In photovoltaic (PV) modules, an initial drop in efficiency is a well-known phenomenon. Known as light induced degradation, it has long been included in the performance guarantees offered by producers in the industry or the calculations of project developers and plant operators. Light induced degradation can cause an approximate 2 per cent decrease in system performance in the first few hours of operation of any new PV installation.

In 2006, a new form of performance degradation began to be noticed. The effect was first discovered in solar plants whose modules used a certain type of high-performance cells. Initial suspicions turned to the specific technology of these cells, which differed substantially from industry-standard cells. In these cases, a concentration of charge carriers at the cell surface was suggested to be the cause of the potential difference between the cells and the ground potential.

It has now been established that this new type of degradation – known as potential induced degradation (PID) or high voltage stress – is indeed promoted by the special technology used in these cells. However, the phenomenon has also been observed in standard and thin-film cells.

As these panels are serially interconnected modules of all cell types, the more modules you connect, the higher the voltage gets. It is this high voltage that causes PID.

The comparatively late discovery of PID in standard solar cells is a direct product of the boom of solar power and the increasing size of solar plants – until approximately five years ago, most solar power installations took the form of domestic, small-scale rooftop plants. PID only became apparent when the first solar power plants were constructed and put into operation with their typically high system voltages.

SOLON SE and PID

SOLON first came across the problem in early 2009. SOLON’s business revolves around the production of solar modules and PV systems, as well as the development and construction of large-scale rooftop installations and ready-for-use solar power plants. The company also offers maintenance services for SOLON plants.

In SOLON’s research, the company encountered a growing number of instances of PID. SOLON immediately understood the risk of PID for the successful growth of large-scale solar power plants with their great promise to help aid the switch to a renewable energy future. By affecting the long-term sustainability of system performance, PID touches on a key element of solar technology that virtually all financing concepts in the industry rely on.

High voltage is stressing the cells

In order to understand PID, it is important to understand how a solar cell works and how it interacts with other materials in the module. In simple terms, a standard cell consists of a thin film of negatively doped (polarised) silicon on top of a thicker layer of positively doped silicon. When exposed to sunlight, so called electron-hole pairs are produced in the space between the two layers – the depletion zone or space-charge region. Positively charged holes move in the direction of the positively doped semiconductor, whereas negatively charged electrons move to its negatively doped counterpart. The charge carriers are then conducted to the next cell.

The serial array of the cells means that the voltage increases from cell to cell in the module. The same applies to the individual modules in the system, also connected in series. The maximum voltage in such a system can easily reach up to 1,000 volts – this is basically a positive effect, because the higher the voltage is, the lower the electrical resistance. In this way, high voltage helps to increase the capacity of the system.

At the same time, these high system voltages can lead to unwanted leakage currents between the solar cells, the bedding materials, glass, and the grounded module frame. This allows a positive charge to build up on the anti-glare coating at the surface of the cells. The result is a temporary short circuit in the affected cells, which means a decrease in cell voltage and a drop in efficiency – an effect that is reinforced by high temperatures or humidity around the modules.

Possible solutions for PID

PID can be tackled in different ways, ideally preventing the phenomenon from occurring at all. The ideal solution would be to prevent the problem by modifying the cell itself and thus grasping the problem at its roots.

Solving the problem on the level of the system is another option, albeit one with some noticeable drawbacks.

One such solution would be to ground the negatively charged modules in the systems that are subject to the risk of PID. This would concern the half of the module string that is closest to the negative pole of the converter. However, this is only an option in installations that use inverters with integrated transformers, which is usually only the case in older systems, since the lower efficiency of these inverters has virtually removed them from the market.

Systems that use transformerless inverters can be depolarised by using the so-called PV offset boxes that build up a temporary counter-potential in the PID-affected parts of the system – a solution that produces additional costs.

PID protection module-by-module

Such solutions at a system level might have their place in established plants, but they will not be the final answer for the issue of PID. The ball is now in the court of the cell producers who can tackle the problem at its root by modifying the anti-glare coating on their cells. But a solution might be a long time coming.

As a leading producer of solar modules, SOLON is tackling the problem where we know best – on the individual module. By using specially-developed bedding materials that offer higher levels of insulation, the problem of leakage currents is avoided and SOLON able to offer is effective and sustainable prevention of PID. Since January 2011, SOLON has been using these new materials, which have passed all tests and certifications in the industry with flying colours.

SOLON believes that PID can now be considered a problem of the past – but will continue to research and will let the producers of solar cells benefit from its findings. SOLON wants to learn more about the mechanisms at work in PID and pave the way for optimum performance on the level of individual modules and, in particular, whole systems.

PID – Affecting Australia

PID is not limited to any particular region. The problem will occur wherever solar plants operate at high system voltages. This is not only the case in large-scale power plants, as it can also affect any on-grid system above a 1 kilowatt peak which, have been installed all over Australia. PID is highly relevant for Australian solar installations, since many factors that contribute to PID, such as high temperatures or humidity, are a common occurrence across Australia.

As a market for solar power, Australia is taking its first steps towards a promising future. Now that the problem of PID has been understood and suitable countermeasures have been found, the planners and operators of PV projects will benefit from considering the phenomenon and its solution in their future projects.

Dr Lars Podlowski is the Chief Technology Officer at SOLON SE, and Daniel Hundmaier is the PR Manager for SOLON SE, located in Berlin, Germany.