Crystalline Inorganic Semiconductors Research Articles

Schottky-Contact Formation between Metal Electrodes and Molecularly Doped Disordered Organic Semiconductors

We study using three-dimensional kinetic Monte Carlo (KMC) simulations to what extent the formation of Schottky contacts between a metal electrode and a molecularly doped disordered organic semiconductor can be understood from the theory for crystalline inorganic semiconductors, adapted to include the effects of the localized nature of the states in which the charge carriers reside and the hopping transport in between these states. The thickness of the Schottky-contact depletion region is found to be significantly smaller than as expected when the energetical disorder is neglected. The presence of energetic disorder is also found to influence the voltage dependence of the width of the depletion regions near the contacts of single-layer double-Schottky-contact devices. The voltage drop over the two depletion regions and the remaining charge-neutral bulk layer is shown to be described successfully by a semianalytical model, based on an accurately parameterized bulk mobility function of the dopant concentration, energetic disorder, and the electric field. We furthermore find that the mobility in the depletion regions is drastically reduced. As a result, the depletion-region formation process can be ultraslow, with a characteristic time scale ranging from microseconds to beyond milliseconds.

Perovskite Solar Cell-Gated Organic Electrochemical Transistors for Flexible Photodetectors with Ultrahigh Sensitivity and Fast Response.

Photodetectors (PDs) are the building block of various imaging and sensing applications. However, commercially available PDs based on crystalline inorganic semiconductors cannot meet the requirements of emerging wearable/implantable applications due to their rigidity and fragility, which creates the need for flexible devices. Here, a high-performance flexible PD is presented by gating an organic electrochemical transistor (OECT) with a perovskite solar cell. Due to the ultrahigh transconductance of the OECT, the device demonstrates a high gain of ≈106 , a fast response time of 67µs and an ultrahigh detectivity of 6.7×1017 Jones to light signals under a low working voltage (≤0.6V). Thanks to the ultrahigh sensitivity and fast response, the device can track photoplethysmogram signals and peripheral oxygen saturation under ambient light and even provide contactless remote sensing, offering a low-power and convenient way for continuous vital signs monitoring. This work offers a novel strategy for realizing high-performance flexible PDs that are promising for low-power, user-friendly and wearable optoelectronics.

Heat Transfer Enhancement of n-Type Organic Semiconductors by an Insulator Blend Approach.

The transfer of heat energy in organic semiconductors (OSCs) plays an important role in advancing the applications of organic electronics, especially for lifetime issues. However, compared with crystalline inorganic semiconductors, the thermal transport of OSCs is less efficient and a relevant understanding is very limited. In this contribution, we show that the heat conduction of OSCs can be enhanced by blending with a “commodity” insulator (both thermal and electrical). PC71BM, a well-known electron transporter but poor thermal conductor, was selected as the host OSC material. The blending of a small amount of polystyrene (PS), a commonly used insulating polymer, can facilitate the heat transfer of PC71BM films, as substantiated by the scanning photothermal deflection technique and an infrared thermal camera. The phase thermodynamics of PC71BM/PS blends indicates that the efficient heat transfer preferably occurs in the OSC/insulator blends with better intimate mixing, where isolated PC71BM domains can be effectively bridged by PS that thread through the regions. The applicability of this approach can be observed in blends with another host material—ITIC. This work provides a facile strategy for designing thermally durable organic electronic devices.

Crystallographic Orientation Control of 1D Sb2Se3 Nanorod Arrays for Photovoltaic Application by In Situ Back‐Contact Engineering

Low‐dimensional (LD) crystalline inorganic semiconductors have attracted increasing interest due to their unique electrical and optical properties. The crystallographic orientation in the LD films is one of the critical parameters that determine their strong anisotropic physical properties and device performance. Antimony selenide (Sb2Se3), a 1D crystalline semiconductor, is a promising absorber material for emerging photovoltaic technologies. The suitably oriented Sb2Se3 absorbers can offer excellent carrier transport and trap‐free grain boundaries, facilitating high‐efficiency devices. The crystallographic orientation of the Sb2Se3 light‐absorbing layer is found to be governed by the underlying layers, i.e., the back contact in substrate‐type solar cells. Herein, an in situ surface selenization treatment to the tungsten (W) back contact is applied to change the growth of Sb2Se3 layer from a layer‐like growth mode to an island‐like growth mode. As a result, highly [hk1]‐oriented Sb2Se3 nanorod arrays growing perpendicular to the W back‐contact surface are achieved. Moreover, the resulting tungsten selenide (WSe2) thin layer also acts as a hole transport layer and promotes hole extraction. Consequently, a conversion efficiency as high as 8.46% in Sb2Se3 solar cells with substrate configuration is achieved.

Electronic Structure Characterization of Soft Semiconductors

AbstractSoft semiconductors are a class of materials that have seen increased interest in terms of both basic research and development of technology, in particular optoelectronic devices. These materials, organic semiconductors and metal halide perovskites, are defined by being more mechanically malleable than the traditional crystalline inorganic semiconductors and with thin film fabrication done at lower temperatures and often from solution. In this short perspective article, basic properties of the materials are introduced, as well as their typical applications and a number of advanced characterization techniques that offer distinct advantages for studying soft semiconductor thin films.

Flexible Organic Thermoelectric Materials and Devices for Wearable Green Energy Harvesting.

In the past few decades, organic thermoelectric materials/devices, which can exhibit remarkable potential in green energy conversion, have drawn great attention and interest due to their easy processing, light weight, intrinsically low thermal conductivity, and mechanical flexibility. Compared to traditional batteries, thermoelectric materials have high prospects as alternative power generators for harvesting green energy. Although crystalline inorganic semiconductors have dominated the fields of thermoelectric materials up to now, their practical applications are limited by their intrinsic fragility and high toxicity. The integration of organic polymers with inorganic nanoparticles has been widely employed to tailor the thermoelectric performance of polymers, which not only can combine the advantages of both components but also display interesting transport phenomena between organic polymers and inorganic nanoparticles. In this review, parameters affecting the thermoelectric properties of materials were briefly introduced. Some recently developed n-type and p-type thermoelectric films and related devices were illustrated along with their thermoelectric performance, methods of preparation, and future applications. This review will help beginners to quickly understand and master basic knowledge of thermoelectric materials, thus inspiring them to design and develop more efficient thermoelectric devices.

Rational In Silico Design of an Organic Semiconductor with Improved Electron Mobility.

Organic semiconductors find a wide range of applications, such as in organic light emitting diodes, organic solar cells, and organic field effect transistors. One of their most striking disadvantages in comparison to crystalline inorganic semiconductors is their low charge-carrier mobility, which manifests itself in major device constraints such as limited photoactive layer thicknesses. Trial-and-error attempts to increase charge-carrier mobility are impeded by the complex interplay of the molecular and electronic structure of the material with its morphology. Here, the viability of a multiscale simulation approach to rationally design materials with improved electron mobility is demonstrated. Starting from one of the most widely used electron conducting materials (Alq3 ), novel organic semiconductors with tailored electronic properties are designed for which an improvement of the electron mobility by three orders of magnitude is predicted and experimentally confirmed.

(Invited) Thermoelectric Thin Films and Nanocomposites Synthesized Via Solution-Phase Chemistry

Solution-phase processing routes to thermoelectric materials have the potential to decrease costs and enable novel device architectures. The best thermoelectric materials are crystalline inorganic semiconductors, which makes finding solution-phase routes to these materials of high interest. Unfortunately, inorganic semiconductors are generally insoluble due to their strong covalent bonds. One way around this hurdle is to create soluble semiconductor precursors that can be transformed into crystalline semiconductors after deposition. Furthermore, these precursor chemistries can be combined with colloidal nanocrystals to create nanostructured thin films with controlled composition and microstructure. We first present the synthesis of CuSeS and Ag-doped CuSeS thin films via soluble precursors and then report their thermoelectric properties. Specifically, we study the effect of Cu vacancies, Se:S ratio, and Ag-doping. We find that Ag-doping leads to appreciable improvements in thermoelectric performance by increasing Seebeck coefficient and decreasing thermal conductivity, while having little to no change in electrical conductivity. Overall, we find that the room temperature thermoelectric properties of these solution-processed materials are comparable to measurements on CuSeS alloys made via conventional thermoelectric material processing methods. Achieving parity between solution-phase processing and conventional processing is an important milestone and demonstrates the promise of this approach to making thermoelectric materials. Next, we present new chemical syntheses for soluble PbSe, PbTe, SnSe, and SnTe precursors. These precursors are especially relevant because PbSe and PbTe are the best-performing conventional thermoelectric materials in the moderate-to-high temperature regime. This chemical discovery is also important because soluble precursor routes to PbSe and PbTe thin films have not been reported in the literature. In addition, SnSe has the highest thermoelectric figure of merit of any known material, which makes our SnSe synthesis of interest as well. Lastly, we combine these precursors with colloidal nanocrystals to create nanostructured thin films with controlled composition and microstructure. Colloidal nanocrystals are made via solution-phase chemistry and consist of an inorganic core with short ligand molecules bound to its surface. These nanocrystals can be made with excellent control over size, shape, and composition. We demonstrate that our precursors can replace the typical organic ligands used in colloidal nanocrystal synthesis. This creates new pathways to inorganic nanostructured thermoelectric thin films with controlled composition and microstructure, and we report on the thermoelectric properties of said films.

Solution-processed semiconductors for next-generation photodetectors

Efficient light detection is central to modern science and technology. Current photodetectors mainly use photodiodes based on crystalline inorganic elemental semiconductors, such as silicon, or compounds such as III–V semiconductors. Photodetectors made of solution-processed semiconductors — which include organic materials, metal-halide perovskites and quantum dots — have recently emerged as candidates for next-generation light sensing. They combine ease of processing, tailorable optoelectronic properties, facile integration with complementary metal–oxide–semiconductors, compatibility with flexible substrates and good performance. Here, we review the recent advances and the open challenges in the field of solution-processed photodetectors, examining the topic from both the materials and the device perspective and highlighting the potential of the synergistic combination of materials and device engineering. We explore hybrid phototransistors and their potential to overcome trade-offs in noise, gain and speed, as well as the rapid advances in metal-halide perovskite photodiodes and their recent application in narrowband filterless photodetection. Conventional photodetectors, made of crystalline inorganic semiconductors, are limited in terms of the compactness and sensitivity they can reach. Photodetectors based on solution-processed semiconductors combine ease of processing, tailorable optoelectronic properties and good performance, and thus hold potential for next-generation light sensing.

Extended carrier lifetimes and diffusion in hybrid perovskites revealed by Hall effect and photoconductivity measurements

Impressive performance of hybrid perovskite solar cells reported in recent years still awaits a comprehensive understanding of its microscopic origins. In this work, the intrinsic Hall mobility and photocarrier recombination coefficient are directly measured in these materials in steady-state transport studies. The results show that electron-hole recombination and carrier trapping rates in hybrid perovskites are very low. The bimolecular recombination coefficient (10−11 to 10−10 cm3 s−1) is found to be on par with that in the best direct-band inorganic semiconductors, even though the intrinsic Hall mobility in hybrid perovskites is considerably lower (up to 60 cm2 V−1 s−1). Measured here, steady-state carrier lifetimes (of up to 3 ms) and diffusion lengths (as long as 650 μm) are significantly longer than those in high-purity crystalline inorganic semiconductors. We suggest that these experimental findings are consistent with the polaronic nature of charge carriers, resulting from an interaction of charges with methylammonium dipoles.

Study of Interfacial Charge Transfer Loss in Hybrid Solar Cells by Impedance Spectroscopy

Organic solar cells have been much studied because of the simplicity in their fabrication process in comparison to solar cells based on crystalline inorganic semiconductors. However, the actual power conversion efficiency is still much lower than the theoretical prediction, which may indicate the effect of inefficient charge transfer or charge recombination loss inside the cell. We have studied the photovoltaic characteristics of hybrid solar cells with transition metal doped ZnO layer as electron transport layers. Under dark condition, their J-V curves show a deviation from an ideal diode characteristic, which is indicated by current flow below the threshold voltage. Further investigations to clarify the photovoltaic characteristics of those cells were then carried out by the measurements of impedance spectroscopy. The impedance characteristics are dependent on the transport layer used, where the Nyquist plots of cells with TiZO exhibit larger impedance than those of cells with ZnO. However, those cells with larger impedance show better working performances. This fact was then interpreted as the result from smaller interfacial charge transfer loss, which may take place via charge carrier recombination at the interface of the transport layer and active layer, in the cells with TiZO.

Template-Free Preparation of Crystalline Ge Nanowire Film Electrodes via an Electrochemical Liquid–Liquid–Solid Process in Water at Ambient Pressure and Temperature for Energy Storage

The direct electrodeposition of crystalline germanium (Ge) nanowire film electrodes from an aqueous solution of dissolved GeO(2) using discrete 'flux' nanoparticles capable of dissolving Ge(s) has been demonstrated. Electrodeposition of Ge at inert electrode substrates decorated with small (<100 nm), discrete indium (In) nanoparticles resulted in crystalline Ge nanowire films with definable nanowire diameters and densities without the need for a physical or chemical template. The Ge nanowires exhibited strong polycrystalline character as-deposited, with approximate crystallite dimensions of 20 nm and a mixed orientation of the crystallites along the length of the nanowire. Energy dispersive spectroscopic elemental mapping of individual Ge nanowires showed that the In nanoparticles remained at the base of each nanowire, indicating good electrical communication between the Ge nanowire and the underlying conductive support. As-deposited Ge nanowire films prepared on Cu supports were used without further processing as Li(+) battery anodes. Cycling studies performed at 1 C (1624 mA g(-1)) indicated the native Ge nanowire films supported stable discharge capacities at the level of 973 mA h g(-1), higher than analogous Ge nanowire film electrodes prepared through an energy-intensive vapor-liquid-solid nanowire growth process. The cumulative data show that ec-LLS is a viable method for directly preparing a functional, high-activity nanomaterials-based device component. The work presented here is a step toward the realization of simple processes that make fully functional energy conversion/storage technologies based on crystalline inorganic semiconductors entirely through benchtop, aqueous chemistry and electrochemistry without time- or energy-intensive process steps.