Last July, Dr Ahmad R. Kirmani published the following article the same day he defended his PhD. He has worked with Quantum Dot Solar Cells at the King Abdullah University of Science and Technology (Saudi Arabia), on a work I invited him to discuss here: Molecular Doping of the Hole-Transporting Layer for Efficient, Single-Step-Deposited Colloidal Quantum Dot Photovoltaics. This paper deals with a way to block a back-flow of photogenerated electrons, care band-bending issues in n-i-p architecture thin film photovoltaics and enhance the efficiency of these solar cells.

What are Quantum Dot Solar Cells?

Quantum dot solar cells (QDSCs) work almost the same way as Perovskite or Dye Sensitized solar cells: an active material produces excitons when illuminated. These excitons are then separated and the carriers are collected by hole and electron transport layers to generate a current. Here the active material used is composed of quantum dots. These are nanoscale semiconductors endowed with the quantum size effect, which allows their bandgap to be tuned by changing the size of the dots.

The efficiency of QDSC has recently reached 10%, that is comparable to organic solar cells.

Quantum dots are semiconducting particles that have the exceptional particularity to have finite energies. This is allowed by their very small size that is below the Exciton Bohr radius. In the article Ahmad proposed, solar cells are composed of Colloidal Quantum Dots that are easy to process and fabricate

Starting with a few ideas

The article presented can actually be seen as an extension of another article: Remote Molecular Doping of Colloidal Quantum Dot Photovoltaics. In this first part of the work, Ahmad introduced a platform to dope colloidal QD solids, that can be used as absorber layers in solar cells. He explains his ideas referring to the QD community:

To go very briefly in the historical aspect, colloidal quantum dots doping started in a very comprehensive manner, by a few researchers around the globe. Scientists begun with the introduction of dopants inside of the lattice of the quantum dots, or decorating the surface of the nanocrystal. However neither these applications, nor these proceedings made it into realistic solar devices.

After having considered all of this, his idea in the first article was to introduce a novel protocol for doping quantum dots that could have been used directly in QDSCs.

In fact, once the quantum dot absorber layer was processed, he deposited a dopant solution on top of the cell:

In this follow-up study, the paper published a couple of weeks ago about the molecular doping of the hole transporter, I have expanded the applcation gamut of the remote doping scheme to the more modern, higher-efficiency archticture, mimicing the current standards of the thin film photovoltaics. We fabricated n-i-p cells, the device architecture employ the molecular dopants to effectively p-dope the hole transporter, pushing its expect of a doping technique. In essence, doing so removes the unfavourable ‘kink’ in the energy bands near the hole-collecting junction, allowing smoother charge flow and more efficient charge-collection. Our efforts finally result in an optimally doped hole-transporter and enhanced device performances!

We believe that the challenge Ahmad addressed will soon be faced by the perovskite community and that they can draw on the experience we present here. The significance of Ahmad’s study was justified a few days ago, as Alba Pellaroque from Prof. Henry Snaith’s group at Oxford University published a paper employing these molecular dopants to cure similar interfacial band bending issues in the n-i-p perovskite solar cells.

A stability issue?

Doping the quantum dot solar cells is easy in the way that quantum dots are all inorganic.

I definitely think that in the case of perovskite, this is currently a problem. The use of organic material as the hole transport layer, Spiro-O-Me-TAD, leads to many issues. People tried to use undoped spiro, but it never worked and led to low devices performances. The current strategy is tu use lithium-doped Spiro. Importantly, Spiro is hygroscopic and hence the perovskite photovoltaics faces stability concerns. I strongly believe that this thin organic hole transporter is a barrier to the development of the perovskite technology. A switch to inorganic hole-transporters, such as those based on quantum dots, might be a game-changer for thin-film photovoltaics!

Organic materials are not known to be very stable. We previously showed stability concerns with <Organic and Perovskite Solar Cells, mostly due to organic materials. Future will tell us if Ahmad’s vision is right!

To conclude, the doping that has been achieved in colloidal quantum dots could be transposed to perovskite techniques if scientists first find a solution to current issues with these devices. That may increase their efficiency, and the use of 100% inorganic materials could enhance the stability of perovskites.