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Semiconductor band theory goes a long way to explain how different types of semiconducting devices operate. Determining the energies of electronic states (energy bands) in semiconducting materials allows us predicting which way positive and negative charges will move in order to achieve the most energetically stable state and which pathway imposes the lowest energy barrier. Solid materials contain electronic bands that are either filled with electrons or are empty. The filled energy band with the highest energy is called the valence band (VB) and the unfilled energy band with the lowest energy is called the conduction band (CB).
In the case of organic semiconductors and conducting polymers, these bands arise from molecular orbitals on the conjugated π-system of the material and are referred to as the highest occupied molecular orbital (HOMO) band and the lowest unoccupied molecular orbital (LUMO) band. The difference in energy between the VB and CB, or HOMO and LUMO band is the bandgap (Eg). Placing an extra electron in the CB or LUMO band creates a mobile negative charge called an electron, while removing one electron from the VB or HOMO creates a positively charged vacancy called a hole that is also mobile.
Photons (light) are able to interact with electrons in semiconductors in a few ways. If the energy of a photon is greater than Eg, it may promote a bound electron from the VB to the CB, or from the HOMO to the LUMO. If the excited electron is able to undergo electron transfer to a different material, or is swept away by an electric field before it is able to decay back to the VB or HOMO, then it can be extracted from the semiconductor as an electrical current. This is the basis of solar cell devices. The opposite process, called electroluminescence, occurs when free electrons and holes are injected from electrodes into the CB (or LUMO) and VB (or HOMO) of a semiconductor, and the electrons decay to the lower energy VB or HOMO bands with the emission of photons. The energy of the photons is equal to the Eg of the material and this process forms the basis for light emitting diode (LED) devices.
Energy bands for different materials can be measured by techniques like ultraviolet photoelectron spectroscopy, cyclic voltammetry or calculated by techniques like density functional theory (DFT). In general, electrons tend to move down in energy while holes move up. Applying a postive or negative electrical potential will cause charge carriers to move towards or away from the electrode depending on the sign of their charge. These principal govern how electrical charges move in semiconducting devices.
Charge carriers (electrons and holes) exist in organic semiconductors and conjugated polymers as chemical moieties called radical cations and radical anions. The annihilation of radical cations and radical ions in the solid state constitutes the fundamental mechanism behind light emission in devices like light-emitting electrochemical cells (LECs) and organic light-emitting diodes (OLEDs). Conversely, organic solar cells function by converting photo-generated excited states into pairs of radical cations and radical anions. Despite the fundamental importance of these chemical species, relatively little synthetic work has been done in working with this interesting class of compounds.
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Solar Cells
All types of solar cell function by the same basic principal of converting photons into separated electrical charges, and extracting the electrical charges to two electrodes. Each class of solar cell differs a little bit in how the charges are generated and separated. This animation summarizes how organic polymer solar cells generate and separate electrical charges.
All solar cells have the same basic electrical parameters which can be measured by applying a varying electrical potential to the device and measuring the current. The most important quantity is the power conversion efficiency, which quantifies how efficiently the device converts sunlight into electrical energy and can be calculated by dividing the power generated at the maximum power point by the power of incident light being shined on it.
LEDs
LEDs perform essentially the opposite function of solar cells, converting positive and negative electrical charges into electronically excited states which are able to emit photons.
FETs
Field effect transistors are 3-electrode devices which use the voltage applied to one electrode (the gate electrode) to control the current between two other electrodes (the source and drain electrodes. This device may seem complicated at first, but it is only a little more complicated than a diode. This animation shows schematically how organic field effect transistors operate.
Light Emitting FETs
A light emitting field effect transistor is able to emit light when current passes through an emissive layer after passing through the transport channel of an FET. In this way, a small signal applied to the gate electrode is able to regulate a large current through the emissive layer and cause the device to turn on and off. This has the potential to allow flat-panel displays to be fabricated without any silicon back-plane.
Air Sensitive Chemistry
Tips on how to carry out air sensitive chemistry here.
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