Synthetic Principles Directing Charge Transport in Low-Band-Gap Dithienosilole–Benzothiadiazole Copolymers

Abstract

Given the fundamental differences in carrier generation and device operation in organic thin-film transistors (OTFTs) and organic photovoltaic (OPV) devices, the material design principles to apply may be expected to differ. In this respect, designing organic semiconductors that perform effectively in multiple device configurations remains a challenge. Following "donor-acceptor" principles, we designed and synthesized an analogous series of solution-processable π-conjugated polymers that combine the electron-rich dithienosilole (DTS) moiety, unsubstituted thiophene spacers, and the electron-deficient core 2,1,3-benzothiadiazole (BTD). Insights into backbone geometry and wave function delocalization as a function of molecular structure are provided by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. Using a combination of X-ray techniques (2D-WAXS and XRD) supported by solid-state NMR (SS-NMR) and atomic force microscopy (AFM), we demonstrate fundamental correlations between the polymer repeat-unit structure, molecular weight distribution, nature of the solubilizing side-chains appended to the backbones, and extent of structural order attainable in p-channel OTFTs. In particular, it is shown that the degree of microstructural order achievable in the self-assembled organic semiconductors increases largely with (i) increasing molecular weight and (ii) appropriate solubilizing-group substitution. The corresponding field-effect hole mobilities are enhanced by several orders of magnitude, reaching up to 0.1 cm(2) V(-1) s(-1) with the highest molecular weight fraction of the branched alkyl-substituted polymer derivative in this series. This trend is reflected in conventional bulk-heterojunction OPV devices using PC(71)BM, whereby the active layers exhibit space-charge-limited (SCL) hole mobilities approaching 10(-3) cm(2) V(-1) s(-1), and yield improved power conversion efficiencies on the order of 4.6% under AM1.5G solar illumination. Beyond structure-performance correlations, we observe a large dependence of the ionization potentials of the polymers estimated by electrochemical methods on polymer packing, and expect that these empirical results may have important consequences on future material study and device applications.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

“…The polymer exhibits an even more remarkable hole mobility of 9.0 cm 2¨V´1¨s´1 when using a high-k P(VDF-TrFE) ferroelectric polymer as a gate dielectric. These mobility values are significantly larger than those reported previously for its non-fluorinated equivalent (3ˆ10´3 cm 2¨V´1¨s´1 ) [77]. Such a pronounced effect of fluorination may be attributed to the higher backbone planarity, which allows stronger π-stacking in solid state.…”

“…As photovoltaic devices require high charge carrier mobilities in the out-of-plane direction, SCLC (Space Charge Limited Current) devices have been frequently used to probe the charge carrier mobility in photovoltaic materials. Peng et al [77] reported a SCLC study on two equivalent isoindigo polymers, the PBDTT-ID and the fluorinated PBDTT-FID derivative (see Figure 13). Fluorination resulted in a vertical mobility enhancement by a factor of five (from 6.5 × 10 −4 to 3.2 × 10 −3 cm 2 ·V −1 ·s −1 ) and in a significant increase in Jsc and Voc (see Entry 1, Table 5).…”

Double acceptor donor–acceptor alternating conjugated polymers containing cyclopentadithiophene, benzothiadiazole and thienopyrroledione: toward subtractive color organic photovoltaics

“…The short wavelength shifts within each series may be attributed to the weakened D-A interaction resulting from the insertion of thiophene(s); consequently the donor part dominates. Reynold and co-workers 67 reported a similar series of copolymers using the fused ring donor dithieno[3,2-b:2′,3′-d]silole (DTS) in place of CPDT, but the order of λ max was not in accordance with the increasing donor ratio in the backbone as is observed for the CPDT-BT-CPDT series. The DTS-BT-DTS series showed absorptions that were more blueshifted than members of the CPDT-BT-CPDT series that had the same connecting units because the smaller electronegativity of silicon compared with carbon (Si = 1.9oC = 2.5) and the reduced π-electron bond-forming ability of the Si atom makes DTS a stronger donor than CPDT.…”

“…Contradictorily, the DTS-BT-DTS copolymer connected with a bithiophene unit showed a longer λ max , at 617 nm, than the copolymer connected with one thiophene unit (λ max = 602 nm); this is a specific phenomenon induced by DTS. 67 Polymer films were prepared by drop casting chloroform solutions of the desired polymer onto glass substrates, and the λ max values Table 2 Optical and electrochemical properties of polymers Abbreviations: E g-opt, optical band gap; Ox, oxidation; Rd, reduction; Δλ, shift between film and solution. recorded in the ultraviolet-visible spectra of the films were compared with those obtained in solution (Table 2 and Supplementary Figure S19 in Supplementary Information).…”

How intermolecular geometrical disorder affects the molecular doping of donor–acceptor copolymers

“…D-A copolymers have revolutionized the field of organic electronics by showing single-junction solar cells reaching close to 10% power conversion efficiency 17 , carrier mobility 410 cm 2 V À 1 s À 1 in field effect transistors 18 and bipolar transport 19 . The most important feature of D-A copolymers is the possibility of finely tuning the energy bandgap via the selection of the two moieties 20 .…”