OPV stability, an intriguing mechanical approach

Two weeks ago, we discussed the stability issues of Perovskite Solar Cells. I invite you to keep connection with stability, but with organic solar cells. I found a very interesting work combining different techniques and analysis, from Dr. Sylvain Chambon. Let’s go back to Bordeaux, the southwestern French Organic Electronics’ City where he based his research with a mechanical approach of stability!

His work is composed of two articles, that are part of his latest PhD student. He used to work on the degradation of OPV, with a focus on interfaces: Interfacial thermal degradation in inverted organic solar cells (Applied Physics Letter) and Improved mechanical adhesion and electronic stability of organic solar cells with thermal ageing: the role of diffusion at the hole extraction interface (Materials Chemistry A).

The origin of stability issues

Stability should always be put in relation with a stress. An architecture that is stable when stored in the air can show large instabilities when heated or photosensitized. The mechanisms leading to instability are very diverse.

As we saw last week the structure of Organic Solar Cells is a bit different from the structure of Perovskite solar cells. The active layer is composed of two organic semiconductors, polymers or organic materials. The amorphous structure of the bulk polymers is often very stable when no stress is applied but thermal stress can modify the fine morphology of the active layer.

On the one hand stability with OPV issue is chemical. It deals with the role of oxygen, of water and interactions with electrodes. I will not give the details of the degradation mechanisms in this article but the diffusion of oxygen and water degrade the polymers, that can be enhanced under light illumination: that is photooxydation and photochemistry.

On the other hand OPV also face physical degradation. Two types of physical degradation can co-exist: on the one hand clusters of materials like PCBM can appear leading to performance losses. That is called morphological degradation. On the other hand OPV are composed of stacked thin layers that have different elastic modulus, that can crack upon bending or delaminate due to differences in adhesion. That is flexibility degradation. Such a delamination can be tracked by measuring the fracture energy between the different layers.

The progress of findings

The story of the creative study Dr. Sylvain Chambon supervised begins with the constitution of devices with different hole transport layers and electrodes. They witnessed that some structures were more stable than others, particularly oxides combinations were more stable to air storage. However, when carrying out thermal ageing, they found that the silver-molybdenum oxide device had much lower performances compared with silver-pedot. They correlated it to a VOC drop. The most astonishing is that neither molibdenum nor silver trigger any VOC drop by themselves.

This leads to the hypothesis that the interfaces play a major role in these observations. It might be due to diffusion of the species, but also to change of surface roughness. This two hypothesis have been proved possible in the Applied Physics Letter article with RBS measurements.

Here the second article is of astounding interest. The scientists decided to analyse the interfaces after having cut the samples. Fracture studies have been carried out from the cut. It’s very interesting to see such analysis being used in solar cells. The article brought together probe techniques and communities that do not usually meet up: mechanical analysis and organic electronics.

illustration of fracture study

Illustration of the solar cell structure and the mechanical fracture study. The measured fracture energy leads to the nature and the position of the degradation.

Thus large devices have been processed for the scientists to carry out research measurements at Stanford University, in Prof. Reinold Dauskardt’s group. I find appealing that all the usually process methods have been changed to adapt such fracture study. The sizes involved were different that the one used typically for solar cells and scaling concepts with the conservation of the same morphology must involve long thoughts.

Then the idea was to understand if the electrode dewetting will foster or not the delamination of the electrode under a thermal stress. The fracture analysis correlates the resistance energy of the crack to the position where it appears, could it be in the bulk or at the interface. They showed that initially the crack spreads in the hole transport layer, and after thermal ageing, it does in the active layer of the solar cells. In fact the HTL/active layer interface becomes more resistant while, concomitantly the active layer becomes less resistant when thermally stressed.

That lets scientists puzzled

Hence Dr. Sylvain Chambon’s team has shown an interdiffusion could happen in the different layers and that it leads to an increase of the adhesion between the active layer and the HTL/electrode. However, all the probe techniques that were used can not prove with certainty that diffusion phenomenon is only responsible of the observed VOC loss. When I talked to him about his articles, he felt bewildered with their results. On the one hand they observed hints that diffusion happens, but another degradation mechanism must occur in silver-molybdenum oxide devices to fully explain the VOC drop. We can though be optimistic for finding new clues in other projects with other pathways. I’m sure Sylvain is on the way!

The cover photo is from Cambridge University on Flickr. The illustration is from S.R. Dupont et al.

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