Inclusion of a variety of compounds such as DMSO

Inclusion of a variety of compounds such as DMSO, EG in the aqueous dispersion of PEDOT:PSS has enabled an increase in the electrical conductivity of the films. Si/PEDOT:PSS hybrid solar cell also offers freedom to the use of additives such as dimethyl sulfoxide (DMSO) or ethylene glycol (EG) to PEDOT:PSS solution, which reportedly enhances the PCE (Power conversion efficiency) of the device structure significantly by reducing the thickness of PSS shell of PEDOT particle. Besides DMSO, Nitric acid (HNO3) has also been used to modify the resistivity of hybrid semiconductor heterojunction 4. Application of PEDOT:PSS thin film is not limited to organic devices but can also be extended to organic/inorganic hybrid semiconductor devices. Simplest of these devices which enable a perfect understanding of the nature of electrical behavior of the polymers are Metal-polymer-semiconductor (MPS) Schottky diodes 9, 10.
Current study focuses on the fabrication of PEDOT: PSS/n-Si heterostructures with excellent electrical characteristics for optoelectronic applications. The organic layers are coated by spin coating technique offering a cost effective fabrication method. Effect of addition of DMSO in PEDOT: PSS solution on the thickness and characteristics of the films are evaluated. Further, the as fabricated devices are treated with HNO3 to enhance the conductivity of the organic layers. The devices were systematically characterized using various techniques and optimized results are presented.
PEDOT:PSS/n-Si hybrid heterojunctions were fabricated as follows: the n-Si wafer with 100 orientation and resistivity ~ 3-5 ?-cm and thickness 525 µm was cleaned in acetone, isopropanol, and distilled water respectively in an ultrasonic bath for 10 min each and then immersed in dilute hydrofluoric acid (10 %) for one min to remove the oxide layer. The cleaning procedure was repeated and the substrate was dried. PEDOT:PSS (Sigma Aldrich – 1.3 %) and PEDOT:PSS mixed with DMSO (1:5 wt %) were spin coated on n- type Si wafer with 5000 rpm for 30 s. In order to obtain a homogenous film, the spin coated layers were annealed on a hot plate for 20 min at 120 °C in ambient atmosphere. The procedure was repeated multiple times to obtain the desired thickness. Finally Nitric acid vapor treatment for 3 min was carried out to modify the resistivity of hybrid heterostructures. Schematic diagram of PEDOT:PSS/n-Si heterojunction with and without treatment is shown in figure 2.The as prepared heterostructures were characterized using Cross-sectional High Resolution Scanning Electron Microscopy (HRSEM-FEI Quanta FEG 200; range of magnification: 60000 X; Voltage: 20 kV), Atomic Force Microscopy (AFM – Park XE 100) as well as Raman spectroscopy. Transmittance was measured by a UV-visible spectrophotometer (V-760, JASCO). The electrical characteristics were investigated by Vander Pauw-Hall measurements using Ecopia HMS-5000 to determine the carrier concentration, resistivity, conductivity and mobility of PEDOT:PSS/n-Si with and without treatment. An I-V probe station (Keithley source meter 2250) was used to record Current (I) – Voltage (V) characteristics of the heterojunction. Cross-sectional SEM images of PEDOT:PSS/n-Si heterojunctions are shown in Figure 3. The measured thicknesses are 200 nm for S1, 150 nm for S2, 130 nm for S3, 230 nm for S4 and 94 nm for S5 respectively. From cross sectional SEM image, formation of PEDOT:PSS layer on n-Si was confirmed. The reduced thickness of sample (S2) is attributed to the washing-away of unassociated PSS–rich layer on the film surface upon dipping 2. For samples S3 and S5, PSS-rich domains disappear post the HNO3 vapor treatment 12 which accounts for the reduced thickness. The removal of PSS chains, post –treatment with DMSO has also been confirmed by the reduction in the thickness of the film (S4) 13.

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