shell nanoparticles into all organic layers

Nano Energy (]]]]) ], ]]]–]]]

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

RAPID COMMUNICATION

Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers Boxue Chena, Wenfeng Zhanga, Xinghao Zhoua, Xiao Huanga, Xuemei Zhaoa, Haitao Wanga, Min Liua, Yalin Lua,b,⁎, Shangfeng Yanga,⁎ a

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion & Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China b Laser Optics Research Center, Department of Physics, United States Air Force Academy, CO 80840, USA Received 27 January 2013; accepted 16 March 2013

KEYWORDS

Abstract

Polymer solar cells; Gold nanoparticles; Core/shell structure; Surface plasmon; Power conversion efficiency

We report the incorporation of Au@SiO2 core/shell nanoparticles (NPs) into bulk heterojunction polymer solar cell (BHJ-PSC) devices, leading to an obvious efficiency enhancement due to the localized surface plasmon resonance (LSPR) effect. The Au@SiO2 core/shell NPs comprise of large Au NPs with an approximate size of 70 nm coated by a ∼50 nm thick SiO2 shell. Such NPs were doped into the poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer of P3HT:PCBM BHJPSCs, resulting in that the large NPs penetrate into all organic layers including the PEDOT:PSS buffer layer and P3HT:PCBM active layer, and are partially embedded in the Al cathode layer. The power conversion efficiency (PCE) of the P3HT:PCBM BHJ-PSC devices incorporated with Au@SiO2 NPs increased from 3.29% to 3.80%, and such a ∼16% efficiency enhancement can be primarily attributed to the light absorption enhancement, originating from the LSPR effect induced by Au NPs as confirmed by the UV–vis absorption spectrocopic study. Contrarily, the analogous P3HT:PCBM BHJ-PSC devices incorporated with bare Au NPs exhibited a lower efficiency enhancement, indicating that the coating of dielectric SiO2 shell is beneficial for the LSPR effect. & 2013 Elsevier Ltd. All rights reserved.

⁎

Corresponding authors at: Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion & Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China. Fax: +86 551 63601750. E-mail addresses: [email protected] (Y. Lu), [email protected] (S. Yang).

Introduction Polymer solar cells (PSCs) have been attracting much interest as a renewable energy source because of its advantages of

2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.03.011 Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

2

B. Chen et al.

light-weight, low-cost, and mechanical flexibility [1,2]. Among the reported types of PSCs, bulk heterojunction (BHJ) structure with the photoactive layer comprising an interpenetrating network of a conjugated polymer donor such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and a soluble fullerene acceptor like [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been demonstrated to be the most popular and efficient architecture of PSCs [3–5], and the power conversion efficiency (PCE) of BHJ-PSCs based on the solution-processible blend of P3HT and PCBM has reached up to 5% [3,6]. One of the critical problems limiting the device performance of BHJ-PSCs is the insufficient photon absorption by the thin photoactive layer (100–200 nm) that is however the prerequisite of reducing the probability of charge recombination after the exciton dissociation due to the comparatively low carrier mobility of organic conjugated active materials [2,7,8]. To enhance the photon absorption within such a thin active layer, a few strategies including the incorporation of an optical spacer have been implemented in the past so as to spatially redistribute the light intensity [9–11]. Recently, much attention have been paid to localized surface plasmon resonance (LSPR) effect for effective lighttrapping [12–26], which could enhance the light absorption directly and increase the optical path length by scattering light in PSCs as well. Typically, LSPR effect has been applied to PSCs either by building periodic, metallic nanoarrays or nanograting to trigger surface plasmon polaritons along the metal-dielectric interface [15,16,27] or by incorporating noble metallic nanoparticles (NPs) to generate localized surface plasmons [13,17,23–26,28]. The latter could be more technically feasible due to that the NPs' incorporation is solution-processible and is not necessarily required to be nanofabricated for being periodic. In 1995, Stenzel et al. incorporated small metal clusters such as Ag, Au and Cu into organic solar cells as the interface layer to improve the photovoltaic conversion performance [29]. Choy et al. incorporated relatively small Au NPs (18 and 35 nm in diameter) into both poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and P3HT:PCBM layers, resulting in an improvement of PCE by ∼22% [25]. They concluded that Au NPs in PEDOT:PSS mainly contributed to the better hole collection whereas Au NPs in P3HT:PCBM active layer contributes to the enhanced optical absorption, realizing a more balanced charge-transport. In a similar way, Chen et al. doped octahedral Au NPs with a larger size (∼45 nm) only into the PEDOT:PSS anodic buffer layer, and concluded that the incorporation triggered the LSPR, which enhanced the PCE of the BHJ-PSCs without dramatically sacrificing their electrical properties [13]. Very recently, Heeger et al. used an even larger truncated octahedral Au NPs (∼70 nm) to dope the active layers composed of different polymer donors and PC70BM and achieved a dramatic enhancement on shortcircuit current (Jsc), fill factor (FF), and PCE, which were attributed to the enhanced light absorption caused by the light scattering of Au NPs inside the active layer. However, in that work the plasmon-induced light concentration at specific wavelengths was not observed [17]. Very favorably, this work suggested the advantages of using larger Au NPs, including more efficient hole transport and light harvesting due to enhanced reflection and scattering of the incident light [17]. More recently, Chen et al. incorporated different sized (20, 35, 50, and 75 nm) Au NPs into P3HT:PCBM BHJ-PSCs

by spin-coating Au NPs onto ITO anode prior to the fabrication of PEDOT:PSS layer and demonstrated a maximum PCE improvement of ∼23% by incorporating 35 nm Au NPs. They concluded that the smaller sized Au NPs do not enhance the light absorption but are beneficial for the improved hole collection efficiency, whereas the larger Au NPs can obviously enhance the light absorption, enlarge the interface areas between PEDOT:PSS and P3HT:PCBM, and reduce the exciton quenching [26]. All the previous studies came to a similar conclusion that an efficiency enhancement of the BHJ-PSCs can be achieved by incorporating bare Au NPs with different sizes and shapes, while the enhancement mechanism by such bare Au NPs is however still opaque. For example, the expected LSPR effect could only be proved in few previous cases, and this fact suggests that Au NPs-induced LSPR effect in BHJ-PSCs is sensitively dependent on size and shape of Au NPs and on the organic layer in which Au NPs were doped. Furthermore, questions such as how those doped Au NPs affect the electrical conducting properties should be understood in more details. At last, in all previous reports it was generally necessary to cap the bare Au NPs with surfactants to avoid the exciton quenching by non-radiative energy transfer between the active layer and Au NPs. The impact of such a dielectric environment changing around the Au NPs was seldom studied. Despite of the aforementioned successes on utilizing LSPR effect induced by bare Au NPs in PSCs for effective lighttrapping, two critical issues should be considered when using such bare metallic NPs inside a practical PSC, including the fading of the plasmon-enhanced efficiency with time as a result of the interaction of metallic NPs with their dielectric environment and the increasing recombination rate of lightgenerated charge carriers at the surface of such metallic NPs [30,31]. As a practical solution for these problems, coating metallic NPs with one or more dielectric shells to form a socalled core/shell structure has been demonstrated to be beneficial to reduce defects or to optimize the interfaces between NPs and the dielectric environments, thus inhibiting the loss of localized surface plasmons [14,32,33]. For instance, Snaith et al. incorporated Au@SiO2 core/shell NPs (∼15 nm Au core coated by ∼3 nm SiO2 shell) into dye-sensitized solar cells (DSSCs) and achieved strong surface plasmon resonance effect [14]. The authors concluded that the utilization of Au@SiO2 core/shell NPs overcame four main shortcomings of using the bare Au NPs, including charge recombination within the Au metal, thermal stability during processing, chemical stability, and control of Au NP/dye chromophore separation to inhibit non-radiative quenching [14]. However, to the best of our knowledge, up to now such a strategy of utilizing core/shell NPs has not been applied in PSCs yet. In this paper, large Au@SiO2 core/shell NPs comprising of large Au NPs core coated by thick dielectric SiO2 shells were incorporated into P3HT:PCBM BHJ-PSCs, resulting in an obvious efficiency enhancement. Despite of the unusual device structure with Au@SiO2 NPs penetrating into all organic layers including the PEDOT:PSS buffer layer and P3HT:PCBM active layer and partially embedded in the Al cathode layer, LSPR effect is still observed unexpectedly and responsible for the efficiency enhancement of BHJPSCs. The role of the dielectric SiO2 shell on the LSPR effect induced by Au NPs is discussed in details on the basis of a comparative study of incorporating bare Au NPs.

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

Surface plasmon enhancement of polymer solar cells

Experimental Materials Analytic grade (AR) chloroauric acid (HAuCl4·4H2O, 47.8% Au), trisodium citrate dihydrate (C6H5Na3O7·2H2O, 99%), hydroxylammonium chloride (HONH3Cl, 98.5%), ammonia solution (NH3, 25%), ethyl silicate (TEOS, 28.4% SiO2) were from the Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. Preparation of Au@SiO2 core/shell NPs Au NPs were synthesized by a modified stepwise seedmediated growth method [34]. First, Au NPs with an approximate size of 13 nm were obtained by using a method reported previously [35]. In brief, 1 ml trisodium citrate dihydrate (1 wt%) was added into 50 mL boiling HAuCl4 aqueous solution (2.06×10−4 M) under 145 °C, with rapid stirring for 5 min. The color of the mixture solution changed from light yellow to wine-red. In the next step, Au NPs with an approximate size of 30 nm were prepared using the 13 nm Au NPs as seeds. 1 ml 0.5 M hydroxylamine hydrochloride was added into 100 mL diluted Au (13 nm) seed solution under vigorous stirring. Then 2 ml 14.1 mM HAuCl4 was dropwise and slowly added into the mixture. After 30 min, the Au NPs grew to 30 nm as confirmed from transmission electron microscopy (TEM) measurements. The light red mixture solution then turned to dark red. Finally, Au NPs with an approximate size of 70 nm were synthesized using the 30 nm Au NPs as seeds repeating the similar method described above, and the mixture solution turned to brick-red in the last step. The coating of Au NPs (∼70 nm) by a thick SiO2 shell (∼50 nm) was accomplished by using the literature method, and the thickness of SiO2 shell can be readily tuned by changing the amount of TEOS [36]. In brief, the as-prepared Au NPs solution (1.5 mL) was mixed with isopropanol (5 mL) and ammonia (0.1 mL, 30 wt%). Then TEOS in isopropanol (0.4 mL, 10 mM) was added into the mixture for four times with a time interval of 2 h. After shaking for 18 h, another 0.4 mL of 10 mM TEOS was added in the same way followed by another 18 h shaking. After several rounds of centrifugation and redispersion using water and ethanol, pure Au@SiO2 core–shell NPs were obtained. Devices fabrication Our detailed fabrication procedure of the P3HT:PCBM BHJPSCs has been reported previously [37–41]. Briefly, the ITOcoated glass substrate (8 Ω/□, purchased from Shenzhen Nan Bo Group, China) was cleaned by sonication in detergent, deionized water, acetone and isopropanol for 15 min each. After dried, it was treated with UV-ozone for 12 min. prior to spin-coating. A thin film (∼35 nm) of Baytron P (PEDOT:PSS, obtained from SCM Industrial Chemical Co., Ltd.) was first spin-coated at 4500 rpm for 60 s and then annealed at 120 °C for 30 min. For the Au NPs-incorporated devices, Au (Au@SiO2) NPs were added into Baytron P solution with an optimized doping concentration of 3×1010 particles/ml (∼2 wt%). The P3HT:PCBM (from Luminescence Technology Corp and Nichem Fine Technology Co., Ltd., respectively) blend solution (1:0.8 w/w) was prepared by

3 stirring at 40 °C until both were completely dissolved. The blend films were spin-coated at 850 rpm for 60 s to form a 90 nm thick active layer. After all of the solution process were carried out in air, the device was transferred into a vacuum chamber (∼10−5 Torr) to deposit an Al electrode (∼100 nm) and a shadow mask was used to define the device active area (2×5 mm2). Finally, the device was annealed at 135 °C for 10 min in a nitrogen atmosphere. Measurements Au@SiO2 NPs were characterized by scanning electron microscopy (SEM, JSM-6700 M, JEOL, Japan) and then their core/shell structures were further observed by TEM with an accelerating voltage of 200 kV (JEOL-2010, Japan). The absorption spectra of polymer solar cells were measured using an UV/vis/NIR spectrometer (UV-3600, Shimadzu, Japan). The surface morphologies were investigated by atomic force microscopy (AFM) performed on a Digital Instruments system and AFM images were obtained in the contact mode using a Nanoscope III (Digital Instruments, USA). J–V characteristics of solar cells were measured using a Keithley 2400 source measurement unit. The photocurrent response was obtained under simulated AM 1.5 irradiation (100 mW cm−2) with a xenon-lamp-based solar simulator (Oriel Sol 3A, USA). More than ten devices were fabricated independently under each experimental condition and measured to ensure the reproducibility of the data, and the average data were used in the following discussions. Simulation First, the sizes of Au and Au@SiO2 NPs were considered using Mie scattering theory [42]. For a particle with the size far smaller than the wavelength of light, Mie theory could lead to relatively accurate results. When the size of Au (Au@SiO2) NPs doped in Baytron P changes from 10 to 100 nm, its scattering, absorption and extinction cross sections were then calculated under the light irradiation of 550 nm using those optical constants available in literatures (see supporting information Figs. S1) [43–45].

Results and discussion Optimization of the size of Au@SiO2 core/shell NPs To determine the optimum sizes of Au core and SiO2 shell within Au@SiO2 core/shell NPs, simulations of light absorption, scattering and extinction (the combined effect of absorption and scattering) of Au and Au@SiO2 NPs were carried out as the first step. Clearly, for pure Au NPs doped in Baytron P, both absorption and scattering cross sections increase with increasing the Au NPs size, and the tendency of the absorption cross section increasing becomes flat when the size of Au NPs is beyond ∼70 nm (see Supporting Information Figure S1). Although both light absorption and scattering have positive impacts on the performance of PSCs, the former is considered to be more important because it could couple with some other effects such as up-conversion [46]. Therefore, to use excessively large NP (larger than 70 nm, for example) is hardly beneficial technically. Moreover, larger NPs are easier to aggregate during the processing, and this would make it more difficult to

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

4

B. Chen et al.

incorporate them into PSCs. The above two considerations suggest an optimum size of ∼70 nm for the Au core, and this size coincide with a previous study by Heeger et al. in which the doping of truncated octahedral Au NPs with an average size of ∼70 nm into the photoactive layers resulted in a dramatic enhancement on PCE [17]. As revealed in previous studies, the dielectric environment plays an important role in the LSPR of Au NPs [47,48]. Once the optimum size of Au core was determined (∼70 nm), we try to introduce a dielectric shell in order to limit the conducting properties of Au. Here a SiO2 shell was preferably chosen because it has been used in metal-oxide–semiconductor (MOS) system for quite a long time [49,50]. When the NP is less than 100 nm, its' scattering and absorption cross sections can be expressed by Eqs. (1) and (2), respectively [51,52]: 8π 4 6 k a ðε−εm Þ=ðε þ 2εm Þj 3   ε−εm Cabs ¼ 4πka3 Im ε þ 2εm

Csca ¼

effect, it is necessary to optimize the thickness of SiO2 shell. The simulation of light intensity, which is closely related to the absorption, in BHJ-PSC devices is obtained by finite-difference time-domain method (FDTD). Since the Au NPs were incorporated in Baytron P layer randomly, only an individual NP is considered and the simulation results are shown in Figure 1. In the case of incorporating bare Au NPs, the LSPR localizes only in Baytron P layer. Importantly, the LSPR penetrates into the P3HT:PCBM active layer and is dramatically enhanced with the increase of the thickness of SiO2 shell from 10 to 50 nm (Figs. 1c to e). However, further increasing the thickness of SiO2 shell to 70 nm will shift the dominant absorption region beyond the P3HT:PCBM active layer (Figure 1f). Therefore, we determine the optimum thickness for the SiO2 shell to be 50 nm.

ð1Þ ð2Þ

where Csca, Cabs is the scattering and absorption cross section, respectively; k ¼ 2π=λ is the wavenumber of light; a is the diameter of Au NP; ε and εm are the permittivity of NPs and environment, respectively. According to Eq. (1), when the bare Au NPs were put into absorbing medium (εm o0 while ε40) directly, Im ½ðε−εm Þ=ðε þ 2εm Þwill be very large, and this will lead to a small scattering cross section. When Au NPs exist in other dielectric environments such as P3HT (the pristine environment within active layer) and SiO2, we found that the scattering cross section of Au NPs in SiO2 was 4 times higher than that in P3HT medium, whereas the absorption cross section for the two cases only changes slightly. Since a SiO2 shell have a large impact on the scattering cross section of Au NPs which is directly correlated to LSPR

Figure 2 UV–vis spectra of Au@SiO2 NPs and Au NPs with different sizes. Inset: TEM image of the Au@SiO2 core/shell NPs.

Figure 1 Simulations of E-field intensity distribution across the organic layers of P3HT:PCBM BHJ-PSC devices without (a) and with Au (b) or Au@SiO2 (c–f) NPs incorporation. The regions of the Baytron P (PEDOT:PSS) and P3HT:PCBM active layer are set at (0 to 40) and (40 to 130) nm, respectively. The size of Au NPs is fixed at 70 nm. Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

Surface plasmon enhancement of polymer solar cells According to the optimized NP structure of Au core (70 nm) and SiO2 shell (50 nm), Au@SiO2 core/shell NPs were synthesized by a modified stepwise seed-mediated growth method [34–36]. The morphology and size of the asprepared Au@SiO2 core/shell NPs were measured by both TEM (see inset of Figure 2) and SEM (not shown), indicating that the average diameter of Au core and the thickness of SiO2 shell are approximately 70 and 50 nm, respectively. Figure 2 shows UV–vis absorption spectra of the bare Au and Au@SiO2 NPs. Interestingly, while for bare Au NPs (70 nm) a maximum of the absorption band is observed at∼540 nm, this absorption maximum red-shifts to ∼556 nm in Au@SiO2 NPs.

Performance of P3HT:PCBM BHJ-PSC devices with Au@SiO2 NPs incorporated A layered structure ITO/PEDOT:PSS (40 nm)/P3HT:PCBM (90 nm)/Al (100 nm) optimized from our previous works was used for the P3HT:PCBM BHJ-PSC device [37–41]. Because the diameter of Au@SiO2 core/shell NPs (∼170 nm)

Scheme 1

5 exceeds the sum of both organic layers' thicknesses determined experimentally by a surface profilometer, Au@SiO2 core/shell NPs were preferably doped into the PEDOT:PSS (Baytron P) layer, as a result the large Au@SiO2 NPs penetrated into all organic layers and were partially embedded in the Al cathode layer (see Scheme 1). Such an unprecedented structure was experimentally confirmed by AFM as shown in Figure 3. As clearly seen from the AFM images of the PEDOT: PSS layer with and without the incorporation of Au or Au@SiO2 NPs (images a–c), Au and Au@SiO2 NPs are both partially exposed on the surface, and the average exposed height contrasts of Au and Au@SiO2 NPs are around 32 and 140 nm, respectively, which are consistent with their diameters determined by TEM (see inset of Figure 2). Furthermore, upon spincoating the P3HT:PCBM active layer with the thickness of ∼90 nm, the surface morphology of the reference P3HT:PCBM film only changes slightly (see images a, d). However, the films with Au or Au@SiO2 NPs incorporation both exhibit dramatic changes: the exposed Au NPs are completely covered by P3HT: PCBM (image e) while the average height contrast of Au@SiO2 NPs decreases to 50–60 nm. These results illustrate a successful incorporation of Au@SiO2 NPs into all organic layers.

Schematic structures of the P3HT:PCBM BHJ-PSC devices with the incorporation of Au@SiO2 NPs (a) or bare Au NPs (b).

Figure 3 AFM images of PEDOT:PSS (I) and (PEDOT:PSS)/(P3HT:PCBM) bilayer (II) films without (a, d) and with bare Au NPs (b, e) or Au@SiO2 NPs (c, f) incorporation. Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

6

B. Chen et al.

A slight aggregation of doped Au@SiO2 NPs in the device (images c, f) will not affect the expected LSPR according to our simulations (see Supporting Information Figure S2). Different doping weight ratios of Au@SiO2 NPs were used in the experiments in order to determine the optimum doping ratio (see Scheme 1 and Supporting information Table S1), and it is about 2 wt% for the structure used in the present study. The current density–voltage (J–V) curves of the P3HT:PCBM BHJ-PSC devices with and without the incorporation of Au@SiO2 NPs are compared in Figure 4, and the measured parameters (Jsc, open-circuit voltage (Voc), FF, PCE) are summarized in Table 1, which also includes those of the reference and the device incorporated with bare Au NPs for comparison. The reference P3HT:PCBM BHJPSC device without Au@SiO2 NPs exhibited a PCE of 3.29%, which is comparable to the reported values with devices fabricated under similar conditions (chlorobenzene as solvent, all spin-coating procedures were carried out in air atmosphere)[13,37–41,53–57]. When 2 wt% Au@SiO2 NPs was incorporated into the devices, PCE dramatically enhances to 3.80% with an enhancement ratio of ∼16%. When bare Au NPs was incorporated, PCE enhances to 3.69% as well but the enhancement ratio is slightly lower (∼13%). To better reveal the factor(s) accounting for the enhancement of PCE with the incorporation of Au@SiO2 (Au) NPs, all photovoltaic parameters including Voc, Jsc and FF as well as series resistance (Rs) and shunt resistance (Rsh) were

analyzed and compared in Table 1. With the incorporation of Au or Au@SiO2 NPs, clearly Voc of the devices keep almost constant (0.61–0.62 V) compared to the reference P3HT: PCBM device, whereas both Jsc and FF increase. Besides, the obvious decrease of Rs and increase of Rsh are consistent with the increase of FF. It should be noted that, since the device structure with the penetration of Au@SiO2 NPs into all organic layers is unprecedented, an intuitive question whether or not such a penetration would affect the phase separation of P3HT and PCBM molecules and consequently the transports of charge carriers should be addressed. To study the influence of such a large NP and the role of Au core on the performance of PSC device, homogenous SiO2 NPs with a comparable size of ∼170 nm were synthesized following the literature method [58], and were doped into the PEDOT:PSS layer with the same doping ratio (2 wt%) under the identical conditions. The J–V curve measurement of this reference SiO2 NPsincorporated device indicates only marginal increase of the PCE from 3.29% to 3.32% (see curve d in Figure 4 and Table 1), which could be explained simply by the light scattering of SiO2 NPs. This experiment not only confirms the exclusive contribution of the Au core to the observed enhancement of PCE, but also suggests that unexpectedly the penetration of large NPs into all organic layers does not lead to a dramatic influence on the transports of charge carriers.

The effect of Au@SiO2 NPs on the enhancement of device performance

Figure 4 J–V curves of the P3HT:PCBM BHJ-PSC devices after thermal annealing with and without bare Au, SiO2 or Au@SiO2 NPs incorporation. The measurements were carried out under AM 1.5 illumination at an irradiation intensity of 100 mW cm−2.

To understand the increase of Jsc and FF upon the incorporation of Au or Au@SiO2 NPs, the dark J–V curve was measured (Figure 5), providing valuable information of the inherent electrical characteristics of the BHJ-PSC devices including the series resistance (Rs), the shunt resistance (Rsh), the leakage current, and the saturation current density [3,58,59]. As clearly seen from Figure 5, the dark current of Au NPs-incorporated device is dramatically higher than that of the reference device in both reverse and forward regions, and these results are in good accordance with the decreases of Rs for the Au NPs-incorporated device (see Table 1), which is resulted from the good electrical conducting properties of Au. For the case of Au@SiO2 NPsincorporated device, in the reverse region the dark current is comparable to that of the reference device, whereas it is much higher in the forward region. A plausible explanation

Table 1 Device performances of P3HT:PCBM BHJ-PSC devices with and without Au@SiO2 (Au) NPs incorporations after thermal annealing. Doping NPsa

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

ΔPCEb

Rsh (Ω cm2)

Rs (Ω cm2)

Reference Au NPs Au@SiO2 NPs SiO2 NPs

0.61±0.01 0.62±0.01 0.62±0.01 0.62±0.01

10.0±0.2 10.6±0.2 10.6±0.3 10.1±0.2

54±1 56±1 57±1 53±1

3.29±0.13 3.69±0.16 3.80±0.17 3.32±0.16

– 13% 16% 1%

10.2 9.8 8.7 11.5

341 385 405 323

a b

Doping ratio of NPs is 2 wt%. ΔPCE is the enhancement ratio of PCE relative to the reference P3HT:PCBM BHJ-PSC devices.

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

Surface plasmon enhancement of polymer solar cells

7

for this phenomenon is that SiO2 shell served as a block layer which limits the recombination of charge carriers. According to previous reports claiming the LSPR effect induced by Au NPs, the increase of Jsc has been mainly attributed to the enhanced light absorption as evidenced by UV–vis spectrocopic characterizations [13]. Indeed, according to the comparison of the UV–vis spectra of the devices with and without Au@SiO2 (Au) NPs (Figure 6), the light absorption is enhanced slightly for the bare Au NPsincorporated device relative to the reference one, while

Figure 5 J–V curves of the P3HT:PCBM BHJ-PSC devices in the dark with and without bare Au or Au@SiO2 NPs incorporation.

Figure 6 UV–vis spectra of P3HT:PCBM films with and without bare Au or Au@SiO2 NPs incorporation.

Table 2

the Au@SiO2 NPs-incorporated device exhibits much higher enhancement of light absorption. Noteworthy, despite of the difference on the enhancement ratio of light absorption for the bare Au and Au@SiO2 NPs-incorporated devices, the enhancement ratio of Jsc is nearly the same (see Table 1), this suggests that the Jsc enhancement mechanism is different for Au and Au@SiO2 NPs-incorporated devices. While the good electrical conducting property of Au NPs is presumed to be responsible for compensation of the lower light absorption enhancement, for Au@SiO2 NPs the role of SiO2 shell plays undoubtedly an important role in the light absorption enhancement as discussed further below. Considering that PSS as the crucial component of PEDOT: PSS (Baytron P) enabling its aqueous dispersibility may behave as a surfactant to partially coat Au or Au@SiO2 NPs, we fabricated several additional comparative devices to rule out the influence of PSS and to elucidate further the role of SiO2 shell on the efficiency enhancement of P3HT: PCBM BHJ-PSCs. It has been reported that some organic solvents like dimethylformamide (DMF) can be used to treat PEDOT:PSS leading to an efficiency enhancement mainly due to the improvement of its conductivity [39,60]. Thus we used DMF to treat Au or Au@SiO2 NPs prior to doping in PEDOT:PSS layer, and the characteristic photovoltaic parameters (Jsc, Voc, FF, PCE) are summarized in Table 2 (see Supporting information Figure S3 for the measured J–V curves). When Au NPs were dispersed in DMF without using the surfactant PSS and then doped into PEDOT:PSS layer, the fabricated P3HT:PCBM BHJ-PSC device shows the resistance behavior (i.e. J–V curve is a straight line). This is obviously caused by the severe aggregation of Au NPs which have superior electrical conducting properties leading to shortcircuiting. However, with the similar DMF-treatment for Au@SiO2 NPs, the PCE of the device incorporating Au@SiO2 NPs with DMF treatment increases to 4.22%, which is higher than that of the device with the DMF treatment only (4.06%, see Table 2). It was further enhanced by ∼11% relative to that of the Au@SiO2 NPs-incorporated device without the DMF-treatment (3.80%). These results reveal that the DMFtreatment provides a simple and facile strategy to enhance the efficiency of Au@SiO2 NPs-incorporated BHJ-PSC device. Since the possible influence of PSS on Au@SiO2 NPs could be almost eliminated by DMF in this case, prohibition of the aggregation of Au NPs by the SiO2 shell should account for the further efficiency enhancement. In this sense, SiO2 shell plays the role of “surfactant” fulfilling the good dispersion of Au NPs in both the PEDOT:PSS and P3HT:PCBM layers, which is crucial for LSPR to be further discussed below.

Device performances of P3HT:PCBM BHJ-PSC devices with and without DMF treatment of Au@SiO2 (Au) NPs.

Treatment

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

ΔPCEa

Rsh (Ω cm2)

Rs (Ω cm2)

Reference Reference+DMF Au NPs+ DMF Au@SiO2 NPs+DMF

0.61±0.01 0.63±0.01 0.03±0.02 0.63±0.01

10.0±0.2 11.6±0.2 4.8±0.5 11.8±0.2

54±1 56±1 21±5 57±1

3.29±0.13 4.06±0.18 0.03±0.03 4.22±0.18

– 23% −99% 28%

10.2 10.6 –b 8.5

341 476 –b 332

a b

ΔPCE is the enhancement ratio of PCE relative to the reference P3HT:PCBM BHJ-PSC devices. Not given by the measurement software.

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

8

B. Chen et al.

Since the roles of Au core and SiO2 shell were elucidated respectively above, here we then discuss the LSPR effect on the efficiency enhancement of BHJ-PSC device. Comprehensive understanding of LSPR effect of bare Au NPsincorporated in BHJ-PSCs has been well established in many previous reports, featuring the local enhancement of the electromagnetic field surrounding the Au NPs [13,24,61]. In our case, since SiO2 shell plays the role of “surfactant”, the superior electrical conducting properties of the Au NPs would be prohibited by the insulating SiO2 shell, thus the enhancements on both Jsc and FF should be purely attributed to the LSPR effect induced by the Au NP core. As discussed before, Rs decreases for both Au and Au@SiO2 NPs-incorporated devices when compared to that of the reference one (see Table 1), while the good conducting properties of Au NPs is directly correlated to the improvement of conductivity of PEDOT:PSS as evidenced by the decrease of Rs, for the case of Au@SiO2 NPs incorporation it is the pure LSPR effect that results in local enhancement of the electromagnetic field surrounding the NPs and consequently the conductivity of PEDOT:PSS improves as well. Besides, such a LSPR effect would presumably induce the interplay between the surface plasmons and excitons as proposed in Ref. [13], which may facilitate the exciton dissociation and contribute additionally to the efficiency enhancement.

Conclusions In summary, for the first time we incorporated large Au@SiO2 core/shell NPs into BHJ-PSC devices, resulting in efficiency enhancement by ∼16% which is originated from the LSPR effect induced by Au NPs. The optimum size of Au core (∼70 nm) and thickness of SiO2 shell (∼50 nm) were determined by simulations, and the large Au@SiO2 core/ shell NPs with the optimized size were experimentally prepared by a modified stepwise seed-mediated growth method. The as-prepared Au@SiO2 NPs were doped into the PEDOT:PSS layer of P3HT:PCBM BHJ-PSCs, resulting in that Au@SiO2 NPs penetrated into all organic layers and were partially embedded in Al cathode layer. Under the optimum doping ratio of 2 wt%, PCE of the P3HT:PCBM BHJPSC devices dramatically enhances to 3.80% with an enhancement ratio of ∼16%, which is higher than that obtained by incorporating bare Au NPs (∼13%). Although the efficiency enhancement was attributed to the increases of Jsc and FF for both cases of Au@SiO2 and Au NPs incorporations, the enhancement mechanism of Jsc is proposed to be different based on the UV–vis absorption spectrocopic study. The Au@SiO2 NPs-incorporated device exhibits a much higher pure LSPR-induced enhancement on both light absorption and Jsc, while the good electrical conducting property of Au NPs should be responsible for the enhancement of Jsc for the bare Au NPs-incorporated device. The SiO2 shell provides an insulating barrier to limit the good electrical conducting properties of Au NPs and plays the role of “surfactant” fulfilling the good dispersion of Au NPs in both the PEDOT:PSS and P3HT:PCBM layers as well. With the effectiveness of Au@SiO2 core/shell NPs on inducing the pure LSPR effect, this study provides not only a new insight into the LSPR effect but also a simple and

practical strategy toward efficiency enhancement of polymer solar cells.

Acknowledgments This work was partially supported by the National Basic Research Program of China (2010CB923300, 2011CB921400 (to S.Y.) and 2012CB922001 (to M.L. and Y.L.)), National Natural Science Foundation of China (Nos. 21132007 and J1030412), “100 Talents Program of CAS” from Chinese Academy of Sciences, and the Fundamental Research Funds for the Central Universities (WK2060140005).

Appendix A.

Supporting information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.03.011.

References [1] C. Deibel, V. Dyakonov, Reports on Progress in Physics 73 (2010) 096401. [2] B.C. Thompson, J.M.J. Fréchet, Angewandte Chemie International Edition 47 (2008) 58–77. [3] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nature Materials 4 (2005) 864–868. [4] H. Tang, G. Lu, L. Li, J. Li, Y. Wang, X. Yang, Journal of Materials Chemistry 20 (2009) 683–688. [5] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. McCulloch, C.S. Ha, Nature Materials 5 (2006) 197–203. [6] G. Dennler, M.C. Scharber, C.J. Brabec, Advanced Materials 21 (2009) 1323–1338. [7] P. Schilinsky, C. Waldauf, C.J. Brabec, Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors, Applied Physics Letters 81 (2002) 3885–3887. [8] P. Peumans, S.R. Forrest, Applied Physics Letters 79 (2001) 126–128. [9] F.-C. Chen, J.-L. Wu, Y. Hung, Applied Physics Letters 96 (2010) 193304. [10] D.H. Ko, J.R. Tumbleston, L. Zhang, S. Williams, J.M. DeSimone, R. Lopez, E.T. Samulski, Nano Letters 9 (2009) 2742–2746. [11] S.-I. Na, S.-S. Kim, J. Jo, S.-H. Oh, J. Kim, D.-Y. Kim, Advanced Functional Materials 18 (2008) 3956–3963. [12] H.A. Atwater, A. Polman, Nature Materials 9 (2010) 205–213. [13] J.L. Wu, F.C. Chen, Y.S. Hsiao, F.C. Chien, P. Chen, C.H. Kuo, M.H. Huang, C.S. Hsu, ACS Nano 5 (2011) 959–967. [14] M.D. Brown, T. Suteewong, R.S.S. Kumar, V. D’Innocenzo, A. Petrozza, M.M. Lee, U. Wiesner, H.J. Snaith, Nano Letters 11 (2011) 438–445. [15] M.-G. Kang, T. Xu, H.J. Park, X. Luo, L.J. Guo, Advanced Materials 22 (2010) 4378–4383. [16] W. Sha, W. Choy, W. Chew, Optics Express 18 (2010) 5993–6007. [17] D.H. Wang, D.Y. Kim, K.W. Choi, J.H. Seo, S.H. Im, J.H. Park, O.O. Park, A.J. Heeger, Angewandte Chemie International Edition 50 (2011) 5519–5523. [18] S. Vedraine, P. Torchio, D. Duché, F. Flory, J.-J. Simon, J. Le Rouzo, L. Escoubas, Solar Energy Materials and Solar Cells 95 (2011) S57–S64.

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

Surface plasmon enhancement of polymer solar cells [19] D. Duche, P. Torchio, L. Escoubas, F. Monestier, J.J. Simon, F. Flory, G. Mathian, Solar Energy Materials and Solar Cells 93 (2009) 1377–1382. [20] S.S. Kim, S.I. Na, J. Jo, D.Y. Kim, Y.C. Nah, Applied Physics Letters 93 (2008) 073307. [21] N. Yang, Q. Yuan, J. Zhai, T. Wei, D. Wang, L. Jiang, ChemSusChem 5 (2012) 572–576. [22] S.Y. Chou, W. Ding, Optics Express 21 (2013) A60–A76. [23] X. Li, W.C.H. Choy, L. Huo, F. Xie, W.E.I. Sha, B. Ding, X. Guo, Y. Li, J. Hou, J. You, Advanced Materials 24 (2012) 3046–3052. [24] C.C.D. Wang, W.C.H. Choy, C. Duan, D.D.S. Fung, E. Wei, F. X. Xie, F. Huang, Y. Cao, Journal of Materials Chemistry 22 (2012) 1206–1211. [25] F.-X. Xie, W.C.H. Choy, C.C.D. Wang, W.E.I. Sha, D.D.S. Fung, Applied Physics Letters 99 (2011) 153304. [26] X. Chen, L. Zuo, W. Fu, Q. Yan, C. Fan, H. Chen, Solar Energy Materials and Solar Cells 111 (2013) 10.1016/j. solmat.2012.12.016. [27] W. Wang, S. Wu, K. Reinhardt, Y. Lu, S. Chen, Nano Letters 10 (2010) 2012–2018. [28] D.C. Lim, K.D. Kim, S.Y. Park, E.M. Hong, H.O. Seo, J. Lim, K.H. Lee, Y. Jeong, Y.D. Kim, C. Song, Energy & Environmental Science 5 (2012) 9803–9807. [29] O. Stenzel, A. Stendal, K. Voigtsberger, C. Von Borczyskowski, Solar Energy Materials and Solar Cells 37 (1995) 337–348. [30] H. Qi, D. Alexson, O. Glembocki, S.M. Prokes, Nanotechnology 21 (2010) 215706. [31] C. Hagglund, M. Zach, B. Kasemo, Applied Physics Letters 92 (2008) 013113. [32] Z.W. Lei, M. Liu, W. Ge, Z.P. Fu, K. Reinhardt, R.J. Knize, Y.L. Lu, Applied Physics Letters 101 (2012) 083903. [33] M.A. Noginov, G. Zhu, A.M. Belgrave, R. Bakker, V.M. Shalaev, E.E. Narimanov, S. Stout, E. Herz, T. Suteewong, U. Wiesner, Nature 460 (2009) 1110–1113. [34] C.D. Keating, K.M. Kovaleski, M.J. Natan, Journal of Physical Chemistry B 102 (1998) 9404–9413. [35] K.C. Grabar, R.G. Freeman, M.B. Hommer, M.J. Natan, Analytical Chemistry 67 (1995) 735–743. [36] S. Liu, M. Han, Advanced Functional Materials 15 (2005) 961–967. [37] W. Zhang, Y. Xu, H. Wang, C. Xu, S. Yang, Solar Energy Materials and Solar Cells 95 (2011) 2880–2885. [38] W. Zhang, H. Wang, B. Chen, X. Bi, S. Venkatesan, Q. Qiao, S. Yang, Journal of Materials Chemistry 22 (2012) 24067–24074. [39] H. Wang, W. Zhang, B. Chen, S. Yang, Journal of University of Science and Technology of China 42 (2012) 775–784. [40] M. Chen, M. Li, H. Wang, S. Qu, X. Zhao, L. Xie, S. Yang, Polymer Chemistry 4 (2013) 550–557. [41] H. Wang, W. Zhang, C. Xu, X. Bi, B. Chen, S. Yang, ACS Applied Materials & Interfaces 5 (2013) 26–34. [42] H.C. Hulst, H. Van De Hulst, Light Scattering by Small Particles, Dover Publications, 1957. [43] P.B. Johnson, R.W. Christy, Physical Review B 6 (1972) 4370–4379. [44] E. Lioudakis, A. Othonos, I. Alexandrou, Y. Hayashi, Journal of Applied Physics 102 (2007) 083104. [45] H. Hoppe, N.S. Sariciftci, D. Meissner, Molecular Crystals and Liquid Crystals 385 (2002) 113–119. [46] A. Tao, P. Sinsermsuksakul, P. Yang, Nature Nanotechnology 2 (2007) 435–440. [47] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The Journal of Physical Chemistry B 107 (2003) 668–677. [48] M.M. Miller, A.A. Lazarides, The Journal of Physical Chemistry B 109 (2005) 21556–21565. [49] F. McLean, IEEE Transactions on Nuclear Science 27 (1980) 1651–1657. [50] K.C. Lee, J.G. Hwu, IEEE Electron Device Letters 18 (1997) 565–567.

9 [51] C.F. Bohren, Journal of Physics 51 (1983) 323–327. [52] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer Science + Business Media LLC, New York, 2007. [53] J. Liu, S. Shao, H. Wang, K. Zhao, L. Xue, X. Gao, Z. Xie, Y. Han, Organic Electronics 11 (2010) 775–783. [54] S. Sharma, G. Sharma, J. Mikroyannidis, Solar Energy Materials and Solar Cells 95 (2011) 1219–1223. [55] H. Gao, X. Zhang, Z. Yin, H. Tan, S. Zhang, J. Meng, X. Liu, Applied Physics Letters 101 (2012) 133903–133904. [56] Q. Wei, T. Nishizawa, K. Tajima, K. Hashimoto, Advanced Materials 20 (2008) 2211–2216. [57] W. Wang, H.B. Wu, C.Y. Yang, C. Luo, Y. Zhang, J.W. Chen, Y. Cao, Applied Physics Letters 90 (2007) 183512–183513. [58] M. Darbandi, R. Thomann, T. Nann, Chemistry of Materials 17 (2005) 5720–5725. [59] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Advanced Functional Materials 15 (2005) 1617–1622. [60] C. Winder, N.S. Sariciftci, Journal of Materials Chemistry 14 (2004) 1077–1086. [61] D.D.S. Fung, L. Qiao, W.C.H. Choy, C. Wang, E. Wei, F. Xie, S. He, Journal of Materials Chemistry 21 (2011) 16349–16356.

Boxue Chen is currently an undergraduate student in Department of Materials Science and Engineering of University of Science and Technology of China. His research is focused on synthesis of noble metal nanoparticles and their application in polymer solar cells.

Wenfeng Zhang received his BS degree in 2007 from Southwest Jiaotong University. He is currently a Ph.D. candidate under the supervision of Prof. Shangfeng Yang in University of Science and Technology of China. His work is focused on optimization of polymer solar cells.

Xinghao Zhou is currently an undergraduate student in Department of Materials Science and Engineering of University of Science and Technology of China. His research is focused on synthesis of novel conjugated polymers and their application in organic solar cells.

Xiao Huang is currently an undergraduate student in Department of Materials Science and Engineering of University of Science and Technology of China. His research is focused on applications of nanomaterials in energy devices such as solar cells and Li-ion battery.

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011

10

B. Chen et al. Xuemei Zhao received her BS degree in 2011 from Sichuan University. She is currently pursuing her master degree under the supervision of Prof. Shangfeng Yang in University of Science and Technology of China. Her research is focused on applications of nanomaterials in polymer solar cells.

Haitao Wang received his BS and master degrees from Anhui University (2009) and University of Science and Technology of China (2012), respectively. His research is focused on optimization of polymer solar cells.

Min Liu received her BS in Wuhan University of Technology (2002) and Ph.D. in Shanghai Institute of Ceramics, Chinese Academy of Sciences (2007), then worked as a Postdoc in Nanyang University of Technology and National University of Singapore (2007– 2010). She is now an Associate Professor of Department of Materials Science & Engineering, University of Science and Technology of China (USTC). Her research interests mainly focus on energy, optical and multiferroic materials.

Yalin Lu currently is the Deputy Director of CAS Hefei Center for Matter Science and Technology, Executive Head of Materials Science and Engineering Department and Director of CAS Key Laboratory of Materials for Energy Conversion at University of Science and Technology of China. He obtained his Ph.D. in Solid State Physics from Nanjing University in China in 1991. He was a Visiting Professor in Lawrence Berkeley National Laboratory in 1996, a Research Professor in EE with Tufts University (1998–2000) and a Professor in Physics with USAFA (2003-). His research group in USTC works on energy materials, THz metamaterials, optoeelctronics and lasers, and materials physics of complex oxides. Shangfeng Yang received his Ph.D. from Hong Kong University of Science and Technology (HKUST) in 2003. He then joined Leibniz-Institute for Solid State and Materials Research (IFW) Dresden, Germany as an Alexander von Humboldt (AvH) Fellow. Since 2005 he had been a Guest Scientist in IFWDresden. In December 2007 he joined University of Science and Technology of China (USTC) as an awardee of “Hundreds of Talents Programme” of Chinese Academy of Sciences. He is now a Professor of Hefei National Laboratory for Physical Sciences at Microscale & Department of Materials Science and Engineering. His current research interests include the synthesis and characterization of novel carbon nanostructures and fullerene-based organic solar cells.

Please cite this article as: B. Chen, et al., Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.03.011