Mahui

Hexamethyl‐mono‐n‐butyl‐substituted zinc phthalocyanine (Me₆Bu‐ZnPc) is synthesized through a ring‐expansion method. The favored face‐on molecular alignment is observed for Me₆Bu‐ZnPc on the perovskite layer. Perovskite solar cells using Me₆Bu‐ZnPc as the dopant‐free hole‐transporting material achieve the highest power‐conversion efficiency (PCE) of 17.41% and retain over 90% of their initial PCE after 1400 h storage at 25 °C and with a relative humidity of 75%.

Efficient and stable hole‐transporting materials (HTMs) are necessary for perovskite solar cells (PSCs) with excellent efficiency and long‐term stability. Here, two A₃B‐type metal phthalocyanine (MPc) compounds are prepared as dopant‐free HTMs for conventional n‐i‐p structured PSCs. Mono‐n‐butyl‐substituted zinc phthalocyanine and hexamethyl‐mono‐n‐butyl‐substituted zinc phthalocyanine (Me₆Bu‐ZnPc) are synthesized through ring‐expansion method, and their exact structures are characterized using nuclear magnetic resonance and mass spectroscopy. The molecular orientation of the developed HTM thin films against the underlying surface is studied using X‐ray diffraction. Different substituents in MPcs can strongly affect their molecular orientation, resulting in different hole mobilities. The favored face‐on molecular alignment is only observed for Me₆Bu‐ZnPc on the perovskite layer, proving the crucial role of methyl substituents in controlling the molecular alignment through the special interactions between the MPc molecule and different sites of perovskite material on the surface. PSCs using Me₆Bu‐ZnPc as a dopant‐free HTM yields the highest reported power‐conversion efficiency (PCE) of 17.41%. With its high hydrophobicity and good coverage, Me₆Bu‐ZnPc HTM thin film acts as an encapsulation layer, which leads to significantly increased long‐term stability. The Me₆Bu‐ZnPc‐based devices retain over 90% of their initial PCE after 1400 h storage at 25 °C and with a relative humidity of 75%.

Surface treatments of SnO₂, such as UV–ozone (UVO) treatment, for 60 min, are shown to enhance efficiency and reduce hysteresis. UVO treatment improves contact charge selectivity, with a decrease in the recombination rate of the perovskite solar cells.

Tin oxide (SnO₂) is widely used as an electron transporting layer (ETL) in perovskite solar cells (PSCs) because of its good transparency, band alignment to perovskite, and stability. The interface between the ETL and the perovskite in the PSCs affects the charge extraction process and influences the optoelectronic properties. Surface treatment of SnO₂, such as the UV–ozone (UVO) treatment, is shown to enhance the efficiency and reduce the light soaking effect of the PSCs. Herein, the success in control and suppressing hysteresis reaching the highest photoconversion efficiency of 19.4% with negligible hysteresis for the growth of the devices on SnO₂ treated with UVO for 60 min is reported. The wettability of the treated SnO₂ is well matched with the polar solvent of the perovskite solution, leading to complete coverage of the substrate, although the treatment does not affect the morphology and the crystallinity of the perovskite thin films. Impedance spectroscopy measurement analysis clearly indicates the decrease in the recombination rate after the UVO treatment and the reduction in low frequency capacitance causing a reduction in the current–potential curve hysteresis.

An asymmetric crystal growth method is described for the synthesis of mixed‐cation perovskite single‐crystal micro‐plates. The thickness of the crystals is controlled by the difference between the thicknesses of the two spacers. The surface of the spaces weakens the attraction between the gap and the precursor complexes, thereby relieving the limit imposed by the low diffusion rate of the precursor ions.

The synthesis of certain asymmetric perovskite single crystals (SCs)—including CH₃NH₃PbI₃, which is used most commonly—for application in high‐performance perovskite solar cells (PeSCs), remains very challenging. Herein, a promising but general method, differential space‐limited crystallization (DSLC), is described for synthesizing high‐quality perovskite single‐crystal micro‐plates. The thickness of the perovskite SCs is controlled by the difference between the thicknesses of two sets of polytetrafluoroethylene (PTFE) spacers. Because the DSLC method does not require very thin spacers, it simplifies the procedure of crystal growth. More importantly, the hydrophobicity of the PTFE spacers weakens the attraction between the surfaces of the confined space and the precursor complexes, thereby increasing the rate of diffusion of the precursor ions. Accordingly, the critical nucleation step is not limited by the low rate of diffusion of the ions. This approach is used to prepare mixed‐cation lead iodide single‐crystalline micro‐plates for solar cell applications. The device performance of single‐crystal PeSCs improves after introducing formamidinium ions. The stability of the single‐crystal devices also improves relative to that of conventional thin‐film counterparts. It is anticipated that this DSLC method can also be used to synthesize different types of asymmetrical perovskite SCs for other optoelectronic applications.

A series of new spiro‐arranged hole‐transporting materials based on an orthogonal dibenzosuberene core unit incorporated with substituted phenylpyrazole are synthesized, namely, THY‐1 to THY‐5. The THY‐5 exhibits almost identical photovoltaic performance when compared with spiro‐OMeTAD, because of its homogeneous film morphology, which is attributed to the presence of bulky ^tBu and hydrophobic CF₃ moieties, facilitating the solubility in the casting solvent.

A series of spiro‐arranged hole‐transporting materials are designed and synthesized by incorporating a substituted phenylpyrazole unit to the orthogonal dibenzosuberene core unit, which are named as THY‐1 to THY‐5. All of them exhibit good optical, electrochemical, and electronic properties as needed for HTMs, despite the distinctive morphologies observed for the spin‐casted thin film. A perovskite solar cell based on THY‐5 exhibits the highest power conversion efficiency of 15.83%, which is comparable to that of the N2,N2,N2′,N2′,N7,N7,N7′,N7′‐octakis(4‐methoxyphenyl)‐9,9′‐spirobi[9H‐fluorene]‐2,2′,7,7′‐tetramine (Spiro‐OMeTAD) reference device (16.22%) fabricated using identical architecture.

The performance of inverted perovskite solar cells (i‐PSCs) is significantly improved using bulk‐heterojunction electron transport layers. The high efficiency originates from reduced trap‐assistant recombination due to the shortened electron capture region, high electron mobility, and suitable energy level of electron transport layers. The ultrahigh stability is attributed to effectively prevented moisture permeation due to more hydrophobic electron transport layers.

The power conversion efficiency (PCE) of inverted perovskite solar cells (i‐PSCs) is lower than that of the normal structures. The low efficiency is mainly ascribed to the inferior properties of commonly used [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) electron transport layers (ETLs) such as complexity in achieving high‐quality films, low electron mobility, imperfect energy level for electron extraction, and large electron capture region. Herein, the bulk heterojunction (BHJ) ETLs composed of PCBM and polymers are developed. The electron mobility of the BHJ film is enhanced by more than three times compared with PCBM, leading to efficient electron extraction. The electron capture region of the BHJ film decreases to 1.20 × 10⁻¹⁸ from 3.70 × 10⁻¹⁷ cm⁻³ for PCBM due to increased relative permittivity, which reduces the trap‐assistant recombination at the interface. Meanwhile, the devices with BHJ exhibit good stability regardless of illumination and dark storage conditions owing to the more hydrophobic BHJ films and full coverage of perovskite surface, which effectively prevent the moisture permeation into the perovskite devices. It is believed that this breakthrough provides a suitable approach to improve the efficiency and stability of i‐PSCs.

Four delicately designed dyes are synthesized. The fine‐tuning of energy levels and 3D structures of the dyes greatly reduces the energy losses of dye‐sensitized solar cells (DSSCs). Finally, DSSCs made with the NB6 dye and a cobalt‐tris(4‐methoxyphenyl)amine tandem electrolyte show one of the highest reported open‐circuit voltages of 1.03 V and an impressive power conversion efficiency of ≈12.1%.

Four weak donor backbones (BT, BTP, BT2, and BT3), featuring stepwise enhanced electron‐donating capacities, are designed and synthesized. The sp ³ type carbons introduced are tethered with auxiliary groups to generate a better electron‐blocking stereoscopic structure. A series of NB dyes are subsequently synthesized from these central cores by end‐capping a strong diphenylamine donor and a planar heterocyclic acceptor 4‐(benzo[c][1,2,5]thiadiazol‐4‐ylethynyl)benzoic acid. The fine‐tuning of steric configurations and energy levels of the resulting dye molecules reduces the energy losses significantly when applied in dye‐sensitized solar cells. These devices offer one of the highest open‐circuit voltages (≈1.03 V) reported so far, and high power conversion efficiencies of 9.6%–12.1% using the NB dyes in combination with a well‐developed cobalt‐tris(4‐methoxyphenyl)amine‐based tandem electrolyte.

The factors affecting the V _OC in 2D perovskite cells with different [PbI₆]⁴⁻ layer sheets (n = 2–4) are elucidated. Nonradiative recombination at the perovskite/C₆₀ interface is found to dominate except for the n = 2 system where the bulk recombination determines the properties of the cell. Substantial V _OC gains through suppression of interfacial recombination at the top interface are expected.

Abstract

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1} perovskite cells with different numbers of [PbI₆]⁴⁻ sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V _OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V _OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

Through a design strategy of fluorine modification, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄ is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization and ferroelectric photovoltaic effect. The discovery of such a material provides a great platform for the fundamental study of lead halide perovskite solar cells and other optoelectronic applications.

Abstract

Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead‐iodide‐based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH₃NH₃PbI₃ is still under debate, and obtaining ferroelectric lead iodide perovskites is still challenging. In order to avoid the randomness and blindness in the conventional method of searching for perovskite ferroelectrics, a design strategy of fluorine modification is developed. As a demonstration, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄, 1, is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization. The direct bandgap of 2.38 eV with strong photoluminescence also guarantees the direct observation of polarization‐induced FEPV. More importantly, the 2D structure and fluorination are also expected to achieve both good stability and charge transport properties. 1 is not only a 2D fluorinated lead iodide perovskite with confirmed ferroelectricity, but also a great platform for studying the effect of ferroelectricity and FEPV in the context of lead halide perovskite solar cells and other optoelectronic applications.

Highly efficient perovskite light‐emitting diodes are achieved by implementing a simple and cost‐effective method for efficient outcoupling of waveguided light. A record external quantum efficiency of 28.2% is realized for the device based on cesium lead bromide (CsPbBr₃), while retaining the same spectral response for broad viewing angles.

Abstract

Perovskite light‐emitting diodes (PeLEDs) show great application potential in high‐quality flat‐panel displays and solid‐state lighting due to their steadily improved efficiency, tunable colors, narrow emission peak, and easy solution‐processing capability. However, because of high optical confinement and nonradiative charge recombination during electron–photon conversion, the highest reported efficiency of PeLEDs remains far behind that of their conventional counterparts, such as inorganic LEDs, organic LEDs, and quantum‐dot LEDs. Here a facile route is demonstrated by adopting bioinspired moth‐eye nanostructures at the front electrode/perovskite interface to enhance the outcoupling efficiency of waveguided light in PeLEDs. As a result, the maximum external quantum efficiency and current efficiency of the modified cesium lead bromide (CsPbBr₃) green‐emitting PeLEDs are improved to 20.3% and 61.9 cd A⁻¹, while retaining spectral and angular independence. Further reducing light loss in the substrate mode using a half‐ball lens, efficiencies of 28.2% and 88.7 cd A⁻¹ are achieved, which represent the highest values reported to date for PeLEDs. These results represent a substantial step toward achieving practical applications of PeLEDs.

The conjugated polyelectrolytes (CPEs) with K⁺ and tetramethylammonium (TMA⁺) are introduced as a multifunctional passivating and hole‐transporting layer for perovskite light‐emitting diodes. TMA⁺ improves significant growth of perovskites with suppressed interfacial defects, resulting in dramatically enhanced emitting properties and device performance. The lower formation energy of Pb_i‐TMA than that of Pb_i‐K suggests that passivation by TMA⁺ ions is more favorable than K⁺ ions.

Abstract

Metal halide perovskites (MHPs) have attracted significant attention as light‐emitting materials owing to their high color purities and tunabilities. A key issue in perovskite light‐emitting diodes (PeLEDs) is the fabrication of an optimal charge transport layer (CTL), which has desirable energy levels for efficient charge injection while blocking opposite charges and enabling perovskite layer growth with reduced interfacial defects. Herein, two poly(fluorene‐phenylene)‐based anionic conjugated polyelectrolytes (CPEs) with different counterions (K⁺ and tetramethylammonium (TMA⁺)) are presented as multifunctional passivating and hole‐transporting layers (HTLs). The crystal growth of MHPs grown on different HTLs is investigated through X‐ray photoelectron spectroscopy, X‐ray diffraction, and density functional theory calculation. The CPE bearing the TMA⁺ counterions remarkably improves the growth of perovskites with suppressed interfacial defects, leading to significantly enhanced emission properties and device performance. The luminescent properties are further enhanced via aging and electrical stress application with effective rearrangement of the counterions on the interfacial defects in the perovskites. Finally, efficient formamidinium lead tribromide‐based quasi‐2D PeLEDs with an external quantum efficiency of 10.2% are fabricated. Using CPEs with varying counterions as a CTL can serve as an effective method for controlling the interfacial defects and improving perovskite‐based optoelectronic device properties.

A thermodynamically favored crystal preferable orientation growth along the (001) crystal plane is explored in formamidinium/methylammonium mixed perovskites, and the origin is found to be the reduction of surface energy. Combined with the (001) plane lying parallel to the substrate, it affects the charge transportation and collection in the resultant perovskite solar cells, resulting in a power conversion efficiency of 21.2%.

Abstract

Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed‐cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed‐cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed‐cation perovskite (FA_{1– x} MA _x PbI_2.87Br_0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed‐cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

The Fano resonances created in oligomers of gold nanoparticles are exploited to generate a strongly localized temperature distribution. Luminescent carbon quantum dots are fabricated by irradiating a poly(vinyl alcohol) film doped with gold nanoparticles by using femtosecond laser light. The controllable formation of carbon quantum dots exhibits potential applications in optical data storage, optical display, and sensing.

Abstract

Rapid and controllable formation of fluorescent carbon quantum dots (CQDs) is highly desirable in the fields of nanophotonics and biophotonics. Here, a novel strategy for creating CQDs, which emit white light efficiently under the excitation of either laser light or a mercury lamp, is proposed and demonstrated. The luminescent CQDs are generated by irradiating a poly(vinyl alcohol) (PVA) film doped with dense gold nanoparticles (AuNPs) with femtosecond laser pulses. The creation of CQDs from PVA is a two‐step dehydration process mediated by AuNPs which act not only as heat sources but also as catalytic agents. The formation of CC, CC, and CO bonds is confirmed by infrared Fourier transformation spectroscopy and X‐ray photoelectron spectroscopy. It is revealed both numerically and experimentally that a spatially localized temperature distribution at the deep subwavelength scale can be achieved in oligomers of AuNPs by resonantly exciting the Fano resonances formed in the oligomers of AuNPs, enabling the generation of CQDs with small diameters. As one of the potential applications, it is demonstrated that optical display and optical data storage with ultralow energy can be realized by selectively introducing luminescent CQDs in the AuNP/PVA film.

A new equivalent ligand strategy with a strong ionic sulfonate head is demonstrated and the purification and storage problems of perovskite nanocrystals are overcome. Both theoretical and experimental results prove the elimination of nonradiative recombination and high quantum efficiency are maintained throughout purification, storage, and irradiation.

Abstract

The stability and optoelectronic device performance of perovskite quantum dots (Pe‐QDs) are severely limited by present ligand strategies since these ligands exhibit a highly dynamic binding state, resulting in serious complications in QD purification and storage. Here, a “Br‐equivalent” ligand strategy is developed in which the proposed strong ionic sulfonate heads, for example, benzenesulfonic acid, can firmly bind to the exposed Pb ions to form a steady binding state, and can also effectively eliminate the exciton trapping probability due to bromide vacancies. From these two aspects, the sulfonate heads play a similar role as natural Br ions in a perfect perovskite lattice. Using this approach, high photoluminescence quantum yield (PL QY) > 90% is facilely achieved without the need for amine‐related ligands. Furthermore, the prepared PL QYs are well maintained after eight purification cycles, more than five months of storage, and high‐flux photo‐irradiation. This is the first report of high and versatile stabilities of Pe‐QD, which should enable their improved application in lighting, displays, and biologic imaging.

Nature, Published online: 05 June 2019; doi:10.1038/d41586-019-01710-9

2D crystalline membranes are easily made from some materials, but not from those with strong 3D lattices, such as technologically useful perovskite oxides. Free-standing perovskite monolayers have finally been made.

Large excitonic optical nonlinearity in single‐crystalline 2D Ruddlesden–Popper perovskite (RPP) nanosheets characterized by a microscopic Z‐scan setup is reported. A room‐temperature excitonic Mott transition occurs near the exciton resonance of the thinnest quantum‐well RPPs, boosting the nonlinear response. The magnitude and sign of the nonlinear coefficients vary strongly with the excitation wavelength offering various nonlinear functionalities in the visible waveband.

Abstract

Materials with large optical nonlinearity, especially in the visible spectral region, are in great demand for applications in all‐optical information processing and quantum optics. 2D hybrid Ruddlesden−Popper‐type halide perovskites (RPPs) with tunable ultraviolet‐to‐visible direct bandgaps exhibit large nonlinear optical responses due to the strong excitonic effects present in their multiple quantum wells. Using a microscopic Z‐scan setup with femtosecond laser pulses tunable across the visible spectrum, it is demonstrated that single‐crystalline lead halide RPP nanosheets possess unprecedentedly large nonlinear refraction and absorption coefficients near excitonic resonances. A room‐temperature insulator (exciton)–metal (plasma) Mott transition is found to occur near the exciton resonance of the thinnest qunatum‐well RPPs, boosting the nonlinear response. Owing to the rapidly changing refractive index near resonance, a single RPP crystal can exhibit different nonlinear functionalities across the excitation spectrum. The results suggest that RPPs are efficient nonlinear materials in the visible waveband, indicating their potential use in integrated nonlinear photonic applications such as optical modulation and switching.

The efficiency and photostability of all‐inorganic mixed‐halide perovskite solar cells (PVSCs) can be simultaneously enhanced by introducing an amino‐functionalized polymer PN4N as a novel cathode interlayer and dopant‐free PDCBT hole‐transporting layer. The favorable interaction between perovskite crystal and PN4N/PDCBT can effectively improve CsPbI₂Br film quality, with power conversion efficiency over 16%.

Abstract

A synergic interface design is demonstrated for photostable inorganic mixed‐halide perovskite solar cells (PVSCs) by applying an amino‐functionalized polymer (PN4N) as cathode interlayer and a dopant‐free hole‐transporting polymer poly[5,5′‐bis(2‐butyloctyl)‐(2,2′‐bithiophene)‐4,4′‐dicarboxylate‐alt‐5,5′‐2,2′‐bithiophene] (PDCBT) as anode interlayer. First, the interfacial dipole formed at the cathode interface reduces the workfunction of SnO₂, while PDCBT with deeper‐lying highest occupied molecular orbital (HOMO) level provides a better energy‐level matching at the anode, leading to a significant enhancement in open‐circuit voltage (V _oc) of the PVSCs. Second, the PN4N layer can also tune the surface wetting property to promote the growth of high‐quality all‐inorganic perovskite films with larger grain size and higher crystallinity. Most importantly, both theoretical and experimental results reveal that PN4N and PDCBT can interact strongly with the perovskite crystal, which effectively passivates the electronic surface trap states and suppresses the photoinduced halide segregation of CsPbI₂Br films. Therefore, the optimized CsPbI₂Br PVSCs exhibit reduced interfacial recombination with efficiency over 16%, which is one of the highest efficiencies reported for all‐inorganic PVSCs. A high photostability with a less than 10% efficiency drop is demonstrated for the CsPbI₂Br PVSCs with dual interfacial modifications under continuous 1 sun equivalent illumination for 400 h.

Perovskite solar cells based on polarized ferroelectric polymers are fabricated by doping the ferroelectric polymer into the perovskite layer with different polarizing electric fields and different doping concentrations, different polarized ferroelectric polymers' interlayers between the perovskite and the hole‐transporting layer, and both doping and interlayer. After these treatments, the fabricated devices show a maximum power conversion efficiency of 21.38%.

Abstract

In hybrid organic–inorganic lead halide perovskite solar cells, the energy loss is strongly associated with nonradiative recombination in the perovskite layer and at the cell interfaces. Here, a simple but effective strategy is developed to improve the cell performance of perovskite solar cells via the combination of internal doping by a ferroelectric polymer and external control by an electric field. A group of polarized ferroelectric (PFE) polymers are doped into the methylammonium lead iodide (MAPbI₃) layer and/or inserted between the perovskite and the hole‐transporting layers to enhance the build‐in field (BIF), improve the crystallization of MAPbI₃, and regulate the nonradiative recombination in perovskite solar cells. The PFE polymer‐doped MAPbI₃ shows an orderly arrangement of MA⁺ cations, resulting in a preferred growth orientation of polycrystalline perovskite films with reduced trap states. In addition, the BIF is enhanced by the widened depletion region in the device. As an interfacial dipole layer, the PFE polymer plays a critical role in increasing the BIF. This combined effect leads to a substantial reduction in voltage loss of 0.14 V due to the efficient suppression of nonradiative recombination. Consequently, the resulting perovskite solar cells present a power conversion efficiency of 21.38% with a high open‐circuit voltage of 1.14 V.

A new equivalent ligand strategy with a strong ionic sulfonate head is demonstrated and the purification and storage problems of perovskite nanocrystals are overcome. Both theoretical and experimental results prove the elimination of nonradiative recombination and high quantum efficiency are maintained throughout purification, storage, and irradiation.

Abstract

The stability and optoelectronic device performance of perovskite quantum dots (Pe‐QDs) are severely limited by present ligand strategies since these ligands exhibit a highly dynamic binding state, resulting in serious complications in QD purification and storage. Here, a “Br‐equivalent” ligand strategy is developed in which the proposed strong ionic sulfonate heads, for example, benzenesulfonic acid, can firmly bind to the exposed Pb ions to form a steady binding state, and can also effectively eliminate the exciton trapping probability due to bromide vacancies. From these two aspects, the sulfonate heads play a similar role as natural Br ions in a perfect perovskite lattice. Using this approach, high photoluminescence quantum yield (PL QY) > 90% is facilely achieved without the need for amine‐related ligands. Furthermore, the prepared PL QYs are well maintained after eight purification cycles, more than five months of storage, and high‐flux photo‐irradiation. This is the first report of high and versatile stabilities of Pe‐QD, which should enable their improved application in lighting, displays, and biologic imaging.

Through a design strategy of fluorine modification, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄ is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization and ferroelectric photovoltaic effect. The discovery of such a material provides a great platform for the fundamental study of lead halide perovskite solar cells and other optoelectronic applications.

Abstract

Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead‐iodide‐based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH₃NH₃PbI₃ is still under debate, and obtaining ferroelectric lead iodide perovskites is still challenging. In order to avoid the randomness and blindness in the conventional method of searching for perovskite ferroelectrics, a design strategy of fluorine modification is developed. As a demonstration, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄, 1, is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization. The direct bandgap of 2.38 eV with strong photoluminescence also guarantees the direct observation of polarization‐induced FEPV. More importantly, the 2D structure and fluorination are also expected to achieve both good stability and charge transport properties. 1 is not only a 2D fluorinated lead iodide perovskite with confirmed ferroelectricity, but also a great platform for studying the effect of ferroelectricity and FEPV in the context of lead halide perovskite solar cells and other optoelectronic applications.