Cuihong Li

Reversible switching of the emission of diphenyldibenzofulvenes by thermal and mechanical stimuli.

The development of stimuli-responsive materials has attracted much attention. Of particular interest are those molecules whose emissions can be repeatedly switched between different colors or between dark and bright states by external stimuli in the solid state, because of their potential applications in such fi elds as sensors, [ 1 ] memories, [ 2 ] and security inks. [ 3 ] Although modifi cation of molecular structures is the most common approach to tune emissions of dyes, limited success has been made in switching the luminescence of solid-state materials with high effi ciency and reproducibility, because of insuffi cient conversion or irreversible reactions. [ 4 ] Some dyes exhibit a morphology dependent emission, thus the emission of these dyes may be repeatedly switched in the solid state provided that the molecular arrangement could be reversibly tuned in the solid state. [ 5 ] Most of the reported materials respond to chemicals, heat, and vapor, [ 1 , 5 ] however, the mechanochromic fl uorescence of organic materials is seldom reported and a molecular-level understanding of the mechanochromic mechanisms remains unclear. [ 6 ] Switching of emission colors is frequently reported, however, the tuning of fl uorescence effi ciency is rarely examined. Although some copper, [ 7 ] gold, [ 8 ] and platinum [ 9 ] complexes, derivatives of oligo( p -phenylene vinylene) with cyano groups and long alkyl chains, [ 10 ] pyrene [ 11 ] and anthracene [ 12 ]

A Planar Copolymer for High Efficiency Polymer Solar Cells

An alternating copolymer, poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1), was designed, synthesized, and used as the donor material for high efficiency polymer solar cells. The close packing of the polymer chains in the solid state was confirmed by XRD. A J(sc) of 9.6 mA/cm(2), a V(oc) of 0.81 V, an FF of 0.69, and a PCE of 5.4% were achieved with HXS-1 and [6,6]-phenyl C(71)-butyric acid methyl ester (PC(71)BM) as a bulk heterojunction active layer spin-coated from a solvent mixture of 1,2-dichlorobenzene and 1,8-diodooctane (97.5:2.5) under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW/cm(2).

Malachite green derivative-functionalized single nanochannel: light-and-pH dual-driven ionic gating.

A highly efficient and perfectly reversible ionic gate that can be activated by pH or UV light is demonstrated. Switching between the OFF state and the ON state is mainly dependent on the surface charge transition brought about by a malachite green derivative attached to the interior surface of an ion track-etched conical nanochannel, which makes it suitable for confined spaces. Applications in electronics, actuators, and biosensors can be foreseen.

Triindole-cored star-shaped molecules for organic solar cells

Two new triindole-cored star-shaped molecules SM-1 and SM-2 have been designed and synthesized, and their optical, electrochemical, thermal, transport and photovoltaic properties have been investigated in detail. SM-1 and SM-2 exhibited good thermal stability, intensive absorption in a broad region, and relatively high hole mobility. Photovoltaic performances of these two molecules were investigated by fabricating bulk heterojunction solar cell devices with a blend film of SM-1:PC71BM or SM-2:PC71BM as the active layer. Organic solar cells (OSCs) based on SM-1:PC71BM and SM-2:PC71BM gave power conversion efficiencies (PCEs) of 2.05% and 2.29%, respectively. A PCE of 2.29% is the best result for all the reported triindole-based photovoltaic materials, indicating that triindole-based small molecules could become promising donor materials for solution-processed OSCs.

Switching the emission of tetrakis(4-methoxyphenyl)ethylene among three colors in the solid state

Emission of a luminogen could be switched among three colors in the solid state by transformation among three different aggregation states. The partly amorphous solid of the luminogen exhibits excitation dependent emission due to the contribution of both amorphous and crystalline parts to the photoluminescence intensity.

4-Alkyl-3,5-difluorophenyl-Substituted Benzodithiophene-Based Wide Band Gap Polymers for High-Efficiency Polymer Solar Cells.

Two novel polymers PTFBDT-BZS and PTFBDT-BZO with 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as the donor unit, benzothiadiazole or benzooxadiazole as the acceptor unit, and thiophene as the spacer have been synthesized and used as donor materials for polymer solar cells (PSCs). These two polymers exhibited wide optical band gaps of about 1.8 eV. PSCs with the blend of PTFBDT-BZS:PC71BM (1:2, by weight) as the active layer fabricated without using any processing additive and any postannealing treatment showed power conversion efficiency (PCE) of 8.24% with an open circuit voltage (Voc) of 0.89 V, a short circuit current (Jsc) of 12.67 mA/cm(2), and a fill factor (FF) of 0.73 under AM 1.5G illumination, indicating that PTFBDT-BZS is a very promising donor polymer for PSCs. The blend of PTFBDT-BZO:PC71BM showed a lower PCE of 5.67% with a Voc of 0.96 V, a Jsc of 9.24 mA/cm(2), and an FF of 0.64. One reason for the lower PCE is probably due to that PTFBDT-BZO has a smaller LUMO offset with PC71BM, which cannot provide enough driving force for charge separation. And another reason is probably due to that PTFBDT-BZO has a lower hole mobility in comparison with PTFBDT-BZS.

1,8-Naphthalimide-Based Planar Small Molecular Acceptor for Organic Solar Cells

Four small molecular acceptors (SM1-4) comprising a central benzene core, two thiophene bridges and two 1,8-naphthalimide (NI) terminal groups were designed and synthesized by direct C-H activation. SM1 has a planar chemical structure and forms H-aggregation as films. By attachment of different substituents on the central benzene ring, the dihedral angles between the two NI end groups of SM1-4 gradually increased, leading to a gradual decrease of planarity. SM1-4 all possess a high-lying LUMO level, matching with wide band gap (WBG) polymer donors which usually have a high-lying LUMO level. When used in OSCs, devices based on SM1 and WBG donor PCDTBT-C12 gave higher electron mobility, superior film morphology and better photovoltaic performance. After optimization, a PCE of 2.78% with a V(oc) of 1.04 V was achieved for SM1 based devices, which is among the highest PCEs with a V(oc) higher than 1 V. Our results have demonstrated that NI based planar small molecules are potential acceptors for WBG polymer based OSCs.

Highly Efficient Planar Perovskite Solar Cells Via Interfacial Modification with Fullerene Derivatives

Planar heterojunction perovskite solar cells with a high efficiency up to 17.76% are fabricated by modifying the compact TiO2 (c-TiO2) with a [6,6]-phenyl-C61-butyric acid (PCBA) monolayer. High quality CH3NH3PbI3 films can be easily fabricated on PCBA-modified c-TiO2 substrates by a one-step solution processing method. Significant improvements of the device parameters are observed after PCBA modification. A high open-circuit voltage (Voc) of 1.16 V has been achieved, indicating that the PCBA monolayer can act as a hole blocking layer to reduce the trap site density atop the c-TiO2 and the hole recombination at the c-TiO2 /perovskite interface. The enhancement of the fill factor, as well as the partial quenching of the fluorescence of perovskite after modification with PCBA, reveals that the charge extraction is improved.

A nonfullerene acceptor for wide band gap polymer based organic solar cells

A new 1,8-naphthalimide based planar small molecular acceptor and two benzothiadiazole based wide band gap (WBG) polymer donors P1 and P2 were synthesized for nonfullerene organic photovoltaic cells (OPVs). Devices based on fluorinated polymer P2 achieved a highly improved PCE of 3.71% with an open circuit voltage (V(oc)) of 1.07 V, which is beyond the currently known levels for nonfullerene OPVs with the V(oc) higher than 1 V.

Enhancing the performance of polymer photovoltaic cells by using an alcohol soluble fullerene derivative as the interfacial layer.

Alcohol soluble fullerene derivative (FN-C60) has been synthesized and used as a cathode interfacial layer for high-efficiency polymer solar cells (PSCs). To examine the function of the FN-C60 interfacial layer, polymer solar cells were fabricated with blends of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM as the active layer. In comparison to the bare Al electrode, power conversion efficiencies (PCEs) of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM based PSCs were increased from 3.50 to 4.64%, 4.69 to 5.25%, 2.70 to 4.60%, and 1.52 to 2.29%, respectively, when FN-C60/Al was used as the electrode. Moreover, the overall photovoltaic performances of PSCs with the FN-C60/Al electrode were better than those of cells with LiF/Al electrode, indicating that FN-C60 is a potential interfacial layer material to replace LiF.