Polyoxometalate-based Inorganic-Organic Hybrid [Cu(phen)2]2[(Î±

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Polyoxometalate-based Inorganic-Organic Hybrid [Cu(phen)2]2[(#-Mo8O26)]: a New Additive to SpiroOMeTAD for Efficient and Stable Perovskite Solar Cells Yayu Dong, Yulin Yang, Lele Qiu, Guohua Dong, Debin Xia, Xianda Liu, Mengru Li, and Ruiqing Fan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00477 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Energy Materials

Polyoxometalate-based Inorganic-Organic Hybrid [Cu(phen)2]2[(α-Mo8O26)]: a New Additive to Spiro-OMeTAD for Efficient and Stable Perovskite Solar Cells Yayu Dong,a Yulin Yang,*a Lele Qiu,a Guohua Dong,b Debin Xia,a Xianda Liu,a Mengru Li,a Ruiqing Fan*a aMIIT

Key Laboratory of Critical Materials Technology for New Energy Conversion

and Storage School of Chemistry and Chemical Engineering Harbin Institute of Technology Harbin 150001, P. R. China E-mail: [email protected]; [email protected] bCollege

of Chemistry and Chemical Engineering Qiqihar University Qiqihar 161006,

P. R. China KEYWORDS: polyoxometalates, perovskite solar cells, oxidation, conductivity, long-term stability

ABSTRACT: It is quite meaningful for improving the properties of Spiro-OMeTAD based hole transport materials (HTMs) to realize high-efficiency and stable perovskite solar cells. In this work, a polyoxometalate (POM) based inorganic-organic hybrid [Cu(phen)2]2[(α-Mo8O26)](CMP) with strong electron-accepting properties was

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successfully synthesized, and for the first time introduced into Spiro-OMeTAD to achieve quantitative and controllable oxidation of Spiro-OMeTAD, which presents obviously increased conductivity in comparison to its counterpart. Accordingly, the corresponding perovskite solar cells (PSCs) performance shows a substantial improvement. The optimal device achieves a power conversion efficiency (PCE) as high as 18.72%, which is higher than that of the ever-reported POMs-based PSCs. Meanwhile, CMP based PSCs also show long-term stability. This work provides a feasible and efficient approach for further development of PSCs.

1. INTRODUCTION Organic-inorganic perovskites have been proved as prominent solar energy materials 1, mainly ascribing to their properties of strong light absorption, tunable band gaps, and long charge diffusion length.2,3 During a relatively short period of time, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) was increased from 3.81% to a record value of 23.7%

4-6,

which is dramatically higher

than that of organic solar cells and on par with that of polycrystalline silicon.7 Undoubtedly, PSCs has been regarded as the most promising next-generation thin film photovoltaic cells. It is well-known that hole transport material (HTM) is one of the crucial components of PSCs device, which is mainly responsible for the extraction and transport of photogenerated holes from perovskite light-harvesting materials, thus having an important impact on photovoltaic performance.8,9 As

far

as


we

know,

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2,2′,7,7′-tetrakis(N,N-dimethoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is one of the most commonly-used HTM for PSCs. However, as an organic semiconductor, Spiro-OMeTAD has many notorious inherent drawbacks, such as low conductivity and hole mobility, which seriously restrict further performance improvement of PSCs device.10,11 FK209, lithium bis(trifluroromethanesulfonyl)mide (Li-TFSI) and 4-tert-butylpyride (TBP) are the most common additives for spiro-OMeTAD. In which, Li-TFSI actually promotes the reaction between spiro-OMeTAD and oxygen, TBP could optimize the morphology to enhance the open circuit voltages and spiro-OMeTAD could be oxidized by FK209.12,13 Therefore, major efforts have been invested to overcome the above-mentioned limitations. Demonstratively, doping of chemical additives, such as metal-based complexes14-17, F4-TCNQ18, FeCl3,19 SnCl4,20 CuI,21 silver bis(trifluoromethane-sulfonyl)imide22 and tris-(pentafluorophenyl)borane23 has been widely recognized as an extremely effective strategy for enhancing the holes transportation and extraction of Spiro-OMeTAD. However, these additives commonly suffer from complex synthesis and purification process, together with high cost. Therefore, it is essential to explore novel and easily synthesized additives that can directly oxidize Spiro-OMeTAD for improving the performance of HTMs in PSCs. Recently, many works have been demonstrated that polyoxometalates (POMs) can exert positive influence for improving the photoelectric properties of thin-film solar cells, such as dye-sensitized solar cells (DSSCs)24,25 or organic solar cells (OSCs).26,27 Additionally, our previous work also proved that POMs can directly oxidize



Spiro-OMeTAD for improving the performance of PSCs.28 Noted that there is still rare report of POMs-based PSCs and chemical structure of POMs is confined to the Keggin type.29-33 To further explore the POMs applications in PSCs, POMs-based inorganic-organic hybrid materials might be good candidates. As we know, POMs-based inorganic-organic hybrid materials have already gathered significant interest in the domain of materials science, which manifest numerous potential applications in catalytic oxidation, magnetism, nonlinear optics and adsorption due to its structure feature, such as three-dimensional pore structure, highly-charged metal ions(Mo6+, W6+) and abundant functional groups(PO43-).34-38 Expectedly, the introduction of metal complexes into POMs readily might generate hydrogen bonding and π-π interactions,39 which is quite beneficial for enhancing the conductivity and charge transport ability. Taking the above-mentioned considerations, in this work, we have designed and synthesized POMs-based inorganic-organic hybrid material [Cu(phen)2]2[(α-Mo8O26)] (CMP) , whose structure was determined by single-crystal X-ray diffraction analysis. Furthermore, CMP, as an effective additive, was introduced into HTM for the first time, which could realize oxidation of Spiro-OMeTAD to replace oxygen and reduce the harmful from moisture, and the conductivity of resultant HTM was effectively enhanced due to its strong electron-accepting property. Consequently, the hole extraction was enhanced and charge separation was also accelerated from the perovskite to the HTM, effectively impeding charge recombination in the interface between perovskite and HTM. These results are beneficial to enhancement of FF and


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Voc values, consistently resulting in an improved efficiency of the PSCs. Finally, after careful optimization of the CMP doping concentration, the corresponding PSCs not only give an obvious increased PCE, but also exhibit better stability. 2. EXPERIMENTAL SECTION 2.1 Synthesis of CMP: The [Cu(phen)2]2[(Mo8O26)] was synthesized using the hydrothermal method according to previous work.40 A mixture of MoO3, H2MoO4, Cu(Ac)2H2O, 1,10-phenanthroline and H2O in the molar ratio 1.0:1.0:0.84:1.68:520 was stirred for 2 h at room temperature. The suspension was put into a 25 mL Teflon-lined stainless-steel autoclave and kept under autogenous pressure at 170°C for 3 days. After slowly cooling to room temperature, deep purple crystals were filtered

and

washed

with

distilled

water.

Then

target

product

[Cu(phen)2]2[(α-Mo8O26)] was dried at room temperature in a yield of 30.1%. 2.2 Device Fabrication: Perovskite solar cells were prepared with FTO/C-TiO2/ /Cs0.05FA0.81MA0.14PbI2.55Br0.45/HTM/Au-based planar structure. Initially, FTO glass (NSG Company, 15 Ω/m2 resistance) was etched by 4M hydrochloric acid and zinc powder, followed by washing with detergent, deionized water, acetone and isopropanol in ultrasonic bath for 15 minutes, respectively. After that, the glass was blowed to dry by nitrogen and treated by ozone for 10 minutes. The blocking titanium dioxide layer was prepared by spin-coating a mixture solution composed of 175 μL titanium (IV) isopropanol, 17.5 μL 3 M hydrochloric acid and 1.25 mL isopropanol at 3000 rpm for 30 s. After sintering at 500 ℃ for 30 minutes, the prepared film was immersed into a 20 mM aqueous TiCl4 aqueous solution at 70 ℃ for 30 minutes to



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form a dense blocking layer. Then, the 1.2 M perovskite precursor solution were prepared from a precursor solution containing FAI, MABr, PbI2, PbBr2 and CsI powder in DMF:DMSO (4:1, v/v) and kept at 65 ℃ for 2 hours under vigorous stirring before using. Perovskite solutions are successively spin-coated on the ETL substrates at 1000 rpm for 10 s and 4000 rpm for 30 s, respectively. 200 μL of chlorobenzene was dropped in 20 s at 4000 rpm. Afterward, the films were annealed at 100 °C for 10 min on a hot plate.41 The pristine hole transport material (HTM) was prepared

by

spin-coating

the

mixture

of

72.3

mg

2,2',7,7'-Tetrakis(N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD), 29 μL 4-tert-butylpyride (TBP), 17.5 μL solution of 520 mg lithium bis(trifluroromethanesulfonyl)mide (Li-TFSI) dissolving in 1 mL acetonitrile and 10 μL FK209 (300 mg/mL in acetonitrile). 20 μL HTM solution was dropped onto the perovskite-coated substrates and spin-coated at 4000 rpm for 20 s. Finally, 80 nm of Au electrode was thermally sublimated onto the HTM. To form the HTM-C solution, CMP with different concentration were added into the pristine HTM solution.

For experiencing the long-term stability of PSCs, the relevant PSCs were

prepared with different hole transport layers under humidity about 30% and temperature about 25 °C in the dark in air atmosphere. 3. RESULTS AND DISCUSSION Single-crystal X-ray diffraction analysis revealed that CMP is constricted by two principal

structural

building

units

(SBUs):

centrosymmetric

β-octamolybdate-supported two [Cu(phen)2]2+ cations (SBU-1) and [Mo8O26]4- anion


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(SBU-2) (Figure 1a and S1 in Supporting Information). SBU-1 contains two crystallographically independent Cu atoms, whose coordination environments are entirely similar and coordinated by four N atoms from different phenanthroline monohydrate ligands (Figure 1b). Clearly, from Figure 1c, SBU-2 consists of a ring of six MoO6 octahedra in an edge-sharing manner, which tightly bind two MoO4 tetrahedra in the opposite faces by sharing oxygen atoms.

Figure 1. (a) Ball-and-stick representation of the nanocluster of CMP, (b) The phenanthroline monohydrate ligands, (c) The [α-Mo8O26]4- anion. H atoms and lattice water molecules are omitted for clarity

As can be seen in Figure 1a, the [α-Mo8O26]4- anion tightly couple two neighboring [Cu(phen)2]2+ cations in the two opposite sides by a single Cu-O bond, and both hydrogen bonds and π-π stacking interactions play a significant role in bridging the adjacent clusters, which interacts together to give a 3D supramolecular network.(Figure S2) Notably, even though the synthesis method is the same, the structure of CMP is different from that reported in the literature.40 To begins with, [α-Mo8O26]4- anion supersedes [β-Mo8O26]4- anion. Next, anion bridges two



[Cu(phen)2]2+ cations by a single Cu-O bond instated of couple Cu-O bonds. Finally each Cu atom connects to two ligands rather than one ligand. Therefore, a novel α-octamolybdate-supported complex was generated. As reported in previous

literatures, the π-π stacking interactions is beneficial for charge transport in some light response devices.17,39 Additionally, CMP structure was also verified by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra (Figure S3).

Fig. 2 (a) Change of the absorption spectra of HTM solution upon the gradual addition of CMP, (Insert) Photographs of the color variation before and after doping CMP, (b) XPS spectra of HTM-B and HTM-C, (c) ESR spectra of HTM-B and HTM-C, (d) I−V curves of the different HTM solutions

For better verifying the effect of CMP doping on Spiro-OMeTAD, the UV-vis


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absorption spectra were initially performed on different concentrations HTM solutions doped by CMP. In following text, HTM-A represents pristine Spiro-OMeTAD, and HTM-B is the abbreviation for Spiro-OMeTAD doping with lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tertbutylpyridine (TBP). HTM-C is designated as the introduction of CMP into HTM-B solution. The corresponding results are shown in Figure 2a, and the inset of Figure 2a is the photograph of the HTM solutions. Notably, it is easily found that there is no obvious absorption peak in the range of entire wavelength for HTM-B solution. Evidently, the presence of Li-TFSI and TBP only plays an auxiliary role during the oxidizing process in air atmosphere because they are unable to oxidize Spiro-OMeTAD directly. By comparison, the introduction of CMP for HTM-C solution gives rise to a distinctive new absorption peak at 526 nm, ascribing to oxidized Spiro-OMeTAD+ proved by previously reported literatures.42-44 And the peak intensity shows an obvious enhancement with increase of the amount of the CMP dopant, which means that levels of the Spiro-OMeTAD+ increase simultaneously. As can be seen in the inset of Figure 2a, Spiro-OMeTAD, Li-TFSI (dissolving in acetonitrile) and TBP could well dispersed in chlorobenzene (CB) and finally result in an uniform intrinsic light yellow solution.45,46 However, after the CMP addition, the above-mentioned solution shows a remarkable dark red. As the literatures confirmed, we therefore conclude that the introduction of CMP can indeed oxidize Spiro-OMeTAD.8 X-ray photoelectron spectra (XPS) of HTM-B and HTM-C were carried out to further understand the effect of energy level shifts. From Figure 2b, it could be found that



shift of 0.7 eV toward the lower binding energy of the C1s peak was clearly observed compared with that of the HTM-B suggested that Spiro-OMeTAD molecules are directly oxidized with the CMP, which is well consisted with the previous literatures.20 The electron spin resonance spectroscopy (ESR) is another effective technique to detect free radicals in the oxidized sprio-OMeTAD solution.47 Figure 2c shows the measured ESR spectra. Obviously, the blank HTM-B solution exhibits an extremely weak, symmetric radical signal on the ESR spectrum, mainly due to the formation of a small amount of Spiro-OMeTAD+ triggered by the exposure to oxygen during the measuring process.48 In contrast, an obvious signal is detected at a magnetic field of around 3480 G when the introduction of CMP, which efficiently manifests that the electron could easily transmit from the nitrogen atoms of Spiro-OMeTAD to the molybdenum atoms of CMP, that is oxidation of Spiro-OMeTAD. For exploring the effect of CMP doping on the conductivity of Spiro-OMeTAD films, current-voltage measurements were carried out under protection with inert atmosphere and dark, and the corresponding results were displayed in Figure 2d. Compared to HTM-A and HTM-B, the higher slope of the HTM-C corresponds to the higher conductivity, illustrating that the chemical additives of CMP could effectively enhance the conductivity of resultant HTM. For better illustrating this enhancement of the conductivity of HTM, a mechanism based on oxidation process was proposed according to the previous literatures.11,44 Specifically, CMP acts as a strong electron


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acceptor that could effectively promote electron transfer from Spiro-OMeTAD to CMP. Moreover, it is well known that Li-TFSI is unable to oxidize the Spiro-OMeTAD, but facilitate the reaction of Spiro-OMeTAD and CMP.48 Typically, the following equation illustrates the oxidation process of Spiro-OMeTAD by CMP with the simultaneous presence of Li-TFSI. CMP + Spiro-OMeTAD + Li-TFSI → Spiro-OMeTAD+TFSI- + Li+CMPTo further study the hole mobility of Spiro-OMeTAD affected by CMP, hole-only

devices

with

the

structure

of

(ITO/PEDOT:PSS/HTM-B

or

HTM-C/MoO3/Ag) were fabricated and space-charge-limited current (SCLC) were used to calculate hole mobility. As shown in Figure S4, the hole mobility of HTM-C (4.7 × 10-4 cm2 V-1 S-1) is higher than that of HTM-B (2.9 × 10-4 cm2 V-1 S-1), respectively. The higher hole mobility indicates the effectiveness of oxidation and also facilitates the transport of holes. Sequentially, IR spectra were collected to further prove the molecular structure of HTM-B and HTM-C. The results are shown in Figure S5. Clearly, the IR spectrum of HTM-C basically in line with that of HTM-B, indicating that the CMP doping does not change the self-molecular structure of Spiro-OMeTAD, but the internal conjugated electrons are transferred from the boundary orbit of the Spiro-OMeTAD molecule to CMP.



Fig. 3 PL spectra of the perovskite and coated HTM-B and HTM-C, (a) The UV-vis spectra and PL emission spectra, (b) PL lifetime decay spectra

For further checking the charge transfer process and quenching behavior between perovskite and Spiro-OMeTAD based HTM-B and HTM-C, we employed steady-state and time-resolved photoluminescence (PL) measurements. Figure 3a indicates that single-layer perovskite film has the strongest fluorescence emission intensity at 790 nm after exciting with a light of 470 nm. On contrary, the perovskite films coating two diverse hole transport layers show significantly weaken PL intensity, especially the weakest PL emission intensity for HTM-C, indicating that CMP doped HTM could be more conducive to extraction and dissociation of the holes from perovskite. Undoubtedly, the oxidation of Spiro-OMeTAD is the primary reason. Substantially, the charges extraction ability of the HTM in PSC can also be assessed by the PL lifetime decay spectra.51,52 Figure 3b displays PL lifetime decay spectra of the perovskite film with or without coating diverse hole transport layers. Obviously, the perovskite layers coated with HTM-C shows a significantly reduced PL lifetime, indicating that doping of CMP could enhance hole extraction and


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accelerate charge separation from the perovskite to the HTM. Furthermore, Table S1 provides the charges lifetime fitted with a double exponential mode. We can further observe that the average PL delay time reduced from 3.57 ns for the perovskite-HTM-B to 3.14 ns for the perovskite-HTM-C (Table S1). Accordingly, it is confirmed that the hole extraction at the interfacial of perovskite/HTM-C is more efficient than that of perovskite/HTM-B. As a consequence, the fast and efficient hole extraction could impede charge recombination, thus resulting in an improved efficiency of the PSCs.

Fig. 4 (a) Cross-section SEM image of the PSCs, (b) Energy diagram of the different components in a perovskite solar cell, (c) The current density-voltage (J-V) curves of the optimal PSCs with different HTMs under AM 1.5G (100 mW·cm−2) illumination, (d) Statistical histogram of PCEs



for each of 20 PSCs based on HTM-B, HTM-C and HTM-FK209

Experimentally, in view of the improved properties of HTM with CMP doping, the planar PSCs consist of FTO/C-TiO2/Perovskite/HTM/Au were structured by means of one-step coating method with an anti-solvent treatment strategy.41 Figure 4a displays the cross-section scanning electron microscopy (SEM) image of the final PSCs. Clearly, the fabricated PSCs is mainly composed of fluorine-doped tin oxide (FTO), a compact layer of TiO2, an approximate 500 nm thick of perovskite layer, a Spiro-OMeTAD based HTM and Au electrode, which are annotated with the corresponding yellow color designations. The UV−vis and cyclic voltammetry (CV) are used to obtain the energy level values of CMP, which are shown in Figure S6a and S6b. Thereby, the highest occupied molecular orbital (HOMO) level of the CMP is calculated to be -5.60 eV, which shows a good matching with the HOMO values of perovskite and Spiro-OMeTAD. Figure 4b schematically shows the energy level for each of the components for the PSCs. Initially, the conjugated electrons of Spiro-OMeTAD molecule can transferred to CMP in the solution owing to the strong electron-accepting ability of CMP, which is also the main reason for the oxidation of the Spiro-OMeTAD. Meanwhile, we can further infer that the CMP can also play a bridge connecting role between the perovskite and the HTM in the final PSCs, ensuring that photo-generated holes could be effectively transported from the interface of perovskite to HTM, which can lead to a reduced charge recombination and increased hole transportation in the PSCs. Additionally, Figure S7 shows the SEM image and relevant XRD pattern of perovskite film, confirming that the dense


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and homogeneous perovskite film with high crystallinity can be obtained and the PbI2 can also be completely converted by the one-step method used in our work, which is very crucial for the high efficiency of the PSCs.41,53 From Figure S8, It is clearly that the characteristic absorption of perovskite are similar between 400 to 800 nm, but exist slight fluctuation due to the deviation of the perovskite fabrication process. Table 1 The photovoltaic parameters of the above-mentioned PSCs

Devices

Jsc (mA·cm-2)

Voc (V)

FF

PCE (%)

HTM-B HTM-C HTM-FK209

23.85 23.86 23.38

1.04 1.05 1.07

0.68 0.74 0.70

16.91 18.72 17.77

The photocurrent density-voltage (J-V) curves are shown in Figure 4c. The related photovoltaic parameters are summarized in Table 1. Noted that FK209, a Co(III)-complex, is one of the best Spiro-OMeTAD dopants for improving its charge-transport properties.54 For comparison, HTM-B, HTM-C or HTM-FK209 based PSCs were fabricated. The PSC fabricated with HTM-B and HTM-FK209 yield low PCE of 16.91% and 17.77% with a short circuit photocurrent density (Jsc) of 23.85 and 23.38 mA cm-2, an open-circuit voltage (Voc) of 1.04 and 1.07 V and a fill factor (FF) of 0.68 and 0.70, respectively. On the contrary, the HTM-C-based PSC presents a higher PCE of 18.72% together with a Jsc of 23.86 mA cm-2, a Voc of 1.05 V and a FF of 0.74. Quite obviously, the enhanced FF is the main reason for the PCE improvement, ascribing to the increased conductivity and hole extraction efficiency of the doped Spiro-OMeTAD since CMP accelerates the oxidation process. The



hysteresis phenomenon is a critical issue that concern PCE and stability of PSCs.55 Thus, we have performed the I-V measurements with the forward (from short circuit to open circuit) and reverse (from open circuit to short circuit) scanning directions, respectively. And the results have been shown in Figure S9. Quite manifestly, it is clearly find that the CMP based PSCs shows a lower hysteresis factor of 0.069 than that (0.103) of the pristine one, which manifests that appropriate CMP addition can effectively suppress hysteresis. It need to be noted that the hysteresis factor (HI) is expressed to HI = (PCEreverse - PCEforward)/PCEreverse. Then, the statistical distribution of PCEs for the PSCs with varied doping ratio of CMP were investigated. As shown in Figure S10, it can be easily found that when the doping concentration of CMP is 1.5 mg/mL, the FF is significantly improved, and then the PCE also reaches the maximum value. However, when increasing the concentration of CMP, the PCE of device decline dramatically, which may ascribe to excessive oxidation of Spiro-OMeTAD. Therefore, 1.5 mg/mL is considered as the optimal doping concentration to realize quantitative and controllable oxidation and used on the rest of experiments. In order to investigate the reproducibility of the devices, a batch of 20 devices based on the HTMs with different additions were fabricated under the same condition and characterized by the I-V measurements under AM 1.5 G simulated solar light, individually. The statistical histogram is shown in Figure 4d. Expectedly, the CMP based devices achieved an average efficiency of 17.8% in comparison with that of the HTM-C and HTM-FK209, whose efficiency reaches only 16.0% and 16.9%,


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respectively. Quite obviously, apart from high efficiency, CMP doping can lead to a higher PCE reproducibility, which can mainly be attributed to the efficient charge transfer from perovskite to the oxidized Spiro-OMeTAD based HTM doped by CMP. Figure S11 shows the dark current-voltage curves of the above-mentioned PSCs. It is clearly analyzed that the charges recombination at the interface of perovskite and HTM can be effectively reduced by the CMP addition which can be demonstrated by the significant reduction of dark current in comparison with the pristine HTM based one. The incident photon-to-electron conversion efficiency (IPCE) curves present in Figure S12a for further evaluating the performance of PSCs. Distinctly, both devices show a nearly coincident tendency of the IPCE curves regardless of CMP addition. This corresponds to the analogous integrated J values of 23.05 mA cm−2 for the device based on HTM-B and 23.18 mA cm−2 for device based on HTM-C, which are well in accordance with Jsc attained from the J–V curves. Additionally, for giving a deep understanding of the PCE enhancement for the PSCs with CMP addition, the interfacial charge transfer and recombination processes in the PSCs were investigated by measuring the electrical impedance spectroscopy (EIS).56,57 It is initially noted that EIS measurement was carried out in an electrochemistry workstation at frequency from 0.1 Hz to 105 Hz under AM 1.5 G illumination. Figure S12b shows the Nyquist plots of the PSCs coating with HTM-B or HTM-C. Obviously, as confirmed in the previous reports, EIS curves basically consist of a small arc high-frequency range which corresponds to the charge transfer resistance (Rrt) at the interface between HTM and perovskite, while a big semicircle in low-frequency range belongs to the



charge recombination resistance (Rrec) at the interface between electron transfer layer and perovskite, respectively. Due to the same electron transfer layer used in the PSC, the similar Rrec can easily be certified. Significantly, the smaller Rrt value for the CMP based PSC means a large charge transfer rate, which can also easily be interpreted by the enhanced conductivity and the hole extraction efficiency from perovskite to HTM due to CMP addition. Consequently, the EIS analysis further demonstrated that the suitable amounts of CMP doping for the HTM can effectively improve conductivity and hole extraction from perovskite to HTM, which are beneficial to higher FF and Voc values, consistently with the increased changing tendencies obtained from photovoltaic analysis. Besides, the film-quality of the HTM is another important factor for obtaining high efficiency of the PSC. For further understand the influence of the CMP addition on the HTM film, high-resolution SEM and atomic-force microscopy (AFM) images were collected and shown in Figure S13 and S14. The morphology of HTM-C film has not changed significantly compared with HTM-B, attesting that the introduction of CMP does not destroy the structure of the HTM film. From Figure S14, it is obviously that neat perovskite was coated by dopant-free HTM producing 8.09 nm of the surface roughness. After doping CMP, roughness was significantly decreased to 3.91 nm. As a result, the surface was generated by coating HTM with CMP doping on the perovskite is smoother than that of dopant-free HTM, which would be conducive to enhancing hole transportation efficiency and result in heightening the fill factor.


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Fig. 5 (a) Transient photocurrent and (b) transient photovoltage measurements for HTM with and without CMP

The charge extraction and recombination process in the devices was further evaluated by the transient photovoltage (TPV) and transient photocurrent (TPC) measurements. As depicted in Figure 5a, the TPC spectra show that the device based on HTM-C exhibits a smaller charge-extraction time (1.43 μs) than that of the counterpart HTM-B (1.66 μs), testifying that the CMP doped HTM has strong electron-accepting properties and more efficient carrier extraction ability from perovskite to HTM. Additionally, Figure 5b displays that photovoltage decay time is significantly reduced from 3.41 μs to 0.69 μs, also reflecting the electron lifetime in the perovskite and provides insight into carrier recombination rates in the cell. Therefore, we can conclude that the CMP addition in HTM can simultaneously lead to the slower charge recombination and faster carrier extraction at the interface of the perovskite and HTM, finally resulting in an improved photovoltaic performance.58,59



Figure 6. Long-term stability of PSCs based on different HTMs (humidity about 30% and temperature about 25 °C in the dark in air atmosphere) (a) JSC, (b) VOC, (c) FF, (d) PCE and water contact angle of (e) HTM-B and (f) HTM-C.

The long-term stability is a crucial indicator for commercial applications of PSCs.60,61 Therefore, we experienced the long-term stability of PSCs prepared with different hole transport layers (humidity about 30% and temperature about 25 °C in the dark in in air atmosphere). The corresponding experimental results are shown in Figure 6a-d. Under the same storage conditions, the PCE of PSCs based on HTM-C still maintain about 85% of its initial PCE. However, the PCE lost more than 40% for PSCs prepared by HTM-B. As we know, Li-TFSI possesses properties of deliquescent


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and hygroscopic, easily to attract water molecules and result in serious degradation for HTM. Therefore, the results show that the long-term stability of the device could be significantly improved after CMP doping HTM. We speculate that the increase in stability is mainly due to the formation of a more stable HTM film in the presence of organic groups, which may prevent the damage of water molecules in the environment. Due to the doping of CMP into HTM, the instantaneous water contact angle enhanced from 66.1° to 72.5° (Figure 6e and 6f). The larger water contact angle means that the introduction of CMP could serve as a barrier to water molecules. 4. CONCLUSIONS In summary, POM-based inorganic-organic hybrid [Cu(phen)2]2[(Mo8O26)] was successfully synthesized and firstly used as a new additive for Spiro-OMeTAD. Due to the strong electron-accepting, the PSCs based on CMP addition shows a significantly enhanced conductivity and charge extraction property. Thereby, due to the significant enhanced FF and Voc, the corresponding PSCs give rise to an enhanced photovoltaic performance with the maximum PCE as high as 18.72%, which is higher than that of the ever-reported PSCs based on POMs. Meanwhile, the long-term stability of PSCs is also improved after doping CMP. Unambiguously, we believe that these results not only significantly certify promise of POMs-based inorganic-organic hybrid material as an effective dopant for HTM, but also further encourage us to research other POMs-based materials with novel structures for further improving the performance of the PSCs.



ASSOCIATED CONTENT *Supporting Information Additional characterization including structures of CMP, FT-IR spectra, XRD, hole mobility, UV–vis absorption spectra, CV, hysteresis, SEM images, AFM images, IPCE spectra, EIS measurement and crystallographic data of CMP. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] NOTE The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21571042, 21873025, 21371040 and 51603055), the Natural Science Foundation of Heilongjiang Province (Grant No. QC2017055), the China Postdoctoral Science Foundation (Grant No. 2016M601424, 2017T100236), the Postdoctoral Foundation of Heilongjiang Province (Grant No. LBH-Z16059). The Science and Technology Plan Project of Qiqihar (Grant No. GYGG-201705). REFERENCES (1) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an


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Polyoxometalate-based Inorganic-Organic Hybrid [Cu(phen)2]2[(Î±

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