Ligang Yuan

Efficient and deep‐blue perovskite light‐emitting diodes (LEDs) are achieved by using Ruddlesden–Popper phase (PEA)₂PbBr₄ nanoplates. The device exhibits an external quantum efficiency up to 0.31% with a corresponding luminance of 147.6 cd m⁻² at the wavelength of 408.8 nm, presenting one of the best performing deep‐blue perovskite LEDs.

Abstract

Ruddlesden–Popper perovskite, (PEA)₂PbBr₄ (PEA = C₈H₉NH₃), is a steady and inexpensive material with a broad bandgap and a narrow‐band emission. These features make it a potential candidate for deep‐blue light‐emitting diodes (LEDs). However, due to the weak exciton binding energy, LEDs based on the perovskite thin films usually possess a very low external quantum efficiency (EQE) of <0.03%. Here, for the first time, the construction of high‐performance deep‐blue LEDs based on 2D (PEA)₂PbBr₄ nanoplates (NPs) is demonstrated. The as‐fabricated (PEA)₂PbBr₄ NPs film shows a deep‐blue emission at 410 nm with excellent stability under ambient conditions. Impressively, LEDs based on the (PEA)₂PbBr₄ NPs film deliver a bright deep‐blue emission with a maximum luminance of 147.6 cd m⁻² and a high EQE up to 0.31%, which represents the most efficient and brightest perovskite LEDs operating at deep‐blue wavelengths. Furthermore, the LEDs retain over 80% of their efficiencies for over 1350 min under ≈60% relative humidity. The steady and bright deep‐blue LEDs can be used as an excitation light source to realize white light emission, which shows the potential for light communication. The work provides scope for developing perovskite into efficient and deep‐blue LEDs for low‐cost light source and light communication.

The use of large organic cations made of stilbene derivatives is reported to generate new variants of air stable Sn‐based two‐dimensional hybrid organic–inorganic perovskites (HOIPs), [(FSA)₂(MA)_{( n –1)}Sn _n I_{(3 n +1)}] (n = 1 and 2; (2‐(4‐(3‐fluoro)stilbenyl)ethanammonium iodide = FSAI and methylammonium iodide = MAI). The single crystal of Sn‐HOIP shows excellent air stability and can be fabricated into field effect transistors and fast photodetectors.

Abstract

Hybrid organic–inorganic perovskites have recently emerged as potential disruptive photovoltaic technology. However, the toxicity of lead used in state‐of‐the‐art hybrid perovskites solar cell prevents large‐scale commercialization, which calls for lead‐free alternatives. Sn‐based perovskites have been considered as alternatives but they are limited by rapid oxidation and decomposition in ambient air. Here, an Sn‐based two‐dimensional hybrid organic–inorganic perovskites [A₂B_{( n ‐1)}Sn _n I_{(3 n +1)}] (n = 1 and 2) are reported with improved air stability, using bulky stilbene derivatives as the organic cations (2‐(4‐(3‐fluoro)stilbenyl)ethanammonium iodide (FSAI)). The moisture stability of the [(FSA)₂SnI₄] perovskites is attributed to the hydrophobic properties of fluorine‐functionalized organic chains (FSA), as well as the strong cohesive bonding in the organic chains provided by H bonds, CH···X type H bonds, weak interlayer F···F interaction, and weak face‐to‐face type π‐π interactions. The photodetector device fabricated on exfoliated single crystal flake of [(FSA)₂SnI₄] exhibits fast and stable photoconductor response.

High‐performance single‐component organic light‐emitting transistors (OLETs) are constructed based on two high‐mobility emissive organic semiconductors of 2,6‐diphenylanthracene (DPA) and 2,6‐di(2‐naphthyl) anthracene (dNaAnt). Strong and spatially controlled light emission are demonstrated with high external quantum efficiency approaching 1.61% and 1.75% for DPA‐ and dNaAnt‐based OLETs, respectively, which shows the great potential of OLETs for science and technology investigations and novel optoelectronic logic applications.

Abstract

Construction of high‐performance organic light‐emitting transistors (OLETs) remains challenging due to the limited desired organic semiconductor materials. Here, two superior high mobility emissive organic semiconductors, 2,6‐diphenylanthracene (DPA) and 2,6‐di(2‐naphthyl) anthracene (dNaAnt), are introduced into the construction of OLETs. By optimizing the device geometry for balanced ambipolar efficient charge transport and using high‐quality DPA and dNaAnt single crystals as active layers, high‐efficiency single‐component OLETs are successfully fabricated, with the demonstration of strong and spatially controlled light emission within both p‐ and n‐ conducting channels and output of high external quantum efficiency (EQE). The obtained EQE values in current devices are approaching 1.61% for DPA‐OLETs and 1.75% for dNaAnt‐based OLETs, respectively, which are the highest EQE values for single‐component OLETs in the common device configuration reported so far. Moreover, high brightnesses of 1210 and 3180 cd m⁻² with current densities up to 1.3 and 8.4 kA cm⁻² are also achieved for DPA‐ and dNaAnt‐based OLETs, respectively. These results demonstrate the great potential applications of high mobility emissive organic semiconductors for next‐generation rapid development of high‐performance single‐component OLETs and their related organic integrated electro‐optical devices.

A remarkable high efficiency of 17.34% is achieved for low‐dimensional Ruddlesden–Popper perovskite (RPP) solar cells (n ≤ 5) by using a fluorinated phenylethalammonium (4‐fluoro‐phenethylammonium (4FPEA)) organic cation. These fluorinated devices also show the better humidity and thermal stability as compared to nonfluorinated phenylethlammonium (PEA) devices. These findings provide a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.

Abstract

Low‐dimensional Ruddlesden–Popper perovskites (RPPs) exhibit excellent stability in comparison with 3D perovskites; however, the relatively low power conversion efficiency (PCE) limits their future application. In this work, a new fluorine‐substituted phenylethlammonium (PEA) cation is developed as a spacer to fabricate quasi‐2D (4FPEA)₂(MA)₄Pb₅I₁₆ (n = 5) perovskite solar cells. The champion device exhibits a remarkable PCE of 17.3% with a J _sc of 19.00 mA cm⁻², a V _oc of 1.16 V, and a fill factor (FF) of 79%, which are among the best results for low‐dimensional RPP solar cells (n ≤ 5). The enhanced device performance can be attributed as follows: first, the strong dipole field induced by the 4‐fluoro‐phenethylammonium (4FPEA) organic spacer facilitates charge dissociation. Second, fluorinated RPP crystals preferentially grow along the vertical direction, and form a phase distribution with the increasing n number from bottom to the top surface, resulting in efficient charge transport. Third, 4FPEA‐based RPP films exhibit higher film crystallinity, enlarged grain size, and reduced trap‐state density. Lastly, the unsealed fluorinated RPP devices demonstrate superior humidity and thermal stability. Therefore, the fluorination of the long‐chain organic cations provides a feasible approach for simultaneously improving the efficiency and stability of low‐dimensional RPP solar cells.

Stable and efficient perovskite solar cells (PSCs) are achieved via introducing PbPyA₂ as an additive. Benefiting from the strong interaction, incorporating PbPyA₂ can lower the defects, suppress ion migration and component volatilization of perovskite, resulting in great improvements in heat and humidity tolerance. More importantly, the resulting PSC maintains 93% of initial efficiency after maximum power point tracking for 540 h.

Abstract

Stability has become the main obstacle for the commercialization of perovskite solar cells (PSCs) despite the impressive power conversion efficiency (PCE). Poor crystallization and ion migration of perovskite are the major origins of its degradation under working condition. Here, high‐performance PSCs incorporated with pyridine‐2‐carboxylic lead salt (PbPyA₂) are fabricated. The pyridine and carboxyl groups on PbPyA₂ can not only control crystallization but also passivate grain boundaries (GBs), which result in the high‐quality perovskite film with larger grains and fewer defects. In addition, the strong interaction among the hydrophobic PbPyA₂ molecules and perovskite GBs acts as barriers to ion migration and component volatilization when exposed to external stresses. Consequently, superior optoelectronic perovskite films with improved thermal and moisture stability are obtained. The resulting device shows a champion efficiency of 19.96% with negligible hysteresis. Furthermore, thermal (90 °C) and moisture (RH 40–60%) stability are improved threefold, maintaining 80% of initial efficiency after aging for 480 h. More importantly, the doped device exhibits extraordinary improvement of operational stability and remains 93% of initial efficiency under maximum power point (MPP) tracking for 540 h.

Publication date: 16 October 2019

Source: Joule, Volume 3, Issue 10

Author(s): Le Yang, Vincent Kim, Yaxiao Lian, Baodan Zhao, Dawei Di

Context & Scale

Summary

Graphical Abstract

Efficient Zn‐SnO _x electron transport layers (ETLs) by the low‐temperature (100 °C) electron beam (E‐beam) method are prepared. Doping Zn²⁺ in SnO₂ improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO₂, leading to an improved power conversion efficiency from 18.95% to 20.16%. The low‐temperature preparation of ETLs and the excellent performance of devices present great commercial potential for future applications.

Perovskite solar cells (PSCs) attract tremendous interest due to their feasibility, high power conversion efficiency (PCE), light weight, and flexible architecture. However, some challenges are still present for cheap mass fabrication in commercial applications. Herein, efficient Zn‐SnO _x electron transport layers (ETLs) are used by the low‐temperature (100 °C) electron beam (E‐beam) method. Doping Zn²⁺ in SnO₂ improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO₂, leading to an improved PCE from 18.95% to 20.16%. More importantly, the PCE of the modified device is more than 80% of its initial values for 800 h in ambient air with a relative humidity of ≈40%. The flexible device exhibits a PCE of 15.25% and remains at an initial PCE of 83% after 100 bending cycles. The efficient and flexible PSCs are potentially used as wearable energy power sources. The low‐temperature preparation of ETL and the excellent performance of devices present great commercial potential for future applications.

Herein, Pb‐site doping in organic–inorganic hybrid perovskite (OIH‐LHP) and inorganic CsPbX₃‐based materials is discussed, elucidating the functions of doping on lead halide perovskite (LHP) crystallization, optoelectronic property, and stability. Perspectives for further investigation are also presented.

Although great success has been achieved in perovskite solar cells (PSCs), it still suffers from several drawbacks in terms of stability and higher efficiency. Doping as an effective method to modify the optical and electronic properties of the materials is extensively studied in lead halide perovskites (LHPs). Herein, Pb‐site doping in organic–inorganic hybrid perovskites (OIH‐LHPs) and inorganic CsPbX₃‐based materials is discussed. Doping has three functions toward PSCs: participating in the crystalline process, modifying the energy states in LHPs, and contributing to the stability of PSCs. Issues about further improvements are raised, and perspectives for further investigation are presented.

The location and distribution of fullerenes in the perovskite:fullerene hybrid phase are confirmedly visualized by the conductive atomic force microscopy and Kelvin probe force microscopy measurements. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current is avoided in perovskite solar cells based on the hybrid perovskite:fullerene phases.

In perovskite solar cells (PSCs), hybrid perovskite:fullerene phases are proposed to suppress macroscopic current hysteresis behavior by alleviating ion migration. However, the understanding of how fullerenes exactly alleviate the current hysteresis and what is the influence of fullerenes in such hybrid phases are still unclear from a microscopic viewpoint. Herein, the intentional incorporation of fullerene into perovskite is used to examine how fullerene exactly reduces the macroscopic current hysteresis. The location and distribution of fullerenes in the hybrid phase are confirmedly visualized using conductive atomic force microscopy and Kelvin probe force microscopy measurements. Fullerenes located at grain boundaries function as a source of beneficial effect on choking the channels of ion migration and also as the electron traps that compromise the photocarrier extraction. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current signals is avoided in PSCs based on the hybrid perovskite:fullerene phases. These results not only provide a strong correlation between nanoscopic and macroscopic current hysteresis behaviors but also clearly clarify how fullerenes play a role in reducing the current hysteresis in hybrid phases and thus prototype devices.

A comprehensive theoretical analysis of two‐terminal and four‐terminal perovskite/copper indium gallium selenide (CIGS) tandem solar cells is investigated from optical and electrical aspects. According to different optical absorptions, the current matching points of different halide components are obtained. Under the condition of current matching, an optimal performance up to 31.13% can be obtained by using two‐terminal CH₃NH₃PbI₂Br/CIGS tandem structure.

Perovskite/copper indium gallium selenide (CIGS) tandem solar cells represent an attractive configuration to obtain ultrahigh efficiency. A detailed theoretical analysis is crucial for further improving the performance of tandem solar cells. Herein, four‐terminal and two‐terminal perovskite/CIGS tandem solar cells are intensively researched. For four‐terminal perovskite/CIGS tandem solar cell, the optimal thicknesses of CH₃NH₃PbI₃ and CIGS are 0.5 and 3 μm, respectively, according to the simulation result. Reducing the thickness of TiO₂ and Spiro‐OMeTAD can minimize the short‐wavelength parasitic absorption and long‐wavelength parasitic absorption, respectively. Meanwhile, using antireflection coating, such as 100 nm MgF₂, is beneficial to increase the photon absorption. For two‐terminal perovskite/CIGS tandem solar cells, the thicknesses of perovskite and CIGS are tuned to meet the current matching. To further improve the efficiency of two‐terminal tandem cells, FTO thickness is reduced to minimize reflection, and the optimal doping concentration of CIGS (1 × 10¹⁸ cm⁻³) is used. In addition, results show that the quality of perovskite films should be improved by enlarging the grain size to decrease the trap states at grain boundary. Finally, the optimal efficiency of two‐terminal CH₃NH₃PbI₂Br/CIGS tandem solar cells reaches 31.13%.

A general strategy based on an “anti‐colloidal‐solution” is shown to embed a variety of laser‐generated nanocrystals in a solution processed perovskite film. The capability of such universal embedding is also demonstrated for improved film performance in photovoltaics, e.g., power conversion efficiency of up to 21.41%, and moisture stability over 5000 h in relative humidity of 40%.

Abstract

Regulating the chemical/physical features of solution processed metal halide perovskite films by integrating sub‐10 nm nanocrystals is a highly promising strategy to advance their outstanding optoelectronic performance. However, significant challenges remain for the universal embedding of the well‐defined nanocrystals in the film matrix. By generating nanocrystals in desired solvents via pulsed laser irradiation in liquid, the authors demonstrate the effective decoration of sub‐10 nm nanocrystals in perovskite films for enhanced optoelectronic performance. It is believed that this improved performance is due to the modification of the widely adopted “antisolvent” to a novel “anti‐colloidal‐solution” (ACS). Exemplified by a typical ACS; carbon dots in chlorobenzene, its encouraging superiority in regulating, not only the films morphology, but also the electronic structure, is demonstrated. This results in perovskite solar cells with a champion efficiency of 21.41% as well as a pronounced stability over 5000 h in relative humidity of 40%. The capability of nanocrystal embedding for boosted photovoltaic performance is further exploited by employing other laser generated ACSs. Such a strategy may open up a route to regulating hybrid perovskite film performance via nanocrystal embedding for photovoltaics or even beyond optoelectronic applications.

This review aims at presenting a comprehensive overview of the latest progress on perovskite solar cells (PSCs), especially the strategies toward enhancing their near‐infrared light harvesting. An in‐depth understanding of the working mechanism of tandem solar cells (TSCs) and integrated perovskite/organic solar cells (IPOSCs) is presented, and the recent developments of perovskite/Si, perovskite/Cu(In_1–x , Ga _x )Se₂ TSCs, and IPOSCs are further highlighted.

The emerging perovskite materials present great opportunities for cost‐saving and efficient photovoltaic devices. However, perovskite solar cells (PSCs) suffer from the limitation of short optical absorption edge, resulting in most of the near‐infrared (NIR) light being wasted. Recently, strategies toward broadening the NIR spectra response and further improve the power conversion efficiency of PSCs have attracted extensive attention. In this review, the unique features of perovskite materials are first introduced; subsequently, the current developments of organic–inorganic hybrid PSCs and all‐inorganic PSCs are highlighted. Then, a detailed summary of the strategies toward enhancing the NIR light harvesting of PSCs, namely, perovskite/Si and perovskite/Cu(In_1–x , Ga _x )Se₂ tandem solar cells (TSCs) and the integrated perovskite/organic solar cells (IPOSCs), is presented. After an in‐depth understanding of the working mechanism of TSCs and IPOSCs, a comprehensive overview about their recent developments, key detrimental factors restricting their further performance enhancement, and feasible countermeasures to conquer these scientific and technological problems are given. In the end, the perspectives on the related materials and devices are addressed.

The introduction of funtional molecular self‐assembled monolayers (SAMs) atop of zinc oxide (ZnO) effectively optimizes the energetic and heterojunction properties of the organic–metal oxide interface to improve the performance and photostability of nonfullerene polymer solar cells.

Abstract

Charge events across organic–metal oxide heterointerfaces routinely occur in organic electronics, yet strongly influence their overall performance and stability. They become even more complicated and challenging for the heterojunction conditions in polymer solar cells (PSCs), especially when nonfullerene acceptors with varied energetics are employed. In this work, an effective interfacial strategy that utilizes novel small molecule self‐assembled monolayers (SAMs) is developed to improve the electronic and electric, as well as chemical properties of organic–zinc oxide (ZnO) interfaces for nonfullerene PSCs. It is revealed that the tailored SAMs with well‐controlled energy levels and molecular dipoles can effectively optimize the energetic barrier and work function (WF) of heterointerface for optimal electron extraction. In addition, the introduction of SAMs atop of ZnO facilitates not only acceptor segregation near the n‐contact interface, but also passivation of the photocatalytic activities for ZnO, to improve overall performance and photo stability of the derived nonfullerene PSCs. Overall, the methodology and structure–property relationship revealed herein would be beneficial for a wide range of hybrid electronics.

When crosslinking and nonfullerene acceptors are introduced in organic photovoltaics, the burn‐in loss due to thermal aging and light soaking is dramatically suppressed because of the frozen morphology and high miscibility of the acceptor. The resulting crosslinked device shows 9.4% power conversion efficiency, which is the highest value reported to date for crosslinked active materials, in the first green processing approach.

Abstract

This work deals with the investigation of burn‐in loss in ternary blended organic photovoltaics (OPVs) prepared from a UV‐crosslinkable semiconducting polymer (P2FBTT‐Br) and a nonfullerene acceptor (IEICO‐4F) via a green solvent process. The synthesized P2FBTT‐Br can be crosslinked by UV irradiation for 150 s and dissolved in 2‐methylanisole due to its asymmetric structure. In OPV performance and burn‐in loss tests performed at 75 °C or AM 1.5G Sun illumination for 90 h, UV‐crosslinked devices with PC₇₁BM show 9.2% power conversion efficiency (PCE) and better stability against burn‐in loss than pristine devices. The frozen morphology resulting from the crosslinking prevents lateral crystallization and aggregation related to morphological degradation. When IEICO‐4F is introduced in place of a fullerene‐based acceptor, the burn‐in loss due to thermal aging and light soaking is dramatically suppressed because of the frozen morphology and high miscibility of the nonfullerene acceptor (18.7% → 90.8% after 90 h at 75 °C and 37.9% → 77.5% after 90 h at AM 1.5G). The resulting crosslinked device shows 9.4% PCE (9.8% in chlorobenzene), which is the highest value reported to date for crosslinked active materials, in the first green processing approach.

A facile low‐energy‐cost one‐pot scalable preparation strategy is developed to achieve homogeneously dispersed organic–inorganic perovskites nanoparticles in a freestanding gel with superior stability and high color purity even in water. The modular material design allows for a broad range of mechanical properties tunable from high elasticity stretchable gel in LEDs to rigid arbitrary 2D/3D structures printed by fast 3D‐printing technology.

Abstract

Metal‐halide perovskites have become appealing materials for optoelectronic devices. While the fast advancing stretchable/wearable devices require stability, flexibility and scalability, current perovskites suffer from ambient‐environmental instability and incompatible mechanical properties. Recently perovskite−polymer composites have shown improved in‐air stability with the protection of polymers. However, their stability remains unsatisfactory in water or high‐humidity environment. These methods also suffer from limited processability with low yield (2D film or beads) and high fabrication cost (high temperature, air/moisture‐free conditions), thereby limiting their device integration and broader applications. Herein, by combining facile photo‐polymerization with room‐temperature in‐situ perovskite reprecipitation at low energy cost, a one‐step scalable method is developed to produce freestanding highly‐stable luminescent organogels, within which CH₃NH₃PbBr₃ nanoparticles are homogeneously distributed. The perovskite‐organogels present a record‐high stability at different pH and temperatures, maintaining their high quantum yields for > 110 days immersing in water. This paradigm is universally applicable to broad choices of polymers, hence casting these emerging luminescent materials to a wide range of mechanical properties tunable from rigid to elastic. With intrinsically ultra‐stretchable photoluminescent organogels, flexible phosphorous layers were demonstrated with > 950% elongation. Rigid perovskite gels, on the other hand, permitted the deployment of 3D‐printing technology to fabricate arbitrary 2D/3D luminescent architectures.

A comprehensive and in‐depth understanding of fundamental polycrystalline perovskite film formation is first summarized, which provides a guidance base for various solution processing methods. Benefitting from the development of film manufacture, small‐ and large‐scale perovskite films with high quality are obtained, which contribute to the excellent performance in photovoltaics and stability of perovskite solar cells.

In recent years, tremendous research interest has been devoted to organic–inorganic halide perovskites because of their excellent optical and electrical properties, which make them intriguing photovoltaic materials. The recorded efficiency of Pb‐based halide perovskite solar cells (PSCs) has gone beyond 24%, thus fulfilling their potential for industrialization. The photovoltaic performance of PSCs is predominantly determined by the quality of the perovskite film, which in turn, is controlled by the fabrication process. Therefore, a comprehensive and in‐depth understanding of fundamental polycrystalline perovskite film formation is imperative for further development of PSC manufacturing. This review summarizes recent advances in the field of PSCs and mainly reviews the fundamental knowledge of nucleation and growth during perovskite crystallization from solution processing methods and promising small area and large‐scale solution manufacturing methods combined with their properties and relevant PSC performance. A brief overview of stabilization strategies and cost discussion is then presented. Finally, the challenges and outlooks of PSC development for upcoming photovoltaic technology for industrial application are discussed.