This report focuses on the renewable energy source, solar cells, particularly their function and new designs such as the multi-junction solar cell, soft solar cells and dye-sensitized solar cells. This report also explains brief concepts of light, Bohr's theory of atoms and how this idea led to the conduction of semiconductor materials. This report assumes that readers have reasonable knowledge to understand the basics of physics and science. This report was written originally by Jack Lee and most of the information was researched by on the internet.
Introduction
Solar cells are simply devices that convert the energy given by light, to electricity for our usage. They are used in everyday life devices such as calculators but are also used to give power to homes and satellites. While the fossil fuel resources are likely to shrink in the future, the energy by the solar cells would always be provided from sunlight. The function of solar cells is based on the simple process of photovoltaic effect which is to be explained later on.
Light and Quantum Theory
Light is an electromagnetic radiation in the visible spectrum that can be observed by human eye. The nature of light was continually debated throughout the history of physics due to its ambiguous properties; some suggested that light existed as particles while others suggested that it is a wave. By the twentieth century, light was defined to be in a wave nature until the confusion arose when the experiments involving black body radiation showed contradicting result, inconsistent with the theoretical assumptions. In reality, when the actual radiation emitted from a black-body was measured, it was seen not to shoot toward in infinity at the ultraviolet region as the classical theories suggested, but rather was highest toward the middle of the visible range of the spectrum. (Also the concept of infinite energy by the classical physicists violated the Law of Conservation of Energy) This discrepancy is known as the "Ultraviolet catastrophe". As a solution, the German physicist, Max Planck hypothesised that light was "quantised", meaning that it existed in a discrete packets or units called "quanta" (singular: quantum). Also he hypothesized that light emitted possesses a certain frequency that defined it which follows his equation of E=hf (where E: energy of quantum, h: "Planck's constant", 6.626and f: frequency of the wave). In 1905, Albert Einstein extended Planck's concept of quantisation to electromagnetic waves. He postulated that light (or any electromagnetic wave) can be considered to be a stream of photons. Einstein maintained that the energy of light is not distributed evenly over the classical wavefront, but is concentrated in discrete particles called photons. He utilised the photoelectric effect to explain his theory and suggested that a photon not gives a portion but all its energy, hf during an interaction with an electron.
Band Structure and Doping of Semiconductors
â…°. Bohr's Quantum Model an Atom
In 1913, scientists thought that electrons orbited the nucleus like the solar system. However there was a major problem with this theory: electrons orbiting this way would inevitably radiate their energy and spiral into the nucleus, consequently destroying the atom. A young Danish physicist, Niels Bohr followed the quantum theory to set a solution. Bohr's Quantum Model of an Atom states that when electromagnetic radiation interacts with matter, energy is exchanged, and atoms and molecules may absorb this energy.
When this process occurs, electrons within the atom excites from one energy level to another, undergoing a change of quantum state. In returning to its more stable lower energy state, an atom or molecule then emits a photon in a certain part of the electromagnetic spectrum.
â…± . Band Structure in Semiconductors
-All the rectangular shapes are the energy bands.
-The highest occupied energy band is called the valence band (top blue). This is partly or completely full.
-Above the valence band is the conduction band (bottom yellow) and it is empty.
-Between the valence band and the conduction band is the forbidden energy gap (band gap) which in semiconductors is ≒ 1.14 eV. It is very small for a conductor and very large for an insulator.
According to Bohr's model of the atom, electrons cannot occupy energy states between the stable energy states, forbidden energy gaps. This means that in order for electrons to flow they must move from one allowed energy state to another allowed energy state. For a conduction of electricity, the electrons in the valence band must jump up to the conduction band where they can move freely to other atoms. In a semiconductor at room temperature, there are a number of electrons with sufficient energy in the valence band to move into the conduction band and there are a small number of electrons in the conduction band (semiconducting). However, at low temperature there are no electrons in the conduction band. At high temperatures, there is more thermal energy for electrons to absorb and therefore, thermal excitation across the narrow energy gap is more probable. Thus conduction is dependent of temperature.
â…² . Electron-hole pairs
In a semiconductor there are both negative and positive charge carriers. When an electron moves from the valence band into the conduction band, it leaves behind a vacant crystal site called a "hole". This hole, or an electron-deficient site, appears as a positive charge. It acts as a charge carrier in the sense that a valence electron from a nearby bond can move into the hole, filling it and leaving a hole behind in the electron's original place. Thus the holes and electrons migrate through the material.
In a pure semiconductor there are equal numbers of conduction electrons and holes which are called "electron-hole pairs" and the semiconductor itself is called intrinsic semiconductor. (e.g. silicon, germanium). In the presence of an electric field, the holes move in the direction of the field and the conduction electrons move opposite to the field.
â…³. Doped semiconductors - P-type, N-type semiconductors
The conductivity of semiconductors can be improved by introducing a small amount of suitable replacement atoms (impurities) into the pure semiconductor lattice - a process called "doping". All practical semi-conducting devices today are doped materials. When the impurities are added to the semiconductor lattice, the semiconductor either becomes an n-type semiconductor or a p-type semiconductor depending on the number of valence electrons of the dopant.
n-type semiconductors
Semiconductors doped with donor atoms, which are atoms with five valence electrons (group five in the periodic table. E.g. - arsenic), are called n-type semiconductors. They are called n-type because most of the charge carriers are electrons. Consider an arsenic atom, which has five valence electrons and a silicon atom which is a semiconductor with four valence electrons. When it is added to the silicon atom, it participates in the formation of covalent bonds with four silicon atoms. Therefore one electron of the arsenic atom is left over.
The fifth electron from the arsenic atom cannot fit into the valence band as it is full. It has an energy level called "donor level" in the band diagram that lies within the energy gap, just below the conduction band.
Valence band
Conduction band
Donor level
The energy spacing between the conduction band and the donor level is very small and the presence of the donor level improves the conductivity of silicon by the donated extra energetic electron which is able to move into the conduction band with the supply of small amount of thermal energy.
p-type semiconductors
Semiconductors doped with acceptor atoms, which are atoms with three valence electrons (group three in the periodic table. E.g. - aluminium), are called p-type semiconductors. They are called p-type as the majority of thencharge carriers are the positive holes. Consider an aluminium atom, which has three valence electrons added to a silicon semiconductor lattice. The three valence electrons of alumnium form three covalent bonds with the neighbouring atoms, leaving an electron deficiency, hole.
Electron deficiency or a hole
The energy levels of such impurities lie within the energy gap, just above the valence band, as indicated in the figure below.
Valence band
Conduction band
Acceptor level
holes
Acceptor level is a new energy level just like the donor level but it is close to the valence band. If a small amount of thermal energy were supplied to the electrons in the valence band, the electrons have enough thermal energy at room temperature to fill some of the acceptor levels since the energy difference between the acceptor level and the valence band is small. When an electron jumps into these empty acceptor levels, it leaves behind a hole in the valence band. The holes drift as a positive flow from atom to atom through the crystal lattice.
Solar Cell Anatomy and Function
â…°. The P-N Junction
P-N junction is a simple two-layer device formed by sticking a piece of p-type semiconductor to a piece of n-type semiconductor. The interface between the two materials is referred as a p-n junction. The difference in electrical properties between the materials (due to the difference in band structure) results in a depletion layer forming at the junction. The depletion layer has two defining properties:
-It has no charge carriers
-It has an electric field
At the p-n junction, the n-type semiconductor is close to the p-type semiconductor, so the conduction electrons from the n-side can fill the holes in the valence band of p-side. This movement of electrons from the n-type semiconductor to the p-type semiconductor creates the depletion layer.
The n-type has lost electrons and becomes positively charged while the p-type has gained electrons and becomes negatively charged. Thus, an electric field is formed across the depletion layer, in the direction pointing from n to p. The electric field repels electrons and holes from crossing the junction and thus the depletion layer stops from expanding.
P-type N-type
â…±. Anatomy of Solar cell
The anatomy of a conventional solar cell consists of the front electrode, anti-reflection coating, n-type silicon, p-type silicon (the two combined to form the p-n junction) and the back electrode.
â…². The photovoltaic effect and how a solar cell works
Conventional solar cells rely on the photovoltaic effect to create electron-hole pairs and thus achieve conduction of current. The photovoltaic effect is the process of creating voltage in a material upon an exposure to an electromagnetic radiation.
When the light strikes the solar cell, part of it is reflected, just passed through or it is absorbed. If the light is absorbed by falling on the p-n junction, the photons excite the bound electrons into a higher energy state, making them free electrons. The electrons excited from the p-type material would move up due to the influence of the electric field in the depletion layer where they can jump from atom to atom eventually reaching the exit wire.
The holes they leave in the p-type material are filled with electrons from atoms below, which leave holes behind. Thus the holes move down and the electrons move up.
Electrons flow in this direction
Hole created but soon will be filled
Through this movement of holes and electrons, photo-voltage (potential difference) is created and work is done when the electrons travels through the external circuit and return to the p layer uniting with the holes.
New Designs in Solar Cells - The Third Generation Solar Cells
The third generation solar cells are continually being developed in many countries as an approach to overcome the limited efficiency of the first and second generation solar cells (crystalline solar cells and thin film solar cells). The third generation solar cells expand on much greater range of technologies which is currently being developed and researched. Thus the third generation solar cells can be seen as a target or an alternative to current solar cells.
â…°. Multi-junction Solar Cells
Although publicly unavailable for the population, multi-junction solar cells have reached the highest maximum efficiency of 40.7% that no other solar cells could achieve. Multi-junction solar cells consist of multiple junctions with different energy bands to absorb maximum amount of photons which have broad range of varying energy from one another, whereas a single energy band solar cell can only absorb a certain photon that has the corresponding energy with the energy bands. Thus any other photon would be passed through the solar cell or converted to heat. Each semiconductor has its characterized band gap energy which causes it to only absorb certain portion of the solar spectrum, thus it is critical to choose the right semiconductor to achieve the maximum efficiency. However, the multiple junctions with different band gap energy capture more photons from sunlight and convert them to electricity at a greater efficiency. The top layer of the multi-junction solar cell has the largest band gap and it absorbs only the most energetic photons. Less energetic photons would pass through the top layer(s) as it does not have the enough energy to excite the electron to the conduction band and create electron-hole pairs. Therefore each layer would absorb the photons with higher energies than the band gap of that layer and less than the band gap of the previous layer. Unfortunately, this highly efficient solar cell is much too expensive to manufacture and are only used in high performance applications such as, satellites and spacecrafts.
â…±. Soft Cells (Flexible Solar Cells)
Soft cells consist of similar structure to 3d solar cell which has the three-dimensional nanowire architecture to catch the light. However soft cells are very flexible compared to other solar cells and they are cheap to produce. The main feature of this solar cell is the unusual structure of the cell.
The single flat layer of a conventional solar cell is replaced with a film of vertically grown nanoscale silicon wires. When light comes in, it gets trapped within this three dimensional forest of silicon wires, bouncing between each wires. Also the polymer including, light-reflecting nanoparticles is poured between each wire so that the unabsorbed photons are scattered back to the wires. Along with the silver back-reflector at the bottom which supports this process, this new design of flexible solar cell can absorb up to 85% of incoming light. However the imperfections due to the crystal structure of the wires reduces the efficiency to about 20%. Although it's efficiency is not perfect, this design requires only about a hundredth of material of conventional solar cells. Also their high flexibility can allow them to be peeled off and just stuck almost anywhere, such as windows. Another unique feature is that the transparent polymers of the device allow the cell to be near transparent. Scientists and engineers are currently striving forward with these slim, flexible soft solar cells to achieve its maximum efficiency to be available for the everyday usage.
â…². Dye-Sensitized Solar Cells
According to Dyesol (2010, technology section para.2), dye-sensitized solar cells can be best described as 'artificial photosynthesis', using an electrolyte, a layer of titania (a pigment used in tooth pastes and white paint) and ruthenium dye sandwiched between conductive glass. [1] Compared to conventional solar cells, the dye-sensitized solar cells are much simple in design consisting of only four major components: the dye, the two conductive glasses, a layer of titania and an electrolyte.
When the light enters and passes through the conductive glass, the photons with enough energy are absorbed by the dye layer close to the titania layer which excites the electrons of the dye to jump across to the conduction band of the titanium dioxide. Then the freed electrons flow through the conductive glass as a current for external power. When these electrons re-enter to the bottom conductive glass, they flow through the electrolyte which then transports it back to the dye layer for continuous usage. Dye-sensitized solar cells are currently available in public and are relatively low cost compared to crystalline solar cells. Yet, it has low conversion efficiency of 11% while the scientists assume the future designs' efficiency to be around 20%.
â…³. Design Comparison chart
Crystalline
Silicon Solar Cell
(conventional)
Multi-junction Solar Cell
Soft Solar Cell
(flexible solar cell)
Dye-sensitized Solar Cell
Efficiency
15%~20%
40.7%
12%~20%
11%
Cost
Expensive
(depending on efficiency)
Extremely expensive
Very cheap
Very cheap and easy to produce
Materials needed
Silicon is second most abundant element on earth's surface
Gallium arsenide substrate, Germanium substrate, Indium phosphide substrate (scarce element required)
Only requires hundredth of materials needed for crystalline silicon solar cells
Can be even made at homes using titanium dioxide
Conclusion
The better understanding of light and atoms, especially the discovery of the quantisation led to the production and development of solar cells which are still being developed to achieve a higher efficiency for everyday use. Solar cells are now considered as a viable energy source as a replacement of fossil fuel energy and in the near future it is highly probable that solar cells may be the energy source the population depends upon.
Bibliography and Reference
Text citation
[1] - Dyesol. (2010). Dyesol. Webpage, available: http://www.dyesol.com/page/Technology Date accessed: 23,April 2010
Figures
[1] - Brona, G. B., Głębocka, G. G. & Helbing, T. H. (1998). Atom the Incredible World. Webpage available: http://library.thinkquest.org/19662/low/eng/model-bohr.html Date accessed: 22, May 2010
[2] - Wikipedia. (2007). Semiconductors. Webpage, available: http://en.wikipedia.org/wiki/File:Electronic_band_diagram.svg Date accessed: 20, May 2010
[3] - Self-drawn by Jack Lee
[4] - Self-drawn by Jack Lee
[5] - Self-drawn by Jack Lee
[6] - Self-drawn by Jack Lee
[7] - Self drawn by Jack Lee
[8] - Self drawn by Jack Lee
[9] - Speedace. (2006). Solar Car Anatomy, Solar Arrays. Webpage, available: http://www.speedace.info/solar_car_photovoltaic_cells.htm Date accessed: 23, May 2010
[10] - Self-drawn by Jack Lee
[11] - Michael Kelzenberg of California Institute of Technology. (2010). Caltech Researchers Create Highly Absorbing, Flexible Solar Cells with Silicon Wire Arrays. Webpage, available: http://media.caltech.edu/press_releases/13325 Date accessed: 7, June 2010
[12] - Dyesol. (2010). Dyesol. Webpage, available: http://www.dyesol.com/index.php?page=How+DSC+Works Date accessed: 23, April 2010
