Investigating Carbon Nanotubes

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 nanostructured materials.

 The cover photo is from NASA Goddard Space Flight Center on Flickr.

Understanding degradation of perovskites

Degradation of perovskite has been a well-developed topic on this blog. Actually, it represents one of the greatest challenges to address. Scientists try to tackle the degradation issues from many approach angles. Here I showcase two articles I’ve read in the past months about how the lattice evolves and how it leads to cell degradation: “Effects of Small Polar Molecules on Degradation Processes of Perovskite Solar Cells”, from Prof. Chun-Sing Lee et al. and “Study on degradation mechanism of perovskite solar cell and their recovering effect by introducing CH₃NH₃I layers” from Prof Masanori Ozaki et al. I find important links between those two articles, that is actually what I wanted to present.

As we all know, degradation of perovskite materials can be caused by humidity, oxygen, light, or heat. After being exposed to humidity, the perovskite’s colour changes, making it a perfect moisture sensitive material. In this very interesting article, Prof. Eugene A. Katz and co-workers found that the decomposition of perovskite is induced by illumination in the presence of water. In fact, concentrated sunlight experiments have shown that light triggers degradation and the mechanism depends highly on the temperature and the composition of the materials.

Water has been considered as the main cause of the instability of the perovskite, causing hydrolysis reactions that degrade the solar cells. Prof. Aron Walsh and co-workers have shown in 2014 that upon illumination and heating, hydrated MAPI (methylammonium lead triiodide) perovskite phases, such as MAPbI₃.H2O and MA₄PbI₆.2H₂O are formed and are followed by the release of a gas phase, composed of MAI. Those hydrated phases present no optoelectronic properties, therefore, this effect is considered responsible for the degradation along which the cell proceeds.

The release of methylammonium ignites the degradation

Prof. Nam-Gyu Park and co-workers reported last year that mobile MA⁺ ions appear in the perovskite film during degradation. Here Prof. Lee’s article is of particular importance. It appears that polar molecules, such as MA⁺ or H₂O are closely related to the degradation processes. Knowing that there are mobile MA⁺ ions in the perovskite film during degradation, the authors investigated the degradation processes of MAPI perovskite solar cells with different PbI₂/MAI ratios. They show with X-ray diffraction that the perovskite lattice is expanded by incorporation of water molecules. This mechanism is followed by a crystal breakage, in which MA⁺ and I⁻ ions are released. Those ions diffuse to form PbI₂ crystals.

The formed lead iodide crystals do not present electronic properties thus they are blocking the conducting channels. Hence the ion diffusion coefficient is lowered with the formation of those crystals. Later on, the scientists have demonstrated that the adsorption of water leads to a decrease in the Jsc along with a bulk degradation that accompanies an ion diffusion process. It becomes important when putting in relation with Prof. Ozaki’s article.

Effects of MAI in recovering the performances of degraded structures

In this interesting article, MAPI perovskite materials are evaluated before and after degradation due to moisture and sunlight. It has been reported that during degradation, the crystalline structure changed with peaks assigned to planes of hexagonal PbI₂ and orthorhombic I₂. Simultaneously, the colour of the perovskite changed, indicating its degradation. Because they observed such a phenomenon, Ozaki et al. tried to add MAI layers on top of the perovskite. They have shown that with this layer addition, the MAPI solar-cell performance recovered. The hypothesis developed is that PbI₂ formed with the degradation allows recrystallization of perovskite after introducing MAI layers.

Thus, with the assumption that evaporated MAI cannot be recycled, the authors tried to add it directly to the surface. That allows the cell to be “refurbished”. This article shows an example of what can be done with MAI to reverse the degradation process. This could have many derived applications in the future.

Those two examples show how scientists have been able to understand, in a very basic way, how degradation happens. These understandings have led them to ideas of better, more stable and more efficient cells. That was to say that the perovskite field is rapidly evolving and we will rapidly have a better understanding of the degradation processes that will allow us to develop more stable and more efficient solar cells.

The cover photo has been taken from Swansea University on Flickr

Defect tolerance within halide perovskite

The properties of crystalline semiconductors, and particularly those giving rise to the photoelectric effect, are mainly due to the presence of defects inside a crystalline lattice. Two weeks ago, at the annual Perovskite Solar Cells and Optoelectronics Conference (#PSCO17) in Oxford, one of the main topic developed was the exceptional defect tolerance found in the perovskites and the doping allowed by it. I contacted Prof. Aron Walsh, from the Department of Materials at Imperial College London (United Kingdom) that published recently a an interesting commentary in Nature Materials: ‘Instilling defect tolerance in new compounds’ and presented new results at the conference.

The role of defects in crystalline semiconductors

There are different type of defects in crystalline semiconductors: interruptions (crystallographic defects) or foreign atoms in the lattice (impurities). Those interruptions can be present in the form of point defects, including atomic vacancies, interstitials or anti-site substitutions, and higher dimensional defects like dislocations, grain boundaries or precipitates. Defects are formed in the lattice because there are thermodynamically favourable: their equilibrium concentration depends highly on their formation energy.

All of these defects will have different impact on the electronic and optoelectronic structures. Defects with energy levels inside the bandgap will induce electron or hole trapping. In other cases, defects can imply electron or hole emission: one hole and electron can be captured by a recombination centre. To sum it up, defects in a crystalline lattice produce recombination or emission centres that are at the heart of the optoelectronic properties of a lattice.

Due to these defects, semiconductors create unintentional n- or p-type doping, leading to the formation of p-n junctions. Also, from Fermi level pinning by defect states, unfavourable band alignment can be created that limit the voltage of solar cells.

Representation of possible implication of defect states within the bandgap (a-e), scattering of a charge array (f) and possible band diagram deformation (h,i) in comparison with the ideal p-n junction band diagram (g)

Defect tolerance properties within perovskites

Perovskites are also crystalline semiconductors. For this reason, defects exist and are active within the lattice. Their implications are not negligible and contribute significantly to many properties like carrier recombination. Also, in some cases, the presence of defects seems to have no major implication on recombination that leads to astonishing performance of perovskite solar cells. Prof. Aron Walsh explains this defect tolerance:

Practically speaking, in halide perovskite we expect many point defects to be present, but they do not destroy the performance of solar cells. In optoelectronic devices, it is essential to avoid non-radiative processes that convert electronic to thermal energy via defects. These events lower efficiency and can trigger device breakdown. It turns out that many of the low energy point defects in halide perovskite are “inert”, which is one of the reasons for the materials success. In other technologies such as kesterite (Cu₂ZnSnS₄) solar cells, non-radiative recombination is high, limiting the solar cell efficiencies to below 15%. Such defect intolerance is common in many materials that have been studied for solar cells (e.g. SnS, Cu₂O, FeS₂), which is why the unusual behaviour of halide perovskites is so important to understand.

I think that understanding the reason leading to such properties will lead to ways to enhance not only the properties of perovskites, but also the properties of other semiconductors. That is one of the reasons why research with perovskite is of high importance.

There are other properties that are in relation with the presence of defects. The mobile ions issue, which was previously discussed on this blog, is said to be due to the defect properties within perovskite structures. The presence of halogen interstitial or vacancy within the structure plays a major role: it has been observed that under light exposure, iodide and bromide species migrate along the cell, creating iodide- and bromide-rich domains, that act as recombination centres. Many scientist think that improving the crystal quality would lower the separation in halogen-rich domains. Personally, I think that a better understanding of the defect tolerance will allow us to develop better materials, that will not present hysteresis cycles lowering the performance of the cell.

Doping the perovskites

In typical crystalline semiconductors, defects are very important for the reason they allow the doping of the structure. Doping means that a given material can be tuned to be a better electron- or hole-acceptor, due to the presence of charged defects. Therefore, the question arises as to whether the perovskite materials can be tuned or not. That is a real question. Aron has an enticing opinion on it:

In principle, doping of perovskite is simple: partial substitution of Pb²⁺ by a metal with a higher charge could generate excess electrons, while replacing it by a singly charged metal could generate excess holes. The challenge is achieving this without forming other defects that compensate the charge.

He explains the process in a clear and precise manner in his latest publication, even which there are now limitations. Working with these limitations, and trying to overcome these are a challenge most of the scientists in the perovskite community are working on.

Going further with p-n junctions

If one manages to dope perovskite with tangible results, these could lead to the creation of p-n junction and therefore to field-effect transistors made with perovskite. The perspective of Prof. Walsh is very interesting at this point:

It will be possible once we would have understood how to control defect concentrations and transport. The current generation of perovskite solar cells are based on a p-i-n architecture, where the perovskite layer is intrinsic (i) with low carrier concentrations. Alternative p-n junctions would enable lower cost perovskite devices with fewer layers of materials, and potentially higher efficiency comparable to the best silicon solar cells. If realised, this would be a major breakthrough in the field.

Thus, the challenge to address is of great importance. I am sure we will understand in the year ahead the main reasons why perovskite are defect-tolerant, and will find solutions to create p-n junctions that will improve both the quality, the performance of solar cells, but also will allow us to create new electronic devices based on perovskites that will revolutionise the semiconductor industry.

The cover photo and the figure have been drawn by James M. Ball and Prof. Annamaria Petrozza in Nature.

Are DSSC still growing ?

First reported in 1991 by Prof. Michael Graetzel from EPFL, Lausanne (Switzerland), Dye sensitised Solar Cells (DSSC) have known important developments in the late 1990s ans early 2000s. Then the field began to falter, the community having massively preferred turning to organic and then Perovskite solar cells. Here I present how DSSCs are working and with the example of an article from Anders Hagfeldt and co-workers published in Nature last June, show that the topic is still evolving.

How do DSSCs work?

The behavior of DSSC is mostly an electrochemical behaviour. The current generation is based on the absorption of light by a dye molecule. Just after the absorption of photons, charge carriers move to a mesoporous semiconductor, generally TiO2 nanoparticles, where electrons and holes are separated and collected at hole-conducting or electron-conducting electrodes. A good engineering of the energy levels is necessary, to confine recombinations and enhance the photoelectric effect.

scheme of DSSC
Architecture of a DSSC

In original DSSCs, the system needs an electrolyte solution, typically Iodide/iodine, to enhance the conductivity to the cathode. Nowadays, most of the DSSCs are composed of a solid electrolyte.

energy diagram
Energy diagram of a DSSC.

Perovskite solar cells have been, at first, in 2009 used in dye sensitzed solar cells, as the dye molecule. But rapidly it has been shown that the dye molecule could be used alone, thanks to its exceptional semiconducting properties. For this reason, all the research that had been carried out on solid state DSSC (ssDSC) had been transferred to the field of perovskite solar cells, allowing its unprecedented growth.

The latest evolutions of DSSCs have been focused on the stability and the efficiency of solar cells, particularly in indoor conditions. In 2005, Fukuzumi et al. reported that copper-based electrolytes worked well in indoor (low-light) conditions. Later on, in 2012, Cao and co-workers have shown that ruthenium-based dyes used with iodide-based electrolytes perform well in ambient conditions. That is what I want to share.

What efforts are done to improve their efficiency?

Nowadays, only a few articles each year are dealing with the field of DSSCs, because it is admitted in the community that the field is mature enough. Most of the research is carried out by Prof. Graetzel or Prof. Nazeeruddin teams at EPFL in Switzerland. That is where Marina Freitag carried out research  reported in his Nature Article: “Dye-sensitised solar cells for efficient power generation under ambient lighting”.

The authors show that a proper design of DSSCs can result in outstanding results in term of power onversion efficiency, and particularly in indoors applications. They introduce a DSSC design that uses a copper organic complex as a redox shuttle, with D35 and XY1 sensitisers. These materials are based on the one previously known to perform well in indoor applications.

In the article, many reasons are discussed to explain the good performance of the produced cell. The most important, to me, is that the good engineering of the cell is responsible of its behaviour. The binding of the dye to the TiO₂ has been well controlled, as well as the position of the arylamine donor group, tho that the energy loss is minimized during the electronic process. The great matching of the potentials are of utter importance for the application with DSSCs, as well as the good choice of sensitizers.

 

This example shows that DSSCs developments are still going towards more stable and more efficient cells, particularly in indoor applications. I think that the good engineering of materials must be done in other solar cells applications, from Organic Solar Cells to Perovskite, that also present tremendous performance in indoor conditions.

The cover photo is from QITS2012 on Flickr. The energy diagram is from MD McGehee et al., the DSSC architecture figure from N Anscombe.

Exploring Excitonic Binding Energy in Perovskites

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 optimization factor, leading to fewer constraints on the device’s design. Secondly, 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 characterizations before we sent the samples to colleagues at Laboratoire National des Champs Magnétiques Intenses (Toulouse) who are specialized in high magnetic field optical measurements.

Before starting the research, I had this confidence that the binding energy would not change noticeablywith 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 bandgap 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 optimize. 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 optimization 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 bandgap does not change either, because it depends on a larger scale. As Arman told me:

The bandgap 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 bandgap. Nevertheless, the binding energy and excitonic reduced mass of the charge-carriers seems to remain unchanged. Relatedly, replacing methylammonium by formamidinum, and keeping the lead iodide cage, the binding energy remains almost similar because you haven’t changed the inorganic octahedra, contrary to the bandgap.

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 bandgap 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 adress to push the device at its limits. Long term stability and ion migration are absolutely essential to adress to push them into the market. We work with very good candidates with very good charge transport or good charge properties. We eare 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…

Contact information:

Dr Arman M. Soufiani, University of New South Wales, Sydney: a.mahboubisoufiani@unsw.edu.au

Dr Samuel D. Stranks, Optoelectronics Group, Cavendish Laboratory, Cambridge University: sds65@cam.ac.uk

Perspective: Is Perovksite Fever Over?

I started this blog two months ago to share my opinion and enthusiasm about third generation photovoltaic technologies. In fact last year I used to ask questions to some of my teachers about perovskite solar cells, and each time, they were very sceptic about it and told me that the topic exploded some years ago, but that it could rapidly fall back. In the field, we call it the Perovskite Fever. I have never agreed with the fact perovskites could easily fall back, telling this was the original aim of this blog. In this perspective, I give my opinion about the past, current and future developments of perovskite solar cells and my vision on how to push it further.

Historical background: the Perovskite Fever

The perovskite technology has received tremendous interest in photovoltaic applications for five years. At the very beginning, this material was tin-based and its semiconducting properties were reported in 1994 by Mitzi et al. Actually, in 2009, Miyazaki et. al were the first to use perovskite materials into solar cells. At the time, they used lead-based perovskites in DSSCs to enhance their properties. Later on, in 2012, Kim et al. employed an all-solid configuration, reporting a 9% efficiency.

Since then, the topic received substantial attention from researchers all around the globe. Hence the performance of perovskite solar cells has been dramatically improved to more than 20% efficiency, boosted by material and interfaces engineering. Today, I consider perovskites as one of the most promising semiconducting material, that will boost the realm of optoelectronics in a very near future. Its low processing cost, abundance and excellent optical properties including a high absorption coefficient, strong photoluminescence, low trap-state density and long carrier diffusion length make it a perfect candidate for applications in solar cells, lasers, light emitting diodes, etc.

The crystalline structure

Before trying to talk about future developments, I think it is very important to understand the crystalline structure of perovskite. The term perovskite originated from the similarity with the CaTiO3 mineral, called Perovskite. In this structure, usually referred as ABX3 perovskite, a divalent metal B (lead in optoelectronics applications) is surrounded by six halogen atoms (iodide or bromide) in an octahedral network, and the cation A (methylammonium, formamidinum or caesium) is either located in the centre of an eight BX6 octahedral network (3D perovskite) or sandwiched between corner-sharing BX6 octahedral layers (2D perovskite). In fact, each layer of BX6 octahedra is only connected to each other by weak van der Waals forces. This structure is very important to understand because all the physical properties are induced by the crystallographic lattice.

picture: different perovskite structures
Different ABX₃ structures. Perovskite cubic(a), tetragonal (b) and orthorhombic (c) structure have PV properties. (d) has no PV property

It has recently been demonstrated that this crystalline structure evolves with the temperature. One must understand the phase dynamics leading to different lattice structure. Below 150K, lead halide perovskites are stable in an orthorhombic phase (Pnma). Behind 150K, the tetragonal phase (I4/mcm) is more stable and at room temperature or higher, the cubic phase is stable (Pm3̅m). Only the cubic phase presents optoelectronic properties. It means that at low temperature, the perovskite cannot absorb or emit light. However, there is no clear barrier: some scientists predicted metal halide perovskites to be metastable: particularly between 200 and 300K, both the tetragonal and the cubic phases can coexist. It is a problem to address: if this material cannot be represented on an equilibrium phase diagram, it means it can only be synthesised with a finite lifetime.

The lattice evolves

The metastability is often associated with a very small energy barrier for the cation rotation. It has been predicted that molecular sub-lattice is dynamically disordered due to the ability for the cation to rotate, but scientists found that Bragg scattering does not probe such a local disorder, even if these vibrations are observed with the vibrational IR and Raman spectra. That lets a bunch of physical and chemical properties to understand.

Ion migration processes are also of utter importance. In fact, there are charged point defects in the bulk that allow the transport of ions, thus ensuring spatial fluctuations of the electrostatic potential of the solar cells. Schottky defects are prevalent in the crystallographic structure. Therefore it has been observed with X-ray diffraction that the ions segregate into crystalline phases under illumination: one iodide rich and one bromide-rich. The accumulation of charge carriers in different zones increases the lattice strain and is a path toward more and more segregation. That “doping” leads to a hysteresis cycle of the IV-curve, that reduces the photovoltaic performance. However, this is a reversible process. That means that in the dark, the iodide- and bromide-rich phases blend one each other. To date, the mobile ion issue is the one to address in priority before commercialisation. A way to find solutions is to focus on the crystallisation process, that is still unknown. Understanding this will surely lead to more reproducible, better performance solar cells, and will mitigate the hysteresis issue, providing greater assets to address the problem of long-term stability. During the crystallisation process, the role of chloride ions remains still unknown. It could be the source of a doping effect that would explain the large carrier diffusion length of perovskites because it is challenging to find chloride ions in the final perovskite. The reduction of ion diffusion is also important: the understanding of doping processes and exciton confinement will allow scientists to understand the dopant states and the impact of halide ion exchange.

The crystalline structure and the lattice vibrations drive directly the degradation of the materials. Even if there have never been any rigorous attempts to model the decomposition path of hybrid perovskites, including chemical reactions, anharmonic lattice vibration and thermal conductivity, many scientists are on their way. I think these are important things to be understood for the sustainability of the perovskites.

The electronic properties

All of the lattice behaviour changes the electronic properties, and particularly the electron-hole recombination. There are three types of possible recombination: non-radiative, radiative, and Auger recombination. Only the non-radiative recombination limits the efficiency of the solar cells, according to the Shockley-Queisser law. Hence the efficiency of perovskite solar cells is important due to the low number of non-radiative processes.

Moreover, a lot of work to understand the electronic properties have been carried out by simulations and theoretical calculation, often based on first principle calculations. These simulations allow scientists to compare different models and therefore to identify the crystalline structure based on measurable deductions. It has been shown that the valence and conduction band structure are quite sensitive to changes in the crystal structure. The band are also perturbed by the vibrations of the lattice and electron-phonon coupling, that increase the rate of non-radiative recombinations, thus decreasing the efficiency of the solar cells.

Furthermore, there are defect levels in the bandgap, and working further on finding and reducing these levels can allow scientists to understand the low rate of non-radiative recombination. I am sure that this will have a broad impact in solid state physics.

Another field of deep interest lie at the surfaces, the grain boundaries and the interfaces of perovskites. A lot of work has currently been achieved by many scientists to understand the influence of these parameters in the device performance and to push its limit further in terms of long-term stability.

Moving towards luminescence

Applications for perovskites have started in the solar cells field, but the research realm is moving to other optoelectronic properties.
The first is light emission allowed by metal halide perovskites. Working in this field is very important because these properties are due to radiative recombination. Enhancing the radiative recombination would enhance both the absorption and emission properties of the cells. That may seem counter-intuitive, but the perovskite solar cells that would present the best performances will also emit light when working.

Thus the optical properties, including tunable emission are important within perovskite solar cells. As we just saw, the material presents excellent exciton and carrier properties, leading to a high performance in the solar cell research area. The same properties make it an outstanding light emitter that can be used in LEDs and lasers. If we compare it with conventional III-V and II-VI semiconductors, an attractive feature of perovskite is the ability one can have to tune the emission wavelength, throughout the whole visible range. This can be achieved by controlling the stoichiometry, by substituting halide elements, for example, chloride or iodide. Replacing lead by other metal ions also enable a bandgap tuning, that can be achieved in the near infrared or ultraviolet.

image: tunable emission
Tunable emission of perovskite: (a) PL emission bands, (b) bandgap VS halide composition of the perovskite, (c) images of perovskite under UV light

Electronic properties of luminescence

Perovskites are excellent light emitters. Their high quantum efficiency is the result of a clear bandgap with a low rate of charge-trapping states, that promotes the exciton radiative recombination efficiency. Some scientists have discussed how the unique electronic band structure of these perovskites contribute to the formation of defect-tolerant CsPbX3 nanocrystals. One of the future challenges in modifying optical properties is by doping the crystals with Mn2+ or Bi3+, creating heterostructures with metal nanoparticles.

Luminescence has been mostly observed in perovskite nanocrystals. In fact, in that crystal the quantum confinement effect — described by a crystal size that is too small to be comparable to the Bohr radius of exciton — allows the energy levels to be discrete (instead of continuous as in solids). Thus, the optical absorption and emission properties can easily be tuned by changing the size of the semiconductor, i.e. the nanocrystal. Hence, when reducing the size of the nanocrystal, scientists have observed a blue shift in the bandgap. If most of these effects have been observed in all-inorganic CsPbBr3 perovskite nanocrystals, the improvement of this structure and use in organo-metal halide perovskites will surely lead to enhancement of the absorption properties of perovskite solar cells.

Main applications in light emission

There are two main applications for luminescence with perovskite: optical lasing and light emitting diodes. Due to the high absorption coefficient and strong photoluminescence properties, optical lasing is possible. In fact, when a high gain material is put in an optical cavity, one can expect lasing. Exploration of amplified spontaneous emission in perovskite allowed scientists to design some perovskite-based lasers. We understood in this perspective that the high crystallinity of perovskites was the base of many properties. Here, nanocrystals can act as a Fabry-Pérot cavity, allowing lasing in the materials. This will lead to applications in nanoscale optoelectronics, with the ability to tune the emission wavelength to achieve lasing in the full visible spectrum.

Light emitting diodes applications make perovskite one of the most promising material. The fact it is low cost, easy to prepare, with high performance and a high colour tunability, with very precise emission rays make it a technology we will hear about in the coming years. The main issue is overcoming the stability issue. I am sure that a combined work from two different communities (the one that works on solar cells and the one working with LEDs) will be the key towards these applications.

Forthcoming challenges

I do not think that the compound design should be application-driven. Perovskite is truly a multifunctional semiconductor, and the understanding of its properties for particular applications should drive the design for the whole applications. The understanding of the functions of each component, such as organic cations, metal halide octahedra network, and effect of each on the entire crystal structure in order to formulate better compound for each application is of paramount importance.

One of the biggest challenges for metal halide perovskite is their stability. Their vulnerability surrounding environments, such as moisture and oxygen or polar molecules, affect dramatically the performance of the devices. This casts a shadow towards their practical applications and commercialisation. Thus we need to develop a method to stabilise the material, both by chemical approaches, (capping agents, for example, stabilizers) or by physical methods such as device encapsulation.

Perovskite may have reached their high development peak, but we saw in this perspective there is still many years of research for chemist and physicist to improve the optical and electronic properties of perovskite by tailoring the crystallographic sciences and designing new devices. I think we are at the beginning of a new Perovskite era when scientists will not try to improve their devices based on existing knowledge of organic and inorganic semiconductors but will try to deeper understanding. And there is plenty of room for this!

I am deeply convinced that the world rapidly needs stable, efficient and low-cost photovoltaic solutions. Tackling climate change is a matter of days. A better understanding of Perovskite emission properties and their degradation mechanism will lead to a better understanding of this incredible material, that makes the Perovskite Fever not over yet. I am sure that we will understand all of their physical properties so that Perovskites will power what will become the major source of energy within 10 years, and I have a glimmer of hope that Perovskite will lead us to a sustainable future.

The cover photo is taken from NREL on flickr. The Perovskite structure illustration has been drawn by Walsh et al., the luminescence illustration by Pathak et al.

Tuning Quandum Dot Solar Cells

In July, Dr Ahmad R. Kirmani published a very interesting article the day he defended his PhD. He has worked with Quantum Dot Solar Cells at the King Abdullah University of Science and Technology (Saudi Arabia) while publishing the article I invited him to talk about: Molecular Doping of the Hole-Transporting Layer for Efficient, Single-Step-Deposited Colloidal Quantum Dot Photovoltaics, 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

Beginning 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 quandum 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.

The cover photo is from U.S. Department of Energy and isn’t considered to be a US gov. Work.

Engineering interfaces with gold nanoparticles

When discussing interfaces issues with perovskite solar cells, Prof. Iván Mora-Seró from Institute of Advanced Materials, Castelló (Spain) is one of the scientists with the stronger background. Some month ago, he explored the use of gold nanoparticles at the interface between the electron sensitive contact and the perovskite material TiO₂ in perovskite solar cells.

Eliminating the plasmonic effect hypothesis

Previous studies used gold nanoparticles to show the existence of a plasmonic effect leading to enhancements of the solar cells efficiencies. During our discussion, Iván was very sceptic about the fact that a real plasmonic effect was behind the efficiency enhancement in some of the previous works in perovskite solar cells:

I do not think it is the real interpretation in some cases leading to increase in efficiencies of perovskite solar cells. When my visiting student Naemeh Aeineh wanted to analyse such an issue with gold nanoparticles, I was reticent but finally I gave her a half-hearted green light. She realized that no significant effect could be found by focusing on mixing the nanoparticles with the TiO₂ mesoporous scaffold as would be expected from a plasmonic effect. However, she observed a significant effect when the nanoparticles were deposited at the interface between two layers of TiO₂.

The nanoparticles (NPs) used were core/shell Au@SiO₂. They placed them at the interface between the electron collector and the perovskite material. Thus, by affecting these interfaces, they managed to change the electrostatic potential, that created free carriers at the interfaces.

Observing different phenomena

With the work they carried out, they observed different phenomena. The first was that the addition at the electron selecting interface of Au@SiO₂ leads to an increase in the performances of the perovskite solar cells. A deeper observation with different techniques allowed Prof. Iván Mora-Seró and his team to understand why.

Light scattering plays a crucial role in this enhancement:

When illuminating materials, one can observe very often light scattering. The nanoparticles diffuse the incident light, leading to an increase in the efficiency. However, we are also observing a quenching of the photoluminescence, also associated with the fact we improved the charge separation properties, indicating that the effect of the interfacial NPs is not just a scattering effect.

The quenching means the electrons collected are not involved in photoluminescence properties of perovskite solar cells. That is due to the fact that the nanoparticles enhance the electrons collection hence fewer photons are involved in luminescence, quenching the luminescent spectrum.

energy diagrams
Energy diagram at the considered interface with (b) or without (a) Au@SiO₂ NPs. Gold nanoparticles produce an enhancement of Velec, resulting in an increase of the Voc.

That electron collection increase is shown by an increase in the electrostatic potential and an increase in the open circuit potential (VOC), but there are much research to be done in this field:

We are sure that we observe light scattering effects, but also something else, because we increase the injection properties in our system. Because we are increasing the VOC, electron probably would be injected in channels into the perovskite. We need further study to prove this and are working with colleagues on numerical simulations to see if it is such important to manage the electric field. But it is just a hypothesis for the moment!

No wonder he will manage to find shreds of evidence!

A size effect

When working with nanoparticles, one must always think about their size-effects. Because nanoparticles are small objects, their ratio surface/volume is very important. That enhances the role of interfaces within these materials. Moreover, their small size lead in this case to small channels, that is very important for the perovskite-electrode contacts. In fact, it could lead to a decrease in ion mobility.

In other applications, nanoparticles are used for the fact they help the crystallisation of the perovskite. If we manage to control these process with nanosized objects, it could lead to lower ion mobility and more stable solar cells by affecting recombination at the interfaces.

As a conclusion, we will still hear about the role of interfaces in solar cells for a long time. Last week, Iván has published a review about it. I am sure it will be a hit in the community, leading the though towards more stable and more efficient perovskite solar cells.

The cover photo is from Li et al.

The challenges of encapsulation

In previous articles, I presented instabilities within organic and perovskite solar cells. Most are due to the presence of water and oxygen. Therefore contrary to silicon modules, third generation solar cells degrade very fast when exposed to an oxygen and moisture atmosphere. For instance, an typical organic solar cell can degrade in just a few hours, meaning that its Power Conversion Efficiency decreases.

Solutions do exist: new architectures and new active materials can be found to overcome this issue. However, it might not allow the research of higher performances, that also needs new architectures and active materials. Most of the time it is hard to combine those two properties.

Barrier Layers

For purposes of finding a compromise between high efficiency and low degradation, encapsulation of the cells is usually seen as the best opportunity. The principle is very simple: adding barrier layers for moisture and oxygen not to diffuse up to the solar module (Cros et al.). However, there are many constraints: the materials should be transparent, flexible, with low permeation rates, chemically and physically stable and compatible with the cells. It might seem antagonist, but the use of thin films is a way to overcome the issue.

Thus the protection against moisture and oxygen requires the development of protective materials with sufficiently low water and oxygen permeation. To do so, industrials like Armor or Heliatek usually laminate films on their optoelectronic devices.

Measuring permeation

Measuring permeation requires specific techniques. The principle is simple: comprising a thin film between two chambers and measuring the rate of water that goes through the film. In the upstream side chamber, we set a partial atmosphere saturated with D2O vapour at a fixed temperature. In the downstream side chamber, we fix the partial pressure to high vacuum, maintained with a high pumping system, and detect D2O flow with a mass spectrometer. The use of D2O is justified because the abundance of D2O is much lower than H2O then the measures are more precise by reducing the noise. That process has been patented.

When measuring permeation, we use two values: Water Vapour Transmission Rate (WVTR) and Oxygen Transmission Rate (OTR). For the needs of OPV encapsulation, the WVTR must be lower than 10-3 gm-2d-1 and the OTR lower than 10-3 cm3 m-2 d-1.

Multilayers films

When devising transparent and flexible films, one can think about PET. Yet it is far from fulfilling the requirements I exposed with permeation. In facts, water and gazes can diffuse inside polymers. Hence it requires the addition of inorganic films to block that diffusion. However, the barrier properties are very limited inside inorganic films, due to possible defects and the fact that inorganic films are prone to present pinholes or to crack and then delaminate. For this reason, even if it might seem counter-intuitive, increasing the thickness of an inorganic film leads to higher permeability.

For this reason, Affinito et al. had the idea to develop dense organic-inorganic multilayers barrier films. Inorganic films can present defects but the presence of organic layers induces a decorrelation of the defects. Hence gases molecules need to diffuse laterally within each organic layer to go through the next effect. That idea increases the tortuosity, resulting in a rise in the time needed for the molecules to cross the membrane (time lag), and diminishes the WVTR. This inorganic films can be deposited via plasma-enhanced chemical vapour deposition (PE-CVD).

Multilayer stack

Yet the multilayers deposited via PE-CVD are expensive to process because it requires the deposition to be made in a high vacuum. The organic layers are solution-processable and that cannot be done in a high vacuum. It means that one must process an organic layer in the air, then process an inorganic layer in high vacuum conditions, and repeat. A way to address the issue is to achieve solution-processed inorganic layers. Hence scientists need to find a solution precursor of those inorganic layers. It is a promising topic on which Dr Arnaud Morlier worked when he did his PhD at CEA.

Edge permeation

Here we described solutions for gases and water not to go through the solar cells, by stacking protective layers on top of the devices. However, edge permeation exists and it could represent more than 50% of the whole permeation. Water and gases diffusing through the interfaces cannot be neglected. For this reason, there is a need to consider the couple gas-barrier film + adhesive. It is positive to say that on the edges, the requirements are much lower: because the layer can be made larger on the edges, the required WVTR is comprised between 1 to 10 g m-2 d-1.

To measure properties and permeability of the edge materials, one can at first, measure permeation on thin films. Yet scientists have developed a modelization of the optoelectronic devices, by using a calcium test. We encapsulate a piece of calcium as if was is a solar cell. Calcium is opaque and with thermal/water ageing, it can evolve to Ca(OH)2, that is a transparent material. Measuring the transparency of the piece of calcium leads to understanding if water permeates or not.

Designing edge materials will be a key issue for the commercialization of durable OPV materials.

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 to 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 photooxidation and photochemistry.

On the other hand, OPV also faces 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 is composed of stacked thin layers that have a different elastic modulus, that can crack upon bending or delaminate due to differences in adhesion. That is flexibility degradation. Such 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 molybdenum nor silver triggers 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 has 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 usual process methods have been changed to adapt such fracture study. The sizes involved were different than 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 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 in the adhesion between the active layer and the HTL/electrode. However, all the probe techniques that were used cannot prove with certainty that diffusion phenomenon is only responsible for 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 about 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.

The principles of organic solar cells

Organic and perovskite photovoltaics are the solar cell research fields with the greatest interest from the community. Many articles are published each week about it and for this reason, I will certainly write a lot about those two kinds of solar cells. Here I chose to present how do Organic Solar Cells work.

Organic Materials?

The particularity of organic solar cells in comparison with silicon solar cells is the use of organic semiconductors. Many advantages are related to organic materials and the greatest is their price. Organic materials are abundant and easy to process. Thus organic solar cells are a low-cost technology to harness solar energy. In addition, the weak intermolecular Van der Waals interactions in organic materials enable to process them with a large variety of techniques, notably thin films on the large surface. Many of the processes are easy to carry out (in comparison with silicon that has to be purified, that is very expensive and energy-consuming). That allows organic solar cells to be coated on flexible substrates with very large areas.

Moreover, organic materials are significantly attractive because they usually have a high absorption coefficient combined with high transparency. Their properties are also tunable with efficient solution processes.

Therefore organic materials are very versatile in term of processing methods, in term of substrates and in term of applications. For many reasons, including stability under direct sunlight, organic solar cells researchers now channel their efforts into indoor applications.

The photoelectric materials

In such solar cells, the active layer is composed of amorphous organic materials with semiconductive properties. These materials are often conjugated polymers. Conjugation may be described as the overlap of p-orbitals that leads to the delocalization of pi-electrons across the chain of aligned p-orbitals. The absence of any crystallographic structure, compared with inorganic materials that wring their semiconductive properties out of their crystallographic structure, leads to a low dielectric constant (εr=2-4). Thus it results in tightly bonded excitons (electron-hole pair). The Frenkel exciton binding energy is comprised between 0.3 and 1eV: that is large and it prevents exciton dissociation by an electrical field.

The active materials were firstly designed by Dr Ching W. Tang from Kodak in 1979 by superposing two layers of n and p-type semiconductors, as it was done with silicon. Researchers used to process donor-acceptor bilayer planar heterojunctions that achieved 1%PCE. Do to so, the energy difference between the LUMO (lowest unoccupied molecular orbital) of the donor material and the HOMO (highest occupied molecular orbital) of the acceptor leads the dissociation of Frenkel excitons when absorbing light. Then, the separated holes and electrons are collected to the anode and the cathode respectively. However, such planar structures had many issues scientists had to deal with. The main was that for such planar junction, the surface area between the donor and acceptor interface was tightly restricted and the carrier lifetime was too small so that the electrons and holes could recombine before reaching their respective electrodes.

energy diagram of an OPV
energy diagram of a polymer solar cell. PEDOT:PSS composes the hole transport layer. Al composes the electron transport layer.

That leads in 1995 to the development of bulk heterojunctions, simultaneously with polymer-fullerene by Prof. Alan Heeger in UC Santa Barbara and with polymer-polymer blends by Prof. Sir Richard Friend in Cambridge University. Bulk heterojunction involves the mix of a donor and acceptor materials as an active layer. As shown on the figure, distances electrons and holes have to travel before being collected are much reduced and the surface of such nanostructured materials is largely enhanced. That lead to much higher performances.

representation of a bulk heterojunction
representation of a bulk heterojunction

The architecture

Traditional structures were made using the model of silicon solar cells in which the photons were going through the anode before being converted to exciton in the bulk heterojunction. However, it has been shown that an inverted structure behaves much better and was more stable.

The working principle of these cells is very easy: in the Bulk heterojunction, incident photons are absorbed, that leads to the creation of an exciton. These excitons can diffuse within the material, before being separated as a hole-electron pair. Each carrier is transported to a transport layer toward its respective electrode.

The bulk heterojunction is composed of a donor polymer, P3HT for example and an acceptor organic material. Owing to their strong electronegativity and high electron mobility, C60 derivatives like PCBM have become standard acceptor molecules in OPV devices. Today, the inverted structure with fullerene derivatives has become a standard in organic solar cells.

representation of normal and inverted structure.
Representation of the normal (left) and inverted (right) structures. The arrows represent the incident photons

Research perspective

Since the advent of perovskite solar cells, many researchers in the field of organic photovoltaics have drifted to this new field. That lets much fewer resources to the organic field. However, This is now a much mature technology. Scientists focus on ensuring their stability and improving their efficiency. Many companies are trying to commercialize it, including Eight19 in Cambridge (UK) and Armor in Nantes (France). Their developments may become great commercial hits in the coming years depending on what they chose as market segments.

The cover photo is under CC-by-nc-sa license, from BASF on Flickr. The figures are from Ameri T. et al. and Chen L.M. et al.

Stay positive about stability of perovskite solar cells!

Some weeks ago, I had a conversation of deep interest with Dr Tomas Leijtens from Stanford University. Tomas has important research expertise in perovskite stability. Thus he wrote some month ago a review about it: Towards enabling stable lead halide perovskite solar cells; the interplay between structural, environmental, and thermal stability.

The point to understand before reading this review is that Tomas was among the first to design perovskite solar cells in 2012. At the time, he was a PhD candidate in Prof. Henry Snaith’s laboratory where was discovered the photovoltaic properties of metal halide perovskites. For this reason, he has spent 5 years observing their degradation and how it happens. This review is, therefore, a bit special: it resonates as his own perspective on stability.

To what extent are Perovskite Solar Cell unstable?

Perovskite solar cells major problem is their instability. It is actually the one to solve in the coming years before it can hit the marketplace. Because of their rapid evolution, many of the problems are evolving and today’s problem with stability are not the one perovskite used to face.

ABX₃ organometal halide materials can crystallise in the so-called perovskite structure (the name was inspired from the Perovskite, a piezoelectric mineral). In this configuration, PV properties have been measured and are about to exceed silicon performances. Under uncontrolled chemical, temperature or illumination conditions, the metal halide compounds might evolve in other crystalline phases or into any structure that has no optical property. Therefore, scientists observe losses in the PCE.

picture: different perovskite structures
Different ABX₃ Perovskite structure. The cubic (a), tetragonal (b) and orthorhombic (c) group spaces have PV properties. (d) has no PV property.

There are other ways to deteriorate the PV properties of the perovskite solar cells. Thermal decomposition is one of them. In fact, temperature disrupts the crystal lattice, and acceleration of this phenomenon has been observed with moisture and oxygen. Organic material like methylammonium has been shown to be the source of this thermal instability because it is acidic and volatile. Replacement with Cesium has shown enhanced thermal stability.

Furthermore, moisture and oxygen are the most harmful for the perovskite solar cells stability. Oxygen leads to photo-oxidation of the devices that worsen the PV properties. Moisture can create hydrogen bonds with the acidic Methylammonium cation. That deteriorates the structure.

Encapsulation is a way to lower such stability deficiency.

Tomas has shown that these stability concerns are linked to each other. To fix one of these, one should take into consideration the others, because they can have a positive influence. Devising a less acidic or aprotic cation, for example, can overcome both the moisture and the thermal issues.

Stability, but at which timescale?

When talking about his work, Tomas had a valuable view of the stability problem:

I think that the community we are focuses too much on quick test on stability. We have to move to relevant stability tests. A solar panel usually lasts 25 years on a rooftop. Even if it is not our primary goal, it is necessary to carry out long scale research.

To do so, we could find inspiration by looking at existing thin films PV manufacturers. How do Firstsolar, Miasolé, or others package their module, what layers do they use? Finding inspiration from these could lead to a lasting solution.

Moreover, we have to focus on things that cannot be done by other technologies. For a long time, perovskite solar cells researchers have capitalized on what was done with other PV technologies like OPV. However, there are problems that can not be solved by comparing to other technologies. The big question is about mobile ions. The crystalline structure implies we have to solve this problem ourselves with new and distinctive ideas.

In the next several years, moisture, oxygen and mechanical are the problem to solve. His idea on it will lead to a much mature technology.

Hope drives the research

I found Tomas’ conclusion strong: “This[…] makes us optimistic that perovskite solar cells can reach the level of stability required for commercial applications.” Currently, no sign shows that this will be the case. But he is very positive and explains why:

In this field more than anywhere else, there are lots of sceptical readers. This strong finishing sentence helps us to keep motivated. We all know that with more work we can do it. It does not especially mean it will last 25 years on a rooftop. Yet the big challenges we were looking at 2 years ago are solved and we have good strategies. I promise surprising steps forward towards commercialization.

I have no doubts that with such a perspective, perovskite will still be a long-term topic of conversation!

Contact Information

Dr Tomas Leijtens, Department of Materials Science, Stanford University, leijtens@stanford.edu

The cover photo is taken from the Institute for Future Environment on flickr. The Perovskite illustration has been drawn by Walsh et al.