Broad-band plasmonic Cu-Au bimetallic nanoparticles for organic bulk heterojunction solar cells

Organic Electronics 38 (2016) 213e221

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Broad-band plasmonic Cu-Au bimetallic nanoparticles for organic bulk heterojunction solar cells Mingliu Tang a, 1, Bingbing Sun a, 1, Dongying Zhou a, Zhenggen Gu a, Kai Chen a, Jun Guo b, Lai Feng a, *, Yi Zhou c a College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China b College of Chemistry, Testing and Analysis Center, Soochow University, Suzhou 215006, China c Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215163, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 June 2016 Received in revised form 18 August 2016 Accepted 24 August 2016

In this work, a facile preparation of Cu-Au bimetallic nanoparticles (NPs) with core-shell nanostructures is reported. Importantly, as-prepared Cu-Au NPs are highly stable, solution-processable and exhibit a broad localized surface plasmon resonance (LSPR) band at long wavelengths of 550e850 nm. Highly efficient plasmonic organic solar cells (OSCs) were fabricated by embedding Cu-Au NPs in an anodic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer. The average power conversion efficiency (PCE) was enhanced from 3.21% to 3.63% for poly(3-hexylthiophene) (P3HT):phenylC61-butyric acid methyl ester (PC61BM) based devices, from 6.51% to 7.13% for poly[(ethylhexylthiophenyl)-benzodithiophene -(ethylhexyl)-thienothiophene](PTB7-th):PC61BM based devices and from 7.53% to 8.48% for PTB7-th:PC71BM based devices, corresponding to 9.5e13.4% PCE improvement. Such an improvement is very comparable to that (12.5%) obtained in those with plasmonic Au NPs but achieved at lower cost. This study thus demonstrates a novel and cost-effective approach to enhance the photovoltaic performance of OSCs, in combination with the broad-band plasmonic Cu-Au bimetallic nanostructures. © 2016 Elsevier B.V. All rights reserved.

Keywords: Organic solar cells Plasmonics Copper-gold nanoparticles Core-shell nanostructures Broad-band absorption Hole transport layer

1. Introduction In the past decade, many of efforts have been devoted to the organic bulk heterojunction devices (BHJ-OSCs) because of their advantages of easy fabrication, light weight and high mechanical flexibility [1e3]. Very recently, both the single-junction and tandem OSCs reported the record power conversion efficiencies (PCEs) exceeding 10%, indicative of their fast development [4,5]. Nevertheless, further PCE enhancement and cost reduction are desired for commercialization of OSCs. To realize these targets, various techniques have been developed for fabricating high-performance and cost-effective OSCs. For example, a variety of plasmonic nanomaterials have been developed to enhance the incident light absorption in the OSCs based on polymer donors and fullerene

* Corresponding author. E-mail address: [email protected] (L. Feng). 1 These authors contribute equally to this work. http://dx.doi.org/10.1016/j.orgel.2016.08.023 1566-1199/© 2016 Elsevier B.V. All rights reserved.

acceptors [6e11], which otherwise suffer from insufficient light harvesting due to the low light absorption and short optical path length (~200 nm) in the active layer. The most attractive aspect of this technique is that the optical properties of these nanostructures can be readily tailored through modifications in their sizes, shapes and compositions [12e14], thus permitting their applications in not only OSCs [15e21] but also other optoelectronics, such as Si solar cells and light emitting diodes [22e25]. In particular, gold (Au), silver (Ag) and their hybrid nanostructures have been of the subject of intensive research owing to their excellent optical properties in the visible regime, including the localized surface plasmon resonance (LSPR) and surface enhanced Raman scattering (SERS). Previous studies [3,7,26e35] have verified that the incorporation of plasmonic Au or/and Ag nanostructures into OSC remarkably increases the incident light absorption efficiency, yielding significant enhancement in PCE. Nevertheless, the high cost of Au and Ag might limit their wide use in fabricating the cost-effective OSCs. Copper (Cu) is more abundant and much cheaper than Au and

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Ag. Cu nanostructures also exhibit LSPR typically at ~590 nm though it is somewhat inferior to those reported for Au and Ag [36,37]. Thus, Cu nanostructures have been considered as attractive alternatives to Au in the field of cost-effective optoelectronics. However, Cu nanostructures suffer from low chemical stabilities and readily undergo oxidation in air or embedding condition, which degrade their optical properties and intrinsically limit their applicability. In recent years, though a variety of Cu-based nanostructures were reported [38e40], they have been rarely applied to plasmonic OSCs probably due to their unresolved stabilities [41e43]. Thus, our knowledges about how the Cu-based nanostructures boost the photovoltaic performance of OSC are still limited. Herein, we report a facile preparation of Cu-Au NPs with coreshell nanostructures. Importantly, these Cu-Au NPs are highly stable, solution processable, and exhibit a broad spectral response at a long wavelength region (i.e., 550e850 nm), which all allow their use in OSCs. Subsequently, we incorporated the NPs into a poly(3,4ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) layer, which was applied to the OSC based on poly(3hexylthiophene) (P3HT) or low bandgap polymer donor poly [(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thieno thiophene] (PTB7-th) and fullerene acceptor. The devices with CuAu NPs exhibited a remarkable increase in PCE by 9.5e13.4%, mainly by improving the photocurrent but not FF. Apparently, the incorporation of Cu-Au NPs significantly improved the light harvesting efficiency of OSC, which contributes to the increase in PCE.

2.3. Characterizations Scanning electron microscopy (SEM) images were obtained with a Hitachi SU8010 instrument. TEM images, energy dispersive X-ray spectroscopy (EDS) data and selective area electron diffraction (SAED) data were collected on a FEI Tecnai G220 instrument transmission electron microscope operating at 200 kV. EDS linescan analysis and element mapping were performed on a FEI Tecnai G2F-20 transmission electron microscope equipped with a scanning TEM (STEM) unit and a high-angle annular dark-field (HAADF) detector. For TEM measurements, a few droplets of the NP suspension were deposited onto amorphous carbon-coated 400 mesh nickel grids and vacuum dried. The X-ray diffraction (XRD) pattern of the isolated Cu-Au NPs on the glass substrate was recorded on a Rigaku D/Max 2000 powder diffractometer with Cu Ka radiation (40 kV, 20 mA). Vis-NIR absorption or extinction spectra were obtained using a UV-Vis spectrometer (Shimazu UV2600). FT-IR spectrum was recorded on a Perkin-Elmer FT1730 instrument, using KBr pellets. Raman spectra were measured by an instrument of Horiba Jobin Yvon using an Arþ ion CW laser of 633 nm as an excitation source. The laser spot is about 1 mm and the sample structure is glass/ITO/PEDOT:PSS with or without NPs. The Raman signals were averaged over three different sites for each sample. Morphology images of the PEDOT:PSS layers were obtained using MFP-3D-BIO (Asylum Research) atomic force microscopy (AFM) in tapping mode. 2.4. Device fabrication and characterization

2. Experimental 2.1. Chemicals Hydrogen tetrachloroaurate trihydrate (99.99%, HAuCl4$3H2O), 1-octadecene (tech. 90%), and oleic acid (tech. 90%) were purchased from Alfa-Aesar. Copper(II) oxide (99þ%, CuO) and oleylamine (tech 80e90%) were purchased from Sigma Aldrich. All glasswares were dried in an oven at 120  C before use, and the reaction was carried out under Ar atmosphere.

2.2. Synthesis of Cu-Au NPs Cu-Au NPs were prepared using a method modified from the procedures for preparing Cu [44] and Co@Au [45], Ni@Au NPs [46]. In a typical experiment, 8 mg (0.1 mmol) CuO, 1.6 mL of oleic acid and 1.6 mL of 1-octadecene were mixed in a 10 mL three-neck flask under inert gas (Ar). The flask was then rapidly heated to 250  C with stirring and kept for 20 min. Next, 1.4 mL (4.3 mmol) of oleylamine was slowly dropwise added into the hot mixture and the reaction was hold at 230  C for 10 min to produce Cu NPs. Next, 120 mL of toluene solution containing 0.8 mg (0.002 mmol) H4AuCl4 was quickly injected into the flask and the reaction mixture was cooled to 140  C. Next, 4 mL of 1,2-dichlorobenzene (o-DCB) solution containing 35 mg (0.07 mmol) [(C6H5)3P]AuCl [46] was injected into the flask and the mixture was kept at 140  C for 30 min. Next, the reaction mixture was cooled to room temperature to obtain Cu-Au bimetallic NPs. After dispersing the reaction mixture in 20 mL toluene/ethanol mixture (1:1, v:v), the NPs were isolated by centrifuging at 10000 rpm for 5 min. This washing procedure was repeated three times to remove unreacted reagents and solvents. Finally, the obtained Cu-Au NPs were further washed using 10 mL N-methylpyrrolidone (NMP). The isolated Cu-Au NPs were stored in NMP or ethanol before use.

The OSCs were fabricated with a configuration of the traditional sandwich structure (i.e., ITO/PEDOT:PSS with or without NPs/active layer/Ca/Al). The ITO glass was cleaned by a sequential ultrasonic treatment in detergent, deionized water, acetone and isopropanol. Next, to prepare PEDOT:PSS (Al 4083, Clevios) layer with NPs, 1 mL NMP solution containing various amount of NPs was blended with 1 mL PEDOT:PSS aqueous solution. The mixture was spin-coated on the ITO glass, followed by baking at 150  C for 30 min in air. The thickness of the PEDOT:PSS layer is ca. 40 nm. Then, a photoactive layer (z150 nm) was prepared by spin-coating the blend of P3HT:PC61BM (20 mg:20 mg in 1 mL o-dichlorobenzene) at 800 rpm for 40 s on the top of the PEDOT:PSS layer. Next, a slow evaporation of active-film was carried out by keeping the film in a petri dish overnight, followed by thermal treatments (150  C for 10 min). For high-performance OSCs, a dichlorobenzene solution (1 mL) containing a mixture of PTB7-th:PC61BM or PTB7-th:PC71BM (10 mg:15 mg) and 1,8-diiodooctance (3 vol %) was spin-cast on the prepared substrate at 1000 rpm for 90 s. Finally, the Ca (20 nm)/Al (80 nm) electrode was vacuum evaporated on the photoactive layer with a shadow mask under a vacuum of ca. 105 Pa. The active area of the device was 4 mm2. The current densityevoltage (JeV) measurement of the device was performed with a Keithley 2400 SourceMeter under simulated Air Mass 1.5 Global (AM. 1.5 G) solar illumination with an intensity of 100 mW cm2. The light source was calibrated using a standard silicon solar cell before use. In addition, to prepare the film of PEDOT:PSS with or without NPs/ active layer for UV-vis absorption measurement, a thinner active layer was deposited by spin-coating the active blend at 1500 rpm for 40 s (P3HT:PC61BM) or 2000 rpm for 90 s (PTB7-th:PC61BM or PTB7-th:PC71BM). The incident photon-to-current conversion efficiency (IPCE) was measured in air by a solar cell spectral response measurement system QE-R3011 (Enli Technology Co., Ltd.). The light intensity was calibrated using a standard silicon solar cell as a reference. The SCLC (hole-only) devices were fabricated with a configuration of ITO/PEDOT:PSS with or without NPs/P3HT:PC61BM/MoO3/

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Ag. The PEDOT:PSS layer with or without NPs (z40 nm) was firstly spin-coated on an ITO glass substrate. Then, a thin active layer (z100 nm), an MoO3 layer (10 nm) and a Ag cathode (100 nm) were deposited in sequence on the PEDOT:PSS layer. J-V curves were measured in dark using Keithley 2400 SourceMeter. 3. Results and discussion 3.1. Synthesis and characterizations Cu-Au bimetallic NPs were synthesized in a hot organic media (i.e., a mixture of oleic acid, 1-octadecene and oleylamine) using CuO and a complex of [(C6H5)3P]AuCl as metal precursors (see experimental section for synthesis details), respectively. In the initial stage, Cu NPs were synthesized according to the previously reported method. In the next stage, the addition of Au precursors triggered the galvanic displacement of Cu by Au owing to the lower reduction potential of Cu2þ/Cu (0.34 V vs. SHE) relative to that of Au3þ/Au pair (0.99 V vs. SHE), yielding Cu-Au bimetallic NPs. The as-prepared Cu-Au NPs were first characterized using electron microscopy techniques. Fig. 1a shows their SEM image, exhibiting nearly spherical Cu-Au NPs with an average size of ~50 nm. The uniformity of the NPs is not well controlled using the present synthesis method. Fig. 1b provides a typical TEM image of the products. It is clear seen that most of NPs show brightness difference between their inner and outer regions, probably indicating a core-shell bimetallic nanostructure. Moreover, high-angle annular dark-field (HAADF) imaging that is sensitive to the atomic number was conducted. As shown in Fig. 1c (inset), the center portion of the NP is darker relative to the edge. The

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corresponding compositional profile (Fig. 1c) obtained by an energy-dispersive X-ray spectroscopy (EDS) line scan demonstrates that the Cu signal in the core region is much stronger than those of Au. This was confirmed by the EDS element mapping performed on a single NP. As shown in Fig. 1e and f, Au is much less abundant than Cu in the center region, while both Au and Cu are observed in the edge region. On the basis of these observations, we believe that the particle edge is Cu-Au bimetallic while the core is Cu-rich. Thus, unlike the previously reported formations of Co@Au and Ni@Au NPs, in our case, the reaction of Cu NPs and [(C6H5)3P]AuCl precursors yielded the NPs with bimetallic nanoshell and Cu-rich core. Additional high-resolution TEM (HRTEM) image (Fig. 2a) revealed the polycrystalline nature of the as-prepared Cu-Au NPs. Within either the shell or core region, multiple crystalline domains can be identified with the measured lattice fringes of 0.238 and 0.208 nm, corresponding to Au (111) and Cu (111) planes, respectively. A selected area electron diffraction (SAED) pattern (inset in Fig. 2a) recorded from a single NP displays mixed diffraction rings involving both fcc-Au and fcc-Cu, in good agreement with the HRTEM observations. Besides, the XRD patterns of the Cu-Au bimetallic NPs are shown in Fig. 2b. The peaks were assigned to the (111), (200), (220), (311) and (222) planes of fcc-Au, whereas the peak corresponding to the (111) plane of fcc-Cu is almost negligible probably due to the overlap with that of Au-(220) plane. The diffraction pattern also does not show the presence of CuO or Cu2O, indicating the fact that the Cu-Au bimetallic NPs are oxidationresistant. The FT-IR spectrum (Fig. S1 in SI) displays characteristic peaks at 3500-3200 cm1 (NeH stretching mode), 3000-2850 cm1 (CH2 stretching modes), 1600-1700 cm1 (C]O stretching mode) and

Fig. 1. (a,b) SEM and TEM images of the as-prepared Cu-Au bimetallic NPs. (c) STEM-HAADF image (inset) and corresponding elemental line profiles. (d,e,f) STEM-HAADF and corresponding EDS elemental mapping images. The scale bar is 50 nm.

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on the surface of NPs, which act as steric stabilizers to prevent NPs from aggregation when dispersed in solution. Fig. 3 displays the UV-vis absorption spectrum of the asprepared Cu-Au NPs in comparison with those of Au NPs and Cu NPs with similar size. In particular, LSPR bands at 530 nm and 590 nm were typically observed for Au NPs and Cu NPs, respectively, fully consistent with the previous reports [28,37], while CuAu NPs exhibited a broad absorption band with the maximum at long wavelengths of 650e700 nm. The additional inset photography in Fig. 3 shows a red solution of Au NPs and a brown solution of Cu NPs whereas a dark blue solution for Cu-Au NPs. It is also noted that this LSPR band is remarkably red-shifted and broadened relative to those of pure Au NPs and Cu NPs as well as Au-Cu alloy NPs (572 nm) [41]. However, it is closely comparable to those of Au nanoshells having either hollow interiors (634 nm) [47] or a dielectric core such as Co@Au (680 nm) [45], Ni@Au (600 nm) NPs [46] and Fe@Au (680 nm) [48]. These spectral features also indicate that the as-prepared Cu-Au NPs might have core-shell nanostructures, in good agreement with the TEM observations. Thus, the increase in LSPR band width may be explained by inhomogeneous polarization of the Cu-Au shell in the electromagnetic field as well as multipole excitation effects, due to the fact that the bimetallic shell is asymmetrical and unlike the symmetrical sphere which has only one plasmon resonance [46]. In addition, these coreeshell nanoparticles show tunable plasmon resonance that depends on the ratio of Cu/Au precursors (see SI for details), similar to those reported for other core-shell NPs [46,48]. 3.2. Performances of plasmonic OSCs

Fig. 2. (a) HRTEM image along with SAED pattern (inset). The lattice fringes can be assigned to the Au and Cu components, respectively. (b) XRD patterns.

around 1059 cm1 (CeN stretching mode) for the Cu-Au NPs washed subsequently in toluene/ethanol, NMP and ethanol. These spectral features suggest the presence of oleylamine and/or NMP

Fig. 3. UV-vis spectrum of the as-prepared Cu-Au NPs dispersed in NMP solution in comparison with those of Au NPs and Cu NPs. Inset shows their photography: (a) Au NPs, (b) Cu NPs and (c) Cu-Au NPs.

To evaluate the plasmonic property of the NPs embedded in PEDOT:PSS layer, the UV-vis spectra of the PEDOT:PSS:NPs/ P3HT:PC61BM layers (Fig. 4a) were measured and compared to that of pristine layer (PEDOT:PSS/P3HT:PC61BM). Their similar absorption features indicate that the crystalline structure of P3HT remains unchanged after NP-doping. Nevertheless, the absorbance is remarkably enhanced in the range of 300e650 nm, as compared to the pristine layer. The layer with 0.88 wt% NPs exhibits the largest absorption enhancement among all layers. Noteworthy is that the absorbance enhancement is observed in a wide range beyond the plasmonic band of Cu-Au NPs, indicating that not only the LSPR absorption but also light scattering term plays an important role in the improvement of light harvesting. Moreover, Raman spectra of the PEDOT:PSS layers with or without NPs were measured to evaluate the light scattering effect of Cu-Au NPs. Particularly, the Raman spectrum of pristine PEDOT:PSS layer (Fig. S3 in SI) presents characteristic peaks with low or moderate intensity near 991, 1264, 1368, 1441, 1533 and 1567 cm1 [49]. In comparison, the PEDOT:PSS:NPs layer displays a similar spectrum with remarkable signal enhancement over the thiophene-ring stretching region of 1400e1600 cm1, confirming the plasmonic light scattering effect of the embedded Cu-Au NPs. The NP-doped PEDOT:PSS layers were then introduced into OSCs to study the plasmonic effect of Cu-Au NPs on the photovoltaic performance of device. A typical device with structure of ITO/ PEDOT:PSS:NPs/P3HT:PC61BM/Ca/Al was applied in this study (Fig. 4b). Fig. 4c shows the current density-voltage (J-V) curves of the devices without or with various concentrations of NPs (i.e., 0.44, 0.88 and 1.76 wt%). The key parameters are summarized in Table 1. Obviously, the best photovoltaic performance was obtained from the device with 0.88 wt% Cu-Au NPs. The average PCE increases by 13.4% from 3.20% (without NPs) to 3.63% (with 0.88 wt% NPs). This improvement can be primarily attributed to the enhancement of Jsc from 8.24 mA cm2 to 10.91 mA cm2. Meanwhile, the Voc is almost constant and the FF decreases from 61.1% to

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Fig. 4. (a) Schematic of a plasmonic OSC with a structure of ITO/PEDOT:PSS:NPs/active layer/Ca/Al. (b) UV-vis spectra of the layer of PEDOT:PSS with or without NPs/P3HT:PC61BM. (c) Current density-voltage (J-V) curves and (d) Incident photon-to-current efficiency (IPCE) spectra of the device with or without NPs under illumination of simulated solar light (AM1.5, 100 mW cm2). (e) Current density-voltage (J-V) characteristics of the devices with or without NPs measured in the dark condition. (f) Current density-voltage (J-V) characteristics of the SCLC (hole-only) devices with configuration of ITO/PEDOT:PSS with or without NPs/P3HT:PC61BM/MoO3/Ag.

52.3% in response to the change in PCE. Moreover, the incident photon-to-current efficiency (IPCE) curves of the devices with or without Cu-Au NPs are compared in Fig. 4d. It is obvious that the IPCE values of the NP-doped devices are larger than the reference one in the range of 350e700 nm, which agrees well with the increase of Jsc for NP-doped devices. To explore the hole transport characteristics of NP-doped PEDOT:PSS layer, the hole mobility (mh) was measured by using the space charge limited current (SCLC) model according to the

MottGurney equations [54e56]: JSCLC ¼ 9ε0εtmh(VVBI)2/(8L3), where ε0 is the free space permittivity, εt is the dielectric constant of the material, V is the applied voltage, VBI is the build-in voltage, L is the thickness of the active layer and mh is the hole mobility (the details of the carrier mobility calculations are described in SI). As compared to the mh of the device without NPs (2.89  104 cm2 V1 s1), higher value of 1.09  103 cm2 V1 s1 was obtained for the NP-doped device (0.88 wt% NPs). The improvement of mh agrees well with the reduced series resistances

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Table 1 Device performancea of P3HT-based OSCs with or without Cu-Au NPs under 100 mW cm2 AM 1.5G illumination. Device type (active layer/NPs)

Voc (mV)

Jsc (mA cm2)

FF (%)

PCE (%)b

Rsc (U cm2)

Rshc (U cm2)

P3HT:PC61BM/w/o NPs P3HT:PC61BM/0.44 wt% NPs P3HT:PC61BM/0.88 wt% NPs P3HT:PC61BM/1.76 wt% NPs

639 644 638 641

8.24 8.62 10.91 9.54

61.1 60.4 52.3 55.9

3.21(3.28) 3.34(3.41) 3.63(3.71) 3.41(3.52)

12.7 12.8 11.4 10.8

1394.5 1055.2 280.7 762.7

a b c

Averaged photovoltaic characteristics were obtained from 10 identical devices. The best PCE values are in brackets. The series and shunt resistances (Rs and Rsh) are estimated from the J-V curves using the method demonstrated in the literatures [50e53].

(Rs) from 12.7 U cm2 (without NPs) to 11.4 U cm2 (with 0.88 wt% NPs), indicating a better hole-transport through the NP-doped PEDOT:PSS layer. The dark J-V curves were investigated to further evaluate the electrical characteristics of the NP-doped PEDOT:PSS layer. As shown in Fig. 4e, NP-doped device presents higher dark current density under the reverse bias, compared to the reference (undoped) device, thus impacting the shunt resistance (Rsh), which was reduced from 1349.5 U cm2 (without NPs) to 280.7 U cm2 (with 0.88 wt% NPs). The reduction of Rsh upon NP-doping, which sacrifices FF, might suggest that the doping of Cu-Au NPs in PEDOT:PSS layer causes larger leakage current or higher charge recombination at the PEDOT:PSS:NPs/active layer interface [57e59]. To figure out the reason for the decreased FF or reduced Rsh of NP-doped device, the morphologies of the PEDOT:PSS layers with or without NPs were investigated by means of AFM technology. As shown in Fig. 5, the PEDOT:PSS layer with 0.44 wt% or 0.88 wt% NPs presents more rough surface (with the surface root-mean-square (RMS) roughness of 1.20 or 1.25 nm), compared to the pristine

PEDOT:PSS layer (RMS roughness of 0.85 nm) and the layer with 1.76 wt% NPs (RMS roughness of 0.89 nm). It is also observed that there are slightly clustered or largely aggregated NPs on the surfaces of the NP-doped PEDOT:PSS layers, indicating that partial doped Cu-Au NPs are in contact with the active layer. It is thus supposed that the holes could be trapped by the defects of polycrystalline Cu-Au NPs and are inclined to undesired recombination with the electrons of the active layer [60e62]. In comparison, the PEDOT:PSS layer with 0.88 wt% NPs might have more interface traps due to its moderate NP concentration and higher RMS roughness relative to others, which probably accounts for the smaller Rsh as well as the lower FF in the corresponding device.

3.3. High-performance plasmonic solar cells with optimized condition To further verify the photovoltaic contribution of plasmonic CuAu NPs, we applied the NP-doped PEDOT:PSS layer to highperformance OSCs, in which low band-gap polymer PTB7-th was

Fig. 5. AFM topographic images of PEDOT:PSS layers containing Cu-Au NPs of a) 0.44 wt%, b) 0.88 wt%, c) 1.76 wt%, and d) 0%, respectively. The imaging area is 2 mm  2 mm.

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Table 2 Device performancea of PTB7-th-based OSCs with or without Cu-Au NPs under 100 mW cm2 AM 1.5G illumination. Device type (active layer/NPs)

Voc (mV)

Jsc (mA cm2)

FF (%)

PCE (%)b

Rsc (U cm2)

Rshc (U cm2)

PTB7-th:PC61BM/w/o NPs PTB7-th:PC61BM/0.88 wt% NPs PTB7-th:PC71BM/w/o NPs PTB7-th:PC71BM/0.88 wt% NPs

796 800 794 791

13.13 15.50 14.55 17.78

62.3 57.9 65.2 60.1

6.51(6.56) 7.13(7.23) 7.53(7.56) 8.48(8.55)

8.0 5.7 6.4 5.0

589.1 207.6 604.6 232.7

a b c

Averaged photovoltaic characteristics were obtained from 10 identical devices. The best PCE values are in brackets. The series and shunt resistances (Rs and Rsh) are estimated using the method employed in the literatures [50e53].

PTB7:PC71BM-based devices doping with plasmonic Ag or Au NPs [63,64]. Correspondingly, the related photovoltaic characteristics (i.e., Jsc, FF, Rs, Rsh) of PTB7-th:PC71BM-based devices all varied with the trends similar to those observed for PTB7-th:PC71BM-based devices. Furthermore, for both PTB7-th:PC61BM and PTB7th:PC71BM-based devices, the increase of Jsc upon NP-doping was confirmed by the IPCE results, which show an obvious increase of the IPCE values in the region of 350e750 nm (see Figs S4a and S5a in SI). Since the enhancement of light absorption in the similar region was also observed (see Figs S4b and S5b in SI), the increase of Jsc as well as the enhancement of IPCE might be mainly attributed to the plasmonic effect of the doped Cu-Au NPs, which is consistent with that discussed above for P3HT:PC61BM-based devices.

4. Conclusions

Fig. 6. Current densityevoltage (JeV) curves of the devices with and without Cu-Au NPs based on (a) PTB7-th:PC61BM (1:1.5, w/w) and (b) PTB7-th:PC71BM (1:1.5, w/w), respectively.

employed as electron donor. As listed in Table 2 and shown in Fig. 6, upon the incorporation of 0.88 wt% NPs, the average PCE of PTB7th:PC61BM-based devices was improved from 6.51% to 7.13%, corresponding to 9.5% improvement. Particularly, the Jsc increases by 18% from 13.13 to 15.50 mA cm2, whereas FF drops by 7% from 62.3 to 57.9%, both in response to the change of PCE. Meanwhile, Rs is reduced from 8.0 to 5.7 U cm2, and Rsh reduced from 589.1 to 207.6 U cm2, confirming the dual electrical effects of the doped NPs as discussed above for P3HT:PC61BM-based devices. Similar to the case of PTB7-th:PC61BM-based devices, for PTB7-th:PC71BM-based devices, the average PCE was increased by 12.6% from 7.53% to 8.48% upon the NP-incorporation, which is very comparable to those (i.e., 8.9e12.5%) obtained from previously reported

In summary, we have prepared solution-processable Cu-Au bimetallic NPs using a one-pot reaction. Importantly, these Cu-Au NPs exhibited high stability as well as a broad LSPR band covering a long wavelength region of 550e850 nm, which is believed to be closely correlated with their core-shell nanostructures. By incorporating 0.88 wt% Cu-Au NPs in an anodic PEDOT:PSS layer, the average PCEs of the P3HT:PC61BM, PTB7th:PC61BM or PTB7-th:PC71BM based BHJ-OSC devices reach 3.63%, 7.13% and 8.48%, corresponding to 13.4%, 9.5% and 12.6% improvements, respectively. Such PCE improvements are even comparable to that (12.5%) obtained from the similar device doping with Au NPs but achieved at lower cost. Furthermore, it is suggested that the PCE improvement of the NP-doped devices can be primarily attributed to the increase of Jsc or the plasmonic effect of the Cu-Au NPs, as confirmed by IPCE and optical absorption measurements. Besides, it is also proposed that the NP-doping not only improves the hole-transport but also induces more chargerecombination between the hole-transport layer and active layer, leading to remarkable sacrifice in FF as well as PCE. With a success on the first preparation of solution-processable and broad-band plasmonic Cu-Au bimetallic nanostructures, this study thus demonstrates a novel and cost-effective approach to enhance the photovoltaic performance of OSCs.

Acknowledgments This work is the supported in part by the NSFC (51372158), Jiangsu Specially Appointed Professor Program (SR10800113), the Project for Jiangsu Scientific and Technological Innovation Team (2013).

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.08.023.

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