a novel photodiode made of hybrid organic/inorganic nanocomposite

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    A novel photodiode made of hybrid organic/inorganic nanocomposite

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    2009 J. Phys. D: Appl. Phys. 42 155502


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    J. Phys. D: Appl. Phys. 42 (2009) 155502 (6pp) doi:10.1088/0022-3727/42/15/155502

    A novel photodiode made of hybridorganic/inorganic nanocompositeWaleed E Mahmoud1

    Faculty of Science, Physics Department, King Abdulaziz University, Jeddah, Saudi Arabia

    Received 12 April 2009, in final form 16 June 2009Published 7 July 2009Online at stacks.iop.org/JPhysD/42/155502

    AbstractNovel hybrid organic/inorganic nanocomposites made of metal oxide and conjugated polymernanocomposite and its application in bulk-heterojunction solar cells were studied. Thecomposite was composed of different concentrations of strontium titanate (SrTiO3) andpolyaniline doped phosphoric acid. The optimum concentration of strontium titanate wasfound to be 0.2 v/v. An inorganicorganic photovoltaic device with a structure ofAg/PaniH3PO4SrTiO3/Al has been fabricated. The ideality factor value of the diode wasfound to be 1.8. This n value of the diode implies a deviation from ideal junction behaviour.The barrier height b value for the diode was found to be 0.56 eV. TheAg/PaniH3PO4SrTiO3/Al diode shows a photovoltaic behaviour with a maximumopen-circuit voltage Voc of 2.49 V, and short-circuit current Isc of 5.6 mA under lightillumination = 460 nm. The conversion efficiency was found to be 5.2%. It is evaluated thatthe Ag/PaniH3PO4SrTiO3/Al diode is a good photodiode with calculated electronicparameters.

    1. Introduction

    The inorganicorganic nanocomposites have attracted muchattention due to their large potential applications in thefield of optics [1, 2], electronics [3], mechanics [46]and photoconductors [7]. The synthesis of -conjugatedconducting polymers that are stable in air has stimulated theiruse as active components in electronic devices. These devicesare considered by many in the field to shape the next generationof cheap and disposable electronic inventions. A betterpolymer based device to fabricate is a hybrid organic/inorganicSchottky diode in which a junction is formed between ap-doped polymer and an n-doped inorganic semiconductor.

    The challenge in organic photovoltaic devices is toachieve efficient charge separation of electrons and holescreated by the absorption of photons. Charge separationin polymer/metal oxide systems is achieved by fast transferof photoexcited electrons from the polymer to the metaloxide. To further enhance the efficiency of devices, anovel structure is suggested. This suggestion is a differentapproach to create a bulk-heterojunction between polyaniline(electron donor) and strontium titanate (electron acceptor) byblending the two materials. This structure should enhance

    1 Permanent address: Faculty of Science, Physics Department, Suez CanalUniversity, Ismaillia, Egypt.

    the transport of electrons and holes through the acceptor anddonor material, respectively, in a preferred direction to theextracting electrodes. In addition, this structure is believed toenhance rectification of the photovoltaic devices by preventingdirect pathways for the charge carriers from the cathode to theanode through either of the materials. Better rectification willimprove the fill factor and with that the efficiency of the solarcells [8].

    2. Experimental

    2.1. Materials

    Aniline monomer was distilled under reduced pressure andammonium peroxydisulfate ((NH4)2S2O8), phosphoric acid(H3PO4), nitric acid (HNO3), titanium oxide (TiO2 2H2O),strontium nitrate (Sr(NO3)2 6H2O) were used as received.These materials were purchased from Aldrich and Alpha Aesar.

    2.2. Syntheses of SrTiO3 nanocrystals

    Firstly, 2.1 g of TiO2 powder was dissolved in 50 ml of HNO3 toform TiO(NO3)2. Sr(NO3)2 was dissolved in distilled water toform a solution. Then TiO(NO3)2 and Sr(NO3)2 solutions wereadded to each other and left on a magnetic stirrer. Secondly,

    0022-3727/09/155502+06$30.00 1 2009 IOP Publishing Ltd Printed in the UK


  • J. Phys. D: Appl. Phys. 42 (2009) 155502 W E Mahmoud

    1M of NH4OH solution was added to the mixture drop by dropfor 30 min under vigorous stirring at 70 C until a brown gelformed. Then the gel was dried in an oven at 100 C for 2 h.Finally, the xerogel was calcined at 700 C for 3 h until whitepowders were finally obtained.

    2.3. Polymerization

    Freshly distilled aniline was dissolved in 1.5M phosphoric acid(H3PO4) solution. Then, SrTiO3 was added into the mixtureand stirred for 30 min. Ammonium peroxydisulfate dissolvedin water was added dropwise to it with continuous stirringfor 45 h. The polymerization temperature was maintainedat around 10 C. The resulting precipitate was washed witha mixture of water and methanol several times. Finally,the product was dried at room temperature for 24 h. In allexperiments, the molar ratio of aniline to phosphoric acidwas fixed, but the concentration of SrTiO3 nanoparticles waschanged to understand the effect of the SrTiO3 nanoparticleson the morphology, structure and electrical properties of theresulting PaniH3PO4/SrTiO3 nanocomposites.

    2.4. Characterization

    The microstructures and the particle distribution wereinvestigated by TEM (Zeiss EM 10) operating at 100 kV.The x-ray diffraction (XRD) pattern was measured using anx-ray diffractometer (Philips PW 1370) operating at 35 kVand 15 mA, using a monochromated Ni filter, Co radiation( = 0.1789 nm), a scanning rate of (2 min1) and arange 10 2 70.

    The prepared specimens used for electrical measurementswere in the form of discs of 1 104 m2 area and 0.01 mthickness. Aluminium electrode at one side was attached toone face of the samples during press (Karl Kolb, heating press,Germany), the other face coated with a silver paste.

    2.5. Photocurrent and IV curve measurements

    A 300 W xenon arc lamp from Oriel Instruments is used as anillumination source. The light passes through a CVI AB 301automated filter wheel, which is equipped with appropriatefilters and an aluminium disc to block the light beam. ACVI CM110 monochromator with 2400 grooves/mm, blazedat 400 nm is used to select a wavelength between 300 and700 nm, which is then focused on the device by an UV lens.The resulting currents in the device are recorded by a Keithley485 Autoranging Picoammeter. To correct the photocurrentspectra for the intensity spectrum of the Xe lamp, a calibrationmeasurement is needed. This is done by using a calibratedSi photodiode from Oriel Instruments to determine the powerPsource of the light at each selected wavelength, using theknown photoresponsivity spectrum of the photodiode. Thesame illumination setup is used to measure the IV curves ofthe devices. The picoammeter is replaced by a Keithley 236Source Measure Unit. The IV curves were measured in thedark and under illumination at selected wavelengths.

    Figure 1. XRD of SrTiO3 calcined at 700 C, PaniH3PO4 andPaniH3PO4SrTiO3.

    3. Results and discussion

    3.1. XRD analysis

    Figure 1 depicts the XRD pattern of the prepared SrTiO3xerogel calcined at 700 C, Pani doped with phosphoric acidand PaniH3PO4/SrTiO3, respectively. For the preparedSrTiO3 xerogel calcined at 700 C, it is clear that all peakscan be indexed to SrTiO3 with a cubic structure (JCPDS fileno 36-734) [9]. No peaks corresponding to any of the sourcematerials or other impurities are found, suggesting that a purecrystalline compound exists. The XRD pattern of Pani dopedwith phosphoric acid shows two crystalline peaks at 25 and21 and an amorphous peak at 17. Since phosphoric aciddoped Pani shows the sharpest and the most crystalline peakat 25, it has the longest order -conjugation [10]. Thismeans that the benzenoid and quinonoid units are more orderlyarranged in phosphoric acid doped Pani compared with Panidoped with the other acids [11]. The XRD pattern of Panidoped with phosphoric acid/SrTiO3 indicates that the metaloxide is intercalated with the polymer.

    3.2. TEM and SEM analysis

    The typical TEM image of the obtained SrTiO3 powderscalcined at 700 C is presented in figure 2(a). It can be seenthat most particles are the perovskite phase of SrTiO3 andtheir sizes are in the range 62 9 nm. The typical TEMimage of the PaniH3PO4 is presented in figure 2(b). Thisfigure depicts that most particles are fine and nearly sphericalwith some agglomeration and their sizes are in the range41 7 nm. Figure 2(c) depicts the SEM of PaniH3PO4SrTiO3. It indicates that the SrTiO3 nanoparticles have anucleus effect on the polymer and caused a homogeneous Panishell around them, resulting in an increase in the blend grainsize to about 120 10 nm.

    3.3. Currentvoltage (IV ) characteristics of PaniSrTiO3

    Figure 3 depicts the currentvoltage (IV ) characteristicsof PaniH3PO4SrTiO3 at ambient temperature. From thisfigure, it is found that the IV curve exhibited a strongasymmetry between the forward and reve