A Review on Interface Engineering of MXenes for Perovskite Solar Cells.

With an excellent power conversion efficiency of 25.7%, closer to the Shockley-Queisser limit, perovskite solar cells (PSCs) have become a strong candidate for a next-generation energy harvester. However, the lack of stability and reliability in PSCs remained challenging for commercialization. Strategies, such as interfacial and structural engineering, have a more critical influence on enhanced performance. MXenes, two-dimensional materials, have emerged as promising materials in solar cell applications due to their metallic electrical conductivity, high carrier mobility, excellent optical transparency, wide tunable work function, and superior mechanical properties. Owing to different choices of transition elements and surface-terminating functional groups, MXenes possess the feature of tuning the work function, which is an essential metric for band energy alignment between the absorber layer and the charge transport layers for charge carrier extraction and collection in PSCs. Furthermore, adopting MXenes to their respective components helps reduce the interfacial recombination resistance and provides smooth charge transfer paths, leading to enhanced conductivity and operational stability of PSCs. This review paper aims to provide an overview of the applications of MXenes as components, classified according to their roles as additives (into the perovskite absorber layer, charge transport layers, and electrodes) and themselves alone or as interfacial layers, and their significant importance in PSCs in terms of device performance and stability. Lastly, we discuss the present research status and future directions toward its use in PSCs.

Double GaAs/Si Heterojunction Layers in Si Solar Cells Fabricated by Electron Beam Evaporation.

We investigated silicon solar cells with double gallium arsenide heterojunctions. Both p-type and n-type GaAs were deposited on the back and front side of the Si pn junction using an electron beam evaporator under a high vacuum condition to fabricate ppnn cells, respectively. The ppnn cell with a micro-textured Si surface showed enhanced quantum efficiency by about 10% in the shorter wavelength region and by about 1% in the longer wavelength region compared to ppnn cell with nanotextured Si surface. Moreover, the average minority carrier lifetime was increased by 1 µs and conversion efficiency was increased by 0.74% for the micro-ppnn cell despite its high series resistance. Morphological analysis showed that GaAs thin-film on Si surface was As-rich in the as-deposition case. It became distorted at higher annealing temperatures, leading to formation of cracks. Moreover, annealing of the deposited GaAs on a nano-textured Si surface at 100 °C for 30 min resulted in formation of GaAs nanodots in an amorphous GaAs matrix.

Low Energy Effect of Electron Beam Irradiation on Si Solar Cells.

We investigated the change in the cell efficiency resulting from electron beam accelerated irradiation to understand the possibility of using silicon solar cells with nano-surfaces for satellites. The Si solar cells were made of 6″ micro-textured p-type Si wafers. The Si solar cells were fabricated using a standard cell fabrication process, including POCl3 diffusion, anti-reflection coating layer deposition, and metal contact and co-firing heat treatment. After manufacturing the cell, we irradiated the cell with a low-energy electron beam to determine its effect. We analyzed the changes in the cell resulting from the irradiation with the electron beam by evaluating the photo-reflectance, transmission electron microscopy, electroluminescence and current-voltage curve.

Nanoporous Anodic Edge Passivation of Si Solar Cells.

We investigated the anodization effect on edge passivation of Si solar cells. The Si anodization allowed SiO2 formation on the edges of the cell for electrical passivation. The edge passivated cell showed enhanced conversion efficiency with reduced carrier recombination which was observed from photoluminescence and electroluminescence images. The luminescences were reduced at the edges indicating prevention of edge current leakage. However, when the rear Al paste layer of a sample was contacted to the solution during the anodization process, the conversion efficiency of the cell was reduced. We characterized oxide thin films by performing the anodization process for front Al thin film layer deposited by evaporation and rear Al paste layer. The front anodic aluminum oxide covering the Si emitter layer showed the excellent phototransmission with small photoreflectance lower than 5% and the anodization of Al paste showed the formation of a thin SiO2 film as well as nanoporous Al2O3 layer originating from the microspherical Al paste. The rear Al paste anodization allowed the Al microspheres to be filled with the nanopores in the inner empty space.