Developing a reliable service life for solar

What makes developing new applications with thin-film solar cells so difficult? In this series we look at the work behind these new developments. A peek into the kitchen of a research institute. In this first episode we ask Dorrit Roosen why reliability is so difficult to investigate.

When people install traditional silicon-based solar panels, they can expect service life of 20 to 30 years. The solar panel is well protected against the influences of water and light. In recent decades, much knowledge has been gained on reliability of solar panels. This technology is also used with thin-film solar panels on glass, but a different approach is needed for flexible solar cells.

Protection is crucial

Glass is an ideal encapsulation material for most solar panels: it is transparent, inexpensive, does not allow moisture or gasses through and can easily be recycled. But the disadvantage of glass is that it is heavy [1]. It is also stiff and fragile and therefore not suitable for flexible solar cells [2].

High demands are placed on the material that must protect flexible solar cells. It must protect the vulnerable layers of the solar cell against external influences, both perovskite and CIGS lose their conversion efficiency due to degradation under the influence of water and oxygen. For high efficiency, the encapsulation material must be sufficient transparent and of course, be available at acceptable costs.

“To protect solar cells well, a film is required that allows 1,000 times less moisture than the best films used to package food,” says Dorrit Roosen, researcher in the Shared Research Program Integration, “Also at the vulnerable edges and on the area where the connectors leave the foil, the design and materials must be good enough to achieve its requested service life”. With the material you are not there yet, you must also be able to process it well.

Solliance Solar Research foresees in its technology roadmap solar cells will be produced on the roll, without expensive vacuum or high-temperature processes. Therefore the research on the protective materials focuses on flexible, polymer films, whether or not in combination with inorganic layers.

Combination of layers provides synergy

There is not a single material in which all properties come together. However, a smart combination of different materials can create a suitable barrier for thin-film solar cells. A thin inorganic layer can make the film impenetrable for oxygen and water. “Remember that if you bend such a pile of different materials, the layers are not allowed to crack, otherwise the barrier loses its function,” emphasizes Dorrit Roosen.

The research by Solliance Solar Research and TNO also focuses on technologies to deposit high quality layers quickly on a solar cell. Solliance strives to combine process steps as much as possible in one production process. This keeps costs low and often results in higher quality. With spatial atomic layer deposition, for example, it is possible to make roll-to-roll, fast thin layers of an extremely high quality.

And what about testing lifetime?

Measuring lifetime takes time. Only after many years in the open air can it be determined whether the solar panel achieves the required lifespan or whether an improvement was effective enough.

Fortunately, there are standard test methods, that help predict lifetimes in real life, Accelerated Lifetime Tests (ALT). These test methods try to accelerate the dominant failure mechanism. They are very important to be able to quickly measure results in the research process. ALT tests are normalized and are used by the industry. See also boxed area.

As more data becomes available in real life, ALT tests can be verified. A statistical link can be made between the outcome of an ALT test and the actual lifespan. The lifespan can then be predicted with this mathematical model. What makes is really complicated is that all materials and processes have influence on the lifespan of solar modules. Changes in one of these variables influence the life span and the accuracy of  your predictive mathematical model. Therefore materials and processes are researched and validated by Solliance Solar Research.

Results

The operating conditions for flexible solar cells are different from those for conventional, rigid panels. Flexible solar cells are mechanically and thermally stressed as a flexible product. Because flexible solar cells can be processed industrially in existing (building) materials, we also investigate the effect of mechanical and thermal stress during this production process.

Figure 1 shows the efficiency of CIGS in a Damp Heat test. For cells with a barrier layer and without barrier layer, the conversion efficiency was regularly measured after staying in a climate cabinet at 85 degrees Celsius and a relative humidity of 85%. See also boxed area for a brief overview of a number of ALT tests.

This is even more true in the research and development of commercial perovskite solar cells (PSCs). Shared Research Program PSC investigates systematically the degradation mechanisms of PSCs. Besides intrinsic issues, how different materials react on each other and influence stability, Solliance  focuses also on stability to extrinsic factors like UV-light or moisture.

The perovskite team found that device configuration, perovskite composition, type of layers and electrodes, all can be modified to decrease the influence of external effects on the operating lifetime. All these factors to be considered to achieve higher stability.

At this moment, the best configuration, cells with s-ALD barrier layer, retained 93% of initial stabilized power output after 3000 hours aging at 85°C in an inert atmosphere, significantly better than cells without the protective barrier. “Based on our knowledge this is the best word wide result so far for thermal stability of inverted perovskite cells, both in terms of aging time and final retained efficiency after aging”, according to Mehrdad Najafi, researcher in the Shared Research Program PSC.

Figure 1. Test results of samples CIGS with PET foil protection during Damp Heat test, plain PET foil (blue) and PET with barrier layer (orange)

Figure 2. Test results of Heat Load test on perovskite solar cells with electron transport s-ALD barrier

Further developments in testing

Flexible solar cells have unique properties that are simply not provided for in the test for rigid modules. Yet there are no separate standardized tests for flexible solar modules, so that the tests for conventional panels are also used for flexible semi-finished products or panels. Solliance Solar Research investigates whether new test methods can be used (see Flexible solar gets new test), first of all to investigate failure mechanisms and to substantiate lifetime research. Later this research can contribute to the development of standard test for flexible solar modules.

Footnotes