Figure1. Cross-section view of solar cell

Working of Solar Cell. There are three important phenomenon which takes place inside the solar cell when sunlight hits on its surface:

1. Excitation of free charge carriers due to sun light absorption.

2. Separation of charge carriers as electron and hole.

3. Collection of charge carriers by metal contacts.

The pn junctions inside solar cell results in an electric field at junction. When sunlight hits the solar cell, an electron springs up and attracted towards n type. Also hole created by this electron move towards p side and electron combine with hole after  travelling through external circuit which generates current.

Module. Since PV cells operation is based on sunlight. So it is necessary to install them in open area. During operation, these cells would exposed to many harsh conditions such as humidity, dew, rain, and high temperature. It has been known that temperature and humidity are some important factors determining performance and reliability of semiconductor devices. This apply to PV cell as well. Hence to secure them from these conditions, PV cells are generally encapsulated with additional materials such as glass and backsheet. These encapsulated constructions having solar cell are known as PV module shown in Figure 2. The material used as a protective glass and base in PV module should have to be resistant to temperature changes and UV rays exposure. It also should resist mechanical stress up to an optimum level. An aluminium frame is attached to the structure in order to make the structure strong and easy to mountable.

Figure 2. Components of a PV module

Figure 3. Different degradation in PV module over time

In 2005, a new reliability issue was introduced which is known as PID. It got attention because of its rapid degradation. It can result in huge output power drop (>40 %) in few months as shown in Figure 3. Due to its serious impact on the performance of PV module, this issue should be addressed for all PID-experienced or susceptible modules. It is a process which occurs only a few years after installation and damage expands exponentially with time. It affects the cells negatively and leads to power loss in a module by electrically modifying solar cell resulting in leakage current.

PID mechanism is associated with the drift of ions across the pn junction under high voltage stress which add to the leakage current. Therefore, leakage current along with its influencing parameters can be utilised as deterministic for PID. In some cases, it leads to permanently damage of a module. PID problem will be more serious in future as PV industry are trying to increase the maximum system voltage to decrease overall cost. The maximum permissible limits of direct current system voltage is 600 V in the USA and 1000 V in Europe. These limits are specified by the manufacturers [4]. Now a days domestic as well as commercial roof top PV plants operates voltages around 400-800 V. At such voltage danger of PID must be assessed since mostly such installations are transformer less as well as not maintained regularly, hence techniques like PID recovery also not possible to implement. In the direction of PID resistant design the part of encapsulation responsible for PID should be identified and rectified. This project is also working in the field of identifying culprit and to make some suggestions in order to minimize PID degradation.

Potential Induced Degradation

PID is a degradation mechanism that results in a significant drop in power in a short duration. This mechanism can be understood as, in a PV system, many modules are connected in series and parallel arrays. This arrangement of modules results in high voltage stress on the solar cell as the modules are grounded through frames for safety reasons. Under this high voltage stress, ions, i.e., sodium (Na+) present on the surface of a solar cell, drift towards the cell along with leakage current and accumulate inside the solar cell. This accumulation of ions penetrates the p-n junction and establish a metallic contact that behaves as a shunt. This shunting is known as potential induced degradation (PID-s) shunting. It results in a significant drop in the performance of a solar cell because it provides an alternative path for current to flow. It is happening along with leakage current; therefore, leakage current is also used as a determinant for PID.