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

Sangwon Ko, Do Hwan Kim, Alexander L. Ayzner, Stefan C. B. Mannsfeld, Eric Verploegen, Alexander M. Nardes, Nikos Kopidakis, Michael F. Toney, Zhenan Bao

Research output: Contribution to journalArticlepeer-review

12 Scopus citations


© 2015 American Chemical Society. We observed a thermotropic phase transition in poly[3,4-dihexyl thiophene-2,2′:5,6′-benzo[1,2-b:4,5-b′]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 (μ) in organic field-effect transistors (OFETs) and photovoltaic characteristics, such as exciton diffusion length (LD) and power conversion efficiency (PCE), in organic solar cells (OSCs) using PDHBDT. The μ was continuously improved with increasing annealing temperature and PDHBDT films annealed at 270 °C resulted in a maximum μ up to 0.46 cm2/(V s) (μavg = 0.22 cm2/(V s)), which is attributed to the well-ordered lamellar structure with a closer - stacking distance of 3.5 Å 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-voltage characteristics of PDHBDT/C60 bilayer solar cells. This observation corresponds to an enhanced dark current density (JD) and a decreased LD 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.
Original languageEnglish (US)
Pages (from-to)1223-1232
Number of pages10
JournalChemistry of Materials
Issue number4
StatePublished - Feb 11 2015
Externally publishedYes

Bibliographical note

KAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-C1-015-21
Acknowledgements: This work was supported by the Center for Advanced Molecular Photovoltaics, Award KUS-C1-015-21, made by King Abdullah University of Science and Technology (KAUST). GIXS measurements were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. S.K. acknowledges financial support by Korea Railroad Research Institute through the project "Development of Improvement Technology of Railroad Environment (PK1504C)" D.H.K. acknowledges financial support by the Center for Advanced Soft-Electronics under the Global Frontier Project (CASE-2014M3A6A5060932) and the Basic Science Research Program (2014R1A1A1005933) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning. The photoluminescence measurements (A.M.N. and N.K.) were carried out under funding from the Energy Frontier Research Center "Molecularly Engineered Energy Materials (MEEMs)" funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-SC0001342:001.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.


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