The PEDOT:PSS brand Clevios PH500 (Series number:81076210) from Heraeus Precious Metals GmbH & Co. was used without any alteration. The solutions were filtered by using 0.5µm nylon filter on the glass syringe before being mixed it with the dopant materials.
3.1.1 Synthesis of glycerol and ethylene glycol doped PEDOT:PSS
Glycerol and ethylene glycol (EG) are the two alcohol based materials which have the identical physical properties such as colourless, odorless and liquidity. Both were used without any alteration and mixed with PEDOT:PSS solution. Glycerol and EG were purchased from Sigma Aldrich Sdn. Bhd. with code number of G7893 and 324558respectively.The doping amount of glycerol and EG were determined by volume rate ratio. For example, 2% doped glycerol mixed to PEDOT:PSS was based on the volume of PEDOT:PSS, e.g: 5ml. Hence, the volume of 2% glycerol is:-
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3.1
The amount of glycerol and EG doped to PEDOT:PSS were varied from 0% (which also named pristine PEDOT:PSS) to 8% with 2% interval. Each doping material was mixed to 5ml PEDOT:PSS and was placed under ultrasonic bath at room temperature for 1 hour.
Subsequently all solutions were aged for another 24 hours before the deposition process.
3.1.2 Synthesis Of Zinc Oxide (ZnO) and aluminum zinc oxide (AZO) doped PEDOT:PSS.
The aim of this process is to synthesize zinc oxide solution and three aluminum doped zinc oxide solutions with different aluminum doping percentage from 2% to 6% with 2% interval. Then the solutions were mixed with PEDOT:PSS liquid in a simple process of ultrasonic bath to assure complete mixing between the metal oxide and semiconducting polymer materials.
The ZnO and AZO solutions of each aluminum percentage were prepared using sol-gel technique. First, the precursor solution of zinc oxide (ZnO) was prepared using zinc acetate dehydrate, Zn(CH3CO2)2.2H2O as astarting material and dissolved in 2-methoxyetahnol. Few drops of diethanolamine (DEA) were used as stabilizer. This solution was then divided into four different vials. While one of the solution remains undoped, the other three were added with aluminum which obtained from aluminum nitrate Al(NO3)3.9H2O precursor with variation of atomic percentage, 2.0, 4.0 and 6.0wt %. Then all solutions including the pristine (undoped aluminum) were stirred at 3000 rpm under magnetic stirrer for 2 hours, and annealed at 60oC. The resultant solutions which were very
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clear and transparent were then kept ageing in vials for more than 48 hours before film deposition process.
3.1.3 Preparation of doped PEDOT:PSS thin film.
The solution of all doped PEDOT:PSS were used in preparing the thin film on the cleaned glass substrate by spin coating technique. Since the solution is based on organic substance that tends to decompose with a heating treatment, this method is suitable owing to its process which operates only under room temperature. Moreover, this method is identified to produce a uniform and controllable thickness of the organic thin film.
The WS400B-6NPP-Lite spin coater instrument was used in this work which required a nitrogen gas to control the pressure to hold the sample on its disk plate. The spin speed, spin duration and spin mode can be adjusted and programmed in order to obtain the desired thickness of doped PEDOT:PSS thin film.
In the present work, the thicknesses of all studied doped PEDOT:PSS were intended to be approximately constant since the focus is to study the effect on different type of material doping and their doping amount to PEDOT:PSS. Thus the spin speed for all doping PEDOT:PSS solutions were maintained at 3000 rpm and spun for about 60 s which expected to form doped PEDOT:PSS thin films with thicknesses only in the range of 100 nm to 150 nm. This range is the optimum thickness for buffer layer component in order to assist the absorption of phonon light to the active layer region and as hole transport layer to anode electrode.
Firstly, the cleaned glass substrate was placed at the middle of the disk plate inside the spin coater. The glass substrate was holding on the disk plate under vacuum condition while the spinning process is in progress. The instrument was set-up at the desired speed and duration before the spin coating process started. After the deposition process completed
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the glass substrate was dried at 120C for 20 minutes to remove the moisture in the deposited thin film. Later, all doped PEDOT:PSS thin films were ready for the characterization.
3.1.4 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) as n-type in thermoelectric device
PTCDA powder was purchased from Sigma Aldrich and consists of a red powder in colour. It was deposited on cleaned glass substrates under vacuum environment by a thermal evaporation technique. The deposition pressure is 1 x 10-3 Mbar and the time taken for a deposition process was around 3 min. This process produced PTCDA thin films with the thickness approximately ~100 nm.
3.1.5 Tris(8-hydroxyquinoline) aluminum (III) (Alq3) as n-type in thermoelectric device
The process of Alq3 deposition is very much alike with PTCDA, where the yellowish Alq3 powder which was purchased from Sigma Aldrich was deposited under vacuum pressure of 10-3 Mbar for about 3 min. The Alq3 thin films with thickness around 100 nm were then deposited on the cleaned glass substrates under the thermal evaporation process.