Effect of Fluorine Substitution on the Charge Carrier Dynamics of Benzothiadiazole‐Based Solar Cell Materials

Abstract

The femtosecond transient absorption (TA) characterization of a new benzothiadiazole (BT)-based donor-acceptor conjugated copolymer, poly[(2,6-dithieno[3,2-b:2',3'-d]thiophene)-alt-(4,7-di(4-octyldodecylthiopen-2-yl)-2,1,3-benzo[c][1,2,5]thiadiazole (PBT), as well as its fluorinated derivatives, PFBT and PDFBT, is carried out. Additionally, bulk heterojunction (BHJ) films consisting of the copolymers and [6,6]-phenyl-C71 -butylic acid methyl ester (PC70 BM) are examined using TA spectroscopy. Both the singlet excited state dynamics in the copolymers and the charge transfer state dynamics in the BHJs are investigated in terms of fluorination dependency; the fluorinated copolymers exhibit less singlet exciton recombination rate than the fluorine-free copolymer, and the BHJs including the fluorinated copolymers display slower monomolecular recombination than the fluorine-free analogue. Furthermore, the excitation-intensity-dependent TA dynamics of the copolymers and BHJs is investigated, revealing that, when sufficiently high excitation intensity is used to induce annihilation processes, the fluorinated copolymers and BHJs incorporating the fluorinated copolymers show more rapid TA decay ascribable to morphological enhancement. These TA spectroscopic findings are found to correlate with the device characteristics with respect to fluorinated content in the polymer solar cells. In particular, both the short-circuit current density and fill factor of BHJ solar cells correspond closely with the fast decay parameters of the BHJ films under high excitation intensity.

The Effect of Fluorine Substitution on the Molecular Interactions and Performance in Polymer Solar Cells

“…All three systems show broad transmission signals (Δ T /T) from 500 to 750 nm that are attributed to photobleaching (PB) that includes the information on all excited carrier recombination to ground state. The PB decay signals can be presented in multiphasic dynamics as shown in a previous report . However, as depicted in Figure a, the PB at 620 nm shows fairly monotonic decay with the lack of a fast component shorter than 10 ps, making it possible to consider only long-lived excited species; the 620 nm PB decays of both PFBT:​PC 71 ­BM and PD­FBT:​PC 71 ­BM are much slower than that of PBT:​PC 71 ­BM.…”

“…To identify the underlying excited nature, the additional TA kinetic experiments at several wavelengths have been conducted under various excitation fluences and were fitted using the multiexponential decay function. 43 As a result, fluence-independent decay components with a time scale of several hundred picoseconds was observed in each polymer:PC 71 BM blend, and this can be ascribed to monomolecular charge recombination. 44 The fluence-independent time constants of PBT:PC 71 BM, PFBT:PC 71 BM, and PDFBT:PC 71 BM were extracted to 405, 475, and 495 ps, respectively, therefore indicating that there is longer-lasting monomolecular species in the F-containing systems than the nonfluorinated blend.…”

“…The PB decay signals can be presented in multiphasic dynamics as shown in a previous report. 43 However, as depicted in Figure 4a, the PB at 620 nm shows fairly monotonic decay with the lack of a fast component shorter than 10 ps, making it possible to consider only long-lived excited species; the 620 nm PB decays of both PFBT:PC 71 BM and PDFBT:PC 71 BM are much slower than that of PBT:PC 71 BM. To identify the underlying excited nature, the additional TA kinetic experiments at several wavelengths have been conducted under various excitation fluences and were fitted using the multiexponential decay function.…”

Chlorinated Polymers for Efficient Solar Cells with High Open Circuit Voltage: The Influence of Different Thiazole Numbers

“…To address the above issue, for many donor–acceptor (D–A) systems, the introduction of a heteroatom could be one effective approach to lower HOMO energy levels, including in fluorine, chlorine, and sulfur atoms . However, the drawbacks of a relatively low yield from the fluorination reaction and the strong reducibility of sulfur atoms restrict their application .…”

Fabrication-Method-Dependent Excited State Dynamics in CH3NH3PbI3 Perovskite Films

“…As shown in Table 1, the time scale of accelerated decay by high-order processes in the perovskite films is several tens to hundreds of picoseconds, which is markedly longer than that in polymer films 30–32 . These are associated with the previously reported high-order recombination rate constants for perovskite systems (~1 × 10 −10 to ~1 × 10 −9 cm 3 s −1 ) 29,33,34 and polymers semiconducting materials (~4 × 10 −9 to ~3 × 10 −8 cm 3 s −1 ) 35,36 .…”