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Solar Energy Materials & Solar Cells 90 (2006) 1705–1714

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Bi-layer photovoltaic devices with PPQ as theelectron acceptor layer

E. Kymakisa,�, E. Koudoumasa, I. Franghiadakisb

aElectrical Engineering Department, TEI of Crete, P.B. 1939, Heraklion 71004, Crete, GreecebSciences Department & Photovoltaic Park, TEI of Crete, Heraklion 71500, Crete, Greece

Received 15 July 2005; received in revised form 22 September 2005; accepted 22 September 2005

Available online 2 November 2005

Abstract

Photovoltaic cells made from bi-layer thin-film heterojunctions having poly(3-octylthiophene)

(P3OT) as the electron donor and poly(phenylquinoxaline) (PPQ) as the electron acceptor have been

investigated. The photovoltaic characteristics of the heterojunction device are considerably enhanced

from those in a single-layer device. The photocurrent is improved by almost two orders of magnitude,

and the open-circuit voltage corresponds to the estimated energy level difference of the two polymers.

The observed enhancement of the heterojunction device has been discussed in terms of photoinduced

charge transfer between P3OT and PPQ. The results showed that PPQ can be regarded as an efficient

electron acceptor material.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Organic and polymer photovoltaics; Solar cells; Bi-layers; p–n junction

1. Introduction

The renewable energy sources and especially the photovoltaic technology have recentlyattained extensive attention and become increasingly critical as an environmentallysustainable path to world socioeconomic development. In order for a major breakthroughto take place, a significant improvement in the device efficiency-to-cost ratio is essential.Here is where the polymeric solar cells come into picture. The conjugated polymers arevery attractive in this aspect, since they can be easily spin coated from solution and do not

see front matter r 2005 Elsevier B.V. All rights reserved.

.solmat.2005.09.006

nding author. Tel.: +302810 394121; fax: +30 2810 254858.

dress: kymakis@stef.teiher.gr (E. Kymakis).

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require ultra-clean environment and high-temperature steps during any processingprocedure. Moreover, these materials can be specifically designed to have a wide rangeof chemical and optoelectronic properties and deposited in large-area, flexible and light-weight substrates. These advantages have created great interest albeit they exhibitsignificantly smaller power conversion efficiencies than inorganic cells, mainly caused bythe fundamentally different photocurrent generating process in the polymers, such as theelectric field required to break up the photogenerated excitons and the low charge carriermobility.The most fundamental process in organic solar cells is the separation of the

photogenerated excitons (bound electron–hole pairs) at interfaces and their transport tothe appropriate electrodes. The excitons can be normally separated by charge transfer atinterfaces between molecules of different electron donating and accepting properties. Theelectron is accepted by the electron acceptor (A) with a larger electron affinity and the holeby the electron donor (D) with a lower ionization potential [1–4]. Intensive research inorganic photovoltaics in recent years has led to a wide variety of different structure designsand different types of hole and electron acceptor materials [5–7]. Typical electron donormaterials efficiently used are phthalocyanines, polythiophenes and poly(phenyleneviny-lenes). Representative electron acceptors are perylenes, nanorods, fullerenes andnanotubes [8–11].In this study, we use poly(phenylquinoxaline) (PPQ) as the electron acceptor (n-type)

and poly(3-octylthiophene) (P3OT) as the electron donor (p-type) to fabricate a donor/acceptor (D/A) heterojunction solar cell with the configuration ITO/P3OT/PPQ/Al. Theirchemical structures are shown in Fig. 1(a), and the device structure, similar to a p–n diode,

Sn

CH2(CH

2)6CH

3

N N

NN

O

nP3OT PPQ(a)

(b)

ExternalCircuit

A1

n-type polymer PPQ

p-type polymer P3OT

Glass or PolyesterITO

Light

Fig. 1. (a) Chemical structures of the regioregular P3OT and PPQ and (b) the photovoltaic device architecture.

ARTICLE IN PRESSE. Kymakis et al. / Solar Energy Materials & Solar Cells 90 (2006) 1705–1714 1707

is shown in Fig. 1(b). The objective of the electron acceptor layer is to generate a path forthe electrons through the layer and at the same time provide a barrier for the passage ofholes. So, the electron transporting material layer must not only be able to transportelectrons, but also have a relatively large energy gap to provide a significant barrier to thetransport of holes at the junction between the hole transport layer and itself.

PPQ is already identified as being a potential electron transporting material due to thepresence of electron-deficient nitrogen containing quinoxaline moieties in the polymerchains [12]. The inclusion of these functional groups increases the electron affinity in thepolymer and thus provides an environment which favours the transport rather than thetrapping of electrons [13]. The estimated electron affinity of PPQ is approximately 3.6 eV[14], a value quite appropriate for a suitable electron transporting polymer material.

2. Experimental

The difference in solubility between P3OT and PPQ was used for the fabrication of theheterojunction solar cell. PPQ is deposited from a chloroform solution, while P3OT from atoluene solution, in which PPQ is insoluble. The bi-layer heterojunction solar cells, ITO/P3OT/PPQ/Al, were fabricated by sequential spin coating of the polymer layers on glass orpolyester substrates precoated with a 1500 A thick transparent ITO anode with a sheetresistance of 10O/square. P3OT thin films were spin coated from a toluene solution andwere dried inside a nitrogen-filled glove box at 60 1C for 1 h. On top of the resulting P3OTthin film, a layer of PPQ was spin coated from its solution in chloroform. The P3OT/PPQbi-layer thin films were further dried in the glove box at the same conditions beforevacuum evaporation of the aluminium electrode. The device structure is shown in Fig.1(b). The thickness of the layer was determined by optical absorption measurements (i.e.,the optical absorbance of the film showed a linear relation with the film thickness).

All the photovoltaic properties were evaluated in ambient conditions at roomtemperature. The current–voltage characteristics of the photodiodes in dark and undersimulated AM1.5 sunlight illumination were measured using a HP 4140B picoammeter.Finally, the AM1.5 sunlight was produced with a filtered tungsten halogen lamp.

3. Results and discussion

Fig. 2 presents the absorption spectra of 30 nm thick P3OT and PPQ films, together withthe spectrum of the P3OT/PPQ (60 nm) bi-layer film. The absorption maximum of P3OT isobserved at 2.4 eV (500 nm), while the optical gap, which is the onset of the opticalabsorption, amounts to 1.9 eV. The PPQ shows an optical gap of 2.9 eV and an absorptionmaximum at 3.2 eV. Finally, the corresponding absorption spectrum of the P3OT/PPQ bi-layer film is simply a superposition of the P3OT and PPQ layers, suggesting that nodetectable ground-state interaction is taking place between the donor and acceptor.

Fig. 3 shows typical I–V characteristics of the ITO/P3OT(30 nm)/PPQ(50 nm)/Al solarcell in dark and illumination. Forward bias is defined as a negative potential applied to theAl contact. As shown, a rectifying curve is recorded in dark with a ratio over 104 at 3V,indicating the formation of a heterojunction as proposed. When illuminated by thesimulated solar light, the device shows a photovoltaic behaviour with a maximum open-circuit voltage Voc of 0.9V and a short-circuit current Isc of 40 mA/cm2, whereas thecalculated fill factor FF ¼ (VmIm)/(IscVoc) is 0.48 and the power conversion efficiency

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0.8

0.7

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0.5

0.4

0.3

0.2

0.1

0.01.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Energy (eV)

Abs

orpt

ion

(a.u

)P3OT

P3OT/PPQ

P3OT/PPQ

PPQ

PPQ

P3OT

Fig. 2. Absorption spectra of the 30 nm films of P3OT, PPQ and the bi-layer.

50

40

30

20

10

-10

-20

-30

-40

-50

0

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Voltage (V)

Cur

rent

Den

sity

(µµA

/cm

2 )

darkphoto

Fig. 3. I–V characteristics of an ITO/P3OT/PPQ/Al device in dark and under white illumination.

E. Kymakis et al. / Solar Energy Materials & Solar Cells 90 (2006) 1705–17141708

Z ¼ (FF IscVoc)/Pin is 0.017%. Comparing this device with a single-layer one preparedusing pure P3OT [11] (Voc ¼ 0.4V, Jsc ¼ 0:7mA=cm2, FF ¼ 0.3, Z ¼ 10�4%), assummarized in Table 1, we can see that the photovoltaic characteristics of the solarcell have been notably enhanced through the insertion of an n-type PPQ layer. The

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‘Table 1

Performance of the single- and bi-layer photovoltaic devices

Isc (mA/cm2) Voc (V) FF Z (%)

ITO/P3OT/Al [11] 0.7 0.4 0.3 10�4

ITO/P3OT/PPQ/Al 40 0.9 0.48 0.017

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short-circuit current in the heterojunction device is larger by almost two orders ofmagnitude than that in the single-layer device. Moreover, the open-circuit voltageand FF in the heterojunction device are almost doubled than those in the single-layerdevice. It is worth mentioning that the open-circuit voltage is considerably high for an entire polymer device (typical values are around 0.4V). Also, a small shoulderbetween 0.5 and 1V can be observed in the illuminated I–V, which is probably due to thegranular nature of the polymer films. The photogenerated bound electron–hole pairs(excitons) in the P3OT are diffused at the PPQ interface, where they are separated byelectron transfer to the PPQ layer. Also, the photocurrent shows a considerablesmall increase in magnitude with increasing reverse bias, in contrast to the behaviour ofthe single-P3OT device. This suggests that the dissociation of excitons mainly occurs at thep–n junction.

Furthermore, the effect of PPQ layer thickness on the device photovoltaic characteristicsis presented in Fig. 4. Several devices were fabricated with PPQ layer thickness rangingfrom 20 to 80 nm (changing either the solution concentration or speed of spin coating),while keeping the P3OT layer constant at 30 nm. The open-circuit voltage Voc iscomparable (0.95V) regardless of film thickness, with the exception of the 20 nm film,which had a Voc of 0.4V, which is equal to the Voc of the single-layer P3OT device. Thissuggests that there is a minimum PPQ thickness (30 nm) necessary to redistribute the p–njunction’s electric field.

In contrast, the short-circuit current density Isc and the fill factor FF are stronglyaffected by the PPQ layer thickness and exhibit the best performance at around 50 nm, butvalues decrease for both thinner and thicker films. This behaviour can be explained bytaking into account that an increase in PPQ thickness results in an increase in absorbance,resulting in an increase in the photogeneration rate and hence the photocurrent.Nonetheless, in thicker samples, transport of the dissociated charges is limited due to anincrease in the recombination rate and series resistance, which the experimental dataelucidates.

In order to further evaluate the device characteristics using the p–n junctionsemiconductor theory, the dependence of the short-circuit current, open-circuit voltageand fill factor on incident AM1.5 white light intensity (P) were measured in the 1–100mW/cm2 range and are shown in Fig. 5. As shown in Fig. 4(a), Jsc scales linearly with incidentlight intensity. Regarding Voc, this also increases with the incoming light intensity and Fig.4(b) presents its variation with Isc. The variation of Voc with Jsc (points) seems to followquite well (solid line) a relation valid for a conventional inorganic semiconductorphotovoltaic cell with a p–n junction [15,16]:

Voc ¼nkT

e½lnðJsc=JsÞ þ 1�, (1)

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ηFF

0.30

0.32

0.34

0.36

0.38

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0.48

0.000

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0.004

0.006

0.008

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0.016

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0.020

ISC

VOC

I SC (

µA /c

m2 )

VO

C (V

)

0.60

0.65

0.70

0.75

0.80

0.85

0.90F

ill F

acto

r

PPQ layer thickness (nm)80706050403020

PPQ layer thickness (nm)80706050403020

10

15

20

25

30

35

40

45

50

Fig. 4. (a) Isc (squares) and Voc (open circles) b) FF (open squares) and Z (circles) dependence on PPQ layer

thickness for an ITO/P3OT(30nm)/PPQ/Al device.

E. Kymakis et al. / Solar Energy Materials & Solar Cells 90 (2006) 1705–17141710

where k is the Boltzmann constant; e, the electron charge; T, the temperature; Js, thesaturated reverse current and n, the diode ideality factor and is equal to 1 for an ideal cell.An ideality factor of 1.5 is calculated from the fitting, a result that can be explained by thepresence of defects at the junction, which facilitate e–h recombination. The variation ofpower conversion efficiency with illumination intensity is also shown in Fig. 5(a). FFremains constant (0.48) throughout the range of illumination levels, whereas efficiencyincreases with light intensity for low illumination levels and reaches saturation at 60mW/

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VO

C (V

)

0.0

0.2

0.4

0.6

0.8

1.0

706050403020100

ηη (%

)

η

Jsc

Jsc (µA /cm2)

J sc (

µA /c

m2 )

0.008

0.010

0.012

0.014

0.016

0.018

Light Intensity (mW/cm2)220200180160140120100806040200

0

10

20

30

40

50

60

70

80

Fig. 5. (a) Light intensity dependence of the short-circuit current density. (b) Plot of Voc with Jsc for a p–n

photovoltaic cell. The solid line is obtained from Eq. (1).

E. Kymakis et al. / Solar Energy Materials & Solar Cells 90 (2006) 1705–1714 1711

cm2. In higher intensities (4120mW/cm2), there is a small decrease in efficiency, which canbe correlated to charge injection effects.

The observed maximum open-circuit voltage of 0.9V at 100mW/cm2 cannot be directlyrelated to the electrode work function difference between ITO and Al electrodes (0.4 eV)according to the MIM model [17], since this is only about half the observed Voc. Moreover,the maximum Voc is approximately equal to the energy difference between the HOMO ofP3OT and LUMO of PPQ (LUMOPPQ�HOMOP3OT). In order to investigate the effect ofmetal work function on photovoltaic properties, three different metal electrodes instead ofAl ((FAl ¼ 4:3 eV) were used: gold (FAu ¼ 5:1 eV), copper (FCu ¼ 4:65 eV) and chromium(FCr ¼ 4:5 eV). Table 2 summarizes the photovoltaic characteristics of the devices.

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Table 2

Performance of the photovoltaic devices with different metal electrodes

Device configuration DF Isc (mA/cm2) Voc (V) FF Z (%)

ITO/P3OT/PPQ/Al 0.4 40 0.9 0.48 0.017

ITO/P3OT/PPQ/Aua 0.4 28 0.65 0.4 0.007

ITO/P3OT/PPQ/Cu 0.05 31 0.8 0.48 0.012

ITO/P3OT/PPQ/Ca 0.2 40 0.85 0.45 0.015

aPolarity of the applied voltage is reversed in the Au device for comparison.

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As expected, the work function of the metal has no significant effect on Voc, contrary towhat would be if Voc was determined only by the difference in contact metal workfunctions according to the MIM model. Therefore, we suggest that the internal potentialfor charge separation arises from the donor/acceptor polymer/polymer interfaces inagreement with previous investigations in polymer/polymer [18], polymer/fullerene [19]and polymer/nanotube [20] solar cells. The best characteristics are obtained for the Aldevice due to its larger internal bias.The performance of the p–n junction photovoltaic devices under consideration strongly

depends on HOMO and LUMO energy differences of p- and n-materials since these are theimpetus for production of charges from an exciton. A schematic energy diagram for aP3OT/PPQ device with Al and ITO electrodes, before the materials are placed in contact,is shown in Fig. 6. The energies of the LUMO and HOMO levels for P3OT are 2.85 and5.25 eV [20], while for PPQ are 3.6 and 6.8 eV, respectively [8,14]. The work functions of Aland ITO electrodes are taken be about 4.3 and 4.7 eV, respectively. The device principlescan be explained as follows: the excitons photogenerated in P3OT are diffused to theinterface between the two polymer layers. The LUMO of P3OT lies above that of PPQ, sothe excitons are expected to efficiently dissociate by electron transfer to PPQ. In the samemanner, a photoinduced hole transfer from PPQ to P3OT is expected, i.e., thephotoinduced charge transfer by the dissociation of excitons in holes in P3OT andelectrons in PPQ. Furthermore, since electrons are carried to one contact by PPQ, andholes are transported by the P3OT layer to the opposite contact, the paths of oppositelycharged carriers are separated and recombination should be suppressed.Analytically, the interfacial barrier in the P3OT/PPQ heterojunction is 0.75 eV for the

holes and 1.55 eV for electrons. Hence, the energetic position of the LUMO at 3.6 eVsuggests that in the case of PPQ, a significant enhanced electron transport from the P3OTto the PPQ layers is to be expected. Furthermore, the offset in the HOMO level at theP3OT/PPQ interface is significantly larger than the barrier at the ITO/P3OT interface, andthus the hole current is limited by the P3OT/PPQ interface. Therefore, since the energybarrier for holes between the P3OT/PPQ interface is larger than for electrons, PPQ will actas an effective electron transport and a hole-blocking layer. Finally, due to significantdifference between the barriers, recombination is only possible within the hole transportingP3OT layer.Finally, another interesting aspect of the double-layer devices is the possibility of an

enhancement in the absorption of light. In order for a diode to act as an efficient solar cell,it must absorb as much light of the solar emission spectrum as possible. In a single-layerdevice, materials with a small band gap such as the silicon (Eg[Si] ¼ 1.1 eV) are successfullyused. However, a disadvantage of using a medium with a small band gap is that the extra

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0

-1

-2

-3

-4

-5

-6

-7

(eV

)

4.7

HOMO

LUMO

LUMO

HOMO

5.25

6.8

3.6

2.85

4.3

∆h = 1.55 eV

∆e = 0.75 eV

e-

ITO PPQ A1P3OT

Vaccum level

Fig. 6. Band diagram with HOMO/LUMO levels of donor polymer P3OT and acceptor PPQ in relation to work

functions of ITO and Al.

E. Kymakis et al. / Solar Energy Materials & Solar Cells 90 (2006) 1705–1714 1713

energy carrier by shorter wavelength radiation is wasted, as the neutral photoexcited statesin the P3OT relax thermally before dissociating into the oppositely charged carriers.Therefore, a proposed solution is to place the material with the higher band gap (PPQ)closest to the transparent contact. In this way, the high-energy photons will be absorbed byPPQ, while those with energy lower than the PPQ band gap can pass through it and beabsorbed by the underlying P3OT layer. Therefore, a greater portion of the absorbedphoton energy will be involved in the creation of electron–hole pairs, and the excitons, ineach layer, will dissociate at the interface between them.

A device with the reverse configuration ITO/PPQ/P3OT/Al was also fabricated andtested. The absorption of the device was significantly improved; however, the photovoltaiccharacteristics were much worse: I sc ¼ 50mA=cm2, Voc ¼ 0:7V and FF ¼ 0.35. Inaddition, the open-circuit voltage showed the opposite polarity as compared with thatof the ITO/P3OT/PPQ/Al configuration. The main problem in the characteristics of thisdevice is the mismatching of energy level between the work function of the ITO and theHOMO of PPQ.

4. Conclusions

The photovoltaic characteristics of a P3OT/PPQ heterojunction photovoltaic cell areconsiderably improved from those in a P3OT single-layer photovoltaic cell. The observed

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enhancement in the photovoltaic effect of the heterojunction device, treated as a p–ndevice, can be explained in terms of a photoinduced charge transfer between P3OT andPPQ. In conclusion, PPQ is an efficient electron transport layer and can be utilized as then-layer in photovoltaic devices.

Acknowledgments

The authors would like to acknowledge E. Aperathitis and P. Tzanetakis forexperimental support and fruitful discussions. Financial support by EPEAEK ArchimedesI ‘‘Novel low-cost solar cells’’ is acknowledged.

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