Fullerene, a versatile material for PV applications

Some of my readers know me for my tweets about #OrganicElectronics. In this field, fullerene and its derivative have been for a long time used in many applications for their conductive properties. Last December, I was allured by a paper about the use of fullerene for perovskite interlayers: Cross-Linkable Fullerene Derivatives for Solution-Processed n–i–p Perovskite Solar Cells. In this post, I want to show you how this research is inspiring!

I’ve been in contact with one of its authors, Guillaume Wantz, Professor at the University of Bordeaux (France), since then. I wanted to invite him to present here his work and his views about the use of fullerene derivatives in OPV and perovskite solar cells. We shared a very interesting conversation leading to this blog post.

What is fullerene and why is it fabulous?

Neat fullerene is a conjugated sphere of 60 carbons. It is an amazing electron acceptor material. That is why we use its derivatives, such as PCBM, in Organic Photovoltaics. In the bulk heterojunction, it is usually blended with an electron donating polymer. It helps the charge separation and ensures the exciton does not recombine. However, the mechanism is not clear. Most of past studies with fullerene in solar cells are empirical: scientists observed fullerene is a good material without bringing theoretical hypothesis. Maybe someone knows why it is that good, but no one revealed it! I predict the understanding of its physical properties in the bulk heterojunctions will become a research field of paramount importance.

For now, what we can be sure about fullerene is that it is a very versatile molecule. We can crosslink it, photo-oxydize it, or anything! That is exactly what Guillaume did.

Representations of fullerene and its most known derivative: PCBM

Proceeding by trials and errors

Research is really interesting, for that it is full of twists and turns. It is a tortuous path towards a finding and discoveries. About his paper, Guillaume tells it like anyone else:

Some years ago, within the ELORGA group in Bordeaux, we had developed C60 crosslinking strategies for organic photovoltaics. Our idea was to freeze the nanometric morphology of bulk heterojunctions thanks to crosslinking. To do so, we had worked for several years trying to bind C60 one on each other in situ − a kind of polymerization of C60. We finally came up with some azide derivatives to be blended with PCBM for a stable morphology.

Prof. Henry Snaith (Oxford) read our study and contacted us. At the time, he was looking for some crosslinkable C60 derivatives to use as interlayer in his perovskite solar cell. Crosslinked because he wanted to avoid the dissolution of the interlayer by the subsequently coated layers. The project was to coat a layer of fullerene and then make it insoluble before continuing the construction of the solar cells. In fact, there is a real problem with solubility. Neat C60 it is not really soluble, one cannot coat it sucessfully without making pellets. That is why derivatives, like PCBM, that are more soluble are better for this application. However, they can diffuse and their solubility may affect chemically the layers of the solar cell.

Firstly tried, the azide crosslinkers chemistry affected the C60 structure and lowered its electron accepting ability. Thus we proposed a novel sol-gel processed C60 coming from Dr. Olivier Dautel, an inspiring CNRS reasearcher from Montpellier. I did not know what to do with this material before! In 5 years, we kept this material in our “drower of unsuccessful materials” until having the problem with azide crosslinkers. I had the idea to make sol-gel processable layers that could be insoluble. Eureka! It works very well!

The cross-linkable C60 from Dr Olivier Dautel

Nonetheless, this material is not really easy to process: the way we had to think the sol-gel catalysis was not obvious. The initialization of the reaction must be done with acid to start the hydrolysis, for the polycondensation to happen. Imagine a real sol-gel ink to be spin-coated, it is reactive and will evolve eventually quickly. The truth is you never know what you’re spin-coating! Then we had the idea to spin-coat the pristine molecules, to make a layer and then expose it to acid vapors for initializing the hydrolysis. We used a soft acid not to ruin the ITO electrode: we exposed the fullerene layer to trifluoroacetic acid vapors. It was successful, films were finally insoluble in every organic solvents.

The insoluble crosslinked sol-gel fullerene

The real problem is that during the sol-gel process, SiOx moities are created. These are neither conducting nor semiconducting, they are the essence of the known glassy insulator (SiO2). We carried out mobility measurements on organic field effects transistors and on vertical sandwiched structures using SCLC models. When comparing to PCBM, we lost 2 orders of magnitude in term of electrons mobility! That was astonishing. However, the result was not that bad on vertical sandwichs. That is the clue! We use just a thin interlayer of that material so that the carriers do not travel long distance through it.

Then they managed to put only a tiny part of fullerene that worked well for the application in n-i-p perovskite solar cells! Is that not exciting?

Beyond Fullerene

Following this article and the work that was done, one might want to go further. But that needs a deeper understanding of the phenomena that make fullerene that versatile. One could think about carbon nanotubes: they are a good electron acceptor, fitting the requirement we outlined with good conducting properties. However, carbon nanotubes are harshly processable. Many tried to make films with it. We do not believe in it for the OPV applications as fullerene substitutes.

Nevertheless, as we did with the sol-gel C60, one can find new molecules, non-fullerene acceptors (NFA). As with many other groups in the world, Guillaume is on this new promising topic!

To conclude, there is a real problem with fullerene: it does photo-oxide on its cage in presence of oxygen. Then when one processes it at an industrial scale, it is necessary to expose it to the air. Thus a tiny part of oxygen can fix the cell and, one day will kill it. We don’t know when. That is the topic of Guillaume’s forthcoming article, which we are eagerly awaiting.

Contact information:

Prof. Guillaume Wantz, Bordeaux Institute of Technology (Bordeaux INP / ENSCBP)
guillaume.wantz@ims-bordeaux.fr

The cover photo is under CC-by license, from fdecomite on Flickr. The molecules representations are courtesy of Prof. Guillaume Wantz.

Beyond the Shockley-Queisser limit

As I said in the introduction, I will publish presentation, that are blog posts in which I will expose research articles or specific fields of research. This article is the first of the series and is about Xu et al article: The generalized Shockley-Queisser limit for nanostructured solar cells, Sci. Rep. As I said last week, the Shockley-Queisser theory limits the efficiency of solar cells. In this article, we will discuss these phenomena, in relation to Xu et al. article.

How to measure a solar cell’s performances?

For a good understanding of the Shockley-Queisser limit, we first need to define the main figures PV scientists are working with. These figures allow us to compare different solar cells.

Measuring a solar cell is usually done by scanning a voltage and measuring the current response under simulated illumination. Typical solar cells have a response as shown in this figure:

example IV curve for a solar cell

Some data are easily understandable: VOC is the open circuit current, the maximum voltage the cell will supply. The short circuit current JSC is the maximum current at zero resistance load.

Pmax is the maximum power the cell can deliver, defined by Pmax = VPmax × JPmax. Hence we can define the Fill Factor FF:

FF = Pmax /(VOC × JSC)

The Fill Factor compares the maximum power with the maximum current and voltage. It is always comprised between 0 and 1 and gives the information about how large the maximum power is in comparison with these two parameters.

Therefore, we define the Power Conversion Efficiency as the ratio of generated electric power and incoming light power:

PCE = Pmax / Plight = ISC×VOC×FF/Plight

Those parameters allow us to compare the efficiencies of different solar cells, independently of their sizes. When we talk about the efficiency of a solar cell, we always mean the Power Conversion Efficiency.

Theoretical limits for solar conversion

In a classic PN junction solar cell, the conversion of light into energy is based on the photoelectric effect. It is actually an interaction between an electromagnetic wave and matter, that can be modelled as atom chains, metals, semiconductors or dielectrics. One can picture the problem with Hamiltonians in a realistic potential. In that way, Shockley and Queisser discovered a mismatch between absorption and emission angles. In their 1961 article, they set a balance between incoming photon flux and the emission of other radiation. The calculus is based on a model in which the recombination of a hole-electron-pair is radiative. It is the main source of losses.

They make the assumption that solar cells and the sun behave as black bodies. For a planar solar cell, they compute the maximum efficiency that is found to be 30% for a 1.1eV gap. The model of a planar solar cell is of paramount importance: it is considered that the photons hit a planar surface without any delay or difference.

Considering this hypothesis, one can wonder whether nanostructured solar cells (that cannot be considered as planar) behave with the same law or not. Xu et al. proved the Shockley-Queisser limit efficiency was still valid and proposed a model to adapt it.

Is the Shockley-Queisser limit still valid for third generation PV?

The main hypothesis of the Shockley-Queisser model is that photon ram a planar surface, restricting the maximum power conversion efficiency. A third-generation technology allows us to consider more photons, called Concentrated PV. In fact, a concentration device — like an optical lens or a parabolic mirror — is put above the cell, concentrating more photons upon it. Hence the photons hit the surface with a multitude of different angles. It boosts the efficiency of the considered solar cell. In this case, it is easy to understand that only one hypothesis that changes, expanding the PCE of the concentrator-solar cell device without changing the equations established by Shockley and Queisser in 1961.

Now consider a solar cell without any concentration device but with a structure that is not planar and that is composed of nanostructures. Because of the roughness of the surface, there is also a multitude of angles with which the photon can hit the surface. It allows magnification of the Power Conversion Efficiency the same way concentrated devices work.

Moreover, Xu et al. found that the emission angle is much closer to the acceptance angle for nanostructured devices. Then the entropy generated by this mismatch is lowered resulting in a voltage improvement. For this reason, Nanostructured solar cells have an efficiency limit of about 42%. I find the understanding of this phenomenon of outstanding significance for the improvement of high-efficiency solar cells.

As a conclusion, if the Shockley-Queisser limit seems to minimize the maximum efficiency of a solar cell, the article from Xu et al. shows that third generation photovoltaic technologies found ways to subvert it to boost it. In this example, the understanding of the limiting phenomena helped scientists to find ways to overcome it. That is what I find really interesting.

The cover photo is taken from École Polytechnique on flickr. The plot is under CC-By-Sa license: you can copy it, share it as long as you quote solarcell.science

The History of Emerging PV

Less than a month ago, president Donald Trump took the outrageous decision to get out the Paris Agreement. Climate change is real. Climate change is disastrous for our lives, and not only for our children’s and grandchildren’s as we used to say. The Paris agreement aims at limiting global warming below +2°C.

Renewable energies

According to Pye et al. (Nature Energy, 2017), the only way to meet the agreement is switching globally from now on to renewable energies. To do so, there are different sources of energy: solar, wind, waves, tidal… The fact is that we currently need a bit less than 20TW of energy. Scientists predict we will need 30TW in 2050 and 50TW in 2100. Let’s put these figures in relation with available sources of renewable energy. Biomass is can produce from 5 to 7TW. Hydroelectric can produce up to 1.2TW. Geothermal energy is worth 1.9TW, tide: 0.7 TW and wind: 14TW. Solar energy received at earth surface accounts for 10⁵TW. Therefore solar energy can feed the demand entirely. I will focus on the way to produce electricity from solar energy.

First Generation Solar Cells

In 1921, Albert Einstein received the Nobel Prize of physics for the discovery of the law of the photoelectric effect. This effect describes the emission of electrons when the light is absorbed onto a material. It is the basis of the first generation of solar cells that still represent 90% of the market.

This technology is based on a crystalline semiconductor (silicon). One layer of crystalline n-doped semiconductor (in which a doping agent has been added so that the electron concentration is higher than the holes concentration: it has electrons to give away) is stuck to a layer of crystalline p-doped semiconductor (with higher hole concentration than electrons: it has electrons to be given), creating a P-N junction. Properties of P-N junctions can be well and easily understood using stationary states of the Schödinger Equation but I will not explain it here.

The electrons should be excited from a valence band into a conduction band to produce electricity as a current. These bands are separated by a gap. It means that, for a photon to excite an electron into a higher energy state, the energy must be greater than or equal to the energy of the gap. The energy can be tuned when doping differently the two regions. It means first generation solar cells have a broad spectral absorption range. However, if a photon has an energy higher than the gap, the extra energy is wasted as heat. It defines a limit in the yield, known as the Shockley-Queisser limit. Moreover, producing crystalline silicon requires expensive manufacturing technologies.

Why do we need other technologies?

Silicon is a pretty good technology. However, a huge amount of energy is needed for the production of solar cells and in their use to produce energy. That is why we need to take into account other parameters: energy needed for preparation, energy output, lifetime, availability and costs to understand whether the technology can be used for 100% of the energy production or not. For a clear demonstration, I will only estimate orders of magnitude.

Photovoltaics is a fast growing business, therefore availability of the technology is a real problem. Let’s say, for example, that we want to replace all the electrical power output of a nuclear power plant with a nominal 1GW production. We use crystalline silicon with a module efficiency ηsc = 0.1, exposed to an average central European insolation Φexp = 100W/m².

We assume we need 1mm of silicon to produce a mean absorber thickness of the silicon wafer dSi = 500µm, (we need another 500µm that accounts for Si powder produced by slicing the ingots), the total volume of silicon is VSi = 10⁸ × 10⁻³ = 10⁵ m³. The energy needed to produce crystalline silicon wafers is around 1,000 kWh/m², which corresponds to about 100GWh for the 1GW PV plant example.

Lifetime of a silicon solar cell is about 30years (τ = 10⁴days), therefore for one 1GW plant, we need a production rate of Vprod = VSi/τ = 10m³/day and then Pprod = 10 kWh/day = 100kW/day of production. Because the annual production of a 1GW plant is Uprod = 10TWh then it is Utot = 10¹⁴ Wh over 30 years and yields P = 1GW. Then tpayback = Utot/P = 10¹⁴ / 10⁹ = 10⁵ h = 10 years. It means the effective production of the solar plant is only 20 years.

Replacing 1GW would require a total module area : Asc = P / (ηsc · Φexp) = 10⁸ m² = 10km×10km = 6miles×6miles.

Now, if we consider we need to replace not only 1 power plant but all earth production. For 30TW (expected needs in 2050), we would need 3·10¹² m² of solar panels, that represents India’s area.

Would you imagine using such a surface with such an energy payback time (and therefore such a cost)? I don’t. That is the reason why I consider we need to find new and emerging technologies for PV devices. That is why scientists have tried designing new materials and new technologies.

Second Generation Solar Cells

A solution towards reducing the cost and the price of active materials is diminishing their quantities in solar cells. That is the principle of Thin Films devices, known as the Second Generation of Solar Cells. This second generation photovoltaic reduces the price per watt by removing the unnecessary materials thereby reducing the cost equation. The use of thin films allows lowering the number of materials without losing efficiency.

These solar cells are single junction semiconductors that aim to use fewer materials. The semiconductor of the most used cells is amorphous silicon, CuInGaSe₂ (CIGS), CdTe-CdS or polycrystalline Si on a low-cost substrate. This technology absorbs solar spectrum with greater efficiency than crystalline silicon and uses only 1 to 10µm of active material. However, their conversion efficiency is lower than crystalline silicon.

Because Thin Film Solar cells are semitransparent, they can be applied as window glazing for the Building Integrated Photovoltaics (BIPV) market.

Nonetheless, there are still many ways to enhance the efficiency of solar cells. This leads us to the third generation.

Third Generation Solar Cells

At the beginning of the 21st century, scientists have tried new techniques to overcome the Shockley-Queisser limit. With this objective begins a true race to design materials able to overcome it. Scientists have tried to optimize charge collection and thereby enhance energy capture.

With the discovery of organic materials, solar cells had the potential for low cost and high optical absorption. The use of innovative semiconductors defines the third generation of solar cells. These cells are mostly known under the Emerging PV name

  • photoelectrochemical cells that produce energy in a process similar to the electrolysis of water;
  • dye-sensitized solar cells in which the active material is a complex formed by a dye on a highly porous semiconductor material;
  • organic and polymer solar cells, working with a P-N junction made with an electron donor polymer and a hole donor polymer;
  • quantum dot solar cells, that use quantum dots as the semiconductor layer;
  • copper zinc tin sulfide (CZTS) and derivatives, thin film-like technology;
  • perovskite and kesterite solar cells, in which the semiconductor is a crystalline material.

Other techniques allow to surpass the Shockley-Queisser limit:

  • multijunction cells that are cells composed of many layers with the ability to absorb different energies;
  • intermediate band cells which have multiple bandgaps to increase their energy;
  • hot carrier cells that convert the excess energy of above-bandgap photons into electricity;
  • spectrum conversion that converts the incoming polychromatic sunlight into a narrower distribution.

Research evolution

This plot shows the highest conversion efficiency for different types of solar cells:

plot: best research cell efficiency
Best Research Cell Efficiency

We see on this plot that even if the efficiencies of third generation solar cells (Emerging PV) are lower than current efficiencies of first and second solar cells, the evolution is much faster. Hence this field is of growing interest, I will show it in future articles.

The cover photo is taken from École Polytechnique on flickr. The plot is courtesy of the National Renewable Energy Laboratory, Golden, CO

Emerging photovoltaics: Overview of the blog’s outline

Welcome to this blog. I’m Julien Barrier, you might know me for my tweets about organic electronics, perovskite solar cells and emerging photovoltaics.

In addition with my tweets, I’ll issue articles about new and emerging photovoltaic technologies from July, 1st. I’ll publish regularly, i.e. each saturday.

The articles

In particular there will be four categories of articles:

  • briefs, that will present the state-of-the-art of global topics
  • presentations, where I will expose research articles or specific fields of research
  • opinions, where I will comment, probe and dissect articles or give perspective about a field
  • invited researchers: I will present, with a researcher, the last advances in its field of research

To enumerate I will cover a range of topics around emerging photovoltaics, including:

  • physical effects within solar cells
  • chemistry of solar cells
  • dye-sensitized solar cells
  • organic solar cells
  • perovksite solar cells
  • thin film solar cells
  • emerging businesses
  • concentrator photovoltaics
  • etc.

Miscellaneous

You might also want to know more about who I am… Don’t hesitate to check the about page or contact me!

Cheers!

 

The cover photo is licensed under cc-by-nc-nd, belonging to Berkeley lab.