Jens Hauch

Simulation of light intensity dependent current characteristics of polymer solar cells

An extended replacement circuit describing the current–voltage characteristics of bulk heterojunction polymer solar cells at different light bias levels is introduced and discussed. A one diode-model is expanded by an extraction model for photogenerated carriers taking into account the effective reduction of the mean distance which the charge carriers cover when sweeping the electrical bias through the fourth quadrant of the solar cell. The model properly describes the current–voltage behavior of bulk heterojunction solar cells over more than three orders in light intensity with one set of parameters.

GMR sensors for contactless position detection

Abstract Magnetic sensors are ideal for all kinds of contactless position registration, e.g. distance, speed, angle, rotational speed and sense of rotation. They work even under adverse and dirty environmental conditions. These basic advantages have resulted in the widespread use of all kinds of magnetic sensors in contactless position detection. The magnetic sensors based on the giant magneto resistance (GMR) effect developed for position detection are now finding their way into industrial and automotive applications. They overcome a weakness in conventional magnetoresistors and Hall sensors, because they are less sensitive to air gap deviations apparent in many applications. This paper presents the basic concepts of GMR sensors for contactless position detection. The advantages of the sensor and relevant examples for industrial and automotive applications are discussed.

Adapting GMR sensors for integrated devices

Abstract Magnetoelectronic devices like current sensors or magnetocouplers which integrate the signal generator and the sensor element on one substrate represent an area of major commercial importance for GMR sensors. Due to their intrinsic properties exchange biased GMR spin valves [Phys. Rev. B 42 (1990) 10583] [1] are particularly well suited for these devices. However, the adaptation of the spin valve sensors to such applications is difficult, since only the low field part of the signal characteristic is utilized, which is sensitive to the composition of the GMR stack as well as domain structure. While a large amount of research has focused on the influence of layer composition on the properties of GMR sensors [Annu. Rev. Mater. Sci. 25 (1995) 357] [2] , little is known about the effect of sensor geometry, which influences the domain structure of the sensing layer. In this paper, we introduce the design of an integrated magnetocoupler device and examine the influence of geometry on sensor properties in a design suitable to magnetic field detection.

Performance of bulk-heterojunction organic photodetectors

The performance of organic photodetectors has already reached a high level. In the near future organic photodetectors can be found in applications like imaging and sensing. The core of organic photodetectors is the bulk-heterojunction. Fullerenes and conjugated polymers forming islands in nanometer size are causing the outstanding quantum efficiencies. The conjugated polymers have a high absorption coefficient and the fullerenes are efficiently transferred electrons from the polymer. This leads to a high sensitivity to light. Organic photodetectors with low dark currents, external quantum efficiencies of 80% in the visible, linearity over several decades and fast transient behavior are demonstrated.

Statistical overview of findings by IR-inspections of PV-plants

First statistical evaluation of IR-inspections of PV-plants reveals that 86% of the installed PV-plants show IR-abnormalities. More than 120 PV-plants with more than 160,000 PV-modules were inspected and evaluated statistically. Main IR-abnormalities or failures in modules and string installations are analyzed, respectively. The average failure rate for PV-modules is about 8% and for module strings approximately 4%. The differentiation between the installation locations reveals that small residential installation show relatively more defective modules than large field installations. – Therefore, IR-imaging is a valuable method to give fast and reliable information about the actual quality and failure rate in inspected PV-installations.

Technology-dependent analysis of the snow melting and sliding behavior on photovoltaic modules

In this study, the snow melting behavior of several photovoltaic technologies, all installed at the same location under identical conditions, is analyzed based on the time-dependent changes of the snow cover, which is extracted from images of a monitoring webcam, for various temperature and irradiation conditions. From this study, conclusions can be drawn for the optimum module technology for a given location with respect to snow-dependent yield losses. In particular, the melting behavior is analyzed regarding its dependence on the ambient temperature and the irradiation level. Finally, the relevance of snow cover-related losses is discussed. The study shows that comparably large frameless modules exhibit the highest snow shedding rates. Hence, they are snow-free for longer periods, thereby increasing their potential for electricity generation in snowy regions. In summary, this paper reveals the beneficial snow removal properties of large frameless modules for snowy areas by applying a novel image process...

Influence of pre-existing damages on the degradation behavior of crystalline silicon photovoltaic modules

In this paper, the influence of pre-existing crystalline damage, such as cracked or broken cells or soldering failures, as they are frequently observed in operating photovoltaic (PV) plants, on the degradation behavior of mono- and polycrystalline silicon PV modules is investigated. In particular, it is analyzed if and to what extent pre-damage introduced prior to lamination propagates upon stress exposure. Therefore, the pre-damaged modules are exposed to various accelerated aging conditions in order to analyze the impact of the pre-damage on the degradation behavior under the respective aging scenario. In order to separate the influence of the pre-damage from composition-induced influences, the choice of materials used in the modules is varied. These investigations reveal that none of the accelerated aging tests causes any change in the pre-existing damage. In fact, the degradation behavior and rate rather depended on the choice of the module components than on the nature of the pre-damage. Finally, the...

High speed signal transmission with magneto-couplers

Couplers in general serve for galvanically isolated signal transmission between sensitive equipment. Standard opto-couplers are generally limited to a bandwidth of 25 MHz. However, with ever-increasing data transfer rates the need for couplers with much higher bandwidth is steadily growing. For this purpose magneto-couplers are very promising because their bandwidth is expected to be limited only by the Larmor precession of the magnetic moments with frequencies in the range of 1–10 GHz. In magneto-couplers the signal to be transmitted is converted into a current through a micro-coil. The resulting magnetic field is detected by a giant magnetoresistance(GMR)sensor element separated by an isolation layer. In the present case this element is a Wheatstone bridge consisting of four spin valves. The voltage across the bridge resulting from the magnetization change of the soft layers due to the field pulses is detected with a 1 GHz storage oscilloscope. Present experiments indicate a bandwidth larger than 500 MHz. After proper termination of the signal paths and applying a dc-bias field the rise time of the input signal of less than 1 ns is not increased by the coupler. Furthermore we show that the driving pulse may be completely reproduced in shape by applying external dc-bias fields. It will also be discussed how capacitive and inductive crosstalk between the coil and the spin valves and nonideal termination of the signal paths affect the achievable bandwidth.

High through-put, outdoor characterization of silicon photovoltaic modules by moving electroluminescence measurements

Fast and non-destructive quality control tools are important to assess the reliability of photovoltaic plants. On-site inspection is essential to minimize the risk of module damage and electrical yield losses. This may only be achieved by using highly sensitive imaging techniques such as luminescence or infrared thermography imaging. Nowadays, electroluminescence is used to detect defects such as local cell changes, series resistances and shunts in solar cells and modules which can cause electrical losses. However, the drawback of this method is the relatively low measurement throughput. To increase the throughput InGaAs cameras with a resolution of 640 × 512 pixels are used, for which low integration times are possible to acquire electroluminescence images. For such low integration times even moving image acquisition and movie recording are feasible to detect the mentioned defects. In this paper, an outdoor electroluminescence setup is presented for mobile handheld recording. Experiments showed that 5 ms integration time is a good compromise between low contrasts for lower integration times and motion blurring for higher integration times. The camera prototype has an onboard computer to avoid image transmission losses. It was controlled and visualized over Wi-Fi and remote desktop connection. The energy supply was provided from LiPo-batteries for improved mobility. In comparison to conventional electroluminescence measurements we can decrease the measurement time of a 20 module string from 5 min to 20 s.

Analyzing the degradation of pre-damaged PV-modules

Cracked PV-modules are frequently detected in PV-systems. The impact of cracked cells on the energy yield and module performance under real operating conditions is not yet understood but of great relevance. Standard tests cannot reveal the relevant information conclusively. Therefore, we conducted a twofold analysis. 1) field exposure (global analysis on string level as well as detailed analysis on module level), and 2) load testing in the lab. Here, we present comparative electroluminescence (EL-) images recorded in an outdoor test stand and during load testing. Additionally, infrared (IR-) images as well as power data obtained from loaded and operating (field) conditions are discussed.