PbI2 Films Research Articles

Optimization of the MA/FA Ratio in the Structure of Absorber Layers Based on MA(1-x)FAxPbI3 Perovskites for Stable and Efficient Solar Cells

Herein, we investigate the mixture of methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3). The perovskite films were coated onto fluorine-doped tin oxide (FTO) glass substrates using a spin-coating method, with the spin-coater set at 4,000 rpm for 20 s. We studied the influence of incorporation in MA(1-x) FA(x)PbI3 films. The crystal structure of the perovskite films was characterized using X-ray diffraction (XRD). Optical properties were assessed using UV-Vis spectroscopy with a spectrophotometer, and photoluminescence (PL) was characterized using a He-Cd Si-CCD laser source and a Hamamatsu detector. Images depicting the characteristic morphology of the films were captured with a scanning electron microscope (SEM). Our measurements reveal that the crystallinity of the MAPbI3 thin film improved with the incorporation of FAPbI3. In the case of 70% FAPbI3, the morphology of the MA30FA70PbI3 mixture also improved, exhibiting a rough surface with pores. The optical properties were enhanced, and the mixed thin film demonstrated better stability compared to pure MAPbI3 and FAPbI3 thin films. These film characteristics indicate that the mixtures are particularly suitable for use in photovoltaic applications.

Using Low‐Cost Additive to Improve Perovskite Crystallization for Efficient Solar Cells

In the two‐step sequential deposition, the reaction of PbI2 and organic halides for conversion to perovskite is crucial for achieving high‐performance perovskite solar cells, which heavily depends on the morphology of the PbI2 film. The highly structured and dense PbI2 film is not conducive to the diffusion of organic halides in the planar heterojunction. To solve this problem, acetone is introduced into the PbI2 precursor as the regulator of the film morphology. The addition of acetone weakens the interaction between dimethyl sulfoxide and PbI2, leading to the appropriate morphology of PbI2 film. Therefore, PbI2 can react with organic halides and convert to perovskite more efficiently. The high‐quality FA1−xMAxPbI3 perovskite film with reduced defect density is achieved. The device delivers an outstanding power conversion efficiency enhancement with a low‐cost additive.

Natural drying yields efficient perovskite solar cells

Natural drying (without spin coating or assistance of antisolvent, gas, or vacuum) might be the least-cost drying method to make perovskite films for solar cells. However, perovskite films made without quenching generally show undesirable morphology and low photovoltaic performance. In this work, we developed a high-throughput screening method to find the perovskite compositions that can form high-quality film without quenching process. We found that composition-graded perovskite films can be easily made by using a solution interdiffusion process, which produces perovskite films with continuously changing composition and the resulting film morphology, enabling fast screening of desired perovskite compositions. Several perovskites (CsPbI2Br, FA0.4Cs0.6PbI3, FA0.2MA0.8PbI3, and FA0.95Cs0.05PbI3) which can form high-quality films without quenching were successfully found. FA0.95Cs0.05PbI3 films yield a PCE of 23.28%, which is the highest PCE for perovskite solar cells with naturally-dried perovskite layer.

Characterization of perovskite films prepared with different PbI2 deposition rates

The perovskite (CH3NH3PbI3) films prepared using PbI2 film deposited at controlled deposition rates were evaluated. In this process, PbI2 films were deposited by vacuum evaporation process and then converted into CH3NH3PbI3 films by annealing in methylammonium iodide vapor. The grain size of CH3NH3PbI3 films were successfully tuned from 90 to 125 nm by controlling the PbI2 deposition rate from 0.025 to 0.4 nm s−1. Furthermore, by using the controlled CH3NH3PbI3 film as the light harvesting layer, inverted planar perovskite solar cells were fabricated, and the improvement in power conversion efficiency was confirmed.

Two-step perovskite crystallization assisted with F-containing additives enables over 23% efficiency in perovskite solar cells

The two-step preparation of perovskite solar cells (PSCs) has attracted the interest of many researchers because this method can better control the nucleation and growth of perovskite thin films. This work proposes a crystallization management strategy to improve the quality of active layer with the assistant of F-containing additives with low boiling point. Hexafluorobenzene (HFB) is introduced into lead iodide (PbI2) precursor solutions, and HFB aggregation forms due to its low surface energies. As the result, nano porous PbI2 films are obtained after the evaporation of HFB during annealing process, leading to pinhole free perovskite films with large grain size and low defect states. As the consequence, the best power conversion efficiency (PCE) of 23.01% is obtained for the PSCs with HFB assisted perovskite films. Meanwhile HFB modified unencapsulated device maintains 88% of its initial PCE after aging for over 1440 h at 25 °C and 30% relative humidity in ambient conditions.

Structural, Optical, and Chemical Characteristics of High‐Quality PbI2 Thin Films via Chemical Solution Deposition with Thermal Annealing

Herein, a synthesis method for obtaining lead iodide through chemical bath deposition, a type of chemical solution deposition without the use of organic solvents, is presented; the process consists of three stages. In the first stage, tin hydroxide films are produced which act as a seed layer allowing for a better adherence of the film that is to be deposited. The second stage consists of the synthesis of the lead iodide films. Finally, annealing is done (100, 150, and 200 °C) with the purpose of promoting the polytypic transitions of the material and to study the effect of these structures on the properties of the material. Assessment of the structural and optical properties of the films is done by means of scanning electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, optical spectroscopy, Raman spectroscopy, and transmission electron microscopy, where a direct influence of the annealing in the induction of polytypes is observed. The resultant PbI2 films have a large‐area coverage across the substrate, wide‐bandgap (2.38 eV), and a structure that belongs to the 2H and 4H polytypes. Hence, it is suggested from these results that the chemically deposited PbI2 thin films are promising candidates for large‐area optoelectronic applications.

Valine-Modified PbI2 for the Growth of Pinhole-Free Lead Halide Perovskite Thin Films by Vapor–Solid Reaction

In recent years, the efficiency of perovskite solar cells (PSCs) has rapidly increased, making PSCs commercially viable and able to compete with silicon solar cells. However, the technology for large-scale production of perovskite thin films still poses a significant challenge to the commercialization of PSCs. This is mainly because the thickness of the perovskite light-absorbing film is less than 1 μm and easily produces pinholes during large-scale production, causing serious charge recombination and reducing the efficiency of the device. As a result, the efficiencies of most large-scale PSC modules are significantly lower than those of small-size devices. Vapor-reaction technology is considered a crucial method to enhance the uniformity of perovskite thin films. However, current techniques are not yet advanced enough to deposit pinhole-free perovskite thin films on a large scale. One major challenge is that lead iodide (PbI2) precursor thin films have a tendency to develop a sheet-like structure with a loose morphology, which contributes to the formation of pinholes in the perovskite film. Additionally, uncoordinated Pb2+ could be easily generated in the PbI2 film during thermal evaporation, causing an increase in trap-states in the perovskite crystal. To address this, we developed a valine treatment process in this study to change the preferred orientation of PbI2 from a sheet-like to a particle-like aggregate and obtained compact PbI2 thin films. Meanwhile, this valine vapor treatment can significantly reduce the uncoordinated Pb2+ in PbI2 films. Based on this, we successfully prepared pinhole-free perovskite thin films using a vapor–solid reaction. Perovskite films based on these valine-treated PbI2 exhibited a reduced trap density and significantly improved PSCs’ performance, with a maximum efficiency exceeding 21%. Large-area PSC mini-modules (5 cm × 5 cm) produced from the perovskite film (30 cm × 30 cm) showed uniformly distributed power conversion efficiency values exceeding 17%, with the highest value reaching 18.78%. We believe that this facile valine vapor treatment holds great potential for the large-scale production of stable and high-efficiency PSC modules.

Enhanced photostability of multication lead halide perovskites through the use of azaadamantane-based modifiers

Complex lead halides incorporating simultaneously a few univalent cations such as cesium (Cs+), methylammonium (MA+) and formamidinium (FA+) represent highly promising materials for perovskite solar cells, delivering record-high power conversion efficiencies. Unfortunately, these materials demonstrate insufficient stability, which limits their practical implementation; therefore, improving the photochemical and thermal stability of multication lead halide perovskites is an urgent task that needs to be addressed to achieve sufficiently long operational lifetimes of perovskite solar cells to facilitate their commercial real-life applications. Herein, we present a comparative study of two azaadamantane-based molecular modifiers as stabilizing additives for Cs0.10MA0.15FA0.75PbI3 and Cs0.12FA0.88PbI3 perovskite formulations. An iodide of N-methylated 1,3,5,7-tetraazaadamantane known as urotropine (MAdI) showed a modest stabilizing effect in Cs0.10MA0.15FA0.75PbI3, though it stabilized effectively Cs0.12FA0.88PbI3. On the contrary, 4,6,10-trihydroxy-3,5,7-trimethyl-1,4,6,10-tetraazaadamantane hydrochloride (NAdCl) strongly enhanced the stability of both lead halide perovskites and prevented the formation of metallic lead Pb0 as a photolysis product. Different stabilizing effects caused by MAdI and NAdCl were related to their different localization within the perovskite films, as evidenced by nanoscale infrared (IR) s-SNOM microscopy. Density functional theory (DFT) calculations confirmed the enhanced stability granted by NAdCl by demonstrating a much higher adsorption energy towards undercoordinated lead atoms on the surface of the perovskite. Finally, the optimal molecular modification of Cs0.10MA0.15FA0.75PbI3 and Cs0.12FA0.88PbI3 films with azaadamantanes made them completely tolerant to light and heat within at least 5000 and 16 000 h of aging, thus setting new perovskite stability records and presenting the newly-developed material formulations as highly promising absorbers for efficient and stable perovskite solar cells.

Multifunctional Metal-organic Frameworks Capsules Modulate Reactivity of Lead Iodide toward Efficient Perovskite Solar Cells with Ultraviolet Resistance.

The two-step sequential deposition process is demonstrated as a reliable technology for the fabrication of efficient perovskite solar cells (PVSCs). However, the complete conversion of dense PbI2 to perovskite in planar PVSCs is tough without mesoporous titanium dioxide as support. Herein, multifunctional capsules consisting of zeolitic imidazolate framework-8 (ZIF-8) encapsulant and formamidinium iodide (FAI) are introduced between tin oxide (SnO2 ) and lead iodide (PbI2 ) layer. Intriguingly, the capsule dopant interlayer benefits the formation of porous PbI2 film due to the porous nanostructure of ZIF-8 which is favorable for the subsequent intercalation reaction. Furthermore, the constituent of the perovskite precursor in ZIF-8 pores can convert into the crystal nuclei of perovskite by reacting with PbI2 first, thereby promoting further perovskite crystallization. Significantly, the incorporation of ZIF-8 can enhance the resistance of perovskite against ultraviolet (UV) illumination due to down-conversion effect. Consequently, the modified device achieves a champion power conversion efficiency (PCE) of 24.08% and displays enhanced UV stability, which can sustain 83% of its original PCE under 365nm UV illumination for 300h. Moreover, the unencapsulated device maintains 90% of initial PCE after 1500h storage in dark ambient conditions with a relative humidity range of 50∼70%. This article is protected by copyright. All rights reserved.

Intermediate Phase Engineering with 2,2-Azodi(2-Methylbutyronitrile)for Efficient and Stable Perovskite Solar Cells.

Sequential deposition has been widely employed to modulate the crystallization of perovskite solar cells because it can avoid the formation of nucleation centers and even initial crystallization in the precursor solution. However, challenges remain in overcoming the incomplete and random transformation of PbI2 films with organic ammonium salts. Herein, a unique intermediate phase engineering strategy has been developed by simultaneously introducing 2,2-azodi(2-methylbutyronitrile) (AMBN) to both PbI2 and ammonium salt solutions to regulate perovskite crystallization. AMBN not only coordinates with PbI2 to form a favorably mesoporous PbI2 film due to the coordination between Pb2+ and the cyano group (C≡N), but also suppresses the vigorous activity of FA+ ions by interacting with FAI, leading to the full PbI2 transformation with the preferred orientation. Therefore, perovskites with favorable facet orientations are obtained, and the defects are largely suppressed owing to the passivation of uncoordinated Pb2+ and FA+ . As a result, a champion power conversion efficiency over 25% with a stabilized efficiency of 24.8% is achieved. Moreover, the device exhibits an improved operational stability, retaining 96% of initial power conversion efficiency under 1000 h continuous white-light illumination with an intensity of 100mW cm-2 at ≈55°C in N2 atmosphere.

Effects of Lead Iodide Crystallization on Photovoltaic Performance of Perovskite Solar Cells by the Vapor-Solid Reaction Method.

The vapor-solid reaction method (VRM) is one of the promising techniques to prepare high-performance perovskite solar cells. Herein, PbI2 precursor films were prepared by vacuum evaporation. It was found that the PbI2 precursor films exhibit high crystallinity and orderly morphology at the substrate temperature of 110 °C. On this basis, the precursor films were prepared by VRM to obtain high-quality perovskite films and the power conversion efficiency (PCE) of perovskite solar cells (PSCs) devices reached 17.1%. In contrast, the PbI2 film precursor was prepared on the substrate without being heated and the PCE of the final PSCs devices was only 13.04%.

Crystal Growth Regulation of α‐FAPbI3 Perovskite Films for High‐Efficiency Solar Cells with Long‐Term Stability

AbstractThe two‐step sequentially deposition strategy has been widely used to produce high‐performance FAPbI3‐based solar cells. However, due to the rapid reaction between PbI2 and FAI, a dense perovskite film forms on top of the PbI2 layer immediately and blocks the FAI diffusion into the bottom of the PbI2 film for a complete reaction, which results in a low‐efficiency and limited reproducibility of perovskite solar cells (PSCs). Here, high‐quality α‐FAPbI3 perovskite films by crystal growth regulation with 4‐fluorobenzamide additives is fabricated. The additives can interact with FAI to suppress the fast reaction between the FAI and PbI2 and effectively passivate the under‐coordinated Pb2+ or I‐ defects. As a result, α‐FAPbI3 perovskite films with low trap density and large grain size are prepared. The modified PSCs present a high‐power conversion efficiency of 24.08%, maintaining 90% of their initial efficiency after 1400 h in high humidity. This study provides an efficient strategy of synergistic crystallization and passivation to form high‐quality α‐FAPbI3 films for high‐performance PSCs.

Suppressed Voltage Deficit and Degradation of Perovskite Solar Cells by Regulating the Mineralization of Lead Iodide.

Both the uncoordinated Pb2+ and excess PbI2 in perovskite film will create defects and perturb carrier collection, thus leading to the open-circuit voltage (VOC ) loss and inducing rapid performance degradation of perovskite solar cells (PSCs). Herein, an additive of 3-aminothiophene-2-carboxamide (3-AzTca) that contains amide and amino and features a large molecular size is introduced to improve the quality of perovskite film. The interplay of size effect and adequate bonding strength between 3-AzTca and uncoordinated Pb2+ regulates the mineralization of PbI2 and generates low-dimensional PbI2 phase, thereby boosting the crystallization of perovskite. The decreased defect states result in suppressed nonradiative recombination and reduced VOC loss. The power conversion efficiency (PCE) of modified PSC is improved to 22.79% with a high VOC of 1.22V. Moreover, the decomposition of PbI2 and perovskite films is also retarded, yielding enhanced device stability. This study provides an effective method to minimize the concentration of uncoordinated Pb2+ and improve the PCE and stability of PSCs.

Zwitterionic Ionic Liquid as Additive for High‐Performance FAPbI3 Perovskite Solar Cells with Negligible Hysteresis

Imidazolium‐based ionic liquids have been established as promising candidates for additives in perovskite solar cells. Herein, 1‐(1‐ethyl‐3‐imidazolium)propane‐3‐sulfonate (EIMS) zwitterionic ionic liquid is studied as additive to enhance the performance of the FAPbI3 perovskite solar cells. The addition of EIMS leads to the generation of porous PbI2 film, which promotes the reaction of PbI2 with FAI solution and thus improves the quality of FAPbI3 perovskite film during the two‐step sequential spin coating. It is revealed that EIMS accelerates the extraction of electron, inhibits the formation of pinhole, and reduces the defect density, which leads to an increased efficiency of 22.1%. The hysteresis index is also reduced from 0.142 to 0.009, which indicates the addition of EIMS can realize a negligible hysteresis behavior in the solar cells. Moreover, devices prepared with EIMS retain above 96% of the original power conversion efficiency (PCE) value after being stored in dry air for 45 days. All these results demonstrate that the imidazolium‐based zwitterionic ionic liquid additive strategy is a promising way for high‐performance perovskite solar cells.

Resolving Complex Photoconductivity of Perovskite and Organic Semiconductor Films Using Phase-Sensitive Microwave Interferometry

Complex transient photoconductivity (Δσ) contains rich fingerprints of charge recombination dynamics in photoactive films. However, a direct measure of both real (Δσ′) and imaginary (Δσ″) components has proven difficult using conventional cavity-based time-resolved microwave conductivity approaches. Here, we present a novel approach to resolve Δσ′ and Δσ″ parts of Δσ by using a nonresonant coplanar transmission line and a microwave interferometric detection scheme. The use of a phase-sensitive microwave interferometer greatly increases the measurement sensitivity and eliminates the requirement of a resonant cavity. This broadband detection scheme allows for direct measurement of Δσ. The relationship between the experimental phase shift and Δσ′ and Δσ″ components is decoded through an in situ electron spin resonance (ESR) measurement. ESR line shape analysis is used to confirm the assignment of the transients to the Δσ′ and Δσ″ components. We demonstrate the utility of this technique on thin films of poly(3-hexylthiophene): [6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) and perovskite MA0.85FA0.15PbI3 films on glass.

Crossover from strong to weak exciton confinement in thickness-controlled epitaxial PbI2 thin films

Spatially confined excitons undergo two distinct quantization effects depending on the confinement length, the enhanced binding energy under strong confinement, and the center-of-mass quantization under weak one. However, the transition between them has not been experimentally identified in two-dimensional (2D) materials due to the lack of thin films satisfying large-scale uniformity and atomic-level flatness in a wide thickness range. Here, we reveal the crossover in high-quality epitaxial thin films of a 2D semiconductor PbI2 grown by molecular beam epitaxy. The absorption spectra exhibit oscillatory structures manifesting the exciton center-of-mass quantization, and the quantization energies show an additional blue shift associated with the strong confinement effect below five-layer thickness (35 Å). The precise control of exciton quantum states will lead to the further development of optoelectronic functionalities of 2D materials.

Solvent-vapor assisted conversion process for hybrid perovskites coupling thermal evaporation and slot-die coating

The present paper reports for the first time on the fabrication of highly efficient solar cells based on a perovskite layer produced by a sequential deposition method coupling thermal evaporation and slot-die coating. Thermally evaporated PbI2 films were converted into perovskite by slot-die coating a solution containing the organic MA and FA cations. Careful optimization of the coating parameters resulted in uniform double-cation perovskite films with an optical band gap of 1.64 eV. This work highlights the need for a DMSO (Dimethyl Sulfoxide) vapor treatment performed on the evaporated PbI2 film to obtain high-quality perovskite layers after the conversion step. Thus, a simple and short pre-treatment has been designed to induce the formation of porous PbI2(DMSO)x complex phases enabling a fast and complete perovskite conversion process. Thereby, the champion device delivers a power conversion efficiency of 19.8% with enhanced short-circuit current, open-circuit voltage, and fill factor compared to control devices without treatment.

Metal-organic framework nanocrystals enabled efficient and durable two-step perovskite photovoltaics

The complete conversion of PbI2 into high-grade perovskite material with less trap-state density and long-term durability still remains challenging for two-step perovskite photovoltaics. In this work, nanoscale UiO-66, as a classical Zr-MOFs with respectable stability and high specific surface area, has been innovatively introduced into the PbI2 layer to facilitate the conversion process by morphology tuning and also ultimately improve the durability of the resultant perovskite photovoltaic devices. The added UiO-66 has impressively changed the morphology of PbI2 film from a pristine compact one into a porous structure, providing better contact with organic salt during second-step deposition, consequently benefitting the thorough and uniform perovskite conversion process. Importantly, the addition of UiO-66 has effectively suppressed the halide vacancies formation due to the increased halide vacancy formation energy at the interface, as evidenced by density functional theory (DFT) calculation, leading to noticeably enhanced charge transport property and greatly improved optoelectronic property. Furthermore, the crystal orbital Hamilton population (COHP) calculation result directly proves that the incorporation of UiO-66 has enhanced the stability of perovskite materials by intensifying the bonding interaction of Pb-I bonds at the interface. Consequently, the UiO-66-assisted devices deliver a champion power conversion efficiency of 23.05% with noticeably improved stability, significantly outperforming the pristine devices. This work demonstrates the brightening potential of MOFs to assist the consequential manufacture of highly efficient and stable perovskite solar cells for upscaling process and deployment in the near future.

Temperature−Dependent Raman Scattering Investigation on vdW Epitaxial PbI2/CrOCl Heterostructure

Van der Waals (vdW) epitaxial growth provides an efficient strategy to prepare heterostructures with atomically and electronically sharp interfaces. Herein, PbI2 was in situ thermally deposited onto exfoliated thin−layered CrOCl nanoflakes in high vacuum to fabricate vdW PbI2/CrOCl heterostructures. Optical microscopy, atomic force microscopy, X−ray diffraction, and temperature−dependent Raman spectroscopy were used to investigate the structural properties and phonon behaviors of the heterostructures. The morphology of PbI2 films on the CrOCl substrate obviously depended on the substrate temperature, changing from hemispherical granules to 2D nanoflakes with flat top surfaces. In addition, anomalous blueshift of the Ag1 and Au2 modes as the temperature increased in PbI2/CrOCl heterostructure was observed for the first time. Our results provide a novel material platform for the vdW heterostructure and a possible method for optimizing heterostructure growth behaviors.

Bidirectional Modification of Buried Interface Reduces Energy Loss for Planar Perovskite Solar Cells with Efficiency >23%

Planar n–i–p perovskite solar cells (PSCs) based on a SnO2 electron transport layer dominate the certified high‐efficiency devices. However, the defects located at the SnO2/perovskite interface (i.e., buried interface) or inside the perovskite films impede the further improvement of power conversion efficiency (PCE). Herein, nickel acetate (NiAc2) is introduced on buried interface as a bidirectional modifier to improve electron extraction of SnO2 and the crystal growth of perovskite for the first time. First, NiAc2 is chemically anchored on SnO2 to passivate oxygen vacancies, increase conductivity, and optimize the energy level alignment of the buried interface. Second, the porous morphology of PbI2 film deposited on NiAc2‐modified SnO2 endows more sufficient permeation and reaction of organic amine salts (formamidinium iodide [FAI] and methylammonium iodide [MAI]), forming high‐quality perovskite film with reduced PbI2 residues. Meanwhile, NiI2 and MAAc/FAAc may be produced via in situ reaction between NiAc2 and organic amine salts, which serve as interface modifier and crystallization regulator to further reduce defects located at the buried interface or inside the perovskite film, respectively. Consequently, an improved PCE of 23.02% for SnO2‐based PSCs with an ultrahigh open‐circuit voltage of 1.17 V is obtained. In addition, long‐term storage and light stability of the optimized PSCs are improved.