Chen Weijie

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Sheng Dong, Tao Jia, Kai Zhang, Jianhua Jing, Fei Huang

Nature Materials, Published online: 24 August 2020; doi:10.1038/s41563-020-0774-9

Ultralong charge‐carrier lifetimes >6 μs are achieved in polycrystalline halide perovskites by decorating the grain boundaries with a trace amount of electron‐rich anchors, which benefits from weak excitonic effects and the weakening of electron–phonon couplings in passivated films, fulfilling reduced voltage deficits and enhanced efficiencies in perovskite photovoltaics. This finding provides a new insight into realizing superior carrier properties of polycrystalline perovskite films and high‐performance perovskite optoelectronics.

Abstract

Lead halide perovskite films have witnessed rapid progress in optoelectronic devices, whereas polycrystalline heterogeneities and serious native defects in films are still responsible for undesired recombination pathways, causing insufficient utilization of photon‐generated charge carriers. Here, radiation‐enhanced polycrystalline perovskite films with ultralong carrier lifetimes exceeding 6 μs and single‐crystal‐like electron–hole diffusion lengths of more than 5 μm are achieved. Prolongation of charge‐carrier activities is attributed to the electronic structure regulation and the defect elimination at crystal boundaries in the perovskite with the introduction of phenylmethylammonium iodide. The introduced electron‐rich anchor molecules around the host crystals prefer to fill the halide/organic vacancies at the boundaries, rather than form low‐dimensional phases or be inserted into the original lattice. The weakening of the electron‐phonon coupling and the excitonic features of the photogenerated carriers in the optimized films, which together contribute to the enhancement of carrier separation and transportation, are further confirmed. Finally the resultant perovskite films in fully operating solar cells with champion efficiency of 23.32% are validated and a minimum voltage deficit of 0.39 V is realized.

All‐inorganic hole‐transport layer (HTL)‐free CsPbBr₃‐based perovskite solar cells doped with Eu²⁺ are studied. The decrement in trap‐state density and suppression of nonradiative recombination after doping is achieved with a higher power conversion efficiency (PCE) of 7.28% and V _OC of 1.45 V.

All‐inorganic perovskite of CsPbBr₃ thin‐films solar cells has attracted increasing interest in recent years due to its potential long‐term stability over the generally used hybrid perovskites. Herein, all‐inorganic CsPbBr₃ perovskites are doped with Eu²⁺ to enhance the efficiency of perovskite solar cells (PVSCs). The perovskite films exhibit a better crystallinity with smooth morphology after the introduction of rare‐earth elements. Hence, the hole‐transport layer‐free device with presence of Eu²⁺ and low‐cost carbon electrode achieves both enhanced efficiency and stability. In particular, the power conversion efficiency (PCE) enhances from 5.66% to 7.28% with high V _OC of 1.45 V by optimizing the doping concentration of Eu²⁺. In addition, the storage stability measurements reveal excellent performances of PCE without encapsulation in air with relative humidity of 70–80%. These results can pave changes in future inorganic PVSCs.

A modified monomolecular layer strategy (m‐MLS) enables high‐quality perovskite films formation on the hydrophobic polymer hole transporting layer (HTL), and minimizes the ohmic loss induced by the HTL. The perovskite solar cells (PSCs) based on m‐MLS‐modified HTL (F‐PSCs) give a superior reproducibility and a champion efficiency of 19.7% with a fill factor of over 80%.

The hole transport materials that interact with the indium tin oxide (ITO) surface can be processed into monomolecular layers (MLs), which often exhibit different surface and electronic properties than their thin‐film counterparts. Herein, it is found that poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) films (R‐PTAA) can be easily processed into ML (M‐PTAA) due to the van der Waals interaction between ITO and PTAA. However, compared with R‐PTAA, the work function (WF) and conductivity of M‐PTAA are simultaneously reduced by the charge transfer at the ITO/PTAA interface. To address this issue, a modified monomolecular layer strategy (m‐MLS) is developed, where a small amount of 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) is introduced to enhance the interaction force between ITO and PTAA. PTAA treated by m‐MLS (F‐PTAA) has a hydrophilic physical surface, closely matching electronic energy level with the perovskite layer and smaller bulk resistance. As a result, the efficiency and reproducibility of perovskite solar cells (PSCs) are substantially improved. PSCs based on F‐PTAA demonstrated the highest power conversion efficiency (PCE) of 19.7% with a fill factor of over 80%. This study inspires the development of novel interface modification materials, and provides a simple and convenient direction for the fabrication of high‐performance and reproducible inverted PSCs with high fill factors.

This work discusses the fundamental understanding of the built‐in potential on efficient operation of the perovskite solar cells (PSCs) and the approach for enhancing the built‐in potential in the PSCs. The outcomes of this work are very inspiring, providing a commercially viable and cost‐effective approach for attaining high‐performance solution‐processable PSCs.

The performance of perovskite solar cells (PSCs) has been improved substantially over the past few years. However, the related fundamental understanding of improving the built‐in potential on the efficiency of the PSCs is still far from adequate. A combination of morphology, charge extraction, and built‐in potential studies would help us to gain an insight on efficient operation of the PSCs. Herein, the effect of the hybrid hole extraction layer (HEL), comprising a mixture of tungsten oxide (WO₃) and poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) (WO₃–PEDOT:PSS), on the growth of the perovskite photoactive layer and built‐in potential in PSCs is investigated using structural analyses, photoelectron spectroscopy, and transient photocurrent (TPC) measurements. It shows that the use of hybrid HEL is an effective approach for enhancing the built‐in potential across the photoactive layer in the PSCs, leading to >20% increase in power conversion efficiency as compared to that of a control PSC prepared using a pristine PEDOT:PSS HEL. PSCs with a higher built‐in potential are favorable for efficient cell operation, as manifested by the charge extraction analyses and TPC measurements.

A dual‐ion‐migration phenomenon and its underlying possible mechanism are reported for the lead‐free double perovskite Cs₂AgBiBr₆, where the diffusive behavior of both Ag and Br contribute significantly to the degradation of the perovskite thin‐film and long‐term operational stability of the Cs₂AgBiBr₆ solar cells.

Abstract

Lead‐free double perovskite Cs₂AgBiBr₆ has attracted increasing research interest in addressing the toxicity and stability challenges confronted by lead halide perovskites. While most of the studies on this Cs₂AgBiBr₆ material have been focusing on photovoltaic performance and potential applications, its long‐term stability and degradation mechanism are well under‐explored. Herein, high‐quality Cs₂AgBiBr₆ thin‐films are developed for lead‐free double perovskite solar cells with a decent efficiency of 1.91%. By exploring the ambient stability of these photovoltaic devices, it is found that the Cs₂AgBiBr₆ exhibits a unique dual‐ion‐migration phenomenon, where Ag and Br ions gradually diffuse through the hole‐transporting layer in the long‐term operation. This phenomenon leads to the degradation of the Cs₂AgBiBr₆ perovskite and subsequent device failure. Theoretical calculations indicate that low formation energies of the Ag and Br vacancies, and low diffusive energy barriers contribute to the dual‐ion‐migration effect. A possible mechanism involving a vacancy‐mediated ion‐migration is proposed to explain this phenomenon. These key findings are essential for halide double perovskites not only in providing a new knowledge base for further addressing the challenge of double perovskite stability, but also in extending their optoelectronic/electronic applications where mixed electronic, ionic and photonic properties may be desired.

A naphthalene diimide based double‐cable conjugated polymer provided a record efficiency of 8.4 % in single‐component organic solar cells. It simultaneously facilitates exciton separation and charge transport via miscibility control.

Abstract

A record power conversion efficiency of 8.40 % was obtained in single‐component organic solar cells (SCOSCs) based on double‐cable conjugated polymers. This is realized based on exciton separation playing the same role as charge transport in SCOSCs. Two double‐cable conjugated polymers were designed with almost identical conjugated backbones and electron‐withdrawing side units, but extra Cl atoms had different positions on the conjugated backbones. When Cl atoms were positioned at the main chains, the polymer formed the twist backbones, enabling better miscibility with the naphthalene diimide side units. This improves the interface contact between conjugated backbones and side units, resulting in efficient conversion of excitons into free charges. These findings reveal the importance of charge generation process in SCOSCs and suggest a strategy to improve this process: controlling miscibility between conjugated backbones and aromatic side units in double‐cable conjugated polymers.

CsSn_0.6Ge_0.4I₃ nanocrystals have been synthesized for the first time by a B‐site co‐alloying strategy. The introduction of Ge effectively decreases the high density of intrinsic Sn defects, resulting in an extended excitonic lifetime and enhanced solar cell performance. The stability of the new nanocrystals also improves owing to the effective protection of Sn²⁺ against oxidation.

Abstract

Colloidal lead‐free perovskite nanocrystals have recently received extensive attention because of their facile synthesis, the outstanding size‐tunable optoelectronic properties, and less or no toxicity in their commercial applications. Tin (Sn) has so far led to the most efficient lead‐free solar cells, yet showing highly unstable characteristics in ambient conditions. Here, we propose the synthesis of all‐inorganic mixture Sn‐Ge perovskite nanocrystals, demonstrating the role of Ge²⁺ in stabilizing Sn²⁺ cation while enhancing the optical and photophysical properties. The partial replacement of Sn atoms by Ge atoms in the nanostructures effectively fills the high density of Sn vacancies, reducing the surface traps and leading to a longer excitonic lifetime and increased photoluminescence quantum yield. The resultant Sn‐Ge nanocrystals‐based devices show the highest efficiency of 4.9 %, enhanced by nearly 60 % compared to that of pure Sn nanocrystals‐based devices.

The A‐D‐A′‐D‐A strategy is applied to develop two new Y6‐type unfused‐ring acceptors. The resulting fluorinated unfused acceptors lock more planar conformation, thus exhibiting red‐shifted absorption and better aggregation properties, leading to high device efficiencies of over 12%.

Unfused‐ring acceptors (UFAs) have gained considerable research attention as they offer simple chemical structures through simplified synthesis methods, which would boost the commercialization of organic solar cells (OSCs). Recently, a new small molecule acceptor (SMA) named Y6 was reported, yielding high‐performance OSCs. Herein, the Y6‐like A‐DA′D‐A framework is developed to A‐D‐A′‐D‐A‐type backbone adopted in constructing UFAs. Two new Y6‐like UFAs are synthesized within four steps and the effect of noncovalent atoms at the central electron‐deficient core on material properties and device performances is studied. It is found that the introduction of fluorine atoms can bring larger red‐shift in the absorption spectra and better aggregation of the resulting UFA film states compared with those of oxygen atoms. Interestingly, the variations in the noncovalent interaction atoms induce different intermolecular charge transfer between donors and UFAs. When blended with another economical donor, PTQ10, F substitution at the benzothiadiazole ring is more effective than O substitution, leading to the increased short‐circuit current density (J _SC) and higher efficiency of over 12%, among the best performances of UFA‐based OSCs. This contribution demonstrates the appropriate introduction of noncovalent interaction is a promising method for tuning energy levels, absorption, and aggregation of UFAs for high‐performance OSCs.

A new side chain engineering strategy is developed to rationally adjust the crystallinity of small molecule acceptors. Through restricting the rotation of bulky side chain on carbazole‐based acceptor, the crystallinity and morphology of the heterojunction are optimized, leading to a high power conversion efficiency of 12.06%.

Herein, three carbazole‐based small molecule acceptors (SMAs), named 4TC‐4F‐C8C8, 4TC‐4F‐C6C8, and 4TC‐4F‐C16, are synthesized to study the influence of side chain conformation on SMAs. The three acceptors exhibit similar optical and electrochemical properties, but different crystallization properties. 4TC‐4F‐C16 shows a high crystallinity due to the small steric hindrance of linear n‐hexadecyl (C16) side chain. The large steric hindrance and free rotation for the 2‐hexydecyl (C6C8) side chain seriously disturb the molecular packing and result in a low crystallinity for 4TC‐4F‐C6C8. Despite the large steric hindrance for the 1‐octylnonyl (C8C8) side chain, 4TC‐4F‐C8C8 shows a moderate crystallinity due to the large torsion barrier restricting the rotation of C8C8 side chain. Attributing to the ideal morphology and better crystalline ordering in blend film, organic solar cell based on poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT):4TC‐4F‐C8C8 displays a power conversion efficiency of 12.06%, higher than PBTIBDTT:4TC‐4F‐C6C8 (2.38%)‐ and PBTIBDTT:4TC‐4F‐C16 (9.53%)‐based devices. The work indicates that controlling the conformation of bulky side chain can tune the molecular packing of SMAs and the morphology of blend film, providing a new insight into the molecular design of SMAs.

This Minireview describes developments in all‐polymer solar cells containing a new type of n‐type conjugated polymer, polymerized small‐molecule acceptors (PSMAs). PSMAs combine the merits of small‐molecule acceptors (narrow band gap, strong absorption, and suitable electronic energy levels) with the good film formation, higher morphology and light‐irradiation stability of polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have drawn tremendous research interest in recent years, due to their inherent advantages of good film formation, stable morphology, and mechanical flexibility. The most representative and most widely used n‐CP acceptor was the naphthalene diimide based D‐A copolymer N2200 before 2017, and the power conversion efficiency (PCE) of the all‐PSCs based on N2200 reached over 8% in 2016. However, the low absorption coefficient of N2200 in the near‐infrared (NIR) region limits the further increase of its PCE. In 2017, we proposed a strategy of polymerizing small‐molecule acceptors (SMAs) to construct new‐generation polymer acceptors. The polymerized SMAs (PSMAs) possess low band gap and strong absorption in the NIR region, which attracted great attention and drove the PCE of the all‐PSCs to over 15% recently. In this Minireview we explain the design strategies of the molecular structure of PSMAs and describe recent research progress. Finally, current challenges and future prospects of the PSMAs are analyzed and discussed.

Ion migration plays critical roles in the functionalities of metal halide perovskites. Herein, using newly developed time‐resolved time‐of‐flight secondary ion mass spectrometry, hysteretic CH₃NH₃ ⁺ and I⁻ migrations in CH₃NH₃PbI₃ are observed, where CH₃NH₃ ⁺ migration hysteresis is illumination‐dependent. The ion redistribution in CH₃NH₃PbI₃ can lead to a permanent spontaneous current after the application of electric bias.

Abstract

The gap in understanding how underlying chemical dynamics impact the functionality of metal halide perovskites (MHPs) leads to the controversy about the origin of many phenomena associated with ion migration in MHPs. In particular, the debate regarding the impact of ion migration on current–voltage (I–V) hysteresis of MHPs devices has lasted for many years, where the difficulty lies in directly uncovering the chemical dynamics, as well as identifying and separating the impact of specific ions. In this work, using a newly developed time‐resolved time‐of‐flight secondary ion mass spectrometry CH₃NH₃ ⁺ and I⁻ migrations in CH₃NH₃PbI₃ are directly observed, revealing hysteretic CH₃NH₃ ⁺ and I⁻ migrations. Additionally, hysteretic CH₃NH₃ ⁺ migration is illumination‐dependent. Correlating these results with the I–V characterization, this work uncovers that CH₃NH₃ ⁺ redistribution can induce a remanent field leading to a spontaneous current in the device. It unveils that the CH₃NH₃ ⁺ migration is responsible for the illumination‐associated I–V hysteresis in MHPs. Hysteretic ion migration has not been uncovered and the contribution of any ions (e.g., CH₃NH₃ ⁺) has not been specified before. Such insightful and detailed information has up to now been missing, which is critical to improving MHPs photovoltaic performance and developing MHPs‐based memristors and synaptic devices.

The application of a low frequency alternating electric field on hybrid perovskite can evidently induce irreversible change of grains/crystal domains and distribution of ionic species in the bulk, leading to tunable opto‐electronic properties and significantly increased efficiency in inverted MAPbI₃ cells without additional treatments and impressive intrinsic long‐term and thermal stability in air without encapsulation.

Abstract

With the rapid development of hybrid metal halide perovskites, controlling and understanding their growth processes have become an important but challenging task. In this paper, alternating electric field as an effective modulation method that acts on the intermediate state in perovskite formation under ambient conditions is introduced. The morphology and microstructure of the as‐formed perovskites can be effectively controlled by tuning simple physical parameters such as the frequency and amplitude, which have shown strong impact on the motion of ionic species and thus influences the formation of materials. Furthermore, the optic and electronic properties of the perovskite (such as the band position) can also be easily tuned by the field parameters. Finally, a conversion efficiency of 19.08% can be achieved in MAPbI₃ device without any doping or additional treatment, with impressive ambient and thermal stability without encapsulation. This result has not only illustrated a new physical approach for material fabrication, but also facilitates deeper understanding of the formation mechanism and generally shed light to the development of more devices and materials.

A unique strategy of “transparent hole‐transporting frameworks” is proposed. A hole‐transporting large‐bandgap polymer, PTAA, is employed to partially replace the polymer donors in the active layer. As a result, semitransparent organic photovoltaic devices with power conversion efficiencies ≈12% and average visible transmittances ≈20% are achieved both on rigid and flexible substrates.

Abstract

Thanks to the nature of molecular orbitals, the absorption spectra of organic semiconductors are not continuous like those in traditional inorganic semiconductors, which offers a unique application of organic photovoltaics (OPVs): semitransparent OPVs. Recently, the exciting progress of materials design has promoted the development of semitransparent OPVs. However, in the perspective of device engineering, almost all reported works reduce the thickness of back/reflected electrode to obtain high average visible transmittance (AVT), which is a trade‐off between power conversion efficiency (PCE) and the transmittance of the whole solar spectrum (visible and infrared), and therefore limit the further development. Herein, a unique strategy of “transparent hole‐transporting frameworks” is proposed. A hole‐transporting large‐bandgap polymer (poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine (PTAA)) is employed to partially replace polymer donors in the active layer of PBDB‐T/Y1. PTAA is a p‐type polymer with a large bandgap of 2.9 eV; the partial substitution of PBDB‐T by PTAA reduces the absorption of the active layer only in the visible region, keeping the hole‐transporting pathways as well as the optimized film morphology. As a result, semitransparent OPVs with PCEs of 12% and AVTs of 20% are achieved, both on rigid and flexible substrates. To demonstrate the generality, this strategy is also used in three different active layers.

Highly efficient temperature‐dependent‐aggregation polymer‐based organic solar cells are fabricated by hot slot‐die coating with hydrocarbon solvents. Power conversion efficiencies of 15.2%, 15.4%, and 15.6% are obtained when chlorobenzene, 1,2,4‐trimethylbenzene (TMB), and ortho‐xylene are used, respectively.

Abstract

Organic solar cells (OSCs) have made rapid progress in terms of their development as a sustainable energy source. However, record‐breaking devices have not shown compatibility with large‐scale production via solution processing in particular due to the use of halogenated environment‐threatening solvents. Here, slot‐die fabrication with processing involving hydrocarbon‐based solvents is used to realize highly efficient and environmentally friendly OSCs. Highly compatible slot‐die coating with roll‐to‐roll processing using halogenated (chlorobenzene (CB)) and hydrocarbon solvents (1,2,4‐trimethylbenzene (TMB) and ortho‐xylene (o‐XY)) is used to fabricate photoactive films. Controlled solution and substrate temperatures enable similar aggregation states in the solution and similar kinetics processes during film formation. The optimized blend film nanostructures for different solvents in the highly efficient PM6:Y6 blend is adopted to show a similar morphology, which results in device efficiencies of 15.2%, 15.4%, and 15.6% for CB, TMB, and o‐XY solvents. This approach is successfully extended to other donor–acceptor combinations to demonstrate the excellent universality of this method. The results combine a method to optimize the aggregation state and film formation kinetics with the fabrication of OSCs with environmentally friendly solvents by slot‐die coating, which is a critical finding for the future development of OSCs in terms of their scalable production and high‐performance.