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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

(http://iopscience.iop.org/0022-3727/42/15/155502)

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

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 inorganic–organic photovoltaic device with a structure ofAg/Pani–H3PO4–SrTiO3/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 φb value for the diode was found to be 0.56 eV. TheAg/Pani–H3PO4–SrTiO3/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 λ = 460 nm. The conversion efficiency was found to be 5.2%. It is evaluated thatthe Ag/Pani–H3PO4–SrTiO3/Al diode is a good photodiode with calculated electronicparameters.

1. Introduction

The inorganic–organic nanocomposites have attracted muchattention due to their large potential applications in thefield of optics [1, 2], electronics [3], mechanics [4–6]and photoconductors [7]. The synthesis of π -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 4–5 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 Pani–H3PO4/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(λ = 0.1789 nm), a scanning rate of (2θ◦ min−1) and arange 10 � 2θ � 70.

The prepared specimens used for electrical measurementswere in the form of discs of 1 × 10−4 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 I–V 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 I–V curves ofthe devices. The picoammeter is replaced by a Keithley 236Source Measure Unit. The I–V curves were measured in thedark and under illumination at selected wavelengths.

Figure 1. XRD of SrTiO3 calcined at 700 ◦C, Pani–H3PO4 andPani–H3PO4–SrTiO3.

3. Results and discussion

3.1. XRD analysis

Figure 1 depicts the XRD pattern of the prepared SrTiO3

xerogel calcined at 700 ◦C, Pani doped with phosphoric acidand Pani–H3PO4/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 π -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 Pani–H3PO4 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 Pani–H3PO4–SrTiO3. 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. Current–voltage (I–V ) characteristics of Pani–SrTiO3

Figure 3 depicts the current–voltage (I–V ) characteristicsof Pani–H3PO4–SrTiO3 at ambient temperature. From thisfigure, it is found that the I–V curve exhibited a strongasymmetry between the forward and reverse bias. The I–V

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

Figure 2. (a) TEM of SrTiO3 calcined at 700 ◦C, (b) TEM of Pani doped phosphoric acid and (c) SEM of Pani–H3PO4–SrTiO3.

-5

-3

-1

1

3

5

7

9

11

13

-10 -5 0 5 10

V (Volt)

I (m

A)

Figure 3. I–V characteristic curve of Pani–(0.3 v/v) SrTiO3.

curve is the characteristic of a Schottky diode. In order toquantitatively analyse the diode characteristics we assume thestandard thermionic emission model of a Schottky junction asfollows [12]:

J = Js

[exp

(q V

n k T

)− 1

], (1)

Js = A∗T 2 exp

(−qφb

kT

), (2)

where J is the current density, Js is the saturation currentdensity, q is the electron charge, k is the Boltzmann constant,T is the absolute temperature, φb is the barrier height, n is theideality factor and A∗ is Richardson’s constant.

Figure 4 shows a relation between the logarithm of thediode current versus the applied voltage under forward biasconditions at 300 K. At low bias a linear variation of the currentis observed consistent with equation (1) while the deviationfrom linearity at higher bias voltages is generally related toOhmic losses due to the diode series resistance. Extrapolatingthe linear portion of the semi-log plot to zero bias yields asaturation current density of 5.95×10−2 A cm−2 and the diodeideality factor calculated from the slope of the linear portionof the plot as follows:

n = q

kT

(∂V

∂ ln J

). (3)

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0 2 4 6 8 10 12

V (Volt)

Lo

g I

303K

333K

363K

393K

Figure 4. The semi-logarithmic plot of the I–V curves of theAg/Pani–SrTiO3 diode at various temperatures.

The ideality factor (n) which is equal to 1 for an idealdiode usually has a value greater than unity. High values ofn (=1.8) can be attributed to the presence of the interfacialthin native oxide layer (SrTiO3), series resistance, barrierinhomogeneity, etc [13, 14].

It is known that A∗ in equation (2) should dependon the effective mass of the charge carrier, which determinesthe mobility of carrier transport in the device. Hence,the values of A∗ were evaluated for Ag/Pani–SrTiO3 as1.42 × 10−7 A cm−2 K−2. The values of A∗ for the deviceare very low in comparison with A∗ for free electron(120 A cm−2 K−2) implying that thermionic mechanismoperates predominantly.

In order to get further insight into the mechanism ofcurrent transport through the Ag/Pani–SrTiO3 contact, the I–V

characteristics were acquired at different temperatures in therange 303–393 K with a temperature step �T of 30 K. Figure 4shows the semi-logarithmic plot of the I–V curves of theAg/Pani–SrTiO3 diode at various temperatures.

From a linear fit of the semi-log forward I–V plots infigure 4, the barrier height (φb) and n values were calculatedby the slopes and intercepts of the linear fits. The determinedvalues of n and φb are presented in figure 5 as a functionof temperature. As generally expected, while φb increaseswith increasing temperature, n decreases. It can be seen fromfigure 5 that n decreases from 1.8 to 1.65 by increasing thetemperature. This can also be seen from the I–V curvesof the as-fabricated diode in figure 4. The diode shows analmost ideal behaviour at high temperatures while it presents adeviation from linearity at low bias and low temperatures. On

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

1.5

1.55

1.6

1.65

1.7

1.75

1.8

300 320 340 360 380 400

T (K)

Ideality

facto

r (n

)

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Bar

rier

hie

gh

t ( φ

b)

n bφ

Figure 5. Ideality factor and barrier height as a function oftemperature.

Figure 6. The short-circuit current (Isc) and open-circuit voltage(Voc) as a function of SrTiO3 concentration.

the other hand, the φb increases from 0.56 eV to 1.13 eV byincreasing the temperature from 303 K to 373 K, respectively.

Since current transport across the metal/semiconductorinterface is a temperature activated process, electrons at lowtemperatures are able to surmount the lower barriers andtherefore current transport will be dominated by the currentflowing through the patches of lower barrier heights [15, 16].When the temperature increases, more and more electronshave sufficient energy to surmount the higher barrier. As aresult, the dominant barrier height will increase with increasingtemperature and bias voltage [17].

3.4. Effect of SrTiO3

Figure 6 shows short-circuit current (Isc) and open-circuitvoltage (Voc) changes at different SrTiO3 concentrations. Isc

was dependent on the concentration of SrTiO3 nanoparticles.Jsc increased up to a concentration of 0.2 v/v and thendecreased. This shows that the SrTiO3 nanoparticles networkhas a good connection with Pani for better transfer of theelectrons injected from the Pani molecules. One can concludethat a concentration of 0.2 v/v of SrTiO3 is an optimal conditionfor Pani–H3PO4–SrTiO3 solar cell for both particle connectionand for fast electrons transfer.

Figure 7. I–V characteristics of Pani–H3PO4 with and withoutSrTiO3.

In order to determine the effect of SrTiO3, the photovoltaiccharacteristics of Pani–H3PO4 and Pani–H3PO4–SrTiO3 wereinvestigated. As shown in figure 7, the electrical propertieswere very much improved by the addition of 0.2 v/v SrTiO3

to Pani compared with Pani alone, and the electric powerconversion was much enhanced. Since the lowest unoccupiedmolecular orbital (LUMO) of Pani is above the SrTiO3

conduction band (CB) [18], the ELUMO − ECB (SrTiO3)difference presents an enthalpic driving force for electroninjection from Pani to SrTiO3. Similarly, since the highestoccupied molecular orbital (HOMO) of Pani is below that ofsilver [19], it offers a driving force for hole injection into theelectrode. This is the major mechanism for charge separationin the Pani–H3PO4–SrTiO3 solar cells.

The open-circuit voltage (Voc) of Pani–H3PO4–(0.2 v/v)SrTiO3 is 2.49 V and the short-circuit current (Isc) is 5.6 mA.This representation gives information about the fill factor of thephotovoltaic and therefore gives some essential informationabout the efficiency of the cell. The fill factor FF can becalculated according to the following relation [20]:

FF = IMaxVMax

IscVoc. (4)

The sample shows FF that is equal to 0.42. The overallefficiency of a photovoltaic is called the power conversionefficiency. It can also be expressed in terms of Isc, Voc andFF according to [21]

η = VocIscFF

Plight. (5)

The power conversion efficiency is 5.2%. The slope ofthe characteristics around the open-circuit point (I = 0 mA,V = 2.49 V) indicates a rather high-series resistance, whichprobably lies at the SrTiO3 grains–polymer interface.

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

Figure 8. PR and EQE of Pani–H3PO4–(0.2 v/v) SrTiO3 as afunction of wavelength.

(This figure is in colour only in the electronic version)

3.5. Photoresponsivity, external quantum efficiency andrectification

An important property of a solar cell is the wavelengthspectrum for which it absorbs light and produces aphotocurrent. This spectrum helps in determining whethera solar cell is capable of converting the available lightilluminating it into electric energy. One possibility isto measure the photoresponsivity of the solar cell. Thephotoresponsivity is defined as [22]

PR = Isc(λ)

Psource(λ).

The photoresponsivity spectrum PR(λ) of a solar cell canbe determined by scanning through the wavelength rangeavailable from the light source. A different quantity oftenused to characterize the spectral behaviour of solar cells isthe external quantum efficiency EQE. The external quantumefficiency is defined as [23]

EQE = hc

e· PR

λ.

Figure 8 depicts the photoresponsivity and EQE of thedevice Pani–H3PO4–(0.2 v/v) SrTiO3 as a function of incidentphotons wavelength. From this figure it is clear that bothPR and EQE show a peak of photocurrent 23 mA W−1 andquantum efficiency 5.16%, respectively, at a wavelengthof 460 nm.

The I–V characteristics of the blend device are plottedon a semi-log scale in figure 9. The device has four ordersof magnitude rectification in the dark and three orders ofmagnitude rectification under illumination.

One can conclude that after photon absorption, it is likelythat the created electron–hole pair forms a singlet exciton.This means that the electron and hole remain on the samepolymer chain and are bound to each other by their electrostaticattraction and there are no free charge carriers.

Since the electron and hole are bound together, amechanism must be found to efficiently separate electron andhole and to prevent recombination of the two. A possibilityto achieve this charge separation is by introducing an electronacceptor that dissociates the exciton by transferring the electron

Figure 9. I–V characteristic curve of Pani–H3PO4–(0.2 v/v) SrTiO3

under dark and illumination.

from the polymer (therefore being the electron donor) to themetal oxide (the electron acceptor material). As a result, thepolymer is left with a P+ polaron that can drift through the filmto the anode while the electron is in the acceptor material andcan be transported to the cathode. The main condition for theexciton dissociation to occur is that the electron affinity of theacceptor is larger than the ionization potential of the donor.

4. Conclusion

Polyaniline doped different concentrations of strontiumtitanate were successfully prepared. The nanocompositesare characterized by the XRD and TEM studies. Theaverage particle size of the nanocomposite was about120 nm. The optimum volume fraction of the synthesizednanocomposites was 0.2 v/v. This nanocomposite exhibitsSchottky diode-like behaviour. The ideality factor decreasedwith temperature and the barrier height increased withtemperature. The I–V characteristic curve of the Pani–SrTiO3 film, under illumination, shows photovoltaic cell-likebehaviour. The device shows photovoltaic behaviour witha maximum open-circuit voltage Voc of 2.49 V and a short-circuit current Isc of 5.6 mA at λ = 460 nm. This confirms thatAg/Pani–H3PO4–SrTiO3/Al diode is a promising photodiode.

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