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Advanced Modes of Electrostatic and Kelvin Probe Force Microscopy for Energy Applications
Published in Cai Shen, Atomic Force Microscopy for Energy Research, 2022
Martí Checa, Sabine M. Neumayer, Wan-Yu Tsai, Liam Collins
As an example, in Figure 2.8a–c, we highlight the work by Sadewasser et al.,167 who utilized KPFM in UHV to map potential profiles across individual grain boundaries for a CuGaSe2 thin film grown on Mo-covered glass by physical vapor deposition (PVD). The samples showed a typical granular topographic structure, and by comparing with KPFM potential captured under dark/light conditions, it was possible to deduce a reduced surface band bending upon illumination. In addition, as shown in Figure 2.8c, the work function dip with respect to the dark measurement is different for individual grain boundaries, suggesting differences in electronic behaviors exist.167,168 In the context of solar cell operation, knowledge of the band bending and nanoscale variation is extremely important as it can have a real impact on the transfer and collection of the charge carriers. KPFM has also been extensively applied to map variations in SPV, i.e., the change in the work function with illumination (Φilluminated − Φdark). Glatzel et al.164 performed KPFM under both dark and illuminated conditions necessary to extract locally resolved SPV for conjugated polymer/fullerene organic solar cells, as shown in Figure 2.8d. Recently, Garrett et al.169 combined KPFM under illumination with high speed KPFM was realized using heterodyne KPFM (see Figure 2.8e). They demonstrated mapping of the SPV (or open circuit potential) on hybrid organic/inorganic perovskite films at an imaging rate of ~16 frames/second.
Electrical Spin Injection and Transport in Semiconductors
Published in Evgeny Y. Tsymbal, Igor Žutić, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
One avenue is to take advantage of the band bending which occurs at the metal/semiconductor interface. This approach exploits a natural characteristic of the interface, and avoids the use of a discrete barrier layer and the accompanying problems with pinholes and thickness uniformity. Schottky contacts are also routine ingredients in semiconductor technology. In the case of an n-type semiconductor, electrons are transferred into the metal, depleting the semiconductor interfacial region and causing the CB to bend upward, forming a pseudo-triangular-shaped barrier with a quadratic falloff with distance into the semiconductor [101]. The depletion width associated with the Schottky barrier depends upon the doping level of the semiconductor, and is generally far too large to allow tunneling to occur. For example, in n-GaAs, the depletion width is on the order of 100 nm for n ∼ 1017 cm−3, and 40 nm for n ∼ 1018 cm−3 [102]. However, this width can be tailored by the doping profile used at the semiconductor surface [103]. Heavily doping the surface region can reduce the depletion width to a few nanometers, so that electron tunneling from the metal to the semiconductor becomes a highly probable process under reverse bias.
Effect of Dye Concentration on Series Resistance of Thionin Dye-Based Organic Diode
Published in Nazmul Islam, Satya Bir Singh, Prabhat Ranjan, A. K. Haghi, Mathematics Applied to Engineering in Action, 2021
Pallab Kumar Das, Swapan Bhunia, Sarmistha Basu, N. B. Manik
The LUMO level shifts down near the ITO/thionin interface due to the transfer of holes from ITO to thionin. The LUMO level also goes up near the thionin/Al interface due to the transfer of electrons from Al to thionin. The band bending effect for the migration of charge carriers produce a potential barrier near interfaces. Due to this barrier, there is no flow of charge carriers at equilibrium. In this case, a forward bias is applied by connecting the positive terminal of the source to ITO and the negative terminal to Al. When the applied voltage is greater than the potential barrier of the interface the carriers are transported. But as organic semiconductors are disorder in nature and very much prone to traps, a large part of the charge carriers injected from the electrode are being trapped in these trap energy states at the interface which is shown in Figure 6.6. So the carriers will not come out from the electrode and the current, as well as the mobility of the charge carriers, becomes very low. At higher voltage, these trapped carriers in between HOMO and LUMO may be released or recombined with the opposite charge carriers which increase the current for the device. But due to the disorder of the organic material, presence of traps, recombination of carriers at the metal-semiconductor interface, the current flow through the device is very low. So the values of electronic parameters η, Rs of this organic device become unusually high values. Also, it is expected that the charge trapping in the metal-organic dye interface is responsible for this high value of Rs. By increasing the dye concentration, the no of charge carriers are increased which in turn increase the device performance.
Understanding the fundamentals of TiO2 surfaces
Published in Surface Engineering, 2022
Band bending effects resulting from water adsorption on rutile (110) surface. The shift of the upper edge of the O2p VB Δ (Ev − EF) relative to the Fermi level, due to H2O exposure, is shown in the insert in Figure 22a [99]. The direction of this band bend is consistent with the presence of positively charged surface species (see Figures 22 and 23) [99,134]. The electrons will transfer from the donor molecules, such as H2O, to the semiconductor, such as TiO2-x, which due to this process is enriched with electrons [99,134]. The existence of a positive charge at the interface enhances the concentration of electrons near the surface within a so-called accumulation layer. In this case, an electric field is set up thereby causing downward band bending.