Multilayer organic solar cells with wet-processed ...

Multilayer organic solar cells with wet-processed polymeric bulk heterojunction film and dry-processed small molecule films Youngkyoo Kim, Minjung Shin, Inhyuk Lee, Hwajeong Kim, and Sandrine Heutz Citation: Appl. Phys. Lett. 92, 093306 (2008); doi: 10.1063/1.2890169 View online: http://dx.doi.org/10.1063/1.2890169 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v92/i9 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 92, 093306 共2008兲

Multilayer organic solar cells with wet-processed polymeric bulk heterojunction film and dry-processed small molecule films Youngkyoo Kim,1,a兲 Minjung Shin,1 Inhyuk Lee,1 Hwajeong Kim,1 and Sandrine Heutz2 1

Organic Nanoelectronics Laboratory, Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea 2 Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom

共Received 3 December 2007; accepted 26 January 2008; published online 5 March 2008兲 Here, we report multilayer organic solar cells fabricated using a mix of solution 共wet兲 and thermal evaporation 共dry兲 techniques, which consist of indium-tin oxide 共ITO兲, poly共3,4-ethylenedioxythiophene兲:poly共styrenesulfonate兲共PEDOT:PSS兲, poly共3-hexylthiophene兲共P3HT兲:1-共3-methoxycarbonyl兲-propyl-1-phenyl-共6 , 6兲C61 共PCBM兲, C60, bathocuproine 共BCP兲, and aluminum layers. Results show that the short circuit current density 共JSC兲 of a ITO/ PEDOT: PSS/ C60/BCP/Al device was greatly improved by inserting a pristine P3HT light-absorbing layer between the PEDOT:PSS and C60 layers. Addition of PCBM to the P3HT layer lowered the JSC in the devices compared to the pristine P3HT layer and, in general, the JSC continued to decline with increasing PCBM content. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2890169兴 Recently, the performance of organic solar cells has been improved since the early attempts of employing organic semiconducting thin films.1–7 Most of the best devices to date employ fullerene or its derivatives as an electron-accepting component.2–7 The reason for this is understood to be due to the sophisticated band structure of fullerenes, which is shifted toward higher energies compared to that of most light-absorbing electron-donating organic materials.3,4 The shifted band structure, namely, the lowest unoccupied molecular orbital 共LUMO兲 and the highest occupied molecular orbital 共HOMO兲, of fullerenes enables an efficient charge separation of the excitons generated in the associated lightabsorbing and electron-donating materials.8 An additional benefit of using fullerene is the high electron mobility, which results in efficient electron collection.9,10 As a light-absorbing and electron-donating material, both copper phthalocyanine 共CuPc兲 and regioregular poly共3hexylthiophene兲共P3HT兲 have shown their robustness with exhibiting power conversion efficiency 共PCE兲 of ⬎4% under simulated solar light illumination 关air mass 共AM兲 1.5兴.3,7 Here, we note that the CuPc is commonly used to make small molecule devices in which the organic layers are made via thermal evaporation in vacuum 共namely, a dry process兲, while P3HT is used to make polymer solar cells in which the P3HT/soluble fullerene blend films are coated from their solutions 共namely, a wet process兲. In the case of all small molecular devices, a multilayer structure has been widely employed but only few attempts have been reported for polymer/small molecule layered devices that combine wet and dry processes to achieve a multilayer device.11 To date, no studies have been reported on multilayer devices where the major charge generating layer consists of P3HT/soluble fullerene bulk heterojunction 共BHJ兲 film and a thermally evaporated fullerene 共C60兲 film. In this work, we attempt to make multilayer organic solar cells with a polymeric BHJ layer that is made using P3HT and 1-共3a兲

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

0003-6951/2008/92共9兲/093306/3/$23.00

methoxycarbonyl兲-propyl-1-phenyl-共6 , 6兲C61 共PCBM兲. Regioregular P3HT with hydrogen end groups 共⬃96% regioregularity兲 was used as received from Merck Chemicals, Ltd.4,7 C60 and PCBM were used as received from Nano-C. Bathocuproine 共BCP兲 was purchased from Aldrich and used without further purification. Blend solutions 共P3HT: PCBM= 1 : 0.1, 1:0.5, 1:1, and 1:2 by weight兲 were prepared using chlorobenzene as a solvent at a solid concentration of ⬃12 mg/ ml. For the measurement of UV-visible spectrum, the C60 solution was prepared using 1,2dichlorobenzene at a solid concentration of 7.2 ⫻ 10−5 mg/ ml. To fabricate devices, the same process for coating poly共3,4-ethylenedioxythiophene兲:poly共styrenesulfonate兲共PEDOT:PSS兲共Baytron P standard grade, HC Stark兲 onto indium tin oxide 共ITO兲-coated glass substrates 共⬃25 ⍀ / 䊐兲 was employed as described in our previous work.4,7 The pristine P3HT or blend 共P3HT:PCBM兲 films were spin coated on the annealed PEDOT:PSS layer and then soft baked at 50 ° C for 15 min 共the polymeric films were controlled to be 30 nm thick兲. Next, these samples were loaded into a vacuum chamber and then C60 共40 nm兲 and BCP 共12 nm兲 layers were deposited on top of the polymeric film side of the samples. Finally, an Al top electrode 共⬃100 nm thick兲 was deposited on the BCP layer at ⬃3 ⫻ 10−6 Torr. The active area of devices was 0.16 cm2 关see Fig. 1共a兲–1共c兲兴. UV-visible spectrophotometer 共V-650, Jasco兲 was used to measure the optical absorption spectra of solutions and films. The device performance under AM 1.5 simulated solar light illumination 关incident light intensity 共PIN兲 = 85 mW/ cm2兴 was measured using a solar simulator system 共Oriel兲 equipped with Xe lamp, AM 1.5 spectral filter, and an electrometer 共Keithley 2400兲. As shown in Fig. 2, before inserting the P3HT layer, the device exhibited quite a low short circuit current density 共JSC兲 due to the poor overlap of C60 absorption spectrum with the solar spectrum 关see the inset of Fig. 2共d兲 and 2共e兲兴,12,13 though the open circuit voltage 共VOC兲 was fairly high 关see Fig. 1共a兲兴. After inserting the P3HT layer, JSC was remarkably increased 共approximately eightfold兲, indicative

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Appl. Phys. Lett. 92, 093306 共2008兲

FIG. 1. 共Color online兲 Chemical structure of materials and flat energy band diagrams of devices: 共a兲 ITO/ PEDOT: PSS/ C60 / BCP/ Al, 共b兲 ITO/ PEDOT: PSS/ P3HT/ C60 / BCP/ Al, and 共c兲 ITO/ PEDOT: PSS/ BHJ共P3HT: PCBM兲 / C60 / BCP/ Al.

of the enhanced light harvesting by the P3HT layer insertion 关see the inset of Fig. 2共c兲兴. However, VOC was considerably reduced, which can be attributed to the narrowed band offset between the LUMO of C60 and the HOMO of P3HT 关see Fig. 1共b兲兴. We consider that the charge separation phenomenon is restricted to the interfacial region between the P3HT layer 共donor兲 and the C60 layer 共acceptor兲共note that the exciton diffusion length is less than 10 nm for most conjugated polymers兲.14 Hence, the pristine P3HT and C60 layers, away from the charge separation region, produce no charges but act only as a charge carrier transporting medium. We can expect that broadening the charge separation zone into the deeper parts of the light-absorbing P3HT layer away from the C60 interface would increase the amount of mobile charges generated, leading to enhanced JSC. In order to test this, devices with a P3HT:PCBM layer of different compositions were fabricated 关see Fig. 1共c兲兴. As shown in Fig. 3 共top panel, inset兲, adding only a small amount of PCBM to the P3HT layer 共P3HT: PCBM = 1 : 0.1兲 resulted in a slight decrease in JSC. This negative effect of JSC is attributed to the lowered absorption coefficient due to the inclusion of ⬃10 wt % PCBM molecules in the light absorbing layer compared to the pristine P3HT layer of the same thickness. Interestingly, however, we note that

FIG. 2. 共Color online兲 J-V characteristics of devices under simulated solar light illumination 共AM1.5, PIN = 85 mW/ cm2兲: 共a兲 ITO/ PEDOT: PSS/ C60 / BCP/ Al and 共b兲 ITO/ PEDOT: PSS/ P3HT/ C60 / BCP/ Al. Inset shows the normalized optical density as a function of wavelength: 共c兲 pristine P3HT film, 共d兲 C60 film, and 共e兲 solution of C60 in 1,2-dichlorobenzene.

the addition of ⬃10 wt % PCBM slightly improved the series resistance 共RS兲 of the device 共see Table I兲 in accord with the increase in fill factor 共FF兲. Further adding of PCBM molecules to the P3HT layer did, in general, reduce the JSC of the devices 共see Fig. 3 top panel兲, except for the device with the P3HT:PCBM 共1:0.5兲 layer where it was very slightly increased. This result can be similarly explained as the effect of the reduced absorption coefficient, as shown in Fig. 3 共bottom panel兲. However, further changes to JSC beside those from changes to the absorption coefficient could also be given such as the influence of the P3HT:PCBM film morphology, particularly at the vicinity of the P3HT/ C60 interface. For instance, the charge blocking resistance in the P3HT layer is expected to be increased by adding PCBM molecules, assuming that the PCBM molecules were dispersed uniformly, adversely affecting the series resistance.15 This last effect can clearly be seen from the current density-voltage 共J-V兲 characteristics above the open circuit condition. In particular, the J-V curve

FIG. 3. 共Color online兲共Top panel兲 J-V characteristics of devices having BHJ layer 共P3HT:PCBM兲 under simulated solar light illumination 共AM1.5, 85 mW/ cm2兲. Inset shows the comparison of J-V characteristics between 共a兲 ITO/ PEDOT: PSS/ P3HT/ C60 / BCP/ Al and 共b兲 ITO/ PEDOT: PSS/ P3HT: PCBM共1 : 0.1兲 / C60 / BCP/ Al devices. 共Bottom panel兲 Correlation between the short circuit current density 共JSC兲 of BHJ devices and the absorption coefficient 共␣兲 of the BHJ 共P3HT:PCBM兲 films at 520 nm as a function of the PCBM weight ratio.


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TABLE I. Summary of device characteristics measured under simulated solar light illumination 共AM1.5, PIN = 85 mW/ cm2兲. Active layer structure

JSC 共mA/ cm2兲

VOC 共V兲

FF 共%兲

RS 共k⍀兲

PCE 共%兲

C60 / BCP P3HT/ C60 / BCP P3HT: PCBM/ C60 / BCP 共1:0.1兲 P3HT: PCBM/ C60 / BCP 共1:0.5兲 P3HT: PCBM/ C60 / BCP 共1:1兲 P3HT: PCBM/ C60 / BCP 共1:2兲

0.27 2.19

0.48 0.21

36.4 38.4

5.34 0.35

0.055 0.207

1.95

0.19

45.2

0.31

0.193

2.01

0.18

30.9

0.46

0.132

1.11

0.19

27.9

0.99

0.071

0.92

0.24

23.3

1.63

0.055

shape in the fourth quadrant became worse, leading to the gradual decrease in the FF, as the PCBM content increased. As a result, PCE was monotonically decreased with the PCBM content 共see Table I兲. In summary, the present result on the composition dependence of the P3HT:PCBM layer is different from a previous publication where the highest PCE was achieved with a 1:1 共P3HT:PCBM兲 composition layer.16 This discrepancy can be explained by the fact that, in case of the present device structure, the charge separation is made both in the bulk and at the BHJ-C60 interface, which could enhance JSC at lower PCBM contents compared to the reference device 共ITO/PEDOT:PSS/P3HT:PCBM/Al兲, in which the BHJ-C60 interface effect is absent.16 Finally, the present result suggests that the device performance could be further improved if the PCBM content is optimized below 10 wt % since this composition showed improved series resistance at the slight expense of JSC. The authors thank Merck Chemicals, Ltd. for supplying P3HT materials. This work was supported by the Korea Science and Engineering Foundation 共KOSEF兲 grant funded by the Korea government 共MOST兲共No. R01-2007-000-108360兲. C. W. Tang, Appl. Phys. Lett. 48, 183 共1986兲. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 270,

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