Iron Pyrite And Solar Cells
Whilst silicon is by far and away the market leader for semiconductor materials in solar cells, there has been much discussion over the years on the utility of iron pyrite in solar cells for electricity generation owing to its appealing photoelectrochemical properties and inexpensive nature.
Overview Of Solar Power Generation
Broadly speaking, solar power generation is when electricity is produced by the sun shining onto a specific material. In a little more detail, semiconductor materials are used in solar panels. When solar radiation hits these panels, the photons are absorbed by the semiconductor material, which causes electrons to rise to a higher-energy state. The electron may relax and return to its original energy state (with associated heat loss), or it may travel through the solar cell to an electrode, where a current is generated. Solar cell efficiencies rely on several aspects, which include:
- The materials comprising them having suitable (i.e. low) bandgaps, meaning less energy is required to ‘excite’ an electron
- A highly efficient electron acceptor, a typical example is titanium dioxide
- Robust and productive hole transporting materials, which are responsible for the movement of electrons to the electrode and the separation and inhibition of charge - this is the role that pyrite typically acts in
- Deposition/film production being an efficient process with as few defects as possible
The most commonly used semiconductor in solar panels is silicon, but high-purity silicon is expensive to produce especially at the scale needed to produce a solar array considering the layers of silicon need to be relatively thick. Pyrite is a material that has found burgeoning successes in research trials in the solar generation space. Inexpensive, widely available and naturally highly pure straight out of the ground, interesting semiconductor-like properties have been observed.
Pyrite The Material
Iron pyrite (iron(ii) disulfide, FeS2) is a widely and naturally occurring sulfide of iron. Pyrite is found in quartz seams, in sedimentary and metamorphic rocks, and often beside coal deposits. Many of its industrial and scientific uses stem from its high natural purity, moderate hardness and easiness to work with.
Crucially for solar applications, pyrite has suitable chemical and electronic properties. FeS2 has an indirect bandgap of ca. 0.95 eV, a direct bandgap of 1.05-1.10 eV, which is comparable to silicon (1.10 eV), and a solar absorption coefficient that is two orders of magnitude greater than silicon(1). Furthermore, such absorption is over a wide range of the electromagnetic spectrum, from near infrared all the way through the visible light spectrum(2). Such broadness contributes to the fact that even thinner layers of pyrite can be used compared to silicon.
Pyrite In Solar Cells
Many of the higher performing solar cell constructions are when pyrite is used alongside other semiconductors. The use of pyrite may enhance the electrical producing properties of the panel, or it may decrease the cost of manufacture, or oftentimes both. Notwithstanding this, pyrite has been shown to be effective as a photoelectrochemical cell in its own right. The first research was published in the area in 1984(3), but the pyrite solar cell only had an efficiency of ca. 2.8%.
Single Crystal Devices
Using pyrite alone for the production of a solar cell is limited in scope. The highest efficiency levels found by researchers for lone pyrite are 2%(4). Although pyrite can be deposited into a film that is 1,000 times thinner than a film of silicon and still absorb sunlight, research has shown that on the nano-scale, small surface defects can occur in the deposition of the pyrite, whereby some sulfur atoms are ‘missing’(5).
The performance of solar cells composed of pyrite is subject to ongoing research. Studies suggest that the chief limiting factor is ‘sulfur vacancies’, where there are missing sulfur atoms in the crystal structure of the films produced for cell manufacture(6). This is viewed as less of a problem when pyrite is not used on its own.
Perovskite Solar Cells
A perovskite solar cell is a cell where the primary absorber of light is a material with a perovskite-structured layer, such as tin halides. In perovskite-type solar cells, pyrite sheets have been used as hole transporting materials. Pyrite is particularly adept at this function, with some research reporting power conversion efficiencies of 11.2% where perovskite is the intrinsic semiconductor, titanium dioxide is the electron acceptor and pyrite the hole transporter(7). The same report claimed that per square metre, pyrite film is 300 times cheaper to produce than other hole transporting materials such as poly-triarylamine polymer. Furthermore, conventional hole transport materials require chemical doping to ensure sufficient conductivity levels and optimal ionic potentials(8) - doping is not required for pyrite.
Research has shown the long term stability of pyrite as a hole transporting material, with one study showing an initial power conversion efficiency for pyrite/perovskite cells of 12.8%, dropping by only 8% after 1,000 hours in laboratory conditions(9). Pyrite nanoparticles for these purposes can be obtained from the microwave-assisted hydrothermal treatment of pyrite in high purity(10).
A subset of perovskite cells, heterojunction cells often employ cadmium compounds. Cadmium sulfide is a high performing semiconductor and can be built into a solar cell as a film alongside pyrite(11), although such technology is still in the early stages of development. Pyrite is used in this sense as a solar absorbing layer in the heterojunction cell - relying on pyrite’s superior optical absorption and photo responsive properties. Under simulated daylight, 94 mV of electricity was generated at a current density of 0.4 mA/cm2 - although low numbers, it is clear of the potential of pyrite as a solar absorber(12), even with films as thin as 100 nm. Similar data have been observed for cadmium telluride-iron pyrite heterojunction cells(13), again with iron pyrite as the hole transporter.
Solar efficiencies of 13.3% have been reported where pyrite (as a hole transport medium) has been deposited as a thin film alongside cadmium sulfide/cadmium telluride in a perovskite cell(14), here, pyrite films are utilised at a 1 μm thickness so as to avoid any film porosity related issues. One of the many reasons, it has been reported, that thin pyrite films behave so well as a hole transport medium is that they have a high free hole density and a relatively deep work function around 5 eV(15).
Dye Sensitised Solar Cells (DSSC)
DSSCs are another class of solar cells which have been shown to benefit from the inclusion of iron pyrite in their manufacture. DSSCs are solar cells where a semiconductor is sandwiched between a photosensitised anode and an electrolyte(16). Such thin film cells have been in development since the early 1990s. For the purposes of this work, the identity of the ‘dye’ is largely irrelevant. One of the reasons for the high cost of such cells is the platinum electrodes which are used. Recent research replaces platinum with pyrite, and not only reduces cost of manufacture but displays conversion efficiencies of 7.27% under a 100 mW/cm2 lamp(17). Pyrite’s high quantum efficiency(18) and long term stability in a DSSC’s corrosive iodide liquid electrolyte(19) are viewed as valuable contributory factors. Such work builds on the use of hybrid iodine/cobalt redox cells with an iron pyrite film as the counter electrode(20).
A subclass of DSSC is where the pyrite is in nanoparticle form and is coated with a thin layer of carbon before being made into a film, which acts as the counter electrode along with a redox electrolyte, a material like titanium dioxide as the primary electron acceptor, and all assembled atop a substrate such as fluorinated tin oxide. Such structures utilise pyrite in a different manner to the sheet forms as seen in other cell types, i.e. coated in carbon, but reported efficiency data is as good as if the counter electrode is the conventional platinum(21). One of the advantages of DSSCs is in their production on a roll - a manifold to which pyrite sheets fit well - meaning the solar cell produced is incredibly thin and flexible.
- Pyrite is a highly pure naturally occurring sulfide of iron with interesting chemical and electronic properties that can be suitable for solar energy production
- As a single crystal solar cell, early research showed low solar generation efficiencies
- When used in a perovskite solar cell, pyrite comes into its own as a highly efficient hole transport medium - with solar efficiencies of up to 13.3%; in heterojunction set-ups, pyrite is also used as a solar absorber
- In dye sensitised solar cells, pyrite can be used effectively to replace platinum at the (counter)electrode
- Cost of production decreases with more pyrite compared to silicon - paving the way for a more sustainable future
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