Recent progress in spiro-type hole transport materials for efficient and stable perovskite solar cells

Abstract

Hole transport materials (HTMs), as an important part of n–i–p perovskite solar cells (PSCs), are one of the main bottlenecks to further improving the efficiency and stability of devices. Since the introduction of Spiro-OMeTAD, it has shown remarkable power conversion efficiency (PCE) in n–i–p PSCs due to its high film quality and matched energy level. However, the high cost, low carrier mobility and poor stability of Spiro-OMeTAD greatly limit the practical application and commercialization of PSCs. Therefore, a large number of HTMs have been reported to replace the traditional Spiro-OMeTAD. Among them, spiro-type HTMs are one class of the most promising competitors with photovoltaic performance comparable to that of Spiro-OMeTAD. But few reviews have summarized spiro-type HTMs. In this review, we summarize efficient spiro-type HTMs reported in the last 8 years and discuss their structure–property relationships and photovoltaic performance. Also, we conclude the design points of spiro-type HTMs. It is hoped that this review can provide guidance for developing low-cost and efficient HTMs.

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Gang Xie

Gang Xie is currently a PhD candidate in the College of Chemistry and Materials of Jiangxi Normal University. He is mainly engaged in the research of efficient and stable perovskite solar cells. He has published 4 papers as the first author, including Angew. Chem. Int. Ed., Chin. J. Chem., Chem. Eur. J. and Eur. J. Inorg. Chem.

Ling Chen

Ling Chen is currently studying for a master's degree in the College of Chemistry and Materials of Jiangxi Normal University. She is mainly engaged in the research of efficient and stable perovskite solar cells.

Aihui Liang

Aihui Liang is a full professor at Jiangxi Normal University. He received his PhD from South China University of Technology in 2013. He worked as a visiting scholar from 2019 to 2021 at Prof. Letian Dou's group at Purdue University. He has published more than 50 research papers and 6 patents. His research interests mainly focus on photoelectric functional materials and devices.

Yiwang Chen

Yiwang Chen is a foreign academician of the Russian Academy of Sciences. His research interests include polymer solar cells, perovskite solar cells, supercapacitors, electrocatalysis for zinc-air batteries and fuel cells, and intelligent elastomers and fibers. He has published more than 500 research papers and 40 invention patents as well as 4 books. His research project has been awarded “Second Class Prize of Science and Technology in Universities of China” in 2019 and the Gold Medal of the Geneva Salon International Des Inventions in 2022.

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1. Introduction

With the increasing demand for energy and the depletion of traditional fuels, the development and utilization of inexhaustible and clean energy such as solar energy has attracted great interest from researchers all over the world. Solar cells are one of the most important technologies for solar energy utilization. Perovskite solar cells (PSCs), as the third generation of new cells, have attracted much attention because of their simple manufacturing process, low cost and excellent power conversion efficiency (PCE).¹ Especially after more than 10 years of development, the certified PCE of PSCs has now reached 26.7%, which is comparable to crystalline silicon solar cells.² More importantly, the stability of PSCs has also been significantly improved.^3–5 All the above shows that PSCs have great research value and development prospects and are the most promising candidates for silicon-based photovoltaic technology, which is dominant in the market.

The general chemical formula of an ideal three-dimensional (3D) perovskite crystal is ABX₃, in which A is a monovalent cation, such as CH₃NH₃⁺(MA⁺), (NH₂)₂CH⁺ (FA⁺) or Cs⁺; B is a divalent cation, such as Pb²⁺ or Sn²⁺; and X is a monovalent halide anion, such as Cl⁻, Br⁻ or I⁻, as shown in Fig. 1(a).⁶ The 3D perovskite skeleton is composed of the [BX₆]⁴⁻ octahedron with shared corners and A cations in the cuboctahedron cavities.⁷ This special structure makes perovskite materials have a large absorption coefficient, low exciton binding energy, adjustable composition and absorption edge, long carrier diffusion length and effective bipolar charge transport.^8–12

Fig. 1 (a) Crystal structure of a typical perovskite ABX₃. (b) The common PSC device architectures.

A typical PSC device is mainly composed of a perovskite active layer, a charge transport layer and an electrode, where the charge transport layer includes an electron transport layer (ETL) and a hole transport layer (HTL).¹³ According to the physical position of the HTL and the ETL, the PSCs can be divided into n–i–p (conventional) and p–i–n (inverted) structures, as shown in Fig. 1(b). In addition, depending on the morphology of the transport layer at the bottom of the device, the n–i–p type PSCs include mesoporous and planar structures. Up to now, the highest authentication efficiencies of mesoporous n–i–p PSCs (FTO/meso-ETM (electron transport materials)/perovskite/HTMs (hole transport materials)/Au), planar n–i–p PSCs (FTO/ETM/perovskite/HTM/Au) and planar p–i–n PSCs (ITO/HTM/perovskite/ETM/Ag) are 26.3%,¹⁴ 26.2%¹⁵ and 26.7%,² respectively. The working principle of PSCs is shown in Fig. 2. When PSCs are irradiated, holes/electrons generated by the absorption of light in the perovskite active layer are transferred to the HTM/ETM and selectively collected by the anode/cathode.¹⁶ It can be seen that HTMs play an important role in extracting and transporting holes as well as inhibiting charge recombination. Additionally, HTMs can also prevent external moisture from invading the perovskite layer, which affects the stability of PSCs.

Fig. 2 Simple working principle of PSCs.

Therefore, the design of HTMs is very important and generally needs to meet the following requirements: (i) they should be aligned with the appropriate energy level of perovskite to increase the extraction of holes and minimize the energy loss at the interface; (ii) they should have high hole mobility and conductivity to promote carrier migration at the perovskite/HTL interface and reduce charge recombination; (iii) they should have good photo-thermal stability; (iv) they should have excellent solubility and membrane quality to form a smooth and pinhole-free film morphology; and (v) they are low-cost. Additionally, different PSC device structures have different requirements for the HTL.⁷ For n–i–p PSCs, HTMs should have good hydrophobicity and film compactness to protect the perovskite layer from the external environment. In contrast, for p–i–n PSCs, the HTL requires good wettability and general hydrophobicity to promote the spread and crystallization of perovskite. Moreover, the HTL should be prevented from being dissolved by perovskite precursor solution during the preparation of the perovskite layer.

Doped 2,2′,7,7′-tetrakis [N,N-di(4-methoxyl phenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD) is the most commonly used HTM to realize efficient n–i–p PSCs, whose highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbital (LUMOs) and optical bandgap are −5.2 eV, −2.3 eV and 2.9 eV, respectively (Fig. 3).¹⁷ Spiro-OMeTAD is a semi-crystalline p-type semiconductor with a glass transition temperature (T_g) of 125 °C and a melting temperature of 248 °C.¹⁸ Because of the orthogonal structure of spirofluorene and the propeller-like diarylamine (DPA), which prevents the formation of a compact π–π layer and increases the intermolecular distance, the original Spiro-OMeTAD exhibits low hole mobility (≈10⁻⁵ cm² V⁻¹ s⁻¹), low conductivity (≈10⁻⁶ mA cm⁻²), and good solubility in nonpolar solvents.¹⁹ Therefore, dopants are needed to improve the electronic and physical properties of Spiro-OMeTAD. The commonly used dopants are 4-tert-butylpyridine (t-BP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)tri(bis(trifluoromethane)sulfonimide) (FK209).²⁰ The basic p-doping process is the charge transfer reaction between Spiro-OMeTAD and the dopant.²¹ The electrons extracted from the HOMO of spiro metal molecules by dopants with high electron affinity become single occupied molecular orbitals (SOMO), which leads to the increase of hole density and conductivity of Spiro-OMeTAD.²² In addition, dopants can deepen the HOMO energy level and improve the membrane morphology of Spiro-OMeTAD.²⁰ However, dopants are a double-edged sword, which can improve the hole transport characteristics but also cause some serious problems: (1) dopants are hygroscopic, which can accelerate the degradation of perovskite.²³ (2) Though t-BP has a high boiling point, it is volatile and corrosive to perovskite.²⁴ Also, t-BP can decrease the T_g of Spiro-OMeTAD significantly (from 125 °C to ∼50 °C).²⁵ (3) Spiro-OMeTAD needs oxidation after doping, but the oxidation will damage perovskite.^26–28 The oxidation level is difficult to control accurately, which will also cause reproducibility problems.²⁹ (4) The dopants connect with Spiro-OMeTAD by a non-covalent bond, which leads to ion diffusion and is not conducive to long-term stability.³⁰ (5) Most importantly, Spiro-OMeTAD is very expensive (∼$500 g⁻¹ from Sigma) due to multiple synthetic steps, low yield and difficulty in purification.³¹ Moreover, the extra doping process further increases the complexity and cost, which hinders the large-scale fabrication of PSCs with Spiro-OMeTAD.

Fig. 3 Chemical structures of Spiro-OMeTAD, t-BP, LiTFSI, and FK209. An integral charge transport mechanism of Spiro-OMeTAD with molecular p-type dopants. Copyright 2022, The Royal Society of Chemistry.³²

In order to solve the above problems, researchers have developed thousands of HTMs to replace Spiro-OMeTAD, which can be divided into doped HTMs and dopant-free HTMs.^33–35 Although dopant-free HTMs have solved many problems caused by dopants and improved the stability of PSCs,^36,37 their efficiency still lags behind that of doped Spiro-OMeTAD, as shown in Fig. 4. Recently, many doped HTMs have been reported with device efficiency even exceeding that of Spiro-OMeTAD, and a large part of them are optimized, improved and redesigned with reference to the Spiro-OMeTAD structure. Because of their spiro-like structure, they are named spiro-type HTMs. Unfortunately, although there are many reports on spiro-type HTMs, few reviews have summarized them. Therefore, we collected the efficient spiro-type HTMs reported in the last 8 years and categorized them into spirobifluorene (SBF)-core HTMs, spiro(fluorene-9,9′-xanthene) (SFX)-core HTMs and spiro-like HTMs according to the similarity of their intermediate cores and structures to study their design principles, structural characteristics and photovoltaic performances. Finally, the design principles and perspectives of spiro-type HTMs are briefly discussed and summarized.

Fig. 4 The highest efficiency of PSCs based on Spiro-OMeTAD, dopant-free HTMs and spiro-type HTMs in corresponding years.^2,14,35,38

2. SBF-core HTMs

Although researchers have developed a large number of HTMs with different cores to replace Spiro-OMeTAD, Spiro-OMeTAD has been a leader in realizing high-performance n–i–p PSCs since it was introduced into PSCs in 2012.^39,40 This conversely triggers further research on the design, modification and optimization of Spiro-OMeTAD to adjust its energy level, stability, hole conductivity, film morphology and defect passivation properties. As shown in Fig. 5 and Table 1, SBF-core HTMs retain the SBF core and modify the end groups of Spiro-OMeTAD by molecular engineering, such as the introduction of heteroatoms, the modification of side chains and building blocks, and the increase of coupling length.

Fig. 5 Chemical structures of SBF-core HTMs.

Table 1 Overview of properties of SBF-core HTMs

HTM	Device configuration	HOMO/LUMO [eV]	Hole mobility [cm^2 V^−1 s^−1]	V OC [V]	J SC [mA cm^−2]	FF [%]	PCE [%]	T g (°C)	Stability conditions	Times [h]	PCE [%] retention	Cost [$ per g]	Ref.
a The unencapsulated devices were stored in an N2-filled glove box or ambient air under continuous 1 sun equivalent illumination.
Spiro-E	ITO/HTM/CH3NH3PbI3/PCBM/ZnO	−4.81/−1.91	1.26 × 10^−5	1.07	18.24	80.00	15.75	255					41
Spiro-N	ITO/HTM/CH3NH3PbI3/PCBM/ZnO	−4.42/−1.42	2.50 × 10^−4	0.96	16.55	75.00	11.92						41
Spiro-S	ITO/HTM/CH3NH3PbI3/PCBM/ZnO	−4.92/−1.97	1.90 × 10^−5	1.06	19.15	78.00	15.92						41
Spiro-TTB	ITO/HTL/perovskite/PCBM/BCP/Ag	−5.30/−2.23	1.97 × 10^−3	1.07	22.02	78.00	18.38						42
Spiro-O SMeTAD	FTO/bl-TiO2/mp-TiO2/Cs0.05(FA0.85–MA0.15)0.95Pb(I0.85Br0.15)3/HTM/Au	−5.18/−2.17		1.10	21.31	78.00	18.24	266	In air, RH 20%	1920	97		43
Spiro-4TFETAD	ITO/SnO2/(FAPbI3)0.97(MAPbBr3)0.03/HTL/Ag	−5.25/−2.35	2.04 × 10^−4	1.16	24.27	73.13	20.53	140	In air, RH 60%	250	83		44
									In N2^a	100	82
Spiro-mF	FTO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.19/−2.23	7.47 × 10^−3	1.16	26.35	80.90	24.82		In air, RH 50%	500	87		45
Spiro-oF	FTO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.06/−2.04	7.29 × 10^−3	1.16	26.34	80.15	24.50		In air, RH 50%	500	87		45
Spiro-mF-A	ITO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.33/−2.34	4.93 × 10^−3	1.15	25.91	79.46	23.80	146	In air	1000	>90		46
									In N2	600	>90
Spiro-mF-P	ITO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.37/−2.37	1.13 × 10^−2	1.17	25.94	81.62	24.85	144	In air	1000	>90		46
									In N2	600	>90
SC	FTO/SnO2/(CsPbI3)x(FAPbI3)y(MAPbBr3)1−x−y/HTM/Ag	−5.26/−2.35	3.15 × 10^−3	1.15	23.47	80.62	21.76	164	In air, RH 30 ± 5%	720	90		47
									In N2, at 60 °C	500	87
ST	FTO/SnO2/(CsPbI3)x(FAPbI3)y(MAPbBr3)1−x−y/HTM/Ag	−5.31/−2.34	1.77 × 10^−3	1.06	23.05	74.39	18.18	160	In air, RH 30 ± 5%	720	64		47
									In N2, at 60 °C	500	68
DM	FTO/bl-TiO2/mp-TiO2/(FAPbI3)0.95(MAPbBr3)0.05/HTM/Au	−5.27/−2.36	>1.0 × 10^−4	1.14	24.91	81.29	23.2	161	In air, at 60 °C	500	95		48
									In N2, encapsulated^a	300	>90
Spiro-DBF	FTO/bl-TiO2/mp-TiO2/(CsPbI3)0.05(FAPbI3)0.95/HTM/Au	−5.22/−2.34	6.31 × 10^−3	1.12	24.21	79.00	21.43	155	In air, RH 60%	400	>70		49
Spiro-4	FTO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.29/−2.34	6.98 × 10^−3	1.17	24.98	80.00	23.38	163	In air	2400	97		50
									In N2, at 60 °C	150	80
SBF-FC	ITO/SnO2/FAPbI3/HTL/Au	−5.06/	1.20 × 10^−5	1.18	25.90	80.90	24.70	272	In N2, at 85 °C	500	92		51
SF-MPA-MCz	FTO/TiO2/perovskite/HTM/Au	−5.26/−2.38	4.50 × 10^−5	1.18	26.24	79.20	24.53	154	In N2^a	500	90		52
									In N2, at 85 °C	1000	85
Spiro-Naph	ITO/SnO2/FAPbI3/HTL/Au	−5.05/−2.18	8.08 × 10^−3	1.16	25.97	80.60	24.43	147	In air	2000	89.4		53
									In N2, at 60 °C	400	78.5
									In air, encapsulated^a	300	80
Spiro-cyclOMe	FTO/c-TiO2/mp-TiO2/Cs0.05FA0.95PbI3/HTM/Au	−5.09/−2.14	2.25 × 10^−3	1.18	24.86	79.00	23.10	142	In air	1300	89		54
									In N2, at 60 °C	240	65
Spiro-tBuBED	ITO/SnO2/FAPbI3/HTL/Au	−5.30/−2.72	2.29 × 10^−4	1.10	22.99	73.50	18.60	258	In air, RH 30%	672	96	167	55
DP	FTO/c-TiO2/SnO2/perovskite/OAI/HTL/Au	−5.18/−2.25	5.19 × 10^−3	1.14	26.13	84.90	25.24	149	In air, encapsulated^a	2560	95.1		56
									RH 85%, at 60 °C^a	600	87
Spiro-OMeIm	FTO/bl-TiO2/mp-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/HTL/Au	−5.13/−1.93	1.60 × 10^−5	1.10	20.46	76.00	17.10					15	57
V1307	ITO/SnO2/FAPbI3/HTL/Au	−5.46/−2.63	6.40 × 10^−4	1.07	23.21	77.00	19.20	169	In N2^a	12	95	37	58

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The first exploration was the influence of changing the methoxy group of Spiro-OMeTAD on the properties of HTMs. In 2016, Meng et al. developed three SBF-core HTMs by replacing p-methoxy with methyl sulfanilamide (Spiro-S), N,N-dimethylamino (Spiro-N) and ethyl (Spiro-E).⁴¹ Both Spiro-N and Spiro-E show high thermal stability, while Spiro-S has a deeper HOMO energy level and higher hole mobility. In addition, Spiro-S shows good hydrophobicity, which is beneficial to the grain growth of the perovskite layer. Finally, Spiro-S-based p–i–n PSCs achieved a PCE of 15.92%, which is more than 38% higher than that of Spiro-MeOTAD. However, SBF-core HTMs are not ideal for the application of p–i–n PSCs because they are easily dissolved in N,N-dimethylformamide (DMF), which is a commonly used solvent for preparing perovskite precursors. Therefore, in 2019, Guo's group used an orthogonal solvent of γ-butyl lactone (GBL) and dimethyl sulfoxide (DMSO) as a perovskite precursor solution to avoid damaging the HTL.⁴² The PCE of the device is 18.38% based on methyl-substituted Spiro-TTB. In the application of n–i–p PSCs, Qu's group found that the HTM (Spiro-OSMeTAD) with the semi-methoxy group and semi-methylsulfanyl group showed better device performance and less hysteresis than the HTMs with a full methoxy group or a full methylsulfanyl group, which was attributed to the better film-forming ability and the stronger hole extraction ability of Spiro-OSMeTAD.⁴³ After device optimization, the PCE of Spiro-OSMeTAD-based devices is 20.18%, which is comparable to that of Spiro-OMeTAD (PCE = 20%).

Considering that fluorination can induce dipoles to lower the HOMO energy level and enhance the molecular packing and hydrophobicity of conjugated materials. Guo et al. developed a fluorinated isomer HTM (Spiro-4TFETAD) substituted by trifluoroethoxy.⁴⁴ While retaining the control of the methoxy group on oxidation potential and jumping hole transport, the substituent of fluorine was introduced to adjust the energy level, promote charge transport and improve hydrophobicity. As a result, the PCE of the Spiro-4TFETAD-based PSCs reached 21.11% with an open-circuit voltage (V_OC) as high as 1.17 V. In 2020, Yang's group developed the meta- and ortho-fluorinated isomer HTMs of Spiro-OMeTAD, named Spiro-mF and Spiro-oF.⁴⁵ It was found by simulation that Spiro-mF exhibited a more unfolded structure regarding the angle between the center of SBF and two fluorinated methoxyphenyl groups, while the fluorine atom of Spiro-oF hindered the free movement of methoxyphenyl. Moreover, Spiro-mF is adsorbed on the perovskite surface in a layered way, while Spiro-oF and Spiro-OMeTAD are randomly stacked on the surface, which makes the hole transport of Spiro-mF more efficient. Therefore, Spiro-mF-based PSCs achieved a considerable PCE of 24.82% (certified PCE is 24.64%, with V_OC = 1.18 V, J_SC = 26.18 mA cm⁻², and FF = 79.6%), and the unencapsulated devices based on Spiro-mF showed excellent long-term stability at high relative humidity. In addition, the Spiro-mF-based device achieved an excellent PCE of 22.31% in a large area (1 cm²) device. Jin et al. designed and synthesized spiro-mF-A and spiro-mF-P with allyloxy and propargyloxy side chains while retaining the advantages of meta-fluorination to improve hole mobility and match the energy levels of the perovskite layer.⁴⁶ Notably, spiro-mF-A and spiro-mF-P showed good solubility and high-quality film morphology in a non-halogenated solvent (toluene) system. When using toluene to prepare environmental-friendly PSCs, spiro-mF-P-based devices achieved an excellent PCE of 24.85%, which is the highest PCE reported so far for n–i–p PSCs, whose HTMs were prepared by non-halogenated solvents.

Rigid building blocks are used to replace terminal DPA in different degrees to extend π-conjugation, so as to adjust energy level, stability and hole extraction ability. In 2018, Seo's group synthesized SBF-core HTM (DM) with fluorene end-capping.⁴⁸ The introduction of a fluorene unit deepens the HOMO level and increases the T_g of DM, making it superior to Spiro-OMeTAD in terms of V_OC and thermal stability. The champion device based on DM achieved a PCE of 23.2% (certified as 22.6%), a J_SC of 24.9 mA cm⁻², an FF of 81% and a V_OC of 1.14 V. A PCE of 21.7% (certified as 20.9%) was also achieved for devices with an effective area of 1 cm². In 2021, Chen's group designed SC and ST with N-ethylcarbazole and dibenzothiophene and used a π-conjugated structure with weak electron-donating ability to reduce the HOMO level and increase V_OC.⁴⁷ Ethyl chain increased the solubility and decreased the crystallinity of SC, which made SC have good miscibility with Li-TFSI to prevent phase separation. Hence, the film morphology of SC is smoother, more uniform and pinhole-free compared to ST and Spiro-OMeTAD, and the PCE of SC-based devices is as high as 21.76%. Compared with Spiro-OMeTAD and HTMs with the dibenzothiophene group, Spiro-DBF with dibenzofuran designed by Dai's group displayed higher charge transfer ability and better film-forming ability.⁴⁹ And the champion PCE of the Spiro-DBF-based PSC is 21.43%. Interestingly, Dai's group recently reported the influence of different substitution positions of dibenzofuran on the properties of spiro-type HTMs.⁵⁰ The results show that Spiro-4 with substituents at position 4 has a better matching energy level, higher T_g, higher hole mobility and better film-forming ability. The PCE of spiro-4-based devices increased to 23.38%. Wang's group designed asymmetric DM and SBF-FC with fluorene and fluorocarbazolamide as terminal electron donors.⁵¹ The highly asymmetric SBF-FC showed a rather high HOMO energy level. But its intrinsic and doped T_g are 222 °C and 176 °C, respectively, which are much higher than that of Spiro-OMeTAD. This is favourable to inhibiting the corrosion and decomposition of perovskite films, which can significantly improve the high-temperature durability of PSCs. Therefore, SBF-FC-based PSCs have achieved an average initial efficiency of 24.5% and excellent long-term thermal stability at 85 °C. Recently, Wang et al. reported SF-MPAMCz with diphenylamine and carbazole as end groups by anisotropic adjustment strategy.⁵² The introduction of rigid carbazole units improves the thermal stability, hole mobility, energy level matching, membrane morphology and interface contact of SF-MPAMCz. The device based on SF-MPAMCz achieved a remarkable PCE of 24.53% and obviously improved stability. The PCE remained above 90% after 500 h of continuous illumination.

Small-scale modifications are more cost-effective than drastic adjustments of the edge segments. In 2022, Yang's group developed Spiro-Naph with asymmetric phenylnaphthylamine end groups by using a symmetry-tuned strategy at the molecular level.⁵³ The inherent properties of HTM, such as π-electron delocalization, effective conjugate length and structural robustness, are enhanced due to the larger naphthyl alkyl group than the phenyl group. In addition, density functional theory (DFT) reveals that the π orbitals of phenylethylamine fragment have higher directionality and larger area, which increases the intermolecular van der Waals interaction and results in a more compact and stable molecular packing of Spiro-Naph. Spiro-Naph-based PSCs not only achieved an ultra-high PCE of 24.43% but also showed excellent stability (the PCE remained at 21.12% after 2000 h in air with a relative humidity of ∼25%,). Most importantly, the large-area module (25 cm²) based on Spiro-Naph achieved an excellent PCE of 21.83%. Later on, Liu's group developed Spiro-cyclOMe by methoxy cyclization to improve hole mobility and thermal stability.⁵⁴ Spiro-cyclOMe-based devices have achieved a PCE of 23.10%, with a high V_OC of 1.18 V.

It is also possible to introduce π-units between the core and the end group to extend conjugation. In 2019, Liu's group designed and synthesized Spiro-tBuBED with 3,4-ethylenedioxythiophene as the π-spacer and t-butyl as the terminal group.⁵⁵ Spiro-tBuBED was synthesized by a direct C–H bond activation/arylation reaction, which requires only two steps and does not require metal catalysis. Importantly, Spiro-tBuBED-based PSCs showed a PCE of 18.6% without oxidation treatment. In 2023, Ding et al. developed a new HTM (DP) with SBF-core by partially replacing the anisole unit with 4-methoxybiphenyl.⁵⁶ Through extending the π-conjugation, the energy level, hole mobility and T_g of DP were improved. The highest PCE of DP-based PSCs is 25.24% for a small area (0.06 cm²) device and 21.78% for a large area (27.86 cm²) device. In addition, the packaged devices based on DP maintained 95.1% of their initial performance after 2560 hours under iso-l-1 conditions and 87% of initial performance after 600 hours under iso-l-3 conditions. Among them, iso-l-1 and iso-l-3 are international standard conditions for the stability test of perovskite solar cells, aiming to standardize the experimental process and ensure the consistency and comparability of data.⁵⁹

Unfortunately, the complete replacement of DPA with new donor moieties does not seem to be satisfactory. In 2020, Shahroosvand et al. synthesized Spiro-OMeIm with benzimidazole as the terminal group.⁵⁷ However, the energy level and hole mobility of Spiro-OMeIm are not as good as those of Spiro-OMeTAD, and its device only achieved a low PCE of 17.10%. Nazeeruddin et al. synthesized V1307 by an enamine condensation reaction under ambient conditions with the only by-product of water.⁵⁸ This avoids the cross-coupling reaction for the synthesis of Spiro-OMeTAD, which required inert reaction conditions, precious metal catalysis and difficult purification. As a result, the cost of the laboratory-scale synthesis of V1307 is ∼$37 g⁻¹, which is two-fifths of the price of Spiro-OMeTAD. And the maximum PCE of V1307-based PSCs was 19.2%, and its stability was comparable to that of Spiro-OMeTAD.

3. SFX-core HTMs

The synthetic route of 4Br-SBF is shown in Fig. 6, which consists of four steps: Suzuki–Miyaura cross-coupling reaction, Grignard reaction, cyclization reaction and bromination reaction.⁶⁰ The complex synthetic route, harsh reaction conditions and difficult purification of the 4Br-SBF core make its synthesis cost high, which greatly limits the large-scale commercial production of Spiro-OMeTAD. In this regard, researchers have developed many different types of spiro-cores to replace SBF. Adding oxygen atoms to convert SBF to SFX can not only improve its solubility and processability but also adjust its optical properties. In addition, SFX has the advantages of an electrophilically substituted heteroanthracene ring, a 3D conjugated framework and outstanding intramolecular charge transfer (ICT) characteristics. Therefore, SFX is the most common alternative to SBF with significant potential in the field of optoelectronics shown in Fig. 7 and Table 2.^61,62

Fig. 6 Synthetic routes of 4Br-SFX and 4Br-SBF.

Fig. 7 Chemical structures of SFX-core HTMs.

Table 2 Overview of properties of SFX-core HTMs

HTM	Device configuration	HOMO/LUMO [eV]	Hole mobility [cm^2 V^−1 s^−1]	V OC [V]	J SC [mA cm^−2]	FF [%]	PCE [%]	T g (°C)	Stability conditions	Times [h]	PCE [%] retention	Cost [$ per g]	Ref.
a The unencapsulated devices were stored in an N2-filled glove box or ambient air under continuous 1 sun equivalent illumination.
X60	TiO2/nc-TiO2/perovskite/HTM/Au	−5.61/−2.56	1.90 × 10^−4	1.14	24.20	71.00	19.84					1.12	63
mp-SFX-2PA	FTO/TiO2/MAPbI3/HTM/Au	−4.92/−1.98	3.00 × 10^−5	1.08	20.87	78.30	17.70		In air	2000	90		64
HTM-FX’	ITO/TiO2-Cl/perovskite/HTM/Au	−5.16/	4.80 × 10^−4	1.17	21.70	78.00	19.70	137					65
pDPA-SFX	ITO/SnO2/FACsPbI3/HTMs/Ag	−5.01/−2.11	2.31 × 10^−4	1.16	24.96	78.20	22.50	71	In N2^a	2238	80	13.4	60
									In air, RH 40%	500	87
mDPA-SFX	ITO/SnO2/FACsPbI3/HTMs/Ag	−5.06/−2.16	5.74 × 10^−4	1.18	25.50	82.54	24.80	105	In N2^a	1815	80	15.1	60
									In air, RH 40%	500	67
XDB	ITO/SnO2/C60/CH3NH3PbI3/HTM/MoO3/Ag	−5.15/−2.11	7.30 × 10^−5	1.02	19.90	76.10	15.50						66
XPP	ITO/SnO2/C60/CH3NH3PbI3/HTM/MoO3/Ag	−5.15/−2.11	1.60 × 10^−4	1.06	21.30	78.40	17.70		In air, RH 40–50%	1080	>70		66
2mF-X59	FTO/TiO2/CH3NH3PbI3/HTM/Au	−5.19/−2.40	7.14 × 10^−5	1.01	24.97	71.60	18.13	113	In air, RH 30%	500	95	21.9	67
P65	FTO/c-TiO2/SnO2/MAPbI3/HTM/Au	−5.12/−2.09	4.80 × 10^−5	1.09	21.70	75.00	17.70	266	In air, RH 40–65%	720	84		68
SP-SMe	ITO/SnO2/perovskite/HTL/Au	−5.36/−2.64		1.16	24.23	77.65	21.95	117	In air, RH 35 ± 5%	500	90	26.3	69
									In N2, at 65 °C	160	85
									In N2^a	200	82
BTPA-6	FTO/CH3NH3PbI3/HTM/Au	−5.34/−2.43		1.03	20.61	64.70	13.81	143	In air, RH 25%	240	72		70
SFX-FM	FTO/c-TiO2/Cs0.05(FA0.83MA0.17)0.95Pb (I0.83Br0.17)3/HTMs/Au	−5.24/−2.28	1.26 × 10^−4	1.11	22.89	68.00	17.29	154	In air	648	70	24.0	71
T2	FTO/SnO2/perovskite/HTL/Au	−5.31/−2.32		1.18	26.47	84.94	26.41		In air	2800	95	14	15
									In N2^a	600	80
									In Ar, at 60 °C	1600	84
X55	FTO/c-TiO2/mp-TiO2/FA0.85MA0.15Pb(I0.85Br0.15)3/HTM/Au	−5.23/−2.26	6.81 × 10^−4	1.15	23.4	77.0	20.8	174					72
SFXDAnCBZ	FTO/bl-TiO2/mp-TiO2/(FAPbI3)0.095(MAPbBr3)0.05/HTM/Au	−4.95/−2.17	4.28 × 10^−4	1.09	23.10	83.00	20.87	122	In air, RH 30%	720	77		73
M6-F	ITO/SnO2/MA0.16FA0.84PbI3/HTM/MoO3/Ag	−4.99/−2.55	2.10 × 10^−4	1.15	24.45	78.56	22.17	150	In air	816	86		74
									In N2^a	240	59
SFX-DM-DPA	FTO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.09/−2.24	2.52 × 10^−4	1.15	24.50	80.60	22.70		In air, RH 45%, 60 °C	1000	80		75
SFX-DM	FTO/c-TiO2/SnO2/(FAPbI3)0.992 (MAPbBr3)0.008/SFX-DM/Au	−5.07/−2.15		1.13	24.91	80.46	22.70	152	In air	2000	74.8		76
									In air^a	500	48
SFX-DM-F	FTO/c-TiO2/SnO2/(FAPbI3)0.992 (MAPbBr3)0.008/interlayer/SFX-DM HTL/Au	−5.13/−2.22		1.16	25.15	82.79	24.20	155	In air	2000	87.7		76
									In air^a	500	83
SFX-DM-Cl	FTO/c-TiO2/SnO2/(FAPbI3)0.992 (MAPbBr3)0.008/interlayer/SFX-DM HTL/Au	−5.22/−2.16		1.17	25.20	84.03	24.80	159	In air	2000	94		76
									In air^a	500	83

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In 2016, Sun et al. synthesized X60 by replacing the SBF core of Spiro-OMeTAD with the SFX core.⁶³ The PCE of X60-based devices is 19.84%, slightly lower than that of Spiro-OMeTAD (20.8%). Zhan et al. found that the DPA arm at the meta position (3,6) has better photovoltaic performance than the para position (2,7), but unfortunately the efficiency is not as good as X60.⁶⁴ In 2018, Berlinguette et al. also found that HTMs (HTM-FX') with a DPA arm at the meta position had better thermal stability and higher hole mobility, and a PCE of 20.8% was obtained based on the HTM-FX' device.⁶⁵ Recently, Yan et al. also synthesized pDPA-SFX and mDPA-SFX to explore the effect of different substituent sites on the performance of HTMs.⁶⁰ The difference is that trifluoromethyl is introduced to partially replace methoxy to further enhance the electron-donating ability and improve the dipole moment. Moreover, trifluoromethyl enhances the interfacial bonding of HTMs on the perovskite surface, inhibits ion movement and improves the photostability of PSCs. Compared with pDPA-SFX, meta-substituted mDPA-SFX exhibited a larger dipole moment, more ordered molecular packing and better charge transport, and the efficiency of PSCs based on mDPA-SFX was significantly improved from 22.5% to 24.8%. The corresponding cell maintained 80% of its initial efficiency under the maximum power point tracking (MPPT) of 2238 hours.

Berlinguette et al. found that the DPA group located near the highly conjugated fluorene fragment is the relevant electrochemical active unit, while the DPA group located in the heteroanthracene part tends to affect the molecular structure and thermal properties.⁶⁵ Therefore, the functional groups were introduced at the meta position of heteroanthracene to functionalize HTMs. In 2017, Jen's group designed an HTM (XPP) with para-substituted pyridine to avoid doping of t-BP and enhance the interface contact with the perovskite layer.⁶⁶ Without t-BP doping, the PCE of the XPP-based device is 17.2%, which is much higher than that of XDB (5.4%) and Spiro-OMeTAD (5.5%). In 2019, Guo et al. synthesized a low-cost spirofluoride [fluorene-9,9′-heteroanthracene]-based HTM (2mF-X59).⁶⁷ The addition of fluorine atoms can deepen the HOMO energy level and improve hole mobility and hydrophobicity. After doping with 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the PCE of 2mF-X59-based PSCs reached 18.13%. In 2021, Sun's group synthesized an HTM (P65) with 3D structure by mild crosslinking of styrene.⁶⁸ P65 has good solubility and excellent film-forming performance, and the PCE of 17.7% was achieved by dopant-free P65-based PSCs. Recently, Singh et al. synthesized SP-SMe by replacing the methoxy group of DPA with methylsulfanyl.⁶⁹ “Lewis soft” S atoms increase the interaction with the perovskite surface, and SP-SMe obtained a suitable energy level and enhanced thermal stability and humidity resistance. The maximum PCE of PSCs based on dopant-free SP-SMe is 21.95%.

The energy level, thermal properties and photoelectric properties of SFX HTMs can be easily adjusted by expanding the π-conjugation of edge plates. HTMs with TPA as the terminal group and SFX as the core were synthesized by Otsuki et al.⁷⁰ It was found that the charge recombination resistance at the perovskite/HTM interface gradually increases with the increase of TPA units on SFX, leading to an increase in V_OC. The BTPA-6-based device had achieved a PCE of 13.81%. Wang et al. synthesized SFX-FM by partially replacing anisole with dimethylfluorene.⁷¹ The π-expansion of the edge plate gives SFX-FM a suitable HOMO level (−5.24 eV) and a fairly high T_g (154 °C). In addition, SFX-FM was synthesized at a cost of $23.97 g⁻¹, which is one-twentieth of that of Spiro-OMeTAD. Recently, Yi's group also synthesized T2 by partially replacing anisole with dimethyl fluorene, but the difference was that sulfur methyl was also introduced to lower the HOMO energy level and enhance defect passivation and hydrophobicity.¹⁵ Compared with Spiro-OMeTAD, T2 not only greatly reduces the synthetic cost, but also shows better energy level matching and interfacial contact with the perovskite layer. In addition, T2 can form a strong binding force with the dopant of Li-TFSI, resulting in a uniform and smooth HTL without pinholes, which is conducive to improving the stability of PSCs. The PCE of T2-based PSCs was 26.41% (26.21% certified) for an area of 0.1 cm², 24.88% (certified) for an aperture area of 1.0 cm² and 21.45% for a large area (mask area of 14.4 cm²). Notably, 26.21% is one of the highest certified efficiencies of n–i–p PSCs so far. Most importantly, T2 has a good effect on inhibiting ion migration, which leads to its device stability better than that of Spiro-OMeTAD-based devices. Thus, T2 with low cost and high performance is expected to replace Spiro-OMeTAD and realize industrial scalable manufacturing.

Considering that the most important charge transport performance and electrochemical activity of SFX-type HTMs depend on the fluorene fragment, in order to further simplify the structure and reduce the cost, the modification of the heteroanthracene fragment was abandoned. In 2018, Sun et al. synthesized X55 with three SFX groups through a one-pot Buchwald–Hartwig cross-coupling reaction.⁷² Compared with Spiro-OMeTAD, X55 has a deeper HOMO energy level, higher hole mobility and conductivity, as well as better thermal stability and film-forming properties. The PSC device based on X55 realized a PCE of 20.8% with excellent stability. The efficiency maintained 93% of its initial efficiency after long-term aging for 6 months in a controlled atmosphere. Seok et al. only modified the fluorene fragment of SFX with a carbazole group to synthesize low-cost SFXDAnCBZ.⁷³ The device prepared from SFXDAnCBZ with low solubility (28.57 mg mL⁻¹) achieved a PCE of 20.87%. Liu et al. designed and synthesized M6-F by using a linearization strategy and conjugate engineering strategies.⁷⁴ M6-F introduces dithiopyrrole (DTP) with a coplanar skeleton as a π-bridge to improve the inherent hole mobility, hydrophobicity and defect passivation effect. On the other hand, the fluorinated SFX core ensures that M6-F has good film-forming properties. The PCEs of dopant-free and doped devices based on M6-F are 22.17% and 21.21%, respectively. Cheng's group synthesized SFX-DM-DPA by extending the π-conjugation of SFX-DM by connecting DPA units to fluorene.⁷⁵ Expanding π-conjugation improves the charge transfer ability and film-forming ability, and thus PSCs based on SFX-DM-DPA achieved a PCE of 22.7%. However, the introduction of DPA raises the HOMO energy level of SFX-DM, which leads to a mismatch with the energy level of the perovskite layer. Therefore, in order to deepen the HOMO energy level, Ding et al. introduced halogen atoms into SFX-DM to synthesize SFX-DM-F and SFX-DM-Cl.⁷⁶ As the interface layer between perovskite and HTM, SFX-DM-F and SFX-DM-Cl improve the interface contact and charge transfer efficiency. And the sandwich structure similar to HTMs also improves the effective transport of interface holes. The PCEs of SFX-DM-Cl-based PSCs were 24.8% (0.0625 cm²) and 23.1% (1 cm²), respectively. Moreover, the unencapsulated devices still maintain their initial efficiency of 94% after 2000 hours at a relative humidity of 15-20%.

4. Spiro-like HTMs

In addition to the common SBF and SFX cores, researchers have also developed a large number of HTMs with spiro-like cores in the hope of replacing Spiro-OMeTAD, as shown in Fig. 8 and Table 3.

Fig. 8 Chemical structures of spiro-like HTMs.

Table 3 Overview of properties of spiro-like HTMs

HTM	Device configuration	HOMO/LUMO [eV]	Hole mobility [cm^2 V^−1 s^−1]	V OC [V]	J SC [mA cm^−2]	FF [%]	PCE [%]	T g (°C)	Stability conditions	Times [h]	PCE [%] retention	Cost [$ per g]	Ref.
a The unencapsulated devices were stored in an N2-filled glove box or ambient air under continuous 1 sun equivalent illumination.
G1	FTO/c-TiO2/mp-TIO2/FA0.85MA0.15Pb(I0.85Br0.15)3/HTM/Au	−5.14/−2.22	1.03 × 10^−4	1.08	22.01	65.00	15.50					1.12	77
Dispiro-OBuTAD	FTO/TiO2/MAPbI3/HTM/Au	−5.16/−2.23	1.02 × 10^−5	1.08	22.79	75.00	18.46	98					78
D-OC6	FTO/c-TiO2/SnO2/PVK/HTM/Au	−5.10/−2.26	2.22 × 10^−4	1.17	25.27	83.71	24.80		In air, RH 20–30%	4000	90	10.2	79
									In air^a	400	96
CuH	FTO/c-TiO2/mp-TiO2/perovskite/HTM/Au	−5.38/−2.93	9.76 × 10^−4	1.05	21.71	69.00	15.75		In air, RH 40–45%	720	79		80
sDTG-tpa	FTO/TiO2/CH3NH3PbI3/HTM/Ag	−4.95/−2.62		1.06	19.58	70.34	14.67		In air, RH 28%	480	>75		81
Si-Spiro-MeOTAD	ITO/SnO2/FAPbI3/HTM/Au	−4.88/−2.23	1.86 × 10^−4	1.12	25.90	77.10	22.50		In N2^a	120	>90		82
KR216	FTO/c-TiO2/SnO2/PVK/HTM/Au	−5.09/−2.68	7.00 × 10^−4	1.02	22.3	77.00	17.80	157					83
H11	FTO/TiO2/perovskite/HTM/Au	−5.00/−2.08	1.58 × 10^−4	1.15	24.20	71.00	19.80						84
WH-1	FTO/c-TiO2/m-TiO2/perovskite/WH-1/Au	−5.14/−2.06	3.5 × 10^−5	1.08	23.37	77.54	19.57	107	In air, RH 50–70%	1440	87	23.6	85
BFM-C1	FTO/c-TiO2/mp-TiO2/Cs0.05(FA0.9MA0.1)0.95Pb(Br0.1I0.9)3/HTM/Au	−5.06/−2.10	1.01 × 10^−4	1.08	24.27	77.62	20.51	144	In air, RH 40–60%	720	88.6		86
									In N2, at 60 °C	384	85.5
BFM-C6	FTO/c-TiO2/mp-TiO2/Cs0.05(FA0.9MA0.1)0.95Pb(Br0.1I0.9)3/HTM/Au	−5.05/−2.08	9.71 × 10^−5	1.12	23.96	77.86	20.87	113	In air, RH 40–60%	720	90.4		86
									In N2, at 60 °C	384	86.1
2,7BCz-OMeTAD	FTO/mp-TiO2 filled perovskite/perovskite/HTM/Au	−5.15/−2.25	9.50 × 10^−5	1.08	22.38	72.50	17.60	145	In air	2040	90	14	87
2,7-BCz A4OCCF3	FTO/dense TiO2/perovskite/HTM/Au	−5.21/−2.26	1.71 × 10^−4	1.07	25.10	76.15	20.53	183	In air, RH 30%	720	92.6	34.5	88
V1366	FTO/SnO2/perovskite/HTM/Au	−4.77/−1.94	3.50 × 10^−5	1.09	24.38	79.10	21.00	173	In air	550	>98		89
BCzSPA	ITO/SnO2/perovskite/HTM/Au	−5.38/−2.38	6.03 × 10^−4	1.17	25.99	83.81	25.42	256	In N2^a	2400	82.2	28.3	38
									encapsulated, at 85 °C	2400	80.3
FDT	FTO/c-TiO2/mp-TiO2 filled perovskite/perovskite/HTM/Au	−5.16/−2.28		1.15	22.70	76.00	20.2	110				60	31
SFDT-TDM	ITO/SnO2/CsxFA1−xPbI3/HTM/MoO3/Ag	−5.25/−2.36	1.97 × 10^−4	1.13	24.10	79.50	21.70		In air, RH 35%	480	80	12.1	90
Z-W-03	FTO/c-TiO2/mp-TiO2/Cs0.045MA0.045FA0.91PbI3/HTMs/Au	−5.36/−2.30	2.24 × 10^−4	1.18	24.72	82.50	24.02	165	In N2^a	1400	95.5		91
									In air	1440	95.1
SCZF-5	FTO/TiO2/MAPbI3/HTM/MoO3/Ag	−5.17/−2.45	4.69 × 10^−4	1.11	24.40	74.00	20.10	133	In air, RH 30%	168	84		92
TPE	FTO/TiO2/(en)FASnI3/HTL/Au	−4.99/−2.22	1.50 × 10^−3	0.46	22.39	68.01	7.02	159				30	93
WP1	ITO/SnO2/perovskite/HTM/Ag	−5.24/−2.23	6.13 × 10^−4	1.15	25.29	82.80	24.04	172	In air	1728	87		94
YSH-oF	FTO/SnO2/perovskite/HTM/Ag	−5.19/−3.88	8.87 × 10^−4	1.15	25.56	80.24	23.59	180	In air	1000	88	92.2	95
									In N2, at 85 °C	500	84
									In N2^a	500	>70
FTPE-OSMe	FTO/SnO2/Cs0.05FA0.95PbI3/HTMs/Au	−5.03/−2.36	2.43 × 10^−4	1.14	26.31	83.37	24.94		In air, at 60 °C	1000	90		96
									In N2^a	800	>90

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Different from the single spiro-ring with a vertical or orthogonal structure, the di-spiro-ring usually presents an H configuration with two parallel arms. The enlargement of the π-conjugation of cores can improve the rigidity and crystallinity of HTMs. In addition, the larger π-conjugation length of di-spiro-based HTMs is expected to provide better π–π stacking, and thus improve hole mobility. In 2018, Jen's group designed and synthesized G1 with a di-spiro core.⁷⁷ The good symmetry of G1 made its solubility and film-forming properties poor, so the G1-based device achieved a low PCE of 15.5%. Differently, Dispiro-OBuTAD with butyl as the tail synthesized by Li's group showed a rigid stepped structure and a large steric hindrance, which can reduce molecular symmetry and improve amorphous properties and film-forming properties.⁷⁸ The PCE of the Dispiro-OBuTAD-based PSC was 18.46%, which was higher than that of the Spiro-OMeTAD-based device (17.82%). Considering the low cost and green solvent processability, Chen et al. designed and synthesized D-OC6 with alkoxy side chains and di-SFX skeleton.⁷⁹ Compared with the structure of di-SBF, the core of di-SFX has the advantages of safe synthesis, low cost and simple purification. The introduction of an alkoxy side chain increases the polarity of the molecule, making it easier to purify and can be processed with green solvent (2-methoxytoluene). In addition, D-OC6 can chelate with Pb²⁺ and Li⁺, thus inhibiting lead leakage and ion migration. The excellent PCE of the D-OC6-based PSC was 24.80%, while the efficiency of the perovskite solar module (the active area is 17.1 cm²) was 21.00%.

It is believed that the introduction of heavy elements can enhance the intermolecular interaction and thus improve the carrier transport performance of the films. And changing the central atom can significantly alter the photoelectric properties of HTMs. In 2018, Kloo's group synthesized a metal–organic composite HTM (CuH) in which the central atom was replaced by copper.⁸⁰ Doping a large volume of CuH into Spiro-OMeTAD can effectively improve hole mobility and inhibit self-aggregation during film deposition. The PCE of PSCs based on the composite HTM system is 18.83%, which is higher than that of the PSCs based on single CuH (15.75%) or Spiro-OMeTAD (14.47%). Ohshita et al. synthesized sDTG-tpa with a spiro-condensed dithiengermane system to enhance its conjugation.⁸¹ The high flatness of the conjugated system makes sDTG-tpa have high hole mobility. Unfortunately, sDTG-tpa-based PSCs achieved a low PCE of 14.67%. In 2023, Jang et al. synthesized Si-Spiro-OMeTAD by replacing the central carbon atom of Spiro-OMeTAD with a silicon atom.⁸² X-ray photoelectron spectroscopy (XPS) shows that Si-Spiro-OMeTAD exhibits strong intermolecular interaction, which is beneficial to the enhancement of π–π stacking and the prevention of metal ion migration. Meanwhile, the hole extraction efficiency at the Si-Spiro-OMeTAD/perovskite interface is also improved. The PSCs based on Si-Spiro-OMeTAD achieved a PCE of 22.5%, which is better than that based on Spiro-MeOTAD.

The introduction of heavy atoms to replace the central carbon atom of SBF does not seem to achieve ideal photovoltaic performance, but increases the difficulty and cost of synthesis. Therefore, researchers have developed a simple synthetic method to replace the central carbon atom of SBF and obtain HTMs with high-performance. In 2016, Nazeeruddin's group synthesized KR216 with a 9,9′-difluoroethylene core in two steps. KR216 doesn’t require an expensive synthetic process, yet has the same properties as Spiro-OMeTAD.⁸³ However, the competition between planarization and repulsive steric hindrance leads to the pseudo-helical conformation and different twist angles of the 9,9′-difluoroethylene core, which hinders the close face-to-face overlap. However, a shorter π–π distance and C··O contact are still expected to promote charge transfer. A KR216-based PSC achieved a PCE of 17.8%. Johansson et al. synthesized H11 by destroying the rigid structure of the spiro core, and changing the “monoatomic“ configuration and the “C=C” configuration into the “C–C” configuration.⁸⁴ The destruction of the rigid structure further weakened the molecular interaction between conjugated systems, which favored the formation of high-quality thin films and further promoted hole extraction at the perovskite/H11 interface. The PCE of H11-based PSCs was 19.8%, which was superior to that of H12- and Spiro-OMeTAD-based devices. Jia's group designed and synthesized WH1 by replacing the central carbon atom of Spiro-OMeTAD with cyclohexane.⁸⁵ The spatial configuration of WH1 is changed from an orthogonal conformation to a parallel arrangement, which enhances the orderly packing of molecules and thus improves the charge transfer performance. As a result, the PCE of dopant-free WH-1-based PSCs was 19.57%. Yin et al. synthesized two HTMs (BFM-C1 and BFM-C6) with intramolecular π–π interaction by adding alkyl groups to the Spiro skeleton.⁸⁶ Different from the orthogonal conformation of Spiro-OMeTAD, the single crystal results show that both BFM-C1 and BFM-C6 adopt a “face-to-face” stacking orientation with obvious intramolecular π–π interaction, which is conducive to obtaining high hole mobility. In addition, the long alkyl chains can improve membrane morphology and hydrophobicity, leading to improved charge extraction and water stability. Finally, the PSCs based on dopant-free BFM-C6 achieved a PCE of 19.06% with good long-term stability.

Carbazole is widely used in the design of HTMs because of its low cost, multiple modification sites and easy adjustment of electronic and optical properties. The N,N′-bicarbazole core that connects two carbazole units by the “N–N” bond is a vertically twisted core, which is similar to the Spiro core and has more active sites than a single carbazole. In 2018, Huang's group synthesized a low-cost 2,7 BCz-OMeTAD with N,N′-bicarbazole as the core.⁸⁷ 2,7 BCz-OMeTAD has excellent film-forming ability and matching energy level. And the 2,7 BCz-OMeTAD-based PSC (area of 1 cm²) achieved a PCE of 17.6%. Subsequently, Guo et al. synthesized 2,7-BCzA4OCCF₃ by modifying the terminals with trifluoroethoxy to deepen the HOMO level and improve hole mobility, hydrophobicity and thermal stability.⁸⁸ The 2,7-BCzA4OCCF₃-based PSC achieved a PCE of 20.53%. Getautis et al. synthesized V1366 with carbazoles as the outer end by introducing a cyclobutane between bicarbazole.⁸⁹ Rigid steric hindrance leads to the competition between planarization and repulsive steric hindrance, which makes V1366 show a pseudo-spiral arrangement and a variety of torsion angles. V1366-based PSCs achieved an excellent PCE of 21%, and the efficiency of the large area (30.24 cm²) PSC module exceeded 19%. Recently, Chen et al. designed and synthesized BCzSPA with a 3D conjugated extended “L”-type configuration by using a molecular clipping strategy.³⁸ The results of single crystal show that BCzSPA molecules can be stacked in order, realizing a dense 3D mosaic self-assembly with intermolecular coupling and multi-channel charge jump characteristics. In addition, the incorporation of F atoms promotes the multi-directional intermolecular interaction, enhances the morphological stability and improves the interface contact. Thus, the PCE of BCzSPA-based PSCs was 25.42% (certified 24.53%) for small-area (0.1 cm²) devices and 24.01% for large-area (1 cm²) devices. Notably, this is the highest PCE realized by PSCs based on dopant-free HTMs.

In addition, other types of spiral structures have been reported. In 2016, Nazeeruddin's group designed FDT with an asymmetric spiral fluorene-dithiene core by molecular engineering.³¹ FDT has the advantage of low cost and can be processed with non-halogenated solvent (toluene). Through simulation and crystallographic analysis, it was found that the additional thiophene-iodine interaction can improve the charge transfer at the perovskite/FDT interface. The FDT-based PSC devices achieved an excellent PCE of 20.2%. Zhu et al. synthesized SFDT-TDM with spirofluorene–dithioalkane as the core.⁹⁰ The small steric hindrance of the core and the intermolecular C–H⋯π interaction of SFDT-TDM are beneficial for high hole mobility. In addition, the S atom in SFDT-TDM can be used as a Lewis base to passivate the defects of perovskite films and inhibit the non-radiative recombination at the interface. The application of dopant-free SFDT-TDM to MA-free n–i–p PSCs achieved the champion PCEs of 21.7% and 20.3% for 0.04 cm² and 1.0 cm² devices, respectively. Most importantly, the cost of the synthesis of SFDT-TDM is extremely low, at $12.03 g⁻¹, far lower than that of Spiro-OMeTAD. In 2018, Jiang et al. designed and synthesized a novel carbazole-based HTM with a single helix (SCZF-5).⁹¹ The incorporation of a carbazole block hardens the screw skeleton, which leads to the enhancement of hole transport ability and the decrease in the HOMO level. SCZF-5-based PSC devices achieved a PCE of 20.10%. Recently, Shao et al. synthesized Z-W-03 with a thiophene helix.⁹² The quasi-planar core of Z-W-03 and the stacking between several CH/π and π–π molecules can improve the hole hopping. In addition, compared with the propeller-like TPA in Spiro-OMeTAD, the planar carbazole segment of Z-W-03 can improve the charge transfer more effectively, thus improving V_OC and output stability. The PCEs of dopant-free and doped devices based on Z-W-03 are 22.92% and 24.02%, respectively.

The pseudo-planar nature of the spiral structure leads to the low intrinsic hole mobility of spiro-type HTMs. Therefore, it is an important way to improve the intrinsic hole mobility by developing rigid large planar cores to enhance π–π stacking of molecules. In 2018, Kanatzidis et al. synthesized TPE with tetraphenylethylene as the core and TPA as the end cap.⁹³ The rigid planar structure of the tetraphenylene core enables TPE to have high hole mobility and does not require dopants. TPE can be used as the HTL for tin-based PSCs, and achieves a PCE of 7.23%. Recently, Hua's group also synthesized WP1 with a tetra-styrene structure, but with methylthio diarylamine as the end cap.⁹⁴ Compared to Spiro-OMeTAD, WP1 has a more matched energy level and higher hole mobility and conductivity. Femtosecond transient absorption shows that the hole transfer rate of WP1 at the perovskite/HTM interface is four times higher than that of Spiro-OMeTAD. Therefore, the PCE of PSCs increased from 22.85% (Spiro-OMeTAD) to 24.04% (WP1). Lin's group designed and synthesized a new HTM (YSH-oF) with fluorinated pentafulvalene.⁹⁵ The pentafulvalene-fused core has a large rigid π-conjugated structure, which is beneficial to charge transfer. Fluorination can enhance the π–π stacking interaction and defect passivation ability of molecules, so as to improve hole mobility and inhibit non-radiation recombination. As a result, YSH-oF-based PSCs obtained a PCE of 23.59%. Recently, Ding et al. synthesized FTPE-OSMe with a large planar dibenzo[g,p]chrysene core.⁹⁶ The peripheral methoxy group and the methylthio group enable FTPE-OSMe to have matching energy levels, high hole mobility and enhanced defect passivation ability. The PCEs of dopant-free and doped FTPE-OSMe-based devices are 22.40% and 24.94%, respectively.

5. Conclusions and perspectives

In 2012, the introduction of solid Spiro-OMeTAD solved the serious degradation problem caused by liquid electrolytes, which was an important milestone in the rapid development of PSCs.^39,40 To date, Spiro-OMeTAD is still the most important and widely used HTM in the field of n–i–p PSCs, thanks to the good solubility, film forming, energy level matching and device reproducibility of Spiro-OMeTAD. The helical orthogonal configuration of Spiro-OMeTAD enhances the morphological stability, but the electronic properties of the electroactive part remain unchanged. The advantages of this molecular structure are: (i) high stereoscopic structure to stimulate molecular rigidity; (ii) high T_g; (iii) minimized crystallization below T_g and improved amorphous stability; and (iv) weakened intermolecular π–π interaction and increased solubility.⁹⁷ However, this structure will also lead to low intrinsic hole mobility and conductivity of Spiro-OMeTAD, which needs to be improved by dopants. Unfortunately, dopants will seriously affect the long-term stability of PSCs and increase the complexity and cost of the process. Therefore, the high cost of Spiro-OMeTAD also hinders the mass production of PSCs.

To this end, researchers have developed a large number of HTMs to replace Spiro-OMeTAD. Among them, spiro-type HTMs are one type of the most promising competitors with photovoltaic performance equivalent to that of Spiro-OMeTAD. We categorized the efficient spiro-type HTMs reported in the past eight years according to similar cores and structures in an attempt to summarize the general rules of designing HTMs with high PCE, great stability and low cost. We found that:

1. Improvement of the hole mobility and conductivity. Based on the intrinsic properties of materials, hole mobility and conductivity are enhanced by expanding π-conjugation, introducing halogen atoms and adjusting structural units to increase the intermolecular π–π interaction. However, the limitations of the spiro-type structure doomed the introduction of dopants. Although some spiro-type HTMs (such as SP-SMe, SFDT-TDM, Z-W-03 and BCzSPA) can be applied to undoped PSCs, their photovoltaic performance still lags behind that of the doped devices. Therefore, the future design direction should not be limited to improving the intrinsic hole mobility and conductivity of materials, but to developing HTMs that can achieve stable doping with dopants. Just like the original Spiro-OMeTAD, the low hole mobility and conductivity can be improved by doping, leading to significant improvements in photovoltaic performance.^98,99

2. Appropriate energy level. The V_OC of the device is closely related to the HOMO level of HTMs. The HOMO level of ideal HTMs should be slightly higher than the valence band maximum (VBM) of the perovskite layer. Different perovskite compositions will change the valence band and conduction band accordingly, resulting in different requirements for the HOMO energy levels of HTMs. Therefore, proper energy level alignment between HTMs and perovskite is indeed crucial for minimizing energy losses, which is essential for achieving high V_OC. Although V_OC is also affected by other factors, including the alignment of the Fermi levels, the bandgap of the perovskite, and the quality of the interfaces. The methods for tuning the energy levels of HTMs include adjusting building blocks, expanding π-conjugation, introducing electron-absorbing groups, and doping. In addition, HTMs also need a wide band gap to block electrons and reduce back-transfer recombination loss. Fortunately, most spiro-type HTMs have this condition.

3. High quality film. The film of HTMs should be smooth, uniform and pinhole-free, which can completely cover the perovskite layer to avoid direct contact between the perovskite layer and the electrode and form good ohmic contact. The high-quality film can minimize the series resistance and thus increase J_SC of the device, which can be improved by enhancing the solubility of HTMs and using dopants. In addition, the film also needs to have high hydrophobicity to block the invasion of moisture. In order to reduce the interfacial barrier and non-radiative recombination, the thin films of HTMs need to form good interfacial contact with the perovskite surface. Lewis acids and bases can be introduced into HTMs to passivate the defects of under-coordinated Pb²⁺ and halide anions at the interface through chemical bonds or interactions. For example, S atoms were incorporated into Spiro-OSMeTAD, T2 and WP1 to form a stronger Pb²⁺–S interaction for more effective passivation of interface defects and extraction of holes. And passivation groups can also be selected as the core of HTMs, such as SFX, spirofluorene-dithiene and so on.

4. Improvement of the stability in water, oxygen, heat, light and other conditions. In fact, the stability of water and oxygen is the easiest to address, and encapsulation technology can isolate PSCs from the corrosion and damage caused by water and oxygen. Thermal stress is an important stability test for PSCs, because they may experience high temperatures (60–80 °C) due to the influence of environment, light and heat. Thermal stability is mainly to test the T_g of HTMs, especially paying attention to the fact that the addition of dopants will greatly reduce the T_g. Generally, rigid building blocks are selected to increase the T_g of HTMs. For example, fluorene and carbazole were used as terminal electron donors to make the intrinsic and doped T_g of SBF-FC as high as 222 °C and 176 °C. The light stability test is generally to track the steady-state power output at the maximum power point (MPP) under continuous illumination. The photostability of PSCs can be improved by inhibiting ion movement.

5. Low cost. In addition to efficiency and stability, another key factor limiting HTMs from realizing large-scale commercial production of PSCs is cost. The cost of Spiro-OMeTAD, which is the most widely used in n–i–p PSCs, is about $500 g⁻¹, which hinders the commercialization of PSCs. Therefore, it is necessary to reduce the synthesis cost of HTMs by simplifying the synthesis route and adopting cheap raw materials and easy purification methods. Although some spiro-like HTMs are low cost, their efficiency is much lower than that of doped Spiro-OMeTAD. Fortunately, SFX-HTMs exhibited the characteristics of low cost and excellent photovoltaic performance. As recently reported, T2-based PSCs have achieved an amazing PCE of 26.41% (26.21% certified), but the total cost of raw materials for T2 is ∼$14.0 g⁻¹, much lower than Spiro-OMeTAD.

In a word, an ideal HTM should have high hole mobility and conductivity, appropriate energy level, high quality film, good stability and low cost, as shown in Fig. 9. This is very difficult for the pristine spiro-type HTMs, and dopants are needed to help improve them. Therefore, the way forward should be to develop HTMs that can achieve stable doping of dopants, rather than blindly eliminating or reducing the addition of dopants. Finally, with the continuous improvement of the efficiency and stability of spiro-type HTM-based PSCs, we believe that it is possible to completely replace Spiro-OMeTAD in the near future. Meanwhile, we also hope that this review can provide a good reference value for the design and research of spiro-type HTMs, so as to promote the commercialization of n–i–p PSCs.

Fig. 9 Five-element golden law for developing ideal spiro-type HTMs.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 52363026 and U20A20128), the Jiangxi Provincial Natural Science Foundation (no. 20224ACB213002), the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology) (no. 2024-skllmd-06), the Open Fund for Key Lab of Guangdong High Property and Functional Macromolecular Materials (20240005) and the Jiangxi Provincial High-level and High-skilled Leading Talents Project.

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Footnote

† These authors contributed equally to this work.

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