The efficiency of organic-inorganic pervoskite solar cell is increasing rapidly, however the stability is a big issure. The device is extremely sensitive to moisture. For the device with a planar stucutre, 30 mins in air is enough to kill it. But for the device with a mesoporous structure, it could last a few days in air. Just want to know the exact reason. Any clue will be appreciated.
I do agree that stability is a great issue at the moment in this research field, and no satisfying answer have been found to that problem yet. In fact, as the device architecture, processing routines and materials in use are still so diverse at the moment, that every combination of materials may have different problems and difficulties coming with them.
While moisture definitely is an issue, I agree that this can be handled by good encapsulation. But to my feeling, moisture is not the only problem, as devices in inert gas or dry atmhosphere do degrade also (although slower to some extent).
See for example the recent record device paper in Science by Huanping Zhou et al from the group of Prof. Yang Yang at the UCLA.:
Huanping Zhou et al.: DOI: 10.1126/science.1254050
There is still a long way to go in this sense, I suppose.
Best,
Paul Pistor
Thank you, Paul. I just doubt that device degradation more likely is interface-related. (The perovskite material is not stable though. ) It could be the reason that mesoporous structure is more stable than the planar one.
Dear Sheldon, I agree that interface may play a crucial role in degradation mechanism. Actually, there are a variety of levers where degradation can tackle the device and to my feeling nobody knows the answer to this at the moment.
That interfaces do play an important role can be seen as stability is enhanced by replacement of TiO2 by Al2O3 for example in ref. DOI:10.1038/ncomms3885, or Al2O3 is added to the interface to the HTM (ref.: DOI: 10.1039/c3ta13606j).
What is a little counterintuitive is that the interface (of the perovskite to the electron conductor TiO2) is greatly enhanced in mesoscopic solar cells. To my humble opinion, it is still early to compare stabilities of solar cells prepared at different sites due to many variations in cell architecture, processing, measuring and storing conditions and many factors more to be taken into account. Therefore the decision on stability of planar and mesoscopic cells might still not be conclusive.
Sorry for not being able to give a conclusive answer to your question, but I am happy to learn more about it any time.
@ Fadi: We have been investigating this research area and found out that water ingress is not the major player to destabilize the perovskite devices. We have investigated planar and various type of scaffold cells/modules which were encapsulated. We have observed nearly similar degradation in encapsulated planar to that of an open device.
We believe it is related to crystallanity of the scaffold layer and also the morphology of the perovksite grown. Unfortunately I can't share the publications now (as they are in review process), Will share here once they are accepted.
Dear Sheldon, I came across this question again after a long time and meanwhile we have carried out research on planar, mesoporous (employing TiO2 NPs) and also Nanorods as electron transport layer and compared the performance for over 2500 h. We found that three possible reasons that can also answer your question (to some extent).
See the papers for details.
http://www.sciencedirect.com/science/article/pii/S0378775315003183
and
http://pubs.acs.org/doi/abs/10.1021/acsnano.5b03265
1. Physical encapsulation. It appears that a mesoporous layer acts as a physical barrier to moisture as perovksite layer seems to penetrate within the pore of TiO2.
2. Surface degradation: Among the three ETL we compared we found that degradation follows the trend, planar>TiO2 Nanoparticles> TiO2 Nanorods; i.e., inverse to their surface area. This could suggest that there is surface degradation owing to the higher surface area of planar (ETL is composed of particle of ~1 - 3 nm) and m-TiO2 (size ~25 nm). Nanorods (diameter >100 nm, length ~600 nm) survived surface degradation. This is evident from the XRD over aging and also the amount of perovksite phase fraction left over.
3. It also seems the chemical nature of ETL playes an important role, remember m-TiO2 NPs are anatase and Tio2 Nanorods are Rutile in nature. For details see the second paper in the links provided.
I hope it helps.
Azhar, thank you so much. You gave the convincing reasons. They do make sense!
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