量子ドット太陽電池とペロブスカイト太陽電池における界面エンジニアリングと界面電荷移動ダイナミクス

電気通信大学201

Strategies for controlled electronic doping of colloidal quantum dots

Over the last several years tremendous progressed progress has been made in incorporating Colloidal Quantum Dots (CQDs) as photoactive components in optoelectronic devices. A significant part of that progress is associated with significant advancements made in achievingon controlled electronic doping of the CQDs and thus improving the electronic properties of CQDs solids. Today, a variety of strategies exists towards that purpose and this minireview aims at surveying major published works in this subject. Additional attention is given to the many challenges associated with the task of doping CQDs as well as to the optoelectronic functionalities and applications being realized when successfully achieving light and heavy electronic doping of CQDs.Peer ReviewedPostprint (author's final draft

Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells

The implementation of a wide range of novel concepts for next-generation high-efficiency solar cells is based on nanostructures with configuration-tunable optoelectronic properties. On the other hand, effective nano-optical light-trapping concepts enable the use of ultra-thin absorber architectures. In both cases, the local density of electronic and optical states deviates strongly from that in a homogeneous bulk material. At the same time, non-local and coherent phenomena like tunneling or ballistic transport become increasingly relevant. As a consequence, the semi-classical, diffusive bulk picture conventionally assumed may no longer be appropriate to describe the physical processes of generation, transport, and recombination governing the photovoltaic operation of such devices. In this review, we provide a quantum-kinetic perspective on photovoltaic device operation that reaches beyond the limits of the standard simulation models for bulk solar cells. Deviations from bulk physics are assessed in ultra-thin film and nanostructure-based solar cell architectures by comparing the predictions of the semi-classical models for key physical quantities such as absorption coefficients, emission spectra, generation and recombination rates as well as potentials, densities and currents with the corresponding properties as given by a more fundamental description based on non-equilibrium quantum statistical mechanics. This advanced approach, while paving the way to a comprehensive quantum theory of photovoltaics, bridges simulations at microscopic material and macroscopic device levels by providing the charge carrier dynamics at the mesoscale.Comment: 22 pages, 8 figures; review article based on an invited talk at the MRS Spring Meeting 2017 in Phoeni

Copper Indium Sulfide Quantum Dots for Light Selective Nanocomposite Thin Films and Solar Cell Applications

Nanotechnology is allowing the solar energy industry to advance at an accelerating rate, although new materials and processes are required for developing new types of solar cells. Similar to other industries, it is desirable to develop the most environmentally friendly and cost-effective solutions on how to make these next generation materials. Of these new materials, quantum dots (QDs) are of current scientific interest which provide record-breaking increases in efficiency and a new approach for harnessing solar radiation. However, most previous QD work has focused on lead or cadmium based materials, which are not earth friendly and have low thresholds in both California and European legislation. For this reason, this work examines the earth friendly and abundant materials Copper Indium Sulfide (CIS) QDs, i.e. CIS-QDs, which have favorable emission properties. These materials were prepared and examined for use in solar harvesting in photovoltaic (PV) devices. Copper Indium Sulfide (CIS) QDs were synthesized using three different synthesis techniques, then compared based on their optical and size-dependent properties. Two techniques followed a hydrothermal batch reaction process, referred to as hot injection (HI) and heat up (HU) techniques, that are differentiated by the time at which the sulfur component is added to the reaction medium. The third technique was based on a continuous microfluidic approach. Results showed that the QDs produced from the HU and HI methods have a chalcopyrite structure, with their optical properties being highly dependent on their size and elemental composition. QDs produced from the microfluidic approach were found to agglomerate quickly and had a resulting weak photoluminescent response. This work examined these QDs in two separate solar applications, both for use in light spectrum conversion with solar films and for use in third generation solar cells. For application in light spectrum conversion, the QDs were melt-mixed with ethylene-vinyl-acetate plastic, using a twin-screw extruder and pressed into thin films using a Carver hydraulic press and Universal film maker. QDs were also reviewed for their use in third generation solar cell configurations. Based on the optimal configuration, QD sensitized solar cells were fabricated and tested. Resulting current-voltage (IV) curves and solar cell data showed a direct relation between QD composition and cell efficiency

Photo-FETs: phototransistors enabled by 2D and 0D nanomaterials

The large diversity of applications in our daily lives that rely on photodetection technology requires photodetectors with distinct properties. The choice of an adequate photodetecting system depends on its application, where aspects such as spectral selectivity, speed, and sensitivity play a critical role. High-sensitivity photodetection covering a large spectral range from the UV to IR is dominated by photodiodes. To overcome existing limitations in sensitivity and cost of state-of-the-art systems, new device architectures and material systems are needed with low-cost fabrication and high performance. Low-dimensional nanomaterials (0D, 1D, 2D) are promising candidates with many unique electrical and optical properties and additional functionalities such as flexibility and transparency. In this Perspective, the physical mechanism of photo-FETs (field-effect transistors) is described and recent advances in the field of low-dimensional photo-FETs and hybrids thereof are discussed. Several requirements for the channel material are addressed in view of the photon absorption and carrier transport process, and a fundamental trade-off between them is pointed out for single-material-based devices. We further clarify how hybrid devices, consisting of an ultrathin channel sensitized with strongly absorbing semiconductors, can circumvent these limitations and lead to a new generation of highly sensitive photodetectors. Recent advances in the development of sensitized low-dimensional photo-FETs are discussed, and several promising future directions for their application in high-sensitivity photodetection are proposed.Peer ReviewedPostprint (author's final draft

Emerging Thin Film Solar Panels

Utilizing of photovoltaics (PVs) has been rapidly developing over the past two decades due to its potential for transition from fossil fuels to renewable energy based economies. However, PVs as fuel less energy sources will be sustainable if some issues such as raw materials abundance, production cost, and environmental impacts carefully addressed in their value chains. Among PV technologies, thin film solar panels have been illustrated the potential to reach the sustainability. In this chapter we review some studies about environmental impacts of thin film PVs through life cycle assessment (LCA) and some environmental fate modeling. For the PV technologies, LCA studies need to be conducted to address environmental and energy impacts and encourage the development of PV technologies in a better sustainable way. Three methods of impact assessment in LCA are reviewed and compared, namely, Energy Payback Time (EPBT), Cumulative Energy Demand (CED), and Greenhouse Gases (GHG) emission rate, owing to data and information published in the literature. Generally, most results show promising potential of emerging thin film PVs, especially perovskite solar cells, to reach the best sustainable solution among PV technologies in near future

Thermal and Mechanical Energy Harvesting Using Lead Sulfide Colloidal Quantum Dots

The human body is an abundant source of energy in the form of heat and mechanical movement. The ability to harvest this energy can be useful for supplying low-consumption wearable and implantable devices. Thermoelectric materials are usually used to harvest human body heat for wearable devices; however, thermoelectric generators require temperature gradient across the device to perform appropriately. Since they need to attach to the heat source to absorb the heat, temperature equalization decreases their efficiencies. Moreover, the electrostatic energy harvester, working based on the variable capacitor structure, is the most compatible candidate for harvesting low-frequency-movement of the human body. Although it can provide a high output voltage and high-power density at a small scale, they require an initial start-up voltage source to charge the capacitor for initiating the conversion process. The current methods for initially charging the variable capacitor suffer from the complexity of the design and fabrication process. In this research, a solution-processed photovoltaic structure was proposed to address the temperature equalization problem of the thermoelectric generators by harvesting infrared radiations emitted from the human body. However, normal photovoltaic devices have the bandgap limitation to absorb low energy photons radiated from the human body. In this structure, mid-gap states were intentionally introduced to the absorbing layer to activate the multi-step photon absorption process enabling electron promotion from the valence band to the conduction band. The fabricated device showed promising performance in harvesting low energy thermal radiations emitted from the human body. Finally, in order to increase the generated power, a hybrid structure was proposed to harvest both mechanical and heat energy sources available in the human body. The device is designed to harvest both the thermal radiation of the human body based on the proposed solution-processed photovoltaic structure and the mechanical movement of the human body based on an electrostatic generator. The photovoltaic structure was used to charge the capacitor at the initial step of each conversion cycle. The simple fabrication process of the photovoltaic device can potentially address the problem associated with the charging method of the electrostatic generators. The simulation results showed that the combination of two methods can significantly increase the harvested energy

Stabilization of Black Perovskite Phase in FAPbI3 and CsPbI3

Although halide perovskites allow a great versatility, the application on single-absorber solar cells restricts significantly the number of available materials. In this context, CsPbI3 and FAPbI3 (FA, formamidinium) present a huge potential for the inorganic approach with enhanced stability and narrow bandgap, respectively. However, for these materials, Cs+ and FA+ are relatively too small and too big to stabilize the perovskite black phase at room temperature, both presenting a nonphotoactive yellow phase as the most stable phase. This fact limits dramatically their application and also helps in the understanding of the main research lines in the halide perovskite photovoltaic field in the quest for the stabilization of FAPbI3. In this Perspective, we present an overview of different strategies for the stabilization of the perovskite black phase of these two materials. We evaluate the stability approaches envisioning efficient and stable materials, with a particular focus on the positive and limiting aspects of the exploitation of low dimensionality and chemi-structural mechanisms

Bandgap engineering in semiconductor alloy nanomaterials with widely tunable compositions

Over the past decade, tremendous progress has been achieved in the development of nanoscale semiconductor materials with a wide range of bandgaps by alloying different individual semiconductors. These materials include traditional II-VI and III-V semiconductors and their alloys, inorganic and hybrid perovskites, and the newly emerging 2D materials. One important common feature of these materials is that their nanoscale dimensions result in a large tolerance to lattice mismatches within a monolithic structure of varying composition or between the substrate and target material, which enables us to achieve almost arbitrary control of the variation of the alloy composition. As a result, the bandgaps of these alloys can be widely tuned without the detrimental defects that are often unavoidable in bulk materials, which have a much more limited tolerance to lattice mismatches. This class of nanomaterials could have a far-reaching impact on a wide range of photonic applications, including tunable lasers, solid-state lighting, artificial photosynthesis and new solar cells

High Open Circuit Voltage Solar Cells based on bright mixed-halide CsPbBrI2 Perovskite Nanocrystals Synthesized in Ambient Air Conditions

Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution processed semiconductors since they possess high photoluminescence quantum yield (PLQY), and colour tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a high PLQY in film (65%). Solar cell devices operating in the wavelength range 350-660 nm were fabricated in air with two different deposition methods. The solar cells display a photo-conversion efficiency of 5.3% and open circuit voltage (Voc) up to 1.31V, among the highest reported for perovskite based solar cells with band gap below 2eV, clearly demonstrating the potential of this material.Peer ReviewedPostprint (author's final draft