B. Cells, Modules, Arrays, and Systems
A photovoltaic cell is a specially-designed large-area p-n junction semiconductor diode, with the junction located very close to the top surface. Suitable metal contacts, usually on the top and bottom-a grid structure on the top and a sheet structure in the bottom-collect the minority carriers crossing the junction under irradiation and serve as the output terminals.
The output of an individual cell is rather low, about a watt or two at 0.5-0.6 V. Therefore, several cells must be connected in a series-parallel configuration to obtain practical outputs. Several cells (typically around 40-50) are connected in series-parallel to form a module. Many such modules are usually combined (again in a series-parallel arrangement) together to constitute an array (string). For larger industrial or utility installations a collection of several arrays are connected in a segment (or subfield). Today's large-scale plant or system will consist of several segments feeding into a bank of inverters, which convert the d.c. input into utility-grade a.c., for injection into the grid.
Several fabrication stages exist between a laboratory cell and a PV system. They are: production cells, production modules, operating array, and operating systems. At each stage, a certain decrease in efficiency is experienced. The overall efficiency of conversion of insolation into electrical energy of a system could be as low as 60% of the efficiency of a laboratory cell. Figs. 1 and 2 illustrate the progress achieved in the efficiencies of solar cells and modules respectively in the recent past.