Effect of ZnSe nanoparticles in the PEDOT: PSS photodiodes
Abstract
In this article we report the photovoltaic properties of devices made using a highly conducting polymer electrode, from poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) incorporated with ZnSe nanoparticles on ITO substrate as an anode and a Poly(3-hexylthiophene-2,5-diyl) (P3HT) mixed with Phenyl-C61-butyric acid methyl ester (PC61BM) in the ratio of 1:1 as the active layer. The device performance was compared with that of devices made with PEDOT-PSS without ZnSe nanoparticles on ITO substrates. The IV characteristics were studied and the surfaces of PEDOT:PSS with ZnSe were imaged using Transmission electron microscopy (TEM). It shows that the functions of the photodiode devices increase with the addition of ZnSe nanoparticles. The improvement is linked to two physical effects: the increased scattering efficiency of the fine ZnSe nanoparticles and .
Keywords: PEDOT:PSS; light scattering; hybrid organic-inorganic composite; hybrid photodiodes, ZnSe.
Introduction
Improving the efficiency of photo detectors are always a popular research issue due to its impending applications in various fields like visible light communication, chemical analysis, water purification, flame detection, cameras, camcorders and plasma displays [1,2]. Addition of nano particles like GaN, SiC, ZnO, TiO2, CdSe and ZnSe has shown considerable increase in the efficiency of photo detectors [3-9]. Among these several kinds of semiconductor nanocrystals, CdS, and CdSe nanocrystals are the most extensively studied ones, whereas researches on ZnSe nanocrystals are comparatively limited. ZnSe is considered as an n-type semiconductor material with wider band gap of ~2.7 eV at room temperature and transmittance range (0.5–22 µm). It demonstrates tunable blue-ultraviolet (UV) luminescence. This UV range is virtually inaccessible for cadmium-based systems such as CdSe, other than this the toxicity of cadmium represents an additional disadvantage. Thus ZnSe is more appropriate for the fabrication of short-wavelength photodiodes than other semiconductor materials.
ZnSe is an important semiconductor that is widely applied in various industries. ZnSe is highly suitable for short-wavelength lasers, light-emitting diodes, blue laser diodes, and tunable mid-IR laser sources (Chen et al., 2011). Optoelectronics operating in the blue-green region is one of the areas in which ZnSe has been widely used. Semiconductors made of Zinc Selenide blue lasers have also been developed in the recent years (Chang and Lii, 1998). Besides, semiconductor has recently been used under high temperature conditions.
ZnSe has also been recently used in the preparation of a uniform, well-defined ZnSe-(diethylenetriamine) 0.5 ([ZnSe](DETA)0.5) nanobelts. In this case, solvothermal reactions in a ternary solution facilitated by hydrazine hydrate has been used (Chen et al., 2011). Recent studies have also demonstrated the synthesis of nitrogen-doped graphene/ZnSe (GN-ZnSe) nanocomposites using ZnSe nanoparticles supplied by [ZnSe]-(DETA)0.5. In this case, ZnSe is deposited on the nitrogen-doped graphen (GN) surface in the shape of nanorods composed of ZnSe nanoparticles (Chen et al., 2011). Apart from its application as an optoelectronic material, it is also likely to be suitable for use in windows, lenses, beam expanders,output couplers, and optically-controlled switching (Zhang, 2009). Studies on the chemical and physical properties of ZnSe have not been explored widely. Consequently, not many insights have been gained into the properties and several applications of ZnSe. The current study focuses on how ZnSe can influence the properties of photodiodes. Studies have also shown thatZnSe has widey helped in improving the efficiency of photodiodes. In this research, the efficiency of Solar Cell with PEDOT:PSS enhanced using ZnSe and that which not enhanced with ZnSe were compared. Efficiency was measured in terms of the density of current. Studies have shown that normalized efficiency vary depending on the concentration of ZnSe in PEDOT: PSS. Based on this fact, the study sought to determine this relationship.
Experimental
The aqueous solution of CLEVIOSTM PH-1000 PEDOT:PSS [poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)] with hole conductivity of 900–1000 S/cm was commercially obtained from H.C. Starck (Goslar, Germany). Poly(3-hexylthiophene-2,5-diyl) (P3HT) and Phenyl-C61-butyric acid methyl ester (PC61BM) were purchased from Sigma Aldrich. ZnSe nanoparticles. In this research work, all commercially purchased photoactive materials were used as received without any further purification. The chemical structures of PEDOT:PSS , P3HT and PC61BM are depicted in Fig. 1. For the fabrication of the OPDs, first the pre etched ITO anode substrates (25 mm × 25 mm) were washed properly by sonication in ample amount of soap water, DI water, acetone, ethanol, isopropyl alcohol and DI water sequentially. The substrates were blown to dry in nitrogen stream. 5 wt. % slurry of PEDOT:PSS with average size 40 nm of ZnSe nanoparticles was prepared by mixing ZnSe nanoparticles in PEDOT-PSS aqueous solution. It was stirred by using a magnetic stirrer for one hour. After filtering using 0.45 µm PVDF filter, PEDOT-PSS:ZnSe slurry in various ratio (1, 0.8, 0.6, 0.2 weight %), was coated onto ITO substrate using spin coating followed by annealing at ~ 100 °C for 30 mins. Solutions of P3HT and PC61BM were prepared independently in chloroform by stirring 10 mg of each material in 1 ml chloroform (via magnetic stirrer at room temperature overnight). The organic photoactive layer (thickness ~ 140 nm) of P3HT: PC61BM mixed in an optimum blend ratio was deposited on PEDOT:PSS-ZnSe buffer layer. Organic photoactive layer deposition was later annealed at 100 °C for 30 min. Aluminium (Al) cathodes were finally thermally deposited on top of the photoactive layer using shadow mask (active area ~ 0.045 mm2 and thickness ~100 nm). The devices were subjected to post-fabrication annealing at 100 °C for 30 min. The entire fabrication process of the hybrid organic-inorganic PD was carried out in ambient condition.
Photocurrent density-voltage (J-V) characteristics of the photodiode in dark and under illumination were recorded by programmable Keithley 236 source measuring unit (SMU). For photocurrent response measurement a Xe and Hg(Xe) lamp was used as light source whose output power was controlled by NEWPORT 69907 Oriel Digital Arc Lamp Power Supply.
(a) (b)
(c)
This experiment finds that ZnSe nanoparticles exhibit a significant influence on the PEDOT: PSS photodetector. Precisely, the experiment finds that the efficiency of the PEDOT: PSS photodetector actually varies with the concentration of ZnSe nanoparticles. In table 1, the data shows that the concentration of ZnSe is inversely proportional to the efficiency of the photodetector. In this case, the PEDOT: PSS photodetector recorded the highest efficiency when ZnSe concentration of 0.2 mg/mL was used. On the other hand, the lowest efficiency was recorded when the concentration of ZnSe used was 0.8mg/mL (0.44%).
The results also show that the current density increase with the increase in voltage as shown in figure 4. However, the different concentration values of ZnSe show varying trends. To illustrate, the curve representing 0.2mg/mL of the ZnSe showed that the concentration is highly responsive to the changes in the current density. The figure shows that 0.2mg/mL is the least responsive to changes in voltage. In figure 5, an increase in voltage increases with the increase in power up to the voltage of between 0.2 and 0.3. This shows that the optimum current for the photodetector is between 0.2 and 0.3 volts. The concentration of 0.2 mg/ML of ZnSe shows the highest power at the optimum current. However, the concentration at 0.8 mg/mL exhibits the lowest power. Figure 5 shows the relationship between normalized efficiency and ZnSe concentration. According to the results, the highest efficiency was achieved when the concentration of ZnSe used was 0.2. Therefore, this experiment finds that ZnSe influences the efficiency of PEDOT: PSS photodetector. In this case, the optimum concentration of ZnSe that can result into the highest efficiency is 0.2mg/mL. Therefore, ZnSe should be used in enhancing the efficiency of PEDOT: PSS photodetector. It is one of the strategies that should be used to improve the efficiency of the photodetector.
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