Mahui

Zero‐dimensional (0D) perovskites are emerging as a class of optoelectronic materials due to their outstanding inherent optoelectronic properties and better stability. Herein, the origin of green emission is revealed using the femtosecond transient absorption measurements, and the lasing performance is realized from 0D perovskite Cs₄PbBr₆ microdisks prepared using a room‐temperature reverse microemulsion method.

Zero‐dimensional (0D) perovskites are emerging as a class of optoelectronic materials due to their unprecedented strong excitonic properties and high stability. Although the photoluminescence properties of 0D perovskites (Cs₄PbX₆) are investigated, the origin of green emission is still opaque, and their lasing performances are not reported. Herein, using the femtosecond transient absorption measurements to study the photophysical properties of Cs₄PbBr₆, the presence of polarons in Cs₄PbBr₆ is revealed, which provides the evidence that the green emission is contributed from the intrinsic behavior of Cs₄PbBr₆ rather than CsPbBr₃ impurities. The successful lasing achieved from Cs₄PbBr₆ microdisks (MDs) by a room‐temperature reverse microemulsion method is demonstrated. The as‐prepared MDs with a smooth surface and a regular geometric structure can act as ideal whispering‐gallery‐mode microcavities. Optically pumped single‐mode lasing with a low threshold and high‐quality factor is successfully achieved from MDs under both one‐ and two‐photon excitation at room temperature. The MDs display an excellent stability while stored under ambient conditions for several months. In addition, the phase transformation between CsPbBr₃ and Cs₄PbBr₆ can be easily achieved via tuning the amounts of surfactants. This work suggests that 0D perovskites can be promising materials toward the development of miniaturized lasers and other optoelectronic devices.

Herein, the open‐circuit voltage losses and bias‐dependent photo‐ and electroluminescence of high‐performance 2D/3D perovskite solar cells, which exhibit outstanding optoelectronic properties, are investigated. These are state‐of‐the‐art photovoltaic devices. Results suggest that by reducing nonradiative recombination processes in the absorber, the power conversion efficiency of the studied photovoltaic devices can be improved.

Herein, the optoelectronic properties of interface‐engineered perovskite 2D|3D‐heterojunction structure solar cells are reported. The reciprocity theorem is applied to determine the maximum open‐circuit voltage (V _oc) the device can deliver under solar illumination. A V _oc of 1.295 V is found, analyzing the measured external quantum efficiency and assuming only radiative recombination. For comparison, the experimental open‐circuit voltage found for the studied 2D|3D heterojunctions is 1.15 V. The contribution of nonradiative recombination is explored by measuring the electroluminescence quantum yield. A quantum yield of 0.4% is found at current densities equivalent to 1 sun illumination. This translates into a V _oc loss of ≈140 mV, which is in very good agreement with the experimental findings. In addition, the fundamental correlation between luminescence intensity and the chemical potential predicted by the generalized Planck law is confirmed for the photoluminescence measured at different light intensities when the device is operated under open‐circuit conditions and for the electroluminescence when operated under a forward bias. The investigations in this study suggest that further efficiency improvements can be achieved by reducing the nonradiative recombination in the studied solar cell. At the same time, a high‐performance near IR light emitting diode can be realized.

Utilizing multiple donor–acceptor pairs for organic solar cells (OSCs) is a very effective strategy for overcoming the limitations of conventional OSCs based on a single donor–acceptor pair. Recent cases of OSCs with multiple donor–acceptor pairs are not only summarized but their perspectives are also presented.

Abstract

Compared with conventional organic solar cells (OSCs) based on single donor–acceptor pairs, terpolymer‐ and ternary‐based OSCs featuring multiple donor–acceptor pairs are promising strategies for enhancing the performance while maintaining an easy and simple synthetic process. Using multiple donor–acceptor pairs in the active layer, the key photovoltaic parameters (i.e., short‐circuit current density, open‐circuit voltage, and fill factor) governing the OSC characteristics can be simultaneously or individually improved by positive changes in light‐harvesting ability, molecular energy levels, and blend morphology. Here, these three major contributions are discussed with the aim of offering in‐depth insights in combined terpolymers and ternary systems. Recent exemplary cases of OSCs with multiple donor–acceptor pairs are summarized and more advanced research and perspectives for further developments in this field are highlighted.

Embracing perovskite grains in a soft fullerene network represents a new and scalable approach, to make perovskite mechanically stable and thus compatible with flexible substrates. The method is demonstrated to prepare flexible perovskite solar cells with the highest ever reported power conversion efficiency. The superior mechanical stability from device performance under working conditions is characterized in situ.

Abstract

Halide perovskite films processed from solution at low‐temperature offer promising opportunities to make flexible solar cells. However, the brittleness of perovskite films is an issue for mechanical stability in flexible devices. Herein, photo‐crosslinked [6,6]‐phenylC₆₁‐butyric oxetane dendron ester (C‐PCBOD) is used to improve the mechanical stability of methylammonium lead iodide (MAPbI₃) perovskite films. Also, it is demonstrated that C‐PCBOD passivates the grain boundaries, which reduces the formation of trap states and enhances the environmental stability of MAPbI₃. Thus, MAPbI₃ perovskite solar cells are prepared on solid and flexible substrates with record efficiencies of 20.4% and 18.1%, respectively, which are among the highest ever reported for MAPbI₃ on both flexible and solid substrates. The result of this work provides a step improvement toward stable and efficient flexible perovskite solar cells.

Embracing perovskite grains in a soft fullerene network represents a new and scalable approach, to make perovskite mechanically stable and thus compatible with flexible substrates. The method is demonstrated to prepare flexible perovskite solar cells with the highest ever reported power conversion efficiency. The superior mechanical stability from device performance under working conditions is characterized in situ.

Abstract

Halide perovskite films processed from solution at low‐temperature offer promising opportunities to make flexible solar cells. However, the brittleness of perovskite films is an issue for mechanical stability in flexible devices. Herein, photo‐crosslinked [6,6]‐phenylC₆₁‐butyric oxetane dendron ester (C‐PCBOD) is used to improve the mechanical stability of methylammonium lead iodide (MAPbI₃) perovskite films. Also, it is demonstrated that C‐PCBOD passivates the grain boundaries, which reduces the formation of trap states and enhances the environmental stability of MAPbI₃. Thus, MAPbI₃ perovskite solar cells are prepared on solid and flexible substrates with record efficiencies of 20.4% and 18.1%, respectively, which are among the highest ever reported for MAPbI₃ on both flexible and solid substrates. The result of this work provides a step improvement toward stable and efficient flexible perovskite solar cells.

Molecular doping is demonstrated as a powerful strategy for enhancing electrical conductivity in arrays of electronically coupled CsPbI₃ nanocrystals. The high surface‐area‐to‐volume ratio enables physisorbed organic redox molecules to inject charge carriers into a nanocrystal array. p‐Type doping improves both ground‐state conductivity and photoconductivity, enabling pronounced performance enhancements to both transistors and phototransistors.

Abstract

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead‐halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface‐area‐to‐volume ratio, are particularly well‐suited for doping via ground‐state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI₃ nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI₃ nanocrystal films are found to be slightly p‐type with increasing conductivity achieved by incorporating the electron‐accepting dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) and decreasing conductivity for the electron‐donating dopant benzyl viologen. Time‐resolved spectroscopic measurements reveal the time scales of Auger‐mediated recombination in the presence of excess electrons or holes. Microwave conductance and field‐effect transistor measurements demonstrate that both the local and long‐range hole mobility are improved by F₄TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p‐type arrays leads to a pronounced enhancement in phototransistors.

In article no. 1900011, Gongqiang Li, Haibo Ma, Aung Ko Ko Kyaw, and co‐workers synthesize and systemically characterize a bifunctional saddle‐shaped small molecule, α, β‐COTh‐Ph‐OMeTAD, as a dopant‐free hole transporting material and interfacial layer in perovskite solar cells.

For the first time, isomer‐pure 2,9,16,24‐tetra‐n‐butyl‐Zn(II) phthalocyanine (RE‐ZnBu₄Pc) is synthesized through ring expansion of symmetric tri‐n‐butyl‐substituted boron subphthalocyanine and is used as hole transporting material (HTM) in planar conventional perovskite solar cells (PSCs), offering higher efficiencies and better long‐term stabilities for these devices. Using isomer‐pure compounds as HTMs in PSCs can also improve the reproducibility of the device fabrication process.

Herein, the important role of the isomer purity of hole‐transporting materials (HTMs) in achieving high‐performance perovskite solar cells (PSCs) is highlighted. The isomer‐pure 2,9,16,24‐tetra‐n‐butyl‐Zn(II) phthalocyanine (RE‐ZnBu₄Pc) is directly synthesized through a ring expansion method, without any further purification. The ground‐state absorption, fluorescence and thermal properties of RE‐ZnBu₄Pc and the isomer mixture ZnBu₄Pc, along with their hole mobilities and film morphologies are investigated, proving that RE‐ZnBu₄Pc can be the more efficient HTM. The devices based on RE‐ZnBu₄Pc, as dopant‐free HTMs, achieve a higher average power conversion efficiency (PCE of 11.49% ± 0.67%) and more stability at 25 °C and under 75% relative humidity than that of isomer mixture ZnBu₄Pc (PCE of 9.51% ± 1.15%). RE‐ZnBu₄Pc‐based PSCs also show better reproducibility in the fabrication process. This study demonstrates that better device performance can be expected for PSCs with isomer‐pure HTM materials.

Defects in metal halide perovskites contribute to nonradiative recombination of photo‐carriers. On device level, such recombination undesirably inflates the open‐circuit voltage deficit and acts as a significant roadblock toward the theoretical efficiency limit of 30% perovskite solar cells. Such voltage‐limiting mechanisms are assessed by focusing on their origin and possible mitigation strategies.

Abstract

Metal‐halide perovskites are rapidly emerging as an important class of photovoltaic absorbers that may enable high‐performance solar cells at affordable cost. Thanks to the appealing optoelectronic properties of these materials, tremendous progress has been reported in the last few years in terms of power conversion efficiencies (PCE) of perovskite solar cells (PSCs), now with record values in excess of 24%. Nevertheless, the crystalline lattice of perovskites often includes defects, such as interstitials, vacancies, and impurities; at the grain boundaries and surfaces, dangling bonds can also be present, which all contribute to nonradiative recombination of photo‐carriers. On device level, such recombination undesirably inflates the open‐circuit voltage deficit, acting thus as a significant roadblock toward the theoretical efficiency limit of 30%. Herein, the focus is on the origin of the various voltage‐limiting mechanisms in PSCs, and possible mitigation strategies are discussed. Contact passivation schemes and the effect of such methods on the reduction of hysteresis are described. Furthermore, several strategies that demonstrate how passivating contacts can increase the stability of PSCs are elucidated. Finally, the remaining key challenges in contact design are prioritized and an outlook on how passivating contacts will contribute to further the progress toward market readiness of high‐efficiency PSCs is presented.

Nature Photonics, Published online: 08 May 2019; doi:10.1038/s41566-019-0462-y

A betaine‐based zwitterionic polymer poly sulfobetaine methacrylate (PSBMA) is employed as interfacial material in p‐i‐n perovskite solar cells. Through improving the interfacial affinity and regulating the energy level at the anode and cathode, respectively, the power conversion efficiency as well as storage stability of the devices greatly improve. In addition, PSBMA also shows advantages in large active area devices.

To improve the performance of perovskite solar cells (Pero‐SCs), a betaine‐based zwitterionic polymer poly(sulfobetaine methacrylate) (denoted by PSBMA) is employed as interlayers at both the anode and cathode in p‐i‐n Pero‐SCs. 1) At the anode side, PSBMA acts as a glue to stitch the two interfacially unfavorable materials: perovskite and poly(bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine), by which the quality of perovskite films as well as the corresponding device performance greatly improve. 2) At the cathode side, PSBMA smoothes the energy levels between PC₆₁BM and Al, and thus facilitates the electron injection efficiency. The power conversion efficiency (PCE) is promoted from 17.31% to 19.16% after PSBMA is introduced as both anode and cathode sides of the p‐i‐n Pero‐SCs. More importantly, PSBMA also shows great potential for large active area (1 cm × 1 cm) Pero‐SCs, and a PCE as high as 15.7% is achieved.

Black phosphorus quantum dots (BPQDs)‐assisted growth of a perovskite film is reported. Serving as heterogeneous nucleation centers, the BPQDs assist in the crystallization of the perovskite film, achieving perovskite films with higher crystallinity and less defects. Consequently, the perovskite solar cells made with BPQDs achieve a maximum power conversion efficiency of 20% and an encouraging improved thermal stability.

Crystallinity and trap‐state density of a perovskite film play a critical role in the performance of corresponding perovskite solar cells (PVSCs). Herein, liquid‐phase‐exfoliated black phosphorus quantum dots (BPQDs) are incorporated into the perovskite precursor solution as additives to direct the formation of the perovskite film, i.e., methylammonium lead iodide (MAPbI₃). It is found that the perovskite films made with BPQDs have higher crystallinity and less nonradiative detects compared with the pristine ones, leading to longer carrier lifetime and higher carrier collection efficiency. Time‐of‐flight secondary‐ion mass spectra and surface density calculation of BPQDs reveal that the improvement of the perovskite film quality may be related to the heterogeneous nucleation of the perovskite film at the BPQDs. PVSCs using MAPbI₃ films made with BPQDs achieve a maximum power conversion efficiency of 20.0% and an encouraging thermal stability of T ₈₀ = 100 h at 100 °C. Both values are remarkably higher than the devices with pristine perovskite films. Therefore, this work demonstrates the potential of the 2D materials quantum dots‐assisted growth method for high‐performance PVSCs.

An oxidized poly(3,4‐ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) monolayer is constructed to demonstrate the in‐plane movement of charge carriers in the charge transfer layer, which possibly leads to severe charge recombination at the interfaces. Consequently, a perovskite solar cell fabricated on the oxidized PEDOT:PSS monolayer yields a power conversion efficiency of 18.8% with a high fill factor of 82%.

Charge extraction at the active layer‐electrode interfaces is critical in obtaining highly efficient planar perovskite solar cells (PSCs). It is commonly achieved by enhancing the charge carrier mobility of the charge transfer layer (CTL) that possesses a desirable energy level. Nevertheless, the in‐plane movement of charge carriers in the CTL possibly leads to severe charge recombination in the presence of defects at the interfaces. To verify this overlooked possibility, herein, an oxidized monolayer of poly(3,4‐ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) hole transfer layer (HTL) is constructed by water rinsing followed by H₂O₂ oxidation. The oxidized PEDOT:PSS monolayer ensures a high charge transfer ability from perovskite to electrode, but at the same time limits in‐plane charge transport. An inverted planar PSC fabricated on the oxidized PEDOT:PSS monolayer yields a power conversion efficiency (PCE) of 18.8%, higher than 17.0% of the control device based on a pristine PEDOT:PSS monolayer. The main contribution comes from the fill factor (FF), which is as high as 82%. Characterizations indicate that the conjugation length of PEDOT chains is decreased after H₂O₂ oxidation, which lowers the conductivity of PEDOT:PSS HTL in the in‐plane direction. This study suggests that the charge recombination at the electrode interfaces due to in‐plane charge transport in the CTLs is not to be neglected.

The development of lead‐free perovskites has attracted increasing attention. The design rules for lead‐free perovskite materials with diverse structures are presented. The structure–property relationships and optical‐, electric‐, and magnetic‐related applications of these lead‐free perovskites are summarized. Based on these structure–property relationships, strategies for multifunctional perovskite design are proposed.

Abstract

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high‐performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less‐toxic, elements while maintaining the superior properties. Here, the design rules for lead‐free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead‐free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic‐related properties. 3D perovskites, especially A₂B⁺B³⁺X₆‐type double perovskites, demonstrate very promising optoelectronic prospects, while low‐dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure–property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead‐free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead‐free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.

The conjugated polyelectrolytes (CPEs) with K⁺ and tetramethylammonium (TMA⁺) are introduced as a multifunctional passivating and hole‐transporting layer for perovskite light‐emitting diodes. TMA⁺ improves significant growth of perovskites with suppressed interfacial defects, resulting in dramatically enhanced emitting properties and device performance. The lower formation energy of Pb_i‐TMA than that of Pb_i‐K suggests that passivation by TMA⁺ ions is more favorable than K⁺ ions.

Abstract

Metal halide perovskites (MHPs) have attracted significant attention as light‐emitting materials owing to their high color purities and tunabilities. A key issue in perovskite light‐emitting diodes (PeLEDs) is the fabrication of an optimal charge transport layer (CTL), which has desirable energy levels for efficient charge injection while blocking opposite charges and enabling perovskite layer growth with reduced interfacial defects. Herein, two poly(fluorene‐phenylene)‐based anionic conjugated polyelectrolytes (CPEs) with different counterions (K⁺ and tetramethylammonium (TMA⁺)) are presented as multifunctional passivating and hole‐transporting layers (HTLs). The crystal growth of MHPs grown on different HTLs is investigated through X‐ray photoelectron spectroscopy, X‐ray diffraction, and density functional theory calculation. The CPE bearing the TMA⁺ counterions remarkably improves the growth of perovskites with suppressed interfacial defects, leading to significantly enhanced emission properties and device performance. The luminescent properties are further enhanced via aging and electrical stress application with effective rearrangement of the counterions on the interfacial defects in the perovskites. Finally, efficient formamidinium lead tribromide‐based quasi‐2D PeLEDs with an external quantum efficiency of 10.2% are fabricated. Using CPEs with varying counterions as a CTL can serve as an effective method for controlling the interfacial defects and improving perovskite‐based optoelectronic device properties.