Direct observation of the potential distribution within organic light emitting diodes under operation

Abstract

Kelvin probe measurements and orientation polarization Previous Kelvin probe investigations of Alq 3 films have shown a constant surface potential increase with film thickness (up to 5 V per 100 nm) that is linked to spontaneous orientation polarization of the polar Alq 3 molecules but vanishes for various external stresses such as visible light or elevated temperature. [1,2] This permanent polarization density is around -1 mC/m 2 . [3][4][5][6] SKPM measurements reproduced the surface potential shift on bare Alq 3 films as well as in the areas that had been behind a shadow mask during metal evaporation. It corresponds to the potential of the film surface against the substrate in the case of depletion at the transition voltage. After deposition of a metal contact onto the Alq 3 layer no such shift was found neither on the metal surface (before FIB cutting) nor in the Alq 3 beneath (after FIB cutting with floating top contact). This means that deposition of a metal layer on top of the Alq 3 film triggers compensation of the orientation polarization by charge accumulation. SKPM raw dataThe SEM image of the cross section of an OLED was captured by a high resolution and therefore high dose electron beam after exposure of the cross section but before the polishing to avoid electron beam damage to the final, polished cross section. After a final polishing step the probe tip was positioned at the edge of the milled hole using the SEM with high scan rate to avoid carbon deposition. Subsequently, topography and SKPM images were taken. The SEM allowed for accurate identification of the different layers and alignment and calibration of the SPM. The electron beam was blanked for further data acquisition. Figure S1 shows the CPD profiles across the cross section of an OLED with an active layer of 224 nm Alq 3 and a) 224 nm NPB and b) 112 nm NPB at external bias voltage between a) -10 V and 5 V and b) -10 V and +10 V. The Al contact was kept on ground potential and the bias voltage was applied to the ITO contact. The 0 V profiles correspond to the work function difference between the cantilever tip and the sample. As electrostatic force is long-range, not only the very end of the probe tip interacts with the sample. The measured signal is also influenced by the sample interaction with the whole cantilever. [7][8][9] Although contacted to ground potential the measured CPD at the Al contact slightly changes its value upon different biases on the ITO. The corresponding measurement artifact on the ITO contact is even more pronounced because the FIB milled hole is significantly smaller than the cantilever so that most of the cantilever is positioned above and interacting with the Al contact. Therefore, the measured potential at the ITO contact is skewed towards the potential of the Al contact and smaller than the applied bias voltage. To facilitate the comparison of predicted and measured values shown in Fig. 2, we accounted for this measurement artifact by setting the ITO and Al on the nominal applied potential and extending the ...

Study of the Interface in a GaP/Si Heterojunction Solar Cell

“…The fill factor can be increased by highly n-doping the Si at the GaP/Si interface as also observed by others [24]. We performed cross-sectional KPFM measurements [49]- [51] that revealed a strong photovoltage drop at the GaP/Si interface. Altogether, these findings suggest a charge transport barrier at the interface between Si and GaP-caused by unfavorable band alignment-being the culprit of poor GaP/Si heterojunction solar cell performance.…”

“…Here, we report on an experimental investigation to determine the band alignment at the GaP/Si heterojunction interface, with GaP grown using methods similar to those which could be employed in a solar cell fabrication process. We present an extensive X-ray photoelectron spectroscopy (XPS), Kelvin probe (KP) [46], and cross-sectional KP force microscopy (KPFM) [47]- [51] study of GaP grown on high-carrier lifetime silicon by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). We found that the conduction band offset is significantly higher than predicted by the Anderson model, which provides a possible explanation for the observed device characteristics.…”

Watching Space Charge Build Up in an Organic Solar Cell

“…We point out that a direct observation of the potential distribution and the collection probability, as shown in Figure , would in principle be possible with scanning probe techniques such as Kelvin probe microscopy and electron‐beam‐induced currents . These techniques, however, are complex to conduct and usually not well suited to organic materials.…”

The Role of Cation Coordination in the Electrical and Optical Properties of Amorphous Transparent Conducting Oxides

“…However, the microscopic effects of compositional variation in ZTO on local and long-range order and on transport propertiescarrier generation, N e , and μ e are unclear. Studies of ZTO have primarily been in developing ZTO as a thin-film transistor or buffer layer for device incorporation, ,,,− post-deposition treatments, − or in developing low-cost, low-temperature growth methods. − Only recently have some authors begun to inspect the origins of transport properties from an electronic structure view of ZTO. ,,,− Many of these studies consider a narrow range of ZTO compositions, investigate ZTO films with carrier densities lower than those suitable for widespread optoelectronic applications, or do not satisfactorily provide information on the local coordination changes of atomic species with changing metal composition. In this study, we use a combinatorial sputtering approach for deposition and characterization of ZTO spanning 100% SnO 2 to 100% ZnO with compositional libraries containing a large set of varied Zn:Sn ratios, sputtered at 300 and 400 °C, resulting in films with optoelectronic device-relevant electrical performances.…”