Introduction and optoelectronic properties of Inorganic semiconductors. Especially

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Last updated: September 28, 2019

IntroductionWelive a life completely surrounded by electronic appliances. Currently, theimpact of electronic technology is prominent in health, economy, security,broadcasting, and in almost every other field of life. Electronics technologyflourished rapidly after the advent of semiconducting materials. Inorganicsemiconducting material like silicon, germanium and gallium arsenide remainedprime focus of material scientists and engineers due to their remarkableproperties. Inorganic semiconducting materials are fulfilling most of thetechnological requirements of modern world but still there exists some areaswhere efficiency of inorganic semiconducting materials is limited.

 Recently, organic semiconducting materialsare  found promising alternative toinorganic semiconductors due to potential advantages they offer such as, lowcost and low temperature processing, availability of number of solvents,environment friendly nature, physical flexibility and  compatibility with almost all kinds ofsubstrates.1, 2. Organic semiconductors canefficiently replicate the semiconducting and optoelectronic properties ofInorganic semiconductors. Especially the organic semiconductors have attracteda lot of attention as active photosensitive materials due to their strongsensitivity to a particular range of  wave lengths only 2.Organic electronic structures offer more compatibility with bio systems ascompared to inorganic devices in terms of flexibility, stretch ability andextent of mechanical softness which is not possible with Inorganic devices3. Fromtechnological point of view, photodetectors have various applications likedigital cameras, automatic brightness adjustment of the high dimensional LCDs,camcorders and plasma displays in varying ambient light levels2.

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Organic materials and polymers have been studied extensively for fabrication ofoptoelectronic devices as these materials show sensitivity only to a particularwavelength of spectrum so these materials can be used as wavelength detectorsin photosensitive devices. The use of soluble conjugated polymers as activematerials in optoelectronic and photonic applications has opened up thepossibility of fabricating optoelectronic devices with remarkable properties4.Relative advantages offered by soluble conjugated polymers include lowerprocessing cost, flexibility and higher absorption coefficient 4.

The functional limitations of organic photodetectors can be overcome byemploying bulk heterojunction (BHJ)5.BHJ technique involves blending of donor (D) and acceptor (A) materials2.D material has relatively low ionization potential and the A material possessconsiderable higher electron affinity and charge mobility6.Interpenetration of the D and A materials enables efficient flow of chargesthrough the device5.The performance of device can be further improved by precisely controlling theconcentration of D and A materials in the blend5.Amongthe organic semiconducting materials, Phthalocyanines (PCs) exhibit remarkablechemical and physical properties7.

The PCs exhibit versatile properties due to their unique molecular structure.Two hydrogen atoms in the central cavity of PCs molecule can be replaced bymore than 70 substituent materials. The optical band gape of PCs stronglydepends upon the material bound in their central cavity8.Their molecular structure possesses ? electron system due to which goodphotosensitivity in the visible spectral region can be obtained. PCs exhibitstrong absorption in the visible spectral region (400-800nm) which makes themsuitable for visible light optoelectronic applications 7.It is previously reported that the electron mobility in PCs can be as high as theholes mobility. Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO),an important member of PCs family, shows high absorption in upper region ofvisible spectra( 600nm-800nm)7.

VOPcPhO has another advantage over PCs that, it can be readily dissolved  innumber of organic solvents2Poly(2-methoxy-5(2′-ethyl) Hexoxy-Phenylenevinylene) (MEH-PPV) has been consideredas an important p-type conjugated polymers for optoelectronics and photovoltaicapplications4.MEH-PPV acts as an electron donor material with potential advantages likesignificant conducting properties and environmental stability4.Analysis of optical spectrum of MEH-PPV shows strong absorption in 500-600nmrange of the spectrum5.

The mobility of conjugated polymers can be enhanced significantly byincorporating organic or inorganic nanoparticles9.Inorganic semiconducting nanoparticles like TiO2,ZnO and CdSe show excellent performance when used as electron acceptor alongwith conjugated polymers9.  Currently, composites comprising inorganicsemiconductor nanocrystals and organic semiconductor are widely used inphotonic and optoelectronic applications10.The device based on organic-inorganic composites combines potential advantagesof both types of material like ease of fabrication and solution processabilityof organic semiconductors and strong carrier mobility of inorganic nanoparticles11.

CdSenanoparticles exhibit efficient charge transport properties when used forfabrication of visible light detectors. The potential advantages of CdSenanoparticles when incorporated with organic materials include strongphotoluminescence quenching and fast switching between the operational statesof device. Both of these qualities are of prime concern for the performance ofa photosensitive device9, 12.

  PreviouslyCdSe nanoparticles have been used as an acceptor with P3HT and MEH-PPV for the fabricationof photovoltaic and photosensitive devices 9, 11-13. This research project will focuson the effects of CdSe nanoparticles on the photosensitivity of donor acceptorblend of MEH-PPV and VOPcPhO. Figure 1 shows molecular structures of MEH-PPV,VOPcPhO and Hexagonal structure of CdSe Nanoparticles.                                                                                                                                                   (C)                                                                                   Figure 1:  (a) Molecular structure of MEH-PPV (b) VOPcPhO(c) Hexagonal structure of CdSe Aims and ObjectivesFollowing are the objectives ofthis study:1.      Toinvestigate the photo-detective properties of organic inorganic ternary blendbased on vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO),Poly(2-methoxy-5(2′-ethyl) Hexoxy-Phenylenevinylene) (MEH-PPV) and Cadmiumselenide (CdSe).2.      Toexplore photosensitivity of the ternary system to visible wavelengths of the spectrum(400-790nm).

3.      Toinvestigate the effect of CdSe nanoparticles on the n-type mobility of blend Research MethodologyThe research plan will be executed in the followingsteps.Preparation of solutions:Thevery 1st step of intended research project is the preparation ofsolutions. 10mg/ml standard solution of both MEH-PPV and VOPcPhO will beprepared separately. Literature reveals that all the three materials show goodsolubility in chlorobenzene and in chloroform.

Therefore, any of these twosolvents would be used as a common solvent for preparation of blend in thefollowing Volumetric ratios.           MEH-PPV        :    VOPcPhO        :                 CdSe               2                    1                1 mg               2                    1                2 mg               2                     1                3 mg               2                    1                4 mg        Photoluminescence CharacterizationForthe analysis of PL spectra, all these concentration will be spin coated onsimple glass substrates. PL spectroscopy will be performed by using Ramanspectrophotometer. The best ratio can be selected only after the analysis of PLspectra of all these concentrations. The PL spectra showing best quenching willthen be selected further for fabrication of the device.

Device Fabrication Onthe basis of PL spectra, the best concentration will be then used for thefabrication of photodetector. Indium tin oxide (ITO) coated glass will be usedas a substrate. ITO will act as a bottom electrode. Aqueous solution ofPoly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS) will bespin coated as a buffer layer. The PEDOT:PSS will ensure the smooth flow ofcharge carriers due to its good conductive properties.

Post depositionannealing will be employed to smoothen the deposited film. The active layerwill then be spin coated over PEDOT:PSS layer. Aluminum top electrode, actingas cathode, will be deposited by vacuum evaporation. Figure 2 shows schematicstructure of the intended device.              Figure2: Cross sectional view of ITO/PEDOT:PSS/MEH:PPV,VOPcPhO,CdSe/Al based   BHJ  Photo detector CharacterizationPerformanceof photodetector can be analyzed on the basis of various optoelectronicparameters. Characterization techniques like UV-VIS spectroscopy, current-voltageresponse, calculation of response and recovery time will be performed tooptimize various operational parameters of the device.

Analysis of the resultsof these characterization techniques will reveal operational efficiency of thedevice.Required Apparatus andEquipmentTheintended research work will be executed at Material Research Laboratory, CentralResource Laboratory, University of Peshawar. For further technical support andhelp, other universities and institutes engaged in technical research will becontacted as required. Following are the major equipments to be used during thecourse of research work:1)      DigitalMicro balance2)      Ultrasonicbath3)      Hotplate electric heater4)       Magnetic stirrer5)      Spincoater 6)      Thermalevaporator7)      RamanSpectrometer8)      UV-VisSpectrometer9)      Current-voltageanalysis apparatus 10)   External quantum efficiency analysis apparatus        Flow ChartThe overall researchplan can be presented in the form of flow chart as shown below.     Preparation of solutions                                                                                                                                                               Preparation of Blends with different volumetric Ratios         Deposition and PL                        characterization of thin films         Analysis of PL Results                                                                                                                                         Suitable Results?                                                                                                       NO                                                                                                                                                            YES           Fabrication and                Characterization of Device  

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