PID (potential induced degradation) is a phenomenon where leakage currents cause power loss in solar PV modules over time. It occurs when high voltages between the solar cells and module frame allow electrons to migrate through the encapsulant material. This can reduce a module's power output by up to 20%. Researchers are working to optimize cell coatings, materials, and system grounding to prevent PID and ensure solar modules maintain high performance over their 25+ year lifetimes. Testing and certification standards may need to be updated to account for PID.

1. PERSPECTIVE PERSPECTIVE
and module frames via the encapsulation effects similar to PID. In
The PID
materials. This can be tackled by modify- the worst case scenario,
ing the interfaces or by changing the high voltages can lead to
encapsulation material. The root cause of electrochemical corro-
this degradation effect, however, lies with- sion in the layers of the
Effect
in the cell. Hence, the issue needs to be material, the thickness of
addressed by optimising every step of the which is a few micro-
cell production process. metres. According to the
latest research, PID is
Power loss in solar PV modules Temperature and humidity caused by the diffusion
The PID effect occurs when the module of positive sodium ions
surface is wet while being exposed to a and water vapour pene-
Kapil Maheshwari, Engineering and Projects, Q-Cells Systems India higher air temperature. High humidity also tration into the boundary
fosters the development of PID. Therefore, between the glass and
O
ver the past decade, many photo- PID can reduce the module output by up while PID is becoming widely recognised the transparent conduc-
voltaic (PV) plant operators have to 20 per cent. Under the existing inter- in Europe, it is probably a greater threat in tive oxide on the front of the thin film cell. standards can be improved to check a
observed the unexplained degrada- national module testing standards (IEC India due to much higher average temper- in the form of electrons or ions, will result in phenomenon like PID.
tion and power loss of PV modules. Recent 61215), PID is not monitored. However, atures and humidity. an increased charge concentration above Solution to PID
research shows that this is due to potential many thought leaders in Europe and the the solar cell in the encapsulation material Multiple companies are working on a Intensive testing of the encapsulation
induced degradation (PID), also known as US are already working on eliminating or Degraded performance (Fig. 2). The electric field of these charge solution for PID, including Q-Cells, which material or vapour permeability should
high voltage stress. reducing PID in solar cells and modules. Cell level carriers influences the p-n junction in such has analysed and researched the prob- also be implemented. This could be
Fig. 1 shows the evolution of the current- a way that the junction gets more conduc- lem for two years. During this time, it was accomplished by measuring the charge
The effect of PID on the long-term stabil- In simple terms, PID is a loss of PV sys- voltage (I-V) curve with the ongoing PID. In tive and the local shunt resistance drops. discovered that the anti-reflective coating flow through the encapsulation material
ity of solar modules, depending on the tem power caused by leakage current at the case of PID, shunt resistance as well Both multicrystalline and monocrystalline (ARC) of a cell has a role not only in influ- during a climate chamber test conducted
leakage current between the solar cells high voltage. The electrical voltage as the reverse bias current is the first to be cells are affected by this phenomenon. encing PID but also in preventing it. under high voltage.
and the ground, was first addressed by between the cells and the frame can affected, followed by the fill factor. Finally, Crystalline modules are not tested for how Hence, Q-Cells carried out the research
the National Renewable Energy Labora- cause the electrons to come loose from Voc (open-circuit voltage) decreases, they respond to PID under the established on different ARC process variations with Conclusion
tory in 2005. In the early growth phase of the material used in the PV module, which causes the junction to be less likely certification procedures, as specified in the the goal of designing a PID-resistant It is important to address PID in PV sys-
grid-connected PV, this phenomenon migrate into the electric field, and then be to separate holes and electrons. international standard IEC 61215. As high- ARC that retains the optimal anti-reflec- tems with higher system voltages as the
was very rare. This was because PV sys- discharged through the frame. This lighted by PHOTON International, PID can tion properties necessary for outstanding power loss due to PID is significant.
tems seldom exceeded 120 V. Since sys- results in the formation of an electrical The short-circuit current is the parameter lead to performance losses of up to 20 per cell performance. Although the origin of PID is on the cell
tem voltages today typically range from charge (polarisation) within the cell, that is least affected, but with advancing cent over the life of a solar module. level, it can be minimised on all levels –
600 V to 1,000 V, it is expected that PID which reduces the yield from the PV PID, it too degrades. Depending on the Q-Cells also found that to minimise PID on system, module and cell. The solution on
will affect the performance of the plants effect and thus results in power loss. The degree of PID, the junction will lose its Module level the module level, it is important to select the system level is choosing an appropri-
being set up now. According to solar extent of voltage bias degradation is blocking characteristics under reverse bias As seen in the electroluminescence (EL) suitable material combinations and opti- ate grounding scheme of the string poles,
power publication PHOTON International, linked to the leakage current or charge or totally break down. The leakage current, images before and after the PID test (Fig. mise the design of the module to ensure while on the module level the properties of
passed from the solar 3), the general brightness of the second low leakage currents, which subsequently the encapsulation material determine the
cell active layer th- image is lower. Also single cells are not increase the lifetime and reliability of the level of leakage current due to PID.
rough the encapsulant uniformly affected. Some cells seem to module. In short, Q-Cells has adjusted the
and glass to the mod- have degraded heavily and have short-cir- cell metallisation, the ARC, the emitter On the cell level, many parameters influ-
ule frame. cuited, while others appear to be stable. design and the doping of its cell. ence the PID stability of solar cells.
The reasons for this variation must be Besides base material resistivity and emit-
Factors leading to investigated on the cell level. Anti-PID module verification ter sheet resistance, the most important
performance loss Announced in June 2011, the Associa- parameter was found to be ARC.
Voltage System level tion of German Engineers (VDE) devel-
The standard system When the potential turns from negative to oped the new VDE Quality Seal. At pre- Long-term stability and reliability of solar
architecture of PV positive with respect to ground, degrada- sent, only two manufacturers in the world modules can be significantly improved by
installations exposes tion stops (Fig. 4). In case the potential is have received this certification and Q- adapting the process on all levels in order
solar modules to bias fixed by grounding the negative pole of Cells is one of them. The inspection of to minimise PID and, therefore, optimise
voltages of several the string, PID can be effectively avoided. solar modules during assembly elimi- the energy output of the PV system over a
hundred volts, result- nates potentially faulty components to 25-year period. Testing and certification
ing in leakage currents PID in thin film modules ensure that only VDE quality-tested mod- standards for solar cells and modules
between solar cells Thin film modules are also subjected to ules leave the manufacturing site. IEC should also monitor PID. ■
46 ● Renewable Watch ● January 2012 January 2012 ● Renewable Watch ● 47