When we think of the long term future for perovskite solar cells, there are basically two strategies. The first is to enhance their efficiency by making tandem solar cells. The second is to create flexible devices with roll to roll technologies, as industrials currently do with organic solar cells. This latter strategy has an enormous potential, but shows an important issue: to be scalable, we absolutely need enhanced stability, especially at the interface between the electrodes and the active layer. One way to enhance this stablity is to use carbon nanotubes (CNTs).

To present CNTs for solar cell applications, let me introduce Dr Severin N. Habisreutinger (@sevnhabis) who, during and after his Ph.D. at the University of Oxford, worked extensively with carbon nanotubes for perovskite applications. He has now ben awarded the Director’s fellowship at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, for investigating the nature of CNT-perovskite interface.

About CNTs

To imagine a (single-wall) carbon nanotube think of a sheet of paper being rolled up such that it forms a tube. If you replace the sheet of paper with a monoatomic sheet of carbon atoms (known as graphene) you get a carbon nanotube. Commonly the diameter is around one nanometer (4-5 orders of magnitude smaller than the diameter of a single strand of hair). CNTs exhibit several very interesting properties. On one hand, their mechanical strength make it one of the strongest materials we know. In addition they are chemically quite resilient, which means they do not readily form strong chemical bonds with other materials. This is important, in particular, at interfaces with very reactive materials. And finally, the property which makes them particularly interesting for photovoltaic application is their extraordinary charge-transport characteristics, above all the high charge carrier mobility. This means that a charge carrier can travel a long distance through a CNT before it is lost when it recombines with a charge carrier of the opposite polarity. The basic idea how to exploit this particular property is to use CNTs at the interface with an absorber material in which light generates charge carriers. These photogenerated carriers can then be rapidly transferred away from the interface through the CNTs which act like high-speed channels for those charges moving them away from a region, namely the illuminated absorber, in which they can readily recombine and would thus be lost.

A challenge for using CNTs is the fact that they have the tendency to form bundles, which makes it more difficult to process them out of solution and it negates some of their excellent electronic properties. A strategy which successfully overcomes this challenge is wrapping individual CNTs with conjugated polymers, known from organic electronics.

With this approach, CNT thin films can be used in photovoltaic applications, such as perovskite solar cells, for example as hole-selective layers. As opposed to conventional solar cells, charge separation in a perovskite device does not rely on an internal electric field, instead charge selective contacts introduce a driving force for electrons on one side, and holes on the other side.

For CNTs, the mechanism leading to the collection of holes is a good alignment of the valence band of the CNTs and the HOMO of the perovskite active layer. Another aspect is their ability to block the electrons. The combination of these characteristics makes them a good hole-transport layer. Severin explains:

Actually, the exact underlying mechanism is still a bit contested, but we know that when CNTs are in contact with ambient oxygen, they become more p-type therefore there is a higher concentration of holes available in the carbon nanotubes. That makes them an interesting hole-transporting material which is the main aspect most recent publications are currently looking at.

Transport properties

However, in some conditions, he explains, PCBM can be used to dope the material and make it more n-type:

I think there is a lot of potential in using CNTs both as anode and cathode with the active layer sandwiched in between. Looking forward, I would imagine that quite a few researchers will look into using CNTs in such a capacity, but for now we still need to answer a range of questions.

Specifically for the use in perovskite devices, we would like to know: what exactly happens at the interface in terms of charge transfer kinetics? Do the metallic-type CNTs have a positive or a negative effect? What about hybrid layers comprising CNTs? We expect that the answers will lead to interesting new concepts and ideally to more stable and more efficient contacts.

Large-scale use of CNTs in this capacity still needs a lot of work to be done to better understand the interface interactions. Guess what? That is exactly what Severin is looking at. Therefore we expect a deeper understanding leading to higher quality charge selective layers and more stable solar cells.

Sci-Fi ideas

Our discussion evolved to more futuristic ideas, such as direct inclusion of CNTs into the perovskite absorber. The basic idea is to extract photogenerated charges even more directly thus further reducing recombination losses. At the moment, it seems hard to tell whether this would be beneficial since the charge transport in perovskite is already remarkably good. But we imagined grafting carbon nanotubes on perovskite crystals to avoid surface recombination. In fact, this concept of a heterojunction is essential for organic solar cells because the electron diffusion length is limited, and one needs to provide a direct conducting path between the active layer and the CNT electrode. As mentioned above, this is less of a concern for the perovskite bulk. However, it could also be an approach to avoid recombination at grain boundaries, if CNTs were to penetrate a perovskite film. There is one interesting study, Severin mentioned, by Prof Horváth and co-workers from EPFL who tried some interesting in that direction. In fact, they grew a perovskite single crystal around carbon nanotubes, achieving a vertical protogenic inclusion of CNTs into a MAPbBr₃ single-crystal.

We also talked about using graphene layers:

In general, if you have a pristine graphene layer, it will have better charge transport characteristics than a CNT film. There is a common issue with the charge transfer between individual carbon nanotubes within a conductive film. The inter-tube junctions create obstacles for the charge-carriers often due to the formation of Schottky barriers, limiting the transport of charges on a macro-scale. That is not present in pristine graphene. However, there is still a big challenge for graphene: fabricating it large scale. One interesting way to implement graphene nonetheless is to make a hybrid layer of graphene flakes and CNTs. This way we could use the excellent electrode properties of graphene in combination with the charge selectivity of CNTs.

Thus, we might expect synergistic effects from the combination of both graphene and CNTs. That may be the kind of idea to explore in the future, especially combined with other nano-structured materials.