Abstract
We demonstrate the improved device performances by using the structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film coated onto the indium tin oxide (ITO) anodic electrode annealed at 400 °C under the normal ambient. The ITO thin films show the improved film quality with decreased dislocation density and lattice strain as annealing temperature increases. The spin-coated PEDOT:PSS film smoothens the wrinkle kind of surface morphology of the ITO film annealed at 400 °C. The annealed ITO (400 °C) with PEDOT:PSS interlayer improves the hole-current density in the hole-only devices (HODs) having the device structure of ITO/PEDOT:PSS/N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine as hole-transporting layer/Al. It enhances the efficiency of organic photovoltaic devices [ITO (annealed)/PEDOT:PSS/P3HT:PCBM (active layer)/LiF/Al] by three times higher (1.69 %) when compared to that (0.48 %) of pristine ITO based OPV device. These results show that the annealing of ITO film at the high temperature of 400 °C under the normal ambient improves the film quality and lowers the potential energy barrier at ITO/PEDOT:PSS interface for effective hole injection/extraction process, resulting in the enhanced device electrical performances. • Annealed ITO films improve film quality with decreased dislocation density and lattice strain. • PEDOT:PSS smoothens the wrinkle kind of surface morphology of ITO annealed at 400 °C. • 400 °C annealed ITO with PEDOT:PSS interlayer lowers the potential barrier and improves hole-current density in HODs. • It enhances the efficiency of OPVs by three times (1.69%) compared to that (0.48%) of pristine ITO based OPV. • 400 °C annealed ITO film lowers potential barrier at ITO/PEDOT:PSS interface for effective hole injection/extraction process.
Highlights
The annealed indium tin oxide (ITO) (400 °C) with PEDOT:PSS interlayer improves the hole-current density in the hole-only devices (HODs) having the device structure of ITO/PEDOT:PSS/N,N'-Bis(3-methylphenyl)-N,N'diphenylbenzidine as hole-transporting layer/Al. It enhances the efficiency of organic photovoltaic devices [ITO /PEDOT:PSS/P3HT:PCBM/Lithium fluoride (LiF)/Al] by three times higher (1.69 %) when compared to that (0.48 %) of pristine ITO based organic photovoltaic cells (OPVs) device. These results show that the annealing of ITO film at the high temperature of 400 °C. The annealed ITO (400 °C) under the normal ambient improves the film quality and lowers the potential energy barrier at ITO/PEDOT:PSS interface for effective hole injection/extraction process, resulting in the enhanced device electrical performances
In support of the above results, our earlier studies on ITO substrates annealed at different temperatures under the normal ambient revealed that as annealing temperature increases upto 300°C, the ITO film quality improves with slight increase in the sheet resistance (92 Ω/€ for pristine and 105 Ω/€ for 300°C), enhancing the hole injection properties at the ITO/organic interface in hole-only devices, whereas the annealing of ITO at 400°C forms the wrinkle kind of surface morphology with higher sheet resistance of 185 Ω/€, limiting the hole current through the interface in the device [24]
The transmittance of PEDOT:PSS/annealed (400°C) ITO bilayer structure is considerably decreased in the UV region (33 % at 325 nm) when compared to other films (58 % for pristine, 61 % for 200°C and 57 % for 300°C at 325 nm) without any significant change in its transmittance in the visible region (97 % for 400°C at 550 nm), which may be attributed to the significant change in the ITO film and/or in the ITO/PEDOT:PSS interface properties upon annealing of ITO at high temperature of 400°C [23, 24]
Summary
The recent technological developments and commercial applications of organic optoelectronic devices such as organic light emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) have sparked wide research interest in order to improve the device efficiency and device durability for their potential use in the generation flat panel displays, solid state lighting, and renewable solar energy harvesting owing to its ease of large area device fabrication by roll-to-roll printing, light weight, the possibility to fabricate flexible device, and cost effectiveness [1,2,3,4,5,6,7,8,9,10,11]. In support of the above results, our earlier studies on ITO substrates annealed at different temperatures under the normal ambient revealed that as annealing temperature increases upto 300°C, the ITO film quality improves with slight increase in the sheet resistance (92 Ω/€ for pristine and 105 Ω/€ for 300°C), enhancing the hole injection properties at the ITO/organic interface in hole-only devices, whereas the annealing of ITO at 400°C forms the wrinkle kind of surface morphology with higher sheet resistance of 185 Ω/€, limiting the hole current through the interface in the device [24] These reports show that the film quality (bulk), surface chemical and morphological properties of ITO thin film are the important parameters to make it more efficient to be used as anodic electrode in the optoelectronic devices. This bilayer structure is utilized in the OPVs for the improved device performances
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