A facile molecularly engineered copper (II) phthalocyanine as hole transport material for planar perovskite solar cells with enhanced performance and stability

Author’s Accepted Manuscript A Facile Molecularly Engineered Copper (II) Phthalocyanine as Hole Transport Material for Planar Perovskite Solar Cells with Enhanced Performance and Stability Guang Yang, Yu-Long Wang, Jia-Ju Xu, HongWei Lei, Cong Chen, Hai-Quan Shan, Xiao-Yuan Liu, Zong-Xiang Xu, Guo-Jia Fang

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S2211-2855(16)30534-1 http://dx.doi.org/10.1016/j.nanoen.2016.11.039 NANOEN1630

To appear in: Nano Energy Received date: 18 September 2016 Revised date: 14 October 2016 Accepted date: 20 November 2016 Cite this article as: Guang Yang, Yu-Long Wang, Jia-Ju Xu, Hong-Wei Lei, Cong Chen, Hai-Quan Shan, Xiao-Yuan Liu, Zong-Xiang Xu and Guo-Jia Fang, A Facile Molecularly Engineered Copper (II) Phthalocyanine as Hole Transport Material for Planar Perovskite Solar Cells with Enhanced Performance and Stability, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.11.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Facile Molecularly Engineered Copper (II) Phthalocyanine as Hole Transport Material for Planar Perovskite Solar Cells with Enhanced Performance and Stability

Guang Yanga1, Yu-Long Wangb1, Jia-Ju Xub1, Hong-Wei Leia, Cong Chena, Hai-Quan Shanb, Xiao-Yuan Liub, Zong-Xiang Xub*, Guo-Jia Fanga*

a

Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of

China, School of Physics and Technology, Wuhan University, Wuhan 430072, China b

Department of Chemistry South University of Science and Technology of China, Shen Zhen, Guangdong, P. R. China, 518000.

[email protected]

[email protected]

*

Corresponding authors.

1

Guang Yang, Yu-Long Wang and Jia-Ju Xu contributed equally.

Abstract Perovskite solar cells (PSCs) demonstrate huge potential in photovoltaic conversion, yet their practical applications face one major obstacle: their instability. As to conventional hole transport materials (HTMs) such as spiro-OMeTAD, their future commercialization maybe hampered for the cost and instability. Here, we report a new HTM of copper (II) phthalocyanine with octamethyl-substituted function groups (CuMe2Pc). Unlike the normal edge on orientation of pristine copper (II) phthalocyanine (CuPc), we found that CuMe2Pc could form face-on molecular alignment when deposited on perovskite via vacuum thermal evaporation, resulting in higher hole mobility, more condense thin film structure and more hydrophobic surface. These properties are more favorable for hole transport and moisture generated applications in PSCs. PSCs with planar structure were fabricated and tested, utilizing different phthalocyanines and spiro-OMeTAD as HTMs. PSCs with CuMe2Pc showed 25% higher power conversion efficiency (PCE) compared with those with CuPc. Furthermore, beneficial from the hydrophobic nature of CuMe2Pc, the devices with CuMe2Pc as HTM show improved stability and retained over 95% of their initial efficiencies even after storage in the humidity about 50% for 2000 h without encapsulation. This study demonstrates that CuMe2Pc is a potential HTM for fabricating low-cost and efficient PSCs with long-term stability.

Graphical Abstract

Planar perovskite solar cells employed CuMe2Pc as hole transport material shows high performance with PCE up to 15.73% and excellent operation stability.

Keywords: hole transport materials, copper (II) phthalocyanine, octamethylsubstituted, molecular alignment, long-term stability

1. Introduction In recent years, organic-inorganic lead halide perovskite solar cells (PSCs) are attractive photovoltaic devices in view of their low cost, easy fabrication, high efficiency.[1-3] Unfortunately, although highly efficient perovskite solar cells have been demonstrated, some important issues such as hysteresis, long-term stability and high cost of synthesis of hole transport materials (HTMs) in perovskite solar cells are still open problems and need to be concerned.[4-7] The most currently studied CH3NH3PbI3 tends to degrade when it was exposed to moisture, oxygen and heat stress, giving rise to reduced device performance.[8-13] In view of this, many strategies have been applied to make further progress in enhancing stability of perovskite solar cells, such as perovskite materials engineered with hydrophobic molecules, modifying perovskite with crosslinks neighbouring grain surface, or adopting a polymer or metal oxide-scaffold structure and so on.[14-18] Apart from these, improved contacts and interfaces, including electron selective layer (ESL)/perovskite interface and perovskite/hole selective layer (HSL) interface, demonstrate to be effective approaches to enhance stability of PSCs.[9, 19-23] Among these approaches, perovskite/HSL interface engineering in conventional PSCs is a

simple method to protect perovskite from degradation from moisture to some extent.[11, 22, 24]

The most efficient PSCs have usually utilized HSL, which is responsible for the hole extraction and simultaneously serves as an electron blocking layer to retard charge recombination.[25-27] An ideal HSL for efficient and stable PSCs should be desirably cost-effective, high-mobility, highly stable (against moisture and thermal stress). The most employed HTM in efficient PSCs is 2,2’,7,7’-tetrakis-(N,N-di-pmethoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD), which inherently suffer from low hole mobility (110-5 to 110-4 cm2/Vs).[28] And p-doped spiro-OMeTAD exhibited enhanced mobility after adding lithium bis(trifl uoromethanesulfonyl)imide (LiTFSI) and 4-tert–butyl pyridine (TBP) as additives. However, these additives could accelerate the degradation of devices performance due to their deliquescent property.[29, 30] Moreover, it has been found that LiTFSI and TBP could evaporate at 85C, which limits device’s thermal stability.[31] Besides, the complex synthetic route of spiro-OMeTAD also renders its potential large-scale application in commercial photovoltaics. Recently, an interesting p-type small molecule, copper phthalocyanine (CuPc), which is well known to be cost-effective, thermally and chemically stable, relatively high mobility and long exciton diffusion length, is successfully introduced into PSCs as a HTM.[32-35] Our previous work showed that CuPc modification by introducing a tetra methyl substituted variant could enhance hole mobility of CuPc due to a stronger - interaction.[36,

37]

But methyl substitution of CuPc is not an

efficient way to optimize its properties such as the degree of mobility enhancement

and the change of molecular orientation (from edge-on to face-on). In this regard, we further optimized the optical, electrical and physical properties of CuPc by introducing octamethyl-substituted function groups (CuMe2Pc), which contributes to the enhancement of device efficiency and long-term stability. We demonstrated that such a clear improvement of device’s long-term stability was correlated to the superior stability and moisture-proof performance of CuMe2Pc. In this paper, we report the synthesis and characterization of a new copper (II) phthalocyanine with octamethyl-substituted function groups (CuMe2Pc). Unlike the normal edge on orientation of pristine copper (II) phthalocyanine (CuPc), we found that CuMe2Pc could form lying down (face on) molecular alignment when deposited on perovskite via vacuum thermal evaporation, resulting in higher hole mobility, more hydrophobic surface and more condense thin film structure. These properties are more favorable for hole transport and moisture resistance applications in PSCs. Perovskite solar cells with conventional n-i-p planar structure were fabricated and tested, utilizing tin dioxide (SnO2)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bilayer as electron selective layer (ESL), organic-inorganic hybrid perovskite (CH3NH3PbI3) as active absorber layer and phthalocyanine as hole transport layer (HTL), respectively (Figure 1a). Both phthalocyaines’ applications in HTL for perovskite solar cells (PSCs) were examined. As expected, CuMe2Pc based planar PSCs exhibited higher power conversion efficiency (PCE) of 15.73% than that of CuPc based devices (12.55%) and better long-term stability.

Fig 1. (a) Schematic structure of perovskite solar cell utilizing phthalocyanine as hole transport layer and the chemical structures of CuMe2Pc and CuPc; (b) Schematic energy level diagram of materials used in the fabricated perovskite solar cell. 2. Materials and methods 2.1.Materials. Unless specified otherwise, all materials were purchased and used as received. CuPc (Sublimed grade 99%), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and organic solvents were purchased from Aldrich. 4,5-dimethylphthalonitrile was purchased from Shanghai Run Biotech. CH3NH3I and PbI2 were purchased from TCI.

Materials Synthesis of Octal-methyl substituted Copper (II) Phthalocyanine (CuMe2Pc) (Scheme S1). A mixture of 4,5-dimethylphthalonitrile (1.0 g, 6.4 mmol), copper (II) chloride (0.28 g, 2.1 mmol), 2,3,4,6,7,8,9,10-octahydropyrimido[1,2a]azepine (DBU) (0.2 ml), and 1-pentanol (5 ml) was put in a two-neck round bottom flask. The reaction mixture was stirred at 135˚C under argon overnight, and then allowed to cool down to room temperature. A 50 ml ethanol was introduced to the resulting mixture to precipitate the product. The suspension was kept stirring for 5 min, and then filtered. The filter paper was washed with DI water, ethanol, and CH2Cl2. The solid was dried at 50˚C in oven and combined. To fulfill the requirements of electronic applications, the product was further purified by vacuum sublimation in a sublimation machine (Technol VDS-80) operated at 475˚C and ~5×10−4 Pa. Elemental analysis calcd (%) for C40H32CuN8: C, 69.80; H, 4.69; N, 16.28; found: C, 69.81; H, 4.68; N, 16.33. UV-Vis (CH2Cl2): λmax = 679, 612 nm. IR (KBr): ν = 2966, 2916, 2850, 1614, 1504, 1464, 1406, 1342, 1311, 1176, 1101, 1028, 877, 810, 748, 717, 667 cm-1.

2.2.Characterization UV-Vis spectra of solution (in CH2Cl2) were recorded on a PerkinElmer Lambda750S spectrophotometer. Elemental analysis was performed by Vario MICRO analyzer. Thermogravimetric analysis (TGA) was recorded with a Discovery TGA analyzer. A few milligrams of solid sample were loaded on a platinum crucible. Weight loss of each sample between 50 and 850˚C at rate of 10 ˚C /min under a flowing N2 stream was examined. Electrochemical measurements were carried out in

a three-electrode glass cell at room temperature. A solution of 0.1 M tetrabutyl ammonium tetrafluoroborate in acetonitrile (MeCN) was used as the supporting electrolyte with ferrocenium/ferrocene couple (Fc/Fc+) as the internal standard in all measurements. Commercially available glassy carbon disk (CH Instruments, Inc.) served as the working electrode, Ag/AgCl electrode as the reference electrode, and Pt wire as the counter electrode. The glassy carbon disk was polished with a 1 μm αalumina slurry (Linde) and then with a 0.3 μm α-alumina slurry (Linde) on a microcloth (Buehler Co.). All the solutions were degassed with nitrogen for 10 min prior to measurement. The cyclic voltammetry was performed using a scan rate of 100 mV/s. The HOMO levels were estimated based on the onset oxidation potentials (Eox). 2.3.Perovskite solar cell fabrication and characterization PSCs were fabricated with the structure FTO/SnO2/PCBM/Perovskite/CuMe2Pc or CuPc/Au on patterned FTO glass with a sheet resistance of 14 Ω/□. The patterned FTO glass was precleaned in an ultrasonic bath of acetone and isopropyl alcohol, and treated in an ultraviolet–ozone chamber (Novascan Company, USA) for 20 min. SnO2 ESLs were prepared by a low-temperature solution process reported in our previous work.[38] In short, 0.1 M precursor solutions of SnCl2·2H2O in ethanol were spin coated on the clean FTO substrates with a spin rate of 2000 rpm for 45 s to obtain dense and uniform SnO2 ESLs. And then, the SnO2 coated FTO substrates were annealed on a hotplate in air at 180˚C for 1h. A thin layer of PCBM (10 nm) was prepared on the FTO surface at a spin-coating rate of 2000 rmp and baked at 100˚C for 10 min. The PCBM solutions were prepared by dissolving 15 mg PCBM in 1 mL

chlorobenzene. The perovskite film (about 500 nm) was deposited via a solvent engineering method.[39] The thickness of the photosensitive layer was measured using an Ambios Technology (Santa Cruz, CA) XP-2 profilometer. Hole transporting layers of CuMe2Pc or CuPc (60 nm) and Au electrode (100 nm) were grown by thermal evaporation under a vacuum of ~1 × 10−6 Torr, and the layer thickness was monitored in-situ using quartz crystal monitors during deposition. The resulted device has an active area of 0.09 cm2. The J-V curve was measured with a standard ABET Sun 2000 Solar Simulator. A xenon lamp coupled with AM 1.5 solar spectrum filters was used as the light source, and the optical power at the sample was 100 mW/cm2. The light intensity of the solar simulator was calibrated using a standard silicon solar cell. The IPCE spectrum was measured in air by a QE-R 3011 system (Enli) in the range of 300-800 nm wavelength. Thin film characterization. Thin film samples of PCBM/Perovskite, PCBM/Perovskite/CuMe2Pc and PCBM/Perovskite/CuPc deposited on FTO glass were fabricated following the same procedure as PSCs fabrication process. Grazing incidence X-ray diffraction patterns of deposited films formed on FTO/PCBM substrates were recorded using a Smartlab 9 kW diffractometer with a Göbel mirror attachment. Irradiation of the parallel CuKα1,2 Xray beam was fixed at a grazing incident angle of 1.000°(θ) and the detector was independently moved to collect the diffraction data in 2θ range of 4°–10° with a stepsize of 0.02° (2θ) at a fixed speed of ca. 5 s/step. UV/Vis spectra of as fabricated films were recorded on a PerkinElmer Lambda750S spectrophotometer. Morphologies of the thin film samples were recorded by atomic force microscopy (AFM) using a

Keysight Technologies (5500AFM/STM) scanning probe in tapping mode. Steadystate photoluminescence (PL) spectra were measured by FLS980 SpectrometerEdinburgh Instruments. The wavelength of excitation light was 470 nm and the samples were excited from the perovskite or phthalocyanine layer. The contact angle measurement of CuMe2Pc and CuPc deposited on perovskite were recorded by Drop Shape Analyzer (DSA 25S，KRUSS). Hole mobility was measured using a HP4155A semiconductor parameter analyzer (Yokogawa Hewlett-Packard, Tokyo, Japan). The carrier mobility was extracted by fitting the J-V curves according to the modified Mott–Gurney equation.[40] J=

9 V ε εμ 8 d

(1)

where J is the current density, ε0 is the permittivity of free space, εr is the relative permittivity, µ is the zero-field mobility, V is the applied voltage, d is the thickness of active layer.

3. Results and Discussion 3.1. Synthesis and Characterization of CuPc and CuMe2Pc The general synthesis of CuMe2Pc is outlined in supporting information and involves a one-step synthesis from 4,5-dimethylphthalonitrile according to a modified reported procedure (Scheme 1).[41] The final product was purified by vacuum sublimation to ensure adequate purity for electronic applications. Purity was determined by elemental analysis.

Scheme 1. Synthetic route to octamethyl substituted copper (II) phthalocyanine (CuMe2Pc).

As the stability and durability of HTM is an important factor, its thermal property usually needs to be concerned. To determine the thermal properties of molecularly modified CuMe2Pc, thermogravimetric analysis (TGA) was carried out (Figure S1). It was found that CuMe2Pc exhibited a better thermal stability with a 6% weight loss occurring at decomposition temperature (Td) of 607˚C, which is higher than that of CuPc (Td of 555˚C), suggesting that the addition of octamethyl-substituted function groups increases the thermal stability of CuPc. The optical and electrochemical properties of CuMe2Pc were characterized by UV– vis absorption spectroscopy and cyclic voltammetry (CV). The UV–vis absorption spectra of CuMe2Pc and CuPc compounds measured in CH2Cl2 solution are depicted in Figure S2a. Similar to CuPc, CuMe2Pc showed a strong absorption in the near UV region (Soret band), and a strong absorption band in the visible region (Q-band). The intense absorption Q-band at around 679 nm is attributed to π→π* transition (a1u→eg*) from the highest occupied molecular orbital (HOMO) to the lowest

unoccupied molecular orbital (LUMO) of the Pc ring. The higher energy Soret band at about 348 nm is attributed to π→π* transition (a2u→eg*) from HOMO to LUMO. Substituted methyl group has insignificant effect on the absorption spectrum of the phthalocyanine in solution while the results are different for solid state absorption spectra. The UV-vis absorption spectra of films of both phthalocyanie are depicted in Figure S2b. Both absorptions are broad with a good coverage in 500-800 nm spectral regions. There are two absorption peak maxima at 632 and 693 nm for CuMe2Pc, and 622 and 695 nm for CuPc. It should be noted that the absorption peak maximum at ~690 nm of CuMe2Pc is more prominent when compared with that of CuPc, rendering the former to have broad absorption in the 500-800 nm spectral region which could be more advantageous for photovoltaic application. As demonstrated in previous report, CuPc, as a p-type donor, exhibited a considerable photovoltaic performance in organic photovoltaics (OPV).[42] Some work reported that the weak absorbance in the visible light region was favorable, because the loss of incident photons absorbed by HTM could be reduced.[43] However, different from this explanation, a relatively stronger absorption of CuMe2Pc in the visible spectrum may partly contribute to increasing the number of photo-generated carriers and thus enhance the short-circuit current density (Jsc). The optical bandgap of CuMe2Pc was estimated using the absorption edge of the solution and found to be 1.7 eV. The HOMO level of the CuMe2Pc was estimated from the onset of the oxidation waves, referred to the Ag/AgCl oxidation couple, and was found to be 5.1 eV (Figure S3). The LUMO level of the CuMe2Pc was calculated

using the CuMe2Pc optical bandgap and its HOMO level and found to be 3.4 eV. The energy levels of different layers were obtained from the literature and listed in Figure 1b.[33, 44] As showed in Figure 1b, the HOMO levels of both phthalocyanines match well with that of perovskite (5.4 eV) and work function of Au electrode (5.1 eV), suggesting them to be an efficient hole extraction layer from perovskite and hole injection layer to Au. Photoluminescence (PL) spectra (Figure S4) also verified this. As showed in Figure S4, for the reference sample without Pc layer, a strong PL peak at 772 nm was observed (Figure S4a). Clearly, the PL spectra of perovskite films can be effectively quenched by inserting a thin layer of both phthalocyanines, indicating that photo-induced excitons can be separated and transferred effectively. Moreover, we can find that the more quenching efficiencies for the perovskite/CuMe2Pc sample, indicating a more efficient hole extraction from the perovskite film to CuMe2Pc than to CuPc and leading to higher JSC, which suggested CuMe2Pc to be more favorable for photovoltaic application.[45] In order to further understand the charge transfer process in perovskite solar cells with different HTMs, time-resolved photoluminescence (TRPL) spectroscopy was carried out. Figure S4b presents the TRPL spectra of perovskite films with CuPc and CuMe2Pc HTMs. It is clearly seen that the PL decay lifetime for the device with CuMe2Pc is shorter than the device with CuPc, which also suggest a more efficient hole extraction from perovskite to CuMe2Pc. Figure 2 shows the grazing incidence X-ray diffraction (GIXRD) patterns of thin film samples deposited on FTO glass substrate. When deposited on FTO, each phthalocyanine film showed one diffraction signal with a 2θ value of 5.8° and 6.8°

(Figure 2a), corresponding to the interplanar distances of 15.2 Å for CuMe2Pc and 13 Å for CuPc, respectively. The d-spacing value is consistent with the estimated molecular length of each phthalocyanine, indicating that both molecules orient nearly perpendicular to the substrate (edge on). It is noted that the diffraction signal of 5.8° for CuMe2Pc is relatively weak which suggests a poor crystallinity of CuMe2Pc film structure. However, no diffraction signal was observed in the 2θ value range between 25° to 28° for both phthalocyanines deposited on FTO (Figure 2b). When CuPc deposited on perovskite layer, the molecular orientation remained the same as standing up orientation indicating by the same diffraction peak at 2θ of 6.8°, while for the CuM2Pc deposited on perovskite, diffraction peak of 2θ of 5.8° was not observed (Figure 2c), indicating that no CuMe2Pc molecule continue to adopt the edge-on orientation on perovskite. Interestingly, within the 2θ value range between 25° to 28° no diffraction signal can be found for CuPc, but a strong diffraction peak of 26.1° was revealed for CuMe2Pc, corresponding to the interplanar distances of 3.4 Å, which indicates that the CuMe2Pc adopts a lying flat (face-on) orientation on the perovskite (Figure 2d). This result is consistent with the reported phenomenon of ZnPc or CuPc, both adopted edge-on orientation on ITO substrate and changed to face-on orientation when employing a CuI or PTCDA template on the substrate.[46]

Fig 2. Thin film X-ray diffraction patterns of thin film samples deposited on FTO glass: (a) CuPc or CuMe2Pc (2θ of 3-10°), (b) CuPc or CuMe2Pc (2θ of 25-27°), (c) PCBM/Perovskite/CuPc or CuMe2Pc (2θ of 3-10°) and (d) PCBM/Perovskite/CuPc or CuMe2Pc (2θ of 25-27°) (Inset figure is the edge-on orientation model for CuPc and face-on orientation for CuMe2Pc deposited on perovskite, respectively).

3.2. Photovoltaic performance of PSCs with CuPc and CuMe2Pc-based HTM As is known, vertical molecular alignment with molecules standing up, or edgeon is beneficial for organic thin film transistors application because of the parallel π-π stacking direction to the substrate as well as the conducting channel.[47] However, for

the application of solar cell with diode structure, planar π-conjugated molecules comprising a face-on orientation is more favorable for facilitating vertical charge transport. The PSCs based on CuMe2Pc or CuPc were fabricated and characterized using conventional planar devices architecture of FTO/SnO2/PCBM/Perovskite/CuMe2Pc or CuPc/Au. The photovoltaic performance of perovskite solar cell employing CuMe2Pc and CuPc as HTL are measured under AM 1.5, 100 mW/cm2 simulated light illumination. Herein, SnO2 layer with high electron mobility combined with effective fullerene passivation layer were corporately worked as ESL. As expected, more balanced electron/hole transport can be achieved by using high electron and hole mobility transporting materials (SnO2 and CuMe2Pc), leading to suppressed interfacial recombination.[30, 48] The photovoltaic parameters including Jsc, open-circuit voltage (Voc), fill factor (FF) and PCE are summarized and shown as Table 1. For each type of PSCs, 10 devices were examined. Compared with CuPc, device based on CuMe2Pc exhibited increased Jsc as well as higher Voc and FF, resulting in a significantly increased PCE. As it is clearly depicted in Figure 3, the device statistics (Voc, Jsc, FF, PCE) of 10 CuPc-based and 10 CuMe2Pc–based devices as collected from the same batch are presented. We note improvements in all device parameters, demonstrating good reproducibility of PSCs employing CuMe2Pc as HTM. Figure 4a depicted the current-voltage (J-V) characteristics obtained from the best cells based on each HTL. The CuPc based device exhibited a Voc of 1.045 V, Jsc of 19.37 mA/cm2 and FF of 62%, yielding a PCE of 12.55%. The device based on CuMe2Pc had a larger Jsc of 21.32 mA/cm2, higher Voc (1.085 V) and higher FF

(68%), yielding an enhanced PCE of 15.73%. It was reported that Jsc showed superlinear increase as HTM oxidation potential decreases (HOMO level upward shift). Compared to the CuPc, CuMe2Pc shows lower HOMO level and enough over potential, which is expected to promote efficient photo-generated charge transfer at the interface and induce a higher Jsc.[49] Previous reports revealed that lowering the highest occupied molecular orbital (HOMO) of the HTM could enhance the Voc of PSCs.[50, 51] In our study, we found that CuMe2Pc based devices exhibited higher Voc than that of CuPc, although CuPc possesses the relatively deeper HOMO level. This is possibly because more balanced charge transport can result in reduced charge recombination, leading to less voltage loss.[52] Besides, J-V hysteresis of devices with CuMe2Pc as HTM was also measured by scanning the applied voltage at 0.1 V/s in reverse and forward scan under simulated solar illumination (AM 1.5, 100 mW/cm2). And the corresponding results are depicted in Figure 4c, which confirms the little hysteresis in the CuMe2Pc based device. To further confirm the device reliability of the PSCs employing CuMe2Pc as HTM, the stabilized efficiency measured at a constant bias of 0.85 V near the maximum power point is also performed and presented in Figure 4d. In planar devices that employed CuMe2Pc as HTM, we observed highly stable steady-state photovoltaic performance with a steady PCE of 15.02%.

Fig 3. Photovoltaic parameters statistics of 10 CuPc-based and 10 CuMe2Pc–based devices as collected from the same batch.

Table 1. Summary of photovoltaic parameters of PSCs employing HTM of CuPc and CuMe2Pc.

HTM

Hole Mobility (cm2/Vs)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

CuPc

7.25×10-4

1.03±0.01

17.88±0.61

57.9±0.05

10.73±1.16

-2

1.08±0.01

20.36±0.74

65.4±0.02

14.34±0.67

CuMe2Pc

4.79×10

To get deep insight into the effects of HTMs on device performance, the incident photon-to-electron conversion efficiencies (IPCEs) of both types of perovskite devices was measured to certify the photovoltaic performance (Figure 4b). The IPCE

results were in good agreement with the electrical characteristics of each type of device. The CuMe2Pc based device exhibited enhanced IPCE over the whole range of visible wavelength from 300 nm to 800 nm, indicating its better charge transport ability. It has been mentioned, face-on molecular orientation is more favorable for charge transport in photovoltaic than that of edge-on orientation. We performed hole mobility characterization for the related devices and the results are depicted in Table 1. As shown in Figure S5, due to its face-on orientation, the hole mobility of CuMe2Pc was calculated to be μh=4.79×10-2 cm2/Vs, which is over 2 orders of magnitude higher than that of CuPc (7.25×10-4 cm2/Vs) with edge-on orientation. And the hole mobility of CuMe2Pc is also superior to the most utilized spiro-OMeTAD and Poly(triarylamine) PTAA (between 10−5 and 10−4 cm2/Vs).[53] The higher charge carrier mobility is consistent with the higher Jsc and FF of CuMe2Pc based perovskite solar cell. This result is consistent with our reported finding, tetramethyl function group leads to stronger π-π stacking of copper (II) phthalocyanine and results in higher charge mobility and better photovoltaic performance.[41]

Fig 4. (a) J-V characteristics and (b) IPCE spectra of the best devices with hole conductor of CuMe2Pc or CuPc. (c) J-V characteristics of a champion device using CuMe2Pc HTM. (d) Steady-state efficiencies of perovskite solar cells using CuMe2Pc HTM measured at constant bias voltages of 0.85 V.

3.3. Long-Term stability of PSCs with CuPc and CuMe2Pc-based HTM The long-term stability of PSCs remaining challenge is another important issue need to be addressed. Relating to their commercial viability, it is necessary to achieve PSCs with high stability in addition to high efficiencies. Metal phthalocyanine exhibits high thermal and chemical stability, which could work as encapsulation layer on perovskite. The air stability of unsealed devices with both phthalocyanine based hole transporting layers was examined every 500 hours. They were stored in ambient environment with temperature of 25˚C and relative humidity of 50%, and their

corresponding performance change is summarized in Figure 5. As seen in Figure 5a, the PCEs of PSCs with CuPc as HTM retained 76% of their initial efficiencies after 2000-h stability test. Nevertheless, more surprisingly, the CuMe2Pc -based devices kept 95% of their initial efficiencies. It needs to be mentioned is that the Voc, Jsc and FF of CuMe2Pc based device exhibited higher stability compared with those of CuPc one (Figure 5b-c). During the 2000-h stability test, the Voc and FF exhibited a slight fluctuation for CuMe2Pc based devices. While for the CuPc, the fluctuation of Voc and Jsc were 10% and 6%, respectively (Figure 5b-c). The FF of CuPc based device showed a decrease trend during the test, which suggests a lower stability compared with that of CuMe2Pc one. This stability difference could be induced by the protecting effect of phthalocyanine hole transporting layer and will be explained in next section. The degradation of the PSCs performance is mainly ascribed to a reaction of the perovskite film with moisture from the atmosphere, and it is examined every 24 hours (one day) by UV-vis absorption and X-ray diffraction degradation measurements for perovskite layer with and without phthalocyanine deposited. As showed in Figure S7, all the as-prepared samples exhibited the reported full absorption from 400-800 nm and an absorption onset at approximately 780 nm. Before exposing to atmosphere, the optical images of all samples exhibit shinny brown color (Figure S6 inset). It is apparent that prominent degradation of the perovskite film without phthalocyanine encapsulation takes place just after 2-day exposure to atmosphere (Figure S6a) and the thin film becomes gray and lusterless after 10-day exposure (Figure S6a inset). However, only very slight degradation can be observed for perovskite covered by

CuMe2Pc even after 10-day exposure, which is more stable than the one covered by CuPc (Figure S6b-c). This result is consistent with the X-ray diffraction measurements. As shown in Figure S8a, all fresh thin film samples exhibited distinguish diffraction signal of perovskite CH3NH3PbI3. In addition, strong diffraction peak at 2θ of 6.8° corresponding to edge-on orientation of CuPc (Figure S8b) and a shoulder peak at 2θ of 26.1° corresponding to face-on orientation of CuMe2Pc (Figure S7c) were observed, respectively. As exhibited in Figure S8, the perovskite without Pc encapsulation reacted with moisture and decomposed fast, which can be indicated by increase intensity of the diffraction peak of PbI2 impurity at 2θ value of 12.7°. However, after covering with CuPc, the enhancement of this diffraction signal became much less (Figure S7e). And only very slight impurity signal was found for CuMe2Pc encapsulated perovskite even after 10-day expose to ambient environment, which suggests its highest stability (Figure S7f).

Fig 5. (a) PCE, (b) Jsc, (c) Voc and (d) Fill Factor values as a function of ambient storage time of the devices with different hole conductor.

To investigate the encapsulation effect of each phthalocyanine, the morphology of the perovskite and CuMe2Pc or CuPc deposited on perovskite were explored by atomic force microscopy (AFM). The grain sizes of the perovskite film were clearly shown in Figure 6a-b, which exhibited densely packed morphology with large grain boundaries. The average grain size and root mean square (RMS) were determined to be ~200 nm and 7.15 nm respectively. Figure 6c-f compared the top surface images of CuMe2Pc and CuPc deposited on perovskite. Due to the different molecular orientation during thin film formation on perovskite via vacuum thermal deposition,

the grain size and morphology of CuMe2Pc and CuPc exhibited great difference. It is noted for CuPc films, larger worm like grains with size of ~100 nm and bigger grain boundaries were obtained. However, CuMe2Pc formed particle like grains with size of ~20 nm and exhibited more condense film structure.

Although the RMS (6.24 nm) of CuMe2Pc is large than CuPc one (5.71 nm), condense film structure with smaller grain boundaries of CuMe2Pc is expected to provide better protection of perovskite from decomposition of moisture. As is known, carbon chain has effect on the static contact angles, in other words, CuMe2Pc with methyl chain results in higher hydrophobicity, which can prevent the atmospheric moisture penetration into the perovskite layer. This may be another reason of CuMe2Pc based perovskite solar cell having higher stability compared with that of CuPc based devices. The wettability of phthalocyanine on perovskite surface was crucial for fabricating photovoltaic device with high stability and it was investigated using a contact angle measurement. Figure 6d-f (inset) shows the optical microscopy images of water droplets dripped onto the surfaces of CuPc and CuMe2Pc deposited on perovskite layer. As shown in Figure 6d (inset), the contact angle of water was measured as 81.2° on CuPc surface. CuMe2Pc deposited on perovskite exhibited a very big water contact angle of 119.6° (Figure 6f inset), which suggested a highly decreased wettability and improved hydrophobicity and led to a higher stability of its corresponding PSCs. For comparison, AFM topography images of spin-coated spiroOMeTAD film is showed in Figure S8c. Consistent with some recent reports, a large density of pinholes in spiro-OMeTAD film were detected, which is found to be

detrimental to the perovskite stability.[54] To further check the HTM protection effect of perovskite layer against moisture, we fabricated a PSC based on spiro-OMeTAD. It exhibited a good photovoltaic performance, with a PCE of 16.37%.(Figure S8a) A 2000-h stability test in the humidity about 50% was also carried out. The PCE for PSC with spiro-OMeTAD as HTM dropped 18%, 27%, 45%, and 79% after 500, 1000, 1500, 2000 h storage, respectively.(Figure S8b) The rapid degradation of spiroOMeTAD-based device under certain humidity is in good agreement with previous reports.[55-57] The hydrophilic additive in spiro-OMeTAD and pinholes existence may account for the instability of device based on spiro-OMeTAD.[22, 54]

Fig 6. AFM images of (a-b) SnO2/PCBM/Perovskite, (c-d) SnO2/PCBM/Perovskite/CuPc and (e-f) SnO2/PCBM/Perovskite/CuMe2Pc deposited on FTO. (Inset: Droplets of water on (d) CuPc and (f) CuMe2Pc).

In summary, a novel phthalocyanine CuMe2Pc with octamethyl substituents was designed and synthesized. Compared with pristine copper (II) phthalocyanine, the methyl substituents help to change the molecular orientation from edge-on to face-on when deposited on perovskite layer, which is more favorable for charge transport in photovoltaic device. Besides, CuMe2Pc exhibits two orders of magnitude higher of carrier mobility than pristine CuPc or spiro-OMeTAD and more hydrophobic surface, leading to better hole transport properties and moisture-proof performance. When employed as hole transporting layer in planar perovskite solar cell, CuMe2Pc based device showed high performance with PCE up to 15.73% and higher operation stability. We believe that our easily-synthesized, highly stable CuMe2Pc is potentially promising HTM for efficient and stable PSCs.

Acknowledgements The work at Wuhan University was supported by the National High Technology Research and Development Program (2015AA050601), the National Basic Research Program (No.2011CB933300) of China, the National Natural Science Foundation of China (61376013, 91433203, J1210061). The work at South University of Science and Technology of China was supported by National Natural Science Foundation of China (No. 210381), and Special Funds for the Development of Strategic Emerging Industries in Shenzhen (JCYJ20150630145302239).

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Authors Biography

Guang Yang is a Ph.D. candidate studying in the group guided by Prof. Guojia Fang at school of Physics and Technology, Wuhan University. His current research interest includes the design of perovskite solar cells with high efficiency and stability.

Yulong Wang is currently a research assistant with Prof. Zongxiang Xu at the Department of Chemistry, Southern University of Science and Technology. He obtained his master degree in organic chemistry from Nanjing University of Aeronautics and Astronautics, China in 2013. His research interests are focused on metal phthalocyanine materials and their application in optoelectronic semiconductor devices (OFETs, OLEDs and OPVs). He has published the first few works on organic photovoltaic cells and hybrid perovskite solar cells.

Xu Jiaju received his B.S. degree in Xiamen University in 2001, M.S. degree in Zhejiang University in 2004, and Ph.D degree in City University of Hong Kong in 2013. Now he conducts post-doctoral studies in South University of Science and Technology of China. His research interests focus on design and synthesis of organic semiconductors as well as their application in organic light-emitting diodes, fieldeffect transistors and organic solar cells.

Hongwei Lei received his Bachelor's degree from School of Physics and Technology at Wuhan University in 2012. He is currently pursuing his Ph.D. under the supervision of Prof. Guojia Fang at Wuhan University. His research interests mainly focus on polymer solar cells and perovskite solar cells.

Cong Chen was educated at Wuhan University where he received a bachelor's degree in 2016. Now he is a Ph.D. guided by Prof. Guojia Fang in School of Physics and

Technology at Wuhan University. His research interest mainly focuses on perovskite solar cells.

Haiquan Shan obtained his M.E. degree from the Institute of Microstructure and Property of Advanced Materials and Beijing Key Lab of Microstructure and Property of Advanced Materials in Beijing University of Technology under the supervision of Prof. Xiaodong Han in 2014. Since 2014 he works as research assistant in Dr. Xu Zongxiang’s group in Southern University of Science and Technology. His research interests focus on the characterizations of semiconductors and graphene nanocomposites.

Liu Xiaoyuan received his B.S. degree in Henan University of Science and Technology in 2013. Now he is a Ph.D student of Nanjing Tech University and a visiting student of South University of Science and Technology of China. His research interests focus on design and synthesis of organic semiconductors as well as their application in perovskite solar cells and organic solar cells.

ZongXiang Xu is an Associate Professor in Chemistry Department of South University of Science and Technology of China. Dr. Xu received his B.S. and M.S. in Chemistry from Xiamen University. In 2008 he received PhD Degree with Prof. ChiMing Che in Chemistry from the University of Hong Kong, where he completed postdoctoral research between 2009-2010. His research is focused on molecular materials design and application on new energy harvesting, such as organic solar cell, organic light-emitting diode and perovskite solar cell.

Guojia Fang received his Ph.D. in Physical Electronics from Huazhong University of Science& Technology in 2001. He became a research associate at the department of electrical & electronic engineering of Imperial College London in 1996~1997. He was a postdoctoral associate in the department of electronic engineering at Tsinghua University (Beijing) from 2001 to 2003. In 2003, he joined the Wuhan University, Hubei, China, where he is currently a Professor. He became a visiting professor at Department of Electrical Engineering of Imperial College from 2003 to 2004. His research interests include thin film photovoltaic device, light emitting diode and related devices.

Highlights 

Copper (II) phthalocyanine was substituted with octamethyl function groups (CuMe2Pc)



CuMe2Pc can form lying down (face on) molecular alignment



CuMe2Pc possesses high hole mobility, condense structure and hydrophobic surface



The device with CuMe2Pc as HTM shows improved stability and enhanced performance