Namsu Kim

A hybrid encapsulation method for organic electronics

We report a thin-film encapsulation method for organic electronics that combines the deposition of a layer of SiOx or SiNx (100 nm) by plasma enhanced chemical vapor deposition followed by a layer of Al2O3 (10–50 nm) by atomic layer deposition and a 1-μm-thick layer of parylene by chemical vapor deposition. The effective water vapor transmission rates of the encapsulation was (2±1)×10−5 g/m2 day at 20 °C and 50% relative humidity (RH). The encapsulation was integrated with pentacene/C60 solar cells, which showed no decrease in conversion efficiency after 5800 h of exposure to air demonstrating the effectiveness of the encapsulation methodology.

Thermal transport properties of thin films of small molecule organic semiconductors

A series of harmonic Joule-heating experiments have been employed to determine the thermal conductivities of thin films of pentacene, N,N′-diphenyl−N,N′-di(3-methylphenyl)−(1,1′-biphenyl)-4,4′-diamine, and tris(8-hydroquinolinato)aluminum, three widely used organic semiconductors. Room-temperature thermal conductivity values of 0.51, 0.24, and 0.48W∕mK were measured for films of these three compounds, respectively. These values are over two orders of magnitude lower than those of inorganic semiconductors. While amorphous films were found to display only small thermal conductivity changes over the temperature range of 228–350 K, pentacene exhibited stronger variations that are typical of phonon-phonon scattering observed in polycrystalline semiconductors.

The development of thin film barriers for encapsulating organic electronics

In this work, we investigate several approaches to the development of encapsulation of organic electronics. A combination of plasma enhanced chemical vapor deposition, atomic layer deposition, and physical vapor deposition are used to make single layer and multilayer thin films and study the impact of structure on effective water vapor transmission rates. It was found that multilayer thin films consisting of organic and inorganic layers as well as inorganic nanolaminates provided the highest performance barrier films with effective water vapor transmission rates less than 5 × 10−5 g/m2/day. Materials such as atomic layer deposition deposited Al2O3 also showed excellent initial performance, but were found to be susceptible to corrosion from water. Combining alumina with other materials was found to improve the long-term performance of the alumina films. Integration of these films into organic solar cell platforms was shown to effectively maintain shelf lifetime performance for more than 7000 h.

Determination of moisture ingress through various encapsulants used in CIGS PV module under continuously varying environment

It has been reported that water vapor ingress and saturation of encapsulant materials dominantly impact on longterm reliability of copper indium gallium selenide PV cells. To understand diffusion mechanism of water vapor into PV module, diffusion coefficient and solubililty of various encapsulants and their temperature dependency were investigated. Based on experimentally determined permeation properties, governing equation for time dependent diffusion were solved using finite element method software to suggest design guideline for packaging of PV module for long-term reliability.

Fabrication and Characterization of High-Performance Thin-Film Encapsulation for Organic Electronics

Continued advancements in organic materials have led to the development of organic devices that are thin, flexible, and lightweight and that can potentially be used as low-cost energy-conversion devices. While these devices have many advantages, the environmentally induced degradation of the active materials and the low-work-function electrodes remain a valid concern. Hence, many vacuum deposition processes have been applied to develop low-permeation barrier coatings. In this work, we present the results pertaining to the developed thin-film encapsulation. Multilayer encapsulation involves the use of or with parylene. The effective water vapor transmission rates were investigated using a Ca-corrosion test. The integration of the developed barrier layers was demonstrated by encapsulating pentacene/ solar cells, and the results are presented.

Degradation of Backsheets for Crystalline Photovoltaic Modules under Damp Heat Test

The reliability of crystalline silicon photovoltaic (PV) modules is determined not only by the efficiency loss of the silicon cells but also by the degradation of other components (ethyl vinyl acetate, ribbon, glass, and backsheet). Among these components, the backsheet provides electrical insulation and physical support, and acts as a barrier against moisture and weathering of the PV module. We investigated the degradation of different types of backsheets under damp heat conditions [95°C/85% relative humidity (R.H), 85°C/85% R.H]. During the damp heat treatment, tensile tests were carried out to investigate the mechanical degradation of the backsheets. The molecular weight of the polyethylene terephthalate (PET) in the backsheets was measured by gel permeation chromatography (GPC) to understand the intrinsic mechanism of degradation. After damp heat test, mechanical degradation of backsheet occurred significantly beyond passing the specified molecular weight region of PET film in backsheet materials. By finding the correlation between tensile strength and the molecular weight of the backsheets, our results make it possible to improve the reliability of backsheets.

Developing accelerated life test for backsheet used in PV module and correlation study between field and accelerated conditions

Packaging materials are critical not only to maintain the performance of photovoltaic module in harsh environment but also to ensure the long-term stability of photovoltaic module. Among packaging materials, backsheet protects module from ultraviolet, dust, moisture, and other gases. Hence, to guarantee the lifetime of modules, backsheet should not fail for the same period of time. This paper presents results on the development of accelerated life test method and correlation study between actual use and accelerated field conditions. The main purpose of this study is to develop the method which can guarantee 25 year lifetime of backsheet.

A correlation study between the total permeated water vapor and lifetime of an encapsulated OPV

Barrier films play a critical role in the packaging and protection of solar cells, especially organic photovoltaics. While high performance barrier films are needed to limit the ingress of water vapor and oxygen into the cells, little has been done to correlate the performance of such cells with their barrier films. In this work, a study is presented with correlates the shelf lifetime of an encapsulated organic solar cells with barrier films having differing water vapor transmission rates. The study shows that overall barrier performance of multilayer thin films and the extended shelf lifetime of encapsulated organic solar cells can be related through the calculation of the total permeated water vapor through the barrier films. The total permeated water vapor was calculated separately in the transient and steady-state regions considering the lag time effect of the multilayer barriers. The efficiency of pentacene/C60-based solar cells encapsulated with one or two pairs of SiNX/parylene dropped to 50% after permeation of about 1.63 g/m2 of water vapor regardless of transmission rate. From these calculations, the lifetime of encapsulated devices with three pairs are predicted.

Fabrication and Characterization of SiOx/Parylene and SiNx/Parylene Thin Film Encapsulation Layers

Successful commercialization of flexible organic electronic devices is largely dependent on proper encapsulation that protects them from permeation of oxygen and water vapor. At present, low permeation encapsulation materials generally consist of multilayer films of organic/inorganic materials which can be deposited by plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), and vapor phase deposition. For this study, we report the effective water vapor transmission rates (WVTR) for multilayer thin films consisting of low temperature PECVD deposited SiNx and SiOx combined with a parylene organic layer. The effective WVTR was measured as a function of the number of bilayer pairs using Ca corrosion tests. The effective WVTR at 20 °C and 50% relative humidity [RH] for three bilayer pairs of SiOx/parylene ranged between 4.4–8.0 × 10−4 g/m2 /day while SiNx/parylene had a transmission rate 1.3×10−4 g/m2 /day. In general, additional layers were found to decrease the permeation rates to as low as 3.9×10−5 g/m2 /day, while the SiNx/parylene coatings performed the best overall.© 2007 ASME