title: Pronounced Cosolvent Effects in Polymer:Polymer Bulk Heterojunction Solar Cells with Sulfur-Rich Electron-Donating and Imide-Containing Electron-Accepting Polymers.
abstract: The performance of solar cells with a polymer:polymer bulk heterojunction (BHJ) structure, consisting of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) donor and poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) acceptor polymers, was investigated as a function of cosolvent (p-xylene:chlorobenzene (pXL:CB)) composition ratio. A remarkable efficiency improvement (∼38%) was achieved by spin-coating the photoactive blend layer from pXL:CB = 80:20 (volume) rather than pXL alone, but the efficiency then decreased when the CB content increased further to pXL:CB = 60:40. The improved efficiency was correlated with a particular PTB7-Th:P(NDI2OD-T2) donor-acceptor blend nanostructure, evidenced by a fiber-like surface morphology, a red-shifted optical absorption, and enhanced PL quenching. Further device optimization for pXL:CB = 80:20 films yielded a power conversion efficiency of ∼5.4%. However, these devices showed very poor stability (∼15 min for a 50% reduction in initial efficiency), owing specifically to degradation of the PTB7-Th donor-component. Replacing PTB7-Th with a more stable donor polymer will be essential for any application potential to be realized.
authors.affiliation: †Organic Nanoelectronics Laboratory, School of Applied Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea.|‡Center for Plastic Electronics, Department of Physics, Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom.
authors.forename: Sungho
authors.initials: S
authors.lastname: Nam
authors.affiliation: §Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Republic of Korea.
authors.forename: Sungho
authors.initials: S
authors.lastname: Woo
authors.affiliation: †Organic Nanoelectronics Laboratory, School of Applied Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea.
authors.forename: Jooyeok
authors.initials: J
authors.lastname: Seo
authors.affiliation: §Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Republic of Korea.
authors.forename: Wook Hyun
authors.initials: WH
authors.lastname: Kim
authors.affiliation: †Organic Nanoelectronics Laboratory, School of Applied Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea.|∥Research Institute of Advanced Energy Technology, Kyungpook National University, Daegu 702-701, Republic of Korea.
authors.forename: Hwajeong
authors.initials: H
authors.lastname: Kim
authors.affiliation: ⊥Department of Materials Engineering, Monash University, Victoria 3800, Australia.
authors.forename: Christopher R
authors.initials: CR
authors.lastname: McNeill
authors.affiliation: #SAXS Beamline, Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea.
authors.forename: Tae Joo
authors.initials: TJ
authors.lastname: Shin
authors.affiliation: ‡Center for Plastic Electronics, Department of Physics, Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom.
authors.forename: Donal D C
authors.initials: DD
authors.lastname: Bradley
authors.affiliation: †Organic Nanoelectronics Laboratory, School of Applied Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea.
authors.forename: Youngkyoo
authors.initials: Y
authors.lastname: Kim
keywords: cosolvents; degradation; nanostructure; polymer:polymer solar cells; stability