ConPro – Concentrator Project - All-Purpose MWT Concept for Solar Cells of Variable Dimensionsolarzellen

Laufzeit: June 2013 - August 2014
© Fraunhofer ISE
Fig. 1: Front and back views of the AP-MWT solar cells on a Si wafer.
© Fraunhofer ISE
Fig. 2: Schematic cross-sections through AP-MWT solar cells which have been optimized for high (Al-BSF, upper) and low (PERC, lower) irradiance levels.
© Fraunhofer ISE
Fig. 3: Efficiency values versus irradiance for the two technological routes currently being pursued for AP-MWT solar cells.

In addition to the conventional usage of Si solar cells in flat solar panels, there are applications which are particularly optimized for radiation concentrated by either low or, more commonly, high concentration factors. This application is distinguished by the requirement that the solar cells used must transport either very low or particularly high current densities. In addition, this type of application demands variable dimensions and a flexible current-voltage ratio from the finished product. For this context, we have developed the All-Purpose- Metal-Wrap-Through (AP-MWT) concept. Peak efficiency values of 20.2 % have been achieved for an irradiance level of 1W/cm² (C = 10), 21.0 % at C = 1 and 18.3 % for C = 0.1.

The AP-MWT concept features an elementary cell design, which allows cell dimensions from a minimum of 2.25 cm² to a maximum of 189 cm² to be fabricated (Fig. 1). On a single Si wafer, 84 elementary cells are located, which each have an external contact for negative and positive polarity on the back surface. With this configuration, we minimize shading losses due to the front metal grid and maximize flexibility by connections which are made only on the back. With this approach, the current-voltage ratio can be adapted by choosing the appropriate number and dimensions of the cells to be connected. We only use industrially relevant processes to produce the AP-MWT solar cells. Depending on the intended application, technology is used which is particularly suitable for the anticipated irradiance levels. The upper cross-section in Fig. 2 shows a solar cell which has been contacted over the entire back surface (Al-BSF), which is particularly well suited for applications with high irradiance levels. The lower crosssection shows a passivated emitter and rear contact (PERC) solar cell, which is used particularly for irradiance levels of between one and a tenth of a sun.

In the PV-TEC located at Fraunhofer ISE, small series of up to 1000 wafers can be produced, the elementary cells can be separated from the wafers, I-V measurements are made at the required irradiance level and the cells are sorted into performance classes. Furthermore, we offer R&D services concerning connector technology, surface mount technology (SMT) processes, encapsulation and theoretical modelling in addition to electrical characterization of modules.