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Improved Optical Field Distribution and Charge Extraction Through an Interlayer of Carbon Nanospheres in Polymer Solar Cells Zhiqi Li, Jiajun Dong, Chunyu Liu, Xulin Zhang, Xinyuan Zhang, Liang Shen, Wenbin Guo, Liu Zhang, and Yongbing Long Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b05307 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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Chemistry of Materials

Improved Optical Field Distribution and Charge Extraction Through an Interlayer of Carbon Nanospheres in Polymer Solar Cells Zhiqi Li,1 Jiajun Dong,2 Chunyu Liu,1 Xulin Zhang,1 Xinyuan Zhang,1 Liang Shen,1 Wenbin Guo,*1 Liu Zhang,*3 and Yongbing Long4 1

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering , Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 2

State Key Laboratory on Superhard Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 3

College of Instrumentation & Electrical Engineering, Jilin University, 938 Ximinzhu Street, Changchun 130061, People’s Republic of China

4

School of Electronic Engineering, South China Agricultural University, Guangzhou, 510642, China

ABSTRACT Nanostructured carbon is a low-cost, economic and elementally abundant candidate for manufacturing high-conductivity counter electrodes of organic photoelectric devices. Herein, we prepare onion–like carbon nanosphere:silver (OLCNS:Ag) composite electrodes for efficient, invertedarchitecture polymer solar cells (PSCs) via a simple, solution-processed approach. The optical electric field distribution from the OLCNS:Ag nanocomposite layer opens up the possibility of additional light harvesting of the entire visible spectrum resulting from synergies between both components. The large effective specific surface area and high conductivity of OLCNS allow significant charge transfer and collection, resulting in a remarkably enhanced power conversion efficiency (PCE) of 9.81 % in PTB7:PC71BM PSCs, and 6.95 % in PCDTBT:PC71BM PSCs, compared with control devices with PCEs of 7.76 % and 5.31 %, respectively. These consequences indicate that OLCNS:Ag composite electrodes constitute a valid and versatile method to realize high-performance organic photovoltaic devices.

1. Introduction Recently, there has been intensive research interest in polymer solar cells (PSCs) with inverted architectures comprising an anode of a noble metal (gold and silver) and the cathode of indium tin oxide (ITO).1-5Compared to conventional forward devices, inverted architecture PSCs make the utmost of vertical phase separation and the concentration gradient of the active layer, and exhibit better longterm environmental stability. To balance charge injection and transport, various metal oxides have been applied as the top contact hole extraction layers of inverted structure PSCs, such as sol-gel or thermal evaporative MoO37,8 and WO39, a soluble version of MoS,10 and functionalized carbon nanotubes,11 which not only balance charge injection and transport but also show advantages that include improved air-stability, non-toxicity, low-cost and exceptional high transparency. Therefore, the inverted configuration is an ideal device type for all kinds of PSCs. Despite these merits, the traditional counter electrodes of inverted configuration PSCs are fabricated from more costly metals, and the metal thickness is usually more than 100 nm to ensure good contact, which significantly adds to the cost burden and hinders practical application.12-19 Hence, it is worth developing a new high efficient counter electrode for PSCs to obtain enhanced performance. 1

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As a green substitute for semiconductor quantum dot nanomaterials, carbon nanoparticles have attracted extensive attention of both companies and academies because of its excellent advantages, including fantastic optical, electrical, and mechanical properties.20-22 Carbon-based materials have also been is actually exploited for tribological microfilms coating, bio-imaging, sensors, photocatalysts, energy transformation/storage, light-emitting diodes (LED), drug/gene delivery, and high-performance composites.23-25 In addition, carbon materials exhibit superior mobility to transfer electrons or holes and can act as exciton dissociation centers.26 Thus, it is appealing to incorporate carbon particles into organic photovoltaic cells to improve device performance. In previous literature sources, carbon-based materials including carbon dots, carbon nanotubes, carbon nanorods have been reported as a good dopant to enable both high efficiency and stabilization, which presents a more encouraging prospect for the commercialization in photovoltaic technology.27-30 Carbon nanotubes and graphene with excellent conductive property have also been employed as a new type of the hole collector and electrode materials with small resistance.31-33 However, most of carbon materials used in PSCs are amorphous structure, and crystalline framework carbonaceous substances are in demand to obtain a better capability.34,35 Considering its structure and size, graphitic framework with its peculiar performance including carbon atoms bonding vibration (sp2–sp3), the graphitic interlayer distance variation and carbon rings structure disorder can generate porous cave, which could create a famous microstructure and/or admirable electrochemical features.36-39 Therefore, developing new prepared technology to produce carbonaceous nanomaterials with crystalline structure and establishing rational application in photovoltaic devices have been a realistic and challenging issue. Herein, we demonstrate a simple synthetic route of onion-like ordered carbon nanospheres (OLCNS) and properties study of OLCNS:Ag counter electrodes for high-efficiency PSCs. The carbon counter electrodes contain OLCNS graphite as the main components, an ultrathin silver coating on surface was used to fill up the mesoporous structure of OLCNS, which acted as bridge to closely contact with carbon flakes and improved interfacial adhesion of OLCNS and buffer layer, leading to the improvement of charge extraction capacity of electrode. Compared to only Ag electrode, the OLCNS:Ag counter electrodes not only reduced the requirement of precious metals but also improved the charge collection ability due to the high conductivity of OLCNS. Therefore, an optimal power conversion efficiency of 9.81% for PTB7:PC71BM PSCs and 6.95 % for PCDTBT:PC71BM PSCs were achieved under AM 1.5 illumination of 100 W/m2. This study provides an efficient approach to develop nanostructure electrode, which will be helpful in exploiting the potential improvement of organic photovoltaic devices with carbon counter electrode in the future.

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Figure 1 (a) Chemical structures of PTB7, PCDTBT, and PC71BM, (b) the bulk heterojunction solar cells, (c) energy levels of PSCs.

2. Results and discussion In this section, we present the characteristic of OLCNS:Ag based PSCs (o-PSCs) and Ag based PSCs (a-PSCs), including the morphology of the OLCNS, the interface of OLCNS:Ag electrode, and the transport of charge carrier in the pristine and the OLCNS:Ag blend. The device structure (Figure 1(b)) of o-PSCs is based on glass/ITO/TiO2/active-layer/MoO3/OLCNS:Ag (20 nm). Meanwhile, aPSCs based on glass/ITO/TiO2/active-layer/MoO3/Ag (100 nm) were also fabricated. The active layer (Figure 1(a)) was prepared from a chlorobenzene (CB) solution of PTB7:PC71BM (1:1.5 w/w ratio) containing 3 vol% of 1,8-diiodooctane (DIO) and a dichlorbenzene (DCB) solution of PCDTBT:PC71BM (1:4 w/w ratio). Energy levels of all materials used in inverted device are presented in Figure 1(c). All the details about the fabrication and characterization of the samples are described in the experimental section. 2.1 Properties of the OLCNS The morphology of the OLCNS was characterized using transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM). The TEM images of the product are shown in Figure 2 (a) and 2(b). It can be seen that the size of OLCNS is very uniform, and the size distribution histogram of the nanospheres of OLCNS was acquired from the statistic analysis of TEM images (Figure 2(g)). The histogram indicates that OLCNS are monodispersed in the scope of 20–60 nm with an average size of ∼41 nm. The HRTEM images in Figure 2(c) and 2(d) exhibit that the circular shape of the carbonspheres has a concentric rings structure (onion), which consists of the same multilayer graphene-like shells with visible crystalline lattice fringes (Figure 2(d)). These multilayer graphene-like shells can work as the holes transport channel, which plays as efficient charge/discharge centers in the anodes to boost charge collection of the device. Carefully observing, it can be found in Figure 2(e) that turbostratic crystal can be observed in some graphitic layers due to low crystallization, while amorphous carbon still exists in the outermost enclosure of sample. The electron diffraction pattern of the specimen in Figure 2(f) reveals several weak rings, corresponding to low crystallinity. The dspacings distribution of graphitic layers is corresponding to the broad diffraction rings in Figure 2(e). 3



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Powder XRD analysis of the final carbon onion sample was employed to explore the degree of crystallinity. The standard PDF card of 75-1621 was selected for comparison. As shown in Figure 2h, there is a distinct peak between 25o and 32o, mapping the hexagonally packed structure of graphite carbon. Additionally, a small amount of disturbance peak can be found in the XRD image, which is result from the SiO2 substrate and a random error.

Figure 2 TEM images of OLCNS in 100 nm (a) and 50 nm (b); HRTEM images of OLCNS in 20 nm (c) and 10 nm (d, e), (f) the selected-area electron diffraction patterns; (g) Size histogram of the OLCNS; (h) XRD patterns for the nanospheres.

Figure 3 The (a) Raman and (b) IR spectra of OLCNS. Raman spectra of the OLCNS is shown in Figure 3(a), whose two emblematic peaks at 1356 and 1585 cm−1 represent D-band and G-band in the carbon sample, respectively. The D-band roots in the breathing vibration of carbon rings, which is consistent with multilayer graphene-like shells of OLCNS. 4


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The full width at half maximum (FWHM) of D-band is large due to the various small graphitic fragments of OLCNS. The G-band is assigned to the C–C bonds stretching vibration of the in-plane sp2, and the small FWHM shows the low crystalline in graphitic layer.40-42 Meanwhile, the high intensity ratio of two bands indicates a high disorder of graphite sample, which coincides with the TEM and XRD spectra. In addition, the IR spectra of OLCNS was used to further understand degree of hydrogenation of OLCNS. As is displayed in Figure 3(b), the peak at 1637 cm−1 refers to C–C vibrations of graphite rings, while the bands at around 1085 cm−1 correspond to C–O groups. The absence of the distinct bands at the range of 2580-2980 cm−1 presents that none of C–H bonds are preserved, which exhibits a low hydrogenation for OLCNS.43,44 2.2 Characterization of devices with and without carbon interlayer Table 1. Stabilized device parameters of control and optimized PSCs, which are average values of 32 individual devices. Voc (V)

Device

Jsc (mA/cm2)

FF (%)

0.776 16.589 57.41 Ag 19.709 61.92 OLCNS:Ag 0.791 0.833 11.307 54.14 Ag PCDTBT:PC71BM 14.471 57.02 OLCNS:Ag 0.841 The stabilized current-density–voltage (J–V) characteristics PTB7:PC71BM

illumination

(100

mW

cm−2)

are

exhibited

in

Figure

PCE (ave) (%) 7.71±0.06% 9.79±0.02% 5.26±0.05% 6.93±0.03% of PSCs under 4(a)

PCE (best) (%)

Rs (ohm/mm2)

7.76 9.81 5.31 6.95 simulated

(PTB7:PC71BM)

19.58 12.06 25.06 21.31 AM 1.5G and

4(b)

(PCDTBT:PC71BM), and the device data are summarized in Table 1. The PSCs incorporating a OLCNS:Ag film as the electrode give a maximum PCE (PCEmax) of 9.81% (PTB7:PC71BM, PCEave=9.79±0.02%) and 6.95% (PCDTBT:PC71BM, PCEave=6.93±0.03%). While the reference aPSCs give a PCEmax of 7.76% (PTB7:PC71BM) and 5.31% (PCDTBT:PC71BM). The main difference between the a-PSCs and the o-PSCs is the improved short-circuit current density (Jsc) from 16.589 to 19.709 mA cm−2 (PTB7:PC71BM) and 11.307 to 14.471 mA cm−2 (PCDTBT:PC71BM). Additionally, FF are increased from 57.41 % to 61.92 % (PTB7:PC71BM) and 54.14% to 57.02 % (PCDTBT:PC71BM), whilst open-circuit voltage (Voc) only shows little improvement.

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Figure 4 Stabilized J–V curves of the PSCs consisted of (a) PTB7:PC71BM and (b) PCDTBT:PC71BM under standard irradiation of 100 mW cm–2; IPCE of the PSCs based on (c) PTB7:PC71BM and (d) PCDTBT:PC71BM; dark J–V characteristics of the PSCs consisted of (e) PTB7:PC71BM and (f) PCDTBT:PC71BM. The work function (WF) of the OLCNS was −4.4 eV as determined from Kelvin probe measurements (Figure 1(c)). It is worth noting that the WF of the Ag is -4.26 eV, hence, the increased WF can reduce the energy barrier and accelerate holes injection from the photoactive layer to electrode, which balances charge carrier transport and reduces internal recombination in active layer (Figure 5(a) and 5(b)). The results of Voc are in accordance with these observations. Figure 4(c) and 4(d) are IPCE spectra of the relevant devices, Jsc enhancement was triggered by IPCE enhancement in the range 350– 700 nm, whose variation tendency is consistent with J-V spectrum. Moreover, Figure 4(e) and 4(f) display the stabilized J–V characteristics of the PSCs based on PTB7:PC71BM and PCDTBT:PC71BM under dark, the current density at reverse bias and leakage current region is obviously reduced, which 6


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shows an increased diode rectifying ratio. The improved dark current characteristics and increased shunt resistance (Rsh) can prevent the leakage current, leading to the increase of Voc and FF. While the dark current in the forward bias region (0.5−1V) changed quickly, indicating that OLCNS:Ag electrode facilitated holes transport in PSCs.

Figure 5 Proposed schemes for band alignment and charge transport processes (towards anode) in (a) active layer /MoO3/Ag junction and (b) active layer /MoO3/OLCNS:Ag junction. 2.3 Microstructure analysis of OLCNS:Ag electrode The high performance of OLCNS based PSCs could be ascribed to especial structure for good charge and discharge abilities and other electricity property. Figure S1 (Supporting Information) presents TEM and HRTEM images of the OLCNS as the composite electrode. High electronic conductivity of carbon shells promotes charge transfer between carbon spheres and Ag. Multilayer graphene-like shells inside the sphere work as electrochemically active holes storage locations during the current carrier insertion and extraction. The concentric carbon shells also buffer well against the local volume change during the illumination, thus retaining the structural stability and leading to good charge collection. The OLCNS surface area, pore diameter, pore volume were also measured by Brunauer–Emmett–Teller (BET) method and shown in Figure S2 (Supporting Information). The active area is 45.336 m2/g, pore volume is 0.125 cm3/g, and pore diameter is 9.623 nm. The smaller size and bigger BET active area of the nanospheres could boost charge collection and provide accessible transporting sites inside the concentric carbon shells.

Figure 6 Water contact angles of PTB7:PC71BM, MoO3 coated PTB7:PC71BM, Ag coated PTB7:PC71BM and Ag/OLCNS coated PTB7:PC71BM films. Furthermore, water contact angle (WCA) characterizations were employed to gain deep insight into the correlation of device performance. The surface wettability affects the contact way of another material subsequently deposited on the surface and the device stability in the air. WCAs of all film 7



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were measured and shown in Figure 6, corresponding parameters are listed in Table S1 (Supporting Information). As shown in Figure 6, the bare PTB7:PC71BM substrate shows a WCA of 86°, while MoO3-modified PTB7:PC71BM shows a decreased WCA of 35°. Hence, MoO3 modification is of benefit to the deposition of water-soluble OLCNS, and this kind of close unite can decreased the density of interfacial trap and recombination center for charge transport. Simultaneously, it is known that the rough surface of a material can enhance the hydrophobicity of a hydrophobic surface. Microscopy on the carbon onion sample revealed an extended micro-porosity, which is believed to contribute to the surface roughness. The carbon onions were found to exhibit superhydrophobicity with a WCA of 134o when coated on PTB7:PC71BM. As expected, OLCNS:Ag film exhibits a high WCA of 89o compared to the bare Ag film of 31o. Increased hydrophobicity of top electrode can effectively insulate the device from the water vapor and allow a better and stable device performance.46 We also investigated the use of only OLCNS as electrode for PSCs, and the device structure is glass/ITO/PTB7:PC71BM/MoO3/OLCNS. Other than the MoO3 was deposited by vacuum thermal evaporation, all other layers were sequentially deposited onto ITO using spin-coating and blade-coating. Unlike PSCs with OLCNS:Ag electrode exhibited a high PCE, the devices with only OLCNS film as the electrode had a poor device performance. To explore the reason this observation, the surface morphology of OLCNS electrode was carried out, which included pure carbon material. Microstructure analysis of SEM (Figure S3) indicated that the OLCNS particles were conglobate and layered, some of the powder agglomerated and resulted in discontinuities in surface. OLCNS electrode with too many pores may be not beneficial for the charge transport, leading to a poor device performance. In order to remove part of the aggregated grains, the sample was dispersed with ultrasonic dispersion instrument and spin-coated on the surface of MoO3 rather than blade-coating. On this basis, the concentration of the material, spin-coating speed and time were compared. Figure S4 presented that OLCNS became gradually dispersed with increased spin-coating speed. Samples still kept agglomerating under low spin-speed (Figure S4(d)), whilst surface spacing increased at a higher spin-speed (Figure S4(a) and S4(b)). Figure S4(c) showed that OLCNS granulates were distributed evenly in the material mentioned above. Uniformly dispersed Ag (~20 nm) filled up the pores of OLCNS and acted as bridge to closely contact with carbon flakes. A complete coverage of Ag on the OLCNS would provide improved interfacial adhesion, hence the composite electrode could effectively enhance the capability of charge transfer. To deeply confirm the reduced internal resistance and the enhanced electrical conductivity of OLCNS:Ag, the J-V characteristics of the diodes with the structures of ITO/TiO2/MoO3/Ag (Structure A) and ITO/TiO2/MoO3/OLCNS:Ag (Structure B) were tested. The different slopes of J-V curves imply different electrical conductivities of devices. As shown in Figure 7, the slopes of J-V curves for Structure B with a OLCNS:Ag film is larger, which present a higher conductivity. The enhanced electrical conductivity would boost charge transport, resulting in a reduced internal resistance. In consequence, the improved electrical conductivity of the OLCNS:Ag interface mentioned above reduced the energy barrier and the energy loss, therefore could lead to a remarkable improvement of the device performance.

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Figure 7 Current-voltage characteristic of diodes of Structure A and Structure B. 2.4 Optical property measurements To explore the shift of the IPCE and the Jsc enhancement caused by the OLCNS layer, we measured optical-absorption spectra of photoactive layers. As shown in Figure 8(a) and 8(b), the incorporation of OLCNS layer enhanced absorption over the wavelength range of 300–700 nm in both PCDTBT:PC71BM and PTB7:PC71BM devices, which is consistent with IPCE spectra. To probe the reason for improved light-absorption of PSCs, the optical electric field distribution of the device with OLCNS layer were simulated. The simulated distribution of normalized modulus square of the optical electric field (|E(x)|2) for the device is presented in Figure 8(c). The optical electric field distribution regulation of OLCNS:Ag nanocomposite layer further limits the light intensity within active layer, which opens up the possibility of improved light harvesting in whole visible spectrum due to the synergistic effect of both components. Additionally, according to the TEM results, there are many possibilities regarding the distribution of OLCNS itself. Accordingly, we performed a simulation of optical field for varying distributed OLCNS. It can be observed that evenly distributed OLCNS on the interfacial layer makes the optical electric field distribution between carbon sphere stronger (Figure 8(d)and 8(e)), resulting in an improved light absorption and broadly red shifted spectra. These simulation results are obviously consistent with experimental absorption spectrum enhancement. Photo-generated charges through near-infrared light harvesting of OLCNS transfer to the inherent interface-trap at the MoO3 and OLCNS interface could overcome the high activation barrier of charge as well as intrinsic defect of semiconductor oxide.47-49 Figure 8(f) gives schematic illustration of the charge transfer from this process. Consequently, interfacial charge transfer channel and coherence are both enhanced, which caused an obviously performance enhancement of PSCs.

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Figure 8 The absorption spectrum of (a) PTB7:PC71BM based device and (b) PCDTBT:PC71BM based device; The optical electric field distribution of (c) whole PSC device, (d) simple layer OLCNS, (e) double layer OLCNS, (f) schematic illustration of the charge transfer by an electron acceptor, donor, and OLCNS.

3. Conclusions In conclusion, we have exhibited the synthesis of OLCNS and its application as OLCNS:Ag nanocomposite electrode for inverted PSCs. OLCNS graphite worked as the main components of the counter electrode, an ultrathin silver was coated on the OLCNS interval to fill up the mesoporous structure of OLCNS and acted as bridge to closely contact with carbon flakes. Compared to the only Ag electrode, the OLCNS:Ag counter electrodes not only reduce the requirement of precious metals but also improve the charge collection ability due to the high conductivity of OLCNS. The optical electric field distribution from OLCNS:Ag nanocomposite layer makes it possible to enhance light harvesting in whole visible spectrum due to the synergistic advantages of both components. Large effective specific surface area and high conductivity of OLCNS allow significant charge collection and injection capability, leading to a remarkably enhanced PCE up to 9.81 % for PTB7:PC71BM and 6.95 % for PCDTBT:PC71BM based PSCs.

4. Experiment methods 4.1. Sample preparation The carbon material is prepared by a typical synthesis using naphthalene (M&B Chemicals) as precursor 0.5 g of naphthalene (M&B Chemicals) was put into a dish and burned smoothly in air. Then, the carbon material is collected on top of the flame using a glass beaker and sintered in a vacuum oven at 200 oC for 12 h to remove remaining naphthalene. 4.2. Device fabrication 10


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Electron donor materials of PTB7, PCDTBT and electron acceptor PC71BM were purchased from 1-material Chemscitech and Aldrich used as received, respectively. The inverted device structure is ITO/TiO2/PTB7:PC71BM(1:1.5 by weight) or PCDTBT:PC71BM(1:4 by weight)/MoO3/C/Ag and the regular device structure is ITO/TiO2/PTB7:PC71BM or PCDTBT:PC71BM/MoO3/Ag. The TiO2 interlayer material was prepared by a sol-gel process and its solution was spin-coated on top of the precleaned ITO substrate, which was treated by UV ozone for 10 min. The PCDTBT:PC71BM (concentration of 35 mg/mL ) active layer, with the thickness of 150 nm, was prepared by spin-coating at 2,000 rpm for 20 s and annealed at 70 oC for 20 min. The PTB7:PC71BM (concentration of 25 mg/mL ) active layer, with a nominal thickness of 100 nm (with a variation of 20 nm over the entire film), was prepared by spin-coating a mixed solvent of chlorobenzene/1,8- diiodoctane (97:3% by volume) solution at 1,000 rpm for 2 min. A 4 nm MoO3 layer was subsequently evaporated on the top of active layer. The carbon material was dissolved in isopropanol and then spin-coated on top of the MoO3. And the thickness was controlled to be 60 nm, which was measured by Ellipsometer. Finally, a 20 nm Ag layer was subsequently evaporated through a shadow mask to define the active area of the devices and formed a top OLCNS:Ag anode. For reference devices, a 100 nm Ag layer was evaporated on top of MoO3 to form a top anode. 4.3. Sample characterization Scanning electron microscopy (SEM) images were taken using a Hitachi S900 SEM. TEM images were recorded using a Philips CM200 at 200 kV and was driven by a field emission gun with an extraction voltage of 4.48 kV. Samples were prepared by ultrasonication of a small amount of the carbon sample for several minutes in ethanol, before dropping by pipette onto a carbon grid and allowing to air dry. X-ray diffraction (XRD) was performed on a Philips X’pert Multipurpose X-ray Diffraction System using a Cu source (k=0.154056 nm). Contact angle measurements were measured by the sessile drop method, and the water droplets are introduced using a micro-syringe and images are captured to measure the angle of the liquid–solid interface. PCE values were determined from J–V curve measurements (using a Keithley 2400 source meter) under a 1 sun, AM 1.5G spectrum from a solar simulator (Oriel model 91192, 100 mW cm-2). ASSOCIATED CONTENT Supporting Information Brief statement in nonsentence format listing the contents of the material supplied as Supporting Information. AUTHOR INFORMATION Corresponding Author (W. B. Guo) E-mail: [email protected]. (L. Zhang) E-mail: [email protected].

Acknowledgements The authors are grateful to National Natural Science Foundation of China (61275035, 61370046, 11574110), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF10), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant

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No.2014A030306005), Foundation for High-level Talents in Higher Education of Guangdong Province, China (Yue Cai-Jiao [2013]246, Jiang Cai-Jiao[2014]10) for the support to the work.

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