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Chemical Physics 330 (2006) 501–505

High open-circuit voltage in UV photovoltaic cell based onpolymer/inorganic bilayer structure

Hui Jin, Yanbing Hou *, Xianguo Meng, Feng Teng

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology,

Beijing Jiaotong University, Beijing 100044, China

Received 15 July 2006; accepted 20 September 2006Available online 1 October 2006

Abstract

Polymer/inorganic hybrid ultra violet (UV) photovoltaic device is fabricated by using poly(N-vinyl-carbazole) (PVK) and zinc sulfide(ZnS). The device shows promising photovoltaic characteristics with a high open-circuit voltage of 1.65 V, and a short-circuit current of46.8 lA/cm2 under the illumination of 340 nm UV light with the intensity of 14 mW/cm2. Besides, much correlative photocurrent spectraof the device under forward and reverse applied bias manifest the transport mechanism of charge carriers in PVK/ZnS bilayer systems.� 2006 Elsevier B.V. All rights reserved.

PACS: 72.40.+w; 72.80.Le

Keywords: Ultra violet; Polymer; Photovoltaic

1. Introduction

As photoelectronic materials, semiconducting polymershave many advantages, such as low cost, ease of fabrica-tion, and high absorption. Due to these remarkable advan-tages, electronic components made out of semiconductingpolymers have aroused the interest of many researchers[1–3]. In this perspective, photovoltaic (PV) devices withp-type conjugated polymer as an active component becomeone of the most attractive fields [4,5]. In these PV devices,polymers act as both a light absorber and carrier transportlayer. Upon irradiating, bound electron–hole pairs are gen-erated in the polymer and diffuse towards interfaces wherethey are dissociated into free electrons and holes. However,single layer polymer photodiodes are usually inefficient dueto the difficulty in dissociating the tightly bound excitonsproduced by photoexcitation [6,7]. Efficient exciton dissoci-ation can be achieved at the interface between electron-

0301-0104/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemphys.2006.09.034

* Corresponding author. Tel.: +86 10 51688605; fax: +86 10 51688018.E-mail address: ybhou@science.njtu.edu.cn (Y. Hou).

accepting and hole-accepting materials, which may bearranged in an intermixed [8,9] or a bilayer structure[10,11]. Although intermixed systems can increase dissocia-tion interfaces, transport of carriers remains to be a chal-lenging problem. Hybrid devices based on somecombination of organic and conventional inorganic thinfilms have attracted much attention, because organic andinorganic materials can take their own advantages in thesebilayer hybrid devices [12,13]. Inorganic materials havehigh carrier mobility, good chemical and structural stabil-ity. After many decades of researches, a wide variety ofinorganic semiconductor materials have been well devel-oped, and their fabrication technique and properties arerefined significantly. Besides, cost for fabricating devicescan be reduced by replacing one or more of their compo-nents with an organic analogue. It is found that organic/inorganic composites are expected to be used in variousoptoelectronic fields, such as photoconductive detectorsor solar cells [14,15], and electroluminescence devices[16,17].

In present work, the bilayer thin-film devices based onthe hybrid of poly(N-vinyl-carbazole) (PVK) and zinc

502 H. Jin et al. / Chemical Physics 330 (2006) 501–505

sulfide (ZnS) were fabricated. PVK is selected as polymercomponent because it is a traditional hole-transportingpolymer and has high absorption within ultra violet (UV)region. The best performance is currently obtained insolid-state dye-sensitized photovoltaic cell with PVK holetransporter, yielding power conversion efficiencies of2.0% under AM1.5 illumination and the open-circuit volt-age of 0.72 V [18]. ZnS is chosen as inorganic componentbecause it is one kind of direct gap semiconductors withhigh optical transition probability for absorption [19]. Itis also is suitable for ZnS to serve as an electron acceptordue to its conduction and valance band matching theLUMO and HOMO energy level of PVK. The absorptionof ZnS in UV region is high and partly overlaps the absorp-tion region of PVK (as shown in Fig. 1). Substantially, theacceptor of ZnS, in the hybrid devices, is expected toenhance the dissociation of photogenerated excitons andthe electron transport, and also to improve open-circuitvoltage. In this work, attributing to electronic characteris-tics and photogeneration of carriers in ZnS, the layer ofZnS plays an important role in transport of free carriersafter hole–electron pairs dissociate at the interfacesbetween PVK and ZnS.

2. Experimental

Polymer-inorganic bilayer hybrid devices, ITO/PEDOT/PVK/ZnS/Al were fabricated on 30 mm · 40 mmITO-glass coated with poly(3,4-ethylene dioxythiophene)doped with poly(styrene sulfonate) (PEDOT: PSS), whichcan decrease the roughness of ITO and improves the qual-ity of the contact. The devices with the structure of ITO/PEDOT/PVK/Al and ITO/PEDOT/ZnS/Al were fabri-cated for comparison. PVK film was spin-cast from thechloroform solution of PVK and uniform with the thick-ness about 700 A. ZnS film of 50 nm thickness was depos-ited by electron-beam evaporation. During deposition, thefilm thickness is monitored with a quartz crystal microbal-

325 350 375 4000.0

0.2

0.4

0.6

0.8

1.0

PVK/ZnS

PVK

ZnS

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

Fig. 1. Absorption spectra of ZnS, PVK, and the hybrid of ZnS and PVK.

ance. The substrate temperature was set at 100 �C. Al topelectrode was evaporated to form devices with an activearea of 2 mm2. All samples were encapsulated in a glovebox filled with dry nitrogen. In the glove box, the concen-trations of both O2 and H2O were lower than 5 ppm.

Current–voltage curves were recorded by a Keithely2410 Source Measure Unit both in the dark and under illu-mination. The illumination light was located at the wave-length of 340 nm with an intensity of 14 mW/cm2. Thephotocurrent spectra were measured by the combinationof a Keithley 2410 source-measure unit and a Xe lamp(450 W) dispersed through a monochromator as a photoex-citation source. The photocurrent spectra were correctedaccording to the spectral distribution of illumination light.Positive voltage is defined as ITO electrode biased posi-tively and the samples were illuminated through the ITOelectrode.

3. Results and discussion

I–V characteristics of the hybrid device with the struc-ture of ITO/PEDOT/PVK/ZnS/Al in the dark and underillumination of 340 nm UV light with the intensity of14 mW/cm2 are shown in Fig. 2. Under the illuminationof UV light, the hybrid device exhibits an obvious photo-voltaic activity. Table 1 shows the short-circuit currentdensity (Isc), open-circuit voltage (Voc), fill factor (FF),and power conversion efficiencies (g) of the investigateddevice and the single layer PVK device. g is calculatedaccording to the expression of g ¼ ðIVÞmax

LA , where L is theintensity of the illumination, and A is the active area ofthe device. The bilayer PVK/ZnS hybrid device presentsmuch better properties, especially a high open-circuit volt-age of 1.65 V. This higher Voc causes a significant enhance-ment of Isc. However, due to the poor carrier transport ofPVK, the g obtained for the hybrid device still is low,although it is much higher than that of the single layerdevice.

-2 -1 0 1 2

1E-3

0.01

0.1

1

10

100

Cur

rent

Den

sity

(μΑ

/cm

2 )

Voltage (V)

1.65V

Fig. 2. The current–voltage characteristics under illumination (solid line)and in the dark (square + line) for the hybrid photocell with the structureof ITO/PEDOT/PVK/ZnS/Al.

Table 1Photovoltaic devices parameters of the bilayer device of ITO/PEDOT/PVK/ZnS/Al and the single layer device of ITO/PEDOT/PVK/Al (underk = 340 nm with the light intensity of 14 mW/cm2)

Isc

(lA/cm2)Voc

(V)FF

ðIVÞmax

I scV oc

� �g ðIVÞmax

LA

� �

ITO/PEDOT/PVK/ZnS/Al 46.8 1.65 0.20 0.11%ITO/PEDOT/PVK/Al 2.5 0.8 0.25 0.004%

H. Jin et al. / Chemical Physics 330 (2006) 501–505 503

The hybrid device shows the Voc of 1.65 V, which indi-cates that Voc is not simply determined by the differencebetween the work-function of electrodes or donor andacceptor energy levels, but is highly dependent on themolecular properties of semiconducting materials and/orthe physical/chemical interfacial arrangement [20]. How-ever, the similar feature is unavailable in such bilayer sys-tems as ITO/PEDOT/PVK/CdS/Al and ITO/PEDOT/MEH-PPV/ZnS/Al, whose Voc is much lower (�0.7 V).Accordingly, the higher Voc may be caused by the interfa-cial action between PVK and ZnS rather than that at theinterface between ITO/PEDOT and PVK or between ZnSand Al. In our views, when ZnS layer is deposited on top

300 325 350 375 400

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1.0

Nor

mal

ized

Sho

rt-C

ircui

t Pho

tocu

rren

t (a.

u.)

Wavelength (nm)

PVK ZnS PVK/ZnS

Fig. 3. The short-circuit photocurrent spectra of hybrid device, ITO/PEDOT/PVK/ZnS/Al (triangle). The PC spectra of PVK (square) andZnS (circle) also are shown for comparison.

Fig. 4. The band energy diagram of ZnS and PVK. (a)–(c) illustrates the traPEDOT/PVK/ZnS/Al under short-circuit conditions, in reverse and forward b

of PVK, there is a profound effect on the final value ofthe Voc. A possible explanation is suggested that a dipolelayer at the interface between PVK/ZnS occurs becauseof polar molecule properties of ZnS. As well known, dipolelayer would change the effective band alignment and addcomplication to the determination of Voc [21]. In the pro-cess of depositing ZnS, polar molecules of ZnS begin togrow and interact with PVK molecules so as to build updipole bonds, and thus dipole layers at the interfacebetween PVK and ZnS are formed. The direction of thedipole layer is same with that of the electric field providedby the build-in potential, and the Voc of the devices withZnS is obviously raised.

Fig. 3 shows the normalized spectral response of short-circuit photocurrent (PC) of the hybrid device with thestructure of ITO/PEDOT/PVK/ZnS/Al. For the sake ofcomparison, the PC spectra of the devices with single layerof PVK and with single layer of ZnS are shown too. Asshown in Fig. 3, the PC spectrum of the PVK/ZnS hybriddevice is much wider than that of PVK, and the PC peaksignificantly shifts towards short wave. These featuresresult from photogeneration and transport mechanism ofcharge carriers in PVK/ZnS bilayer hybrid devices. Fig. 4

nsport processes of photogenerated carriers in the hybrid device of ITO/ias, respectively.

300 325 350 375 4000

100

200

300

400

Pho

tocu

rren

t (μΑ

/cm

2 )

Wavelength (nm)

-3V

3V

0V

Fig. 5. The spectral response of photocurrent at applied potential of ±3 Vfor hybrid devices with the structure of ITO/PEDOT/PVK/ZnS/Al. ThePC spectra under short circuit conditions are also shown for comparison.

Fig. 6. The (IV)max values of various thickness combinations for PVK/ZnS hybrid devices.

504 H. Jin et al. / Chemical Physics 330 (2006) 501–505

illustrates the photogeneration and the transport of chargecarriers in PVK/ZnS hybrid devices. There are two pre-mises which must be considered. First, ZnS in the bilayerdevices is not transparent for the incident light of340 nm. Second, the incident light illuminates through theITO electrode. Under these two considerations, the mostof incident light is absorbed by the PVK layer, and themain excitons are generated in PVK. When they diffuseto the interfaces of PVK/ZnS, the photoexcited excitonsare partly dissociated into separate electrons and holes.After the dissociation of excitons, electrons entering tothe conduction band of ZnS and the holes remaining inPVK are pushed to two electrodes efficiently by the highbuild-in field. Furthermore, the ZnS layer absorbs residualincident light, and electrons and holes generated in ZnStraverse the interface of PVK/ZnS or semiconductor/metalwithout potential barrier (as shown in Fig. 4a). Therefore,since the photoexcited carriers in ZnS contribute to photo-current of the hybrid device, its PC spectrum of hybriddevice is broadened and shifts towards the photocurrentpeak of ZnS.

The characteristics of PC spectra in opposite appliedbias (0 V, and ±3 V) in Fig. 5 further prove our above anal-ysis about the processes of photogeneration and transportof charge carriers in PVK/ZnS hybrid devices. The highestPC occurs at the reverse bias of �3 V, and the lowerincrease of PC appears at the forward bias of 3 V. It isbecause that the reverse external electric field helps thebuild-in potential to extract carriers to respective contacts(as illustrated in Fig. 4b). At forward bias, the lowerincrease in PC is ascribed to the counteraction betweenthe external and built-in electric field, which weakens thetransport of charge carriers to contacts. Furthermore, atforward bias, the energy offset between ZnS and PVKbecomes a potential barrier for the electrons in ZnS trans-

porting to Al electrode via PVK, and for the holes remain-ing in PVK to ITO electrode (Fig. 4c). Accordingly, theoutput of PC in forward bias is lower than that in reversebias.

In fact, the energy conversion efficiency of the hybriddevice can be influenced by the thickness of ZnS andPVK. In order to find out this optimum point or range,the hybrid devices with various thicknesses of ZnS andPVK were investigated. The thickness of ZnS was chosenas 30, 50, 100, and 200 nm, respectively, and the thicknessof PVK was as 40, 50, 70, 90, and 110 nm, respectively.The (IV)max values of various thickness combinationsare expressed by color mapping in Fig. 6. The red starsare lower and indicates lower efficiencies, while the bluestars represent the higher one. According to Fig. 6, thehigher conversion efficiencies mainly belong to the deviceswith the ZnS layer of 50 nm. Comparing with ZnS, theinfluence of the thickness of PVK on the efficiency isweaker. The optimum thickness of ZnS layer should beabout 50 nm, and that of PVK can be optionally around100 nm.

4. Conclusions

Polymer/inorganic hybrid UV photovoltaic devices werefabricated. The device designed to deposit an inorganicmaterial (ZnS) on top of a polymer (PVK) achieves a highopen-circuit voltage. Under the illumination of 340 nmwith the intensity of 14 mW/cm2, the device shows theVoc up to 1.65 V, the Isc of 46.8 lA/cm2, a fill factor of0.20 and a monochromatic conversion efficiency of 0.1%.The dipole layer between ZnS and PVK is believed to bethe reason which causes the higher Voc. To conclude,PVK/ZnS bilayer structures can be potentially used as akind of UV photovoltaic devices.

H. Jin et al. / Chemical Physics 330 (2006) 501–505 505

Acknowledgements

This work is supported by Trans-Century Training Pro-gram Foundation for the Talents of Natural Science by theState Education Commission, Key Project of Chinese Min-istry of Education (No. 105041), National Natural Science& Foundation Committee of China (NSFC) (project No.90401006, 10434030 and 90301004), State key project ofbasic research (2003CB314707) and the Key Laboratoryof Specially Functional Materials and Advanced Manufac-turing Technology, South China University of Technology,Ministry of Education, China. The authors greatly appreci-ate Prof. Gang Yu’s help. One of the authors(Hui Jin)isalso grateful to the Doctor Innovation Foundation of Bei-jing Jiaotong University for financial support.

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