Kimmo Leppänen

Electrical heating synchronized with IR imaging to determine thin film defects

Measuring conductive thin film properties during production and in end products is a challenge. The main demands for the measurements are: production control, reliability and functionality in final applications. There are several ways to measure thin film quality in a laboratory environment, however these methods are poorly applicable for production facilities. In order to bypass the limitations of existing methods, a simple synchronized heating and IR-imaging based system was implemented. To demonstrate the proposed method, Indium Tin Oxide (ITO) was selected as an example of conductive thin films. PET-ITO films were bent to obtain samples with defects. The proposed method was used and automated signal processing was developed. The results show that the system developed here is suitable for defining breakage types and localizing defects.

Synchronized thermography for multi-layer thin film characterization

Organic solar cells and organic LEDs are typically made of conductive and semi-conductive thin films. The uniformity requirement for these films is exceptionally high. In the case of multi-layer structures, surface characterization based methods (e.g. profilometer, atomic force microscope, scanning electron microscope) encounter certain challenges when attempting to detect the defects inside the structure. One way to overcome this drawback is by using synchronized thermography (ST). In this work ST is used to study multi-layered thin film structures. Indium Tin Oxide (ITO) was used as an example of conductive thin film and poly(3,4-ethylenedioxy-thiopene):poly(styrene-sulfonate) (PEDOT:PSS) was used as an example of a hole transporting layer. Uniformity differences were generated in these layers and ST was used to detect them. The results show that ST is capable of localizing small defects in the stack using a single infrared (IR) image. It can often be deduced from the same image in which layer the defect is located. This shows that ST is capable of profiling the structures of multi-layer thin films.

IR-imaging based system for detecting the defects of conductive materials

Uniformity of conductive materials is an important property which is measured during manufacturing and in finished products, especially in electronics applications such as organic solar cells. Differences in uniformity are often very small, invisible or below the surface of the sample. Therefore, they are not always detectable even by high-resolution imaging systems. Respectively, electrical conductivity measurements are limited to those mainly between the measuring probes. Uniformity difference measurements are time-consuming in the case of a large area characterization. To bypass the described limitations, a simple heating and IR-imaging based system was designed and demonstrated with conductive materials. Samples with different defects were used to investigate the correlation of conductance and defect positioning. By making punched holes in the samples, it was possible to demonstrate how the local resistances of thin films have functions to each other and how this may be observed on an IR-figure. Thermographs of punched thin films confirm that those areas where the holes prevented the current flow have lower heat emissions. Therefore, it can also be concluded that, generally, the temperature is highest at the areas where current density is highest. When comparing the defects of bent samples to these punctured ones, the correlations of resistance and breakage areas were comparable. The applied system is capable of localizing small defects in large-area samples using a single IR-image. This is a significant advantage from the manufacturing process measurement point of view.

Detecting Defects in Photovoltaic Cells and Panels and Evaluating the Impact on Output Performances

This paper investigates the ways to detect defects in photovoltaic (PV) cells and panels. Here, two different methods have been used. First, the output behavior was characterized by measuring the amount of current at different voltage levels to obtain the current-voltage and power-voltage curves. Second, infrared emissions of forward-biased nonilluminated PV cells and panels were measured by the use of synchronized thermography. From these measurements, temperature maps can be derived, which indicate that the temperature within a given PV cell unevenly rises due to the defects in the cell. Uneven temperature distribution indicates defects and reduced output power.

Detecting defects in photovoltaic modules with the help of experimental verification and synchronized thermography

In this paper we investigate defects in photovoltaic modules which cause variations in output performances. Here, we concentrate on two possible ways to verify potential output power levels. Firstly, we characterise the output behaviour by measuring the amount of current at different voltage levels to obtain the I-V (Current-Voltage) and P-V (Power-Voltage) curves. Secondly, we measured the infrared (IR) emissions of photovoltaic modules by the use of synchronized thermography. From those measurements, temperature maps can be derived which indicate that the temperature rises differently in photovoltaic modules due to defects. As a result, we are able to establish quantitative and qualitative verifications of photovoltaic modules.

Thermography based online characterization of conductive thin films in large-scale electronics fabrication

Flexible electronics is an emerging thin film based technology enabling completely new types of products and applications compared to conventional electronics. Since the quality of films defines the functionality of fabricated devices, the lack of suitable online manufacturing quality assessment tools has been identified to be a critical bottleneck while upscaling the volume and the yield of thin film electronics manufacturing. In order to solve that problem, a synchronized thermography (ST) based online measurement system was built. Applicability of proposed roll-to-roll compatible ST based system was demonstrated by characterizing a moving plastic film with conductive indium tin oxide on top. Obtained results show that ST can be utilized for online homogeneity characterization and sheet resistance estimation of large area thin films which are not possible with other existing methods.

Estimating the impact of defects in photovoltaic cells and panels

This paper investigates defects in photovoltaic cells and panels which cause notable losses in output performances. Here, the focus lies on the impact of hairline cracks which result in a remarkable drop of the available output current and, thus, the available output power. Firstly, samples were characterised with the help of synchronized thermography (ST) in order to localise and analyse the defects. Secondly, samples were measured with the help of electrical verification to obtain the characteristic I-V (Current-Voltage) curve. Finally, the geometric area of PV cells was calculated which corresponds to the effective area for energy production due to the presence of a defect. Results show the correlation between the available power of PV cells with temperature variations in IR-emissions. Proposed methods are capable of detecting defects in PV cells and quantise the impact on output performances.

Defect localisation in photovoltaic panels with the help of synchronized thermography

This paper investigates defects in photovoltaic (PV) panels, more precisely, the location of defects in PV panels. With the help of electrical verification, it is possible to verify the impact of defects on output performances. However, it is not possible to determine the location of defects in order to address problems, for example in the manufacturing process of PV panels. In this paper, the focus lies on finding similarities in the location of defect areas in PV panels. Samples were characterised with the help of synchronized thermography (ST) in order to obtain infrared (IR) images of PV panels. IR-images are helpful to get a visual image on the health of PV panels and identify the position of defects. This information can be useful, for example to improve the fabrication process of PV panels.