Small Molecule–Polymer Composite Hole-Transporting Layer for

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Small Molecule−Polymer Composite Hole-Transporting Layer for Highly Efficient and Stable Perovskite Solar Cells Jin-Miao Wang,† Zhao-Kui Wang,*,† Meng Li,† Ke-Hao Hu,† Ying-Guo Yang,‡ Yun Hu,† Xing-Yu Gao,‡ and Liang-Sheng Liao† †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China ‡ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China S Supporting Information *

ABSTRACT: Effective and stable hole-transporting materials (HTMs) are necessary for obtaining excellent planar perovskite solar cells (PSCs). Herein, we reported a solution-processed composite HTM consisting of a polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and a small-molecule copper phthalocyanine−3,4′,4″,4‴-tetrasulfonated acid tetrasodium salt (TS−CuPc) with optimized doping ratios. The composite HTM is crucial for not only enhancing the hole transport and extraction but also improving the perovskite crystallization. In addition, the composite HTM can weaken the indium tin oxide erosion by reducing the acidity and increasing the dispersibility of the PEDOT:PSS aqueous dispersion via incorporating suitable TS−CuPc. Consequently, a highly efficient device was fabricated with a power conversion efficiency (PCE) of 17.29%. Its short-circuit current (JSC) is 22.23 mA/cm2, and its open-circuit voltage (VOC) is 1.01 V. Meanwhile, it exhibited a higher fill factor (FF) of 77% and improved cell stability. The developed composite HTM provides a good potential anode interfacial layer for fabricating outstanding PSCs. KEYWORDS: perovskite solar cells (PSCs), small molecule-polymer, hole-transporting layer, high efficiency, high stability

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INTRODUCTION Advances have been made for perovskite solar cells (PSCs) with a remarkably increased power conversion efficiency (PCE) of more than 20%.1−5 Mesoscopic and planar structures are mainly used to fabricate high-performance PSCs because of prominent perovskite materials.6−13 In particular, the planar structure has the advantages of a simple device architecture and fabrication procedure, low temperature and solution processing compared with the mesostructured devices.9−13 In a planarstructured PSCs, a hole-transporting layer (HTL) between the light-absorption layer (perovskite) and the anode is necessary for fabricating high-performance solar cells. With a suitable HTL, the devices can realize the aims of improving Schottky contact, promoting the separation of exciton, reducing the charge recombination, and facilitating the transport of holes.14−18 Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is a universal HTL in PSCs with a p-i-n structure. 19−21 Some other hole-transporting materials (HTMs), such as poly(triaryl amine) (PTAA),22 nickel oxide (NiO),23,24 germanium dioxide (GeO2),25 molybdenum trioxide (MoO3),26 and graphene oxide (GO),27 have also been used in PSCs with a p-i-n structure. Nevertheless, except for the PEDOT:PSS, the PSCs based on these reported HTMs presented unsatisfactory device performance. In addition, © 2017 American Chemical Society

PEDOT:PSS has some shortcomings when acting as an HTL in PSCs with a p-i-n structure. One is its limited work function, which would cause a lower open-circuit voltage of around 0.9 V.28−30 The other is that the indium tin oxide (ITO) electrode is liable to erosion by the acidity possessed by the PEDOT:PSS aqueous solution, and the acidity restricts the stability of PSCs.31−33 Therefore, great efforts are required to seek efficient and stable HTMs in PSCs with a p-i-n structure. Copper phthalocyanine (CuPc) is ubiquitous in organic photovoltaic devices because of its merits such as ease of synthesis, low band gap, and high hole mobility.34,35 Recently, vacuum thermally evaporated CuPc has been used in PSCs with an n-i-p structure.36,37 However, its poor solubility hinders its application in solution-processable devices. TS−CuPc (copper phthalocyanine−3,4′,4″,4‴-tetrasulfonated acid tetrasodium salt) is an excellent solution-processable HTM with merits such as the alkaline nature of aqueous solution, physical and chemical stabilities, remarkable photovoltaic properties, high work function, and good water solubility.38,39 Consequently, an excellent hole interfacial layer by doping PEDOT:PSS with Received: February 15, 2017 Accepted: March 23, 2017 Published: March 23, 2017 13240

DOI: 10.1021/acsami.7b02223 ACS Appl. Mater. Interfaces 2017, 9, 13240−13246

Research Article

ACS Applied Materials & Interfaces TS−CuPc based on solution processing is expected to fabricate PSC devices. In this work, we combine the advantages of PEDOT:PSS and TS−CuPc by mixing them based on an aqueous solution processing. The composite films act as the HTL in the PSCs in which CH3NH3PbI3−xClx, PC61BM (phenyl-C61-butyric acid methyl ester), and bathophenanthroline (Bphen) are used as the active layer, the electrontransporting layer, and the cathode modification layer, respectively. The preparation technology results in a higher VOC, and thus a more outstanding PCE, and in an improved cell stability compared with devices without the incorporation of TS−CuPC. As a consequence, the device has a PCE of 17.29%, a VOC of 1.01 V, a JSC of 22.23 mA/cm2, and an FF of 0.77. Moreover, improved cell stability is achieved through simultaneous improvements of the cell parameters VOC, JSC, and FF.

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Figure 1. Schematic diagram of the planar perovskite solar cell structure and the chemical structures of TS−CuPc and PEODT:PSS.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION First, pristine TS−CuPc HTL-based cell devices were acquired. Unfortunately, a low PCE of 6.28% was obtained (Figure S1). The reason is the poor film-forming ability of TS−CuPc on the ITO substrate. Actually, perovskite crystallization strongly depends on its underlayer.39−41 The device performance could be obviously improved when adopting a bilayer HTL of PEDOT:PSS/TS−CuPc regardless of their deposition sequence (Figure S1). Interestingly, TS−CuPc-doped PEDOT:PSS composite films on ITO demonstrated a better film uniformity with lower roughness (Figure S2), which is beneficial for the growth of perovskite crystal films. Figure 2a−c presents the AFM images of PEDOT:PSS, TS−CuPC, and TS−CuPc-doped PEDOT:PSS (50 wt %) films. Their rootmean-square (rms) values are 1.55, 2.90, and 1.26 nm, respectively. It means that a smooth composite film, which is favorable to the growth of perovskite films, could be formed by doping PEDOT:PSS with TS−CuPC. The corresponding SEM images of the perovskite films are presented in Figure 2d−f. Perovskite films prepared using PEDOT:PSS as the HTL exhibit smaller perovskite grains with many small pinholes and voids between the crystalline domains. However, pristine TS− CuPc-based perovskite films present poor film coverage because of the appearance of many large voids. By adopting TS−CuPc-doped PEDOT:PSS as an underlayer, the perovskite film possesses largely improved crystal quality with a good coverage area and fewer pinholes and voids. Figure 3a shows the film conductivities of different HTLs including PEDOT:PSS, TS−CuPc, and the optimized composite film of TS−CuPc-doped PEDOT:PSS (50 wt %). The corresponding device structure is ITO/HTL (40 nm)/ MoO3 (10 nm)/Ag (100 nm).42 The HTLs are PEDOT:PSS, TS−CuPc, and the composite film of TS−CuPc-doped PEDOT:PSS (50 wt %), respectively. The composite film possesses the highest conductivity among the three samples. The PEDOT:PSS composite film doped by TS−CuPc also plays a role in manipulating the carrier behavior in PSCs because of the better energy alignment.43 Figure 3b shows the UPS spectra of the PEDOT:PSS film, the TS−CuPc film, and the composite film of TS−CuPc-doped PEDOT:PSS (50 wt %). The photoemission cutoffs are also shown in Figure 3b. The work functions of PEDOT:PSS, TS−CuPc, and TS− CuPc-doped PEDOT:PSS (50 wt %) are 4.8, 4.9, and 5.1 eV, respectively. The doped composite film presents an increased work function by 0.3 eV than PEDOT:PSS, which would

Materials and Preparation. TS−CuPc (99%, Aldrich) was fabricated by string over 10 h in deionized water. The concentration of the TS−CuPc aqueous solution is 0.5 wt %. The prepared TS− CuPc aqueous solution and the purchased PEDOT:PSS (Clevios PVP AI 4083, Heraeus, Germany) were blended at different volume ratios. A molar ratio of 3:1 was used in the perovskite precursor solution of CH3NH3I and PbCl2 (99.9%, Alfa-Aesar) with an N,N-dimethylformamide (DMF) (amine free, anhydrous, 99.9%, Alfa-Aesar) solvent. Device Fabrication. The sheet resistance of the glass/ITO substrates is 15 Ω/sq. The glass/ITO substrates were cleaned in an ultrasonic bath for 15 min using detergent, acetone, and ethanol. The dried ITO-coated glass substrates were placed in an ultraviolet ozone machine for approximately 20 min. The TS−CuPc-doped PEDOT:PSS layers were obtained using spin-coating and annealing. The rotation speed was 4500 rpm/40 s, and the annealing temperature was 110 °C. After spin-coating, the annealing was sustained for 20 min. The bilayer HTLs of ITO/PEDOT:PSS/TS−CuPc and ITO/TS− CuPc/PEDOT:PSS were deposited by spin-coating their precursors under the same condition with corresponding deposition order. The CH3NH3PbI3−xClx solution (30 wt %) was deposited with a mixed solution of CH3NH3I and PbCl2 in a 3:1 molar ratio at 4000 rpm/40 s onto the HTL in a N2 glove box. After annealing from 60 to 100 °C gradually, the perovskite films were acquired on a hot plate. After that, 20 mg/mL PC61BM (Nichem Fine Technology Co. Ltd., Taiwan) is in chlorobenzene and 0.5 mg/mL Bphen (Nichem Fine Technology Co. Ltd., Taiwan) exists in ethanol. They were spin-coated at 2000 rpm/40 s and 4000 rpm/40 s, respectively. After this process, there is no further annealing. Ag was vacuum-deposited onto the Bphen layer under 10−6 Torr with a mask area of 7.25 mm2. ITO/PEDOT:PSS (with or without TS−CuPc)/CH3NH3PbI3−xClx/PC61BM/Bphen/Ag (100 nm) is the final device structure. Figure 1 displays different functional layers of the p-i-n structure and the molecular structures of PEDOT:PSS and TS−CuPc. Characterization. The HTL film surface morphologies were evaluated using atomic force microscopy (AFM; Veeco Multimode V instrument). Scanning electron microscopy (SEM) was used to test the perovskite film surface morphologies (Quanta 200 FEG, FEI Co.). The J−V characteristics of PSCs were obtained. Then, a Keithley 2400 source meter is put into use. The perovskite films also can be analyzed using X-ray diffraction (XRD); using Empyrean (PANalytical 80), with Cu Kα radiation. PerkinElmer Lambda 750 was used to carry out UV− vis spectrophotometry analyses. Electrochemical impedance spectroscopy (EIS) was performed under dark conditions using an IM6e Electrochemical Workstation (Zahner). UPS (ultraviolet photoelectron spectroscopy) was carried out to obtain the work function of different HTLs. The steady-state photoluminescence (PL) spectra of the perovskite films were collected. A Horiba Jobin-Yvon LabRAM HR800 was used with a 520 nm excitation wavelength under dark conditions. A MarCCD was used to obtain 2D GIXRD images. 13241

DOI: 10.1021/acsami.7b02223 ACS Appl. Mater. Interfaces 2017, 9, 13240−13246

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Figure 2. AFM images of different HTLs: (a) PEODT:PSS, (b) TS−CuPc, and (c) TS−CuPc-doped PEODT:PSS (50 wt %). SEM images of the CH3NH3PbI3−xClx perovskite films deposited on (d) PEODT:PSS, (e) TS−CuPc, and (f) TS−CuPc-doped PEODT:PSS (50 wt %).

performance: 17.29% (PCE), 22.23 mA/cm2 (JSC), 1.01 V (VOC), and 0.77 (FF). The hysteresis tested at a speed of 100 ms delaying time is not obvious in the optimally performed device (Figure S4).44 Interestingly, the composite HTL of PEDOT:PSS also delivered an obviously improved cell efficiency in PSCs based on CH3NH3PbI3 and (NH2CH NH2PbI3)0.83(CH3NH3PbBr3)0.17 (Figures S5 and S6). For the CH3NH3PbI3−xClx PSCs, PCE and FF improved by 30%and 13%, respectively. The JSC and VOC also increased by around 8%. The obvious improvement in performance after inducting the new technology is on account of facilitating the charge transport and reducing charge recombination owing to appropriate energy alignment and desired crystallinity of the perovskite light absorption layer on the composite HTL. To better understand the reason behind the improved device performance, further investigations were carried out. Figure 4b presents the PL spectra of ITO/PEDOT:PSS/ CH3NH3PbI3−xClx, ITO/TS−CuPc/CH3NH3PbI3−xClx, and ITO/TS−CuPc-doped PEDOT:PSS (50 wt %)/CH3NH3PbI3−xClx samples. By doping TS−CuPc into PEDOT:PSS, the PL quenching efficiency of CH3NH3PbI3−xClx on the composite HTL is increased effectively, which is beneficial for the hole transport and extraction of the ITO electrode.45 The UV−vis absorption spectra of the perovskite light absorption layer deposited on three different HTL films are also tested (Figure 4c). The characteristic absorbance wavelength range of the three samples is rarely changed. The TS−CuPc can harvest photons for the improvement of PSCs46−48 (Figure S7). Nevertheless, the perovskite film deposited on the composite HTL exhibits the perfect light absorption across the entire visible light, which is ascribed to the enhanced JSC and thus improved PCE, as shown in Figure 4a. The interfacial properties at the anode side were also studied using EIS. The impedance of devices was tested under the conditions of no light, as shown in Figure 4d. The corresponding circuit diagram of the planar structure PSCs can also be seen in Figure 4d. The equivalent circuit diagram is a parallel circuit, which includes series resistance (Rs), charge transfer resistance (RCT), and capacitor (CCT). The RCT values in devices based on the PEDOT:PSS, TS−CuPc, and TS− CuPc-doped PEDOT:PSS (50 wt %) were 238.58, 573.54, and 159.92 Ω, respectively (Table S3). The minimum RCT value of 159.92 Ω among the three devices indicates that holes could be transported and extracted effectively in the composite HTL

Figure 3. (a) Film conductivities of the different HTLs by evaluating the J−V characteristics in the devices ITO/HTLs (40 nm)/MoO3 (10 nm)/Ag. (b) UPS spectra of the different HTLs on ITO substrates. Inset is the magnified region of the photoemission cutoffs.

facilitate an efficient hole transport and extraction between ITO and the CH3NH3PbI3−xClx active layer. Figure 4a presents the J−V characteristics of PSCs using different HTLs [PEDOT:PSS, TS−CuPc, and TS−CuPcdoped PEDOT:PSS (50 wt %)]. Table 1 lists the performances of the PSCs based on different HTLs. The photovoltaic performance is strongly dependent on the ratio of TS−CuPc in PEDOT:PSS, and a best doping ratio of 50 wt % is decided based on the device performance (Figure S3). The control device using pristine PEDOT:PSS exhibits a regular performance: 13.29% (PCE), 20.47 mA/cm2 (JSC), 0.94 V (VOC), and 0.68 (FF). The doping strategy delivers an optimal device 13242

DOI: 10.1021/acsami.7b02223 ACS Appl. Mater. Interfaces 2017, 9, 13240−13246

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ACS Applied Materials & Interfaces

Figure 4. (a) J−V characteristics of the perovskite solar cells based on different HTLs under AM 1.5G illumination with a light intensity of 100 mW/ cm2. (b) PL spectra of the CH3NH3PbI3−xClx perovskite films on different HTLs. (c) Absorption spectra of the CH3NH3PbI3−xClx perovskite films deposited on different HTLs. (d) Nyquist plots of the different HTL-based perovskite solar cells measured in the dark and at an open voltage. (e) Radially integrated intensity plots along the ring at q = 10 nm−1, assigned to the (110) plane of the CH3NH3PbI3−xClx perovskite films deposited on different HTLs. (f) J−V characteristics of the hole-dominated devices with the ITO/HTLs/CH3NH3PbI3−xClx/MoO3 (10 nm)/Ag structure.

Table 1. Performance Parameters in the Perovskite Solar Cells Based on Different HTLs HTL TS−CuPc PEDOT:PSS PEDOT:PSS/TS−CuPc TS−CuPc/PEDOT:PSS PEDOT:PSS + TS−CuPc PEDOT:PSS + TS−CuPc PEDOT:PSS + TS−CuPc PEDOT:PSS + TS−CuPc PEDOT:PSS + TS−CuPc PEDOT:PSS + TS−CuPc

(25 (40 (45 (50 (55 (60

wt wt wt wt wt wt

%) %) %) %) %) %)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

Rs (Ω)

0.77 0.94 0.91 0.95 0.98 0.98 1.00 1.01 1.01 0.98

17.17 20.47 19.76 21.33 20.97 21.92 22.12 22.23 22.14 21.38

51 68 60 65 69 72 75 77 76 71

6.28 13.29 10.78 13.17 14.11 15.56 16.59 17.29 16.99 15.11

160.34 91.25 120.65 92.26 58.91 42.56 33.64 25.72 38.55 48.30

Figure 5. (a) Normalized PCEs as a function of time in the doped and nondoped PEDOT:PSS-based devices without encapsulation and under identical storage conditions (in atmosphere and at room temperature). (b) pH values of the PEDOT:PSS, TS−CuPc, and PEDOT:PSS + TS−CuPc (50 wt %) aqueous solutions.

(TS−CuPc-doped PEDOT:PSS)-based devices. Furthermore, the crystallization of the perovskite films grown onto different HTLs is evaluated using XRD. One-dimensional (1D) XRD

presents the main crystal planes of (110), (220), and (310), whose peaks are located at 14.2°, 28.5°, and 31.9°, respectively (Figure S8).49 However, no large difference in the 1D XRD 13243

DOI: 10.1021/acsami.7b02223 ACS Appl. Mater. Interfaces 2017, 9, 13240−13246

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ACS Applied Materials & Interfaces patterns is observed among the three samples. To obtain a deep understanding of the perovskite crystallization, the real-time grazing-incidence X-ray diffraction (GIXRD) data are recorded (Figure S9). Figure 4e shows the radially integrated intensity plots along the ring at q = 10 nm−1 (scattering vector, q = 4π sin (θ)/λ), which assigns to the (110) plane of the CH3NH3PbI3−xClx perovskite films. The image of the perovskite films using the optimal composite film as HTL possesses two characteristics. The one is obvious spots and ring, and the other is a narrow full width at half-maximum. They suggest the large grain size and good crystal orientation.50−52 Figure 4f exhibits the J−V curves in the hole-dominated devices. The structure ITO/HTL (40 nm)/CH3NH3PbI3−xClx (300 nm)/ MoO3 (10 nm)/Ag is used. The symbol μ represents the hole mobility and can be calculated using space charge limited current (SCLC) using the following equation J=

9 V2 ε0εrμ 3 8 d

(1)

where ε0 (=8.8542 × 10−14 F/cm) is the permittivity of free space, εr (=6.5)53 is the dielectric constant of the perovskite film, and d (=300 × 10−7 cm) is the film thickness of the perovskite film. The calculated hole mobility is summarized in Table S4. The composite HTL-based perovskite film presents a hole mobility of 5.91 cm2 V−1 s−1, which is the highest among the pristine PEDOT:PSS, TS−CuPc-based perovskite films, and TS−CuPc-doped PEDOT:PSS (50 wt %)-based perovskite films. The stability is also an advantage of these modified devices. It is more prominent for the composite HTL [TS−CuPc-doped PEDOT:PSS (50 wt %)]-based device than the control devices. Figure 5a shows the time-dependent PCEs in the doped devices and control devices as well as the normalized PCEs. These devices were analyzed without encapsulation and under identical storage conditions (in atmosphere and at room temperature). The half-life of the PCE degradation is 80 h in the PEDOT:PSS-based device, whereas it takes 240 h in the composite HTL (TS−CuPc-doped PEDOT:PSS)-based device. The remarkable improvement in the cell stability is ascribed to the stable composite HTL consisting of PEDOT:PSS and TS− CuPc. As can be seen from Figure 5b, the PEDOT:PSS aqueous dispersion is acidic with a pH value of 2.1, in contrast to the alkalescent nature of the TS−CuPc aqueous solution with a pH value of 10.0. The composite aqueous solution [TS− CuPc-doped PEDOT:PSS (50 wt %)] still maintains a pH value of 3.3, which can weaken the ITO erosion to some extent and result in an improved device stability. The improved device stability can be seen from the crosssectional SEM images of the aged (288 h) devices, as shown in Figure 6. In the pristine PEDOT:PSS-based device, the interface between the perovskite layer and the HTL was badly deteriorated with many large voids (Figure 6b). By contrast, the composite HTL-based device still maintained a stable interface contact between the CH3NH3PbI3−xClx layer and the HTL, with a good cross-sectional morphology (Figure 6d). The influence of the HTLs on the cell stability can also be seen from the variation in the J−V characteristic curves (J ∝ Vm) in the fresh and aged devices.54 Figure 6e shows the double logarithmic J−V curves of the fresh and aged (288 h) control devices and the optimal devices in dark. For the fresh PSCs, the J−V characteristics demonstrated stable rectification behaviors with three distinct regions: in the low bias region ( 8), associated with an SCLC with an exponential distribution of traps;54−57 and in the high bias region, the current goes through toward a V2 dependence, which could be described as the SCLC model. In particular, the region related to the SCLC with exponentially distributed traps strongly depends on the interface condition such as the interfacial trap states.50 After 288 h of aging, the current transition region toward V2 dependence disappeared in the control devices because of the serious interface degradation. However, an obvious transition region still existed in the device based on PEDOT:PSS + TS−CuPc owing to the suppressed interface deterioration by the composite HTL. The obtained water and DMF contact angles of the PEDOT:PSS, TS−CuPc, and PEDOT:PSS + TS−CuPc (50 wt %) films implied that the optimal film possessed bigger contact angles. The bigger contact angles indicated a better stability (Figure S10). We ascribed it to the weakened ITO erosion owing to reduced acidity and increased dispersibility (by testing the zeta potential, Figure S11) of PEDOT:PSS aqueous dispersion after introducing TS−CuPc.

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CONCLUSIONS In summary, we have demonstrated a composite film consisting of PEDOT:PSS and TS−CuPc in a suitable doping ratio that can act as a pre-eminent and stable HTL for PSCs. The perovskite film grown onto the composite HTL presented a 13244

DOI: 10.1021/acsami.7b02223 ACS Appl. Mater. Interfaces 2017, 9, 13240−13246

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ACS Applied Materials & Interfaces high-quality crystallization with a large grain size, good film coverage, and increased carrier mobility. Compared with that of the reference, the PCE increased from 13.29 to 17.29%. Besides its roles in enhancing the hole transport and extraction and improving the perovskite crystallization, the composite HTL can improve the device stability owing to the reduced acidity and increased dispersibility of the PEDOT:PSS aqueous dispersion after introducing TS−CuPc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02223. Properties of different HTL films and the device performance based on different HTL films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhao-Kui Wang: 0000-0003-1707-499X Liang-Sheng Liao: 0000-0002-2352-9666 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (no. 61674109) and from the China Postdoctoral Science Foundation (no. 2015M580460). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology (Nano-CIC) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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