Shi-Yong Liu

C–H activation: making diketopyrrolopyrrole derivatives easily accessible

Diketopyrrolopyrrole (DPP) derivatives are an important class of high-performance pigment used in inks, paints, plastics, and organic electronics. Until now, DPP derivatives containing sophisticated aryl units at the DPP core have usually been obtained via Suzuki, Stille, or Negishi cross-coupling reactions, which require organometallic precursors. In this work, a series of DPP-based π-conjugated molecules bearing diverse aryl substituents on the thiophene- or benzene-DPPs were facilely synthesized in moderate to excellent yields through the Pd-catalyzed direct arylation of C–H bonds. The synthetic procedures feature advantages over traditional C–C cross-coupling reactions such as: (1) avoidance of the use of organometallic reagents in the starting materials leading to simpler byproducts and higher atom economy, (2) fewer synthetic steps, (3) higher yields, (4) better compatibility with chemically sensitive functional groups, and (5) simpler catalytic systems free of phosphine ligands. These advantages make the present protocol an ideal and versatile strategy for the synthesis of DPP derivatives, especially for structurally complicated DPPs that may possess chemically sensitive functionalities. The optical and electrochemical properties of the synthesized DPPs (17 compounds) were systematically investigated using UV-vis spectroscopy, steady-state fluorescence spectroscopy, and cyclic voltammetry (CV).

A Tetraperylene Diimides Based 3D Nonfullerene Acceptor for Efficient Organic Photovoltaics

A nonfullerene acceptor based on a 3D tetraperylene diimide is developed for bulk heterojunction organic photovoltaics. The disruption of perylene diimide planarity with a 3D framework suppresses the self‐aggregation of perylene diimide and inhibits excimer formation. From planar monoperylene diimide to 3D tetraperylene diimide, a significant improvement of power conversion efficiency from 0.63% to 3.54% can be achieved.

Pyrene and Diketopyrrolopyrrole-Based Oligomers Synthesized via Direct Arylation for OSC Applications

In this report, an atom efficient and facile synthetic strategy for accessing multi-diketopyrrolopyrrole (DPP)-based oligomers used in solution-processed organic field effect transistors (OFETs) and organic solar cells (OSCs) has been developed. The DPP units were successfully installed onto benzene and pyrene cores via palladium-catalyzed dehydrohalogenative coupling of mono-capped DPPs with multi-bromo-benzene or -pyrene (direct arylation), affording four oligomer small molecules (SMs 1-4) containing bis-, tri-, tri-, and tetra-DPP, respectively, in high yields of 78-96%. All the designed linear or branched DPP-based oligomers exhibit broad light absorptions, narrow band-gaps (1.60-1.73 eV), deep highest occupied molecular orbital (HOMO) levels (-5.26∼-5.18 eV), and good thermal stability (Td=390-401 °C). OFETs based on SMs 1-4 showed hole mobilities of 0.0033, 0.0056, 0.0005, and 0.0026 cm2 V(-1) s(-1), respectively. OSCs based on SMs 1-4 under one sun achieved power conversion efficiencies of 3.00%, 3.71%, 2.47%, and 1.86% accordingly, along with high open-circuit voltages of 0.86-0.94 V. For OSC devices of SM 1, SM 3, and SM 4, the solvent CHCl3 was solely employed to the formation of active layers; neither high boiling point additives nor annealing post-treatment was needed. Such a simple process benefits the large-scale production of OSCs via roll to roll technology.

A direct arylation-derived DPP-based small molecule for solution-processed organic solar cells

A diketo-pyrrolo-pyrrole (DPP) oligomer containing three DPP cores (Ph4Th4(DPP)3) was synthesized via direct arylation of C-H bonds (DACH). Ph4Th4(DPP)3 has good solubility in many organic solvents, and shows a broad absorption band from the visible to near-infrared region as well as a field-effect hole mobility as high as 0.006 cm(2) V(-1) s(-1). Solution-processed bulk heterojunction organic solar cells based on blends of Ph4Th4(DPP)3 as electron donor and fullerene derivative as electron acceptor were fabricated. An optimized power conversion efficiency of 3.76% with a high open-circuit voltage of 0.85 V was achieved after finely tuning the morphology by changing the blend ratio and by adding additives. These results indicate that DACH is an effective way to produce π-conjugated oligomers for organic solar cells.

Roll-coating fabrication of flexible large area small molecule solar cells with power conversion efficiency exceeding 1%

All solution-processed flexible large area small molecule bulk heterojunction solar cells were fabricated via roll-coating technology. Our devices were produced from slot-die coating on a lab-scale mini roll-coater under ambient conditions without the use of spin-coating or vacuum evaporation methods. Four diketopyrrolopyrrole based small molecules (SMs 1–4) were utilized as electron donors with (6,6)-phenyl-C61-butyric acid methyl ester as an acceptor and their photovoltaic performances based on roll-coated devices were investigated. The best power conversion efficiency (PCE) of 1.01%, combined with an open circuit voltage of 0.73 V, a short-circuit current density of 3.13 mA cm−2 and a fill factor of 44% were obtained for the device with SM1, which was the first example reported for efficient roll-coating fabrication of flexible large area small molecule solar cells with PCE exceeding 1%. In addition, roll-coated devices based on SMs 2–4 also showed good performances with PCEs of 0.41%, 0.54%, and 0.31%, respectively. Our results prove that small molecules have the potential for use in industries for large scale production of efficient organic solar cells.

Electron acceptors with varied linkages between perylene diimide and benzotrithiophene for efficient fullerene-free solar cells

In this work, we present two new electron acceptors, TriPDI and Fused-TriPDI, in which three perylene diimide (PDI) moieties are tethered to a benzotrithiophene (BTT) core via either single bonds (TriPDI) or ring-fusion (Fused-TriPDI). TriPDI connects three PDIs to BTT via carbon–carbon single bonds, resulting in a rotatable and highly twisted skeleton. Instead, Fused-TriPDI, made through oxidative ring-fusion of TriPDI, exhibits good structural rigidity and planarity, as well as effective conjugation between PDI and BTT. As a result, the fused molecule shows up-shifted energy levels, and enhanced absorption and charge mobility over the unfused one. The polymer solar cells (PSCs) with a PTB7-Th:Fused-TriPDI blend show the best power conversion efficiency of 6.19%, which is around three times higher than that with PTB7-Th:TriPDI.

Three-dimensional molecular donors combined with polymeric acceptors for high performance fullerene-free organic photovoltaic devices

Non-fullerene acceptor based organic photovoltaic devices (OPVs) reported so far are inferior to those derived from fullerenes. This increases the speculation on whether donors need to be tailored for advancing non-fullerene OPVs. We explored herein two direct arylation-derived diketopyrrolopyrrole (DPP)-based three-dimensional (3D) donors that can deliver respectable power conversion efficiencies (PCEs) of 4.64% and 4.02% with polymeric acceptor N2200 blends, surpassing those obtained from PC71BM (3.56% and 3.22%, respectively). It is found that these 3D-shaped molecular donors can yield improved photo-to-current conversion and balanced charge transport when blending with the linear N2200 polymer. This finding suggests that engineering molecular geometry can be a promising approach for developing high-performance materials.

A-D-A small molecule donors based on pyrene and diketopyrrolopyrrole for organic solar cells

Three new electron donating small molecules (SMs), Pyr(EH-DPP)2, Pyr(HD-DPP)2 and PyrA(EH-DPP)2, are designed and synthesized through coupling electron rich pyrene core with electron deficient diketopyrrolopyrrole (DPP) terminals, of which the derived organic solar cells (OSCs) exhibit interesting structure-performance correlation. It shows that the tune of their solubilizing side chains and π-bridge for the acceptor-donor-acceptor (A-D-A) SMs can significantly alter the resultant short-circuit current density and power conversion efficiency (PCE) in OSCs. The Pyr(EH-DPP)2 with short side chains displays broader absorption and higher hole mobility than the Pyr(HD-DPP)2 with long side chains. Although showing planar structure, the acetylene bridge-incorporated PyrA(EH-DPP)2 adapts an undesired edge-on packing and strong aggregation in film, leading to non-ideal morphology and poor miscibility with fullerene acceptors. As a result, the PCE of the solar cell based on Pyr(EH-DPP)2 is several times higher than those based on Pyr(HD-DPP)2 and PyrA(EH-DPP)2, indicating the A-D-A combination of polyaromatics with DPP would be the promising skeleton for developing photovoltaic semiconductors.