Work function of ZnO

It must be considered the work function reduction was closely relative to the thickness of the polymer interlayer. For example, the work function of ZnO increased from 2.47 eV (4 nm PEI) to 3.39 eV (16 nm PEI). This was because with the increasing polymer interlayer thickness, the random molecule dipoles coimtervailed with each other and thus weakened the interfacial dipole effect between ZnO and the interlayer. When polymer interlayer was used in the inverted structure, the device performance was obviously enhanced. Fig. 7.4D demonstrates the L-Vcurves of PLEDs. The tum-on voltage (voltage at luminance of 1 cdm and... 

ZnO is one of the materials most commonly used as the ETL in OSCs owing to its attractive electrical properties, low work function, ease of preparation and good air stability. The low work function of ZnO allows the ETL to form ohmic contact with the active layers and ZnO is compatible with simple chemical bath methods such as roll-to-roll coating. It is a non-toxic semiconductor and has a wide bandgap of 3.37 eV. ... 

Excrements show that all the alkyl, hydroxyl, and amine radicals which we have studied considerably reduce the conductivity and increase the work function of oxide semiconductors like ZnO, Ti02, CdO, WO2, M0O3, etc. during chemisorbtion. It should be noted that the revealed effects are rather profound especially if we are dealing with the effect of chemisorbtion of active particles on conductivity of a thin (less than 1 pm) sintered polycrystal semiconductor films. Thus, conductivity of such films in the presence of free CH3-radicals with the concentration of even 10 cm and less may change from initial value by dozens or hundreds percent depending on experimental conditions. 

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

According to an increase of (EF -Ey) in the depletion layer in NiO, the activation energy on NiO is raised by the influence of Ag support. The activation energy on ZnO on Ag support is decreased according to the decrease of (Ec - EF) in the accumulation layer in ZnO. The electron work function of Co304 is smaller than that of Ag, so that even in the dark, electrons flow into the silver, generating an accumulation layer of defect electrons. (EF - Ev) is decreased and so is the activation energy. 

The work functions and ionization potentials of sputter-deposited ZnO and ZnO Al films are shown in Fig. 4.13. The different Fermi level positions of ZnO and ZnO Al for deposition at room temperature in pure Ar are also observed in the work function. The undoped films prepared under these conditions have a work function of 4.1eV, while the Al-doped films show values of 3.2eV. The difference is almost of the same magnitude as for the Fermi level position and, therefore, explained by the different doping level. Also the ionization potentials are almost the same under these preparation conditions. The work function of the undoped material is close to the value reported by Moormann et al. for the vacuum-cleaved Zn-terminated (0001) surface [20], The same authors report a work function of 4.95 eV for the oxygen terminated ZnO(OOOl) surface, which is in good agreement with the values obtained for films deposited with >5% oxygen in the sputter gas. Since the Fermi level position of the undoped ZnO films does not depend on the oxygen content in the sputter gas (Fig. 4.12), the different work functions correspond to different ionization potentials. 

Figure 4.31 shows ultraviolet photoelectron spectra recorded during the same interface experiment shown in Fig. 4.26. A clear transition from the Cu(In,Ga)Se2 valence band structure with a valence band maximum at 0.8eV binding energy to the ZnO valence band structure with a valence band maximum at 3eV is observed with increasing ZnO deposition. The well-resolved valence band features are enabled by the in situ sample preparation. Also very sharp secondary electron cutoffs are obtained, which allow for an accurate determination of work functions. The work functions of Cu(In,Ga)Se2 and ZnO are determined as 5.4 and 4.25 eV, respectively. These result in ionization potentials of 6.15 and 7.15 eV for Cu(In,Ga)Se2 and ZnO.

The n-type layer in the inverted configuration had mostly been ZnO, until Kippelen et al. introduced ethoxylated polyethylenimine (PEIE), which is deposited from water and 2-methoxyethanol on a thermally annealed PEDOT PSS to give a coating of only a few nanometers. This layer induces a modification of the work function of the PEDOT PSS at the top surface from 4.9 to 3.6 eV, enabling selective electron collection. The effect is thought to originate from a molecular dipole and interface dipole. ... 

It should be noted that dissociation of surface complexes of oxygen in polar solvents on semireduced ZnO films is presumably justified from the thermodynamic point of view as oxygen adsorption heat on ZnO and electron work function are [58] 1 and approximately 5 eV respectively while the energies of affinity of oxygen molecules to electron, to solvation of superoxide ion and surface unit charge zinc dope ions are 0.87, 3.5, and higher than 3 eV, respectively [43]. 

Electronic surface properties including Fermi level positions, work functions, and ionization potentials of sputter-deposited ZnO and Al-doped ZnO films in dependence on deposition parameters. The results provide insight into aspects of doping, surface chemistry, and terminations. 

Fig. 4.13. Work function and ionization potential of magnetron sputtered ZnO and ZnO Al films in dependence on oxygen content in the sputter gas for samples deposited at room temperature (left) and in dependence on substrate temperature for deposition in pure Ar (right). The values are derived from He I excited valence band spectra. All films were deposited using a total pressure of 0.5 Pa, a sputter power density of 0.74 Worn 2 and a substrate to target distance of 10 cm...

Covalent semiconductors, such as Si or GaAs, have a large density of surface states in the band gap, causing pinning of the barrier [83]. On the other hand, ionic semiconductors, such as ZnO, have low density of surface states in the band gap, and the barrier height does indeed depend on the work function difference between the metal and the semiconductor. It has been shown that for strongly ionic semiconductors the barrier height will indeed be equal to or proportional to the... 

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