Solid State Communications 149 (2009) 771–774

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

A ZnO/PEDOT:PSS based inorganic/organic hetrojunctionBhupendra K. Sharma a, Neeraj Khare a,∗, Shahzada Ahmad ba Department of Physics, IIT Delhi, New Delhi-110016, Indiab National Physical Laboratory, New Delhi-110012, India

a r t i c l e i n f o

Article history:Received 7 December 2008Received in revised form22 January 2009Accepted 27 February 2009 by D.D. SarmaAvailable online 9 March 2009

PACS:73.40.Lq73.40.Ei73.30.+y78.55.Et

Keywords:A. HetrojunctionsA. SemiconductorsA. PolymersD. Photoluminescence

a b s t r a c t

An inorganic/organic hetrojunction has been fabricated by spin coating the p-type polymer poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on an n-type zinc oxide (ZnO) film. TheZnO film was deposited by the ultrasonically assisted chemical vapor deposition method on a patternedindium tin oxide (ITO) coated glass substrate. Goldwas deposited on top of the PEDOT:PSS film by thermalevaporation. The current–voltage (I–V ) characteristic of ZnO/PEDOT:PSS shows diode-like behavior. TheI–V characteristic was examined in the framework of the thermionic emission model. The ideality factorand barrier height were obtained as 3.8 and 0.63 eV respectively.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Zinc oxide (ZnO), a direct wide band gap (≈3.3 eV) semicon-ducting material having large exciton binding energy (≈60 meV)at room temperature, is a very promising material for short wave-length optoelectronic devices and transparent electronics [1–5]. Torealize high performance ZnO based optical and electronic devices,high quality Schottky junctions or p–n junctions are required. Thereproducible growth of p-type ZnO is still a problem which is cre-ating a hindrance in realizing ZnO p–n homojunctions. Alternativeapproaches such as the fabrication of metal–ZnO Schottky junc-tions have been attempted. There are several reports on the fab-rication of Schottky contacts using high work function metals (Au,Pd, Pt, etc.) and n-type ZnO [6–11]. The presence of high donor con-centration in the surface region due to native defects (oxygen va-cancies/zinc interstitials) quite often poses problems in realizinggood quality metal–semiconductor Schottky junctions. It has beenobserved that the junction properties are highly dependent on thepreparation methods of metal deposition and surface treatmentconditions [12]. Another alternative approach can be to use a p-type polymer for fabricating p–n junctions of ZnO [13,14]. The fab-rication of inorganic/organic hetrojunctions using a solution based

∗ Corresponding author. Tel./fax: +91 11 2659 1352.E-mail address: nkhare@physics.iitd.ernet.in (N. Khare).

0038-1098/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2009.02.035

process for depositing a polymer film on an n-type semiconduc-tor is expected to avoid trap states at the interfaces due to smallphysical/chemical stress at the interface.In this paper we report the formation of an inorganic/organic

hetrojunction by depositing the p-type polymer poly(3,4 ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on an n-type ZnO film. The method employed here offers a extremely sim-ple and scalable route to form inorganic/organic hybrid films.PEDOT:PSS is one of the most widely studied electroactive

polymers because of its excellent film forming ability and highconductivity [15]. PEDOT:PSS is transparent and has good photostability. PEDOT:PSS has been used as the hole transport layerbetween an ITO anode and the absorption/emissive layer inpolymer solar cells and LEDs [16,17]. In the present study wehave used PEDOT:PSS as the p-type polymer for fabricatingZnO/PEDOT:PSS hetrojunctions.

2. Experimental details

ZnO film was deposited on a patterned indium tin oxide (ITO)coated glass substrate by the ultrasonically assisted chemical vapordeposition technique. Prior to depositing the ZnO film, a singlenarrow ITO strip on the glass substrate was patterned using wetetching. The patterned substrate was cleaned successively withacetone, deionized water and propanol, using an ultrasonic bath.

772 B.K. Sharma et al. / Solid State Communications 149 (2009) 771–774

Fig. 1. Schematic of (a) the molecular structure of PEDOT:PSS and (b) ITO/ZnO/PEDOT:PSS/Au multilayers.

A 0.1 M solution of zinc acetate in deionized water was used asthe spraying solution. Pyrolysis of the ultrasonically created misttook place at 450 ◦C on the patterned ITO coated glass substrate.The deposition was carried out for 45 min, resulting in a ZnO filmof thickness∼500 nm. After the deposition, the film was cooled toroom temperature at the rate of 2.5 ◦C per min. The as-depositedfilm was annealed in vacuum (∼4 × 10−5 torr) at 450 ◦C forone hour. The resultant n-type semiconducting film has resistivity∼0.15� cm.PEDOT:PSS was used for the fabrication of a p–n hetrojunction

with ZnO. Fig. 1(a) shows the molecular structure of PEDOT:PSSin which the π-conjugated system is responsible for the holetransport in PEDOT. An aqueous colloidal dispersion of PEDOT:PSS(Baytron P) was purchased from Bayer AG. Baytron P is an aqueousdispersion of the intrinsically conductive polymer PEDOT/PSSand the formulation consists of the PEDOT:PSS ratio as 1:2.5(by weight). The dispersion consists of submicrometer sized gelparticles which upon drying form a continuous film which isconducting and transparent.Fig. 1(b) shows the schematic diagram of a ZnO/PEDOT:PSS/Au

device. The p-type polymer PEDOT:PSSwas spin coated (2000 rpm,60 s) on the vacuum annealed ZnO film. The film (thickness∼150 nm) was then slowly heated at 80◦C for one hour and thenleft overnight to cool gradually, to avoid cracks. A thin layer of goldwas deposited on top of the PEDOT:PSS by the thermal evaporationtechnique. Two contacts, one from a narrow ITO strip and theother from gold, were made for current–voltage measurements. Inaddition to this we also fabricated an ITO/PEDOT:PSS/Au structurefor the measurement of the conductivity of PEDOT:PSS.Structural characterization of ZnO film was carried out by

X-ray diffraction (model: X/pert pro XRD MPD PANalytical).Photoluminescence spectra of the ZnO film were studied witha spectrofluorometer. The surface morphology and roughnessof the ZnO film were observed using atomic force microscopy(AFM). A Keithley source meter (model: 2400) was used for thecurrent–voltage (I–V ) measurements.

3. Results and discussion

Fig. 2 shows the X-ray diffraction (XRD) pattern of the ZnO filmdeposited on the glass substrate. It exhibits the characteristic peaksfor ZnO having the wurtzite structure with lattice parameters a =b = 3.3155 Å and c = 5.1853 Å.Fig. 3 shows photoluminescence (PL) spectra of vacuum

annealed ZnO film at room temperature. A small peak at 383 nmand a bigger broad peak at 495 nm were observed. The peak at383 nm in the UV emission region is related to near band edgeemission. It originates from the recombination of excitons nearthe band edge. The strong green emission peak corresponding to495 nm is related to defects such as oxygen vacancies or/and Zninterstitials. It has been observed that the possible mechanism

Fig. 2. X-ray diffraction pattern of ZnO film.

Fig. 3. Photoluminescence spectra of vacuum annealed ZnO film. The inset showsthe absorption spectra of the same ZnO film.

for the green band in ZnO is due to the donor–acceptor pairrecombination [18,19] which is governed by the transition fromthe conduction band to acceptor oxygen vacancies. The opticalabsorption spectra of the ZnO film is shown as a inset of Fig. 3.There is a sharp absorption at ∼383 nm indicating its band gap,∼3.2 eV.We annealed the ZnO film at 450 ◦C in vacuum (10−4 torr).The appearance of a broad peak at 495 nm indicates that thevacuum annealed ZnO film has a lot of oxygen vacancies/Zninterstitial defects. The resistivity of our as deposited ZnO filmwas∼500� cm, and after vacuum annealing the resistivity of the filmdecreased to 0.15� cm. The decrease in the resistivity of ZnO filmis due to the presence of oxygen vacancies which introduce donorelectrons and make it an n-type semiconductor.Fig. 4 shows an AFM image of the ZnO film. The surface

morphology of the film is reasonably smooth, with uniform graindistribution. The surface roughness was found to be∼17 nm.Fig. 5 shows the current–voltage (I–V ) characteristic of

ITO/ZnO/PEDOT:PSS/Au. A positive voltage was applied to the goldelectrode and a negative voltage was applied to the ITO electrode.The I–V characteristic shows diode-like behavior. In order tocheck that the observed nonlinearity is due to the ZnO/PEDOT:PSSinterface and not due to transport in PEDOT:PSS, we measuredthe current–voltage characteristic of ITO/PEDOT:PSS/Au. The insetof Fig. 5 shows the linear behavior of the current–voltagecharacteristic of the ITO/PEDOT:PSS/Au structure in the voltagerange −1 to +1 V, indicating that the observed nonlinearity in

B.K. Sharma et al. / Solid State Communications 149 (2009) 771–774 773

Fig. 4. AFM image of vacuum annealed ZnO film.

Fig. 5. I–V characteristic of a ZnO/PEDOT:PSS hetrojunction. The inset shows theI–V characteristic of ITO/PEDOT:PSS/Au.

the ITO/ZnO/PEDOT:PSS/Au structure is due to the ZnO/PEDOT:PSSinterface.Fig. 6(a) shows the energy band diagramof ITO, ZnO, PEDOT:PSS

and Au. The work functions of Au (φAu ≈ 5.1 eV) and PEDOT:PSS(φPEDOT:PSS ≈ 5.2 eV) are similar, leading to formation of an Ohmiccontact. There is also not much difference in the work function ofITO and the electron affinity of ZnO, and an Ohmic contact alsoforms there. Due to difference in energy levels of PEDOT:PSS andZnO, a barrier is formed at the interface which leads to diode-like characteristics, as observed by us in the experiment. Fig. 6(b)shows the barrier formation at the interface and the flowof chargesin the forward bias situation.

The diode-like behavior of the ZnO/PEDOT:PSS hetrojunctionwas examined using the thermionic emission model [20]. Accord-ing to this model, the junction under forward bias has the I–V re-lation as

I = Is

[exp

qVnKT− 1

]where Is is the saturation current, q is the elementary charge,V is the applied forward voltage, n is the ideality factor, K isthe Boltzmann constant and T is the absolute temperature. Thesaturation current Is is expressed as

Is = AA∗T 2 exp(−qΦbKT

)where A is the junction area, A∗ is the effective Richardson constantand Φb is the barrier height at the ZnO/PEDOT:PSS interface. Thevalue of A∗ for ZnO is ≈36 A cm−2 K−2 [21] and the junctionarea in the present case is ≈6.0 × 10−2 cm2. In order to find outthe values of n and Φb of the junction a graph of ln(I) againstV was plotted. Fig. 7 shows the semilogarithmic plot of currentas a function of bias voltage. The slope and the intercept fromthe linear fit to the semilog plot yield the ideality factor, n =3.8, and the barrier height, Φb = 0.63 eV. The resultant valueof the ideality factor is comparable to the recently reported nvalue for a polyaniline/ZnO p–n junction [13]. According to theSah–Noyce–Shockley model [22], the value of the ideality factoris expected to be 1.0 at low voltage and 2.0 at higher voltage.The higher value of n observed in our case indicates that thebehavior of ZnO/PEDOT:PSS junction diode deviates from the idealbehavior. This may be due to the presence of surface states in ZnO.These surface states provide additional energy states which areresponsible for the existence of multiple current pathways [23–26]. In our case lot of defects has been introduced into the ZnOfilm by vacuum annealing the ZnO film as verified by the largedefect related PL intensity of the vacuum annealed ZnO film.These defects provide additional energy states and hence multiplecurrent pathways, yielding A higher value of THE ideality factor.

4. Conclusion

In summary, a ZnO/PEDOT:PSS hetrojunction exhibiting diode-like characteristics has been successfully fabricated. The idealityfactor and barrier height were calculated by applying thethermionic emission model and were found to be 3.8 and 0.63eV, respectively. The higher value of ideality factor is attributed tothe existence of multiple current pathways due to the presence ofdefects in the vacuum annealed ZnO film.

Acknowledgments

Financial support from the Council of Scientific and IndustrialResearch (CSIR) is gratefully acknowledged. One of us (BKS) isthankful to CSIR, New Delhi, India, for awarding a Junior ResearchFellowship.

Fig. 6. Schematic of (a) the energy band diagrams of ITO, ZnO, PEDOT:PSS and Au, and (b) band bending due to the interface formed between PEDOT:PSS and ZnO.

774 B.K. Sharma et al. / Solid State Communications 149 (2009) 771–774

Fig. 7. Semilogarthimic plot of current (ln(I)) and voltage (V ) for a ZnO/PEDOT:PSShetrojunction.

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