Semiconductors

Energy demand is rising globally, and the transition to a clean energy economy cannot only rely on populations reducing their consumption. Instead, we need technologies that deliver the same services, such as lighting, computing, communication, transportation, while using dramatically less energy. Semiconductors are fundamental to this goal. They are the materials that enable solar panels to convert sunlight into electricity, LEDs to produce light far more efficiently than incandescent bulbs, and electronic devices to process information while consuming minimal power. Advances in semiconductor materials and devices directly translate to reduced energy generation requirements and lower costs.

What makes semiconductors special? Semiconductors occupy a middle ground between conductors (which carry electricity freely) and insulators (which block electricity). This intermediate behavior arises from their electronic structure: semiconductors have a "bandgap", an energy difference between the filled electron states and the empty states that electrons can move into. By absorbing energy (from light, heat, or electrical voltage), electrons can jump across this bandgap, enabling the material to conduct electricity or emit light.

The size of this bandgap determines what the semiconductor can do. A bandgap matching visible light energies allows efficient absorption or emission of light, this is critical for solar cells and LEDs. A smaller bandgap enables room-temperature electrical conductivity for transistors and circuits. A larger bandgap provides stability at higher temperatures or voltages. By choosing or engineering semiconductor materials with specific bandgaps, researchers can optimize performance for different applications.

Crucially, semiconductor properties can be controlled through composition (what elements the material contains), structure (how atoms are arranged), doping (adding small amounts of other elements), and nanoscale engineering (quantum dots, thin films, nanostructures). This tunability makes semiconductors extraordinarily versatile for energy applications.

RASEI research spans multiple semiconductor material classes, each with distinct advantages for different applications.

Inorganic semiconductors include traditional materials like silicon (the foundation of most solar panels and electronics) and compound semiconductors combining multiple elements. Silicon dominates because it's abundant, well-understood, and highly efficient. Modern silicon solar cells exceed 26% efficiency and silicon transistors enable all computing devices. However, silicon has limitations: it requires high-temperature processing (energy-intensive), is rigid and brittle (limiting applications), and has a fixed bandgap that isn't optimal for all uses.

RASEI researchers explore compound semiconductors, materials like gallium arsenide (GaAs), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS), which offer advantages for specific applications. These materials can achieve higher efficiencies than silicon in solar cells, emit light efficiently for LEDs and lasers, or operate at higher speeds for advanced electronics. The tradeoff is often higher cost or scarcer elements, making materials selection application dependent.

Metal oxide semiconductors include materials like zinc oxide (ZnO), titanium dioxide (TiO₂), and tin oxide (SnO₂). These oxides offer unique properties: transparency (enabling transparent electronics and solar cells), abundance and low toxicity (important for large-scale deployment), and stability in harsh environments. Metal oxides serve as transparent conductors in displays and solar cells, photoactive layers in sensors and photocatalysts, and electron transport materials in various devices. RASEI research addresses improving oxide conductivity, controlling defects that limit performance, and engineering interfaces for better charge extraction in solar cells and other devices.

Organic semiconductors are carbon-based materials, typically conjugated polymers or small molecules, that combine semiconducting properties with the advantages of organic materials: solution processability (can be printed or coated from liquids), mechanical flexibility, and low-temperature manufacturing. These properties enable applications difficult or impossible with rigid inorganic semiconductors: flexible displays, lightweight solar panels, wearable electronics, and large-area devices manufactured by printing rather than expensive vacuum deposition.

The challenge with organic semiconductors is performance, they typically conduct charge less efficiently and are less stable than inorganic materials. RASEI research focuses on molecular design to improve charge transport, understanding how molecular packing affects electronic properties, and addressing degradation mechanisms that limit device lifetime.

Quantum-confined semiconductors exploit nanoscale effects. When semiconductor structures are made small enough (quantum dots, nanowires, thin films just nanometers thick), quantum mechanical effects dominate, and material properties become size-dependent. A quantum dot's bandgap, and therefore what color light it absorbs or emits, can be tuned simply by changing its diameter. This tunability enables applications from precisely color-matched LEDs and displays to solar cells engineered to absorb specific portions of the solar spectrum. RASEI research explores quantum dot synthesis with precise size control, understanding how quantum confinement affects charge transport, and integrating quantum materials into devices.

RASEI's semiconductor research integrates theoretical understanding, materials synthesis, advanced characterization, and device fabrication. Computational modeling predicts how semiconductor structures will behave and guides materials design. Advanced synthesis techniques create semiconductors with precisely controlled composition and structure. Cutting-edge characterization, including cutting-edge spectroscopy, microscopy, and electrical measurements, reveals how electrons move through materials, where energy losses occur, and how structure affects performance. Device fabrication translates materials into functioning solar cells, LEDs, or transistors, providing real-world performance data that feeds back into materials design.

This integrated approach, from understanding semiconductor physics at a fundamental level to a maintained focus on device performance and energy impact, enables RASEI researchers to develop materials and architectures that push the boundaries of what semiconductors can do for energy applications. Semiconductor research directly affects how much energy is required to power modern society.