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Impact of the Incorporation of Au Nanoparticles into Polymer/Fullerene Solar Cells† K. Topp,‡,| H. Borchert,*,§,| F. Johnen,§ A. V. Tunc,§ M. Knipper,§ E. von Hauff,§ J. Parisi,§,| and K. Al-Shamery‡,| Institute of Pure and Applied Chemistry, UniVersity of Oldenburg, Carl-Von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany, Department of Physics, Energy and Semiconductor Research Laboratory, UniVersity of Oldenburg, Carl-Von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany, and Center of Interface Science (CIS) ReceiVed: October 26, 2009; ReVised Manuscript ReceiVed: NoVember 24, 2009
The addition of small amounts of dodecylamine-capped Au nanoparticles into the active layer of organic bulk heterojunction solar cells consisting of poly(3-octylthiophene) (P3OT) and C60 was recently suggested to have a positive impact on device performance due to improved electron transport. This issue was systematically further investigated in the present work. Different strategies to incorporate colloidally prepared Au nanoparticles with a narrow size distribution into organic solar cells with the more common donor/acceptor system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were pursued. Au nanoparticles were prepared with either P3HT or dodecylamine as ligands. Additionally, efforts were undertaken to incorporate nearly ligand-free Au nanoparticles into the system. Therefore, a procedure was successfully developed to remove the dodecylamine ligand shell by a postpreparative ligand exchange with pyridine, a much smaller molecule that can later partly be removed from solid films by annealing. However, for all types of nanoparticles studied here, the performance of the P3HT/PCBM solar cells was found to decrease with the Au particles as an additive to the active layer, meaning that adding Au nanoparticles is not a suitable strategy in the case of the P3HT/PCBM system. Possible reasons are discussed on the basis of detailed investigations of the structure, photophysics and charge transport in the system. 1. Introduction Organic solar cells based on a bulk heterojunction of conductive polymer and fullerene derivates have high potential for large-scale application due to the possibility to use flexible substrates and to employ low-cost production processes such as printing technologies.1,2 However, the efficiency of this class of solar cells is still limited and reaches about 5-6% in the best cases.3,4 The device performance is strongly dependent on the morphology of the bulk heterojunction layer.3,5-11 On the one hand, the electron donor and acceptor materials need to be finely intermixed with domain sizes of only a few tenths of nanometers to take into account the low exciton diffusion length and thus to enable efficient charge separation at the donor/ acceptor interface.6-8 On the other hand, the domains of each phase need to form connected networks (percolation pathways) to enable efficient hole transport through the donor (polymer) phase and electron transport through the acceptor (fullerene) phase.6-8 Kim and Carroll reported a few years ago that adding small amounts of Ag or Au nanoparticles (∼5-6 nm in diameter with a standard deviation of ∼20%) with a ligand shell of dodecylamine (DDA) to P3OT/C60 blends is a suitable way to improve the device performance and attributed this effect to improved electron transport in the system.12 Although of strong potential ‡
Institute of Pure and Applied Chemistry, University of Oldenburg. Center of Interface Science (CIS). Department of Physics, Energy and Semiconductor Research Laboratory, University of Oldenburg. † Part of the special issue “Green Chemistry in Energy Production Symposium”. * Corresponding author. E-mail address: holger.borchert@ uni-oldenburg.de. |
§
interest, this issue has not been further investigated, possibly because the concentration of metal nanoparticles (up to ∼3 wt % with respect to P3OT) and the resulting effects were rather small in the initial work.12 Furthermore, current research in organic photovoltaics concentrates more on other donor/acceptor systems such as P3HT/PCBM.2,13-16 Another motivation to introduce Au nanoparticles into organic solar cells relates to their absorption properties. Au nanoparticles exhibit a strong plasmon absorption resonance in the visible range.17-21 Possibilities to use coatings of Ag or Au nanoparticles to enhance the absorption of thin film solar cells due to surface plasmons and light-trapping are topics of current research activities.22-24 Furthermore, the introduction of a thin Au layer into dyesensitized solar cells was proposed as a new concept to physically separate the processes of light-absorption and charge carrier transport.25,26 In the present work, the idea of Kim and Carroll was revisited, methods to incorporate suitable amounts of colloidally prepared and well-defined Au nanoparticles into blends of the more commonly used P3HT/PCBM system were developed, and the effect on the performance of corresponding bulk heterojunction solar cells was carefully evaluated. Different types of Au nanoparticles were used for this purpose. First, Au particles directly stabilized with P3HT were employed. With this approach Au nanoparticles can be added to the donor/acceptor system without simultaneously adding any other organic components. Furthermore, one could expect that P3HT ligands strongly bound to the Au particles might lead to direct electron transfer from the polymer to the Au. Next, DDA-capped particles were used as in the earlier work on P3OT/C60.12 In a third attempt, a method was developed to replace the ligand shell of DDA-capped particles with pyridine with the intention
10.1021/jp910227x 2010 American Chemical Society Published on Web 12/23/2009
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to incorporate larger amounts of Au nanoparticles without a thick organic ligand shell into the P3HT/PCBM blends. With all types of nanoparticles, solar cells were prepared and characterized by current-voltage measurements. For deeper understanding, additional characterization methods were applied. UV-vis spectroscopy was used to investigate molecular order in the polymer phase, organic field effect transistors were prepared to study charge carrier mobility in the blends, and photoinduced absorption spectroscopy was applied to gain insight into possible charge transfer processes. 2. Experimental Section 2.1. Synthesis of Au Nanoparticles Stabilized with P3HT. Au nanoparticles stabilized with P3HT were prepared following a procedure developed by Zhai and McCullough.27 The procedure is briefly described here for P3HT-stabilized Au nanoparticles containing 3 and 16 wt % Au, respectively. Tetraoctylammonium bromide (0.02 mmol for 3 wt % and 0.0815 mmol for 16 wt %, respectively) was dissolved in toluene (2 mL, 3 mL), and an aqueous solution of HAuCl4 with a concentration of 30 mmol/mL was added (0.20 mL, 0.82 mL). After the phase transfer to the organic phase was completed, P3HT (40 mg, 14 mg) dissolved in toluene (3.5 mL) was added. The reaction mixture was stirred for 30 min before adding sodium borohydride (0.06 mmol, 0.25 mmol) as a reducing agent. After formation of the Au nanoparticles, indicated by a color change to dark brown, and stirring for 1 h the organic phase was separated and cleaned by precipitation with methanol. The cleaned Au nanoparticles were dried to constant weight under reduced pressure. 2.2. Synthesis of Au Nanoparticles Stabilized with DDA. Au nanoparticles stabilized with dodecylamine as ligands were prepared following a procedure developed by Jana and Peng for noble metal nanoparticles.28 Briefly, 1.0 mmol of DDA solved in 2.5 mL of toluene was added to 0.025 mmol goldtrichloride (AuCl3), which was dissolved by ultrasonication. Next, 0.1 mmol of tetrabutylammonium borohydride (TBAB) was dissolved in a solution of 0.1 mmol didodecyldimethylammonium bromide (DDAB) in 1 mL of toluene. This solution was added to the DDA/AuCl3 solution under vigorous stirring. An immediate color change from orange to dark red indicated the formation of the Au nanoparticles. After some further stirring, the Au nanoparticles were cleaned by precipitation with methanol. The dark precipitate was solved in toluene, and the solution was then dried to constant weight under ambient conditions. For some experiments this synthesis procedure was successfully scaled up by a factor of 8.5. 2.3. Ligand Exchange with Pyridine. To obtain Au nanoparticles without a thick organic ligand shell, a ligand exchange procedure with pyridine was developed for Au nanoparticles stabilized after synthesis with DDA ligands. The DDA-capped nanoparticles were in this case prepared as follows: DDAB was dissolved in toluene with a concentration of 100 mg/mL. Next, 0.025 mmol of AuCl3 and subsequently 0.50 mmol of DDA were dissolved by ultrasonication in 2.5 mL of the DDAB solution. Separately, 0.10 mmol of TBAB was dissolved in 1 mL of DDAB solution and then rapidly injected to the yellow AuCl3/DDA/DDAB solution under vigorous stirring. An immediate color change to dark brown indicated the formation of the Au nanoparticles. After further stirring, 2 mL of the reaction mixture was dried under ambient conditions. For cleaning, the residue was washed with methanol twice. Using these carefully cleaned Au nanoparticles, different procedures were applied to remove the DDA shell by a ligand
Topp et al. exchange with pyridine. In the first procedure (treatment Pyr1) the cleaned Au nanoparticles were dissolved in 1.5 mL of pyridine and stirred for 3 h at 65 °C and then several more hours at ambient temperature. The resulting dark precipitate was isolated and, as it was not soluble in hexane or pyridine anymore, suspended in hexane for characterization by TEM. The second procedure (treatment Pyr2) also implied solving the cleaned Au nanoparticles in 1.5 mL of pyridine, but only stirring for 30 s at 65 °C, whereas there was an immediate change in color from red to violet-blue. The Au nanoparticles were then precipitated by pouring the reaction mixture in 9 mL of hexane. After the precipitate was isolated, it was redissolved in pyridine with the help of ultrasonication to form a blue solution. In the third procedure (treatment Pyr3) the cleaned Au nanoparticles dissolved in 1.5 mL of pyridine were stirred for 15 min at ambient temperature, still showing a red solution. The precipitation with 9 mL of hexane resulted in an instant color change to blue. After the precipitate was isolated, it was redissolved in pyridine to form a blue solution. Finally, in the fourth procedure (treatment Pyr4) the cleaned Au nanoparticles were dissolved in 1.5 mL of pyridine and stirred for 15 min at ambient temperature. For the precipitation only 1.5 mL of hexane were added slowly to the reaction mixture, resulting in a color change from red to blue. After the precipitate was isolated, it was redissolved in 200 µL of pyridine to form a blue solution. 2.4. Characterization of Au Nanoparticles. A Zeiss EM 902A microscope operating at 80 kV was used for transmission electron microscopy (TEM), a Varian Cary 100 spectrometer was used for UV-vis absorption spectroscopy, and thermogravimetric analysis (TGA) was performed with a TGA/SDTA851e instrument from Mettler-Toledo. 2.5. Preparation of Solar Cells. Laboratory solar cells were fabricated in a layered structure. ITO-coated glass substrates (15 mm × 15 mm) were structured by removing the ITO on half of the substrate with nitrohydrochloric acid. After cleaning with different solvents and plasma etching, a layer of PEDOT: PSS was deposited as a hole transport layer by spin-coating. Afterwards, the substrates were transferred into a nitrogen-filled glovebox and annealed at 180 °C for 10 min. Next, the active layer was deposited by spin-coating, followed by annealing at 140 °C for 10 min. The devices were finished by thermal evaporation of three Al contacts as cathodes on top of the active layer, so that each substrate contained three individual solar cells. In some cases the Al cathode was deposited before the annealing step at 140 °C. The active area, defined as the overlap of the ITO anode with the Al cathode, ranged from 0.04 to 0.09 cm2. In the case of P3HT/PCBM bulk heterojunction solar cells the active layer consisted of a 1:1 (wt:wt) solution of P3HT (regioregular, purchased from Rieke Metals) and PCBM (purchased from Solenne) in chlorobenzene. The absolute concentrations were 1 wt % of P3HT and 1 wt % of PCBM, giving an overall concentration of 2 wt % P3HT/PCBM in chlorobenzene. In the case of the Au-containing solar cells, the composition of the active layer is mentioned in the respective cases. The weight ratio of P3HT and PCBM was kept constant at 1:1 throughout the work. 2.6. Characterization of Solar Cells and Au-Containing P3HT/PCBM Blends. Solar cells were characterized by standard current-voltage (I-V) measurements using a class B solar simulator (K. H. Steuernagel) for measurements under AM 1.5 conditions. Spectral mismatch due to calibration of the intensity of the solar simulator with a calibrated Si solar cell
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was not taken into account. The thickness of the active layer was measured with a profilometer (Veeco Dektak 6M). Charge carrier mobilities were measured in organic field effect transistors (OFETs) that were prepared using highly doped p-Si wafers as the gate electrode. The gate insulator was thermally grown SiO2 with a thickness of ∼200 nm. Substrates were cleaned with acetone and isopropyl alcohol in an ultrasonic bath and finally with O2 plasma prior to use. The source and drain contacts were patterned on the SiO2 layer using photolithography. The semiconducting layer was applied to the substrates and annealed following the same recipe used for the solar cells. The OFETs were measured under vacuum (10-6 mbar) and in the dark. Photoinduced absorption (PIA) spectroscopy was carried out with a PIA-setup, which is described in more detail elsewhere.29 Briefly, a chopped cw-Nd:YAG laser (532 nm) was used for excitation, and continuous white light illumination was done with a halogen lamp. Both light sources were focused on the sample, and the transmission was measured using a Si-diode (UV-vis-NIR) and a cooled InSb-detector (IR). Appropriate filters were used to suppress second order interference effects. All PIA measurements were performed at 110 K using a chopping frequency of ∼120 Hz. 3. Results and Discussion 3.1. Solar Cells with Au Nanoparticles Directly Stabilized with P3HT. A promising approach to incorporate Au nanoparticles into P3HT/PCBM solar cells is to use nanoparticles directly stabilized with P3HT and not possessing any other ligand shell. Such Au nanoparticles, in the following named Au/P3HT, can be synthesized by colloidal chemistry,27 and Figure 1a shows a typical TEM image of the obtained particles. The particles have an average diameter of 3.7 nm with a standard deviation of 35%. The size distribution is not as narrow as can typically be achieved with other ligands in the synthesis of Au nanoparticles,28 but the approach has the advantage that Au particles can be added to the P3HT/PCBM blends without simultaneously introducing any other ligand molecules into the system. Furthermore, one could expect the strong bonds between the P3HT and Au nanoparticles to enable efficient electron transfer from the polymer to the Au nanoparticles at the P3HT/ Au interface. The concentration of Au nanoparticles that can be stabilized with P3HT as ligands turned out to be limited. At high Au concentration, precipitation was observed. Thermogravimetric analysis was used to determine the weight fraction of Au in the P3HT-stabilized particles. Figure 1b shows the results for a highly concentrated sample. Upon annealing under nitrogen atmosphere, part of the organics was lost at ∼480 °C, and after switching to an oxygen atmosphere at 600 °C, the total weight loss increased to 84%, meaning that a metal content of 16 wt % could be achieved with this synthesis method. The influence of Au nanoparticles on the performance of organic solar cells was now studied by preparing solar cells with blends of Au/P3HT and PCBM as active layer and reference cells with blends of pure P3HT and PCBM in parallel. Chlorobenzene was used as solvent, and the total amount of P3HT was the same in both cases. Several substrates with each of them carrying three solar cells were prepared with all of the blends to get enough statistics. Figure 2a shows representative I-V curves, and Table 1 summarizes characteristic values. Without illumination, the typical behavior of a rectifying diode was observed for all of the cells. Under illumination at standard conditions, i.e., illumination with 100 mW/cm2 AM 1.5 radia-
Figure 1. (a) Typical TEM image of Au nanoparticles stabilized with P3HT. (b) Thermogravimatric analysis of a Au/P3HT sample. At 600 °C, the atmosphere was switched from N2 to O2.
tion, all cells showed a clear photoresponse. The reference cells reached open circuit voltages (VOC, voltage at zero current) of ∼0.6 V and short circuit current densities (JSC, current density at zero voltage) of ∼8 mA/cm2. The fill factors (FF), which describe the shape of the I-V curves (FF ) 1 for an ideal solar cell), were about 0.55, and power conversion efficiencies were about 2.5%. Solar cells containing 3 wt % Au showed a slightly lower performance, the efficiency being decreased to about 1.5%. The metal loading of 3 wt %, which is of similar order of magnitude as in the earlier work by Kim and Carroll,12 corresponds to a quite low molar ratio of Au nanoparticles to PCBM molecules of about 1:50000. The introduction of higher amounts of Au nanoparticles (16 wt %) led to a strong decrease in the solar cell performance. All parameters, the current density, the open circuit voltage, and the fill factor were decreased. Thus, a beneficial effect of Au nanoparticles could not be confirmed in this series of experiments. A reason for the decreased performance might be that the properties of the P3HT changed during the synthesis of Au nanoparticles, because water as well as HAuCl4 (oxidizing agent) and NaBH4 (reducing agent) were present during the synthesis of Au/P3HT. To study the influence of these chemicals, we performed a blank test where P3HT was exposed to conditions similar to the synthesis of Au/P3HT, but with H2O2 instead of the Au precursor HAuCl4. The resulting P3HT solution was also blended with PCBM and processed to solar cells (labeled “P3HT/PCBM+chem.” in Figure 2a and Table 1). The performance was not found to be decreased with respect to the reference cells. In contrast, the results obtained were even slightly better, but we do not consider this effect as significant due to a too limited number of experiments performed with the
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Figure 2. (a) IV curves in the dark (solid lines) and under 100 mW/ cm2 AM 1.5 irradiation (symbols) of representative solar cells with different active layers. Two reference samples with a pure P3HT/PCBM blend (black squares, gray squares), two samples containing different amounts of Au (prepared with P3HT as stabilizer; 3 wt %, blue diamonds; 16 wt %, red circles), and a blank sample where P3HT was exposed to some chemicals used in the synthesis of Au nanoparticles (green triangles) are compared. (b) UV-vis spectra of the same samples as in part (a). These spectra were measured of the complete solar cells, i.e., on top of the PEDOT:PSS/ITO-coated glass substrates. The spectra were corrected for a vertical offset (set to zero at 800 nm) and normalized to the maximum around 500 nm.
blank samples. However, we can conclude that the chemicals present during the synthesis of Au/P3HT are unlikely to change the properties of P3HT in a manner detrimental for the performance of the solar cells. More likely, the Au nanoparticles themselves seem to lower the performance of the organic solar cells. As another reason for the reduced performance, it might be possible that the presence of Au nanoparticles, especially of particles with a very broad size distribution, disturbs the ordering of the P3HT phase, which in turn can influence the hole transport properties. Therefore, we studied structural properties of the active layer by UV-vis spectroscopy and investigated also the hole transport in these materials. Figure 2b shows absorption spectra of the same solar cells that were characterized by I-V measurements in Figure 2a. All spectra show the typical shape for P3HT/PCBM films with a maximum around ∼500 nm and two shoulders at ∼555 and ∼605 nm, which indicate the establishment of molecular order (π-π stacking) in the P3HT phase.16,30 In the case of Aucontaining cells, distinct features (Plasmon band) corresponding to the Au nanoparticles could not be resolved, meaning that the absorption is still clearly dominated by the polymer at ∼16 wt % Au content (corresponding to a volume fraction of ∼0.4%
Topp et al. Au in the composite film). This is in good agreement with another study where even absorption spectra of P3HT/Au nanoparticle films with a weight ratio of 1:1 were still dominated by the polymer absorption.31 In the case of the P3HT/PCBM solar cell labeled “reference 2”, the shoulders indicative for molecular order appear as pronounced as typically observed in the literature for P3HT/ PCBM blends,16,30 whereas the cell labeled “reference 1” had obviously a less ordered structure. A preparative reason for this variance cannot be deduced from the limited number of experiments carried out within this study. However, it is interesting to note that both of the cells had approximately the same efficiency (see Table 1), with the active layer being thicker in the case of “reference 1”, which probably compensated the deficiency of lower molecular order. Moreover, it is noteworthy that the presence of the chemicals used during synthesis of the Au/P3HT nanoparticles does not seem to affect the ability of P3HT to form an ordered phase, because the corresponding UV-vis spectrum (“P3HT/PCBM + chem.”) is almost identical to that of the well-ordered reference cell. Turning now to the Au-containing solar cells, the shoulders indicative for structural order are almost as pronounced as in the case of the well-ordered reference solar cell. Therefore, a disturbed order in the Aucontaing solar cells is unlikely to be the reason for the decrease in efficiency. This was confirmed by field effect measurements of the hole mobility in the blends. Figure 3 shows the I-V characteristics of a P3HT OFET and an Au/P3HT OFET for gate voltages between 0 and -60 V. Though it can be seen that the P3HT OFET has a slightly larger current, the field effect mobility of holes in both cases was found to be ∼1 × 10-4 cm2/(V s). The mobility was determined from the slope of the transfer characteristics according to a procedure described in ref 32 at a gate voltage of -20 V and a constant drain-source voltage of -5 V. These results indicate that the Au nanoparticles do not influence the macroscopic transport properties in P3HT. In conclusion from these experiments the structure of the polymer phase and the hole transport do not seem to be strongly influenced by the presence of the Au nanoparticles. As there are reports on electron transfer between photoexcited chromophores and small Au nanoparticles (diameter below ∼5 nm),33 photoinduced absorption spectroscopy (PIA) was used, if charge transfer is also possible between the P3HT and the Au nanoparticles in our case. This method measures differences in the absorption before and after light excitation and is capable of detecting the light-induced appearance of long-lived polarons (lifetimes in the range of ∼1 µs to ∼1s), i.e., isolated charges on the donor or acceptor entity after a charge transfer.34 Figure 4 shows PIA spectra of a pure P3HT film and a P3HT film containing ∼15-20 wt % Au nanoparticles. (It is noted that the metal content was not exactly determined in this case. Au (47 wt %) was used in the synthesis, but part of the material precipitated during synthesis. The indicated value of 15-20 wt % is a rough estimation based on TGA data obtained for another sample.). Primarily one signal can be observed at ∼1.24 eV in both cases. The assignment of this feature is not exactly clear. Singlet excitons should give a signal at ∼1.1 eV, and polarons should give a pair of peaks at ∼1.25 and ∼0.4 eV.34,35 Although the data are quite noisy, one can conclude that indications for Au-enhanced formation of long-lived polarons in the P3HT phase are absent. Since only long-lived polarons can be detected by this method, it can, however, not be excluded that an electron transfer from the polymer to the Au takes place, but that this or another electron is rapidly transferred back and recombines with
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TABLE 1: Characteristic Values of the Solar Cells Represented in Figure 2a under 100 mW/cm2 AM 1.5 Irradiationa sample
dact.layer (nm)
VOC (V)
JSC (mA/cm2)
FF
η (%)
act. area (cm2)
tempering before/after evaporation
P3HT/PCBM (ref 1) P3HT/PCBM (ref 2) 3 wt % Au/P3HT/PCBM 16 wt % Au/P3HT/PCBM P3HT/PCBM + chem
∼90 ∼60 ∼80 ∼75 ∼60
0.56 0.59 0.54 0.40 0.61
8.1 8.5 6.0 2.6 11.0
0.56 0.51 0.45 0.35 0.49
2.5 2.5 1.5 0.4 3.3
0.07 0.07 0.07 0.07 0.09
after before after before after
a Two reference samples with a pure P3HT/PCBM blend, two samples containing different amounts of Au (prepared with P3HT as stabilizer) and a blank sample where P3HT was exposed to some chemicals used in the synthesis of Au nanoparticles are compared. dact.layer is the thickness of the active layer, VOC is the open circuit voltage, JSC is the short circuit current density, FF is the fill factor, η is the power conversion efficiency, “act. area” denotes the active area, and the last column indicates whether the cells were annealed at 140 °C before or after deposition of the Al contact.
Figure 3. I-V characteristics for a P3HT OFET and an Au/P3HT OFET. The gate voltage was varied from 0 to -60 V as shown by the arrow.
Figure 4. PIA spectra of a pure P3HT film without annealing (black squares) and P3HT films containing ∼15-20 wt % of Au nanoparticles without annealing (blue circles) and after annealing for 60 min at 100 °C (red diamonds).
the hole on the polymer. In this case, the presence of Au nanoparticles would provide a pathway for quenching of the excited state (excitons in the polymer). This could be a reason for the decrease of the efficiency. A similar process was also discussed to play a role at the interface of Au with organic dyes.26 On the other hand, photoluminescence quenching experiments performed by Nicholson et al. did not provide evidence for quenching of the excited state in blends of P3HT and Au nanoparticles capped with dodecanethiolates,31 but in that case the strongly passivating ligand shell may have prevented this process. Since other explanations, such as disturbed order in the polymer phase, were ruled out, we consider quenching of the excited state as a likely explanation, but a direct experimental proof would require ultrafast spectroscopy methods. An alternative explanation could be that the Au nanoparticles segregate at the interface between the active layer and the Al
contact. In this case, the structure of the P3HT/PCBM blend and the associate hole mobility would remain unchanged, explaining the absence of significant differences in the UV-vis spectra and the measured hole mobility. In contrast, such a segregation effect should lower the work function of the cathode and thus reduce the build-in field of the solar cells. In a systematic study of bulk heterojunction solar cells with PCBM and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) where different metals were used as cathode with ITO as the anode, a Au cathode lowered the open circuit voltage by ∼0.1 eV with respect to an Al cathode.36 This value does not directly correspond to the difference between the work functions of Al and Au due to Fermi level pinning effects but still leads to a reduced VOC.36 The short circuit current density was found to be lowered by ∼50% with Au instead of Al, which was explained by the ability of Au to form an ohmic contact not only with PCBM but also with the polymer, which in turn should enhance recombination of charge carriers at the interface.36 The effects observed in the cited study36 seem to be a bit too small to explain the strong decrease of the device performance observed in our case. Furthermore, the interaction between the S-containing polymer and the Au nanoparticles is expected to be rather strong, so that demixing and segregation effects seem unlikely. However, segregation phenomena of the Au nanoparticles at the interfaces could not be analyzed within this study and can therefore not be excluded to contribute to the reduced device performance. 3.2. Solar Cells with Au Nanoparticles Stabilized with DDA Ligands. In the next step, we used another synthesis route for Au nanoparticles with dodecylamine as ligands, i.e., with the same ligands that were used in the work on P3OT/C60 solar cells where a positive effect on the device performance was reported.12 Figure 5a shows a typical TEM image of DDAcapped Au nanoparticles (Au/DDA) that form ordered 2D superstructures due to the narrow size distribution with an average diameter of 7.0 nm and only 10% standard deviation in this case. These nanoparticles are of comparable size as in the earlier work12 but have a much narrower size distribution. The DDA-capped Au nanoparticles are soluble in chlorobenzene and can simply be added to the P3HT/PCBM solutions to obtain solar cells with incorporated Au/DDA nanoparticles. Figure 5b shows representative I-V curves and Table 2 summarizes characteristic values of the obtained solar cells. The reference solar cell shown in this series of experiments had a slightly higher efficiency (∼3.2%) than those discussed before (∼2.5%). This variance may be due to the smaller active area of the cell shown in Figure 5b which can lead to a slight overestimation of the photocurrent due to edge effects that become more important at reduced size.37 The addition of small amounts of Au (3 wt % with respect to P3HT) turned out to have only a small, but again not beneficial, effect on the device
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Figure 5. (a) Typical TEM image of Au nanoparticles stabilized with DDA. (b) IV curves in the dark (solid lines) and under 100 mW/cm2 AM 1.5 irradiation (symbols) of representative solar cells with different active layers. A reference sample with a pure P3HT/PCBM blend (black squares) and two samples containing different amounts of DDA-capped Au nanoparticles (3 wt %, blue diamonds; 52 wt %, red circles) are compared.
performance. Adding high amounts of Au (∼50 wt % with respect to P3HT) resulted in a strongly reduced performance, meaning that also the addition of Au/DDA does not have a beneficial effect on the P3HT/PCBM solar cells. Although in contrast to the results found for the P3OT/C60 system,12 this seems not surprising, because the DDA ligand shell is in principle expected to act as an insulating barrier for charge transport. Therefore, one cannot expect a positive effect due to improved electron transport, here. The insulating behavior of the DDA shell is also evidenced by the dark I-V curve of the solar cell with high Au/DDA content (see Figure 5b), where the current under forward bias is strongly reduced with respect to the reference cell. Comparing the results from the two synthesis routes with P3HT and DDA, despite the insulating DDA ligand shell, we found that solar cells containing DDA-capped Au nanoparticles seem to be better than those containing Au nanoparticles stabilized with P3HT. If quenching of the excited state in the polymer is the major mechanism for decreasing the efficiency, it seems possible that the insulating DDA shell reduces the quenching probability. 3.3. Solar Cells with Au Nanoparticles Stabilized with Pyridine. In a third approach we intended to incorporate Au nanoparticles without a thick organic ligand shell into the P3HT/ PCBM devices, following strategies known from organic-based bulk heterojunction solar cells with semiconductor nanoparticles
Topp et al. as electron acceptors.38,39 For example, CdSe nanoparticles can be prepared in high quality with a narrow size distribution by the use of organic ligands containing carbon chains with typically ∼8-20 carbon atoms.40 However, such ligand shells were found to prevent efficient charge transfer between conductive polymers and the CdSe nanoparticles, because they act as an insulating barrier.29,41 To overcome this difficulty, the initial ligand shell after synthesis can be replaced by ligand exchange with pyridine prior to the use of the nanoparticles in hybrid solar cells.42,43 Pyridine is a small molecule, and Huynh et al. could show that part of the pyridine is even removed from the CdSe surface upon annealing of the active layer to ∼120 °C,43 so that the ligand exchange with pyridine is a suitable way to incorporate nanoparticles with a narrow size distribution into the active layer of organic-based solar cells and simultaneously to enable efficient charge transfer.29,34,41 In the first attempt, a direct transfer of the ligand exchange procedure with pyridine from the CdSe to the Au(DDA) system was tried. This procedure (treatment Pyr1; see Experimental Section for details) involves heating at 65 °C for several hours. Figure 6a shows a TEM image of the resulting Au particles. Obviously, this procedure is not suitable for the Au system but leads to strong sintering of the Au particles. Therefore, the annealing time at 65 °C was reduced to 30 s in the next attempt (Pyr2). Figure 6b shows the corresponding TEM image, which still shows strong sintering effects. In the treatment Pyr3, the ligand exchange was performed at room temperature with an exchange time of only 15 min. The resulting nanostructure was already smaller (see Figure 6c), but still sintering occurred. However, reducing the amount of hexane used to precipitate the particles after the ligand exchange (treatment Pyr4) helped to obtain finally Au nanoparticles, which were not too strongly sintered together. In the corresponding TEM image (Figure 6d), one can still see individual Au nanoparticles with an average size of 14.9 nm ( 21%. Obviously, the Au nanoparticles have a strong tendency to aggregation and sintering after removal of the DDA ligand shell. This behavior was studied in more detail by absorption spectroscopy. Figure 7 shows UV-vis spectra of a sample at different stages during a pyridine treatment carried out at room temperature. The DDA-capped nanoparticles initially have a plasmon resonance around 510 nm. After addition of pyridine and stirring for 15 min this resonance feature became more pronounced and shifted to ∼540 nm. Addition of a small amount of hexane led to precipitation, which is a sign of successful ligand exchange, because the DDA-capped nanoparticles were initially soluble in hexane. The absorption spectrum recorded after redissolution of the precipitate in pyridine shows, in addition to the plasmon resonance around 540 nm, a second broad absorption feature at ∼650 nm. This is a spectroscopic indication for aggregation, as was shown in studies of Au nanoparticle aggregates.44,45 When larger amounts of hexane are used for precipitation, the second absorption feature is shifted to higher wavelength indicating stronger aggregation. It is noteworthy that the broad absorption feature indicative for aggregation occurred only after precipitation (see Figure 7). This means that the ligand exchange can be carried out at room temperature in solution without aggregation of the nanoparticles. Aggregation and sintering seem to occur after precipitation, i.e., when the solvent is removed. Therefore, it should in principle be better to find another way than precipitation to remove the exchanged DDA ligands from the pyridine solution after ligand exchange. This could, however, not be achieved within the
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TABLE 2: Characteristic Values of the Solar Cells Represented in Figure 5b under 100 mW/cm2 AM 1.5 Irradiationa sample
dact.layer (nm)
VOC (V)
JSC (mA/cm2)
FF
η (%)
act. area (cm2)
tempering before/after evaporation
P3HT/PCBM 3 wt % Au/P3HT/PCBM 52 wt % Au/P3HT/PCBM
∼70 ∼90 ∼85
0.65 0.59 0.27
9.7 10.8 5.3
0.52 0.46 0.25
3.2 2.9 0.4
0.04 0.04 0.06
before before before
a A reference sample with a pure P3HT/PCBM blend and two samples containing different amounts of DDA-capped Au nanoparticles are compared. dact.layer is the thickness of the active layer, VOC is the open circuit voltage, JSC is the short circuit current density, FF is the fill factor, η is the power conversion efficiency, “act. area” denotes the active area, and the last column indicates whether the cells were annealed at 140°C before or after deposition of the Al contact.
Figure 6. TEM images of DDA-capped Au nanoparticles after ligand exchange with pyridine according to different procedures: (a) Pyr1 (3 h at 65 °C); (b) Pyr2 (30 s at 65 °C); (c) Pyr3 (15 min at RT); (d) Pyr4 (15 min at RT, precipitation with reduced amount of hexane).
Figure 7. UV-vis absorption spectra of colloidal solutions of Au nanoparticles at different stages of a pyridine treatment carried out at room temperature: before ligand exchange (thick black line), after ligand exchange with pyridine, but before precipitation (thick red line), after precipitation with 1.5 mL of hexane (thin green line), and after precipitation with 9 mL of hexane (thin gray line).
present work. Here, we used the treatment Pyr4 to obtain Au nanoparticles stabilized with pyridine that are not too strongly aggregated. To incorporate these Au nanoparticles into solar cells, the Au particles dissolved in pyridine were mixed with P3HT/ PCBM dissolved in chlorobenzene. Thus, the solution used for spin coating was a binary solvent mixture of chlorobenzene and pyridine (with a volumetric ratio of 5:2 of chlorobenzene:
Figure 8. (a) IV curves in the dark (solid lines) and under 100 mW/ cm2 AM 1.5 irradiation (symbols) of representative solar cells with different active layers. A reference solar cell with a pure P3HT/PCBM layer processed from chlorobenzene (black squares), a cell with a P3HT/ PCBM layer processed from a solvent mixture of chlorobenzene and pyridine (5:2, v:v) (green triangles), and a solar cell containing 23 wt % of pyridine-treated Au nanoparticles (red circles) are compared. (b) UV-vis spectra of pure P3HT/PCBM films processed from chlorobenzene and from a chlorobenzene/pyridine mixture and a spectrum of a P3HT/PCBM film containing Au nanoparticles. These spectra were measured of the complete solar cells, i.e., on top of the PEDOT:PSS/ ITO-coated glass substrates. The spectra were corrected for a vertical offset (set to zero at 800 nm) and normalized to the maximum around 500 nm.
pyridine). This cannot be avoided, because P3HT/PCBM is not soluble in pure pyridine, and the Au nanoparticles are not soluble in pure chlorobenzene after the ligand exchange. Figure 8a shows representative I-V curves of solar cells with incorporated, pyridine-capped Au nanoparticles and about 23 wt % of Au with respect to P3HT. The corresponding characteristic values are given in Table 3. Also this approach was found to fail in improving the performance of the solar cells. To ensure that the decreased efficiency is not simply due to the addition of pyridine to the solvent, P3HT/PCBM solar cells were prepared from chlorobenze/pyridine mixtures as well (see Figure 8a and Table 3). No significant differences with respect
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TABLE 3: Characteristic Values of the Solar Cells Represented in Figure 8a under 100 mW/cm2 AM 1.5 Irradiationa sample
dact.layer (nm)
VOC (V)
JSC (mA/cm2)
FF
η (%)
act. area (cm2)
tempering
P3HT/PCBM (chlorobenzene) P3HT/PCBM (chlorobenzene/pyridine) ∼23 wt % Au/P3HT/PCBM (chlorobenzene/pyridine)
∼60 ∼65 ∼45
0.59 0.62 0.42
8.5 8.7 7.1
0.51 0.45 0.31
2.5 2.4 1.1
0.07 0.05 0.08
before before before
a A reference solar cell with a pure P3HT/PCBM layer processed from chlorobenzene, a cell with a P3HT/PCBM layer processed from a solvent mixture of chlorobenzene and pyridine (5:2, v:v), and a solar cell containing 23 wt % of pyridine-treated Au nanoparticles are compared. dact.layer is the thickness of the active layer, VOC is the open circuit voltage, JSC is the short circuit current density, FF is the fill factor, η is the power conversion efficiency, “act. area” denotes the active area, and the last column indicates whether the cells were annealed at 140°C before or after deposition of the Al contact.
to the reference cells processed from pure chlorobenzene were observed. Furthermore, UV-vis spectra showed no significant influence of the addition of pyridine on the establishment of molecular order in the P3HT phase (see Figure 8b). Note that this result is generally of interest for the field of hybrid polymer/ nanoparticle solar cells, where solvent mixtures such as chlorobenzene/pyridine are frequently used.46,47 Concerning the decrease of the efficiency, quenching of the excited state could be a reason again. Segregation of Au to the interface with the Al electrode might contribute as well. Furthermore, all Au-containing solar cells from this series of experiments had a low parallel resistance (see the slope of the dark I-V curve at 0 V). Possibly, a network of aggregated Au nanoparticles leads to local shunts, i.e., local short circuits, in this case. 4. Summary and Conclusion The influence of Au nanoparticles as an additive to the active layer in P3HT/PCBM bulk heterojunction solar cells was investigated, the work being motivated by indications given in the literature that suggested a positive effect on the electron transport properties in P3OT/C60 blends.12 In a first approach, Au nanoparticles stabilized with P3HT were prepared and incorporated into P3HT/PCBM solar cells. However, the efficiency was found to be strongly decreased. Reduced hole mobility due to disturbed order of the polymer phase could be ruled out as explanation. As another possibility, we propose that the Au nanoparticles enable efficient quenching of excited states in the polymer phase. Segregation of Au nanoparticles at the interface between the active layer and the Al contact might contribute to the decreased efficiency as well. In a second approach, Au nanoparticles were synthesized with a narrow size distribution using DDA ligands. With the ligand shell present on the nanoparticle surface the performance of solar cells was found to decrease as well due to the DDA shell which acts as an insulating barrier. Therefore, procedures were developed to remove the DDA shell by ligand exchange with pyridine. The Au nanoparticles were found to have a strong tendency for aggregation and sintering after removal of the initial ligand shell, but an adopted pyridine treatment carried out at room temperature was found to be suitable to obtain Au particles that were not strongly aggregated. However, also with this type of nanoparticles incorporated into P3HT/PCBM bulk heterojunction solar cells, a decrease of the device performance was observed. Here, indications were found that the strong tendency to aggregation of the metal nanoparticles leads to local shunts in the thin films. As an important side result, it was found that adding nearly 30 vol % of pyridine to the main solvent chlorobenzene does not have a significant effect on the performance of pure P3HT/ PCBM solar cells. In conclusion, a positive effect of introducing Au nanoparticles into P3HT/PCBM bulk heterojunction solar cells could
not be observed. This result stays in contrast to the earlier work on P3OT/C60. Although the materials are not as different, it might be possible that the interaction with Au nanoparticles differs in both systems. Anyhow, it can be concluded that the strategy to use Au nanoparticles as an additive to polymer/ fullerene solar cells fails in the case of the P3HT/PCBM system, most likely due to quenching of the excited state in the polymer and segregation phenomena. Acknowledgment. We thank M. Ahlers (University of Oldenburg) for assistance with the TGA measurements. References and Notes (1) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30, 10. (2) Brabec, C.; Dyakonov, V.; Scherf, U.(Eds.) Organic PhotoVoltaics; Wiley-VCH: Weinheim, 2008. (3) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mat. 2005, 15, 1617. (4) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297. (5) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45. (6) Yang, X.; Loos, J. Macromolecules 2007, 40, 1353. (7) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (8) Chen, L.-M.; Hong, Z.; Li, G.; Yang, Y. AdV. Mater. 2009, 21, 1434. (9) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. Appl. Phys. Lett. 2001, 78, 841. (10) Hoppe, H.; Glatzel, T.; Niggemann, M.; Schwinger, W.; Schaeffler, F.; Hinsch, A.; Lux-Steiner, M. C.; Sariciftci, N. S. Thin Solid Films 2006, 511, 587. (11) Zhang, F.; Jespersen, K. G.; Bjo¨rstro¨m, C.; Svensson, M.; Andersson, M. R.; Sundstro¨m, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Ingana¨s, O. AdV. Funct. Mater. 2006, 16, 667. (12) Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 203113. (13) Moule´, A. J.; Meerholz, K. AdV. Mater. 2008, 20, 240. (14) Moon, J. S.; Lee, J. K.; Cho, S.; Byun, J.; Heeger, A. J. Nano Lett. 2009, 9, 230. (15) van Bavel, S. S.; Sourty, E.; de With, G.; Loos, J. Nano Lett. 2009, 9, 507. (16) Cook, S.; Katoh, R.; Furube, A. J. Phys. Chem. C 2009, 113, 2547. (17) Turkevich, J. Gold Bull. 1985, 18, 125. (18) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678. (19) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (20) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (21) Alvarez, M. M.; Khoury, J. T.; Schaaf, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (22) Westphalen, M.; Kreibig, U.; Rostalski, J.; Lu¨th, H.; Meissner, D. Sol. Energy Mat. Sol. Cells 2000, 61, 97. (23) Catchpole, K. R.; Polman, A. Appl. Phys. Lett. 2008, 93, 191113. (24) Akimov, Y. A.; Ostrikov, K.; Li, E. P. Plasmonics 2009, 4, 107. (25) McFarland, E. W.; Tang, J. Nature 2003, 421, 616. (26) Gra¨tzel, M. Nature 2003, 421, 586. (27) Zhai, L.; McCullough, R. D. J. Mater Chem. 2004, 14, 141. (28) Jana, N. R.; Peng, X. J. Am. Chem. Soc. 2003, 125, 14280. (29) Pientka, M.; Dyakonov, V.; Meissner, D.; Rogach, A.; Talapin, D.; Weller, H.; Lutsen, L.; Vanderzande, D. Nanotechnology 2004, 15, 163. (30) Li, L.; Lu, G.; Yang, X. J. Mater. Chem. 2008, 18, 1984. (31) Nicholson, P. G.; Ruiz, V.; Macpherson, J. V.; Unwin, P. R. Chem. Commun. 2005, 1052.
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