to the excitonic absorption. Our devices exhibit unexpectedly efficient charge carrier separation with an external quantum efficiency of ≤8.9% at the excitonic resonance, which we trace back to the involvement of exciton polarons. Our photodetector achieves response times of 150 μs and a maximum specific detectivity of 2.5 × 1010 Jones at the excitonic peak.

The optical properties of strontium manganite thin films prepared by novel phototreatment technique

SrMnO3 thin films were successfully prepared by spin coating the viscous solution obtained from the autocombustion synthesis. To ensure the transformation of the precursor solution into the manganite phase, conventional thermal treatment and novel phototreatment were applied. The thermal treatment resulted in stable hexagonal SrMnO3, while phototreatment yielded cubic SrMnO3 phase which is usually not stable at ambient conditions. The optical properties of the prepared materials were measured revealing the optical gaps of 1.5 eV for thermally treated material and about 1.3 – 1.4 eV for the phototreated samples. Comparison with ab initio result shows a higher theoretical value of 1.9 eV, ascribed to high exciton binding energy. Since these values are similar to the optical gap (1.48 eV) of widely studied formamidium lead iodide thin films, SrMnO3 thin films are promising candidates for active layers in inorganic perovskite solar cells.

Amine-Thiol/Selenol Chemistry for Efficient and Stable Perovskite Solar Cells

Controlling the crystallization of perovskites is imperative to reduce defect densities in perovskite thin films and extend device lifetimes. In this work, combinations of amine and chalcogenide ligands were introduced in the sequential deposition method to fabricate highly crystalline and oriented formamidinium lead iodide thin films with reduced defect densities and increased charge carrier lifetimes. The dual additives can tune the perovskite intermediate state and control the crystallization, leading to devices with improved efficiencies and stabilities. While thiophenol failed to prevent the amine ligand from degrading the perovskite precursors, benzene selenol combinations with amine ligands drastically changed the solution chemistry to increase the PbI2 conversion to highly crystalline and oriented α-FAPbI3 films with lower defect densities. X-ray photoelectron spectroscopy studies reveal benzene selenol evaporates from the thin film, leaving behind a modified surface, which is associated with the amine additive. These results indicate the amine selection can be used to tune the surface properties. Lastly, we propose a highly tunable I2 reduction strategy using chalcogenide chemistry to help enable the realization of perovskite solar cells with high performance and stability.

Alcohol assistant surface passivated perovskites for efficient perovskite solar cells

The surface passivation has been intensively applied for diminishing surface defects, restricting leakage current and suppressing surface charge recombination, aimed to boost device performance of perovskite solar cells (PSCs). In this work, we report the utilization of alcohol to treat the surface of methylammonium lead iodide (CH 3 NH 3 PbI 3 , MAPbI 3 ) thin film for generating excess PbI 2 as a surface passivation layer. X-ray diffraction, grazing-incident wide-angle x-ray scattering, and morphological studies reveal that additional PbI 2 is indeed formed at the surface of MAPbI 3 thin films with alcohol treatment. Photoluminescence studies demonstrate that the non-radiative charge recombination is suppressed within the butanol-treated MAPbI 3 thin film. The optimal film morphology is obtained from the butanol-treated MAPbI 3 thin film. The enlarged recombination resistance, reduced trap density, and improved build-in potential are observed from the PSCs based on the butanol-treated MAPbI 3 thin film compared to those based on either pristine MAPbI 3 or the ethanol- or isopropanol-treated MAPbI 3 thin films, respectively, indicating the surface charge recombination is restricted and the interface defects are suppressed within the butanol-treated MAPbI 3 thin film. As a result, a power conversion efficiency (PCE) of 20.44% is observed from the PSCs based on the butanol-treated MAPbI 3 thin film in comparison with a PCE of 17.24% from the PSCs based on the pristine MAPbI 3 thin film. Our findings demonstrate that we have developed a facile way to approach efficient PSCs through PbI 2 surface passivation of MAPbI 3 thin films with alcohol treatment. • Development of efficient perovskite solar cells (PSCs) through surface passivation layer. • We report the utilization of various alcohols to treat the surface of methylammonium lead iodide (CH 3 NH 3 PbI 3 , MAPbI 3 ) thin film for generating excess PbI 2 as a surface passivation layer. It is found that the optimal film morphology is obtained from the butanol-treated MAPbI 3 thin film. • The enlarged recombination resistance, reduced trap density, and improved build-in potential are observed from the PSCs based on the butanol-treated MAPbI 3 thin film compared to those based on either pristine MAPbI 3 or the ethanol- or isopropanol-treated MAPbI 3 thin films, respectively, indicating the surface charge recombination is restricted and the interface defects are suppressed within the butanol-treated MAPbI 3 thin film. • A power conversion efficiency (PCE) of 20.44% is observed from the PSCs based on the butanol-treated MAPbI 3 thin film in comparison with a PCE of 17.24% from the PSCs based on the pristine MAPbI 3 thin film. Our findings demonstrate that we have developed a facile way to approach efficient PSCs through PbI 2 surface passivation of MAPbI 3 thin films with alcohol treatment.

Impact of Sb-insertion on structural, optical, and dielectric characteristics of the PbI2 thin film

Antimony (Sb) has been widely used as an additive in solar cells for enhanced performance. We described a simple, affordable, solution-processed approach for producing pure and antimony (Sb3+)-doped lead iodide (PbI2) thin films with varying doping ratios (0.5, 1.0, 2.0, and 3.0%). Structural, optical, and dielectric properties were investigated using XRD and UV–Vis spectroscopy. The XRD spectra illustrated that the synthesized PbI2 thin films have a preferred orientation along the c-axis (001) with a hexagonal structure. The crystalline sizes of PbI2 increased by increasing the Sb-doping concentration up to 1%, then decreased with 2% and 3% Sb-doping. In the visible and NIR regions, the optical transparency of the grown films were in between 70 and 80%. The optical band gap changes inversely with the crystalline sizes; it decreases with 0.5% and 1% Sb-doping and increases with further doping. As the energy gap is spanned between 2.39 and 2.27 eV, therefore, Sb: PbI2 films are ideal for use in solar cells. The resulting refractive index values were found to rise as the Sb doping concentration increased. Sb-doping decreases optical conductivity and dielectric constant.

Sequential surface passivation for enhanced stability of vapor-deposited methylammonium lead iodide thin films

Understanding the correlation between surface passivation and degradation kinetics in vapor-deposited organic–inorganic hybrid perovskite thin films is crucial for developing strategies to enhance their structural integrity for long-term operational stability. Here, we demonstrate that the sequential introduction of vapor-phase phenethylammonium iodide (PEAI) and methylammonium (MAI) into methylammonium lead iodide (MAPbI3) thin films, which is obtained from PbI2, suppresses their degradation and enhances the ambient stability. Compared with the simultaneous PEAI introduction during the conversion of PbI2 to MAPbI3, the MAPbI3 films with sequential treatment exhibited higher PEAI surface coverage, which imparts superior resistance to degradation. Combined analyses of the exposure-time-dependent crystal structure evolution, photoemission profiles, and photocurrent generation characteristics of the films reveal that conformal PEAI passivation suppresses the volatile release of methylammonium (MA) species and the inclusion of O2/H2O molecules. This study highlights the importance of developing a vapor-phase passivation strategy for innate polycrystalline perovskite thin films and their optoelectronic applications with long-term stability.

Electrochemical characterization of halide perovskites: Stability & doping

As halide perovskite materials have emerged at the forefront of optoelectronics development, there is an ongoing need to understand their underlying physics and control their emergent properties. Electrochemistry shows promise to both access fundamental parameters of perovskites and to enhance their performance through doping. However, halide perovskites pose a significant challenge to solution-based electrochemistry, as both aqueous solutions and organic solvents are often destructive to thin films of these materials. This work provides a perspective of recent approaches to electrochemical measurements and modifications of halide perovskites with a specific focus on stability and doping. We also report the electrochemistry of methylammonium lead iodide (MAPbI3) thin films relevant for solar applications with a solvent toolkit based on hydrofluoroethers. Both oxidation and reduction peaks are revealed from electrochemical characterization exhibiting the characteristic HOMO/LUMO levels and additional features. This electrochemical driving is found to have little impact on the photoluminescence or underlying morphology of the thin films. Finally, electrochemical lithium salt doping with this solvent toolkit enhanced the conductivity of a thin film device by nearly two orders of magnitude, demonstrating the utility of this approach for optoelectronic applications.

Gamma ray effects on the properties of PbI2 thin films

This study reports the impact of γ-ray exposure on the nanostructural, morphological, and optical characteristics of lead iodide (PbI2) thin films (TFs). These PbI2 were deposited on glass substrates using a spin coating technique. The γ-ray irradiation was carried out using Co-60 gamma source with dose range from 0 to 100 kGy. The X-ray diffraction (XRD) results revealed a decrease in crystallinity and an increase in crystallite size of PbI2 as a function of γ-ray dose. Field emission scanning electron microscopy (FESEM) exhibited the variations in surface morphology with increasing γ-ray dose. The elemental composition of as prepared TFs was confirmed by energy dispersive X-rays (EDX) analysis. The optical transmission of PbI2 TFs was evaluated with varying the γ-ray doses. The estimated energy band gap (Eg) reduced from 2.54 to 2.45 eV with γ-ray dose. Photoluminescence (PL) spectra represent a broad peak centered at 507.64 nm with a hump present at 519.73 nm in the as prepared sample, which grow gradually with increasing the γ-ray dose. The maximum value of mobility was found to be 10.48 cm2 V−1 s−1 at 50 kGy. The change in the mobility is associated with the density of structural defects influenced by γ-ray doses. These effects conclude that γ-irradiation played a key role in structural, electrical and optical properties of the PbI2 TFs and proposes that the γ-irradiation response makes it a good candidate for radiation detecting material application.

Intermediate Phase-Free Process for Methylammonium Lead Iodide Thin Film for High-Efficiency Perovskite Solar Cells.

Solvent engineering by Lewis‐base solvent and anti‐solvent is well known for forming uniform and stable perovskite thin films. The perovskite phase crystallizes from an intermediate Lewis‐adduct upon annealing‐induced crystallization. Herein, it is explored the effects of trimethyl phosphate (TMP), as a novel aprotic Lewis‐base solvent with a low donor number for the perovskite film formation and photovoltaic characteristics of perovskite solar cells (PSCs). As compared to dimethylsulfoxide (DMSO) or dimethylformamide (DMF), the usage of TMP directly crystallizes the perovskite phase, i.e., reduces the intermediate phase to a negligible degree, right after the spin‐coating, owing to the high miscibility of TMP with the anti‐solvent and weak bonding in the Lewis adduct. Interestingly, the PSCs based on methylammonium lead iodide (MAPbI3) derived from TMP/DMF‐mixed solvent exhibit a higher average power conversion efficiency of 19.68% (the best: 20.02%) with a smaller hysteresis in the current‐voltage curve, compared to the PSCs that are fabricated using DMSO/DMF‐mixed (19.14%) or DMF‐only (18.55%) solvents. The superior photovoltaic properties are attributed to the lower defect density of the TMP/DMF‐derived perovskite film. The results indicate that a high‐performance PSC can be achieved by combining a weak Lewis base with a well‐established solvent engineering process.

Chemical and Structural Degradation of CH3NH3PbI3 Propagate from PEDOT:PSS Interface in the Presence of Humidity

AbstractUnderstanding interfacial reactions that occur between the active layer and charge‐transport layers can extend the stability of perovskite solar cells. In this study, the exposure of methylammonium lead iodide (CH3NH3PbI3) thin films prepared on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)‐coated glass to 70% relative humidity (R.H.) leads to a perovskite crystal structure change from tetragonal to cubic within 2 days. Interface‐sensitive photoluminescence measurements indicate that the structural change originates at the PEDOT:PSS/perovskite interface. During exposure to 30% R.H., the same structural change occurs over a much longer time scale (>200 days), and a reflection consistent with the presence of (CH3)2NH2PbI3 is detected to coexist with the cubic phase by X‐ray diffraction pattern. The authors propose that chemical interactions at the PEDOT:PSS/perovskite interface, facilitated by humidity, promote the formation of dimethylammonium, (CH3)2NH2+. The partial A‐site substitution of CH3NH3+ for (CH3)2NH2+ to produce a cubic (CH3NH3)1−x[(CH3)2NH2]xPbI3 phase explains the structural change from tetragonal to cubic during short‐term humidity exposure. When (CH3)2NH2+ content exceeds its solubility limit in the perovskite during longer humidity exposures, a (CH3)2NH2+‐rich, hexagonal phase of (CH3NH3)1−x[(CH3)2NH2]xPbI3 emerges. These interfacial interactions may have consequences for device stability and performance beyond CH3NH3PbI3 model systems and merit close attention from the perovskite research community.

Controlled solution-based fabrication of perovskite thin films directly on conductive substrate

Organometallic perovskites are one of the most investigated materials for high-efficiency thin-film devices to convert solar energy and supply energy. In particular, methylammonium lead iodide has been used to realize thin-film perovskite solar cells, achieving an efficiency higher than 20%. Different fabrication procedures based on the spin-coating technique have been proposed, which do not ensure homogenous morphologies.In this work, we present a scalable process to fabricate methylammonium lead iodide thin films directly on conductive substrates, consisting of electrodeposition and two subsequent chemical conversions. A thorough investigation of the morphological, structural and compositional properties of the layer is performed after each fabrication step. It is demonstrated that this method allows fine control of the thickness of the layer by tuning the cell parameters during the electrodeposition step. X-ray diffraction patterns and energy-dispersive X-ray analysis indicate the achievement of high-purity methylammonium lead iodide layers. Micro-Raman analyses were used to demonstrate the formation of methylammonium lead iodide. Finally, ultraviolet-visible absorption spectra were acquired to determine the optical band edge of the layer (̴ 1.56 eV) and the absorbance of methylammonium lead iodide as a function of the film thickness. As expected, the material exploits excellent optical properties, achieving an absorption ≥ 99.9% in the entire visible range for a layer thickness of 1.3 µm. The results presented here pave the way for the application of cost-friendly solution-based processes to fabricate high-quality perovskite solar cells.

Benzodithiophene-Based Spacers for Layered and Quasi-Layered Lead Halide Perovskite Solar Cells.

Incorporating extended pi‐conjugated organic cations in layered lead halide perovskites is a recent trend promising to merge the fields of organic semiconductors and lead halide perovskites. Herein, we integrate benzodithiophene (BDT) into Ruddlesden–Popper (RP) layered and quasi‐layered lead iodide thin films (with methylammonium, MA) of the form (BDT)2MAn−1PbnI3n+1. The importance of tuning the ligand chemical structure is shown as an alkyl chain length of at least six carbon atoms is required to form a photoactive RP (n=1) phase. With N=20 or 100, as prepared in the precursor solution following the formula (BDT)2MAN−1PbNI3N+1, the performance and stability of devices surpassed those with phenylethylammonium (PEA). For N=100, the BDT cation gave a power conversion efficiency of up to 14.7 % vs. 13.7 % with PEA. Transient photocurrent, UV photoelectron spectroscopy, and Fourier transform infrared spectroscopy point to improved charge transport in the device active layer and additional electronic states close to the valence band, suggesting the formation of a Lewis adduct between the BDT and surface iodide vacancies.