Advances in Hole Transport Materials for Layered Casting Solar Cells

Abstract

Huge energy consumption and running out of fossil fuels has led to the advancement of renewable sources of power, including solar, wind, and tide. Among them, solar cells have been well developed with the significant achievement of silicon solar panels, which are popularly used as windows, rooftops, public lights, etc. In order to advance the application of solar cells, a flexible type is highly required, such as layered casting solar cells (LCSCs). Organic solar cells (OSCs), perovskite solar cells (PSCs), or dye-sensitive solar cells (DSSCs) are promising LCSCs for broadening the application of solar energy to many types of surfaces. LCSCs would be cost-effective, enable large-scale production, are highly efficient, and stable. Each layer of an LCSC is important for building the complete structure of a solar cell. Within the cell structure (active material, charge carrier transport layer, electrodes), hole transport layers (HTLs) play an important role in transporting holes to the anode. Recently, diverse HTLs from inorganic, organic, and organometallic materials have emerged to have a great impact on the stability, lifetime, and performance of OSC, PSC, or DSSC devices. This review summarizes the recent advances in the development of inorganic, organic, and organometallic HTLs for solar cells. Perspectives and challenges for HTL development and improvement are also highlighted.

Keywords: hole transport layer; inorganic HTL; organic HTL; perovskite solar cell; polymer solar cells.

Figure 1

(a) Conventional and (b) inverted structures of an organic solar cell; (c) band gap alignment of active material with electron/hole transport; (d) current density (J)–voltage (V) curves of the solar cell with the inset of the fill factor equation.

Figure 2

(a) Common organic hole transport materials (OHTMs) and (b) their energy levels. Reproduced with permission from ref. [14]. Copyright 2021, Springer Nature.

Figure 3

Performance of PSCs based on spiro-OMeTAD(TFSI)₂ in the absence (red) and existence (blue) of LiTFSI: (a) PCE, (b) $V_{OC}$, (c) $J_{SC}$, (d) FF. Reprinted with permission from ref. [38] Copyright 2019, Wiley-CH.

Figure 4

(a) The structure of PSCs based on PTAA/M2, (b) current density–voltage characteristics, (c) incident IPCE spectra, (d) stabilized power output, tracked at MPP under AM 1.5G illumination. Reprinted with permission from ref. [57] Copyright 2022, American Chemical Society.

Figure 5

(a) Atomic force microscopy image of GO; (b) energy band alignment of GO in OSC structure; (c) J–V curves of GO-, PEDOT:PSS-, and bare ITO-based OSCs. Reproduced with permission from ref. [101] Copyright 2010, American Chemical Society. (d) Structure; (e) energy band alignment; and (f) J–V curves of flexible GQDs and graphene-based MAPbI₃ PSCs. Reprinted with permission from ref. [114] Copyright 2019, American Chemical Society.

Figure 6

(a) Cell structure; (b) external quantum efficiency; (c) band alignment of energy; and (d) J–V curves of NiO/PEDOT:PSS HTL-based OSCs. Reproduced with permission from ref. [136] Copyright 2019, MDPI.

Figure 7

(a) Scheme of PSCs based on 2D MoS₂; (b) energy band alignment of different layers in PSC; (c) J–V curves of devices with different treatments P1, P2, and P3, corresponding with MoS₂ on perovskite without annealing, annealing MoS₂ on pre-annealed perovskite, and annealing MoS₂ on perovskite without annealing, respectively. Reproduced with permission from ref. [185] Copyright 2020, Springers Nature. (d) Energy band alignment work function of WS₂ with/without UV treatment and PEDOT:PSS in OSCs; and (e) J–V curves of devices based on WS₂-UVO and PEDOT:PSS as HTLs. Reprinted with permission from ref. [188] Copyright 2014, Wiley-CH.

Figure 8

(a) Chemical structures of ZnP and CuP with (b) their frontier orbitals; (c) energy band alignment of perovskite and HTLs; and (d) J–V curves measured in reverse and forward voltage scans of CuP, ZnP, and spiro-OMeTAD-based HTL PSC devices. Reproduced with permission from ref. [213] Copyright 2017, American Chemical Society.

Figure 9

Energy level diagrams for commonly used inorganic HTLs in PSCs. Reproduced with permission from ref. [207] Copyright 2021, Wiley-CH.