Thermotropic Phase Transition of Benzodithiophene Copolymer Thin Films and Its Impact on Electrical and Photovoltaic Characteristics

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• 作者：Sangwon Ko ; Do Hwan Kim ; Alexander L. Ayzner ; Stefan C. B. Mannsfeld ; Eric Verploegen ; Alexander M. Nardes ; Nikos Kopidakis ; Michael F. Toney ; Zhenan Bao
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：February 24, 2015
• 年：2015
• 卷：27
• 期：4
• 页码：1223-1232
• 全文大小：540K
• ISSN：1520-5002

文摘

We observed a thermotropic phase transition in poly[3,4-dihexyl thiophene-2,2鈥?5,6鈥?benzo[1,2-b:4,5-b鈥瞉dithiophene] (PDHBDT) thin films accompanied by a transition from a random orientation to an ordered lamellar phase via a nearly hexagonal lattice upon annealing. We demonstrate the effect of temperature-dependent molecular packing on charge carrier mobility (渭) in organic field-effect transistors (OFETs) and photovoltaic characteristics, such as exciton diffusion length (L_D) and power conversion efficiency (PCE), in organic solar cells (OSCs) using PDHBDT. The 渭 was continuously improved with increasing annealing temperature and PDHBDT films annealed at 270 掳C resulted in a maximum 渭 up to 0.46 cm²/(V s) (渭_avg = 0.22 cm²/(V s)), which is attributed to the well-ordered lamellar structure with a closer 蟺鈥撓€ stacking distance of 3.5 脜 as shown by grazing incidence-angle X-ray diffraction (GIXD). On the other hand, PDHBDT films with a random molecular orientation are more effective in photovoltaic devices than films with an ordered hexagonal or lamellar phase based on current鈥搗oltage characteristics of PDHBDT/C60 bilayer solar cells. This observation corresponds to an enhanced dark current density (J_D) and a decreased L_D upon annealing. This study provides insight into the dependence of charge transport and photovoltaic characteristics on molecular packing in polymer semiconductors, which is crucial for the management of charge and energy transport in a range of organic optoelectronic devices.