Modulation of recombination zone position for quasi-two-dimensional blue perovskite light-emitting diodes with efficiency exceeding 5

In recent years, substantial progress has been made in developing perovskite light-emitting diodes with near-infrared, red and green emissions and over 20% external quantum efficiency. However, the development of perovskite light-emitting diodes with blue emission remains a great challenge, which retards further development of full-color displays and white-light illumination based on perovskite emissive materials. Here, firstly, through composition and dimensional engineering, we prepare quasi-two-dimensional perovskite thin films with improved blue emission, taking advantages of reduced trap density and enhanced photoluminescence quantum yield. Secondly, we find a vertically non-uniform distribution of perovskite crystals in the PEDOT:PSS/perovskite hybrid film. Through modulating the position of the recombination zone, we activate the majority of quasi-two-dimensional perovskite crystals, and thus demonstrate the most efficient blue perovskite light-emitting diode to date with emission peak at 480 nm, record luminance of 3780 cd m-2 and record external quantum efficiency of 5.7%.

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.

Porous and Intercrossed PbI2-CsI Nanorod Scaffold for Inverted Planar FA-Cs Mixed-Cation Perovskite Solar Cells

Depth-dependent growth of perovskite crystals remains challenging for high-performance perovskite solar cells made by a two-step spin-coating method. Effective morphology engineering approaches that enable depth-independent perovskite crystals growth and facile characterization technique to monitor subtle yet influential accompanying changes are urgently required. Here, a porous and intercrossed PbI₂-(CsI)_0.15 nanorods scaffold is prepared by integrating CsI incorporation with toluene dripping in ambient air, and the underlying mechanism is uncovered. With this porous scaffold and moisture-assisted thermal annealing, depth-independent growth of FA_0.85Cs_0.15PbI₃ is achieved, as evidenced in the photoluminescent (PL) spectra acquired by exciting the perovskite film from the top and bottom individually. It is of broad interest that PL spectroscopy is demonstrated as a sensitive technique to monitor the depth-dependent growth of perovskite. Moreover, the resulting inverted planar FA_0.85Cs_0.15PbI₃ perovskite solar cells deliver an efficiency of 16.85%, along with superior thermal and photostability. By incorporating 2% large-sized diammonium cation, propane-1,3-diammonium, the efficiency is further increased to 17.74%. Our work not only proposes a unique porous PbI₂-(CsI)_0.15 nanorods scaffold to achieve high-quality perovskite films in a two-step method but also highlights the distinctive advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films.

Effect of chloride substitution on interfacial charge transfer processes in MAPbI3 perovskite thin film solar cells: planar versus mesoporous

For photovoltaic devices based on hybrid organic-inorganic perovskite thin films, the cell architecture is a vital parameter in defining the macroscopic performance. However, the understanding of the correlation between architecture and carrier dynamics in perovskite thin films has remained elusive. In this work, we utilize concerted materials characterization and optical measurements to investigate the role of chloride addition in PSC devices with two different architectures. Perovskite thin films, prepared with varying ratios of methylammonium halide MACl : MAI (0 : 1, 0.5 : 1, 1 : 1, and 2 : 1), were coated on either planar or mesoporous TiO₂/FTO substrates. X-ray diffraction analysis reveals that with increasing the ratio of the Cl^- precursor, there is an increasing preferential directional growth of the perovskite film in both configurations. Time-resolved photoluminescence spectroscopy was applied to investigate the electron injection dynamics from the photoexcited perovskites to the TiO₂. It is found that the interfacial electron injection rate from perovskite to planar TiO₂ is accelerated with increasing Cl^- content, which explains the increased power conversion efficiencies using Cl^--modified perovskites as photoactive materials. In contrast, Cl^- addition demonstrate no discernable influence on electron injection to mesoporous TiO₂, suggesting the interfacial charge recombination rather than electron injection give rise to the improved performance observed in the mesoporous configuration. The results presented here, provide a deeper understanding of the mechanism of chloride addition to MAPbI₃ solar cells with different architectures.

Green Solution-Processed Tin-Based Perovskite Films for Lead-Free Planar Photovoltaic Devices

The eco-friendly Sn-based perovskites have attracted more and more attention in lead-free perovskite photovoltaic field. However, the device performance and reproducibility are greatly challenged in preparing high-quality perovskite films. Here, we fabricated uniform and dense Sn-based perovskite films via a green gas pump treatment technology. Remarkably, we successfully fabricated a large-area (>20 cm²) Sn-based perovskite film with a mirror-like surface, which is the largest Sn-based perovskite film ever reported. Besides, we found that the phase separation phenomenon induced by excess SnF₂ was eliminated when the pressure is 1500 Pa. Finally, we fabricated highly reproducible Sn-based solar cells and obtained an inspiring efficiency of 1.85%, which is the highest reported efficiency for Sn-based devices with a configuration of fluorine-doped tin oxide/compact TiO₂/perovskite/hole transport material/electrode. Our results demonstrate the feasibility of using gas pump treatment technique to prepare high-quality Sn-based perovskite films, which paves a way for large-scale green manufacturing of Sn-based perovskite solar cells in the future.

Light Processing Enables Efficient Carbon-Based, All-Inorganic Planar CsPbIBr2 Solar Cells with High Photovoltages

Inorganic halide perovskite CsPbIBr₂ possesses the most balanced band gap and stability characters among all of the concerned analogs for carbon-based, all-inorganic solar cells that are free of any hole-transporting layers and noble-metal electrodes. Yet, the current CsPbIBr₂ solar cells seem to deliver the lowest record efficiency. This is originally plagued by a serious energy loss ( E_loss) in the cells, which thus limits their open-circuit voltages ( V_oc) severely. Herein, we demonstrate a light-processing technology that can overcome this obstacle successfully, by enabling the full-coverage, pure-phase CsPbIBr₂ films featured with large grains, high crystallinity, and preferential [100] grains orientation, along with favorable electronic structure. It is achieved by the exposure of CsPbIBr₂ precursor film formed in a conventional one-step spin-coating route to a simulated AM 1.5 G illumination before thermal annealing. The resulting carbon-based, all-inorganic planar cells give an optimized power conversion efficiency (PCE) of 8.60% with the V_oc of 1.283 V. Notably, such an impressive V_oc stands the highest value among all of the previously reported CsPbIBr₂ solar cells; hence, its PCE exceeds nearly all of them. Therefore, our work suggests a new route to further improve the efficiency of low-cost, stable, and simple-fabrication CsPbIBr₂ solar cells.

A peri-Xanthenoxanthene Centered Columnar-Stacking Organic Semiconductor for Efficient, Photothermally Stable Perovskite Solar Cells

Modulating the structure and property of hole-transporting organic semiconductors is of paramount importance for high-efficiency and stable perovskite solar cells (PSCs). This work reports a low-cost peri-xanthenoxanthene based small-molecule P1, which is prepared at a total yield of 82 % using a three-step synthetic route from the low-cost starting material 2-naphthol. P1 molecules stack in one-dimensional columnar arrangement characteristic of strong intermolecular π-π interactions, contributing to the formation of a solution-processed, semicrystalline thin-film exhibiting one order of magnitude higher hole mobility than the amorphous one based on the state-of-the art hole-transporter, 2,2-7,7-tetrakis(N,N'-di-paramethoxy-phenylamine 9,9'-spirobifluorene (spiro-OMeTAD). PSCs employing P1 as the hole-transporting layer attain a high efficiency of 19.8 % at the standard AM 1.5 G conditions, and good long-term stability under continuous full sunlight exposure at 40 °C.

N-Methyl-2-pyrrolidone as an excellent coordinative additive with a wide operating range for fabricating high-quality perovskite films

N-Methyl-2-pyrrolidone (NMP), forming only one PbI₂·NMP complex, is demonstrated as an excellent coordinative solvent for the fabrication of high-quality perovskite thin films.

In situ epitaxial growth of Ag3PO4 quantum dots on hematite nanotubes for high photocatalytic activities

Surface-adsorbed phosphate anions on Fe₂O₃ nanotubes can guide the in situ epitaxial growth of Ag₃PO₄ quantum dots on the nanotubes, efficiently improving the photogenerated charge transfer and photocatalytic activity.

Furrowed hole-transport layer using argon plasma in an inverted perovskite solar cell

In this study, a novel process was found to be effective using the argon-plasma treatment, in which the ion cluster was used to scour the PEDOT:PSS surface instead of the traditional bombardment method. The photoelectric conversion efficiency of the device reaches 14.8%.

Nickel phthalocyanine as an excellent hole-transport material in inverted planar perovskite solar cells

Pristine nickel phthalocyanine (NiPc) was introduced as a hole transporting material (HTM) in inverted planar perovskite solar cells (PSCs) for the first time. A power conversion efficiency of 14.3% was achieved, outperforming the values obtained in the solar cells based on the CuPc HTM, which is a typical representative of metal phthalocyanine based HTMs. Moreover, inverted planar PSCs based on NiPc HTMs show a very weak hysteresis behavior.

Lithium and Silver Co-Doped Nickel Oxide Hole-Transporting Layer Boosting the Efficiency and Stability of Inverted Planar Perovskite Solar Cells

In this work, a lithium and silver co-doping strategy has been successfully implied to prepare NiO _x films for high performance inverted planar perovskite solar cells (PSCs). Compared to the pristine and single-doped NiO _x, the Li and Ag co-doping approach exhibits the synergistic effect and can endow NiO _x films with higher electrical conductivity, higher hole mobility and better interface energy band alignment with perovskite active layers. Moreover, the perovskite film with enhanced crystallinity can be obtained induced by the Li,Ag:NiO _x film. The PSC with Li,Ag:NiO _x HTL shows a high power conversion efficiency (PCE) up to 19.24% and less hysteresis effect, which outperforms the devices with the pristine NiO _x or single-doped NiO _x HTLs. Meanwhile, the Li,Ag:NiO _x device can retain 95% of its initial PCE after storage at the relative humidity of 30 ± 2% in 30 days without encapsulation. Our work demonstrates that lithium and silver co-doping is a promising route for realizing efficient p-type NiO _x HTL, which provides a simple way to boost the efficient and stable of inverted planar PSCs.

Diboron-Assisted Interfacial Defect Control Strategy for Highly Efficient Planar Perovskite Solar Cells

Metal halide perovskite films are endowed with the nature of ions and polycrystallinity. Formamidinium iodide (FAI)-based perovskite films, which include large cations (FA) incorporated into the crystal lattice, are most likely to induce local defects due to the presence of the unreacted FAI species. Here, a diboron-assisted strategy is demonstrated to control the defects induced by the unreacted FAI both inside the grain boundaries and at the surface regions. The diboron compound (C₁₂ H₁₀ B₂ O₄ ) can selectively react with unreacted FAI, leading to reduced defect densities. Nonradiative recombination between a perovskite film and a hole-extraction layer is mitigated considerably after the introduction of the proposed approach and charge-carrier extraction is improved as well. A champion power conversion efficiency of 21.11% is therefore obtained with a stabilized power output of 20.83% at the maximum power point for planar perovskite solar cells. The optimized device also delivers negligible hysteresis effect under various scanning conditions. This approach paves a new way for mitigating defects and improving device performance.

Design of an Inorganic Mesoporous Hole-Transporting Layer for Highly Efficient and Stable Inverted Perovskite Solar Cells

The unstable feature of the widely employed organic hole-transporting materials (HTMs) (e.g., spiro-MeOTAD) significantly limits the practical application of perovskite solar cells (PSCs). Therefore, it is desirable to design new structured PSCs with stable HTMs presenting excellent carrier extraction and transfer properties. This work demonstrates a new inverted PSC configuration. The new PSC has a graded band alignment and bilayered inorganic HTMs (i.e., compact NiO_x and mesoporous CuGaO₂ ). In comparison with planar-structured PSCs, the mesoporous CuGaO₂ can effectively extract holes from perovskite due to the increased contact area of the perovskite/HTM. The graded energy alignment constructed in the ultrathin compact NiO_x , mesoporous CuGaO₂ , and perovskite can facilitate carrier transfer and depress charge recombination. As a result, the champion device based on the newly designed mesoscopic PSCs yields a stabilized efficiency of ≈20%, which is considered one of the best results for inverted PSCs with inorganic HTMs. Additionally, the unencapsulated PSC device retains more than 80% of its original efficiency when subjected to thermal aging at 85 °C for 1000 h in a nitrogen atmosphere, thus demonstrating superior thermal stability of the device. This study may pave a new avenue to rational design of highly efficient and stable PSCs.

Preparation and Two-Photon Photoluminescence Properties of Organic Inorganic Hybrid Perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4

Organic inorganic hybrid perovskites have potential applications in solar cells, electroluminescent devices and radiation detection because of their unique optoelectronic properties. In this paper, the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 were synthesized by solvent evaporation. The crystal structure, morphology, absorption spectrum, laser power dependence of the photoluminescence (PL) intensity and lifetime were studied. The results showed that the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 display a layered stacking structure of organic and inorganic components. The absorption peaks are located at 392 nm (3.16 eV) and 516 nm (2.40 eV), respectively. It was observed that the PL intensity and photoluminescence quantum yield (PLQY) increases with increasing laser power, and that the PL lifetime decreases with increasing laser power, which is mainly due to the non-geminate recombination.

A Compact and Smooth CH₃NH₃PbI₃ Film: Investigation of Solvent Sorts and Concentrations of CH₃NH₃I towards Highly Efficient Perovskite Solar Cells

Four solvents (isopropanol (IPA), n-butyl alcohol (NBA), n-amyl alcohol (NAA), and n-hexyl alcohol (NHA)) were investigated to prepare CH₃NH₃I (methylammonium iodide, MAI) solutions to transform PbI₂ film into CH₃NH₃PbI₃ (MAPbI₃) film. It was found that the morphology of the perovskite MAPbI₃ film was not only affected by the chain of the solvent molecule, but also by the concentration of MAI. The use of solvents with a long alkyl chain (NAA and NHA) allowed the MAPbI₃ to grow via an in situ transformation step, which easily made the perovskite films compact, but with a high surface roughness due to the growth of unexpected nanorods/nanoplates. The solvent with a short alkyl chain (IPA) led to the dissolution−crystallization growth mechanism, resulting in rapid generation of perovskite films with a number of pinholes. A high-quality (compact, smooth, pinhole-free) perovskite film was obtained with NBA and an optimized MAI concentration of 8 mg/mL. The corresponding perovskite solar cells achieved a maximum power conversion efficiency (PCE) of 16.66% and average PCE of 14.76% (for 40 cells).

Repairing Defects of Halide Perovskite Films To Enhance Photovoltaic Performance

On account of the low-temperature solution fabrication, the much high defect density at the interfaces and grain boundaries of halide perovskite films is recognized as one of the big obstacles toward high-efficiency solar cells. Here, the time-resolved photoluminescence (TRPL) with incident light exciting from the upper surface and bottom of halide perovskite films, respectively, showed very different results, verifying the much more surface trap states in the film. To eliminate the defects and enhance the photovoltaic properties of perovskite solar cells (PSCs), we designed a facile and effective method to repair the defects of the perovskite film using formamidinium iodine (FAI) solution. The dissociative FA⁺ and I^- ions could compensate for the loss of volatile organic cations and also fill the I^- vacancies of halide perovskites. After repairing defects with proper concentration of FAI solution, the TRPL curves obtained by light exciting from the different sides of the perovskite film nearly overlap together, indicating the reduction of surface traps. As a result, both the total carrier lifetime and charge extractions were improved by removing the nonradiative channels (surface traps), which universally enhanced the power conversion efficiency and stability of the planar heterojunction structural PSCs.

Challenges for commercializing perovskite solar cells

Perovskite solar cells (PSCs) have witnessed rapidly rising power conversion efficiencies, together with advances in stability and upscaling. Despite these advances, their limited stability and need to prove upscaling remain crucial hurdles on the path to commercialization. We summarize recent advances toward commercially viable PSCs and discuss challenges that remain. We expound the development of standardized protocols to distinguish intrinsic and extrinsic degradation factors in perovskites. We review accelerated aging tests in both cells and modules and discuss the prediction of lifetimes on the basis of degradation kinetics. Mature photovoltaic solutions, which have demonstrated excellent long-term stability in field applications, offer the perovskite community valuable insights into clearing the hurdles to commercialization.