Enhanced Performance of Perovskite Solar Cells with Zinc Chloride Additives

The structural, optical and photovoltaic properties of Zn-doped MAPbI2Br perovskite solar cells

The spin coating method was used to deposit MAPbI2Br films on FTO-glass substrates. Zn2+ (zinc) doping was used for these films at intensity rates of 2% and 4%, respectively. XRD analysis proved that MAPbI2Br films had a cubic structure and a crystalline character. 2% Zn doping into the MAPbI2Br film had a modest large grain size (38.09 nm), Eg (1.95 eV), high refractive index (2.66), and low extinction coefficient (1.67), according to XRD and UV-vis analyses. To facilitate and enhance carrier transit, at contacts as well as throughout the bulk material, the perovskite's trap-state densities decreased. The predicted MAPbI2Br valence and conduction band edges are -5.44 and -3.52, respectively. The conduction band (CB) edge of the film that was exposed to Zn atoms has been pressed towards the lower value, assembly it a better material for solar cells. EIS is particularly useful for understanding charge carrier transport, recombination mechanisms, and the influence of different interfaces within the device structure. Jsc is 11.09 mA cm-2, Voc is 1.09, PCE is 9.372% and FF is 0.777. The cell made with the 2% Zn doped into the MAPbI2Br film demonstrated a superior device.

Iodide manipulation using zinc additives for efficient perovskite solar minimodules

Interstitial iodides are the most critical type of defects in perovskite solar cells that limits efficiency and stability. They can be generated during solution, film, and device processing, further accelerating degradation. Herein, we find that introducing a small amount of a zinc salt- zinc trifluoromethane sulfonate (Zn(OOSCF3)2) in the perovskite solution can control the iodide defects in resultant perovskites ink and films. CF3SOO̶ vigorously suppresses molecular iodine formation in the perovskites by reducing it to iodide. At the same time, zinc cations can precipitate excess iodide by forming a Zn-Amine complex so that the iodide interstitials in the resultant perovskite films can be suppressed. The perovskite films using these additives show improved photoluminescence quantum efficiency and reduce deep trap density, despite zinc cations reducing the perovskite grain size and iodide interstitials. The zinc additives facilitate the formation of more uniform perovskite films on large-area substrates (78-108 cm2) in the blade-coating process. Fabricated minimodules show power conversion efficiencies of 19.60% and 19.21% with aperture areas of 84 and 108 cm2, respectively, as certified by National Renewable Energy Laboratory (NREL), the highest efficiency certified for minimodules of these sizes.

Advances in chloride additives for high-efficiency perovskite solar cells: multiple points of view

Chloride (Cl) additives are rather effective in improving the performance of perovskite solar cells (PSCs) through the modulation of crystallization process and surface morphology. After incorporating Cl-containing additives, the optoelectrical properties of perovskite films, such as the electron/hole diffusion length and carrier lifetime, are greatly enhanced. However, only a trace amount of Cl has been identified in the resultant perovskite film, and the mechanism of efficiency improvement induced by Cl remains unclear. In this review, we discuss organic and inorganic Cl additives systematically from the perspective of their solubility, volatility, cation size and chemical groups. In addition, the roles of residual Cl anions and cations are analyzed in detail. Finally, some valuable future perspectives of Cl additives are proposed.

Enhanced stability of triple-halide perovskites CsPbI3-x-yBrxCly (x and y = 0-0.024): understanding the role of Cl doping from ab initio calculations

Doping of chloride in mixed iodide-bromide perovskites has been shown experimentally to suppress the photo-induced halide-ion segregation and enhance the stability of triple-halide perovskites (THP). However, a fundamental understanding of the effects of Cl doping is yet to be achieved especially when the doping concentration is low. Here we report the results of a state-of-the-art ab initio study of the atomic structure of THP by considering small doping concentrations of Br and Cl in CsPbI3. We find a reduction in the Pb-I bond lengths and tilting of PbI6 octahedra with Cl doping which lead to exothermic heat of mixing and therefore higher stability of THP. Moreover, using quasi-chemical approximation, our results show that there is a very small contribution of configurational entropy to Gibbs free energy at such low doping concentrations and at the operational temperature of 50 °C. This suggests that the favorable heat of mixing value is more important for the stability at low doping concentrations of Cl while a higher concentration of Cl increases the risk of halide segregation. Further calculations on Frenkel defect formation energy of I or Br-interstitial shows that the doping of Cl in I/Br mixed binary-compounds hinders the formation of Frenkel defects. These results support experiments and help to understand the role of chloride in suppressing the halide ion mobility with only a slight increase in the band gap. Accordingly, the THPs manifest a promising pathway for developing single-phase perovskites for solar cells and light-emitting diodes with improved performance and enhanced stability.

Mechanistic insights into the key role of methylammonium iodide in the stability of perovskite materials

The possible mechanisms damaging perovskite solar cells have attracted considerable attention in the photovoltaic community. This study answers specifically open problems regarding the critical role of methylammonium iodide (MAI) in investigations as well as stabilizing the perovskite cells. Surprisingly, we found that when the molar ratio between PbI₂ : MAI precursor solution increased from 1 : 5 to 1 : 25, the stability of perovskite cells dramatically increased over time. The stability of perovskite in the air without any masking in the average stoichiometry was about five days, while when the amount of MAI precursor solution increased to 5, the perovskite film was unchanged for about 13 days; eventually, when the value of MAI precursor solution enhanced to 25, the perovskite film stayed intact for 20 days. The outstanding XRD results indicated that the intensity of perovskite's Miler indices increased significantly after 24 h, and the MAI's Miler indices decreased, which means that the amount of MAI was consumed to renew the perovskite crystal structure. In particular, the results suggested that the charging of MAI using the excess molar ratio of MAI reconstructs the perovskite material and stabilizes the crystal structure over time. Therefore, it is crucial that the main preparation procedure of perovskite material is optimized to 1 unit of Pb and 25 units of MAI in a two-step procedure in the literature.

Sr-Doping All-Inorganic CsPbBr3 Perovskite Thick Film for Self-Powered X-ray Detectors

The all-inorganic perovskite cesium lead bromine (CsPbBr₃) has attracted much attention in the field of X-ray detectors because of its high X-ray absorption coefficient, high carrier collection efficiency, and easy solution preparation. The low-cost anti-solvent method is the main method to prepare CsPbBr₃; during this process, solvent volatilization will bring a large number of holes to the film, leading to the increase of defects. Based on the heteroatomic doping strategy, we propose that Pb²⁺ should be partially replaced by Sr²⁺ to prepare leadless all-inorganic perovskite. The introduction of Sr²⁺ promoted the ordered growth of CsPbBr₃ in the vertical direction, increased the density and uniformity of the thick film, and achieved the goal of CsPbBr₃ thick film repairing. In addition, the prepared CsPbBr₃ and CsPbBr₃:Sr X-ray detectors were self-powered without external bias, maintaining a stable response during on and off states at different X-ray dose rates. Furthermore, the detector base on 160 µm CsPbBr₃:Sr had a sensitivity of 517.02 µC Gy_air^-1 cm^-3 at zero bias under the dose rate of 0.955 µGy ms^-1 and it obtained a fast response speed of 0.053-0.148 s. Our work provides a new opportunity to produce cost-effective and highly efficient self-powered perovskite X-ray detectors in a sustainable way.

Understanding the Water-in-Salt to Salt-in-Water Characteristics across the Zinc Chloride : Water Phase Diagram

Using a series of time- and temperature-resolved synchrotron diffraction experiments, the relationship between multiple polymorphs of ZnCl₂ and its respective hydrates is established. The δ-phase is found to be the pure anhydrous phase, while the α, β, and γ phases result from partial hydration. Diffraction, gravimetric, and calorimetric measurements across the entire ZnCl₂·R H₂O, 0 > R > ∞ composition range using ultrapure, doubly sublimed ZnCl₂ establish the ZnCl₂ : H₂O phase diagram. The results are consistent with the existence of crystalline hydrates at R = 1.33, 3, and 4.5 and identify a mechanistic pathway for hydration. All water is not removed from hydrated ZnCl₂ until the system is heated above its melting point. While hydration/dehydration is reversible in concentrated solutions, dehydration from dilute aqueous solutions can result in loss of HCl, the source of hydroxide impurities commonly found in commercial ZnCl₂ preparations. The strong interaction between ZnCl₂ and water exerts a significant impact on the solvent water such that the system exhibits a deep eutectic at a composition of about R = 7 (87.5 mol %) and a eutectic temperature below -60 °C.

Insight into the Interface Engineering of a SnO2/FAPbI3 Perovskite Using Lead Halide as an Interlayer: A First-Principles Study

The interfacial properties of the perovskite photovoltaic layer and electron transport layer (ETL) are critical to minimize energy losses of perovskite solar cells (PSCs) induced by interfacial recombination. Herein, the interface engineering of the SnO₂/FAPbI₃ perovskite using PbX₂ (X = Cl, Br, or I) as an interlayer is extensively studied using first-principles calculations. The results reveal that the thickness of the PbI₂ interlayer needs to be finely controlled, which may limit charge transport if there is a large amount of PbI₂ precipitation at the interface. The high lattice mismatch of the PbBr₂ with the SnO₂/FAPbI₃ interface makes PbBr₂ an unfavorable passivation material. Due to the strong coupling of the PbCl₂ with both SnO₂ and FAPbI₃, an efficient electron transport pathway could be built after applying PbCl₂ as an interlayer. Meanwhile, the PbCl₂ interlayer could also effectively passivate interface defects, therefore lowering the energy losses of PSCs.

Progress in Perovskite Solar Cells towards Commercialization-A Review

In recent years, perovskite solar cells (PSCs) have experienced rapid development and have presented an excellent commercial prospect as the PSCs are made from raw materials that are readily and cheaply available depending on simple manufacturing techniques. However, the commercial production and utilization of PSCs remain immature, leading to substantial efforts needed to boost the development of scalable fabrication of PSCs, pilot scale tests, and the establishment of industrial production lines. In this way, the PSCs are expected to be successfully popularized from the laboratory to the photovoltaic market. In this review, the history of power conversion efficiency (PCE) for laboratory-scale PSCs is firstly introduced, and then some methods for maintaining high PCE in the upscaling process is displayed. The achievements in the stability and environmental friendliness of PSCs are also summarized because they are also of significance for commercialization. Finally, this review evaluates the commercialization prospects of PSCs from the economic view and provides a short outlook.

Chlorides, other Halides, and Pseudo-Halides as Additives for the Fabrication of Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) are attracting a tremendous attention from the scientific community due to their excellent power conversion efficiency, low cost, and great promise for the future of solar energy. The best PSCs have already achieved a certified power conversion efficiency (PCE) of 25.5 % after an unprecedented rapid performance rise. However, high requirements with respect to large area, high-efficiency devices, and stability are still the challenges. Major efforts, especially for achieving a high degree of chemical control, have been made to reach these targets. The use of halide additives has played a critical role in improving the efficiency and stability. The present paper reviews the important breakthroughs in PSC technologies made by using halide additives, especially chloride, and pseudo-halide additives for the preparation of the perovskite layers, other layers, and interfaces of the devices. These additives help perovskite (PVK) crystallization and layer morphology control, grain boundary reduction, bulk and interface defects passivation, and so on. Normally, these halide additives play different roles depending on their categories and their location. Herein, recent progresses made due to additives employment in every possible layer of PSCs are reviewed, with focus on chloride, other halides, and pseudo-halides as additives in PVK films, halide additives in carrier transport layers, and at PVK-contact interfaces. Finally, an outlook of engineering of these additives in PSC progress is given.

Huge upconversion luminescence enhancement by a cascade optical field modulation strategy facilitating selective multispectral narrow-band near-infrared photodetection

Since selective detection of multiple narrow spectral bands in the near-infrared (NIR) region still poses a fundamental challenge, we have, in this work, developed NIR photodetectors (PDs) using photon upconversion nanocrystals (UCNCs) combined with perovskite films. To conquer the relatively high pumping threshold of UCNCs, we designed a novel cascade optical field modulation strategy to boost upconversion luminescence (UCL) by cascading the superlensing effect of dielectric microlens arrays and the plasmonic effect of gold nanorods, which readily leads to a UCL enhancement by more than four orders of magnitude under weak light irradiation. By accommodating multiple optically active lanthanide ions in a core-shell-shell hierarchical architecture, developed PDs on top of this structure can detect three well-separated narrow bands in the NIR region, i.e., those centered at 808, 980, and 1540 nm. Due to the large UCL enhancement, the obtained PDs demonstrate extremely high responsivities of 30.73, 23.15, and 12.20 A W^-1 and detectivities of 5.36, 3.45, and 1.91 × 10¹¹ Jones for 808, 980, and 1540 nm light detection, respectively, together with short response times in the range of 80-120 ms. Moreover, we demonstrate for the first time that the response to the excitation modulation frequency of a PD can be employed to discriminate the incident light wavelength. We believe that our work provides novel insight for developing NIR PDs and that it can spur the development of other applications using upconversion nanotechnology.

DFT screening of metallic single-replacements for lead-free perovskites with intrinsic photovoltaic functionalities

Methylammonium lead triiodide perovskites, CH₃NH₃PbI₃ (MAPbI₃), are solution-processable materials with photovoltaic properties capable of surpassing those of silicon solar cells. However, concerns over lead toxicity and lack of exploration into transition metal perovskites drove this ab initio Density Functional Theory screening for environmentally friendly perovskite materials by incorporating transition and post-transition metals at the B-site of MAPbI₃. This revealed fourteen replacements to be suitable: their band structures are highly dispersive while band gaps of such materials fall within ideal ranges for single-junction and tandem cells. Transition metal monoreplacements are shown to be viable perovskites after reducing the size of the halide, corroborating that tunability of the band gap is observed in halide replacement at the X-site. Strong peaks in the imaginary output of the dielectric function below 3.5 eV indicate high sunlight absorption efficiency for select materials. Excellent carrier mobility is expected of studied materials as their effective mass is low. This work helps gain further insight into the viability of transition metals for lower toxicity and higher absorption divalent perovskites.

DFT calculations revealed MAZnCl₃ as a suitable replacement of MAPbI3, and revealed new low band-gap transition metal perovskites

Minimizing Defect States in Lead Halide Perovskite Solar Cell Materials

In order to reach the theoretical efficiency limits of lead-based metal halide perovskite solar cells, the voltage should be enhanced because it suffers from non-radiative recombination. Perovskite materials contain intrinsic defects that can act as Shockley–Read–Hall recombination centers. Several experimental and computational studies have characterized such defect states within the band gap. We give a systematic overview of compositional engineering by distinguishing the different defect-reducing mechanisms. Doping effects are divided into influences on: (1) crystallization; (2) lattice properties. Incorporation of dopant influences the lattice properties by: (a) lattice strain relaxation; (b) chemical bonding enhancement; (c) band gap tuning. The intrinsic lattice strain in undoped perovskite was shown to induce vacancy formation. The incorporation of smaller ions, such as Cl, F and Cd, increases the energy for vacancy formation. Zn doping is reported to induce strain relaxation but also to enhance the chemical bonding. The combination of computational studies using (DFT) calculations quantifying and qualifying the defect-reducing propensities of different dopants with experimental studies is essential for a deeper understanding and unraveling insights, such as the dynamics of iodine vacancies and the photochemistry of the iodine interstitials, and can eventually lead to a more rational approach in the search for optimal photovoltaic materials.

Reducing Detrimental Defects for High-Performance Metal Halide Perovskite Solar Cells

In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film-growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.

Unraveling the Dual-Functional Mechanism of Light Absorption and Hole Transport of Cu2CdxZn1-xSnS4 for Achieving Efficient and Stable Perovskite Solar Cells

Broadening the near-infrared (NIR) spectrum of device is critical to further improve the power conversion efficiency (PCE) of the perovskite solar cells (PSCs). In this work, novel Cu₂CdZn_1-xSnS₄ (CZTS:Cd) film prepared by thermal evaporation method was employed as the NIR light-harvesting layer to complement the absorption of the perovskite. At the same time, Au nanorods (NRs) were introduced into the hole-transporting layer (HTL) to boost the utilization of CZTS:Cd to NIR light through localized surface plasmon effect. The perovskite/CZTS:Cd and Au NR-integrated PSCs can extend the photoelectric response to 900 nm. And more, the well-matched energy levels between CZTS:Cd and perovskite can effectively extract holes from perovskite and depress the charge carrier recombination. As a result, the champion PSC device insulating with CZTS:Cd and Au NRs demonstrates a remarkably increased PCE from 19.30 to 21.11%. The modified PSC devices also demonstrate highly improved long-time stability. The device retains a PCE of 87% after 500 h even under air with a relative humidity of 85%, implying the superior humidity stability of the devices with CZTS:Cd. This work suggests that perovskite/inorganic-integrated structure is a promising strategy to broaden and boost the NIR response of the PSCs.

Incorporation of Nickel Ions to Enhance Integrity and Stability of Perovskite Crystal Lattice for High-Performance Planar Heterojunction Solar Cells

Enhancement of integrity and stability of crystal lattice are highly challenging for polycrystalline perovskite films. In this work, a strategy of incorporation of nickel (Ni) ions is presented to modulate the crystal structure of the CH₃NH₃PbI₃ perovskite film. A broad range of experimental characterizations reveal that the incorporation of Ni ions can substantially eliminate the intrinsic halide vacancy defects, since Ni ions have a strong preference for octahedral coordination with halide ions, resulting in significantly improved integrity and short-range order of crystal lattice. Moreover, it is also demonstrated that the stronger chemical bonding interaction between Ni ions and halide ions as well as organic group can improve the stability of the perovskite material. Simultaneously, the surface morphology of the perovskite thin film is also improved by the incorporation of nickel ions. As a result, a planar heterojunction perovskite solar cell incorporated with 1.5% Ni exhibits a power conversion efficiency of 18.82%, which is improved by 25% compared with 14.92% for the pristine device. Simultaneously, the device formed incorpration of 1.5% Ni shows remarkable stability with 90% of the initial efficiency after storage in an air environment for 800 h. The studies provide a new insight into metal-incorporated perovskite materials for various optoelectronic applications.

Bulk Organic-Inorganic Methylammonium Lead Halide Perovskite Single Crystals for Indirect Gamma Ray Detection

Scintillators that convert ionization radiation photons to UV-visible photons have attracted extraordinary attention. Traditional scintillators are associated with a vacuum photomultiplier tube that faces strict constraints of fragility, magnetic fields, and operated voltage, or coupled to a silicon photomultiplier (SiPM) with optical silicone grease. Here, we report a high-performance radiation detector with an indirect photon-to-photon conversion radiation detection model based on perovskite single crystals (SCs), where perovskite SCs have been directly integrated into the window of SiPM by using the solution growth method at low temperature. Tunable X (γ)-ray excited light emission in the range of 414 to 600 nm is obtained with different concentrations of Br doping, which greatly matches the response wavelength of SiPM. Small Br-doped CH₃NH₃PbBr_0.05Cl_2.95 SCs exhibit high transmittance and weak self-absorption, resulting in improved scintillation light emissions. Moreover, we have successfully collected a ¹³⁷Cs source gamma-ray pulse height spectrum with the SiPM readout. The MAPbBr_0.05Cl_2.95 scintillator exhibits a decay time of 0.14 ± 0.02 ns and a light yield of 18 000 photons/MeV with an energy resolution of 10.5 ± 0.4% at 662 keV. The results indicate that the CH₃NH₃PbBr_xCl_3-x perovskite SCs could enable the next generation of low-cost, fast, and fine-energy resolution scintillators.

Zn-Alloyed All-Inorganic Halide Perovskite-Based White Light-Emitting Diodes with Superior Color Quality

Recently, lead halide perovskite nanocrystals (NCs) have gained tremendous attention in optoelectronic devices due to their excellent optical properties. However, the toxicity of lead limits their practical applications. Here, the synthesis of Zn2+-alloyed CsZnxPb1-xX3 (up to 15%) NCs is reported to achieve lead-reduced white light-emitting diodes (WLEDs). The incorporation of Zn2+ into CsPbX3 host NCs results in a lattice contraction, without altering the structure and morphology, which has a direct effect on the optical properties. The blue-shifts in the photoluminescence emission and increase in bandgap is observed while retaining high photoluminescence quantum yield. Then by engineering the different compositions of halides for 15% Zn2+-alloyed CsZnxPb1-xX3 NCs, tunable emission (411-636 nm) is obtained. Notably, the WLEDs are experimentally demonstrated employing the lead-reduced NCs (blue, green, yellow, and red). By varying the ratios of the amount of NCs, white lights with a tunable correlated-color temperature (2218-8335 K), an exemplary color-rendering index (up to 93) and high luminous efficacy of radiation (268-318 lm·W-1) are obtained. Best of our knowledge, these are superior to other reported WLEDs based on CsPbX3 NCs doped with transition metal ions. This work places the halide perovskite NCs one-step closer in designing the environmentally benign and energy-efficient WLEDs.

Metal Cations in Efficient Perovskite Solar Cells: Progress and Perspective

Metal halide perovskite solar cells (PVSCs) have revolutionized photovoltaics since the first prototype in 2009, and up to now the highest efficiency has soared to 24.2%, which is on par with commercial thin film cells and not far from monocrystalline silicon solar cells. Optimizing device performance and improving stability have always been the research highlight of PVSCs. Metal cations are introduced into perovskites to further optimize the quality, and this strategy is showing a vigorous development trend. Here, the progress of research into metal cations for PVSCs is discussed by focusing on the position of the cations in perovskites, the modulation of the film quality, and the influence on the photovoltaic performance. Metal cations are considered in the order of alkali cations, alkaline earth cations, then metal cations in the ds and d regions, and ultimately trivalent cations (p- and f-block metal cations) according to the periodic table of elements. Finally, this work is summarized and some relevant issues are discussed.

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Crystal Orientation and Grain Size: Do They Determine Optoelectronic Properties of MAPbI3 Perovskite?

Growing large, oriented grains of perovskite often leads to efficient devices, but it is unclear if properties of the grains are responsible for the efficiency. Domains observed in SEM are commonly misidentified with crystallographic grains, but SEM images do not provide diffraction information. We study methylammoinium lead iodide (MAPbI3) films fabricated via flash infrared annealing (FIRA) and the conventional antisolvent (AS) method by measuring grain size and orientation using electron back-scattered diffraction (EBSD) and studying how these affect optoelectronic properties such as local photoluminescence (PL), charge carrier lifetimes, and mobilities. We observe a local enhancement and shift of the PL emission at different regions of the FIRA clusters, but we observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite substantial differences in grain size between the two systems, we find similar optoelectronic properties. These findings show that optoelectronic quality is not necessarily related to the orientation and size of crystalline domains.

High-Performance CsPbIBr2 Perovskite Solar Cells: Effectively Promoted Crystal Growth by Antisolvent and Organic Ion Strategies

Growing attention has been paid to CsPbIBr₂ perovskite solar cells (PSCs) after balancing the band gap and stability features of the interested full-inorganic perovskites. However, their power-conversion efficiency (PCE) still lags behind that of the PSCs using hybrid halide perovskite and how to increase the corresponding PCE is still a challenge. Herein, antisolvents and organic ion surface passivation strategies were systematically applied to precisely control the growth of CsPbIBr₂ crystals for constructing a high-quality full-inorganic perovskite film. Through careful adjustments, a CsPbIBr₂ film with a pure phase, full coverage, and high crystallinity with preferable (100) orientation was successfully obtained by introducing diethyl ether as the antisolvent followed by guanidinium surface passivation. The optimal CsPbIBr₂ film was composed by a large grain with an average size of 950 nm, few grain boundaries, and higher hydrophobic property. Planer PSC using the optimal CsPbIBr₂ film and electron-beam-deposited TiO₂ compact layer exhibits a PCE of 9.17%, which ranks among the highest PCE range of the reported CsPbIBr₂ PSCs. Besides, the designed CsPbIBr₂ PSC exhibited good long-term stability, which could maintain 90% of the initial PCE in 40% humidity ambient, which remained constant after heat treatment at 100 °C for 100 h. Based on the optimal CsPbIBr₂ film, the flexible and large-area (up to 225 mm²) PSCs were further fabricated. The adopted film improvement methods were further extended to other kinds of full-organic PSCs, which demonstrated the universality of this strategy.

Air-Stable and Oriented Mixed Lead Halide Perovskite (FA/MA) by the One-Step Deposition Method Using Zinc Iodide and an Alkylammonium Additive

We present a one-step method to produce air-stable, large-grain mixed cationic lead perovskite films and powders under ambient conditions. The introduction of 2.5 % of Zn(II), confirmed by X-ray diffraction (XRD), results in stable thin films which show the same absorption and crystal structure after 2 weeks of storage under ambient conditions. Next to prolonged stability, the introduction of Zn(II) affects photophysical properties, reducing the bulk defect density, enhancing the photoluminescence (PL), and extending the charge carrier lifetime. Furthermore, 3-chloropropylamine hydrochloride is applied as the film-forming agent. The presence of this amine hydrochloride additive results in highly oriented and large crystal domains showing an ulterior improvement of PL intensity and lifetime. The material can also be prepared as black precursor powder by a solid-solid reaction under ambient conditions and can be pressed into a perovskite pellet. The prolonged stability and the easy fabrication in air makes this material suitable for large-scale, low-cost processing for optoelectronic applications.

Perovskite Thin Film Materials Stabilized and Enhanced by Zinc(II) Doping

Recent work of ten different groups shows that the application of zinc-halides in lead perovskite materials results in a contraction of the d-space, stronger interaction with the organic cation, improved crystallization with larger crystal domains, a Goldschmidt factor closer to unity, smoother and denser thin films and an even distribution of Zn(II) (at the Pb(II) sites) throughout the material. These combined effects may lead to: (1) a substantially higher stability (even at ambient or high humidity conditions); (2) enhanced luminescent properties; (3) a higher power conversion efficiency (PCE) of the corresponding solar cell devices (up to PCE ~20%, with enhancement factors of 1.07 to 1.33 relative to undoped material).

Enhancing Photostability of Perovskite Solar Cells by Eu(TTA)2(Phen)MAA Interfacial Modification

Organic-inorganic lead halide perovskite solar cells (PSCs) exhibit spectacular changes in the photovoltaic area, but they still face the challenges of full spectral utilization and photostability under continuous light irradiation. The ultraviolet (UV) part in sunlight could induce oxygen vacancy in the mesoporous TiO₂ (m-TiO₂) layer, resulting in the degradation of perovskite photoactive films and the rapidly decreased device performance. In this work, we demonstrate that an effective luminescent downconversion material, Eu(TTA)₂(Phen)MAA (ETPM), can be used as an interfacial modifier between the m-TiO₂ layer and the perovskite photoactive layer to improve the power conversion efficiency (PCE) from 17.00 to 19.07%. The improved device performance can be ascribed to the effective utilization of incident UV light and reduced carrier recombination. Meanwhile, the conversion of the UV light by ETPM could inhibit the stability loss of the device under irradiation. As a result, the modified PSCs can maintain 86% of their initial value under continuous light soaking for 100 h, higher than that of 40% for the control device. This work indicates that the introduction of the luminescent downconversion material ETPM can successfully improve the PCE and photostability of PSCs.

Lead-Free Halide Double Perovskite Materials: A New Superstar Toward Green and Stable Optoelectronic Applications

Lead-based halide perovskites have emerged as excellent semiconductors for a broad range of optoelectronic applications, such as photovoltaics, lighting, lasing and photon detection. However, toxicity of lead and poor stability still represent significant challenges. Fortunately, halide double perovskite materials with formula of A₂M(I)M(III)X₆ or A₂M(IV)X₆ could be potentially regarded as stable and green alternatives for optoelectronic applications, where two divalent lead ions are substituted by combining one monovalent and one trivalent ions, or one tetravalent ion. Here, the article provides an up-to-date review on the developments of halide double perovskite materials and their related optoelectronic applications including photodetectors, X-ray detectors, photocatalyst, light-emitting diodes and solar cells. The synthesized halide double perovskite materials exhibit exceptional stability, and a few possess superior optoelectronic properties. However, the number of synthesized halide double perovskites is limited, and more limited materials have been developed for optoelectronic applications to date. In addition, the band structures and carrier transport properties of the materials are still not desired, and the films still manifest low quality for photovoltaic applications. Therefore, we propose that continuing efforts are needed to develop more halide double perovskites, modulate the properties and grow high-quality films, with the aim of opening the wild practical applications.

Improving the light harvesting and colour range of methyl ammonium lead tri-bromide (MAPbBr3) perovskite solar cells through co-sensitisation with organic dyes

Co-sensitisation of methylammonium lead tri-bromide perovskite solar cells with red (D205) and blue (SQ2) organic dyes improves device efficiencies and allows device colour tuning. Sensitising the film after perovskite crystallisation produces higher device efficiencies (2.6% SQ2, 3.1% D205) than perovskite-only devices (2%) and devices sensitised before the perovskite layer deposition (1.5% SQ2, 2.0% D205).

Magnesium-Doped MAPbI3 Perovskite Layers for Enhanced Photovoltaic Performance in Humid Air Atmosphere

Despite the high efficiency of MAPbI₃ perovskite solar cells, the long term stability and degradation in humid atmosphere are issues that still needed to be addressed. In this work, magnesium iodide (MgI₂) was first successfully used as a dopant into MAPbI₃ perovskite prepared in humid air atmosphere. Mg doping decreased the valence band level, which was determined from photoelectron yield spectroscopy. Compared to the pristine MAPbI₃ perovskite film, the 1.0% Mg-doped perovskite film showed increased crystal grain size and formation of pinhole-free perovskite film. Performance of the solar cell was increased from 14.2% of the doping-free solar cell to 17.8% of 1.0% Mg-doped device. Moreover, 90% of the original power conversion efficiency was still retained after storage in 30-40% relative humidity for 600 h.

Dependence of Acetate-Based Antisolvents for High Humidity Fabrication of CH3NH3PbI3 Perovskite Devices in Ambient Atmosphere

High-efficiency perovskite solar cells (PSCs) need to be fabricated in the nitrogen-filled glovebox by the atmosphere-controlled crystallization process. However, the use of the glovebox process is of great concern for mass level production of PSCs. In this work, notable efficient CH₃NH₃PbI₃ solar cells can be obtained in high humidity ambient atmosphere (60-70% relative humidity) by using acetate as the antisolvent, in which dependence of methyl, ethyl, propyl, and butyl acetate on the crystal growth mechanism is discussed. It is explored that acetate screens the sensitive perovskite intermediate phases from water molecules during perovskite film formation and annealing. It is revealed that relatively high vapor pressure and high water solubility of methyl acetate (MA) leads to the formation of highly dense and pinhole free perovskite films guiding to the best power conversion efficiency (PCE) of 16.3% with a reduced hysteresis. The devices prepared using MA showed remarkable shelf life stability of more than 80% for 360 h in ambient air condition, when compared to the devices fabricated using other antisolvents with low vapor pressure and low water solubility. Moreover, the PCE was still kept at 15.6% even though 2 vol % deionized water was added in the MA for preparing the perovskite layer.

CH3NH3Pb1−xEuxI3 mixed halide perovskite for hybrid solar cells: the impact of divalent europium doping on efficiency and stability

The crucial role of the impact of divalent europium doping in perovskite solar cells is investigated in this work.