No abstract

Transparent optoelectronics can enable a new class of applications such as transparent displays, smart windows, and invisible sensors. Here, we demonstrate all-transparent NO 2 gas sensors based on aluminum-doped zinc oxide (AZO) freestanding hollow nanofibers. Freestanding AZO nanofibers are fabricated by sputtering AZO on template polyvinylpyrrolidone (PVP) nanofibers, which are electrospun on a glass frame with indium zinc oxide (IZO) transparent electrodes, followed by a heat treatment to remove the PVP template nanofibers. Not only the gas-sensing active material but also other components such as the substrate and electrodes are all transparent in the visible region. The optical transparency of the nanofibers is controlled by changing the AZO nanofibers density without compromising the sensitivity. The gassensing measurements of the transparent sensor depict n-type response behavior with full recovery even at low NO 2 concentrations (0.5 ppm). The high sensitivity of the transparent sensors is attributed to the higher surface area of the hollow nanofibers and the high impact frequency of trapped NO 2 gas inside the hollow compared to solid counterpart nanofibers. The unique combination of transparency and high sensitivity can potentially have applications in advanced sensor systems that can be attached to windows integrated with the Internet of Things.

Transparent solar cells (TSCs) are emerging devices that combine the advantages of visible transparency and light-to-electricity conversion. Currently, existing TSCs are based predominantly on organics, dyes, and perovskites; however, the rigidity and color-tinted transparent nature of those devices strongly limit the utility of the resulting TSCs for real-world applications. Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a freestanding form of n-silicon microwires (SiMWs). Flat-tip SiMWs with controllable spacing are fabricated via deep-reactive ion etching and embedded in a freestanding transparent polymer matrix. The light transmittance can be tuned from ~10 to 55% by adjusting the spacing between the microwires. For TSCs, a heterojunction is formed with a p-type polymer in the top portion of the n-type flat-tip SiMWs. Ohmic contact with an indium-doped ZnO film occurs at the bottom, and the side surface has an Al2O3 passivation layer. Furthermore, slanted-tip SiMWs are developed by a novel solvent-assisted wet etching method to manipulate light absorption. Finite-difference time-domain simulation revealed that the reflected light from slanted-tip SiMWs helps light-matter interactions in adjacent microwires. The TSC based on the slanted-tip SiMWs demonstrates 8% efficiency at a visible transparency of 10% with flexibility. This efficiency is the highest among Si-based TSCs and comparable with that of state-of-the-art neutral-color TSCs based on organic–inorganic hybrid perovskite and organics. Moreover, unlike others, the stretchable and transparent platform in this study is promising for future TSCs.

PEG-treated poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) exhibited an enhanced power factor with increased strain.

Room‐temperature (RT) gas sensitivity of morphology‐controlled free‐standing hollow aluminum‐doped zinc oxide (AZO) nanofibers for NO2 gas sensors is presented. The free‐standing hollow nanofibers are fabricated using a polyvinylpyrrolidone fiber template electrospun on a copper electrode frame followed by radio‐frequency sputtering of an AZO thin overlayer and heat treatment at 400 °C to burn off the polymer template. The thickness of the AZO layer is controlled by the deposition time. The gas sensor based on the hollow nanofibers demonstrates fully recoverable n‐type RT sensing of low concentrations of NO2 (0.5 ppm). A gas sensor fabricated with Al2O3‐filled AZO nanofibers exhibits no gas sensitivity below 75 °C. The gas sensitivity of a sensor is determined by the density of molecules above the minimum energy for adsorption, collision frequency of gas molecules with the surface, and available adsorption sites. Based on finite‐difference time‐domain simulations, the RT sensitivity of hollow nanofiber sensors is ascribed to the ten times higher collision frequency of NO2 molecules confined inside the fiber compared to the outer surface, as well as twice the surface area of hollow nanofibers compared to the filled ones. This approach might lead to the realization of RT sensitive gas sensors with 1D nanostructures.