MXenes for Solar Cells

Abstract

Application of two-dimensional MXene materials in photovoltaics has attracted increasing attention since the first report in 2018 due to their metallic electrical conductivity, high carrier mobility, excellent transparency, tunable work function and superior mechanical property. In this review, all developments and applications of the Ti₃C₂T_x MXene (here, it is noteworthy that there are still no reports on other MXenes' application in photovoltaics by far) as additive, electrode and hole/electron transport layer in solar cells are detailedly summarized, and meanwhile, the problems existing in the related studies are also discussed. In view of these problems, some suggestions are given for pushing exploration of the MXenes' application in solar cells. It is believed that this review can provide a comprehensive and deep understanding into the research status and, moreover, helps widen a new situation for the study of MXenes in photovoltaics.

Keywords: Additives; Electrodes; Hole/electron transport layers; Solar cells; Ti3C2Tx MXene.

Fig. 1

Roles of the Ti₃C₂T_x MXene played in application of varying solar cells. The areas correspond to the publication numbers for each application

Fig. 2

a Nucleation and growth routes of the MAPbI₃-based perovskite films with and without adding the Ti₃C₂T_x MXene. b Nyquist plots of the PV devices with and without 0.03 wt% Ti₃C₂T_x addition measured in the dark with a bias of 0.7 V. Copyright © 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. c Schematic illustration of the preparation process of surface-decorated MAPbBr₃ nanocrystals by few-layer Ti₃C₂T_x MXene nanosheets, i.e., MAPbBr₃/Ti₃C₂T_x heterostructures. d Energy-level alignment and electron transfer between the MAPbBr₃ crystals and the coated Ti₃C₂T_x nanosheets. Copyright © 2020 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. e Device architecture and cross-sectional scanning electron microscopy (SEM) image, and f energy-level alignment of the perovskite solar cell with the embedded ultrathin Ti₃C₂T_x quantum dots in the perovskite layer and the ETL/TiO₂ interface and Cu_1.8S in the Spiro-OMeTAD HTL. Copyright © 2020 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 3

a Device architecture, b cross-sectional SEM image and c schematic energy-level diagram of each component for the PV device of ITO/Ti₃C₂T_x MXene-added SnO₂ ETL/MAPbI₃/Spiro-OMeTAD/Ag. Copyright © 2019 The Royal Society of Chemistry. d Device architecture, e cross-sectional SEM image and f schematic energy-level diagram of each component for the PSCs using MDCN as the ETL

Fig. 4

a Schematic illustration of morphological and structural modification in PEDOT:PSS with incorporation of the Ti₃C₂T_x MXene nanosheets. b Electrical conductivity of PEDOT:PSS with varying Ti₃C₂T_x additions on bare glass. c Device configuration and d energy-level diagram of each component for the OSC using Ti₃C₂T_x-modified PEDOT:PSS as the HTL. e Stability test for the devices with varying Ti₃C₂T_x additions based on the PBDB-T:ITIC photoactive layer measured in a N₂ glove box. Copyright © The Royal Society of Chemistry. f Device configuration and g energy-level diagram of each component for the OSC using ZnO/Ti₃C₂T_x as the ETL. h Stability test of the devices based on the PBDB-T:ITIC photoactive with varying Ti₃C₂T_x additions without encapsulation in air. Copyright © 2020 Elsevier B.V

Fig. 5

a Fabrication process of the MAPbI₃-based PSCs with a Ti₃C₂T_x back electrode prepared using a hot-pressing method. b Cross-sectional SEM image and c energy-level alignment of each component for the MAPbI₃-based PSCs. Copyright © 2019 The Royal Society of Chemistry. d Device configuration of the CsPbBr₃-based solar cells using the mixed carbon electrode and cross-sectional SEM images of the mixed carbon electrode. e Illuminated J–V curves of the solar cells with different types of electrodes. Copyright © 2020 The Royal Society of Chemistry

Fig. 6

a Fabrication process of the flexible transparent electrodes based on Ti₃C₂T_x MXene nanosheets and Ag nanowire networks. Copyright © 2019 American Chemical Society. b Schematic of the preparation (left), optical photograph (upper right) and cross-sectional demonstration (lower right) of the Ti₃C₂T_x transparent flexible electrode for the PV supercapacitors. c Working principle of the semitransparent, flexible PV supercapacitor in the charge (left) and discharge states. Copyright © 2020 The Royal Society of Chemistry

Fig. 7

a Schematic illustration for the n⁺–p–p⁺ Si solar cell using the Ti₃C₂T_x MXene as the electrode contacted with the n⁺ emitter. SEM images of grooves on the n⁺ side surface b before and c after MXene coating. d Energy-level alignment. Φ is a work function; E_g, E_c and E_v are the energy bandgap, conduction band and valence band of Si. e Illuminated J-V curves before and after 30 s RTA treatment at varying temperatures. f Series resistance values deduced from the J–V measurement for the samples before and after the RTA process. Copyright © 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 8

CsPbBr₃-based PSCs using the Ti₃C₂T_x nanosheet layer as the HTL: a Cross-sectional SEM image, b energy-level alignment and c carrier transport mechanism at illumination. Copyright © 2019 The Royal Society of Chemistry. d Device architecture and e cross-sectional SEM image, f energy-level alignment of each layer. Copyright © 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. g Device configuration and h energy-level alignment of the device. i Cross-sectional SEM image of the device (left), first-principle optimized structures and electron density differences for SnO₂/Ti₃C₂(OH)₂ (upper right), and spatially resolved mapping of the current transport efficiency of the MXene-inserted device (lower right). Copyright © 2020 American Chemical Society

Fig. 9

PBDB-T:ITIC-based OSCs using the UV–O₃ and/or N₂H₄-treated Ti₃C₂T_x MXene as the ETL/HTL: a Energy levels of the main components and b illuminated J–V curves of the PBDB-T:ITIC-based OSCs. Here, U-MXene and UH-MXene denote the MXene treated only by UV–O₃, and first by UV–O₃ and then by N₂H₄, respectively. Moreover, U-MXene is used for the hole collection in the normal OSCs, and the UH-MXene is for the electron collection in the inverted OSCs; and c V_oc versus the treatment duration. Copyright © 2019 The Royal Society of Chemistry. PBDB-T:ITIC-based OSCs using Ti₃C₂T_x nanosheets as the HTL: d Energy-level alignment of each component, e illuminated J–V curves, and f stability test under the atmosphere condition without any encapsulations. Copyright © 2019 The Royal Society of Chemistry

Fig. 10

Ti₃C₂T_x MXene/n-Si solar cells: a Energy-level alignment of the main components (the blue strip indicates the SiO₂ thin layer), b illuminated J–V curves for the devices fabricated via floating and oven transfer methods and c illuminated J–V curves for the devices fabricated by the oven transfer method and after the two-step (HCl + AuCl₃) chemical treatment and further coating the PDMS antireflection film. Copyright © 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim