Solar Energy Materials & Solar Cells 98 (2012) 208–211

Contents lists available at SciVerse ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

Researc

Science

E-m1 Th

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

New series connection method for bulk-heterojunction polymer solarcell modules

Jongjin Lee a,n,1, Hyungcheol Back b,1, Jaemin Kong b, Dong-Won Park a, Kwanghee Lee a,b,n

a Heeger Center for Advanced Materials (HCAM) & Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology,

Gwangju 500-712, South Koreab Department of Materials Science and Engineering, Gwangju Institute of Science and Technology 500-712, South Korea

a r t i c l e i n f o

Article history:

Received 6 September 2011

Received in revised form

1 November 2011

Accepted 6 November 2011Available online 18 November 2011

Keywords:

Bulk-heterojunction

Polymer solar cell

Module

Series connection

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.11.008

esponding authors at: Heeger Center for Ad

h Institute for Solar and Sustainable Energies

and Technology, Gwangju 500-712, South Ko

ail addresses: bandy1@gist.ac.kr (J. Lee), klee@

ese authors contributed equally to this work

a b s t r a c t

We propose and demonstrate a new series connection method that employs alternating conventional

and inverted bulk-heterojunction (BHJ) solar cells. The two adjacent cells in each submodule are

connected through top or bottom ‘continuous terminals’ made of aluminum or indium tin oxide (ITO),

which are shared by cells with different polarities in either a top-to-top or a bottom-to-bottom manner.

In comparison with the conventional series-interconnection method, our new structure offers unique

advantages in that both printing methods and efficient coating methods can be applied in the

manufacturing of series-connected BHJ photovoltaic modules.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Over the last decade, much attention has been focused on thedevelopment of organic photovoltaics (OPVs) because of theirsimple structure and their potential for low-cost manufacturing[1,2]. The current state-of-the-art devices are known as bulk-heterojunction (BHJ) devices in which an OPV active layer is formedfrom a blend of donor polymer and acceptor fullerene-derivativematerials. The power conversion efficiencies (PCEs) of BHJ polymersolar cells are close to commercialization; through optimization ofthe materials and processing parameters, PCEs exceeding 8% haverecently been achieved [3–5]. In recent years, researchers have alsoattempted to enlarge these solar cells (modules) for practical use.However, the conductivity of transparent conducting oxides (TCOs),e.g., the widely used indium tin oxide (ITO), on a glass substrate isapproximately 6�103 S/cm, which is at least one order of magni-tude smaller than that of thin metal films. Thus, a simple scaling upof the active area results in ohmic losses of the photogeneratedcharge carriers in the TCO substrate [6,7]. Two different methods arecommonly adopted to circumvent the limited conductivity of TCOfilms. One approach for large-area cells is the use of current-collecting metal grids. Depending on the positions of the transparent

ll rights reserved.

vanced Materials (HCAM) &

(RISE), Gwangju Institute of

rea.

gist.ac.kr (K. Lee).

.

metal grids, front- and back-side illuminated structures have beenemployed [8–12]. Many groups have successfully demonstratedlarge-area OPV cells with the same PCEs as small-scale laboratorycells [8–10]. Large-area roll-to-roll production with ITO-free struc-tures has also been demonstrated [11,12]. However, the fabricatedcells should be interconnected in series or parallel to be used inmore large-scale modules, as this monolithic approach is wellestablished in crystalline silicon solar module manufacturing.

Another approach is to connect arrays of parallel single cells inseries to enlarge a single module. This technique is widely used inthin film solar cells, including OPVs, to boost the voltage of themodule. Voltage boosting is favored over current boosting becausevoltage boosting produces lower ohmic losses in low-conductivitysubstrates. However, excessively increasing the number of connec-tions per unit area causes aperture losses in the photoactive areabecause a substantial area is required to connect the adjacent cells.Tipnis et al. demonstrated the fabrication of a series-interconnectedmodule using spin coating and laser ablation [13]. However,although the PCE of the active area was 2.4%, the PCE of the totalarea was only 1.1% because approximately 54% of the aperture ofthe photoactive area was lost for connections. Krebs et al. demon-strated the elaboration of an all-printed module using the slot-diemethod. This module also suffered a 40% aperture loss because eachprinted layer of a BHJ solar cell must be shifted relative to theunderlying layers to avoid undesirable fill-over and to ensure seriesinterconnections [14,15]. Thus, a series-interconnection methodthat is able to avoid or at least suppress substantial aperture losswould be highly desired.

Submodule I

Glass

ITO

TiOx

P3HT:PCBM

PEDOT:PSS

PEDOT:PSS

Al (Continuous Terminal 1)

ITO (Continuous Terminal 2)

PEDOT:PSS

Al

Submodule II

4.8 eV

4.3 eV

TiOx

4.2 eV

ITO P3HT:PCBM

PEDOT:PSS

4.8 eV

8.1 eV

3.2 eV

4.3 eV

5.1 eV

6.1 eV

5.2 eV

P3HT: PCBM

3.2 eV

4.3 eV

5.1 eV

6.1 eV

Al PEDOT:PSS

5.2 eV

ITO P3HT:PCBM

PEDOT:PSS

3.2 eV

4.3 eV

5.1 eV

6.1 eV

5.2 eV

4.2 eV

Al

Submodule I Submodule II

Fig. 1. (a) Schematic diagram of a new series-connected structure showing submodules I and II. (b) The corresponding energy level diagrams of the structures.

J. Lee et al. / Solar Energy Materials & Solar Cells 98 (2012) 208–211 209

In this study, we propose a new series connection method forfuture large-scale modules, which is achieved by alternatingconventional and inverted BHJ solar cells. Fig. 1(a) shows aschematic cross-section of the proposed module structure thatis composed of two components: submodules I and II. Theoperation of this device is illustrated by the corresponding energylevel diagram in Fig. 1(b). The structure provides neighboringcells with alternate biases. Two different continuous terminals, B(an aluminum electrode) and C (an ITO electrode), spontaneouslyconnect neighboring cells in series. Another advantage of thisstructure is that the BHJ layer, which is the most sensitive to thePCE, does not have to be patterned or scribed for the seriesconnections.

2. Experimental

In this work, submodules I and II, which were connected byshared ‘continuous terminals’ made of aluminum or ITO electrodes,respectively, were demonstrated using a printing method. For thefabrication of the BHJ solar cells, (patterned) ITO substrates werefirst cleaned with detergent, ultrasonicated in distilled water,

acetone and isopropyl alcohol, subsequently dried in an oven andtreated with a 10 min UV ozone exposure. Poly(3,4-ethylenediox-ythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP AI4083, H. C. Starck) was coated on the left half of the ITO substrateas a hole transport layer (HTL) using a doctor blade. The films wereannealed at 120 1C for 10 min in air. A sol–gel processed titaniumoxide (TiOx) precursor solution diluted in methanol to a concen-tration of 1:200 was coated on the remaining right half of the ITOsubstrate as an electron transport layer (ETL) by the same methodand heated at 80 1C for 10 min in air [16]. A 1 wt% solution ofpoly(3-hexylthiophene) (P3HT) and [6] 6-phenyl-C61-butyric acidmethyl ester (PCBM) dissolved in chlorobenzene at a weight ratioof 1:0.8 was spin cast on top of the underlying PEDOT:PSS and TiOx

layers, followed by annealing at 80 1C for 10 min under N2 to forma thin film with the thickness about 80 nm . Afterwards, on top ofthe right half region where absorber layer and bottom TiOx

layer were deposited previously, PEDOT:PSS diluted in isopropyl-alcohol was coated by doctor blade, followed by annealing at120 1C for 10 min. The solar cells were completed by evaporatingaluminum (Al) electrodes through a shadow mask in high vacuum(10�6 mbar). Unit cell area defined by patterned ITO or aluminumwas 4.64 mm2. Current density–voltage (J–V) characteristics of the

J. Lee et al. / Solar Energy Materials & Solar Cells 98 (2012) 208–211210

unit cells and the submodules were measured using a Keithley 236Source Measure Unit under Air Mass 1.5 Global (AM 1.5 G)simulated solar illumination at 100 mW cm�2. The incident-photon-to-current-efficiency (IPCE) measurements for the conven-tional and inverted BHJ solar cells were performed with a Solar CellSpectral Response/QE/IPCE measuring system.

Table 1Summary of the performance characteristics of Submodules I and II, including

each unit cell.

Voc (V) Jsc (mA/cm2) FF PCE (%)

Submodule I 1.23 3.78 0.49 2.28

Conventional (A–B) 0.62 6.99 0.45 1.96

Inverted (B–C) 0.62 8.12 0.47 2.35

Submodule II 1.18 2.96 0.55 1.95

Inverted (B0–C0) 0.65 6.41 0.59 2.44

Conventional (C0–D) 0.55 6.73 0.52 1.93

3. Results and discussion

Fig. 2(a) shows the structure of submodule I, which is com-posed of conventional and inverted BHJ solar cells. Two singlecells with different bias polarities were built from a seamlesssingle active layer by alternating the positions of the chargeselective functional layers. Each cell was connected by a shared‘continuous terminal’ of aluminum. The cathode buffer layer in aconventional cell was omitted in this work, because Al is a goodcathode material. Fig. 2(b) plots the J–V characteristics in the darkand under illumination. The two terminals (A–B) of the conven-tional cell of submodule I exhibited an open-circuit voltage (Voc)of 0.62 V, a short-circuit current density (Jsc) of 6.99 mA/cm2, and apower conversion efficiency (PCE) of 1.96%. In the inverted cell, thetwo terminals (B–C) yielded a Voc of 0.62 V, a Jsc of 8.12 mA/cm2,and a PCE of 2.31%. The PV performance of the SELECT submodulebetween two terminals (A–C) achieved a Voc of 1.23 V, a Jsc of3.78 mA/cm2 (derived from doubled photoactive area), and a PCEof 2.28%. While the Voc of the submodule was approximately equalto the sum of the two Voc values, the short circuit current density ofthe submodule reduced to a value between the half of the currentdensities of the two unit cells [17].

Submodule II has a structure similar to that of submodule I, butITO was used as the continuous terminal rather than aluminum, asshown in Fig. 2(c). The J–V characteristics of the individual cells

Glass

ITO

TiOx

P3HT:PCBM

PEDOT:PSS

PEDOT:PSS

Al (Continuous Terminal 1)

ITOA

B

C

-0.2

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

Conventional (A-B) Inverted (B-C) Sub Module I (A-C)

0.0 0.2 0.4 0.6 0.8 1.0 1.2-10

-8

-6

-4

-2

0

2

4

Fig. 2. (a) and (b) are a schematic diagram of submodule I and its corresponding J–V ch

in series by the aluminum electrode (continuous terminal B). (c) and (d) are the schem

terminal C0 . The unit cell area was 4.64 mm2. The open symbols represent dark curren

and the submodule are presented in Fig. 2(d). The continuous ITOterminal provided a good series connection between the twodifferent cells. The characteristic J–V parameters of submodules Iand II, including each unit cell, are listed in Table 1. Although wecould not sharply define the overlapping area between the doctor-bladed ETL and HTL, the fill factors of the submodules were almostunchanged. Because the in-plane sheet resistances of the PED-OT:PSS and TiOx layers used in this study were estimated to be onthe order of 109 O/sq, which were large enough to avoid cell-to-cell cross-talk, the leakage currents through the overlapping ETLand the HTL between two separated in-plane electrodes werenegligible. Additionally, on the continuous terminals, because boththe ETL and HTL lie on an equipotential surface of aluminum orITO, leakage through these interfacial layers can be neglected.

Optimum series operation can only be achieved by currentmatching between the cells. However, it is difficult to match thecurrent densities of two different cell types, as evidenced by theIPCE spectra of the unit cells of submodule I shown in Fig. 3(a).This problem can be circumvented by adjusting the ratio of theareas of the two cell types, although we did not employ thisapproach in this work. This area matching will not be a process

Glass

TiOx

P3HT:PCBM

PEDOT:PSS

Al

ITO (Continuous Terminal 2)

PEDOT:PSS

Al B’

C’

D

-10

-8

-6

-4

-2

0

2

4

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

Conventional (B'-C') Inverted (C'-D) Sub Module II (B'-D)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

aracteristics. The constituent conventional and inverted solar cells were connected

atic and J–V curves for submodule II. The cells were connected by the continuous

ts.

350 400 450 500 550 600 650 700 750 8000

10

20

30

40

50

60

70

IPC

E (%

)

Wavelength (nm)

ConventionalInverted

Fig. 3. (a) IPCE spectra of the conventional and inverted P3HT:PCBM solar cells of submodule I. (b) A photograph of submodule II.

J. Lee et al. / Solar Energy Materials & Solar Cells 98 (2012) 208–211 211

hurdle because the active area can be easily controlled byadjusting the metal areas, which are shown in Fig. 3(b). However,to employ this structure in mass production, the most criticalpoint will be reducing the number of patterning steps. When theconventional top-to-bottom connection method was employedand both an ETL and an HTL are used, five layers (ITO\ETL\acti-ve\HTL\metal) must be patterned, including two electrodes. In ourproposed structure, five layers must also be patterned (ITO\ETL\H-TL\active (no pattern)\HTL\metal); however, if we adopt an ETL toenhance cell performance, the number of patterning layers willincrease to six. Further efforts toward reducing the number ofpatterning are thus required for this structure.

4. Conclusion

In summary, we fabricated a new method for the series-connection of submodules by alternating conventional andinverted BHJ solar cells, and using seamless spin cast BHJ films.The unit cells were connected by continuous terminal electrodesshared by adjacent cells with different polarities. The powerconversion efficiencies of the submodules were between thoseof the unit cells, with values of approximately 2% for the twotypes demonstrated here. The novel structure of a seamless activelayer and a continuous terminal electrode is a suitable moduleconcept for connecting two adjacent solar cells. More impor-tantly, this new structure eliminates the need to pattern thesensitive photoactive materials.

Acknowledgments

This work was supported by the Core Technology Develop-ment Program for Next-Generation Solar Cells of the ResearchInstitute of Solar and Sustainable Energies (RISE), GIST and theBasic Science Research Program through the National ResearchFoundation of Korea (NRF) funded by the Ministry of Education,Science and Technology (R15-2008-006–02001-0).

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