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Unraveling the Morphology of High Efficiency Polymer Solar Cells Based on the Donor Polymer PBDTTT‐EFT
Author(s) -
Huang Wenchao,
Gann Eliot,
Thomsen Lars,
Dong Cunku,
Cheng YiBing,
McNeill Christopher R.
Publication year - 2015
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201401259
Subject(s) - materials science , polymer , polymer solar cell , microstructure , crystallization , chemical engineering , substrate (aquarium) , stacking , absorption (acoustics) , solar cell , fullerene , composite material , optoelectronics , organic chemistry , oceanography , chemistry , engineering , geology
The microstructure of the polymer PBDTTT‐EFT and blends with the fullerene derivative PC 71 BM that achieve solar conversion efficiencies of over 9% is comprehensively investigated. A combination of synchrotron techniques are employed including surface‐sensitive near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy and bulk‐sensitive grazing‐incidence wide angle X‐ray scattering (GIWAXS). A preferential “face‐on” orientation of PBDTTT‐EFT is observed in the bulk of both pristine and blend thin films, with π–π stacking largely normal to the substrate, which is thought to be beneficial for charge transport. At the surface of the blend, a slight “edge‐on” structure of the polymer is observed with side‐chains aligned normal to the substrate. The effect of the solvent additive 1,8‐diiodooctane (DIO) on solar cell efficiency and film microstructure is also investigated, where the addition of 3 vol% DIO results in an efficiency increase from ≈6.4% to ≈9.5%. GIWAXS studies indicate that the addition of DIO improves the crystallization of the polymer. Furthermore, atomic force microscopy and transmission electron microscopy are employed to image surface and bulk morphology revealing that DIO suppresses the formation of large PC 71 BM aggregates.