吴晓晓

Publication date: May 2021

Source: Nano Energy, Volume 83

Author(s): Wen Liang Tan, Nigel M. Kirby, Yi-Bing Cheng, Christopher R. McNeill

A Ni phthalocyanine (NiPc) decorated by four methoxyethoxy units with a strong intramolecular electric field is prepared and used as hole‐transporting materials (HTMs) in perovskite solar cells (PSCs). The best PSCs with NiPc as dopant‐free HTM show a record efficiency of 21.23 % (certified 21.03 %). The PSCs also exhibit the excellent stability.

Abstract

Low conductivity and hole mobility in the pristine metal phthalocyanines greatly limit their application in perovskite solar cells (PSCs) as the hole‐transporting materials (HTMs). Here, we prepare a Ni phthalocyanine (NiPc) decorated by four methoxyethoxy units as HTMs. In NiPc, the two oxygen atoms in peripheral substituent have a modified effect on the dipole direction, while the central Ni atom contributes more electron to phthalocyanine ring, thus efficiently increasing the intramolecular dipole. Calculation analyses reveal the extracted holes within NiPc are mainly concentrated on the phthalocyanine core induced by the intramolecular electric field, and further to be transferred by π‐π stacking space channel between NiPc molecules. Finally, the best efficiency of PSCs with NiPc as dopant‐free HTMs realizes a record value of 21.23 % (certified 21.03 %). The PSCs also exhibit the good moisture, heating and light stabilities. This work provides a novel way to improve the performance of PSCs with free‐doped metal phthalocyanines as HTMs.

The electronic nature of Sb‐ and Bi‐doped lead halide perovskites is an area of current scientific debate. Model compounds featuring [PbE₂I₁₆]⁸⁻ (E=Sb, Bi) anions that represent precise cut‐outs of doped perovskites are presented. The compounds display surprisingly low band gaps owing to an excellent electronic match between PbI₆ and EI₆ units and represent the first members of a promising new class of metal halide materials.

Abstract

Doping and alloying are valuable tools for modifying and enhancing the properties and performance of lead halide perovskites. However, the effects of heterovalent doping with Sb³⁺ and Bi³⁺ cations are still a matter of current investigation. Due to the different charge of the dopants compared to the constituting Pb²⁺ ions, a simultaneous creation of defects is unavoidable and the influence of these defects and the actual metal substitution become entangled. Herein, we present the first 14–15 iodido metalates, (BED)₄PbE₂I₁₆ (BED=N‐benzylethylenediammonium; E=Sb (1), Bi (2)), which are model compounds for doped lead iodide perovskites and display surprisingly low band gaps of 2.01 (1) and 1.88 eV (2). Quantum chemical investigations show that this stems from a good electronic match between the PbI₆ and EI₆ units of the compounds. Our results provide a model system for doped perovskites, but also represent the first examples of a promising new class of metal halide materials.

An ionic liquid, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆), was used to passivate a perovskite to decrease the defects of Pb‐cluster and Pb‐I antisite, thereby reducing the energy barrier between the perovskite and hole transport layer. A perovskite solar cell attained a 23.25 % efficiency with a high stability due to hydrophobic DMIMPF₆.

Abstract

Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆) ionic liquid (IL) is adopted to passivate the surface of a formamidinium‐cesium lead iodide perovskite (Cs_0.08FA_0.92PbI₃) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb‐cluster and Pb‐I antisite defects can be effectively passivated by [DMIM]⁺ bonding with the Pb²⁺ ion on the perovskite surface, leading to significantly suppressed non‐radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF₆‐treated perovskite device demonstrated long‐term stability because the hydrophobic DMIMPF₆ layer blocked moisture permeation.

Semiconducting oxide overlayer materials (SOOMs) can offer a new way for low-cost and highly-stable halide perovskite solar cells (HPSCs) compared to organic semiconducting overlayer materials. The effective deposition of SOOMs on top of the perovskite layer is expected to contribute to the commercialization of single-junction as well as multi-junction HPSCs.

Abstract

Halide perovskite solar cells (HPSCs) contain charge transport layers (CTLs) both above and below the photoactive perovskite layer. These semiconducting CTLs are just as important as the perovskite layer to fully realizing the potential of perovskite materials. In particular, semiconducting oxide overlayer materials (SOOMs) are expected to lower costs and provide better long-term stability compared to the organic semiconducting materials commonly used for the upper layer. However, SOOM-based HPSCs are currently less efficient than conventional devices owing to SOOM's deposition constraints imposed by the underlying perovskite layer. This progress report focuses on the recent evolution of SOOM-based HPSCs by describing the key issues and recent advances in SOOM deposition methods. Finally, remaining challenges and future research directions for SOOMs are discussed to provide guidance toward the commercialization of HPSCs.

A layer‐by‐layer (LbL) deposition technique is used to successfully fabricate the high‐performance all‐polymer solar cells by synergistically controlling additive dosages in donor and acceptor solutions.

Abstract

In this work, an efficiency of 15.17% in the PBDB‐T/PYT all‐PSCs fabricated by a layer‐by‐layer (LbL) deposition technique is achieved by synergistically controlling additive dosages, which is not only higher than that (14.06%) of the corresponding bulk heterojunction (BHJ) device, but also the top efficient for all‐PSCs. Through the studies of physical dynamics and morphological characteristics, it is found that the LbL film can effectively improve optical and electronic properties, ensure exciton separation, charge generation and extraction, reduce trap‐assisted recombination, and facilitate hole transfer in LbL blends, thus achieving higher performance compared to its BHJ counterpart. Notably, the synergistic regulation of additive dosages in donor and acceptor solutions is also confirmed in the other three photovoltaic systems. Of particular note is that over 15% device performance is also achieved in the PBDB‐T/PYT LbL all‐PSCs fabricated via a blade‐coating technique, further demonstrating the great significance of this synergistic additive‐doping strategy for the printing fabrication of organic photovoltaics.

Recently, metal halide perovskites have attracted great interest in laser applications. The fundamental photophysical characteristics of perovskite gain materials, the effects of charge‐carrier dynamics on the optical gain process, and the recent advances of laser applications are discussed here.

Abstract

Metal halide perovskites have drawn tremendous attention in optoelectronic applications owing to the rapid development in photovoltaic and light‐emitting diode devices. More recently, these materials are demonstrated as excellent gain media for laser applications due to their large absorption coefficient, low defect density, high charge carrier mobility, long carrier diffusion length, high photoluminescence quantum yield, and low Auger recombination rate. Despite the great progress in laser applications, the development of perovskite lasers is still in its infancy and the realization of electrically pumped lasers has not yet been demonstrated. To accelerate the development of perovskite‐based lasers, it is important to understand the fundamental photophysical characteristics of perovskite gain materials. Here, the structure and gain behavior in various perovskite materials are discussed. Then, the effects of charge carrier dynamics and electron–phonon interaction on population inversion in different types of perovskite materials are analyzed. Further, recent advances in perovskite‐based lasers are also highlighted. Finally, a perspective on perovskite material design is presented and the remaining challenges of perovskite lasers are discussed.

A triple axial chirality, high glass transition temperature, racemic molecular semiconductor based on thiahelicene and ethylenedioxythiophene is used as hole transport material for 21%‐efficiency, 60 °C stable perovskite solar cells. High glass transition temperature molecular semiconductor damps the motion of diffusive perovskite component for better control of perovskite decomposition.

Abstract

For the low‐cost fabrication of large‐area, durable perovskite solar cells, it is of pivotal importance to engineer organic semiconducting films with a combined property of matched energy level, sufficiently large conductivity, high glass transition temperature, and excellent solution processability. Toward this goal, herein an in silico tailored molecular semiconductor (T5HE‐OMeTPA) with triple axial chirality, by joint use of thia[5]helicene and ethylenedioxythiophene, is reported. T5HE‐OMeTPA with a reduced reorganization energy of hole transfer can be exploited as the hole transport layer for perovskite solar cells with 21% efficiency, which also display excellent long‐term stability at 60 °C. The doped, sufficiently conductive T5HE‐OMeTPA composite with a glass transition temperature of 121 °C not only exhibits persistent film morphology under thermal stress, but also surprisingly damps the motion of diffusive components of perovskite for a better control of the degradation of photoactive layer. The translational motion of both ions and molecules is intrinsically associated with the glass transition of a doped molecular semiconductor composite, which is in stark contrast to the microscopic fashion for the glass transition of an undoped molecular semiconductor, that is, thermally activated rotation of diphenylamine.

Using newly developed high‐quality FlexAgNEs, flexible OPV devices are fabricated and studied with the newly emerging star acceptor Y6 and its derivatives. Comparable performance with rigid counterparts is achieved for all the tested materials. The flexible devices display superior and robust mechanical stability under extreme bending or even folding conditions. Furthermore, the mechanism underlying the super mechanical robustness of these flexible devices is thoroughly investigated.

Abstract

Among the various advantages of organic photovoltaics (OPVs), the key one is their ability to be a highly flexible renewable energy source. However, the power conversion efficiencies for flexible OPV devices still lag behind those of their rigid counterparts, and their mechanical stability cannot meet the requirements for practical applications at present. These, in particular, depend on flexible transparent electrodes (FTEs). Here, a high‐quality FTE (called FlexAgNE), with the simultaneously combined excellent characteristics, has been tested with a series of efficient active materials for flexible OPV devices, and high performance comparable with rigid counterparts has been achieved. In addition, due to the synergistic effect of FlexAgNE and the upper ZnO transport layer, including strong binding between the polyethylene terephthalate substrate and a hydrophilic polyelectrolyte (the key component of FlexAgNE), together with the capillary force effect of crossed silver nanowires and tight filling of ZnO, the flexible devices demonstrate robust mechanical stability even under extreme bending or folding conditions.

The modulation of the excitonic properties of 2D perovskites by coherent optical phonons is investigated using comprehensive optical spectroscopy and theoretical modeling. A clear understanding of these phonon modulation mechanisms injects fresh insights into the exciton–phonon coupling in 2D perovskites, which may help unlock new optoelectronic applications.

Abstract

Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light‐emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier‐phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI₆ octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two‐step process leads to a unique pump‐ and probe‐energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton–phonon coupling of 2D perovskites relevant for emergent optoelectronics development.

Sunlight can be converted to electricity via solar cells, with which then light can be generated the other way around. In this regard, skyscrapers performing light show during night or the bright Karst landscape under the ground are exemplified, echoing the geometry of the TiO₂ nanopillars which are fabricated via a low‐temperature dry process and work as the efficient electron‐transporting layer in flexible perovskite solar cells. More details can be found in article number 2001512 by Zhifeng Huang, Zijian Zheng, and co‐workers.

Recently, double-halide Cs₂SnI₆ perovskite emerged as a star material due to its favorable optoelectronic properties, stable nature, and environmental friendliness. Thus, an in-time review to recapitulate the recent advances of Cs₂SnI₆ is critical to provide viable theoretical and experimental strategies. This literature survey is intended to reveal the merits and demerits of double-halide Cs₂SnI₆ perovskite.

Since the booming research on perovskite solar cells (PSCs), organic–inorganic hybrid halide perovskites have triggered widespread research attention. This is seen in the unprecedented improvement of the power conversion efficiency (PCE) from an initial 3.8% to a remarkable 25.5%. Despite the fascinating improvement in PCEs, the toxicity of the detrimental lead element is a major limiting factor that hampers the commercialization prospect of lead-based materials. Extensive efforts have been dedicated to the progress of lead-free, stable, and ecofriendly perovskite materials for green-energy applications. Recently, double-halide Cs₂SnI₆ perovskite emerged as a star material due to its favorable optoelectronic properties, stable nature, and environmental friendliness. Thus, an in-time review to recapitulate the recent advances of Cs₂SnI₆ is critical to provide viable theoretical and experimental strategies for synergic optimization of perovskite films. Herein, the theoretical and experimental understandings of the properties of Cs₂SnI₆ are summarized and the different fabrication methodologies and their influences on the properties of Cs₂SnI₆ are discussed. The application potential of Cs₂SnI₆ is further reviewed and the limiting factors that influence the performance of Cs₂SnI₆ devices are highlighted. In the end, prospective research directions to improve the optoelectronic properties are presented for developing efficient Cs₂SnI₆ devices.

An alkyl linker‐free, fully conjugated aromatic 2,2′‐biimidazolium cation‐based quasi‐2D perovskite shows enhanced hole and electron mobilities and subsequently improved performance compared with the well‐known organic cation phenylethylammonium‐based quasi‐2D perovskite. A high power conversion efficiency (PCE) of 11.4% (n = 5) is achieved with random‐orientated crystal growth.

Quasi‐2D perovskites are attractive because of their improved stability compared with 3D perovskites counterparts; however, they suffer from poor performance due to the insulating organic cation spacers. To resolve this issue, a strategy of replacing the insulating spacer with conducting spacer is proposed which successfully converts the spacer from a charge‐transporting “barrier” to charge‐transporting “bridge.” Specifically, an alkyl linker‐free, fully conjugated aromatic 2,2′‐biimidazolium (BIDZ) cation is introduced as a spacer to compose quasi‐2D perovskites. Density functional theory (DFT) simulation results show that the lowest unoccupied molecular orbital (LUMO) level localizes on BIDZ and the highest occupied molecular orbital (HOMO) level is on the perovskite. However, both HOMO and LUMO levels localize on perovskite slabs for the well‐known phenethylammonium (PEA)‐based 2D perovskites. The strong electronic coupling between BIDZ and 3D perovskite slabs improves carrier mobilities even for a low‐weak‐crystallinity and random‐orientated quasi‐2D perovskite film. As a result, a remarkable power conversion efficiency up to 11.4% (n = 5) is achieved, which is much higher than that of PEA‐based random‐orientated quasi‐2D perovskites with the same processing condition (6.5%). The strategy paves the way to highly efficient and stable quasi‐2D perovskites solar cells through designing new organic spacer cations.

An intrinsic amorphous silicon layer is introduced on the rear side of bifacial silicon heterojunction (SHJ) solar cells, together with transparent conductive oxides, functioning like a distributed Bragg reflector. This design can improve current density by 0.4 mA cm⁻² by increasing internal reflectance in infrared range, maintaining a cell bifaciality of 55% in >23.5% efficiency SHJ solar cells.

To improve the infrared (IR) response, a high‐refractive‐index intrinsic amorphous silicon (a‐Si:H) layer is introduced after metallization of bifacial silicon heterojunction (SHJ) solar cells, resulting in a transparent conductive oxide (TCO)/a‐Si:H back reflector, which functions like distributed Bragg reflector (DBR). This concept is demonstrated by both Sentaurus Technology Computer‐Aided Design (TCAD) simulation and experimental methods. The TCO/a‐Si:H back reflector can increase rear internal reflectance by reducing the transmission loss, thus improving the IR external quantum efficiency. The using of Sn‐doped In₂O₃ (ITO)/a‐Si:H back reflector in >23.5% efficiency SHJ solar cells can improve short‐circuit current density by 0.4 mA cm⁻² which is quite similar as using the more expensive ITO/Ag back reflector, while keeping a cell bifaciality of 55%. This brings its advantage for monofacial application case. Future studies would be nice to work on higher transparent back reflectors to broaden the application in bifacial case. This back‐reflector design promotes IR response of SHJ solar cells with transferring to a wide variety of TCOs.

A facile structure design is proposed for preparing perovskite solar cells module, a power conversion efficiency of 18.82% on designated area of 21.06 cm² is achieved, and the module demonstrates its practical application on operating different loads. Moreover, it can still keep working even parts of the device are broken.

The fabrication of large area perovskite solar modules (PSMs) is attracting increasing attention. Traditionally, thin film solar modules are prepared by laser‐engraving several isolated lines to create a series of subcells. This process inevitably leads to an increase in series resistance when combining the subcells. Herein, a facile structure combining series and parallel cell connections is designed and developed for constructing PSMs. A champion power conversion efficiency (PCE) of 18.82% is achieved for perovskite composite modules with a designated area of 21.06 cm². Furthermore, the PSMs can continue to operate even when parts of the device are shaded or broken. Because of the repeated structural arrangement, these modules may be suitable for fabrication on a broader scale for commercial applications.

Sulfur can passivate trap states, suppress charge recombination and inhibition migration, thereby enhancing the stability of perovskite solar cells (PSCs). PbS bonds provide new channels for carrier extraction. This review summarizes the sulfur‐based compounds utilized in PSCs by their functions in each layer, which can help others understand the intrinsic phenomena of sulfur‐based PSCs and motivate additional investigations.

In the past decade, organic–inorganic hybrid perovskite solar cells (PSCs) have begun to be increasingly studied worldwide owing to the superior properties of perovskite material. However, some issues have delayed their commercialization, such as their long‐term stability, cost reduction, scale‐up ability, and efficiency. The introduction of sulfur to PSCs can relieve the above issues because sulfur can passivate interfacial trap states, suppress charge recombination, and inhibit ion migration, thereby enhancing the stability of PSCs. Furthermore, PbS bonds provide new channels for carrier extraction. Herein, the sulfur‐based compounds utilized in PSCs are summarized and classified according to their functions in the different layers of PSCs. The results indicate that these sulfur‐based compounds have efficiently promoted the commercialization of PSCs. It is hoped that this review can help others understand the intrinsic phenomena of sulfur‐based PSCs and motivate additional investigations.

Recent years have witnessed substantial progress in nanostructured‐perovskite‐based devices. Patterning and integration techniques for nanostructured perovskites are summarized and recent progress in novel nanostructured‐perovskite‐based applications is presented.

Abstract

In the past decade, lead halide perovskites have been intensively explored due to their promising future in photovoltaics. Owing to their remarkable material properties such as solution processability, nice defect tolerance, broad bandgap tunability, high quantum yields, large refractive index, and strong nonlinear effects, this family of materials has also shown advantages in many other optoelectronic devices including microlasers, photodetectors, waveguides, and metasurfaces. Very recently, the stability of perovskite devices has been improved with the optimization of synthesis methods and device architectures. It is widely accepted that it is the time to integrate all the perovskite devices into a real system. However, for integrated photonic circuits, the shapes and distributions of chemically synthesized perovskites are quite random and not suitable for integration. Consequently, controlled synthesis and the top‐down fabrication process are highly desirable to break the barriers. Herein, the developments of patterning and integration techniques for halide perovskites, as well as the structure/function relationships, are systematically reviewed. The recent progress in the study of optical responses originating from nanostructured perovskites is also presented. Lastly, the challenges and perspective for nanostructured‐perovskite devices are discussed.

A sequential coevaporation technique of Sb₂Se₃ and sulfur powders is developed for the deposition of antimony selenosulfide (Sb₂(S,Se)₃) thin film, from which it is discovered that the interfacial properties, source–substrate distance, and temperature of the substrate conjugatedly affect the structural and electrical properties. The corresponding device delivers a champion efficiency of 8.0% in the vapor‐deposition‐derived alloy‐type Sb₂(S,Se)₃ films.

Abstract

Antimony selenosulfide (Sb₂(S,Se)₃) is an emerging low‐cost, nontoxic solar material with suitable bandgap and high absorption coefficient. Developing effective methods for fabricating high‐quality films would benefit the device efficiency improvement and deepen the fundamental understanding on the optoelectronic properties. Herein, equipment is developed that allows online introduction of precursor vapor during the reaction process, enabling sequential coevaporation of Sb₂Se₃ and S powders for the deposition of Sb₂(S,Se)₃ thin films. With this unique ability, it is revealed that the deposition sequence manipulates both the interfacial properties and optoelectronic properties of the absorber film. A power conversion efficiency of 8.0% is achieved, which is the largest value in vapor‐deposition‐derived Sb₂(S,Se)₃ solar cells. The research demonstrates that multi‐source sequential coevaporation is an efficient technique to fabricate high‐efficiency Sb₂(S,Se)₃ solar cells.

Inspired by an ancient topography technology, a thin (15 µm) and flexible Li–Sn alloy anode is achieved with excellent electrochemical performance and good safety. A range of printed patterns on various substrates can be obtained using the stamping process. The printed Li–Sn alloy has an areal capacity of 3 mAh cm^–2, which can match that of the most typical commercial cathodes.

Abstract

Li metal holds great promise to be the ultimate anode choice owing to its high specific capacity and low redox potential. However, processing Li metal into thin‐film anode with high electrochemical performance and good safety to match commercial cathodes remains challenging. Herein, a new method is reported to prepare ultrathin, flexible, and high‐performance Li–Sn alloy anodes with various shapes on a number of substrates by directly stamping a molten metal solution. The printed anode is as thin as 15 µm, corresponding to an areal capacity of ≈3 mAh cm^–2 that matches most commercial cathode materials. The incorporation of Sn provides the nucleation center for Li, thereby mitigating Li dendrites as well as decreasing the overpotential during Li stripping/plating (e.g., <10 mV at 0.25 mA cm^–2). As a proof‐of‐concept, a flexible Li‐ion battery using the ultrathin Li–Sn alloy anode and a commercial NMC cathode demonstrates good electrochemical performance and reliable cell operation even after repetitive deformation. The approach can be extended to other metal/alloy anodes such as Na, K, and Mg. This study opens a new door toward the future development of high‐performance ultrathin alloy‐based anodes for next‐generation batteries.