Silaindacenodithiophene-Based Molecular Donor: Morphological Features and Use in the Fabrication of Compositionally Tolerant, High-Efficiency Bulk Heterojunction Solar Cells

详细信息    查看全文

• 作者：John A. Love ; Ikuhiro Nagao ; Ye Huang ; Martijn Kuik ; Vinay Gupta ; Christopher J. Takacs ; Jessica E. Coughlin ; Li Qi ; Thomas S. van der Poll ; Edward J. Kramer ; Alan J. Heeger ; Thuc-Quyen Nguyen ; Guillermo C. Bazan
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：March 5, 2014
• 年：2014
• 卷：136
• 期：9
• 页码：3597-3606
• 全文大小：600K
• 年卷期：v.136,no.9(March 5, 2014)
• ISSN：1520-5126

文摘

A novel solution-processable small molecule, namely, benzo[1,2-b:4,5-b]bis(4,4鈥?dihexyl-4H-silolo[3,2-b]thiophene-2,2鈥?diyl)bis(6-fluoro-4-(5鈥?hexyl-[2,2鈥?bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole (p-SIDT(FBTTh₂)₂), was designed and synthesized by utilizing the silaindacenodithiophene (SIDT) framework as the central D² donor unit within the D¹AD²AD¹ chromophore configuration. Relative to the widely studied 7,7鈥?[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b鈥瞉dithiophene-2,6-diyl]bis[6-fluoro-4-(5鈥?hexyl-[2,2鈥?bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole] (p-DTS(FBTTh₂)₂), which contains the stronger donor fragment dithienosilole (DTS) as D², one finds that p-SIDT(FBTTh₂)₂ exhibits a wider band gap and can be used to fabricate bulk heterojunction solar cells with higher open circuit voltage (0.91 V). Most remarkably, thin films comprising p-SIDT(FBTTh₂)₂ can achieve exceptional levels of self-organization directly via solution deposition. For example, high-resolution transmission electron microscopy analysis shows that p-SIDT(FBTTh₂)₂ spin-cast from chlorobenzene organizes into crystalline domains with lattice planes that extend over length scales on the order of hundreds of nanometers. Such features suggest liquid crystalline properties during the evolution of the film. Moreover, grazing incidence wide-angle X-ray scattering analysis shows a strong tendency for the molecules to exist with a strong 鈥渇ace-on鈥?orientation relative to the substrate plane. Similar structural features, albeit of more restricted dimensions, can be observed within p-SIDT(FBTTh₂)₂:PC₇₁BM bulk heterojunction thin films when the films are processed with 0.4% diiodooctane (DIO) solvent additive. DIO use also increases the solar cell power conversion efficiencies (PCEs) from 1.7% to 6.4%. Of significance from a practical device fabrication perspective is that, for p-SIDT(FBTTh₂)₂:PC₇₁BM blends, there is a wide range of compositions (from 20:80 to 70:30 p-SIDT(FBTTh₂)₂:PC₇₁BM) that provide good photovoltaic response, i.e., PCE = 4鈥?%, indicating a robust tendency to form the necessary continuous phases for charge carrier collection. Light intensity photocurrent measurements, charge selective diode fabrication, and internal quantum efficiency determinations were carried out to obtain insight into the mechanism of device operation. Inclusion of DIO in the casting solution results in films that exhibit much lower photocurrent dependence on voltage and a concomitant increase in fill factor. At the optimum blend ratio, devices show high charge carrier mobilities, while mismatched hole and electron mobilities in blends with high or low donor content result in reduced fill factors and device performance.