In situ-prepared composite materials of PEDOT: PSS buffer layer-metal nanoparticles and their application to organic solar cells
© Woo et al.; licensee Springer. 2012
Received: 20 July 2012
Accepted: 15 October 2012
Published: 23 November 2012
The Erratum to this article has been published in Nanoscale Research Letters 2014 9:506
We report an enhancement in the efficiency of organic solar cells via the incorporation of gold (Au) or silver (Ag) nanoparticles (NPs) in the hole-transporting buffer layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which was formed on an indium tin oxide (ITO) surface by the spin-coating of PEDOT:PSS-Au or Ag NPs composite solution. The composite solution was synthesized by a simple in situ preparation method which involved the reduction of chloroauric acid (HAuCl4) or silver nitrate (AgNO3) with sodium borohydride (NaBH4) solution in the presence of aqueous PEDOT:PSS media. The NPs were well dispersed in the PEDOT:PSS media and showed a characteristic absorption peak due to the surface plasmon resonance effect. Organic solar cells with the structure of ITO/PEDOT:PSS-Au, Ag NPs/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/LiF/Al exhibited an 8% improvement in their power conversion efficiency mainly due to the enlarged surface roughness of the PEDOT:PSS, which lead to an improvement in the charge collection and ultimately improvements in the short-circuit current density and fill factor.
KeywordsIn situ preparation PEDOT:PSS-metal NPs Enhanced light absorption Organic solar cells
Organic solar cells (OSCs) have recently been studied intensively for use in the next-generation photovoltaic devices due to their lower material cost, large area, flexibility, and simple solution processability [1, 2]. Although their power conversion efficiency (PCE) levels have been improved to 5% based on the bulk heterojunction of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), further improvement of up to 10% is required for successful commercialization [3, 4]. The main drawback of OPV is the limitation of the exciton diffusion length to about 10 to approximately 20 nm and the poor charge transport property. Moreover, the difficulty of increasing the active layer thickness leads to insufficient absorption of incident light [5, 6]. To improve light absorption in the organic photoactive layer, several methods, including the introduction of new low-band-gap polymers , the use of an inorganic optical spacer between active layer and the metal electrode , and the application of surface plasmons (SPs) based on metal nanoparticles (NPs) incorporated in the active layer or deposited onto an indium tin oxide (ITO) electrode [9, 10] have been reported. Among these methods, plasmonic enhancement is considered to be one of the best approaches due to its simple process, effectiveness, and controllability of the plasmon absorption wavelength by adjusting the particle size, shape, and compositions. SPs are collective surface oscillations of conduction electrons within metal nanostructures that tend to trap optical waves near their interface between the metal and a dielectric medium under electromagnetic field excitation. They can create strong near-field electromagnetic fields and far-field propagating waves, which can be used for the enhancement of the light absorption and photocurrent of OSCs . Three methods using the SP phenomena of metal NPs in OSCs have been reported, but they all have some drawbacks. The first involves the use of metal NPs incorporated into the photoactive layer, but in this case the PCE can be restricted by exciton quenching with nonradiative energy transfer and the differences between the electronic properties of the metal NPs and the conjugated photoactive molecules [12, 13]. Recently, however, several reports on the successful application of Au or Ag NPs into a photoactive layer have been published, demonstrating the enhancement of optical and electrical properties based on SPs as well as other additional effects (e.g., increased surface roughness and balanced charge mobilities) [14–17]. The second method involves the formation of a metal NP layer between the ITO electrode and the poly 3,4-ethylenedioxy-thiophene:polystyrene sulfonate (PEDOT:PSS) buffer layer, which typically requires an extra deposition process such as the vacuum deposition of a metal nanolayer with a sequential thermal treatment or the spin-coating of a metal NP precursor solutions [18, 19]. The third method relies on a metal-NP-incorporated PEDOT:PSS layer [20–22]. Both the first and third methods require a two-step experimental process, including the aqueous synthesis of metal NPs using organic capping agents such as poly(ethylene glycol) (PEG) or poly(vinyl pyrrolidone) (PVP) and the mixing/dispersion of synthesized metal NPs with a photoactive or PEDOT:PSS solution.
Here, we suggest a simpler method to introduce metal NPs into PEDOT:PSS without the need for organic capping agents or an additional mixing process via in situ-prepared composite materials of PEDOT:PSS-metal NPs in which Au or Ag NPs are stably incorporated. We also investigate the effects of metal NPs on the device properties as to whether they act as SPs or another mechanism.
Synthesis of PEDOT:PSS-Au or Ag NPs composite materials
Results and discussion
Au and Ag NPs are among the most widely used nanomaterials in biological and electronic applications. Such applications, however, require these particles mostly to be water-dispersible and/or suspended in water without a loss of their physical or chemical properties over long periods of time. To obtain stable NPs in media with a high ionic strength (water), organic capping stabilizers such as PEG, PVP, cetyltrimethylammonium bromide, or sodium dodecylsulfate, which cannot easily be removed during processing for further applications, are required. While synthetic methods of NPs in organic media were reported by several groups with a well-defined size and shape, they are water-immiscible and have limited application range . Recently, research on combining these NPs with PEDOT:PSS has been conducted in an effort to prepare hybrid materials for use as a surface plasmon source [20–22].
Prior to checking the device performance, we measured the sheet resistance of the PEDOT:PSS film and the effect of residual impurities remaining in PEDOT:PSS-metal NPs solution on the device performance. The sheet resistance measured by a four-point probe was slightly reduced for PEDOT:PSS-Au NPs (0.81 MΩ/sq) or increased for PEDOT:PSS-Ag NPs (1.39 MΩ/sq) compared to pristine PEDOT:PSS (1.12 MΩ/sq). We also noted that dissolving each small amounts of NaBH4, HAuCl4, or AgNO3 into PEDOT:PSS did not affect the device performances (data not shown here). This indicates that the effects of the electrical conductivity and residual impurities in the PEDOT:PSS-metal NPs are not likely to be important factors in our device system.
Summary of device performances
With Au NPs
With Ag NPs
In conclusion, we successfully synthesized composites materials of PEDOT:PSS-Au or Ag NPs by a simple in situ preparation method. The synthesized NPs with a size of 20 to approximately 40 nm were well dispersed in PEDOT:PSS media and showed a characteristic absorption peak due to the SPR effect. According to our J-V and UV–vis findings, the 8% improvement of the PCE is mainly caused by the increased hole collection to the anode due to the increased surface roughness and increased interface area of the buffer layer. Moreover, the optical effects of metal NPs are a minor factor when the metal NPs are embedded fully into the PEDOT:PSS layer.
Our in situ method provides an easy solution to stabilize metal NPs dispersed in water-based functional polymers as it does not require the use of an organic capping agent. This can be a suitable means of incorporating metal NPs for the future practical application of organic electronics such as organic memory, organic thin film transistors, and organic solar cells without any change to the conventional device structure or fabrication process.
This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (12-EN-02). It was also supported by Korean government grant (Priority Research Center-2009-0093819).
- Dennler G, Scharber MC, Brabec CJ: Polymer-fullerene bulk-heterojunction solar cells. Adv Mater 2009, 21: 1323–1338. 10.1002/adma.200801283View Article
- Thompson BC, Frechet JMJ: Polymer-fullerene composite solar cells. Angew Chem Int Ed 2007, 47: 58–77.View Article
- Kim Y, Cook S, Tuladhar SM, Choulis SA, Nelson J, Durrant JR, Bradley DDC, Giles M, McCulloch I, Ha CS, Ree M: A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells. Nat Mater 2006, 5: 197–203. 10.1038/nmat1574View Article
- Scharber MC, Muhlbacher D, Koppe M, Denk P, Waldauf C, Heeger AJ, Brabec CJ: Design rules for donors in bulk-heterojunction solar cells - towards 10% energy-conversion efficiency. Adv Mater 2006, 18: 789–794. 10.1002/adma.200501717View Article
- Clarke TM, Durrant JR: Charge photo-generation in organic solar cells. Chem Rev 2010, 110: 6736–6767. 10.1021/cr900271sView Article
- Coakley KM, McGehee MD: Conjugated polymer photovoltaic cells. Chem Mater 2004, 16: 4533–4542. 10.1021/cm049654nView Article
- Park SH, Roy A, Beaupre S, Cho S, Coates N, Moon JS, Moses D, Leclerc M, Lee K, Heeger AJ: Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nat Photonics 2009, 3: 297–302. 10.1038/nphoton.2009.69View Article
- Kim JY, Kim SH, Lee HH, Lee K, Ma W, Gong X, Heeger AJ: New architecture for high-efficiency polymer photovoltaic cells using solution-based titanium oxide as an optical spacer. Adv Mater 2006, 18: 572–576. 10.1002/adma.200501825View Article
- Shen H, Bienstman P, Maes B: Plasmonic absorption enhancement in organic solar cells with thin active layers. J Appl Phys 2009, 106: 073109. 10.1063/1.3243163View Article
- Lee JH, Park JH, Kim JS, Lee DY, Cho K: High efficiency polymer solar cells with wet deposited plasmonic gold nanodots. Org Electron 2009, 10: 416–420. 10.1016/j.orgel.2009.01.004View Article
- Pillai S, Green MA: Plasmonics for photovoltaic applications. Sol Energy Mater Sol Cells 2010, 94: 1481–1486. 10.1016/j.solmat.2010.02.046View Article
- Rand BP, Peumans P, Forrest SR: Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters. J Appl Phys 2004, 96: 7519. 10.1063/1.1812589View Article
- Kim K, Carroll DL: Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C60 bulk heterojunction photovoltaic devices. Appl Phys Lett 2005, 87: 203113. 10.1063/1.2128062View Article
- Wang CCD, Choy WCH, Duan C, Fung DDS, Sha WEI, Xie FX, Huang F, Cao Y: Optical and electrical effects of gold nanoparticles in the active layer of polymer solar cells. J Mater Chem 2012, 22: 1206–1211. 10.1039/c1jm14150cView Article
- Xie FX, Choy WCH, Wang CCD, Sha WEI, Fung DDS: Improving the efficiency of polymer solar cells by incorporating gold nanoparticles into all polymer layers. Appl Phys Lett 2011, 99: 153304. 10.1063/1.3650707View Article
- Wang DH, Kim DY, Choi KW, Seo JH, Im SH, Park JH, Park OO, Heeger AJ: Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles. Angew Chem Int Ed 2011, 50: 5519–5523. 10.1002/anie.201101021View Article
- Wang DH, Kim JK, Lim GH, Park KH, Park OO, Lim B, Park JH: Enhanced light harvesting in bulk heterojunction photovoltaic devices with shape-controlled Ag nanomaterials: Ag nanoparticles versus Ag nanoplates. RSC Adv 2012, 2: 7268–7272. 10.1039/c2ra20815fView Article
- Morfa AJ, Rowlwn KL, Reilly TH III, Romero MJ, Lagemaat JVD: Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics. Appl Phys Lett 2008, 92: 013504. 10.1063/1.2823578View Article
- Yoon WJ, Jung KY, Liu J, Duraisamy T, Revur R, Teixeira FL, Sengupta S, Berger PR: Plasmon-enhanced optical absorption and photocurrent in organic bulk heterojunction photovoltaic devices using self-assembled layer of silver nanoparticles. Sol Energy Mater Sol Cells 2010, 94: 128–132. 10.1016/j.solmat.2009.08.006View Article
- Qiao L, Wang D, Zuo L, Ye Y, Qian J, Chen H, He S: Localized surface plasmon resonance enhanced organic solar cell with gold nanospheres. Appl Energy 2011, 88: 848–852. 10.1016/j.apenergy.2010.09.021View Article
- Chen FC, Wu JL, Lee CL, Hong Y, Kuo CH, Huang MH: Plasmonic-enhanced polymer photovoltaic devices incorporating solution-processable metal nanoparticles. Appl Phys Lett 2009, 95: 013305. 10.1063/1.3174914View Article
- Fung DDS, Qiao L, Choy WCH, Wang C, Sha WEI, Xie F, He S: Optical and electrical properties of efficiency enhanced polymer solar cells with Au nanoparticles in a PEDOT:PSS layer. J Mater Chem 2011, 21: 16349–16356. 10.1039/c1jm12820eView Article
- Wiley B, Sun Y, Chen J, Cang H, Li ZY, Li X, Xia Y: Shape-controlled synthesis of silver and gold nanostructures. MRS Bull 2005, 30: 356–361. 10.1557/mrs2005.98View Article
- Rycenga M, Cobley CM, Zeng J, Li W, Moran CH, Zhang Q, Qin D, Xia Y: Controlling the synthesis and assembly of silver nanostructures for plasmonic applications. Chem Rev 2011, 111: 3669–3712. 10.1021/cr100275dView Article
- Hsu MH, Yu P, Huang JH, Chang CH, Wu CW, Cheng YC, Chu CW: Balanced carrier transport in organic solar cells employing embedded indium-tin-oxide nanoelectrodes. Appl Phys Lett 2011, 98: 073308. 10.1063/1.3556565View Article
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