preliminary photovoltaic response from a polymer containing p-vinylenephenylene amine backbone

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doi:10.1016/j.so

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Solar Energy Materials & Solar Cells 91 (2007) 1289–1298

www.elsevier.com/locate/solmat

Preliminary photovoltaic response from a polymer containingp-vinylenephenylene amine backbone

Jianyuan Suna, Zhiqun Hea,�, Linping Mua, Xiao Hana, Junjie Wanga, Bin Wanga,Chunjun Lianga, Yongsheng Wanga, Yingliang Liub, Shaokui Caob,��

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

Beijing Jiaotong University, Beijing 100044, PR ChinabSchool of Materials Science and Engineering, Zhengzhou University, 75 Daxue Road, Zhengzhou 450052, PR China

Received 12 December 2006; received in revised form 11 April 2007; accepted 26 April 2007

Available online 13 June 2007

Abstract

Optoelectronic properties from a novel polymer, poly(p-phenylene N-4-n-butylphenyl-N,N-bis-4-vinylenephenylamine) (PNB) have

been investigated. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the

material were estimated to be �5.18 and �2.75 eV, respectively, measured with cyclic voltammetry. A single-layer device structure was

prepared by spin-coating PNB thin films from a solution on top of an indium–tin oxide (ITO) substrate while aluminum was used as a

top electrode. Current density–voltage (J–V) characteristic was measured which showed a typical rectifying behavior. Photovoltaic from

a single-layered device was observed under a white arc lamp illumination. This was improved via a double-layer structure comprising

vacuum evaporated copper phthalocyanine (CuPc) or N,N0-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) as an

additional layer. The open-circuit voltage, short-circuit current and hence the efficiency were improved in the double-layer devices. An

ITO/PNB/PTCDI-C13/Al device was estimated to have external quantum efficiency (EQE) around 1% at 330 nm. In a comparison of

optical absorption and photocurrent spectra, it was demonstrated that the excitons could be separated and further, generated carriers

drifting to the opposite electrodes more efficiently in the double-layer cells.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Poly(p-phenylene N-4-n-butylphenyl-N,N-bis-4-vinylenephenylamine); Copper phthalocyanine; N,N0-ditridecylperylene-3,4,9,10-tetracarboxylic

diimide; Photovoltaic; Photocurrent spectra

1. Introduction

Since 1839, when Becquerel first observed photovoltaiceffect in an electrolytic solution, great progress has beenmade in photovoltaic cells utilizing clean and renewableenergy sources from sunlight. Among which, inorganicsolar cells made from silicon materials have reached theapplication requirement and have been manufactured intoproducts. However, they are expensive in fabrication andrequire complex manufacture processes, which limit theirfuture developing. One potential alternative to inorganic

e front matter r 2007 Elsevier B.V. All rights reserved.

lmat.2007.04.026

ing author. Tel.: +8610 51688675; fax: +86 10 51683933.

sponding author.

sses: zhqhe@bjtu.edu.cn (Z. He), caoshaokui@zzu.edu.cn

solar cells is to use organic or polymer solar cells [1–3],which may offer a low cost in fabrication over a large areaand also a potential of being made onto flexible substrates[4–11].In early 1980s, the polymeric cells investigated comprised

of polyacetylenes, polythiophene, etc. as an active material,where only low quantum efficiency (QE) and low open-circuit voltage (VOC) were obtained [12,13]. It was suggestedthat the low VOC maybe due to the formation of thepolarons and the energy relaxation which reduced thep�p* gap. The first investigation of a poly(p-phenylenevinylene) (PPV) single-layer photovoltaic device withindium–tin oxide (ITO) and aluminum as electrodes wasmade in 1993 which reported a VOC of 1V and a powerconversion efficiency (PCE) of 0.1% under a white lightillumination [14], and a PPV and buckminsterfullerene,

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C60, double-layered device was reported in the same year[15]. A large number of researches in polymeric photo-voltaic cell have been focused on PPV or its derivatives. Tooptimize device performance, it is required to improveexciton dissociation efficiency and the charge transportingof the materials. Using poly(2-methoxy-5-(2-ethyl-hexy-loxy)-1,4-phenylene vinylene) (MEH-PPV) and C60, anefficient charge separation was achieved by Yu [16] with aninternal heterojunctions approach which improve the PCEto ca. 2.9%. A fine control of the domain size comparableto the charge diffusion length would improve the perfor-mance [17]. In addition, a number of other polymermaterials have been reported to show photovoltaicresponses [18–20]. Recent reported photovoltaic devicesbased on polythiophene–fullerene composites reached aPCE approaching 4–5% in laboratory [21–23]. However,organic and polymer photovoltaics are still in an immaturedevelopment stage. The interests in screening and devel-oping a wide range of polymeric materials for photovoltaicapplication, understanding physical mechanism of thedevice, as well as the product development towardscommercialization are high.

Poly(p-phenylene N-4-n-butylphenyl-N,N-bis-4-vinyle-nephenylamine) (PNB) contains PPV segment in its back-bone but that was separated by amine group. Theconjugation was modified along the chain. In a previousstudy, this polymer demonstrated a super thermal stabilityand interesting photoactive properties [24]. Electrolumi-nescence device has been prepared which emitted greenlight upon electrical excitation. However, photovoltage wasalso observed upon photoexcitation in a single-layereddevice [25]. Our preliminary results suggested a possibilityof dual exciton sites in this polymer, which formed thecompetition between the recombination of carriers to formexcitons and the dissociation of excitons. Both optoelec-tronic processes are reversed in mechanism. Manipulationof either the materials or the structure of the devices mayoptimize the performance and enable to gain a properunderstanding. It is a key to learn how these processes are

Fig. 1. Chemical structures of P

related and converted to each other. In addition, thispolymer is solution processable. It is an advantage inpolymer thin film processing. It can be a promisingmaterial for future device development.In this paper, a preliminary investigation is performed,

focused on optical and electronic properties of PNB, inparticular, the photovoltaic behavior of this polymer.Photocurrent spectral response as well as the currentdensity–voltage (J–V) characteristics of the PNB-basedheterojunction devices were analyzed to reveal the under-neath mechanism. Double-layered devices were prepared inconjunction with two low molecular weight organicmolecule materials, copper phthalocyanine (CuPc) andN,N0-ditridecylperylene-3,4,9,10-tetracarboxylic diimide(PTCDI-C13). Photovoltaic response, the VOC or theshort-circuit current (ISC), from double-layer cells werestudied. Our result demonstrated that the PNB polymer isa good photovoltaic material and has the potential for usein photovoltaic applications.

2. Experimental

PNB was synthesized as in previous work [24]. CuPc andPTCDI-C13 were purchased from Aldrich. Molecularstructures of the three materials are shown in Fig. 1.Optical absorption spectra were recorded using a

Shimadzu UV-3101PC spectrometer. Cyclic voltammetry(CV) measurement was performed on a CHI660A electro-chemical workstation. A three-electrode configuration cellin one compartment was used: a Platinum (Pt) diskworking electrode of 3mm in diameter, a Pt wire counter-electrode and a Ag/AgCl reference electrode. A solution(0.1M) of tetrabutylammonium perchlorate in acetonitrilewas used as medium. The apparatus was purged with anitrogen flow. The polymer films were deposited on theworking electrode by solution casting and the scan rate was20mV/s. Ferrocene was used as a reference material.Thin films of PNB were prepared from a tetrahydrofuran

(THF) solution using spin coating (KW-4A Spin Coater)

NB, CuPc and PTCDI-C13.

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onto ITO-coated glass substrates, in which the latter werepreviously cleaned with acetone, ethanol and de-ionizedwater in an ultrasonic bath, respectively, and were dried bynitrogen flow. The polymer films were then dried in avacuum oven for 1 h at 120 1C to remove the remainingsolvent. All operations mentioned above were taken inambient condition. For devices having a double-layeredstructure, a CuPc or PTCDI-C13 layer was vacuumevaporated using a ZZX-500 apparatus at 1� 10�3 Paand a growth rate of 0.03–0.06 nm/s, which was monitoredby an oscillating quartz thickness control. Aluminum topelectrode was also prepared through vacuum evaporationat 3� 10�4 Pa. Thickness of the films was determined by asurface profiler (Ambios Technology XP-2).

Devices of different structures were prepared forinvestigation. (1) PNB single-layered device: ITO/PNB/Al, (2) CuPc–PNB double-layered devices: ITO/CuPc/PNB/Al and ITO/PNB/CuPc/Al, and (3) perylene-PNBdouble-layered devices: ITO/PNB/PTCDI-C13/Al andITO/PTCDI-C13/PNB/Al. These were constructed asdepicted in Fig. 2(a). For ITO/organic molecule/PNB/Alstructure, spin coating of PNB film was operated after theorganic molecular film had been evaporated. A shadowmask was used in Al deposition to define active areas thatwere about 0.09 cm2 in size, with six devices active areas perslide and several samples tested for each device type forreproducibility. These films were also deposited on quartzslides in parallel for absorption measurement.

Dark and illuminated J–V characteristics of the deviceswere measured using a Keithley 2410 source meter. Thelight was illuminated from the ITO side unless otherwisestated, by a 35W compact white arc lamp. The emissionspectrum from the lamp covers a wavelength range of380–680 nm consisting mainly of partial xenon arc spec-

Fig. 2. Configuration of photovoltaic cell and energy

trum overlapped with a high proportion of line spectrafrom mercury and possibly other elements. Photocurrentspectra were measured by an illumination scan to thedevices with a range of wavelengths from a monochro-mator (Hitachi 850 with 150W Xenon source or SPEXFluorolog-3 fluorescence spectrophotometer with 450WXenon source) and the photocurrent was synchronicallymonitored using a Keithley 6430 or 2410 source meter.The output light power was estimated with a calibrated

optical meter (Newport 1830C). J–V characteristics weremeasured via our self-programmed PC control software.All measurements described above were carried out atambient condition.

3. Results and discussion

3.1. HOMO/LUMO measurement of PNB film

To characterize material properties, energy levels of thePNB material were measured using cyclic voltammetry, anelectrochemical technique. Material parameters, the high-est occupied molecular orbital (HOMO) and the lowestunoccupied molecular orbital (LUMO), which representthe ability to remove an electron from the molecule or fillan electron to the molecule, respectively, were calculatedfrom the onset oxidation and reduction potentials.A typical CV curve of PNB thin solid film is shown in

Fig. 3. Two distinct double oxidation peaks at positivevoltage scans were observed with good reproducibility. Theonset of the first oxidation peak was at 0.84V and thesecond was at about 1.15V. These correspond to energylevels of �5.18 and �5.49 eV, respectively. The energyvalues were referenced against ferrocene (0.46V from ourexperiment) as an internal standard where acetonitrile was

level diagram of PNB-based double-layer devices.

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Fig. 3. Cyclic voltammogramm of a PNB film on top of the Pt work electrode in a 0.1M solution of tetrabutylammonium perchlorate in acetonitrile. The

scan rate was 20mV/s.

J. Sun et al. / Solar Energy Materials & Solar Cells 91 (2007) 1289–12981292

used as a solvent [26]. However, no distinct reduction peakof PNB film was found in the CV measurement, whichindicated irreversible processes. If E ¼ �5.18 eV was takenas the HOMO level of the PNB polymer, the LUMO levelof PNB can be estimated using the optical band gapdetermined from the absorption edge of the opticalabsorption spectra. The optical band gap, Eg

opt, obtainedfrom the solid-state spectra was 2.43 eV and the LUMOwas estimated to be �2.75 eV. The two distinct oxidationpeaks in PNB polymer were an indication of two levelelectron processes involved in the material, which mayrelate to localization characteristics in molecular struc-tures, such as the triphenyl amide or phenylene vinylenemoieties. These are at same level to the HOMO from PPVor TPA, which may imply a relatively low level of electrondelocalization along the polymer chain. Energy diagramsof device structures are shown in Fig. 2(b), where theenergy levels of CuPc and PTCDI-C13 were referred fromthe literature [27,28].

3.2. Photovoltaic properties

(a) From PNB only device: Initial photovoltaic responsesof a PNB single-layer device were measured using J–Vcharacterization when monochromatically illuminated witha variety of wavelengths as shown in Fig. 4. It was seen thatillumination produced a strong photoconductive response,but the photovoltage is relatively small. Three character-istics can be observed: (1) illumination caused a strongincreases of the current in the reverse bias direction,(2) illumination caused a reduction of Voc in the forwardbias direction, (3) the photovoltaic effect and the Voc

reached maximum around 420 nm, where the PNB polymerhas an absorption maximum. It reasonably dropped tothe dark current level at 540 nm (not shown), where

the polymer PNB has almost no absorption. Factorsthat determine the value of the Voc appeared to be relatedto the wavelengths of the excitation. These observationsare somehow different to what were reported in mostinorganic or some polymer PV cells. The general increase incurrent, especially in the reverse bias direction, may becaused by the photoexcitation and then the chargegeneration in this material. The reduction of Voc

(as compared to the dark threshold voltage) in theforward bias direction indicated a reduced built-in poten-tial. There might have been two possibilities for these:the illumination may alter the interface potential barrierbetween the polymer and one of the contact electrodes inaddition to the charge generation. If this happened, itmeans a change in electronic structure of the polymer andas a result, a shift in the energy level of the polymer inrelation to the electrode Fermi level. Alternatively, theremay exist a barrier inside the polymer matrix, which wasreduced by the illumination. As the polymer was ahomopolymer, the latter case was less possible. If theexcitation generated a large amount excitons or polaronsin the polymer layer, these could be strongly associatedwith the structure of the polymer as a whole, whichsetup a new equilibrium and hence cause a change in theenergy level of the polymer in some degree. Similarobservation was also reported in PPV-based single-layerdevice [29,30].(b) From double-layer device: J–V characteristics of the

PNB-based multiple devices had been measured both indark and under illumination as shown in Fig. 5. In thiswork, small molecule polymer double-layer devices wereprepared, which were in the form of ITO/molecule/PNB/Al(abbr. molecule/PNB device), and ITO/PNB/molecule/Al(abbr. PNB/molecule device) and under a white lightillumination.

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Fig. 4. Current density–voltage characteristics of ITO/PNB(100 nm)/Al in dark and under monochromatic illumination at various wavelengths as

indicated.

Fig. 5. Current density–voltage characteristics of ITO/PNB(100 nm)/Al (squares), ITO/CuPc(45 nm)/PNB(40 nm)/Al (triangles), ITO/PNB(50 nm)/

CuPc(45nm)/Al (stars) devices, ITO/PTCDI-C13(20 nm)/PNB(70nm)/Al (diamond), and ITO/PNB(70nm)/PTCDI-C13(20 nm)/Al (circles). [Solid

symbols represent the results measured at dark and open symbols measured under a white light illumination.]

Table 1

Characteristics of PNB photovoltaic devices illuminated under a compact

white arc lamp

Cell structure Jsc (mA/

cm2)

Voc

(V)

ff

ITO/PNB (100 nm)/Al 0.005 0.26 0.28

ITO/CuPc (45 nm)/PNB (40 nm)/Al 0.006 0.8 0.19

ITO/PNB (50 nm)/CuPc (45 nm)/Al 0.011 0.6 0.18

ITO/PTCDI-C13 (20 nm)/PNB

(70 nm)/Al

0.00873 0.65 0.21

ITO/PNB (70 nm)/PTCDI-C13

(20 nm)/Al

0.00944 0.48 0.16

J. Sun et al. / Solar Energy Materials & Solar Cells 91 (2007) 1289–1298 1293

Main parameters of photovoltaic response from double-layer device are listed in Table 1. Open-circuit voltages Voc

were improved from 0.26V observed for single-layerdevice, to 0.8V [ITO/CuPc(45 nm)/PNB(40 nm)/Al], 0.6 V[ITO/PNB(50 nm)/CuPc(45 nm)/Al], 0.65V [ITO/PTCDI-C13(20 nm)/PNB(70 nm)/Al] and 0.48V [ITO/PNB(70 nm)/PTCDI-C13(20 nm)/Al] of the double-layered devices.Their corresponding Isc were also improved (Table 1).In single-layered PNB device, the open-circuit voltage

(0.26V) can be understood with the metal–insulator–metal(MIM) diode model [31], where the upper limit of the Voc

was defined by the work function difference between the

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top and bottom electrodes divided by the charge of anelectron. This can be calculated to be about 0.4V byassuming the work function of 4.3 and 4.7 eV for Aland ITO, respectively, and can be used as an estimationto compare with our single-layered PNB device. However,it was occasionally reported that the Voc of polymerMIM diodes had been found larger than the workfunction difference [32]. A number of other importantfactors may also contribute to Voc, such as non-negligible dark current [33], variation in work function ofITO [34], etc.

Above results also demonstrated that the open-circuitvoltages could be improved in double-layered devices. Theobtained Voc of 0.8 and 0.6V from CuPc–PNB double-layer devices were significantly higher than the single-layered one. Together with the fact that the magnitude ofthe Voc in both cases was about the same, this indicatedthat the photovoltage could not be totally controlled by theelectrodes in the double-layered devices, similar to anearlier observation by Gregg [35]. The polarities of the Voc

in both devices were the same no matter what the order ofthe two layers coated while keeping same electrodes. This isenergetically consistent with the energy levels of PNB andCuPc. As can be seen in the energy level diagram, theHOMO level of PNB is close to that of CuPc and theenergy barrier between ITO and the two organics willgreatly facilitate hole transporting. Actually in Tang’s firsttwo layers organic molecule solar cell, the Voc was reportedto be relatively dependent on the choice of the particularpair of organic layers [36]. The origin of the open-circuitvoltage in heterojunction or bulk heterojunction organicsolar cells is still under debate. Besides the MIM modeldepicted above, it was also found that in polymer–fullerenebulk heterojunction solar cells, the Voc is related to theacceptor strength, i.e. it is related directly to the energydifference between the HOMO level of donor and theLUMO level of the acceptor [37]. In polyfluorene-baseddouble-layer devices, additional contribution to Voc wasfound attributed to a counterbalanced drift currentopposing to a diffusive current at open circuit [38].According to the literature [39], unlike inorganic PV cells,the Voc in organic PV cells is expected to be determined byboth built-in potential and the photoinduced generation ofcarriers across the interface giving rise to a chemicalpotential energy gradient that complements the photo-voltage. For the CuPc–PNB double-layer devices, reversalof the order of the CuPc and PNB layers somewhatchanged the Voc (CuPc/PNB for 0.8V and PNB/CuPc for0.6V), but to a lesser degree than the difference betweensingle- and double-layered device. This indicates that morepredominant factors that influence the photovoltage wereexisting additionally in the heterojunction structure ratherthan in the pure material layer.

From the chemical structure of PNB, we expected it tobe a hole transporting material similar to CuPc. A changein the order of the layers modified the barrier heights. Itwould be interesting to see the effect if CuPc was replaced

by an electron transporter, such as a perylene derivative,PTCDI-C13. Further double-layered devices of PTCDI-C13-PNB were prepared. For a pair of two devices made ofPTCDI-C13(20 nm)–PNB(70 nm) active layers, a generallyhigher Voc was observed in PTCDI-C13/PNB device thanthat in reversed structure, PNB/PTCDI-C13 (Table 1). Thedouble-layered devices also demonstrated an increase in theVoc similar to the CuPc–PNB devices. Devices with avariety of PNB thickness were also prepared. It was foundthat Voc of the device PNB(xnm)/PTCDI-C13(20 nm)increased as the PNB thickness increased, whereas in thereversed structure PTCDI-C13(20 nm)/PNB(x nm), Isc wasreduced when the thickness of PNB increased. The way ofthe increase in Voc in PNB/PTCDI-C13 devices might berelated to the charge photogeneration and the creation ofcharge concentration gradient at the junction. Thedependence to the Isc in PTCDI-C13/PNB devices can beexplained by the reduction in charge transportation as thethickness of PNB layer increases.(c) DC characteristics: Dark J–V characteristics of the

devices were also investigated. It can be best described witha power law relation between the current and voltage,JpV n. Fig. 6 shows the J–V characteristics on a doublelogarithmic scale. In the single-layered device, n variedfrom 1.5 at low-voltage bias (obeying Child’s law) to 2.6 inthe higher bias region. Such a power dependence suggeststhat J is a transition from a trap free [40] to a trappedlimited space-charge-limited current (SCLC). In the dou-ble-layered devices, it was found that n was almost equal tounity at low forward bias, which was followed by aquadratic (JpV2 for PTCDI-C13/PNB) and superquadra-tic dependence of current on voltage (JpV2.5 for PNB/PTCDI-C13, JpV3 for CuPc/PNB and JpV3.8 for PNB/CuPc) at higher voltage bias. The latter indicated a trap-filled limit (TFL) conduction [41]. In this case, the numberof carriers captured by deep traps exceeds that in shallowtraps compared with the pure device. We notice that noohmic regime was present in the pure PNB device orPTCDI-C13-PNB devices, indicating a change in mechan-ism to CuPc–PNB devices, which could be contributedfrom the inclusion of small molecule. Inserting of moleculein both double-layered structures whether close to the ITOelectrode or Al somewhat changed the barrier height whichallowed the ohmic regime by a contact carrier injectionprocess.(d) Equivalent circuit analysis: As shown in Table 1, fill

factor from a single-layer PNB device was about 0.28.Double-layer devices improved considerably either on theopen-circuit voltage or on the short circuit current or theboth. However, the fill factors were remaining at a lowlevel, which could pull down the efficiency eventually.Fill factors of the devices may relate to device circuitry,in particular, the series and parallel resistors of thedevice structure. For analysis, an equivalent circuitdiagram approach is used. According to the pioneeringresearch on inorganic solar cells [42], I–V characteristicsof a photovoltaic device under illumination can be

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Fig. 6. The double logarithmic plot of the forward-biased J–V characteristics of ITO/PNB/Al, ITO/molecule/PNB/Al and ITO/PNB/molecule/Al devices.

Fig. 7. Equivalent circuit diagram for a typical solar cell.

J. Sun et al. / Solar Energy Materials & Solar Cells 91 (2007) 1289–1298 1295

generalized as follows:

I ¼ I0 exp eU � IRs

nkT� 1

�� �þ

U � IRs

Rsh� Iph, (1)

where I0 is the dark current, e the electron charge, U theapplied voltage, Rs the series resistance, Rsh the shuntresistance, and Iph the photocurrent. The simplifiedequivalent circuit diagram is shown in Fig. 7. To improvecell performance, devices require a large Rsh and a small Rs,which prevent leakage currents and get sharp rise in theforward bias. These may affect the shape of I–V curves andreveal various mechanisms responsible for the cell perfor-mance, which may help to detect problems and suggest away to improve device performance.

As the intrinsic low conductivity of the materials used,higher Rs values may certainly be responsible partially to thedevice performance. Under illumination, strong alternationsof the I–V curves were observed in all cases, a reducedthreshold voltage and a substantial increase in reverse

current, which reflected a change in the built-in potential atthe electrode interfaces. Alternatively, the energy levels oforganic layers may be shifted vertically upon illumination,which reduced the potential barriers and created newconductive paths in the devices. These caused a reductionin Rsh values and an increase in Rs values. Both arecontributed to the reduction of the fill factors. However, theexact mechanism to this change related to the materials isnot yet clear. To what degree that the Rs value relates to theundesirable film quality in the formation of double layers isalso a factor to be investigated [43].

3.3. Optical absorption and photocurrent spectra

Optical absorption spectra from the pure PNB, CuPcand PTCDI-C13 thin films and the films in bilayer ofmolecule/PNB and PNB/molecule (on quartz) were mea-sured and these are shown in Figs. 8 and 9. As seen inFig. 8, PNB showed a broad absorption band in visiblespectrum range, peaked at 414 nm with a good transpar-ency in the long wavelength range (500–800 nm) andPTCDI-C13 mainly absorbed light in the wavelength rangeof 400–600 nm. While CuPc displayed two strong absorp-tion bands, one at 336 nm in the ultraviolet range and theother a Q-band that showed a double peak at 629 and694 nm in the long wavelength range [44,45]. The absorp-tion spectra from the materials compensate to each other.The double-layered device could therefore cover a widerspectral range. This may potentially enhance the totalcharge generation in the device if it is optimized.

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Fig. 8. Absorption spectra of PNB (solid line), CuPc (dash line) and PTCDI-C13 (dot line) in thin films.

J. Sun et al. / Solar Energy Materials & Solar Cells 91 (2007) 1289–12981296

The photocurrent spectra of PNB-based devices weremeasured and were compared with their optical absorptionspectra (see Fig. 9). The ability to generate substantialphotocurrent in PNB and produce an open-circuit voltageindicates that charge generation following photoexcitationcan be an important process in PNB material. Generallyspeaking, for single-layered organic photovoltaic devices, therelationship between the photocurrent and absorptionspectra can be classified into symbatic and antibatic response[46,47]. In the former case, the maximum photocurrentcorresponds to the peak absorption, while in the latter case,photocurrent is reduced in regions of strong absorption andthe maximum shifted to the absorption edges. It can beinterpreted due to a relationship between the absorptioncoefficient and the penetration depth as follows: When thelight intensity is relatively constant, the generation of photo-carrier is dominated by the absorption efficiency in weakabsorption range. The photocurrent increases with theabsorption coefficient. But at the same time, the penetrationdepth will be reduced and the thickness of the unilluminatedregions of film may be greater than the free mean path ofthe carriers. This causes a reduction in photocurrent since thecarriers are unable to traverse the film. In other words, thephotocurrent spectra is limited by the competition processbetween the amount of light absorbed to produce enoughcarriers and the penetration depth to allow carriers to escapefrom the bulk, which can be reflected by the position of thephotocurrent peak [29]. Therefore, photocurrent spectra canbe a useful tool to monitor the photocurrent generationprocess in a photovoltaic device.

In our experiment, the photocurrent spectra from thesingle-layered device of ITO/PNB(100 nm)/Al (Fig. 9(a))demonstrated an antibatic-type response and peaked at343 nm, which was about 60 nm away from the absorptionpeak. It may be due to the dissociation of excitons takingplace near the interface with cathode rather than in thebulk region of PNB. However, penetration depth of

incident light may not be enough at strong absorptionwavelength for this to happen. The photocurrent aroseonly when absorption coefficiency was somewhat reduced.Photocurrent spectra of ITO/molecule/PNB/Al and ITO/PNB/molecule/Al were also measured and these are shownin Figs. 9(b)–(e), alongside with absorption spectra forcomparison. In contrast to the photocurrent spectra fromsingle-layered PNB, photocurrent spectra from double-layer devices match the absorption to a greater extent.We notice that in the ITO/PNB(50 nm)/CuPc(45 nm)/Aldevice, i.e. CuPc was interfaced with Al, photocurrentspectra resemble mainly to the CuPc absorption, whilethere is only a small contribution from PNB absorption. Ina reversed double-layered structure of ITO/CuPc(45 nm)/PNB(40 nm)/Al device, i.e. PNB was interfaced with Al,however, the situation was reversed. The photocurrentspectra were closely matched to the PNB absorptionspectrum and hardly any photocurrent responded fromCuPc absorption. It is probably that excitons were trappedor recombined in the bulk or interfaces before theydissociated and traversed to the electrode. During theseprocesses, PNB may somehow act as an electron blockingmaterial (consistent with energy diagram analysis inFig. 2(b)), which prevents the current generated fromCuPc side to reach Al electrode. Similar behavior has alsobeen observed in the perylene–PNB-based devices. Thisinformation can be useful in guiding device optimization ofheterojunction photovoltaic devices. The photocurrentspectra experiments from both single-layered PNB anddouble-layered devices indicated that total photocurrent inthe devices were mainly controlled by electron transportprocess to the Al interface. This may indicate a higherinternal field in the vicinity of the Al electrode.External quantum efficiency (EQE) was estimated after

measuring light intensity using an optical power meter.A device ITO/PNB(50 nm)/PTCDI-C13(20 nm)/Al was pre-pared in order to estimate EQE value. From this device,

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Fig. 9. The spectral photocurrent response (solid lines) of: (a) ITO/PNB(100 nm)/Al, (b) ITO/CuPc(45 nm)/PNB(40nm)/Al, (c) ITO/PNB(50 nm)/

CuPc(45nm)/Al, (d) ITO/PNB(50nm)/PTCDI-C13(20 nm)/Al, and (e) ITO/PTCDI-C13(20 nm)/PNB(40 nm)/Al. The absorption spectra (dash line) of

each structure are plotted for comparison. Xenon light source used was 150W for (a)–(c) and 450W for (d)–(e).

J. Sun et al. / Solar Energy Materials & Solar Cells 91 (2007) 1289–1298 1297

Voc ¼ 0.453, Isc ¼ 0.4, ff ¼ 0.21 can be obtained. A maxi-mum EQE at 330 nm was observed which was around 1%.In general, current device structures are not optimized.Further work is required to optimize photovoltaic perfor-mance of PNB polymer. To improve VOC, Isc and ff are thekey issues for the next stage optimization. Explanation for asubstantial improvement in open-circuit voltage in bothCuPc–PNB double-layered structures is attempted here. Insemiconductor device, minority carriers control the totalcurrent flow and these were electrons in our observation.Majority hole generated in the active layer might beaccumulated at the interface and generated a potentialdifference within the device and displayed as an open-circuit

voltage. As shown in the energy level diagram (Fig. 2(b)),the HOMO levels of CuPc and PNB are very close, while theLUMO levels differ by about 0.75 eV. When PNB wasinterfaced with Al, the relatively higher barrier may some-what limit the electron injection at forwards bias, but favourthe reversed current transporting. The short-circuit currentof the double-layer device was improved when utilizing theelectron transporting material PTCDI-C13. However, fillfactors of current device constructions are still low whichlimit the device efficiency. To improve fill factors requiresengineering a device with a proper contact resistance. Thispreliminary work demonstrated that the PNB material is apromising candidate for photovoltaic application.

ARTICLE IN PRESSJ. Sun et al. / Solar Energy Materials & Solar Cells 91 (2007) 1289–12981298

4. Conclusions

In summary, a novel polymer poly(p-phenylene N-4-n-butylphenyl-N,N-bis-4-vinylenephenylamine) (PNB) has beenfound to be a promising material to be used in photovoltaicdevice. HOMO and LUMO levels of this polymer wereestimated to be �5.18 and �2.75 eV, respectively, measuredwith cyclic voltammetry. Small molecule material copperphthalocyanine (CuPc) and N,N0-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) were used in double-layered device construction, in order to monitor currentresponse action and to improve photovoltaic device perfor-mances. Double-layered devices showed a much better open-circuit voltages around 0.4–0.8V, while short-circuit currentswere also enhanced. The ITO/PNB(50nm)/PTCDI-C13(20nm)/Al device was estimated to have an EQE valuearound 1% at 330nm. The open-circuit voltages of thedouble-layered devices were greatly enhanced as well as animprovement in the photocurrent. Interface barrier changewas insufficient to explain open-circuit voltage change. Theimprovement in the open-circuit voltage appeared to berelated to the photogeneration and the charge accumulationat the interface. Photocurrent spectra were investigated whichfound the dominated layer to near Al interface. At the sametime, PNB was found to have electron-blocking character-istics which maybe useful in the device construction.

Acknowledgments

Authors would like to thank to Dr. Xiping Jing ofPeking University for his assistance in partial spectrummeasurements. This work was funded by the grants fromNSFC (20674004 and 10434030), BNSF (3062016), BJTUGrant (2006XM043), the state key program for basicresearch of China (2003CB314707) and National Hi-TechR&D Program of China (2006AA03Z412).

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