Hybrid Perovskite Films Research Articles

All green solvent engineering of organic–inorganic hybrid perovskite layer for high-performance solar cells

Solvent engineering technology is an important approach for fabrication of high quality perovskite films, and thereby high-performance perovskite solar cells (PSCs). However, it is strongly dependent on the usage of toxic solvents, including the host solvent and anti-solvent to prepare perovskite solutions and control the crystallization of perovskite films, respectively. The usage of toxic solvents poses a serious threat to the health of manufactures and the safety of environment. Therefore, the solvent toxicity has become a burning issue before the forthcoming commercialization of PSCs. Here, we report an all green solvent engineering to prepare high quality (FAPbI3)1-x(MAPbBr3)x (x = 0, 0.05) films, where a green Lewis base solvent of triethyl phosphate (TEP) is used to prepare (FAPbI3)1-x(MAPbBr3)x/TEP solution, and a non-polar solvent of dibutyl ether (DEE) is used as anti-solvent. In this approach, a new intermediate phase film is formed after spin-coating the (FAPbI3)1-x(MAPbBr3)x/TEP solution on substrates, followed by dropping of DEE during spinning, and subsequently convert into perovskite films by annealing. Finally, it enables a PCE of 18.65% (x = 0) and 20.13% (x = 0.05) basing on a typical device configuration of FTO/SnO2/Perovskite/Spiro-OMeTAD/Au, which is the highest value for the all green solvent processed PSCs. More importantly, the all green solvent engineering approach can also be used to prepare another typical perovskite of MAPbI3 films for efficient solar cells. Therefore, this work not only provides an universial approach to prepare typical organic–inorganic hybrid perovskite films, but also addresses the solvent toxicity during the production of perovskite films. The green solvent engineering may pave the way for future industrial scale production of PSCs under environment friendly conditions.

Grain‐Boundaries‐Engineering via Laser Manufactured La‐Doped BaSnO3 Nanocrystals with Tailored Surface States Enabling Perovskite Solar Cells with Efficiency of 23.74%

AbstractGrain boundaries (GBs) engineering of hybrid perovskite films is of significance for accessing high performance perovskite solar cells (PSCs), owing to the abundant defect states existed therein originating from the low temperature film processing. Nanocrystals embedding at GBs has shown profound advantages in carrier dynamics modulation, while the surface defects on nanocrystals in turn lead usually to the trapping of carriers at GBs. The authors herein demonstrate the efficient GBs engineering via laser generated nanocrystals with tailored surface states for improved carriers dynamics and environmental stability of PSCs. The embedding of La doped BaSnO3 (LBSO) nanocrystals with bare surfaces in perovskite provides an additional channel to facilitate the effective carrier extraction and reduce the carrier recombination, leading to a maximum power conversion efficiency (PCE) of 21.11% with negligible hysteresis for the mixed‐cation PSCs. To clarify the influence of surface defect states of the laser generated nanocrystals on the performance of PSCs, 1H,1H‐perfluorooctylamine is grafted on LBSO nanocrystals during the laser irradiation, resulting in improved champion PCE up to 21.65% and pronounced environmental stability. The universal embedding of the LBSO nanocrystals with tailored surface states in different perovskite by fabricating FAPbI3 PSCs with a champion PCE of 23.74% is further demonstrated.

Giant magneto field effect in up-conversion amplified spontaneous emission via spatially extended states in organic-inorganic hybrid perovskites

Up-conversion lasing actions are normally difficult to realize in light-emitting materials due to small multi-photon absorption cross section and fast dephasing of excited states during multi-photon excitation. This paper reports an easily accessible up-conversion amplified spontaneous emission (ASE) in organic-inorganic hybrid perovskites (MAPbBr₃) films by optically exciting broad gap states with sub-bandgap laser excitation. The broad absorption was optimized by adjusting the grain sizes in the MAPbBr₃ films. At low sub-bandgap pumping intensities, directly exciting the gap states leads to 2-photon, 3-photon, and 4-photon up-conversion spontaneous emission, revealing a large optical cross section of multi-photon excitation occurring in such hybrid perovskite films. At moderate pumping intensity (1.19 mJ/cm²) of 700 nm laser excitation, a significant spectral narrowing phenomenon was observed with the full width at half maximum (FWHM) decreasing from 18 nm to 4 nm at the peak wavelength of 550 nm, simultaneously with a nonlinear increase on spectral peak intensity, showing an up-conversion ASE realized at low threshold pumping fluence. More interestingly, the up-conversion ASE demonstrated a giant magnetic field effect, leading to a magneto-ASE reaching 120%. In contrast, the up-conversion photoluminescence (PL) showed a negligible magnetic field effect (< 1%). This observation provides an evidence to indicate that the light-emitting states responsible for up-conversion ASE are essentially formed as spatially extended states. The angular dependent spectrum results further verify the existence of spatially extended states which are polarized to develop coherent in-phase interaction. Clearly, using broad gap states with spatially extended light-emitting states presents a new approach to develop up-conversion ASE in organic-inorganic hybrid perovskites.

Strain-free hybrid perovskite films based on a molecular buffer interface for efficient solar cells

We demonstrated that a multi-functional molecular buffer interface could release the tensile stress of films and attain an efficiency of 22.14%.

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.

Temperature-induced strain management in MAPbI3-xClx hybrid perovskite films

Strain management in organometal halide perovskite materials is one of the important strategies to improve charge transport, ion migration and crystallinity of these materials. This work reveals the internal relationship between the perovskite processing temperature and the optoelectronics properties of MAPbI3-xClx perovskite films. The MAPbI3-xClx films have fabricated on various temperature ranges from 80 °C to 180 °C using hot casting technique and explored their structural, surface morphological, optical, and electrical properties. Lattice strain has observed and found to be reduced on increasing the processing temperature and at a particular processing temperature, perovskite films exhibit free of strain. Urbach energy and trap levels have significantly reduced at higher temperatures which confirm the superior optoelectronic feature of perovskite semiconductors. Further, Schottky diodes with device configuration FTO/MAPbI3-xClx/Ag were fabricated and ideality factor, barrier height and reverse saturation current density have been determined in order to explore the charge transport properties of the films.

Hybrid Perovskite Degradation from an Optical Perspective: A Spectroscopic Ellipsometry Study from the Deep Ultraviolet to the Middle Infrared

AbstractA quantitative analysis of the thermally induced degradation of various device‐relevant multi‐cation hybrid perovskite films is performed using spectroscopic ellipsometry, for temperatures between 80 and 120 °C. The studied compositions are a triple cation perovskite Cs0.05(MA0.17FA0.83)0.95Pb(Br0.17I0.83)3, a Rb‐containing variant Rb0.05Cs0.05(MA0.17FA0.83)0.90Pb(Br0.17I0.83)3, and a methylammonium‐free Rb0.05Cs0.10FA0.85PbI3 composition. A very wide combined spectral range of 200 nm to 25 μm is covered by combining the data from two separate instruments. The relative changes in organic cation concentrations are quantified from the middle infrared molecular absorption bands, leveraging the use of point‐by‐point fitting for increased sensitivity. Additionally, the formation of PbI2 and non‐perovskite δ‐CsPbI3 phases is evidenced from Bruggemann effective medium fits to the visible and ultraviolet complex refractive indices. Methylammonium is almost completely depleted from the relevant compositions within 100 to 285 min of thermal annealing. The MA‐free perovskite degrades faster at intermediate temperatures, which is attributed to phase instability due to the formation of δ‐CsPbI3 in addition to PbI2.

Luminescence and Structural Characteristics of Lead Halide Perovskite Films Deposited In Situ by a Versatile Multisource Aerosol Assisted Chemical Vapor Deposition (AACVD) Method

AbstractMost solution‐based synthetic routes for perovskite film deposition take place under highly controlled atmospheres and cannot be easily escalated for mass production. Here, the control in situ over luminescent, morphological, and structural characteristics of hybrid and inorganic perovskite films deposited on top of hot glass substrates by multisource AACVD is reported. By simply adjusting the methylammonium chloride (MACl) to lead bromide (PbBr2) molar ratio in the N‐N dimethylformamide (DMF) precursor solution, a gradual transformation from orthorhombic PbBr2 to cubic MAPbBr2Cl hybrid perovskite films is observed, and the generation of highly luminescent MAPbBr3−xClx nanoparticulate domains in a PbBr2 matrix is easily achieved when a specific ratio is used. Additionally, the crystallization of the films is performed in situ modified by the implementation of an antisolvent stream source during deposition which in turn yields films with superior luminescence properties. Finally, an NIR emitting CsPbBr2Cl:Yb inorganic perovskite film is deposited to probe the versatility of the multisource AACVD system, by the simultaneous deposition of separated PbBr2/YbCl3·6H2O/DMF and CsCl/methanol solutions. The high degree of control on the final properties of the films indicates that AACVD could be a breakthrough for low‐cost, scalable, high‐throughput metal‐halide perovskite film deposition.

Growth and Degradation Kinetics of Organic–Inorganic Hybrid Perovskite Films Determined by In Situ Grazing‐Incidence X‐Ray Scattering Techniques

Organic-inorganic halide perovskite (OIHP) solar cells hold a great promise for commercial breakthrough since their power conversion efficiency has been pushed beyond the mark of 25%, making them capable of competing with traditional crystalline silicon solar cells. The key to achieve efficient and stable perovskite solar cells is inherently related to the film morphology. The understanding of the kinetic processes of film formation and degradation opens up possibilities to tailor the film morphology via the regulation of precursor and processing parameters. In situ grazing-incidence X-ray scattering (GIXS) techniques allow for tracking the morphology evolution of thin films at different length scales and with high temporal resolution. In this review, the selected examples for application of in situ grazing-incidence wide-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques to the growth and stability of OIHPs are summarized after a brief introduction to both techniques, highlighting particularly the morphological evolution of perovskite films over time. Then the correlated mathematical models are reviewed to give a toolbox for analyzing the mechanisms of film formation and degradation. Thus, an overview on the in situ GIXS methods is linked to the research of OIHP kinetics.

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.

PTB7 as additive in Anti-solvent to enhance perovskite film surface crystallinity for solar cells with efficiency over 21%

Low temperature solution-processed organic–inorganic hybrid perovskite films inevitably encounter defects especially on surface and at grain boundaries. Anti-solvent washing has been widely practiced in fabricating uniform and dense perovskite films, but unfortunately it could cause even more defects on surface. Herein, polymer PTB7 was introduced into anti-solvent as an additive to significantly enhance perovskite film surface crystallinity with fewer defects, as evidenced by depth sensitive grazing incidence X-ray diffraction. Such improvements could be attributed to the Lewis base coordination bonds formed between PTB7 and Pb atoms in the surface region which slow down crystallization dynamics for enhanced perovskite surface crystallinity and effectively passivate defects at the perovskite grain boundary and film surface. With PTB7 in anti-solvent, the fabricated perovskite solar cells (PSCs) achieved a champion power conversion efficiency (PCE) of 21.41% from that of 19.79% for the control devices without PTB7. In addition, the hydrophobic alkyl chains in PTB7 were shown to improve film surface hydrophobicity with enhanced environmental stability of both the fabricated films and PSCs. Thus, addition of PTB7 in anti-solvent is proven to effectively improve the performance and stability of PSCs which could be easily applied in the fabrication of PSCs and other perovskite-based optoelectronics.

Growth-Controlled Broad Emission in Phase-Pure Two-Dimensional Hybrid Perovskite Films

Author(s): Kennard, Rhiannon M; Dahlman, Clayton J; Chung, Juil; Cotts, Benjamin L; Mikhailovsky, Alexander A; Mao, Lingling; DeCrescent, Ryan A; Stone, Kevin H; Venkatesan, Naveen R; Mohtashami, Yahya; Assadi, Sepanta; Salleo, Alberto; Schuller, Jon A; Seshadri, Ram; Chabinyc, Michael L

Over 23% power conversion efficiency of planar perovskite solar cells via bulk heterojunction design

The planar organic–inorganic hybrid perovskite solar cells (PSCs) have in recent years had remarkable success due to their superior optoelectronic performance. However, their power conversion efficiency is significantly inferior compared to mesoporous structure PSCs. Unlike most other advances focusing on non-perovskite materials to improve device performance, herein a multifunctional double perovskite material, Cs2PtI6, is proposed as the grain boundary modifier of organic–inorganic hybrid perovskite films. Results show three main benefits of introducing Cs2PtI6 into perovskite films: (1) it prompts growth of perovskite crystals, resulting in improved crystallinity and enlarged crystals; (2) it suppresses the trap assisted recombination at grain boundaries thanks to the formation of heterojunction and interface passivation; (3) it raises the efficiency of carrier collection and transport at grain boundaries owing to the high carrier mobility of Cs2PtI6. Consequently, a PCE of 23.56% is achieved. The unencapsulated devices show less than 10% degradation compared to the initial performance after storage in ambient (≈ 30%) humidity for 2000 h. This study outlines a simple yet effective strategy for boosting the performance of planar PSCs.

New Methylamine-Iodine-Mediated Solvent-Free Approach of Hybrid Perovskite Synthesis via the Redox Conversion of Metallic Lead Films.

The fast progress of lab-scale perovskite solar cells makes the problem of upscaling of perovskite thin-film deposition more and more acute; therefore, the development of new methods for perovskite deposition is highly desired. In this work, we proposed a new solution-free preparation approach for hybrid perovskite films based on the in situ generation of methylammonium iodide from methylamine and iodine vapors in the presence of an organic reducing agent conjugated with a redox process of metallic lead conversion with iodine vapor. At first, we demonstrated that either metallic lead or lead iodide powders can readily react with the solution of methylamine and iodine in the presence of isopropyl alcohol acting as a reducing agent, resulting in a phase-pure polycrystalline CH3NH3PbI3 perovskite. A possible mechanism of underlying chemical processes has been proposed. In order to convert the predeposited thin films of metallic lead to CH3NH3PbI3 perovskite, a protocol of sequential treatment by iodine and methylamine vapors was proposed. Finally, we revealed the optimal conditions of processing, which were proven to be facile and robust for the sake of the better control of perovskite grain morphology.

Aligning Transition Dipole Moment toward Light Amplification and Polarized Emission in Hybrid Perovskites

AbstractOrientational manipulation of transition dipole moment (TDM) plays an important role in controlling the polarization of excited states in light emission as well as lasing actions. The present work discovers vertically aligned TDMs in hybrid perovskite films through angle‐resolved photoluminescence (PL) measurements, which show enhanced emission through the film edge. With increasing excitation intensity, the edge emission induced by these vertically aligned TDMs becomes dominant and eventually leads to amplified spontaneous emission (ASE) through the edge view. Meanwhile, polarized emission of both PL and electroluminescence (EL) provides further evidence for vertically aligned TDMs. Surprisingly, the degree of polarization (DOP) through the film edge is increased when grain boundary defects are passivated through either stochiometric engineering or self‐passivation by mobile ions under working conditions. With increasing DOP, ASE threshold of the perovskite film is reduced owing to enhanced collective behaviors of light‐emitting states. This work presents a useful method to manipulate TDMs in organic–inorganic hybrid perovskites.

Grain Boundary Perfection Enabled by Pyridinic Nitrogen Doped Graphdiyne in Hybrid Perovskite

AbstractThe solution processing in hybrid perovskite films inevitably results in the formation of detrimental defects at grain boundaries (GBs) that deteriorate the optoelectronic properties and bring about severe hysteresis as well as operational instability. Here, an effective scenario to alleviate the imperfection issue at perovskite GBs via incorporating pyridinic nitrogen‐doped graphdiyne (N‐GDY) is proposed. Taking full advantage of periodic acetylenic linkages and introduced pyridinic N atoms, the deep‐level trap states like Pb–I antisite defects and under‐coordinated Pb atoms are considerably passivated, thus diminishing the undesired non‐radiative recombination. Additionally, the spatial confinement coupling with electrostatic repulsion effect originated from the intrinsic 2D structure of N‐GDY, has been identified to deal with the halide ion migration behavior. Such contributions are further theoretically evidenced with the charge density delocalization as well as the ion migration energy barrier elevation. The authors unprecedentedly verified the superiorities based on the flexible chemical‐tailorability of atomic crystal GDY materials toward polycrystalline perovskite related energy conversion devices.

Using pulsed mode scanning electron microscopy for cathodoluminescence studies on hybrid perovskite films

The use of pulsed mode scanning electron microscopy cathodoluminescence (CL) for both hyperspectral mapping and time-resolved measurements is found to be useful for the study of hybrid perovskite films, a class of ionic semiconductors that have been shown to be beam sensitive. A range of acquisition parameters is analysed, including beam current and beam mode (either continuous or pulsed operation), and their effect on the CL emission is discussed. Under optimized acquisition conditions, using a pulsed electron beam, the heterogeneity of the emission properties of hybrid perovskite films can be resolved via the acquisition of CL hyperspectral maps. These optimized parameters also enable the acquisition of time-resolved CL of polycrystalline films, showing significantly shorter lived charge carriers dynamics compared to the photoluminescence analogue, hinting at additional electron beam-specimen interactions to be further investigated. This work represents a promising step to investigate hybrid perovskite semiconductors at the nanoscale with CL.

Mobile Media Promotes Orientation of 2D/3D Hybrid Lead Halide Perovskite for Efficient Solar Cells.

2D/3D hybrid perovskites have shown great environmental stability for use as solar cells. Yet insulating organic cation spacers affect the formation of vertically oriented crystalline grains to hamper carrier transport. To date, the selection of cation spacers is limited because crystal growth kinetics should be strictly obeyed. Here, we adopted a mobile-media-assisted method to fabricate highly oriented 2D/3D hybrid perovskite films, wherein bulky molecules of (C(CH3)3NH3, TBA) are used as cation spacers. We found the compositional volatile salt promote the orientation of perovskite polycrystalline films with various compositions universally. And we obtained efficient solar cells with a photoelectric conversion efficiency (PCE) of 17.38% and a certified PCE of 19.30% based on 2D/3D hybrid perovskites an overall stoichiometry matching n = 10 and 20, respectively, which are one of the highest efficiencies among all 2D/3D hybrid perovskite based solar cells so far. These findings largely broaden the selection category of bulky cations and promote further exploration of more functional organic cations.

Rapid hybrid perovskite film crystallization from solution

Rapid hybrid perovskite film crystallization from solution

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

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