Since solar energy generation is getting more and more important worldwide PV systems and solar parks are becoming larger consisting of an increasing number of solar panels being serially interconnected. As a consequence panels are frequently exposed to high relative potentials towards ground causing High Voltage Stress (HVS). The effect of HVS on long term stability of solar panels depending on the leakage current between solar cells and ground has been first addressed by NREL in 2005 [1). This potential degradation mechanism is not monitored by the typical PV tests listed in lEG 61215 [2). Depending on the technology different types of Potential Induced Degradation (PIO) occur. This paper is focusing on PIO of wafer based standard p-type silicon technology aiming on increasing life times for solar panels once exposed to external potentials in the field. A test setup is presented for simulation of the PIO in the lab and the influence of cell properties on PIO is demonstrated in order to reveal the cell being the precondition for the PIO. However, PIO can also be stopped or minimized on panel and system level as shown in the paper.
BACKGROUND
Silicon heterojunction solar cells (SHJ) have been increasingly attracting attention to the PV community in the last years due to their high efficiency potential and the lean production process. We report on the development of a stable baseline process for SHJ cells with a focus on the optical improvement of the solar cells' front side. An amorphous silicon oxide layer (a-SiO 2 ) was used as an anti-reflective coating (AR) on top of the finished SHJ devices. Both optical simulations and experimental results demonstrate a short circuit current density (J sc ) improvement of 0.4 mA/cm 2 when applying the a-SiO 2 AR, yielding maximum conversion efficiencies of 23.0 %. Full-size cells with 244-cm 2 total area have been produced using three front contact stacks: ITO as reference, ZnO:Al and ZnO:Al/SiO 2 showing the J sc improvement with the double AR configuration. Damp-heat tests on those samples are currently being carried out.
We present a comprehensive overview over infrared imaging techniques for (electrical) silicon solar cell characterization. Recent method development in local series resistance imaging is reviewed in more detail and new results in local breakdown investigations on multicrystalline (mc) silicon solar cells are reported. We observe local junction breakdown sites on industrial mc-cells at reverse voltages as low as -7V and breakdown in great areas of the cell at voltages around -14V. As these breakdown sites (as well as local shunts) can cause hot spots which can damage the cell and the module, we also present an ultra-fast, simple and quantitative method for hot-spot detection. Typical measurement times in the order of 10 milliseconds are achieved
Results of optical carrier lifetime measurements like carrier density imaging significantly depend on surface conditions of the sample under test. Rough or textured surfaces have a severe impact on the measurement quality since they cause blurring and overestimation of the lifetime measurement. We propose a correction method for both, the adjustment of the absolute value and the restoration of the spatial distribution of the recombination lifetime. The absolute value is corrected by taking the emissivity of the sample into account. The unblurred signal distribution is obtained by mathematical deconvolution via Wiener filtering. For this purpose an appropriate point spread function is experimentally determined.
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