Optical Transmittance Enhancement of Flexible Copper Film Electrodes with a Wetting Layer for Organic Solar Cells

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.

Smooth, Chemically Altered Nucleating Platform for Abrupt Performance Enhancement of Ultrathin Cu-Layer-Based Transparent Electrodes

Rapid advances in flexible optoelectronic devices necessitate the concomitant development of high-performance, cost-efficient, and flexible transparent conductive electrodes (TCEs). This Letter reports an abrupt enhancement in the optoelectronic characteristics of ultrathin Cu-layer-based TCEs via Ar⁺-mediated modulation of the chemical and physical states of a ZnO support surface. This approach strongly regulates the growth mode for the subsequently deposited Cu layer, in addition to marked alteration to the ZnO/Cu interface states, resulting in exceptional TCE performance in the form of ZnO/Cu/ZnO TCEs. The resultant Haacke figure of merit (T¹⁰/R_s) of 0.063 Ω^-1, 53% greater than that of the unaltered, otherwise identical structure, corresponds to a record-high value for Cu-layer-based TCEs. Moreover, the enhanced TCE performance in this approach is shown to be highly sustainable under severe simultaneous loadings of electrical, thermal, and mechanical stresses.

Application of Solution Method to Prepare High Performance Multicomponent Oxide Thin Films

Capacitors play an increasingly important role in hybrid integrated circuits, while the MIM capacitors with high capacitance density and small thickness can meet the needs of high integration. Generally speaking, the films prepared with a single metal oxide dielectric often achieve a breakthrough in one aspect of performance, but dielectric layers are required to be improved to get better performance in leakage current, capacitance density, and transmittance simultaneously in modern electronic devices. Therefore, we optimized the performance of the dielectric layers by using multiple metal oxides. We combined zirconia, yttria, magnesium oxide, alumina, and hafnium oxide with the solution method to find the best combination of these five metal oxides. The physical properties of the multi-component films were measured by atomic force microscopy (AFM), ultraviolet-visible spectrophotometer, and other instruments. The results show that the films prepared by multi-component metal oxides have good transmittance and low roughness. The thicknesses of all films in our experiment are less than 100 nm. Then, metal–insulator–metal (MIM) devices were fabricated. In addition, we characterized the electrical properties of MIM devices. We find that multi-component oxide films can achieve good performances in several aspects. The aluminum-magnesium-yttrium-zirconium-oxide (AMYZO_x) group of 0.6 M has the lowest leakage current density, which is 5.03 × 10⁻⁸ A/cm² @ 1.0 MV/cm. The hafnium-magnesium-yttrium-zirconium-oxide (HMYZO_x) group of 0.8 M has a maximum capacitance density of 208 nF/cm². The films with a small thickness and a high capacitance density are very conducive to high integration. Therefore, we believe that multi-component films have potential in the process of dielectric layers and great application prospects in highly integrated electronic devices.

Additive-Induced Vertical Component Distribution Enables High-Performance Sequentially Cast Organic Solar Cells

Modulation of the active layer morphology to form a vertical component distribution structure is an effective way of improving the efficiency of organic solar cells (OSCs). In this paper, a layer-by-layer (LbL) spin-coating method was adopted combined with an additive strategy to achieve the purpose of precisely adjusting the morphology, and finally, high-performance OSCs based on a D18-Cl/Y6 system were achieved. After adding n-octane in D18-Cl, D18-Cl+/Y6 devices realized a PCE of 17.70%, while with the incorporation of 1-fluoronaphthalene (FN) in Y6, D18-Cl/Y6+ devices obtained a power conversion efficiency (PCE) of 17.39%, both higher than the control devices (16.66%). The former resulted in a more orderly arrangement of D18-Cl, forming a suitable phase separation morphology, and the latter improved the crystallization of Y6, which facilitated carrier transport. Furthermore, the dual-additive-treated D18-Cl+/Y6+ bilayer devices with n-octane doping in the donor and FN in the acceptor had a more desirable vertical morphology, exhibiting an excellent PCE of 18.16% with an improved J_SC of 27.17 mA cm^-2 and FF of 76.88%, one of the highest efficiencies for LbL OSCs. The results demonstrated that combining the LbL spin-coating method with the additive strategy is a valid way to achieve hierarchical morphology control and enhance device performance, which is of great significance for the fabrication and development of OSCs.

Strategy for Fabricating Ultrathin Au Film Electrodes with Ultralow Optoelectrical Losses and High Stability

A vital objective in the wetting of Au deposited on chemically heterogeneous oxides is to synthesize a completely continuous, highly crystalline, ultrathin-layered geometry with minimized electrical and optical losses. However, no effective solution has been proposed for synthesizing an ideal Au-layered structure. This study presents evidence for the effectiveness of atomic oxygen-mediated growth of such an ideal Au layer by improving Au wetting on ZnO substrates with a substantial reduction in free energy. The unexpected outcome of the atomic oxygen-mediated Au growth can be attributed to the unconventional segregation and incorporation of atomic oxygen along the outermost boundaries of Au nanostructures evolving in the clustering and layering stages. Moreover, the experimental and numerical investigations revealed the spontaneous migration of atomic oxygen from an interstitial oxygen surplus ZnO bulk to the Au-ZnO interface, as well as the segregation (float-out) of the atomic oxygen toward the top Au surfaces. Thus, the implementation of a 4-nm-thick, two-dimensional, quasi-single-crystalline Au layer with a nearly complete crystalline realignment at a mild temperature (570 K) enabled exceptional optoelectrical performance with record-low resistivity (<7.5 × 10^-8 Ω·m) and minimal optical loss (∼3.5%) at a wavelength of 700 nm.

Ultrathin Metals on a Transparent Seed and Application to Infrared Reflectors

Ultrathin metal films (UTMFs) are widely used in optoelectronic applications, from transparent conductors to photovoltaic cells, low emissivity windows, and plasmonic metasurfaces. During the initial deposition phase, many metals tend to form islands on the receiving substrate rather than a physically connected (percolated) network, which eventually evolves into continuous films as the thickness increases. For example, in the case of Ag and Au on dielectric surfaces, percolation begins when the thickness of the metal film is at least about 5 nm. It is known that the type of growth can be changed when a proper seed layer is used. Here, we show that a CuO layer directly deposited on a substrate can dramatically influence surface wetting and promote early percolation of polycrystalline Ag and Au UTMFs. We demonstrate that the proposed seed is effective even when its thickness is sub-nanometric, in this way maintaining the full transparency of the receiving substrate. The Ag and Au films seeded with CuO showed a percolation thickness close to 1 nm and were morphologically and optically characterized from an ultraviolet (λ = 300 nm) to a midinfrared (λ = 15 μm) wavelength. Infrared reflectors, a mirror and a resonant plasmonic structure, were also demonstrated and uniquely tuned by electrical gating, this being possible owing to the small thickness of the constituting Au UTMF.

Over 14% Efficiency Folding-Flexible ITO-free Organic Solar Cells Enabled by Eco-friendly Acid-Processed Electrodes

Environment-friendly manufacturing and mechanical robustness are imperative for commercialization of flexible OSCs as green-energy source, especially in portable and wearable self-powered flexible electronics. Although, the commonly adopted PEDOT:PSS electrodes that are treated with severely corrosive and harmful acid lack foldability. Herein, efficient folding-flexible OSCs with highly conductive and foldable PEDOT:PSS electrodes processed with eco-friendly cost-effective acid and polyhydroxy compound are demonstrated. The acid treatment endows PEDOT:PSS electrodes with high conductivity. Meanwhile, polyhydroxy compound doping contributes to excellent bending flexibility and foldability due to the better film adhesion between PEDOT:PSS and PET substrate. Accordingly, folding-flexible OSCs with high efficiency of 14.17% were achieved. After 1,000 bending or folding cycles, the device retained over 90% or 80% of its initial efficiency, respectively. These results represent one of the best performances for ITO-free flexible OSC reported so far and demonstrate a novel approach toward commercialized efficient and foldable green-processed OSCs.

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Highly conductive PEDOT:PSS electrodes based on eco-friendly acid were exploited

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14.17% folding-flexible organic solar cells were realized

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The bending performance was significantly improved by interface bonding engineering

Pinhole-free TiO2/Ag(O)/ZnO configuration for flexible perovskite solar cells with ultralow optoelectrical loss

Perovskite solar cells (PSCs) fabricated on transparent polymer substrates are considered a promising candidate as flexible solar cells that can emulate the advantages of organic solar cells, which exhibit considerable freedom in their device design thanks to their light weight and mechanically flexibility while achieving high photocurrent conversion efficiency, comparable to that of their conventional counterparts fabricated on rigid glasses. However, the full realization of highly efficient, flexible PSCs is largely prevented by technical difficulties in simultaneously attaining a transparent electrode with efficient charge transport to meet the specifications of PSCs. In this study, an effective strategy for resolving this technical issue has been devised by proposing a simple but highly effective technique to fabricate an efficient, multilayer TiO₂/Ag_(O)/ZnO (TAOZ) configuration. This configuration displays low losses in optical transmittance and electrical conductivity owing to its completely continuous, ultrathin metallic Ag_(O) transparent electrode, and any notable current leakage is suppressed by its pinhole-free TiO₂ electron transport layer. These features are a direct consequence of the rapid evolution of Ag_(O) and TiO₂ into ultrathin, completely continuous, pinhole-free layers owing to the dramatically improved wetting of metallic Ag_(O) with a minimal dose of oxygen (ca. 3 at%) during sputtering. The TAOZ configuration exhibits an average transmittance of 88.5% in the spectral range of 400–800 nm and a sheet resistance of 8.4 Ω sq⁻¹ while demonstrating superior mechanical flexibility to that of the conventional TiO₂ on ITO configuration. The photocurrent conversion efficiency of flexible PSCs is significantly improved by up to 11.2% thanks to an optimum combination of optoelectrical performance and pinhole-free morphologies in the TAOZ configuration.

A TiO₂/Ag_(O)/ZnO configuration is developed for flexible perovskite solar cells to provide a pinhole-free electron transport layer and a transparent electrode.

Nitrogen-Mediated Growth of Silver Nanocrystals to Form UltraThin, High-Purity Silver-Film Electrodes with Broad band Transparency for Solar Cells

Controlling the shape and crystallography of nanocrystals during the early growth stages of a noble metal layer is important because of its correlation with the final layer morphology and optoelectrical features, but this task is unattainable in vapor deposition processes dominated by artificially uncontrollable thermodynamic free energies. We report on experimental evidence for the controllable evolution of Ag nanocrystals as induced by the addition of nitrogen, presumed to be nonresidual in the Ag lattice given its strong float-out behavior. This atypical formation of energetically stable Ag nanocrystals with significantly improved wetting abilities on a chemically heterogeneous substrate promotes the development of an atomically flat, ultrathin, high-purity Ag layer with a thickness of only 5 nm. This facilitates the fabrication of Ag thin-film electrodes exhibiting highly enhanced optical transparency over a broad spectral range in the visible and near-infrared spectral range. An Ag thin-film electrode with a ZnO/Ag/ZnO configuration exhibits an average transmittance of about 95% in the spectral range of 400-800 nm with a maximum transmittance of over 98% at 580 nm, which is comparable with the best transparency values so far reported for transparent electrodes. This degree of optical transparency provides an excellent chance to improve the photon absorption of photovoltaic devices employing an Ag thin film as their window electrode. This is clearly confirmed by the superior performance of a flexible organic solar cell with a power conversion efficiency of 8.0%, which is far superior to that of the same solar cell using a conventional amorphous indium tin oxide electrode (6.4%).

Ultrathin Silver Film Electrodes with Ultralow Optical and Electrical Losses for Flexible Organic Photovoltaics

Improving the wetting ability of Ag on chemically heterogeneous oxides is technically important to fabricate ultrathin, continuous films that would facilitate the minimization of optical and electrical losses to develop qualified transparent Ag film electrodes in the state-of-the-art optoelectronic devices. This goal has yet to be attained, however, because conventional techniques to improve wetting of Ag based on heterogeneous metallic wetting layers are restricted by serious optical losses from wetting layers. Herein, we report on a simple and effective technique based on the partial oxidation of Ag nanoclusters in the early stages of Ag growth. This promotes the rapid evolution of the subsequently deposited pure Ag into a completely continuous layer on the ZnO substrate, as verified by experimental and numerical evidence. The improvement in the Ag wetting ability allows the development of a highly transparent, ultrathin (6 nm) Ag continuous film, exhibiting an average optical transmittance of 94% in the spectral range 400-800 nm and a sheet resistance of 12.5 Ω sq^-1, which would be well-suited for application to an efficient front window electrode for flexible solar cell devices fabricated on polymer substrates.