Electret Modulation Strategy to Enhance the Photosensitivity Performance of Two-Dimensional Molybdenum Sulfide.

Due to the limited light absorption efficiency of atomic thickness layers and the existence of quenching effects, photodetectors solely made of transition metal dichalcogenides (TMDs) have exhibited an unsatisfactory detection performance. In this article, electret/TMD hybridized devices were proposed by vertically coupling a MoS2 channel and the PTFE film, which reveals an optimized photodetection behavior. Negative charges were generated in the PTFE layer through the corona charging method, akin to applying a negative bias on the MoS2 channel in lieu of a traditional voltage-driven back gate. Under a charging voltage of -6 kV, PTFE/MoS2 devices reveal improved photodetection performance (Rhybrid = 67.95A/W versus Ronly = 3.37 A/W, at 470 nm, 1.20 mW cm-2) and faster recovery speed (τd(hybrid) = 2000 ms versus τd(only) = 2900 ms) compared to those bare MoS2 counterparts. The optimal detection performance (2 orders of magnitude) was obtained when the charging voltage was -2 kV, limited by the minimum of the carrier density in MoS2 channels. This study provides an alternative strategy to optimize optoelectronic devices based on the 2D components through non-voltage-driven gating.

Enhancing Photodetection Ability of MoS2 Nanoscrolls via Interface Engineering.

Van der Waals semiconductors have been really confirmed in two-dimensional (2D) layered systems beyond the traditional limits of lattice-matching requirements. The extension of this concept to the 1D atomic level may generate intriguing physical functionalities due to its non-covalent bonding surface. However, whether the curvature of the lattice in such rolled-up structures affects their optoelectronic features or the performance of devices established on them remains an open question. Here, MoS2-based nanoscrolls were obtained by virtue of an alkaline solution-assisted method and the 0D/1D (BaTiO3/MoS2) strategy to tune their optoelectronic properties and improve the light sensing performance was explored. The capillary force generated by a drop of NaHCO3 solution could drive the delamination of nanosheets from the underlying substrate and a spontaneous rolling-up process. The package of BaTiO3 particles in MoS2 nanoscrolls has been evident by TEM image, and the optical characterizations were mirrored via micro-Raman spectroscopy and photoluminescence. These bare MoS2 nanoscrolls reveal a reduced photoresponse compared to the plane structures due to the curvature of the lattice. However, such BaTiO3/MoS2 nanoscrolls exhibit a significantly improved photodetection (Rhybrid = 73.9 A/W vs Ronly = 1.1 A/W and R2D = 1.5 A/W at 470 nm, 0.58 mW·cm-2), potentially due to the carrier extraction/injection occurring between BaTiO3 and MoS2. This study thereby provides an insight into 1D van der Waals material community and demonstrates a general approach to fabricate high-performance 1D van der Waals optoelectronic devices.

Colorful Luminescence of Conjugated Polyelectrolytes Induced by Molecular Weight.

Due to their distinctive intrinsic advantages, the nanoaggregates of conjugated polyelectrolytes (CPEs) are fascinating and attractive for various luminescence applications. Generally, the emission luminescence of CPEs is determined by the conjugated backbone structure, i.e., different conjugated backbones of CPEs produce emission luminescence with different emission wavelength bands. Here, we polymerized the bis(boronic ester) of benzothiadiazole and an alkyl sulfonate sodium-substituted dibromobenzothiatriazole to provide PBTBTz-SO3Na with different molecular weights via controlling the ratio of the monomer and the catalyst. Theoretically, the CPEs with the same molecular structure usually display similar photoelectronic performances. However, the resulting PBTBTz-SO3Na reveal a similar light absorption property, but different luminescence. The higher molecular weight is, the stronger the fluorescence intensity of PBTBTz-SO3Na that occurs. PBTBTz-SO3Na with different molecular weights have different colors of luminescence. It is well known that the molecular aggregates often led to weaker luminescent properties for most of the conjugated polymers. However, PBTBTz-SO3Na exhibits a higher molecular weight with an increasing molecular chain aggregation, i.e., the nanoaggregates of PBTBTz-SO3Na are beneficial to emission luminescence. This work provides a new possible chemical design of CPEs with a controllable, variable luminescence for further optoelectronics and biomedicine applications.

Epitaxial Growth of Lead-Free 2D Cs3 Cu2 I5 Perovskites for High-Performance UV Photodetectors.

The all-inorganic lead-free Cu-based halide perovskites represented by the Cs-Cu-I system, have sparked extensive interest recently due to their impressive photophysical characteristics. However, successive works on their potential application in light emission diodes and photodetectors rely on tiny polycrystals, in which the grain boundaries and defects may lead to the performance degradation of their embodied devices. Here, 2D all-inorganic perovskite Cs3 Cu2 I5 single crystals are epitaxially grown on mica substrates, with a thickness down to 10 nm. The strong blue emission of the Cs3 Cu2 I5 flakes may originate from the radiative transition of self-trapped excitons associated with a large Stocks shift and long (microsecond) decay time. Ultravioelt (UV) photodetectors based on individual Cs3 Cu2 I5 nanosheets are fabricated via a swift and etching-free dry transfer approach, which reveal a high responsivity of 3.78 A W-1 (270 nm, 5 V bias), as well as a fast response speed (τrise  ≈163 ms, τdecay  ≈203 ms), outperforming congeneric UV sensors based on other 2D metal halide perovskites. This work therefore sheds light on the fabrication of green optoelectronic devices based on lead-free 2D perovskites, vital for the sustainable development of photoelectric technology.

In-Situ Synthesis of TiO2@GO Nanosheets for Polymers Degradation in a Natural Environment.

Plastic photodegradation naturally takes 300-500 years, and their chemical degradation typically needs additional energy or causes secondary pollution. The main components of global plastic are polymers. Hence, new technologies are urgently required for the effective decomposition of the polymers in natural environments, which lays the foundation for this study on future plastic degradation. This study synthesizes the in-situ growth of TiO2 at graphene oxide (GO) matrix to form the TiO2@GO photocatalyst, and studies its application in conjugated polymers' photodegradation. The photodegradation process could be probed by UV-vis absorption originating from the conjugated backbone of polymers. We have found that the complete decomposition of various polymers in a natural environment by employing the photocatalyst TiO2@GO within 12 days. It is obvious that the TiO2@GO shows a higher photocatalyst activity than the TiO2, due to the higher crystallinity morphology and smaller size of TiO2, and the faster transmission of photogenerated electrons from TiO2 to GO. The stronger fluorescence (FL) intensity of TiO2@GO compared to TiO2 at the terephthalic acid aqueous solution indicates that more hydroxyl radicals (•OH) are produced for TiO2@GO. This further confirms that the GO could effectively decrease the generation of recombination centers, enhance the separation efficiency of photoinduced electrons and holes, and increase the photocatalytic activity of TiO2@GO. This work establishes the underlying basic mechanism of polymers photodegradation, which might open new avenues for simultaneously addressing the white pollution crisis in a natural environment.

Alternative alcohol-soluble conjugated small molecule electrolytes for high-efficiency inverted polymer solar cells.

New alcohol-soluble conjugated small molecule electrolytes (CSMEs), 3,6-bis-(5-benzoic acid-thiophen-2-yl)-2,5-bis-(2-ethylhexyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione liquid crystalline (DPP-COOH) and di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II) dye (N719), are developed as interfacial modification in inverted polymer solar cells (PSCs). Further optimization of the device architecture by combining the electrolytes as hole and electron buffer layers can significantly promote the photovoltaic performances of PSCs due to the integrated advantages of excellent alcohol processability, hole and electron mobility, interfacial dipole effect and good energy level alignment with electrodes. Moreover, the PSCs with the CSMEs interlayers based on narrow band-gap PTB7:PC71BM active layers show considerable improvement in power conversion efficiency (PCE), compared with P3HT:PCBM active layer-based devices. Devices with DPP-COOH and N719 modifications after thermal treatment at 120 °C exhibit the PCE of 8.0% and 7.6% under AM 1.5G irradiation, respectively, improving from 6.7% PCE of the pristine device without any interfacial layer. Encouragingly, the simultaneous use of CSMEs as hole and electron modification layers can boost the PCE to 8.2%. These findings demonstrate that the utilization of alcohol-soluble small molecule conjugated electrolytes with lower band gaps as interfacial modification layers is an effective and practical strategy for improving photovoltaic performance in PSCs.

Dye-sensitized nanoarrays with discotic liquid crystals as interlayer for high-efficiency inverted polymer solar cells.

The well-aligned and highly uniform one-dimensional ZnO with organic dyes core/shell (ZNs) and ZnO with dyes and liquid crystals core/double-shells nanoarrays (ZNLs) with controllable lengths were fabricated as electron transport layers (ETLs) in inverted polymer solar cells (PSCs). Ditetrabutylammonium cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II) dye (N719) was presented to reduce the surface defects of ZnO nanoarrays (NAs). In addition, the shell modification could decrease the electron injection barrier between ZnO and active layer, thereby facilitating electron injection effectively and forming a direct electron transport channel into the cathode. Due to the orientation of nanoarrays and the self-organization of 3,6,7,10,11-pentakis(hexyloxy)-2-hydroxytriphenylene liquid crystals (LCs) in liquid crystalline mesophase and isotropic phase transition, the components of active layer would be driven rearrange and infiltrate among the interspaces of nanoarrays more orderly. The increased interfacial contact between cathode and active layer would benefit charge generation, transportation and collection. On the basis of these advantages, it was found the N719 shell and N719/LCs double-shells modifications of ZnO NAs could boost the photovoltaic performance of PSCs with the best power conversion efficiency (PCE) of 7.3% and 8.0%, respectively.

Hybrid bulk heterojunction solar cells based on the cooperative interaction of liquid crystals within quantum dots and diblock copolymers.

In this article, the conjugated rod-rod polythiophene diblock copolymers comprising a regioregular poly(3-hexylthiophene) (P3HT) segment and a side-chain liquid-crystalline polythiophene segment bearing cyanobiphenyl mesogenic pendants (PTcbp), polythiophene-b-poly{3-[10-(4'-cyanobiphenyloxy)decyl]thiophene} (P3HT-b-PTcbp), were rationally designed and synthesized. It was observed that the diblock copolymers could self-assemble into high crystalline and oriented nanofibrils upon 1,2-dichlorobenzene solvent vapor annealing, originating from the crystallization of two segments and the orientation of cyanobiphenyl side-chain mesogens. Hybrid bulk heterojunction (BHJ) solar cells were then fabricated using P3HT-b-PTcbp as electron donors and ZnO and CdS quantum dots (QDs) modified by 4'-hydroxy-[1,1'-biphenyl]-4-carbonitrile (cbp) liquid-crystalline ligands (cbp@ZnO and cbp@CdS) as electron acceptors. The interaction between the cbp ligands on the surface of ZnO and CdS QDs and cyanobiphenyl side-chain mesogens of diblock copolymers promoted the cooperative self-assembly and controllable well-dispersion of QDs in the polymer matrix and, as a consequence, yielded an intimately contacted polymer-QD nanocomposites. The power conversion efficiency (PCE) of the device based on P3HT-b-PTcbp/cbp@ZnO hybrids was improved by 2.6 times compared with that of P3HT/ZnO hybrids from 0.58 to 0.97. In addition, an overall PCE of a homologous device based on the P3HT-b-PTcbp/cbp@CdS hybrid active layer reached 2.3%. The research paved the way for the further development of high-efficiency hybrid BHJ solar cells by introducing block copolymer nanofibrils with favored crystalline domain orientations and liquid-crystalline organization properties.