A paper recently published in the journal ACS Applied Energy Materials demonstrated the use of MXenes in transparent photoelectric applications, specifically in transparent photovoltaic devices (TPVDs).
Study: MXene-Integrated Metal Oxide Transparent Photovoltaics and Self-Powered Photodetectors. Image Credit: Baloncici/Shutterstock.com
Background
TPVDs can play a significant role in emerging optoelectronics, such as photovoltaic devices (PVs). In contrast to conventional opaque PVs, TPVDs can be integrated into the windows of buildings, vehicles, and displays as invisible power supplies as they can convert incident light into electric power without obstructing human vision.
Diverse materials, including perovskite semiconductors, dye, and inorganic materials, were used for TPVDs. Recently, wide-bandgap inorganic materials have gained significant prominence for application in TPVDs as they are chemically stable, nontoxic, and low cost. Several studies have demonstrated the effectiveness of using these materials as optimizing photoactive materials for TPVDs.
The charge collecting layer also plays a critical role in the efficient functioning of TPVDs. Indium tin oxide (ITO) is typically utilized as a transparent conducting layer for TPVDs. However, ITO is expensive, environmentally hazardous, and not compatible with next-generation human interface electronics and optoelectronics.
Two-dimensional (2D) materials, such as graphene, with unique mechanical, electrical, and optical properties, can be suitable as charge collecting layers for TPVDs. However, several complicated transfer processes are required to synthesize these 2D materials in large quantities, which hinders their use in real-world PV applications.
MXenes, an emerging group of 2D nanomaterials, have recently attracted considerable attention in optoelectronics research owing to their hydrophilic nature, which provides an opportunity to achieve direct and large-scale integration of 2D materials into PV applications.
The hydrophilicity of MXenes due to the existence of surface functional groups makes them suitable for solution processing by different methods, including ink jet printing, spin coating, and drop-casting. Additionally, MXenes are optically transparent and composed of nonhazardous and earth-abundant elements, which enable the fabrication of low-cost and environmentally safe PV devices, a crucial requirement for solar cells.
Organic-dispersible titanium carbide (Ti3C2Tx) MXene ink was coated successfully on stretchable substrates using the spray-coating technique after alkyl phosphonic acid functionalization. However, the conductivity of bare MXene decreased significantly due to the surface incorporation of organic functional groups.
Recently, trifluoroacetic acid (TFA)-treated MXene demonstrated a high conductivity of 8900 S cm−1, an oxidation resistance during processing and storage, good dispersion stability in ethanol, modulation of surface functional groups, and an etching byproduct. Thus, researchers in this study used TFA-treated ethanolic dispersion of Ti3C2Tx MXene as a transparent conducting layer of TPVDs.
The Study
In this study, researchers fabricated a Ti3C2Tx MXene-based inorganic TPVD and characterized the fabricated samples, specifically the PV effect of the TVPD. Bare Ti3C2Tx MXene was synthesized using modified minimally intensive layer delamination. The TFA-treated Ti3C2Tx MXene was prepared by treating the bare MXene solution with TFA in deionized (DI) water at 0 °C.
Fluorine doped tin oxide (FTO)/glass substrates were cleaned with acetone, methanol, and deionized water under 10 min ultrasonication in a sequence and then dehydrated with flowing nitrogen gas. Subsequently, the titanium dioxide (TiO2) layer was initially deposited on the FTO substrates, followed by the deposition of the nickel oxide (NiO) layer on the deposited TiO2 layer using the reactive sputtering technique to obtain the TiO2/NiO heterostructure.
The TFA-treated Ti3C2Tx MXene solution was then deposited on the transparent NiO/TiO2/FTO device as a transparent electrode using the spin coating method. The MXene films were dried at 100°C. The optical transparency and conductivity of the MXene layer were adjusted by controlling the number of MXene coatings.
Field emission scanning electron microscopy (FE-SEM) was used to characterize the MXene/NiO/TiO2/FTO structure, while the transmittance spectra of the MXene/NiO/TiO2/FTO device and MXene film were obtained using an ultraviolet (UV)−visible diffused reflectance spectrophotometer.
A potentiostat/galvanostat was utilized to measure the current−voltage (I−V) characteristics of the fabricated device. The transient photocurrent and photovoltage were obtained using an oscilloscope.
Observations
A transparent MXene/NiO/TiO2/FTO device/ MXene-integrated TPVD was fabricated successfully using TFA-treated Ti3C2Tx MXene. MXene film was uniformly embedded in the inorganic NiO/TiO2 heterostructure, creating a seamless platform for transparent photodetectors and PVs.
The transparent metal oxide NiO/TiO2 heterojunction produced power from UV light, which showed the energy-efficient operation of the device. The reduction in the sheet resistance of MXene and the improvement in the contact between MXene and NiO/TiO2 heterojunction led to the generation of 30 μW cm−2 electric power from UV light and selective transmittance of visible light for high transparency.
The highest transparency of the MXene films was 55%, while the overall transmittance of the MXene/NiO/TiO2/FTO device/ MXene-integrated TPVD was 39.73%. The photovoltage of the fabricated TPVD was strongly governed by the MXene film, and the maximum photovoltage was 0.62 V. The MXene electrode with a nanosheet structure completely covered the parallel NiO/TiO2/FTO surface, leading to an efficient collection of photogenerated carriers.
The MXene-transparent device spacing at color-neutral coordinates in the color maps indicated the invisibility of the device and the feasibility of using these transparent devices for the windows of vehicles and buildings as onsite power supply systems.
The PV effect induced a high photovoltage of 0.45 V that enabled the device to function in self-powered mode. Moreover, fast photoresponses were obtained in the self-powered mode, with a fall time of 130 μs and a rise time of 80 μs, allowing the self-reliant photoelectric applications of MXene-embedded transparent photodetectors. The MXene-embedded transparent photodetector also demonstrated a high detectivity of 1.6× 1010 Jones.
Taken together, the findings of this study demonstrated the feasibility of using MXene on a large scale as a seamless platform for transparent electronics of photodetectors and PVs.
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Source
Patel, M., Murali, G., Kim, J. et al. MXene-Integrated Metal Oxide Transparent Photovoltaics and Self-Powered Photodetectors. ACS Applied Energy Materials 2022. https://doi.org/10.1021/acsaem.2c00458
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