JIANG ZHI XUAN

2D materials based on main group element compounds exhibit rich stoichiometries and structure motifs. The synthesis of these multiphase materials is controllable using new methodologies. Furthermore, their properties are revealed based on theoretical predictions and experimental exploration, and then developed for various potential applications.

Abstract

2D materials based on main group element compounds have recently attracted significant attention because of their rich stoichiometric ratios and structure motifs. This review focuses on the phases in various 2D binary materials including III–VI, IV–VI, V–VI, III–V, IV–V, and V–V materials. Reducing 3D materials to 2D introduces confinement and surface effects as well as stabilizes unstable 3D phases in their 2D form. Their crystal structures, stability, preparation, and applications are summarized based on theoretical predictions and experimental explorations. Moreover, various properties of 2D materials, such as ferroelectric effect, anisotropic optical and electrical properties, ultralow thermal conductivity, and topological state are discussed. Finally, a few perspectives and an outlook are given to inspire readers toward exploring 2D materials with new phases and properties.

Nature Communications, Published online: 11 August 2020; doi:10.1038/s41467-020-17868-0

Space-charge-limited currents are widely used to characterize charge transport in semiconductors. Here, the authors characterize space-charge-limited electron and hole currents in metal-halide perovskites, applicable in emerging solar cells. The currents are strongly influenced by the high permittivity and ion motion.

Recent research is a testimony to the fact that perovskite material based solar cells are most efficient since they exhibit high power conversion efficiency and low cost of fabrication. Various perovskite materials display hysteresis in their current-voltage characteristic which accounts for memory behaviour. In this paper, we demonstrate efficient non-volatile memory devices based on hybrid organic-inorganic perovskite (CH 3 NH 3 PbI 3 ) as a resistive switching layer on a Glass/Indium Tin Oxide (ITO) substrate. Our perovskite solar cells have been developed over the fully solution processed electron transport layer (ETL) which is a combination of SnO 2 and mesoporous (m)-TiO 2 scaffold layers. Hysteresis behaviour was observed in the current-voltage analysis achieving high ratio of ON & OFF current under dark and ambient conditions. Proposed perovskite-based Glass/ITO/SnO 2 /m-TiO 2 /CH 3 NH 3

An ultra‐flexible and transparent biomass‐derived conductive substrate is fabricated from polylactic acid. It exhibits high mechanical durability even when subjected to 15 000 bending cycles. Perovskite solar cells based on the biomass electrodes show good mechanical stability, retaining over 86% of its original power conversion efficiency after bending 1500 times at a curvature radius of 5 mm.

Biomass substrates are urgently needed to develop green electronics. Herein, an ultra‐flexible and transparent biomass‐derived conductive substrate is originated from nature polylactic acid (PLA) with silver nanowires (AgNWs) modified by PH1000. The composite electrode exhibits low sheet resistance of 25 Ω sq⁻¹, high transmittance (over 82% in the region of 400–800 nm), and excellent mechanical durability. After bending tests of 15 000 times at a curvature radius of 3 and 5 mm, the sheet resistances of the composite electrodes only increase to 89 and 51 Ω sq⁻¹, respectively. Biomass electrode–based flexible perovskite solar cells are demonstrated with a champion power conversion efficiency (PCE) of 11.44% and high bending tolerance with preserving over 86% of the initial PCE after 1500 bending cycles at a curvature radius of 5 mm. The biomass electrode exhibits great potential for the development of green flexible devices.

The development of the bulk heterojunction, in terms of materials design, device engineering, and the underpinning physical understanding, has led to significant improvements in organic photovoltaics. Looking forward, the bulk heterojunction concept is likely to allow even greater solar cell efficiencies and interestingly, can be applied to other organic electronic applications, such as organic photodetectors and photocatalysts.

Abstract

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction‐based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar‐cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Nature Communications, Published online: 07 August 2020; doi:10.1038/s41467-020-17693-5

Here, the authors synthesize a variety of ultra-thin III–V single crystals, ranging from ultra-narrow to wide bandgap semiconductors, through enhancing the interfacial interaction between the III–V crystals and the growth substrates.

Antisolvent engineering is employed to tune the crystal nucleation and grain growth of perovskite for achieving efficient perovskite solar cells. The engineering of perovskites treated with the green antisolvent MABr‐Eth, suppressing defects‐induced nonradiative recombination in perovskite solar cells, is developed. As expected, the device delivers over 21% power conversion efficiency and a better storage and light‐soaking stability.

Abstract

Organic–inorganic hybrid perovskites have attracted considerable attention due to their superior optoelectronic properties. Traditional one‐step solution‐processed perovskites often suffer from defects‐induced nonradiative recombination, which significantly hinders the improvement of device performance. Herein, treatment with green antisolvents for achieving high‐quality perovskite films is reported. Compared to defects‐filled ones, perovskite films by antisolvent treatment using methylamine bromide (MABr) in ethanol (MABr‐Eth) not only enhances the resultant perovskite crystallinity with large grain size, but also passivates the surface defects. In this case, the engineering of MABr‐Eth‐treated perovskites suppressing defects‐induced nonradiative recombination in perovskite solar cells (PSCs) is demonstrated. As a result, the fabricated inverted planar heterojunction device of ITO/PTAA/Cs_0.15FA_0.85PbI₃/PC₆₁BM/Phen‐NADPO/Ag exhibits the best power conversion efficiency of 21.53%. Furthermore, the corresponding PSCs possess a better storage and light‐soaking stability.

Antisolvent engineering is employed to tune the crystal nucleation and grain growth of perovskite for achieving efficient perovskite solar cells. The engineering of perovskites treated with the green antisolvent MABr‐Eth, suppressing defects‐induced nonradiative recombination in perovskite solar cells, is developed. As expected, the device delivers over 21% power conversion efficiency and a better storage and light‐soaking stability.

Abstract

Organic–inorganic hybrid perovskites have attracted considerable attention due to their superior optoelectronic properties. Traditional one‐step solution‐processed perovskites often suffer from defects‐induced nonradiative recombination, which significantly hinders the improvement of device performance. Herein, treatment with green antisolvents for achieving high‐quality perovskite films is reported. Compared to defects‐filled ones, perovskite films by antisolvent treatment using methylamine bromide (MABr) in ethanol (MABr‐Eth) not only enhances the resultant perovskite crystallinity with large grain size, but also passivates the surface defects. In this case, the engineering of MABr‐Eth‐treated perovskites suppressing defects‐induced nonradiative recombination in perovskite solar cells (PSCs) is demonstrated. As a result, the fabricated inverted planar heterojunction device of ITO/PTAA/Cs_0.15FA_0.85PbI₃/PC₆₁BM/Phen‐NADPO/Ag exhibits the best power conversion efficiency of 21.53%. Furthermore, the corresponding PSCs possess a better storage and light‐soaking stability.

Herein, allylammonium cations (CH₂CH₂CH₃NH₃ ⁺) are incorporated into tin‐based perovskite to induce a preferred crystal orientation, leading to enhanced carrier transport and reduced trap density in the lead‐free films. The photovoltaic devices with an optimized perovskite layer exhibit a high power conversion efficiency of 9.48% and improved stability.

The low power conversion efficiency (PCE) of tin‐based perovskite solar cells is mainly caused by severe Sn²⁺ oxidation and trap state formation. Herein, the reduction of Sn⁴⁺ content and trap density is realized through the formation of quasi‐2D tin‐based perovskite by incorporating allylammonium (ALA) cations into formamidinium tin iodide (FASnI₃). The composition modification substantially enhances the crystallinity and morphology of the perovskite films, leading to a preferred crystal orientation that facilitates charge carrier transport. After optimization, a high maximum PCE of 9.48% is achieved in a planar p‐i‐n photovoltaic device based on (ALA)₂(FA) _n−1Sn _n X_3n+1 (<n> = 25). In the meantime, the device also shows improved stability compared with the FASnI₃‐based one.

High‐temperature degradation of perovskite solar cells with spiro‐OMeTAD hole transport layer is investigated. The postdoping of the spiro‐OMeTAD layer by iodine released from an iodine‐containing perovskite layer at high temperature is discovered as one reason for the high‐temperature degradation. Using an iodine‐free perovskite absorber, thermally stable perovskite solar cells are demonstrated.

Organic–inorganic halide perovskites are promising as the light absorber of solar cells because of their efficient solar power conversion. An issue frequently occurring in perovskite solar cells (PSCs) with a hole transport layer of N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD) is a quick performance degradation at high temperature. Herein, it is discovered that postdoping of the spiro‐OMeTAD layer by iodine released from the perovskite layer is one possible mechanism for the high‐temperature PSC degradation. Iodine doping leads to the highest occupied molecular orbital level of the spiro‐OMeTAD layer becoming deeper and, therefore, induces the formation of an energy barrier for hole extraction from the perovskite layer. It is demonstrated that it is possible to suppress the high‐temperature degradation by using an iodine‐blocking layer or an iodine‐free perovskite in PSCs. These findings will guide the way for the realization of thermally stable perovskite optoelectronic devices in the future.

The M13 bacteriophage functions as an effective perovskite growth template and a passivator in perovskite solar cells. This is owing to its filamentous and uniform dimension, as well as the amino acids on its surface. These effects enhance when the M13 viruses are denatured at high temperature. The efficiency increases from 17.8% to 20.1% upon addition of the denatured viruses.

Abstract

The M13 bacteriophage, a nature‐inspired environmentally friendly biomaterial, is used as a perovskite crystal growth template and a grain boundary passivator in perovskite solar cells. The amino groups and carboxyl groups of amino acids on the M13 bacteriophage surface function as Lewis bases, interacting with the perovskite materials. The M13 bacteriophage‐added perovskite films show a larger grain size and reduced trap‐sites compared with the reference perovskite films. In addition, the existence of the M13 bacteriophage induces light scattering effect, which enhances the light absorption particularly in the long‐wavelength region around 825 nm. Both the passivation effect of the M13 bacteriophage coordinating to the perovskite defect sites and the light scattering effect intensify when the M13 virus‐added perovskite precursor solution is heated at 90 °C prior to the film formation. Heating the solution denatures the M13 bacteriophage by breaking their inter‐ and intra‐molecular bondings. The denatured M13 bacteriophage‐added perovskite solar cells exhibit an efficiency of 20.1% while the reference devices give an efficiency of 17.8%. The great improvement in efficiency comes from all of the three photovoltaic parameters, namely short‐circuit current, open‐circuit voltage, and fill factor, which correspond to the perovskite grain size, trap‐site passivation, and charge transport, respectively.

A recast strategy is proposed to optimize the spatial distribution of components in organic bulk‐heterojunction (BHJ) films in an all‐inorganic perovskite/BHJ integrated solar cells, leading to extended photoresponse, enhanced ambipolar charge transport, and suppressed charge carrier recombination. A record power conversion efficiency of 11.08% and robust thermal stability are obtained.

Abstract

All‐inorganic CsPbIBr₂ perovskite solar cells (pero‐SCs) exhibit excellent overall stability, but their power conversion efficiencies (PCEs) are greatly limited by their wide bandgaps. Integrated solar cells (ISCs) are considered to be an emergent technology that could extend their photoresponse by directly stacking two distinct photoactive layers with complementary bandgaps. However, rising photocurrents always sacrifice other photovoltaic parameters, thereby leading to an unsatisfactory PCE. Here, a recast strategy is proposed to optimize the spatial distribution components of low‐bandgap organic bulk‐heterojunction (BHJ) film, and is combined with an all‐inorganic perovskite to construct perovskite/BHJ ISCs. With this strategy, the integrated perovskite/BHJ film with a top‐enriched donor‐material spatial distribution is shown to effectively improve ambipolar charge transport behavior and suppress charge carrier recombination. For the first time, the ISC is not only significantly extended and enhanced the photoresponse achieving a 20% increase in current density, but also exhibits a high open‐circuit voltage and fill factor at the same time. As a result, a record PCE of 11.08% based on CsPbIBr₂ pero‐SCs is realized; it simultaneously shows excellent long‐term stability against heat and ultraviolet light.

An efficient photon downshifting layer is developed based on ultrasonic spray deposition of a platinum(II) complex, and considerable improvements in both the performance and stability of perovskite solar cells are observed. The photon downshifting layer is demonstrated to be applicable to various types of perovskite solar cells, achieving a maximum device performance of 22.0%.

Abstract

Despite a rapid increase in light harvesting efficiencies, organic–inorganic hybrid perovskite solar cells (PSCs) exhibit relatively inefficient photocurrent generation in the UV region and severe degradation when exposed to UV light and humidity. Herein, to enhance UV and humidity stability as well as photocurrent generating efficiency, a water‐repellent platinum(II) complex, Pt‐F, is developed as a luminescent photon downshifting layer (PDL) for PSCs. The Pt‐F PDL is fabricated on the glass substrate of a PSC using ultrasonic spray deposition, resulting in a considerably higher crystallinity and photoluminescence quantum yield (PLQY) than those fabricated by conventional spin‐coating processes (PLQYs of 77% and 19%, respectively). A maximum device performance of 22.0% is achieved through the addition of a PDL coating to a 21.4% efficient PSC owing to the long‐range photon downshifting effect of Pt‐F, as confirmed by the enhanced spectral response of the device in the UV region. Moreover, remarkable improvements in UV and humidity stability are observed in Pt‐F‐coated PSCs. The versatile effects of the Pt‐F‐based PDL, when fabricated by ultrasonic spray deposition, suggest wide ranging applicability that can improve the performance and stability of other optoelectronic devices.

Three homologous small molecule donors with hydrogen, fluorine, and chlorine substitution afford organic solar cells with efficiencies over 10% in combination with a common acceptor. The chlorinated derivative exhibits a more crystalline nanomorphology with relatively pure domains and provides more than 12% efficiency.

Abstract

Compared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.

A robust strategy for constructing flexible perovskite solar cells that can be conveniently biodegraded is introduced. The results signify the great potential of meniscus‐assisted solution printing for controllably assembling aligned conductive nanomaterials for biodegradable electrodes. As such, it represents an important endeavor toward environmentally friendly, multifunctional, and flexible electronics.

Abstract

Increasing performance demand associated with the short lifetime of consumer electronics has triggered fast growth in electronic waste, leading to serious ecological challenges worldwide. Herein, a robust strategy for judiciously constructing flexible perovskite solar cells (PSCs) that can be conveniently biodegraded is reported. The key to this strategy is to capitalize on meniscus‐assisted solution printing (MASP) as a facile means of yielding cross‐aligned silver nanowires in one‐step, which are subsequently impregnated in a biodegradable elastomeric polyester. Intriguingly, the as‐crafted hybrid biodegradable electrode greatly constrains the solvent evaporation of the perovskite precursor solution, thereby generating fewer nuclei and in turn resulting in the deposition of a large‐grained dense perovskite film that exhibits excellent optoelectronic properties with a power conversion efficiency of 17.51% in PSCs. More importantly, the hybrid biodegradable electrode‐based devices also manifest impressive robustness against mechanical deformation and can be thoroughly biodegraded after use. These results signify the great potential of MASP for controllably assembling aligned conductive nanomaterials for biodegradable electrodes. As such, it represents an important endeavor toward environmentally friendly, multifunctional and flexible electronic, optoelectronic, photonic, and sensory materials and devices.