Ailing Tang

Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications.

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

The effect of conjugated π-bridge and fluorination on the properties of asymmetric-building-block-containing polymers (ABC polymers) based on dithienopyran donor and benzothiadiazole acceptors

To investigate the effect of conjugated π-bridge and fluorination on the properties of asymmetric-building-block-containing polymers (ABC polymers), hexylthiophene (HT) was inserted into D–A-type polymers based on asymmetric dithieno[3,2-b:2′,3′-d]pyran (DTPa) donor and benzothiadiazole (BT), mono-fluorinated benzothiadiazole (fBT) or di-fluorinated benzothiadiazole (ffBT) acceptors, respectively. In comparison with non-bridged analogs, introducing the HT π-bridge between DTPa and BTs significantly reduced the intramolecular charge transfer and consequently affected the absorption spectra, frontier energy levels, and electrochemical and charge transport properties of the resulting ABC polymers. Interestingly, HT π-bridge enlarged the fluorination effect on photovoltaic properties, and the power conversion efficiency (PCE) increased gradually from 1.56% (DTPa-HTBT) to 4.39% (DTPa-HTfBT) and 6.09% (DTPa-HTffBT), significantly different with non-bridged polymers. Most importantly, with the increase in the number of the fluorine atoms on the BT unit, all of the photovoltaic parameters, including PCE, Voc, Jsc, and FF, improved gradually, which provides a useful guide for the further design of ABC photovoltaic polymers.

Effect of fluorination and symmetry on the properties of polymeric photovoltaic materials based on an asymmetric building block

Conjugated polymers based on an asymmetric dithieno[3,2-b:2′,3′-d]pyran (DTPa) donor and benzothiadiazole (BT), mono-fluorinated benzothiadiazole (fBT) or di-fluorinated benzothiadiazole (ffBT) acceptors were designed and synthesized. The introduction of fluorine substituents in the BT unit could not only enhance the electronegativity of the acceptors, but also change the symmetry of the BT derivatives, and thus affect the optical, electrochemical, and optoelectronic performance of the final polymers. With the increase of fluorine atoms in the BT unit, the peaks of the absorption spectra for these three polymers hypsochromic shift gradually, combined with the decrease of the HOMO energy levels. The asymmetric structure of fBT results in more complex multichromophore systems and consequently shows a broader absorption FWHM (full-width-at-half-maximum) of 229 nm in chloroform, as well as low absorption intensity and charge carrier mobility for polymer PDTPa-fBT. Finally, polymer solar cells based on these polymers demonstrate power conversion efficiency varying from 4.01% for PDTPa-BT to 3.70% for PDTPa-fBT and to 5.26% for PDTPa-ffBT. These results indicate that the symmetry of both electron-donating and electron-accepting building blocks in conjugated polymers could evidently influence the optical and photovoltaic properties, which might pave the way for the further development of novel photovoltaic polymers based on asymmetric building blocks.

Design and Synthesis of a Novel n-Type Polymer Based on Asymmetric Rylene Diimide for the Application in All-Polymer Solar Cells

A novel n-type polymer of PTDI-T based on asymmetric rylene diimide and thiophene is designed and synthesized. The highest power conversion efficiency of 4.70% is achieved for PTB7-Th:PTDI-T-based devices, which is obviously higher than those of the analogue polymers of PPDI-2T and PDTCDI. When using PBDB-T as a donor, an open-circuit voltage (VOC ) as high as 1.03 V is obtained. The results indicate asymmetric rylene diimide is a kind of promising building block to construct n-type photovoltaic polymers.

Inside-fused perylenediimide dimers with planar structures for high-performance fullerene-free organic solar cells

For perylenediimide derivatives it seems that twisted structures are essential to avoid excessive aggregation tendencies and realize high-performance fullerene-free solar cells. However, in this communication, we designed and synthesized two planar inside-fused perylenediimide dimers, TDI2 and BDT-TDI2. Theoretical calculations reveal that both TDI2 and BDT-TDI2 have a highly planar molecular conformation with small dihedral angles of 0.02° and 11.4° between two TDI segments, respectively. By using BDDT as the donor polymer, power conversion efficiencies (PCEs) of the photovoltaic cells reached 5.80% for TDI2 and 4.52% for BDT-TDI2, with high open-circuit voltages (VOC) of ∼1.0 V. These results indicate planar PDI-dimer derivatives are also possible electron acceptors to realize high-performance fullerene-free solar cells.

A perylenediimide dimer containing an asymmetric π-bridge and its fused derivative for fullerene-free organic solar cells

Recent success stories of perylenediimide (PDI) derivatives have stimulated tremendous interest. They are under intense investigation to replace fullerenes as the electron acceptor in organic solar cells. In priciple, the strategy of linking two PDI rings to get a PDI dimer could efficiently suppress the excessive π–π stacking tendency and avoid the large phase separation. However, almost all the reported PDI dimers are symmetric structures, asymmetric building blocks as the π-bridge are rare. Here, we used an asymmetric 6-(thiophen-2-yl)benzo[b]thiophene (T-BTh) group as the π-bridge to connect two PDI units, affording compound A101. Through annulation of A101, we further synthesized the fused PDI dimer of A102. Fused A102 shows increased effective π-conjugation and reduced twist angle between the two PDI subplanes, resulting in a favorable morphology and strong π–π stacking. As a result, A102 showed an improved power conversion efficiency of 5.65% in comparison with that of A101 (3.57%), due to the higher open-circuit voltage (VOC) and short-circuit current (JSC). The higher VOC results from the higher lowest unoccupied molecular orbital (LUMO) level of A102 and the higher JSC can be explained on the basis of better charge carrier mobilities. These results provide important information and open a new way to design novel PDI-based or other small molecular electron acceptors.

Utilizing Benzotriazole and Indacenodithiophene Units to Construct Both Polymeric Donor and Small Molecular Acceptors to Realize Organic Solar Cells With High Open-Circuit Voltages Beyond 1.2 V

Devolopment of organic solar cells with high open-circuit voltage (VOC) and power conversion efficiency (PCE) simutaniously plays a significant role, but there is no guideline how to choose the suitable photovoltaic material combinations. In our previous work, we developed “the Same-Acceptor-Strategy” (SAS), by utilizing the same electron-accepting segment to construct both polymeric donor and small molecular acceptor. In this study, we further expend SAS to use both the same electron-accepting and electron-donating units to design the material combination. The p-type polymer of PIDT-DTffBTA is designed by inserting conjugated bridge between indacenodithiophene (IDT) and fluorinated benzotriazole (BTA), while the n-type small molecules of BTAx (x = 1, 2, 3) are obtained by introducing different end-capped groups to BTA-IDT-BTA backbone. PIDT-DTffBTA: BTAx (x = 1–3) based photovolatic devices can realize high VOC of 1.21–1.37 V with the very small voltage loss (0.55–0.60 V), while only the PIDT-DTffBTA: BTA3 based device possesses the enough driving force for efficient hole and electron transfer and yields the optimal PCE of 5.67%, which is among the highest value for organic solar cells (OSCs) with a VOC beyond 1.20 V reported so far. Our results provide a simple and effective method to obtain fullerene-free OSCs with a high VOC and PCE.

Quinoxaline-Containing Nonfullerene Small-Molecule Acceptors with a Linear A2-A1-D-A1-A2 Skeleton for Poly(3-hexylthiophene)-Based Organic Solar Cells

We used the quinoxaline (Qx) unit to design and synthesize two nonfullerene small-molecule acceptors of Qx1 and Qx1b with an A2-A1-D-A1-A2 skeleton, where indacenodithiophene (IDT), Qx, and rhodanine (R) were adopted as the central donor (D), bridge acceptors (A1), and terminal acceptors (A2), respectively. Qx1 and Qx1b contain different side chains of 4-hexylphenyl and octyl in the central IDT segment to modulate the properties of final small molecules. Both small molecules show good thermal stability, high solubility, and strong and broad absorption spectra with optical band gaps of 1.74 and 1.68 eV, respectively. Qx1 and Qx1b exhibit the complementary absorption spectra with the classic poly(3-hexylthiophene) (P3HT) and the high-lying lowest unoccupied molecular orbital energy levels of -3.60 and -3.66 eV, respectively. Polymer solar cells based on P3HT:Qx1 showed a high open-circuit voltage ( Voc) of 1.00 V and a power conversion efficiency (PCE) of 4.03%, whereas P3HT:Qx1b achieved a Voc of 0.95 V and a PCE of 4.81%. These results demonstrate that the Qx unit is also an effective building block to construct promising n-type nonfullerene small molecules to realize a relatively high Voc and PCE for P3HT-based solar cells.

A2-A1-D-A1-A2 type non-fullerene acceptors based on methoxy substituted benzotriazole with three different end-capped groups for P3HT-based organic solar cells

In the last three years, the A2–A1–D–A1–A2 skeleton has become increasingly popular in the design of non-fullerene acceptors (NFAs), and it could match particularly well with the classic p-type polymer of poly(3-hexylthiophene) (P3HT). In this manuscript, we successfully synthesized three NFAs with this skeleton, named BTA100, BTA101 and BTA103, where BTA units were substituted by methoxy groups and used as the middle electron-accepting unit (A1). To fine-tune the energy levels of the final BTA-based NFAs, three different electron-deficient building blocks, thiazolidine-2,4-dione (TD), rhodanine (R) and 2-(1,1-dicyanomethylene)rhodanine (RCN), were used as the end groups (A2), respectively. The introduction of methoxy groups into BTA can upshift the lowest unoccupied molecular orbital (LUMO) energy level of NFAs and realize high open-circuit voltage (VOC) organic solar cells. In addition, the O atom shows a weak interaction with the S atom in the neighbouring thiophene ring, which might be able to facilitate intramolecular charge transfer. The organic solar cell (OSC) device based on P3HT:BTA103 shows a high PCE of 5.31% with a VOC of 0.94 V, a JSC of 8.56 mA cm−2 and a FF of 0.66. In addition, it is worth noting that the VOC of P3HT:BTA100 reached 1.34 V, which is one of the highest values for P3HT based solar cells. These results indicate that RCN is also an effective end group to construct NFAs and methoxy substitution is a simple method to improve the VOC for P3HT-based OSC devices.

Recent progress in porphyrin-based materials for organic solar cells

This article is written to provide an up-to-date review of porphyrin-based materials used in organic solar cells (OSCs). During the past two decades, OSCs have been the subject of extensive research and significant efforts have been devoted to developing low-cost OSCs, and they are not far from commercialization. Porphyrin and its analogues have been successfully applied to different optoelectronic devices, especially attaining remarkable fame when applied in dye sensitized solar cells. Despite the initial failures of their application in OSCs, porphyrins still attract much attention because of their structural versatility and recently realized significant improvement. In this review, we focus on summarizing the recent progress in porphyrin-based photovoltaic materials, including polymers, dyads, triads, small-molecules, and so on. We hope this paper could provide an in-depth study on the structure–property–performance relationship and provide a guideline for the further development of porphyrin-based and even other photovoltaic materials.