Photoluminescence Enhancement in Formamidinium Lead Iodide Thin Films

Formamidinium lead iodide (FAPbI₃) has a broader absorption spectrum and better thermal stability than the most famous methylammonium lead iodide, thus exhibiting great potential for photovoltaic applications. In this report, the light‐induced photoluminescence (PL) evolution in FAPbI₃ thin films is investigated. The PL intensity evolution is found to be strongly dependent on the atmosphere surrounding the samples. When the film is exposed to air, its photoluminescence intensity is enhanced more than 140 times after continuous ultraviolet laser illumination for 2 h, and the average lifetime is prolonged from 17 to 389 ns. The enhanced photoluminescence implies that the trap density is significantly reduced. The comparative study of the photoluminescence properties in air, nitrogen, and oxygen/helium environment suggests that moisture is important for the PL enhancement. This is explained in terms of moisture‐assisted light‐healing effect in FAPbI₃ thin films. With this study, a new method is demonstrated to increase and control the quality of hybrid perovskite thin films.     

Photovoltaic and Amplified Spontaneous Emission Studies of High‐Quality Formamidinium Lead Bromide Perovskite Films

This study demonstrates the formation of extremely smooth and uniform formamidinium lead bromide (CH(NH₂)₂PbBr₃ = FAPbBr₃) films using an optimum mixture of dimethyl sulfoxide and N,N‐dimethylformamide solvents. Surface morphology and phase purity of the FAPbBr₃ films are thoroughly examined by field emission scanning electron microscopy and powder X‐ray diffraction, respectively. To unravel the photophysical properties of these films, systematic investigation based on time‐integrated and time‐dependent photoluminescence studies are carried out which, respectively, bring out relatively lower nonradiative recombination rates and long lasting photogenerated charge carriers in FAPbBr₃ perovskite films. The devices based on FTO/TiO₂/FAPbBr₃/spiro‐OMeTAD/Au show highly reproducible open‐circuit voltage (V_oc) of 1.42 V, a record for FAPbBr₃‐based perovskite solar cells. V_oc as a function of illumination intensity indicates that the contacts are very selective and higher V_oc values are expected to be achieved when the quality of the FAPbBr₃ film is further improved. Overall, the devices based on these films reveal appreciable power conversion efficiency of 7% under standard illumination conditions with negligible hysteresis. Finally, the amplified spontaneous emission (ASE) behavior explored in a cavity‐free configuration for FAPbBr₃ perovskite films shows a sharp ASE threshold at a fluence of 190 μJ cm^?2 with high quantum efficiency further confirming the high quality of the films.     

Flexible Piezoelectric Nanocomposite Generators Based on Formamidinium Lead Halide Perovskite Nanoparticles

Organic–inorganic lead halide perovskite materials have recently attracted much attention in the field of optoelectronic devices. Here, a hybrid piezoelectric nanogenerator based on a composite of piezoelectric formamidinium lead halide perovskite (FAPbBr₃) nanoparticles and polydimethylsiloxane polymer is fabricated. Piezoresponse force spectroscopy measurements reveal that the FAPbBr₃ nanoparticles contain well‐developed ferroelectric properties with high piezoelectric charge coefficient (d₃₃) of 25 pmV⁻¹. The flexible device exhibits high performance with a maximum recordable piezoelectric output voltage of 8.5 V and current density of 3.8 μA cm⁻² under periodically vertical compression and release operations. The alternating energy generated from nanogenerators can be used to charge a capacitor and light up a red light‐emitting diode through a bridge rectifier. This result innovatively expands the feasibility of organic–inorganic lead halide perovskite materials for application in a wide variety of high‐performance energy harvesting devices.     

Rethinking the Role of Excess/Residual Lead Iodide in Perovskite Solar Cells

It is widely believed that excess/residual lead iodide (PbI₂) can affect the performance of perovskite solar cells . Moderate PbI₂ can enhance efficiency by passivating defects, while extremely active PbI₂ leads to non-negligible hysteresis effects and reduces device stability. Although several efforts are made to investigate the role of excess PbI₂, its impact is still underestimated. Recent advances further demonstrate the extraordinary potential of modifying excess PbI₂; however, a comprehensive study is required to obtain a deeper understanding. Herein, the important breakthroughs regarding excess PbI₂ are reviewed and the mechanism of excess PbI₂ in terms of efficiency and stability is rethought. In addition, the origins, verification, and regulation of residual PbI₂ are summarized.     

Finite-Temperature Dynamics in Cesium Lead Iodide Halide Perovskite

Lattice dynamics are often regarded as signatures of the underlying crystal structure. Here, a first-principle-based effective Hamiltonian method combined with molecular dynamics simulations is used to study dynamical behaviors of CsPbI₃ perovskite across temperature and structural phase transitions. A single (short-range tilting) parameter in this effective Hamiltonian is varied in order to make the temperature range of the intermediate tetragonal P4/mbm phase, existing in-between the cubic Pm$\overline{3}$m and orthorhombic Pnma phases, either broader than observed or completely disappearing. Comparing the dynamics of these different cases allows one to conclude that real CsPbI₃ perovskite should have i) two iodine-octahedral-tilt related modes that differ in frequency but both significantly soften as the temperature decreases within the cubic phase toward the Pm$\overline{3}$m-to-P4/mbm transition; and ii) one mode that maintains a very low frequency (of the order of 1.0 cm⁻¹) in the entire region of P4/mbm stability, as a result of the temporal exploration of various structural states. Such latter sub-THz mode mixes fluctuations of antiphase iodine tiltings and Cs antipolar motions because of a trilinear energetic coupling.     

Low-Dimensional Single-Cation Formamidinium Lead Halide Perovskites (FAm+2PbmBr3m+2): From Synthesis to Rewritable Phase-Change Memory Film

Synthesis of 2D perovskites often demands long and bulky organic spacer cations, but they hamper optoelectronic properties of the resulting 2D perovskites. Novel low-dimensional single-cation perovskites with a general formula of FA_m+2Pb_mBr_3m+2 are prepared by using a quenching-assisted solution process, which leads to the wide dimensional control over 1D FA₃PbBr₅ (m = 1), 2D FA_m+2Pb_mBr_3m+2 (m ≥ 2), and 3D FAPbX₃ (m = ∞) perovskites simply by changing the composition of precursors. In this case, formamidinium (FA) acts as both an A-site cation and spacer cation in FA_m+2Pb_mBr_3m+2. Unlike conventional 2D perovskites, FA_m+2Pb_mBr_3m+2 perovskites have (110) orientation. PVDF (poly(vinylidene fluoride)) preferentially stabilizes the low-dimensional FA_m+2Pb_mBr_3m+2 phases, which is utilized to fabricate the stable FA_m+2Pb_mBr_3m+2-PVDF composite films. The phase transitions from 1D and 2D to 3D are investigated in response to various stimuli, including humidity, ultraviolet, oxygen, and solvents, are exploited for rewritable phase-change memory films.     

Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching

CsPbX₃ (X = halide, Cl, Br, or I) all‐inorganic halide perovskites (IHPs) are regarded as promising functional materials because of their tunable optoelectronic characteristics and superior stability to organic–inorganic hybrid halide perovskites. Herein, nonvolatile resistive switching (RS) memory devices based on all‐inorganic CsPbI₃ perovskite are reported. An air‐stable CsPbI₃ perovskite film with a thickness of only 200 nm is successfully synthesized on a platinum‐coated silicon substrate using low temperature all‐solution process. The RS memory devices of Ag/polymethylmethacrylate (PMMA)/CsPbI₃/Pt/Ti/SiO₂/Si structure exhibit reproducible and reliable bipolar switching characteristics with an ultralow operating voltage (<+0.2 V), high on/off ratio (>10⁶), reversible RS by pulse voltage operation (pulse duration < 1 ms), and multilevel data storage. The mechanical flexibility of the CsPbI₃ perovskite RS memory device on a flexible substrate is also successfully confirmed. With analyzing the influence of phase transition in CsPbI₃ on RS characteristics, a mechanism involving conducting filaments formed by metal cation migration is proposed to explain the RS behavior of the memory device. This study will contribute to the understanding of the intrinsic characteristics of IHPs for low‐voltage resistive switching and demonstrate the huge potential of them for use in low‐power consumption nonvolatile memory devices on next‐generation computing systems.     

Macroscopic and Microscopic Structures of Cesium Lead Iodide Perovskite from Atomistic Simulations

A first‐principles‐based effective Hamiltonian is developed and employed to investigate finite‐temperature structural properties of a prototype of perovskite halides, that is CsPbI₃. Such simulations, when using first‐principles‐extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic $Pm\overline{3}m$ upon heating, unlike measurements that reported the occurrence of an intermediate long‐range‐tilted tetragonal P4/mbm phase in‐between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first‐principle methods can be trusted to mimic hybrid perovskites, can be resolved by “only” changing one short‐range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic $Pm\overline{3}m$ phase is highly locally distorted and develops strong transverse antiphase correlation between first‐nearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling.     

Hierarchical Chiral Self-Assembly of Lead Halide Perovskite Nanocrystals by Oriented Phase Transition

Self-assembly of lead halide perovskite nanocrystals (NCs) into close-packed, long-range-ordered nanostructures can effectively modulate their photoelectronic properties, yet significantly challenging. Herein, an efficient approach is reported to induce the hierarchical self-assembly of perovskite CsPbBr₃ NCs by phase transition using chiral cysteine ligands, yielding asymmetric Cs₄PbBr₆ nanorods (NRs) with the circularly polarized luminescent response. An interfacial phase transition process is found during the conversion of CsPbBr₃ nanocubes to Cs₄PbBr₆ NCs initiated by cysteine molecules. Then the Cs₄PbBr₆ NCs aggregate sequentially to form nanoclusters, which further self-assemble into the chiral Cs₄PbBr₆ NRs. Molecular dynamics simulations reveal that the Cs₄PbBr₆ nanochains gradually approach each other to achieve an asymmetric structure, and the simulated circular dichroism spectrum further supports the formation of a chiral structure. This work offers a facile method for the hierarchical chiral self-assembly of lead halide perovskite nanostructures, which brings new insights to explore chiral nanostructures by modulating the surface chemistry and post self-assembly.     

Perovskite Photovoltaics for Dim‐Light Applications

The use of photovoltaic cells with an organometallic perovskite as the active layer for indoor dim‐light energy harvesting is evaluated. By designing the electron‐transporting materials and fabrication processes, the traps in the perovskite active layers and carrier dynamics can be controlled, and efficient devices are demonstrated. The best‐performing small‐area perovskite photovoltaics exhibit a promising high power conversion efficiency up to ≈27.4%, no hysteresis behavior, and an exceptionally low maximum power point voltage variation of ≈0.1 V under fluorescent lamp illumination at 100–1000 lux. The 5.44 cm² large‐area device also shows a high efficiency of 20.4% and a promising long‐term stability. Compared with the most efficient inorganic and organic solar cells nowadays, the competitive efficiency, low fabrication cost, and low raw material costs make perovskite photovoltaics ideal for indoor light harvesting and as Internet of Things power provider.     

Inter-Sample and Intra-Sample Variability in Electronic Properties of Methylammonium Lead Iodide

While the challenges associated with the stability of metal halide perovskites are well known and intensely studied, variability in electronic properties represents an equally significant, yet seldom studied, challenge that could potentially slow or inhibit the commercial viability of these systems. In this work, the contactless characterization technique time-resolved microwave conductivity (TRMC) is used to quantify the variability in electronic properties of the prototypical perovskite, methylammonium lead iodide (MAPbI₃) both between different samples, and at different locations within the same sample. Using scanning electron microscopy (SEM) and a quasi-automated image-analysis strategy, it is possible to evaluate the metrics of heterogeneity in surface microstructure and correlate them with the electronic properties as obtained by TRMC. Substantial intra-sample and inter-sample variation is observed in the mobility-yield product in samples prepared following differing protocols, and in samples prepared following identical protocols.     

Exceptional Long Electron Lifetime in Methylammonium Lead Iodide Perovskite Solar Cell Made from Aqueous Lead Nitrate Precursor

Studies on the photoelectronic properties of perovskite solar cells (PSCs) made from non‐PbI₂ precursors are seldom reported. In this study, a series of transient techniques are applied to investigate the charge recombination and trap distribution in an efficient PSC fabricated using a low‐toxicity Pb(NO₃)₂/water protocol. A device with identical conversion efficiency fabricated using a conventional PbI₂/dimethylformamide protocol is also studied for comparison. Transient photovoltage and time‐resolved photoluminescence analysis reveal that the Pb(NO₃)₂/water‐based device exhibits a long lifetime in both bimolecular and trap‐assisted recombination. However, differential capacitance and differential charging analysis indicate that there are more charges stored in the Pb(NO₃)₂/water‐based perovskite layer, which stretches the energy tail from band edge to midband and should provoke serious trap‐assisted recombination. The exceptional long electron lifetime in the Pb(NO₃)₂/water‐based device is explained by a benign defect inactivation, which originates from water and NO₃^? residues from the aqueous precursor solution and is involved in the formation of perovskite crystal. Consequently, despite the perovskite film made from Pb(NO₃)₂/water protocol possessing high trap density, its photovoltaic device still exhibits a long electron lifetime and superior photovoltaic properties.     

Air‐Stable Highly Crystalline Formamidinium Perovskite 1D Structures for Ultrasensitive Photodetectors

State‐of‐the‐art optoelectronic devices based on metal‐halide perovskites demand solution‐processed structures with high crystallinity, controlled crystallographic orientation, and enhanced environmental stability. Formamidinium lead iodide (α‐FAPbI₃) possesses a suitable bandgap of 1.48 eV and enhanced thermal stability, whereas perovskite‐type polymorph (α‐phase) is thermodynamically instable at ambient temperatures. Stable α‐FAPbI₃ perovskite 1D structure arrays with high crystallinity and ordered crystallographic orientation are developed by controlled nucleation and growth in capillary bridges. By surface functionalization with phenylethylammonium ions (PEA⁺), FAPbI₃ wires sustain a stable α‐phase after 28 day storage in the ambient environment with a relative humidity of 50%. Enhanced photoluminescence (PL) intensity and elongated PL lifetime demonstrate suppressed trap density and high crystallinity in these 1D structures, which is also reflected by the enhanced diffraction density and pure (001) crystallographic orientation in the grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) pattern. Based on these high‐quality 1D structures, sensitive photodetectors are achieved with average responsivities of 5282 A W^?1, average specific detectivities of more than 1.45 × 10¹⁴ Jones, and a fast response speed with a 3 dB bandwidth of 15 kHz.     

Highly Sensitive Low‐Bandgap Perovskite Photodetectors with Response from Ultraviolet to the Near‐Infrared Region

It is a great challenge to obtain broadband response perovskite photodetectors (PPDs) due to the relatively large bandgaps of the most used methylammonium lead halide perovskites. The response range of the reported PPDs is limited in the ultraviolet–visible range. Here, highly sensitive PPDs are successfully fabricated with low bandgap (≈1.25 eV) (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite as active layers, exhibiting a broadband response from 300 to 1000 nm. The performance of the PPDs can be optimized by adjusting the thicknesses of the perovskite and C₆₀ layers. The optimized PPDs with 1000 nm thick perovskite layer and 70 nm thick C₆₀ layer exhibit an almost flat external quantum efficiency (EQE) spectrum from 350 to 900 nm with EQE larger than 65% under ?0.2 V bias. Meanwhile, the optimized PPDs also exhibit suppressed dark current of 3.9 nA, high responsivity (R ) of over 0.4 A W^?1, high specific detectivity (D* ) of over 10¹² Jones in the near‐infrared region under ?0.2 V. Such highly sensitive broadband response PPDs, which can work well as self‐powered conditions, offer great potential applications in multicolor light detection.     

Centimeter-Sized Single Crystals of Two-Dimensional Hybrid Iodide Double Perovskite (4,4-Difluoropiperidinium)4AgBiI8 for High-Temperature Ferroelectricity and Efficient X-Ray Detection

Ferroelectricity and X-ray detection property have been recently implemented for the first time in hybrid bromide double perovskites. It sheds a light on achieving photosensitive and ferroelectric multifunctional materials based on 2D lead-free hybrid halide double perovskites. However, the low T_c, small P_s, and relatively low X-ray sensitivity in the reported bromide double perovskites hinder practical applications. Herein, the authors demonstrate a novel 2D lead-free iodide double perovskite (4,4-difluoropiperidinium)₄AgBiI₈ (1) for high-performance X-ray sensitive ferroelectric devices. Centimeter-sized single crystal of 1 is obtained and exhibits an excellent ferroelectricity including a high T_c up to 422 K and a large P_s of 10.5 μC cm⁻². Moreover, due to a large X-ray attenuation and efficient charge carrier mobility (μ)–charge carrier lifetime (τ) product, the crystal 1 also exhibits promising X-ray response with a high sensitivity up to 188 μC·Gy_air⁻¹ cm⁻² and a detection limit below 3.13 μGy_air·s⁻¹. Therefore, this finding is a step further toward practical applications of lead-free halide perovskite in high-performance photoelectronic devices. It will afford a promising platform for exploring novel photosensitive ferroelectric multifunctional materials based on lead-free double perovskites.     

Copper(I) Iodide as Hole‐Conductor in Planar Perovskite Solar Cells: Probing the Origin of J–V Hysteresis

Organic–inorganic lead halide perovskite solar cells are promising alternatives to silicon‐based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole‐conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 ± 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI‐based devices with a planar structure. In contrast to related planar 2,2′,7,7′‐tetrakis‐(N,N ‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based devices, the CuI‐based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open‐circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro‐OMeTAD. A slow response on a timescale of 10–100 s is observed for the spiro‐OMeTAD‐based devices. In comparison, the CuI‐based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open‐circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current–voltage hysteresis.