Ligang Yuan

The adaptation of the two‐step deposition method is demonstrated, which enables the direct probe into the growth dynamics of perovskites using in situ diagnostics. A detailed view of the effects of solvent, lead halide film solvation, and Br incorporation and alloying on the transformation behavior is presented.

Inorganic−organic hybrid perovskites MAPb(I _x Br_1−x )₃ (0 < x < 1) hold promise for efficient multi‐junction or tandem solar cells due to tunable bandgap and improved long‐term stability. However, the phase transformation from Pb(I _x Br_1−x )₂ precursors to perovskites is not fully understood which hinders further improvement of optoelectronic properties and device performance. Here, adaptation of the two‐step deposition method, which enables the direct probe into the growth dynamics of perovskites using in situ diagnostics, and a detailed view of the effects of solvent, lead halide film solvation, and Br incorporation and alloying on the transformation behavior is presented. The in situ measurements indicate a strong tendency of lead halide solvation prior to crystallization during solution‐casting Pb(I _x Br_1−x )₂ precursor from a dimethyl sulfoxide (DMSO) solvent. Highly‐efficient intramolecular exchange is observed between DMSO molecules and organic cations, leading to room‐temperature conversion of perovskite and high‐quality films with tunable bandgap and superior optoelectronic properties in contrast to that obtained from crystalline Pb(I _x Br_1−x )₂. The improved properties translate to easily tunable and a relatively higher power conversion efficiency of 16.42% based on the mixed‐halide perovskite MAPb(I_0.9Br_0.1)₃. These findings highlight the benefits that solvation of the precursor phases, together with bromide incorporation, can have on the microstructure, morphology, and optoelectronic properties of these films.

A facile and effective additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM) molecules into perovskite (PVK) layer. The synergistic effect of the SM⁻ anions and the Gua⁺ cations are demonstrated, which effectively reduces the trap density and the recombination in PVK, so that the photovoltaic performance and stability of the perovskite solar cells are improved noticeably.

Abstract

High efficiency and good stability are the challenges for perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high defect density and internal nonradiative recombination of perovskite (PVK) limit its development. In this work, a facile additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM; CH₆N₃ ⁺, Gua⁺; H₂N−SO₃ ⁻, SM⁻) into PVK. The size of Gua⁺ ion is suitable with Pb(BrI)₂ cavity relatively, so it can participate in the formation of low‐dimensional PVK when mixed with Pb(BrI)₂. The O and N atoms of SM⁻ can coordinate with Pb²⁺. The synergistic effect of the anions and cations effectively reduces the trap density and the recombination in PVK, so that it can improve the efficiency and stability of PSCs. At an optimal concentration of GuaSM (2 mol%), the PSC presents a champion power conversion efficiency of 21.66% and a remarkably improved stability and hysteresis. The results provide a novel strategy for highly efficient and stable PSCs by bifunctional additive.

A new, universal vapor‐phase anion‐exchange strategy is developed to realize the precise phase and bandgap control of large‐scale inorganic perovskites by using gas injection cycle. Ab initio calculations unveil the mechanism accounting for the impact of anion exchange on the structural and electronic properties. Tunable photodetectors with wide‐range response (≈100 nm) and ultrahigh spectral resolution (≈1 nm) are fabricated.

Abstract

Anion exchange offers great flexibility and high precision in phase control, compositional engineering, and optoelectronic property tuning. Different from previous successful anion exchange process in liquid solution, herein, a vapor‐phase anion‐exchange strategy is developed to realize the precise phase and bandgap control of large‐scale inorganic perovskites by using gas injection cycle, producing some perovskites such as CsPbCl₃ which has never been reported in thin film morphology. Ab initio calculations also provide the insightful mechanism to understand the impact of anion exchange on tuning the electronic properties and optimizing the structural stability. Furthermore, because of precise control of specific atomic concentrations, intriguing tunable photoluminescence is observed and photodetectors with tunable photoresponse edge from green to ultraviolet light can be realized accurately with an ultrahigh spectral resolution of 1 nm. Therefore, a new, universal vapor‐phase anion exchange method is offered for inorganic perovskite with fine‐tunable optoelectronic properties.

The perovskite solar cells have emerged as one of the most promising candidates for next‐generation solar cells. However, their instability remains a grand challenge for practical applications. Here, we aim to enhance the stability and efficiency simultaneously by tuning the organic components in Ruddlesden−Popper perovskites (2D‐RPPs). Four groups of 2D‐RPPs are prepared and the influence of 4‐fluorophenethylammonium (FPEA) and formamidinium (FA) cations on the film properties and device performances are investigated. The (FPEA)₂(FA)₈Pb₉I₂₈ film is found to be exceptionally vertically orientated, showing enhanced charge transport and lower defect density. Its absorption edge substantially extends in infrared region, which greatly increases the photocurrent. A high efficiency of 16.15% along with a V _oc of 1.07 V and a J _sc of 20.88 mA cm⁻² is achieved for the (FPEA)₂(FA)₈Pb₉I₂₈ solar cell. Notably, the (FPEA)₂(FA)₈Pb₉I₂₈ film exhibits good humidity stability and remarkably enhanced thermal stability. Its unencapsulated device maintains 95% of its stating PCE after 2112 h when exposed to ambient air with 30‐70% RH, which is more superior than the reported (PEA)₂(MA)₈Pb₉I₂₈ and (FPEA)₂(MA)₈Pb₉I₂₈ solar cells. Our study demonstrates that enhanced performances of 2D‐RPPs can be obtained by strategically designing organic compositions, which paves an avenue towards the commercialization of 2D‐RPP devices.

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The defect landscape in metal–halide perovskites is described. This Review highlights the promise of the compounds, explains defects as an outstanding problem, and discusses the background of defects, methods to probe defects, and various passivation strategies used successfully to date.

The rise of hybrid metal–halide perovskites as potential solar energy materials has revolutionized research on next‐generation solar cells. According to recent studies, the rationale behind such success is the rich defect physics of materials. Studies on the origin of different types of prevailing defects, their formation, and mechanism of defect passivation have hence become decisive avenues. Herein, the possible origins of defects and different defect analysis techniques in hybrid halide perovskites are discussed. While initiating the discussion with the archetypal methylammonium lead halide, perovskites beyond the conventional ABX₃ structure are included. In this direction, some major advancements to date on defect formation in the bulk of hybrid halide perovskites, at the grains and grain boundaries, are summarized. Numerous effective methods to passivate the defects and the adverse effect of defects on device efficiency are further highlighted. Hence, the prospect of defect engineering in perovskite materials is pointed toward improving the power conversion efficiency and long‐term stability of perovskite solar cells (PSCs). The discussion rightfully addresses that the in‐depth exploration of defect engineering is anticipated to have a gigantic impact toward the achievement of predicted efficiency in metal–halide PSCs.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.

Nature Photonics, Published online: 07 December 2020; doi:10.1038/s41566-020-00727-1

A facile and efficient strategy of gradient doping is adopted for optimizing inverted CsPbI₂Br‐based perovskite solar cells (PeSCs) using a bicationic iodine salt, namely BFBAI₂, as the dopant. The doped PeSCs exhibit significantly improved photovoltaic performance and stability, which is attributed to efficient defect passivation and enhanced electric field upon the gradient doped BFBAI₂.

Cesium‐based all‐inorganic perovskites (PVKs) are prized for their high thermal stability and wide bandgap suitable for the top layer of tandem solar cells. To further boost the photovoltaic performance of inorganic PVK solar cells (PeSCs), a variety of strategies aiming to either passivate defects or enhance the electric field are developed. Nevertheless, a double‐aim strategy is less explored. Herein, a facile strategy of gradient doping is adopted for optimizing the inverted CsPbI₂Br PeSCs. Particularly, a bicationic iodine salt, namely 2,2′‐bis(trifluoromethyl)‐[1,1′‐biphenyl]‐4,4′‐diamine iodine (BFBAI₂), is used to realize gradient doping in PVK and ZnO layers, respectively. As a result, the inverted PeSCs with the doped PVK/ZnO bilayer deliver improves power conversion efficiency (PCE) up to 14.38% along with enhanced device stability under ambient or thermal aging conditions, greatly surpassing the pristine devices. The improvements are attributed principally to the low‐defect PVK layer as well as enhanced electric field across the inverted PeSCs upon gradient doping. This work thus demonstrates an efficient bifunctional strategy toward highly efficient and stable CsPbI₂Br PeSCs with inverted configuration.

Here, a straightforward polyTPD passivation is introduced to reduce the defect‐mediated recombination by elucidating the imperfections on the surface and grain boundaries of perovskite materials. Suppressed non‐radiative recombination and improved interfacial hole extraction result in perovskite solar cells with stabilized efficiency exceeding 21%. Moreover, ultra‐hydrophobic and thermally robust polyTPD passivated devices retain 94% of the initial efficiency after 800 h under operational conditions.

Abstract

The failure of perovskite solar cells (PSCs) to maintain their maximum efficiency over a prolonged time is due to the deterioration of the light harvesting material under environmental factors such as humidity, heat, and light. Systematically elucidating and eliminating such degradation pathways are critical to imminent commercial use of this technology. Here, a straightforward approach is introduced to reduce the level of defect‐states present at the perovskite and hole transporting layer interface by treating the various perovskite surfaces with poly(N,N′‐bis‐4‐butylphenyl‐N,N′‐bisphenyl)benzidine (polyTPD) molecules. This strategy significantly suppresses the defect‐mediated non‐radiative recombination in the ensuing devices and prevents the penetration of degrading agents into the inner layers by passivating the perovskite surface and grain boundaries. Suppressed non‐radiative recombination and improved interfacial hole extraction result in PSCs with stabilized efficiency exceeding 21% with negligible hysteresis (≈19.1% for control device). Moreover, ultra‐hydrophobic polyTPD passivant considerably alleviates moisture penetration, showing ≈91% retention of initial efficiencies after 300 h storage at high relative humidity of 80%. Similarly, passivated device retains 94% of its initial efficiency after 800 h under operational conditions (maximum power point tracking under continuous illumination at 60 °C). In addition to interfacial passivation function, hole‐selective role of dopant‐free polyTPD is also evaluated and discussed in this study.

Reversible phase transitions between Cs₄PbBr₆, CsPbBr₃ and CsPb₂Br₅ can be controlled by varying the amount of water and dimethylsulfoxide (DMSO) in the water–dimethylformamide (DMF)–DMSO system. Compared with the traditional anhydrous DMF/DMSO system, the three‐solvent system provides an ideal solution environment for the direct growth of pure‐phase emissive Cs₄PbBr₆ crystals without going through the CsPbBr₃ intermediate phase.

Abstract

All‐inorganic cesium lead bromide perovskites have attracted a lot of attention because of their excellent optical properties that can potentially be applied in various optical devices. However, large‐scale preparation of cesium lead bromide perovskites with outstanding optical performance is still hindered by the poor solubility of CsBr in polar aprotic solvents. In this work, a water/dimethylsulfoxide (DMSO)/dimethylformamide (DMF) system is demonstrated for the synthesis of inorganic cesium lead bromide perovskites, where the introduction of water can effectively address the dissolution issue of Cs⁺ ions. Large‐scale synthesis of pure‐phase emissive Cs₄PbBr₆ with a product yield of up to 73% is achieved and the photoluminescence quantum yield of resulting product reaches as high as 76%. The origin of the light emission is further revealed by the photoluminescence evolution of the colored Cs₄PbBr₆ single crystals. Furthermore, reversible phase transitions between Cs₄PbBr₆, CsPbBr₃ and CsPb₂Br₅ are achieved by changing solely the water content, which paves the way for the recycle of the mother solution of Cs₄PbBr₆ without generation of additional hazardous waste. The three‐solvent‐based synthetic approaches enable an economical, robust, and large‐scale production for all‐inorganic cesium lead bromide perovskites.

An all‐inorganic perovskite photodetector (PD) is designed with a sandwich structure of NiO _x /CsPbBr₃/TiO₂ (NCT). The optimized NCT PD exhibits outstanding performance with ultralow dark current (≈10⁻¹¹ A), broad linear dynamic range (186.7 dB), and ultralow detection limit (857 pW cm⁻²). The satisfactory imaging results show promising applications of the all‐inorganic perovskite PD serving as a weak‐light imaging sensor, with unique potential for low‐cost manufacturing of large‐area optoelectronic devices.

Abstract

Linear dynamic range (LDR) and weak‐light detection limit of photodetectors (PDs) directly determine the imaging resolution and signal‐to‐noise ratio (SNR) in weak‐light condition. Herein, an all‐inorganic perovskite PD is designed with a sandwich structure of NiO _x /CsPbBr₃/TiO₂ (NCT). The CsPbBr₃ perovskites are designed by a spin‐coating method, and NiO _x and TiO₂ layers are introduced by an atomic layer deposition (ALD) method. The optimized NCT PD exhibits outstanding performance with ultralow dark current (≈10⁻¹¹ A), broad LDR (186.7 dB), and ultralow detection limit (857 pW cm⁻²). Significantly, a proof‐of‐concept optical imaging system has been demonstrated using the NCT PD as the sensor element, which shows high resolution and high SNR even under weak‐light condition. Furthermore, as a demo application, a solution is provided to the identification problem of 2D barcode under a weak‐light environment as low as 4 nW cm⁻². The results show promising applications of the all‐inorganic perovskite PD serving as a weak‐light imaging sensor, with unique potential for low‐cost manufacturing of large‐area optoelectronic devices.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.

A new, universal vapor‐phase anion‐exchange strategy is developed to realize the precise phase and bandgap control of large‐scale inorganic perovskites by using gas injection cycle. Ab initio calculations unveil the mechanism accounting for the impact of anion exchange on the structural and electronic properties. Tunable photodetectors with wide‐range response (≈100 nm) and ultrahigh spectral resolution (≈1 nm) are fabricated.

Abstract

Anion exchange offers great flexibility and high precision in phase control, compositional engineering, and optoelectronic property tuning. Different from previous successful anion exchange process in liquid solution, herein, a vapor‐phase anion‐exchange strategy is developed to realize the precise phase and bandgap control of large‐scale inorganic perovskites by using gas injection cycle, producing some perovskites such as CsPbCl₃ which has never been reported in thin film morphology. Ab initio calculations also provide the insightful mechanism to understand the impact of anion exchange on tuning the electronic properties and optimizing the structural stability. Furthermore, because of precise control of specific atomic concentrations, intriguing tunable photoluminescence is observed and photodetectors with tunable photoresponse edge from green to ultraviolet light can be realized accurately with an ultrahigh spectral resolution of 1 nm. Therefore, a new, universal vapor‐phase anion exchange method is offered for inorganic perovskite with fine‐tunable optoelectronic properties.

A passivation process for all‐inorganic cesium lead iodide (CsPbI₃) perovskite quantum dots (QDs) was developed by using a bifunctional ligand, L‐phenylalanine (L‐PHE). The in situ treated CsPbI₃ QDs display significantly reduced surface states, increased vacancy formation energy, higher photoluminescence quantum yields, and much improved stability.

Abstract

To fine‐tune surface ligands towards high‐performance devices, we developed an in situ passivation process for all‐inorganic cesium lead iodide (CsPbI₃) perovskite quantum dots (QDs) by using a bifunctional ligand, L‐phenylalanine (L‐PHE). Through the addition of this ligand into the precursor solution during synthesis, the in situ treated CsPbI₃ QDs display significantly reduced surface states, increased vacancy formation energy, higher photoluminescence quantum yields, and much improved stability. Consequently, the L‐PHE passivated CsPbI₃ QDs enabled the realization of QD solar cells with an optimal efficiency of 14.62 % and red light‐emitting diodes (LEDs) with a highest external quantum efficiency (EQE) of 10.21 %, respectively, demonstrating the great potential of ligand bonding management in improving the optoelectronic properties of solution‐processed perovskite QDs.

Besides achieving large grains by reducing the nucleation centers, controlling the crystal growth rate is another key issue to pursue high crystallinity. The crystal quality of quasi‐2D perovskite can be substantially improved by adding a small amount of growth inhibitors (i.e., Rb⁺ ions) in the precursor solution, which dynamically accumulate in the crystal growth front and retard the crystal growth rate.

Abstract

Tailoring the organic spacing cations enables developing new Ruddlesden–Popper (RP) perovskites with tunable optoelectronic properties and superior stabilities. However, the formation of highly crystallized RP perovskites can be hindered when the structure of organic cations become complex. Strategies to regulate crystal growing process and grains quality remain to be explored. In this study, mixing Rb⁺ ions in precursor solution is reported to significantly promote the crystallinity of phenylethylammonium (PEA⁺) based RP perovskites without impacting on the major orientation of perovskite grains, which leads to increased power conversion efficiencies from 12.5% to 14.6%. It is found that the added Rb⁺ ions prefer to accumulate at crystal growing front and form Rb⁺ ions‐rich region, which functions as mild crystal growth inhibitor to retard the absorption and diffusion of organic cations at growing front and hence regulates crystal growing rate. The retarded crystal growth benefits PEA‐based RP perovskite films with elevated crystal qualities and prolonged carrier recombination lifetimes. Similar increased crystallinity and photovoltaic performance are achieved in other RP perovskites with non‐linear organic cations such as phenylmethylammonium (PMA⁺), 1‐(2‐naphthyl)‐methanammoniun (NMA⁺) by adding Rb⁺ ions, demonstrating using a small amount of growth inhibitor as a general route to regulate crystal growth.

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Aili Wang, Xiaoyu Deng, Jianwei Wang, Shurong Wang, Xiaobin Niu, Feng Hao, Liming Ding

By introducing moderate phenylethylammonium iodide and lead acetate in CsPbI₃ perovksite, moiture resistance and charge recombination are optimized. The device achieves a 17% power conversion efficiency, a 1.33 V open‐circuit voltage (V _OC) and the smallest 0.38 V V _OC deficit. Meanwhile, the device maintains 94% of its efficiency after 2000 h storage in ambient environment.

Abstract

Cesium lead iodide (CsPbI₃) perovskite has gained great attention due to its potential thermal stability and appropriate bandgap (≈1.73 eV) for tandem cells. However, the moisture‐induced thermodynamically unstable phase and large open‐circuit voltage (V _OC) deficit and also the low efficiency seriously limit its further development. Herein, long chain phenylethylammonium (PEA) is utilized into CsPbI₃ perovskite to stabilize the orthorhombic black perovskite phase (γ‐CsPbI₃) under ambient condition. Furthermore, the moderate lead acetate (Pb(OAc)₂) is controlled to combine with phenylethylammonium iodide to form the 2D perovskite, which can dramatically suppress the charge recombination in CsPbI₃. Unprecedentedly, the resulted CsPbI₃ solar cells achieve a 17% power conversion efficiency with a record V _OC of 1.33 V, the V _OC deficit is only 0.38 V, which is close to those in organic‐inorganic perovskite solar cells (PSCs). Meanwhile, the PEA modified device maintains 94% of its initial efficiency after exceeding 2000 h of storage in the low‐humidity controlled environment without encapsulation.

Photoinduced degradation can happen in each functional layer in perovskite solar cells, including the active layer, electronic transport layer, hole transport layer and their interfaces. An overview of these degradation categories and the corresponding solutions is proposed in this review, in the hope of encouraging further research and optimization of the devices.

Abstract

Solar cells based on metal halide perovskites have reached a power conversion efficiency as high as 25%. Their booming efficiency, feasible processability, and good compatibility with large‐scale deposition techniques make perovskite solar cells (PSCs) desirable candidates for next‐generation photovoltaic devices. Despite these advantages, the lifespans of solar cells are far below the industry‐needed 25 years. In fact, numerous PSCs throughout the literature show severely hampered stability under illumination. Herein, several photoinduced degradation mechanisms are discussed. With light radiation, the organic–inorgainc perovskites are prone to phase segregation or chemical decomposition; the oxide electron transport layers (ETLs) tend to introduce new defects at the interface; the commonly used small molecules‐based hole transport layers (HTLs) typically suffer from poor photostability and dopant diffusion during device operation. It has been demonstrated the photoinduced degradation can take place in every functional layer, including the active layer, ETL, HTL, and their interfaces. An overview of these degradation categories is provided in this review, in the hope of encouraging further research and optimization of relevant devices.

It is found that the dynamic redistribution of mobile ions modifies the injection and transport property of charge carriers in the emissive layer, which can well explain the hysteresis in external quantum efficiency (EQE)– and radiance–voltage curves, as well as the rise phenomena of EQE and radiance under low constant driving voltages.

Abstract

Despite quick development of perovskite light‐emitting diodes (PeLEDs) during the past few years, the fundamental mechanisms on how ion migration affects device efficiency and stability remain unclear. Here, it is demonstrated that the dynamic redistribution of mobile ions in the emissive layer plays a key role in the performance of PeLEDs and can explain a range of abnormal behaviours commonly observed during the device measurement. The dynamic redistribution of mobile ions changes charge–carrier injection and leads to increased recombination current; at the same time, the ion redistribution also changes charge transport and results in decreased shunt resistance current. As a result, the PeLEDs show hysteresis in external quantum efficiencies (EQEs) and radiance, that is, higher EQEs and radiance during the reverse voltage scan than during the forward scan. In addition, the changes on charge injection and transport induced by the ion redistribution also well explain the rise of the EQE/radiance values under constant driving voltages. The argument is further rationalized by adding extra formamidinium iodide (FAI) into optimized PeLEDs based on FAPbI₃, resulting in more significant hysteresis and shorter operational stability of the PeLEDs.