Benzo[c]thiophene-based Dipolar Molecules for Organic Solar Cells

• Main Authors: Hui-Ning Wan, 萬惠寧
• Other Authors: none
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/93544269518396668845

碩士 === 國立臺灣科技大學 === 工程技術研究所 === 98 === A series of linear dipolar compounds containing benzo[c]thiophene unit have been synthesized and applied for dye-sensitized solar cells or photovoltaics. These molecules consist of 9,10-diethyl or 9,10-bis(2-ethylhexyl) fluorene as the conjugated spacer, a diphenylamine as the electron donor and an cyano-containing group as the electron acceptor. Compound N1 adopting a 2-cyanoacrylic acid as the electron acceptor and the anchoring group can be used as the sensitizer of dye-sensitized solar cells, while compounds N2−N5 utilizing a dicyanovinylene group as the acceptor can be used as the sensitizer of photovoltaic cells. All of these compounds have fairly high molar absorption coefficients (21,900–35,300M-1cm-1) at the region between 487–556 nm. Compound N4 which has two 2-ethylhexyl substitutuents at 9-position of the fluorene segment exhibits higher molar extinction coefficient and more red-shifted absorption wavelength than analogues N1 and N2 which have two ethyl substituents at the same position. Though compound N5 absorbs at the highest-energy region because of the absence of an amino donor, it still possesses a large absorption coefficient due to the two 2-ethylhexyl substituents. Theoretical calculations indicate that these compounds N1−N5 possess fairly small dihedral angles between the neighboring aromatic rings of the conjugated spacer (0.5−1.3o) except that between benzothiophene and the neighboring fluorene group (39.4−42.0o). The lowest-lying transitions of these compounds have considerable charge-transfer character with high oscillator strength (0.62−1.16). The bulk-heterojunction photovoltaic devices utilize the benzothiophene-containing compounds as the donor material and PCBM (phenyl-C61-butyric acid methyl ester) as the acceptor were also investigated. The photovoltaic cell fabricated with N2 with PCBM blending ratio of 50 wt% turn out to have the highest power conversion efficiency (?? reaching 0.56%. Furthermore, all the devices fabricated with the benzothiophene molecules exhibit large open-circuit voltage (VOC) in the range between 0.67−0.98 V. This can be attributed to the more stabilized HOMO levels of the benzothiophene compounds. OPVs based on N2 and N3 have the smallest short–circuit current density (JSC) when the concentration of PCBM is ~80 wt%. In comparison, the JSC values of N4 and N5 increase as the blending ratio of PCBM increases. The experiments using tapping–mode atomic–force microscopy (AFM) were conducted to probe the surface morphologies of the active layers of N2 or N5 devices. The N2/PCBM films are very uniform, and the r.m.s. surface roughness (σ) values are 0.320, 0.266 and 0.212 nm for blends of 1:1, 1:2 and 1:4 weight ratios, respectively. In comparison, the N5/PCBM films have surface roughness of 0.387, 0.302 and 0.205 nm, respectively, for blends of 1:1, 1:2 and 1:4 weight ratios. They have higher surface roughness values compared to the N2/PCBM films.