Typical defects of PV-cells G. Acciani, O. Falcone, S. Vergura Dept. of Electrotechnics Politecnico di Bari, Italy acciani@poliba.it falcone@deemail.poliba.it vergura@deemail.poliba.it Abstract-The paper introduces the issue of the typical defects in PhotoVoltaic (PV) cells and focuses the attention on two specific defects: linear edge shunt and hole. These defects are modeled by means of Finite Element Method (FEM) and implemented in Comsol Multiphysics environment in order to analyze the temperature distribution in the whole defected PV-cell. Usually the defects reveal themselves as hot spots and can be pointed out by means of thermo-graphy. Simulation results are compared with the thermo-grams of real cases and the accuracy of the models is confirmed. I. INTRODUCTION The abnormal temperature increase of PV-cell causes a drastic efficiency loss and then a reduction of the totally produced energy. This problem reduces further the low efficiency of PV cells and represents another obstacle for the large spread of PV technology. The efficiency of common solar cells is in a range between 5% to 19 % (mono-Silicon PV cells). This low value is strongly affected by the temperature. According to NOCT (Nominal Operative Cell Temperature), the best operating temperature for solar cells is about 46°C ± 2°C, depending on manufacturer specifications. Then, a temperature increase of 10°C of the cell surface causes about 4 % power loss (the hot spot is said light), while 18°C temperature increase reduces the power of about 10% (the hot spot is said strong). As the system cannot be investigated point-to-point through a thermometer, the issue is overcome by means of thermo-graphy. This not destructive technique allows highlighting hot spots, if present, but does not give information about their origins. Typical defects inside PV-cells, strongly connected to the hot spot presence, are reported in [1]-[2]-[3]. From a physical point of view hot spots can be classified as process-induced or material- induced. Instead, from the I-V characteristic point of view they can be divided in resistive-like or diode-like. The shunts for mono-Si cells are classified in nine different typologies. The paper introduces them and focuses attention on two specific shunts (linear edge shunt and hole) which will be modeled and implemented in Comsol Multiphysics environment. The models of the shunts are integrated into the 3-D model of a well-operating PV cell [4]. The aim of the paper is to implement the shunt models of the two defects and to compare the simulation results with the real temperature distributions usually observed by thermo-graphy. II. CHARACTERISTIC DEFECTS AND THERMO-GRAPHY Thermo-graphical techniques allow performing an efficient and systematic investigation on shunts in solar cells [5]. A division of typical shunts in two categories is possible according to their I-V characteristic: the shunt is defined resistive-like if it is highlighted under forward bias as well as under reverse bias. If the shunt compares only under forward bias it is defined diode-like. From a physical point of view another classification is possible. The shunts are now grouped in two categories: process induced and material induced. In this paper the attention is focused on the second one, i.e. linear edge shunt and hole. The former one comes from a bad edge insulation of the solar cell: when this incomplete insulation occurs, a low resistivity walk stats between top and bottom electrodes of the cell. This low resistivity walk acts like a short circuit on one side of the cell. Figures 1 and 2 report such a shunt highlighted through lock-in thermo- graphy under both forward and reverse bias respectively. Through lock-in thermography a thermal evaluation of I-V shunt characteristic can be done [6]. Thermo-grams analysis allows to evaluate the temperature increasing of a specific part of the cell with respect to the remaining device improving the evaluation of the power losses. It is important to observe that while V OC (open circuit voltage) strongly decreases when temperature increases, the short circuit current, I SC , shows only slight variation [7]. Also the efficiency η and the output power of the PV cell are strongly influenced by temperature variations as well as the fill factor FF [8]. Fig. 1. Linear edge shunt visible under forward bias in lock-in thermography.