APPLIED PHYSICS LETTERS 97, 103503 共2010兲
Organic solar cells with submicron-thick polymer:fullerene bulk heterojunction films Seungsoo Lee, Sungho Nam, Hwajeong Kim,a兲 and Youngkyoo Kimb兲 Department of Chemical Engineering, Organic Nanoelectronics Laboratory, Kyungpook National University, Daegu 702-701, Republic of Korea
共Received 15 June 2010; accepted 19 August 2010; published online 8 September 2010兲 We report the viability of organic solar cells with submicron-thick bulk heterojunction films, which were fabricated by mixing poly共3-hexylthiophene兲 and 关6,6兴-phenyl-C61-butyric acid methyl ester at a solid concentration of 90 mg/ml. To elucidate the physics behind the thick film solar cells, optical transmittance and electrical characteristics were compared for film thicknesses between 520 and 1000 nm. Results showed that the device 共520 nm thick film; efficiency= 3.68%兲 exhibited similar performance to that of a control device 共170 nm thick兲. A decreasing device performance was measured for much thicker films 共efficiency= 0.34% for the 1000 nm thick device兲. © 2010 American Institute of Physics. 关doi:10.1063/1.3488002兴 Encouraging breakthroughs in polymer:fullerene solar cell efficiency have been achieved by the solvent-aided nanomorphology control1 and the thermal/solvent annealing process optimization processes.2–6 Recent efficiencies have been improved up to 7%–8%.7,8 This latest improvement attributed to the use of new conjugated polymers and C70 derivatives, which enabled simultaneous increase in both open circuit voltage 共VOC兲 and short circuit current density 共JSC兲 compared to the well-known combination of poly共3hexylthiophene兲 共P3HT兲 and 关6,6兴-phenyl-C61-butyric acid methyl ester 共PC61BM兲.7–10 Interestingly, to date, most polymer solar cells exhibiting high efficiencies have been fabricated with blend films of which thicknesses are typically much less than approximately 250 nm.1–13 Considering the principle of bulk heterojunction 共BHJ兲 concept, which compensates the drawback of short exciton diffusion lengths of organic semiconductors,14,15 the thickness of blend films can be further increased for maximizing the sunlight absorption if an optimum charge percolation path can be still maintained.16,17 In particular, the film coating process can be much more industrially feasible if the blend films become thicker because the tolerance 共standard deviation兲 range of thicker films is milder than that of thinner films 共i.e., ⫾5 nm for 100 nm thick films versus ⫾25 nm for 500 nm thick films in the case of ⫾5% tolerance in terms of thickness兲. In this letter, we demonstrate polymer solar cells with thick P3HT: PC61BM blend films 共520–1000 nm兲 which were prepared by using concentrated blend solutions. Results show that the device with the 520 nm thick film exhibited slightly higher efficiency than the control device 共170 nm兲. P3HT 共regioregularity ⬎95%; weight-average molecular weight⬵ 6 ⫻ 104; polydispersity index= 2兲 and PC61BM 共purity ⬎99%兲 were used as received from Lumtec and Nano-C, respectively. Blend solutions 共P3HT: PC61BM= 1 : 0.8 by weight兲 were prepared using chlorobenzene at a solid concentration of 90 mg/ml 共actually ⬃45 mg per 0.5 ml兲. Formerly at Institute of Biomedical Engineering 共IBE兲, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom. b兲 Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: ⫹82-53-950-5616. a兲
0003-6951/2010/97共10兲/103503/3/$30.00
On top of cleaned indium-tin oxide-coated glass substrates 共⬃11 ⍀ / 䊐兲, poly共3,4-ethylenedioxythiophene兲: poly共styrenesulfonate兲 共PEDOT:PSS兲 共Clevios PH500, HC Starck兲 was spin-coated.18 Next, the P3HT: PC61BM blend films were deposited on the PEDOT:PSS layer by spincoating with various spin speeds. Here we note that the film thickness was quite reproducible even for second or third films made from the stock solutions. Finally, Al electrodes were deposited at ⬃1 ⫻ 10−6 Torr 共active area= 0.09 cm2兲. All devices 共films兲 were thermally annealed at 140 ° C for 30 min. The optical absorption spectra were measured using a UV-visible spectrophotometer 共Optizen 2120UV, Mecasys兲. The current density-voltage 共J-V兲 characteristics were measured using a solar cell measurement system equipped with a solar simulator 共Newport-Oriel兲 and an electrometer 共Keithley 2400兲. The external quantum efficiency 共EQE兲 spectra were measured in the same way as in our previous report.6,18 The cross-sectional morphology of devices 共broken inside liquid nitrogen兲 was examined using a fieldemission scanning electron microscope 共FESEM兲 共S-4800, Hitachi兲. As shown in Fig. 1共a兲, the optical density 共OD兲 of the films gradually increased with increasing the film thickness,
FIG. 1. 共Color online兲 OD change in the P3HT: PC61BM blend films as a function of thickness: 共1兲 520 nm, 共2兲 630 nm, 共3兲 720 nm, 共4兲 1000 nm, and 共0兲 170 nm. Inset shows the transmitted light intensity 共ITR兲 at 510 nm passed through the P3HT: PC61BM blend films where the incident intensity of simulated solar light was 100 mW/ cm2.
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TABLE I. Solar cell characteristics with various P3HT: PC61BM thicknesses 共t兲 under simulated solar light illumination 关Air Mass 1.5 Global 共AM1.5G兲, PIN = 100 mW/ cm2兴. t 共nm兲
CSOL a 共mg/ml兲
Speedb 共rpm兲
JSC 共mA/ cm2兲
VOC 共V兲
FF 共%兲
RS c 共k⍀兲
PCE 共%兲
170 520 630 720 1000
60 90 90 90 90
1500 2000 1500 1000 700
11.87 11.45 9.49 5.72 2.82
0.59 0.58 0.57 0.54 0.44
51 55 51 47 28
0.15 0.12 0.16 0.33 1.61
3.54 3.68 2.76 1.46 0.34
a
Concentration of P3HT: PC61BM solutions in chlorobenzene. Spin-coating speed. c The RS values were calculated at around open circuit voltages under illumination. b
FIG. 2. 共Color online兲 共a兲 Light and 共b兲 dark 共inset: normalized EQE spectra兲 J-V characteristics of devices under simulated solar light illumination 共AM1.5, 100 mW/ cm2兲: 共1兲 520 nm, 共2兲 630 nm, 共3兲 720 nm, 共4兲 1000 nm, and 共0兲 170 nm. 共c兲 Relation between JSC 共circles兲 and light absorption 共squares 510 nm兲 as a function of film thicknesses 共see arrows for the increasing deviation with the film thickness兲.
whereas the absorption range was almost unchanged with the film thickness. The two absorption shoulders at the wavelengths of 550 and 600 nm were more pronounced for the 720 and 1000 nm thick films than for the other thinner films. In particular, the 1000 nm thick film showed a plateau-like absorption at the wavelength 共⬃500 nm兲, indicating the instrumental limitation that this film is too thick to be properly measured with the present UV-visible spectrometer. This plateau absorption roughly implies that the photons 共from sun兲 with the energy of ⬃2.5 eV 共⬃500 nm兲 can be almost perfectly absorbed by the 1000 nm thick film. To understand how many photons can reach the Al cathode region for the films with various thicknesses, we simply calculated the transmitted intensity of light which passes through the films by using ITR = IIN ⫻ e−␣·t where IIN, ITR, ␣, and t denote the incident light intensity 共100 mW/ cm2兲, the transmitted light intensity, the film absorption coefficient, and the film thickness, respectively 共The average absorption coefficient of ␣ = 1 ⫻ 105 cm−1 at the wavelength of 510 nm was applied兲. As shown in Fig. 1 共inset兲, the transmitted light intensity became less than 1% of the incident light intensity for the thicker films 共t ⬎ 500 nm兲, which reflects that the amount of charge generation close to the Al electrode-film interface is as small as ⬍1%. Interestingly, all the thicker films, except the 1000 nm film, exhibited quite good light J-V curves 关Fig. 2共a兲兴. In particular, the J-V curve shape was slightly better for the device with the 520 nm thick film than the control device 共170 nm thick film兲 even though the JSC value was margin-
ally lower 共but comparable兲 for the 520 nm thick film than for the control device. However, further increasing the film thickness resulted in noticeably lowered JSC. Furthermore, the VOC value was also lowered as the film thickness was increased further 共Table I兲. In particular, the device with the 1000 nm thick film exhibited the lowest JSC and VOC, which can be attributed to the relatively poor charge transport 共under illumination兲 by the presence of the less chargegenerated 共photodoped兲 zone close to the Al electrode compared to the other devices with the thinner films 关see the significantly low dark current density in Fig. 2共b兲兴. This result indicates the higher 共transport-limited兲 recombination loss of separated charges for the thicker films despite the better light absorption 关Fig. 2共c兲兴. We note that a balance 共tradeoff兲 between the improved light harvesting and the limited charge transport might be made for the 520 nm thick film. The transport-limited charge recombination is clearly seen from the shape of EQE spectra in which the EQE intensity at the wavelengths of 450–600 nm gradually decreased as the film thickness increased. In terms of microscopic morphology, no particularly different morphology such as large size aggregation of nanoparticles was found for the device with the 1000 nm thick film in the given resolution of the FESEM instrument 关Fig. 3兴 even though their nanomorphology on a much lower nanoscale18 could be slightly different because of the different spin-coating speeds 共see Table I兲. In conclusion, the P3HT:PCBM devices with different film thicknesses 共520–1000 nm兲 were fabricated. Interestingly, the 520 nm thick device showed similar and even slightly better performance than the control device 共170 nm thick兲 while for thicknesses greater than 520 nm the device efficiency was lower than the control device. This result
FIG. 3. 共Color online兲 FESEM images of cross-sections of devices: 共left兲 520 nm and 共right兲 1000 nm 关note that the devices were slightly deformed during sampling 共breaking兲 process inside liquid nitrogen兴.
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demonstrates that organic solar cells utilizing BHJ active layers of submicron thickness, which would be easier to industrially fabricate than thin films of ⬍100 nm, are viable. This work was financially supported by Korean Government grants 共Priority Research Center Program Grant No. 2009-0093819, Pioneer Research Center Program Grant No. 2010-0002231, NRF_20090072777, KETEP-2008-N-PV08J-01-30202008, and NRF_20100004164兲. 1
Appl. Phys. Lett. 97, 103503 共2010兲
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