High-performance floating-gate organic phototransistors based on n-type core-expanded naphthalene diimides

In the field of organic phototransistor, achieving both broad-spectral and high photosensitivity has always been a big challenge. The innovation of device structure has previously proven to be a possible solution to this problem. Here in this study, a novel organic phototransistor based on a high mobility n-type small molecule as the conducting layer and an isolated bulk heterojunction light-absorbing layer as the floating gate has been demonstrated in this study. With the special designed device structure, the phototransistor shows extremely high sensitivity to broad spectral and weak light irradiation, and the photoresponsivity and photocurrent/dark-current ratio of the device can reach up to 4840 mA/W and 1.8×10~5 respectively.For conclusion, this study suggests a potential way to obtain high-performance phototransistors at room temperature, which will further promote the commercial application of organic phototransistors.

Impact of hydraulic perforation on fracture initiation and propagation in shale rocks

To enhance the oil and gas recovery rate, hydraulic fracturing techniques have been widely adopted for stimulation of low-permeability reservoirs. Pioneering work indicates that hydraulic perforation and layout could significantly affect fracture initiation and propagation in low-permeability reservoir rocks subjected to complex in-situ stresses. This paper reports on a novel numerical method that incorporates fracture mechanics principles and the numerical tools FRANC3 D and ANSYS to investigate the three-dimensional initiation and propagation behavior of hydro-fracturing cracks in shale rock. Considering the transverse isotropic property of shale rocks, the mechanical parameters of reservoir rocks attained from laboratory tests were adopted in the simulation. The influence of perforation layouts on the 3D initiation of hydro-fracturing fractures in reservoir rocks under geo-stresses was quantitatively illuminated. The propagation and growth of fractures in three dimensions in different perforating azimuth values were illustrated. The results indicate that: 1) the optimal perforation direction should be parallel to the maximum horizontal principal stress, 2) the crack plane gradually turns toward the direction of the maximum horizontal principal stress when they are not in parallel, 3) compared with the linear and symmetric pattern, the staggered perforation is the optimal one, 4) the proper perforation density is four to six holes per meter, 5) the optimal perforation diameter in this model is 30 mm, and 6) the influence of the perforation depth on the fracture initiation pressure is low.

The effect of alkyl chain branching positions on the electron mobility and photovoltaic performance of naphthodithiophene diimide(NDTI)-based polymers

Conjugated polymers are widely used in organic optoelectronic devices due to their solution processability,thermal stability and structural diversity.Generally,alkyl side chains must be utilized to increase the solubility of final polymers in the processing solvent.However,the effects of different type alkyl chains on the properties of type photovoltaic polymers have rarely been investigated.In this article,we synthesized three naphthodithiophene diimide(NDTI)based polymers containing bulky alkyl chains with different branching position,named as NDTI-1,NDTI-2 and NDTI-3,respectively.We systematically investigated the effect of different branching point on the molecular packing,charge transport and photovoltaic performance.When moving the branching point away from the backbone,the intermolecular interaction became stronger,which could be proved by 2D grazing incidence wide angle X-ray scattering(GIWAXS)measurement.Therefore,the electron mobilities in organic field-effect transistors gradually increased from 2.11×10^3 cm^2 V^-1 s^-1 for NDTI-1 to 4.70×10^-2 cm^2 V^-1 s^-1 for NDTI-2 and 9.27×10^-2 cm^2 V^-1 s^-1,for NDTI-3,which are quite high values for polymers with face-on orientation.In addition,the NDTI-2 and NDTI-3 thin films exhibited redshifted absorption spectra compared with NDTI-1.When blending with three classic donor polymers PBDB-T,PTB7-Th and PE61,NDTI-2 based devices always showed the higher power conversion efficiencies(PCEs)than the other two polymers(beside the comparable result of PTB7-Th:NDTI-3 combination)as a result of the high photocurrent response and high fill factor.Our results indicate that bulky alkyl chain with branching point at 2-position should be a good and safe choice for the design of naphthodithiophene diimide-based and even naphthalene diimide-based n-type photovoltaic polymers.