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Controlling Solution‐Phase Polymer Aggregation with Molecular Weight and Solvent Additives to Optimize Polymer‐Fullerene Bulk Heterojunction Solar Cells
Author(s) -
Bartelt Jonathan A.,
Douglas Jessica D.,
Mateker William R.,
Labban Abdulrahman El,
Tassone Christopher J.,
Toney Michael F.,
Fréchet Jean M. J.,
Beaujuge Pierre M.,
McGehee Michael D.
Publication year - 2014
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.201301733
Subject(s) - polymer solar cell , materials science , fullerene , polymer , solubility , chemical engineering , solvent , phase (matter) , solar cell , organic solar cell , polymer chemistry , organic chemistry , chemistry , composite material , optoelectronics , engineering
The bulk heterojunction (BHJ) solar cell performance of many polymers depends on the polymer molecular weight ( M n ) and the solvent additive(s) used for solution processing. However, the mechanism that causes these dependencies is not well understood. This work determines how M n and solvent additives affect the performance of BHJ solar cells made with the polymer poly(di(2‐ethylhexyloxy)benzo[1,2‐ b :4,5‐ b ′]dithiophene‐ co ‐octylthieno[3,4‐ c ]pyrrole‐4,6‐dione) (PBDTTPD). Low M n PBDTTPD devices have exceedingly large fullerene‐rich domains, which cause extensive charge‐carrier recombination. Increasing the M n of PBDTTPD decreases the size of these domains and significantly improves device performance. PBDTTPD aggregation in solution affects the size of the fullerene‐rich domains and this effect is linked to the dependency of PBDTTPD solubility on M n . Due to its poor solubility high M n PBDTTPD quickly forms a fibrillar polymer network during spin‐casting and this network acts as a template that prevents large‐scale phase separation. Furthermore, processing low M n PBDTTPD devices with a solvent additive improves device performance by inducing polymer aggregation in solution and preventing large fullerene‐rich domains from forming. These findings highlight that polymer aggregation in solution plays a significant role in determining the morphology and performance of BHJ solar cells.