Performance characteristics of polymer photovoltaic solar cells with an additive-incorporated active layer
© Kim et al; licensee Springer. 2012
Received: 5 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
We have investigated the performance characteristics of bulk-heterojunction polymer solar cells based on poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester by adding 1,8-octanedithiol as a processing agent in an active layer. The effects of the additive, 1,8-octanedithiol, on the device performance parameter characteristics have been discussed. The current density-voltage measurements, UV-Vis absorption spectra, X-ray diffraction spectra, and scanning probe microscope images have been used to discuss the performance characteristics of polymer solar cells.
Keywordsbulk heterojunction power conversion efficiency polymer solar cell excitons
Clean and renewable energies have been considerable issues in the last decade. For this reason, organic photovoltaic cells have been attractive devices as next-generation substitute energy sources [1–4]. Currently, the power conversion efficiencies of organic photovoltaic cells have been steadily improved around 6% through polymer solar cells . There have been reports that polymer solar cells have many advantages of cost-effectiveness in the fabrication process, and the mechanical flexibility and polymeric materials provide a wide field of applications [6, 7].
Bulk-heterojunction [BHJ] solar cells, based on phase-separated blends of polymer semiconductors and fullerene derivatives, typically consist of a conjugated polymer, poly(3-hexylthiophene-2,5-diyl) [P3HT] as an electron donor, and fullerene derivatives, [6,6]-phenyl C61 butyric acid methyl ester [PCBM] as an electron acceptor [8–12]. Especially, P3HT has attracted lots of interest due to its high crystallinity and self-assembling property. In supporting P3HT crystallite formation, PCBM should be dispersed between P3HT chains . For this, thermal and solvent annealing can be used to improve their roles between P3HT and PCBM [14, 15]. Recently, a small volume ratio of additives such as 1,8-octanedithiol has been incorporated into the P3HT:PCBM system to improve the interactions between P3HT and PCBM .
In this work, we have fabricated BHJ solar cells based on P3HT and PCBM, which were dispersed using a single solvent, chlorobenzene and 1,2-dichlorobenzene. The effects of the additive, 1,8-octanedithiol, on the performance characteristics of polymer solar cells have been investigated. The results of current density-voltage [J-V] measurements, UV-Visible [UV-Vis] absorption spectra, X-ray diffraction [XRD] spectra, and scanning probe microscope [SPM] images will be intensively used to discuss the performance characteristics of polymer solar cells fabricated in this study.
BHJ films were prepared via a solution process. P3HT (Rieke Metals, Inc., Lincoln, NE, USA) and PCBM (Nano-C, Westwood, MA, USA) with a 1:1 wt/wt ratio was dissolved in chlorobenzene and 1,2-dichlorobenzene to make a 2.4 wt.% solution. The blend solution was stirred for 24 h at 40°C in a shaking incubator. 1,8-Octanedithiol (formula C8H18S2, molecular weight 178.36 g/mol, boiling point 269°C to 270°C, density, 0.97 g/mL at 25°C, Sigma-Aldrich Corporation, St. Louis, MO, USA) and 1,8-diiodooctane (formula C8H16I2, molecular weight 366.02 g/mol, boiling point 167°C to 169°C, density 1.84 g/mL at 25°C, Sigma-Aldrich Corporation) were selected as additives, and 2.5 vol.% additives were then added into the base solution. The solution containing a mixture of P3HT:PCBM with processing additives was stirred for 10 min. Polymer solar cells were fabricated on the pre-patterned indium tin oxide [ITO] glass substrate. Poly(3,4-ethylenedioxyhiophene):poly(styrenesulfonate) [PEDOT:PSS] was spin-coated onto the ITO substrate at 3,000 rpm for 30 s, and the prepared thin film was then baked at 120°C for 10 min on a hot plate in air. The prepared solution was spin-coated onto the PEDOT:PSS layer at 1,000 rpm for 30 s, and then, the spin-coated thin film was dried in a Petri dish. As a final step, an Al electrode was deposited onto the spin-coated layer by thermal evaporation. The fabricated devices were annealed at 120°C for 30 min. An active area of the device, 2 mm × 2 mm in dimension, was made using a shadow mask. The J-V and power conversion efficiency (ηe) characteristics were measured using a 2400 multi-source meter unit (Keithley Instruments, Inc., Seoul, South Korea). A xenon lamp (100 mW/cm2) was used as a light source, and the light intensity has been measured by a silicon photodiode calibrated for an AM 1.5 spectrum. The absorption spectrum were taken using an Optizen 2120UV spectrophotometer (Mecasys Co., Ltd., Daejeon, South Korea); XRD images were obtained using a high-resolution X-ray diffractometer (Philips, Amsterdam, The Netherlands); and SPM images were obtained using a SPM (Multimode, Digital Instruments, Inc., Tonawanda, NY, USA).
Results and discussion
The performance characteristics of BHJ polymer solar cells based on P3HT and PCBM can be improved by introducing a processing additive, 1,8-octanedithiol, to a solution-based film formation process, and an optimized amount of 1,8-octanedithiol can be determined. As the amount of 1,8-octanedithiol was increased, the intensity of the UV-Vis absorption and the crystallinity of P3HT significantly increased, and the PL intensity also increased simultaneously, consequently exhibiting the improved performances of the BHJ polymer solar cells. By employing the processing additive, 1,8-octanedithiol, the PCE was increased from 2.16% to 3.46% in this study.
HK, SO, and HC are students of a Master's course and YC is a professor in the Chemical Engineering Department of Pusan National University, South Korea.
indium tin oxide
[6,6]-phenyl C61 butyric acid methyl ester
power conversion efficiency
scanning probe microscope
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0003825) and the Brain Korea 21 project.
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