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Understanding How Processing Additives Tune the Nanoscale Morphology of High Efficiency Organic Photovoltaic Blends: From Casting Solution to Spun‐Cast Thin Film
Author(s) -
Shao Ming,
Keum Jong Kahk,
Kumar Rajeev,
Chen Jihua,
Browning James F.,
Das Sanjib,
Chen Wei,
Hou Jianhui,
Do Changwoo,
Littrell Kenneth C.,
Rondi Adam,
Geohegan David B.,
Sumpter Bobby G.,
Xiao Kai
Publication year - 2014
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201401547
Subject(s) - materials science , casting , thin film , organic solar cell , chemical engineering , ternary operation , phase (matter) , energy conversion efficiency , neutron reflectometry , polymer blend , photoactive layer , fullerene , polymer , nanotechnology , small angle neutron scattering , neutron scattering , composite material , scattering , organic chemistry , optoelectronics , copolymer , optics , computer science , engineering , chemistry , physics , programming language
Adding a small amount of a processing additive to the casting solution of photoactive organic blends has been demonstrated to be an effective method for achieving improved power conversion efficiency (PCE) in organic photovoltaics (OPVs). However, an understanding of the nano‐structural evolution occurring in the transformation from casting solution to thin photoactive films is still lacking. In this report, the effects of the processing additive diiodooctane (DIO) on the morphology of the established blend of PBDTTT‐C‐T polymer and the fullerene derivative PC 71 BM used for OPVs are investigated, starting in the casting solution and tracing the effects in spun‐cast thin films by using neutron/X‐ray scattering, neutron reflectometry, and other characterization techniques. The results reveal that DIO has no observable effect on the structures of PBDTTT‐C‐T and PC 71 BM in solution; however, in the spun‐cast films, it significantly promotes their molecular ordering and phase segregation, resulting in improved PCE. Thermodynamic analysis based on Flory‐Huggins theory provides a rationale for the effects of DIO on different characteristics of phase segregation due to changes in concentration resulting from evaporation of the solvent and additive during film formation. Such information may help improve the rational design of ternary blends to more consistently achieve improved PCE for OPVs.