Hole-Transporting Materials for Printable Perovskite Solar Cells

Abstract

Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in their power conversion efficiency from less than 5% to over 22% in just a few years. Hole-transporting materials (HTMs) are an essential building block of PSC architectures. Currently, 2,2',7,7'-tetrakis-(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene), better known as spiro-OMeTAD, is the most widely-used HTM to obtain high-efficiency devices. However, it is a tremendously expensive material with mediocre hole carrier mobility. To ensure wide-scale application of PSC-based technologies, alternative HTMs are being proposed. Solution-processable HTMs are crucial to develop inexpensive, high-throughput and printable large-area PSCs. In this review, we present the most recent advances in the design and development of different types of HTMs, with a particular focus on mesoscopic PSCs. Finally, we outline possible future research directions for further optimization of the HTMs to achieve low-cost, stable and large-area PSCs.

Keywords: hole-transporting material; hybrid; inorganic; perovskite solar cells; polymer; printable; small-molecule.

Figure 1

(a) ABX₃ perovskite structure showing the BX₆ octahedral and larger A cation occupied in the cubo-octahedral site; (b) unit cell of cubic CH₃NH₃PbI₃ perovskite (reproduced with permission from [13], published by Elsevier under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives Licence (CC BY NC ND)).

Figure 2

Schematic illustration of the (a) mesoscopic and (b) planar perovskite solar-cell configurations. In the mesoscopic architecture (a), a smooth perovskite capping layer covers the top of the mesoporous TiO₂ layer. The hole-transporting material (HTM), typically 2,2’,7,7’-tetrakis-(N,N’-di-p-methoxyphenylamine)-9,9’-spirobifluorene) (spiro-OMeTAD), is spin-coated atop the perovskite film. The most frequent film thicknesses reported for the layers of perovskite solar cell (PSC) structures are 50 nm (blocking TiO₂ layer, bl-TiO₂), 300 nm (mesoporous TiO₂, mp-TiO₂), 4–500 nm (perovskite), 1–200 nm (spiro-OMeTAD capping layer) and 80 nm (gold/silver). Please note that a systematic layer thickness optimization is still missing in the literature. In the planar configuration (b), the perovskite film is deposited directly on top of the electron-transporting layer (ETL), commonly a TiO₂ dense hole-blocking layer (the original figure in (a) was reproduced with permission from [24], published by Nature Publishing Group). FTO, fluorine-doped tin oxide; TCO, transparent conducting oxide.

Figure 3

Charge-transfer processes in perovskite solar cells. The valence band energy and the conduction band (CB) of methyl ammonium lead iodide (MAPbI₃) perovskite are at −5.43 eV and −3.7 eV, respectively. The CB of TiO₂ lies at −4.2 eV, and the HOMO energy level of the widely-used HTM spiro-OMeTAD is at −5.22 eV [5] (reproduced with permission from [5], published by John Wiley & Sons, Inc.).

Figure 4

Molecular structures of spiro-OMeTAD, the most widely-used HTM in PSCs, and its dopants (lithium bis(trifluoromethanesulfonyl) imide salt (LiTFSI), 4-tert-butylpyridine (TBP), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK 209).

Figure 5

Structures of pyrene-based HTMs: PY-1, PY-2, PY-3.

Figure 6

HTMs based on a truxene (Trux) and a triazatruxene (Triazatrux) core.

Figure 7

Phenothiazine-based HTMs.

Figure 8

Acridine-, thiophene-, biphenyl-, bithiophene-, tetrathiophene- and phenyl-based HTMs.

Figure 9

Triazine-based HTMs.

Figure 10

Benzotrithiophene (BZTR)- and squaraine (SQ)-based HTMs.

Figure 11

Fluorene-based HTMs for high-performance PSCs.

Figure 12

Spiro-fluorene-based HTMs (1).

Figure 13

Spiro-fluorene-based HTMs (2).

Figure 14

Carbazole-based HTMs (1).

Figure 15

Carbazole-based HTMs (2).

Figure 16

Carbazole-based HTMs (3).

Figure 17

Carbazole-based HTMs (4).

Figure 18

Other organic HTMs.

Figure 19

Other organic HTMs (2).

Figure 20

Polymer-based HTMs.

Figure 21

(a) Flexible PSC employing NiOx as HTM; (b) figures of merit of the flexible PSC (reproduced with permission from [143], published by the Royal Society of Chemistry).

Figure 22

Current-voltage curves for PSC with copper thiocyanate (CuSCN) HTM and without any HTM (reproduced with permission from [148], published by the American Chemical Society).

Figure 23

(a) Photo of freestanding CNT film lifted by tweezers; (b) mesoscopic CH₃NH₃PbI₃ perovskite solar cell with CNT film electrode; (c) top view SEM images of CH₃NH₃PbI₃ perovskite substrate before and (d) after CNT transfer; (e) tilted SEM image of CH₃NH₃PbI₃ perovskite substrate (blue) partly covered by CNT film (purple) (reproduced with permission from [151], published by the American Chemical Society).

Figure 24

Schematic illustration of the perovskite solar cell with carbon nanotube/polymer composite as the non-hygroscopic hole-transporting structure, referred as HTL in the figure (reproduced with permission from [155], published by the American Chemical Society).

Figure 25

Maximum power point (MPP) tracking of Au and SWCNT-contacted devices at high temperatures (reproduced with permission from [156], published by John Wiley & Sons, Inc.).

Figure 26

Summary of the most efficient HTMs discussed in this work, together with their cost (when available).