Stepwise heating in Stille polycondensation toward no batch-to-batch variations in polymer solar cell performance

For a given ??-conjugated polymer, the batch-to-batch variations in molecular weight (Mw) and polydispersity index (??) can lead to inconsistent process-dependent material properties and consequent performance variations in the device application. Using a stepwise-heating protocol in the Stille polycondensation in conjunction with optimized processing, we obtained an ultrahigh-quality PTB7 polymer having high Mw and very narrow ??. The resulting ultrahigh-quality polymer-based solar cells demonstrate up to 9.97% power conversion efficiencies (PCEs), which is over 24% enhancement from the control devices fabricated with commercially available PTB7. Moreover, we observe almost negligible batch-to-batch variations in the overall PCE values from ultrahigh-quality polymer-based devices. The proposed stepwise polymerization demonstrates a facile and effective strategy for synthesizing high-quality semiconducting polymers that can significantly improve device yield in polymer-based solar cells, an important factor for the commercialization of organic solar cells, by mitigating device-to-device variations

Toward Green Synthesis of Graphene Oxide Using Recycled Sulfuric Acid via Couette-Taylor Flow

Developing eco-friendly and cost-effective processes for the synthesis of graphene oxide (GO) is essential for its widespread industrial applications. In this work, we propose a green synthesis technique for GO production using recycled sulfuric acid and filter-processed oxidized natural graphite obtained from a Couette-Taylor flow reactor. The viscosity of reactant mixtures processed from Couette-Taylor flow was considerably lower (???200 cP at 25 ??C) than that of those from Hummers' method, which enabled the simple filtration process. The filtered sulfuric acid can be recycled and reused for the repetitive GO synthesis with negligible differences in the as-synthesized GO qualities. This removal of sulfuric acid has great potential in lowering the overall GO production cost as the amount of water required during the fabrication process, which takes a great portion of the total production cost, can be dramatically reduced after such acid filtration. The proposed eco-friendly GO fabrication process is expected to promote the commercial application of graphene materials into industry shortly

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Department of Materials Science and Engineeringclos

Solution-Processed Molybdenum Oxide with Hydroxyl Radical-Induced Oxygen Vacancy as an Efficient and Stable Interfacial Layer for Organic Solar Cells

The interfacial layer (IL) in organic solar cells (OSCs) can be an important boosting factor for improving device efficiency and stability. Herein, a facile and cost-effective approach to form a uniform molybdenum oxide (MoO3) film with desirable stability is provided, based on solution processing at low temperatures by simplified precursor solution synthesis. The solution-processed MoO3 (SM) film, with oxygen vacancies induced by the hydroxyl group, functions as an efficient anode IL in conventional OSCs. The hole-transporting performance of SM is well demonstrated in nonfullerene-based OSCs exhibiting over 10% of power conversion efficiency. The enhanced device performance of SM-based OSCs over that of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is investigated by analyzing the morphology, electronic state, and electrical conductivity of such a hole-transporting layer, as well as the charge dynamics in the completed devices. Furthermore, the high stability of the SM films in OSCs is examined under various environmental conditions, including long-term and thermal stability. In particular, fullerene-based OSCs with SM maintain over 90% of their initial cell performance over 2500 h under inert conditions. It is shown that solution-processed metal oxides can be viable ILs with high functionality and versatility, overcoming the drawbacks of conventionally adopted conducting polymer interlayers

Locking-In Optimal Nanoscale Structure Induced by Naphthalenediimide-Based Polymeric Additive Enables Efficient and Stable Inverted Polymer Solar Cells

Operational stability and high performance are the most critical issues that must be addressed in order to propel and advance the current polymer solar cell (PSC) technology to the next level, such as manufacturing and mass production. Herein, we report a high power conversion efficiency (PCE) of 11.2%, together with an excellent device stability in PTB7-Th:PC71BM-based PSCs in the inverted structure by introducing the n-type P(NDI2OD-T2) macromolecular additive (>75% PCE retention at high temperature up to 120 degrees C, >97% PCE retention after 6 months in inert conditions, >93% PCE retention after 2 months in air with encapsulation, and >80% PCE retention after 140 h in air without encapsulation). The PCE is the highest value ever reported in the single-junction systems based on the PTB7 family and is also comparable to the previously reported highest PCE of inverted PSCs. These promising results are attributed to the efficient optimization and stabilization of the blend film morphology in the photoactive layer, achieved using the P(NDI2OD-T2) additive. From the perspective of manufacturing, our studies demonstrate a promising pathway for fabricating low-cost PSCs with high efficiency as well as long-term stability

The effect of the graphene integration process on the performance of graphene-based Schottky junction solar cells

With the rise of graphene, its applications as the active component in various types of solar cells, such as transparent conductors, additives, or interfacial charge transport layers, have been intensively investigated. Among them, graphene-based Schottky junction solar cells have been rapidly developed due to their relatively simple device structures compared to conventional p-n junction type solar cells. Through various modifications such as chemical doping, antireflection coating, and interfacial oxide layer control, a power conversion efficiency of over 15% was successfully reported. However, graphene-based Schottky junction type solar cells often suffer from s-shaped current density-voltage characteristics, which leads to the inevitable performance degradation, particularly for the fill factor. In this work, we investigate the origin of such aforementioned behaviors and propose a facile approach to suppress the s-shape character in the operation of graphene-based Schottky junction solar cells. Through the careful modulation of the graphene integration process, the interfacial charge recombination seemed to be significantly suppressed leading to a notably improved device performance (from 0.8% to 12.5%). Our findings shall provide valuable insights into the operating principle of graphene-based Schottky junction solar cells, which can play an important role as one of the primary suppliers of next-generation renewable clean energy

Development of Annealing-Free, Solution-Processable Inverted Organic Solar Cells with N-Doped Graphene Electrodes using Zinc Oxide Nanoparticles

An annealing-free process is considered as a technological advancement for the development of flexible (or wearable) organic electronic devices, which can prevent the distortion of substrates and damage to the active components of the device and simplify the overall fabrication process to increase the industrial applications. Owing to its outstanding electrical, optical, and mechanical properties, graphene is seen as a promising material that could act as a transparent conductive electrode for flexible optoelectronic devices. Owing to their high transparency and electron mobility, zinc oxide nanoparticles (ZnO-NP) are attractive and promising for their application as charge transporting materials for low-temperature processes in organic solar cells (OSCs), particularly because most charge transporting materials require annealing treatments at elevated temperatures. In this study, graphene/annealing-free ZnO-NP hybrid materials were developed for inverted OSC by successfully integrating ZnO-NP on the hydrophobic surface of graphene, thus aiming to enhance the applicability of graphene as a transparent electrode in flexible OSC systems. Chemical, optical, electrical, and morphological analyses of ZnO-NPs showed that the annealing-free process generates similar results to those provided by the conventional annealing process. The approach was effectively applied to graphene-based inverted OSCs with notable power conversion efficiencies of 8.16% and 7.41% on the solid and flexible substrates, respectively, which promises the great feasibility of graphene for emerging optoelectronic device applications

Improved Interface Control for High-Performance Graphene-Based Solar Cells

The demand for high-efficiency flexible optoelectronic devices is ever-increasing because next-generation electronic devices that comprise portable or wearable electronic systems are set to play an important role. Graphene has received extensive attention as it is considered to be a promising candidate material for transparent flexible electrode platforms owing to its outstanding electrical, optical, and physical properties. Despite these properties, the inert and hydrophobic nature of graphene surfaces renders it difficult to use in optoelectronic devices. In particular, commonly used charge transporting layer (CTL) materials for organic solar cells (OSCs) cannot uniformly coat a graphene surface, which leads to the inevitable devices failing. Herein, this paper proposes an approach that will enable CTL materials to completely cover a graphene electrode; this is done with the assistance of commonly accessible polar solvents

Bio-Inspired Catecholamine-Derived Surface Modifier for Graphene-Based Organic Solar Cells

Owing to the growing interest in next-generation solar cells as a clean and renewable energy source, the demand for alternative transparent conducting electrodes (TCEs) has also increased. Although indium tin oxide (ITO) has been widely used as the standard TCE, its chemical and mechanical instabilities limit its widespread use in emerging photovoltaics. Graphene has attracted much attention as a potential alternative TCE owing to its excellent physical, optical, and electrical properties. However, owing to the inert nature of graphene with a hydrophobic surface, a significant amount of research has been devoted to resolve the nonwetting issue of charge-transporting materials on graphene. In this study, a thin layer of norepinephrine, an amphiphilic catecholamine derivative, was applied to graphene as a hydrophilic surface modifier to enable efficient surface modification without significantly decreasing the optical transmittance or the electrical conductivity. This modification allowed a commonly used hole-transporting material to be applied uniformly to the surface. Thus, the power conversion efficiency (PCE) of organic solar cells (OSCs) fabricated with this poly(norepinephrine)-coated graphene electrode was 7.93%, which is approaching close to that of the ITO-based reference device with a PCE of 8.73%. This work represents the first demonstration of an adhesive biomaterial as an efficient surface modifier for chemically inert graphene and its successful application in OSCs, which shows promise for the future development of bio-inspired graphene systems for applications to various optoelectronic devices