Interfacial Engineering Importance of Bilayered ZnO Cathode Buffer on the Photovoltaic Performance of Inverted Organic Solar Cells

The role of cathode buffer layer (CBL) is crucial in determining the power conversion efficiency (PCE) of inverted organic solar cells (IOSCs). The hallmarks of a promising CBL include high transparency, ideal energy levels, and tendency to offer good interfacial contact with the organic bulk-heterojunction (BHJ) layers. Zinc oxide (ZnO), with its ability to form numerous morphologies in juxtaposition to its excellent electron affinity, solution processability, and good transparency is an ideal CBL material for IOSCs. Technically, when CBL is sandwiched between the BHJ active layer and the indium–tin-oxide (ITO) cathode, it performs two functions, namely, electron collection from the photoactive layer that is effectively carried out by morphologies like nanoparticles or nanoridges obtained by ZnO sol–gel (ZnO SG) method through an accumulation of individual nanoparticles and, second, transport of collected electrons toward the cathode, which is more effectively manifested by one-dimensional (1D) nanostructures like ZnO nanorods (ZnO NRs). This work presents the use of bilayered ZnO CBL in IOSCs of poly­(3-hexylthiophene) (P3HT)/[6, 6]-phenyl-C₆₀-butyric acid methyl ester (PCBM) to overcome the limitations offered by a conventionally used single layer CBL. We found that the PCE of IOSCs with an appropriate bilayer CBL comprising of ZnO NRs/ZnO SG is ∼18.21% higher than those containing ZnO SG/ZnO NRs. We believe that, in bilayer ZnO NRs/ZnO SG, ZnO SG collects electrons effectively from photoactive layer while ZnO NRs transport them further to ITO resulting significant increase in the photocurrent to achieve highest PCE of 3.70%. The enhancement in performance was obtained through improved interfacial engineering, enhanced electrical properties, and reduced surface/bulk defects in bilayer ZnO NRs/ZnO SG. This study demonstrates that the novel bilayer ZnO CBL approach of electron collection/transport would overcome crucial interfacial recombination issues and contribute in enhancing PCE of IOSCs

Polycrystalline and Mesoporous 3‑D Bi₂O₃ Nanostructured Negatrodes for High-Energy and Power-Asymmetric Supercapacitors: Superfast Room-Temperature Direct Wet Chemical Growth

Superfast (≤10 min) room-temperature (300 K) chemical synthesis of three-dimensional (3-D) polycrystalline and mesoporous bismuth­(III) oxide (Bi₂O₃) nanostructured negatrode (as an abbreviation of negative electrode) materials, viz., coconut shell, marigold, honey nest cross section and rose with different surface areas, charge transfer resistances, and electrochemical performances essential for energy storage, harvesting, and even catalysis devices, are directly grown onto Ni foam without and with poly­(ethylene glycol), ethylene glycol, and ammonium fluoride surfactants, respectively. Smaller diffusion lengths, caused by the involvement of irregular crevices, allow electrolyte ions to infiltrate deeply, increasing the utility of inner active sites for the following electrochemical performance. A marigold 3-D Bi₂O₃ electrode of 58 m²·g^–1 surface area has demonstrated a specific capacitance of 447 F·g^–1 at 2 A·g^–1 and chemical stability of 85% even after 5000 redox cycles at 10 A·g^–1 in a 6 M KOH electrolyte solution, which were higher than those of other morphology negatrode materials. An asymmetric supercapacitor (AS) device assembled with marigold Bi₂O₃ negatrode and manganese­(II) carbonate quantum dots/nickel hydrogen–manganese­(II)–carbonate (MnCO₃QDs/NiH–Mn–CO₃) positrode corroborates as high as 51 Wh·kg^–1 energy at 1500 W·kg^–1 power and nearly 81% cycling stability even after 5000 cycles. The obtained results were comparable or superior to the values reported previously for other Bi₂O₃ morphologies. This AS assembly glowed a red-light-emitting diode for 20 min, demonstrating the scientific and industrial credentials of the developed superfast Bi₂O₃ nanostructured negatrodes in assembling various energy storage devices

Annealing environment effects on the electrochemical behavior of supercapacitors using Ni foam current collectors

Nickel (Ni) foam-based symmetric/asymmetric electrochemical supercapacitors benefit from a randomly 3D structured porous geometry that functions as an active material support and as a current collector. The surface composition stability and consistency of the current collector is critical for maintaining and consistency supercapacitor response, especially for various mass loading and mass coverage. Here we detail some annealing environment conditions that change the surface morphology, chemistry and electrochemical properties of Ni foam by NiO formation. Air-annealing at 400 and 800 °C and annealing also in N2 and Ar at 800 °C result in the in situ and ex situ formation of NiO on the Ni foam (NiO@Ni). Oxidation of Ni to NiO by several mechanisms in air and inert atmospheres to form a NiO coating is subsequently examined in supercapacitors, where the electrochemical conversion through Ni(OH)2 and NiOOH phases influence the charge storage process. In parallel, the grain boundary density reduction by annealing improves the electronic conductivity of the foam current collector. The majority of stored charge occurs at the oxidized Ni-electrolyte interface. The changes to the Ni metal surface that can be caused by chemical environments, heating and high temperatures that typically occur when other active materials are grown on Ni directly, should be considered in the overall response of the electrode, and this may be general for metallic current collectors and foams that can oxidize at elevated temperatures and become electrochemically active

Low-Temperature Solution-Processed Thiophene-Sulfur-Doped Planar ZnO Nanorods as Electron-Transporting Layers for Enhanced Performance of Organic Solar Cells

1-D ZnO represents a fascinating class of nanostructures that are significant to optoelectronics. In this work, we investigated the use of an eco-friendly, metal free in situ doping through a pure thiophene-sulfur (S) on low temperature processed (<95 °C) and annealed (<170 °C), planar 1-D ZnO nanorods (ZnRs) spin-coated as a hole-blocking and electron transporting layer (ETL) for inverted organic solar cells (<i>i</i>OSCs). The TEM, HRTEM, XPS, FT-IR, EDS and Raman studies clearly reveal that the thiophene-S (Thi-S) atom is incorporated on planar ZnRs. The investigations in electrical properties suggest the enhancement in conductivity after Thi-S doping on 1-D ZnRs. The <i>i</i>OSCs of poly­(3-hexylthiophene-2,5-diyl) and phenyl-C₆₁-butyric acid methyl ester (P3HT: PC₆₀BM) photoactive layer containing thiophene-S doped planar ZnRs (Thi-S-PZnRs) as ETL exhibits power conversion efficiency (PCE) of 3.68% under simulated AM 1.5 G, 100 mW cm^–2 illumination. The ∼47% enhancement in PCE compared with pristine planar ZnRs (PCE = 2.38%) ETL is attributed to a combination of desirable energy level alignment, morphological modification, increased conductivity and doping effect. The universality of Thi-S-PZnRs ETL is demonstrated by the highest PCE of 8.15% in contrast to 6.50% exhibited by the <i>i</i>OSCs of ZnRs ETL for the photoactive layer comprising of poly­[4,8-bis­(5-(2-ethylhexyl)­thiophene-2-yl)­benzo­[1,2-b;4,5-b]­dithiophene-2,6-diyl-<i>alt</i>-(4-(2-ethylhexyl)-3-fluorothieno­[3,4-<i>b</i>]­thiophene-)-2-carboxylate-2–6-diyl)]: phenyl-C71-butyric acid methyl ester (PTB7-Th: PCB₇₁M). This enhancement in PCE is observed to be driven mainly through improved photovoltaic parameters like fill factor (ff) as well as photocurrent density (<i>J</i>_sc), which are assigned to increased conductivity, exciton dissociation, and effective charge extraction, while; better ohmic contact, reduced charge recombination, and low leakage current density resulted in increased <i>V</i>_oc

Large, Linear, and Tunable Positive Magnetoresistance of Mechanically Stable Graphene Foam–Toward High-Performance Magnetic Field Sensors

Here, we present the first observation of magneto-transport properties of graphene foam (GF) composed of a few layers in a wide temperature range of 2–300 K. Large room-temperature linear positive magnetoresistance (PMR ≈ 171% at <i>B</i> ≈ 9 T) has been detected. The largest PMR (∼213%) has been achieved at 2 K under a magnetic field of 9 T, which can be tuned by the addition of poly­(methyl methacrylate) to the porous structure of the foam. This remarkable magnetoresistance may be the result of quadratic magnetoresistance. The excellent magneto-transport properties of GF open a way toward three-dimensional graphene-based magnetoelectronic devices

Improved Photoelectrochemical Cell Performance of Tin Oxide with Functionalized Multiwalled Carbon Nanotubes–Cadmium Selenide Sensitizer

Here we report functionalized multiwalled carbon nanotubes (<i>f</i>-MWCNTs)–CdSe nanocrystals (NCs) as photosensitizer in photoelectrochemical cells, where <i>f</i>-MWCNTs were uniformly coated with CdSe NCs onto SnO₂ upright standing nanosheets by using a simple electrodeposition method. The resultant blended photoanodes demonstrate extraordinary electrochemical properties including higher Stern–Volmer constant, higher absorbance, and positive quenching, etc., caused by more accessibility of CdSe NCs compared with pristine SnO₂–CdSe photoanode. Atomic and weight percent changes of carbon with <i>f</i>-MWCNTs blending concentrations were confirmed from the energy dispersive X-ray analysis. The morphology images show a uniform coverage of CdSe NCs over <i>f</i>-MWCNTs forming a core–shell type structure as a blend. Compared to pristine CdSe, photoanode with <i>f</i>-MWCNTs demonstrated a 257% increase in overall power conversion efficiency. Obtained results were corroborated by the electrochemical impedance analysis. Higher scattering, more accessibility, and hierarchical structure of SnO₂-<i>f</i>-MWCNTs-blend–CdSe NCs photoanode is responsible for higher (a) electron mobility (6.89 × 10^–4 to 10.89 × 10^–4 cm² V^–1 S^1–), (b) diffusion length (27 × 10^–6), (c) average electron lifetime (32.2 ms), and transit time (1.15 ms)