Flexible All-Solution-Processed Organic Solar Cells with High-Performance Nonfullerene Active Layers

Magnetic Nanocomposite Modified Hybrid Hole-Transport Layer for Constructing Organic Solar Cells with High Efficiencies

An interface modification layer holds paramount significance in reducing interface carrier recombination and improving the ohmic contact between the active layer and the electrode in organic solar cells (OSCs). Modifying or doping the widely used hole-transport layer (HTL) PEDOT:PSS to adjust the work function, conductivity, and acidity has become a common strategy for achieving high-performance OSCs. Metal oxides and two-dimensional materials as secondary dopants into PEDOT:PSS, respectively, as well as a replacement of PEDOT:PSS both exhibit immense potential for achieving high-performance OSCs due to their excellent electrical properties. Herein, we report a method utilizing a Fe3O4/GO magnetic nanocomposite as a secondary dopant for PEDOT:PSS to modulate its inherent properties for constructing high-efficiency OSCs. The magnetic nanocomposite hybrid HTL exhibits a suitable optical transmittance and higher work function. Meanwhile, it is found that the addition of Fe3O4/GO magnetic nanoparticles expands the domain of PEDOT and enhances the phase separation between PEDOT and PSS segments, thereby improving the conductivity of PEDOT:PSS. By fine-tuning the doping ratio of a Fe3O4/GO magnetic nanocomposite in PEDOT:PSS, the best power conversion efficiency of OSCs based on PM6:L8-BO was up to 18.91%. The notable enhancement of the device's performance was due to the enhanced hole mobility and the improved charge extraction, further complemented by the decreased likelihood of interface recombination brought about by the hybrid HTL. Compared with PEDOT:PSS-based OSCs, an enhanced stability of the hybrid HTL-based device was also obtained. In addition, the diverse adaptability of the hybrid HTL was demonstrated in enhancing the performance of OSCs that are based on PM6:Y6 and PBDB-T:ITIC. The effectiveness and versatility of a magnetic nanocomposite hybrid HTL present opportunities for achieving high-performance OSCs.

Constructing Multiscale Fibrous Morphology to Achieve 20% Efficiency Organic Solar Cells by Mixing High and Low Molecular Weight D18

This study underscores the significance of precisely manipulating the morphology of the active layer in organic solar cells (OSCs). By blending polymer donors of D18 with varying molecular weights, a multiscale interpenetrating fiber network structure within the active layer is successfully created. The introduction of 10% low molecular weight D18 (LW-D18) into high molecular weight D18 (HW-D18) produces MIX-D18, which exhibits an extended exciton diffusion distance and orderly molecular stacking. Devices utilizing MIX-D18 demonstrate superior electron and hole transport, improves exciton dissociation, enhances charge collection efficiency, and reduces trap-assisted recombination compared to the other two materials. Through the use of the nonfullerene acceptor L8-BO, a remarkable power conversion efficiency (PCE) of 20.0% is achieved. This methodology, which integrates the favorable attributes of high and low molecular weight polymers, opens a new avenue for enhancing the performance of OSCs.

A Polymeric Two-in-One Electron Transport Layer and Transparent Electrode for Efficient Indoor All-Organic Solar Cells

Transparent electrodes (TEs) are vital in optoelectronic devices, enabling the interaction of light and charges. While indium tin oxide (ITO) has traditionally served as a benchmark TE, its high cost prompts the exploration of alternatives to optimize electrode characteristics and improve device efficiencies. Conducting polymers, which combine polymer advantages with metal-like conductivity, emerge as a promising solution for TEs. This work introduces a two-in-one electron transport layer (ETL) and TE based on films of polyethylenimine ethoxylated (PEIE)-modified poly(benzodifurandione) (PBFDO). These PEIE-modified PBFDO layers exhibit a unique combination of properties, including low sheet resistance (130 Ω sq-1), low work function (4.2 eV), and high optical transparency (>85% in the UV-vis-NIR range). In contrast to commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the doping level of PBFDO remains unaffected by the PEIE treatment, as verified through UV-vis-NIR absorption and X-ray photoelectron spectroscopy measurements. When employed as a two-in-one ETL/TE in organic solar cells, the PEIE-modified PBFDO electrode exhibits performance comparable to conventional ITO electrodes. Moreover, this work demonstrates all-organic solar cells with record-high power conversion efficiencies of >15.1% under indoor lighting conditions. These findings hold promise for the development of fully printed, all-organic optoelectronic devices.

Advances in Stretchable Organic Photovoltaics: Flexible Transparent Electrodes and Deformable Active Layer Design

Stretchable organic photovoltaics (OPVs) have attracted significant attention as promising power sources for wearable electronic systems owing to their superior robustness under repetitive tensile strains and their good compatibility. However, reconciling a high power-conversion efficiency and a reasonable flexibility is a tremendous challenge. In addition, the development of stretchable OPVs must be accelerated to satisfy the increasing requirements of niche markets for mechanical robustness. Stretchable OPV devices can be classified as either structurally or intrinsically stretchable. This work reviews recent advances in stretchable OPVs, including the design of mechanically robust transparent electrodes, photovoltaic materials, and devices. Initially, an overview of the characteristics and recent research progress in the areas of structurally and intrinsically stretchable OPVs is provided. Subsequently, research into flexible and stretchable transparent electrodes that directly affect the performances of stretchable OPVs is summarized and analyzed. Overall, this review aims to provide an in-depth understanding of the intrinsic properties of highly efficient and deformable active materials, while also emphasizing advanced strategies for simultaneously improving the photovoltaic performance and mechanical flexibility of the active layer, including material design, multi-component settings, and structural optimization.

Metal nanowire-based transparent electrode for flexible and stretchable optoelectronic devices

High-quality transparent electrodes are indispensable components of flexible optoelectronic devices as they guarantee sufficient light transparency and electrical conductivity. Compared to commercial indium tin oxide, metal nanowires are considered ideal candidates as flexible transparent electrodes (FTEs) owing to their superior optoelectronic properties, excellent mechanical flexibility, solution treatability, and higher compatibility with semiconductors. However, certain key challenges associated with material preparation and device fabrication remain for the practical application of metal nanowire-based electrodes. In this review, we discuss state-of-the-art solution-processed metal nanowire-based FTEs and their applications in flexible and stretchable optoelectronic devices. Specifically, the important properties of FTEs and a cost-benefit analysis of existing technologies are introduced, followed by a summary of the synthesis strategy, key properties, and fabrication technologies of the nanowires. Subsequently, we explore the applications of metal-nanowire-based FTEs in different optoelectronic devices including solar cells, photodetectors, and light-emitting diodes. Finally, the current status, future challenges, and emerging strategies in this field are presented.

Fullerene-Based Heterojunctions for Non-Selective Absorption Transparent Solar Cells

Transparent photovoltaic (TPV) devices have great potential to be applied as smart windows in construction and agriculture fields. TPVs with an average visible transmission (AVT) exceeding 50% are among the strong candidates to build lighting windows since the champion efficiency has already exceeded 10%. However, it is still a challenge in TPVs that semiconductors are generally expensive and transparency is difficult to further enhance, particularly for device AVT exceeding 70%. In this work, we develop a set of fullerene-based heterojunctions to harvest the light. By utilizing the low-cost fullerene as the light-absorbing material and combining it with the transparent electrode, the fabricated TPV device can achieve an AVT of 72.1% with a PCE exceeding 1%. Notably, the device with an AVT of 82% is also successfully demonstrated. This study provides an effective approach for building low-cost and efficient TPV devices.

Interface Engineering for Highly Efficient Organic Solar Cells

Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

Waterproof and ultraflexible organic photovoltaics with improved interface adhesion

Ultraflexible organic photovoltaics have emerged as a potential power source for wearable electronics owing to their stretchability and lightweight nature. However, waterproofing ultraflexible organic photovoltaics without compromising mechanical flexibility and conformability remains challenging. Here, we demonstrate waterproof and ultraflexible organic photovoltaics through the in-situ growth of a hole-transporting layer to strengthen interface adhesion between the active layer and anode. Specifically, a silver electrode is deposited directly on top of the active layers, followed by thermal annealing treatment. Compared with conventional sequentially-deposited hole-transporting layers, the in-situ grown hole-transporting layer exhibits higher thermodynamic adhesion between the active layers, resulting in better waterproofness. The fabricated 3 μm-thick organic photovoltaics retain 89% and 96% of their pristine performance after immersion in water for 4 h and 300 stretching/releasing cycles at 30% strain under water, respectively. Moreover, the ultraflexible devices withstand a machine-washing test with such a thin encapsulation layer, which has never been reported. Finally, we demonstrate the universality of the strategy for achieving waterproof solar cells.

Anion-substitution interfacial engineering to construct C@MoS2 hierarchical nanocomposites for broadband electromagnetic wave absorption

Developing an effective strategy to regulate the interfacial properties of hierarchical structure is of great significance for preparation of high-performance electromagnetic wave absorption (EMA) materials. Ion-substitution can change intrinsic structure and properties of a materials, but its effect on the interfacial properties of hierarchical structure remained to be explored. Herein, we first constructed a C@MoS2 hierarchical structure via simple hydrothermal reaction, then used the ion-substitution strategy to replace the S atoms in MoS2 with O, F and Se, and finally obtained anion-substituted hierarchical structure (C@X-MoS2, X = O, F, Se). The results show that ion-substitution destroys the MoS2 crystal structure and realizes tunable dielectric properties of C@MoS2, which leads to further enhancement of overall interfacial polarization. After optimization, the absorption strength and width of C@O-MoS2 has been significantly improved. The minimum reflection loss (RLmin) reaches -62.17 dB, and the maximum effective absorption bandwidth (EABmax) is 7.0 GHz. The simulation results show the obtained absorbent can greatly reduce the radar cross section of target, indicating it has broad application potential. Therefore, this work provides a novel method for regulation of EMA performance of hierarchical structure and preparation of high-performance absorbents.

Suppressing electron-phonon coupling in organic photovoltaics for high-efficiency power conversion

The nonradiative energy loss (∆Enr) is a critical factor to limit the efficiency of organic solar cells. Generally, strong electron-phonon coupling induced by molecular motion generates fast nonradiative decay and causes high ∆Enr. How to restrict molecular motion and achieve a low ∆Enr is a sticking point. Herein, the free volume ratio (FVR) is proposed as an indicator to evaluate molecular motion, providing new molecular design rationale to suppress nonradiative decay. Theoretical and experimental results indicate proper proliferation of alkyl side-chain can decrease FVR and restrict molecular motion, leading to reduced electron-phonon coupling while maintaining ideal nanomorphology. The reduced FVR and favorable morphology are simultaneously obtained in AQx-6 with pinpoint alkyl chain proliferation, achieving a high PCE of 18.6% with optimized VOC, JSC and FF. Our study discovered aggregation-state regulation is of great importance to the reduction of electron-phonon coupling, which paves the way to high-efficiency OSCs.

Flexible Solution-Processed Electron-Transport-Layer-Free Organic Photovoltaics for Indoor Application

Organic photovoltaics (OPVs) have unique advantages of low weight, mechanical flexibility, and solution processability, which make them exceptionally suitable for integrating low-power Internet of Things devices. However, achieving improved operational stability together with solution processes that are applicable to large-scale fabrication remains challenging. Their major limitation arises due to the instable factors that occur both inside the thick active film and from the ambient environment, which cannot be completely resolved via the current encapsulation techniques used for flexible OPVs. Additionally, thin active layers are highly vulnerable to point defects, which result in low yield rates and impede the laboratory-to-industry translation. In this study, flexible fully solution-processed OPVs with improved indoor efficiency and long-term operational stability than that of conventional OPVs with evaporated electrodes are achieved. Benefiting from the oxygen and water vapor permeation barrier of the spontaneously formed gallium oxide layers on the exposed eutectic gallium-indium surface, fast degradation of the OPVs with thick active layers is prevented, maintaining 93% of its initial P_max after 5000 min of indoor operation under 1000 lx light-emitting diode (LED) illumination. Additionally, by using the thick active layer, spin-coated silver nanowires could be directly used as bottom electrodes without complicated flattening processes, thereby substantially simplifying the fabrication process and proposing a promising manufacturing technique for devices with high-throughput energy demands.

Solution-Processed Perovskite/Metal-Oxide Hybrid X-Ray Detector and Array with Decoupled Electronic and Ionic Transport Pathways

Lead halide perovskites possess heavy elements and excellent mobility-lifetime (µτ) product, becoming desirable candidates for X-ray detectors. However, current perovskite photoconduction detectors (PCDs) with vertical geometry, where electronic signals and mobile ions share the same conduction path, are facing with extremely challenging ion-migration issue. Herein, a hybrid X-ray detector device structure, in which perovskite is vertically stacked onto an indium oxide (In₂ O₃ ) transistor with lateral transport geometry is designed, perovskite mainly acts as X-ray sensitizer to activate In₂ O₃ conduction channel, the actual electrical signal is conducted and collected in the lateral metal-oxide device. With the decoupled ionic and electronic transportation, hybrid detectors are insensitive to the ionic motion of perovskite, hence demonstrating no hysteresis and almost no shifting of baseline that are often observed in PCDs, hybrid detectors also exhibit reduced dark current, improved response time, and four times higher photocurrent signals. Finally, array integration of hybrid detectors and preliminary X-ray imaging is realized. The work provides an effective device strategy in addition to the mere material alternations to attain high-performance perovskite-based X-ray detectors and arrays.

Mechanically and Ultraviolet Light Stable Ultrathin Organic Solar Cell via Semi-Embedding Silver Nanowires in a Hydrogen Bonds-Based Polyimide

Ultrathin organic solar cells (OSCs) with both high power conversion efficiency (PCE) and operational stability are of great significance for the industrial applications but still challenging. Here, we synthesized a polyimide (PI) substrate for high-performance and stable ultrathin OSCs, which was physically crosslinked via strong hydrogen bonds (denoted as HB-PI) to enhance the mechanical, thermal, solvent-resistant, and UV filtering properties (with a cut-off wavelength of 376 nm). An ultrathin flexible transparent composite electrode (FTCE, ∼7 μm) was fabricated via semi-embedding AgNWs in the HB-PI substrate. The FTCE possesses excellent optoelectronic property, smooth surface, and high mechanical stability simultaneously. Based on this FTCE, an ultrathin OSC was constructed with a PCE of 13.52% (average of 13.22%). Moreover, the ultrathin OSC showed outstanding mechanical stability (PCE decreased by less than 4% after 1000 bending cycles at a small bending radius of 0.5 mm) and superior UV light stability (no evident PCE degradation after irradiation under UV light for 10 h). This work will provide a new avenue for fabricating high-performance and stable ultrathin OSCs. This article is protected by copyright. All rights reserved.

On the interface reactions and stability of nonfullerene organic solar cells

Long-term stability is critical for organic solar cells (OSCs) for practical applications. Several factors affect the stability of OSCs, including materials stability, morphology stability of bulk-heterojunctions and interface stability. In this perspective, we focus on interface stability due to interfacial reactions between the emerging acceptor-donor-acceptor (A-D-A) type nonfullerene active layers and interfacial layers. The description covers the initial phenomena of interfacial instability, mechanism of interfacial reactions, and strategies adopted to suppress interfacial reactions between the nonfullerene active layers and interfacial layers. Methods to test and analyze the chemical instability of nonfullerene acceptors are also included. The C[double bond, length as m-dash]C vinyl linker between the donor moiety and acceptor moiety is chemically or photochemically reactive and is a weak point for interface stability. The interface stability of OSCs could be enhanced by reducing the reactivity of the C[double bond, length as m-dash]C vinyl linker or removing it directly, modifying the surface of interfacial layers, and developing other novel interfacial materials.

Random Copolymerization Strategy for Host Polymer Donor PM6 Enables Improved Efficiency Both in Binary and Ternary Organic Solar Cells

Although breakthroughs have been made in organic solar cells (OSCs) in recent years, the power conversion efficiency (PCE) of OSCs still lags behind inorganic/perovskite solar cells. In this work, two terpolymers were synthesized by introducing the thieno[3,4-c]pyrrole-4,6-(5H)-dione (TPD) block into the host polymer donor PM6. Owing to the lower highest occupied molecular orbital energy level, wider light absorption, optimal molecular packing, and more desirable aggregation morphology by addition of the TPD, the PM6-TPD-5 % : Y6-based device displayed an improved PCE of 16.3 % with an enhanced open-circuit voltage (V_OC ) of 0.860 V, relative to that of PM6-TPD-10 % : Y6 (PCE=14.8 %) and PM6 : Y6-based device (PCE=15.6 %). Interestingly, the V_OC did not always increase in proportion to the third component. Besides, ternary OSCs based on PM6 : PM6-TPD-5 % : Y6 achieved a superior PCE of 17.1 %. This work demonstrated that random copolymerization is a feasible and effective strategy to further increase device performance, and the two polymers that possess similar structure and absorption in ternary devices can also obtain impressive efficiency.

12.42% Monolithic 25.42 cm2 Flexible Organic Solar Cells Enabled by an Amorphous ITO-Modified Metal Grid Electrode

Printed metal nanogrid electrode exhibits superior characteristics for use in flexible organic solar cells (OSCs). However, the high surface roughness and inhomogeneity between grid and blank region is adverse for performance improvement. In this work, a thin amorphous indium tin oxide (ITO) film (α-ITO) is introduced to fill the blank and to improve the charge transporting. The introduction of α-ITO significantly improves the comprehensive properties of metal grid electrode, which exhibits excellent bending resistance and long-term stability under double 85 condition (under 85 °C and 85% relative humidity) for 200 h. Both experimental and simulation results reveal α-ITO with a sheet resistance of 20 000 Ω □^-1 is sufficient to improve the charge transporting within the adjacent grids, leading to a remarkable efficiency of 16.54% for 1 cm² flexible devices. With area increased to 4.00, 9.00, and 25.42 cm² , the devices still display a performance of 16.22%, 14.69%, and 12.42%, respectively, showing less efficiency loss during upscaling. And the 25.42 cm² monolithic flexible device exhibits a certificated efficiency of 12.03%. Moreover, the device shows significantly improved air stability relative to conventional high-conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate-modified device. All these make the α-ITO-modified Ag/Cu electrode promise to achieve high-efficient and long-term stable large-area flexible OSCs.

Producing p-Doped Surface for Hole Transporting Layer-free Nonfullerene Organic Solar Cells

Hole transporting layer-free organic solar cells with simplified device structure are desirable for their mass production. In this work, we adopted a p-dopant of organic molybdenum peroxide (OMP) to dope nonfullerene active layers to produce p-doped surface on the active layer. The OMP can effectively dope widely used polymer donors of nonfullerene organic solar cells, i.e., PTB7-Th, PBDB-T, and, even PBDB-T-2F that has a very deep highest occupied molecular orbital (HOMO) energy level of -5.47 eV. The doping mechanism lies in the strong oxidizing property of peroxide groups of the OMP leading to superior doping properties. In the end, we fabricated hole transporting layer-free nonfullerene organic solar cells with the device structure of ITO/PEI-Zn/PBDB-T-2F:IT-4F/Ag. The cells showed a power conversion efficiency of 12.2% and good thermal stability. This article is protected by copyright. All rights reserved.

Solution-Processed Electron-Transport Layer-free Organic Photovoltaics with Liquid Metal Cathodes

Flexible, lightweight, and large-area solar cells provide new power supply opportunities in the renewable energy field and facilitate the supply of power to internet-of-things devices and wearable devices. The choice of printing process technologies is a key parameter for such flexible power sources because of their energy-saving process technology and high throughput rate. In addition to selecting the appropriate printing method for the active and charge transport layers, the development of printed electrodes is critical. Numerous printable materials have been developed to replace conventional evaporated top electrodes. However, achieving fully solution-processed organic photovoltaics (OPVs) with power conversion efficiency (PCE) comparable to OPVs with vacuum-deposited transparent and top electrodes is challenging. This is because of the difficulty of forming a uniform interface between the top solution-processed electrode and the active layers while preventing deterioration. In this study, an electron transport layer-free, eutectic gallium-indium (EGaIn) top-cathode strategy was developed and a record PCE of 12.7% in fully solution-processed, flexible OPVs was achieved. Direct coating of EGaIn on the active layer, in a nitrogen atmosphere, is conducive for energy band matching and obtaining physically perfect interfaces without any penetrations or voids. An average PCE of 14.1% and enhanced operating stability, comparable to conventional OPVs, were achieved with indium tin oxide transparent electrodes by eliminating the electron-transport layer. The fully solution-processed flexible OPVs fabricated with the embedded silver nanowire strategy in ultrathin transparent polyimide, achieved an average PCE of 12.7%, representing a promising technique to meet green and high-throughput energy demands.

Highly Sensitive and Durable Organic Photodiodes Based on Long-Term Storable NiOx Nanoparticles

Organic optoelectronic devices that can be fabricated at low cost have attracted considerable attention because they can absorb light over a wide frequency range and have high conversion efficiency, as well as being lightweight and flexible. Moreover, their performance can be significantly affected by the choice of the charge-selective interlayer material. Nonstoichiometric nickel oxide (NiO_x) is an excellent material for the hole-transporting layer (HTL) of organic optoelectronic devices because of the good alignment of its valence band position with the highest occupied molecular orbital level of many p-type polymers. Herein, we report a simple low-temperature process for the synthesis of NiO_x nanoparticles (NPs) that can be well dispersed in solution for long-term storage and easily used to form thin NiO_x NP layers. NiO_x NP-based organic photodiode (OPD) devices demonstrated high specific detectivity (D*) values of 10¹²-10¹³ jones under various light intensities and negative biases. The D* value of the NiO_x NP-based OPD device was 4 times higher than that of a conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based device, an enhancement that originated mainly from the 16 times decreased leakage current. The NiO_x NP-based OPD device demonstrated better reliability over a wide range of light intensities and operational biases in comparison to a device with a conventional sol-gel-processed NiO_x film. More importantly, the NiO_x NP-based OPD showed long-term device stability superior to those of the PEDOT:PSS and sol-gel-processed NiO_x-based devices. We highlight that our low-temperature solution-processable NiO_x NP-based HTL could become a crucial component in the fabrication of stable high-performance OPDs.

Ultrathin and Efficient Organic Photovoltaics with Enhanced Air Stability by Suppression of Zinc Element Diffusion

Ultrathin (thickness less than 10 µm) organic photovoltaics (OPVs) can be applied to power soft robotics and wearable electronics. In addition to high power conversion efficiency, stability under various environmental stresses is crucial for the application of ultrathin OPVs. In this study, the authors realize highly air-stable and ultrathin (≈3 µm) OPVs that possess high efficiency (15.8%) and an outstanding power-per-weight ratio of 33.8 W g-1 . Dynamic secondary-ion mass spectrometry is used to identify Zn diffusion from the electron transport layer zinc oxide (ZnO) to the interface of photoactive layer; this diffusion results in the degradation of the ultrathin OPVs in air. The suppression of the Zn diffusion by a chelating strategy results in stable ultrathin OPVs that maintain 89.6% of their initial efficiency after storage for 1574 h in air at room temperature under dark conditions and 92.4% of their initial efficiency after annealing for 172 h at 85 °C in air under dark conditions. The lightweight and stable OPVs also possess excellent deformability with 87.3% retention of the initial performance after 5000 cycles of a compressing-stretching test with 33% compression.

Ultrathin Flexible Transparent Composite Electrode via Semi-embedding Silver Nanowires in a Colorless Polyimide for High-Performance Ultraflexible Organic Solar Cells

Ultraflexible organic solar cells (OSCs) with both high power conversion efficiency (PCE) and good mechanical robustness are still challenging, in which flexible transparent composite electrodes (FTCEs, substrate-cum-electrodes) play critical roles. Here, an ultrathin FTCE (∼9 μm) via semi-embedding a silver nanowire electrode in a colorless polyimide (CPI) substrate was developed, which simultaneously possessed outstanding performance such as low square resistance (R_sq ∼ 12.7 Ω sq^-1), high optical transmittance (T₅₅₀ ∼ 86.3%), smooth surface (root-mean-square ∼ 0.32 nm), and excellent thermal, mechanical, and solution producing stability. Prior to the FTCE fabrication, four CPI samples with the number-average molecular weight ranging from 35.9 to 177.5 kDa were prepared and their optical, mechanical, and thermal properties were studied in detail. Moreover, the effect of the molecular weight on the minimum thickness that can withstand the following solution production of ultraflexible OSCs was investigated, which revealed that the molecular weight of CPI here should be above 81.4 kDa. Based on the FTCE, an ultraflexible OSC with a high PCE value of 14.37% and outstanding mechanical robustness was constructed, in which the PCE could still maintain above 96% of its initial value after 1000 bending cycles at a bending radius of 0.5 mm.

Recent Advances in Hole-Transporting Layers for Organic Solar Cells

Global energy demand is increasing; thus, emerging renewable energy sources, such as organic solar cells (OSCs), are fundamental to mitigate the negative effects of fuel consumption. Within OSC's advancements, the development of efficient and stable interface materials is essential to achieve high performance, long-term stability, low costs, and broader applicability. Inorganic and nanocarbon-based materials show a suitable work function, tunable optical/electronic properties, stability to the presence of moisture, and facile solution processing, while organic conducting polymers and small molecules have some advantages such as fast and low-cost production, solution process, low energy payback time, light weight, and less adverse environmental impact, making them attractive as hole transporting layers (HTLs) for OSCs. This review looked at the recent progress in metal oxides, metal sulfides, nanocarbon materials, conducting polymers, and small organic molecules as HTLs in OSCs over the past five years. The endeavors in research and technology have optimized the preparation and deposition methods of HTLs. Strategies of doping, composite/hybrid formation, and modifications have also tuned the optical/electrical properties of these materials as HTLs to obtain efficient and stable OSCs. We highlighted the impact of structure, composition, and processing conditions of inorganic and organic materials as HTLs in conventional and inverted OSCs.

Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics

Soft electronics are rising electronic technologies towards applications spanning from healthcare monitoring to medical implants. However, poor adhesion strength and significant mechanical mismatches inevitably cause the interface failure of devices. Herein we report a self-adhesive conductive polymer that possesses low modulus (56.1-401.9 kPa), high stretchability (700%), high interfacial adhesion (lap-shear strength >1.2 MPa), and high conductivity (1-37 S/cm). The self-adhesive conductive polymer is fabricated by doping the poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) composite with a supramolecular solvent (β-cyclodextrin and citric acid). We demonstrated the solution process-based fabrication of self-adhesive conductive polymer-based electrodes for various soft devices, including alternating current electroluminescent devices, electromyography monitoring, and an integrated system for the visualization of electromyography signals during muscle training with an array of alternating current electroluminescent devices. The self-adhesive conductive polymer-based electronics show promising features to further develop wearable and comfortable bioelectronic devices with the physiological electric signals of the human body readable and displayable during daily activities.

Patterning of Metal Nanowire Networks: Methods and Applications

With the advance in flexible and stretchable electronics, one-dimensional nanomaterials such as metal nanowires have drawn much attention in the past 10 years or so. Metal nanowires, especially silver nanowires, have been recognized as promising candidate materials for flexible and stretchable electronics. Owing to their high electrical conductivity and high aspect ratio, metal nanowires can form electrical percolation networks, maintaining high electrical conductivity under deformation (e.g., bending and stretching). Apart from coating metal nanowires for making large-area transparent conductive films, many applications require patterned metal nanowires as electrodes and interconnects. Precise patterning of metal nanowire networks is crucial to achieve high device performances. Therefore, a high-resolution, designable, and scalable patterning of metal nanowire networks is important but remains a critical challenge for fabricating high-performance electronic devices. This review summarizes recent advances in patterning of metal nanowire networks, using subtractive methods, additive methods of nanowire dispersions, and printing methods. Representative device applications of the patterned metal nanowire networks are presented. Finally, challenges and important directions in the area of the patterning of metal nanowire networks for device applications are discussed.

Enhanced Efficiency and Excellent Thermostability in Organic Photovoltaics via Ternary Strategy with Twisted Conjugated Compound

The thermal stability of high-efficiency organic photovoltaics (OPVs) is critical to the manufacturing of this technology. Therefore, targeted strategies and approaches shall be developed to improve efficiency and stability, simultaneously. Herein, a seleno twisted benzodiperylenediimides (TBD-PDI-Se) acceptor-doping strategy is taken advantage of to demonstrate the ternary bulk heterojunction OPVs with excellent stability, achieving outstanding power conversion efficiency of 14.43% and 17.25% based on PM6:IT-4F and PM6:Y6 ternary blend devices, respectively, which are superior to the corresponding binary devices. As evidenced by the active layer morphology, exciton dynamic study and the characterizations of the enabled device, the ternary blend device keeps nearly 90% original efficiency (t = 1000 h) under continuous constant heating at 140 °C. Furthermore, the application of acceptor as the third component in PBDB-T:ITIC, J71:ITIC, and PBDB-T:PC₇₁ BM systems is also verified, proving the good universality of acceptor-doping ternary strategy.

Zirconium-Doped Zinc Oxide Nanoparticles as Cathode Interfacial Layers for Efficiently Rigid and Flexible Organic Solar Cells

Low-temperature zinc oxide nanoparticles (ZnO NPs) are widely applied as cathode interfacial layers (CILs) for rigid and flexible organic solar cells. However, the inferior optoelectronic properties of ZnO NPs constrain the improvement in the photovoltaic performance and enhance the thickness sensitivity. Herein, upon application of this ZnO:Zr NP as a CIL for inverted device construction, the maximum power conversion efficiency (PCE_max) is increased to 17.7%, with an enhancement of 12.0% compared to that of the pristine ZnO-based devices (15.8%). A series of optoelectronic characterizations have revealed that the Zr doping methodology would enhance the charge generation and extraction process and suppress trap-assisted recombination, which is beneficial for the synergistic improvement of the thickness tolerance and shelf stability. Encouragingly, ZnO:Zr NPs can be easily fabricated through a doctor-blade coating technique with remarkable performance (16.6%). More critically, this approach can be applied to the development of high-performance flexible solar cells, with a superb PCE of 16.0%.

Benzobis(Thiazole)-Based Conjugated Polymer with Varying Alkylthio Side-Chain Positions for Efficient Fullerene-Free Organic Solar Cells

Alkylthio groups can be used to modulate energy levels and molecular packing of organic semiconductors, which makes it important in the design of materials for organic solar cell. However, its effect has not been sufficiently exploited as most of the studies report introducing an alkylthio group to the donor unit and seldom to the acceptor unit of donor-acceptor conjugated polymers. In this report, two alkylthio-substituted polymers, namely, PBB-TSA and PBB-TSD, with benzo[1,2-d:4,5-d']bis(thiazole) (BBT) as the acceptor unit and benzo[1,2-b:4,5-b']dithiophene (BDT) as the donor unit, were rationally designed, synthesized, and applied in organic photovoltaics. An alkylthio side chain was substituted on the BBT-accepting unit for PBB-TSA, while for PBB-TSD, the alkylthio side chain was substituted on the BDT donor unit. PBB-TSA and PBB-TSD show upshifted and downshifted energy levels, respectively, compared to the nonsulfur-substituted material. Both polymers exhibit dominate face-on orientation, while PBB-TSD exhibits higher crystallinity compared to PBB-TSA. With the contribution of lower energy level and beneficial film morphology, the device based on PBB-TSD/IT-4F has much higher power conversion efficiency (PCE) of 14.6%, whereas the PBB-TSA blend had a lower PCE of 10.7%. 1,8-Diiodooctane can effectively optimize the blend film morphology, and the effect on device performance has also been demonstrated in detail. This result indicates that introducing an alkylthio side chain into the donor or acceptor moieties would result in materials with different energy levels and thus would be utilized to match with various acceptors, achieving optimized performance in organic solar cells.

Boosting Highly Efficient Hydrocarbon Solvent-Processed All-Polymer-Based Organic Solar Cells by Modulating Thin-Film Morphology

Many highly efficient all-polymer-based organic solar cells (OSCs) have been achieved owing to material design and device engineering. However, most of them were achieved by using halogenated solvents to process the active layers, being not beneficial to its nature of green energy technology. In this work, we compared chloroform- and toluene-processed PM6:PY-IT-based all-polymer devices with the same blend solution recipe, same film formation speed, and same postcast treatment. The film cast from toluene exhibited weaker crystallinity. For device performance, toluene enabled a better power conversion efficiency (PCE) of 15.51%, outperforming that of chloroform (15.00%), and it is the highest value for non-halogenated solvent-cast all-polymer-based OSCs to date. Toluene's morphology tuning effect was characterized to increase and balance the charge transport and then suppress the exciton recombination and improve the charge extraction, considered to be the reason for efficiency enhancement. Besides, the toluene-cast active layer-based devices showed slightly better photostability than the chloroform-driven ones. This work provided a new direction for building low-toxicity solvent-treated all-polymer OSCs with cutting-edge performance.

Printable and Semitransparent Nonfullerene Organic Solar Modules over 30 cm2 Introducing an Energy-Level Controllable Hole Transport Layer

For the commercialization of organic solar cells (OSCs), the fabrication of large-area modules via a solution process is important. The fabrication of OSCs via a solution process using a nonfullerene acceptor (NFA)-based photoactive layer is limited by the energetic mismatch and carrier recombination, reducing built-in potential and effective carriers. Herein, for the fabrication of high-performance NFA-based large-area OSCs and modules via a solution process, hybrid hole transport layers (h-HTLs) incorporating WO₃ and MoO₃ are developed. The high bond energies and electronegativities of W and Mo atoms afford changes in the electronic properties of the h-HTLs, which can allow easy control of the energy levels. The h-HTLs show matching energy levels that are suitable for both deep and low-lying highest occupied molecular orbital energy level systems with a stoichiometrically small amount of oxygen vacancies (forming W⁶⁺ and Mo⁶⁺ from the W⁵⁺ and Mo⁵⁺), affording high conductivity and good film forming properties. With the NFA-based photoactive layer, a large-area module fabricated via the all-printing process with an active area over 30 cm² and a high power conversion efficiency (PCE) of 8.1% is obtained. Furthermore, with the h-HTL, the fabricated semitransparent module exhibits 7.2% of PCE and 22.3% of average visible transmittance with high transparency, indicating applicable various industrial potentials.

Significant Enhancement of Illumination Stability of Nonfullerene Organic Solar Cells via an Aqueous Polyethylenimine Modification

Device stability under illumination is the main obstacle of nonfullerene (NF) organic solar cells for moving toward practical application. ZnO, a generally used electron-transporting layer in inverted cells, is prone to induce the decomposition of NF acceptors under illumination with air mass 1.5 (AM1.5) spectrum, resulting in poor device stability. Herein, we report an aqueous polyethylenimine (a-PEI) modification on the ZnO surface could significantly enhance the stability of the NF organic solar cells. After 1000 h of AM1.5 illumination, the efficiency of the cell without a-PEI modification degrades to 43% of its initial value, while the cell with a-PEI modification could maintain 75% of its initial efficiency. The a-PEI modification reduces the number of surface defects with reduced adsorbed oxygen ZnO surface, faster work function recovery kinetics after UV irradiation, and suppressed electron spin resonance response. The reduction of surface defects is beneficial to the stability of NF acceptors on ZnO and also device performance.

Unique Degradation Signatures of Organic Solar Cells with Nonfullerene Electron Acceptors

We investigate the degradation phenomena of organic solar cells based on nonfullerene electron acceptors (NFA) using intensity-modulated photocurrent spectroscopy (IMPS). Devices composed of NIR absorbing blends of a polymer (PTB7) and NFA molecules (COi8DFIC) were operated in air for varying periods of time that display unusual degradation trends. Light aging (e.g., ∼3 days) results in a characteristic first quadrant (positive phase shifts) degradation feature in IMPS Nyquist (Bode) plots that grow in amplitude and frequency with increasing excitation intensity and then subsequently turns over and vanishes. By contrast, devices aged and operated in air for longer times (>5 days) display poor photovoltaic performance and have a dominant first quadrant IMPS component that grows nonlinearly with excitation intensity. We analyze these degradation trends using a simple model with descriptors underlying the first quadrant feature (i.e., trap lifetime and occupancy). The results indicate that the quasi first-order recombination rate constant, k_rec, is significantly slower in addition to lower trap densities in devices exhibiting light aging effects that are overcome by increasing carrier densities (viz. excitation intensity). By contrast, larger trap densities and distributions coupled with larger k_rec values are found to be responsible for the continuous growth of the first quadrant with light intensity. We believe that defect formation and charge recombination at device contact interfaces is chiefly responsible for performance degradation, which offers several directions for materials and device optimization strategies to minimize long-term detrimental factors.

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.

Robust metal ion-chelated polymer interfacial layer for ultraflexible non-fullerene organic solar cells

Achieving high power conversion efficiency and good mechanical robustness is still challenging for the ultraflexible organic solar cells. Interlayers simultaneously having good mechanical robustness and good chemical compatibility with the active layer are highly desirable. In this work, we present an interlayer of Zn²⁺-chelated polyethylenimine (denoted as PEI-Zn), which can endure a maximum bending strain over twice as high as that of ZnO and is chemically compatible with the recently emerging efficient nonfullerene active layers. On 1.3 μm polyethylene naphthalate substrates, ultraflexible nonfullerene solar cells with the PEI-Zn interlayer display a power conversion efficiency of 12.3% on PEDOT:PSS electrodes and 15.0% on AgNWs electrodes. Furthermore, the ultraflexible cells show nearly unchanged power conversion efficiency during 100 continuous compression-flat deformation cycles with a compression ratio of 45%. At the end, the ultraflexible cell is demonstrated to be attached onto the finger joint and displays reversible current output during the finger bending-spreading.

Simultaneously achieving high efficiency and mechanical robustness is challenging for ultraflexible organic solar cells. Here, Qin et al. present a robust interlayer of Zinc-chelated polyethylenimine (PEI-Zn) to facilitate the demonstration of efficient and mechanically robust ultraflexible solar cells.

A novel signal amplification strategy electrochemical immunosensor for ultra-sensitive determination of p53 protein

In this work, we fabricated a novel sandwich-type electrochemical immunosensor for quantitative and ultra-sensitive determination of tumor suppressor protein p53 by signal amplification strategy. Conductive polymers poly (3, 4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) has significantly effect on enhancing charge transfer and markedly increases the sensitivity of electrochemical immunosensing. Gold nanoparticles (AuNPs) as high conductivity nanocarriers were also used to capture monoclonal antibodies (Ab₁) due to their large specific surface areas. In addition, pH responsive zeolitic imidazolate framework (ZIF-8) was used to load the redox probe 2, 3-diaminophenazine (DAP) and the secondary antibodies (Ab₂) to form a sensitive-type ZIF-8-DAP-Ab₂ immunoprobe. After the sandwich-type immunoassay with the free p53 protein, with the release of probe DAP after the electrochemical signal amplificated by PEDOT:PSS and AuNPs, the ultra-sensitive and quantitative determination of p53 protein was realized with working range of 1-120 ng mL^-1 and low detection limit of 0.09 ng mL^-1. Besides, the fabricated electrochemical immunosensor exhibited good recovery, high sensitivity, reliability, and selectivity.