PMMA direct exfoliation for rapid and organic free transfer of centimeter-scale CVD graphene

Despite the various techniques developed for the transfer of large area graphene grown by chemical vapor deposition (CVD), the conventional PMMA transferring technique has been widely applied in laboratories due to its convenience and economical cost. However, the complete removal of PMMA on graphene surface has become a troublesome, and the PMMA residue could degrade the properties of graphene significantly. We report here a facile water assisted technique to directly peel off the PMMA layer over centimeter-sized CVD graphene film for the first time. No organic solvents are involved in the whole transfer process. The transferred graphene film is clean and intact over large area because of the cooperative effect of the capillary force and the van der Waals force which facilitates the conformal contact between graphene film and the substrate. Various types of graphene samples (i.e. monolayer, multilayer, and incomplete domains) can be easily transferred to diverse substrates including silicon wafer, sapphire, and quartz with good integrity. The transferred graphene film is of high cleanliness, and the graphene transistors show higher carrier mobility and lower level of p-type doping comparing to the conventional wet transfer technique.

Demonstration of valley anisotropy utilized to enhance the thermoelectric power factor

Valley anisotropy is a favorable electronic structure feature that could be utilized for good thermoelectric performance. Here, taking advantage of the single anisotropic Fermi pocket in p-type Mg3Sb2, a feasible strategy utilizing the valley anisotropy to enhance the thermoelectric power factor is demonstrated by synergistic studies on both single crystals and textured polycrystalline samples. Compared to the heavy-band direction, a higher carrier mobility by a factor of 3 is observed along the light-band direction, while the Seebeck coefficient remains similar. Together with lower lattice thermal conductivity, an increased room-temperature zT by a factor of 3.6 is found. Moreover, the first-principles calculations of 66 isostructural Zintl phase compounds are conducted and 9 of them are screened out displaying a pz-orbital-dominated valence band, similar to Mg3Sb2. In this work, we experimentally demonstrate that valley anisotropy is an effective strategy for the enhancement of thermoelectric performance in materials with anisotropic Fermi pockets.

Backbone Effects on the Thermoelectric Properties of Ultra-Small Bandgap Conjugated Polymers

Conjugated polymers with narrower bandgaps usually induce higher carrier mobility, which is vital for the improved thermoelectric performance of polymeric materials. Herein, two indacenodithiophene (IDT) based donor–acceptor (D-A) conjugated polymers (PIDT-BBT and PIDTT-BBT) were designed and synthesized, both of which exhibited low-bandgaps. PIDTT-BBT showed a more planar backbone and carrier mobility that was two orders of magnitude higher (2.74 × 10−2 cm2V−1s−1) than that of PIDT-BBT (4.52 × 10−4 cm2V−1s−1). Both exhibited excellent thermoelectric performance after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, where PIDTT-BBT exhibited a larger conductivity (0.181 S cm−1) and a higher power factor (1.861 μW m−1 K−2) due to its higher carrier mobility. The maximum power factor of PIDTT-BBT reached 4.04 μW m−1 K−2 at 382 K. It is believed that conjugated polymers with a low bandgap are promising in the field of organic thermoelectric materials.

Up-Scalable Fabrication of SnO2 with Multifunctional Interface for High Performance Perovskite Solar Modules

Tin dioxide (SnO2) has been demonstrated as one of the promising electron transport layers for high-efficiency perovskite solar cells (PSCs). However, scalable fabrication of SnO2 films with uniform coverage, desirable thickness and a low defect density in perovskite solar modules (PSMs) is still challenging. Here, we report preparation of high-quality large-area SnO2 films by chemical bath deposition (CBD) with the addition of KMnO4. The strong oxidizing nature of KMnO4 promotes the conversion from Sn(II) to Sn(VI), leading to reduced trap defects and a higher carrier mobility of SnO2. In addition, K ions diffuse into the perovskite film resulting in larger grain sizes, passivated grain boundaries, and reduced hysteresis of PSCs. Furthermore, Mn ion doping improves both the crystallinity and the phase stability of the perovskite film. Such a multifunctional interface engineering strategy enabled us to achieve a power conversion efficiency (PCE) of 21.70% with less hysteresis for lab-scale PSCs. Using this method, we also fabricated 5 × 5 and 10 × 10 cm2 PSMs, which showed PCEs of 15.62% and 11.80% (active area PCEs are 17.26% and 13.72%), respectively. For the encapsulated 5 × 5 cm2 PSM, we obtained a T80 operation lifetime (the lifespan during which the solar module PCE drops to 80% of its initial value) exceeding 1000 h in ambient condition.

Analysis of Surface Reaction for Group III-V Compound Semiconductors in Functional Water

Group III–V compound semiconductors are attracting attention as new channel materials that have higher carrier mobility than Si. However, defects easily occur at the interface between the semiconductor and insulator film, which degrades performance. In an earlier study, we demonstrated that the interfacial properties of InP are degraded by the growth of In2O3 and that In2O3 grows better in water than in air. Therefore, it is necessary to suppress the growth of In2O3 to improve the interfacial properties of InP. In this work, we focused on functional water, which can be controlled by adjusting the water conditions, and investigated the growth behavior of In2O3 in functional water. As a result, we found that the growth is suppressed in the low-pH range and in hydrogen water. It is important that H+ ions reduce OH− ions, which contributes to the reaction with InP.

Single Crystal Hybrid Perovskite Optoelectronics: Progress and Perspectives

Organic–inorganic hybrid perovskites, which combine the superior optical and electronic properties and solution-processed manufacturing, have emerged as a new class of revolutionary optoelectronic devices with the potential for various practical applications. Encouraged by the advantages of longer carrier diffusion length, higher carrier mobility and lower trap densities as compared to the polycrystalline counterparts’, increasing research attention has focused on preparation and optimization of perovskite crystal candidates, via using various facile growth techniques, for the development of a wide range of optoelectronic applications. This chapter presents a comprehensive review of recent advances in the field of optoelectronic technologies based on different forms of single crystals, including bulk crystals and thin ones, with emphasis placed on the optimization of crystals and the relationship among the charge-carrier transport, operation mechanism, device architecture, and device performance. First, we introduce the main methods used to prepare bulk and thin single crystals, and analyze several aspects of their properties. Thereafter, the applications of single crystals into solar cells, photodetectors, light-emitting diodes, and lasers, are discussed in depth. Finally, we summarize the challenges of perovskite single crystals and propose further improvements in the synthesis approaches and device applications.

Structure and Doping Optimization of IDT-Based Copolymers for Thermoelectrics

π-conjugated backbones play a fundamental role in determining the thermoelectric (TE) properties of organic semiconductors. Understanding the relationship between the structure–property–function can help us screen valuable materials. In this study, we designed and synthesized a series of conjugated copolymers (P1, P2, and P3) based on an indacenodithiophene (IDT) building block. A copolymer (P3) with an alternating donor–acceptor (D-A) structure exhibits a narrower band gap and higher carrier mobility, which may be due to the D-A structure that helps reduce the charge carrier transport obstacles. In the end, its power factor reaches 4.91 μW m−1 K−2 at room temperature after doping, which is superior to those of non-D-A IDT-based copolymers (P1 and P2). These results indicate that moderate adjustment of the polymer backbone is an effective way to improve the TE properties of copolymers.

Interfacial Built-In Electric Field-Driven Direct Current Generator Based on Dynamic Silicon Homojunction

Searching for light and miniaturized functional device structures for sustainable energy gathering from the environment is the focus of energy society with the development of the internet of things. The proposal of a dynamic heterojunction-based direct current generator builds up new platforms for developing in situ energy. However, the requirement of different semiconductors in dynamic heterojunction is too complex to wide applications, generating energy loss for crystal structure mismatch. Herein, dynamic homojunction generators are explored, with the same semiconductor and majority carrier type. Systematic experiments reveal that the majority of carrier directional separation originates from the breaking symmetry between carrier distribution, leading to the rebounding effect of carriers by the interfacial electric field. Strikingly, NN Si homojunction with different Fermi levels can also output the electricity with higher current density than PP/PN homojunction, attributing to higher carrier mobility. The current density is as high as 214.0 A/m2, and internal impedance is as low as 3.6 kΩ, matching well with the impedance of electron components. Furthermore, the N-i-N structure is explored, whose output voltage can be further improved to 1.3 V in the case of the N-Si/Al2O3/N-Si structure, attributing to the enhanced interfacial barrier. This approach provides a simple and feasible way of converting low-frequency disordered mechanical motion into electricity.

With PBDB-T as the Donor, the PCE of Non-Fullerene Organic Solar Cells Based on Small Molecule INTIC Increased by 52.4%

At present, most high-performance non-fullerene materials are centered on fused rings. With the increase in the number of fused rings, production costs and production difficulties increase. Compared with other non-fullerenes, small molecule INTIC has the advantages of easy synthesis and strong and wide infrared absorption. According to our previous report, the maximum power conversion efficiency (PCE) of an organic solar cell using PTB7-Th:INTIC as the active layer was 7.27%. In this work, other polymers, PTB7, PBDB-T and PBDB-T-2F, as the donor materials, with INTIC as the acceptor, are selected to fabricate cells with the same structure to optimize their photovoltaic performance. The experimental results show that the optimal PCE of PBDB-T:INTIC based organic solar cells is 11.08%, which, thanks to the open voltage (VOC) increases from 0.80 V to 0.84 V, the short circuit current (JSC) increases from 15.32 mA/cm2 to 19.42 mA/cm2 and the fill factor (FF) increases from 60.08% to 67.89%, then a 52.4% improvement in PCE is the result, compared with the devices based on PTB7-Th:INTIC. This is because the PBDB-T:INTIC system has better carrier dissociation and extraction, carrier transportation and higher carrier mobility.

Study on Carrier Mobility Model for PD-Ge Monolithic Optoelectronic Integration Chips

Based on the difference of thermal expansion coefficient between Si and Ge, low-intensity tensile stress can be introduced into Ge epitaxial layer on Si substrate. S-Ge/Si semiconductor (as known as low tensile strained Ge grown on Si substrate) has a higher carrier mobility when compared with unstrained-Ge or Si material, so that s-Ge/Si is appropriate for the production of high-speed circuit. At the same time, transformation from indirect bandgap semiconductor Ge into Pseudo-Direct bandgap semiconductor (which is also called PD-Ge) will be happen after s-Ge/Si is heavy doped, which makes LED produced of PD-Ge material perform a higher luminous efficiency because the radiative recombining probability of carriers in PD-Ge material is greatly improved compared with unstrained one. Taking the advantages referred of s-Ge/Si into account, s-Ge/Si has the potential to PD-Ge monolithic optoelectronic integration. Carrier mobility of the semiconductor is one of the key physical parameters during the design and simulation of PD-Ge monolithic optoelectronic integrated system. While as far as the authors are aware, carrier mobility model of s-Ge/Si is still rarely reported to date. In view of that all above, based on the E-k relation in both conduction band and valence band of s-Ge/Si material, the analytical models of physical parameters in energy band are established, and the models are verified by experiments. Then the s-Ge/Si carrier models are further established based on our band structure model, and the Monte Carlo method is used to verify our s-Ge/Si carrier mobility model. The quantificational results of our paper will help understand s-Ge/Si material physics and provide an important theoretical basis for the design of PD-Ge monolithic optoelectronic integration.