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Power Conversion Efficiency and Device Stability Improvement of Inverted Perovskite Solar Cells by Using a ZnO:PFN Composite Cathode Buffer Layer Xiaorui Jia, Lianping Zhang, Qun Luo, Hui Lu, Xueyuan Li, ZhongZhi Xie, Yongzhen Yang, Yan-Qing Li, Xuguang Liu, and Chang-Qi Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03724 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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ACS Applied Materials & Interfaces

Power Conversion Efficiency and Device Stability Improvement of Inverted Perovskite Solar Cells by Using a ZnO:PFN Composite Cathode Buffer Layer Xiaorui Jia, †,‡ Lianping Zhang, ‡ Qun Luo, *, ‡ Hui Lu, ‡ Xueyuan Li, ‡ Zhongzhi Xie, § Yongzhen Yang, † Yan-Qing Li, § Xuguang Liu,*, † Chang-Qi Ma*, ‡ †

Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan

University of Technology, Ministry of Education, Taiyuan, 030024, P. R. China. ‡

Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics,

Chinese Academy of Sciences (CAS), Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou, 215123, P. R. China. § Institute of Functional Nano & Soft Materials, Soochow University, Suzhou, 215123, P. R. China. E-mail: [email protected]; [email protected]; [email protected] Tel: +86-512-6287-2769, Fax: +86-512-6260-3079 KEYWORDS: Inverted perovskite solar cell; Cathode buffer layer; ZnO:PFN nanocomposites; Enhanced power conversion efficiency; Stability improvement

1 ACS Paragon Plus Environment


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ABSTRACT:

We have demonstrated in this article that both power conversion efficiency (PCE) and performance stability of inverted planar heterojunction perovskite solar cells can be improved by using a ZnO:PFN nanocomposite (PFN: poly[(9,9-bis(3'-(N,N-dimethylamion)propyl)-2,7fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]) as the cathode buffer layer (CBL). This nanocomposite could form a compact and defect-less CBL film on the perovskite/PC61BM surface (PC61BM: phenyl-C61-butyric acid methyl ester). In addition, the high conductivity of the nanocomposite layer makes it works well at a layer thickness of 150 nm. Both advantages of the composite layer are helpful in reducing interface charge recombination and improving device performance. The power conversion efficiency (PCE) of the best ZnO:PFN CBL based device was measured to be 12.76%, which is higher than that of device without CBL (9.00%), or device with ZnO (7.93%) or PFN (11.30%) as the cathode buffer layer. In addition, the long term stability is improved by using ZnO:PFN composite cathode buffer layer when compare to that of the reference cells. Almost no degradation of open circuit voltage (VOC) and fill factor (FF) was found for the device having ZnO:PFN, suggesting that ZnO:PFN is able to stabilize the interface property and consequently improve the solar cell performance stability.


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1. INTRODUCTION: Since the first report in 2009 by Miyasaka et al.,

1

the organolead halide perovskite

(CH3NH3PbX3/PVSK, X = I, Br, or Cl) solar cells have sprung to the forefront of the thin film photovoltaic research, owing to the advantageous characteristics of crystalline CH3NH3PbX3 film, including direct optical bandgap, low exciton bonding energy, long charge carrier diffusion length, and excellent compatibility with cost effective printing process. Tremendous efforts have been made in perovskite solar cell in the last few years, and the power conversion efficiency (PCE) has been improved from 3% to over 20%.1-5 There are two different types of perovskite solar cells depending on the device architecture, i.e. meso-superstructured solar cells and planar heterojunction solar cells. 6 For the planar perovskite solar cells, the inverted architecture with a layer sequence of ITO/PEDOT:PSS/perovskite layer/PC61BM/Al (ITO: indium tin oxide; PEDOT:PSS: poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); PC61BM: phenyl-C61butyric acid methyl ester) showed great commercialization prospects due to the simplicity of device structure, low processing temperature, printing process compatibility, potential of largescale manufacturing, and high PCE.5, 7-14 In the inverted planar heterojunction perovskite solar cells, the charge injection and extraction at cathode interface has significant impact on the electrical properties of the device. To minimize the contact barrier, introduction of proper cathode buffer layer between the perovskite layer and PC61BM is an important strategy to improve device performance. Cathode buffer layer reported in the literatures includes thermally evaporated C60/BCP, polyelectrolyte

layer,

i.e.

ethoxylated

15

LiF,16 or C60/LiF,

polyethylenimine

trimethylammoniumhexyl)thiophene] (P3TMAHT),

18

(PEIE)

or

17

ultrathin

poly[3-(6-

other organic compounds like amio-

functionalized polymer PN4N,19 perylene-diimide,20 thiol-functionalized cationic surfactant,

21



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and metal oxide, such as ZnO 22-23 and TiO2. 24 Among them, the metal oxide nanoparticles have advantages of high charge carrier extraction efficiency and good material stability. However, the aggregation behavior of metal oxide nanoparticles on the surface usually leads to a rough surface and large amount of intra-gap state, which would act as charge recombination centers.

25-26

The

organic cathode buffer layers, on the other hand, show the disadvantage of high thickness dependence,

27-28

making them not compatible with roll-to-roll printing processing. Recently,

nanocomposite of metal oxide nanoparticle and conjugated polymer was emerging as another electrode buffer layer, in which the surface defect of metal oxides can be passivated by polymers. 21, 29-30

This nanocomposite material is usually prepared by simply mixing the nanoparticle with

polymer in solvent, and the composite film can be easily deposited through low-temperature solution process. More importantly, device performance is improved due to enhanced electron selectivity of this composite buffer layer relative to pure metal oxides. In the meanwhile, the device performance was less sensitive to the thickness of the nanocomposite electrode buffer layer. As observed, the thickness of the composite buffer layer can reach to 100 nm without obvious decrease in device performance.

30

Therefore, such nanocomposite electrode buffer

materials are highly compatible with printing processes, and show very promising application prospect in organic solar cells. In our previous study, the composite cathode interfacial layer containing ZnO and PFN (poly[(9,9-bis(3'-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]) was utilized in the organic solar cells, and significant improved device performance due to enhanced electron selective extraction was found.30 Although such nanocomposite materials have been successfully used in organic solar cells, the use of this composite based on metal oxides and polymer in perovskite solar cell has not been reported yet. Herein, we will report the use of a


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ZnO:PFN nanocomposite cathode buffer layer in the perovskite solar cells as the first time. It is known that perovskite film is very sensitive to the solvent used for the deposition the upper layer since solvent corrosion on the PVSK layer could not be fully avoided, a mixture solvent of methanol and n-butanol was developed for this composite system. The power conversion efficiency of inverted perovskite solar cells using ZnO:PFN buffer layer is remarkably increased from 9.00% to 12.76%, which is ascribed to the enhanced electron selectivity and reduced charge recombination. Additionally, the long-term stability of the device is found to be largely improved by using ZnO:PFN relative to w/o CBL, ZnO and PFN cathode buffer layer based device. These advantageous features demonstrate the ZnO:PFN composite layers are suitable for p-i-n planar heterojunction perovskite solar cells.

2. EXPERIMENTAL SECTION: 2.1 Materials: The PEDOT:PSS (AI 4083) was purchased from Heraeus Ltd. Poly[(9,9bis(3'-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene]

(PFN,

Mn

=9,300 g/mol, PDI=1.4) was provided by Sunatech Inc. Poly(sodium p-styrenesulfonate) (PSSNa, Mn =70,000 g/mol) was purchased from Acros Organics. Phenyl-C61-butyric acid methyl ester (PC61BM) was purchased from Solarmer Energy, Inc. (Beijing). PbCl2 (99%) was purchased from Sinopharm Group Co. Ltd. All chemicals were used without purification. 2.2 Synthesis of CH3NH3I: Methylammonium iodide (CH3NH3I, MAI) was synthesized according to the route as reported in literature.

31

In detail, 37 mL CH3NH2, 38 mL HI were

mixed at 0°C for 2 h with stirring. The reaction mixture was concentrated to about 10 mL by a rotary evaporator at 70°C, and white precipitate formed from the solution. The precipitate was



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dissolved in ethanol, re-precipitated from diethyl ether, and then filtered. The MAI powder was collected and dried at 60 oC in a vacuum oven for 24 h. 2.3 The preparation of ZnO:PFN composite inks: ZnO nanoparticles were synthesized with KOH and Zn(OAc)2 as raw materials in methanol,32 and followed by about 12 h standing. After that, the white precipitate was washed three times with methanol, and centrifuged at 4000 rpm for 10 min. Finally, equal volume of methanol and n-butanol were added to disperse the precipitate. After a period of ultrasonic stirring, the final ZnO nanoparticles ink with a concentration of 6 mg/mL was obtained. The PFN ink (1 mg/mL) was prepared by dissolving PFN in methanol. Finally, the ZnO and PFN inks were mixing together to obtain a series of composite inks with different composite ratios. 2.4 Film characterization: Atomic Force Microscopy (AFM) images of films were measured using Dimension 3100. The Cross-sectional scanning electron microscopy (SEM) images were recorded by the Quanta 400 FEG. The thicknesses of PEDOT:PSS layer, perovskite film, ZnO layer, and ZnO:PFN composite films were estimated from the cross-sectional SEM images, while the thicknesses of PFN films were estimated using ellipsometer with a PFN films coated on the Si substrate. 2.5 The fabrication and characterization of the devices: The perovskite solar cell devices were fabricated according to the following procedure. Patterned indium-tin oxide (ITO) substrates were subsequently cleaned in detergent aqueous solution, deionized water, acetone, and isopropanol, and ultraviolet-ozone (UVO) treating for 30 min. PEDOT:PSS aqueous was spin-coated on the ITO at 3500 rpm for 60 s and annealed at 124 oC for 25 min in the glove box. Then PSS-Na aqueous (2 wt%) was spin-coated on the annealed film and then baked at 140 oC for 5 min under the ambient condition.33 The other layers, i.e. the perovskite layers, PC61BM,


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ZnO:PFN layer and the Al electrode were fabricated inside the N2 glove box. In detail, a 30 wt.% CH3NH3PbI3-xClx precursor solution was prepared by dissolving CH3NH3I and PbCl2 (molar ratio of 3:1) in anhydrous DMF, and stirred at 50 oC for 12 h. The precursor solution was spincoated onto the PEDOT:PSS/PSS-Na layer at 6000 rpm for 60 s. After that, the prepared perovskite thin film was annealed at 95 oC for 70 min. After cooling to room temperature, PC61BM solution (20 mg/mL in chlorobenzene) was spin-coated on the CH3NH3PbI3-xClx layer at 1000 rpm for 60 s. The ZnO:PFN composite inks in the methanol and n-butanol binary solvent (1:1, v/v) 34 was spin-coated on the CH3NH3PbI3-xClx/PC61BM film at 1000 rpm for 60 s. Finally, 100 nm -thick Al electrodes were deposited on the top of devices under high vacuum (1×10-4 Pa) by thermal evaporation. The effective device area was 16 or 9 mm2 . The current density-voltage (J-V) testing is carried out using the Keithley 2400 under AM 1.5G solar illumination (100 mW/cm2). Herein, the solar illumination is provide by a white light from halogentungsten lamp after filtering. 30 External quantum efficiencies (EQE) measurements were carried out using a home-made IPCE system consisting of a 150 W tungsten halogen lamp (Osram 64642), a monochromator (Zolix, Omni-λ300), an optical chopper, and an I-V converter. The degradation process of the un-encapsulated perovskite solar devices was monitored by a decay testing system (PVLT-G6100L, Suzhou D&R Instruments) in a glovebox (H2O < 10 ppm, O2 < 10 ppm) under the condition of the ISOS-L-1 standard. 35 The detail information about I-V, EQE and the long-term stability measurement could be read in our previous report. 30

3. RESULTS AND DISCUSSION: Surface morphology analysis



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Heterojunction perovskite solar cells with a configuration of ITO/PEDOT:PSS/PSSNa/CH3NH3PbI3-xClx/PC61BM/CBL/Al were fabricated, where PEDOT:PSS/PSS-Na serves as the hole transport layer, and PC61BM serves as the electron transport layer. Different cathode buffer layers, including: ZnO, PFN, ZnO:PFN composite layer were deposited on the fullerene surface by spin coating. For comparison, reference cells without CBL were also fabricated and tested. It is worth noting that a thin layer of poly(sodium p-styrenesulfonate) (PSS-Na) was inserted between the PEDOT:PSS and perovskite films to improve the device performance and performance reproducibility of the final cells.33 Figure 1c illustrated the cross-section scanning electron microscopy (SEM) image of a typical device without aluminum electrode. As can be seen from this figure, the layer thickness of PEDOT:PSS/PSS-Na, CH3NH3PbI3-xClx (PVSK), PC61BM and ZnO:PFN could be determined to be around 20, 200, 50, 150 nm, respectively.

Figure 1. (a) The schematic structure of the perovskite solar cells, (b) molecular structure of PFN,

and

(c)

cross-sectional

SEM

image

of

a

ITO/PEDOT:PSS/PSS-

Na/PVSK/PC61BM/ZnO:PFN film.


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Figure 2 presents the AFM images of the PVSK/PC61BM film surface with different cathode buffer layers. The surface morphology of the pristine PVSK film was also measured and the AFM image is shown in Figure 2a. As can be seen here, the surface of the perovskite film is relatively rough with a root mean-square (RMS) roughness of 9.5 nm. After the deposition of a thin PC61BM layer (~50 nm), the RMS roughness reduces to 3.0 nm, indicating that the uneven PVSK surface is largely smoothened by PC61BM layer. The RMS roughness of the PVSK/PC61BM/PFN surface further decreases to 2.8 nm, which is ascribed to the better film forming ability of PFN. In contrast, the deposition of ZnO and ZnO:PFN on the PVSK/PC61BM surface leads to a RMS roughness of 5.7 and 5.3 nm, respectively, which are slightly higher than that of PVSK/PC61BM film. Such a surface roughness increase is mainly ascribed to the solvent effect, since n-butanol could partially destroy the perovskite film (Figure S1).34 In addition, quite

a

few

pinholes

are

observed

on

PVSK/PC61BM,

PVSK/PC61BM/ZnO

and

PVSK/PC61BM/PFN films. The detail reason for the formation of such pinholes is not known yet. However, these undesired pinholes might promote the inward and outward diffusion of H2O and O2 molecules in PVSK films, which could greatly lower the device stability. 36 In the case of ZnO:PFN film, no obvious pinhole can be seen, suggesting a compact and smooth cathode buffer layer formed on the PVSK/PC61BM surface, which should be an advantage in improving device stability.



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Figure 2. AFM images of the (a) pristine PVSK, (b) PVSK/PC61BM, (c) PVSK/PC61BM/ZnO, (d) PVSK/PC61BM/ZnO:PFN, and (e) PVSK/PC61BM/PFN films. Perovskite solar cell performance A series of perovskite solar cells with different cathode buffer layers were fabricated. For ZnO:PFN, ZnO, and PFN, the optimized thickness were about 150, 100, and 3.8 nm, respectively. It is worth to note that the optimized thickness of ZnO:PFN is around 150 nm. This high layer thickness tolerance of ZnO:PFN device is originated from the high conductivity and good film quality of the ZnO:PFN film, and this will make it more compatible with roll-to-roll printing process. The current-voltage (J-V) curves measured in the backwards scan direction and external quantum efficiency spectra (EQE) of the best performance cells are shown in Figure 3a and 3b. The device performance data are listed in Table 1. As shown in this table, the control device without cathode buffer layer exhibited an overall PCE of 9.00%, with a VOC of 0.81 V, a JSC of 18.84 mA/cm2, and a FF of 59% (Entry 1), which is among the average values of the p-i-n heterojunction perovskite solar cells reported by the references. 18, 37-38 Deposition of a ZnO layer (~100 nm) on the top of PC61BM led to a decrease of power conversion efficiency to 7.93%. Such a performance decrease was mainly due to the decrease of FF (from 59% to 50%), which 10 ACS Paragon Plus Environment

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could be due to the PVSK film corrosion by n-butanol. After carefully optimization on the PC61BM layer thickness, the performance can reach to 11.04% (enter 3). When a 3.8 nm-thick PFN film is inserted between fullerene and Al electrode, a PCE of 11.30% was obtained. However, the PCE of PFN CBL employing device is obviously sensitive to the thickness of the PFN CBL. Apparently can be seen from Figure S2 and Table S1 that the power conversion efficiency dramatically decreases to 5.15% and 8.02% as the thickness of PFN increase to 13.6 nm or decreases to 1.6 nm. So the thickness of PFN should be strictly controlled within 10 nm, which demonstrates high controlling difficulty and non-compatibility with roll-to-roll printing. For the ZnO:PFN device, significant improvement of VOC and FF was found (Entry 4 in Table 1) when compared to the device without cathode buffer layer, which was similar to the corresponding polymer solar cells,30 Although JSC decreased slightly, a highest PCE of 12.76% was obtained for this type of device. Slightly decrease of JSC for the ZnO:PFN device could be due to the optical space effect of the cathode buffer layer, which could be clearly seen by the EQE spectra (Figure 3b), where low external quantum efficiency in the wavelength ranging from 600 to 800 nm was observed. 39 As for the improvement of VOC and FF, one reason could be the reduced work function of ZnO:PFN composite film (3.73-4.05 eV, as shown in Figure S3) relative to pure ZnO or pure PFN. Obviously, the work function of ZnO:PFN is more close to the LUMO level of PC61BM (-3.99 eV),

30

leading to a reduced electron extraction barrier and

facilitating electron transport at the electrode interface. And the other reason could be the defect passivation effect of PFN on ZnO surface, which will enhance the electron selectivity and reduce the interface charge recombination (vide infra). The ZnO:PFN composite CBL was also utilized in the thicker PC61BM employing device, which showing a PCE of 11.61% (Entry 5c). Additionally, it should be note that the J-V characteristics measured in the backwards scan



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direction is nearly the same as the curves in the forwards scan direction. As a typical example, the J-V curves of a ZnO:PFN CBL based device both in the two different directions are illustrated in Figure S4. To check the reproducibility of the device, a histogram of device performance obtained from 16 devices was summarized in Figure 3c. It shows that around 90% device give conversion efficiency over 12% with standard deviation of 0.29%, showing an excellent reproducibility for the ZnO:PFN devices. In contrast, the control devices without electrode buffer layer showed poorer PCE with large device performance variation. All these results confirmed that the ZnO:PFN composite buffer layer is helpful in improving perovskite solar cell performance and performance reproducibility. From the dark J-V characteristics, it is additionally found that the devices with ZnO:PFN composite layer presents larger rectification ratio and relative smaller leakage current density in the regime from -1 to 0.1 V, which indicates enhanced electron selectivity of ZnO:PFN. Perovskite solar cells with a thicker perovskite layer (~ 350 nm) were also fabricated and tested, and the device performance data are listed in Table S3. For the 350 nm-thick perovskite layer, the surface roughness and topography would be much larger than that of the 200 nm-thick perovskite layer. Therefore, it is required to deposit a thicker PC61BM layer on the thick perovskite films. With a 65 nm-thick PC61BM layer on the perovskite layer, as can be seen here that the thick perovskite solar cell without CBL showed a poor device performance of PCE = 3.82%, with a VOC = 0.37 V, JSC = 16.64 mA/cm2, FF = 62%. Such a low device performance was mainly due to the uneven PVSK surface and the insufficient surface coverage of PC61BM layer on PVSK layer. However, the device performance was significantly improved to PCE = 11.96% with a VOC = 0.85 V, JSC = 18.51 mA/cm2, FF = 76% when using ZnO:PFN CBL, suggesting the insertion of ZnO:PFN composite layer can greatly reduce the amount of PC61BM.


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Figure 3. (a) J-V characteristics of the perovskite solar cells using different cathode buffer layers. (b) EQE spectra of these devices. (c) The statistic distributions of performance parameters. (d) Dark J-V characteristics of these devices.



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Table 1. Device performance parameters of the perovskite solar cells without or with different cathode buffer layers. Cathode

VOC

JSC

FF

PCE

Aver. PCE ±

buffer layer

(V)

(mA/cm2)a

(%)

(%)

std. dev. (%)b

1

w/o

0.81

18.84

59

9.00

8.19±0.72

2

ZnO

0.88

18.02

50

7.93

7.44±0.41

3c

ZnO

0.92

17.91

67

11.04

10.93±0.11

4

ZnO:PFN

0.90

18.41

77

12.76

12.46±0.29

5c

ZnO:PFN

0.87

17.79

75

11.61

11.53±0.19

6

PFN

0.87

18.04

72

11.30

11.15±0.16

Entry

a

b

The current density is obtained based on the EQE spectra. The average PCE was calculated

from 16 devices. c

Device with a thicker PC61BM layer (65 nm). Influence of the composite ratio on the device performance The influence of ZnO:PFN blended ratio on the device performance was also studied through

regulating the ZnO:PFN weight ratios from 24:1 to 1.5:1 (w/w). Figure 4 depicts the correlation between the device performance and the ZnO:PFN blend ratio. The solar cell performance data are listed in Table S2. As can be seen here, both VOC and JSC were found to be almost independent of the blend ratio, while FF is almost stabilized at around 74% for the ZnO:PFN blend ratio ranging from 6:1 to 1.5:1, with a highest FF of 77% at a blend ratio of 2:1. Less PFN leads to a decrease of FF to 64% (24:1). Overall, the PCE of the perovskite solar cells are found to be rather stable in a blend ratio range of 3:1 to 1.5:1. Such a less blend ratio dependence of device performance was also found in polymer solar cells,

30

which should be an advantage in

ink formulation. The device performance of the thick perovskite solar cells is found to be more sensitive to the ZnO:PFN blend ratio. Nevertheless, an optimized ZnO:PFN blend ratio of about


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3:1 was also found for the thick PVSK cells, which is similar to that of the thin PVSK devices (Table S3).

Figure 4. The relationship between the device performance parameters (VOC, JSC, FF and PCE) and the blend ratios of ZnO to PFN. The average value was calculated based on 8 individual devices. The long-term stability of the perovskite solar cells Furthermore, the stability of perovskite solar cells with different composite cathode buffer layers were tested. For comparison, the reference cell without CBL was also tested. Figure 5 shows the evolution of VOC, JSC, FF and PCE of the un-encapsulated devices under continuous illumination condition. The J-V curves after different degradation time were selected and shown 15 ACS Paragon Plus Environment


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in Figure S5. During the stability testing process, all the devices were aged at maximum power output (mpp) with an external load according to the J-V cure scanning results. Since it was similar to the real service condition of solar cells, the results showed here could provide a more reliable “life-time” of these cells. As can be seen from Figure 5, the VOC of all these three devices increased slightly in the first 10 hours. Such a VOC increase could be due to the doping effect of the trace amount of O2 inside the glovebox. Expect for the reference cell without CBL, which showed a slight VOC decrease (around 5%) during aging, the ZnO and ZnO:PFN device showed almost no VOC decrease over a long time illumination (700 h, Figure 5a). The decay of JSC was found to be very fast for the reference cell without CBL (more than 60% JSC degradation after 700 h). Interestingly, the JSC decay rate of the ZnO:PFN was almost identical to the ZnO device while much slower than the reference cell (around 40% over 700 h). It indicates that JSC decay in ZnO and ZnO:PFN device can be mainly ascribed to the degradation in perovskite layer rather than the interfacial layer. Less than 5% decay of FF was found for the ZnO:PFN device under working, whereas around 10% and 30% decay of FF were found for the device w/o CBL and ZnO-based devices under the same testing condition. Surprisingly, the FF decay of ZnO-based device was much faster than the other two cells, which could be due to the photo-transfer reaction between ZnO and perovskite. 40 The improved FF stability for the ZnO:PFN device was speculated to be ascribed to the protection effect of the PFN component, since the electron-rich nitrogen atoms of PFN could on one hand coordinate with the Pb2+,

41

and on the other hand to

passivate the surface hydroxyl groups of ZnO and reduce reaction between ZnO and PVSK.

40

Hence, the alkyl amino might be contribute to the improved stability of ZnO:PFN device. Overall, the ZnO:PFN device showed the highest device stability. The times of aging until 80% (T80) and 50% (T50) of their initial performance 35 were determined to be 140 h and >700 h (the


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PCE decayed to be 53% of initial value after 700 h), which is much higher than that of the CBLfree (52 h and 182 h) and ZnO-based (85 h and 420 h) devices. The PCE of ZnO:PFN device after aging 700 hours was 6.32%, which was much higher than that of without CBL or with ZnO CBL device (Table 2). Overall, one can clearly conclude that ZnO:PFN could be an excellent CBL for use in inverted planar heterojunction perovskite solar cells with higher device performance and stability, as well as low blend ratio sensitivity, and large layer thickness.

Figure 5. The normalized of (a) VOC, (b) JSC, (c) FF, and (d) PCE monitored as a function of time. Table 2. Perovskite solar cell performance degradation data. Entry

CBL

Initial PCE

T80 (h)

T50 (h)

PCE after 700 h (%)

1

w/o

9.00

52

182

4.51

2

ZnO

7.93

85

420

3.97

4

ZnO:PFN

12.76

140

>700

6.32



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CONCLUSIONS In conclusion, we have demonstrated the simultaneous improvement of performance and stability via using an ZnO:PFN nanocomposite buffer layer in the p-i-n heterojunction perovskite solar cells. Specifically, the insertion of the composite layer yields an obvious increase in PCE from 9.00% to 12.76% relative to the control device, which is mainly originated from the increase in fill factor. In addition, since the 150 nm ZnO:PFN composite films are smooth and robust, the interaction between electrode and perovskite might be diminished, resulting in an improved long-term stability. As a consequence, the T80 and T50 lifetime of the ZnO:PFN device extended 2.7 and 3.8 times compared to the control device, respectively. This study show that the utilization of ZnO:PFN composite buffer layer may be a practical strategy to improve the efficiency and stability of perovskite solar cells.

ASSOCIATED CONTENT Supporting Information. Figure S1. (a) Absorption spectra and (b) XRD patters of the pristine ITO/PEDOT:PSS/PSS-Na/PVSK/PC61BM films and films being washed by n-butanol. Figure S2. J-V characteristics of the perovskite solar cells with PFN as cathode buffer layers. Figure S3. UPS spectra of the ZnO, PFN, and ZnO:PFN composite cathode buffer layers. Figure S4. The JV characteristics of a typical ZnO:PFN device from the forward and backward scan direction. Figure S5. J-V curves of the perovskite devices aging with different time, (a) w/o CBL, (b) with ZnO, and (c) with ZnO:PFN as cathode buffer layers. Table S1. Device performance parameters 18 ACS Paragon Plus Environment

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of the PFN cathode buffer layer involved perovskite solar cells with different thicknesses of PFN layers. Table S2. Device performance parameters of the PC61BM/ZnO:PFN composite cathode buffer layer involved perovskite solar cells with different ZnO/PFN blended ratios. Table S3. Device performance parameters of the 350 nm-perovskite solar cells with different cathode buffer layers. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work is financially supported by Jiangshu Provincial Natural Science Foundation (BK20130346), National Natural Science Foundation of China (61306073), the Jiangshu Provincial Major Research Program (BE2015071), and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09020201). REFERENCES (1)

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Table of Contents Graphic


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Figure 1 216x121mm (96 x 96 DPI)

ACS Paragon Plus Environment


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Figure 5 203x157mm (300 x 300 DPI)



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TOC graphic 289x202mm (150 x 150 DPI)


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