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Organic Solar Cells Based on WO2.72 Nanowire Anode Buffer Layer with Enhanced Power Conversion Efficiency and Ambient Stability Longzhen You, Bin Liu, Tao Liu, Bingbing Fan, Yunhao Cai, Lin Guo, and Yanming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15762 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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ACS Applied Materials & Interfaces

Organic Solar Cells Based on WO2.72 Nanowire Anode Buffer Layer with Enhanced Power Conversion Efficiency and Ambient Stability Longzhen You,†,‡ Bin Liu,† Tao Liu,†,‡ Bingbing Fan,†,‡ Yunhao Cai,†,‡ Lin Guo,*† and Yanming Sun*†,‡ †.

School of Chemistry and Environment, Beihang University, Beijing 100191, P. R.

China. E-mail: [email protected]; [email protected]. ‡.

Heeger Beijing Research and Development Center, Beihang University, Beijing

100191, P. R. China.

KEYWORDS: Organic solar cells, anode buffer layer, WO2.72 nanowire, efficiency, stability.

ABSTRACT: Tungsten oxide as an alternative to conventional acidic PEDOT:PSS has attracted much attention in organic solar cells (OSCs). However, the vacuum-processed WO3 layer and high temperature sol-gel hydrolyzed WOX are incompatible with large-scale manufacturing of OSCs. Here, we report for the first time that a specific tungsten oxide WO2.72 (W18O49) nanowire can function well as the anode buffer layer. The nw-WO2.72 film exhibits a high optical transparency. The power conversion efficiency (PCE) of OSCs based on three typical polymer active layers

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PTB7:PC71BM, PTB7-Th:PC71BM, and PDBT-T1:PC71BM with nw-WO2.72 layer were improved significantly from 7.27% to 8.23%, from 8.44% to 9.30%, and from 8.45% to 9.09%, respectively compared to devices with PEDOT:PSS. Moreover, the photovoltaic

performance

of

OSCs

based

on

small

molecule

p-DTS(FBTTh2)2:PC71BM active layer was also enhanced with the incorporation of nw-WO2.72. The enhanced performance is mainly attributed to the improved short-circuit current density (Jsc), which benefits from the oxygen vacancies and the surface apophyses for better charge extraction. Furthermore, OSCs based on nw-WO2.72 show obviously improved ambient stability compared to devices with PEDOT:PSS layer. The results suggest that nw-WO2.72 is a promising candidate for the anode buffer layer materials in organic solar cells.

INTRODUCTION

As a promising candidate for clean energy source, organic solar cells (OSCs) have recently attracted considerable attention and the state-of-the-art power conversion efficiency (PCE) over 12% has been reported for single junction bulk heterojunction (BHJ) OSCs.1 To pursue efficient OSCs, various strategies such as morphology control,2-4 material development,5-7 and device engineering have been developed.8-13 It has been known that the performance of OSCs can be dramatically improved by inserting an appropriate interfacial buffer layer between active layer and electrode in view of tuning work function (WF) of the electrode and preventing undesired recombination of photon-generated carriers.14-16


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Poly(styrenesulfonate)-doped poly(ethylenedioxythiophene) (PEDOT:PSS) is the most frequently used anode buffer layer. However, the strong acidic nature of PEDOT:PSS can cause severe corrosion thus degrade the performance and the stability of OSCs.17 Alternatively, various transition metal oxides have been introduced to substitute PEDOT:PSS, such as molybdenum oxide (MoO3),18,19 vanadium oxide (V2O5),20-22 nickel oxide (NiO),23,24 tungsten oxide (WO3),25-31and rhenium oxide (ReO3).32-34 Among these transition metal oxide, WO3 has been successfully applied in conventional, inverted and tandem OSCs due to its advantages of high photostability, excellent optical and electrical properties.25-31,35 However, most of the WO3 buffer layers applying in OSCs are fabricated by using high-cost techniques, such as vacuum vapor deposition25-31,36 or high temperature annealing sol-gel method,16,37,38 which are incompatible with future roll-to-roll scalable manufacturing. Therefore, efforts should be devoted to achieving low-cost tungsten oxide-based buffer layer that applies in OSCs. Feasibly, tungsten oxide, WO3-X (0≤X≤1), is a kind of chemical composition tunable metal oxide39 and WO2.72 is the only one to form a stable sub-stoichiometric phase.40 Moreover, Zhou et al. have successfully applied WO2.72 as a counter electrode in dye-sensitized solar cells (DSCs).41,42 Thus seeking strategies for expediently utilizing WO2.72 as OSCs anode buffer layer technically is potentially of great significance. WO2.72 (it is also called W18O49) has been synthesized via wet-chemical routes (e.g. hydrothermal method). During preparation, it grows into mainly three types of micro-nano shapes, which can be divided into nanowires, nanorod bundles, microspheres.43, 44 In addition, WO2.72 is the only form that contains largest content of



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oxygen vacancies among the tungsten oxides in WO2.625-WO3,45 and the oxygen vacancies can increase the conductivity of the buffer layer and facilitate the hole transfer. On the other hand it can enhance the interfacial contact.38

The ultrathin WO2.72 nanowire can be homogeneously dispersed in a green solvent. Herein, we used it as an effective anode buffer layer in OSCs. The tungsten hexachloride was firstly added into ethanol and the solution was stirred 20 minutes for sufficient dissolving. Then through a facile and template-free solvothermal approach in a Teflon-lined stainless steel autoclave, the very shallow blue (nearly colorless) transparent WO2.72 nanowire (hereafter simplified as nw-WO2.72) solution can be generated.46 The nw-WO2.72 layer is highly conductive and transparent in the visible region. OSCs based on nw-WO2.72 with three different polymer donors (PTB7, PTB7-Th and PDBT-T1), a small molecule donor (p-DTS(FBTTh2)2) and PC71BM acceptor have been fabricated for investigating the relevant photovoltaic performance. OSCs based on PEDOT:PSS have been fabricated in parallel for comparison. We found that OSCs based on nw-WO2.72 layer exhibit higher photovoltaic performance than devices with PEDOT:PSS. For instance, PCEs of OSCs based on PTB7-Th:PC71BM with the nw-WO2.72 reached 9.30%, which is higher than that (8.44%) of devices with PEDOT:PSS. In addition, OSCs on the flexible polyethylene terephthalate (PET) substrates were also fabricated. It was found that the photovoltaic performance of flexible OSCs was almost unchanged before and after the bending test in different directions, indicating that the WO2.72 nanowires would bring no harm when it comes to the practical application. More importantly, the device with the


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nw-WO2.72 layer also shows much better stability in the ambient than OScs with PEDOT:PSS. The results suggest that nw-WO2.72 is a promising anode interfacial layer for the fabrication of high-efficiency and stable OSCs.

EXPERIMENTAL SECTION

Materials: Patterned ITO glass with a sheet resistance of ≤15 Ω/sq was purchased from South China Xiangcheng science and technology Co., Ltd. (Shenzhen, China). Tungsten hexachloride was purchased from Acros. Ethanol was purchased from Beijing Chemical Works. PEDOT:PSS (Clevious P VP AI 4083) was purchased from H. C. Stark Company. PTB7, PTB7-Th, p-(FBTTh2)2 were purchased from 1-Material Chem-scitech Inc. (Canada). PC71BM was purchased from ADS Inc.

Preparation of nw-WO2.72: The nw-WO2.72 was synthesized by a reported method based on Zhou et al. with a minor change,46 from which the reagent concentration and reaction time were reduced. The concentration of tungsten hexachloride ethanol solution was decreased to 0.4mg ml-1 (w:v). After 20 minutes string for dissolving in the room temperature, the pale yellow solution was transferred to the reaction kettle, then during 18 hours heating at 180℃, the nw-WO2.72 solution formed.

Device fabrication and Measurements: The cleaning procedure of ITO glasses and the preparation of PEDOT:PSS layer were described in our previous work.10 The nw-WO2.72 solution was spin coated atop the ITO glasses at 2000 rpm with subsequently 70℃ a 5 min in air. Then, the active layers were deposited by spin coating (800 rpm) the blend solution (o-DCB:CB 1:1 v/v) of PTB7:PC71BM (1:1.5



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w/w, PTB7 is 10 mg ml-1), the blend solution (o-DCB:CB 1:1 v/v) of PTB7-Th:PC71BM (1:1.8 w/w, PTB7-Th is 10 mg ml-1), the CF solution of PDBT-T1:PC71BM (1:1 w/w, PDBT-T1 is 4 mg ml-1) and spin-coating (1700 rpm) the CB solution of p-DTS(FBTTh2)2:PC71BM (1.3:1 w/w, p-DTS(FBTTh2)2 is 21 mg ml-1) on the modified ITO electrodes for 40 s. The active layers were then dried at room temperature for 20 min (the small molecule/fullerene film was baked at 70 ℃ for 10 min) to remove the residual solvent. At a vacuum level of 5 × 10-5 Pa a thin Ca layer (10 nm) was deposited as the cathode buffer layer, followed by deposition of 100 nm Al as the top electrode. The device photoactive area is 4.5 mm2 which was calibrated by a shadow mask. The measurements of OSCs can be found in our previous work.10

Characterizations: The scanning electron microscopy (SEM) images were measured in a JEOL JSM-7500F. The X-ray diffraction (XRD) measurements were carried out using a Shimadzu XRD-6000. The X-ray photoelectron spectroscopy (XPS) measurements were performed in a Thermo esca lab 250Xi electron spectrometer using 150 W Al Ka (hv =1486.6 eV) radiation. The transmission spectra of the films were measured using a UV-vis spectrophotometer (Shimadzu UV-2000). Atomic force microscopy (AFM) images were performed on a MultiMode 8 AFM microscope (Bruker). The water contact angle measurements were carried out on a OCA40Micro, Dataphysics Instruments GmbH (Germany). The work function was measured with a Kelvin Probe system (KP020, KP Technology) in air.


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RESULTS AND DISCUSSION

The SEM images (Fig. 1a and b) showed that the length of the nw-WO2.72 is up to several micrometers and the diameter is below 30 nm (Fig. S1†a). The solution is stable, homogeneous and transparent (Fig. S1b†) which is necessary for subsequent film processing. The large-area homogeneous nanowire (Fig. 1b) is a necessity in subsequent formation of anode film. Compared with PEDOT:PSS film, the nw-WO2.72 film contains a lot of apophyses because of its microstructure and parts of the apophyses can insert into active layer, which can enlarge the contact area. The XRD is a simple and feasible tool to detect the crystal type of solid crystal. As shown in Fig. 2a, only two strong diffraction peaks are observed, which can be indexed to the W18O49 phase with a corresponding structure (JCPDS No. 36-101). The diffraction peaks at 2θ of 23.4° and 48.0° correspond to WO2.72 (010) and (020) crystal plane, respectively. The remarkable difference between the two peaks indicates that the nw-WO2.72 growth was highly anisotropic along the (010) direction, which agrees well with monoclinic system.47



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Figure 1. SEM images of the nw-WO2.72 thin films with (a) high magnification and (b) low magnification.

X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the chemical composition and purity of nw-WO2.72. The nw-WO2.72 film was fabricated through dropping the ready-made condensed solution on precleaned silicon substrates with being treated on a 70℃ hotplate to remove the solvent. As shown in Fig. 2b, XPS peaks of C, W and O can be clearly observed without any other impurities. The complex energy distribution of W 4f photoelectrons was displayed in Fig. 2c. The W 4f core level spectra can be fitted into three peaks and each peak represents different W atoms oxidation states. The binding energy peaks at 37.7 eV and 35.6 eV are strong spin-obit of W 4f7/2 and W 4f5/2, respectively, which related to W atoms at +6 oxidation state. The lower binding energy peaks at 34.5 eV and 36.6 eV correspond to the W 4f7/2 and 4f5/2 core levels, respectively, which related to W atoms at +5 oxidation state. The remaining third pair of peaks observed at 33.6 eV and 35.7 eV refer to +4 oxidation state of the W atoms. These three oxidation states of W atoms actually are the typical oxidation state in WO2.72 as reported previously. The XPS spectrum of O 1s was shown in Fig. 2d. The highest peak was found at 530.5 eV, which was ascribed to the W-O bond in WO2.72 (W18O49). The peak located at a high binding energy of 532.3 eV might be assigned to O in water or CO2 molecules absorbed in air.


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20

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1200

W 4p W 4p 1000

800

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W 4f

Intensity (a.u.)

(0 2 0)

Intensity (a.u.) 10

O 1s

(b)

(0 1 0)

(a)

0

Binding Energy (eV)

2-Theta (degree)

(c)

(d) O 1s

Intensity (a.u.)

W 4f

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C 1s W 4d W 4d

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W6+ 5+

W 4+

W 42

40

38

36

34

32

Binding Energy (eV)

30

535

534

533

532

531

530

529

528

527

Binding Energy (eV)

Figure 2. (a) X-ray diffraction pattern and XPS spectra: (b) survey scan, (c) W4f spectra, and (d) O spectra of nw-WO2.72. The optical properties of the ITO glass covered with the nw-WO2.72 layer were subjected to optical transmittance measurement. As shown in Fig. 3a, the nw-WO2.72 buffer layer almost has no negative effect of the high transmittance of ITO substrate. As for the ITO/PEDOT:PSS layer, the higher transmittance at wavelengths between 450 to 600 nm was contributed by the modification of PEDOT:PSS layer, in which the light dispersion and extinction were reduced due to the smooth surface.52,53 In the wavelength range between 700 nm and 800 nm, the transmittance of PEDOT:PSS showed a disadvantage compared with nw-WO2.72. It can be seen clearly that the new anode buffer layer with high optical transmittance will not cause additional loss of photons.



(a)100

(b) 200

80

bare ITO PEDOT:PSS nw-WO2.72

60

40

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Current Density (mA/cm 2)

Transmittance (%)

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bare ITO PEDOT:PSS nw-WO2.72

100

0

-100

-200

Wavelength (nm)

-0.2

-0.1

0.0

0.1

0.2

Voltage (V)

Figure 3. (a) Optical transmittance of bare ITO glass, ITO/PEDOT:PSS and ITO/nw-WO2.72 substrates. (b) Current-voltage characteristics of ITO glass, ITO/PEDOT:PSS and ITO/nw-WO2.72 substrates. The resistance of different anode buffer layer plays an important role in determining the charge transfer and charge collection in OSCs. As seen from Fig. 3b, the PEDOT:PSS shows a smallest slope in the J-V curve. It has the largest resistance (1.15 Ω cm2). While the nw-WO2.72 shows a much higher slope in the J-V curve, its resistance is as low as 0.75 Ω cm2 which is basically the same as the resistance of ITO (0.70 Ω cm2). A low resistance is equivalent to a high conductivity. Compared with PEDOT:PSS, the better conductivity of nw-WO2.72 anode buffer layer is beneficial for carrier transport.

The surface morphologies of thin films were studied using an atomic force microscopy (AFM). As shown in Fig. 4a and Fig. S2†, the bare ITO glass presents high root-mean-square (rms) roughness of 3.96 nm. After spin coating PEDOT:PSS atop it, the surface becomes flat. The rms value was decreased to 1.60 nm (Fig. 4b and Fig. S2†). For the ITO glass coated with nw-WO2.72 layer (Fig. 4c and Fig. S2†), the rms roughness was increased to 22.7 nm; the venation shaped surface shows significant ups


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and downs with 100-nm fluctuation range. The result is very different with previous reports on anode buffer layer with tungsten oxide.16,28,36,38 It has been known that the chemical vapor deposition of metal oxide is easy to form dense and smooth films. However, the length of the nw-WO2.72 is several micrometers and the diameter is below 30 nm, which increased the roughness of the ITO substrates. Combining the size and morphology characterizations of SEM measurements, the interlaced nw-WO2.72 film exhibits a large specific surface area, which greatly increases the contact between the active layer and the electrode. In addition, the veined protrusions will be partly embedded into the active layer, which facilitated the extraction of carriers.

The surface energy of ITO, ITO glass coated with PEDOT:PSS and nw-WO2.72 film was investigated with contact angle measurement. As shown in Fig. 4d, 4e, and 4f, the contact angle of the bare ITO is 59.5°, and the ITO/PEDOT:PSS layer is 20.5°. For ITO/nw-WO2.72 layer, the value was increased to 84.3°. The results confirm the nw-WO2.72 layer is remarkably hydrophobic which is different from other solution processed tungsten oxides37, 38 and it will exhibit excellent interfacial contact with photoactive materials being coated onto it38.



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Figure 4. AFM height images of (a) ITO glass (b) ITO/PEDOT:PSS layer and (c) ITO/nw-WO2.72 layer. The scan size is 5µm×5µm. Water contact angle of the substrates: (d) ITO glass (e) ITO/PEDOT:PSS layer and (f) ITO/nw-WO2.72 layer. The molecule structures of donors applied in this study were shown in Fig. 5a. The OSCs with a conventional sandwich-structure of ITO/nw-WO2.72/BHJ/Ca/Al were fabricated (Fig. 5b). Here, PC71BM was used as an acceptor, and three polymers PTB7, PTB7-Th, PDBT-T1 and a small molecule p-DTS(FBTTh2)2 were used as a donor, respectively. The energy level diagram of each material used in the fabrication of OSCs was shown in Fig. 5c. The work function (WF) of nw-WO2.72 was determined by Kelvin Probe system in air. The energy levels of PTB7, PTB7-Th, PDBT-T1, p-DTS(FBTTh2)2, PC71BM were taken from the literature.10,55-58 It was found that the WF of nw-WO2.72 is very close to the HOMO of the active layers. It is expected that a smaller energy level offset between the active layer and the anode will be formed when the nw-WO2.72 was used as the anode buffer layer. It is well known that the a small energy level offset is beneficial for the charge carrier extraction.59,


60

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Therefore,nw-WO2.72 will play a better bridging role in the cascade energy level sequence thus facilitating charge transfer than PEDOT:PSS.

Figure 5. (a) Chemical structures of PTB7, PTB7-Th, PDBT-T1, and p-DTS(FBTTh2)2 (b) Device architecture and (c) Energy levels of eacg material used in this study.

Table 1. Device performance of OSCs fabricated with different anode buffer layers and active layers.

SAM

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Best PCE (%)

Calcd Jsc (mA cm-2)

A (PTB7)

PEDOT:PSS

0.76 ± 0.002

14.00 ± 0.34

67.1 ± 1.8

7.09 ± 0.10

7.27

14.02

B (PTB7)

nw-WO2.72

0.76 ± 0.002

15.29 ± 0.31

70.0 ± 1.0

8.11 ± 0.19

8.23

14.87

C (PTB7-Th)

PEDOT:PSS

0.79 ± 0.003

14.81 ± 0.29

70.5 ± 1.0

8.29 ± 0.11

8.44

14.73

D (PTB7-Th)

nw-WO2.72

0.79 ± 0.003

16.20 ± 0.32

71.0 ± 1.0

9.10 ± 0.11

9.30

15.99

Device



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E (PDBT-T1)

PEDOT:PSS

0.91 ± 0.005

12.47 ± 0.16

72.8 ± 1.3

8.25 ± 0.11

8.45

12.46

F (PDBT-T1)

nw-WO2.72

0.90± 0.006

13.51 ± 0.26

72.2 ± 1.0

8.91 ± 0.11

9.09

13.33

PEDOT:PSS

0.80 ± 0.004

13.79 ± 0.40

65.2 ± 2.2

7.18 ± 0.10

7.30

13.39

nw-WO2.72

0.78 ± 0.005

14.56 ± 0.47

62.1 ± 1.8

7.03 ± 0.19

7.29

14.43

G (p-DTS(FBTTh2)2 H (p-DTS(FBTTh2)2

Eight types (A-H) of OSCs devices were fabricated with nw-WO2.72 and PEDOT:PSS. The optimized device parameters for each type of OSCs from 20 devices are summarized in Table 1 and the J-V curves of OSCs are shown in Fig. 6. In terms of device A, PEDOT:PSS was used as the buffer layer and the PTB7:PC71BM was used as active layer. A high PCE of 7.27% was achieved with an open circuit voltage (Voc) of 0.76 V, a short-circuit current (Jsc) of 13.94 mA cm-2, and a fill factor (FF) of 69.0%, similar to the values reported in the literature.61,62 The photovoltaic performance was obviously improved for device B with the nw-WO2.72 layer. The PCE was improved to 8.23%, mainly due to the increased Jsc (15.38 mA cm-2) and FF values (70.9%). Moreover, device C fabricated with the PEDOT:PSS layer and PTB7-Th:PC71BM exhibited a PCE of 8.44%. It was also found that OSCs based on nw-WO2.72 layer shows a higher PCE of 9.30%. To further probe the universality of the nw-WO2.72 layer in OSCs, a well-developed wide-bandgap donor polymer PDBT-T1 with PC71BM was studied. As shown in Fig. 6c and in Table 1, device E with the PEDOT:PSS layer yielded a PCE of 8.45%, with a Voc of 0.91 V, a Jsc of 12.68 mA cm-2, and a FF of 73.4%. While device F with the nw-WO2.72 layer shows a PCE of 9.09% with a Voc of 0.90 V, a Jsc of 13.72 mA cm-2, and a FF of 73.3%. Furthermore, we extended our research to small molecule donors. Here, p-DTS(FBTTh2)2 was employed and the related J-V


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curves is shown in Fig. 6d. The control device G with PEDOT:PSS layer showed a PCE of 7.30% with a Voc of 0.800 V, a Jsc of 13.41 mA cm-2, and a FF of 68.0%.63-65 We found that device H with nw-WO2.72 layer shows comparable photovoltaic performance with a PCE of 7.29%, a Voc of 0.773 V, a Jsc of 14.97 mA cm-2, and a FF of 63.0%. The results indicate the nw-WO2.72 layer has wide applications in OSCs with different donor materials. (a)

(e)80

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-4

60

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50

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-12

Current Density (mA/cm )

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Figure 6. J-V characteristics and EQE spectra of OSCs based on the active layers of (a, e) PTB7:PC71BM, (b, f) PTB7-Th:PC71BM, (c, g) PDBT-T1:PC71BM and (d, h)



p-DTS(FBTTh2)2:PC71BM, respectively, with different anode buffer layers, respectively.

The external quantum efficiencies (EQEs) of OSCs based on different materials were shown in Fig. 6. It can be seen clearly that all the types of devices fabricated with nw-WO2.72 layer exhibit higher EQE values over a wide spectral range. This explains why the nw-WO2.72-based devices can give higher Jscs. For example, for PTB7-Th:PC71BM solar cells (Fig. 6f), the maximum EQE peak of the device D with nw-WO2.72 layer reached 71% at 560 nm and a good photoresponse in the broad range of 350-450 and 500-730 nm was observed. Similarly, for PTB7:PC71BM, PDBT-T1:PC71BM and p-DTS(FBTTh2)2:PC71BM solar cells with nw-WO2.72 layer, better EQE peaks were observed when compared with PEDOT:PSS layer. (b) 1.0 Normalized Jsc (%)

Normalized Voc (%)

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Figure 7. Ambient degradation of normalized (a) Voc, (b) Jsc, (c) FF, and (d) PCE values of PTB7-Th:PC71BM based devices with PEDOT:PSS or nw-WO2.72.


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In addition, flexible OSCs were fabricated to investigate the possible consequence that the protrusions may lead to short circuit of OSCs when it comes to flexible plastic substrates like PET. Encouragingly, the photovoltaic parameters of OSCs were almost unchanged (Fig. S4† and Table S1†) after 150 times bending test (Fig. S3†), indicating that that the WO2.72 nanowires can be efficiently used even in flexible OSCs.

The stability of OSCs based on PTB7-Th:PC71BM layer with PEDOT:PSS and nw-WO2.72 were tested both in the ambient and vacuum conditions. In both conditions, OSCs based on nw-WO2.72 showed much better stability than PEDOT:PSS-based devices (Fig. S5†-S7†). For the ambient stability test, OSCs were stored and measured in air for a month. The normalized photovoltaic parameters were presented in Fig. 7 and the values were summarized in Table S6†. Apart from the Voc, the fast decay of PEDOT:PSS in other three parameters reveals its evident lower stability. In contrast, OSCs based on nw-WO2.72 layer showed much better ambient stability than PEDOT:PSS-based devices. The improved stability is mainly ascribed to the stable nature of nw-WO2.72. As for PEDOT:PSS layer, it is known to be sensitive to the moisture which can decrease the device stability.

CONCLUSION

In conclusion, we have successfully developed the nw-WO2.72 nanowire as the anode buffer layer in OSCs. The template-free solvothermal processed monoclinic crystal nw-WO2.72 is highly uniform with a quite small diameter that below 30 nm which is essential for film formation. The nw-WO2.72 anode buffer layer shows high



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optical transmittance and high hydrophobic which could not cause additional loss of photon. The OSCs with nw-WO2.72 layer showed improved photovoltaic performance compared with PEDOT:PSS layer. The improved performance is mainly due to the enhanced short-circuit current, which is ascribed to the better interfacial contact and excellent electrical conductivity caused by oxygen vacancies. The enhanced stability of the device is attributed to the inherently neutral and hydrophobic character of the nw-WO2.72. Importantly, the routine for the fabrication of nw-WO2.72 anode buffer layer avoids high temperature annealing or vacuum deposition, and the cost-effective manufacturing process is an important step for the commercialization of organic solar cells. The results show that nw-WO2.72 is an excellent anode buffer layer in the fabrication of high-performance OSCs.

ASSOCIATED CONTENT Supporting Information. Low magnification images of nw-WO2.72, AFM height images of bare ITO, PEDOT:PSS film, and nw-WO2.72 films, the flexible OSCs, J-V curves of OSCs for the stability test and the corresponding device parameters. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Dr. Guo Lin: Email ([email protected]);

*Dr. Sun Yanming: Email ([email protected].)

Notes


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The authors declare no competing financial interest

ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (NSFC) (nos. 51473009, 21674007), and the International Science & Technology Cooperation Program of China (no. 2014DFA52820). The authors thank Fei Qin and Prof. Yinhua Zhou for assistance in performing the work function measurements.

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Organic Solar Cells Based on WO2.72 Nanowire ... - ACS Publications

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