Zhixiang Wei

Self-assembly of two-dimensional nanostructures of linear regioregular poly(3-hexylthiophene)

Two dimensional poly(3-hexylthiophene) (P3HT) nanosheets were prepared by evaporation of a diluted mixture of solvent. The dimension and size of the nanostructures was controlled by tuning the good-to-poor solvent ratio. The morphology and structure of the nanosheets were fully characterized. Atomic Force Microscopy (AFM) characterization showed the nanosheets were lamellar structures with a thickness of about 3–5 nm of a single layer. The anisotropic arrangement of P3HT was clearly defined by selected area electron diffraction. In the nanostructures, P3HT adopted the edge on orientation with the π–π stacking direction parallel to the substrate and the alkyl side chains perpendicular to the substrate. Single nanosheet showed a typical anisotropic charge carrier transport property.

Evolution of morphology and open-circuit voltage in alloy-energy transfer coexisting ternary organic solar cells

The oligomer-type small molecule PDT2FBT-ID is applied in a polymer/small molecule/fullerene ternary system. The PTB7-Th/PDT2FBT-ID/PC71BM ternary active layer shows complementary absorption spectra, enhanced face-on orientation and energy transfer properties. Compared to the binary system, the FF of the ternary system is improved from 66.72% to 76.14% and the Jsc is improved from 17.90 mA cm−2 to 18.92 mA cm−2. The maximum PCE of 11.1% is obtained in the ternary system with a common inverted device structure. Additionally, an alloy-like domain structure and energy transfer between the two donor materials are found to coexist in the ternary system. Based on the combined model, the variation in morphology and Voc is discussed in detail and compared with previous publications. An in-depth interpretation for the selection of a third compound in high performance ternary organic solar cells is provided. Finally, this facile design strategy could also be widely used in other systems.

High open-circuit voltage ternary organic solar cells based on ICBA as acceptor and absorption-complementary donors

Ternary organic solar cells (OSCs) are fabricated with indene-C60 bisadduct (ICBA) as an electron acceptor and the low-band-gap polymer PBDTTT-C-T and the highly crystalline small molecule n-BDT-3T-CNCOO as electron donors. A high open-circuit voltage of 0.98 V is achieved, which is 0.2 V higher than that of ternary OSCs based on phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. Incorporation of n-BDT-3T-CNCOO promotes the power conversion efficiency (PCE) from 5.01% for polymer binary devices to 5.51% for ternary devices. The improved PCE is attributed to the nanofibrous morphology with enhanced crystallinity of the donors and improved aggregation of the ICBA acceptor, which facilitate charge separation and charge transport. This work reveals that the ternary strategy of blending highly crystalline small molecules enhances PCEs of OSCs based on ICBA and other non-fullerene acceptors.

Dialkoxyphenyldithiophene-based small molecules with enhanced absorption for solution processed organic solar cells

Compared to the popular benzodithiophene (BDT) unit, dialkoxyphenyldithiophene (PDT) has the advantages of increased planarity along the molecular backbone (decreased dihedral angles between PDT and π bridges) and stronger donating ability, which may lead to better absorption, higher hole mobility and even higher power conversion efficiency (PCE). In this paper, two small molecules (CNO2TPDT and RD2TPDT) with PDT unit as a donor, bithiophene (2T) as π bridges, and oxoalkylated nitrile (CNO) or 3-ethylrhodanine (RD) as end-capped acceptors were synthesized and characterized. As expected, CNO2TPDT and RD2TPDT showed excellent absorption with edges of 720 and 745 nm, respectively, which are red-shifted by ca. 30 nm compared with their BDT analogues. A CNO2TPDT-based device exhibited relatively high Voc of 0.87 V but a low PCE of 4.16% due to a large phase separation. In contrast, a RD2TPDT-based device exhibited higher PCE of 6.64% with a Jsc of 12.74 mA cm−2 due to its red-shift absorption, better miscibility with [6,6]-phenyl-C71 butyric acid methylester (PC71BM) and higher hole mobility. This study demonstrated rational molecule design and effective morphology control can lead to good absorption ability and hole mobility, indicating the possibility for obtaining higher Jsc and PCE.

Versatile asymmetric thiophene/benzothiophene flanked diketopyrrolopyrrole polymers with ambipolar properties for OFETs and OSCs

Three novel asymmetric thiophene/benzothiophene-flanked diketopyrrolopyrrole (DPP)-based polymers with different π bridges, designated as PBTTDPP-BT, PBTTDPP-TT and PBTTDPP-2FBT, were designed, synthesized and employed in organic solar cells. Compared with the reported thiophene/pyridine-flanked DPP, these thiophene/benzothiophene-flanked DPP polymers exhibited narrower band gaps below 1.5 eV, leading to a broadened absorption that ranged from 500 nm to 850 nm. All polymers displayed promising ambipolar semiconducting properties. PBTTDPP-BT, PBTTDPP-TT and PBTTDPP-2FBT showed hole mobilities of 1.20, 1.68 and 1.50 cm2 V−1 s−1, respectively. Their corresponding electron mobilities were 0.40, 0.14 and 0.35 cm2 V−1 s−1. Interestingly, PBTTDPP-TT and PBTTDPP-2FBT also showed ambipolar properties in organic solar cells. Photovoltaic device based on PBTTDPP-TT as a donor material reached a power conversion efficiency (PCE) of 6.96% with PC71BM as an acceptor, while this device as the acceptor material achieved only 0.28% with poly(3-hexylthiophene) (P3HT) as the donor. In contrast, PCE of PBTTDPP-2FBT-based devices reached 5.62% with PC71BM and 0.44% with P3HT. These results suggest that the adoption of asymmetric flankers in DPP polymers can effectively tune their ambipolar transporting properties for high-performance organic electronic devices.

A novel small molecule based on naphtho[1,2-b:5,6-b′]dithiophene benefits both fullerene and non-fullerene solar cells

Small molecule solar cells have made great progress in recent years. Herein, we synthesized a novel small molecule donor NDTR with naphtho[1,2-b:5,6-b′]dithiophene (NDT) units as the building blocks. NDTR exhibited complementary absorption for both fullerene acceptor PC71BM and non-fullerene acceptor IDIC. Meanwhile, NDTR possessed a HOMO energy level of −5.23 eV and a LUMO energy level of −3.50 eV, which matched well with these two kinds of acceptors. When mixed with PC71BM, a high power conversion efficiency (PCE) of 7.75% was obtained, while the NDTR:IDIC system presented a PCE of 6.60%. The results indicated that NDTR was an all-round small molecule donor which can work well in both fullerene and non-fullerene systems.

Two-dimensional benzo[1,2-b:4,5-b′]difuran-based wide bandgap conjugated polymers for efficient fullerene-free polymer solar cells

Wide bandgap conjugated polymers are important for providing complementary absorption with the state-of-the-art narrow bandgap nonfullerene electron acceptors to maximise the utilization of solar photons. In this work, two wide bandgap two-dimensional conjugated polymers, PBDFT–Bz and PBDFF–Bz, based on benzo[1,2-b:4,5-b′]difuran building block were designed and synthesized. The optical bandgaps of PBDFT–Bz and PBDFF–Bz were found to be 1.90 eV and 1.85 eV, respectively. PBDFF–Bz with 2-ethylhexylfuryl side chains possesses the lower HOMO energy level and shows denser π–π stacking compared to PBDFT–Bz with 2-ethylhexylthienyl side chains. The fullerene-free PBDFF–Bz:ITIC-based polymer solar cell (PSC) showed a PCE of 9.46% with a Jsc of 15.02 mA cm−2, a Voc of 0.94 V and a FF of 67%; by using m-ITIC as an electron acceptor, the PCE was further improved to 10.28% with an enhanced Jsc of 16.57 mA cm−2, a Voc of 0.94 V and a FF of 66%. Under the same condition, the PBDFT–Bz:m-ITIC device gave a PCE of 9.84% with a Jsc of 16.63 mA cm−2, a Voc of 0.85 V and a FF of 70%. In addition, it is exciting that the PBDFF–Bz based devices show a small energy loss (Eloss) of 0.63 eV, while the PBDFT–Bz based devices show an Eloss of 0.72 eV. These results are among the best for fullerene-free devices with BDF-based polymers, and demonstrate that BDF is a very promising candidate for highly efficient polymer solar cells.

Poly(3,4-ethylenedioxythiophene)-coated sulfur for flexible and binder-free cathodes of lithium–sulfur batteries

Flexible energy storage devices are becoming increasingly important for next-generation flexible electronic devices. However, the low energy density of conventional lithium-ion batteries cannot fulfill their high capacity requirement. Therefore, lithium–sulfur (Li–S) batteries with high theoretical capacity and energy density have attracted attention as new-generation energy storage devices. In this work, we designed a freestanding poly(3,4-ethylenedioxythiophene) (PEDOT)-coated diamond-shaped sulfur (D-sulfur)/single-walled carbon nanotube (SWCNT) composite cathode. The conductive PEDOT and SWCNTs effectively confined the dissolution of lithium polysulfides, and a flexible network was successfully fabricated. The D-sulfur/SWCNT composite cathode exhibited a high capacity and stable cycling performance. The initial discharge capacity reached 1520 mA h g−1 at 0.1C, and the capacity retention was 76% after 250 cycles at 0.5C. Furthermore, the D-sulfur/SWCNT was used as a flexible cathode to fabricate a prototype of a flexible Li–S battery, which shows its potential application in flexible integrated systems.