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Interplay Between Side Chain Pattern, Polymer Aggregation, and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk‐Heterojunction Solar Cells
Author(s) -
DyerSmith Clare,
Howard Ian A.,
Cabanetos Clément,
El Labban Abdulrahman,
Beaujuge Pierre M.,
Laquai Frédéric
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.201401778
Subject(s) - polymer solar cell , materials science , side chain , polymer , exciton , dissociation (chemistry) , nanosecond , charge carrier , solar cell , spectroscopy , acceptor , vinyl alcohol , chemical physics , organic solar cell , photochemistry , chemical engineering , chemistry , optoelectronics , optics , laser , physics , quantum mechanics , composite material , condensed matter physics , engineering
Poly(benzo[1,2‐ b :4,5‐ b ′]dithiophene– alt –thieno[3,4‐ c ]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C 71 ‐butyric acid methyl ester (PC 71 BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current ( J sc ). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.