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
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).
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