Photovoltaic sector: the tandem technology's increasing reliability
A study conducted by 3Sun and recently published on leading scientific magazine 'Advanced Science', shows how to bypass the critical issues caused by partial shading on silicon-perovskite technology
How can we prevent the “reverse bias” phenomenon from stressing out silicon-perovskite tandem cells? This question was answered by a study conducted by some of our colleagues of the R&D team, in partnership with the CEA-Liten at INES research centre, and published on leading magazine ‘Advanced Science’, under the title “Silicon/Perovskite Tandem Solar Cells with Reverse Bias Stability down to -40V. Unveiling the Role of Electrical and Optical Design”.
Where did our study set off from?
From 'Reverse bias', which is one of the factors that may compromise the efficiency and integrity of a solar module: even partial shading of the system caused by a tree, by snow or by any other elements can stress out the cells and permanently affect their performance. This may be especially critical on thin-film technology systems, including next-generation models, such as perovskite. 'Reverse bias' has a lower impact on crystalline silicon systems (including heterojunction - HJT systems), which, through special measures - such as integrating bypass diodes inside the PV module - can better adapt to this effect.
Tandem technology cells, high performance and resistance
In its capacity as innovation leader in the photovoltaic sector, 3SUN has analysed the impact of 'reverse bias' on its silicon/perovskite tandem cells, currently under development. The very promising results, recently published in a study on 'Advanced Science', reveal how the tandem coupling with silicon, apart from achieving a cell efficiency higher than 30%, can protect perovskite against the unwanted effects of reverse bias.
In the words of one of the study researchers, our colleague Diego Di Girolamo, 3SUN Advanced R&D Specialist: 'In this study, we analysed tandem cells' reaction to reverse bias, and understood how silicon can reduce the effects on perovskite. At the same time, with it, were able to grasp how to improve the design of some of the key aspects of the tandem technology, to ensure the cell's greatest protection efficiency."
Giuliana Giuliano and Diego Di Girolamo, from our R&D team, participated in the research published in ‘Advance Science’.
What happens to a PV cell in partial shading conditions
'The study - continues Di Girolamo - sums up the activities carried out to analyse the stability and reliability of the silicon/perovskite tandem technology in reverse bias cases. This is a central focus in the photovoltaic sector, and, in general, for any kind of technology, especially the latest ones, such as the tandem system. In particular, the research observed what happens when the solar cells are partially exposed to shading, with some of them generating energy while some others do not, therefore tending to dissipate the energy produced."
So, what does happen to the module, when it is partially shaded? 'We should keep in mind that, usually, a photovoltaic module comprises several solar cells connected to one another in series. This implies that the same current must cross them all. However, if one cell is not exposed to sunlight, since it cannot generate energy, it will end up dissipating the energy produced by the other 'lit up' cells by warming up, and generating a negative potential" - says Di Girolamo. "This causes a thermal and electric stress, which could permanently compromise the solar cell efficiency - an especially relevant aspect for thin-film technologies, such as the PV modules that employ only perovskite as active material".
3SUN: a solution to reverse bias
This is no trivial matter, as it could destabilize the entire system. In our colleague's words: “Perovskite is a recent material, and we know less about it compared to silicon; it tends to suffer from a certain extend of degradation, if subjected to reverse bias. However, with this study, we have shown how silicon can protect it and have identified the aspects to take into account to maximize such protection. We now have some guidelines to design a stable tandem cell when exposed to stress caused by partial shading. Moreover, our study may be used to design more complete evaluation protocols to assess the tandem cells' reliability with respect to reverse bias, which better describe the actual degradation mechanism, which is very relevant, since we are talking about innovative technology."
The central result is the understanding that, by coupling perovskite and silicon, we can use conventional strategies used for silicon also in tandem cells, to preserve the perovskite and its performances through bypass diodes. 'Essentially - says Di Girolamo - we have observed that, even by bringing the tandem to extremely stressful values, for more hours than normal, in likely operating conditions - such as -40V, if the solar cell has been designed properly, it will preserve all of its initial efficiency."