The Central Role of Ligand Conjugation for Properties of Coordination

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The Central Role of Ligand Conjugation for Properties of Coordination Complexes as Hole-Transport Materials in Perovskite Solar Cells Wei Zhang, Yong Hua, Linqin Wang, Biaobiao Zhang, Yuanyuan Li, Peng Liu, Valentina Leandri, Yu Guo, Hong Chen, James M. Gardner, Licheng Sun, and Lars Kloo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01223 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Applied Energy Materials

The Central Role of Ligand Conjugation for Properties of Coordination Complexes as HoleTransport Materials in Perovskite Solar Cells Wei Zhang,a Yong Hua,a Linqin Wang,b Biaobiao Zhang,b Yuanyuan Li,c Peng Liu,a Valentina Leandri,a Yu Guo,a Hong Chen,b James M. Gardner,a Licheng Sun,b, d and Lars Klooa* a)

Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

b)

Department of Chemistry, Organic Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

c)

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden d)

State Key Laboratory of Fine Chemicals, DUT-KTH Joint Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China

ABSTRACT

Two zinc-based coordination complexes Y3 and Y4 have been synthesized, characterized and their performance as hole-transport materials (HTMs) for perovskite solar cells (PSCs) have been

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investigated. The complex Y3 contains two separate ligands, and the molecular structure can be seen as a dis-connected porphyrin ring. On the other hand, Y4 consists of a porphyrin core, and therefore a more extended conjugated system as compared to Y3. The optical and redox properties of the two different molecular complexes are comparable. However, the hole-mobility and conductivity of Y4 as macroscopic material are remarkably higher than that of Y3. Furthermore, when employed as hole-transport materials in perovskite solar cells, cells containing Y4 show a power conversion efficiency (PCE) of 16.05%, comparable to the Spiro-OMeTAD based solar cells with an efficiency around 17.08%. In contrast, solar cells based on Y3 show a negligible efficiency of about 0.01%. The difference in performance of Y3 and Y4 are analyzed and can be attributed to the difference in packing of the non-planar and planar building blocks in the corresponding materials.

KEYWORDS Ligand conjugation, coordination complex, porphyrin, hole-transport material, perovskite INTRODUCTION Perovskite solar cells (PSCs) have attracted substantial attention during the last 5-10 years due to their rapid improvement in power conversion efficiency (PCE).1-5 A perovskite solar cell is usually composed of an electron-transport layer (ETL), a light absorbing layer (the perovskite material), a hole transport layer (HTL) that extracts holes from the perovskite layer, and terminal contacts.6 As one of the most important layers, the HTL has rapidly developed in the past years.78

The most widely used material is Spiro-OMeTAD which comprises two fluorenes connected to

each other via a sp3-hybridized carbon atom in the center. However, even it has proven to be one of the best organic HTMs, Spiro-OMeTAD still suffers from drawbacks, such as low hole-


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mobility9 and high synthesis costs.10 The former has been proven to be crucial for solar cell performance, since low hole-mobility usually increases carrier recombination losses.11-12 Hence, significant research has been devoted to identify better alternatives.13 In order to achieve a high hole-mobility, one strategy is to apply purely inorganic hole-transport materials. These materials may show high stability in ambient air and high hole-mobilities, making them quite attractive as hole-transport materials.14-15 NiOx represents one of the most widely studied and promising purely inorganic HTMs. PSCs from the group of Yang containing p-type NiOx as hole-transport layer have shown a maximum power conversion efficiency of 16.1%.16 Moreover, their long-term stability is much better than that of poly(2,3-dihydrothieno-1,4-dioxin)poly(styrenesulfonate) (PEDOT:PSS), which is commonly used as HTM in PSCs of inverted structure (p-i-n). Copper-based HTMs, such as Cu2O, CuI and CuSCN, have also been widely investigated.17 Cu2O possesses a high p-type conductivity, high carrier mobility up to 100 cm2 V1 s-1,

long diffusion lengths about several micrometers, and is considered as one of best inorganic

candidates.18 PCEs of 13.35% were demonstrated from a solution-process method and high stability at 90% of the initial PCE was observed after 70 days.19 CuI is another interesting material which also displays an electrical conductivity two orders of magnitude higher than that of SpiroOMeTAD. However, the main problem is its fast recombination loss reactions at the interface with perovskite, yielding photovoltaic devices with unsatisfactory efficiencies of about 6%.20 Grätzel et al. showed that replacing I- with SCN- was a good strategy to re-solve this problem.21 They applied CuSCN as hole-transport materials in lead halide perovskites by solution-process deposition, achieving an efficiency of 12.4% with a 65% higher short-circuit current (Jsc) and 9% higher open-circuit potential (Voc) as compared to the devices without HTM. Moreover, a high shunt resistance, instead of problematic recombination, was observed in the devices containing


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CuSCN as HTM. Recent work by Arora and Grätzel et al. further pushed the efficiency of PSCs with CuSCN to 20% and higher together with a highly stable performance.22 Other materials, such as PbS23 and WO3,24 were also studied as HTMs for PSCs. By control of the particle size, the valence band of these materials could be tuned, yielding devices with efficiencies of 7.5% and 7.68%, for PbS and WO3, respectively. Apart from inorganic materials, extensive work has also been conducted on organic materials since they are easier to design and modify at a molecular level.25 In order to increase the holemobility and conductivity, one basic strategy was to convert neutral molecules into ionic organic salts. McGehee et al. designed and synthesized Spiro(TFSI)2 by reacting Spiro-OMeTAD with AgTFSI, and the conductivity of Spiro(TFSI)2 amounted to 10-3 S cm-1.26 Sun et al. made Agbased complexes by reacting a pyridine-based ligand with AgTFSI that resulted in an ionic coordination complex with a conductivity of 10-3 S cm-1, which is two orders of magnitude higher than for non-doped Spiro-OMeTAD. Applied as HTM in perovskite solar cells, it gave devices with an efficiency of 11.98%, which is comparable to that of doped Spiro-OMeTAD, with an efficiency of 12.27% in their work.27 Another way to increase the hole-mobility is by tailoring the design of the molecular structures. Yang et al. chose two different structures as the core of their organic HTMs and found that the one containing an electron-donating substituted benzo[1,2-b:4,5b]dithiophene as core, DERDTS-TBDT, performs much better than the one based on an electronwithdrawing 5,6-difluoro-2,1,3-benzothiadiazole, DORDTS-DFBT, unit in PSCs. This is because of the better matched energy levels and increased hole-mobility of the resulting molecular materials in the PSCs. In particular, as a dopant-free HTM, DERDTS-TBDT renders PSCs with an efficiency around 16.2%.28 In addition, triazatruxene was employed as core for a series of molecular materials studied by Nazeeruddin et al. All the materials show high hole-mobility,


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which was attributed to the triazatruxene two-dimensional π system, promoting π-π stacking between molecules for efficient intermolecular charge transfer in the resulting material. By changing the functional group attached to the triazatruxene core, they could adjust the energy levels of the highest occupied molecular orbitals (HOMOs) and thus obtained an efficiency around 17.7% in PSCs, even out-performing the PSCs containing the reference Spiro-OMeTAD materials showing 17.1% efficiency.29 In recent years, although hole-transport materials have been substantially developed, there have been few articles based on coordination compounds used as HTMs.30-31 Previous work from our group has involved different metals as coordination centers, but none of them showed impressive performance in PSCs well due to low conductivities and low hole mobilities.32 The metal atom in the center did not show extensive influence on the intermolecular charge transfer nor inherent transport properties. In this work, we have designed and synthesized a new molecular material Y4, that is based on a conjugated ring structure from linking the pyrroles at the α positions in the corresponding molecular material Y3 (Figure 1). By linking the two ligands in Y3 to form the single ligand in Y4, we create a conjugated, planar molecular unit building the Y4 material. In comparison, the non-coupled ligands in Y3 demonstrate the importance of ligand conjugation and molecular packing in coordination compounds.

EXPERIMENTAL SECTION Materials. Chemicals and solvents were purchased from Sigma Aldrich and were used without further purification, except when explicitly mentioned below. Lead iodide (PbI2 99.99%) and lead


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bromide (PbBr2 99.99%) were purchased from TCI and Alfa Aesar, respectively. Formamidinium iodide (CH(NH2)2I; FAI) (>98%) and methylammonium bromide (CH3NH3Br; MABr) (>98%) were obtained from Dyenamo and were recrystallized from absolute ethanol before use. Tris(2(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK 209) (98%) was also obtained from Dyenamo. TiO2 paste with an average particle diameter of 30 nm was purchased from Greatcell Solar (30 NR-D). PEDOT:PSS (Al 4083) was purchased from Ossila. Pyrrole was purchased from Sigma Aldrich and was distilled before use. Synthetic Route O O

O

O

O

O

N N H

N

NH HN

1

N

2

HN

HO

H O n InCl3

NH HN

O

N

O

N Zn

N

DMF

HN

O 4

O

O

Zn(OAc)2

N N

N N

Y3

N

N

N Zn

N O

O NH

1, TFA DDQ

N

3

O

N H

N

Zn(OAc)2 CH3OH

DDQ

InCl3 CHO

O

O

N

N

O

O

5

N N

Y4

Figure 1. Synthetic routes of Y3 and Y4. The synthetic routes employed are shown in Figure 1. Details are given below. Zn-Di(4-((1H-pyrrol-2-yl)(2H-pyrrol-2-ylidene)methyl)-N,N-bis(4methoxyphenyl)aniline) (Y3). Details of the synthesis of Y3 was reported in our previous publication.32 1H NMR spectral data (400 MHz, CDCl3): δ 7.49 (d, 4H), 7.38 (d, 4H), 7.18 (d, 8H), 6.96 (d, 4H), 6.90 (d, 4H), 6.88 (d, 8H), 6.42 (d, 4H), 3.82 (s, 12H). 13C NMR spectral data (100 MHz, CDCl3): δ 159.75, 156.33, 149.46, 149.09, 140.77, 140.45, 132.81, 132.17, 130.57, 127.24,


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117.69, 116.65, 114.88, 55.54. MS (ESI) m/z calculated for C58H48N6ZnO4 [M + H]+ 957.30, found 957.10. Details are shown in Figure S1, Figure S2 and Figure S3. Di(1H-pyrrol-2-yl)methane (4). Paraformaldehyde (0.33 g) was dissolved in pyrrole (75 mL) and heated to 50 oC under N2 atmosphere for 10 min. InCl3 (240 mg) was added and the mixture was continuously stirred for 1 hour at 50 oC. The solution was cooled to r.t., and NaOH (240 mg) and alumina (245 mg) were added, the solution was stirred for another 1 hour. When finished, the solution was filtered to remove the catalyst and concentrated to remove excess pyrrole. The residual was purified by silica gel column with petroleum ether (PE)/ethyl acetate (EA) = 5/1. Further precipitation in petroleum ether gave white crystals (0.95 g, 59% yield). 1H NMR spectral data (400 MHz, CDCl3): δ 7.79 (d, 2H), 6.65 (m, 2H), 6.16 (m, 2H), 6.05 (d, 2H), 3.97 (s, 2H). N-(4-(15-(4-(bis(4-methoxyphenyl)amino)phenyl)porphyrin-5-yl)phenyl)-3-methoxy-N(4-methoxyphenyl)aniline (5). 4-(Bis(4-methoxyphenyl)amino)benzaldehyde (1.6 g, 5 mmol) and di(1H-pyrrol-2-yl)methane (0.73 g, 5 mmol) were dissolved in 600 mL dichloromethane (DCM). After purging with N2(g), 0.35 mL trifluoroacetic acid (TFA) was added dropwise. The reaction was stirred at r.t. for 3 hours in the dark. After that, 2,3-dichloro-5,6-dicyano-pbenzoquinone (DDQ) (1.8 g) was added in one portion. The reaction was further stirred at r.t. for another 3 hours, followed by addition of 2 mL triethylamine (TEA) to terminate the reaction. The solution was concentrated by rotary evaporation, the residual was subjected to silica gel column, eluted by DCM. Further recrystallization was made in DCM/MeOH to give a purple solid (340 mg, 8.5% yield). 1H NMR spectral data (400 MHz, CDCl3): δ 10.27 (s, 2H), 9.39 (d, 4H), 9.24 (d, 4H), 8.09 (d, 4H), 7.42 (d, 8H), 7.38 (d, 4H), 7.02 (d, 8H), 3.88 (s, 12H), -2.93 (s, 2H). 13C NMR spectral data (100 MHz, CDCl3): δ 156.25, 148.50, 147.62, 144.98, 141.03, 135.80, 133.06, 131.46, 131.10, 127.19, 119.37, 118.53, 115.00, 105.14, 55.61.


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Porphyrin-Zn (Y4). Compound (5) (91.7 mg, 0.1 mmol) and zinc(II) acetate (Zn(AcO)2) (183.5 mg, 1 mmol) were dissolved in 10 mL dimethylformamide (DMF). The solution then was refluxed for 1 hour. When finished, the solution was cooled to room temperature and poured into 50 mL water. Filtration was used to get the raw product. The raw product was dissolved in DCM and eluted on a silica gel column with (From PE/DCM = 1/1 to DCM). Precipitation in methanol (MeOH) yielded a purple solid (45 mg, 45.9%). 1H NMR spectral data (400 MHz, CDCl3): δ 10.33 (s, 2H), 9.51 (d, 4H), 9.09 (d, 4H), 8.04 (d, 4H), 7.40 (d, 8H), 7.24 (d, 4H), 7.10 (d, 8H), 3.82 (s, 12H).

13C

NMR spectral data (100 MHz, CDCl3): δ 162.81, 156.47, 150.08, 149.23, 148.26,

140.82, 135.84, 134.51, 132.33, 127.64, 119.59, 117.74, 115.68, 106.36, 55.82. HRMS (ESI) m/z calculated for C60H46N6ZnO4 [M + H]+ 978.29, found 978.2853. Details are shown in Figure S4, Figure S5 and Figure S6. Density-functional calculations. In order to elucidate the electronic properties of molecular Y3 and Y4, geometric optimization and single-point energy calculations were performed using the B3LYP hybrid functional and 6-31G* basis sets for the C, H, N and O atoms, without any symmetry constraints. The reorganization energy was determined by four energies using the Nelson four-point method.33-34 All reported calculations were carried out by means of Gaussian 09 (rev. D.01).35 The LANL2DZ effective core potential (ECP) basis set was used for the Zn atom. Same setting was applied for the calculation of a dimer of Y4 molecules. Hole-mobility measurements. The hole-mobilities of the materials in this study were determined according to previous literature methods36-37. Briefly, a hole-conducting-only device with the structure of FTO/PEDOT:PSS/HTM/Au was fabricated. The fluorine-doped tin-oxide (FTO) coated glass substrates (Pilkington TEC15) were etched with Zn powder and 2 M HCl solution. The substrates were then sequentially washed with soap, acetone and alcohol by


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sonication for 30 minutes. The remaining organic residues were removed with plasma cleaner (Harrick Plasma) for 30 min. A roughly 40 nm PEDOT:PSS layer was then spin-coated at 3000 rpm on the substrate, followed by annealing at 120 oC for 20 min. A solution of 20 mg/mL HTM in tetrahydrofuran (THF) was spin-coated on top of the annealed PEDOT:PSS layer, followed by evaporation of 80 nm gold as a counter electrode. J-V characteristics of the devices were recorded with a Keithley 2400 Source-Measure unit, interfaced with a computer. Conductivity measurements. The conductivity of the HTMs formed was investigated according to literature reports38-39. Glass substrates without conductive layer were sequentially washed with soap, acetone, and alcohol by sonication for 30 minutes. A mesoporous TiO2 layer was made by spin-coating at 3000 rpm for 30 s using a solution of 30 nm TiO2 paste (Dyesol DSL 30NR-T) in absolute ethanol (w/w=1/3). After spin-coating, the substrate was immediately placed on a hotplate at 80 oC for 15 min, and then transferred to an oven and sintered at 500 oC for 30 min. 20 mg/mL HTM in THF was subsequently spin-coated on the mesoporous TiO2 layer, followed by evaporation of 80 nm gold as counter electrode. J-V characteristics were recorded in the dark with a Keithley 2400 Semiconductor Characterization System. Fabrication of solar cells. FTO glass substrates with

The Central Role of Ligand Conjugation for Properties of Coordination

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