Free Carriers

Definition

Free carriers are charge carriers, such as electrons or holes, that are mobile within a material, contributing to electrical conductivity. In organic semiconductors, free carriers arise from the excitation of electrons from the valence band to the conduction band, creating electron-hole pairs. The presence and mobility of these free carriers are crucial for the operation of devices like organic photovoltaics, as they facilitate the transport of charge, ultimately leading to energy conversion.

5 Must Know Facts For Your Next Test

1. Free carriers in organic semiconductors are generated when photons excite electrons from the valence band into the conduction band, resulting in electron-hole pairs.
2. The efficiency of organic photovoltaic devices heavily depends on the generation and mobility of free carriers, as they are essential for transporting charge to the electrodes.
3. Free carrier concentration is influenced by factors such as temperature, doping levels, and the energy gap between the conduction and valence bands.
4. In organic materials, free carriers tend to have lower mobility compared to inorganic semiconductors due to greater disorder and lower crystallinity.
5. The presence of defects and impurities in organic semiconductors can trap free carriers, impacting their mobility and ultimately reducing device efficiency.

Review Questions

• How do free carriers contribute to electrical conductivity in organic semiconductors?
  • Free carriers contribute to electrical conductivity by allowing charge transport within the material. When photons excite electrons from the valence band to the conduction band, it generates mobile charge carriers, both electrons and holes. These free carriers can move through the semiconductor when an electric field is applied, facilitating current flow and thus enabling the semiconductor's function in devices like organic photovoltaics.
• Discuss the impact of temperature on the generation and behavior of free carriers in organic semiconductors.
  • Temperature plays a significant role in both the generation and mobility of free carriers in organic semiconductors. As temperature increases, more electrons gain enough energy to jump from the valence band to the conduction band, increasing the concentration of free carriers. However, higher temperatures can also lead to increased lattice vibrations, which can scatter free carriers and reduce their mobility. This creates a trade-off that affects overall device performance.
• Evaluate how defects within organic semiconductors influence the behavior of free carriers and device performance.
  • Defects within organic semiconductors can have a profound impact on free carrier behavior. Defects may act as traps that capture free carriers, effectively reducing their mobility and hindering charge transport. This leads to decreased efficiency in devices like organic photovoltaics since trapped carriers cannot reach electrodes effectively for energy conversion. Understanding and controlling these defects is crucial for improving device performance and optimizing material properties.

Related terms

Conduction Band: The energy band in a semiconductor where electrons can move freely, contributing to electrical conduction.

Valence Band: The energy band in a semiconductor that contains the electrons involved in bonding; electrons in this band are not free to move.

Charge Carrier Mobility: A measure of how quickly charge carriers can move through a semiconductor material when subjected to an electric field.