Semiconductor solar cells
In physics lessons we tend to just look at semiconductor solar cells.
A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell, defined as a device whose electrical characteristics—e.g. current, voltage, or resistance—vary when exposed to light.
Cells can be described as photovoltaic even when the light source is not necessarily sunlight (lamplight, artificial light, etc.) Photovoltaic cells are used as a photodetector (for example infrared detectors), detecting light or other electromagnetic radiation near the visible range, or measuring light intensity.
The operation of a photovoltaic (PV) cell requires 3 basic attributes:
The absorption of light, generating either electron-hole pairs or excitons;
The separation of charge carriers of opposite types;
The separate extraction of those carriers to an external circuit
The majority of current solar cells are made from silicon. Silicon has some special chemical properties, especially in its crystalline form because its outer shell is half full with just four electrons. A silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms to form four covalent bonds. That’s what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.
The only problem is that pure crystalline silicon is a poor conductor of electricity because its electrons are not free to move.
Electrons can jump from anywhere in the valence band to anywhere in the conduction band, so incident light with an energy equal to or greater than the band gap energy can be used to excite the electrons. In the case of crystalline silicon, the band gap energy of silicon (Si) is about 1.1 eV. The Band gap is the energy difference between the top energy level of the valence band and the bottom energy level of the conduction band. The valence band and the conduction band are overlaps of several Si atomic orbitals. For the electrons to jump between the valence band and the conduction band the valence band has to be partially empty and the conduction band has to be partially full.
To solve the poor conductivity problem the silicon in a solar cell has impurities added to it to change its properties.
The image below left shows the phosphorus layer in the conduction band and the image below right shows the boron layer in the conduction band. The impurities reduce the band gap.
Phosphorous is one of the impurities added to the silicon. It has five electrons in its outer shell so adding it to silicon (one atom for every million silicon atoms) allows it to form covalent bonds leaving one phosphorous electron on its own. It doesn’t form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.
Adding phosphorous reduces the energy required to knock these “extra” phosphorous electrons loose because they aren’t tied up in a bond with any neighbouring atoms. As a result, most of these electrons do break free, and there are a lot more free carriers than in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called n-type (“n” for negative) because of the prevalence of free electrons. This makes n-type doped silicon a much better conductor than pure silicon.
The other part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type (“p” for positive) has free openings and carries the opposite (positive) charge.
The two separate pieces of silicon (n and p) are electrically neutral but putting them in contact changes that as an electric field is produced and the free electrons on the n side see all the openings on the P side, and there’s a mad rush to fill them. Not all the free electrons fill all the holes however at the junction they do mix forming a sort of barrier making it harder for electrons on the n side to cross to the p side. Eventually, equilibrium is reached, and there is an electric field separating the two sides.
The image below shows a “Doped” silicon crystalline lattice
This electric field acts as a diode, allowing (and even pushing) electrons to flow from the p side to the n side, but not the other way around. It’s like a hill — electrons can easily go down the hill (to the n side), but can’t climb it (to the p side).
When light, in the form of photons, hits the solar cell, its energy breaks apart electron-hole pairs. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the n side and the hole to the p side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to the p side to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell’s electric field causes a voltage. With both current and voltage, the solar cell has power, which is the product of the two.
There are a few more components left before the cell can be used. Silicon happens to be a very shiny material, which can send photons bouncing away before they’ve done their job, so an antireflective coating is applied to reduce those losses. The final step is to install something that will protect the cell from the elements — often a glass cover plate. PV modules are generally made by connecting several individual cells together to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with positive and negative terminals.
An array of solar cells converts solar energy into a usable amount of direct current (dc) electricity.
The image below shows the basic structure of a silicon based solar cell and its working mechanism.
A silicon solar cell powering a mini-fan
Dye sensitised solar cells
A dye-sensitized solar cell (DSSC, DSC or DYSC) is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system. The modern version of a dye solar cell, also known as the Grätzel cell, was originally co-invented in 1988 by Brian O’Regan and Michael Grätzel at UC Berkeley and this work was later developed by the aforementioned scientists at the École Polytechnique Fédérale de Lausanne until the publication of the first high efficiency DSSC in 1991. Michael Grätzel has been awarded the 2010 Millennium Technology Prize for this invention.
The DSSC has a number of attractive features; it is simple to make using conventional roll-printing techniques, is semi-flexible and semi-transparent which offers a variety of uses not applicable to glass-based systems, and most of the materials used are low-cost. In practice it has proven difficult to eliminate a number of expensive materials, notably platinum and ruthenium, and the liquid electrolyte presents a serious challenge to making a cell suitable for use in all weather. Although its conversion efficiency is less than the best thin-film semiconductor cells, in theory its price/performance ratio should be good enough to allow them to compete with fossil fuel electrical generation by achieving grid parity. Commercial applications, which were held up due to chemical stability problems, are now forecast in the European Union Photovoltaic Roadmap to significantly contribute to renewable electricity generation by 2020.
The above image shows a selection of dye-sensitised solar cells
I have made a dye sensitive solar cell before on the Goldsmiths’ science for society course on sustainable energy but I was more than pleased to have another go.
Making a dye sensitive solar cell
2 conductive, tin dioxide-coated transparent glass plates (which you can buy readymade)
Glass stirring rod
Chem Wipe tissues
Graphite pencil (carbon catalyst)
Iodine electrolyte dropper
Bull dog clips
Distilled water dropper
Hibiscus leaves, black berry juice or black currant black berries, cherries or raspberries, green spinach, algae, and other green leaves
1) I was luckily given two readymade conductive, tin dioxide-coated transparent glass plates and identified the conducting side of the positive electrode (clear glass slide) by placing two probes onto the glass surface and connecting these to a multimeter set to a resistance of 200 Ω. The conducting side was indicated by the display showing a value other than 1.
2) I coated the conducting side of the positive electrode evenly with graphite, using a soft pencil.
3) The negative electrode (TiO2 coated side up) was placed in the dye solution (placed in a petri dish) for 5-10 minutes, rinsed in distilled water and air dried so as to make sure the coating was damaged.
The dye can be obtained from hibiscus leaves, black currants, black berries, cherries, raspberries, green spinach, algae, and other green leaves. You could experiment with other dyes.
4) We put one drop of electrolyte solution between the electrodes, ensuring it spread evenly, and clamped the positive and negative electrodes (leaving a slight offset so as to attach the electrical contacts) glass slides together.
5) Two bull dog clips were used to keep the electrodes together and the cell was connected to a multimeter set on the 200 micro amps. You can see below that 7.6 micro amps were produced.
6) The multimeter was then set on the 2V range and the cell produced 0.161V
7) Putting several cells in series only managed to double the current but this was due to the fact that some of the cells were further from the light source.
Photons pass through the titanium dioxide to dislodge (excite) electrons in the dye. These free electrons move through the titanium dioxide, accumulate at the negative plate (dyed TiO2) and flow through the external circuit. To complete the circuit, the dye is regenerated, regaining lost electrons from the iodide electrolyte. Iodide (I-) ions are oxidised (loss of electrons) to tri-iodide (I3-). The free electrons at the graphite plate then reduce (gain of electrons) the tri-iodide molecules back to their iodide state. The dye molecules are then ready for the next excitation/oxidation/reduction cycle.
The dye solar cell consists of a thin layer (approximately 10 micrometre thickness) of randomly stacked titanium dioxide particles (approximately 20 nm in diameter) to which organic dye molecules are chemically attached. The titanium dioxide particles form a three dimensional network which is electrically conducting when illuminated. Stacked titanium dioxide particles are attached to a TCO coated glass substrate. The TCO layer consists mainly of coated tin oxide on glass and is essential for transporting the current produced by the solar cell to the power consuming device, for example a calculator. The TCO substrate with titanium dioxide and attached dye molecules is called the photo electrode being the negative pole of the solar cell. To complete the solar cell (and the electrical circuit) also a counter electrode and an electrolyte liquid are required.
The transformation of the energy of light into electrical energy works as follows in the dye solar cell. A dye molecule absorbs a small amount of light. The energy present in this light is transferred to one electron in the dye molecule. Upon transferring this energy the electron becomes mobile and is able to leave its defined bond. The electron possesses enough energy to migrate through the titanium dioxide and the TCO to the electricity consuming device. A closed circuit is required in order to have an electrical current run. This means that electrons, after having transferred their energy to a consuming device, must return to the spot where they were released. To realise this, a counter electrode (positive pole) is required which absorbs the electrons from the electricity consuming device. These electrons return via the electrolyte to the dye molecules that are missing an electron. The counter electrode absorbs the electrons and transfers them to ions present in the electrolyte liquid. To let this process run efficiently, a catalyst is required. The catalyst used is a layer of graphite for example from a pencil, deposited onto the TCO of the counter electrode. The “charged” ions carry the electrons through the liquid and the pores of the titanium dioxide network until they meet with a dye molecule missing an electron. The electron is then transferred from the ion to the dye. This final step closes the electrical circuit and the dye is ready again to carry out the process of transforming light into electricity.
The energy gap of relevance here is that between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
For typical dyes the energy gap is 1.55eV or 1.38eV (corresponding to 800nm or 900nm, respectively).
The diagram below left shows energy vs. wavelength (900nm) and below right shows energy vs. wavelength (800nm).
TCO stands for transparent conducting oxide, a thin coating which is electrically conducting like a metal wire. The TCO-coating is present on one side of the glass only. Most of the light of the sun can pass through the glass and the TCO-layer without loss.
Photon is a “packet” of photoelectric energy and this energy depends on the wavelength of the light.
Electron is an elementary particle which is part of an atom, it is negatively charged. Electrical current consists of electrons moving through a conductor, like a metal wire, from the negative to the positive pole of a current source (i.e. a battery or a solar cell).
Titanium dioxide is a mineral that is commonly used as a white pigment in paint, tooth paste, fill material in pills and many other applications.
Electrolyte is an electrically conducting liquid; in this case ions rather than electrons are responsible for the charge transport through the liquid.
Ion is an atom which is missing one or more electrons or has one or more electrons too many and consequently is positively or negatively charged. Originates from the Greek word Ion, ‘Wandering’.
Graphite is a crystalline carbon that can conduct electricity and functions as a catalyst.
1. B. O’Regan, M. Grätzel, Nature 353, 737-739 (1991).
2. A. Kay, M. Grätzel, J. Phys. Chem. 97, 6272 (1993).