Since Mono, Mono PERC and Poly PERC are the new techs coming to the PV world, we thought to educate our readers on their age old problem i.e. LID & LeTID. The solar module is said to generate power when sunlight falls on it. While this may seem simple, in reality there are lot of complex processes going inside a solar module which actually results in power generation. This processes change(s) or more correctly stabilizes when the modules are initially installed on site or (extensively) tested in lab immediately after their production. While this process stabilizes, the module losses its power output to a determinable extent (uncontrollable in few cases). This loss of power is due to phenomenon which is known as Light Induced Degradation (LID) and/or Light enhanced Temperature Induced Degradation (LeTID) (in few cases).

Before diving into the topic, let us first understand how a solar cell actually generates power. A solar cell is a sandwich of p type and n type doped semiconductor with n type facing the sunny side in solar cell. When the light particles (photonto be more specific) fall on the solar cell, they are absorbed inside the cell. This photon excites the (minority charge carriers) holes in n type and electrons in p type semiconductor. This so called “electron-hole” pair is collected through the electrical contacts at the extremities of solar cell. In a solar module, nmbers of solar cells are connected in series and/or in parallel. The generated electrons are collected from each solar cell which leads to generation of proportionate voltage and current.

Figure 1: Working of solar cell (Source: PV Education)

Light Induced Degradation (LID)

LID has been known to researchers for more than 40 years know. LID as the name suggest, is the degradation in the power output (and hence the efficiency) of the (only) crystalline solar cell/module during its initial exposure to irradiance when in field and/or tested in the laboratory. Further understanding its physics, solar cells have stabilized energy levels (called Fermi energy levels) to which electrons are exited. However, while in initial operation there is a shift observed in Fermi level due to many chemical processes which cause degradation in solar cell. This is what is termed as LID. There are two basic and best known mechanisms which cause LID. They are as follows:

Figure 2: Boron-Oxygen LID (Source: Jan Schmidt, “Light-induced Degradation in Crystalline Silicon Solar Cells”) Monoper

Boron-Oxygen (B-O) complexes: One of the extensively studied causes of LID is B-O LID. This type of defect occurs mostly in mono crystalline cell which is manufactured via Czochralski method. This method involves the use of heavy oxygen concentration (up to 1018 atoms/cm3). Occurring mostly on wafer level, the efficiency loss depends on the concentration of boron oxygen level in the cell. During the first hours of illumination on solar cell, the oxygen present in the cell reacts with Boron forming a complex compound (Figure 2: Boron-Oxygen LID (Source: Jan Schmidt, “Light-induced Degradation in Crystalline Silicon Solar Cells”) which while stabilizes quickly (after few hours) forms intermediate energy levels. These intermediate energy levels cause the excited charge carrier to quickly come back to its initial position while releasing heat which causes loss of efficiency.
Dissociation of Iron-Boron (Fe-B) pairs : Another mechanism causing LID is iron-boron dissociation. Iron present during manufacturing (even in the smallest of quantities) reacts with the boron in the dark due to coulomb interaction (force of interaction between positive and negative charged ions). This complex has shallow energy level and does not directly affect the cell efficiency. However when illuminated, the Iron ions are separated which causes the high charge carrier recombination resulting in efficiency loss of solar cell. This efficiency loss is further enhanced with increase in concentration of iron.

Light and elevated Temperature Induced Degradation (LeTID)

A new type of degradation was reported in 2012 where instead of illumination, both light and (elevated) temperature had a role to play.This type of degradation is however more prominent on multi-crystalline silicon Passivated Emitter and Rear Contact (PERC) solar cell instead of the typical Aluminium Back Surface Field (BSF) solar cell. Compared to LID, LeTID occurs at temperature as high as 50 °C to 95 °C. This degradation when compared to LID is observed in the cells after hundreds of hours. Additionally, it is also observed that the concentration of degradation is independent on the dopant and/or oxygen levels. While the exact cause of this degradation is still unknown, it is well known that it causes efficiency reduction of (approximately)10%rel.

It should be clear from above explanation that LID and LeTID is complex. While there exist many solutions on paper, there application to the commercial market of solar cells is still a mystery. However a better approach available is to shift from p-type to n-type solar cell. This is because the n type solar cells are doped with phosphorus and only a part of the cell is doped to form n-p junction. This makes the concentration of Boron in the cell less when compared to the p-type cell thus making an n-typesolar cell immune (to an extent) against LID. While n-type solar cells have started penetrating the market, it would not take much time when n-type almost becomes main stream cell in years to come (Figure 3).

Figure 3: World market share of different wafers (Source: ITRPV) Solar panels

While other researches are on their way to reduce LID and LeTID, it is estimated that these effects may not completely be eradicated from the solar cells. From the current value of 2.5%, LID may only drop to 2.0% by 2020 which while significant shows that LID is there to stay (Figure 4).

Figure 4: Expected trend of product warranty and degradation of crystalline silicon PV module (Source: ITRPV) Solar power

We at Waaree Energies have understood this trend of LID and LeTID (for PERC). While our modules are not LID free, we ensure that all the necessary steps and adequate quality checks are taken such that the module does not lose (more than specified) power output. This is also been backed up by the stringent quality certificates we have received from various national and international certifying agency.

Let us all pledge to make solar energy the primary source of energy in the near future.

RAHE ROSHAN HAMARA NATION