Band bending formation interface

Although the observations for PPV photodiodes of different groups are quite similar, there are still discussions on the nature of the polymer-metal contacts and especially on the formation of space charge layers on the Al interface. According to Nguyen et al. [70, 711 band bending in melal/PPV interfaces is either caused by surface states or by chemical reactions between the polymer and the metal and... 

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

Fig. 4.12 Diagram illustrating space charge layer formation in microcrystalline and nanocrystalline particles in equilibrium in a semiconductor-electrolyte interface. The nanoparticles are almost completely depleted of charge carriers with negligibly small band bending.

The magnitude of the injection barrier is open to conjecture. Meanwhile there is consensus that energy barriers can deviate significantly from the values estimated from vacuum values of the work-function of the electrode and from the center of the hole and electron transporting states, respectively. The reason is related to the possible formation of interfacial dipole layers that are specific for the kind of material. Photoelectron spectroscopy indicates that injection barriers can differ by more than 1 eV from values that assume vacuum level alignment [176, 177]. Photoemission studies can also delineate band bending close to the interface [178]. 

Fig. 4.38. Energy band diagrams at In2S3/ZnO interfaces as determined from photoelectron spectroscopy. The material used as substrate during interface formation is shown to the left. I112S3 films were deposited by evaporation onto substrates held either at room temperature or at 250°C. ZnO Al films were prepared by dc magnetron sputtering at room temperature in pure Ar or with 10% O2 in the sputter gas as indicated at the top. All values are given in electronvolt. Band bending at the interface is <0.2 eV in all experiments. Because of the uncertainty in the band gap, the conduction band positions of In2S3 are given as dotted lines. Reproduced with permission from [136]...

Fig. 9.3. Band bending and space charge layer formation at an n-type semiconductor-electrolyte interface (a) accumulation layer,...

A similarly high Voc for ITO/PPV/Al photovoltaic devices also was observed by other groups. Jenekhe et al. [63, 64] report the observation of a quantum efficiency IPCE of 5% in ITO/PPV/Al photodiodes and of a power conversion efficiency of approximately 0.1% under low light intensities of 1 mW/cm. The typical film thickness of their devices was varied between 100 to 600 nm. The open circuit voltage of these devices, as defined with respect to the ITO electrode, was measured as 1.2 V. The high open circuit voltage was explained by the formation of a Schottky barrier at the Al/PPV interface. The predicted band bending due the PPV/Al interface formation was verified by XPS measurements [65, 66]. 

Mechanism 1 involves two different electron transfer steps, so that the meaning of kt, needs to be clarified. Both mechanisms involve an adsorbed hydrogen intermediate, and this may have consequences for the potential distribution across the interface and hence the band bending. The formation of adsorbed intermediates is a common feature of multi-electron transfer reactions. Other examples are encountered in the photodecomposition of compound semiconductors, for example the photoanodic decomposition of n-CdS ... 

Energy-band diagram of the band bending interface in the presence of surface states for a metal/n-type semiconductor (a) under flat band condition and (b) after contact formation. 

Fig. 18a, b. (a) Band bending for the interface formation of Rb on p- and n-type GaSb(l 10) at room temperature, (b) Band bending for the interface formation of Cs on p and n-type GaSb(l 10) at room temperature. From [94Sch],... 

J. L. Shaw et al.. Chemically Controlled Deep Level Formation and Band-Bending at Metal-CdTe Interfaces, Appl. Phys. Lett. 1988, 53(18), 1723-1725. 

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