Soluble tetra-methoxyltriphenylamine substituted zinc phthalocyanine as dopant-free hole transporting materials for perovskite solar cells

Organic Electronics 69 (2019) 248–254

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Soluble tetra-methoxyltriphenylamine substituted zinc phthalocyanine as dopant-free hole transporting materials for perovskite solar cells

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Zhendong Cui, Yanqing Wang∗, Yan Chen, Xingze Chen, Xinlian Deng, Wangchao Chen, Chengwu Shi School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei University of Technology, Hefei, 230009, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc phthalocyanine Solution-processable Hole transporting materials Dopant-free Perovskite solar cells

The development of solution-processed and dopant-free hole transporting materials (HTMs) are important for the future large-scale application of perovskite solar cells (PSCs). Here, a tetra-methoxyltriphenylamine substituted zinc phthalocyanine (OTPA-ZnPc) has been synthesized and utilized as a dopant-free HTM for PSCs. The introduction of 4,4′-dimethoxytriphenylamine side chains on ZnPc possesses a wonderful solubility in various solvents, such as dichloromethane, chlorobenzene, and N,N-dimethylformamide. The optimized devices achieved a power conversion efficiency of 16.23% under AM1.5 G standard conditions. What's more, due to the hydrophobic nature of the OTPA-ZnPc, the solar cells remained about 80% of its initial efficiency after 720 h of storage in ambient air with a humidity of approximately 45% without encapsulation, which is obvious better stability than the spiro-OMeTAD devices. Our results indicate that solution-processable OTPA-ZnPc is an encouraging dopant-free hole transporting material in perovskite solar cells.

1. Introduction In recent years, organic-metal halide perovskite solar cells (PSCs) have shown great promise in photovoltaic research due to their high efficiency, low cost, and ease of processing. Its power conversion efficiency (PCE) increased quickly from the initial 3.8%–23.7% [1–10]. The most effective PSCs generally employ hole transporting materials (HTMs), which play an important part in hole transport and charge recombination retardation [11,12]. However, the conventional PSCs typically use expensive HTMs such as 2,2′,7,7′-tetrakis-(N,N′-di-4methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) [13] and poly(triarylamine) (PTAA) [2]. It is frustrating that these conventional HTMs have been suffered from several disadvantages of high synthesis cost, low thermal and chemical stability or low conductivity [14–16]. In addition, the commonly used HTMs, especially spiro-OMeTAD, requires the addition of organic dopants such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tert-butylpyridine (t-BP). Hygroscopicity and deliquescence of these additives accelerating degradation of device performance [17–22]. Therefore, the explore of low-cost, dopant-free HTMs is crucial for PSCs technology in the future commercial applications. Phthalocyanine (PCs) have 18-π electron aromatic heterocyclic ring

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system formed by the connection of four isomeric groups, usually studied as a p-type semiconductor in solar cells and organic thin-film transistors in the near-infrared region, because of its high hole mobility, flexibility, outstanding chemical/thermal stability and cheap [23]. Ke et al. achieved the efficiency of 15.4% by use of copper (II) phthalocyanine (CuPc) as the HTM in the fully-vacuum-processed CH3NH3PbI3 PSCs [21]. Wang et al. studied two octamethyl-substituted Pc (HMe2Pc and ZnMe2Pc) as dopant-free HTMs, and achieved PCE of 15.59% and 14.88%, respectively [24]. Guo et al. reported three new dopant-free hole transport materials based on phthalocyanine cores, and the ZnPcNO2-OBFPh based PSC achieved the highest efficiency of 15.74% among them [25]. Other kinds of soluble phthalocyanines, such as amino-functionalized ZnPcs (BI25, BL07, and BL08) [26], tetra(triphenylamine) ZnPc (TPA-Pc) [27], octamethyl-substituted CuPc (CuMe2Pc) [28] and 5-hexyl-2-thiophene modified ZnPc (sym-HTPcH) [22], have been investigated also. With the mixed perovskite (FAPbI3)0.85(MAPbBr3)0.15 as absorber, PSCs based on substituted phthalocyanine as HTMs have displayed a PCE of 9.9% for nickel (II) 1, 4, 8, 11, 15, 18, 22, 25-octabutoxy-29H, 31H-phthalocyanine (NiPc-(OBu)8) [29], 17.5% for tetra-5-hexylthiophene-based ZnPc [30] and 17.1% for copper tetra-(2,4-dimethyl-3-pentoxy) phthalocyanine (CuPc-DMP) [31]. Recently, Jiang et al. reported that by changing the butyl groups

Corresponding author. E-mail address: [email protected] (Y. Wang).

https://doi.org/10.1016/j.orgel.2019.03.035 Received 18 January 2019; Received in revised form 19 March 2019; Accepted 20 March 2019 Available online 23 March 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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to butoxy structure as Pc ring substituent, this change greatly influences the molecular ordering and successfully enhances solar cell performance, and a higher PCE of 17.6% was displayed by the CuPc-OBu compared to that of CuPc-Bu based-device (14.3%) [32]. As a famous electron donor, methoxyltriphenylamine (OTPA) unit has been widespread used in the design of HTMs [13,33]. Furthermore, according to the previous literature, methoxy functional groups in HTMs act as Lewis base for defect healing by forming a Lewis adduct with uncoordinated ion, which could passivate defects in the perovskite and improve the interface to obtain better performances [34–37]. Interesting, though a study on tetra-methoxyltriphenylamine substituted porphyrin was reported, to the best of our knowledge, there is no study on tetra-methoxyltriphenylamine substituted phthalocyanine (OTPAZnPc) [38]. Herein, we designed and synthesized the OTPA-ZnPc complexes, the electrochemical, photophysical and thermal behaviors of this phthalocyanine were studied, and the phthalocyanine was explored as dopant-free HTM in the (FAPbI3)0.85(MAPbBr3)0.15 based PSCs.

was carried out by a U3900H UV–vis spectrophotometer (Hitachi, Japan). The decomposition temperature was carried out in a nitrogen atmosphere on a thermogravimetric analyzer (TGA, Q5000IR) with a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was measured employing Chimaera instrument Q100 DSC under nitrogen, the sample was heated at 10 °C min−1 from 30 °C to 250 °C. The Cyclic voltammetry measurements (CV) measurements were tested by CHI 660D electrochemical workstation (Shanghai Chenhua Device Company, China), with the platinum working electrode, the platinum wire counter electrode and the SCE reference electrode (saturated KCl solution). 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) was added as a supporting electrolyte to the DMF solution, and the redox potential was measured at a scanning rate of 50 mV s−1. Photocurrent-voltage (J-V) curves were obtained under AM 1.5 illumination by a 3A grade solar simulator (Newport, USA, 94043A). IPCE is measured by the QE/IPCE Measurement Toolkit (Newport, USA). Steady-state photoluminescence (PL) measurements were performed on a fluorescence detector (QM400). Field-scan scanning electron microscopy (ZEISS SIGMA) and atomic force microscope (Veeco DiMultiMode V) were used to study the coverage of perovskite films and hole transport materials on the perovskite layer.

2. Experimental 2.1. Synthesis

3. Results and discussion All the chemical reagents were supplied from Aldrich, Acros, or J&K and employed directly without any further purification. The synthesis and characterization of compounds 3–6 were followed those methods detailed in a previous report [23,39], and terminal product OTPA-ZnPc (7) was obtained with the cyclotetramerisation reaction of 6.

The structure of OTPA-ZnPc (7) is shown in Fig. 1 and the detailed synthesis route is shown in Scheme S1 (Supporting Information). The 4iodoptzdzalonitrile (5) was prepared according to literature [39], and the precursor 4,4′-diphenylamino-biphenyl-3,4-dicarbonitrile (6) was obtained by reacting 4-iodophthalonitrile with 4-methoxy-N-(4-methoxyphenyl)eN-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) aniline (4), according to a previous method [22]. Then, the compound 6 was mixed with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and zinc acetate in 1-pentanol. Purification by column chromatography and finally obtained with a green powder of OTPA-ZnPc (7) in 68% yield [23]. As expected, the achieved product showed wonderful solubility in common organic reagents like dichloromethane, chlorobenzene, and N,N-dimethylformamide, which facilitates solution processing on top of perovskite film. The newly synthesized phthalocyanine was characterized by MALDI-TOF Mass Spectrometry, 1H NMR, FT-IR, UV–vis, fluorescence spectroscopies as well as CV (Figs. S1–S6, Supporting Information). TGA of the OTPA-ZnPc exhibits an excellent thermal stability up to 420 °C (Fig. S7, Supporting Information). The glass transition temperature (Tg) of OTPA-ZnPc showed by DSC measurements is around 150 °C, which are substantially higher than that of spiroOMeTAD (Tg = 120 °C), suggesting good morphology stability of OTPAZnPc (Fig. S8, Supporting Information) [41].

2.2. Device fabrication and characterization According to the previous reports we fabricated the PSC devices [40]. FTO coated glass layer was etched employing zinc powder and hydrochloric acid. Then rinsed in acetone, ethanol and deionized water in turn and finally dried with dry air. Next, using a solution of 6 mL anhydrous isopropanol with 0.6 mL titanium diisopropoxide bis(acetylacetonate) deposited on the cleaned FTO electrode by aerosol spray pyrolysis method to obtain a thin layer of compact TiO2. After spraying, the sample was heated at 500 °C for 1 h. Subsequently, a mesoporous TiO2 layer was deposited on the above substrate by spin coating a diluted granular TiO2 slurry (diluted in ethanol with 1:5 w/w) at 5500 r.p.m. for 20 s, then heated at 510 °C for 30 min. The perovskite was deposited on the titanium dioxide film by the following process. The perovskite precursor solution was prepared by mixing PbI2 0.53 g, FAI 0.193 g, PbBr2 0.074 g, MABr 0.022 mg in 0.8 mL DMF and 0.2 mL DMSO, and remained stirring at 80 °C for about 2 h, then deposited on the TiO2 substrate by spin coating with first 1500 r.p.m. for 10 s, second 5000 r.p.m. for 30 s. Then 1 mL of chlorobenzene as anti-solvent was dropped onto the above substrate. Then the substrate was heated at 100 °C for ∼1.5 h, and the color of sample changed from yellow to black. Dopant-free OTPA-ZnPc or doped spiro-OMeTAD was used as HTM and then spin coated at 4000 r.p.m. for 20 s. The OTPA-ZnPc solution was produced by dissolving 55 mg of OTPA-ZnPc solid in 1 mL of chlorobenzene. The spiro-OMeTAD solution including 73 mg of spiroOMeTAD, 28 μL of t-BP and 18 μL of LiTFSI solution (520 mg LiTFSI in 1 mL acetonitrile) in 1 mL of chlorobenzene. The final step in the preparation of the solar cells is to deposit a gold electrode of about 80 nm thickness by thermal evaporation under vacuum. 2.3. Measurements 1

H NMR spectra were obtained on a Bruker DPX using CDCl3 or DMSO‑d6 as solvent. MALDI-TOF mass spectrometry experiments were recorded using MS Bruker Daltonik Reflex III and Bruker solariX spectrometers. The FT-IR spectra were achieved with a SHIMADZU IRTracer-100 spectrometer. UV–vis spectra of OTPA-ZnPc in CH2Cl2

Fig. 1. Structure of OTPA-ZnPc. 249

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and transport the photo-generated holes from the perovskites. The hole mobility of spiro-OMeTAD and pristine OTPA-ZnPc were measured using the space-charge-limited current (SCLC) method, and the corresponding IeV curves of hole-only devices under dark condition for the estimation of hole mobility of HTM were shown in Fig. 3 (d). The extracted hole mobility for pristine OTPA-ZnPc was calculated as 1.08 × 10−5 cm2 V−1 S−1, which is slightly higher than spiro-OMeTAD (1.02 × 10−5 cm2 V−1 S−1). Fig. 4 (a) shows the J–V curves of the champion device based on dopant-free OTPA-ZnPc as HTM and compared with doped spiroOMeTAD. The corresponding values are summarized in Table 2. The OTPA-ZnPc-based PSC provided a PCE of 16.23%, with a short-circuit current density (JSC) of 22.36 mA cm−2, an open-circuit voltage (VOC) of 1.02 V, and a fill factor (FF) of 71.43%. It is considered that the oxygen atoms in methoxy units could act as Lewis bases, passivating the defect sites at the interface between perovskites and HTM, therefore resulting in higher efficiency here, as well as the below mentioned stable PSCs [36]. For comparison, the reference device with the doped spiro-OMeTAD-based PSC obtained a PCE of 18.23%, with a JSC of 22.41 mA cm−2, a VOC of 1.08 V, and a FF of 75.58% under the same condition. Although the PCE is a little lower than the control device, it is worth to notice that the optimal PCE of spiro-OMeTAD is measured with t-BP and LiTFSI as dopants, while the OTPA-ZnPc device is dopantfree one. Fig. 4 (b) shows that the device using OTPA-ZnPc as HTM has a small hysteresis by J–V hysteresis measurement. To confirm the repeatability of devices performance, 30 identical devices were fabricated, and the histograms of PCEs are shown in Fig. 4 (c), which indicating that those PSCs using OTPA-ZnPc-based HTM have good reproducibility. Fig. 4 (d) displays the incident photon-to-current efficiency (IPCE) of the optimal PSC devices to further study the influence of HTMs on the device performance [47]. The OTPA-ZnPc-based device exhibits superior IPCE from 300 to 800 nm and the integrated current density (20.75 mA/cm2) is basically in a good line with the J-V measurements. The steady-state photoluminescence (PL) was used to further investigate the influence of OTPA-ZnPc on the charge transport kinetics of perovskite/HTM interface. In general, the higher degree of interface fluorescence quenching and the greater decrease in lifetime, the stronger hole extraction capability of HTMs and ultimately the performance of device [48]. As shown in Fig. 5, OTPA-ZnPc coated on the perovskite layer quenched the PL emission greatly, indicating that the charge transfer ratio at the perovskite/OTPA-ZnPc interface is more effective than that of pristine perovskite or perovskite/spiro-OMeTAD interface. It is well acknowledged that the structure with a high-grade surface is beneficial for highly efficient PSCs [41]. SEM was utilized to study the distribution of (FAPbI3)0.85(MAPbBr3)0.15 perovskite material on the surface of TiO2 with and without HTMs. As can be seen in Fig. 6 (a), the pristine perovskite layer shows a smooth and dense morphology with no obvious pinholes. The smooth film reveals that the OTPA-ZnPc is uniformly coated on top of the perovskite film (Fig. 6 (b)), and can effectively inhibiting the direct contact between the perovskite film and the Au electrode. Atomic force microscope (AFM) image of the original perovskite film and spin-coated OTPA-ZnPc film are shown in Fig. 6 (c) and Fig. 6 (d), respectively. The root mean square roughness of the perovskite film and the OTPA-ZnPc film are 25.3 nm and 10.7 nm, respectively. The roughness of the perovskite film is reduced after the deposition of 60 nm OTPA-ZnPc film, which can be favorable to the perovskite stability. In addition, we compared the laboratory cost differences between OTPA-ZnPc and spiro-OMeTAD. A flow chart describing the synthesis of 1 g of OTPA-ZnPc is shown in Tables S1–S4 (Supporting Information). For this result does not take into consideration of several important parameters (e.g. labor, waste treatment and energy consumption), it was multiplied by a factor of 1.5 to get a more realistic estimation of lab synthesis costs of ∼$57/g to OTPA-ZnPc. At the same time, the price of

Fig. 2. Optical absorption spectrum of OTPA-ZnPc in CH2Cl2.

UV–vis spectra of the OTPA-ZnPc compound in dichloromethane is shown in Fig. 2, the two typical strong absorption bands were ascribed to the Soret band in the near UV region and the Q-band in the visible region, respectively. Because of the Davydov effect [42,43], double peak absorptions appear at about 654 nm (Q1 band) and 708 nm (Q2 band) in the visible region, resulting with the Q-band splitting, which representing π-π* transitions. The electrochemical properties of OTPA-ZnPc was recorded by CV measurement, and 0.1 M tetrabutylammonium perchlorate was added as a supporting electrolyte in a dichloromethane solution of OTPAZnPc, the result is displayed in Fig. S9 (Supporting Information). The optical band gaps of OTPA-ZnPc was evaluated employing the UV–vis absorption edges of the solution and calculated to be 1.75 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values of the compounds can be achieved by employing the oxidation potentials and optical band gaps, and the data are summarized in Table 1 [31]. The HOMO value of the obtained OTPA-ZnPc is calculated to be −5.58 eV, which is a suitable energy level alignment of OTPA-ZnPc and the mixed perovskite (−5.65 eV), and indicates that the HTM should be able to efficiently extract the generated holes in the perovskite layer [44,45]. To investigate the effect of OTPA-ZnPc HTM in PSCs, we prepared the devices with the structure of FTO/compact TiO2/mesoporous TiO2/ perovskite/HTMs/Au (Fig. 3 (a)), accordingly, the energy level alignment of PSCs was depicted in Fig. 3 (b). The driving force is required to inject holes from the perovskite to the HTMs, and the HOMO of OTPAZnPc is higher than (FAPbI3)0.85(MAPbBr3)0.15 (−5.65 eV) provides this driving force [30,46]. The cross-sectional scanning electron microscopy (SEM) image (Fig. 3 (c)) shows that the device consists of a 200 nm TiO2 layer, a 450 nm (FAPbI3)0.85(MAPbBr3)0.15 absorber, a HTM of OTPAZnPc about 60 nm, and an 80 nm gold electrode. Hole transporting characteristic is an important parameter for HTMs to effectively collect

Table 1 Optical and electrochemical data for OTPA-ZnPc. HTM

λmaxa) solution [nm]

Eoxb) [V]

EHOMOc) [eV]

ELUMOd) [eV]

Ege) [eV]

OTPA-ZnPc

708

0.82

−5.58

−3.83

1.75

Absorption spectrum was measured in dichloromethane solution; b) Cyclic voltammetry measurements were carried out in dichloromethane solutions with TBAPF6 (0.1 M) as supporting electrolyte, referenced against Fc/Fc+; c) HOMO level was determined by the equation EHOMO = −4.44–[(0.67+(Eox–E1/2(Fc/ Fc+)); d) LUMO = HOMO + Eg; e) optical band gap was calculated by the onset of absorption. 250

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Fig. 3. (a) Schematic device architecture of PSCs studied. (b) Energy level diagram of different components in PSCs. (c) Cross-sectional SEM image of the complete PSC device with OTPA-ZnPc as HTM. (d) I-V curves of hole-only devices under dark condition for the estimation of hole mobility of HTM.

Fig. 4. (a) J-V characteristics of the PSC devices studied in this work. (b) Hysteresis J–V characteristics. (c) Histogram of PCEs of 30 PSC devices using OTPA-ZnPc as HTM. (d) IPCE spectra of the PSC devices with OTPA-ZnPc and spiro-OMeTAD as HTMs.

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Table 2 J-V characteristics of PSCs based on OTPA-ZnPc and spiro-OMeTAD as HTMs measured at different scan directions under 100 mW cm−2 illumination (AM 1.5G). HTM

Scan directions

Jsc (mA cm2)

Voc (V)

FF (%)

PCE (%)

Spiro-OMeTAD

forward reverse forward reverse

22.18 22.41 22.30 22.36

1.07 1.07 1.02 1.02

74.99 75.58 68.35 71.43

17.89 18.23 15.58 16.23

OTPA-ZnPc

Fig. 7. Stability test of the PSCs employing OTPA-ZnPc and spiro-OMeTAD without encapsulation, stored under ambient conditions (humidity of ∼45% and temperature of ∼25 °C).

the synthetic spiro-OMeTAD was investigated from earlier literature (∼$500/g, high purity, Merck) [49]. Our newly developed high performance and stability OTPA-ZnPc costs only about one-tenth of spiroOMeTAD. Finally, the long-term stability test of the PSCs utilizing OTPA-ZnPc and doped spiro-OMeTAD as HTM were implemented under ambient conditions (with a humidity of ∼45% and temperature of ∼25 °C). Fig. 7 shows the OTPA-ZnPc-based devices possess an obvious better stability than the spiro-OMeTAD-based ones. After 720 h, the average PCE of the former one retains approximately 80%, whereas the spiroOMeTAD devices retain only 45% of the initial PCE under the same situations. The contact angle of water on OTPA-ZnPc and spiroOMeTAD surface are 97.42° and 82.25°, respectively (Fig. S10,

Fig. 5. Steady-state PL spectra of glass/perovskite/HTMs based on OTPA-ZnPc and spiro-OMeTAD.

Fig. 6. SEM images (top view) of the surface morphology of (a) the mixed-ion perovskite, (b) OTPA-ZnPc; AFM images of (c) FTO/TiO2/perovskite and (d) FTO/ TiO2/perovskite/OTPA-ZnPc. 252

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Supporting Information), which revealing that the surface hydrophobicity of OTPA-ZnPc is better than the spiro-OMeTAD, and can effectively prevents moisture penetration into the perovskite layer [46].

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4. Conclusions

[16]

In conclusion, solution-processable OTPA-ZnPc has been tailormade and explored as high-performance HTM in (FAPbI3)0.85(MAPbBr3)0.15 solar cells. The effective device of OTPAZnPc demonstrated an impressive PCE of 16.23% with a VOC of 1.02 V, a JSC of 22.36 mA cm−2 and a FF of 71.43 under AM 1.5G standard conditions. After storage for 720 h in ambient air, an initial efficiency of about 80% was maintained. It is believed that the easily synthesized OTPA-ZnPc is a promising dopant-free HTM for efficient and stable PSCs.

[17]

[18]

[19]

[20]

Conflicts of interest [21]

The authors declare no conflict of interest. [22]

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51602089, 51472071, and 51272061), the Fundamental Research Funds for the Central Universities (JZ2017HGTB0230, XC2015JZBZ04), and the Talent Project of Hefei University of Technology (75010-037004, 75010-037003).

[23]

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Appendix A. Supplementary data [25]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.03.035. [26]

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