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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 5: 613 - 632

DOI: https://doi.org/10.7536/PC231006

Review

Spiro-Type Small Molecule Hole Transport Materials in Perovskite Solar Cells

  • Ying Zhou ,
  • Xuepeng Liu , * ,
  • Xianfu Zhang ,
  • Mingyuan Han ,
  • Jianlin Chen ,
  • Yongpeng Liang ,
  • Botong Li ,
  • Yong Ding ,
  • Molang Cai ,
  • Songyuan Dai , *
Expand
  • North China Electric Power University, School of New Energy, Beijing Key Laboratory of Novel Thin-Film Solar Cells, Beijing 102206, China
* e-mail: (Xuepeng Liu)；
(Songyuan Dai)

Received date: 2023-10-15

  Revised date: 2023-12-28

  Online published: 2024-02-07

Supported by

National Key R&D Program of China(2020YFB1506400)

National Natural Science Foundation of China(61904053)

National Natural Science Foundation of China(22279033)

111 project(B16016)

Special Foundation for Carbon Peak Carbon Neutralization Technology Innovation Program of Jiangsu Province(BE2022026)

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Abstract

the performance of hole transport materials significantly influences the hole transport and electron-hole recombination in perovskite solar cells,which in turn affects the cells'efficiency.the spiro-type structure has a unique orthogonal molecular conformation.this makes the molecules form good contact on the perovskite film easily.It also leads to uniform charge transport characteristics and a higher glass transition temperature.this material has been widely used as a highly efficient hole transport material skeleton unit in perovskite solar cells.This paper summarizes the advancements in spiro-type hole transport materials,focusing primarily on the optimization of terminal functional groups and spiro-type core regulation in spiro-type small molecule materials.It discusses how changes in molecular structure impact the material’s photophysics,electrochemistry,thermal stability,hole transport characteristics,and overall performance in perovskite solar cells.Additionally,This paper forecasts future developments in This area,examining the trends and research directions of high-performance spiral-type hole transport materials。

Contents

1 Introduction

2 Spiro-type hole transporting materials

2.1 Optimization of terminal groups of spiro-type small molecule HTM

2.2 Molecular nuclear regulation of spiro-type small molecule HTM

3 Conclusion and outlook

Key words： perovskite solar cell; hole transport material; spiro-type; photovoltaic conversion efficiency

Cite this article

Ying Zhou , Xuepeng Liu , Xianfu Zhang , Mingyuan Han , Jianlin Chen , Yongpeng Liang , Botong Li , Yong Ding , Molang Cai , Songyuan Dai . Spiro-Type Small Molecule Hole Transport Materials in Perovskite Solar Cells[J]. Progress in Chemistry, 2024 , 36(5) : 613 -632 . DOI: 10.7536/PC231006

1 Introduction

Perovskite materials are considered to be excellent light-absorbing materials due to their high absorption coefficient,tunable band gap,low exciton binding energy,high carrier mobility,and long carrier diffusion length[1][2⇓~4][5,6][7,8]。 Since Tsutomu's team first used perovskite materials as sensitizers for DSSCs in 2009,the highest certified efficiency of perovskite solar cells(PSCs)with perovskite materials as light-absorbing layers has exceeded 26%after more than a decade of development[9][10]。 in recent years,PSC has become a hot research topic In the field of photovoltaics because of its wide range of materials,low cost,low-temperature solution processing,simple preparation method and high photoelectric conversion efficiency(PCE)[9,11,12]。
High-performance PSCs can be structurally divided into formal devices(n-i-p)and trans-devices(p-i-n),both of which include perovskite light-absorbing layers,electron transport materials(ETM),hole transport materials(HTM),electrodes,etc.Figure 1A below is a schematic diagram of the structure of a common n-i-p device[13]。 When light irradiates the cell,the perovskite layer is excited to generate electron-hole pairs according to the photovoltaic effect,and the electrons migrate to the conduction band of the perovskite and are transported to an external circuit through the electron transport material.Holes remain in the valence band of the perovskite and are transported to the external circuit through the HTM,forming a closed loop with the external circuit to generate current.the charge transport process of the n-i-p device is shown in Figure 1b。
图1 （a）典型n-i-p型PSC结构示意图,（b）n-i-p型PSC电荷传输示意图，（c）n-i-p结构中HTM的空穴转移过程

Fig. 1 (a) Schematic diagram of a typical mesoporous n-i-p type PSC structure. (b) Schematic diagram of charge transfer of n-i-p type PSC. (c) The hole transfer process of HTM of n-i-p type structure

As an important part of perovskite solar cells,HTM is used to extract and transport the photogenerated holes generated by the perovskite absorption layer to the counter electrode,prevent the reverse transport of electrons,and protect the perovskite layer from the influence of moisture and oxygen in the air and the diffusion of metal electrodes.For trans devices,the perovskite layer needs to be deposited on the HTM,which also has the effect of influencing and improving the morphology and crystallization of the perovskite layer[14,15]。 Although HTM-free PSC also exists,the efficiency is generally low[16]。 as shown in fig.1C,there are two processes for the HTM to extract the photogenerated holes generated by the perovskite layer and transfer these photogenerated holes to the electrode in the formal device:(1)the first process is to extract the holes to the perovskite/HTM interface,which is mainly determined by the highest occupied molecular orbital(HOMO)energy level of the HTM and the contact with the perovskite layer,and requires a good ohmic contact with the perovskite layer;(2)the second process is to transport the photogenerated holes to the electrode through the HTM,in which the HTM is generally required to have a high hole mobility.It can be seen that the series resistance of the device can be reduced by using a thinner HTM layer.the thickness of HTM with low hole mobility can be reduced as much as possible,but the surface of perovskite polycrystalline film fluctuates greatly(generally 10~50 nm),so the HTM needs to cover the perovskite layer completely to ensure no leakage[17⇓~19]。 in the trans device,the HTM only needs to cover the TCO with small roughness,and can be prepared very thin(even self-assembled to form a monolayer),so some HTM with strong hole extraction ability but low hole mobility can show better performance In the trans device[20⇓~22]。
In recent years,researchers have done a lot of exploration on the development and design of HTM.HTM has been proved to be one of the key factors to improve the efficiency and stability of PSCs.Obtaining high-performance HTM is an essential process to realize the commercialization of PSCs.At present,the reported HTM mainly includes inorganic materials,Small organic molecules,organic polymers and organometallic complexes.small organic molecules have been widely studied because of their precise molecular weight,easy solution preparation,easy molecular structure control,and generally high efficiency[23,24]。 the small organic molecule HTM can be divided into three categories according to The structural characteristics of its central core:linear,star,and spiro,as shown in Figure 2[25]。 linear HTM is mainly formed by two peripheral groups bonded to the middle connecting unit,and the whole is Linear.This kind of molecule has simple structure,and the common design strategy is D-A-D,D-π-D,D-A-π-A-D structure(where D is an electron-rich donor,A is an electron-deficient acceptor,andπis a conjugated connecting bridge)[26⇓~28]; Star-shaped HTM has three or more groups on the periphery and a non-helical structure in the central core[29,30]; The intermediate core of spirocyclic HTM is a spirocyclic structure with an orthogonal shape。
图2 有机小分子HTM的三种典型分子结构

Fig. 2 Three typical molecular structures of organic small molecule HTM

In general,an excellent HTM should have the following conditions:(1)a HOMO level higher than the valence band of the perovskite layer.In general,the closer the HOMO level of the HTM is to the perovskite material,the less energy is lost to transport holes,and the higher the open-circuit voltage（Voc）of the corresponding device,as well as a higher lowest occupied molecular orbital(LUMO)level to prevent the reverse transport of electrons;(2)high hole mobility or excellent hole extraction ability to meet the most basic requirements for hole extraction;(3)good solubility in common solvents to meet the requirements of solution preparation;(4)（Tg）with high glass transition temperature and excellent light,heat and chemical stability;(5)For the formal device,the HTM is required to have good hydrophobicity to ensure the humidity stability of the device,while for the trans device,the HTM surface is required to have good wettability to the perovskite precursor,which can reduce the energy barrier for nucleation and promote the crystallization of perovskite;(6)Because sunlight will first pass through the HTM of the trans device before reaching the perovskite light absorption layer,the HTM of the trans device is required to have high transmittance to reduce the parasitic absorption of the HTM;(7)In order to be suitable for the growth of the upper perovskite grain,the trans device needs a certain degree of solvent resistance;(8)Finally,HTM requires low cost,easy synthesis,and commercial mass production.Spirocyclic molecules have good photoelectric properties due to their special molecular structure,and their molecular properties can be regulated by molecular tailoring to meet some of the conditions of excellent HTM.At the molecular level,researchers have optimized the properties of helical HTM molecules by regulating the core or peripheral substitution groups,and achieved good research results.Spirocyclic HTM is also by far the most widely used material with excellent properties.In this paper,the effects of structural changes of spiro HTM on molecular properties and device performance in recent years are summarized,which is expected to provide a reference for the research and development of high-performance HTM in PSC,other thin film solar cells and high-performance carriers in various optoelectronic devices 。

2 Spiro hole transport material

Spirocyclic molecules,such as 2,2',7,7'-tetra[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene(spiro-OMeTAD),which is the most commonly used in PSC,have a central core with a cruciform molecular configuration formed by two fluorene units connected by sp3hybridized carbon.The terminal functional groups are four symmetrical methoxydiphenylamines connected to the central spirocyclic nucleus.The molecular structure and the geometry optimization structure based on density functional theory(DFT)calculation are shown in Figure 3A,B below,which shows its unique orthogonal molecular conformation 。
图3 （a）spiro-OMeTAD的结构式;（b）DFT计算的几何构型;（c）spiro-OMeTAD的合成路线

Fig. 3 (a) Chemical structure of spiro-OMeTAD. (b) Geometric configuration for DFT calculation. (c) The synthetic route for spiro-OMeTAD

Spiro molecules usually have the following advantages:(1)they are amorphous at room temperature,their isotropic physical properties ensure the uniformity of hole transport,and their glass transition temperature（Tg）is high,which is helpful to obtain stable films;(2)the non-planar spiro structure has weak intermolecular interaction force and good solubility,is easy to prepare a film by a solution method,and can better fill a perovskite layer;(3)the conjugated system with strong rigidity of the molecules can reduce the aggregation of the molecules in the film forming process so as to form a smooth amorphous film morphology and form good mutual contact with the perovskite layer;(4)The HOMO energy level can basically match well with the valence band of perovskite through molecular modification,etc[25,31⇓⇓ ~34]。 The synthetic route of spiro-OMeTAD is shown in Figure 3C,including Buchwald-Hartwig cross-coupling reaction,Grignard reaction,cyclization reaction and bromination reaction[33,35⇓ ~37]。
as shown in fig.4,with the improvement of PSC efficiency year by year,spiro-OMeTAD with spiro structure is almost dominant in high efficiency PSC.It was first used As a solid-state electrolyte for dye-sensitized solar cells[38]。 in 2012,Kim et al.Used spiro-OMeTAD as the HTM of PSC,which increased the photoelectric efficiency to 9.7%.Since then,spiro-OMeTAD has become a star molecule In PSC by an absolute advantage[39]。
图4 美国可再生能源国家实验室认证钙钛矿太阳电池效率[10]及采用的HTM

Fig. 4 Certified efficiency of perovskite solar cells in National Renewable Energy Laboratory in the United States and the adopted HTM[10]

Although the spiro structure makes the molecule have many advantages,it is also because of the non-planar structure of the molecule,which is not conducive to molecular stacking and reduces the hole mobility,so it is necessary to introduce dopants to enhance the hole mobility[36,40]。 For example,spiro-OMeTAD,the commonly used dopants are mainly 4-tert-butylpyridine(TBP)and lithium bis(trifluoromethylsulfonyl)imide(Li-TFSI).Although spiro-OMeTAD itself has good photothermal stability,the introduction of these chemical dopants reduces the stability of the device and increases the complexity of the process.Li-TFSI has hygroscopicity,which can accelerate water entering perovskite and induce perovskite degradation[41,42]; Lithium ions will diffuse in the battery,further reducing performance and stability;The adverse effect is aggravated by high temperature,and the Tgof spiro-OMeTAD containing dopants is significantly reduced[43⇓~45]; TBP will also accelerate the degradation of perovskite through the formation of complexes[46]。 Recently,the introduction of new additives to stabilize spiro-OMeTAD films has also become a research hotspot in the field[47⇓~49]。
The structure of organic molecules has a great impact on their characteristics.In order to further improve the performance of HTM,many research teams have tried to design analogs of spiro-OMeTAD by optimizing the structure of spiro-core molecules,so as to further improve the performance of HTM while ensuring the inherent advantages of spiro-OMeTAD.For example,designing materials with better compatibility with dopants to reduce instability caused by the presence of dopants;design and develop new central spiro structures or optimize the synthesis steps to achieve the purpose of reducing costs.the design optimization of spirocyclic HTM can be roughly divided into two strategies:terminal group optimization and spirocyclic core structure regulation。

2.1 Optimization of Spiro Small Molecule HTM Terminal Group

The optimization of the terminal group of spirocyclic small molecule HTM refers to focusing on the terminal diphenylamine without changing the central core of spiro-OMeTAD,namely 9,9'-spirobifluorene(SBF),and optimizing its structure,including changing the substitution position,introducing heteroatoms,and introducing new functional groups to regulate the molecular characteristics.In recent years,controlling the terminal substituents of spiro-OMeTAD has been widely used as a method to improve the performance of PSCs.In general,controlling the terminal substituents of spiro-OMeTAD can reduce the symmetry and increase the molecular weight,increase the molecular Tg,and improve the film stability;The HOMO energy level of the molecules is related to the introduction of new functional groups to donate/withdraw electrons,but the Vocof some molecules is also comparable to that of spiro-OMeTAD by improving the contact and other means.However,terminal regulation generally has little effect on molecular absorption and little effect on band size.Table 1 lists the hole mobility,HOMO energy level,and optoelectronic performance parameters of some reported high-performance HTMs based on terminal group optimization,and Figures 5 to 7 show the molecular structures of these HTMs 。
表1 Photovoltaic performance parameters of end-optimized spiro HTMs in PSCs.

Table 1 Photovoltaic performance parameters of terminal optimized spiral HTMs in PSCs

HTM perovskite HOMO
(eV)		 μ
(×10-4 cm2·V-1·s-1)		 Voc
(V)		 Jsc
(mA·cm-2)		 FF
(%)		 PCE
(%)		 Ref
po-spiro-OMeTAD MAPbI3 -5.22 NRa 1.02 21.2 77.6 16.7 51
2,4-spiro-OMeTAD MAPbI3 -5.24 NR 0.956 25.6 70.1 17.2 54
spiro-cyclOMe Cs0.05FA0.95PbI3 -5.09 22.5 1.18 24.86 79 23.10 55
spiro-S MAPbI3 -4.92 0.126 1.06 19.15 78 15.92 56
spiro-OSMeTAD Cs0.05(FA0.85MA0.15)0.95
Pb(I0.85Br0.15)3		 -5.18 NR 1.16 22.81 76 20.18 57
SF48b Cs0.05(FA0.85MA0.15)0.95
Pb(I0.89Br0.11)3		 -4.83 0.17 1.09 22.6 76 18.7 58
spiro-mFc FAPbI3 -5.19 74.7 1.16 26.35 80.9 24.82 59
spiro-4TFETAD (FAPbI3)0.97(MAPbBr3)0.03 -5.32 2.04 1.17 24.31 74.26 21.11 60
spiro-OEtTADc MAPbI3 -5.09 4.76×10-5 1.11 23.93 75.65 20.16 61
spiro-TTB MAPbI3 -5.3 19.7 1.07 22.02 78 18.38 62
spiro-Me-2 CsFA0.9Pb(I0.9Br0.1)3 -4.55 about 50 1.109 22.57 68.7 17.2 63
Spiro-Acid Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 -5.02 NR 0.99 22.20 82.6 18.15 64
DM (FAPbI3)0.95(MAPbBr3)0.05 -5.27 NR 1.14 24.9 81 23.2 65
SC (CsPbI3)x(FAPbI3)y(MA
PbBr3)1-x-y		 -5.26 31.5 1.15 23.47 80.62 21.76 66
spiro-DBF (CsPbI3)0.05(FAPbI3)0.95 -5.22 63.1 1.12 24.21 79 21.43 67
V1267 (FAPbI3)0.87(MAPbBr3)0.13 6.96 1.06 23.41 74 18.3 68
spiro-carbazole Cs0.05FA0.95PbI3 -5.15 64.8 1.18 24.56 76 22.01 69
SBF-FC FAPbI3 -5.06 0.12 1.18 25.9 80.9 24.7 70
spiro-Naphc FAPbI3 -5.05 80.84 1.16 25.97 80.67 24.43 71
spiro-O27 MAPbI3 -5.16 49.96 1.07 22.07 70 16.6 72
DP Cs0.05MA0.05FA0.9PbI3 -5.18 51.9 1.138 26.13 84.9 25.24 73
spiro-tBuBED MAPbI3 -5.3 2.29 1.102 22.99 73.5 18.6 74
spiro-OMeIm (FAPbI3)0.85(MAPbBr3)0.15 -5.13 2.2 1.10 20.46 76 17.10 75
V1307 [(FAPbI3)0.87(MAPbBr3)0.13]0.92
(CsPbI3)0.08		 -5.46 1.4 1.073 23.21 77 19.2 35
CF-Sp-BTh GO(0.5wt%)-MAPbI3 -5.38 96.5 1.07 18.82 71 14.28 76
spiro-PTb MAPbBr3 -5.66 1.68 1.41 7.42 75.23 7.36 77

aNR=not reported ; bDopant-free ; cThe hole mobility of the HTM is obtained through the ITO/PEDOT:PSS/perovskite/HTM/Au structure 。

图5 对spiro-OMeTAD末端甲氧基调控、引入烷基或杂原子官能团调控的HTMs分子结构

Fig. 5 Molecular structures of HTMs regulated by terminal methoxy modulation of spiro-OMeTAD, introduction of alkyl or heteroatom functional groups

图6 采用苯并（杂）环替代spiro-OMeTAD末端苯甲醚的分子结构图

Fig. 6 Molecular structures of spiro-OMeTAD-terminated anisole using benz(hetero)ring substitution

图7 其他末端基团优化的分子结构

Fig. 7 Other terminal-optimized molecular structures

In 2014,Seok et al.,combined with Hammett's research work,found that different substitution positions of methoxy group on the benzene ring of spiro-OMeTAD structure may show different chemical characteristics(meta-electron-withdrawing effect,para-electron-donating effect,and ortho-position will affect the oxidation potential through steric effect)[51][50]。 by changing the position of the methoxy group at the end of spiro-OMeTAD,They Developed two derivatives of spiro-OMeTAD to explore the effects of ortho(o-OMe),meta(m-OMe),and para(p-OMe,the traditional spiro-OMeTAD)methoxy groups on material properties.they found that the ortho-substituted po-spiro-OMeTAD showed better performance than the para and meta,because the steric effect of the ortho-substituted derivative increased the LUMO energy level and better prevented electrons from entering the metal electrode,resulting In higher parallel resistance.the highest PCE of the device was 16.7%,which was the highest efficiency of the device based on spiro-OMeTAD at that time.in 2015,Getautis et al.developed HTM-1 By introducing a methyl group at the ortho position of the methoxy group at the end of spiro-OMeTAD half anisole,and applied it to dye-sensitized solar cells to study the relationship between molecular structure and solar cell performance[52]。 they found that spiro-OMeTAD with high symmetry is easy to crystallize(especially when the film is prepared by solution method),which destroys the contact between spiro-OMeTAD and the light absorption layer,resulting in reduced device performance.the methyl-introduced HTM-1 breaks the high symmetry of the molecule and inhibits the molecular crystallization,thus improving the device stability.the lifetime of the device based on HTM-1 at high temperature is significantly improved,and the initial efficiency is maintained at about 90%after 1000 H at 60℃,while the device based on spiro-OMeTAD degrades rapidly under the same conditions.Subsequently,They further demonstrated that the asymmetric introduction of methyl groups can improve the hole mobility of the molecule[53]。 the subsequent terminal regulation of spirocyclic HTM In PSCs is Also mainly based on the introduction of asymmetric groups to enhance molecular stability.in 2017,Chen et al.also found that the change of methoxy position would affect the HOMO and LUMO energy levels.For this reason,they removed the methoxy group on the terminal half of the anisole of spiro-OMeTAD,and introduced a new methoxy group at the meta or ortho position of the methoxy group on the other part of the anisole,and designed two new compounds,2,4-spiro-OMeTAD and 3,4-spiro-OMeTAD[54]。 Among them,the energy level of 2,4-spiro-OMeTAD is more matched with perovskite materials,and the device efficiency based on it is much higher than that of spiro-OMeTAD In the same batch.in 2023,our group introduced a new methoxy group to the ortho position of methoxy group on spiro-OMeTAD partial anisole to prepare a new compound spiro-biOMe,and cyclized two methoxy groups on the basis of spiro-biOMe to design another compound spiro-cyclOMe[55]。 The cyclized spiro-cyclOMe showed more excellent hole transport ability and higher Tg.Finally,the spiro-cyclOMe based device achieved a champion PCE of 23.10%and showed good device stability 。
In 2016,Huang et al.Introduced functional groups containing heteroatoms into HTM[56]。 They replaced the methoxy group at the end of spiro-OMeTAD with ethyl,methylthio and N,N-dimethyl,respectively,and synthesized three new HTMs,spiro-E,spiro-S and spiro-N.Compared with spiro-OMeTAD,spiro-E and spiro-S have lower HOMO energy levels,which make them better matched with the perovskite layer,and show better hydrophobic properties(resulting In larger perovskite grains and better crystallinity).spiro-S also has a slightly higher hole mobility than spiro-OMeTAD,and the device performance is better than that of spiro-OMeTAD and other compounds as HTM devices.in 2022,our group synthesized spiro-SMeTAD(spiro-S)and spiro-OSMeTAD by partially or completely replacing the methoxy group at the end of spiro-OMeTAD with methylthio[57]。 spiro-OMeTAD was used as a reference to study the effect of heteroatoms on the molecular properties.the methylthio group can adjust the HOMO energy level of the molecule,and the HOMO energy level of the molecule with more methylthio groups is lower.Ultimately,due to the poor incorporation of spiro-SMeTAD with dopants and poor morphology,the device based on spiro-SMeTAD has a low efficiency,while the device based on asymmetric terminal hemimethylthio hemimethoxy is finally optimized to achieve an efficiency of more than 20%.In the same year,Murakami et al.Synthesized two new HTMs,SF27 and SF48,by introducing cyano groups(—CN)at the meta and ortho positions of the N,N-dimethyl group of spiro-N(Huang et al.Proved that the performance of spiro-N was general)[58][57]。 Cyano group is considered to be a group with strong electron-withdrawing ability,and the introduction of cyano group can reduce the HOMO energy level of materials;It also induces dipoles,which enhance intermolecular interactions and promote charge transfer;The cyano group can also react with the uncoordinated Pb2+in the perovskite,resulting in a passivation effect.Finally,under low concentration conditions,the device based on dopant-free SF48 achieved a champion PCE of 18.7%,slightly higher than that of spiro-OMeTAD(18.6%)with dopants,and the thermal stability of the device was also significantly improved 。
In 2020,Yang et al.Designed and synthesized two fluorides of spiro-OMeTAD(spiro-mF and spiro-of)by introducing fluorine atoms at the ortho or meta position oF the methoxy group on the terminal half oF spiro-OMeTAD[59]。 Due to the induced dipole along the C—F bond after fluorination,the HOMO energy level is lowered,the device Vocloss is reduced,and the molecular packing and hydrophobicity are enhanced.spiro-mF as a HTM fabricated device achieved a champion PCE of 24.82%,a Vocof 1.16 V,a Jscof 26.35 mA·cm-2,and a FF of 80.90%,which were the highest efficiencies reported for PSCs at that time.Moreover,the efficiency of the device can still maintain 87%of the initial value after 500 H under wet conditions without packaging.The device performance oF spiro-oF as HTM is not as good as that of spiro-mF,which can be attributed to the steric hindrance caused by the ortho fluorine atom,thus inhibiting the intermolecular interaction.In 2021,Yan et al.Retained the advantages of methoxyl group in controlling the oxidation potential and hole transport ability of materials based on the introduction of fluorine in adjusting energy level,promoting charge transport,improving perovskite phase stability and reducing defects.A new substituent,trifluoroethoxy,composed of alkoxy and fluorine-containing groups was designed.Compared with spiro-OMeTAD,the synthesized spiro-4TFETAD has lower HOMO energy level,higher thermal stability and hole mobility,which improves the device performance[60]。
the introduction of alkyl groups into the terminal group of HTM is also one of the ways to adjust the molecular characteristics.Considering that different carbon chains will produce different molecular packing,in 2019,Li et al.Introduced alkyl chains with different chain lengths into the end of the molecule,and designed spiro-OEtTAD(spiro-E),spiro-OPrTAD,spiro-OiPrTAD and spiro-OBuTAD with different alkoxy chain lengths[61]。 It is found that different alkyl chains lead to different properties of molecules,and the device based on spiro-OEtTAD has the best performance,achieving a champion PCE of 20.16%(higher than 18.64%of spiro-OMeTAD).in the same year,Mai et al.Used three spiro compounds(spiro-TTB,spiro-TAD,spiro-OMeTAD)for trans devices,In which spiro-TTB replaced the methoxy group at the end of spiro-OMeTAD with methyl group,and all three compounds showed hydrophilic surface properties,proving the possibility of depositing perovskite films on them[62]。 Finally,spiro-TTB is the best HTM among the three with a PCE of 18.38%and a high Vocof 1.07 V,which is also one of the commonly used HTMs in perovskite/crystalline silicon tandem solar cells.In 2020,Sini et al.Considered that the two factors affecting the hole mobility were the molecular dipole moment(negative effect)and the intermolecular interaction strength(positive effect).The triphenylamine of spiro-OMeTAD was the key to enhance the intermolecular interaction,but the eight methoxy groups could induce the molecular dipole moment,which had a negative effect on the hole mobility[63]。 to this end,they synthesized two new materials,spiro-Me-1(same as spiro-TTB)and spiro-Me-2,using methyl instead of methoxy.the only difference between the two compounds lies in the number and position of methyl groups.the two new compounds do not enhance the dipole moment of the molecule,and the intermolecular interaction force is stronger and the polarity is smaller.their hole mobility is at least five times higher than that of spiro-OMeTAD.However,based on Their low device efficiency,the authors believe that the methoxy group on spiro-OMeTAD is more conducive To the perovskite layer/HTM interface bonding,resulting in higher device performance。
Due to the three-dimensional structure of spiro-OMeTAD,it is difficult to transfer charge between molecules.Palomares et al.Introduced carboxylic Acid into the molecule to synthesize Spiro-Acid,which can form a self-assembled monolayer on the surface of ITO.Without additives,the PCE of p-i-n devices is equivalent to that of PTAA,and the stability is significantly improved[64]。
the above molecules retain the spiro-OMeTAD skeleton structure,adjust the terminal methoxy group or introduce other functional groups,and replace the anisole at the end of spiro-OMeTAD with a larger volume of functional groups has also been proved to be an effective way to improve the photovoltaic performance parameters of devices.In 2018,Seo et al.Designed and synthesized DM by partially replacing the anisole of spiro-OMeTAD with 9,9-dimethylfluorene[65]。 The weak electron-donating ability of fluorene structure lowers the HOMO energy level(-5.27 eV)and improves the Vocof the device,and the device with DM as HTM has a champion PCE of 23.2%.Moreover,the Tgof DM is also significantly increased to 161℃,nearly 40℃higher than that of spiro-OMeTAD,and the resulting device also shows better thermal stability,maintaining 95%of the initial efficiency at 60℃for 500 H.It is worth noting that the hole mobility of DM is also lower than that of spiro-OMeTAD,and the device still shows good efficiency 。
Carbazole units have become a frequent visitor to HTM design because of their low cost,excellent charge transport ability and ease of modification with a variety of functional groups.In 2020,Chen et al.Introduced extendedπ-conjugated structures with weak electronic capabilities,N-ethylcarbazole and dibenzothiophene,to replace anisole on spiro-OMeTAD,and named the two new materials SC and ST,respectively[66]。 Due to their weak electron-donating properties,the HOMO energy level is reduced and the Vocof the device is improved.The ethyl group in SC increases the solubility of the molecule and reduces the crystallinity,which makes the molecule better miscible with Li-TFSI and suppresses the unfavorable phase separation.The solubility of ST is poor.Finally,the highest efficiency of the device prepared by SC as HTM reaches 21.76%.In 2022,our research group considered that dibenzofuran and dibenzothiophene may have excellent carrier transport ability and strong interaction with perovskite,and introduced them into the molecular structure to replace part of anisole on spiro-OMeTAD,named spiro-DBF and spiro-DBT,respectively[67]。 It was found that spiro-DBF showed higher conductivity,better film forming ability and hole migration ability,and finally the device fabricated by spiro-DBF as HTM achieved a champion efficiency of 21.43%.In the same year,Nazeeruddin et al.Also designed compounds containing only carbazole and semi-carbazole semi-anisole(V1267,V1240)with carbazole group as the terminal group[68]。 Among them,the device prepared by V1267 with only carbazole terminal has the most excellent PCE.In the same year,inspired by the above research,our research group prepared the SC and V1240 reported above.First,we compared the effects of carbazole substitution sites,and further determined that SC had better device performance.Then,we designed and synthesized spiro-carbazole with 2-ethylcarbazole at the end of the molecule[69]。 spiro-carbazole as HTM shows more excellent performance,especially the Tgis up to 196˚C.The device made of spiro-carbazole instead of spiro-OMeTAD has a PCE of 22.01%,and the device has excellent thermal stability and humidity stability.In 2023,Wang et al.Designed and synthesized SBF-FC by replacing the diphenylamine unit in spiro-OMeTAD with highly asymmetric fluorenylcarbazole amine as the electron donor,and replaced the traditional LiTFSI with 4-tert-butylpyridinium bis(trifluoromethanesulfonyl-imide,TBPHTFSI)as the dopant[70]。 The composite prepared by blending SBF-FC and TBPHTFSI at a weight ratio of 85:15 showed a room temperature conductivity of Tgand 49μS·cm-1up to 176°C.Finally,the SBF-FC based device achieved a champion PCE of 24.7%,and an average efficiency of 24.5%and good long-term thermal stability at 85℃aging condition.In 2022,Yang et al.Designed spiro-Naph by replacing the symmetrical diphenylamine in spiro-OMeTAD with an unsymmetrical phenylnaphthylamine[71]。 Since the naphthalene unit is larger than the benzene unit,this design can enhance theπ-electron delocalization,effective conjugation length,and structural stability of the molecule.The PSC prepared as HTM achieved a champion PCE of 24.43%,and the device also had excellent stability.More importantly,the spiro-Naph based large area device achieves a PCE of 21.83%（25 cm2） 。
the above results show that the partial or complete replacement of the terminal anisole structure of spiro-OMeTAD by benzo(hetero)ring structure can effectively improve the performance of PSC devices,In addition,the partial or complete replacement of anisole by other groups has also been proved to be effective.in 2016,Hao et al.Designed a new spiro compound spiro-O27 by replacing two 4,4′-dimethyldiphenylamine at the 2′and 7′positions of the traditional spiro-OMeTAD with two tert-butyl groups[72]。 the molecular structure of spiro-O27 is simpler,and the change of molecular structure does not change the energy level and thermal stability,but enhances the hole mobility,and the synthesis conditions are not stringent,so the device performance based on spiro-O27 is even better than that of spiro-OMeTAD under the same conditions。
Considering that the extended spiro-OMeTAD terminal conjugated structure reported in the literature can improve the device performance,in 2023,our group partially replaced the four anisole units on spiro-OMeTAD with a simple 4-methoxybiphenyl to expand theπ-conjugated system and designed and synthesized DP[73]。 Compared with spiro-OMeTAD,the molecule has better hole mobility,conductivity and stability.The DP-based small-area（0.06 cm2）device achieves a PCE of 25.24%,and the device with a packaged area of 27.86 cm2achieves a certified efficiency of 21.78%.More importantly,the device has more superior stability,with the DP-based device retaining 95%and 81%of its initial performance under ISOSL-1(packaged device measured under continuous illumination with a white LED lamp of 100 mW·cm2at MPP tracking and ambient conditions)and ISOS-L-3(packaged device measured under continuous illumination with a white LED lamp of 100 mW·cm2at MPP tracking and 65˚C,85%RH),respectively,which is significantly better than the device performance of spiro-OMeTAD 。
Spirocyclic compounds are mostly synthesized by cross-coupling reaction,and the high cost of synthesis and complex purification process force researchers to find better synthetic methods.In 2019,Liu et al.Designed a new compound spiro-tBuBED by introducing 3,4-ethylenedioxythiophene at the periphery of spirobifluorene nucleus and replacing the terminal methoxy group with n-butyl group[74]。 Considering the complexity of the traditional synthesis process of spiro HTM,they adopted a new synthesis method,through direct C—H bond activation/arylation,without cumbersome raw material metallation,the target molecule can be easily synthesized In only two steps.Moreover,the spiro-tBuBED-based device does not require a time-consuming oxidation process,and the final device PCE is 18.6%.in 2020,Pilkington et al.Introduced the imidazole group into HTM,replaced the four diphenylamine moieties at the end of spiro-OMeTAD with diphenyl imidazole,and designed and synthesized spiro-OMeIm[75]。 Compared with the traditional spiro-OMeTAD,the preparation of the target precursor 2-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazole does not need a catalyst and a time-consuming purification process,so the synthesis process is easy and the cost is low.spiro-OMeIm has a suitable HOMO energy level(−5.13 eV)and is less negatively affected by the dopant Li-TFSI,thereby favoring increased stability.In 2021,Nazeeruddin et al.Synthesized a series of spiro compounds(V1305,V1306,V1307,V1308)with different numbers of enamine groups as terminals by condensation reaction[35]。 This condensation reaction does not require expensive catalysts,inert reaction conditions,and time-consuming product purification procedures,and the only by-product is water.This undoubtedly makes the synthesis process more green and environmentally friendly,and the synthesis cost is much lower than that of the commonly used spiro-OMeTAD(the synthesis cost of V1307 is~37$/G).This enamine-terminated compound has a higher degree of conjugation,which makes it have better charge transport ability,and can also reduce the HOMO energy level,which is better matched with the valence band of perovskite.They were used to fabricate perovskite solar cells,and the four-arm enamine-terminated V1307 showed the best performance(PCE=19.2%),which was comparable to that of spiro-OMeTAD(PCE=19.7%).In 2019,Shin et al.Introduced 3,5-bis(trifluoromethyl)benzene as an electron acceptor at the end of spirobifluorene to adjust the HOMO energy level,and designed a new HTM,CF-Sp-BTh,which was used as the HTM of trans devices[76]。 A PCE of 14.28%was obtained for the trans-device fabricated using CF-Sp-BTh and graphene oxide(GO)-modified perovskite（MAPbI3）with an optimal concentration of 0.5 wt% 。
Wide-gap perovskite solar cells have attracted more and more attention.They have high Vocand are often used to prepare tandem solar cells.However,common HTM is difficult to match with wide band gap perovskite materials.In 2021,Qin et al.Introduced N-phenothiazine into the end of the spiro-core and designed a new HTM,spiro-PT[77]。 The HOMO energy level of this HTM is very low,−5.66 eV,which can match the valence band(−5.68 eV)of MAPbBr3with a wide band gap,and it can effectively suppress non-radiative recombination.The device fabricated with dopant-free spiro-PT as the HTM achieves a Vocof 1.41 V and a PCE of 7.36%.Moreover,the device still retains 91%of the original efficiency under continuous illumination for 500 H without packaging 。
The above summary shows that some new functional groups partially or completely replace the anisole structural unit on spiro-OMeTAD to form a new spiro-HTM.The introduction of these functional groups can control the molecular energy level,Tg,hole mobility,film morphology,etc.Some of the new HTM show better device performance than spiro-OMeTAD,indicating that spiro-OMeTAD end control is an effective way to improve device performance 。

2.2 Nuclear regulation of spiro small molecule HTM

the cost of spiro-OMeTAD synthesis is high(especially spirobifluorene nucleus),which requires strict reaction procedures,inert reaction conditions,expensive metal catalysts,time-consuming purification process(column chromatography purification)and so on.However,they do not want to abandon its excellent 3D spiro structure,so researchers began to look for new HTM with spiro structure,but different from spirobifluorene,The core is easier to synthesize or has better performance[33,35⇓ ~37]。 This is the molecular nuclear regulation of the spiro small molecule HTM。
the spiro[fluorene-9,9'-xanthene](SFX)is one of the most popular new spiro nuclear structures.Different from the complex synthesis and purification process of spirobifluorene and the high synthesis cost,the SFX core only needs phenol and 9-fluorenone as raw materials to produce parallel fluorene and xanthine units through a palladium-catalyzed one-step Buchwald-Harting cross-coupling reaction.the synthesis routes of the corresponding 2Br-SFX and two 4Br-SFX are shown in Figure 8 below,and the synthesis yield is high without column chromatography for purification[37,78⇓ ~80]。 this aspect has been discussed in detail in the relevant reviews,and only a brief overview is given in This paper.the photovoltaic performance parameters and molecular structures of the relevant HTMs are shown in Figure 9 and Table 2[33,81]。
图8 2Br-SFX和4Br-SFX的合成路线

Fig. 8 Synthetic routes for 2Br-SFX and 4Br-SFX

图9 以SFX作为核心或连接基团的HTMs的分子结构

Fig. 9 Molecular structures of HTMs with SFX as the core or connecting group

表2 Photovoltaic performance parameters of HTMs with SFX as core or linking group

Table 2 Photovoltaic performance parameters of HTMs with SFX as core or linking group

HTM perovskite HOMO
(eV)		 μ
(×10-4 cm2·V-1·s-1)		 Voc
(V)		 Jsc
(mA·cm-2)		 FF
(%)		 PCE
(%)		 Ref
X59 (FAPbI3)1-x(MA
PbBr3)x		 -5.15 0.55 1.13 23.4 73 19.8 79
X60 (FAPbI3)1-x(MA
PbBr3)x		 -5.15 1.9 1.14 24.2 71 19.84 37
X55 FA0.85MA0.15Pb
(I0.85Br0.15)3		 -5.23 6.81 1.15 23.4 77 20.8 82
X26 FA0.85MA0.15Pb
(I0.85Br0.15)3		 -5.08 4.31 1.11 24.3 75 20.2 83
HTM-FX’ Cs0.05FA0.81MA0.14
PbI2.55Br0.45		 -5.16 4.8 1.17 21.7 78 20.8 80
SFXDAnCBZ (FAPbI3)0.95(MA
PbBr3)0.05		 -4.945 4.28 1.09 23.1 83 20.87 84
M6-F MA0.16FA0.84PbI3 -4.99 2.1 1.154 24.45 78.56 22.17 85
mCl-SFXDA Cs0.05FA0.75MA0.20
Pb(I0.96Br0.04)3		 -5.18 1.6 1.14 25.25 75.21 21.34 86
SFX-3 FAPbI3 -5.28 4.86 1.16 25.64 75.51 22.42 87
in terms of HTM research in PSC,in 2016,Hagfeldt and Sun used SFX core as the core unit of HTM,and designed X59 and X60 with triphenylamine as the end.Later,their team further optimized the structure and designed X55 and X54;X26,X36,etc.the 3D spiro structure similar to spiro-OMeTAD enhances the rigidity of the molecule,and the triphenylamine group generates hole transport characteristics.Through the optimization of the molecular structure,a series of materials with SFX as the core have excellent properties,and some of them are even better than spiro-OMeTAD[37,79,82,83]。 Moreover,the synthesis cost of these materials can be at least 30 times lower than that of spiro-OMeTAD.In 2018,Berlinguette et al.,in order to further optimize the structure of SFX core HTM,explored the influence of the position and number of diphenylamine units on the HOMO energy level,thermal performance and photovoltaic performance of HTM,and designed and synthesized HTM-FX(same as X60),HTM-F(same as X59),HTM-X,HTM-X'and HTM-FX'.Finally,the HTM-FX'substituted by diphenylamine has higher Tg（>125℃）and PCE(20.8% )[80]。 In 2020,Seok et al.Synthesized a carbazole-based SFX derivative SFXDAnCBZ,and achieved excellent photoelectric properties(PCE of 20.87%)with a lower concentration(17.5 mM)[84]。 In 2022,Liang et al.Designed three SFX derivatives:H1-F,H2-F and M6-F by introducing different linear conjugated structures and fluorinating the SFX core,among which M6-F with dithienopyrrole(DTP)had the best performance,with a champion PCE of 22.17%for the small-area（0.1 cm2）,a large-area（1.01 cm2）PCE of 20.31%,and a PCE of 21.21%for the undoped device[85]。 Designing bifunctional HTM with transport holes and passivated perovskite/HTM interface defects has become a hot topic in the field in recent years.The uncoordinated Pb2+on the perovskite surface is an important factor affecting the stability of the device,and halides are commonly used to coordinate the functional groups of Pb2+.In 2023,Wei et al.Designed two SFX derivatives,mF-SFXDA containing fluorine and mCl-SFXDA containing chlorine,compared with non-halogenated SFXDA(same as X59),and found that mCl-SFXDA showed better isotropic coordination ability with Pb2+[86]。 The device based on mCl-SFXDA achieves the highest PCE of 22.14%,the Vocis improved from 1.07 V to 1.14 V,and the hysteresis index is also reduced to 0.07%,and the device shows excellent light stability.It can be seen from the above research that the substituent groups on the fluorene structural unit of SFX are all at the 2,7-position.Our research group tried to design and develop SFX-3 substituted at the 3,6-position of the fluorene structural unit,which shows better than SFX-4 at the 2,7-position.The device based on SFX-3 achieves a PCE of 22.42%,indicating that further optimization of the SFX structure is also an effective means to improve the performance of PSC[87]。
in addition to the development of SFX core,in recent years,other new spiro cores with excellent performance have also been designed and synthesized,including Fluorene dithiophene core designed by introducing thiophene into spiro core,Fluorene 9,9′-phenanthrene-10′-one(spiro[Fluorene-9,9′-phenanthrene-10′-one])and double spiro,which will be introduced in detail below.the molecular structure is shown in Figure 10 and Figure 11,and the photovoltaic parameters are shown in Table 3。
图10 核心单元含有噻吩或羰基的HTMs的分子结构

Fig. 10 Molecular structures of HTMs with thiophene or carbonyl group as the core unit

图11 其他螺旋核调控的HTMs的分子结构

Fig. 11 Molecular structures of HTMs regulated by other spiral cores

表3 Photovoltaic Performance Parameters of Novel Spiro-cored HTMs

Table 3 Photovoltaic performance parameters of HTMs with new spiral cores

HTM perovskite HOMO
(eV)		 μ
(×10-4 cm2·V-1·s-1)		 Voc
(V)		 Jsc
(mA·cm-2)		 FF
(%)		 PCE
(%)		 Ref
SCPDT-BiT MAPbI3 -5.07 0.595 0.939 16.54 67 10.39 88
FDT (FAPbI3)1-x(MAPbBr3)x -5.16 NRa 1.148 22.7 76 20.2 89
HTM-1 (CsI)0.05(FAPbI3)0.90
(MAPbBr3)0.10		 -5.01 4.5 1.10 24.7 77 21.0 90
Yih-2 MAPbI3 -5.35 0.496 1.02 22.18 71 16.06 91
NiOx/MS-OC MAPbI3 -5.34 0.727 1.128 22.34 80.8 22.34 92
spiro-1 Cs0.05MA0.2FA0.75Pb(Br0.05I0.95)3 -5.25 8.93 1.10 24.91 79.1 21.67 93
spiro-BC-OMe FA1-xMAxPbI3-yBry -5.13 4.13 1.11 24.98 80.11 22.15 94
DH1 MAPbI3 -5.22 25.8 1.04 22.56 74 17.13 95
SAF-OMe CH3NH3PbI3−xClx -5.07 9.9 1.05 21.07 76 16.73 96
PPyra-TXA MAPbI3 -5.23 0.914 1.10 20.6 79.7 18.06 97
MeSBA-DMPA (FAPbI3)0.85(MAPbBr3)0.15 -4.94 0.498 1.063 22.75 75.3 18.21 98
spiro-CN-OMeTAD (FAPbI3)0.84(MAPbBr3)0.16 -5.16 10.4 1.16 21.97 78.08 19.09 99
DFHb MA0.9FA0.1PbI3-xClx -5.27 10 1.10 22.6 82.9 20.6 100
Dispiro-OBuTAD MAPbI3 -5.16 57.3 1.08 22.79 75 18.46 101
G2 FA0.85MA0.15Pb(I0.85Br0.15)3 -5.22 3.58 1.13 23.52 76 20.2 102
Si-Spiro-MeOTAD FAPbI3 -4.88 1.86 1.12 25.9 77.1 22.5 103

aNR=not reported ; bDopant-free

In 2015,Wang et al.Designed and synthesized a spirothiophene derivative,named SCPDT-BiT,based on 4,4′-spiro[cyclo[2,1-b;3,4-b′]dithiophene](SCPDT)as the core and four 5-octyl-2,2′-dithiophene units as the rigid arms,and the device based on it showed a PCE of 10.39%[88]。 In 2016,Nazeeruddin et al.Replaced the spirobifluorene core of spiro-OMeTAD with an asymmetric fluorene-dithiophene core to synthesize a simpler FDT via a common transition metal-catalyzed cross-coupling reaction[89]。 The potential thiophene-iodine interaction can improve the hole transport at the perovskite/HTM interface;And FDT has good solubility and can be dissolved in toluene which is more environment-friendly;The synthesis cost is low,only 60/G,which is about one-fifth of the cost of purified spiro-OMeTAD(500/G).The FDT-based device finally reaches a champion efficiency of 20.2%.In 2020,Akin et al.Designed a new helical core compound(HTM-1)by using the C2vsymmetric spiroacridine-cyclopentathiophene as the core unit and connecting the commonly used triphenylamine unit on both sides[90]。 Compared with the traditional spiro-OMeTAD,the compound has higher hole mobility and higher Tg（168˚C）.Finally,the HTM-1 based device achieved a PCE of 21.0%.It also shows excellent long-term stability(about 90%of the initial efficiency)under 400 H photoaging conditions 。
Due to the strong dipolar intermolecular interaction and high carrier migration ability,small organic molecules with structures such as donor-acceptor-donor(D-A-D)or donor-π-conjugate-donor(D-π-D)become an effective strategy for constructing HTM with efficient PSC,but they are commonly found In planar linear or star HTM and rarely mentioned in spiro.in 2018,Chang et al.Introduced a carbonyl group into the spiro nucleus and designed two novel D-A-D molecules with fluorene-9,9′-phenanthren-10′-one(spiro[Fluorene-9,9′-phenanthren-10′-one])as the core,which were named Yih-1,Yih-2,respectively,and their synthesis cost was low(<30$/g）、产率高（>55%），纯化过程简单。其中，Yih-2作为钙钛矿太阳电池的HTM，获得16.06%的PCE，与同批次的spiro-OMeTAD相当（16.06%）。2021年，该课题组[92]又将芴-9,9′-菲-10'-酮核心单元的化合物用于反式器件的界面材料，并在末端引入可实现扩展π-共轭的供体，咔唑或二甲基吖啶基团，分别设计了MS-PC（对位咔唑）、MS-PA（对位二甲基吖啶）、MS-OC（邻位咔唑）、MS-OA（邻位二甲基吖啶），其合成成本为18~22 $ /G[91]。 Using the device structure of ITO/NiOx/MS/perovskite/PC61BM/BCP/Ag,they found that MS-OC can not only improve the surface morphology of NiOx,but also help to induce the growth of perovskite with larger grains.Compared with the bare NiOx,the device based on the NiOx/MS-OC showed better hole transport ability,and the final cell obtained a PCE of 20.34%,and still maintained 93.16%of the original PCE after 370 days at room temperature in argon environment.In 2023,Lin et al.Designed and synthesized two D-A-D type spirocyclic nuclear derivatives linked to a triphenylamine unit using 5H-spiro(benzo[1,2-b:6,5-b′]dithiophene-4,4′-cyclo[2,1-b:3,4-b′]dithiophene)-5-one as the core unit,which were named spiro-1 and spiro-2,respectively[93]。 All of them have suitable HOMO energy level,high hole mobility,smooth surface morphology,etc.Finally,the spiro-1 and spiro-2 based devices achieve a champion efficiency of 21.67%and 19.6%,respectively.In the same year,Gao et al.Synthesized three new spirocyclic HTMs,named spiro-BC-OMe(the same as spiro-1),spiro-FP-OMe and spiro-BC-tBu,by one-step rearrangement reaction using fluorenone-type starting materials[94]。 By introducing an electron-withdrawing carbonyl group into the spiro core,all three molecules have tilted orthorhombic configurations and enhanced dipole moment and structural stability.Among them,spiro-BC-OMe has a stronger dipole due to the stronger electron-donating ability of the central core thiophene unit and the terminal methoxy group.They suggest that HTM with stronger dipole moment can be more easily adsorbed on the perovskite surface through electrostatic potential interaction,and the closer distance promotes the easier transfer of holes from perovskite to HTM.Finally,the spiro-BC-OMe based device achieved a champion PCE of 22.15%,comparable to the conventional spiro-OMeTAD(22.05%),and the large area（16.38 cm2）device achieved a PCE of 18.36% 。
the design of hyperbranched macromolecules is beneficial to improve the stability of amorphous,which is an effective design strategy for HTM.In 2020,Li et al.Designed and synthesized a dendritic macromolecular polymer,DH1,which is composed of spirobixanthene(SBX),an analogue of SFX,as the core unit,phenylene as the spacer,and methoxydiphenylamine carbazole as the peripheral group[95]。 DH1 has a hyperbranched structure with a molecular size of up to 1.9 nm,and can form a uniform amorphous film.Therefore,it shows good thermal stability,amorphous nature,full surface coverage with a pinhole-free morphology,and appropriate carrier mobility.Finally,the DH1-based device achieved a PCE of 17.13%and reduced the device hysteresis behavior。
In 2016,Liao et al.Chemically combined the most excellent small molecule spiro-OMeTAD with polymer PTAA,and the main chemical unit of PTAA,N-benzene-N subunit,was introduced into the spiro core of spiro-OMeTAD to construct a new SAF-OMe[96]。 SAF-OMe has better hole transport ability than PTAA and higher thermal stability than spiro-OMeTAD.Finally,the devices fabricated with undoped or doped SAF-OMe as HTM achieved the best efficiency of 12.39%and 16.73%,respectively,while the doped spiro-OMeTAD achieved a PCE of 14.84%.In 2017,Chi et al.Designed a series of new spirocyclic HTMs,PPyra-XA,PPyra-TXA,and PPyra ACD,whose core units are the spirocyclic nuclei of phenylcarbazole-substituted xanthine(O atom),thioxanthine(S atom),and acridine(N atom)respectively[97]。 Among them,PPyra-TXA containing S atoms has a more suitable HOMO level(−5.23 eV)and a higher hole mobility（9.14×10-5cm2·V-1·s-1）due to the strong intermolecular packing induced by the S-S atom interaction.The device based on it achieves a champion PCE of 18.06%,which is higher than that of spiro-OMeTAD(16.15%),and the device based on undoped PPyra-TXA also achieves a PCE of 11.70%.As the simplest structure in organic chemistry,the alkyl chain is often considered the simplest"variable"to reveal structure-property relationships.In 2020,Nazeeruddin et al.Used spirobiacridine(SBA)as the core unit,connected electron-inert alkyl chains and heteroatoms with different lengths and volumes to the in-plane N atoms of SBA,and connected dimethoxyaniline to the end of SBA following the molecular structure of spiro-OMeTAD,and synthesized MeSBA-DMPA,BuSBA-DMPA,FBuSBA-DMPA and EtHexSBA-DMPA,respectively[98]。 They found that SBA core molecules With different carbon chains have little effect on the PCE of PSC,but have a great influence on the long-term stability of the device.MeSBA-DMPA with shorter carbon chain has the best stability performance,which can maintain 88%of the initial efficiency and almost no hysteresis after 1000 H aging experiment under constant light source.with the increase of the alkyl chain,the stability of the device deteriorates。
In 2020,Gao et al.Introduced Lewis base groups(carbonyl,cyano)into the HTM,and designed two new spiro-core HTMs,namely spiro-CN-OMeTAD with dicyanogen in the central core and spiro-PS-OMeTAD with thiocarbonyl,aiming at transporting holes while coordinating the residual Pb2+in perovskite[99]。 Their synthetic routes are simpler and do not change the orthogonal molecular configuration.The compound spiro-CN-OMeTAD can not only reduce the HOMO energy level,but also has better passivation effect than spiro-PS-OMeTAD.Finally,the device based on spiro-CN-OMeTAD achieves a PCE of 19.7%,which is higher than that of the same batch of spiro-OMeTAD.Moreover,the device exhibits higher Voc（1.16 V）and lower hysteresis 。
By simplifying the spirobifluorene core of spiro-OMeTAD,HTM with simpler structure and excellent performance can be designed.In 2019,Berlinguette et al.Reported a dopant-free spirocyclic molecule,DFH,which is suitable for trans devices.its central core is introduced with dioxolane ring with strong intermolecular interaction and connected with fluorene bridge through spirocarbon center.the end is the traditional triphenylamine unit with redox activity,and Its structure is simpler[100]。 The molecule can be synthesized by only two steps of Buchwald-Hartwig amination and acid-catalyzed condensation with ethylene glycol,and is purified by recrystallization,so the purification process is simple,and the cost is as low as 3$/G.As the HTM of the trans device,they also found that DFH is very suitable for the growth of perovskite grains,and the perovskite grains grown on the upper surface after DFH annealing are as high as 1µm.Eventually,the dopant-free trans device of the DFH-based device achieves a PCE of more than 20%.Common dopant-free HTMs are oftenπ-conjugated bridges with planarity and complex donor-acceptor(D-A)structures.This report undoubtedly challenges this confinement and provides a new possibility for the development of dopant-free HTMs 。
Most spiro HTMs have a single spiro ring as the core and an aryl amine as the arm.In 2018,Guo et al.Suggested that increasing the spiro center of the molecule may induce greater steric hindrance and reduce molecular symmetry[101]。 To this end,they designed a new molecule—Dispiro-OBuTAD(abbreviated as Dispiro),which is a double-spiro structure with two fluorene rings connected to an indenylfluorenyl unit through two sp3carbon atoms as a core unit and a tetrakis[N,N-di(4-butoxyphenyl)amino]group as a peripheral substituent.Dispiro with two spiro centers exhibits a rigid ladder-type structure,large steric hindrance,and reduced molecular symmetry.Finally,the device with Dispiro as the HTM achieved a PCE of 18.46%,which is higher than that of the spiro-OMeTAD-based device(17.82%).In the same year,Jen et al.Designed two HTMs with a double spiro ring as the core,named G1 and G2 respectively,where G2 replaced the benzene ring in the center of G1 with a thiophene unit[102]。 the double spiro ring exhibits an H-shaped configuration with two parallel arms and provides the molecule with a largerπ-conjugation system and a longerπ-conjugation length,which is beneficial to improve the hole migration ability of the molecule.G1 and G2 act as HTM,and they both have similar HOMO energy levels and molecular configurations.However,compared with G1,the sulfur atom on G2 can provide higher binding energy,which promotes stronger interfacial coupling between HTM and perovskite,and the poor symmetry of G2 also provides better solubility and film forming ability.G2 exhibits better hole extraction by both PL and TRPL tests.Finally,the G2-based device achieves a PCE of 20.2%,which is much higher than that of G1(15.5%)。
In 2023,Kim et al.Synthesized Si-Spiro-MeOTAD by replacing the central carbon atom of traditional Spiro-OMeTAD with silicon atom[103]。 the molecule has a better hole extraction ability,which they attribute to the fact that the silicon atom of carbon changes the packing characteristics of the HTM.Finally,the device with Si-Spiro-MeOTAD as the HTM achieved a PCE of 22.5%and exhibited excellent long-term stability。

3 Conclusion and prospect

In the past decade,PSC has become one of the research hotspots in the field of photovoltaics.To date,the highest certified efficiency of PSC has exceeded 26%.However,in order to achieve real application,continuous improvement is needed to further achieve high efficiency,stability and low cost.As an important part of PSC,HTM plays an important role in device performance.The spiro small molecule HTM has a unique vertical orthorhombic structure,so that the molecule has high rigidity,high Tg,good solubility in common solvents,good film-forming property,and good contact with the perovskite film.The isotropic charge transport is easy to extract and transport the photogenerated holes generated by the perovskite layer to the electrode,which meets most of the conditions of excellent HTM,and the efficiency of the prepared devices is generally high.spiro-OMeTAD is the most classical spiro-HTM in formal devices.In this review,based on the molecular structure of spiro-OMeTAD,some high-performance spiro-HTMs in recent years are introduced from two aspects of terminal group optimization and spiro-core regulation,and the relationship between molecular structure and device performance is explored 。
Based on spiro-OMeTAD,terminal optimization refers to the structural optimization of terminal triphenylamine without changing the central spirobifluorene nucleus,such as changing the substitution position,introducing heteroatoms,introducing new groups and more complex optimization structures.Considering the complexity and high cost of the central spirobifluorene nucleosynthesis process,nuclear regulation refers to the design and development of new spirocyclic nuclei,which generally have simpler synthetic strategies and lower costs.For example,the SFX core uses a simple one-pot synthesis method,which has the advantages of low cost and high performance,and is expected to become a candidate to replace the spirobifluorene core。
A large number of research works have confirmed that more excellent device performance can be achieved by designing,optimizing the molecular structure,and constructing efficient,low-cost,and stable HTM.(1)In terms of structure,the introduction of electron-withdrawing or weakly electron-donating groups can often reduce the HOMO energy level of the molecule.The lower the HOMO energy level,the lower the energy barrier required for hole transport and the less the voltage loss.The optimal HOMO level should be slightly higher than the valence band of perovskite,preferably not more than 0.3 eV;(2)The introduction of conjugated system or the expansion of conjugated length can often improve the hole transfer ability,while the improvement of the stiffness of conjugated system can often improve the thermal stability.Hole mobility is an effective parameter to characterize the ability of HTM to transfer holes,and the hole mobility of qualified HTM varies from 10-3~10-5cm2·V-1·s-1.The higher the hole mobility,the better the hole transport ability;(3)There are inevitably a large number of defects at the perovskite/HTM interface,and the introduction of passivating groups,such as halogen,cyano,pyridine,etc.,can often achieve the dual function of transporting holes and passivating defects;(4)In addition,the lower the molecular symmetry,the stronger the polarity,and the better the thermal stability of the molecule,so a large number of works often use the structural design strategy of asymmetric substitution 。
On the other hand,from the above research,we can also see that there are some problems in the relationship between molecular structure and molecular characteristics,device performance,and the characterization of molecular characteristics.For example:(1)In view of the fact that the current molecular property simulation methods are very mature,there are relatively few reports on the systematic prediction of molecular properties through theoretical simulation,and the exact results can only be obtained through complex material synthesis and device testing.Therefore,it is necessary to establish a database,give full play to the advantages of theoretical simulation at this stage,and develop high-performance HTM with pertinence;(2)Although the novel spiro molecules reported in the literature show higher performance than the reference spiro-OMeTAD,the highest known NREL certified record in the past five years still uses spiro-OMeTAD as the hole transport layer.It is possible that the new material only has good performance for specific perovskite components or the perovskite layers prepared by different research groups have different surface characteristics,which can not achieve the universality of spiro-OMeTAD,so it is necessary to further develop a new type of spiro-HTM with universality;(3)In the aspect of molecular hole mobility characterization,there are many kinds of test devices(FTO/PEDOT:PSS/HTM/Au,ITO/PEDOT:PSS/perovskite/HTM/Au)and calculation methods(space charge limited current,impedance spectroscopy,charge transit time method,field effect transistor characterization method,DC resistance method,etc.).Only the hole mobility of spiro-OMeTAD varies from 10-5~10-3cm2·V-1·s-1,and some molecules with lower hole mobility have better performance than spiro-OMeTAD,indicating that the hole mobility is not the decisive factor affecting the device performance.However,one of the main functions of hole transport materials is to transport the holes extracted from the perovskite layer to the electrode,so the importance of hole mobility can not be ignored,especially for thick HTM films;(4)The Tgof HTM has an important impact on the thermal stability of the device,but a large number of studies have focused on the thermal aging of the device in the dark state.In fact,there are few studies on the changes of perovskite/HTM interface and HTM itself after illumination,which still need to be further explored;(5)The cost of spiro-OMeTAD is one of the important factors that limit the development of PSC.The laboratory synthesis cost of some reported new HTM is much lower than that of spiro-OMeTAD.The source of HTM raw materials has a great impact on the cost.With the development of PSC and related optoelectronic devices,the cost of spirocyclic nucleus in spiro-OMeTAD has been greatly reduced,and the main cost lies in purification.The application prospect of new materials mainly focuses on performance,and it is believed that the industry can reduce the synthesis cost of a high-performance HTM to a lower level;(6)The literature shows that the HOMO energy level of HTM has a greater impact on the device Voc,in fact,the splitting of the molecular quasi-Fermi level has a deeper impact on the Voc,the HOMO energy level of the reference spiro-OMeTAD reported at present is-5.25~-4.95 eV,and the dopant has a great impact on the molecular Fermi level,the type and amount of dopant used in the new HTM are different,and the microscopic effects of these changes need to be further explored;(7)Ti-TFSI and TBP additives commonly used in HTM at present are classical electrolyte additives as early as the dye-sensitized battery period,and there are many explanations for the mechanism of additives and HTM in PSC research.It is certain that these two additives are not conducive to device stability,and many new additives are not used in certification efficiency,indicating that new additives still need to be developed to improve device performance 。
with the rapid development of PSC,the performance and cost of HTM have become One of the difficulties that must be solved in its commercial application.Although this review summarizes a large number of excellent previous studies,the design and development of high-performance HTM still faces new challenges in order to accelerate the development of PSC.Based on this,the following research directions may be future concerns.(1)in formal devices,defect States will be formed on the surface when the perovskite polycrystalline thin film is formed,which increases the probability of non-radiative recombination of carriers and induces the decomposition of perovskite materials.spiro-OMeTAD can not effectively passivate defects,while the commonly used passivator post-treatment method increases the device preparation process.Moreover,other problems(such as the formation of two-dimensional perovskite materials at high temperature after PEAI post-treatment,the low melting point of long-chain ammonium salts,the poor wettability of common solvents,which is not conducive to the subsequent film deposition and stability,and the difficulty in controlling the optimal amount of passivator)also limit its application.the development of spiro HTM with dual functions of passivating perovskite film surface defect States and transporting holes is expected to be one of the future development directions.Through fine molecular structure regulation,the good isotropic charge transport characteristics of the spiro HTM are maintained,and the defect state density at the interface of the perovskite film and the HTM is reduced under the condition of avoiding the use of a perovskite material passivator.at the same time,the introduction of perovskite defect passivation groups to reduce the symmetry of spiro HTM is also expected to reduce the crystallization characteristics of molecules at high temperature and improve the thermal stability of PSC;(2)From the above summary,It can be seen that spiro-OMeTAD itself has good stability,and when used in formal devices,the electron transport layer and perovskite layer will filter out more ultraviolet light and reduce the photoaging of HTM.one of the main factors limiting its stability is the use of additives,so the development of high-performance additives combined with new HTM is expected to further improve device stability;(3)in the past two years,trans-devices have been developed rapidly,but the low hole mobility of spiro HTM limits its application,and the influence of hole mobility of self-assembled monomolecular films can be almost ignored.By introducing functional groups that can be anchored to the substrate on the spiro HTM,the molecular structure is further optimized,which is expected to promote the application of spiro molecules in trans devices,and this kind of material is only a monomolecular film with a very small amount,which can significantly reduce the cost of devices.(4)it is still necessary to closely integrate the current mature theoretical simulation tools to establish a relatively perfect database of molecular structure,molecular properties and device properties to guide the development of new high-performance spiro HTM。

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