Sihan Li,Yuxuan Yang,Kuo Su,Bao Zhang,2,3,*,Yaqing Feng,2,3,*

1 School of Chemical Engineering and Technology,Tianjin University,Tianjin 300350,China

2 Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),Tianjin 300072,China

3 Jieyang Center of Guangdong Laboratory of Chemistry and Fine Chemicals,Guangdong 515557,China

Keywords:Solar energy Electronic materials Organic compounds Triphenylamine Hole transport materials

ABSTRACT In the past decade,perovskite solar cells have become a promising candidate in the photovoltaic industry owing to their high power conversion efficiency that surpasses 25%.However,there are certain limitations that have hindered the development and full-scale practical application of these cells,including the high cost and degradation of perovskite caused by the dopants.Hence,there is an urgent need to develop dopant-free hole transport materials (HTMs).In recent years,HTMs based on triphenylamine(TPA-HTMs) are receiving growing interest owing to their high hole mobility,excellent film formation,and suitable energy levels.The literature here covers work relevant to TPA-HTMs in the last five years.They have been classified according to different core types.The correlations between performance and structure are summarized,and the future development trend of TPA-HTMs is highlighted.

1.Introduction

Environmental issues such as global warming and air pollution are some of the most severe problems faced by people in the 21st century.These problems are mostly caused by the burning of fossil fuels;therefore,there is an urgent need to develop green energies such as solar energy,which can be converted into electricity by photovoltaic devices (PV).

In the last decade,perovskite solar cells (PSCs) have received widespread attention because of their high power conversion efficiency (PCE) reaching 25.7 % [1-5].Perovskite possesses a crystal structure composed of ABX3(Fig.1b).A can be[6],or Cs+[7];B represents Pb2+and/or Sn2+;and X normally is I-,Br-and/or Cl-.PSC devices can be classified into mesoporous,conventional (n-i-p),and inverted (p-i-n) structures (Fig.1(c)).Holes and electrons are separately injected into the highest occupied molecular orbital (HOMO) of the hole transport layer (HTL) and the lowest unoccupied molecular orbital(LUMO)of the electron transport layer(ETL)from the valence band(VB) and the conduction band (CB) of perovskite and then transferred to the electrodes (Fig.1(d)).Thus,to extract holes and obstruct electrons from the perovskite layer,the HOMO and LUMO levels of HTM should be more positive than the VB and CB of perovskite used in the device,respectively.

During the development of PSCs,hole transporting materials(HTMs),such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spiro-bifluorene (spiro-OMeTAD) [8] and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine](PTAA)[9],are involved as key components,as shown in Fig.1(a).However,dopants such as lithium bis(trifluoromethane)sulfonimide (Li-TFSI) [10],4-tertbutylpyridine (t-BP) [11] and tris(2-(1H-pyrazol-1-yl)-4-tert-butyl pyridine)cobalt(III)tri(bis(trifluoromethane)sulfonimide) (FK209)[12](Fig.1(a))are required to improve the film formation,conductivity,and hole mobility of spiro-OMeTAD.These dopants can corrode perovskite and absorb water and oxygen,consequently resulting in the deterioration of PSCs [13].Furthermore,the high costs of spiro-OMeTAD (500 USD·g-1) and PTAA (2000 USD·g-1)limit their commercial applications.

Due to the presence of an electron lone pair,the nitrogen atom in the triphenylamine (TPA) unit can easily undergo oxidation leading to a nitrogen-based radical cation,which endows TPA derivatives with excellent hole extraction and transport properties(Fig.2(a)).In addition,HTMs based on TPA have a high glass transition temperature,which helps to maintain the amorphous phase at the working temperature,thereby avoiding the production of grain boundaries and the resultant exciton recombination.The propeller geometry (Fig.2(b)) of TPA guarantees its solubility,and the suitable HOMO energy level of TPA makes it an ideal building unit for HTMs.

Fig.1.(a) Chemical structures of spiro-OMeTAD,PTAA and dopants;(b) Crystal structure of perovskite;(c) Scheme illustrating the structure of PSCs;(d) Charge transfer processes in PSCs.Note: 1 eV=1.6 × 10-19 J.

Fig.2.(a) Chemical structure of TPA;(b) 3D structure of TPA.Blue,grey,and white atoms represent N,C and H,respectively;(c) Structures of small-molecule HTMs.

Most small-molecule HTMs are composed of one core group and several terminal groups connecting to the core (Fig.2(c)).The terminal groups are normally responsible for extracting and transporting holes,while the cores play a role in improving the planarity of the molecules and/or passivating the defects on the surface of perovskite.Owing to the excellent features mentioned above,small-molecule TPA derivative-based dopant-free HTMs(abbreviated as TPA-HTMs) possess high hole mobility,which helps to avoid the involvement of hydrophilic dopants.Thus,the stability of PSCs based on TPA-HTMs can be effectively improved compared to those involving spiro-OMeTAD and dopants in general.In addition,the costs of PSC fabricated by TPA-HTMs are also significantly reduced since TPA-HTMs can be readily synthesized and no expensive dopants are required.

Due to the low cost and the superior stability against humidity,oxygen and heat of PSC devices based on dopant-free HTMs,considerable efforts have been focused on developing HTMs to replace spiro-OMeTAD.In recent years,several review articles relating to dopant-free HTMs have been published [14-16].In 2021,Desokyet al.[17] reviewed relationship between structure and performance for polymeric dopant-free HTMs.Most recently,Wanget al.[18] reviewed dopant-free small molecular HTMs based on the role they play in passivation.However,there are few reviews on TPA-HTMs.Therefore,the necessity of summarizing TPAHTMs is highlighted.Herein,TPA-HTMs,which have shown performances similar to those of doped spiro-OMeTAD in the last five years,are reviewed based on different core groups.The characteristics of the different structures were analyzed to illustrate appropriate molecular design strategies for improving the performance of TPA-HTMs.The detailed electrochemical properties and corresponding device performances are summarized in the Supplementary Material (Table S1).

2.TPA-HTMs Based on Spiro Cores

Spiro-OMeTAD is a typical HTM featured with the spiro geometry that possesses a pair of planar fluorene groups connected by the same sp3C atom.The orthogonal configuration of spiro compounds significantly inhibits the intermolecular interaction,leading to high solubility of the HTM in organic solvents,and thereby enhancing the film quality.On the other hand,the hole mobility and charge conductivity of the spiro type HTM are also relatively low due to the poor intermolecular interaction [19].

Strategies to enhance the hole mobility of spiro type HTMs include introducing heteroatoms and extending the conjugation.For example,Guoet al.[20]prepared 2mF-X59 by introducing fluorine atoms into spiro[fluorene-9,9′-xanthene],as shown in Fig.3.Compared to the HTM without fluorination(X59,Fig.3),the HOMO level of 2mF-X59 was down-shifted due to the electronwithdrawing effect of F atoms.Besides,the introduction of F atoms improved the hydrophobicity of 2mF-X59,leading to enhanced device stability.Consequently,the n-i-p devices based on 2mFX59 exhibited a PCE of 15.45%,while the PCE of X59-based devices was 3.95%.Wang and co-workers[21]synthesized X62(Fig.3)by using spiro[dibenzo[c,h]xanthene-7,9′-fluorene] as the core.In comparison to spiro-OMeTAD,the expanded conjugation of X62 improved the molecular packing,and thereby increased hole mobility.The PCE of the X62-based device (15.96 %) was higher than that of spiro-OMeTAD (10.84 %).

3.TPA-HTMs Based on Non-spiro Cores

3.1.Polycyclic aromatic cores

Polycyclic aromatic moieties are outstanding candidates as core groups because of their planarity,which leads to close π-π stacking,and thereby high hole mobility.Using an anthanthrone dye as the core group,Phamet al.[22] prepared TPA-ANT-TPA as shown in Fig.4.In addition to the high hole mobility,the dense stacking of the designed HTM also led to excellent hydrophobicity,which resulted in superior device stability.The corresponding n-ip device exhibited a PCE of 17.5 %.Besides,the stability of devices based on TPA-ANT-TPA was much improved compared with those based on spiro-OMeTAD,whose PCE remained over 80%after storing in ambient air for 50 hours (Fig.5).

Sunet al.[23] used fluoranthene to develop a series of FBAHTMs (Fig.4).The hole mobility of FBA2 was higher than that of FBA1 because the introduction of C=C double bond as a π-bridge extended the conjugation of FBA2.Changing the substitution site of methoxy groups in TPA units down-shifted the HOMO level of HTM molecules from -4.98 eV (1 eV=1.6 × 10-19J) (FBA2) to-5.07 eV (FBA3),which was more matched with the VB of perovskite.Consequently,the n-i-p PSCs based on FBA-HTMs achieved PCEs of 16.80%(FBA1),18.70%(FBA2)and 19.27%(FBA3),respectively.Recently,Sun and co-workers[24]further tuned the HOMO level of the fluoranthene core-based HTM by introducing cyano substituents to the core and synthesized BTF6 (Fig.4),which enabled inverted and flexible PSCs to afford PCEs of 20.34 % and 18.06 %,respectively (Fig.6).

Qiuet al.[25]constructed two HTMs,PYR16 and PYR27,by connecting TPA derivatives to the 1,6-and 2,7-sites of pyrene,as shown in Fig.4.Compared to PYR27,PYR16 possessed a larger dipole moment,which promoted the intramolecular charge transfer of PYR16.Accordingly,the PYR16-based devices showed a PCE of 17.00 %,higher than that of PYR27 (14.67 %).By employing phenanthrene and phenanthroline as the core groups,Zhaoet al.[26] designed YZ18 and YZ22 as the HTMs for PSCs (Fig.4).As a chelating ligand,phenanthroline can effectively passivate perovskite defects,leading to improved hole extraction capability of YZ22,which can be proved by the disappeared peak of Pb0in the XPS spectra (Fig.7(a)).Therefore,the device with YZ22 as dopant-free HTM obtained a notable PCE of 22.4%(Fig.7(b)),much higher than that with YZ18 (18.1 %).

C3hsymmetrical molecules are used in constructing HTMs because of their ability to form face-on stacks and columnar geometries.Huanget al.[27] synthesized Trux-OMeTAD (Fig.4)using truxene as the core group.Trux-OMeTAD exhibited excellent hole mobility (3.6 × 10-3cm2·V-1·s-1) due to the superior molecule packing.The Trux-OMeTAD-based device yielded a champion PCE of 18.6 %.

3.2.Thiophene derivative-based cores

Fig.3.Chemical structures of TPA-HTMs based on spiro-shaped cores.

Fig.4.Chemical structures of TPA-HTMs based on polycyclic aromatic cores.

Fig.5.Stability test of devices based on (a) TPA-ANT-TPA and (b) Spiro-OMeTAD on aging in relative humidity ≥58 %.Reproduced with permission.[22] Copyright 2018,Wiley.

As an electron-rich aromatic ring,thiophene possesses distinctive charge-transfer properties.Moreover,the lone electron pair on S atom in thiophene can passivate the I-defects at the surface and grain boundaries of perovskite,making thiophene derivatives ideal building blocks for high performance HTMs.Upon incorporating S···O interactions into the backbone,Yanget al.[28] improved the planarity of bithiophene coreviaintramolecular noncovalent interactions.The resulting HTM,BTORCNA,exhibited decreased dihedral angle (5.8°) between two thiophenes compared to that of BTRA (57.2°) (Fig.8(a)).The introduction of CN groups led to a larger dipole moment in BTORCNA than that in BTRA,resulting in stronger intramolecular charge transfer.Consequently,the BTRAand BTORCNA-based inverted devices delivered PCEs of 18.40 %and 21.10%,respectively(Fig.8(b)).Using cyclooctatetrathiophene as the core,Menget al.[29] designed a saddle-shaped HTM MF-2(Fig.9).The non-rigid structure of MF-2 allows for good contact with the perovskite,resulting in high hole injection efficiency.Chlorobenzene-and ethyl acetate-processed devices based on MF-2 exhibited PCEs of 18.53 % and 17.25 %,respectively.Shenet al.[30] designed TTE-1 and TTE-2 based on a tetrathienylethylene core (Fig.9).The poor intermolecular interactions ascribed to the orthogonal structure of tetrathienylethylene resulted in the low hole mobility of TTE-1.In contrast,in TTE-2,two thiophenes were fixed by junction with a benzene ring to construct a hybrid orthogonal and planar structure (Fig.8(c)).Compared to TTE-1,the expanded conjugation of TTE-2 enhanced the hole mobility.As expected,an improved PCE was yielded by devices based on TTE-2 (TTE-2,20.04 %;TTE-1,13.68 %;Fig.8(d)).

Fig.6.J-V curves of a) inverted and b) flexible PSCs based on BTF6.Reproduced with permission.[24] Copyright 2021,Elsevier.

Fig.7.(a)XPS spectra of YZ18 and YZ22 stacking on perovskite.(b)J-V curves of YZ18 and YZ22-based PSCs.Reproduced with permission.[26]Copyright 2020,Royal Society of Chemistry.

Fig.8.(a)Molucular geometries of BTORCNA and BTRA from top view and side view.J-V curves of PSCs based on(b)BTORCNA Reproduced with permission.[28]Copyright 2022,Wiley;(c) TTE-1&TTE-2;(d) Molecule structure of core groups of TTE-1 and TTE-2.Reproduced with permission.[30] Copyright 2019,Wiley.

Fig.9.Chemical structures of TPA-HTMs based on thiophene derivatives.

Benzo[1,2-b:4,5-b’]dithiophene(BDT)is widely used in the construction of HTMs.Xueet al.[31]developed a series of BDT-HTMs,among which BDT-TPA-sTh (Fig.9) bearing two thienyl substituents exhibited high hole mobility owing to supramolecular interactions between the thienyl and the benzene rings in TPA units.A PCE of 20.5 % was achieved by conventional devices using BDT-TPA-sTh as a dopant-free HTM.Liet al.[32] designed H2(Fig.9)involving a BDT core.Owing to the introduction of F atoms in the TPA units of H2,H2 exhibited superior hydrophobicity.The resulting device showed a PCE of 18.69 % with high stability.

Zhanget al.[33] designed DBTMT with dibenzo[b,d]thiophene as the core(Fig.9).The hydrophobic HTL fabricated by DBTMT provided little nucleation sites for perovskite crystal to grow on.As a result,perovskite with grain size over 1 μm was formed on the surface of DBTMT film,enabling the inverted devices to exhibit an outstanding PCE of 21.12 % with a high FF of 83.25 %.Leeet al.[34]developed a HTM called CB(Fig.9) by incorporating malononitrile into a cyclopenta[2,1-b;3,4-b’]dithiophene core.The film that CB formed on perovskite was more uniform than that of spiro-OMeTAD owing to the defect passivation effect induced by the malononitrile unit.Thus,an excellent PCE of 21.09%was achieved by CB-based devices fabricated by thermal-assisted blade-coating(TABC) method (Fig.10).Ring fusion between the thiophene units and the benzene moiety in ZT-H1 furnished the polycyclic aromatic core involving four thiophene units in ZT-H2[35] (Fig.9).Compared with ZT-H1,ZT-H2 showed much improved planarity.Therefore,the hole mobility of ZT-H2 was higher than that of ZTH1.The n-i-p PSCs with the dopant-free ZT-H2 as HTM exhibited a higher PCE (19.63 %) than those with ZT-H1 (10.03 %).Haiet al.[36] synthesized Y-T (Fig.9) by linking a large benzothiadiazolebased conjugated electron acceptor with TPA unitsviathiophene π-bridges.PSCs based on Y-T afforded a PCE of 20.29 %.Besides,the resultant devices showed excellent stability owing to the outstanding hydrophobicity of Y-T,maintaining over 90%of the initial PCE after storage in ambient air (relative humidity RH～30 %) for 60 days.

Xuet al.[37] developed YN1 and YN2 based on benzothiadiazole (BT) and thienopyrazine (TP) cores (Fig.9).The stronger electron-withdrawing property of TP resulted in a deeper HOMO level (-5.40 eV) of YN2 than that of YN1 (-5.35 eV).In addition,YN2 possesses higher hole mobility than YN1 due to its stronger intramolecular electron push-pull effect.As expected,the devices based on YN2 exhibited a higher PCE (19.27 %) than those based on YN1 (16.03 %).Guoet al.[38] reported TQ3 and TQ4 constructed by connecting rotatable and chemically fixed thienyl units to quinoxaline as the core(Fig.9).TQ3 and TQ4 both exhibited face-to-face arrangement because thiophene moieties formed intermolecular hydrogen bonds via the C-H···S and S···S interactions (Fig.11(a)).Furthermore,the extended conjugated structure of TQ4 enhanced molecular packing,resulting in high hole mobility of 2.08 × 10-4cm2·V-1·s-1.With TQ4 as HTM,the optimized device delivered an impressively high PCE of 21.03 % (Fig.11(b)).

Fig.10.(a)J-V curves of PSCs based on CB;(b)Schematic of thermal-assisted blade-coating method that used to fabricate CB-based PSCs.Reproduced with permission.[34]Copyright 2022,Elsevier.

Fig.11.(a)The intermolecular interactions in single-crystal structure of TQ4.(b)J-V curves of PSCs based on TQ3&TQ4.Reproduced with permission.[38]Copyright 2021,Wiley.

Fig.12.J-V curves of PSCs based on(a)DTPC13-ThTPA Reproduced with permission.[40]Copyright 2019,Wiley;(b)DTP-C6Th Reproduced with permission.[41]Copyright 2019,Wiley;(d)MPA-BTTI Reproduced with permission.[42]Copyright 2019,Wiley(c)The stability test of devices based on DTP-C6Th and spiro-OMeTAD stored in glovebox and air.The stability test of PSCs employing MPA-BTTI (e) at maximum power point and (f) stored in inert environment at 80 ℃under dark.

As a common electron donor,dithieno[3,2-b:2,3 -d]pyrrole(DTP) has been widely used in organic solar cells [39].Owing to the suitable energy level and high hole mobility,DTP has also been applied in HTMs of PSCs.Tanget al.developed DTPC13-ThTPA[40]and DTP-C6Th [41] by incorporating 7-ethylundecyl and thiophyl moieties into DTP (Fig.9).The introduction of branched alkyl substituents modified the molecular packing of DTPC13-ThTPA,whereas thiophyl expanded the conjugation of DTP-C6Th.Both strategies resulted in high hole mobility.Therefore,the n-i-p devices based on DTPC13-ThTPA and DTP-C6Th delivered PCEs of 20.38 % and 21.0 %,respectively (Fig.12(a),(b)).In addition,the stability of PSCs based on DTP-C6Th were significantly better than those with spiro-OMeTAD(Fig.12(c)).Wanget al.[42]designed an imide-functionalized thiophene-based HTM abbreviated as MPABTTI (Fig.9).MPA-BTTI showed excellent face-to-face molecular stacking,resulting in superior intermolecular charge transfer.Besides,the carbonyl units of imide could passivate the defects on the surface of perovskite,leading to extraordinary device stability.A notable PCE of 21.17 % was obtained by applying MPA-BTTI as dopant-free HTM to the inverted devices (Fig.12(d)).Besides,due to the passivation effect of MPA-BTTI,operational stability and thermal stability of corresponding devices were excellent.The MPA-BTTI-based PSCs maintained 90%of initial PCE after continuous illumination for 500 hours at maximum power point(MPP) or storage at 80 °C for 800 hours (Fig.12(e),(f)).

3.3.Fluorene derivative-involved cores

Fluorene units which can be easily modified exhibit good planarity.FMT (Fig.13) was developed by Zhanget al.[43] using hexyl-substituted fluorene as the core.The S atoms in FMT modified the crystallization of perovskite,leading to a uniform perovskite film.Inverted PSCs based on FMT achieved a PCE of 19.06 %.Zhanget al.[44] introduced 4-vinylbenzyl into fluorene to develop VB-MeO-FDPA (Fig.13),in which the vinyl unit can be thermally crosslinked after annealing.Owing to the antisolvent property of the crosslinked HTL film,the corresponding inverted devices obtained a PCE of 18.7 % with a high fill factor of 77.8 %(Fig.14(a)).By introducing two benzothiadiazole units as bridges,Niuet al.[45] designed a D-A-π-A-D type HTM DTB-FL (Fig.13).High planarity resulted in high hole mobility.Besides,the N and S atoms in the benzothiadiazole unit could effectively passivate the defect present on the surface of perovskite.Owing to the superior film quality,DTB-FL also showed excellent performance in large-area devices Consequently,PCEs of 21.5 % and 19.6 % were achieved for small-area (0.09 cm2) and large-area (1.0 cm2)devices with dopant-free DTB-FL as HTM,respectively (Fig.14(b)).The humidity and operational stabilities of DTB-FL-based PSCs are greatly improved compared to the spiro-OMeTAD-based cells because of the effective defect passivation induced by DTB-FL(Fig.14(c),(d)).The devices based on DTB-FL retained 76 % of initial PCE after storage for 600 h in ambient environment,and retained 87 % of initial PCE after constant illumination at MPP for 600 h.

Caoet al.[46] connected 1,3-dioxolane to fluorene leading to a spiro ring-based a dopant-free HTM abbreviated as DFH (Fig.13).Owing to the presence of the hydrogen bonds of O···H (Fig.15(a)),para crystalline ordering of DFH was formed after annealing(Fig.15(b)),leading to high hole mobility.The DFH-based p-i-n device achieved a high PCE of 20.6 % (Fig.15(c)).In addition,DFH costs only 3 USD·g-1which could significantly reduce the corresponding PSC cost.Subsequently,Wanget al.[47] introduced S atoms into DFH and synthesized SFDT-TDM (Fig.13) based on spiro[fluorene-9,9′-dithiolane].As a Lewis basic site,S atom can form a complex with free Pb2+and thus passivate the defect on the surface of perovskite,which can be indicated by the XPS results(Fig.15(d),15(e)).The PCE of the n-i-p devices based on SFDT-TDM reached 21.7 % (Fig.15(f)).

Fig.13.Chemical structures of TPA-HTMs based on fluorene derivatives.

3.4.Carbazole derivative-involved cores

Carbazole units are commonly involved in the design of HTMs as the core.Yinet al.[48] designed and synthesized 2,7-BCz-OMeTAD by using a bicarbazole unit as the core(Fig.16).The hole mobility of 2,7-BCz-OMeTAD reached 0.95 × 10-4cm2·V-1·s-1,which was close to that of doped spiro-OMeTAD(0.85×10-4cm2-·V-1·s-1).The n-i-p device based on 2,7 BCz-OMeTAD yielded a PCE of 17.6 %.Using an indolo[3,2-b]carbazole core,Caiet al.[49] synthesized C201 and C202 (Fig.16) withp-methoxyphenyl and TPA as the terminal groups,respectively.Compared with C201,C202 exhibited improved film quality because the steric hindrance effect of TPA units in C202 improved its solubility in chlorobenzene.Consequently,the C202-based PSCs attained a PCE of 17.7 %,which was higher than that of C201 (7.8 %).Yanget al.[50] developed Cz-As and Cz-Py (Fig.16) by incorporatingN-p-methoxyphenyl andN-pyridyl carbazole units as the core,respectively.The best PCEs of 20.9 % and 18.2 % were achieved for Cz-As-and Cz-Pybased device,respectively.Owing to the presence of an electron lone pair on the nitrogen of pyridine,Cz-Py could coordinate with the free Pb2+in the perovskite layer.Thus,when employing Cz-As and Cz-Py as the HTL and passivation layer,respectively,a notable PCE of 23.5 % was achieved.

3.5.Porphyrin-and phthalocyanine-based cores

Phthalocyanine and porphyrin derivatives bearing a conjugated macrocyclic core are featured with the planar geometry and tend to undergo molecular stacking due to the strong intermolecular π-π stacking.Thus,they have been regarded as ideal building blocks for interface passivation layers and HTMs for PSCs [51,52].Fenget al.[53]incorporated four TPA units to the copper phthalocyanine core leading to OMe-TPA-CuPc (Fig.17).The strong intermolecular π-π stacking promotes charge transfer,leading to a high PCE of 19.67%for OMe-TPA-CuPc-based PSCs.Chenet al.[54]synthesized ZnP (Fig.17) by integrating TPA units into a zinc porphyrin.Benefiting from the suitable HOMO level and high hole mobility,devices with dopant-free ZnP as HTM exhibited a PCE of 17.78 %.Caoet al.[55] designed an HTM based on a mixture of Co(II) and Co(III) porphyrins,denoted by Co(II)P and Co(III)P(Fig.17).The alkoxy and TPA units resulted in high solubility of porphyrin molecules.Interestingly,the ion migration of I-and MA+was suppressed due to the chemical stability of porphyrin.By mixing Co(II)P and Co(III)P in a molar ratio of 4:6 as the HTM,the optimized devices yielded a PCE of 20.47%with enhanced thermal stability and no considerable change was observed after storage in an 85 °C environment for 1000 h (Fig.18).

Fig.14.J-V curves of (a) VB-MeO-FDPA Reproduced with permission.[44] Copyright 2019,Royal Society of Chemistry;(b) DTB-FL Reproduced with permission.[45]Copyright 2021,Elsevier;Stability test of DTB-FL under (c) ambient environment with ca.40% humidity in the dark and (d) MPP tracking in N2 atmosphere.

Fig.15.(a) The propensity of CH···O interactions between DFH molecules.(b) Powder XRD diffractograms of DFH before and after thermal crystallization (black) and GIXD traces of 150 nm spin-coated DFH thin films before and after annealing at 150°C(blue).J-V curves of(c)DFH Reproduced with permission.[46]Copyright 2019,Royal Society of Chemistry;(f)SFDT-TDM Reproduced with permission.[47]Copyright 2021,Wiley;XPS measurements for pristine SFDT-TDM,CsxFA1-xPbI3,and CsxFA1-xPbI3/SFDT-TDM bilayer films with feature peaks of (d) Pb 4f and (e) S 2p.

Fig.16.Chemical structures of TPA-HTMs based on carbazole derivatives.

4.Conclusions and Outlook

Considerable advancements have been achieved for PSCs in the last 10 years,with the highest reported PCE reaching 25.7 % with the aid of solid TPA-HTMs.However,the exorbitant price and severe instability caused by the dopants added for spiro-OMeTAD and PTAA HTMs hinder the commercialization of PSCs.Therefore,it is important to develop new dopant-free TPA-HTMs to reduce cost and enhance robustness.Several factors could be considered when designing a novel TPA-HTM:

(1) Excellent optoelectrical properties.Suitable HOMO and LUMO levels guarantee effective hole extraction[56,57].High hole mobility and conductivity promote charge transfer inside the HTL[58-60].

(2) Solubility and film formation.Spin-coated HTL requires the HTM to dissolve in solvents (such as chlorobenzene and THF).In particular,for inverted PSCs,the HTM needs resistance against the perovskite precursor solution.Uniform films can enhance interface contact,thus improving charge injection between layers.

(3)Introduction of heteroatoms.Heteroatoms(such as N[61],O[62],and S[63])with lone electron pairs play an important role in the passivation of defects on the perovskite surface,leading to improved hole extraction and cell stability [18].

(4) Superior stability and low cost.For PSCs to be commercialized,the stability and cost of PSCs need to be optimized further.Therefore,a more stable and inexpensive HTM is urgently required.

The design of TPA-HTMs is a systematic project that requires comprehensive consideration.Molecules with good planarity always exhibit poor solubility,while the introduction of functional groups often results in added costs.Therefore,various strategies are employed for different systems to enhance the performance of the TPA-HTM.

In summary,a review of TPA-HTMs with different structures and a summarize on molecule design of TPA-HTMs are presented.We believe that with the improving of the TPA-HTM design,inexpensive and stable TPA-HTMs are expected to accelerate the commercialization of PSCs.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig.17.Chemical structures of TPA-HTMs based on Organometallics.

Fig.18.(a)J-V curves of Co(II)P/Co(III)P.Reproduced with permission.[55]Copyright 2018,Wiley.(b)Stability measured at 85°C in a N2 environment on mesoporous PSCs with Co(II)P/Co(III)P as HTM.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities and the National Key Research and Development Program of China (2020YFB0408002).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.07.027.