Photovoltaic Module Reliability

5Failure Analysis Tools

Regardless of whether a module has degraded from field exposure or accelerated stress testing, it is important to understand what has actually changed within the module that led to lost peak power. If we want to use the results to improve the module construction so that future modules will not degrade, we must understand what particular changes have occurred. In this chapter, we will explore some of the methods used to better understand what has gone wrong within the module. Methods reviewed include, analysis of the I–V parameters, measurement of performance at different irradiances, visual inspection, Infrared (IR) Inspection, Electroluminescence (EL) and evaluation of adhesion. Each will be discussed in the subsections that follow.

5.1 PV Performance – Analysis of Light I–V Curves

Just knowing that the peak power has gone down doesn't tell us why. By analyzing the data from the I–V curve we can often get a better idea of what has degraded. Figure 5.1 shows an I–V curve from a fairly typical PV module. Three points are marked on the curve:

• I_sc is the short‐circuit current of the module or the point at which the module voltage drops to zero.
• V_oc is the open‐circuit voltage of the module or the point at which the current drops to zero.
• P_max is the maximum power point, which is the point on the I–V curve where the product of current and voltage is highest. This is where you would like the module to operate.

One additional parameter of importance is ...