Effect of a Long Alkyl Group on Cyclopentadithiophene as a Conjugated Bridge for D鈥揂鈭捪€鈥揂 Organic Sensitizers: IPCE, Electron Diffusion Length, and Charge Recombination

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• 作者：Qipeng Chai ; Wenqin Li ; Yongzhen Wu ; Kai Pei ; Jingchuan Liu ; Zhiyuan Geng ; He Tian ; Weihong Zhu
• 刊名：ACS Applied Materials & Interfaces
• 出版年：2014
• 出版时间：August 27, 2014
• 年：2014
• 卷：6
• 期：16
• 页码：14621-14630
• 全文大小：470K
• ISSN：1944-8252

文摘

The option of using conjugated 蟺-linkers is critical for rational molecular design toward an energy-level strategy for organic sensitizers. To further optimize photovoltaic performance, methyl- and octyl-substituted 4H-cyclopenta[2,1-b:3,4-b鈥瞉dithiophene (CPDT) are introduced into D鈥揂鈭捪€鈥揂 featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, WS-39 and WS-43 exhibit an extended spectral response due to the excellent conjugation and coplanarity of CPDT. Specifically, we focused on the critical effect of length of the alkyl group linked to the bridging carbon atoms of CPDT on the photovoltaic performances. Octyl-substituted WS-39 shows a broader IPCE onset with an enhanced photovoltage relative to the analogue WS-5. In contrast, WS-43, with methyl substituted on the CPDT moiety, presents a relatively low quantum conversion efficiency within the whole spectral response region, along with low photocurrent density. WS-43 displays a distinctly low IPCE platform, predominately arising from the short electron diffusion length with significant electron loss during the electron transport. The relative movement of the conduction band edge (E_CB) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, WS-39-based solar cells show a promising photovoltaic efficiency of 9.07% (J_SC = 16.61 mA cm^鈥?, V_OC = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide dual protection and exhibit a high propensity to prevent binding of the iodide鈥搕riiodide redox couple, producing an efficient shielding effect to retard the charge recombination and resulting in improvement of V_OC. Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering.

Keywords:

solar cells; organic sensitizers; conjugation bridge; alkyl group; electron diffusion length; photovoltaic efficiency