# Low-Cost Three-Quadrant Single Solar Cell _I-V_ Tracer
## 1 Introduction  
There are many different techniques for characterising photovoltaic (PV) solar cells in various aspects. Some are focused on the microscopical compositions of the cells and the influences on their macroscopical behaviours; others are directly on these macroscopical aspects [1; 2; 3; 4; 5]. From a practical point of view, the cell is used to produce electricity; this may be the most important aspect. Of course, it is very important to know why cells do not perform as well as they should, what problems they currently experience, and how they may evolve. However, we first need to inspect them expeditiously and discover whether they are working as expected. For this purpose, there are techniques, such as electroluminescence (EL) imaging [6; 7; 8], thermal imaging [9; 10], or the _I-V_ curve. The latter is the only one that can directly show the output power of the PV cell with certainty because it measurements the variation of the current flowing through the cell when the voltages across the terminals are forced to change (that is, when the load resistance connected at the output changes). It is not possible to trace the _I-V_ curve of a single cell in a commercial PV module (just the full module _I-V_ curve). Nonetheless, it is possible in a laboratory to use single cells instead of complete modules. This information (at the cell level) is interesting for research purposes and can be used by manufacturers to detect bad cells and discard them before integration into a module, or classify them and put similar cells into a module to avoid mismatching [11]. Various topologies are used to measure the _I-V_ curve, based on different ways of varying the loads seen by the PV cell or module [12]. Most commercial devices use variableloads or capacitor charges to trace the _I-V_ curve. Regarding the systems that are based on various resistive loads, most are designed for standard modules (usually over 60 cells in series) or for an array of modules (usually about 30 modules), where the voltages are in the order of tens or hundreds of volts, and the load resistances are relatively easy to manage. The main difficulty in accurately measuring the _I-V_ curve of a single cell by varying the load is that the voltages implied are very low (less than 1 V) while the currents can be relatively high (in the order of 10 A). With these numbers, [PERSON]'s law yields that we have to manage the load resistances in the order of 0.1 \(\Omega\) or less. In these conditions, achieving the zero-voltage point could be a tough task since any parasitic resistances along the current path will prevent the cell from reaching this point. For example, if we are measuring _I-V_ curve points at \(I\) = 10 A and \(V\) = 0.1 V, an equivalent resistance in the total circuit lower than 10 m\(\Omega\) is needed. Most contact resistances of usual probes, such as the ones in regular multimeters, cable connectors, etc., are higher than this value. Many strategies can be used to solve this: high section cables, soldered connections, wide copper tracks at printed circuit boards (PBCs), short electric paths, gold plated connectors, etc. As will be shown in the following section, these problems are overcome by the special configuration of our circuit: they naturally force voltages across the solar cell from reverse to forward polarizations, inevitably crossing the zero voltage point regardless of the parasitic resistance of the current path, with the added benefits of being very cheap (compared to professional commercial devices) and the possibility of obtaining the complete _I-V_ curve (first, second, and fourth quadrants) of the PV cells measured. Moreover, the electronics can easily be adjusted to measure a small number of cells in a series association (small module) by modifying only the value of a pair of resistors on the PCB. The article is organised as follows: Section 2 presents the method used as well as the available materials. Section 3 presents the results of the scientific work. Section 4 presents the discussion.