Gold nanoparticle-decorated graphene oxides for
plasmonic-enhanced polymer photovoltaic
devices
†
Ming-Kai Chuang,aShih-Wei Lin,aFang-Chung Chen,*aChih-Wei Chub and Chain-Shu Hsuc
In this work, gold nanoparticle/graphene oxide (AuNP/GO) nanocomposites are synthesized and used as anodic buffer layers in organic photovoltaic devices (OPVs). The application of thiol-terminated polyethylene glycol as a capping agent prevents the aggregation of AuNPs on the GO surface and further improves the solubility and stability of these nanomaterials in solutions. When AuNP/GO nanomaterials served as the buffer layers, they introduced localized surface plasmon resonance (LSPR) in the OPVs, leading to noticeable enhancements in the photocurrent and the efficiencies of the OPVs. We attribute the primary origin of the improvement in device performance to local field enhancement induced by the LSPR. We anticipate that this study might open up new avenues for constructing plasmon-enhancing layers on the nanoscale to improve the performance of solar cells.
Introduction
Organic photovoltaic devices (OPVs) incorporating bulk heter-ojunctions are attractive systems for harvesting solar energy because of their lightweight and mechanicalexibility and for their ease of preparation in large-area panels at low cost.1–4 Moreover, the energy pay-back time—the length of time required for an energy-producing system or device to generate as much energy as that required for its manufacture—of an OPV can be amazingly short (on the order of several weeks) because its fabrication can be performed at low temperature, unlike the processes employed to prepare photovoltaic cells based on silicon or other inorganic materials.5Recently, OPVs with power conversion efficiencies (PCEs) of up to approximately 10% have been obtained through the development of novel materials having low band gaps and novel device structures.6 Further improvements in the efficiency and reliability will, however, be required if these OPVs are to compete with inorganic solar technologies.
The internal quantum efficiencies of state-of-art OPVs are approaching close to 100%.7Their photoactive layers are usually thin, typically on the order of 100 nm, so that they can perform high-efficiency charge collection while minimizing the degree of
charge recombination, but, thereby resulting in incomplete absorption of the incident solar irradiation. The use oflms that are too thick will inevitably decrease the charge collection effi-ciency, due to the low carrier motilities of organic materials. Therefore, the challenge remains to achieve OPVs with high absorption efficiencies when using a thin photoactive layer. To solve this problem, many approaches for trapping light have been proposed to improve the absorption efficiency without affecting the charge collection efficiency,8–17for example, using folded device architectures to increase the optical length8,9 or incorporating optical spacers to redistribute the optical elec-trical eld, thereby effectively improving the light harvesting efficiency.10,11 Among the techniques developed for trapping light, exploiting the surface plasmon resonance effect of metal nanostructures has been proved to be an effective strategy.12–16 For example, metal nanoparticles (NPs) that can trigger localized surface plasmon resonance (LSPR) have been introduced into OPVs to enhance their light-trapping ability; incorporating such metal NPs in either the photoactive layers13,14or their vicinity15–18 can improve device efficiencies. To simplify device fabrication, metal NPs are frequently blended with device buffer layers—for example, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS); although such simple structures have been used widely, PEDOT:PSS itself has highly acidic and hygroscopic properties, leading to poor long-term stability.19
Graphene oxide (GO), the oxidized form of graphene, consists of a two-dimensional nanosheet derivatized chemically with oxygen-containing functional groups (e.g., carbonyl, hydroxyl, and epoxide units).20–23GO can be prepared readily through chemical exfoliation from oxide graphite aer ultra-sonication, with the products being well dispersed in water.
aDepartment of Photonics and Display Institute, National Chiao Tung University,
Hsinchu 30010, Taiwan. E-mail: [email protected]
bResearch Center for Applied Sciences, Academic Sinica, Taipei, 11529, Taiwan cDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu 30010,
Taiwan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr05077g
Cite this:Nanoscale, 2014, 6, 1573
Received 23rd September 2013 Accepted 14th November 2013 DOI: 10.1039/c3nr05077g www.rsc.org/nanoscale
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Because GO is solution-processable, thermal or chemical reduction of GO is considered to be a promising approach for the preparation of graphene sheets in large quantities.20,23 Recently, GO and reduced GO (rGO) have been reported as effective interlayers when positioned between the electrodes and the photoactive layer in OPVs.24–29 For example, Li et al. reported the improved efficiencies of OPVs when employing solution-processable GOs as an alternative to PEDOT:PSS in the hole transport layer (HTL);25they found that the GO effectively suppressed the leakage current and lowered the rate of hole– electron recombination. In 2011, Yun et al. used the reduced form of GO to prepare a device having a greater stability than that of the corresponding device incorporating PEDOT:PSS.26
The presence of oxygen functionalities on the surface of GO provides reactive sites for chemical modication.21Therefore, GO can be considered as a promising nanoscale building block, having an ultrahigh surface area, for the preparation of novel composites featuring specic functionalities. Herein, we report the synthesis and characterization of gold nanoparticle/gra-phene oxide (AuNP/GO) nanocomposites for triggering LSPRs in polymer solar cells. We have found that the morphology of the AuNPs on the GO surface can be manipulated by controlling the amount of the capping agent for the NPs added during their synthesis. More importantly, these AuNP/GO nanocomposites can function effectively as anodic buffer layers in OPVs, providing observable enhancements in the photocurrent and overall device efficiency.
Results and discussion
Scheme 1 presents the synthesis of the AuNP/GO nano-composites.30,31First, a mixture of an aqueous GO solution and a HAuCl4 solution was heated at 80 C; a reducing agent,
sodium citrate, was added and then the mixture was main-tained at 80C for 4 h while stirring continuously. Aer cooling the resulting solution to room temperature, thiol-terminated poly(ethylene glycol) (PEG–SH) was added to the solution to modify the surfaces of the AuNPs. Aer stirring for 1 h, the solution was centrifuged and washed with DI water. The resulting AuNP/GO composites were dried through lyophiliza-tion. Fig. 1 and S1† display transmission electron microscopy
(TEM) images of AuNP/GO samples prepared with various amounts of PEG–SH. For all the samples, the AuNPs anchored on the GO surfaces were nearly spherical and highly uniform in size; the average size of the AuNPs attached to the GOs was approximately 18 nm. High-resolution images of the samples (Fig. S2†) conrmed the shapes and sizes of these NPs on the GO surfaces. Because the sizes of the AuNPs were almost identical, we suspect that the NP size was determined during therst step of sodium citrate-mediated reduction of the Au ions. The nucleation and growth of these NPs were complete prior to the addition of PEG–SH, which functioned merely to exchange the weakly capped citrate ions on the surface of the AuNPs. Many previous studies have indicated that the oxygen functionalities on GO surfaces behave as nucleation sites for AuNPs.30–32 In this present investigation, we found that, in addition to individual AuNPs anchoring on the GO surfaces, many free-standing NPs originally produced in solution aggre-gated with those attached to the GO surface. Aer the addition of PEG–SH, the presence of this capping agent prevented further aggregation of the AuNPs. The NPs that were not bound directly to the GO surfaces were easier to remove during the washing step.
In addition to differences in morphology, the solutions of AuNP/GO nanocomposites modied with PEG–SH exhibited better stability. Fig. 2a presents the results of solubility tests of the AuNP/GO nanocomposites. Although all of the nano-composites could be dispersed well in water aer sonication, we occasionally observed small amounts of black powders in solu-tions of the AuNP/GO nanocomposite prepared without PEG–SH aer aging for 15 days. Aer 40 days, however, precipitates began to appear. In contrast, the solutions containing the PEG–SH-modied AuNP/GO composites remained stable even aer aging for 40 days, suggesting their higher solubility.
We used UV-Vis absorption to further characterize the AuNP/ GO nanocomposites. As revealed in Fig. 2b, an aqueous solution of GO exhibited typical absorption peaks at 230 and 305 nm, corresponding top–p* transitions of the C]C bonds and n–p* transitions of the carbonyl groups, respectively.33Upon reduc-tion with sodium citrate, the spectra of the AuNP/GO nano-composites retained these two characteristic absorptions with no shis in their positions, suggesting that any changes in the GO structures were insignicant. Meanwhile, we observed absorption signals near 540 nm, indicating the formation of AuNPs on the GOs.
Raman spectroscopy is a highly effective tool for probing the structural changes of carbon materials.34,35Fig. 3 displays the Raman spectra of our AuNP/GO nanocomposites. Each spec-trum exhibits two characteristic peaks: a D band (ca. 1314 cm 1) corresponding to the breathing mode ofk-point phonons of A1g
symmetry and a G band (ca. 1600 cm 1) arising fromrst-order
scattering of the E2gphonon of sp2-hybridized C atoms.30The D
band could also originate from bonds between sp3-hybridized atoms, formed through oxidation, and other defects, such as vacancies, grain boundaries, and disordered carbon struc-tures.34The ratio of the intensities of the D and G bands (I
D/IG)
is oen used as a parameter for estimating the in-plane sp2
domain size of a graphene structure.33The I
D/IGratio of the GO
Scheme 1 Synthesis of AuNP/GO composites.
was 0.99; it reached approximately 0.90 aer the reduction reactions. Table 1 summarizes the ID/IG ratios. Because the
change in the intensity ratio was less than 10%, the Raman spectra indicated that our GOs were reduced only slightly by sodium citrate under the synthesis conditions we applied in
this study. These results are consistent with the UV-Vis spec-troscopic absorption data.
Fig. 4a displays the device structure incorporating the GO composites and the current density–voltage (J–V) characteristics
Fig. 1 (a–e) TEM images of AuNP/GO nanocomposites prepared at concentrations of PEG–SH of (a) 0, (b) 0.07, (c) 0.13, (d) 0.26, and (e) 0.52 mM. (f) High-resolution image of a typical AuNP; the PEG–SH concentration was 0.26 mM.
Fig. 2 Physical properties and characteristics of AuNP/GO nano-composites. (a) Photograph of an aqueous dispersion of the AuNP/GO nanocomposites prepared (right) with and (left) without PEG–SH after aging for 40 days. A precipitate is evident in the left-hand bottom. (b) UV-Vis absorption spectra of GO and AuNP/GO nanocomposites in water; inset: spectra in the range 450–650 nm.
Fig. 3 Raman spectra of GO and the nanocomposites prepared under different conditions.
Table 1 Peak intensity ratiosID/IGof AuNP/GO materials prepared under various conditions
Sample (PEG–SH concentration, mM) ID/IGratio
GO 0.99 AuNP/GO (0) 0.90 AuNP/GO (0.07) 0.90 AuNP/GO (0.13) 0.89 AuNP/GO (0.26) 0.91 AuNP/GO (0.52) 0.91
of the OPVs obtained under illumination with simulated solar light (AM 1.5G). The device prepared with neat GOs as the HTL exhibited a value of Vocof 0.57 V, a short-circuit current density
( Jsc) of 8.87 mA cm 2, and all factor (FF) of 0.65, resulting in a
PCE of 3.26%. For the devices incorporating AuNP/GO composites, we observed that the direct use of the as-synthe-sized AuNP/GO composites resulted in very poor device performance, probably because of a very strong back-scattering by the AuNPs. Therefore, we further blended the solution with neat GOs to reduce the amount of AuNPs in the devices. The concentrations of AuNP/GO and GO were adjusted to 0.110 and 0.165 mg mL 1, respectively. Aer decoration of the AuNPs on the GO surface, the value of Vocdecreased to 0.55 V, suggesting
that the anodic interface was affected. Although the value of Jsc
increased to 9.37 mA cm 2, the FF also decreased slightly to 0.62 (Table 2); as a result, the PCE was 3.25%. As mentioned above, the AuNP/GO nanocomposites prepared without PEG–SH exhibited lower solubility and stability in solution, possibly resulting in rather poor quality thinlms when deposited on the ITO surface. Therefore, we could not observe signicant device enhancement.
For the device prepared using the PEG–SH-modied nano-composites, however, the value of Vocremained unchanged at
0.57 V, suggesting that the anodic interface was not signicantly affected. The value of Jscand the FF both improved to 10.44 mA
cm 2and 0.67, respectively, resulting in an increase in the PCE to 3.98%. Note, however, that the concentration of AuNPs in the buffer layer also had to be tuned properly with neat GOs (Fig. S3†). This performance suggests that the PEG–SH-modi-ed AuNP/GO nanocomposites introduced their plasmonic
Fig. 4 (a) Current density–voltage (J–V) curves of devices prepared with various types of AuNP/GO composites as the anode buffer layer, under illumination at 100 mW cm 1(AM 1.5G). (b) IPCE curves of OPV devices fabricated with GO and the PEG–SH-modified AuNP/GO composite. (c) The curve of the difference in IPCE (DIPCE) after using the AuNP/GO anode buffer layer compared with the absorption spectrum of the PEG–SH-modified AuNP/GO composite. (d) J–V curves of OPV devices prepared with P3HT:ICBA blends, recorded under illumination.
Table 2 Electrical characteristics of devices fabricated with GO and various AuNP/GO nanocomposites as anode buffer layers
Device condition (PEG–SH concentration, mM) Voc (V) Jsc (mA cm 2) FF (%) PCE (%) Neat GOa 0.57 8.87 64.5 3.26 AuNP/GO (0)a 0.55 9.37 63.0 3.25 AuNP/GO (0.07)a 0.57 9.63 64.9 3.56 AuNP/GO (0.13)a 0.57 10.44 66.8 3.98 AuNP/GO (0.26)a 0.57 10.04 66.9 3.83 AuNP/GO (0.52)a 0.57 9.95 66.7 3.78 Neat GOb 0.81 8.61 57.7 4.02 AuNP/GO (0.13)b 0.81 9.94 62.7 5.05
aPhotoactive materials were P3HT and PCBM.bPhotoactive materials were P3HT and ICBA.
effect without affecting the electrical properties. Furthermore, we observed that the amount of PEG–SH used during the synthesis process affected the device performance. Fig. S4† displays the effect of the amount of PEG–SH on the J–V
char-acteristics of OPVs incorporating the AuNP/GO
nano-composites. We observe that the device performance, especially the photocurrent, improved through the use of PEG–SH. The optimal device was that constructed when the concentration of PEG–SH during the second synthesis step was 0.13 mM; a further increase in the amount of PEG–SH resulted in a decrease in the value of Jsc. Because we could not observe any signicant
differences in morphology among the corresponding TEM images, we infer that the excess of PEG–SH blocked the trans-portation of holes and decreased the charge collection efficiency.
To further investigate the LSPR effect, we measured the incident photon-to-electron conversion efficiencies (IPCEs) of the various devices; Fig. 4b displays the results. The photocur-rent increased aer incorporating the AuNPs. Comparing the curve of the increase in IPCE (DIPCE) with the extinction spectrum of the AuNP/GO nanocomposite (Fig. 4c), we observe that the wavelength regime in which the photocurrent increased coincides with the extinction range of the AuNPs, indicating that excitation of the LSPR was responsible for the improved efficiencies.15Note that the absorption spectrum of the PEG–SH-modied AuNP/GO composite was obtained by subtracting the absorption of GO from that of AuNP/GO nano-composites, which should reect the real plasmonic behavior of the nanocomposites.
To explore the potential plasmonic effects of our PEG–SH-modied AuNP/GO nanocomposites for OPV applications, we further employed other photoactive materials to build photo-voltaic devices. For example, the device Vochas been found to be
linked to the energy difference between the lowest unoccupied molecular orbital (LUMO) of the electron acceptor and the highest occupied molecular orbital (HOMO) of the electron donor in the polymer blends.36,37Therefore, indene–C
60
bisad-duct (ICBA) has a higher-energy LUMO than that of PCBM and can increase values of Vocand PCEs.36,37Fig. 4d displays the J–V
curves of devices fabricated with P3HT:ICBA blends, obtained under standard AM 1.5G illumination conditions. We observed similar improvements in device performance aer using the PEG–SH-modied AuNP/GO nanocomposite as the HTL. The solar cell incorporating the GOs exhibited a value of Vocof 0.81
V, a value of Jscof 8.61 mA cm 2, and an FF of 0.58, yielding a
PCE of 4.02%. As expected, aer decoration of the GO with AuNPs, the value of Jscand the FF both improved to 9.94 mA
cm 2 and 0.63, respectively, while the value of Voc remained
unchanged, thereby increasing the PCE to 5.05%. The IPCE spectra also revealed a similar trend for the photocurrent (Fig. S5†). These results suggest that PEG–SH-modied AuNP/ GO nanocomposites have great potential for OPV applications. Previously, we blended AuNPs into PEDOT:PSS HTLs to improve device efficiencies;15we attributed these improvements in device performance to plasmonic effects. To conrm these effects, we examined the steady state photoluminescence (PL) of
P3HT:PCBM thin lms prepared on various HTLs. The peak
uorescence intensity of the sample prepared on PEG–SH-modied AuNP/GO nanocomposites was enhanced by approxi-mately 55% (Fig. 5). Because the resonance peak of the
nano-composite exhibited signicant overlapping with the
absorption spectrum of P3HT, we infer that the enhanced PL probably resulted from an increased degree of photon absorp-tion due to the near-eld effect. Local enhancement of the electromagnetic eld surrounding the AuNPs can assist with energy dissipation, thereby increasing the number of excitons.15 On the other hand, the scattering cross-section of the AuNP/GO nanocomposites was much lower in the wavelength range cor-responding to the PL spectrum of P3HT. Therefore, the far-eld effect should contribute less to the photocurrent enhancement.
Conclusions
In conclusion, we have synthesized AuNP/GO nanocomposites that improve the efficiency of OPVs by triggering LSPR. The AuNP/GO nanocomposites prepared with a weak capping agent possessed aggregated AuNPs on the GO surface. In contrast, the use of a stronger capping molecule, PEG–SH, prevented aggre-gation of the AuNPs and improved the solubility. When these nanomaterials served as the HTL, they introduced LSPR effects in the OPVs, leading to noticeable enhancements in the photocurrent and the efficiencies of the OPV devices under illumination at 1 sun. We attribute the primary origin of the improvement in device performance to localeld enhancement induced by the LSPR. We anticipate that this study might open up new avenues for constructing plasmon-enhancing layers on the nanoscale to improve the performance of solar cells.
Experimental section
Synthesis of AuNP/GO composites
GO solutions were purchased from UniRegion Bio-Tech. The AuNP/GO composites were synthesized following the procedures reported in the literature.30,31First, an aqueous solution of GO (0.275 mg mL 1, 3 mL) was added to a solution of HAuCl4(0.047
mg mL 1, 10 mL) and then the mixture was aged for 30 min to
Fig. 5 PL spectra of P3HT thinfilms prepared on GO and AuNP/GO substrates.
promote the interaction of the gold ions with the GO surfaces. The solution was heated at 80C before a solution of sodium citrate (0.085 mol dm 3, 188mL), a reducing agent, was added. The mixture was kept at 80C with continuous stirring for 4 h. Next, the solution was cooled to room temperature; while cool-ing, PEG–SH (molecular weight: 294.1 g mol 1) was blended into
the solution to modify the surface of the AuNPs. Aer stirring for 1 h, the resulting solution was centrifuged (6000 rpm) and washed three times with DI water to remove any free gold ions. Finally, the nanocomposites were dried through lyophilization. Characterization
Absorption spectra were measured using an UV-Vis-near-IR spectrometer (PerkinElmer Lambda 950). Raman spectra were acquired on a Horoba high-resolution confocal Raman micro-scope equipped with a HeNe (632.8 nm) laser as the light source. PL spectra were obtained using an Ocean Optics spec-trometer; a 532 nm laser was used as the excitation light source. Device fabrication and testing
OPV devices were fabricated on patterned indium tin oxide (ITO)-coated glass substrates. The ITO-(ITO)-coated glass was treated with UV ozone prior to use. The GO and/or AuNP/GO compositions were then spin-coated onto the ITO substrates and then the samples were baked at 180C for 20 min. To improve thelm quality, a mixed solvent system—H2O and MeOH (1 : 2, v/v)—was adopted
to dissolve the GO materials. The photoactive layer, prepared from a blend of P3HT and PCBM (1 : 1, w/w) in 1,2-dichlorobenzene, was spin-coated onto the GOs or AuNP/GO layer. [Devices were also prepared in which the acceptor (PCBM) was replaced by another C60 derivative, ICBA, which possesses a higher LUMO
energy level,36,37to improve the open-circuit voltage.] The wetlm underwent solvent annealing in a glass Petri dish for at least 20 min.38,39Prior to thermal evaporation of the cathode, consisting of Ca (30 nm) and Al (100 nm), the active blend was thermally annealed at 110 C for 15 min. The J–V characteristics of the devices were measured using a Keithley 2400 source-measure unit. The photocurrent response was obtained under illumination from a 150 W Thermal Oriel solar simulator (AM 1.5G). The intensity of the light source was corrected using a standard Si photodiode.40 The IPCE measurement system (Enli Technology) comprised a quartz tungsten halogen (QTH) lamp as the light source, a monochromator, an optical chopper, a lock-in amplier, and a calibrated silicon photodetector.
Acknowledgements
We thank the National Science Council of Taiwan (Grant no. NSC 102-2221-E-009-130-MY3, NSC 102-3113-P-009-004, and NSC 101-2628-E-009-008-MY3) and the Ministry of Education of Taiwan (through the ATU program) fornancial support.
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