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From Recombination Dynamics to Device Performance: Quantifying the Efficiency of Exciton Dissociation, Charge Separation, and Extraction in Bulk Heterojunction Solar Cells with Fluorine‐Substituted Polymer Donors
Author(s) -
Gorenflot Julien,
Paulke Andreas,
Piersimoni Fortunato,
Wolf Jannic,
Kan Zhipeng,
Cruciani Federico,
Labban Abdulrahman El,
Neher Dieter,
Beaujuge Pierre M.,
Laquai Frédéric
Publication year - 2018
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.201701678
Subject(s) - materials science , polymer solar cell , organic solar cell , exciton , charge carrier , photocurrent , heterojunction , ultrafast laser spectroscopy , chemical physics , analytical chemistry (journal) , optoelectronics , polymer , solar cell , spectroscopy , organic chemistry , chemistry , physics , quantum mechanics , composite material
An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time‐delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two‐pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device‐relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide‐bandgap donor polymers: poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐ b :4,5‐ b′ ]dithiophene‐3,4‐difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐ b :4,5‐ b′ ]dithiophene‐3,4‐thiophene) (PBDT[2H]T) combined with PC 71 BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)‐state energy is shown to lead to significantly lower energy losses (resulting in higher V OC ) during charge generation compared to P3HT:PCBM.