Improving the Light Trapping Efficiency of Plasmonic Polymer Solar

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Improving the Light Trapping Efficiency of Plasmonic Polymer Solar Cells through Photon Management Yu-Sheng Hsiao,† Shobhit Charan,†,‡ Feng-Yu Wu,† Fan-Ching Chien,† Chih-Wei Chu,† Peilin Chen,†,* and Fang-Chung Chen§,* †

Research Center for Applied Sciences, Academia Sinica, 128 Sec. 2, Academia Road, Nankang, Taipei 11529, Taiwan Taiwan International Graduate Program, Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan § Department of Photonics and Display Institute, National Chiao Tung University, Hsiuchu 30010, Taiwan ‡

S Supporting Information *

ABSTRACT: In this study, we have explored how light trapping efficiency can be enhanced by using gold nanoparticles (Au NPs) of various sizes and shapes on the front of polymer solar cells (PSCs) with the active layerblends of poly(3-hexyl thiophene) and [6,6]phenyl-C61-butyric acid methyl ester. The light-concentrating behavior was enhanced after we had incorporated gold nanospheres or nanorods into the anodic buffer layer [based on poly(3,4ethylenedioxythiophene):polystyrenesulfonate] to trigger various localized surface plasmon resonance (LSPR) bands. Comparison of the optical characteristics and the performance of the PSCs prepared with and without Au NPs, and we found that the UV−vis and wavelength-dependent photoluminescent spectral data corroborated with the device performance due to the photon management by considering the light scattering and LSPR effects at the active layer. The presence of Au NPs increased the power conversion efficiency to approximately 4.3% (an enhancement of 24%).

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cells, featuring either continuous metal films [through the excitation of surface plasmon polaritons (SPPs)] or metal nanoparticles [through scattering or localized surface plasmon resonance (LSPR) effects].10−18 In PSC devices using SPPs, the electromagnetic wave propagating along the interface between the active layer and back contact electrode should result in higher light trapping efficiency. The short-range EQE enhancement was observed, however, only at a certain excitation wavelength, which was related to the distance and height of the periodic grating structures of the metallic electrodes.10,11,14,15 In plasmonic PSC devices using LSPR, the optical absorption can be enhanced directly from the electrical field excited by the metallic nanopatterns or nanoparticles; the EQE is, therefore, improved by the plasmonic scattering behavior (forward scattering) and near-field effects.12−15,17,18 At present, the simple solution processing of buffer layers, comprising poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) incorporating gold nanospheres (Au NSs), is the most promising candidate for increasing the light-concentrating ability without significantly altering the energy levels or device architectures of normal PSCs.14,17,18 In previous studies,14,17 we systematically explored how the plasmonic effect, featuring a

INTRODUCTION Polymer solar cells (PSCs) are promising technologies of utilizing renewable energy for mass production because of their lightweight and cost-effective production with simple processability. At present, the best power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) PSCs have reached 6− 8% under AM 1.5G conditions.1−3 After optimizing the thickness and morphology of the donor−acceptor interface of a blended film, consisting of a semiconducting polymer as the donor and a soluble fullerene as the acceptor, a BHJ photoactive layer having a thickness of approximately 200 nm would provide a high fill factor (FF) and an enhanced possibility of exciton dissociation and electrical transportation.4−6 Furthermore, low-bandgap (LBG) materials can be used to further enhance the device performance by extending the absorption region to longer wavelength.7−9 Although LBG materials are often associated with lower hole mobilities than are conventional poly(3-hexylthiophene) (P3HT) materials, these charge-transport problems can be overcome by decreasing the thickness of the photoactive layer, albeit with lower external quantum efficiencies (EQEs). Therefore, it is necessary to develop light-concentrating systems to enhance the light trapping efficiency of thinner active layers, especially for use in normal PSC device architectures. Recently, plasmonic light trapping has been applied to effectively enhance the light harvesting performance of solar © 2012 American Chemical Society

Received: June 21, 2012 Revised: September 6, 2012 Published: September 13, 2012 20731

dx.doi.org/10.1021/jp306124n | J. Phys. Chem. C 2012, 116, 20731−20737

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stirring. During this first step of the growth solution, nanorods were obtained along with spheres; therefore, purification was performed using a centrifugation method. The supernatant was effectively free of nonspherical particles and subsequently used for further growth to achieve gold nanospheres with various diameters − again through reduction of HAuCl4 with ascorbic acid in the presence of CTAB at 30 °C. The growth solution of gold nanorods was prepared as described by Prasad et al.21 In a typical reaction, 0.1 M CTAB (9.5 mL) was mixed with 0.01 M HAuCl4 (0.5 mL), resulting in a dark-yellow-colored growth solution. AgNO3 (0.01 M, 100 μL) was added to the solution at 25 °C. After gentle mixing, freshly prepared 0.1 M ascorbic acid (80 μL) was added to the test tube. Prior to the injection of the seed solution, 0.1 M HCl was added to tune the LSPR band from 680 to 850 nm. All of the Au NS and Au NR solutions were centrifuged twice (7000 rpm, 20 min) and redispersed in doubly distilled water to remove excess CTAB. Using this approach, several Au NSs and Au NRs, featuring LSPR wavelengths of 520, 530, 540, 660, 780, and 850 nm, were obtained for blending in the PEDOT:PSS buffer layer at a doping concentration of approximately 2 × 1011 particles cm−3. Device Fabrication and Instrumentation. The plasmonic PSCs were fabricated with architectures similar to those of the normal devices on indium tin oxide (ITO) glass substrates. Prior to beginning the fabrication process, the ITO glasses were cleaned in an ultrasonic bath sequentially with detergent, deionized water, acetone, and isopropyl alcohol, and then they were treated with UV ozone for 15 min. To prepare the PEDOT:PSS:Au NP composite buffer layer (ca. 30 nm), a Au NP solution was blended into the PEDOT:PSS solution (CLEVIOS P-VP AI 4083, H. C. Starck) and then spin-coated on the ITO. These samples were then dried at 130 °C for 1 h. Subsequently, a solution of P3HT:[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) (1:1, w/w) was spin-coated on the PEDOT:Au NP−modified ITO surfaces. Finally, the devices were covered with a 30 nm thick layer of Ca and a 100 nm thick layer of Al through thermal evaporation under vacuum ( NR780. Excitation sources in the wavelength region (λexc = 580 or 630 nm), however, resulted in the PL enhancements through LSPR effects decreasing in the order NR660 ≈ NR850 > NS540 ≈ NR780. Notably, for the devices incorporating NS540 and NR850, the LSPR effects at different resonance positions were wavelength-dependent and became less pronounced when the excitation wavelength deviated from the resonance peak. Comparing the subtraction EQE spectra of devices and the UV−vis results of Au NPs (Figures 5 and 6), whereas the device D-NS540 exhibited a significant device performance enhancement by fully utilizing the effective LS and LSPR effect near the shorter band edge of active layer, the devices D-NR780 and D-NR850 displayed strong LSPR effects in the region of ca. 400−650 nm near the shorter and longer band edge of active layer. We attribute the poorer performance of the device D-NR660 to a strong LS effect in the front buffer layer in the region of 400−650 nm, thereby directly increasing the optical loss prior to absorption in the photoactive layer (Figure 6). To further extend this photon management approach to improve the light trapping efficiency in devices, we blended NS540 with NR780 or NR850 in PEDOT:PSS films (NS540NR780 or NS540NR850) for use in PSC devices. UV−vis spectra revealed that the two LSPR absorption peaks of NS540NR780 and NS540NR850 could be tailored to cover the absorption band-edge of the active layer, thereby increasing the light harvesting ability of the devices (Figure 2). As revealed in Table 2 and Figure 5, the D-NS540NR850 device could be modulated with the highest PCE of approximately 4.3% − an improvement of 24% relative to that of device D-R. These results were also correlated with the EQE responses with high EQE enhancements covering the full absorption region of the active layer.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], tel.: +886-2-27898000 ext 33, fax: +886-2-27826680 (P.C.); e-mail: [email protected]. edu.tw Tel.: +886-3-5131484; fax: +886-3-5735601 (F.-C. C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by the National Science Council, Taiwan, under contracts 100-2113-M-001-027-MY3, 100-3113-E-009-005, 101-2120-M-001-011 and by the Academia Sinica Research Project on Nano Science and Technology.

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CONCLUSIONS With the knowledge obtained in this study, we have used Au NPs of various sizes and shapes with different LSPR bands to improve the PSC device performance after incorporating them in the front PEDOT:PSS buffer layer. Despite Au NSs and Au NRs exhibiting different absorption bands within the wavelength range from 520 to 850 nm, we observed efficient light absorption as a result of the LSPR-induced local field enhancement and less LS effect, thereby achieving enhanced device performances by to approximately 17%. Furthermore, we used the systematical optical analyses to understand the underlying mechanisms behind the operation of the LSPRinduced PSCs in terms of the LS and LSPR absorption effects. Through the blending of Au NPs and Au NRs into PEDOT:PSS buffer layers, we enhanced the PCE with a high enhancement factor of 24%. We believe that such approaches of photon management will pave the way toward higher-efficiency PSCs. 20736

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dx.doi.org/10.1021/jp306124n | J. Phys. Chem. C 2012, 116, 20731−20737