Improvement of Dynamic Performance and Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors by Incorporating Conjugated Polymers.

Colloidal quantum dots (CQDs) have a unique advantage in realizing near-infrared (NIR) photodetection since their optical properties are readily tuned by the particle size, but CQD-based photodetectors (QPDs) presently show a high dark current density (Jd) and insufficient dynamic characteristics. To overcome these two problems, we synthesized and introduced two types of conjugated polymers (CPs) by replacing the p-type CQD layer in the QPDs. The low dielectric constant and insulating properties of CPs under dark conditions effectively suppressed the Jd in the QPDs. In addition, the energy-level alignment and high-hole mobility of the CPs facilitated hole transport. Therefore, both the responsivity and specific detectivity were highly enhanced in the CP-based QPDs. Notably, the dynamic characteristics of the QPDs, such as the -3 dB cut-off frequency and rising/falling response times, were significantly improved in the CP-based QPDs owing to the sizable molecular ordering and fast hole transport of the CP in the film state as well as the low trap density, well-aligned energy levels, and good interfacial contact in the CP-based devices.

FTO-free counter electrodes for dye-sensitized solar cells using carbon nanosheets synthesised from a polymeric carbon source.

Highly conductive carbon nanosheets (CNSs) are fabricated using a polymeric carbon source and subsequently applied as the counter electrodes (CNS-CEs) for dye-sensitized solar cells (DSSCs). The CNSs have a similar structure to multilayered graphene, and their high electrical conductivity and electrocatalytic activity enable them to have a dual-function as both CEs and charge supporting electrodes. CNSs form a unique CE material that functions successfully while being metal- and fluorine doped tin oxide (FTO)-free and allowing DSSCs to achieve ∼5% power conversion efficiency. The chemical structure, electrical properties, electrocatalytic activity, and work function of the CNS-CEs prepared under various conditions of carbonization are investigated, and their effects on the performance of the corresponding DSSCs are discussed. Carbonization temperature is shown to have influenced the size of graphitic domains and the presence of heteroatoms and functional groups in CNS-CEs. The change in the graphitic domain size has a marginal influence on the work function of the CNS-CEs and the overpotential for the reduction of the redox couples (I(-)/I3(-)). However, the electrical conductivity of CNS-CEs and the charge transfer resistance at CE/electrolyte interfaces in the DSSCs are considerably influenced by the carbonization condition. Our study shows that CNSs serve as efficient, FTO-free CE materials for DSSCs, and they are appropriate materials with which the effects of the chemical/physical properties of graphene-based materials on the electrode performance of various electrochemical devices may be studied.

Development of n-Type Small-Molecule Acceptors for Low Dark Current Density and Fast Response Organic Photodetectors.

Suppressing the dark current density (Jd) while maintaining sufficient charge transport is important for improving the specific detectivity (D*) and dynamic characteristics of organic photodetectors (OPDs). In this study, we synthesized three novel small-molecule acceptors (SMAs) densely surrounded by insulating alkyl side chains to minimize the Jd in OPDs. Introducing trialkylated N-annulated perylene diimide as a terminal moiety to the alkylated π-conjugated core structure was highly efficient in suppressing Jd in the devices, resulting in an extremely low Jd of 4.60 × 10-11 A cm-2 and 10-100 times improved D* values in the devices. In addition, SMAs with a geometrically aligned backbone structure exhibited better intermolecular ordering in the blended films, resulting in 3-10 times as high responsivity (R) values in the OPDs. Outstanding OPD performances with a D* of 8.09 × 1012 Jones, -3 dB cutoff frequency of 205.2 kHz, and rising response time of 16 µs were achieved under a 530 nm illumination in photoconductive mode. Geometrically aligned core-terminal SMAs densely surrounded by insulating alkyl side chains are promising for improving the static and dynamic properties of OPDs.

Enhanced Static and Dynamic Properties of Highly Miscible Fullerene-Free Green-Selective Organic Photodetectors.

We developed p-n junction organic photodetectors (OPDs) composed of a polymer donor and a nonfullerene acceptor (NFA) to increase both the responsivity (R) and detectivity (D*) while maintaining a narrow wavelength selectivity. The selection of the polymer donor and NFA with similar green (G) absorption is important for achieving G-wavelength selectivity in these OPDs, which differentiates them from current fullerene-based OPDs and NFA-based panchromatic absorption OPDs. In addition, mixing the polymer donor and asymmetric NFA was efficient toward increasing the miscibility and decreasing the interfacial energy difference of the blended films, resulting in the formation of a uniform and well-mixed nanomorphology in the photoconductive layer. Two-dimensional (2D) grazing incidence X-ray diffraction and Fourier-transform infrared spectroscopy revealed that the lamellar ordering of the polymer donor was enhanced in the blend film prepared with an asymmetric NFA, whereas the aggregation of a symmetric NFA in the blend films did not increase the lamellar ordering of the polymer donor. Consequently, we achieved an R value of 0.31 A/W and D* value of 2.0 × 1013 Jones with a full width at half-maximum value of 230 nm at -2 V and fast response time of 27 µs without any external bias in the asymmetric NFA-based OPDs. The enhancement in the lamellar ordering and miscibility of the blended films are crucial toward increasing the static and dynamic properties of OPDs.

Enhanced cycle stability of a NiCo2S4 nanostructured electrode for supercapacitors fabricated by the alternate-dip-coating method.

Nanostructured nickel cobalt sulfide (NiCo2S4) electrodes are successfully fabricated using a simple alternate-dip-coating method. The process involves dipping a TiO2 nanoparticles-covered substrate in a nickel/cobalt precursor solution and sulfur precursor solution alternately at room temperature. The fabricated bimetallic sulfide electrode exhibits a synergetic improvement compensating for the disadvantages of the two single metal sulfide electrodes, i.e. the poor cycle stability of the nickel sulfide electrode and the low specific capacitance (Csp) of the cobalt sulfide electrode. The two capacitive properties are optimized by adjusting the ratio of nickel and cobalt concentrations in the metal precursor solution, reaching a Csp of 516 F g-1 at a current density of 1 mA cm-2, with its retention being 99.9% even after 2000 galvanostatic charge-discharge cycles.