Built-in electric field

A schematic representation of a PR apparatus is shown in Figure 2. In PR a pump beam (laser or other light source) chopped at frequency 2 creates photo-injected electron-hole pairs that modulate the built-in electric field of the semiconductor. The photon energy of the pump beam must be larger than the lowest energy gap of the material. A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser. (At elevated temperatures a more powerful pump must be employed.)... 

For sufficiendy high built-in electric fields the electromodulation spectrum can... 

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

In conclusion, nanorods are a potentially interesting material, but present results still do not allow understanding of whether the nanostructure leads to an improvement of the intrinsic photocatalytic behaviour, or whether other factors (accessible surface area, enhanced adsorption, etc) are responsible for the observed differences. In ZnO nanorods have been shown quite recently by surface photovoltage spectroscopy that the built-in electrical field is the main driving force for the separation of the photogenerated electron-hole pairs.191 This indicates that the nano-order influences the photophysical surface processes after photogeneration of the electron-hole pairs. A similar effect could be expected for Titania nanorods. However, present data do not support this suggestion, mainly due to the absence of adequate photo-physical and -chemical characterization of the materials and surface processes. 

While for a solar water splitting cell, light is directly absorbed by the semiconductor electrode (anode or cathode). The separation of electron-hole pairs is achieved in the built-in electric field near the semiconductor surface. The electric field is formed due to the charge transfer between the semiconductor electrode and the electrolyte as schematically shown in Fig. 17.5(b) [28]. Take an n-type semiconductor electrode for example... 

This has implications for the design of high-surface-area solar cells in general If the bulk of the device is essentially field-free at equilibrium, then mobile electrolyte and nanoporosity are required to eliminate the photoinduced electric fields that would otherwise inhibit charge-carrier separation. On the other hand, if the particle size is substantially larger than in the conventional dye cell or if there is no mobile electrolyte, then an interfacial or bulk built-in electric field... 

In most a-Si H solar cells, a built-in electric field (F) assists in the collection of photogenerated carriers, and efficient collection occurs as long as the drift length (/itF) is significantly larger than the film thickness. Crandall (1982) has shown that the transport in p-i-n cells can be charac-... 

Figure 67 The current-field characteristics of a DL electropho-sphorescent organic LED based on the metallo-organic phosphor Ir(ppy)3 (for the molecular structure see Fig. 31). The energy levels of the LED structure are given in the inset. The j(F) curves are well reproduced from run to run except the lowest field region, where the built in electric field (.Fbi = 2 x 105 V/cm), due to the difference in the work functions of the electrodes, becomes comparable with the applied field. After Ref. 304. Copyright 2002 American Physical Society.

Figure VII-3 Band diagram of a polymer LED. At zero bias, the Fermi level must be constant across the device. The asymmetry of the metal work functions causes the energy bands in the polymer to be tilted and results in a built-in electric field, which leads to the photovoltaic effect. With a forward bias, electrons and holes tunnel across the barrier into the TT and tt bands, respectively.

Since the cross section of planar surface cell LECs can be imaged by a microscope, a number of interesting experiments can be done to investigate the operation mechanisms. In a p-n diode, there is a built-in electric field at the p-n junction. This field can be measured by optical beam induced current (OBIC) microscopy, a technique in which a focused laser beam is scanned across the device while the photocurrent is monitored. When the beam excites a region that... 

Devices with luminescent polymer films sandwiched between high and low work function electrodes were originally fabricated to be LEDs. These same devices, however, can be operated as photodetectors or photovoltaic cells. With no externally applied bias, the polymer layer has a built-in electric field, because of the difference in work function of the two electrodes, which tilts the energy bands (Fig. VII-3). When light is absorbed by the polymer, some of the electron-hole pairs that are created are separated by the electric field. The holes are then pushed by the field to one electrode and the electrons are pushed to the other anode. The carriers that reach the electrodes provide a voltage that can either be used as a measure of the light intensity or as a source of energy. 

Many of the photogenerated electron-hole pairs recombine before they are separated by the built-in electric field. To maximize the built-in electric field, one electrode should have a work function which matches the tt- band of the polymer and the other electrode should have a work function which matches the ir -band. Even under these conditions, however, the quantum efficiency is typically only 0.05% [183]. In photodetectors the photosensitivity is usually increased by applying a reverse bias to the device [183,313]. 

The condition to obtain electromodulation (electro- and photo-reflectance) spectra is the existence of a built-in electric field in a structure under investigations. This condition is usually fulfilled in majority structures. Typical CER method utilizes a capacitor-like system with one top semitransparent electrode and one bottom copper-block electrode. The sample is glued to the bottom electrode by using a silver pasta. The front electrode is separated from the sample surface by a spacer (e.g. 0.1 mm). Thus there is nothing in direct contact with the sample. It means that the sample does not conduct any currents and the external electric field is able to change the carriers distribution inside it. Note that the voltage drop appears mainly in the air gap between the front electrode and the sample. The limit for the applied voltage is the electric breakdown in this air gap. It means that the maximal amplitude of EM in the CER technique usually is more limited than the EM amplitude in ER or PR techniques. 

It has been shown that contactless electroreflectance spectroscopy is an excellent experimental technique to study the built-in electric field in AlGaN/GaN heterostructures as well as the band gap discontinuity in GalnNAsSb/GaAs quantum wells. 

The application of MWNTs in the field of polymer solar cells was presented by Ago et al. [311]. Here, a layer of CNTs served as a replacement for the common ITO hole-collecting electrode in a single layer PPV/Al diode (Fig. 64). The authors related the twofold enhancement of the EQE observed with the MWNT based device to the formation of a complex network with an increased interface area between MWNTs and PPV, in addition to a stronger built-in electric field as a result of the higher work function of MWNTs compared to the standard ITO electrode [311,312]. To determine whether electron or energy transfer processes dominate within MWNT PPV blends, their photophysical properties were studied by photoluminescence and PIA spectroscopy. The results confirmed nonradiative energy transfer from PPV singlet excitons to the MWNTs as the main electronic interaction [313]. 

Figure 2a. Band picture with conduction band (CB) and valence band (VB) in an n- and a p-type region with the Fermi level Ef identified, which usually lies close to the donor-tEp) or acceptor level (Ea) respectively. Figure 2b. A pn junction when both regions are joined resulting in the junction region with a barrier height of eVt, of the built-in electric field.

When the contact is produced via an ideal intrinsic semiconductor of thickness d, then a built-in electric field Fbi is established within the semiconductor ... 

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