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Recent Progress on Hole-Transporting Materials forEmerging Organometal Halide Perovskite Solar Cells

Ze Yu and Licheng Sun*

Dr. Z. Yu, Prof. L. SunState Key Laboratory of Fine ChemicalsInstitute of Artificial PhotosynthesisDUT-KTH Joint Education and Research Centeron Molecular DevicesDalian University of Technology (DUT)116024 Dalian, ChinaE-mail: [email protected]

Prof. L. SunDepartment of ChemistryKTH Royal Institute of TechnologySE-100 44 Stockholm, Sweden

DOI: 10.1002/aenm.201500213

strikes the earth in one hour than all ofthe energy currently consumed globally bymankind in an entire year; in other words,harvesting less than 0.02% of this solarenergy would satisfy our present needs.[5]

Photovoltaic (PV) cells, which directlyconvert solar energy to electrical energy,represent promising renewable alterna-tives to fossil fuels. While the current PVmarket is still dominated by the crystallinesilicon based solar cells, “next-generation”

solar cell technologies fabricated throughsolution-processable techniques, such asorganic photovoltaics (OPVs), dye-sensi-tized solar cells (DSCs) and quantum-dotsolar cells (QDSCs), have emerged as low-cost alternatives to replace silicon-basedsolar cells.[6–12]

DSCs have been the representativecandidates among these emerging tech-nologies, and have been intensivelyinvestigated over the past two decades as

promising low-cost alternatives to conventional solar cells.[11–13] A typical liquid-based DSC consists of a photoanode composedof mesoporous TiO2 with a layer of sensitizer attached, an elec-trolyte containing a redox mediator, and a platinized counterelectrode.[12,14] The highest efficiency of liquid-based DSCsreported so far has reached 13% at lab scale and 10% in moduleunder 1 sun irradiation [100 mW cm−2 , air-mass 1.5 global (AM1.5G)].[15,16] However, encapsulations of liquid electrolytes at ele-vated temperatures have become a vital restriction in terms oflong-term stability of DSC devices.[17] In this context, Bach andGrätzel et al. demonstrated the first efficient solid-state DSCs(ss-DSCs) with an overall efficiency of 0.74%, in which liquidelectrolytes were replaced by an organic hole-transporting mate-rial (HTM), 2,2′,7,7′-tetrakis(N ,N -di- p -methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD).[18] Since then, a great dealof effort has been devoted to improving the performances of

ss-DSCs.[12,19–21] Based on spiro-OMeTAD, the overall efficiencyhas been further improved to 7.2% through chemical dopingwith a cobalt (III) complex, in combination with a high-extinc-tion-coefficient organic sensitizer.[22] In 2012, the substitutionof organometallic or oganic dyes with perovskite absorbers inss-DSCs has led to a paradigm shift in the field of thin-filmphotovoltaic technologies.[23,24]

Perovskites refer to a class of compounds that adopt thegeneral chemical formula ABX3 , where X is an anion and Aand B are cations of different sizes (A being larger than B). Asshown in Figure 1 a, in an ideal cubic-symmetry structure, theyform a BX6 octahedra, where the B-cations are located in the

In less than three years, the photovoltaic community has witnessed a rapid

emergence of a new class of solid-state heterojunction solar cells based on

solution-processable organometal halide perovskite absorbers. The energy

conversion efficiency of solid-state perovskite solar cells (PSCs) has been

quickly increased to a certified value of 20.1% by the end of 2014 because

of their unique characteristics, such as a broad spectral absorption range,

large absorption coefficient, high charge carrier mobility and diffusion length.

Here, the focus is specifically on recent developments of hole-transporting

materials (HTMs) in PSCs, which are essential components for achieving

high solar cell efficiencies. Some fundamentals with regard to PSCs are first

presented, including the history of PSCs, device architectures and general

operational principles of PSCs as well as various techniques developed for the

fabrications of uniform and dense perovskite complexes. A broad range of the

state-of-the-art HTMs being used in PSCs are then discussed in detail. Finally,

an outlook on the design of more efficient HTMs for highly efficient PSCs is

addressed.

1. Introduction

By 2050, global energy consumption is predicted to increaseat least two-fold as compared to the world energy consump-tion rate of 13.5 terawattt (TW) in 2001 [1–3] due to the growthof world population and economic developments. The risingglobal energy demand, together with the climate and envi-ronmental concerns associated with the combustion of fossilfuels, have raised increasing public awareness that our societymust urgently explore alternative, renewable and carbon-neu-tral energy sources to replace fossil fuels. Among the variousrenewable energy sources, solar energy is unarguably thelargest exploitable sources that potentially could be scaled up tomeet our future energy necessities. The supply of solar energyto our planet is gigantic: 3 × 1024 J per year.[4] More solar energy

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middle of the octahedra and X-anions reside in the corners. TheA-cations are located at interstices, surrounded by 8 octahedrain the cuboctahedral gap.[25,26] The most commonly used per-ovskites in solar cells are organolead halide perovskites, wherethe organic cations are CH3 NH3

+ (MA) or HC(NH2 )2 + (FA) and

halogens are Cl, Br or I.The first attempt of using organolead halide perovskite

as a sensitizer in liquid-based DSCs dates back to 2009.[27] Miyasaka and co-workers employed CH3 NH3 PbI3 (MAPbI3 )

and CH3 NH3 PbBr3 (MAPbBr3 ) as light absorbers, in combina-tion with an iodine-based liquid electrolyte in DSCs. The power-conversion efficiencies (PCEs) of merely 3.8% and 3.1% wereobtained for the triiodide and tribromide perovskites, respec-tively. In 2011, Park and co-workers further improved the PCEof the liquid-based MAPbI3 solar cell to 6.5% under standardAM 1.5G irradiation through the modification of the titaniasurface and processing method for deposition of perovskite.[28] Unfortunately, these devices were found extremely unstableand degraded rapidly due to the dissolutions of perovskite com-pounds in liquid electrolytes. A big breakthrough was made in2012, when Snaith et al. and Grätzel et al. independently devel-oped solid-state perovskite solar cells (PSCs) using MAPbI3–x Clx and MAPbI3 as light absorbers deposited onto a mesoporousTiO2 layer, in conjunction with the HTM spiro-OMeTAD,leading to PCEs of 7.6% and 9.7%, respectively.[23,24] In lessthan three years, the overall conversion efficiencies of PSCshave quickly jumped to a certified 20.1% by the end of 2014.[29] Tremendous research effort has been devoted to the develop-ments of PSCs, including designs of device architectures, [30–41]

applications of various n-type nanostructures,[42–49]

chemicalmanagements of perovskite compositions,[50–60] various depo-sition techniques for high-quality perovskite films[61–68] etc.HTMs, which extract photogenerated holes from the perovskiteand transport these charges to the back contact metal electrode,play an important role in minimizing recombination losses atthe TiO2 /perovskite/HTM interface, and thus achieving highperformances. Consequently, searching for an efficient HTMhas been one of the hottest research topics in PSCs. This reviewarticle is specifically focused on various HTMs being developedin PSCs.

2. Perovskite Solar Cells2.1. Device Architectures

Typical device architectures of PSCs are analogous to ss-DSCs,as depicted in Figure 1b (left). The fabrication procedure is asfollows: first, a compact TiO2 blocking layer is formed on atransparent-conductive oxide (TCO) substrate, such as fluorine-doped tin oxide (FTO). A mesoporous n-type TiO2 layer is sub-sequently deposited on top of the compact blocking layer. Theperovskite film is then spin-coated on the mesoporous TiO2 layer from a solution using a solvent such as N , N -dimethyl-formamide (DMF), γ -butyrolactone (GBL) or dimethyl sulfoxide(DMSO), followed by the deposition of a thin layer of HTM.

Finally, a metal electrode, such as gold (Au) or silver (Ag), isthermally evaporated on top of the HTM. The thicknesses ofthe HTMs are typically in the range of ≈100–200 nm to cir-cumvent direct contacts between the perovskites and metalelectrodes, thus minimizing charge recombinations. The keycharge-transfer processes involved in PSCs are depicted inFigure 1c.[69] Following the photoexcitation (1), the perovskitelight absorbers inject electrons into titania (2), while holes aretransferred to HTM (3). These photogenerated charge carriersare subsequently collected as photocurrent at the front and backcontacts of the solar cell. Undesirable charge-transfer processesalso occur, including recombinations of photogenerated species

Adv. Energy Mater. 2015, 1500213


Figure 1. a) Crystal structure of cubic perovskite with generic chemicalformula ABX3 . Reprinted with permission.

[25] Copyright 2014, Nature Pub-lishing Group. b) Device configurations of mesoscopic (left) and planar(right) perovskite solar cells. c) Schematic illustration of charge-transferprocesses in perovskite solar cells.

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(4) and back charge-transfer processes at TiO2 /perovskite/HTMinterface (5–7).

The mesoporous TiO2 layer can also be replaced by an inertscaffold, such as Al2 O3 , as demonstrated by Snaith et al.

[24] Inthis case, perovskite acts not only as a sensitizer, but also asan electron-transporter. Thus, the high-temperature processingmesoporous oxide layer seems not to be necessarily required,because of the dual characteristics of perovskite (light harvestingand electron-transporting). The depletion of mesoporous scaf-fold leads to the exploration of planar configurations of PSCs.In planar structures as shown in Figure 1b (right), a thin layerof perovskite is only sandwiched between the compact TiO2 blocking layer and HTM, which makes the fabrication processmuch simpler as compared to the mesoscopic configurations.

More intriguingly, perovskite MAPbI3 was also discoveredas both a light harvester and a hole-transporter in a HTM-freedevice.[35] This ambipolar charge transport propertiy of perovskitemakes the designs of PSCs quite versatile, leading to the emer-gence of so-called inverted planar configurations of PSCs. In aninverted structure, perovskite thin film is deposited on top of an

electron blocking layer, such as poly(3,4-ethylenedioxythiophene)poly(styrene-sulfonate) (PEDOT:PSS) or NiO. An electron acceptor,such as [6,6]-phenyl C61-butyric acid methyl ester (PCBM), is thendeposited on top of the perovskite layer. Inverted mesoscopicconfigurations have also been developed, in which mesoporousNiO or Al2 O3 were incorporated. The inverted cell structures arebeyond the scope of this review, readers who are interested in thistopic please refer to the fruitful literature.[39–41,68,70–75]

2.2. Deposition Techniques for Perovskites

A high-quality perovskite film plays an essential role in deter-mining the photovoltaic performance and reproducibility of aPSC device. With this respect, numerous deposition techniqueshave been developed to fabricate uniform and dense perovskitelayers for both mesoscopic and planar configurations.

2.2.1. Deposition Techniques for Mesoscopic Perovskite Solar Cells

For mesoscopic structures, perovskite films were initiallydeposited using a one-step solution deposition (OSSD) methodvia spin-coating from a precursor solution, in which PbX2 andCH3 NH3 X (X = Cl, Br, I) were dissolved in a polar solvent, suchas GBL, DMF or DMSO.[23,24,59,76] However, this method com-monly led to uncontrolled morphological variations, which

resulted in poor reproducibility of device performances.[77] Inorder to gain a better control of perovskite morphology, a two-step sequential deposition (TSSD) method was subsequentlydeveloped by Grätzel et al. as depicted in Figure 2 a.[61] PbI2 wasfirst spin-coated from a DMF solution onto the mesoporousTiO2 film, followed by dipping it into a solution of methylam-monium iodide (MAI) in iso-propanol (IPA). During the secondstep, perovskite formation occurred in a few seconds within themesoporous TiO2 film. The confinement of the PbI2 within thenanoporous network of the TiO2 film greatly facilitates its con-version to the perovskite. As shown in Figure 2b, a ≈350 nmthick mesoporous TiO2 film is infiltrated with the perovskite

nanocrystals using the TSSD method. In comparison to theone-step deposition method, the sequential one allowed muchbetter control of the perovskite morphology, leading to a PCE of≈15% with high reproducibility.

On the basis of the TSSD method, Park and Grätzel et al. fur-ther developed a two-step spin-coating deposition (TSSCD) tech-nique, in which MAI solution was also spin-coated onto the PbI2 film instead of using the soaking procedure.[63] This TSSCD pro-cedure was found highly reproducible, resulting in an averagePCE as high as 16%, with a small standard deviation. Seok andco-workers developed a solvent-engineering technique (SET) toprepare extremely uniform and dense perovskite layers for one-step solution deposition method.[64] A solvent (e.g. toluene orchloroform) that does not dissolve the perovskite materials and

is miscible with the perovskite precursor solution solvent wasdropped during spin-coating. The role of adding toluene is toinduce a rapid increase of concentration of perovskite precursormaterials uniformly across the entire substrate surface duringspin-coating.[78] This SET method led to a certified PCE of 16.2%for mesoscopic PSCs with no hysteresis effect.

2.2.2. Deposition Techniques for Planar Perovskite Solar Cells

In planar structures, the film morphologies of perovskites areeven crucial for achieving high-performing devices, as pin-hole



Figure 2. a) Two-step sequential deposition method for depositing per-ovskite MAPbI3 . b) Cross-sectional SEM image of a complete photovol-taic device fabricated through TSSD method. Panel (b) reproduced withpermission.[61] Copyright 2013, Nature Publishing Group.

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formation and incomplete coverage of perovskite give rise tolow-resistance shunting paths and loss of light absorption in thesolar cell.[33] To this end, several deposition methods have beenexplored to prepare high-quality, pin-hole free planar perovskitefilms. Snaith and co-workers first demonstrated efficient planarPSCs with a PCE of 15.4% by using a dual-source vapour depo-sition (DSVD) technique to deposit a thin film of mixed-halideperovskite, as depicted in Figure 3 a.[62] The top-view of scan-ning electron microscopy (SEM) images (Figure 3b,c) clearlyhighlight the remarkable differences of film morphologiesbetween vapor-deposited and solution-processed methods. Asshown in Figure 3b, the vapor-deposited films are extremelyuniform with crystalline features on the length scale of hun-dreds of nanometers. In contrast, the solution processed onescoat the substrate only partially, with crystalline platelets on thelength scale of tens of micrometers (Figure 3c).

Yang and co-workers have also prepared a uniform perovs-kite film using a vapor-assisted solution process (VASP) tech-nique, in which a vapor of MAI at a low temperature (150 °C)was exposed to a spin-coated thin film of PbI2 in a N2 atmos-phere.[65] Recently, ambient pressure aerosol-assisted chemical

vapor deposition (AACVD) method has proved to be a prom-ising technique to deposit thin film of MAPbI3 or MAPbBr3 onglass substrate.[79,80] For a typical AACVD, the PbX2 (X = Br orI) and MAX (X = Br or I) are mixed in stoichiometric ratio inDMF. The precursor solution is nebulized and the aerosol isthen carried out into a hot reactor (≈200–250 °C) containing aglass substrate by an inert carrier gas such as Argon. A thinfilm of perovskite is formed through thermal decompositionof the precursor solution. In another report, Qi and co-workersintroduced hybrid chemical vapor deposition (HCVD) methodto deposit perovskite film for planar PSCs.[81] The PbCl2 sub-strate made by thermal evaporation and MAI are loaded into

two separate temperature control zones of the furnace. The fur-nace is then sealed, pumped down to a certain pressure andpurged with an inert gas. Perovskite thin film is thus formedthrough vapor phase deposition of MAI. The perovskite filmfabricated using HCVD technique showed a promising effi-ciency and long-term stability under dry nitrogen gas. Thesechemical vapor deposition techniques provide attractive routesto scale up of perovskite film growth from industrial develop-ments point of view.

Seok and co-workers have also applied the solvent-engi-neering technique to form a uniform and dense planar perovs-kite film.[64] However, large hysteresis effects of the current/voltage curves were observed for planar PSCs fabricated bysuch technique. Huang et al. developed gas-assisted solutionprocessing (GASP) technique to form a uniform perovskite thinfilm consisting of densely packed single crystalline grains.[67] A40 psi Argon gas stream was applied during the spin-coatingof MAPbI3 DMF solution. This gas-assisted method resultedin fast evaporation of solvent, promoting rapid supersaturationand precipitation of the perovskite components. The planar

PSCs constructed from these films produced an impressiveaverage PCE of 15.7% with the highest value reaching 17.0%.

3. Hole-Transporting Materials in PerovskiteSolar Cells

An ideal HTM should fulfil some general requirements tomake it working more efficiently in PSCs, such as a compat-ible HOMO (highest occupied molecular orbital) energy levelof HTM to the valence band energy (VBE) of the perovskite,sufficient hole mobility as well as excellent thermal and pho-tochemical stability etc. A great number of HTMs have beendeveloped and incorporated in PSCs, which are composed oforganic and inorganic hole-conductors. Organic hole-conduc-tors can be essentially divided into three categories: small mole-cule hole-conductors, conducting polymers and organometalliccomplexes.

3.1. Organic Hole-Transporting Materials

3.1.1. Small Molecule Hole-Conductors

Triphenylamine (TPA)-based compounds are the most pop-ular small molecule HTMs in PSCs. Among these TPA-basedHTMs, spiro-OMeTAD (1) (Figure 4 ) has been the most rep-

resentative example being prevalently used in PSCs. The firstattempt of solid-state PSCs was made by the substitution ofliquid electrolyte with this molecular HTM, exhibiting PCEsof ≈10% in mesoscopic cell structures.[23,24] Through judiciousmorphology control of the perovskite absorber by sequentialdeposition methods, the PCEs were remarkably improved toover 15% based on such HTM (Table 1 ).[61,63] High-perfor-mance planar PSCs, where various methods have been appliedto deposit homogeneous and smooth perovskite films, havealso been obtained using spiro-OMeTAD as HTMs, with highPCEs ranging between 15–19%.[62,67,82] Due to the relativelylow hole mobility in its pristine form, additives and chemical



Figure 3. a) Dual-source vapor deposition system for depositing the per-ovskite absorbers; the organic source is MAI and the inorganic sourcePbCl2 . b,c) SEM top views of a vapor-deposited perovskite film and asolution-processed perovskite film, respectively. Reproduced with permis-sion.[62] Copyright 2013, Nature Publishing Group.

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Figure 4. Chemical structures of small molecule HTMs used in PSCs.

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Continued



Table 1. HOMO levels of various HTMs and their photovoltaic parameters in PSCs based on different device configurations.

HTM Acronyma) HOMOb)

[eV]

Device Configurationc) Perovskites Deposition

techniques

Dopantsd) J sc

[mA cm−2 ]

V oc

[V]

FF PCEe)

[%]

Ref.

1 Spiro-OMeTAD –5.22 M MAPbI3 TSSD LiTFSI, TBP, FK209 20.0 0.99 0.73 15.0 [61]

P MAPbI3– x Clx OSSD LiTFSI, TBP, FK209 22.8 1.13 0.75 19.3 [82]

2 pm -spiro-OMeTAD –5.31 M MAPbI3 SET LiTFSI, TBP 21.1 1.01 0.65 13.9 (14.9) [83]

3 po -spiro-OMeTAD –5.22 M MAPbI3 SET LiTFSI, TBP 21.2 1.02 0.77 16.7 (14.9) [83]

4 Py-A –5.41 M MAPbI3 OSSD LiTFSI, TBP, FK209 10.8 0.89 0.35 3.3 (12.7) [84]

5 Py-B –5.25 M MAPbI3 OSSD LiTFSI, TBP, FK209 20.4 0.95 0.64 12.3 (12.7) [84]

6 Py-C –5.11 M MAPbI3 OSSD LiTFSI, TBP, FK209 20.2 0.89 0.69 12.4 (12.7) [84]

7 H101 –5.16 M MAPbI3 TSSD LiTFSI, TBP, FK102 20.5 1.04 0.65 13.8 (13.7) [85]



10 KTM3 –5.29 (PES) M MAPbI3 TSSD LiTFSI, TBP, FK269 13.0 1.08 0.78 11.0 (11.4) [87]

11 T101 –5.29 M MAPbI3 TSSD LiTFSI, TBP, FK102 13.5 1.00 0.63 8.4 (12.9) [88]



14 Triazine-Th-OMeTPA –5.04 M MAPbI3 TSSD — 20.7 0.92 0.66 12.5 (13.5) [89]

15 Triazine-Ph-OMeTPA –5.11 M MAPbI3 TSSD — 19.1 0.93 0.61 10.9 (13.5) [89]

16 OMeTPA-FA –5.15 M MAPbI3 TSSD LiTFSI, TBP, FK209 21.0 0.97 0.67 13.6 (14.7) [90]

17 FA-MeOPh –5.15 M MAPbI3 TSSD LiTFSI, TBP 18.4 0.92 0.70 11.9 (12.8) [91]

18 OMeTPA-TPA –5.13 M MAPbI3 TSSD LiTFSI, TBP, FK209 20.9 0.95 0.62 12.3 (14.7) [90]

19 TPA-MeOPh –5.29 M MAPbI3 TSSD LiTFSI, TBP 17.3 0.99 0.63 10.8 (12.8) [91]

20 Fused-F –5.23 M MAPbI3 TSSD — 17.9 1.04 0.68 12.8 (11.7) [92]

21 — –5.23 M MAPbI3 TSSD — 17.9 0.94 0.69 11.6 (12.1) [93]

22 — –5.35 M MAPbI3 TSSD — 18.1 0.92 0.68 11.3 (12.1) [93]

23 MeO-DATPA –5.02 M(Al2 O3 ) MAPbI3 OSSD H-TFSI, Et4N-TFSIf) 16.4 0.96 0.56 8.8 (12.6) [94]

24 Me2 N-DATPA –4.40 M(Al2 O3 ) MAPbI3 OSSD H-TFSI, Et4N-TFSIf) 18.8 0.87 0.50 8.0 (12.6) [94]

25 TPBS –5.30 M MAPbI3 TSSD — 15.8 0.93 0.70 10.3 (13.3) [95]

26 TPBC –5.33 M MAPbI3 TSSD — 19.3 0.94 0.72 13.1 (13.3) [95]

27 — –4.96 M MAPbI3 OSSD — 16.3 0.94 0.60 9.1 (10.2) [96]

30 CBP –6.00 (PES) M(Al2 O3) MAPbBr3– x Clx OSSD LiTFSI, TBP 4.0 1.50 0.46 2.7 [97]

31 X19 –5.00g) M MAPbI3– x Clx OSSD LiTFSI, TBP 17.1 0.76 0.58 7.6 (10.2) [99]

32 X51 –5.23g) M MAPbI3– x Clx OSSD LiTFSI, TBP 16.8 0.88 0.66 9.8 (10.2) [99]

33 — –5.25g) M MAPbI3 TSSD LiTFSI, TBP,FK209 19.8 0.96 0.70 13.3 (15.2) [100]

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p-dopants, such as lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI), 4-tert-butylpyridine (TBP) and cobalt

(III) complexes, have been routinely incorporated into thespiro-OMeTAD solution as it did in ss-DSCs to enhance thehole conductivity, and thus the conversion efficiencies of PSCdevices.

Methoxy (OMe) groups in spiro-OMeTAD are thought toplay an important role in controlling the electronic propertiesof such HTM. In this context, two spiro-OMeTAD derivatives(2 and 3) were synthesized and tested as HTMs in comparisonto their predecessor in mesoscopic MAPbI3 -based PSCs.

[83] Theposition of one of the two para ( p )-OMe substituent in each ofthe quadrants of spiro-OMeTAD was replaced by meta (m )- orortho (o )-OMe substituent. It is disclosed that the cell perfor-

mance is strongly dependent on the position of the methoxygroup. The ortho-substituted derivative (3) presents a better

overall efficiency (16.7%) as compared to its predecessor(≈15%) under standard AM 1.5G irradiation. This improvedperformance arises from a larger fill factor (FF ) associatedwith a low series resistance and a high shunt resistance. Inthis study, the photovoltaic parameters were taken by averagingthe current–voltage curves measured in forward and reversescan directions because large hysteresis effects were observedfor the PSCs measured under standard AM 1.5 illumination.The preparation of the spirobifluorene core in spiro-OMeTADmolecule involves extensive synthetic processes, which con-siderably increase the production costs. Seok and co-workersreplaced the spirobifluorene core with a pyrene centre based on

Table 1. Continued

HTM Acronyma) HOMOb)

[eV]

Device Configurationc) Perovskites Deposition

techniques

Dopantsd) Jsc

[mA cm−2]

V oc

[V]

FF PCEe)

[%]

Ref.



36 — –5.39 (PES) P MAPbI3– x Clx TSSD — 15.3 0.95 0.60 8.8 (8.9) [101]

37 — –5.26 M MAPbI3 TSSD — 16.4 0.99 0.65 10.5 [102]

38 — –5.10 M MAPbI3 TSSD — 15.2 0.90 0.69 9.5 [102]

39 M1 –5.29 P MAPbI3 TSSD — 19.1 1.02 0.68 13.2 (8.9) [103]

40 — –5.05 M MAPbI3 TSSD — 19.9 0.86 0.64 11.0 (6.2) [104]

41 P3HT –5.20 P MAPbI3– x Clx OSSD — 20.8 0.92 0.54 10.4 [106]

P MAPbI3– x Clx OSSD LiTFSI, D-TBP, 19.1 0.98 0.66 12.4 [107]

42 PCBTDPP –5.40 M MAPbBr3 OSSD — 4.5 1.16 0.59 3.0 [76]

M MAPbI3 OSSD — 13.9 0.83 0.48 5.6 [76]

43 PDPPDBTE –5.40 M MAPbI3 OSSD LiTFSI, TBP 14.4 0.86 0.75 9.2 (7.6) [113]

44 — –5.22 M MAPbI3 OSSD — 15.4 0.89 0.64 8.7 [116]

45 — –5.25 M MAPbI3 OSSD — 14.4 0.83 0.62 7.4 [116]

46 — –5.24 M MAPbI3 OSSD — 12.0 0.84 0.66 6.6 [114]47 PCPDTBT –5.30 M MAPbI3 OSSD LiTFSI, TBP, 10.3 0.77 0.67 5.3 [115]

48 PCDTBT –5.45 M MAPbI3 OSSD LiTFSI, TBP 10.5 0.92 0.44 4.2 [115]

49 PTAA –5.20 (PES) M MAPbI3– x Brx SET LiTFSI, TBP 19.5 1.09 0.76 16.2 [64]

M (FAPbI3 )1– x(MAPbBr3 )x

SET LiTFSI, TBP 22.5 1.11 0.73 18.4 [60]

50 — –5.44 (PES) M MAPbBr3 SET LiTFSI, TBP 6.3 1.36 0.70 6.0 [117]

M MAPbI3 SET LiTFSI, TBP 6.1 1.40 0.79 6.7 [117]

51 — –5.51 (PES) M MAPbBr3 SET LiTFSI, TBP 8.9 0.92 0.56 4.6 [117]

M MAPbI3 SET LiTFSI, TBP 19.0 1.04 0.46 9.1 [117]

52 TFB –5.30 M MAPbI3 OSSD LiTFSI, TBP 17.5 0.96 0.65 10.9 (9.8) [118]

53 PFB –5.10 M MAPbI3 OSSD LiTFSI, TBP 13.8 0.91 0.64 8.0 (9.8) [118]

54 PANI –5.27 M MAPbI3 OSSD LiTFSI, TBP 18.0 0.88 0.40 6.3 [119]55 CuPc –5.20 (N) M MAPbI3– x Clx OSSD — 16.3 0.75 0.40 5.0 [127]

56 CuI –5.20 (N) M MAPbI3 OSSD — 17.8 0.55 0.62 6.0 (7.9) [128]

57 CuSCN –5.30 (N) M MAPbI3 TSSD — 19.7 1.02 0.62 12.4 [131]

a) Full names of the compounds are provided in the Supporting Information; b) From electrochemistry unless stated otherwise, PES: from photoelectron spectroscopy,

N: not mentioned; c) M and P stand for mesoscopic and planar cell configurations, respectively; M(Al 2 O3 ) represent mesoporous Al2 O3 scaffold, others are based on

titania; d) FK209, tris(2-(1H -pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tris(bis(trifluoromethylsulfonyl)imide); FK102, tris(2-(1H -pyrazol-1-yl)pyridine)cobalt(III)

tris(hexafluorophosphate); FK269, bis[2,6-di(1H -pyrazol-1-yl)pyridine] cobalt (III) tris(bis(trifluoromethylsulfonyl)imide); e) The photovoltaic parameters are based on the

best-performing devices. The corresponding efficiencies for spiro-OMeTAD are given in the parentheses; f) H-TFSI: bis(trifluoromethanesulfonyl)imide; Et4 N-TFSI: tetraethyl

bis-(trifluoromethane)sulfonamide; g) Conversion based on spiro-OMeTAD.



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N,N -di- p -methoxyphenylamine substituents.[84] Three pyrene-core arylamine derivatives (4–6) were developed as HTMs inmesoscopic PSCs based on MAPbI3 absorber. Compounds5 and 6 exhibit comparable overall efficiencies (≈12.3%) withrespect to that of spiro-OMeTAD (12.7%). However, a muchlower performance is observed for compound 4, mainly dueto an insufficient driving force for hole injections related to itsdeeper HOMO energy level (–5.41 eV, Figure 5 ) as compared tothe VBE of MAPbI3 (–5.43 eV).

Mhaisalkar and Grimsdale et al. developed a much simplersmall molecule hole-conductor (7) incorporating 3,4-ethylenedi-

oxythiophene (EDOT) as a core structure.[85] Through optimiza-tion of the concentration of cobalt dopant FK102, a maximumefficiency of 13.8% is achieved under 1 sun illumination (AM1.5G), which rivals its counterpart based on spiro-OMeTAD(13.7%). They further synthesized two TPA-based compoundscontaining tetrasubstituted thiophene and tetrasubstituted bith-iophene core units (8 and 9).[86] In comparison to previouslyreported compound 7, the synthesis of these two new com-pounds use cheaper starting materials instead of using EDOT.Moreover, the highest occupied molecular orbital (HOMO)levels of new compounds are deeper than that of compound 7.Deeper HOMO levels are expected to result in higher Voc s, as

the photovoltage is given by the difference in the quasi Fermilevels of both the electron and hole conducting materials. Theca. 80–100 mV deeper HOMO levels as compared to spiro-OMeTAD result in marginally increased Voc s (≈20–30 mVhigher). Overall efficiencies of over 15% are achieved by usingthese two HTMs, which surpass that of spiro-OMeTAD. Thesecompounds (7–9) set good models for designing new smallmolecule HTMs to replace the expensive spiro-OMeTAD giventheir much simpler synthesises and high performances. Byincorporating a swivel-cruciform 3,3′-bithiophene core struc-ture, another TPA-based small molecule HTM (10) was also

tested and showed a competitive overall efficiency relativeto spiro-OMeTAD under the same doping conditions. [87] Ahigher FF is observed for this new HTM, presumably associ-ated with a decrease in charge recombination. The authorstentatively attributed this to the fact that the torsional relaxa-tion in compound 10 may allow for a better interaction withthe perovskite surface and the extended delocalization lengthfor the radical cation. It highlights here that the molecularinteraction between HTM and the perovskite appear to play arole in determining the photovoltaic parameters. In anotherreport, Mhaisalkar and Grimsdale et al. also developed aseries of small molecule HTMs (11, 12 and 13) based on core



spiro-OMeTAD

po-spiro-

OMeTAD

Py-C

Py-A

H101

H112

T102

37

38

40

P3HT

PCBTDPP PDPPDBTE

PCPDTBT

PCDTBT

PTAA

5051

PANI CuPc

CuI

CuSCN

39

PFB

Triazine-Th-OMeTPA

–5.0

–5.1

–5.2

–5.3

–5.4

–5.5

E (eV)

Figure 5. HOMO levels of representative HTMs used in PSCs.

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triptycene in short synthetic routes with high yields, exhibitingpromising conversion efficiencies (12.24% for 12 and 12.38%for 13).[88]

Ko and co-workers designed a series of star-shaped molec-ular TPA-based HTMs incorporating a 1,3,5-triazine core(14 and 15), a fused quinolizino arcidine core (16 and 17) anda triphenylamine core (18 and 19). Reasonable efficienciesbetween 10.79–13.63% are obtained based on these star-shapedHTMs.[89–91] Grätzel and Nazeeruddin et al. synthesized a low-band-gap (1.57 eV) star-shaped molecular HTM 20, which isalso based on a fused quinolizino arcidine core with thiophene-based side arms.[92] A PCE of 12.8% is achieved by using thisHTM alone, which outperforms its counterpart based on spiro-OMeTAD with dopants (11.7%). The device with HTM 20 dis-plays a much higher incident photon-to-current conversionefficiency (IPCE) values in the blue region than that of spiro-OMeTAD, implying that this HTM not only serves as a HTMbut also contributes to the overall photocurrent. Other typesof TPA-based molecular HTMs have also been attempted andshown reasonable performances, including butadiene deriva-

tives (21 and 22), diacetylide-TPA compounds (23 and 24),N ,N ,N ′ ,N ′ -tetraphenyl-benzidine (TPB) derivatives (25 and26) and π -conjugated linear modified TPA-based compounds(27–29).[93–96]

Carbazole derivatives have also been studied as HTMs inPSCs. A commercially available carbazole-based compound (30)was first adopted as a molecular hole-conductor in combinationwith MAPbBr3–x Clx absorber.

[97] By p-doping with this HTM, asignificantly higher Voc up to 1.5 V is obtained, due to the muchdeeper HOMO of compound 30 (–6.0 eV).[98] However, the largerband gap of perovskite used in this work results in very poor pho-tocurrent density with a maximum value of only 4.0 mA cm −2 ,and thus a poor overall efficiency. Sun and co-workers devel-oped and tested two carbazole-based compounds (31 and 32)as HTMs in mesoscopic MAPbI3–x Clx -based PSCs.[99] HTM32 exhibits both high hole mobility (determined by a space-charge-limited current (SCLC) method) and high conductivityas compared to 31, related to a more efficient π –π stacking inthe film of 32. Therefore, a higher PCE of 9.8% is achievedfor HTM 32, which can even rival that of spiro-OMeTAD(10.2%) under optimized conditions. In another report, Leeand co-workers synthesized three carbazole-based moleculeswith two-arm and three-arm type structures, which are linkedthrough phenylene, diphenylene or TPA core units (33, 34and 35).[100] PCEs of over 13% are obtained for these three car-bazole-based HTMs, with the highest value reaching 14.79%for HTM 34.

A hydrophobic oligothiophene derivative 36, containing abackbone structure of a benzodithiophene (BDT) unit as thecentral block and ethylrhodanine as an end group, was appliedas a HTM in planar PSCs based on MAPbI3–x Clx .

[101] Devicesbased on this HTM 36 exhibited an improved stability as com-pared to spiro-OMeTAD under a high humidity condition(>50%). HTM 36 shows a big water contact angle of 107.4°, sothat this hydrophobic HTM probably can prevent the penetra-tion of water into the perovskite layer. More strikingly, HTM36 with additional additives, such as LiTFSI, shows a smallerwater contact angle (ca. 80°) and poor durability, implyingthat the incorporations of hydroscopic ion additives need to

be taken into consideration from long-term durability pointof view. Grätzel and co-workers introduced two low-band-gapacceptor-donor-acceptor (A–D–A) oligothiophenes (37 and 38)as HTMs, consisting of electron-rich thiophene-pyrrole-basedS ,N -heteropentacene central units and terminal dicyanovi-nylene acceptor groups.[102] MAPbI3 -based devices based onthese two HTMs without any additives yield overall efficien-cies of 9.5–10.5% in comparison to a reference device withoutHTM (7.6%). The band gap of HTMs 37 and 38 are 1.49 eVand 1.36 eV, respectively, as shown in the energy diagram of thePSC devices used in this work (Figure 6 a). Therefore, both ofthese two HTMs have strong absorption between 600–800 nmthat is complementary to the perovskite MAPbI3, the absorptionof which becomes weaker after 600 nm (Figure 6b). Deviceswith HTMs 37 and 38 show noticeable enhancements of IPCEvalues (Figure 6c) over the whole region between 400–800 nmas a result of more effective charge extraction and/or light har-vesting. An obvious additional band between 680–800 nm canbe seen for HTM 37, which is correlated well with the absorp-tion maximum in the film (Figure 6b). From the light har-

vesting efficiency (LHE) spectrum (Figure 6d) for devices withand without HTMs, we can see that the LHEs are nearly iden-tical between 400–500 nm, mainly arising from the perovskite.After 550 nm, the LHEs are considerably larger when HTMs arepresent. This result clearly demonstrates that these two HTMsalso contribute to the light harvesting together with the perovs-kite. Photoinduced absorption spectroscopy (PIA) was furthercarried out to scrutinize the charge generation in the devicesbased on HTMs 37 and 38 with and without perovskites. Aspresented in Figure 6e, a TiO2 /37 film without perovskiteshows a negative band at wavelengths shorter than 870 nmand a positive band beyond 950 nm due to the ground statebleaching and absorption of the oxidized species of 37 afterphotoexcitation. This result indicates that charge separationoccurs between TiO2 and 37 even without the presence of per-ovskite. In the presence of the perovskite, the absorption fea-tures of the oxidized species of 37 are stronger extending from800–1400 nm. Meanwhile, the negative band from the emissionof the perovskite is quenched. A similar phenomenon is alsoobserved for HTM 38 (Figure 6f). The PIA results illustrate thateffective charge transfer takes place between the photoexcitedperovskite and HTMs 37 and 38 under formation of long-livedcharged species. Therefore, these two oligomers not only act asHTMs, but also contribute to the light absorption in the lowenergy region of the solar spectrum forming a dual light har-vesting system with the perovskite.

Recently, Sun and co-workers reported another A–D–A

structured small molecule phenoxazine (POZ)-based HTM(39), containing an electron-rich benzo[1,2b:4,5b′]-dithiophene(BDT) unit as a central building block, flanked by POZ units,and end-capped with electron-withdrawing 3-ethylrodanine.[103] Planar devices based on pristine HTM 39 yield an overall effi-ciency of 13.2% under standard AM 1.5G illumination, whichis significantly higher than that of dopant-free spiro-OMeTAD(8.9%). The better performance of devices based on HTM39 should be mainly ascribed to a higher FF , strongly relatedto a relatively higher hole mobility (2.71 × 10−4 cm2 V−1 S−1 ,determined by SCLC method) and a much higher hole conduc-tivity (1.16 × 10−3 S cm−1 ) in its pristine form as compared the



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corresponding values for spiro-OMeTAD (1.32 × 10−4 cm2 V−1 S−1 and 1.57 × 10−4 S cm−1 ). Another dopant-free small moleculeHTM based on a tetrathiafulvalene derivative (40) was also intro-duced in mesoscopic MAPbI3 -based PSCs, showing a compa-

rable efficiency (11.03%) in comparison to spiro-OMeTAD withdopants (11.4%).[104] Moreover, the stability of devices basedon pristine form of this HTM exhibited slight and slow degra-dation under ambient conditions at room temperature with ahumidity of ≈40%, which was more stable than its counterpartspiro-OMeTAD with dopants (Figure 7 ). The authors attributethe improved stability to the avoidance of using the deliquescentadditives. It emphasizes once again that hydroscopic additivesmight accelerate the degradation of the PSC devices. It also mir-rors that the developments of HTMs with high hole mobility intheir pristine forms are of great importance from long-term sta-bility point of view.

3.1.2. Conducting Polymers

Conducting polymers constitute another major class of organicHTMs in PSCs. Polythiophene-based conducting polymer

poly(3-hexylthiophene-2,5-diyl) (P3HT (41), Figure 8 ), has been amodel polymeric HTM being intensively studied, exhibiting rea-sonable results with the highest efficiency of over 10%. [76,104–115] Based on pristine P3HT in conjunction with MAPbI3–x Clx inplanar PSCs, an overall efficiency of exceeding 10% is reachedthrough the optimization of deposition parameters and pre-cursor concentrations.[106] In another report, doped with LiTFSIand a pyridine-based additive (2,6-di-tert -butylpyridine, D-TBP),the efficiency of pristine P3HT is significantly increased from9.2% to 12.4%, owing to a two orders of magnitude larger holeconductivity.[107] Johansson et al. carried out transient photo-voltage decay measurements to compare the electron lifetimes



Figure 6. a) Energy level diagram of the components used in PSCs based on HTMs 37 and 38 . b) UV-vis absorption spectra of HTMs 37 and 38 coatedon mesoporous TiO2 and TiO2 /perovskite films. c) IPCE and d) LHE spectra of PSCs based on HTMs 37 and 38 , and a reference cell without HTMs.e) PIA spectra of mesoporous TiO2 films coated with perovskite, HTM 37 , and perovskite/37 . f) PIA spectra of mesoporous TiO2 films coated withperovskite, HTM 38 , and perovskite/38 .Excitation wavelength: 642 nm. Panels (b–f) adapted with permission. [102] Copyright 2014, the Royal Societyof Chemistry.

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of devices based on P3HT and spiro-OMeTAD. [110] It is foundthat the recombination rate in device with P3HT is more than10 times faster as compared to spiro-OMeTAD, presumablyarising from a closer contact between the flat molecular struc-ture of P3HT and the perovskite surface, which gives rise toa lower photovoltaic performance. It highlights here that themolecular structure should be designed to avoid close contactbetween the HTM and the perovskite. Snaith and co-workersintroduced C60 -substituted benzoic acid self-assembled mon-olayer as an interlayer between mesoporous titania and per-ovskite MAPbI3–x Clx in P3HT-based devices.

[111] The presenceof this interlayer not only significantly enhances the contribu-tion to the photocurrent by light absorbed in P3HT, but alsoenhances the photovoltage by a reduction of energy loss. Basedon pristine P3HT, a PCE of 6.7% is obtained in comparison to3.8% without such interlayer. Recently, Wang et al. developeda HTM composite consisting of P3HT and bamboo-structurednanotubes (BCNs).[112] The introduction of BCNs not onlyimproves the hole conductivity of P3HT, but also reduces chargecarrier recombination at perovskite/HTM interface correlatedto a superior film morphology. These positive effects lead to adramatic enhancement of the overall efficiency from 3.6% (pris-tine) to 8.3% (1 wt% BCNs).

Other types of thiophene-based conducting polymers havealso been incorporated as HTMs in PSCs. Qiu et al. intro-

duced polymer PCBTDPP (42) (Full name is provided in theSupporting Information) as a HTM in MAPbBr3 - and MAPbI3 -based solar cells.[76] Based on MAPbBr3 absorber, this HTM dis-plays a much higher V oc of 1.16 V as compared to P3HT (merely0.50 V), as a result of several factors including a deeper HOMOlevel (–5.4 eV) of 42, a high hole mobility (0.02 cm2 V−1 S−1 ,determined from FET (field effect transistor)), a large offsetbetween the conduction band of MAPbBr3 and the quasiFermi level of TiO2 as well as a possible interaction betweenTiO2 /perovskite/42. The photovoltaic performance is furtherenhanced to 5.55% by substituting MAPbBr3 with its iodidecounterpart, due to the broader light absorption of the latter

absorber. Another diketopyrrolopyrrole-containing thiophene-based polymer PDPPDBTE (43) was also studied in mesoscopicMAPbI3 -based PSCs.

[113] The solar cell device based on this poly-meric HTM displays an PCE of 9.2% exceeding that of spiro-OMeTAD (7.6%), attributable to a deeper HOMO level (–5.4 eV)and high hole mobility of such polymer (10−3 cm2 V−1 S−1 ,estimated from a transient mobility spectroscopy method)associated with a lower series resistance, and thus a high FF .Devices using HTM 43 also exhibited an excellent long-termdurability remaining over 90% of their initial efficiencies after1000 hours under a humidity of ≈20%, whereas spiro-OMeTADbased devices only remained ≈70% (Figure 9 a). As shown inFigure 9b, the water contact angle for HTM 43 is 105°, whileonly 70° for spiro-OMeTAD. This excellent long-term durabilityof 43 should be probably attributed to the hydrophobic proper-ties of such polymer, which prevents water penetration into theperovskite surface. As discussed previously for small moleculeHTM 36, the hydrophobic nature of a HTM seemingly playsa role to improve the long-term durability of PSCs. Otherthiophene-based conducting polymers (44–48) have also been

studied as HTMs in PSCs, giving moderate overall efficienciesranging between 4.2%–8.7%.[114–116]

Grätzel and Seok et al. compared three thiophene-basedpolymeric HTMs (P3HT, 47, 48) with PTAA (49), amongwhich PTAA presents the best performance with a max-imum PCE of up to 12% under standard AM 1.5G con-dition.[115] It has been claimed that the superior perfor-mance obtained with PTAA may be related to a strongerinteraction between the perovskite with PTAA, and itshigher hole mobility (≈1 × 10−2 to ≈1 × 10−3 cm2 V−1 S−1 ,determined from FET) as compared to other polymers.However, further studies are required to confirm the spe-cific chemical interaction between PTAA and the perovskite.In this study, a pillared structure was used, consisting ofthree-dimensional composites of TiO2 /MAPbI3 . Figure 10 apresents a cross-sectional SEM image of the solar cell device.It is clear that the pores of the mesoporous TiO2 are infiltratedwith MAPbI3 and an overlayer of MAPbI3 co-exist on top of theTiO2 film, whereas the polymer PTAA penetrates the scaffoldto a much smaller degree.[77] Figure 10b displays that severalmicrometre-sized MAPbI3 islands are densely formed on themesoporous TiO2 film. Under same conditions, an overall effi-ciency of only 8.4% is obtained for spiro-OMeTAD owing to alower Voc and FF . Based on polymeric HTM PTAA, Seok et al.further fabricated colourful PSCs through the chemical man-agements of MAPbI3–x Brx .

[59] The substitution of iodide withbromide leads to a maximum PCE of 12.3% under 1 sun illumi-

nation (AM 1.5G). More strikingly, devices with MAPbI3–x Brx (x ≥ 0.2) also exhibited improved long-term stability (Figure 10c)presumably associated with their compact and stable structure,because the substitution of larger iodide atoms with smallerbromide atoms leads to the reduction of the lattice constant anda transition to a cubic phase. On the basis of HTM PTAA inconjunction with MAPbI3–x Brx absorber (x = 0.1–0.15), a certi-fied PCE of 16.2% has been achieved in mesoscopic PSCs byusing a solvent-engineering technique to deposit high-qualityperovskite films.[64] Based on this SET technique and PTAA,Seok and co-workers very recently incorporated MAPbBr3 intoFAPbI3 to stabilize the perovskite phase of the latter one, which



Figure 7. Efficiency variation of the PSCs based on HTM 40 (pristine)and spiro-OMeTAD (doped). Unencapsulated cells were stored in air atroom temperature with a humidity of about 40% and were measuredunder standard AM 1.5G illumination. Reproduced with permission. [104]

Copyright 2014, the Royal Society of Chemistry.

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further improved the PCE to more than 18% under standardAM 1.5G irradiation (Figure 10d).[60]

In other work, two triarylamine polymer derivatives (50 and51) with deeper HOMO levels were studied in comparison toPTAA in mesoscopic PSCs based on MAPbI3 and MAPbBr3 .

[117] It is disclosed that the voltage output relies on both the VBE ofthe perovskite absorbers and the HOMO levels of HTMs. In com-bination with MAPbBr3 (VBE, –5.68 eV), deeper HOMO levels

50 and 51 (–5.44 eV for 50, –5.51 eV for 51)as compared to PTAA (–5.14 eV) indeed resultin 70–110 mV increases of photovoltages withthe high Voc up to ≈1.40 V. The larger V oc s ofthese HTMs with deeper HOMO levels coun-teract relatively lower J sc s presumably relatedto lower driving forces for hole injections,giving rise to better overall efficiencies withthe highest value of 6.7% for HTM 51. WhenMAPbI3 is applied, inferior performances areobserved for both 50 (4.6%) and 51 (9.1%) ascompared to PTAA (16.2%), mainly becauseof insufficient hole injections correlated to theenergy misalignments between the VBE ofMAPbI3 and HOMO levels of the HTMs.

Yang and co-workers introduced polyflu-orene derivative polymers that contain fluo-rine and arylamine groups (52 and 53) asHTMs in mesoscopic PSCs based on MAPbI3 absorber.[118] Devices based on HTM 52 pre-

sent promising efficiencies (10.9–12.8%),which are comparable to the correspondingvalues for spiro-OMeTAD (9.8–13.6%). Poly-aniline (PANI, 54) was also tested as a HTMin mesoscopic MAPbI3 -based PSCs.

[119] PANInanoparticles (PANI-NPs) with an averagesize of ≈20 nm were synthesized by the oxida-tive chemical polymerization of aniline mon-omer at 0–5 °C, and then were spin-coatedonto the surface of MAPbI3 /mesoporousTiO2 . Devices based on this polymeric HTMgive satisfactory Jsc of ≈18.0 mA cm

−2 andVoc of 0.87 V. However, a rather low FF (0.40)results in an overall efficiency of merely 6.3%.

3.1.3. Organometallic Compounds

Copper phthalocyanine (CuPc), the struc-ture of which is presented in Figure 11 , hasbeen widely studied as a p-type semicon-ductor in organic thin film transistors andOPVs due to its flexibility, high absorptioncoefficient, high hole mobility as well as itslow production cost.[120–126] Recently, CuPc(55) was employed as a HTM in mesoscopicMAPbI3–x Clx -based PSCs.

[127] CuPc possesses

a large π -conjugated system which makes itdifficult to dissolve in most commonly usedorganic solvent. This complex was there-fore thermally evaporated on top of the sur-

face of MAPbI3–x Clx /mesoporous TiO2 . CuPc devices displaysatisfactory J sc (16.3 mA cm

−2 ) and V oc (0.75 V). However,the FF is rather low (0.40) associated with a low shunt resist-ance, resulting in a maximum efficiency of only 5%. Never-theless, this is the first example employing an organometalliccompound in PSCs. There is still possible room for furtherimprovement of the performance if the charge recombinationlosses at the TiO2 /perovskite/CuPc interface can be suppressed.



Figure 8. Chemical structures of conducting polymers used in PSCs.

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3.2. Inorganic Hole-Transporting Materials

Inorganic p-type semiconductors appear to be good candidatesas alternative HTMs in PSCs due to their high hole mobility,ease of synthesises and thus low production costs. However,only few studies in terms of using inorganic HTMs in PSCshave been reported so far due to limited choices of suitablematerials. Kamat and co-workers reported the first exampleusing inorganic p-type hole-conductor copper iodide (CuI, 56)in mesoscopic PSCs.[128] The deposition of CuI onto the sur-face of mesoporous TiO2 /MAPbI3 film was performed by usinga custom-built apparatus, as shown in Figure 12 a. The FTOsubstrate (2 cm × 1.5 cm) was placed on a hot plate at 80 °Cwith the deposition needle (with a line of small holes, 0.3 mm)aligned parallel approximately 0.5 mm above the surface. CuIsolution (0.1 M in di-n-propyl sulfide and chlorobenzene) was

pumped at a constant rate of 25 µL min−

1 using a syringe pumpuntil a bead of solution formed between the bottom of theneedle and the surface of the solar cell across the entire widthof the FTO. The needle was moved back and forth above thesurface at a rate of 1 mm s−1 while CuI solution slowly infil-trated the TiO2 pores. In this way, a 1.5–2.0 µm thick CuI over-layer was formed on top of the mesoporous TiO2 /MAPbI3 film.CuI-based devices exhibit average efficiencies of 3.7% withthe highest value being 6% under 100 mW cm−2 illumination(AM 1.5G). The V oc (≈0.50 V) is found to be lower as comparedto devices using spiro-OMeTAD (0.78 V), attributed to fastercharge recombination rates in CuI-based devices as determined

Figure 9. a) Long-term stability of spiro-MeOTAD (black), P3HT(red),and PDPPDBTE (43 ) (blue). Cell devices prepared without encap-sulation under a 20% humidity atmosphere. b) Water contact angles ofthe spiro-MeOTAD, P3HT, and PDPPDBTE (43 )-coated films on FTOsubstrates. Reproduced with permission.[113] Copyright 2014, the RoyalSociety of Chemistry.

Figure 10. a) Cross-sectional SEM image of PSCs based on PTAA.b) SEM surface image of MAPbI3 -coated mesoporous TiO2 film. Panels(a,b)reproduced with permission.[115] Copyright 2013, Nature PublishingGroup. c) Efficiency variation of PSCs based on MAPbI3–x Brx using PTAAas HTMs (x = 0, 0.06, 0.20, 0.29) with time stored in air at room tem-perature without encapsulation. The humidity was maintained at 35%,and the cells were exposed to a humidity of 55% for one day on thefourth day to investigate performance variation at high humidity. Repro-duced with permission.[59] Copyright 2013, American Chemical Society.d) Current-voltage curves of forward and reverse bias sweep and theiraveraged curve for (FAPbI3 )0.85 (MAPbBr3 )0.15 -based PSCs using PTAA asa HTM. Reproduced with permission.[60] Copyright 2015, Nature Pub-lishing Group.

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© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1500213 (14 of 17) wileyonlinelibrary.com Adv. Energy Mater. 2015, 1500213


form the electrochemical impedance spectroscopy (EIS). EISalso reveals that CuI exhibits two orders of magnitude higherelectrical conductivity than spiro-OMeTAD, accounting for ahigher FF observed for CuI-based devices.

Another copper-based inorganic p-type hole-conductor

copper thiocyanate (CuSCN, 57) has also been actively studiedin PSCs.[129–132] Ito et al. used doctor-blading technique to

deposit CuSCN on top the mesoporous TiO2 /MAPbI3 .[129]

GuSCN-based device presents an efficiency of only 4.85% with J sc of 14.5 mA cm

−2 , V oc of 0.63 and FF of 0.53, respectively.Based on the same inorganic HTM, Ito and Nazeeruddinet al. further used the sequential deposition method to fabricateMAPbI3 absorber.

[131] In order to maximize the light harvestingby MAPbI3 layer, the loading of the mesoporous TiO2 with PbI2 was carried out twice. As shown in Figure 12b, the depositionof CuSCN by doctor-blading technique forms a ≈600 nm cap-ping layer, efficiently block contacts between the perovskite andAu. A maximum efficiency of 12.4% is achieved based on HTMCuSCN, which has been the highest value reported so far forinorganic HTMs.

4. Conclusion and Future Outlook

A great number of HTMs have been developed and applied inPSCs, including various newly designed small molecule hole-conductors, conducting polymers, as well as inorganic p-type

semiconductors. Spiro-OMeTAD that was well studied in solid-state DSCs, continues to exhibit high performances in PSCs.

Figure 12. a) Automated drop-casting apparatus used for deposition of CuI onto mesoporous TiO2 /MAPbI3 films. Reproduced with permission.[128]

Copyright 2014, American Chemical Society.b) Cross-section SEM image PSC device based on mesoporous TiO 2 /MAPbI3 /CuSCN/Au. Scale bar:200 nm. Reproduced with permission.[131] Copyright 2014, Nature Publishing Group.

Figure 11. Chemical structure of CuPc.

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Due to its relatively low hole mobility and its tedious synthesisstrongly correlated to its production cost, numerous alternativeHTMs have been explored with an aim to replace such a mate-rial. Although considerable progress has been made by alterna-tive HTMs, some key issues still need to be addressed in thefuture in order to further enhance the performances of PSCdevices from designing new HTMs point of view.

Numerous HTMs with deeper HOMO levels were evalu-ated in order to further improve the photovoltages, whereasthe values were marginally improved in most of the cases. Itimplies that other factors beyond energy levels, such as therecombination losses at perovskite/HTM interface, need tobe taken into consideration to sufficiently translate the deeperHOMO levels into higher photovoltages and accordingly betteroverall performances. The comparisons between P3HT witheither spiro-OMeTAD or PTAA,[110,115] which possess com-parable HOMO energy levels, also reflect that the structuralinteraction between the perovskite and HTM seemingly has animpact on the photovoltaic performance. However, the chemicalinteractions between perovskites and the structures of HTMs

have been rarely studied at current stage. A deep understandingof such interaction at perovskite/HTM interface is highly rec-ommended for rational designs of more efficient HTMs in thefuture.

The most efficient perovskites have band gaps of ≈1.5 eV(corresponding to an onset light absorption of ≈820 nm), a pho-tocurrent density of 28 mA/cm2 is theoretically achievable.[133] When considering 15–20% light reflection at the TCO glass,22–24 mA/cm2 of photocurrent density can be envisioned. Thephotocurrent density of the best-performing PSC devices hasalready reached 22 mA cm−2 , which is very close to its theoret-ical maximum. In order to further enhance the photocurrentsof PSCs, the exploration of low-band-gap HTMs appear to offerpossibilities to absorb additional photons at the longer wave-lengths in the near-infrared (NIR) region, where the absorp-tion of the perovskite becomes weaker. Besides being a hole-transporter, the HTM in this case also absorbs additional lightin the low energy region of the solar spectrum, which poten-tially could contribute to an enhancement of the photocurrent.The successful examples of low-band-gap oligothiopheneshave already proven that this strategy should be feasible if theenergy levels of the HTM and the perovskite can be properlyaligned.[102] In another report, PbS quantum dots have beenused as additional light harvesters together with MAPbI3 .

[134] The PbS quantum dots significantly contribute the light absorp-tion in the low energy region (up to 1000 nm), leading to a sur-prisingly high photocurrent density of 24.63 mA cm−2 . All these

findings suggest that the developments of low-band-gap HTMs,the absorptions of which extend to further NIR regions, wouldbe effective strategies to further enhance the photocurrents ofPSCs without suffering significant losses of photovoltages.

Perovskites are considerably conductive, for instance, the elec-tron mobility of MAPbI3 is estimated to be ≈66 cm

2 V−1 S−1 .[135] Such materials directly contact the back contact metal elec-trode will result in undesired shunting paths. A solution tothis problem is the use of thicker HTM layer to avoid pinholes.Thicker HTM layer requires that the conductivity of the HTMshould be sufficient enough to minimize the series resistance;otherwise resulting in a lower FF . Therefore, finding a HTM

with a high hole mobility in its pristine form is significantlycritical. Conducting polymers exhibit high hole mobility, suchas PTAA, appear to be suitable candidates to achieve better FF s.In a recent report, the thickness of PTAA layer was reduced to≈50 nm.[60] Highly conductive inorganic p-type hole-conductors,such as CuI and CuSCN, offer other possibilities, whereas lowFF s are commonly observed for these two materials currently.The suppressions of charge recombination losses between theperovskites and p-type hole-conductors may allow these mate-rials to work more efficiently in PSC devices. In addition todeveloping HTMs with high hole mobility, balancing the chargecarrier transport between the electron and hole transportingmaterials is also of great importance to reduce non-ideal spacecharge distributions within the device as demonstrated in ref.82. A better understanding of the photoelectrical behaviors ofperovskite semiconductors (exciton or free-carrier model)[136,137] will also provide some deep insights into material engineering(electron/hole contacts) and designs of device architectures tofurther enhance the performances of PSCs.

Nevertheless, we have seen significant progress on the

developments of alternative HTMs in PSCs. In stark contrastto the scenario in solid-state DSCs, where spiro-OMeTAD hasexclusively dominated the best-performing devices, alterna-tive HTMs, such as polymer PTAA, have shown competitiveor even better performances in PSCs. One should be optimis-tically confident that there is still a potential room for furtherenhancements of the overall efficiencies in PSCs throughrational designing new HTMs together with judicious combi-nations between these newly designed materials and perovskiteabsorbers.

Supporting Information

Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements

This work was supported by the General Financial Grant from the ChinaPostdoctoral Science Foundation (2014M551093), the National NaturalScience Foundation of China (21120102036, 91233201), the NationalBasic Research Program of China (973 program, 2014CB239402),the Swedish Energy Agency, as well as the Knut and Alice WallenbergFoundation.

Received: January 29, 2015Revised: March 11, 2015

Published online:

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