A polymer photodiode using vapour-phase polymerized PEDOT as an anode
Introduction
Conjugated polymers with heterocyclic structures like polypyrrole, polythiophene, polyaniline, polyphenylenes, and poly(p-phenylene vinylene)s have attracted a great deal of attention because of their great potential of application in a number of electronic devices like displays, smart windows, sensors, capacitors, batteries, and photovoltaic devices [1], [2]. Among the various organic conducting polymers, poly (3,4-ethylenedioxy) thiophene is one of the most widely studied conducting polymer because of its unique properties like high electrical conductivity, almost transparent thin film in the oxidized state and excellent stability at ambient and elevated temperatures [3].
For large-area electronics, it is desirable to use flexible transparent and conducting electrodes. For photovoltaic energy conversion devices, the large active area is a necessity to compensate for the lower energy conversion efficiencies still found in these systems. Deposition of the transparent electrode material should preferably be done by reel-to-reel methods, such as web coating. Also, the difficulties of using transparent oxides on flexible supports during reel-to-reel processing are considerable, arguing the use of flexible metallic polymer conductors. The commercially available polyelectrolyte complex PEDOT-PSS can be easily spin coated resulting in a highly transparent and conducting (0.05–10 S/cm) polymer film. To use this polymer as an anode material in flexible polymeric devices, higher values of conductivities are needed. Electrochemical polymerization is widely used to get high conductivity. However, the method is restricted to depositing the polymer only on conducting substrates. For applications of such materials in large-area electronics, deposition must be done over very large areas, and it is preferable if the deposition methods exclude electropolymerization.
Very recently, several groups are working to improve the conductivity of PEDOT using different approaches. Jönsson et al. significantly improved the conductivity of the commercially available PEDOT-PSS up to 48 S/cm for a film thickness of about 110 nm by mixing the polymer solution with sorbitol as a secondary dopant [4]. Similar works using blending with polyethylene oxide [5] or using other secondary dopants like dimethyl sulfoxide, N,N′-dimethyl formamide, tetrahydrofuran and 2,2′-thiodiethanol also significantly improved the conductivity of PEDOT-PSS in thin films up to 98 S/cm and optical transmission of 84% [6], [7].
PEDOT-PSS modified by glycerol treatment has been reported as anode material for polymer light emitting diodes [8]. Earlier, we reported polymeric photovoltaic cells using polymer anodes made of PEDOT-PSS and PEDOT-PSS modified with sorbitol treatment on a glass substrate, which demonstrated the possibility of using flexible polymer anodes in plastic solar cells [9].
Others have used the chemical oxidative polymerization of PEDOT from its monomer (3,4-ethylenedioxythiophene) by optimizing the ratio of the monomer, the oxidant (iron(III)p-toluenesulfonate (Fe(OTS)3), and a weak base (imidazole) [10], [11], [12]. With this method, conductivity as high as 750 S/cm and 81% transparency have been reported. Further enhancement of the conductivity up to 900 S/cm and 82% transparency has been achieved by using methanol-substituted EDOT [13].
Another approach is to use the vapour-phase polymerization (VPP). Mohammadi et al. [14] first reported this method as chemical vapour deposition (CVD) using FeCl3 and H2O2 as oxidants for polymerizing polypyrrole films. The VPP method to produce conducting polymers like polypyrrole and PEDOT has been reported by Kim et al. [15], [16] using FeCl3 as oxidant. The conductivity of PEDOT obtained by this method reached up to 70 S/cm. Winther-Jensen et al. further developed the method of de Leeuw to improve the conductivity by using base-inhibited VPP of EDOT and reported conductivities as high as 1025 S/cm for a film thickness of 250 nm [17], [18]. Other studies showed that PEDOT could further be electrochemically oxidized to modify its work function and also further enhance the electrical conductivity for improved performance [19], [20], [21].
From all these studies, so far it is the VPP that gives the highest conductivity of PEDOT. Therefore, in this work, we used the base-inhibited VPP method to produce PEDOT anodes for photovoltaic devices. We also made devices from PEDOT-PSS and PEDOT-PSS modified with sorbitol on glass substrate for comparison. In all the devices, a polyfluorene copolymer poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′3′-benzothia- diazole)](APFO-3) mixed with [6,6]-phenyl-C61-butyric acid methylester (PCBM) in 1:4 ratio was used as the active layer.
Section snippets
Experimental
A microscope glass slide was cut into four pieces and cleaned with a 5:1:1 mixture of deionized H2O, 25% NH3 and 28% H2O2 at 85 °C for 5 min to remove organic contaminants. It was further washed with distilled water several times and dried, followed by oxygen plasma treatment. A 40% solution of Fe(III) tosylate (Fe(OTS)3) in butanol (Baytron C) is used as an oxidizing agent. 0.5 mol of pyridine per mole of oxidant was used for base-inhibited vapour-phase polymerization. The mixture of pyridine and
Results and discussion
The VPP PEDOT and PEDOT-PSS doped with sorbitol provide highly conducting anodes. The conductivity values for different anodes made on glass substrate are summarized in Table 1. The transmission properties of the anodes were also studied (Fig. 2) and are summarized in Table 1. The single transmission values refer to the averages of the RGB transmission at , 520 and 480 nm. The morphology of the films was studied by AFM (Fig. 3) and the surface showed rough nanostructure features. This can
Conclusion
The photovoltaic devices with VPP PEDOT and PEDOT-PSS doped with sorbitol anodes showed an order of increase in PCE under sunlight illumination conditions compared to devices with anodes made of PEDOT-PSS. The anodes showed high conductivity and transmission properties, which can be used to replace the indium tin oxide as anodes. The AFM studies showed that the anode surfaces were nanostructured.
Acknowledgements
This work was supported by the Center of Organic Electronics (COE) at Linköping University, financed by the Strategic Research Foundation SSF.
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