Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Nanoscale View Article Online
PAPER
Cite this: Nanoscale, 2014, 6, 6180
View Journal | View Issue
Efficient hybrid plasmonic polymer solar cells with Ag nanoparticle decorated TiO2 nanorods embedded in the active layer Kong Liu,† Yu Bi,† Shengchun Qu,* Furui Tan, Dan Chi, Shudi Lu, Yanpei Li, Yanlei Kou and Zhanguo Wang A hybrid plasmonic polymer solar cell, in which plasmonic metallic nanostructures (such as Ag, Au, and Pt nanoparticles) are embedded in the active layer, has been under intense scrutiny recently because it provides a promising new approach to enhance the efficiency of the device. We propose a brand new hybrid plasmonic nanostructure, which combines a plasmonic metallic nanostructure and onedimensional semiconductor nanocrystals, to enhance the photocurrent of the device through a strong
Received 2nd January 2014 Accepted 10th April 2014
localized electric field and an enhanced charge transport channel. We demonstrate that when Ag nanoparticle decorated TiO2 nanorods were introduced into the active layer of polymer-fullerene based bulk heterojunction solar cells, the photocurrent significantly increased to 14.15 mA cm
DOI: 10.1039/c4nr00030g
cm
www.rsc.org/nanoscale
enhanced to 4.87% from 2.57%.
2
During the past decades, organic solar cells (OSC), based on conjugated polymers, have emerged as potential energy conversion devices having several advantages, including exibility, lightweight, semi-transparent characteristics, and ability for large-scale production at low temperatures.1–3 However, their reported efficiencies are still very low even for laboratory cells. As we know, there are ve main causes of reduced efficiency in OSCs: i. energy level misalignment, ii. insufficient light trapping and absorption, iii. low exciton diffusion length, iv. non-radiative recombination of charges or charge transfer excitons, and v. low carrier mobility.4,5 Among these ve causes, the most crucial one is the non-radiative recombination in a certain material system, which could lead to an energy loss of up to 50% or more.6 The two other crucial problems that many of these devices face are limited light absorption and carrier mobility. To mitigate these problems, some special methods, such as incorporation of inorganic nano-materials with high carrier mobility in the active layer to form hybrid solar cells and introduction of the plasmonic effect in the devices, have been developed for high efficiency.7–18 One-dimensional (1D) semiconductor nanostructures have been widely used in the design of polymer based solar cells.16,19–21 TiO2 has a large exciton binding energy and a wide band gap.22–27 It can be used in effective conducting nanowires Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. E-mail: [email protected]
6180 | Nanoscale, 2014, 6, 6180–6186
from 6.51 mA
without a decrease in the open voltage; thus, the energy conversion efficiency was dramatically
Introduction
† These authors contributed equally to the work.
2
(NWs) or nanorods (NRs) grown by solution based methods; these nanostructures have attracted much attention because they provide a direct path for charge transport and have a high surface to volume ratio.8,28–31 On the other hand, noble metal nanoparticles with surface plasmon resonance have attracted unprecedented attention for their potential to boost the performance of OSCs as they exhibit strong local eld enhancement around the particles, which can increase light scattering and absorption in the organic lm.32–42 Although plasmon enhanced solar cells seem attractive, metal nanoparticles present signicant challenges. Metal nanoparticles embedded in OSC active layers can lead to an enhanced non-radiative decay process and increased carrier recombination.43 For example, an excessive concentration of Ag nanoparticles (AgNPs) added to the active layer may decrease the efficiency of OSCs, because of the increased recombination of carriers trapped on the AgNPs,44 where the increased absorption gained by AgNPs cannot compensate the loss by non-radiative recombination. Here, we propose a new nanostructure, AgNP decorated TiO2 NRs, which can be used instead of adding AgNPs into the active layer of hybrid bulk-heterojunction (BHJ) solar cells to enhance the device performance. This structure can combine the advantages of the electrical property of NRs and the plasmon effect of noble metal nanoparticles to reduce the non-radiative recombination and increase the charge separation due to the strong localized electric eld enhancement around AgNPs. Moreover, AgNP distribution in the active layer can be promoted by depositing these particles on the surface of TiO2 NRs, since aggregation between AgNPs is barely possible because the NPs
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
are selectively deposited along the NR surface. This concept can also be applied to other noble metal NPs with a plasmon effect decorated 1D inorganic semiconductor nanostructure.
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Experimental TiO2 NR synthesis A very simple two-step wet chemical method was used to synthesize stable soluble AgNP decorated TiO2 NRs. First, high quality TiO2 NRs were synthesized according to the literature45 by using a hot injection method, which was conducted in an anaerobic environment. The reactants – 0.01 mol oleic acid, 3.8 mL 1-octadecene (ODE) and 17 mL oleyl amine – were added to a three neck ask. The mixture was heated to 120 C for 30 min and then cooled down to 50 C. Aer that, 0.11 mL TiCl4 was added into the ask and the temperature was increased to 290 C at the rate of 12 C min 1. The reaction was stopped aer it was carried out at 290 C for 30 min. Then, we added an excess amount of acetone to the reactants to separate the NRs from the solution aer the reactants cooled to room temperature. The products were collected by centrifugation and washed with acetone 4 times. Ag–TiO2 NR synthesis The process of decorating TiO2 NRs with AgNPs by using the ethanol reduction method is as follows: rstly, TiO2 NRs were ultrasonically dispersed into a mercaptoacetic acid and ethanol (1 : 1 in v:v) mixed solution for 30 min and stirred at 65 C for 24 h to perform surface modication. Aer that, the TiO2 NRs were collected with 40 mL ethanol by using a centrifugation/wash process. Then, 10 mL AgNO3 water and ethanol (4 : 1 in v:v) mixed solution (1 mmol L 1) was added to the TiO2 NR ethanol solution and stirred at 65 C for 10 h to perform ethanol reduction and then centrifuged. Finally, the obtained dark gray colored precipitate was dispersed in dichlorobenzene for use.
Nanoscale
structures of the materials were characterized by X-ray diffraction (XRD) using copper Ka radiation. The chemical composition was investigated by energy dispersive spectroscopy (EDS). Absorption spectrum (Abs) and photoluminescence (PL) measurements were carried out on Varian U-3000 model ultraviolet-visible spectrophotometer and Varian Cary Eclipse uorescence spectrophotometer, respectively. Current density– voltage (J–V) characteristics were measured with the assistance of AM 1.5 illumination (100 mW cm 2). The quantum efficiency testing was performed on a DH1720A-1250-W bromine tungsten arc source and a Digikrom DK240 monochromator.
Results and discussion Fig. 1 shows transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images of as-obtained TiO2 NRs and as-obtained AgNP-decorated TiO2 NRs. As shown in Fig. 1a, the as-synthesized TiO2 NRs have good dispersity and homogeneity. NRs of 3 nm diameter and length ranging from 10 nm to 50 nm are obtained. The TiO2 NRs of this size are soluble in the dichlorobenzene (DCB), the same solvent dissolving P3HT and PCBM. This makes it possible to dope the NRs in the active layer without a signicant morphology change. At the same time, this size could avoid short circuiting of the active layer (100 nm) by extra-long TiO2 NRs. Fig. 1b shows an HRTEM image of TiO2 NR. It image shows a clear crystal lattice, which illustrates the perfect crystallinity of the NRs. Fig. 1c shows the TEM image of AgNP decorated TiO2 NRs. It can be seen that the NRs were piled up on the copper grids. One reason for this may be that the long chain capping agent (oleic acid) was substituted by a short chain capping agent
Solar cell fabrication In our experiments, we used the organic bulk heterojunction materials poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) at a mass ratio of 1 : 1 (20 mg mL 1). The optimized doping concentration of TiO2 NRs or Ag–TiO2 NRs in the hybrid solution was 3.57%, which was determined by the optical and morphological properties of organic hybrid materials. These methods are reported elsewhere.46 Ag–TiO2 NR-doped P3HT:PCBM hybrid bulk-heterojunction solar cells were fabricated by spin coating the blended solution on poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS)/indium tin oxide (ITO) glass substrates. The devices were baked at 110 C in nitrogen for 20 min aer evaporation of Ca/Al electrodes. We designed four solar cells on each glass substrate. The active area of each cell was 4 mm2. Characterization The morphologies of TiO2 NRs and AgNPs were characterized using transmission electron microscopy (TEM). The crystal
This journal is © The Royal Society of Chemistry 2014
Fig. 1 (a) TEM and (b) HRTEM images of as-obtained TiO2 NRs, (c) TEM and (d) HRTEM images of as-obtained AgNP decorated TiO2 NRs.
Nanoscale, 2014, 6, 6180–6186 | 6181
View Article Online
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Nanoscale
(mercaptoacetic acid) aer the surface treatment, which would decrease the dispersity of the NRs. Another reason may be that some AgNPs deposited on two or more NRs simultaneously when TiO2 NRs were arranged together. However, the dispersity of Ag–TiO2 NRs in dichlorobenzene was excellent. At the same time, although the small diameter TiO2 NRs were not suitable for decorating metal NPs, with the help of surface treatment of as-obtained NRs, 3–5 nm AgNPs were selectively decorated on the NR surface (Fig. 1d). Fig. 2 shows the XRD patterns and energy dispersive spectroscopy (EDS) images of as-obtained TiO2 NRs and Ag–TiO2 NRs. The XRD diffraction peaks (Fig. 2a) can be indexed as the pure anatase phase (JCPDS no. 21-1272). Aer AgNPs were decorated on the NR surface, the XRD pattern was still indexed as the pure anatase phase without a trace of AgNPs; this can be attributed to the low concentration of AgNPs in the TiO2 NRs. Therefore, an EDS analysis was performed and the results are given in Fig. 2b. The peaks around 3.10 keV and 22.16 keV correspond to the binding energies of AgL and Ag Ka1, respectively, while the peaks at binding energies of 0.45 keV, 4.51 keV and 4.93 keV belong to TiL1, Ti Ka and Ti Kb, respectively. Carbon and copper are present in the sample due to the TEM holding grid. No obvious peak belonging to the impurities is detected. The result indicates that the as-fabricated materials are composed of Ag, Ti, and O elements. Fig. 3a shows the absorption spectrum of as-obtained NRs and P3HT:PCBM BHJ composite lms. It can be seen that the absorption range of pure Ag–TiO2 broadened into all the visible wavelengths aer the decoration process, which is due to the plasmonic effect of AgNPs. It should be noted that the plasmon band of AgNPs around 395 nm disappeared. The changes in the absorption spectrum can be attributed to the particle oxidation of the AgNPs, with the formation of an AgO shell on the particle surface.47–50 Notably, the color of the as-obtained NR solution in DCB changed from yellow to a dark color aer the decoration of
(a) XRD patterns, (b) energy dispersive spectroscopy of asobtained TiO2 NRs and Ag–TiO2 NRs.
Fig. 2
6182 | Nanoscale, 2014, 6, 6180–6186
Paper
Fig. 3 (a) Absorption spectrum of as-obtained pure TiO2 NRs, pure Ag–TiO2 NRs, P3HT:PCBM film, TiO2 NR doped P3HT:PCBM film and Ag–TiO2 NR doped P3HT:PCBM film. (b) PL of P3HT:PCBM film, TiO2 NR doped P3HT:PCBM film and Ag–TiO2 NR doped P3HT:PCBM film.
AgNPs, which agreed well with the absorption spectrum. The asobtained solution was quite stable for months without any sign of sedimentation or color change. Plasmon-enhanced absorption in the Ag–TiO2 NR doped active layer was also observed. This clearly shows that the assynthesized TiO2 NRs increase the extinction in the wavelength range from 300 nm to 600 nm. This increase mainly comes from the light absorption of the TiO2 NRs and the light scattering effect, which can lengthen the optical path and increase the capture ratio of photons in an organic lm. Furthermore, the absorption curve of Ag–TiO2 NR doped samples moves up a little more than that of the TiO2 doped samples, indicating the plasmon resonance absorption and scattering effect of AgNPs. However, the absorption enhancement is not noticeable or equivalent compared with the absorption spectrum of pure Ag–TiO2 NRs. The reason for this is that the absorption of P3HT:PCBM is very high. Actually, another effect, strong localized electric eld enhancement of AgNPs, plays the main role in device performance improvement, which we deem important in our experiment design.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Paper
In order to evaluate the charge transport channel effect of TiO2, we tested the photoluminescence (PL) of TiO2 NR and Ag–TiO2 NR doped P3HT:PCBM lms. It can be seen that the PL intensity of TiO2 NR doped samples increased, while that of the Ag–TiO2 NR doped samples decreased. The lower PL intensity indicates more non-radiative recombination, which would lead to improved device performance. Based on the good property of the nanostructure, we designed our hybrid BHJ solar cells. As illustrated in Fig. 4a, AgNP decorated TiO2 NRs were introduced into the active layer of BHJ OSC device instead of AgNPs. With the plasmon enhanced light absorption effect and the strong localized electric eld enhancement effect provided by AgNPs, the light absorption efficiency of the organic layer would be signicantly increased, which means more photo-induced excitons would be generated. To make these excitons contribute to the photocurrent, we need to separate excitons into free carriers within the lifetime with the help of a driving force (temperature and electric eld). Then, the most important effect of plasmon, its strong localized electric eld, comes into play. The bulk distribution of AgNPs in the active layer results in the bulk distribution of a strong localized electric eld in the active layer, where the random BHJ exists. This implies a signicant enhancement of the dissociation efficiency for the photoinduced excitons at the interface of the BHJ. The energy band diagram of our solar cells is shown in Fig. 4c. As shown, the highest occupied molecular orbital (HOMO) of P3HT is positioned to inject holes into PEDOT:PSS and hence into the ITO electrode. The lowest unoccupied molecular orbital (LUMO) of PCBM is well above the conduction band of TiO2. Since 1D TiO2 NRs can provide a naturally direct electron transport path, the extra free carriers trapped in AgNPs
Fig. 4 (a) Schematic representation of the device structure of the hybrid plasmonic BHJ organic solar cell, which introduces AgNP decorated TiO2 NRs into the active layer of polymer-fullerene based solar cell through all solution processes. (b) Proposed electron transfer process in the AgNP decorated TiO2 NR under sunlight. (c) Energy level diagram of the components of the hybrid plasmonic BHJ organic solar cell under flat band condition. (d) Energy band diagram of AgNPs and TiO2 NR contact.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
could transfer to TiO2 NRs, which avoids the charge recombination caused by metal NPs (see Fig. 4d). Therefore, the photoinduced excitons by the plasmon enhanced light absorption and enhanced charge separation by the strong localized electric eld enhancement can efficiently contribute to the photocurrent, which could dramatically improve the performance of OSC devices. It should be pointed out that AgNPs are prone to oxidation even under ambient conditions because of their large surfaceto-volume ratio. The work function of the AgO layer is reported to be 0.4 eV higher than that of Ag.51–54 An ultrathin AgO outer shell around the AgNPs may form an energy step, signicantly decreasing the charge collection efficiency. The experiments that we performed previously under air atmosphere always gave poor performance. Therefore, we carried out all our device fabrication in nitrogen glove box systems. The typical current density–voltage (J–V) characteristics of hybrid plasmonic polymer solar cells and comparable devices without any dopant and with TiO2 NRs in the active layer under AM 1.5 irradiation and in the dark are shown in Fig. 5a and b, respectively. The photovoltaic parameters are summarized in Table 1. As shown from the results, aer introducing the Ag– TiO2 NRs into the active layer, the open voltage (Voc) stayed the same and the short circuit current (Jsc) increased dramatically from 6.51 mA cm 2 to 14.15 mA cm 2, in which the enhancement ratio was as much as 120%. Consequently, the power conversion efficiency (PCE) improved from 2.57% to 4.87%, in which about 89% enhancement was totally due to the contribution of the Jsc increase. As far as we know, there are some
Fig. 5 (a) J–V characteristics of solar cells composed of P3HT:PCBM and TiO2 NRs and Ag decorated TiO2 NR doped P3HT:PCBM composite under AM1.5G illumination from a calibrated solar simulator and (b) in the dark are presented.
Nanoscale, 2014, 6, 6180–6186 | 6183
View Article Online
Nanoscale
Paper
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Table 1 Typical solar cell performance without any dopant, with TiO2, and with Ag–TiO2 in the active layer
Device
Jsc (mA cm 2)
Voc (V)
FF (%)
h (%)
Without TiO2 NRs Ag–TiO2
6.51 5.57 14.15
0.61 0.59 0.61
64.7 59.6 56.5
2.57 1.96 4.87
reports that indicate similar or larger improvements.55,56 For example, Morfa et al. reported that the conversion efficiency of a P3HT:PCBM bulk heterojunction photovoltaic device was found to increase from 1.3% to 2.2% for devices employing thin plasmon-active layers of Ag NP lm. About 69% efficiency enhancement was achieved.57 Kirkeminde et al. obtained a 1.1% conversion efficiency by introducing 110 nm Au nanopyramids (NPYs) into the ITO/PEDOT:PSS/interface of a silole–thiophene (P3):PCBM based solar cell. Efficiency improvement of up to 200% was achieved.18 Although their reported conversion efficiency improvements are similar or larger, their highest conversion efficiency is below the value obtained for our reference devices, which makes our results more signicant. For polymer solar cells with a conversion efficiency of more than 4.8%, the best reported performance enhancement by incorporating plasmonic metal NPs in the active layer is 20%, which is ascribed to the plasmon enhanced absorption in the active layer.55,58 Therefore, it is reasonable to deduce that the dramatic photocurrent enhancement in our hybrid plasmonic polymer solar cell can be attributed to the enhanced charge separation and the promoted charge transfer process with the help of the strong localized electric eld of the plasmonic metalsemiconductor nanostructure. It should be noted that the dark current density for the sample with Ag–TiO2 under the dark condition, shown in Fig. 5b, is increased compared with the reference sample. This is due to the leakage current introduced by AgNPs. There exists current to tunnel through the islands of AgNPs between the cathode and the anode.59 For the device under light illumination, the current density is dominated by the photogenerated carriers, which is signicantly higher than the leakage current. To further demonstrate the function of Ag–TiO2 NRs in the active layer, contrast experiments were carried out. The same amount of TiO2 NRs before decorating AgNPs were doped in the active layer; the corresponding device performance is shown in Table 1. With only TiO2 NRs incorporated in the active layer, Voc decreases slightly from 0.61 V to 0.59 V and Jsc decreases signicantly from 6.51 mA cm 2 to 5.57 mA cm 2, which is probably due to the fact that the extra photo-induced electrons in the conduction band of TiO2 NRs tend to recombine, since there is no effective metal-semiconductor structure to help improve the charge transfer process. At the same time, the PL spectrum of the TiO2 NR doped P3HT:PCBM lm also indicates the reason for the decrease. A higher PL intensity leads to a higher non-radiative recombination, thus resulting in the decrease of photo-induced exciton separation. We report here that we have successfully introduced AgNP decorated TiO2 NRs into the active layer, which can signicantly
6184 | Nanoscale, 2014, 6, 6180–6186
enhance the photocurrent of OSCs with the most intensively studied P3HT:PCBM material system. It is revealed that, in order to further enhance the plasmonic polymer solar cells' performance, it is necessary to utilize the strong localized electric eld of the plasmon besides the plasmon-enhanced absorption to promote the charge separation efficiently and enhance the charge transfer efficiency. Noble metal NP deposited 1-D semiconductor (TiO2, ZnO, etc.) nanostructures with a suitable energy level could promote charge separation, making such composites attractive for solar energy conversion.
Conclusions In summary, AgNP decorated TiO2 nanorods were fabricated by using the hot injection method and the ethanol reduction method. It is revealed that the AgNPs on TiO2 exhibited surface plasmon resonance and increased light absorption, while TiO2 NRs promoted the dispersity of AgNPs in the organic blends and provided the electron transport path. AgNP decorated TiO2/ organic solar cells were also demonstrated, treating them as hybrid solar cells. With the strong localized electric eld enhancement effect provided by AgNPs, the performances of solar cells were improved. The performance was dominated by current enhancement. The short circuit current increased from 6.51 mA cm 2 to 14.15 mA cm 2, with an enhancement of up to 120%. The current gain increased the conversion efficiency from 2.57% to 4.87%, with an enhancement of up to 89%. This enhancement was explained by the surface plasmon resonance effect and the enhanced charge transport channel effect of the AgNP decorated TiO2 NRs.
Acknowledgements We would like to thank Jizheng Wang from Institute of Chemistry, Chinese Academy of Sciences, for his experiment support, and Xingwang Zhang from Institute of Semiconductors, Chinese Academy of Sciences, for fruitful discussions. This work was mostly supported by the National Basic Research Program of China (Grant no. 2012CB934200, 2014CB643503), and National Natural Science Foundation of China (Contract nos 50990064, 61076009, 61204002).
Notes and references 1 M. Kaltenbrunner, M. S. White, E. D. Glowacki, T. Sekitani, T. Someya, N. S. Saricici and S. Bauer, Nat. Commun., 2012, 3, 1772. 2 Z. Ye, T. P. Basel, B. R. Gautam, Y. Xiaomei, D. J. Mascaro, L. Feng and Z. V. Vardeny, Nat. Commun., 2012, 3, 1043. 3 L. Gang, Z. Rui and Y. Yang, Nat. Photonics, 2012, 6, 153–161. 4 K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganas and J. V. Manca, Nat. Mater., 2009, 8, 904–909. 5 Y. Yuan, T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang and J. Huang, Nat. Mater., 2011, 10, 296–302. 6 T. Kirchartz, K. Taretto and U. Rau, J. Phys. Chem. C, 2009, 113, 17958–17966.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Paper
7 M. A. Green and S. Pillai, Nat. Photonics, 2012, 6, 130–132. 8 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Gratzel and S. I. Seok, Nat. Photonics, 2013, 7, 487–492. 9 B. Wu, X. Wu, C. Guan, K. Fai Tai, E. K. L. Yeow, H. Jin Fan, N. Mathews and T. C. Sum, Nat. Commun., 2013, 4, 2004. 10 O. Stenzel, L. J. A. Koster, R. Thiedmann, S. D. Oosterhout, R. A. J. Janssen and V. Schmidt, Adv. Funct. Mater., 2012, 22, 1236–1244. 11 J. H. Seo, D.-H. Kim, S.-H. Kwon, M. Song, M.-S. Choi, S. Y. Ryu, H. W. Lee, Y. C. Park, J.-D. Kwon, K.-S. Nam, Y. Jeong, J.-W. Kang and C. S. Kim, Adv. Mater., 2012, 24, 4523–4527. 12 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338, 643–647. 13 S. Jeong, E. C. Garnett, S. Wang, Z. G. Yu, S. H. Fan, M. L. Brongersma, M. D. McGehee and Y. Cui, Nano Lett., 2012, 12, 2971–2976. 14 J. C. Cardoso, C. A. Grimes, X. J. Feng, X. Y. Zhang, S. Komarneni, M. V. B. Zanoni and N. Z. Bao, Chem. Commun., 2012, 48, 2818–2820. 15 K.-Q. Peng, X. Wang, X.-L. Wu and S.-T. Lee, Nano Lett., 2009, 9, 3704–3709. 16 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295, 2425–2427. 17 R. J. Zhou, Y. Zheng, L. Qian, Y. X. Yang, P. H. Holloway and J. G. Xue, Nanoscale, 2012, 4, 3507–3514. 18 A. Kirkeminde, M. Retsch, Q. Wang, G. Xu, R. Hui, J. Wu and S. Ren, Nanoscale, 2012, 4, 4421–4425. 19 R. Radbeh, E. Parbaile, M. Chakaroun, B. Ratier, M. Aldissi and A. Moliton, Polym. Int., 2010, 59, 1514–1519. 20 M. Yu, Y. Z. Long, B. Sun and Z. Y. Fan, Nanoscale, 2012, 4, 2783–2796. 21 P. Poudel and Q. Q. Qiao, Nanoscale, 2012, 4, 2826–2838. 22 F. Sauvage, D. H. Chen, P. Comte, F. Z. Huang, L. P. Heiniger, Y. B. Cheng, R. A. Caruso and M. Graetzel, ACS Nano, 2010, 4, 4420–4425. 23 Y. Y. Lin, T. H. Chu, S. S. Li, C. H. Chuang, C. H. Chang, W. F. Su, C. P. Chang, M. W. Chu and C. W. Chen, J. Am. Chem. Soc., 2009, 131, 3644–3649. 24 L. E. Greene, M. Law, B. D. Yuhas and P. D. Yang, J. Phys. Chem. C, 2007, 111, 18451–18456. 25 T. Hirakawa and P. V. Kamat, J. Am. Chem. Soc., 2005, 127, 3928–3934. 26 T. Berger, M. Sterrer, O. Diwald, E. Knozinger, D. Panayotov, T. L. Thompson and J. T. Yates, J. Phys. Chem. B, 2005, 109, 6061–6068. 27 M. Lv, D. Zheng, M. Ye, L. Sun, J. Xiao, W. Guo and C. Lin, Nanoscale, 2012, 4, 5872–5879. 28 A. Karpinski, S. Berson, H. Terrisse, M. Mancini-Le Granvalet, S. Guillerez, L. Brohan and M. Richard-Plouet, Sol. Energy Mater. Sol. Cells, 2013, 116, 27–33. 29 X. J. Feng, K. Shankar, O. K. Varghese, M. Paulose, T. J. Latempa and C. A. Grimes, Nano Lett., 2008, 8, 3781– 3786.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
30 K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69–74. 31 D. H. Kim, H. J. Koo, J. S. Jur, M. Woodroof, B. Kalanyan, K. Lee, C. K. Devine and G. N. Parsons, Nanoscale, 2012, 4, 4731–4738. 32 J. F. Zhu, M. Xue, R. Hoekstra, F. X. Xiu, B. Q. Zeng and K. L. Wang, Nanoscale, 2012, 4, 1978–1981. 33 J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White and M. L. Brongersma, Nat. Mater., 2010, 9, 193–204. 34 H. A. Atwater and A. Polman, Nat. Mater., 2010, 9, 205–213. 35 R. A. Pala, J. S. Q. Liu, E. S. Barnard, D. Askarov, E. C. Garnett, S. Fan and M. L. Brongersma, Nat. Commun., 2013, 4, 2095. 36 P. Reineck, G. P. Lee, D. Brick, M. Karg, P. Mulvaney and U. Bach, Adv. Mater., 2012, 24, 4750–4755. 37 X. H. Li, W. C. H. Choy, L. J. Huo, F. X. Xie, W. E. I. Sha, B. F. Ding, X. Guo, Y. F. Li, J. H. Hou, J. B. You and Y. Yang, Adv. Mater., 2012, 24, 3046–3052. 38 N. Kalfagiannis, P. G. Karagiannidis, C. Pitsalidis, N. T. Panagiotopoulos, C. Gravalidis, S. Kassavetis, P. Patsalas and S. Logothetidis, Sol. Energy Mater. Sol. Cells, 2012, 104, 165–174. 39 M. G. Kang, T. Xu, H. J. Park, X. G. Luo and L. J. Guo, Adv. Mater., 2010, 22, 4378–4383. 40 T. W. H. Oates, A. Keller, S. Facsko and A. Mucklich, Plasmonics, 2007, 2, 47–50. 41 D. M. Schaadt, B. Feng and E. T. Yu, Appl. Phys. Lett., 2005, 86, 063106. 42 K. R. Catchpole and A. Polman, Opt. Express, 2008, 16, 21793–21800. 43 A. P. Kulkarni, K. M. Noone, K. Munechika, S. R. Guyer and D. S. Ginger, Nano Lett., 2010, 10, 1501–1505. 44 M. Xue, L. Li, B. J. Tremolet de Villers, H. Shen, J. Zhu, Z. Yu, A. Z. Stieg, Q. Pei, B. J. Schwartz and K. L. Wang, Appl. Phys. Lett., 2011, 98, 253302. 45 R. Buonsanti, V. Grillo, E. Carlino, C. Giannini, T. Kipp, R. Cingolani and P. D. Cozzoli, J. Am. Chem. Soc., 2008, 130, 11223–11233. 46 F. Tan, S. Qu, X. Zhang, K. Liu and Z. Wang, J. Nanopart. Res., 2013, 15, 1–11. 47 Y. Han, R. Lupitskyy, T.-M. Chou, C. M. Stafford, H. Du and S. Sukhishvili, Anal. Chem., 2011, 83, 5873–5880. 48 A. Henglein, Chem. Mater., 1998, 10, 444–450. 49 C.-N. Lok, C.-M. Ho, R. Chen, Q.-Y. He, W.-Y. Yu, H. Sun, P.-H. Tam, J.-F. Chiu and C.-M. Che, J. Biol. Inorg. Chem., 2007, 12, 527–534. 50 O. A. D. Gallardo, R. Moiraghi, M. A. Macchione, J. A. Godoy, M. A. Perez, E. A. Coronado and V. A. Macagno, RSC Adv., 2012, 2, 2923–2929. 51 Y. Yuan, Y. Bi and J. Huang, Appl. Phys. Lett., 2011, 98, 063306. 52 W.-J. Yoon and P. R. Berger, Appl. Phys. Lett., 2008, 92, 013306. 53 U. Kumar Barik, S. Srinivasan, C. L. Nagendra and A. Subrahmanyam, Thin Solid Films, 2003, 429, 129–134. 54 H. Won Choi, S. Young Kim, K.-B. Kim, Y.-H. Tak and J.-L. Lee, Appl. Phys. Lett., 2005, 86, 012104.
Nanoscale, 2014, 6, 6180–6186 | 6185
View Article Online
Nanoscale
57 A. J. Morfa, K. L. Rowlen, T. H. Reilly, M. J. Romero and J. van de Lagemaat, Appl. Phys. Lett., 2008, 92, 013504. 58 L. Lu, Z. Luo, T. Xu and L. Yu, Nano Lett., 2012, 13, 59–64. 59 A. Gloskovskii, D. Valdaitsev, L. V. Viduta, S. A. Nepijko and G. Sch¨ onhense, Thin Solid Films, 2010, 518, 4030–4034.
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
55 E. Stratakis and E. Kymakis, Mater Today, 2013, 16, 133–146. 56 B. Paci, G. D. Spyropoulos, A. Generosi, D. Bailo, V. R. Albertini, E. Stratakis and E. Kymakis, Adv. Funct. Mater., 2011, 21, 3573–3582.
Paper
6186 | Nanoscale, 2014, 6, 6180–6186
This journal is © The Royal Society of Chemistry 2014
Nanoscale View Article Online
PAPER
Cite this: Nanoscale, 2014, 6, 6180
View Journal | View Issue
Efficient hybrid plasmonic polymer solar cells with Ag nanoparticle decorated TiO2 nanorods embedded in the active layer Kong Liu,† Yu Bi,† Shengchun Qu,* Furui Tan, Dan Chi, Shudi Lu, Yanpei Li, Yanlei Kou and Zhanguo Wang A hybrid plasmonic polymer solar cell, in which plasmonic metallic nanostructures (such as Ag, Au, and Pt nanoparticles) are embedded in the active layer, has been under intense scrutiny recently because it provides a promising new approach to enhance the efficiency of the device. We propose a brand new hybrid plasmonic nanostructure, which combines a plasmonic metallic nanostructure and onedimensional semiconductor nanocrystals, to enhance the photocurrent of the device through a strong
Received 2nd January 2014 Accepted 10th April 2014
localized electric field and an enhanced charge transport channel. We demonstrate that when Ag nanoparticle decorated TiO2 nanorods were introduced into the active layer of polymer-fullerene based bulk heterojunction solar cells, the photocurrent significantly increased to 14.15 mA cm
DOI: 10.1039/c4nr00030g
cm
www.rsc.org/nanoscale
enhanced to 4.87% from 2.57%.
2
During the past decades, organic solar cells (OSC), based on conjugated polymers, have emerged as potential energy conversion devices having several advantages, including exibility, lightweight, semi-transparent characteristics, and ability for large-scale production at low temperatures.1–3 However, their reported efficiencies are still very low even for laboratory cells. As we know, there are ve main causes of reduced efficiency in OSCs: i. energy level misalignment, ii. insufficient light trapping and absorption, iii. low exciton diffusion length, iv. non-radiative recombination of charges or charge transfer excitons, and v. low carrier mobility.4,5 Among these ve causes, the most crucial one is the non-radiative recombination in a certain material system, which could lead to an energy loss of up to 50% or more.6 The two other crucial problems that many of these devices face are limited light absorption and carrier mobility. To mitigate these problems, some special methods, such as incorporation of inorganic nano-materials with high carrier mobility in the active layer to form hybrid solar cells and introduction of the plasmonic effect in the devices, have been developed for high efficiency.7–18 One-dimensional (1D) semiconductor nanostructures have been widely used in the design of polymer based solar cells.16,19–21 TiO2 has a large exciton binding energy and a wide band gap.22–27 It can be used in effective conducting nanowires Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. E-mail: [email protected]
6180 | Nanoscale, 2014, 6, 6180–6186
from 6.51 mA
without a decrease in the open voltage; thus, the energy conversion efficiency was dramatically
Introduction
† These authors contributed equally to the work.
2
(NWs) or nanorods (NRs) grown by solution based methods; these nanostructures have attracted much attention because they provide a direct path for charge transport and have a high surface to volume ratio.8,28–31 On the other hand, noble metal nanoparticles with surface plasmon resonance have attracted unprecedented attention for their potential to boost the performance of OSCs as they exhibit strong local eld enhancement around the particles, which can increase light scattering and absorption in the organic lm.32–42 Although plasmon enhanced solar cells seem attractive, metal nanoparticles present signicant challenges. Metal nanoparticles embedded in OSC active layers can lead to an enhanced non-radiative decay process and increased carrier recombination.43 For example, an excessive concentration of Ag nanoparticles (AgNPs) added to the active layer may decrease the efficiency of OSCs, because of the increased recombination of carriers trapped on the AgNPs,44 where the increased absorption gained by AgNPs cannot compensate the loss by non-radiative recombination. Here, we propose a new nanostructure, AgNP decorated TiO2 NRs, which can be used instead of adding AgNPs into the active layer of hybrid bulk-heterojunction (BHJ) solar cells to enhance the device performance. This structure can combine the advantages of the electrical property of NRs and the plasmon effect of noble metal nanoparticles to reduce the non-radiative recombination and increase the charge separation due to the strong localized electric eld enhancement around AgNPs. Moreover, AgNP distribution in the active layer can be promoted by depositing these particles on the surface of TiO2 NRs, since aggregation between AgNPs is barely possible because the NPs
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
are selectively deposited along the NR surface. This concept can also be applied to other noble metal NPs with a plasmon effect decorated 1D inorganic semiconductor nanostructure.
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Experimental TiO2 NR synthesis A very simple two-step wet chemical method was used to synthesize stable soluble AgNP decorated TiO2 NRs. First, high quality TiO2 NRs were synthesized according to the literature45 by using a hot injection method, which was conducted in an anaerobic environment. The reactants – 0.01 mol oleic acid, 3.8 mL 1-octadecene (ODE) and 17 mL oleyl amine – were added to a three neck ask. The mixture was heated to 120 C for 30 min and then cooled down to 50 C. Aer that, 0.11 mL TiCl4 was added into the ask and the temperature was increased to 290 C at the rate of 12 C min 1. The reaction was stopped aer it was carried out at 290 C for 30 min. Then, we added an excess amount of acetone to the reactants to separate the NRs from the solution aer the reactants cooled to room temperature. The products were collected by centrifugation and washed with acetone 4 times. Ag–TiO2 NR synthesis The process of decorating TiO2 NRs with AgNPs by using the ethanol reduction method is as follows: rstly, TiO2 NRs were ultrasonically dispersed into a mercaptoacetic acid and ethanol (1 : 1 in v:v) mixed solution for 30 min and stirred at 65 C for 24 h to perform surface modication. Aer that, the TiO2 NRs were collected with 40 mL ethanol by using a centrifugation/wash process. Then, 10 mL AgNO3 water and ethanol (4 : 1 in v:v) mixed solution (1 mmol L 1) was added to the TiO2 NR ethanol solution and stirred at 65 C for 10 h to perform ethanol reduction and then centrifuged. Finally, the obtained dark gray colored precipitate was dispersed in dichlorobenzene for use.
Nanoscale
structures of the materials were characterized by X-ray diffraction (XRD) using copper Ka radiation. The chemical composition was investigated by energy dispersive spectroscopy (EDS). Absorption spectrum (Abs) and photoluminescence (PL) measurements were carried out on Varian U-3000 model ultraviolet-visible spectrophotometer and Varian Cary Eclipse uorescence spectrophotometer, respectively. Current density– voltage (J–V) characteristics were measured with the assistance of AM 1.5 illumination (100 mW cm 2). The quantum efficiency testing was performed on a DH1720A-1250-W bromine tungsten arc source and a Digikrom DK240 monochromator.
Results and discussion Fig. 1 shows transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images of as-obtained TiO2 NRs and as-obtained AgNP-decorated TiO2 NRs. As shown in Fig. 1a, the as-synthesized TiO2 NRs have good dispersity and homogeneity. NRs of 3 nm diameter and length ranging from 10 nm to 50 nm are obtained. The TiO2 NRs of this size are soluble in the dichlorobenzene (DCB), the same solvent dissolving P3HT and PCBM. This makes it possible to dope the NRs in the active layer without a signicant morphology change. At the same time, this size could avoid short circuiting of the active layer (100 nm) by extra-long TiO2 NRs. Fig. 1b shows an HRTEM image of TiO2 NR. It image shows a clear crystal lattice, which illustrates the perfect crystallinity of the NRs. Fig. 1c shows the TEM image of AgNP decorated TiO2 NRs. It can be seen that the NRs were piled up on the copper grids. One reason for this may be that the long chain capping agent (oleic acid) was substituted by a short chain capping agent
Solar cell fabrication In our experiments, we used the organic bulk heterojunction materials poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) at a mass ratio of 1 : 1 (20 mg mL 1). The optimized doping concentration of TiO2 NRs or Ag–TiO2 NRs in the hybrid solution was 3.57%, which was determined by the optical and morphological properties of organic hybrid materials. These methods are reported elsewhere.46 Ag–TiO2 NR-doped P3HT:PCBM hybrid bulk-heterojunction solar cells were fabricated by spin coating the blended solution on poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS)/indium tin oxide (ITO) glass substrates. The devices were baked at 110 C in nitrogen for 20 min aer evaporation of Ca/Al electrodes. We designed four solar cells on each glass substrate. The active area of each cell was 4 mm2. Characterization The morphologies of TiO2 NRs and AgNPs were characterized using transmission electron microscopy (TEM). The crystal
This journal is © The Royal Society of Chemistry 2014
Fig. 1 (a) TEM and (b) HRTEM images of as-obtained TiO2 NRs, (c) TEM and (d) HRTEM images of as-obtained AgNP decorated TiO2 NRs.
Nanoscale, 2014, 6, 6180–6186 | 6181
View Article Online
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Nanoscale
(mercaptoacetic acid) aer the surface treatment, which would decrease the dispersity of the NRs. Another reason may be that some AgNPs deposited on two or more NRs simultaneously when TiO2 NRs were arranged together. However, the dispersity of Ag–TiO2 NRs in dichlorobenzene was excellent. At the same time, although the small diameter TiO2 NRs were not suitable for decorating metal NPs, with the help of surface treatment of as-obtained NRs, 3–5 nm AgNPs were selectively decorated on the NR surface (Fig. 1d). Fig. 2 shows the XRD patterns and energy dispersive spectroscopy (EDS) images of as-obtained TiO2 NRs and Ag–TiO2 NRs. The XRD diffraction peaks (Fig. 2a) can be indexed as the pure anatase phase (JCPDS no. 21-1272). Aer AgNPs were decorated on the NR surface, the XRD pattern was still indexed as the pure anatase phase without a trace of AgNPs; this can be attributed to the low concentration of AgNPs in the TiO2 NRs. Therefore, an EDS analysis was performed and the results are given in Fig. 2b. The peaks around 3.10 keV and 22.16 keV correspond to the binding energies of AgL and Ag Ka1, respectively, while the peaks at binding energies of 0.45 keV, 4.51 keV and 4.93 keV belong to TiL1, Ti Ka and Ti Kb, respectively. Carbon and copper are present in the sample due to the TEM holding grid. No obvious peak belonging to the impurities is detected. The result indicates that the as-fabricated materials are composed of Ag, Ti, and O elements. Fig. 3a shows the absorption spectrum of as-obtained NRs and P3HT:PCBM BHJ composite lms. It can be seen that the absorption range of pure Ag–TiO2 broadened into all the visible wavelengths aer the decoration process, which is due to the plasmonic effect of AgNPs. It should be noted that the plasmon band of AgNPs around 395 nm disappeared. The changes in the absorption spectrum can be attributed to the particle oxidation of the AgNPs, with the formation of an AgO shell on the particle surface.47–50 Notably, the color of the as-obtained NR solution in DCB changed from yellow to a dark color aer the decoration of
(a) XRD patterns, (b) energy dispersive spectroscopy of asobtained TiO2 NRs and Ag–TiO2 NRs.
Fig. 2
6182 | Nanoscale, 2014, 6, 6180–6186
Paper
Fig. 3 (a) Absorption spectrum of as-obtained pure TiO2 NRs, pure Ag–TiO2 NRs, P3HT:PCBM film, TiO2 NR doped P3HT:PCBM film and Ag–TiO2 NR doped P3HT:PCBM film. (b) PL of P3HT:PCBM film, TiO2 NR doped P3HT:PCBM film and Ag–TiO2 NR doped P3HT:PCBM film.
AgNPs, which agreed well with the absorption spectrum. The asobtained solution was quite stable for months without any sign of sedimentation or color change. Plasmon-enhanced absorption in the Ag–TiO2 NR doped active layer was also observed. This clearly shows that the assynthesized TiO2 NRs increase the extinction in the wavelength range from 300 nm to 600 nm. This increase mainly comes from the light absorption of the TiO2 NRs and the light scattering effect, which can lengthen the optical path and increase the capture ratio of photons in an organic lm. Furthermore, the absorption curve of Ag–TiO2 NR doped samples moves up a little more than that of the TiO2 doped samples, indicating the plasmon resonance absorption and scattering effect of AgNPs. However, the absorption enhancement is not noticeable or equivalent compared with the absorption spectrum of pure Ag–TiO2 NRs. The reason for this is that the absorption of P3HT:PCBM is very high. Actually, another effect, strong localized electric eld enhancement of AgNPs, plays the main role in device performance improvement, which we deem important in our experiment design.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Paper
In order to evaluate the charge transport channel effect of TiO2, we tested the photoluminescence (PL) of TiO2 NR and Ag–TiO2 NR doped P3HT:PCBM lms. It can be seen that the PL intensity of TiO2 NR doped samples increased, while that of the Ag–TiO2 NR doped samples decreased. The lower PL intensity indicates more non-radiative recombination, which would lead to improved device performance. Based on the good property of the nanostructure, we designed our hybrid BHJ solar cells. As illustrated in Fig. 4a, AgNP decorated TiO2 NRs were introduced into the active layer of BHJ OSC device instead of AgNPs. With the plasmon enhanced light absorption effect and the strong localized electric eld enhancement effect provided by AgNPs, the light absorption efficiency of the organic layer would be signicantly increased, which means more photo-induced excitons would be generated. To make these excitons contribute to the photocurrent, we need to separate excitons into free carriers within the lifetime with the help of a driving force (temperature and electric eld). Then, the most important effect of plasmon, its strong localized electric eld, comes into play. The bulk distribution of AgNPs in the active layer results in the bulk distribution of a strong localized electric eld in the active layer, where the random BHJ exists. This implies a signicant enhancement of the dissociation efficiency for the photoinduced excitons at the interface of the BHJ. The energy band diagram of our solar cells is shown in Fig. 4c. As shown, the highest occupied molecular orbital (HOMO) of P3HT is positioned to inject holes into PEDOT:PSS and hence into the ITO electrode. The lowest unoccupied molecular orbital (LUMO) of PCBM is well above the conduction band of TiO2. Since 1D TiO2 NRs can provide a naturally direct electron transport path, the extra free carriers trapped in AgNPs
Fig. 4 (a) Schematic representation of the device structure of the hybrid plasmonic BHJ organic solar cell, which introduces AgNP decorated TiO2 NRs into the active layer of polymer-fullerene based solar cell through all solution processes. (b) Proposed electron transfer process in the AgNP decorated TiO2 NR under sunlight. (c) Energy level diagram of the components of the hybrid plasmonic BHJ organic solar cell under flat band condition. (d) Energy band diagram of AgNPs and TiO2 NR contact.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
could transfer to TiO2 NRs, which avoids the charge recombination caused by metal NPs (see Fig. 4d). Therefore, the photoinduced excitons by the plasmon enhanced light absorption and enhanced charge separation by the strong localized electric eld enhancement can efficiently contribute to the photocurrent, which could dramatically improve the performance of OSC devices. It should be pointed out that AgNPs are prone to oxidation even under ambient conditions because of their large surfaceto-volume ratio. The work function of the AgO layer is reported to be 0.4 eV higher than that of Ag.51–54 An ultrathin AgO outer shell around the AgNPs may form an energy step, signicantly decreasing the charge collection efficiency. The experiments that we performed previously under air atmosphere always gave poor performance. Therefore, we carried out all our device fabrication in nitrogen glove box systems. The typical current density–voltage (J–V) characteristics of hybrid plasmonic polymer solar cells and comparable devices without any dopant and with TiO2 NRs in the active layer under AM 1.5 irradiation and in the dark are shown in Fig. 5a and b, respectively. The photovoltaic parameters are summarized in Table 1. As shown from the results, aer introducing the Ag– TiO2 NRs into the active layer, the open voltage (Voc) stayed the same and the short circuit current (Jsc) increased dramatically from 6.51 mA cm 2 to 14.15 mA cm 2, in which the enhancement ratio was as much as 120%. Consequently, the power conversion efficiency (PCE) improved from 2.57% to 4.87%, in which about 89% enhancement was totally due to the contribution of the Jsc increase. As far as we know, there are some
Fig. 5 (a) J–V characteristics of solar cells composed of P3HT:PCBM and TiO2 NRs and Ag decorated TiO2 NR doped P3HT:PCBM composite under AM1.5G illumination from a calibrated solar simulator and (b) in the dark are presented.
Nanoscale, 2014, 6, 6180–6186 | 6183
View Article Online
Nanoscale
Paper
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Table 1 Typical solar cell performance without any dopant, with TiO2, and with Ag–TiO2 in the active layer
Device
Jsc (mA cm 2)
Voc (V)
FF (%)
h (%)
Without TiO2 NRs Ag–TiO2
6.51 5.57 14.15
0.61 0.59 0.61
64.7 59.6 56.5
2.57 1.96 4.87
reports that indicate similar or larger improvements.55,56 For example, Morfa et al. reported that the conversion efficiency of a P3HT:PCBM bulk heterojunction photovoltaic device was found to increase from 1.3% to 2.2% for devices employing thin plasmon-active layers of Ag NP lm. About 69% efficiency enhancement was achieved.57 Kirkeminde et al. obtained a 1.1% conversion efficiency by introducing 110 nm Au nanopyramids (NPYs) into the ITO/PEDOT:PSS/interface of a silole–thiophene (P3):PCBM based solar cell. Efficiency improvement of up to 200% was achieved.18 Although their reported conversion efficiency improvements are similar or larger, their highest conversion efficiency is below the value obtained for our reference devices, which makes our results more signicant. For polymer solar cells with a conversion efficiency of more than 4.8%, the best reported performance enhancement by incorporating plasmonic metal NPs in the active layer is 20%, which is ascribed to the plasmon enhanced absorption in the active layer.55,58 Therefore, it is reasonable to deduce that the dramatic photocurrent enhancement in our hybrid plasmonic polymer solar cell can be attributed to the enhanced charge separation and the promoted charge transfer process with the help of the strong localized electric eld of the plasmonic metalsemiconductor nanostructure. It should be noted that the dark current density for the sample with Ag–TiO2 under the dark condition, shown in Fig. 5b, is increased compared with the reference sample. This is due to the leakage current introduced by AgNPs. There exists current to tunnel through the islands of AgNPs between the cathode and the anode.59 For the device under light illumination, the current density is dominated by the photogenerated carriers, which is signicantly higher than the leakage current. To further demonstrate the function of Ag–TiO2 NRs in the active layer, contrast experiments were carried out. The same amount of TiO2 NRs before decorating AgNPs were doped in the active layer; the corresponding device performance is shown in Table 1. With only TiO2 NRs incorporated in the active layer, Voc decreases slightly from 0.61 V to 0.59 V and Jsc decreases signicantly from 6.51 mA cm 2 to 5.57 mA cm 2, which is probably due to the fact that the extra photo-induced electrons in the conduction band of TiO2 NRs tend to recombine, since there is no effective metal-semiconductor structure to help improve the charge transfer process. At the same time, the PL spectrum of the TiO2 NR doped P3HT:PCBM lm also indicates the reason for the decrease. A higher PL intensity leads to a higher non-radiative recombination, thus resulting in the decrease of photo-induced exciton separation. We report here that we have successfully introduced AgNP decorated TiO2 NRs into the active layer, which can signicantly
6184 | Nanoscale, 2014, 6, 6180–6186
enhance the photocurrent of OSCs with the most intensively studied P3HT:PCBM material system. It is revealed that, in order to further enhance the plasmonic polymer solar cells' performance, it is necessary to utilize the strong localized electric eld of the plasmon besides the plasmon-enhanced absorption to promote the charge separation efficiently and enhance the charge transfer efficiency. Noble metal NP deposited 1-D semiconductor (TiO2, ZnO, etc.) nanostructures with a suitable energy level could promote charge separation, making such composites attractive for solar energy conversion.
Conclusions In summary, AgNP decorated TiO2 nanorods were fabricated by using the hot injection method and the ethanol reduction method. It is revealed that the AgNPs on TiO2 exhibited surface plasmon resonance and increased light absorption, while TiO2 NRs promoted the dispersity of AgNPs in the organic blends and provided the electron transport path. AgNP decorated TiO2/ organic solar cells were also demonstrated, treating them as hybrid solar cells. With the strong localized electric eld enhancement effect provided by AgNPs, the performances of solar cells were improved. The performance was dominated by current enhancement. The short circuit current increased from 6.51 mA cm 2 to 14.15 mA cm 2, with an enhancement of up to 120%. The current gain increased the conversion efficiency from 2.57% to 4.87%, with an enhancement of up to 89%. This enhancement was explained by the surface plasmon resonance effect and the enhanced charge transport channel effect of the AgNP decorated TiO2 NRs.
Acknowledgements We would like to thank Jizheng Wang from Institute of Chemistry, Chinese Academy of Sciences, for his experiment support, and Xingwang Zhang from Institute of Semiconductors, Chinese Academy of Sciences, for fruitful discussions. This work was mostly supported by the National Basic Research Program of China (Grant no. 2012CB934200, 2014CB643503), and National Natural Science Foundation of China (Contract nos 50990064, 61076009, 61204002).
Notes and references 1 M. Kaltenbrunner, M. S. White, E. D. Glowacki, T. Sekitani, T. Someya, N. S. Saricici and S. Bauer, Nat. Commun., 2012, 3, 1772. 2 Z. Ye, T. P. Basel, B. R. Gautam, Y. Xiaomei, D. J. Mascaro, L. Feng and Z. V. Vardeny, Nat. Commun., 2012, 3, 1043. 3 L. Gang, Z. Rui and Y. Yang, Nat. Photonics, 2012, 6, 153–161. 4 K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganas and J. V. Manca, Nat. Mater., 2009, 8, 904–909. 5 Y. Yuan, T. J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang and J. Huang, Nat. Mater., 2011, 10, 296–302. 6 T. Kirchartz, K. Taretto and U. Rau, J. Phys. Chem. C, 2009, 113, 17958–17966.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
Paper
7 M. A. Green and S. Pillai, Nat. Photonics, 2012, 6, 130–132. 8 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Gratzel and S. I. Seok, Nat. Photonics, 2013, 7, 487–492. 9 B. Wu, X. Wu, C. Guan, K. Fai Tai, E. K. L. Yeow, H. Jin Fan, N. Mathews and T. C. Sum, Nat. Commun., 2013, 4, 2004. 10 O. Stenzel, L. J. A. Koster, R. Thiedmann, S. D. Oosterhout, R. A. J. Janssen and V. Schmidt, Adv. Funct. Mater., 2012, 22, 1236–1244. 11 J. H. Seo, D.-H. Kim, S.-H. Kwon, M. Song, M.-S. Choi, S. Y. Ryu, H. W. Lee, Y. C. Park, J.-D. Kwon, K.-S. Nam, Y. Jeong, J.-W. Kang and C. S. Kim, Adv. Mater., 2012, 24, 4523–4527. 12 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338, 643–647. 13 S. Jeong, E. C. Garnett, S. Wang, Z. G. Yu, S. H. Fan, M. L. Brongersma, M. D. McGehee and Y. Cui, Nano Lett., 2012, 12, 2971–2976. 14 J. C. Cardoso, C. A. Grimes, X. J. Feng, X. Y. Zhang, S. Komarneni, M. V. B. Zanoni and N. Z. Bao, Chem. Commun., 2012, 48, 2818–2820. 15 K.-Q. Peng, X. Wang, X.-L. Wu and S.-T. Lee, Nano Lett., 2009, 9, 3704–3709. 16 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295, 2425–2427. 17 R. J. Zhou, Y. Zheng, L. Qian, Y. X. Yang, P. H. Holloway and J. G. Xue, Nanoscale, 2012, 4, 3507–3514. 18 A. Kirkeminde, M. Retsch, Q. Wang, G. Xu, R. Hui, J. Wu and S. Ren, Nanoscale, 2012, 4, 4421–4425. 19 R. Radbeh, E. Parbaile, M. Chakaroun, B. Ratier, M. Aldissi and A. Moliton, Polym. Int., 2010, 59, 1514–1519. 20 M. Yu, Y. Z. Long, B. Sun and Z. Y. Fan, Nanoscale, 2012, 4, 2783–2796. 21 P. Poudel and Q. Q. Qiao, Nanoscale, 2012, 4, 2826–2838. 22 F. Sauvage, D. H. Chen, P. Comte, F. Z. Huang, L. P. Heiniger, Y. B. Cheng, R. A. Caruso and M. Graetzel, ACS Nano, 2010, 4, 4420–4425. 23 Y. Y. Lin, T. H. Chu, S. S. Li, C. H. Chuang, C. H. Chang, W. F. Su, C. P. Chang, M. W. Chu and C. W. Chen, J. Am. Chem. Soc., 2009, 131, 3644–3649. 24 L. E. Greene, M. Law, B. D. Yuhas and P. D. Yang, J. Phys. Chem. C, 2007, 111, 18451–18456. 25 T. Hirakawa and P. V. Kamat, J. Am. Chem. Soc., 2005, 127, 3928–3934. 26 T. Berger, M. Sterrer, O. Diwald, E. Knozinger, D. Panayotov, T. L. Thompson and J. T. Yates, J. Phys. Chem. B, 2005, 109, 6061–6068. 27 M. Lv, D. Zheng, M. Ye, L. Sun, J. Xiao, W. Guo and C. Lin, Nanoscale, 2012, 4, 5872–5879. 28 A. Karpinski, S. Berson, H. Terrisse, M. Mancini-Le Granvalet, S. Guillerez, L. Brohan and M. Richard-Plouet, Sol. Energy Mater. Sol. Cells, 2013, 116, 27–33. 29 X. J. Feng, K. Shankar, O. K. Varghese, M. Paulose, T. J. Latempa and C. A. Grimes, Nano Lett., 2008, 8, 3781– 3786.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
30 K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007, 7, 69–74. 31 D. H. Kim, H. J. Koo, J. S. Jur, M. Woodroof, B. Kalanyan, K. Lee, C. K. Devine and G. N. Parsons, Nanoscale, 2012, 4, 4731–4738. 32 J. F. Zhu, M. Xue, R. Hoekstra, F. X. Xiu, B. Q. Zeng and K. L. Wang, Nanoscale, 2012, 4, 1978–1981. 33 J. A. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White and M. L. Brongersma, Nat. Mater., 2010, 9, 193–204. 34 H. A. Atwater and A. Polman, Nat. Mater., 2010, 9, 205–213. 35 R. A. Pala, J. S. Q. Liu, E. S. Barnard, D. Askarov, E. C. Garnett, S. Fan and M. L. Brongersma, Nat. Commun., 2013, 4, 2095. 36 P. Reineck, G. P. Lee, D. Brick, M. Karg, P. Mulvaney and U. Bach, Adv. Mater., 2012, 24, 4750–4755. 37 X. H. Li, W. C. H. Choy, L. J. Huo, F. X. Xie, W. E. I. Sha, B. F. Ding, X. Guo, Y. F. Li, J. H. Hou, J. B. You and Y. Yang, Adv. Mater., 2012, 24, 3046–3052. 38 N. Kalfagiannis, P. G. Karagiannidis, C. Pitsalidis, N. T. Panagiotopoulos, C. Gravalidis, S. Kassavetis, P. Patsalas and S. Logothetidis, Sol. Energy Mater. Sol. Cells, 2012, 104, 165–174. 39 M. G. Kang, T. Xu, H. J. Park, X. G. Luo and L. J. Guo, Adv. Mater., 2010, 22, 4378–4383. 40 T. W. H. Oates, A. Keller, S. Facsko and A. Mucklich, Plasmonics, 2007, 2, 47–50. 41 D. M. Schaadt, B. Feng and E. T. Yu, Appl. Phys. Lett., 2005, 86, 063106. 42 K. R. Catchpole and A. Polman, Opt. Express, 2008, 16, 21793–21800. 43 A. P. Kulkarni, K. M. Noone, K. Munechika, S. R. Guyer and D. S. Ginger, Nano Lett., 2010, 10, 1501–1505. 44 M. Xue, L. Li, B. J. Tremolet de Villers, H. Shen, J. Zhu, Z. Yu, A. Z. Stieg, Q. Pei, B. J. Schwartz and K. L. Wang, Appl. Phys. Lett., 2011, 98, 253302. 45 R. Buonsanti, V. Grillo, E. Carlino, C. Giannini, T. Kipp, R. Cingolani and P. D. Cozzoli, J. Am. Chem. Soc., 2008, 130, 11223–11233. 46 F. Tan, S. Qu, X. Zhang, K. Liu and Z. Wang, J. Nanopart. Res., 2013, 15, 1–11. 47 Y. Han, R. Lupitskyy, T.-M. Chou, C. M. Stafford, H. Du and S. Sukhishvili, Anal. Chem., 2011, 83, 5873–5880. 48 A. Henglein, Chem. Mater., 1998, 10, 444–450. 49 C.-N. Lok, C.-M. Ho, R. Chen, Q.-Y. He, W.-Y. Yu, H. Sun, P.-H. Tam, J.-F. Chiu and C.-M. Che, J. Biol. Inorg. Chem., 2007, 12, 527–534. 50 O. A. D. Gallardo, R. Moiraghi, M. A. Macchione, J. A. Godoy, M. A. Perez, E. A. Coronado and V. A. Macagno, RSC Adv., 2012, 2, 2923–2929. 51 Y. Yuan, Y. Bi and J. Huang, Appl. Phys. Lett., 2011, 98, 063306. 52 W.-J. Yoon and P. R. Berger, Appl. Phys. Lett., 2008, 92, 013306. 53 U. Kumar Barik, S. Srinivasan, C. L. Nagendra and A. Subrahmanyam, Thin Solid Films, 2003, 429, 129–134. 54 H. Won Choi, S. Young Kim, K.-B. Kim, Y.-H. Tak and J.-L. Lee, Appl. Phys. Lett., 2005, 86, 012104.
Nanoscale, 2014, 6, 6180–6186 | 6185
View Article Online
Nanoscale
57 A. J. Morfa, K. L. Rowlen, T. H. Reilly, M. J. Romero and J. van de Lagemaat, Appl. Phys. Lett., 2008, 92, 013504. 58 L. Lu, Z. Luo, T. Xu and L. Yu, Nano Lett., 2012, 13, 59–64. 59 A. Gloskovskii, D. Valdaitsev, L. V. Viduta, S. A. Nepijko and G. Sch¨ onhense, Thin Solid Films, 2010, 518, 4030–4034.
Published on 11 April 2014. Downloaded by State University of New York at Stony Brook on 26/10/2014 08:24:15.
55 E. Stratakis and E. Kymakis, Mater Today, 2013, 16, 133–146. 56 B. Paci, G. D. Spyropoulos, A. Generosi, D. Bailo, V. R. Albertini, E. Stratakis and E. Kymakis, Adv. Funct. Mater., 2011, 21, 3573–3582.
Paper
6186 | Nanoscale, 2014, 6, 6180–6186
This journal is © The Royal Society of Chemistry 2014
