Mahui

High‐performance white organic light‐emitting diodes with low efficiency roll‐off, superior color stability, and high color rendering index are achieved by strategic design of exciplex hosts for different color phosphors. The forward‐viewing maximum power efficiency is 59.9 lm W⁻¹, and the values still remain 45.3 and 31.9 lm W⁻¹ at 1000 and 5000 cd m⁻², respectively, exhibiting extremely low efficiency roll‐off.

Abstract

White organic light‐emitting diodes (WOLEDs) are one of the most cogent candidates for environment friendly and healthy solid‐state lighting due to many unique merits. Here, a strategic design of exciplex hosts is applied to simultaneously realize the superior color stability, high color rendering index (CRI), high efficiency, and low efficiency roll‐off properties of WOLEDs. The resulting WOLEDs emit the forward‐viewing maximum power efficiency (PE) of 59.9 lm W⁻¹, and the PE still remains 45.3 and 31.9 lm W⁻¹ at 1000 and 5000 cd m⁻², respectively, exhibiting extremely low efficiency roll‐off. More remarkably, the devices also show high CRI over 80 and extremely stable spectral emission in the luminance ranges from 1000 to 15 000 cd m⁻². The excellent performances indicate the validity of the structure design and provide a new route for the development of high‐performance WOLEDs.

Here, Ho³⁺ doped CNAIC is synthesized via the hydrothermal method for the first time. The doped double perovskite combines efficient self‐trapped exciton (STE) and rare earth associated emission. Furthermore, by means of infrared spectrum and photoluminescence (PL) decay, the associated emission mechanism is explored and the existence of the energy transfer channel from STEs to Ho³⁺ is confirmed.

Abstract

Low dimensional halide perovskites with self‐trapped excitons (STEs) emission have emerged as promising white light phosphors because of their ultrabroadband emission covering the entire visible spectrum from 400 to 800 nm. Such a broad emission from a single material can overcome emission color change and self‐absorption problems within multiple phosphors. However, the color rendering index (CRI) and correlated color temperature (CCT) as two essential parameters of white light quality can hardly be modulated in these perovskite materials. Here, rare earth ion Ho³⁺ is introduced into Cs₂(Na,Ag)InCl₆ for the first time, utilizing the hydrothermal method. Besides the strong warm white STEs emission, the as‐synthesized materials exhibit effective characteristic emission of Ho³⁺ in the visible region. Further, the mechanism of associated emission is explored and the existence of energy transfer from STEs to rare earth is first confirmed. A white light‐emitting diode (LED) prototype is also fabricated by employing the Ho³⁺ doped Cs₂(Na,Ag)InCl₆ as the color conversion material on a commercial 365 nm GaN LED chip, achieving an improved CRI from 70.3 to 75.4 compared to the pure Cs₂(Na,Ag)InCl₆. This result suggests a promising way to achieve high quality single phase all‐inorganic white phosphors and this mechanism has enormous potentials in other optoelectronic applications.

Nature Materials, Published online: 16 September 2019; doi:10.1038/s41563-019-0481-6

Nature Materials, Published online: 16 September 2019; doi:10.1038/s41563-019-0480-7

A 2D metal–organic framework In‐Aipa‐derived N‐rich porous carbon material with rich pyridinic‐N and graphitic‐N is first introduced into the hole transport layers of perovskite solar cells as an auxiliary additive, contributing to the significantly improved power conversion efficiency from 16.47% to 18.51%, as well as the enhanced long‐term stability of over 85% efficiency retention under exposure to air for 720 h.

As the standard bidopants of hole transport layers (HTLs) in perovskite solar cells (PSCs), bis(trifluoromethane)sulfonimide lithium salt (Li‐TFSI) and 4‐tert‐butylpyridine not only induce adverse influence on the quality of thin films, but also seriously impair the long‐term stability of devices. Herein, a metal–organic framework‐derived 2D graphitic N‐rich porous carbon (NPC) is first introduced into the HTLs as an effective auxiliary additive. The introduction of NPC significantly reduces the aggregation of lithium salts and the formation of HTL defects, optimizing film quality for rapid hole extraction and migration. Furthermore, inherent porosity and hydrophobicity of NPCs are extremely beneficial to restrict the permeation of Li⁺ ions and anode metals, and prevent the moisture from eroding the HTLs and perovskite layers, enhancing the stability of PSCs. As expected, the PSCs with NPC realize a satisfactory fill factor of 0.76 and power conversion efficiency (PCE) of 18.51%, apparently higher than that of pristine devices (0.70% and 16.47%). In addition, over 85% of the initial PCE for optimized PSCs is maintained after 720 h of exposure to air. Obviously, an innovative strategy for highly efficient and long‐term stable PSC devices is provided.

The fabrication method of high‐quality (Cs) _x (FA)_1−x PbI₃ perovskite films by varying the amount of cesium chloride (CsCl) in the FAPbI₃ precursor solutions is demonstrated. The best photovoltaic performance with a power conversion efficiency of 19.20% is achieved for the device with 10 mol% excess of CsCl.

The quality of perovskite films plays a crucial role in improving the optoelectronic properties and performance of perovskite solar cells (PSCs). Herein, high‐quality Cs _x FA_1−x PbI₃ perovskite films with different compositions (x = 0, 5, 10, and 15) are achieved by controlling the amount of cesium chloride (CsCl) in the respective FAPbI₃ precursor solution. The effects of CsCl addition on the morphological and optoelectronic properties of the resulting perovskite films and on the performance of the corresponding devices are systematically studied. Introduction of CsCl into FAPbI₃ shows a great potential to stabilize the α‐FAPbI₃ perovskite phase by forming Cs _x FA_1−x PbI₃ films with improved morphology and carrier lifetimes. With an optimal 10 mol% CsCl additive, the average power conversion efficiency (PCE) is increased from 16.83 ± 0.30% for the reference FAPbI₃‐based PSCs to 18.87 ± 0.25% (with a steady‐state PCE of 18.89%). Moreover, the optimized device performance is more stable after 20 days than the controlled one under ≈40% humidity in air.

An efficient control of the film quality and thickness distribution of alternating cations in the interlayer space of 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) quantum wells via incorporation of methylammonium chloride as an additive is demonstrated. The optimized device leads to more efficient charge transport and suppressed nonradiative charge recombination. Consequently, the optimized perovskite solar cell delivers an efficiency of 18.48%.

Abstract

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge‐transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) _n Pb _n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low‐n and high‐n QWs and suppressed nonradiative charge recombination. Therefore, the additive‐treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open‐circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden–Popper, and Dion–Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

Herein, amorphous‐Zn₂SnO₄ (am‐ZTO) is used to provide a large free energy difference (ΔG) to improve electron injection from perovskite to electron transport layers. In addition, the introduction of the am‐ZTO also leads to a dense physical contact between the am‐ZTO and the FTO substrate, leading to decreased leakage current. The optimized device exhibits a power conversion efficiency of 20.04%.

Charge extraction by electron transport layers (ETLs) plays a vital role in improving the performance of perovskite solar cells (PSCs). Here, PSCs with four different types of ETLs, such as SnO₂, amorphous‐Zn₂SnO₄ (am‐ZTO), am‐ZTO/SnO₂, and SnO₂/am‐ZTO, are successfully synthesized. The interface recombination behavior and the charge transport properties of the devices affected by four types of ETLs are systematically investigated. For dual am‐ZTO/SnO₂ ETLs, compact am‐ZTO ETL prepared by the pulsed laser deposition method provides a dense physical contact with FTO than the spin coating films, decreasing leakage current and improving charge collection at the interface of ETL/FTO. Moreover, dual am‐ZTO/SnO₂ ETLs lead to large free energy difference (ΔG), improving electron injection from perovskite to ETLs. One additional electron pathway from perovskite to am‐ZTO is formed, which can also improve electron injection efficiency. A power conversion efficiency of 20.04% and a stabilized efficiency of 19.17% are achieved for the device based on dual am‐ZTO/SnO₂ ETLs. Most importantly, the devices are fabricated at a low temperature of 150 °C, which offers a potential method for large‐scale production of PSCs, and paves the way for the development of flexible PSCs. It is believed that this work provides a strategy to design ETLs via controlling ΔG and interface contact to improve the performance of PSCs.

Upscaling loss for perovskite devices is higher than for any other photovoltaic technology. Herein, electroluminescence, dark lock‐in thermography, microphotoluminescence spectroscopy, and electron spectroscopy are used to investigate upscaling losses, focusing on layer inhomogeneities for modules with an aperture area up to 100 cm². Analysis helps in identification of processing pitfalls and strategies for overcoming or minimizing their effects.

Hybrid metal‐halide perovskite‐based thin‐film photovoltaics (PVs) have the potential to become the next generation of commercialized PV technology with certified power conversion efficiencies reaching 24% on devices having 0.1 cm² area. Recent efforts in upscaling this technology result in an efficiency of 12.6% for 354 cm² modules. However, upscaling loss for perovskite‐based PVs is higher than for any other PV technology. In this study, upscaling losses of devices with aperture area 0.1, 4, and 100 cm² are investigated, with a focus on layer inhomogeneities. Electroluminescence, dark lock‐in thermography, microphotoluminescence spectroscopy, and electron spectroscopy are used to analyze and group layer inhomogeneities with a minimal size of 10 μm and to compare loss mechanisms for radial and linear deposition techniques. Analysis results help to identify current processing pitfalls, where understanding and control of perovskite crystal formation plays the crucial role.

Recent progress in bandgap engineering strategies including the two main, widely used impurity and pressure as well as intermediate band, external electric field, and steric methods are reviewed comprehensively. Their underlying mechanism, achievements, and challenges are outlined. Additionally, future research directions are provided to realize direct and gap size continually tunable perovskites for further enhancing solar cell performance.

Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single‐junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

Using the oxygen‐containing functional group (—COOH and/or —C—OH)‐decorated multiwalled carbon nanotubes as the electrode, the power conversion efficiency of perovskite solar cells shows an improvement after long‐term storage. The reason is confirmed to be the self‐reconstruction ability of perovskite material and the interface reconstruction for its morphology and charge transfer resistance showing significant improvement.

Perovskite solar cells (PSCs) have attracted a lot of interest because of their high efficiency and low cost. However, in commercial applications, standard PSCs suffer from low stability of the cell components, including the hole transportation material (HTM). Owing to their characteristics of high chemical stability, hydrophobicity, and high conductivity, carbon nanotubes (CNTs) can be an alternative electrode to use to form HTM‐free PSCs. Enhancing the interaction with perovskite is vital not only for photovoltaic performance but also for the stability of CNT‐based PSCs. Herein, oxygen‐containing functional groups are introduced into CNTs via acid treatment to enhance the chemical interactions with perovskite. The self‐recrystallization ability of the perovskite material is discovered; its morphology shows significant improvement after long‐term storage. Results show that acid oxidization of CNTs enable the self‐recrystallization characteristics of MAPbI₃‐induced interfacial improvement, such that even with a dispersed initial photovoltaic performance, through storage in an ambient medium with relative humidity of 20–50%, the PSCs possess better interface contact, which results in lower charge transfer resistance, higher photovoltaic performance, and stability. As a result, PSCs with an initial power conversion efficiency range of 3.21–7.89% finally converge to within the range of 9.54–12.14% after long‐term storage.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Tianmin Wu, Jian Wang

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Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jianxing Xia, Junsheng Luo, Hua Yang, Chunlin Sun, Zhongquan Wan, Haseeb Ashraf Malik, Haoli Zhang, Yu Shi, Chunyang Jia

Graphical abstract

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Linxiang Zeng, Zongao Chen, Shudi Qiu, Jinlong Hu, Chaohui Li, Xianhu Liu, Guangxing Liang, Christoph J. Brabec, Yaohua Mai, Fei Guo

Graphical abstract