Tagged: Solar cells

Beginner’s guide to hybrid organic-inorganic halide perovskite (MAPI) solar cells

One challenge of working on hybrid halide perovskites is that the field is moving so rapidly. A second is that there are just too many publications. A third is that many of the older papers use different nomenclature so that it can be difficult to discover them. I started a Mendeley group to track papers, and I think we have done a decent job on the older literature (especially in the key work from the 1980s and 90s).  In 2014, there were over 400 publications on CH3NH3PbI3, and there are likely to be over 1000 in 2015 alone, so it is almost impossible to absorb all available information.

MAPI

From a Web of Science search for “hybrid perovskite OR MAPI OR CH3NH3PbI3 solar cell” with 1564 results – 28th May 2015

Here is a suggested reading list to get started:

1. Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter‐wave spectroscopy (Journal of Chemical Physics, 1987) – Weber reported methylammonium lead iodide in 1978, but the interesting characterisation started here, with the first evidence for disorder due to molecular dipoles.

2. Calorimetric and IR spectroscopic studies of phase transitions in methylammonium trihalogenoplumbates (Journal of Physics and Chemistry of Solids, 1990) – The PhD thesis of Noriko Onoda-Yamamuro is a gold-mine of useful information and careful measurements. It began with a report of the heat capacity and IR spectrum.

3. Conducting tin halides with a layered organic-based perovskite structure (Nature, 1994) – David Mitzi kickstarted interest in the optoelectronic properties of organic-inorganic perovskites beginning here and publishing another 30 or so papers in the following decade.

4. Templating and structural engineering in organic–inorganic perovskites (Dalton Transaction, 2001) – Back to David Mitzi for a review on the chemical and structural diversity of this family of compounds.

5. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells (JACS, 2009) – The first solar cell from Japan, but note the substantial effort that had been put into developing these materials in advance.

6. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites (Science, 2012) – The first high-efficiency solid-state solar cell from Henry Snaith FRS and the point in time when the world took notice.

7. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells (Nano Letters, 2014) – Essentially a think piece, trying to link experimental observations with theory, and the first suggestion that lattice polarisation is important for the photovoltaic action.

8. Compositional engineering of perovskite materials for high-performance solar cells (Nature, 2015) – Things have progressed since 2012 with solar cells made with a variety of materials in a range of architectures. Sang Il Seok’s group hold the record and it is with the complex formulation of meythlammonium/formamidinium – lead – iodide/bromide.

I wouldn’t recommend to start with reading about device hysteresis. While the case is almost closed in my opinion, there is no definitive publication yet (just plenty of data and speculation).

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Beginner’s guide to kesterite (CZTS) solar cells

Life was comfortable back in 2007. While starting to work on quaternary semiconductors for thin-film solar cells, there was very little literature. It was possible to read all of the papers in the field. Since the report of a 10% efficient solar cell made from Cu2ZnSn(S,Se)4 in 2010, interest in the field exploded and there now stands over 1000 publications. I feel sorry for any new graduate student beginning a project…

CZTSFrom a Web of Science search for “Kesterite OR CZTS solar cell” with 1155 results – 28th May 2015

From reading quite a few of these papers and attending conferences and workshops over the years, here is a decent reading list to get started:

1. Development of CZTS-based thin film solar cells (Thin Solid Films, 2009) – An important historical overview of the development of the field. Like many technologies, it all started in Japan.

2. New routes to sustainable photovoltaics: evaluation of Cu2ZnSnS4 as an alternative absorber material (Physica Status Solidi B, 2008) – An important paper from Jonathan Scragg (whose PhD thesis turned into the first book on kesterite solar cells) with layers made by electrodeposition.

3. The crystal structure of kesterite type compounds: A neutron and X-ray diffraction study (Solar Energy Materials and Solar Cells, 2011) – X-ray diffraction has trouble distinguishing between Cu and Zn. Neutron diffraction confirms the ground-state crystal structure (not stannite) and the tendency for cation disorder.

4. Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4 (Advanced Energy Materials, 2012) – The complexity of these materials has provided a fertile ground for theory and simulation, with early efforts reviewed here on structure, defects and band energies.

5. 8.6% Efficient CZTSSe Solar Cells Sprayed from Water–Ethanol CZTS Colloidal Solutions (Journal of Physical Chemistry Letters, 2014) – simple, clean and easy to scale up, with more recent reports of reproducible 10% efficiency from this approach.

6. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency (Advanced Energy Materials, 2014) – The current record device with 12.6% efficiency. The one to beat!

7. Influence of compositionally induced defects on the vibrational properties of device grade CuZnSnSe absorbers for kesterite based solar cells (Applied Physics Letters, 2015) – Precision Raman spectroscopy is becoming increasingly useful for identifying secondary phases and quantifying structural disorder in kesterites. The team at IREC are leading the way.

8. Suns-VOC characteristics of high performance kesterite solar cells (Journal of Applied Physics, 2014) – What is limiting performance to less than 20%? This is one of several important detailed charactertisation papers that point to issues with the back contact.

Hybrid Perovskites Go Bananas?

There have been discussions regarding hysteresis in the performance of hybrid halide perovskite solar cells since the MRS Fall Meeting in 2013 (a brave presentation from the group of Mike McGehee and supplementary slides from the presentation of Henry Snaith). Since then, there has been a flurry of papers reporting and attempting to characterise the behaviour (see a news piece in Chemistry World this week).

A related phenomenon is the low frequency dielectric dispersion of these materials (mentioned in my recent stream of consciousness), where large polarisation features emerge due to build up of charge (e.g. see the Maxwell-Wagner effect).

This effect reminded me of some arugments in the literature several years ago regarding the characterisation of ferroelectric materials (from Bananas go Paralectric to Ferroelectrics go Bananas). The response observed for a banana is remarkably similar to the “giant dielectric effect” reported for the (inedible) hybrid halide perovskites. Quite a chunk of literature can be rationalised through this anology: “With simple experiments, the response of a banana to electric fields is revealed as characteristic for an inhomogeneous paraelectric ion conductor.”

Food for thought…

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Crystal structures of hybrid perovskites are not 0K

Most electronic structure techniques for materials modelling are athermal. Temperature is not treated (i.e. no zero point energy or vibrational entropy). The standard procedure is that all atomic forces (and cell stresses) are minimised to their ground-state configuration before properties are analysed.

It is possible to include temperature effects in various ways, e.g. molecular dynamics (Newtonian dynamics based on quantum mechanical forces) or lattice dynamics (harmonic or quasi-harmonic approximations). A good example of the latter approach is the thermal properties of lead chalcogenides that we published last month.

Perovskite (ABX3) structured materials are a particularly nasty (or interesting, depending on your level of intimacy) case in solid-state chemistry. A series of temperature driven phase transitions are observed based on movements of the structural building blocks. Usually the phase transitions involve rotation or tilting of the corner-sharing network of BX6 octahedra, which correspond to relatively small changes in atomic positions and lattice volumes. Describing and understanding the nature of these phase transitions has kept theorists and crystallographers in business for many decades. Most are second-order displacive transitions, where “soft” phonon modes are associated with ferroelectric or antiferroelectric instabilities.

It is common that at high temperatures a cubic perovskite structure emerges with beautiful octahedral symmetry (a theorist’s dream). Unfortunately, in most cases this structure is an average configuration, which does not represent the local structure at any particular moment in time. For example, Martin T. Dove commented on BaTiO3: “The Ti 4+ atoms appear to occupy a central site in the high temperature cubic phase only on average, whereas in practice that site is always a potential-energy maximum. The potential energy minima for the Ti 4+ cations are located away from the central site along the eight directions, so that in the high-temperature phase the Ti 4+ cations are hopping among the eight different sites.” Nonetheless, for modelling one tends to impose the average space group symmetry which forces occupation of the potential energy maximum or saddle point. This “pseudo-cubic” structure leads to all sorts of peculiarities, e.g. if the symmetry constraints are broken through the formation of a point defect, a spontaneous phase transition can be observed.

I learned about the subtleties of these transitions working at UCL, where a PhD student supervised by Richard Catlow and Alexey A. Sokol was probing phase transitions in SrTiO3 using a combination of density functional theory and interatomic potentials. A common approach used for this type of study is “mode following”: starting from the high temperature cubic phase, lower symmetry phase can be assessed by following the eigenvectors of the imaginary phonon modes (if they are away from the Brillouin zone centre, they involve a supercell expansion). The challenge for the student was that the phase changes are delicate, with meV energy changes that test the limits in the accuracy of the methods and the precision of the codes.

I have written about hybrid perovskites before. The operation of replacing an atomic A site in a perovskite by an isovalent molecule makes matters even worse for materials modelling: the space group operations of standard perovskite are lost. For CH3NH3PbI3 (MAPI), even starting from a cubic basis, the deformation of the PbI3 cages around the molecule are large*. There are also no standard symmetry constrains to stop a tetragonal phase becoming orthorhombic (most molecules break the a = b lattice vector equality for any static configuration). An additional complication is the librational-rotational disorder of the molecules at temperatures relevant to solar cells. Reassuringly, the physically correct behaviour is recovered from molecular dynamics simulations [initially reported here]:

The behaviour of these materials is interesting, complex and challenging. The full implications for the photovoltaic performance remain to be seen.

*In our series of work, we generally use this “pseudo-cubic” basis for our simulations [see GitHub] as, in my opinion, it provides a good representation of the room temperature structure. The larger octahedral distortions observed in the modelled orthorhombic (or “pseudo-tetragonal”) cells are more representative of the behaviour below 160 K, where the molecules are held rigid in the lattice.

The Grätzel cell is dead, long live the Grätzel cell

(1991 – Nature) “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films”

…. 20 years later. A 2011 review: “Although much more is now known about the physical and chemical processes taking place during operation of the DSC [dye-sensitised solar cell], the exponential increase in research effort during this period has not been matched by large increases in efficiency.”

(2012 – Science) “Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites”

(2012 – Nature) “All-solid-state dye-sensitized solar cells with high efficiency”

(2013 – Nature) “Sequential deposition as a route to high-performance perovskite-sensitized solar cells”

(2013 – Nature) “Efficient planar heterojunction perovskite solar cells by vapour deposition”

There has been a rapid year of progress in the area of dye-sensitised solar cells. Many review papers have been appearing that are out of date before page numbers are assigned. The magic word is perovskite (referring to the crystal structure), whether it be inorganic (CsSnI3) or hybrid organic-inorganic (CH3NH3PbI3). These materials absorb a lot of sunlight and can produce voltages approaching the band gaps of the material. Very impressive!

One of the most interesting developments has been the transition from a mesoporous scaffold of TiO2 to a scaffold of Al2O3 to no scaffold at all. It is clear that the technology is converging away from the original dye-cell architecture towards traditional thin-film solar cells. Many key issues remain, including:

  • Pb-free absorber materials.
  • Replace Spiro-OMeTAD as the p-type layer (for the sake of atomistic modelling at least).
  • Make efficient cells with active areas >> 0.2 cm2*.
  • Explain how they work: Electric fields? (Yes), Free carriers? (Yes), Excitons? (No), Ion diffusion? (Yes)

We have been modelling these systems since 2012 thanks to the EU DESTINY network. Our contributions [updated]:

There are no easy answers; standard perovskites are notoriously difficult to model, hybrid perovskites are a nightmare. It is always nice to be challenged! To keep track of the field, I have started a hybrid perovskite paper collection.

*There is something uncomfortable about efficiencies reported with an active area as small as 0.075 cm2. Plenty of room for noise & artifacts, and not in the spirit of a solar cell.

2012 (Part Two)

The second third of the year has involved studies of new photo-active materials (including one previously unknown) and a quaternary high-temperature antiferromagnet.

As the demand for solar cells increases, diversity in the materials (and source elements) involved is essential. ZnSnP2 is one very interesting case, where a single material system has the potential for high-efficiency light-to-electricity conversion. This work resulted from a collaboration with Dr. David Scanlon, currently a Ramsay Fellow at University College London.

My PhD research on lone pairs in the solid-state was reborn during my postdoctoral work, when I came across BiVO4 as a promising photocatalyst for H2 production from water. The Bi(III) ion has a stereochemically active 6s2 lone pair, which results in a reduced ionisation potential for the material. Sn2TiO4 is a novel analog, which combines Sn(II) and Ti(IV), and was one of the first projects for my PhD student Lee Burton.

For his final year undergraduate project at Oxford, Russell Woolley was charged with synthesising a quinternary alloy and measuring its magnetic response. On top of that, he had the energy to perform to electronic structure calculations, before eventually moving to Imperial College for his PhD. This paper  covers one of the end member compounds, which itself is sufficient complex to warrant the input from nine authors, and just as many solid-state techniques.

A subsection of the Kathleen Lonsdale Materials Chemistry group at University College London is the Phantom Fellows. Our investigation of CuF resulted from a side-project of a sub-project of a splinter-project originally conceived by Dr. Alexey A. Sokol. It is the type of work that keeps things interesting when your main research is not going to plan. The full story was kindly covered by Chemistry World.

2012 (Part One)

It has been a busy year in terms of university and network activities, but chemistry comes first. The first third of the year has been a mix of code developments, research reviews and new science.

VASP is the most used electronic structure code on the national supercomputer, but it cannot take full advantage of the thousands of available processors. This paper is the result of a collaboration between the HPC Materials Chemistry Consortium  and NAG  to add a new mode of parallelism into the code. It works, and will hopefully be adopted into the mainstream version of VASP soon. The  approach makes hybrid density functional theory calculations less painful to run!

Solar cells work, but to enable widespread adoption of thin-film technologies, new absorber materials need to be developed that are made from earth abundant elements. Copper based compounds are the current hot topic, and this work presents a theoretical understanding of the structure and bonding in two candidate materials that have recently been synthesised in the Department of Chemistry at Bath.

People are excited about CZTS solar cells, and with good reason. While the elements are abundant and low cost, the challenge is in the complex solid-state chemistry associated with a four-component system. This paper reviews our work performed over the past five years exploring the materials chemistry and physics of the kesterite system.

The development of semiconducting metal-organic frameworks is the primary subject of my recently funded European grant. To complement our predominately theoretical research, we are collaborating with the group of Tom Vaid at the University of Alabama. Novel lead and mercury sulfide networks that show some real promise were synthesized and characterised.

It may be increasingly rare and expensive, but indium makes my favourite oxide material. So simple yet so complex. Last year we predicted the surface structure of In2O3 based on my calculations, and this year we managed to get experimental validation by one of the largest LEED studies performed to date.