B. Plesz

Low cost solar irradiation sensor and its thermal behaviour

This paper presents the working principle, design and thermal characterization of an irradiation sensor. The irradiation measurement device must have accuracy and a reliable construction under outdoor use conditions. The sensor is based on a photoelectric cell and incorporates a read-out circuit for providing steady short circuit conditions for the cell as well as for signal amplification. Thermal tests (HTS, LTOL) were accomplished in a climate chamber in the temperature range of -20 to 80^oC in steps of 10^oC. To determine the fundamentals of the thermal dependence of the sensor cell spectral response measurements under varying temperatures were performed. Measurements show that the self-made solar cells thermal dependence was 0.26%^oC^-^1, revealing higher temperature dependence than in the case of an industrial reference cell.

Improved thermal characterization method of integrated microscale heat sinks

The thermal management of semiconductor devices is still a hot topic. Most designers, who are aware of the thermal aspects of IC design, know that new, cheaper and more efficient methods are required to keep the temperature of electronic systems low. Research by different teams regarding the cooling of stacked die structures is in progress.In this paper an improved thermal characterization method will be presented to determine the flow dependent partial thermal resistance of integrated microchannel based heat sinks. This reliable characterization method does not demand thermal isolation during the measurements, only constant environment conditions. The measurements are based on the industrial standard thermal transient testing method.On the other hand we present an approach to realize an integrated microfluidic channel based heat sink, which can be realized in the backside of the silicon chip itself. The approach is based on a cheap wet etching process instead of reactive ion etching or LIGA technologies, which enables batch processing.

Application of thermal transient testing for solar cell characterization

The current paper deals with the application of thermal transient testing as a characterization tool for solar cells and modules. Based on the measurement of a representative samples -including concentrator and non-concentrator solar cells - we prove the applicability of this measurement technique. Metrics such as junction-to-base plate thermal resistance are derived and can serve as a basis of a model for the accurate prediction of the performance of solar modules.

Integrated microscale cooling for concentrator solar cells

The work presents a new solution proposal to the cooling of concentrator photovoltaic cells. In our concept the microscale channels are integrated into the back surface metallization, the microscale channels are formed by electroplating copper around a photoresist channel pattern. This approach has the advantage that it has no restrictions regarding the solar cell material and technology. In this work we give a description on the process technology, perform mechanical simulations for the feasibility of our approach, optimize the channel geometry for a 20 × 20 mm concentrator solar cell and estimate the cooling performance of the microscale channel structure at different operating conditions. We found, that the proposed cooling solution would have a calculated thermal resistance of 0.26 K/W at pressure drop of 100 kPa. This would result in a temperature raise of less than 8 K in case of a concentration level of 100 suns and a solar cell efficiency of 25 %.

Characterization of solar cells by thermal transient testing

The current paper deals with the application of thermal transient testing as a characterization tool for solar modules. Based on the measurement of a representative sample — a concentrator module — we prove the applicability of this measurement technique. Metrics such as junction-to-base plate thermal resistance are derived and can serve as a basis of a multi domain solar module model. The used technique also enables us to verify the quality of attachment layers in a solar module allowing fair quality control and reliability analysis of these devices.

Influence of the photoactive layer thickness on the device parameters and their temperature dependence in thin crystalline silicon photovoltaic devices

One of nowadays crucial questions with crystalline silicon solar cells is the reduction of manufacturing costs. One possible concept of reducing cost is to produce solar cells with thin photoactive layers, in order to use less amount of good quality and thus expensive raw material. In addition photovoltaic devices are one of the most obvious solutions for on-chip energy harvesting. There are basically two approaches: the monolithically integrated photovoltaic devices, and photovoltaic devices that are attached to the chip surface and connected to the integrated circuit. These devices also feature a thin photoactive layer in the majority of the cases. This paper aims to investigate the influence of the photoactive layer thickness on the on the photocurrent and the spectral response. It was found that the temperature dependence of these parameters increases with decreasing photoactive layer thickness. A possible explanation for this phenomenon is also presented.

Issues of thermal transient testing on photovoltaic modules

The thermal behavior of mass production terrestrial solar modules is a somehow neglected but very important issue of photovoltaics. Despite its significant influence on the power generation, the thermal properties of solar modules are mostly not taken into account in daily applications. In recent literature thermal transient testing was suggested as a possible solution to proper thermal characterization of photovoltaic devices. In this paper we give a short overview on the known issues of the thermal transient testing of photovoltaic devices and address the issue of the variation of the structure function at different heating currents. It is shown that for small current densities the increasing forward driving current (i.e. the heating current) can have a strong influence on the size of the active area that on the other hand determines the volume of the heat path. Due to this an increasing heating current results in the decrease of the Junction to ambient thermal resistance, with the thermal resistance converging to a minimum value at high heating currents. The phenomenon was investigated by Kelvin probe potential mapping, thermal imaging and thermal simulations which uniformly supported our explanation.

Investigation of the thermal behaviour of thin crystalline silicon solar cells

This paper investigates the thermal behaviour of thin crystalline silicon solar cells and determines whether the decrease in cell thickness affects the temperature dependences of the solar cell parameters. For the investigation crystalline solar cells with different photoactive layer thicknesses were processed. Sample cells were formed on n+-substrate wafers with n epitaxial layers where due to the low minority carrier lifetime in the n+-substrate only the epitaxial layer participates effectively in the photocurrent generation. The thin photoactive layers were achieved by the etching of the epitaxial layer. On the samples I-V curves and spectral response functions were measured at different temperatures, and the temperature dependence the solar cell parameters were determined. Most of the parameters showed no differences in their temperature behaviour, but the temperature dependence of the short circuit current and the fill factor differed on all sample cells. From the results it can be concluded, that decreasing the thickness of the solar cells will have practically no effect on the temperature dependence of the performance and the efficiency of crystalline silicon solar cells. However the dependence of in the temperature coefficients for the short circuit currents on the thickness of the photoactive layer could be of interest in sensor applications, e.g. for the thermal compensation of light sensors.