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Phenothiazine Functionalized Multifunctional A-#-D-#-D-#-A Type Hole Transporting Materials via Sequential C-H Arylation Approach for Efficient and Stable Perovskite Solar Cells Chunyuan Lu, Mahalingavelar Paramasivam, Kyutai Park, Chul Hoon Kim, and Hwan Kyu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20646 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

Phenothiazine Functionalized Multifunctional A-π-D-π-D-π-A Type Hole Transporting Materials via Sequential C-H Arylation Approach for Efficient and Stable Perovskite Solar Cells

Chunyuan Lu†a, Mahalingavelar Paramasivam†a, Kyutai Parkb, Chul Hoon Kimb and Hwan Kyu Kim*a a

Global GET-Future Lab, Department of Advanced Materials Chemistry,

Korea University, 2511 Sejong-ro, Sejong 339-700, Korea. E-mail: [email protected] b

Department of Advanced Materials Chemistry,

Korea University, 2511 Sejong-ro, Sejong 339-700, Korea.

+

These authors contributed equally to this work.

*Corresponding author

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ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: Three phenothiazine based A-π-D-π-D-π-A type small molecules containing various terminal acceptor units, which act as Lewis base blocks, have been synthesized via an efficient and step-economical, direct C–H arylation strategy in the aim towards the development of holetransporting materials (HTMs) with multifunctional features (such as efficient hole extraction layer, trap passivation layer and hydrophobic protective layer) for perovskite solar cells (PrSCs). Optical-electrochemical Correlation and DFT studies reveal that dicyanovinylene (DCV) acceptor in SGT-421 downshifted the HOMO level (-5.41 eV) which is more proximal to the VB (-5.43 eV) of the perovskite, whereas N-methyl rhodanine (NMR) in SGT-420 and indanedione (IND) in SGT-422 destabilized the HOMO, leading to an increased interfacial energy level offset. SGT-421 exhibits superior properties in terms of a sufficiently low-lying HOMO level and favorable energy level alignment, intrinsic hole mobility, interfacial hole transfer, hydrophobicity and trap passivation ability over spiro-OMeTAD as a benchmark small molecule HTM. As envisaged the design concept, SGT-421-based PrSC not only yields a comparable efficiency of 17.3% to the state-of-art of spiro-OMeTAD (18%), but also demonstrates the enhanced long-term stability compared over the spiro-OMeTAD due to its multifunctional features. More importantly, synthetic cost of SGT-421 is estimated to be 2.15 times lower than that of spiro-OMeTAD. The proposed design strategy and the study of acceptor-property relationship of HTMs would provide valuable insights and guidelines to the development of new low-cost and efficient multifunctional HTMs for the realization of efficient and long-term stable PrSCs.

Keywords: A-π-D-π-D-π-A, Multi-functional, End-capped acceptor units, Sequential C–H Arylation, Trap passivation, Perovskite solar cells, Hole transporting materials.

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Introduction In recent years, as an emerging photovoltaic technology, perovskite solar cells (PrSCs) have shown remarkable progress due to their record efficiency of 22.7% over a short period.1-2 The impressive photovoltaic performance for a PrSC single-junction device is mainly attributed to its intrinsic photophysical properties, such as long carrier diffusion length, excellent ambipolar charge transport and tunable direct bandgap, etc.3 Hole transporting materials (HTMs), as a key component of PrSCs, can efficiently extract the holes from the VB of perovskite layer and transport them to Au electrode.4 A lot of researches have been dedicated for the design of efficient HTMs based on inorganic components such as CuSCN, organic polymers and small molecules.5 Of these, small-molecule HTMs stand at the forefront in this field over their inorganic and polymeric counterparts due to several advantageous aspects, such as facile synthetic routes, low-cost, facile band gap tuning, and less batch-to-batch synthetic inconsistency. To date, spiro-OMeTAD is widely used as a benchmark small-molecule HTM, but its inferior hole mobility, conductivity without dopant and complex synthetic approach along with tedious purification renders it to be infeasible for commercialization. In this regard, so far, many alternative HTMs, such as pyrene6, triazatruxene,7 3,4-ethylenedioxythiophene,8 carbazole,9-11 azomethine,12 butadiene,13 acene derivatives,14 benzotriselenophene,15 S,N-heteropentacene,16 benzodithiophene,17 thienothiophene,18 thienopyrazine19, or quinoxaline-based derivatives19-20, have been developed in PrSCs. Most of these reported HTMs have been synthesized through traditional multi-step approach which relies majorly on the connection of C-C and C-N bonds using Suzuki, Stille coupling and Buchwald-Hartwig amination as key-transformations reactions. In addition, it demands tedious prefunctionalization synthetic steps such as deprotonation and lithiation to selectively replace the halogenation of the organic compounds using n-butyl lithium

3



and

their

subsequent

transmetalation

reactions

to

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prepare

the

corresponding

organotin/organoboron reagents using highly toxic trimethyl or butyl tin chloride and expensive 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, respectively, along with the formation of toxic byproducts. Overall, they are time-consuming and create a non-benign environment. In this work, we have proposed a versatile, step and atom-economical direct C-H arylation strategy to connect phenothiazine, thiophene 2-carbaldehyde and thiophene segments through Pdcatalyzed sequential C-C bond forming reactions. Employing this sustainable synthetic protocol increases the yield significantly and reduces the number of reaction steps as compared to the traditional methods thereby facilitating the commercialization from synthetic perspectives. One of the persisting challenges encountered in PrSCs is long-term stability because perovskite films undergo degradation upon its exposure to the moisture environment. Except for employing excellent polymer or small molecule with high intrinsic hole mobility as dopant-free HTM for PrSCs,

21-22

another possible solution to surmount this problem is hydrophobicity

enrichment in the HTMs that can act as a capping layer onto perovskite films.23-24 Previous studies25 have demonstrated that under-coordinated Pb ions at grain boundaries are one of the main sources of the trap states in perovskite polycrystalline films. Thus, to minimize the nonradiative charge recombination in PrSCs, it is essential to develop the HTMs possessing Lewis base block units that can passivate the trap sites through non-covalent interactions or coordination with under-coordinated Pb ions.26-27 This grain boundary passivation strategy on perovskite films was significantly alleviated via the treatment by various molecules containing Lewis base blocks.28-31 To design an efficient multifunctional HTM, all these features, such as charge extraction ability, hydrophobicity, and trap-passivation ability, should be taken into consideration to increase the photovoltaic performance together with enhanced long-term

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stability of PrSCs. However, to the best of our knowledge, a single HTM molecule possessing all these features is relatively little explored to date. Herein, to integrate the factors mentioned above, for the first time, we have proposed the multi-functional HTMs based on a generic small molecular A-π-D-π-D-π-A configuration for the realization of efficient and long-term stable PrSCs (Fig. 1). This chemical configuration consists of three parts: 1) a common A-π-D-π-D-π-A architectured backbone end-capped with three various acceptors (Lewis base blocks) on both sides for hole extraction and the passivation of under-coordinated Pb ions; 2) two phenothiazine (PTZ) donor units have been introduced to lower HOMO energy level closer to the VB of the perovskites along with their well-known excellent propensities, including hole-transport and thermal stability and less propensity for aggregation due to their non-planar, butterfly-shaped structure; 3) N-alkylation of two dodecyl chains on two phenothiazine units has been carried out not only for the improvement of solubility and hydrophobicity, but also for increasing the hole mobility of HTM by efficient π–π stacking of the molecules through possible hydrophobic interactions.32 The central thiophene πbridge unit has been introduced between two PTZ units to enhance the planarity by alleviating the torsional distortion between them and strengthen the hole transport. To investigate the acceptor strength-property relationship, various electron acceptors possessing a different electron accepting ability, such as N-methyl rhodanine (NMR), dicyanovinylene (DCV) and 1,3 indanedione (IND), have been introduced and thoroughly characterized through various analytical techniques such as optical-electrochemical studies, DFT-TDDFT simulations, atomic force microscopy (AFM), scanning electron microscopy (SEM) techniques and water contact angle studies, etc. Our investigation demonstrates the developed DCV-based SGT-421 based on our proposed multifunctional HTM design concept possesses multifunctional features for PrSCs,

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such as lowering of HOMO closer to the VB of the perovskites, trap passivation effect toward perovskite films, a greater water contact angle and intrinsic higher hole mobility. All above mentioned multifunctional features of SGT-421 render it to be a high and comparable PCE (17.3%) compared to the state-of-art of spiro-OMeTAD (18.01%). More importantly, the SGT421 based devices exhibit enhanced long-term stability to that of spiro-OMeTAD, which is originated from the effective combination of greater water contact angle, good film-forming ability and trap passivation ability toward the polycrystalline perovskite films. In addition, the synthetic cost of SGT-421 is evaluated to be 2.15 times lower than the synthetic cost of spiroOMeTAD. Therefore, this work would provide insights and guidelines to the development of low-cost and multi-functional HTMs for the realization of efficient and long-term stable PrSCs in the future.

Structure-Property Relationship

A

Planarity enhancement for efficient transport

S

S S

S S N O

S O

N

S NC

CN

A

N

Planarity enhancement by Extending the conjugation

Hole transport & HOMO stabilization

O

SGT-420 SGT-421 SGT-422

Hydrophobicity & solubility

Lewis base blocks for trap passivation

Fig. 1. Design concept and schematic molecular structural representation of the multi-functional HTMs for PrSCs.

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Experimental Synthesis The target HTMs, coded as SGT-420, SGT-421 and SGT-422, were synthesized according to a multistep synthetic pathway through a Pd-catalysed direct C–H arylation approach, as depicted in Scheme 1. This strategy is beneficial in terms of avoided toxic prefunctionalization steps, non-usage of toxic reagents, such as tin and boronic acid derivatives, and required fewer synthetic steps compared to classical carbon–carbon bond coupling via traditional Stille or Suzuki reactions. Moreover, it is more step-economical and eco-friendly. Thiophene and thiophene-2-carbaldehyde have been used as a C-H activation partner in this strategy. The intermediate (1) was synthesized by the direct coupling of thiophene 2-carbaldehyde and 3,7dibromo-10-dodecyl-10H-phenothiazine with the stoichiometric ratio of 1:1 in dry toluene using pivalic acid and Cs2CO3 in the presence of Pd(OAc)2 catalyst and (2-biphenyl)di-tertbutylphosphine (JohnPhos) ligand combination. The mono aldehyde derivative (1) undergoes direct C-H arylation with thiophene under the same reaction conditions using dimethyl acetamide (DMAc) as solvent system. The dialdehyde derivative (2) was obtained with the quantitative yield of 77%. The obtained yield via conventional Suzuki and Stille coupling is comparatively lower than direct C-H arylation. The problem encountered in the purification process is the extensive contact of dialdehyde derivative with the stationary phase during column chromatography techniques. The appropriate selection of eluent combination such as a mixture of hexane, ethyl acetate and dichloromethane are found to be efficient to get the desired product in good yield. Knoevenagel condenzation of 2 with the acceptor units yield the target HTMs in excellent yield. The prolonged stay of the HTMs in the column chromatography yields the mono 7



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derivative i.e., decomposition occurred due to the retro-Knoevenagel reaction.33 All these HTMs were characterized by 1H NMR, 13C NMR, and MALDI-TOF HR-MS. O Br

Br

S

S

Pd(OAc)2

Br

S

Pd(OAc)2 JohnPhos, PivOH

(CH2)11CH3

Cs2CO3, PhCH3

1

H3C(H2C)11

Cs2CO3, DMAc

O S

S

NH4OAc ° DCE, 80 C

O

O

N

S

(CH2)11CH3

N

2

S S S

S S

H3C(H2C)11

N O

S

S

SGT-420

S

N

N

S

77%

80%

(CH2)11CH3

S

S

NC

NC

N

S N

S

2

CN

CN S

S

S H3C(H2C)11

N

86%

O N

S

S

N

JohnPhos, PivOH

(CH2)11CH3

S

S

Thiophene

O

N

O

O

S

(CH2)11CH3

NC

S

CN

NH4OAc ° DCE, 80 C

O

H3C(H2C)11

SGT-421 83%

S

N

N

S

(CH2)11CH3 O

O

O Et3N CHCl3, RT

O

S

H3C(H2C)11

S

S N

S

S N

O

SGT-422 91%

(CH2)11CH3

Scheme 1. Synthetic route for various target HTMs via step-economical direct C–H arylation approach.

Results and Discussion Optical and electrochemical properties The normalized UV-vis absorption and emission spectra of the synthesized HTMs in dichloromethane are shown in Fig. 2a. Their corresponding data are compiled in Table 1. Of these, SGT-420 containing N-methyl rhodanine (NMR) unit shows three absorption bands, whereas SGT-421 and SGT-422, which contain strong acceptor units such as dicyanovinylene (DCV) and 1,3-indanedione (IND), respectively, exhibit two bands ranging from 300 to 650 nm.

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The absorption band at the high energy region at 300–450 nm originated from the localized aromatic π–π* transitions of the A-π-D-π-A skeleton. The absorption band at longer wavelength is assigned to the ICT originated from two phenothiazine donor units to the corresponding terminal acceptor units; the position of the band is considerably red-shifted toward the longer wavelength with respect to their acceptor functionalities. The acceptor order of the HTMs is given based on their absorption profiles: SGT-422 > SGT-421 > SGT-420. The maximum emission bands of HTMs SGT-420, SGT-421 and SGT-422 are obtained at 653, 673 and 672 nm, respectively. The optical band gap (E0-0) values are measured from the intersection of the absorption and fluorescence spectra, and the values are found to be 2.13, 2.08 and 2.06 eV for SGT-420, SGT-421 and SGT-422, respectively. On moving from solution to film state, all the HTMs exhibit an apparent broadening and obvious bathochromic shift of 44~70 nm (Fig. S1). This significant bathochromic shift indicates a strong intermolecular π–π stacking of all new HTM molecules in solid film state, which is expected to improve the hole mobility of HTMs and thus promote power conversion efficiency.34-35 a)

b)

Fig. 2. a) Normalized absorption and emission spectra and b) Cyclic voltammograms of synthesized HTMs (measured in THF). 9



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Cyclic voltammetry (CV) was employed to investigate the redox behaviour of the HTMs that can be used to determine energy levels using a standard three-electrode configuration. The electrochemical properties of new HTMs were measured in dry THF containing 0.1 M TBAPF6 as the supporting electrolyte (Fig. 2b), and the corresponding data are summarized in Table 1. Oxidation potentials in the voltammograms show that all HTMs undergo irreversible oxidation. The oxidation potential (Eox) of the SGT-420 HTM containing NMR unit shows a reversible oxidation peak at 0.74 V (vs. NHE) and the replacement of NMR unit with IND unit leads to a small positive shift of 10–14 mV in both the anodic and cathodic potentials. On the other hand, the incorporation of DCV unit in SGT-421 downshifts the HOMO energy level (0.86 V) with a significant positive shift of 90-94 mV on cathodic and anodic potentials with respect to its congeners SGT-420 and SGT-422. Keeping the π-D-π-D-π framework as common, this study clearly reveals that DCV unit aids in lowering the HOMO energy level when compared to its counterparts such as NMR and IND acceptors. The values obtained are well-matched with energy level pattern influenced by these acceptor units observed from the previous reports.36-37 From the CV curves, the HOMO energy levels of SGT-420, SGT-421 and SGT-422 corresponding to the measured oxidation potentials are calculated to be –5.30, –5.41 and –5.31 eV, respectively. The optical band gaps of the HTMs are found to be in the following order: SGT-420 (2.13 eV) > SGT-421 (2.08 eV) > SGT-422 (2.06 eV). The LUMO energy level is calculated from the optical band gap (E0-0) and HOMO. The values obtained are –3.17, –3.33 and –3.25 eV for SGT-420, SGT-421 and SGT-422, respectively.

Molecular geometry and electron density distribution analysis

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An in-depth knowledge about geometry of the HTMs can be of great help to gain insight about their structure-property relationship. The optimized geometry of all the HTMs using B3LYP/6-311G (d, p) level of theory exhibits the torsional distortion in the range of 19-24˚ between the central thiophene and two PTZ units flanked. A mean angle of ~22˚ between the PTZ donor and auxiliary thiophene units was found (Fig. S2). The terminal acceptors maintain coplanarity with the π-conjugated framework. Among the HTMs, the dipole moment of the SGT-421 is significantly increased from ground state to excited state. The electron density distribution analysis computed for the frontier molecular orbitals showed that the HOMO level delocalization for all the HTMs is nearly identical. On the other hand, LUMO level is majorly governed by acceptor strength of the terminal units which ensure the existence of ICT character in the system. To gain deeper insights about the influence of end-capped acceptor units on the -π-D-π-D-πframework, partial density of state (PDOS) calculations have been performed. All the molecules were divided into four segments as follows: CT: central thiophene, D: phenothiazine, PT: auxiliary thiophene and A: end-capped acceptor units respectively. Molecular orbital density contributions from these segments corresponding to HOMO level are nearly identical for all the molecules. It is worth noting that this contribution from -π-D-π-D-π- framework corresponding to LUMO level is distinctly varied with respect to the terminal acceptor units used (Fig. S3). In general, ICT transitions are directly proportional to the electron accepting strength and the order is based on their absorption profiles and listed here: SGT-422 >SGT-421 > SGT-420. Besides influencing the ICT transitions, end-capped acceptor units also determine the extent of electron density population on -π-D-π-D-π- core. For example, electron withdrawing strength of NMR is lower than both of IND and DCV units but its capability of localizing the electron density (NMR

11



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of SGT-420: 47%) is comparatively high over the other acceptor units (IND of SGT-422: 38%; DCV of SGT-421: 28%). This is mainly attributed to the large (five-membered) ring size of NMR with more heterofunctionalities (C=O, S-C꞊S group with vacant 3d orbitals and a tertamino group). This can accommodate more electron density, which consequently reduces the population on -π-D-π-D-π- framework to a certain extent. IND acceptor unit with two keto functionalities and a benzene ring comes in a second position with the localization of 38% electron density out of the total population. Albeit, DCV unit is a strong electron withdrawing group, it fails to localize the population due to its small size sp hybridized cyano group. Therefore, DCV unit leaves most of the electron density on -π-D-π-D-π- framework in SGT-421. As a consequence, high electron localizing capability of NMR and IND acceptor units leads to HOMO destabilization, whereas DCV stabilized the HOMO level to a considerable extent. The measured energy levels of the previously reported molecules containing these acceptor units follow a consistent qualitative trend with our results.38-39 This clearly reveals that the tuning of HOMO and LUMO energy levels is not only based on the ICT character but also dependent of the capability of end-capped acceptor units to accommodate the electron density on the -π-D-πD-π- core of A-D(π)-A systems.39 To ensure the influence of end-capped acceptor units on the energy levels regulation of HTMs, we have also computed the energy levels of various bare acceptor units and their presence in the corresponding D-π-A (model system) and A-π-D-π-D-π-A configurations. As depicted in Fig. 3, the bare DCV acceptor exhibits a relatively low-lying HOMO (–9.71 eV), compared to NMR and IND units. This behaviour is reflected on the D-π-A (Fig. S4). The simulated trend follows a consistent with the values measured from the optical-electrochemical profiles. Various functionals, such as B3LYP, CAM-B3LYP, PBE0, M06-2X and 6-311G d, p)

12


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basis set, were used to compute the vertical excitation energies in the framework of PCM using CH2Cl2 solvent (Fig. S5). Of these, M06-2X functional simulated excitation energies showed reliable agreement with the experimental observations with small underestimations, whereas other functionals showed more deviations.

Fig. 3. Schematic diagram illustrating the energy levels comparison of various HTMs obtained from the optical-electrochemical profiles vs. values computed from the DFT/B3LYP/6-311G (d, p) level of theory.

The total reorganization energy plays a key role in determining the hole mobility of the HTMs, which is the contribution from the combination of internal and external contributions. The external contribution, such as the medium reorganization/outer sphere reorganization energy arising from the polarization effect of the media, is of little significance and the quantity is not possible to be easily predicted. Thus, we calculated only the internal reorganization energy (intramolecular reorganization energy λ±). Generally, based on Marcus-type electron-transfer mechanism, the lower hole reorganization energy, the faster transfer rate for holes.40 Thus, to elucidate the effect of end-capped acceptors on the hole transport properties of investigated 13



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HTMs from theory, DFT calculations were conducted using Gaussian 09 software at the B3LYP/6-311G(d, p) level of theory. The λh (hole reorganization energy) values of SGT-420, SGT-421 and SGT-422 are 190, 196 and 244 meV, respectively, indicating that SGT-420 and SGT-421 exhibit smaller hole reorganization energy than SGT-422 (Table 1).

Table 1 Optical-electrochemical properties, DFT simulated energy levels and hole reorganization energy of the HTMs. λmaxa HTMs

ε -1

λema -1

Eoxb

HOMO

LUMOc

E0-0d

HOMOe

LUMOe

Ege

λhf

(nm)

(M cm )

(nm)

(V)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(meV)

SGT-420

490 398

76,273 65,302

653

0.74

-5.30

-3.17

2.13

-5.22

-2.87

2.35

190

SGT-421

496 380

53,869 57,279

673

0.86

-5.41

-3.33

2.08

-5.36

-3.16

2.20

196

SGT-422

516 398

43, 446 39,088

672

0.76

-5.31

-3.25

2.06

-5.26

-3.12

2.14

244

a 

μg 2 -1 -1

(cm v s ) -

1.09 × 10–4 -

Absorption and emission spectra measured in CH2Cl2; b Measured in 0.1M TBAPF6 /THF solution (calibrated by

Fc/Fc+ as an external reference); c LUMO=Eg+EΗΟΜΟ; d evaluated from the intersection of normalized absorption and emission spectra; e, fDFT calculation using B3LYP/6-311G (d, p) level; gHole mobility measured by SCLC method.

Thermal properties

The thermal stability of newly developed HTMs was investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques (Fig. 4). TGA results manifest the excellent thermal stability with decomposition temperatures (Td) of 375, 381 and 333 ℃ for SGT-420, SGT-421 and SGT-422, respectively (Table S2).

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a

b

c

d

Fig. 4. Thermogravimetric analysis data (a) and differential scanning calorimetry (DSC) traces (b, c, d) of the HTMs with scan rate of 20 ℃ min–1 under N2 atmosphere.

15



a

b -2.20

-5.25 -5.30

-5.43

SGT-422

FTO

SGT-420

MAPbIxCl3-x

-4.4

SGT-421

-3.93

-4.0

-3.25

Spiro-OMeTAD

-3.17 -3.33

TiO2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5.31

Au -5.1

-5.41

-7.3

d

Multi-functional HTM with suitable HOMO Ⅲ. Hydrophobic unit & solubilizer

c

H2O H2O

Ⅱ. Lewis base block

Ⅰ. HTM

h+ Trap MA+ IPb+ TiO2

e-

Fig. 5. a) Schematic energy level diagram of components used in the device; b) cross-sectional FE-SEM image of representative PrSCs except for Au electrode; c) device structure of normal mesoporous type PrSCs; d) schematic diagram illustrates the multi-functional features such as hole extraction, passivation effect and hydrophobic protection of SGT-421 toward PrSCs.

Film-forming abilities The interface contact of perovskite/HTM and HTM/Au is of crucial importance as it could drastically affect the interfacial hole transfer and charge recombination process in the device (Fig. 5).11,

41

Thus, the morphology of the investigated HTMs coated onto perovskite films were

characterized by FE-SEM. Both SGT-421 and spiro-OMeTAD show a smooth and uniform 16


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capping layer onto perovskite film. However, a large number of pinholes exhibit the noncontinuous capping layer upon the spin-coating of SGT-422 on top of the perovskite film due to its poor solubility in chlorobenzene (Fig. 6). The solubility of developed HTMs in toluene or chlorobenzene are in the order of following: SGT-421 > SGT-422 > SGT-420. Hence, surface morphology study of SGT-420 was not able to be performed. These results demonstrate that end-capped acceptors greatly affect the solubility. Further, surface morphology of various HTM films spin-coated onto perovskite films were characterized by atomic force microscopy (AFM) using non-contact mode. As shown in Fig. 7, bare perovskite film exhibits the largest surface roughness with root-mean-square (RMS) of 11.63 nm. The roughness was obviously decreased after the coating of HTMs onto perovskite films. The RMS values measured upon the spincoating are noticed in the following order: spiro-OMeTAD < SGT-421 < SGT-422. Typically, with smaller RMS values, capping layer formation is appeared to be uniform. In this view, spiroOMeTAD and SGT-421 can form a full-coverage and uniform capping layer onto the underneath perovskite film, which is consistent with the result from FE-SEM images.

17



Page 18 of 36

Fig. 6. SEM images of a) bare perovskite films on FTO/bl-TiO2/mesoTiO2; b) spiroOMeTAD/perovskite; c) SGT-421/perovskite; d) SGT-422/perovskite.

18


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Fig. 7. AFM 3D images of bare MAPbCl3-xIx film and HTMs-coated perovskite films.

Charge transfer and transport properties To study the charge transfer dynamics at the interface of perovskite/HTMs, time-resolved photoluminescence (TR-PL) measurements were performed.7 Considering the absorption characteristics of newly developed HTMs, an excitation laser with a relatively long wavelength of 670 nm was selected to avoid the possible effect from light absorption by HTMs themselves. TR-PL spectra of bare perovskite films and perovskite films spin-coated with various HTMs were displayed in Fig. 8a. The emission lifetimes of bare perovskite films and HTM-coated

19



Page 20 of 36

perovskite films were fitted using a two-exponential decay model, as summarized in Table 2. The bare perovskite layer deposited on glass substrate exhibited an average decay time of 46.2 ns. The PL decay time decreased dramatically when HTMs were deposited on perovskite films. Among them, SGT-421-coated perovskite films showed the shortest average decay time of 15.3 ns. The average decay time for SGT-422 and spiro-OMeTAD-coated films were 17.1 and 18.4 ns, respectively. In this series, SGT-421 exhibits a more efficient hole extraction from the VB of perovskite, as compared to those of SGT-422 and spiro-OMeTAD. This study clearly demonstrates the better hole extraction ability of our designed HTMs of SGT-421 and SGT-422 over the reference spiro-OMeTAD. The hole mobility of spiro-OMeTAD and synthesized HTMs was determined by the spacecharge-limited-current (SCLC) method based on hole-only devices with the configuration of FTO/PEDOT: PSS/HTMs/Au. Hole mobility was evaluated by fitting the J–V curves for various HTMs into the equation (Fig. 8b). The intrinsic superior hole mobility of 1.09 × 10–4 cm2 V–1 s–1 has been found for SGT-421 without any dopant over the spiro-OMeTAD (8.32 × 10–5 cm2 V–  s ).

1 –1 42

Table 2. Time-resolved PL measurement values for the bare perovskites and HTMs spin-coated perovskite materials. A1

τ1 (ns)

A2

τ2 (ns)

τave (ns)

Bare perovskite

0.55174

19.6

0.44826

79.0

46.2

Spiro-OMeTAD

0.75767

11.9

0.24233

38.5

18.4

Perovskite/SGT-421

0.81068

10.6

0.18932

35.3

15.3

Perovskite/SGT-422

0.81548

11.8

0.18452

40.8

17.1

20


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Fig. 8. a) Time resolved-photoluminescence spectra of bare perovskite, perovskite/SGT-421, perovskite/SGT-422 and perovskite/spiro-OMeTAD (λexc: 670 nm and λem: 764 nm); b) J–V characteristics of pristine HTMs in hole-only devices.

The electrostatic surface potential (ESP) of the SGT-421 is shown in Fig. 9a and other two HTMs are displayed in Fig. S7. The red and blue colours indicated the presence of negative and positive charge population on the segments, respectively and green colour represents the neutral charge population. All HTMs showed a groove-shaped backbone with a dominant continuous positive ESP. End-capped acceptor units bear a dense negative charge population which can passivate the trap stimulated by under-coordinated Pb ions via the non-covalent Lewis acid-base interactions between the perovskite and the acceptor unit of the corresponding HTMs.43 To ascertain the interaction between PbI2 and SGT-421 with a Lewis base functional group, Fourier transform infrared (FT-IR)31 was used to analyze bare SGT-421 powders and SGT421/CH3NH3PbI3-xClx blend. The bare SGT-421 powder exhibits one characteristic peak at 2215 cm–1, which is assigned to the stretching vibration mode of ν (C≡N). To distinguish easily the differences between the ν (C≡N) vibration strength of SGT-421 and SGT-421/CH3NH3PbI321



xClx

Page 22 of 36

blend, we normalized their FT-IR peak at 1564 cm–1 (stretching vibration of C=C bonds

(aromatic)), which is not sensitive to Pb ions. As shown in Fig. 9b, the ν (C≡N) vibration strength of the of SGT-421/CH3NH3PbI3-xClx blend is decreased when compared to bare SGT421. To further confirm the passivation mechanism, the passivated perovskite sample and control sample were subjected to high-performance X-ray photoelectron spectroscopy studies. The XPS spectra of the Pb 4f regions are shown in Figure 9c. Both samples showed two peaks, Pb 4f7/2 and Pb 4f5/2 in the Pb 4f spectra. However, compared to control perovskite film, the binding energies of Pb 4f7/2 and Pb 4f5/2 in the passivated perovskite film showed a slight shift toward lower binding energy. In addition, two small peaks around 136.5 and 141.4 eV observed in the control film are assigned to the unsaturated Pb or metallic Pb44-45 and they are disappeared in the passivated perovskite films. This can be mainly ascribed to the effective passivation between the under-coordinated Pb and end-capped acceptor units of the HTMs. And the electron trap density of perovskite film was evaluated by dark I-V curve based on electron-only device. The trap-state density was extracted by the trap-filled limit voltage (VTFL) using the equation: 𝑁𝑡 =

2εε0 𝑉𝑇𝐹𝐿 q𝐿2

,

where ε is relative dielectric constant of perovskite, q is the elemental charge, ε0 is the vacuum permittivity, and L is the perovskite film thickness.46 Compared to the control sample, electron trap density of passivated perovskite film are decreased.

22


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a)

b)

SGT-421

c)

d)

Fig. 9. a) Electrostatic surface potentials (ESP) of SGT-421. Regions of negative and positive electron density population are shown red and blue in colours, respectively; b) FT-IR of the powders of SGT-421, MAPbCl3-xIx and SGT-421/MAPbCl3-xIx blend; c) XPS spectra of Pb atoms in control and SGT-421 passivated perovskite films; d) dark I-V curves of electron-only device.

Photovoltaic performances To explore the potential of the three synthesized phenothiazine-based small molecular HTMs based on A-π-D-π-D-π-A configuration for PrSCs, CH3NH3PbCl3-xIx-based normal-type PrSCs

23



were fabricated. The xIx/HTMs/Au.

device structure is

Page 24 of 36

FTO/compactTiO2/meso-TiO2/CH3NH3PbCl3-

And CH3NH3PbCl3-xIx films were deposited by one-step anti-solvent dripping

process.47 The measured J–V curves of the PrSCs with various HTMs are shown in Fig. 10a. Their detailed photovoltaic parameters are summarized in Table 3, implying the noticeable impact of different terminal acceptors present in the phenothiazine-based small molecular HTMs on the photovoltaic performance of PrSCs. Among this series, SGT-421-based device exhibits an impressive photovoltaic performance of 17.27% with the significant contribution from the opencircuit voltage (Voc) of 1.05 V, a short-circuit photocurrent density (Jsc) of 22.275 mA cm–2, and a fill factor (FF) of 73.8. This is competitive to that of the state-of-the-art spiro-OMeTAD-based device (18.01%) under the same conditions. SGT-422-based PrSC exhibits moderate photovoltaic performance of 10.01% with a Voc of 0.95 V, Jsc of 16.285 mA cm–2, and FF of 64.76. This moderate PCE is majorly ascribed to its limited solubility in chlorobenzene/toluene, which deteriorates the film formation during spin-coating process, and the significant contribution of large hole reorganization energy (244 meV) expected from their DFT calculations. The order of hole reorganization energy of the small molecular HTMs are as follows: SGT-422 > SGT-421 > SGT-420. Nevertheless, the poor solubility of SGT-420, which possesses smaller hole reorganization energy among the series, in comparison to the other congeners, exhibits inferior PCE of 3.62%, with Jsc of 10.83 mA cm–2 and an FF of 41. The poor solubility of SGT-420 and SGT-422 leads to an incomplete and non-uniform thin film formation upon spin-coating method. And the incomplete coverage of HTM film on perovskite seriously affects the hole extraction from perovskite to HTMs.48 Thus, the difference in Voc of PrSCs based on various HTMs mainly results from two reasons: one is the hole extraction at the perovskite/HTMs interface and the charge recombination due to the poor interface of

24


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perovskite/HTMs/Au, and the other reason is the differently resultant interfacial energy level offset to the VB of perovskite.49-50 SGT-421 possesses the most low-lying HOMO but favourable energy level to the VB of perovskite for efficient hole transfer, thus, leading to a higher Voc compared to other HTMs based PrSCs. The hole collection by Au contact is also hampered due to the poor interface of HTMs/Au,48 as a matter of fact, Jsc of PrSCs based on SGT-420 and SGT-422 is dramatically decreased. The measured incident photon-to-current conversion efficiency (IPCE) spectra of PrSCs with various HTMs (Fig. 10b) exhibits a consistent trend with the characteristic J–V curve. The J-V curve of PrSCs under both scan direction was recorded in Fig. S8 a-b. And the SGT-421 based PrSC shows a stabilized power conversion efficiency of 16.93% (Fig. S8 c).

a)

b)

Fig. 10. a) Current density (J)–voltage (V) curves of PrSCs using various HTMs; b) their corresponding IPCE curves.

25



Page 26 of 36

Table 3. Photovoltaic parameters of PrSCs using different HTMs. HTM

Jsc (mA cm–2)

Voc (V)

FF (%) η (%)

Average η (%)

Rs (Ω))

SGT-420

10.831

0.8163

41.00

3.62

2.11

250

SGT-421

22.275

1.0501

73.82

17.27

16.62

40.2

SGT-422

16.285

0.9495

64.76

10.01

7.80

83.1

spiro-OMeTAD

22.293

1.0423

77.50

18.01

17.05

30.2

Note: the average conversion efficiency was calculated based on 18 separate cells, measured under AM 1.5 G illumination (100 mW cm−2).

A relatively hydrophobic surface could help to reduce the penetration of moisture into the perovskite layer. This can be determined from the water contact angles of various HTM films (Fig. 11 a-c). Of these, SGT-421 films exhibit a greater water contact angle (94°) than that of spiro-OMeTAD (78°). However, the small water contact angle of SGT-422 is attributed to the poor film morphology. Moreover, to investigate the effect of trap passivation of new HTMs on long-term stability of PrSCs, perovskite films were subjected to passivation treatment by developed HTMs containing Lewis base blocks and control spiro-OMeTAD, i.e., 300 μL of 0.5 mg mL–1 stock solution containing different HTM molecules (0.5 mg mL–1of spiroOMeTAD/CB, SGT-421/CB and SGT-422/CB were dripped. The ageing of perovskite films treated with various HTMs was performed in ambient environment in the dark without any encapsulation for 50 days. Fig. 11 (e-g) demonstrated that perovskite films passivated by developed HTMs containing Lewis base functional blocks exhibited significantly enhanced longterm stability over the spiro-OMeTAD-treated control sample.

26


Page 27 of 36

a)

b)

Spiro-OMeTAD 78°

SGT-421

SGT-422

94°

d) 1.0

c)

Normalized PCE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47°

0.9 0.8 0.7 SGT-422 SGT-421 spiro-OMeTAD

0.6 0.5 0.4 0

4

8

12

16

20

24

28

Time (day)

e) spiro-OMeTAD f )

SGT-421 g)

SGT-422

Fig. 11. Water contact angles of various HTMs films. a) spiro-OMeTAD; b) SGT-421; c) SGT422; d) the long-term stability of PrSCs in terms of normalized PCE under ambient environment with 46% humidity in the dark condition without encapsulation; e-g) the corresponding photograph of aged perovskite films passivated by various HTMs.

The long-term stability tests of PrSCs based on various HTMs were conducted under ambient environment with 46% humidity in the dark condition without encapsulation. SGT-421 exhibited

27



Page 28 of 36

much better stability as compared to spiro-OMeTAD based PrSCs, while SGT-422-based devices showed the inferior stability (Fig. 11d). The enhanced stability of SGT421-based device could be attributed to the combination effects of the uniform HTM capping layer on perovskite film, the hydrophobicity,51 and the trap passivation ability from the new HTM containing Lewis base functional blocks. In addition, the development of efficient HTMs with low synthetic cost is highly desirable and can render the feasibility of commercialization. The direct C-H arylation approach used in this study precludes the prefunctionalization steps and lowers the cost to a significant extent. According to the model proposed by Osedach and co-workers52, the synthetic cost of SGT-421 is evaluated to be 44.61€g-1, which is 2.15 times lower than that of spiro-OMeTAD (see Table 4). Therefore, the multifunctional HTM SGT-421 has the potential to become an efficient and lowcost alternative to spiro-OMeTAD for PrSCs.

Table 4. Comparison of synthetic cost of a new HTM and spiro-OMeTAD. HTMs

Material cost ((€/g)[a]

Commercial price((€/g)

SGT-421

44.61

-

spiro-OMeTAD

96.09[b]

186-580

[a] Material cost was estimated for the synthesis of 1 g of product. [b] The estimated cost for spiro-OMeTAD was taken from the previous report.12

Conclusion In summary, efficient and step-economical direct C-H arylation strategy was adopted to synthesize a series of three PTZ functionalized HTMs based on A-π-D-π-D-π-A configuration 28


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with varying terminal acceptor units containing Lewis base functional blocks. Acceptor-property relationship of HTMs has been systematically investigated to utilize them for PrSCs. Of these acceptor units, smaller interfacial energy level offset between the HOMO energy level of SGT421 and the VB of perovskite is found to have efficient interfacial hole transfer from the perovskites to HTM. This is mainly attributed to the HOMO level stabilization induced by dicyanovinylene unit, which is a distinct feature observed in SGT-421, whereas N-methyl rhodanine acceptor in SGT-420 and indanedione acceptor in SGT-422 majorly reduced the band gap through remarkable HOMO destabilization, leading to an increased energy level offsets compared to SGT-421. Based on this molecular engineering, developed SGT-421 exhibits a deeper HOMO and favourable energy level alignment to the VB of perovskite and shows more efficient interfacial hole extraction than that of spiro-OMeTAD. In addition, SGT-421 exhibits higher hole mobility than spiro-OMeTAD in the order of 10-4 cm2V-1s-1 and exhibits a greater water contact angle (94°) than that of spiro-OMeTAD film (78°). Moreover, this work found that new developed A-π-D-π-D-π-A structural HTMs containing Lewis base functional blocks can enhance the humidity resistance of perovskite film via the passivation of trap sites through non-covalent interactions or coordination with under-coordinated Pb ions. Therefore, this work demonstrates that the proposed design strategy for multifunctional HTM is effective, and by adopting this design strategy, we successfully developed multifunctional HTM SGT-421 (efficient hole extraction material, trap passivation layer, hydrophobic protective layer) for PrSCs. When applied it to CH3NH3PbI3-xClx-based PrSCs, SGT-421-based PrSC showed a high PCE of 17.3%, which is competitive to that of spiro-OMeTAD (18%). More importantly, the long-term stability of SGT-421 based PrSC was enhanced compared to spiro-OMeTAD based devices due to versatile features of SGT-421. The apparent demonstration of multifunctional

29



Page 30 of 36

features together with high hole mobility and excellent PCE by SGT-421 from the designed A-πD-π-D-π-A structural HTMs shows that it can be a cost-effective and promising alternative to the expensive spiro-OMeTAD for PrSCs. The simple synthesis of multifunctional SGT-421 from cheap and commercially available raw materials opens up a new way for the commercialization of PrSCs.

ASSOCIATED CONTENT Supporting Information. Informative details of regents, characterization, device fabrication process, synthesis cost evaluation, DFT, TD-DFT, 1H NMR and 13C NMR spectra, MALDI-Mass data are available in Supporting Information Section. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions †These two authors contributed equally to this work. ORCID Mahalingavelar Paramasivam: 0000-0001-8416-9986 Hwan Kyu Kim: 0000-0002-6189-5237

30


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Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant, funded by the Korean government (MSIP) through the Mid-career Researcher Program (NRF2017R1A2A1A17069374) as well as Climate Change Program (NRF-2015M1A2A2056543) and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20184030201910). We thank Prof. Chul Hoon Kim and Mr. Kyutai Park for the measurement and data fitting of TR-PL spectra of perovskite films coated with different HTMs.

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35



Graphical Abstract

Multi-functional HTM with suitable HOMO Ⅲ. Hydrophobic & solubilizer units H2O

-2.20

MAPbIxCl3-x

-4.4 FTO

-3.93

-5.25 -5.30

-5.43

SGT-422

-4.0

SGT-420

H2O

SGT-421

Ⅱ. Lewis base block

-3.25

Spiro-OMeTAD

-3.17 -3.33

TiO2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 36

-5.31

Au -5.1

-5.41

Ⅰ. HTM -7.3

h+ Trap MA+ IPb+ eTiO2

36


Phenothiazine Functionalized Multifunctional A-Ï ... - ACS Publications

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