Multifolded polymer solar cells on flexible substrates

Multifolded polymer solar cells on flexible substrates Yinhua Zhou, Fengling Zhang, Kristofer Tvingstedt, Wenjing Tian, and Olle Inganäs Citation: Appl. Phys. Lett. 93, 033302 (2008); doi: 10.1063/1.2957995 View online: http://dx.doi.org/10.1063/1.2957995 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v93/i3 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 93, 033302 共2008兲

Multifolded polymer solar cells on flexible substrates Yinhua Zhou,1,2 Fengling Zhang,1,a兲 Kristofer Tvingstedt,1 Wenjing Tian,2 and Olle Inganäs1 1

Biomolecular and Organic Electronics, Center of Organic Electronics, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden 2 State Key Lab for Supramolecular Structure and Materials, Jilin University, 130012 Changchun, People’s Republic of China

共Received 3 June 2008; accepted 23 June 2008; published online 21 July 2008兲 Arrays of reflective multijunction polymer solar cell were demonstrated by folding four separated cells fabricated on a single plastic substrate using conducting polymer poly共3, 4-ethylene-dioxythiophene兲:polystyrenesulfonate as an anode. The combination of flexible substrate and polymer solar cells 共PSCs兲 makes the construction of multifolded PSCs on one substrate possible. The power conversion efficiency 共PCE兲 of the multifolded reflective PSCs was enhanced by 62% ⫾ 12% with the folded opening angle of 30° compared to the planar cells. In series connection of four solar cells, an open-circuit voltage 共Voc兲 of 3.65 V was obtained. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2957995兴 Conjugated polymers are excellent candidates for use in photovoltaic cells due to its low cost and potential for flexible and large-area application.1 Polymer based solar cells have reached power conversion efficiency 共PCE兲 of 5%–6%.2–4 Optimizing the device performance always relies on a compromise between thick active layers for absorbing more photons and thin layers for efficient collection of photogenerated charge carriers due to low charge carrier mobility of polymers.5,6 To harvest more photons in solar spectrum, one efficient way is to exploit different band-gap absorbers to be stacked in multiple junctions.2,7 The multijunction cells usually possess many layers that make the device preparation more complicated. Another way to enhance photon absorption is to deploy device architectures for light trapping and light redistribution to absorb more photons in thin films.8–10 We have recently demonstrated V-shaped polymer solar cells, where the photons that are not absorbed by one cell are reflected and absorbed by the other adjacent cell.11,12 The optical reflectance of the cells in V-folded geometry indicated an almost complete absorption of photons within the spectral range of the materials at small opening angles. The PCE of the V-shaped solar cells with 60 nm thick active layer can be enhanced by about 80% at small opening angle compared to planar cells. Rim et al. also investigated V-geometry solar cells by both modeling and experiments. A 52% enhancement in PCE was demonstrated for a 170 nm thick polymer cell.13 These V-folded cells were made on indium-tin-oxide 共ITO兲-coated glass as well as plastic substrates. There are several drawbacks using ITO-coated glass as substrates for V-folded cells, as the thick and rigid glass substrate prevents small folded angles for practical reasons, and does not easily allow the assembly of arrays of devices. To extend one-folded cell to multiple folded solar cells, a flexible substrate and anode is necessary. Recently, we demonstrated that efficient small flexible polymer solar cells could be built on polyethylene terephthalate 共PET兲 substrates using bilayer conducting polymer poly共3,4-ethylenedioxythiophene兲共PEDOT兲:polystyrenesulfonate PH500 共hereafter referred to as PH500兲 and PEDOT-EL 共P VP Al a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2008/93共3兲/033302/3/$23.00

4083兲 as anode.14 Here we present larger multiple folded reflective polymer solar cells built on PET substrate using PH500 as an anode, which has potential to be printed for large-area applications. The folded cell exhibited an enhancement in PCE by ⬃60% when the folded opening angle is 30° compared to planar ones. For multiple folded solar cells in series connection, an open-circuit voltage 共Voc兲 of 3.65 V was obtained in these present experimental systems. The materials used in active layers were alternating polyfluorene copolymer 共APFO-3兲 as electron donor and 关6,6兴-phenyl-C61-butyric acid methyl ester 共PCBM兲 as electron acceptor.15 The flexible PET substrate was from Hifi Industrial Film® 共250 ␮m thick兲 and the PH500 was from

FIG. 1. 共Color online兲共a兲 Sketch of the V-shaped cell and light ray in the cell. The V-folded opening angle is defined as 2␣. 共b兲 Pictures of V-geometry and W-geometry cell 共the area not covered by solar cells was covered by black tape during the measurement兲.

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H.C. Starck. The details of polymer anode preparation were described in Ref. 14. The sketch of one-folded cell and light ray in the cell is depicted in Fig. 1共a兲, where the opening angle of one-fold cell is defined as 2␣. To exploit the flexibility of polymer anode on plastic substrate, multiple folded W-shaped 共double V s兲 cells were constructed, which was much easier to be realized than using ITO-coated glass as anode. The adjacent two cells or four cells were prepared on one substrate under the same conditions. The active layer of APFO-3:PCBM 共1:3, weight ratio兲, thickness of about 80 nm, was spin coated from chloroform solution on prepared PET/PH500/PEDOT-EL films. The cathode LiF 共0.6 nm兲 / Al 共60 nm兲 was deposited in vacuum through a mask to define the active area of each cell, which was about 40 mm2, which is almost ten times of the samples used in Ref. 14. The substrate was cut to half depth between two cells and then folded to be V or W shaped before characterization. The samples were characterized using the same setup and under the same conditions as in Ref. 11 except the sample holder required for glass substrate was not needed here because the PET could be free stand at any folding angle. The effective area of the folded cells, utilized for the efficiency calculations for the different folded angles, was taken as the area that the device occupies in the photon stream from the solar simulator.11 The device structure of the solar cells fabricated on plastic substrate was PET/ PH500/ PEDOT-EL/ APFO3 : PCBM/ LiF / Al. To facilitate the connection of electrodes and prevent short circuit during folding, especially at small folded angles, thin copper wires were pasted to PEDOT 共at the top of the cells兲 and Al 共at the bottom of the cells兲 by silver paste, respectively, as shown in Fig. 1共b兲. The singlefolded cell consisted of two separate cells and the multifolded cell contained four cells, which can be connected in either series or parallel. The J-V characteristics from folded cells with different folded opening angles under illumination of a solar simulator 共AM1.5兲 with the intensity of 100 mW/ cm2 are shown in Fig. 2共a兲共one-folded cell兲 and Fig. 2共b兲共multifolded cell兲, respectively. Parallel connected unfolded two cells and four cells 共P180 in Fig. 2兲 had quite similar photovoltaic performance. The planar parallel connected two cells had Voc of 0.96 V, Jsc of 3.4 mA/ cm2, and FF of 41%, which resulted in a PCE of 1.3%. The planar parallel connected four cells had Voc of 0.91 V, Jsc of 3.5 mA/ cm2, and FF of 38%, which resulted in a PCE of 1.2%. The PCEs of the folded cells were lower than that of small area 共⬃5 mm2兲 devices 共PCE of 2.2%兲14 due to the higher series resistance in the PEDOT electrodes in these

FIG. 2. 共Color online兲共a兲 I-V characteristics of V-shaped cell with different folded opening angles 共2␣兲. 共b兲 I-V characteristics of W-shaped cell with different folded opening angles. All the devices measured under simulated illumination of AM 1.5 共100 mW/ cm2兲.

large-area device, 40 mm2 for single cell, 80 mm2 for singlefolded cell, and 160 mm2 for multifolded cell. It is obvious that the performance of the cells is mainly limited by the conductivity of PEDOT, which had stronger impact on big cells than smaller ones. This can be solved either by introducing metal grid beneath PEDOT 共Ref. 16兲 or by using even higher conductivity polymer as anode. Series connected two cells or four cells exhibited a Voc of 1.90 V or Voc of 3.60 V, which is almost double or four times of the Voc of the single cell14 irrespective planar or folded. The photovoltaic data of the single-folded cell and the multifolded cell under different folded angles are summarized in Tables I and II. It was observed that the Voc and FF did not change much in both parallel and series connection 共in Fig. 2 and Tables I and II兲 when the cells were single folded to V-shaped or multiple folded to W-shaped cells in different opening angles, 180° 共planar兲, 120°, 90°, 60°, and 30°. The Voc of V-shaped cell kept at about 1.90 V and the W-shaped cell

TABLE I. The photovoltaic data of V-shaped cell in parallel and series connection with different folded opening angles under illumination of AM 1.5, 100 mW/ cm2. Parallel connection

Series connection

Folded angle 共°兲

Jsc 共mA/ cm2兲

Voc 共V兲

FF

PCE 共%兲

Jsc 共mA/ cm2兲

Voc 共V兲

FF

PCE 共%兲

180 120⫾ 2 90⫾ 2 60⫾ 2 30⫾21

3.4 3.4⫾0.1 0.1 4.4⫾0.1 0.1 5.1⫾0.2 0.2 5.3⫾0.2 0.4

0.96 0.96 0.96 0.94 0.90

0.41 0.41 0.41 0.43 0.44

1.3 1.4⫾0.1 0.1 1.8⫾0.1 0.1 2.0⫾0.1 0.1 2.1⫾0.1 0.2

1.7 1.7⫾0.1 0.1 2.2⫾0.1 0.1 2.4⫾0.1 0.1 2.6⫾0.1 0.2

1.90 1.90 1.90 1.88 1.82

0.41 0.42 0.42 0.44 0.45

1.3 1.4⫾0.1 0.1 1.7⫾0.1 0.1 2.0⫾0.1 0.1 2.1⫾0.1 0.2


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TABLE II. The photovoltaic data of W-shaped cell in parallel and series connection with different folded opening angles under illumination of AM 1.5, 100 mW/ cm2. Parallel connection

Series connection

Folded angle 共°兲

Jsc 共mA/ cm2兲

Voc 共V兲

FF

PCE 共%兲

Jsc 共mA/ cm2兲

Voc 共V兲

FF

PCE 共%兲

180 120⫾ 2 90⫾ 2 60⫾ 2 30⫾21

3.5 3.4⫾0.1 0.1 4.5⫾0.1 0.1 5.0⫾0.2 0.2 5.5⫾0.3 0.5

0.91 0.92 0.92 0.92 0.88

0.39 0.39 0.38 0.40 0.41

1.2 1.2⫾0.1 0.1 1.6⫾0.1 0.1 1.8⫾0.1 0.1 2.0⫾0.1 0.2

0.8 0.9⫾0.1 0.1 1.1⫾0.1 0.1 1.2⫾0.1 0.1 1.3⫾0.1 0.1

3.60 3.65 3.65 3.60 3.45

0.39 0.40 0.39 0.41 0.42

1.2 1.2⫾0.1 0.1 1.6⫾0.1 0.1 1.8⫾0.1 0.1 2.0⫾0.1 0.2

stayed at about 3.6 V in series connections, but the Jsc increased as the opening angle decreased because the smaller folded opening angle more efficiently traps light, which contributes to the photocurrent enhancement. The PCE was enhanced from 1.3% to 2.1% ⫾ 0.1% for the one-folded cell and from 1.2% to 2.0% ⫾ 0.2% for the multifolded cell in both parallel and series connection. The enhancement in the PCEs of the folded cells as functions of the opening angles are depicted in Fig. 3, which show less pronounced enhancement in PCEs with an opening angle larger than 120, due to no recycling of reflected photons between the adjacent cells. However, the PCEs of the folded cells were enhanced significantly by 30% ⫾ 3% compared with the planar one when the opening angle was 90°, which was increased by about 58% ⫾ 10% for V-shaped cells and 62% ⫾ 12% for W-shaped cells with the folded opening angles decreased to 30° compared to the planar cell. In summary, we have demonstrated multiple folded reflective polymer solar cells based on plastic substrates with PEDOT as anode. The PCE of the folded cell was enhanced by ⬃60% with an opening angle of 30° compared with planar cells. In series connection, a high Voc of 3.65 V was

FIG. 3. 共Color online兲 The enhancement in PCEs of single folded cells 共filled symbols兲 and multifolded cell 共open symbols兲 as a function of the folded opening angles where the lines are guides for eyes.

obtained in the multiple folded cells. The simplicity of fabricating multiple folded polymer solar cells on flexible substrate to increase PCE makes the approach feasible for large scale application using roll-to-roll printing. We thank Professor Mats R. Andersson at Chalmers University of Technology for polymer supply. This work was financed by the Center of Organic Electronics at Linköping University, funded by the Strategic Research Foundation SSF. Y. Z. acknowledges the scholarship from Chinese Scholarship Council 共No. 2007U15021兲. S. Gunes, H. Neugebauer, and N. S. Sariciftci, Chem. Rev. 共Washington, D.C.兲 107, 1324 共2007兲. 2 J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante, and A. J. Heeger, Science 317, 222 共2007兲. 3 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G. C. Bazan, Nat. Mater. 6, 497 共2007兲. 4 M. Green, K. Emery, Y. Hishikawa, and W. Warta, Prog. Photovoltaics 16, 61 共2008兲. 5 D. W. Sievers, V. Shrotriya, and Y. Yang, J. Appl. Phys. 100, 114509 共2006兲. 6 S. Lacic and O. Inganas, J. Appl. Phys. 97, 124901 共2005兲. 7 B. de Boer, A. Hadipour, and P. W. M. Blom, Adv. Funct. Mater. 18, 169 共2008兲. 8 M. Niggemann, M. Glatthaar, P. Lewer, C. Muller, J. Wagner, and A. Gombert, Thin Solid Films 511, 628 共2006兲. 9 K. Tvingstedt, M. Tormen, L. Businaro, and O. Inganas, Proc. SPIE 6197, 61970C 共2006兲. 10 J. Y. Kim, S. H. Kim, H. H. Lee, K. Lee, W. L. Ma, X. Gong, and A. J. Heeger, Adv. Mater. 共Weinheim, Ger.兲 18, 572 共2006兲. 11 K. Tvingstedt, V. Andersson, F. Zhang, and O. Inganas, Appl. Phys. Lett. 91, 123514 共2007兲. 12 V. Andersson, K. Tvingstedt, and O. Inganas, J. Appl. Phys. 103, 094520 共2008兲. 13 S. B. Rim, S. Zhao, S. R. Scully, M. D. McGehee, and P. Peumans, Appl. Phys. Lett. 91, 243501 共2007兲. 14 Y. H. Zhou, F. Zhang, K. Tvingstedt, S. Barrau, F. H. Li, W. J. Tian, and O. Inganas, Appl. Phys. Lett. 92, 233308 共2008兲. 15 F. Zhang, K. G. Jespersen, C. Bjorstrom, M. Svensson, M. R. Andersson, V. Sundstrom, K. Magnusson, E. Moons, A. Yartsev, and O. Inganas, Adv. Funct. Mater. 16, 667 共2006兲. 16 K. Tvingstedt and O. Inganas, Adv. Mater. 共Weinheim, Ger.兲 19, 2893 共2007兲. 1
