Keli Shi

The role of conjugated side chains in high performance photovoltaic polymers

Four new D–A type copolymers, namely, PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT, were designed and synthesized to investigate the influence of conjugated side chain pattern on photovoltaic properties of conjugated polymers. All the four copolymers have an identical conjugated backbone comprising benzo[1,2-b:4,5-b′]dithiophene (BDT) donor unit and quinoxaline (Qx) acceptor unit, but with varying conjugated side chains, p-alkoxyphenyl or 2-alkylthienyl, attached to the donor and acceptor units, respectively. As evidenced by UV/Vis absorption spectra, electrochemical cyclic voltammetry, density functional theory (DFT), grazing incidence X-ray scattering (GIXS), transmission electron microscope (TEM) and photovoltaic measurements, the difference in conjugated side chain modulation led to totally different physicochemical properties. Among the four copolymers, PBDT-DFQX-TT exhibits the broadest absorption spectrum, the most close-packed structure as well as a finest fibril structure when blended with PC71BM. After systematic device optimization, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) photovoltaic devices based on the blends of PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT with PC71BM achieved 3.96%, 6.08%, 6.54% and 7.68%, respectively. By systematic varying the side chains of the copolymers from all phenyl groups to all thienyl ones, PCEs was increased by 250% from 3.96% to 7.68%. To date, PBDT-DFQX-TT is one of a few Qx-based PSCs that exhibits PCE exceeding 7.5%, and the results suggest that simultaneously modulating the conjugated side chains on both donor and acceptor units of copolymers could be an effective strategy for constructing high performance photovoltaic copolymers.

Dithieno[3,2-b:2′,3′-d]pyridin-5(4H)-one-based polymers with a bandgap up to 2.02 eV for high performance field-effect transistors and polymer solar cells with an open-circuit voltage up to 0.98 V and an efficiency up to 6.84%

A new electron donor, 4-(2-octyldodecyl)-dithieno[3,2-b:2′,3′-d]pyridin-5(4H)-one (DTPO), for polymer semiconductors is reported. Its homopolymer PDTPO reveals a high hole mobility of 0.19 cm2 V−1 s−1 in field-effect transistors. Its copolymers with benzodithiophenes (BDTO and BDTT), namely PDTPO-BDTO and PDTPO-BDTT, not only show wide optical bandgaps of 2.02 and 1.95 eV, but also possess deep HOMO levels of −5.38 and −5.44 eV, respectively. The polymer solar cell based on PDTPO-BDTO with an inverted architecture achieves a power conversion efficiency (PCE) of 6.84% with a high open-circuit voltage (Voc) of 0.93 V, while the one with PDTPO-BDTT realizes the same PCE with conventional architecture and a reasonably high Voc of 0.96 V. The PCEs are among the highest ever reported for wide bandgap PSCs. Compared to the blend with PDTPO-BDTO having the 2-ethylhexyloxy group, the one with PDTPO-BDTT having the 5-(2-ethylhexyl)thiophene-2yl- group is demonstrated to be superior as a result of faster exciton separation into free charge carriers and larger driving force for exciton dissociation, which results in high short-circuit current and Voc, respectively. The wide optical bandgaps and the excellent device performances make these polymers good candidates for boosting the PCE of the PSCs with a ternary blend layer or tandem structures.

High-performance polymer field-effect transistors fabricated with low-bandgap DPP-based semiconducting materials

A series of new π-conjugated D–A copolymers PDMOTT-n (n = 118, 122, 320, and 420) containing a diketopyrrolopyrrole unit and a 3,6-dimethoxythieno[3,2-b]thiophene moiety was designed and synthesized, and their field-effect properties were characterized. The polymers PDMOTT-n have a low bandgap of 1.27 eV and exhibit a broad absorption. Solution-processed field-effect transistors based on these polymers were fabricated with a bottom-gate/bottom-contact configuration and demonstrated typical p-type charge transporting properties. Of these, PDMOTT-420 with the longest alkyl side chains and having the longest distance between the branching point of the alkyl side chain and π-conjugated backbone of the polymer, exhibited the best carrier transporting performance with a hole mobility of 2.01 cm2 V−1 s−1 and with an on/off current ratio of 104–105. The characterization results of grazing incidence X-ray diffraction and tapping-mode atomic force microscopy showed that the thin film of the polymer PDMOTT-420 forms larger grains and more useful nanofibrillar intercalated structures and owns shorter π–π stacking distance and better crystallization than those of another three polymers. These results could help us to better understand the structure–property relationship of polymer semiconductors and to design novel π-conjugated polymers for high-performance field-effect transistors.

Vinylidenedithiophenmethyleneoxindole: a centrosymmetric building block for donor–acceptor copolymers

Further development of new building blocks is crucial for realizing next-generation, high-performance organic semiconducting materials. In the paper, the design and synthesis of a novel π-extended analogue of isoindigo, vinylidenedithiophenmethyleneoxindole (VDTOI), and two VDTOI-based copolymers is reported. The centrosymmetric VDTOI unit possesses a special acceptor–donor–acceptor structure and contains a highly planar conjugated backbone because of the presence of S⋯O conformational locks. Of special importance is the fact that the VDTOI unit has the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital and energy levels of −5.35 eV/−3.42 eV, which are similar properties to those of the thiophene-flanked diketopyrrolopyrrole building block. The previously mentioned structural features indicate that VDTOI might be a potential building block for constructing polymeric semiconductors. As a trial, two VDTOI-based copolymers were synthesized, namely P1 and P2. The two copolymers show good thermal stability and broad absorption spectra ranging from 330 to 740 nm. Their HOMO energy levels of around −5.40 eV match well with the work function (5.13 eV) of a gold (Au) electrode, which is indicative of effective hole injections from the Au electrodes to polymer semiconductor films. Both the P1- and P2-based field-effect transistors exhibited typical p-type transport characteristics. The highest mobility of 0.35 cm2 V−1 s−1 was achieved in P1-based devices.

Highly planar thieno[3,2-b]thiophene-diketopyrrolopyrrole-containing polymers for organic field-effect transistors

Two novel thieno[3,2-b]thiophene-flanked diketopyrrolopyrrole (TTDPP)-based copolymers PTTDPPTVT and PTTDPPSVS were designed and synthesized in order to investigate their optical, electrochemical, and morphological properties as well as device performance in organic field-effect transistors. By introducing large TTDPP units and highly π-extended moieties di(thiophen-2-yl)ethene or di(selenophen-2-yl)ethene in the conjugated backbone, the copolymers with extremely planar conjugated degree and strongly intermolecular interactions were achieved. The devices based on PTTDPPTVT and PTTDPPSVS showed hole mobilities of 0.437 and 0.067 cm2 V−1 s−1, respectively. These results demonstrated that the TTDPP unit is a promising electron deficient building block for constructing excellent performance semiconducting materials.

Naphtho[1,2b;5,6b′]difuran-based donor–acceptor polymers for high performance organic field-effect transistors

Two naphthodifuran-based donor–acceptor copolymers are presented. Via reasonable main-chain modification and side-chain engineering, remarkably dense π–π stacking spacings (<3.5 A) as well as high “edge-on” orientations are observed. When fabricated as organic field-effect transistors, high hole mobilities exceeding 5 cm2 V−1 s−1 are achieved at a moderate annealing temperature of 120 °C.

Heteroatom substituted naphthodithiophene–benzothiadiazole copolymers and their effects on photovoltaic and charge mobility properties

A novel series of heteroatom-substituted naphthodithiophene and dithienylbenzothiadiazole alternating copolymers namely PNTP, PNBO, PNfTB, and PNffTB were designed and synthesized in which heteroatoms including fluorine, oxygen or nitrogen were incorporated into electron-deficient benzothiadiazole units of the resulting copolymers. In general, with the incorporation of heteroatoms into the acceptor units of the polymer backbone, the absorption spectra would shift to a longer wavelength, indicating a decrease in the optical band gap. In addition, fluorine substitution can effectively lower both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the resulting copolymers; on the other hand, the incorporation of nitrogen or oxygen atoms into the copolymer backbone can only stabilize the LUMO energy level of copolymers. The effect of heteroatoms on the photovoltaic and charge mobility properties of organic solar cells (OSCs) and organic field-effect transistors (OFETs) was also investigated. The bulk heterojunction (BHJ) OSCs fabricated from the blend of heteroatom-substituted copolymers and PC71BM using diiodooctane as a solvent additive with ZnO/Al as a double interlayer in an inverted device structure afforded a power conversion efficiency up to 5.05%. In addition, the solution-processed bottom gate bottom contact based OFETs fabricated from PNfTB exhibited an enhanced hole mobility of 0.14 cm(2) V-1 s(-1) with an on/off current ratio of 10(7). Our results suggested that incorporation of heteroatoms into the donor-acceptor polymer backbone would be a useful strategy to enhance the functional properties for applications in OSCs and OFETs.