Fabrication of Organic Solar Cells Based on a Blend of ... - IEEE Xplore

ICSE 2008 Proc. 2008, Johor Bahru, Malaysia

Fabrication of Organic Solar Cells Based on a Blend of Donor-Acceptor Molecules by Inkjet Printing Technique Ashkan Shafiee1, Muhamad Mat Salleh1, Member, IEEE and Muhammad Yahaya2, Member, IEEE 1 Institute of Microengineering and Nanoelectronics (IMEN) University Kebangsaan Malaysia, 43600 Bangi, Selangor, MALAYSIA 2 School of Applied Physics, Faculty of Science and Technology University Kebangsaan Malaysia, 43600 Bangi, Selangor, MALAYSIA Email: [email protected]

Abstract: Organic solar cells become an interesting research due to their potential for low-cost and flexible power devices. Inkjet printing technique is a promising deposition method to fabricate low-cost and flexible organic solar cells. This paper reports the fabrication of organic solar cells using inkjet printing technique. A blend of electron acceptor and electron donor molecules made of [6, 6]-Phenyl C61 –butyric Acid 3ethylthiophene ester and Poly (3octylthiophene-2, 5-diyl) were printed on an ITO coated glass substrate. Aluminum thin film was used as top electrode. I-V curves of the devices were measured in the dark and under illumination of light. It was found that the device showed rectifier property in the dark and able to generated electrical current under light. Further work to improve the performance of the device is discussed in terms of difference parameters such as number of layers and annealed temperatures. I.

additive patterning, the wide choice of structure designs, compatibility with large-area and flexible substrates7-11, make this new technique become a focus of attention. This paper reports the fabrication of inkjet-printed organic solar cells. The devices consist of an active layer sandwiched between ITO bottom electrode and aluminum as top electrode. The active layer made of a blend of [6, 6]-Phenyl C61 –butyric Acid 3-ethylthiophene ester (PCBE) as acceptor and Poly 3-octylthiophene (P3OT) as the donor.

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EXPERIMENT

The solar cell active materials which were used in this work are [6, 6]-Phenyl C61 – butyric Acid 3-ethylthiophene ester (PCBE) as acceptor and Poly 3-octyl-thiophene (P3OT) as donor. Fig. 1 shows chemical structure of the PCBE and P3OT. The P3OT was purchased from Sigma-Aldrich and PCBE was purchased from American-dye Source and both were used without further purification.

INTRODUCTION

Organic solar cells have become an interesting research due to their potential for low-cost and flexible power devices1. Nevertheless there are still a lot of efforts to improve their poor efficiency to be a reliable alternative for inorganic counterparts. One of the most effective parameters of device fabrication is deposition technique, which may control the quality of thin films and hence device performance. There are a few deposition techniques for fabrication organic solar cells such as spin coating2, screen-printing3, inkjet printing4, 5 and so on. Among all of these techniques, inkjet printing is considered one of the most promising methods since it has capabilities for a controlled deposition of polymers6, 7. Furthermore ability to make

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Fig. 1. a) PCBE (n-type polymer as acceptor), b) P3OT (p-type polymer as the donor of the device).

Fig. 2. shows a schematic diagram of printed organic solar cell. The bottom electrode was indium tin oxide (ITO) thin film coated on glass slide. The sheet resistance of this film was 20 Ώ/⁮. The top electrode was aluminum thin film which was deposited using evaporation technique.

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b Fig 2. Schematic Organic solar cell

To prepare the active layer, at first we made solutions of both donor and acceptor materials in toluene with concentration of 8 mg/ml. The solutions of donor and acceptor were then blended with the ratio of 1:1 by volume using ultrasonic bath for 30 minutes. The blended solution was used as ink where the active layer was deposited on ITO substrate using an ink jet printer. The ink jet printer used in this experiment was “Dimatix Material Printer (DMP) 2831 system”. This is a MEMS based piezoelectric printer. This printer uses special cartridges which includes a reservoir of ink and piezoelectric material in an ink-filled chamber behind each nozzle (Fig. 3.a). The shape of piezoelectric material is changed with application of a voltage pulse and causes the ink chamber to shrink and eject the ink from the nozzle (Fig. 3.b). The magnitude of this voltage determines how far the piezoelectric material bends. This bending makes a force to ink chamber to eject the ink. Hence it may control the volume, form and the velocity of the ink drop. The driving parameters of printer were adjusted to produce the droplets without tail which produce smoother thin films.

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Fig. 3. a) Piezoelectric Cartridge, b) application of voltage and firing

For the devices fabrication we have made a variation in number of active layers printed on ITO, i.e. one, two and four layers. The devices were annealed in air at 50 °C for two hours. The I-V curves of the devices before and after annealing were characterized using Keithley SMU 238 in the dark and under 75 w/m2 illumination. Before preparing the devices two thin films of individual donor and blend of donor/acceptor with the same concentration and ratio with the active layer of the device were tested with Perklin-Elmer Luminescence Spectrometer (LS55) to understand the emission spectra of the thin films.

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RESULTS AND DISCUSSION

The first task in fabrication of the devices using printing technique is understanding the relation between thin film quality and printing parameters such as droplet characteristics. We print the PCBE thin films using 8 mg/ml solution of the material in toluene with variation of driving voltage. Then we studied the surface morphology of the films using AFM and SEM. Fig. 4 shows the images of ink drop with variation of driving voltage. The higher voltages produce longer satellites and higher velocities. In this case 26 V produces longest satellite and fastest droplet. Meanwhile, 10 V produce perfect droplet without tail.


Fig. 4. The images of ink drop with variation of driving voltage a) 26 V, b) 20 V, c) 16 V, d) 10 V

The satellites may separate from main droplet and become new smaller and slower drops which reach the surface and place on the thin film prepared by main drops and change the roughness consequently. For each ink regarding to the surface tension and viscosity there is a respective voltage to fire the perfect droplet hence for each experiment first of all this perfect droplet should be fund then deposit with the same printer parameters.

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c Fig. 5. SEM images of the PBCE thin films prepared with variation driving voltage a) 20 V b) 16 V c) 10V

Fig. 5. compares of SEM images the PCBE thin films printed with variation of driving voltage. The quality of the films depends on shape of the drops that was controlled by the driving voltage. For fabrication of the device the film should has smooth surface without pin holes. In this case the film prepared using 10 V driving voltage which has the shortest tail drop is the smoothest. In preparation of organic solar cells based on blend of acceptor and donor materials, we desire the blend material which has the lowest possible for the generated electron-hole

(exciton) will recombine. This means higher separated electron-hole which results higher current generating. To make sure our blend materials fulfill this requirement we studied the photoluminescence (PL) spectra thin films of the blending materials and individual donor. Fig. 6. shows that PL spectrum of the donor (P3OT) 400% decrease when it blended with PBCE with ratio of 1:1 by volume. This means that the number of recombined excitones decreases dramatically for the blended materials, which fulfills our requirement.

Fig. 6. PL spectra of donor and blended materials

We have fabricated organic solar cells with variation of the number of printed layer of active materials; i.e. 1, 2 and 4 layers. In order to have a better contact between layers the fabricated devices were annealed in air at 50 °C for two hours. The I-V curves of the devices were measured in the dark and under illumination with 75w/m2 in ambient conditions. Although the one-layer device showed diode rectifying effect under dark, they did not generate photovoltaic current. This may be due to the active layer which is too thin to generate the current. Meanwhile as shown in Fig. 7 the un-annealed two-layer device exhibits photovoltaic effect under illumination.

Fig. 7. I-V curves for un-annealed two-layer device in the dark and under illumination.

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The fill factor and efficiency for this device were 0.34 and 4.20×10-6 % respectively. The performance of this device decreased after the annealing process where the fill factor and efficiency for the annealed device were measured 0.33 and 2.0×10-6 % respectively. This decreasing in fill factor and efficiency for annealed samples confirms our previous studies which showed an increase in surface roughness of annealed thin films. Annealing may provide the necessary kinetic energy for materials to crowd around their own stable position which is the lowest energy level and equilibrium and this mass migration may affect on roughness. Four-layer solar cells after annealing were tested to obtain the fill factor and efficiency. Fig. 8. shows I-V characteristics for annealed four- layer solar cells.

Fig. 8 I-V curve for annealed four- layer printed organic solar cells

This device shows decrease in both parameters compare to two- layer counterparts. It exhibits a fill factor equal to 0.2 and efficiency equal to 0.39×10-6 %. This decreasing might be because of thickness of the layers which is too thick for excitons diffusion length to reach the lowest unoccupied molecular orbital (LUMO) of acceptor to be separated and generate the photocurrent. Hence the optimized thickness belongs to the two-layer thin film which is neither too thin to be weak to generate the current nor too thick to lose the excitons because of the distance.

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CONCLUSION:

We successfully printed organic solar cells based on P3OT/PCBE with different number of layers and annealing temperature. It has been shown optimized number of layers obtained by twolayer devices and annealing decrease the device performance.

ACKNOLEDGEMENT: The authors would like to thank Ministry of Higher Education of Malaysia to support this project under Research University Grant UKMGUP-BTT-07-26-178. REFRENCES: [1] M. Al-Ibrahim et.al., Organic Electronics vol. 6, pp. 65-77 (2005) [2] C. He. et.al., Solar Energy Materials & Solar Cells vol. 90, pp. 1815-1827 (2006) [3] S. E. Shaheen, R. Radspinner, N. Peyghambarian, and G. E. Jabbour. APPLIED PHYSICS LETTERS vol 79, pp. 2996-2998 (2001) [4] T. Kawas, H. Sirringhaus, R.H Friend and T. Shimoda, Proc. IEEE International Electron Devices Meeting, 623-626 (2000) [5] H. Sirringhaus, T. Kawas, R.H. Friend, T. Shimoda, M. Inbasekaranand and W. Wu, Science, vol. 290 (5499), pp. 2123-2126 (2000) [6] P. Calvert, Chem. Mater vol. 13, pp. 3299-3305 (2001) [7] B. J. Gans, P. C. Duineveld and U. S. Schubert, Adv. Mater vol. 16, pp. 203-213 (2004) [8] S. R. Forrest, Nature vol. 428, pp. 911-918 (2004) [9] R. Parashkov, E. Becker, T. Riedl, H. Johannes and W. Kowaslasky, Proc. IEEE., 2005, 93, 13211329. [10] U. P. Jang, H. Matt. et al., Nature materials vol. 6, pp. 782-789 (2007) [11] T. Kawas, T. Shimoda, CH. Newsome, H. Sirringhaus, R.H. Friend: Thin Solid Films, vol. 438439, pp. 279- 287(2003)