As usually described on this blog, perovskite solar cells represent a topic that constantly needs optimization. Many studies try to bring new elements for improving their stability or their efficiency. Some month ago, Dr Arman M. Soufiani from UNSW (Sydney) published an interesting article: ‘Impact of microstructure on the electron-hole interaction in lead halide perovskites’ under the supervision of Dr Samuel D. Stranks. I find it appealing for two reasons: first, it shows that parameters previously thought to influence the performance of solar cells are not an optimisation factor, leading to fewer constraints on the device’s design. Second, the work has been carried out involving many teams all around the world, showing the field is in constant evolution.
The genesis of the study
In the 1990s, many scientists used to study the electron-hole interaction in perovskite structure crystals. Since the advent of lead halide perovskites for optoelectronic applications, some scientists have focused on this. In 2015, Prof. Annamaria Petrozza reported in Nature that fabrication procedures and morphology had an important influence on the electronic behaviour of perovskites. The free carriers and excitonic regimes were said to be influenced by the microstructure.
The morphology and size of the crystals (usually called microstructure) are, in the case of perovskites, highly influenced by the deposition techniques. Polycrystalline thin films grown from solution processes do not produce the same crystals as of single crystal growth or nanocrystals. However, the octahedra (e.g. [PbI₆]⁴⁻ in CH₃NH₃PbI₃) in the crystalline structure is expected not to change significantly with the processing method. This idea was the starting point of Arman’s study:
Actually my PhD was composed of two parts. The first was looking at the fundamental properties of perovskites and in particular their excitonic characteristics whereas, the second was dedicated to characterisation of devices. All of my previous study lead me to the idea that the different processing methods might not influence the excitonic binding energy in these semiconductors. We proved it thanks to an international collaboration. We prepared several samples both at UNSW and University of Cambridge and performed few preliminary characterisations before we sent the samples to colleagues at Laboratoire National des Champs Magnétiques Intenses (Toulouse) who are specialised in high magnetic field optical measurements.
Before starting the research, I had this confidence that the binding energy would not change noticeably with the microstructure, because inorganic octahedra remains unchanged. Of course, we expect the grain size to influence the order and the orientation of the organic cation, but I supposed this impact not to be significant.
Observation of the exciton binding energy
In order to properly understand the optoelectronic properties of these materials, the strength of the Coulomb interaction between photo-generated electron in the conduction band and hole in the valence band is of high importance. It is called the exciton binding energy and refers to the fact that once separated the different carriers can recombine or collected in such different ways that can affect the optoelectronic properties. In perovskite semiconductors, the exciton binding energy is small and that contributes to their good device performances; after light absorption, mostly free-carriers are spontaneously generated.
The observation of the exciton binding energy gives insights not only on solar cells performances but also on luminescence applications. Because the binding energy affects the band-gap and the recombination behaviour of charge carriers, light emission following radiative recombination is a topic that deserves to be understood. Dr Samuel D. Stranks works on this in the Cavendish Laboratory (Cambridge, UK):
We are exploring luminescence as a tool not just for solar cells applications but also for LED and lasing applications, so I am looking at developing new optoelectronic materials. At this point in time, hybrid perovskite is one of our key focuses. We are looking at fundamental recombination processes and fundamental sciences in these materials, and using that to learn about how we can improve the devices and the materials, and how we can take them to their limits.
The research Arman achieved shows that the impact of microstructures of perovskite is not significant in the exciton binding energy and charge carrier effective masses
What else could be done?
This result might leave one puzzled. In the research of more and more performant and stable solar cells, knowing that changing a parameter does not affect an electronic property of a material might seem unsettling. With this it lets scientists with no clear idea of what to optimise. In fact it is quite a good result: the processing methods are usually different for manufacturing with the goal of commercialisation and in glovebox fabrication. Whatever the method employed, the described property will not change. The optimisation is thus less constrained.
Following this study, there are many improvement paths. If the octahedral (i.e. the inorganic cage which is the main contributor to the electronic states at the band-edge) are not modified, it does not mean the band-gap does not change either, because it depends on a larger scale. As Arman told me:
The band-gap changes when the unit cell parameters changes. When we move from samples with larger grains to smaller crystals, strains are imposed into the crystal structure which changes the unit cell parameters and thus, affects the band-gap. Nevertheless, the binding energy and excitonic reduced mass of the charge-carriers seems to remain unchanged. Relatedly, replacing methylammonium by formamidinium, and keeping the lead iodide cage, the binding energy remains almost similar because you haven’t changed the inorganic octahedra, contrary to the band-gap.
This can be seen by observing the colour of the material. A change in the crystal size due to the cation size is reflected in changes in the band-gap of the perovskite. That was reported in my previous blog post.
Sam Stranks had a slightly different approach with the future studies that could be carried out from this:
I am looking forward to seeing industrial applications of perovskites. Of course we will see products soon, and certainly from OxfordPV. I am particularly excited about how we can re-imagine what PV can do. We started with silicon, but it will soon be possible to process coloured, lightweight, flexible photovoltaics. All of these would reduce the cost.
However, I think there is still quite a feature to address to push the device at its limits. Long term stability and ion migration are absolutely essential to address to push them into the market. We work with very good candidates with very good charge transport or good charge properties. We are looking for a winner, that is part of the job we are trying to do.
It leads us to learn more about the recombination processes in solar cells. No wonder that it will be a topic we will hear about in the coming months…
- Dr Arman M. Soufiani, University of New South Wales, Sydney: firstname.lastname@example.org
- Dr Samuel D. Stranks, Optoelectronics Group, Cavendish Laboratory, Cambridge University: email@example.com