Size-Tuning of WSe2 Flakes for High Efficiency Inverted Organic Solar Cells

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

Combined Experimental and Computational Insight into the Role of Substrate in the Synthesis of Two-Dimensional WSe2

Synthesis of large-area transition-metal dichalcogenides (TMDs) with controlled orientation is a significant challenge to their industrial applications. Substrate plays a vital role in determining the final quality of monolayer materials grown via the chemical vapor deposition process by controlling their orientation, crystal structure, and grain boundary. This study determined the binding energy and equilibrium distance for tungsten diselenide (WSe2) monolayers on crystalline and amorphous silicon dioxide and aluminum dioxide substrates. Differently oriented WSe2 monolayers are considered to investigate the role of the substrate in the orientation, binding strength, and equilibrium distance. This study can pave the way to synthesizing high-quality two-dimensional (2D) materials for electronic and chemical applications.

Metal oxide-embedded carbon-based materials for polymer solar cells and X-ray detectors

This study examines the effects of hybrid nanoparticles made of NiO@rGO (reduced graphene oxide) and NiO@CNT (carbon nanotubes) on PCDTBT and PCBM active layers in glass/ITO/HTL/active-layer/LiF/Al structured bulk heterojunction (BHJ) polymer solar cells (PSCs) and X-ray photodetectors. These hybrid nanoparticles were used to create BHJ solar cells and photodetectors, and microscopic research was conducted to determine how they affect the structure of the devices. The findings show that compared to conventional matrices, the active layers with NiO@rGO and NiO@CNT incorporation have increased the charge carrier capacities and exciton dissociation properties. In order to assess their impact on the characteristics of charge transport, various weight ratios of these hybrid nanoparticles dispersed in polymer junctions are being investigated. Notably, compared to the pure PCDTBT:PCBM active layer (power conversion efficiency (PCE) = 4.35%), the NiO@CNT device has the highest PCE = 6.42% which, among the tested configurations, demonstrates its superior performance in converting sunlight into electricity. Among the tested X-ray detector materials, "NiO@CNT" achieves the best performance with a sensitivity of 1.92 mA Gy-1 cm-2. Through improved interfacial behaviors and effective charge transport, this work highlights the potential of these cutting-edge hybrid nanoparticles to enhance the performance of organic electronic devices.

Solution-Processed Heterojunction Photodiodes Based on WSe2 Nanosheet Networks

Solution-processed photodetectors incorporating liquid-phase-exfoliated transition metal dichalcogenide nanosheets are widely reported. However, previous studies mainly focus on the fabrication of photoconductors, rather than photodiodes which tend to be based on heterojunctions and are harder to fabricate. Especially, there are rare reports on introducing commonly used transport layers into heterojunctions based on nanosheet networks. In this study, a reliable solution-processing method is reported to fabricate heterojunction diodes with tungsten selenide (WSe2 ) nanosheets as the optical absorbing material and PEDOT: PSS and ZnO as injection/transport-layer materials. By varying the transport layer combinations, the obtained heterojunctions show rectification ratios of up to ≈104 at ±1 V in the dark, without relying on heavily doped silicon substrates. Upon illumination, the heterojunction can be operated in both photoconductor and photodiode modes and displays self-powered behaviors at zero bias.

2D Materials-based Electrochemical Triboelectric Nanogenerators

The integration of 2D materials in triboelectric nanogenerators (TENGs) is known to increase the mechanical-to-electrical power conversion efficiency. 2D materials are used in TENGs with multiple roles as triboelectric material, charge-trapping fillers, or as electrodes. Here, novel TENGs based on few-layers graphene (FLG) electrodes and stable gel electrolytes composed of liquid phase exfoliated 2D-transition metal dichalcogenides and polyvinyl alcohol are developed. TENGs embedding FLG and gel composites show competitive open-circuit voltage (≈ 300 V), instant peak power (530 mW m^-2 ), and stability (> 11 months). These values correspond to a seven-fold higher electrical output compared to TENGs embedding bare FLG electrodes. It is demonstrated that such a significant improvement depends on the high electrical double-layer capacitance (EDLC) of FLG electrodes functionalized with the gel composites. The wet encapsulation of the TENGs is shown to be an effective strategy to increase their power output further highlighting the EDLC role. It is also shown that the EDLC is dependent upon the transition metal (W vs Mo) rather than the relative abundance of 1T or 2H phases. Overall, this work lays down the roots for novel sustainable electrochemical-(e)-TENGs developed exploiting strategies typically used in electrochemical capacitors.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Durable polymer solar cells produced by the encapsulation of a WSe2 hole-transport layer and β-carotene as an active layer additive

WSe2 nanoflakes are obtained by liquid-phase exfoliation. Polymer solar cells with NF-WSe2 as the hole transport layer (HTL) are realized with superior photovoltaic characteristics.

First-principles investigation of potential water-splitting photocatalysts and photovoltaic materials based on Janus transition-metal dichalcogenide/WSe2 heterostructures

In order to evaluate their potential for use in photocatalytic water splitting and photovoltaic solar cells, WSe2/JTMDC heterostructures with various stacking patterns are built. For the intended uses, the materials show promise.

Two-Dimensional Nanomaterials for Boosting the Performance of Organic Solar Cells

The thin-film organic solar cells (OSCs) are currently one of the most promising photovoltaic technologies to effectively harvest the solar energy due to their attractive features of mechanical flexibility, light weight, low-cost manufacturing, and solution-processed large-scale fabrication, etc. However, the relative insufficient light absorption, short exciton diffusion distance, and low carrier mobility of the OSCs determine the power conversion efficiency (PCE) of the devices are relatively lower than their inorganic photovoltaic counterparts. To conquer the challenges, the two-dimensional (2D) nanomaterials, which have excellent photoelectric properties, tunable energy band structure, and solvent compatibility etc., exhibit the great potential to enhance the performance of the OSCs. In this review, we summarize the most recent successful applications of the 2D materials, including graphene, black phosphorus, transition metal dichalcogenides, and g-C3N4, etc., adapted in the charge transporting layer, the active layer, and the electrode of the OSCs, respectively, for boosting the PCE and stability of the devices. The strengths and weaknesses of the 2D materials in the application of OSCs are also reviewed in details. Additionally, the challenges, commercialization potentials, and prospects for the further development of 2D materials-based OSCs are outlined in the end.

A multifunctional 2D black phosphorene-based platform for improved photovoltaics

As one of the latest additions to the 2D nanomaterials family, black phosphorene (BP, monolayer or few-layer black phosphorus) has gained much attention in various forms of solar cells. This is due largely to its intriguing semiconducting properties such as tunable direct bandgap (from 0.3 eV in the bulk to 2.0 eV in the monolayer), extremely high ambipolar carrier mobility, broad visible to infrared light absorption, etc. These appealing optoelectronic attributes make BP a multifunctional nanomaterial for use in solar cells via tailoring carrier dynamics, band energy alignment, and light harvesting, thereby promoting the rapid development of third-generation solar cells. Notably, in sharp contrast to the copious work on revealing the fundamental properties of BP, investigation into the utility of BP is comparatively less, particularly in the area of photovoltaics. Herein, we first identify and summarize an array of unique characteristics of BP that underpin its application in photovoltaics, aiming at providing inspiration to develop new designs and device architectures of photovoltaics. Subsequently, state-of-the-art synthetic routes (i.e., top-down and bottom-up) to scalable BP production that facilitates its applications in optoelectronic materials and devices are outlined. Afterward, recent advances in a diverse set of BP-incorporated solar cells, where BP may impart electron and/or hole extraction and transport, function as a light absorber, provide dielectric screening for enhancing exciton dissociation, and modify the morphology of photoabsorbers, are discussed, including organic solar cells, dye-sensitized solar cells, heterojunction solar cells and perovskite solar cells. Finally, the challenges and opportunities in this rapidly evolving field are presented.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Ultrasonically Processed WSe2 Nanosheets Blended Bulk Heterojunction Active Layer for High-Performance Polymer Solar Cells and X-ray Detectors

Two-dimensional (2D) tungsten diselenide (WSe2) has attracted considerable attention in the field of photovoltaic devices owing to its excellent structure and photoelectric properties, such as ordered 2D network structure, high electrical conductivity, and high mobility. For this test, we firstly prepared different sizes (NS1-NS3) of WSe2 nanosheets (NSs) through the ultrasonication method and characterized their structures using the field emission scanning electron microscope (FE-SEM), Raman spectroscopy, and X-ray powder diffraction. Moreover, we investigated the photovoltaic performance of polymer solar cells based on 5,7-Bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione(PBDB-T):(6,6)-phenyl-C71 butyric acid methyl ester (PCBM) with different WSe2 NSs as the active layer. The fabricated PBDB-T:PCBM active layer with the addition of NS2 WSe2 NSs (1.5 wt%) exhibited an improved power conversion efficiency (PCE) of 9.2%, which is higher than the pure and NS1 and NS3 WSe2 blended active layer-encompassing devices. The improved PCE is attributed to the synergic enhancement of exciton dissociation and an improvement in the charge mobility through the modified active layer for polymer solar cells. Furthermore, the highest sensitivity of 2.97 mA/Gy·cm2 was achieved for the NS2 WSe2 NSs blended active layer detected by X-ray exposure over the pure polymer, and with the NS1 and NS2 WSe2 blended active layer. These results led to the use of transition metal dichalcogenide materials in polymer solar cells and X-ray detectors.

2D WSe2 Flakes for Synergistic Modulation of Grain Growth and Charge Transfer in Tin-Based Perovskite Solar Cells

Tin (Sn)-based perovskites with favorable optoelectronic properties and ideal bandgaps have emerged as promising alternatives to toxic lead (Pb)-based perovskites for photovoltaic applications. However, it is challenging to obtain high-quality Sn-based perovskite films by solution process. Here, liquid-exfoliated 2D transition-metal dichalcogenides (i.e., MoS2 , WS2 , and WSe2 ) with smooth and defect-free surfaces are applied as growth templates for spin-coated FASnI3 perovskite films, leading to van der Waals epitaxial growth of perovskite grains with a growth orientation along (100). The authors find that WSe2 has better energy alignment with FASnI3 than MoS2 and WS2 and results in a cascade band structure in resultant perovskite solar cells (PSCs), which can facilitate hole extraction and suppress interfacial charge recombination in the devices. The WSe2 -modified PSCs show a power conversion efficiency up to 10.47%, which is among the highest efficiency of FASnI3 -based PSCs. The appealing solution phase epitaxial growth of FASnI3 perovskite on 2D WSe2 flakes is expected to find broad applications in optoelectronic devices.

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

The surface sensitivity and lack of dielectric screening in two-dimensional (2D) materials provide numerous intriguing opportunities to tailor their properties using adsorbed π-electron organic molecules. These organic-2D mixed-dimensional heterojunctions are often considered solely in terms of their energy level alignment, i.e., the relative energies of the frontier molecular orbitals versus the 2D material conduction and valence band edges. While this simple model is frequently adequate to describe doping and photoinduced charge transfer, the tools of molecular chemistry enable additional manipulation of properties in organic-2D heterojunctions that are not accessible in other solid-state systems. Fully exploiting these possibilities requires consideration of the details of the organic adlayer beyond its energy level alignment, including hybridization and electrostatics, molecular orientation and thin-film morphology, nonfrontier orbitals and defects, excitonic states, spin, and chirality. This Perspective explores how these relatively overlooked molecular properties offer unique opportunities for tuning optical and electronic characteristics, thereby guiding the rational design of organic-2D mixed-dimensional heterojunctions with emergent properties.

2D MXene-Molecular Hybrid Additive for High-Performance Ambipolar Polymer Field-Effect Transistors and Logic Gates

MXenes are highly conductive layered materials that are attracting a great interest for high-performance opto-electronics, photonics, and energy applications.. Their non-covalent functionalization with ad hoc molecules enables the production of stable inks of 2D flakes to be processed in thin-films. Here, the formation of stable dispersions via the intercalation of Ti₃ C₂ T_x with didecyldimethyl ammonium bromide (DDAB) yielding Ti₃ C₂ T_x -DDAB, is demonstrated. Such functionalization modulates the properties of Ti₃ C₂ T_x , as evidenced by a 0.47 eV decrease of the work function. It is also shown that DDAB is a powerful n-dopant capable of enhancing electron mobility in conjugated polymers and 2D materials. When Ti₃ C₂ T_x -DDAB is blended with poly(diketopyrrolopyrrole-co-selenophene) [(PDPP-Se)], a simultaneous increase by 170% and 152% of the hole and electron field-effect mobilities, respectively, is observed, compared to the neat conjugated polymer, with values reaching 2.0 cm² V^-1 s^-1 . By exploiting the balanced ambipolar transport of the Ti₃ C₂ T_x -DDAB/PDPP-Se hybrid, complementary metal-oxide-semiconductor (CMOS) logic gates are fabricated that display well-centered trip points and good noise margin (64.6% for inverter). The results demonstrate that intercalant engineering represents an efficient strategy to tune the electronic properties of Ti₃ C₂ T_x yielding functionalized MXenes for polymer transistors with unprecedented performances and functions.

Progress in Materials, Solution Processes, and Long-Term Stability for Large-Area Organic Photovoltaics

Organic solar cells based on bulk heterojunctions (BHJs) are attractive energy-conversion devices that can generate electricity from absorbed sunlight by dissociating excitons and collecting charge carriers. Recent breakthroughs attained by development of nonfullerene acceptors result in significant enhancement in power conversion efficiency (PCEs) exceeding 17%. However, most of researches have focused on pursuing high efficiency of small-area (<1 cm² ) unit cells fabricated usually with spin coating. For practical application of organic photovoltaics (OPVs) from lab-scale unit cells to industrial products, it is essential to develop efficient technologies that can extend active area of devices with minimized loss of performance and ensured operational stability. In this progress report, an overview of recent advancements in materials and processing technologies is provided for transitioning from small-area laboratory-scale devices to large-area industrial scale modules. First, development of materials that satisfy requirements of high tolerability in active layer thickness and large-area adaptability is introduced. Second, morphology control using various coating techniques in a large active area is discussed. Third, the recent research progress is also underlined for understanding mechanisms of OPV degradation and studies for improving device long-term stability along with reliable evaluation procedures.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Inherent Resistance of Seed-Mediated Grown MoSe2 Monolayers to Defect Formation

Recent progress in the chemical vapor deposition technique toward growing large-area and single-crystalline two-dimensional (2D) transition metal dichalcogenides (TMDs) has resulted in an electronic/optoelectronic device performance that rivals that of their top-down counterparts, despite the extensive use of hydrogen, a common reducing agent that readily generates defects in TMDs. Herein, we report that 2D MoSe₂ domains containing oxide seeds are resistant to hydrogen-induced defect generation. Specifically, we observed that the etching of the edges of seed-containing MoSe₂ was significantly less than that of pristine MoSe₂, without apparent seed particles, under the same H₂ annealing conditions. Our systematic approach for controlling the H₂ exposure time indicates that the oxidation of Mo and the edge roughening of seedless MoSe₂ coincidentally increase after H₂ exposure owing to the formation of Se vacancy followed by Mo oxidation, which is not the case with seed-containing MoSe₂. An ab initio calculation indicates that hydrogen preferentially adsorbs more onto O bonded to Mo than onto Se, providing further evidence of the resistance of seeded MoSe₂ to hydrogen etching. This finding provides an insight into controlling defect formation in 2D TMDs by employing sacrificial adsorption sites for reactive species (i.e., hydrogen).

Poly(methyl methacrylate)-Assisted Exfoliation of Graphite and Its Use in Acrylonitrile-Butadiene-Styrene Composites

One of the applications of graphene in which its scalable production is of utmost importance is the development of polymer composites. Among the techniques used to produce graphene flakes, the liquid-phase exfoliation (LPE) of graphite stands out due to its versatility and scalability. However, solvents suitable for the LPE process are generally toxic and have a high boiling point, making the processing challenging. The use of low boiling point solvents could be convenient for the processing, due to the easiness of their removal. In this study, the use of poly(methyl methacrylate) (PMMA) as a stabilizing agent is proposed for the production of graphene flakes in a low boiling point solvent, that is, acetone. The graphene dispersions produced in the mixture acetone-PMMA have higher concentration, +175 %, and contain a higher percentage of few-layer graphene flakes (<5 layers), that is, +60 %, compared to the dispersions prepared in acetone. The as-produced graphene dispersions are used to develop graphene/acrylonitrile-butadiene-styrene composites. The mechanical properties of the pristine polymer are improved, that is, +22 % in the Young's modulus, by adding 0.01 wt. % of graphene flakes. Moreover, a decrease of ≈20 % in the oxygen permeability is obtained by using 0.1 wt. % of graphene flakes filler, compared to the unloaded matrix.

Two-Dimensional Bi2O2Se with High Mobility for High-Performance Polymer Solar Cells

Carrier mobility is a critical factor for power conversion efficiency (PCE) of polymer solar cells (PSCs), and the low charge carrier mobility still limits the performance improvement of PSCs. Adding high-mobility material into the active layer is one of the better ways to enhance the PCE of PSCs. Two-dimensional (2D) Bi₂O₂Se can be an ideal additive material for improving the carrier mobility of PSCs because of its ultrahigh mobility and high thermal stability. In this work, the Bi₂O₂Se few-layer 2D nanoflakes are fabricated by combining lithium intercalation with shear force-assisted liquid phase exfoliation and applied as an additive to promote charge transport in PSCs for the first time. The 2D Bi₂O₂Se nanoflakes, when introduced into the active layer, not only provide a new interface between a donor and an acceptor and efficient charge transfer pathways but also induce crystallization of the photosensitive layer and form continuous interpenetrating networks, which promotes the exciton separation and charge transfer in the photosensitive layer. As a result, the PCE of a device based on PBDB-T/ITIC is increased from 10.09% (0 wt %) to 12.22% (2 wt %). Meanwhile, the PCE of a device based on PM6/Y6 is also increased from 14.59% for a binary device to 16.28% for an optimized ternary device (2 wt %). Moreover, the optimized ternary device shows excellent air stability by suppressing the mixing of the two phases. This work provides a good method to enhance the PCE of PSCs and also shows that the Bi₂O₂Se material has a good prospect in photovoltaic devices.

Benzothiadiazole Based Cascade Material to Boost the Performance of Inverted Ternary Organic Solar Cells

A conjugated, ladder-type multi-fused ring 4,7-dithienbenzothiadiazole:thiophene derivative, named as compound ‘T’, was for the first time incorporated, within the PTB7:PC71BM photoactive layer for inverted ternary organic solar cells (TOSCs) realization. The effective energy level offset caused by compound T between the polymeric donor and fullerene acceptor materials, as well as its resulting potential as electron cascade material contribute to an enhanced exciton dissociation, electron transfer facilitator and thus improved overall photovoltaic performance. The engineering optimization of the inverted TOSC, ITO/PFN/PTB7:Compound T(5% v/v):PC71BM/MoO3/Al, resulted in an overall power conversion efficiency (PCE) of 8.34%, with a short-circuit current density (Jsc) of 16.75 mA cm−2, open-circuit voltage (Voc) of 0.74 V and a fill factor (FF) of 68.1%, under AM1.5G illumination. This photovoltaic performance was improved by approximately 12% with respect to the control binary device.

Emphasizing the Operational Role of a Novel Graphene-Based Ink into High Performance Ternary Organic Solar Cells

A novel solution-processed, graphene-based material was synthesized by treating graphene oxide (GO) with 2,5,7-trinitro-9-oxo-fluorenone-4-carboxylic acid (TNF-COOH) moieties, via simple synthetic routes. The yielded molecule N-[(carbamoyl-GO)ethyl]-N′-[(carbamoyl)-(2,5,7-trinitro-9-oxo-fluorene)] (GO-TNF) was thoroughly characterized and it was shown that it presents favorable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels to function as a bridge component between the polymeric donor poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl}) (PTB7) and the fullerene derivative acceptor [6,6]-phenyl-C₇₁-butyric-acid-methylester (PC₇₁BM). In this context, a GO-TNF based ink was prepared and directly incorporated within the binary photoactive layer, in different volume ratios (1%–3% ratio to the blend) for the effective realization of inverted ternary organic solar cells (OSCs) of the structure ITO/PFN/PTB7:GO-TNF:PC₇₁BM/MoO₃/Al. The addition of 2% v/v GO-TNF ink led to a champion power conversion efficiency (PCE) of 8.71% that was enhanced by ~13% as compared to the reference cell.

Hydrogen plasma-treated MoSe2 nanosheets enhance the efficiency and stability of organic photovoltaics

In this paper we report the effect on the power conversion efficiency (PCE) and stability of photovoltaic devices after incorporating hydrogenated two-dimensional (2D) MoSe₂ nanosheets into the active layer of bulk heterojunction (BHJ) organic photovoltaics (OPV). The surface properties of 2D MoSe₂ nanosheets largely affect their dispersion in the active layer blend and, thus, influence the carrier mobility, PCE, and stability of corresponding devices. We treated MoSe₂ nanosheets with hydrogen plasma and investigated their influence on the polymer packing and fullerene domain size of the active layer. For the optimized devices incorporating 37.5 wt% of untreated MoSe₂, we obtained a champion PCE of 9.82%, compared with the champion reference PCE of approximately 9%. After incorporating the hydrogen plasma-treated MoSe₂ nanosheets, we achieved a champion PCE of 10.44%-a relative increase of 16% over that of the reference device prepared without MoSe₂ nanosheets. This PCE is the one of the highest ever reported for OPVs incorporating 2D materials. We attribute this large enhancement to the enhanced exciton generation and dissociation at the MoSe₂-fullerene interface and, consequently, the balanced charge carrier mobility. The device incorporating the MoSe₂ nanosheets maintained 70% of its initial PCE after heat-treatment at 100 °C for 1 h; in contrast, the PCE of the reference device decreased to 60% of its initial value-a relative increase in stability of 17% after incorporating these nanosheets. We also incorporated MoSe₂ nanosheets (both with and without treatment) into a polymer donor (PBDTTBO)/small molecule (IT-4F) acceptor system. The champion PCEs reached 7.85 and 8.13% for the devices incorporating the MoSe₂ nanosheets with and without plasma treatment, respectively-relative increases of 8 and 12%, respectively, over that of the reference. These results should encourage a push toward the implementation of transition metal dichalcogenides to enhance the performances of BHJ OPVs.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Building an Organic Solar Cell: Fundamental Procedures for Device Fabrication

This laboratory experiment is designed to train undergraduate students in the fundamental steps followed in engineering solution-processed organic solar cells and to offer insight on the operating principles of said device. Bulk heterojunction (BHJ) organic solar cells represent a photovoltaic architecture which has attracted a lot of attention due to its promising properties; moreover, this architecture, due to its low cost and potential, is continuously being investigated and improved. This paper is intended as a useful step-by-step guide for students and researchers to learn how to construct such a device. Another primary objective of this article is to highlight the importance of optimizing device performance through enhancing the optical, electrical, and morphological properties of the materials selected as building blocks. Once a completed organic solar cell is made, students will also learn how to investigate and assess its performance through a series of spectroscopic, electrical, and morphological characterization measurements.

Carbon-Based Photocathode Materials for Solar Hydrogen Production

Hydrogen is considered a promising environmentally friendly energy carrier for replacing traditional fossil fuels. In this context, photoelectrochemical cells effectively convert solar energy directly to H₂ fuel by water photoelectrolysis, thereby monolitically combining the functions of both light harvesting and electrolysis. In such devices, photocathodes and photoanodes carry out the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Here, the focus is on photocathodes for HER, traditionally based on metal oxides, III-V group and II-VI group semiconductors, silicon, and copper-based chalcogenides as photoactive material. Recently, carbon-based materials have emerged as reliable alternatives to the aforementioned materials. A perspective on carbon-based photocathodes is provided here, critically analyzing recent research progress and outlining the major guidelines for the development of efficient and stable photocathode architectures. In particular, the functional role of charge-selective and protective layers, which enhance both the efficiency and the durability of the photocathodes, is discussed. An in-depth evaluation of the state-of-the-art fabrication of photocathodes through scalable, high-troughput, cost-effective methods is presented. The major aspects on the development of light-trapping nanostructured architectures are also addressed. Finally, the key challenges on future research directions in terms of potential performance and manufacturability of photocathodes are analyzed.

The Role of Graphene and Other 2D Materials in Solar Photovoltaics

2D materials have attracted considerable attention due to their exciting optical and electronic properties, and demonstrate immense potential for next-generation solar cells and other optoelectronic devices. With the scaling trends in photovoltaics moving toward thinner active materials, the atomically thin bodies and high flexibility of 2D materials make them the obvious choice for integration with future-generation photovoltaic technology. Not only can graphene, with its high transparency and conductivity, be used as the electrodes in solar cells, but also its ambipolar electrical transport enables it to serve as both the anode and the cathode. 2D materials beyond graphene, such as transition-metal dichalcogenides, are direct-bandgap semiconductors at the monolayer level, and they can be used as the active layer in ultrathin flexible solar cells. However, since no 2D material has been featured in the roadmap of standard photovoltaic technologies, a proper synergy is still lacking between the recently growing 2D community and the conventional solar community. A comprehensive review on the current state-of-the-art of 2D-materials-based solar photovoltaics is presented here so that the recent advances of 2D materials for solar cells can be employed for formulating the future roadmap of various photovoltaic technologies.

Synthesis and Characterization of Samarium-Substituted Molybdenum Diselenide and Its Graphene Oxide Nanohybrid for Enhancing the Selective Sensing of Chloramphenicol in a Milk Sample

The electronic conductivity and electrocatalytic activity of metal chalcogenides are normally enhanced by following the ideal strategies such as substitution/doping of heterogeneous atoms and hybridization of highly conductive carbon supportive materials. Here, a rare earth element (samarium) was substituted with MoSe₂ using the simple hydrothermal method. The lattice distortion due to the substitution of Sm³⁺ with MoSe₂ was clearly observed by using high-resolution transmission electron microscopy analysis. As a consequence, the prepared SmMoSe₂ nanorod was encapsulated with graphene oxide (GO) sheets by using ultrasonication process. Furthermore, the GO-encapsulated SmMoSe₂ nanocomposite modified glassy carbon electrode (GO@SmMoSe₂/GCE) was used for the sensing of chloramphenicol. The results showed that the GO@SmMoSe₂/GCE revealed the superior electrocatalytic activity with low detection (5 nM) and sensitivity (20.6 μA μM^-1 cm^-2) to electrochemical detection of proposed analyte. It indicates that the substitution of Sm³⁺ and encapsulation of GO significantly increased both the electrical conductivity and electrocatalytic activity of MoSe₂.

Sonication-assisted liquid-phase exfoliated α-GeTe: a two-dimensional material with high Fe3+ sensitivity

We describe sonication-assisted liquid-phase exfoliation of rhombohedral germanium telluride (α-GeTe) to obtain a good dispersion of α-GeTe nanosheets in ethanol. The thickness of the α-GeTe nanosheets is dependent on the exfoliation conditions, and few-layer α-GeTe nanosheets of 2-4 layers and even monolayer α-GeTe were obtained. We use first-principles calculations to investigate the structural, electronic, and optical properties of monolayer and bulk α-GeTe and compare the optical band gap of centrifugally fractionated α-GeTe nanosheet dispersions with the computational predictions. We demonstrate that few layer α-GeTe nanosheets are purified selectively through centrifugation, and they exhibit high sensitivity to Fe3+. The scalable production of two-dimensional α-GeTe nanosheets can be used in the future optoelectronic industry.

Liquid-Phase Exfoliated Indium-Selenide Flakes and Their Application in Hydrogen Evolution Reaction

Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of β-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g L^-1 . Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. It is demonstrated that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few micrometers, and thicknesses ranging from 1 to 20 nm, with a maximum population centered at ≈5 nm, corresponding to 4 Se-In-In-Se quaternary layers. It is also shown that no formation of further InSe-based compounds (such as In₂ Se₃ ) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for the hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. The dependence of the InSe flakes' morphologies, i.e., surface area and thickness, on the HER performances is highlighted, achieving the best efficiencies with small flakes offering predominant edge effects. The theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes.

Layered Aggregation with Steric Effect: Morphology-Homogeneous Semiconductor MoS2 as an Alternative 2D Probe for Visual Immunoassay

Liquid-phase exfoliation routes unavoidably generate 2D nanostructures with inhomogeneous morphologies. Herein, thickness-dependent sorting of exfoliated nanostructures is achieved via a treatment of differential-zone centrifugation in the surfactant aqueous phase. With this approach, homogeneous MoS₂ nanosheets are obtained, and due to the intrinsic semiconducting characteristics, those 2D nanosheets are endowed with desired optical properties, rivaling classic gold nanoparticles in sensing applications. Furthermore, MoS₂ nanosheets with high uniformity and chemical inertness are coupled with proteins, exhibiting high performance in stability and anti-interferences for bioanalysis. As a consequence of aggregation-induced steric effect, distinguishing running shifts of antibody-anchored conjugates in gel electrophoresis are visually responsive to those specific antigens. This assay enables the easy and fast monitoring of tumor biomarkers just according to "naked-eye" identification of band location in electrophoresis results, which are presented by an alternative visual probe of 2D MoS₂ -protein conjugates. The developed visual immunoassay with the synergistic effect of gel electrophoresis techniques and 2D semiconductors pushes significant progress in "home-made" tests for disease early diagnosis.

Recent Advances of Plasmonic Organic Solar Cells: Photophysical Investigations

The surface plasmon resonance (SPR) of metallic nanomaterials, such as gold (Au) and silver (Ag), has been extensively exploited to improve the optical absorption, the charge carrier transport, and the ultimate device performances in organic photovoltaic cells (OPV). With the incorporation of diverse metallic nanostructures in active layers, buffer layers, electrodes, or between adjacent layers of OPVs, multiple plasmonic mechanisms may occur and need to be distinguished to better understand plasmonic enhancement. Steady-state photophysics is a powerful tool for unraveling the plasmonic nature and revealing plasmonic mechanisms such as the localized surface plasmon resonance (LSPR), the propagating plasmon-polariton (SPP), and the plasmon-gap mode. Furthermore, the charge transfer dynamics in the organic semiconductor materials can be elucidated from the transient photophysical investigations. In this review article, the basics of the plasmonic mechanisms and the related metallic nanostructures are briefly introduced. We then outline the recent advances of the plasmonic applications in OPVs emphasizing the linkage between the photophysical properties, the nanometallic geometries, and the photovoltaic performance of the OPV devices.

Defect Mitigation of Solution-Processed 2D WSe2 Nanoflakes for Solar-to-Hydrogen Conversion

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe₂ thin film photocathodes for solar H₂ production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt-Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100-1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe₂: solar photocurrents for H₂ evolution up to 4.0 mA cm^-2 and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing.

Improving charge transport by the ultrathin QDs interlayer in polymer solar cells

Lead sulfide (PbS) quantum dots (QDs) have been incorporated into PTB7:PC₇₁BM BHJ active layers to fabricate polymer solar cells (PSCs) and gather on the top surface of active layers to form an ultrathin interlayer. The PbS QDs ultrathin interlayer with an appropriate thickness increases the carrier transport capacity, exciton dissociation and reduces the carrier recombination, which leads to a higher short circuit current (J_sc) and fill factor (FF). Finally, the power conversion efficiency (PCE) improves from 7.03% (control devices) to 7.87% with an ultrathin interlayer by doping 5% PbS QDs, while the current density (J_sc) and fill factor (FF) enhances from 13.83 mA cm⁻² to 14.81 mA cm⁻² and from 68.70% to 70.85%, respectively.

Lead sulfide (PbS) quantum dots (QDs) have been incorporated into PTB7:PC₇₁BM BHJ active layers to fabricate polymer solar cells (PSCs) and gather on the top surface of active layers to form an ultrathin interlayer.