Applied Chemistry,
Vol. 14, No. 2, October 2010, 29-32
29
세 가지 물질 블랜딩을 통한 유기태양전지 효율 증대에 관한 연구
김한솔⋅선 욱⋅이경균⋅이성구⋅임은희†
한국생산기술연구원 (ehlim@kitech.re.kr)
Increasing the Efficiency of Organic Photovoltaic Cells of the Ternary Blended System
Han-Sol Kim⋅Wook Sun⋅Kyeong K. Lee⋅Sungkoo Lee⋅Eunhee Lim†
Green Chemistry and Engineering Department, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 331-825, Korea
(ehlim@kitech.re.kr)
Abstract
In this work, new conjugated small molecule and polymer based on benzo [1,2-b: 4,5-b' ] dithiophene (BDT) and thiophene were successfully synthesized via Suzuki and Stillcoupling reactions, respectively. Using these new materials, we introduced the ternary blended system into the donor and acceptor blended films. The device performances of organic photovoltaic cells can be improved by addition of the third component. The effect of third component on the photovoltaic properties was systematically investigated by UV-vis ab- sorption and photoluminescence (PL) spectra.
1. Introduction
During the last decade, the OPV field has progressed remarkably both in terms of device performance as well as understanding of the governing physical processes. [1,2] To date, most efforts have been devoted to developing bulk-heterojunction polymer solar cells com- posed of polymer donors and soluble fullerene, acceptors, because of the easy fabrication and low cost of such cells. The prototypical large band gap material system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) is nearing optimal device performance. However, because the large band gap material system consisted of P3HT can harvest photons with wavelengths below ∼650 nm, new conjugated materials with low band gaps, which can efficiently harvest solar energy over a broader spectrum, are being actively developed[3,4]. The arrangement of the electron-donor and electron-acceptor repeating units alternately along the conjugated backbone is the most successful approach to the synthesis of low-band-gap materials. Meanwhile, there have been several reports on the improved efficiency of P3HT : PCBM system by incorporating additives, so called ternary mixing. The addition of polymers such as electron-donating pol- ymer and region random P3HT led to the increase of open-circuit voltages (VOC)[5,6]. More
30 김한솔⋅선욱⋅이경균⋅이성구⋅임은희
recently, Jeong et. al. reported that the power conversion efficiency (PCE) of P3HT : PCBM device was improved by incorporation of discotic liquid crystal (DLC) additives with self- assembling ability[7]. In spite of low absorbance of DLCs, the devices with DLCs showed a remarkably improved short-circuit current (JSC) owing to increased ordering of P3HT chains.
In this paper, we introduced the ternary blended system into the donor and acceptor blended films. In other words, the third component was incorporated into the donor and ac- ceptor blended systemas an additive to provide a simple method for improving the properties of the active layers. Here, the conjugated polymer and oligomer based on benzodithiophene were newly synthesized and their electrochemical and optical properties are investigated.
The effect of P-BDT on the photovoltaic properties of SM-BDT : PCBM films were studied by UV-vis absorption spectra and photoluminescence (PL) spectra.
2. Result and Discussion
We have synthesized new conjugated small molecule and polymerbased on benzo[1,2-b : 4,5-b' ] dithiophene (BDT) and thiophene, 2,6-bis([2,2' ]bithiophenyl-5-yl)-benzo[1,2-b : 4,5-b' ] dithiophene (SM-BDT) and poly(benzo[1,2-b:4,5-b' ]dithiophene-alt-bithiophene) (P-BDT), via Suzuki and Still coupling reactions, respectively. Fig. 1 shows the chemical structures of the P-BDT and SM-BDT, which were confirmed by 1H-NMR spectroscopy. The weight-average molecular weight (Mw) of P-BDT was 10000, which was determined by gel permeating chromatography (GPC) using tetrahydrofuran (THF) as eluent and polystyrene as standard.
S
S O
O C8H17
C8H17
S S S
S O
O C8H17
C8H17
S S
S S
n
P-BDT SM-BDT
Fig. 1. Chemical structures of the materials.
Fig. 2. (a) UV-vis absorption and (b) photoluminescence (PL) emission spectra of the films.
31
세 가지 물질 블랜딩을 통한 유기태양전지 효율 증대에 관한 연구
As shown in Fig. 2(a), the SM-BDT and P-BDT films show UV-visible absorption max- ima at around 400 and 445 nm, respectively. The SM-BDT and P-BDT were blended with PCBM, which can act as the acceptor for OPV fabrication. For comparison, the films were prepared by spin-coating of blended solution of SM-BDT or P-BDT with PCBM according to the procedure of OPV fabrication. The UV-visible absorption spectra of the blend films of SM-BDT:PCBM and SM-BDT:P-BDT:PCBM are also shown in Fig. 2(a). Because the ab- sorption intensity of P-BDT is higher than that of SM-BDT, the addition of P-BDT into SM-BDT:PCBM (i.e., SM-BDT:P-BDT:PCBM film) showed slightly increased red-shifted UV-absorption spectra that of SM-BDT:PCBM film. The red-shifted absorption may have a positive effect on OPV performance. A positive effect of ternary system is also observed in the PL spectra of the blended films. The PL spectra were obtained by excitation at the cor- responding absorption maxima ((λmax(SM-BDT)=400 nm and λmax(P-BDT)=445 nm). As shown in Fig. 2(b), the SM-BDT and P-BDT films showed the PL emission spectra with maxima at around 550 nm. The intensities of PL spectra of the blended films of SM-BDT:
PCBM and P-BDT:PCBM were reduced than those of SM-BDT and P-BDT films by addition of PCBM. This is well-consistentwith the previous reported results, in which the PL quenching indicates charge transfer from electron-rich donor to electron-deficient acceptor such as PCBM. More interesting results were obtained in the ternary mixing system of SM-BDT:P-BDT:PCBM. The film was excited at the absorption maxima of SM-BDT (400 nm) or P-BDT (445 nm). As shown in Figure 2b, the PL of SM-BDT:P-BDT:PCBM film was completely quenched. In other words, compared to the SM-BDT:PCBM film, the charge transfer from donor to acceptor is clearly more effective in SM-BDT:P-BDT:PCBM film by addition of the third component of P-BDT into SM-BDT:PCBM system. Future works include the fabrication of OPV cells and the optimization of fabrication condition.
3. Conclusion
In summary, we have developed new strategy to improve OPV performance by addition of the third component. The improved absorption and emission properties of the blended films were systematically investigated herein.
Acknowledgement
This research was supported by a grant from Green Smart Card Platform Technologies Based on 3D Printed Electronic Devices Project of MKE and ISTK and the Fundamental R&D Program for Core Technology of Materials funded by MKE of Republic of Korea.
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