Sunday read: the effects of defects


From pv magazine 07/2021

Your recent work represents a big step in understanding how perovskite solar cells work, particularly in terms of stability. Could you please explain a bit about what you’ve found out?

We have been working on the stability of perovskite solar cells for more than 15 years. There are many different strategies in this area; what we have done is the engineering of organic additives within the halide perovskite absorber material.

What often happens in this type of research is that people focus only on improving the efficiency. They assume that if you improve the quality of the perovskite thin film absorber you will improve the efficiency and the stability of the solar cell: that one is linked to the other. We realised that we can separate the two, and this is exciting because now we can work to optimise the efficiency and the stability of the solar cell separately.

The efficiency we achieved was good, at around 21%, but the additive did not improve it as we expected. The stability, however, was extraordinary. We observed almost no degradation after 1,000 hours under continuous irradiation, compared to more than 20% loss for cells without the additive. And next we had to understand why this was happening.

We realised that we were not playing with deep defect passivation, and we were not affecting the voltage and efficiency of the cell. But we were immobilising ions through the passivation of shallow defects. Shallow defects are related to ion migration, one of the primary causes of solar cell instability.

And in terms of commercialising this technology, how important is it to understand the material defects at this level?

It’s very important. Not many have obtained very stable devices that also reach high efficiencies. And even the ones that have didn’t really understand why the stability improved. Now, we can analyse these shallow defects and start to modify them. Eventually we will be able to engineer the material defects to make cells that are much more stable, but the most important thing is that we now know why this is happening. What we have found is only one part of the puzzle, but I think it is a big part. I think now we will see a lot of work going into how to engineer these shallow defects.

In silicon PV, even today a lot of work is still going into understanding defects and degradation mechanisms. Are there comparisons to be made here with perovskites?

Silicon and halide perovskites are very different. In silicon, you focus on deep defects. And because we learned from the silicon PV community, much research in perovskites up to now has focused on this. Silicon is a classical semiconductor, while halide perovskites are soft, ionic/electronic conductors. You don’t see this mixture of ionic and electronic conductivity in silicon.

It is beneficial on one side, and negative on the other. What we have to do is engineer these ions so that they stay together and don’t move. We found hydrogen bonds between the additive and the halide perovskite. The strength of these bonds is what keeps the ions in place. That’s very important, and makes the perovskite look more like a classical semiconductor.

And is stability still the biggest challenge with perovskite solar cells?

Stability is one of the most important, but there are other challenges – for example, halide perovskites use lead, and lead is toxic. So we see a lot of people working on lead-free perovskites, replacing the lead with another metal – tin or bismuth, for example.

There are different views on this, and some say that the amount of lead being used is so small that it’s not a problem. But there is reason for concern, and we are working on lead-free materials as well. Right now, lead free perovskites give you lower efficiency – with lead we have reached above 25%, and with lead-free it’s still 17% to 18%, but we are getting there.

This work was the result of collaboration with quite a long list of scientists and research institutes from all across Europe. How important is this type of collaboration to developing new technologies?

Every analysis we did told us that our solar cells, with or without the additive, were exactly the same. So it was difficult to understand the difference in stability, and it took time and effort to work out that it was down to this shallow defect passivation. The part where we demonstrate the passivation of shallow defects was made in collaboration with groups in Sweden and Switzerland, and we were including people along the way who helped us to understand various parts – the number of people involved just grew and grew.

If you really want to make a big contribution to science, it has to come through collaboration. Because it is very difficult to get all of this characterisation equipment and skills in one laboratory, these tools are very expensive and the techniques are very selective, you have to choose very carefully who you will need to work with.

Where do you see the next steps – both for your group and for perovskite technology more broadly?

We have a project related to outdoor testing for PSCs supported by machine-learning techniques. These results will be very important, because there still aren’t that many people working on testing in outdoor conditions. We aim to predict the PSC lifetime through all the data we will collect in real life.

We should be able to prove many things – for example, people have observed that PSCs can degrade in light and then recover in darkness. This is also related to ion migration, and because we have observed that we can immobilise these ions, maybe this will no longer occur. We expect this technology to reach commercialisation in the coming years.

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