Solar Photovoltaic PhysicsBasic Physics and Materials Science of Solar CellsOriginal Presentation by J. M. Pearce, 2006 Email: profpearce@gmail.com

Basic Physics of Solar Photovoltaic CellsThis presentation will cover the fundamental semiconductor physics that makes solar photovoltaics possible and review the materials needed to make solar cells.

Notes for InstructorFeel free to use all or parts of this presentation but please follow the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License which can be found here:http://creativecommons.org/licenses/by-nc-sa/2.5/

For a basic understanding of solar photovoltaic physics see Solar Electricity, 2nd Edition by Tomas Markvart (Editor), 2000.

For a more advanced discussion of solar photovoltaic physics see The Physics of Solar Cells by Jenny Nelson, Imperial College Press, 2003.

For a detailed discussion of the future of solar photovoltaic physics see Third Generation Photovoltaics : Advanced Solar Energy Conversion by Martin Green, Springer, 2003.

What are Photovoltaics?Photovoltaic (PV) systems convert light energy directly into electricity. Commonly known as solar cells. The simplest systems power the small calculators we use every day. More complicated systems will provide a large portion of the electricity in the near future.PV represents one of the most promising means of maintaining our energy intensive standard of living while not contributing to global warming and pollution.

Basic Physics of Solar Photovoltaic Cells

Photovoltaic (PV) systems convert light energy directly into electricity. They are commonly known as solar cells. The simplest systems power the small calculators we use every day. More complicated systems will provide a large portion of the electricity in the near future. PV represents one of the most promising means of maintaining our energy intensive (with appropriate and technically viable efficiency gains) standard of living while not contributing to global warming and pollution. Solar Photovoltaics is the future of energy!

A Brief History Photovoltaic Technology1839 Photovoltaic effect discovered by Becquerel. 1870s Hertz developed solid selenium PV (2%). 1905 Photoelectric effect explained by A. Einstein. 1930s Light meters for photography commonly employed cells of copper oxide or selenium. 1954 Bell Laboratories developed the first crystalline silicon cell (4%).1958 PV cells on the space satellite U.S. Vanguard (better than expected).

Basic Physics of Solar Photovoltaic CellsHere is the timeline of photovoltaic technology. Solar cell technology is actually pretty old. The photovoltaic effect was discovered in the late 1830s. An explanation of the photoelectric effect won Einstein the Nobel prize. There was a big break through in 1954 because the first crystalline silicon solar cell was developed. Four years later it was used on a space satellite. The good news is that it worked, the bad news is that the solar cells kept working past when NASA expected so then the satellite kept sending data to earth when it was no longer needed and took up E and M bandwidth.

Things Start To Get Interesting...mid 1970s World energy crisis = millions spent in research and development of cheaper more efficient solar cells. 1976 First amorphous silicon cell developed by Wronski and Carlson. 1980s - Steady progress towards higher efficiency and many new types introduced1990s - Large scale production of solar cells more than 10% efficient with the following materials:Ga-As and other III-Vs CuInSe2 and CdTeTiO2 Dye-sensitized Crystalline, Polycrystalline, and Amorphous Silicon Today prices continue to drop and new 3rd generation solar cells are researched.

Basic Physics of Solar Photovoltaic CellsIn the mid 1970s people were getting worried because of the oil crisis and OPEC. Because people were worried, more thought and money was given to find new and cheaper ways of making solar cells. One development that came out of this research push was the first amorphous silicon cell developed by Chris Wronski and Dave Carlson. The discovery of the amorphous silicon solar cells caused a lot of excitement because it was a fundamentally cheaper material than standard crystalline silicon. This material is also used for the thin film transistors that drive modern flat panel displays. They have been mass produced real cheap, and over the years the price for a flat panel has gone down significantly. The same is happening for solar cell prices.

In the 1980s the solar photovoltaic community made steady progress towards higher efficiency and many new types of solar cells were introduced so that by the 1990s there were large scale production of solar cells more than 10% efficient with the following materials: Ga-As and other III-Vs, CuInSe2 and CdTe, TiO2 Dye-sensitized, and finally the largest producers Crystalline, Polycrystalline, and Amorphous Silicon solar cells. Today prices continue to drop and new 3rd generation solar cells are researched.

Types of Solar Photovoltaic Materials

Basic Physics of Solar Photovoltaic CellsThis graph show the record efficiency of different types of solar cells as a function of time. What is important to see here is that all the different types of solar cells follow the same trend as more research is undertaken the efficiencies go up which will drive down the cost of solar electricity relative to fossil fuel fired electricity.

Photovoltaic Materials

Basic Physics of Solar Photovoltaic CellsSolar cells are made from semiconductors - the most important being silicon. Semiconductors have special electronic properties which allow them to be insulating or conducting depending on their composition. In photovoltaic materials you are dealing with the semiconductors (yellow). Most of the doping comes from boron and phosphorous for silcion solar cellsate. There are different types of solar cells such as cadmium telluride (CdTe) made from Cadmium and Tellurium or copper indium diselenide or gallium aresenide. Source:http://www.mse.cornell.edu/courses/engri111//semicon.htm

Electronic Structure of Semiconductors SiliconGroup 4 elemental semiconductorSilicon crystal forms the diamond lattice Resulting in the use of four valence electrons of each silicon atom.

Basic Physics of Solar Photovoltaic CellsMost solar cells are made from silicon. Silicon has the diamond structure -a perfect tetrahedral coordinated crystal. Each silicon atom in the lattice is in an identical position to every other atom and each has four nearest neighbors. The 4 electrons in Si form bonds to the nearest neighbors (as seen by the red bonds in the figure). Silicon is a semiconductor and is the element largely responsible for the integrated circuit, which makes modern computers possible.

CrystallineSiliconAmorphous Silicon

Basic Physics of Solar Photovoltaic CellsLets take a look at silicon technology first because it makes up the majority of the current photovoltaic market. Here is the graphical difference between crystalline silicon and amorphous silicon. The crystalline silicon lattice is perfect, like the swan. The amorphous is messed up with defects (strained and broken bonds), like the ugly duckling. These physical defects lead to electronic defects and lower solar cell performances.

Solar PV Materials:Crystalline & Polycrystalline SiliconAdvantages:High Efficiency (14-22%)Established technology

(The leader)StableDisadvantages:Expensive productionLow absorption coefficientLarge amount of highly

purified feedstock

Basic Physics of Solar Photovoltaic CellsThere are some advantages and disadvantages of crystalline and polycrystalline silicon. Advantages include high efficiency, very established technology and it is stable. The technology for the crystalline silicon is the same technology that drives the semiconductor industry. The disadvantages for crystalline technology include expensive production, low absorption (so you need a lot of material), and a large amount of highly purified feedstock, which of course is expensive. Polycrystalline silicon is cheaper but the electric properties are not as good so the efficiency is lower.

Amorphous SiliconAdvantages:High absorption (dont need a lot of material)Established technology Ease of integration into buildingsExcellent ecological balance sheetCheaper than the glass, metal, or plastic you deposit it on

Disadvantages:Only moderate stabilized efficiency 7-10%Instability- It degrades when light hits itNow degraded steady state

Basic Physics of Solar Photovoltaic CellsThe other kind of silicon solar cell material is amorphous silicon. It has a very high absorption coefficient because it acts like a direct band gap semiconductor instead of an indirect gap like crystalline silicon. It can also come like a sticker, such as the solar cell in the image, where all you do is peal off and stick it to the building and it lasts virtually forever (30 year warranty) . It also does not need any building materials to put it up and it is cheaper than glass, metal or plastic. The disadvantages include efficiency is only 7-10% and it degrades when light hits it this is known as the Staebler-Wronski effect or SWE. All amorphous silicon solar cells sold now have warrantees for the degraded steady state where SWE has stopped. For example you might buy a solar cell with a warrantee of 10% efficient when you first install it your will be happy to find that it will produce electricity at 11% efficient. After a few months it will degrade down to 10% and then stay there the rest of its lifetime.

How do they work?The physics view

Basic Physics of Solar Photovoltaic Cells

Band Theory There are 3 types of materials in Band Theory, which are differentiated by their electronic structure: insulators, conductors, and semiconductors.

EgMetal Insulator SemiconductorEfEfEf

Basic Physics of Solar Photovoltaic CellsBand theory models the behavior of electrons in solids, by postulating the existence of energy bands, continuous ranges of energy which electrons may occupy, and gaps, which they may not. It successfully explains many physical properties of solids, such as electric resistivity and optical absorption. For the conductor, the valence band is above conduction so there are free electrons to conduct electricity. In the insulator, the two bands are far apart from each other and its very hard to get charge carried from the bottom (valence) to top band (conduction band). For the semiconductor there is a gap between the valence and conduction bands this is known as the bandgap and is denoted Eg (Energy of the gap). The Fermi Energy (Ef) is also shown for the insulator and semiconductor. The ground state of a non-interacting fermion system (like one made up of electrons here) is constructed by starting with an empty system and adding particles one at a time, consecutively filling up the lowest-energy unoccupied quantum states. The Fermi-energy is the level (if there is one) where half of the states are occupied with electrons.

Energy Bands in a SemiconductorConduction Band Ec emptyValence Band Ev full of electrons

Basic Physics of Solar Photovoltaic CellsEnergy of an electron in a semiconductor must fall between two defined bands. The valence band are energies of valence orbitals which form bonds between atoms. The conduction band, next higher up, is separated from the valence band by and energy gap, or bandgap. The width of the energy gap, Eg, is defined by the difference in energy of the conduction and valence bands: Ec- Ev. This is the energy band structure for a typical semiconductor in detail like silicon. This is your valence band and conduction band. There is an energy gap noted Eg between the two. The valence band is filled with electrons the energy of electrons is going up. A hole is an absence of an electron. The conduction band is empty at absolute zero but at room temperature some valence electrons are excited into it and free to travel. For the remainder of the presentation, if something is full (almost full) the color will be dark and if something is empty (or almost empty) the color will be light. For example, dark green means full valence band and light green means empty conduction band.

3 Types of SemiconductorsIntrinsicn-typep-type

Types 2 and 3 are semiconductors that conduct electricity - How?by alloying semiconductor with an impurity, also known as dopingcarriers placed in conduction band or carriers removed from valence band.

Note: Color Protocol

Basic Physics of Solar Photovoltaic CellsThere are three types of semiconductors. There is the intrinsic, which we just discussed (green). There are no extra electrons in conduction band and the valence band is full at absolute zero. Then there is the n-type (orange). It has extra electrons, in the conduction band, which are denoted by the blue circles. The extra electrons pull the Fermi level (black line) up towards the conduction band. For clarity the Fermi energy was not shown for the intrinsic semiconductor although in this case it would be in the middle of the gap. The p-type (purple) is when you have extra holes in valence band and the Fermi level is closer to the valence band. You can create n and p type semiconductors by placing atoms of a different element inside the lattice. For example, you can place elements in row three and row five (B and P) and remove electrons or give extra ones respectively for silicon.

Type 1: IntrinsicPure semiconductor (intrinsic): contains the right number of electrons to fill valence band, therefore, conduction band is empty. Because electrons in full valence cannot move, the pure semiconductor acts like an insulator.

Basic Physics of Solar Photovoltaic CellsThe pure intrinsic type contains the right number of electrons to fill the valence band, therefore, the conduction band is empty. Because of that you cannot have any movement of electrons in the valence band. The pure intrinsic type basically acts like an insulator. You have to dope it if you want useful electronic properties.

Type 2: n-Typen-type: current is carried by negatively charged electrons - How?group 5 impurity atoms added to silicon melt from which is crystal is grown4/5 of outer electrons used to fill valence band1/5 left is then put into conduction band. These impurity atoms are called donors.

Within conduction band the electrons are moving, therefore, crystal becomes a conductor

Basic Physics of Solar Photovoltaic CellsFor the n-type you have the group 5 impurities and you are adding an electron. It is also doping with a donor because now there are extra electrons in the conduction band which have room to move. This acts like a conductor, for example, a copper wire. In n-type the current is carried by negatively charged electrons - How? By using group 5 impurity atoms added to silicon melt from which is crystal is grown so 4/5 of outer electrons used to fill valence band and 1/5 left is then put into conduction band. These impurity atoms are called donors.

Type 3: p-Typep-Type: current carried by missing electron holes which act as positively charged particles. How?group 3 added to silicon meltneed 4 out of 5 outer electrons but doping creates lack of electrons in valence band. missing electrons, a.k.a holes, are used to carry current.

Basic Physics of Solar Photovoltaic CellsIn the p-type semiconductor electrons are removed by adding group 3 elements to the silicon. This creates holes or positive charges. There is now room for charge carriers to move around so now you have a hole conductor. Holes are the moving absence of electrons.

What Carries the Current?Prevailing charges are called the majority carriersprevailing charge carrier in n-type: electronsprevailing charge carrier in p-type: holes

Basic Physics of Solar Photovoltaic CellsWhat carries the current?In the n-type material its the electrons. In the p-type material its the holes. This is true for all semiconductors.

Creating a JunctionThere are four main types of semiconductor junctionsp-np-i-nSchottcky barrierHeterojunctionEach has a built in potential

Basic Physics of Solar Photovoltaic CellsYou can create devices by putting two similar semiconductors together. The device is called a junction and there are four different types. There is the p-n junction, p-i-n junction, Schottcky barrier, and the heterojunction. Each has a built in potential (a built in voltage) associated with them.

p-n and p-i-n Junctions

EfEfVbiVbi

Basic Physics of Solar Photovoltaic CellsHere are the first two types of junctions. The p-n and p-i-n junctions. You have the conduction band and valence band. No electrons can exist in band gap, however, semiconductors are not perfect. In thermodynamic equilibrium at room temperature, you actually populate some of the states in this band gap. The solid line is the Fermi level. You can think of it as electrons as a liquid being poured into a bucket (the gap) and being filled up to the Fermi level. In semiconductors, when you put two dissimilar semiconductors together, the Fermi level must stay flat. This is what created the bending of these bands (both the conduction and valence band).

So in the p-n junction you have the p layer and an n layer. In the p-type the Fermi level is very low in the gap an pulled to the valence band. In the n-type semiconductor, the Fermi level is very high and pulled toward the conduction band. There is an enormous bending of the conduction and valence bands due to trying to keep Fermi level flat when they are combined. The bending creates the voltage and the electric field observed when putting two dissimilar (p and n) semiconductors together. The energy between the conduction band in the p-layer and the n-layer is the built in potential.

What happens when you put an intrinsic layer of semiconductor between the p and n types? Well now, this bending of the bands is spaced out. The Fermi level still flat.

Schottky Barriers and Heterojunctions

Basic Physics of Solar Photovoltaic CellsHere are the next two types of junctions that can be used for solar cells: Schottcky Barriers and Heterojunctions. For the Schottcky barrier, there is a metal, filled with electrons, placed right beside the semiconductor. This creates the bend in the band. This type of junction is a way to test semiconductors and is generally not technically practical for solar cells in production.

The heterojunction has two different semiconductors put together that have different energies. When these two semiconductors are put together, they create a bend in the band as well.

Semiconductor JunctionsAll the junctions contain strong electric fieldHow does the electric field occur?When two semiconductors come into contact, electrons near interface from n-type, transfer over to p-type, leaving a positively charged areaHoles from p-type by interface transfer over to n-type leaving a negatively charged area.Because electrons and holes are swapped, a middle potential barrier with no mobile charges, is formed.This potential barrier created does not let any more electrons or holes flow through.Electric field pulls electrons and holes in opposite directions.

Basic Physics of Solar Photovoltaic CellsAll the junctions already shown contain strong electric fields. What is actually happening is that if you have n and p semiconductors, that come into contact, electrons near the interface in the n-type material will be transferred over to the p-type material. What you do is deplete a region of electrons in the n-type right beside the p-type. The same works for holes. The holes move from p-type to n-type and it ends up that there is a region where the negative electrons become positive and positive holes become negative. The changing of charges gives the barrier. Electric field pulls electrons and holes in opposite directions.

Barrier ChangesEquilibrium means there is no net currentReduced barrier height is called forward bias (positive voltage applied to p-side)Result- increases current through diodeIncreased barrier height is called reverse bias.Result- decreases current to a very small amount..

Basic Physics of Solar Photovoltaic CellsThe barrier can be altered by a forward or reverse bias. The motion of the electrons and holes in the far left picture is equal and opposite. If you put on a forward bias, by attaching a battery for example, it forces the electrons to move in the opposite direction which weakens electric field. You can strengthen it by reverse bias. Equilibrium means there is no net current. A reduced barrier height is called forward bias (positive voltage applied to p-side) and results in increases in current through diode. Increased barrier height is called reverse bias.

Electric Currents in p-n Junction Under External Bias

Diode I-V Characteristics

Basic Physics of Solar Photovoltaic CellsThe figure is a typical current (I) and voltage (V) plot for a solar cell known as an IV curve. Applying a voltage from negative to positive and record the current. For the dark current, it is not quite zero and it starts to go up and pushes electrons over creating a massive current. When illuminated, you instantly end up with a current. The power is calculated as the current multiplied by the voltage (P=IV).We will talk about extracting power from a semiconductor p-n junction in a moment.

Current in a Solar CellOutput current = I = Il-Io [ exp(qV/kT)-1]Il=light generated currentq = electric chargeV = voltagek = Boltzmans constant = 1.3807 10-23 J/KWhen in open circuit (I=0) all light generated current passes through diodeWhen in short circuit (V=0) all current passes through external load

2 Important points:1) During open circuit the voltage of open circuit,Voc = (kT/q) ln( Il/Io +1)2) No power is generated under short and open circuit - but Pmax = VmIm=FFVocIsc

Basic Physics of Solar Photovoltaic CellsIn order to calculate current in a solar cell use the following formula: I = Il-Io [ exp(qV/kT)-1]. Where Il=light generated current. Io= dark generated current, q = electric charge, V = voltage, and k = Boltzmans constant = 1.3807 10-23 J/K. Just by looking at this equation you can see one of the problems that occur in solar cells. As temperature increases, the current in the solar cell decreases. You dont get as much energy output from a solar cell if the temperature is higher. This occurs in all types but it hurts the diffusion type of solar cell the most. In amorphous silicon drift type of solar cells, however, you decrease the defects by increasing the temperature and that effect somewhat balances the loss in performance due to temperature increases. The open circuit is defined when I=0. Short circuit conditions occur is when V=0. Rearranging the output current equation and solving for voltage gives the open circuit equation. Voc = (kT/q) ln( Il/Io +1). In addition, no power is generated under short or open circuit but, Pmax = VmIm=FFVocIsc.

I-V Curve for Solar Cells Fourth quadrant (i.e., power quadrant) of the illuminated I-V characteristic defining fill factor (FF) and identifying Jsc and Voc

Basic Physics of Solar Photovoltaic CellsThis is the same graph as before only flipped around to highlight the 4th quadrant of the I-V plot (Current density vs. Voltage). As V=0, you have maximum current, which is called the short circuit current. If you increase voltage, the current decreases. If you continue to increase voltage, the current goes to zero, and this is called an open circuit. The dashed line signifies power density and is shown on the right axis. The maximum current density and voltage is at the maximum power point, because after that, the power drops off, as well as the current. Maximum power for current= Jmp. Maximum power for voltage= Vmp. Fill factor is defined as: FF= (Imax)( Vmax) / short circuit open circuit. A 100% fill factor would have a square angle in the figure.

Light Absorption by a SemiconductorPhotovoltaic energy relies on light.Light stream of photons carries energyExample: On a clear day 4.4x1017 photons hit 1 m2 of Earths surface every second.Eph()=hc/=hfh = planks constant = 6.625 x 10-34 J-s = wavelengthc = speed of light =3 x 108 m/sf = frequency However, only photons with energy in excess of bandgap can be converted into electricity by solar cells.

Basic Physics of Solar Photovoltaic CellsIn order for a solar cell to work, it has to absorb light. Light is basically a stream of photons, which are quantized packets of light energy. There is a huge amount of photons in light. For example, on a clear day 4.4x1017 photons hit 1 m2 of Earths surface every second. Each photon has energy associated with it that depends on the wavelength, or the color of the light. This is shown in the Energy equation for a photon: Eph()=hc/=hf, where h = planks constant = 6.625 x 10-34 J-s, = wavelength, c = speed of light =3 x 108 m/s, and f = frequency. However, only photons with energy in excess of bandgap energy can be converted into electricity by solar cells. This is because, when light shines on a semiconductor the only photons that are absorbed have at least the energy of the bandgap. Photons that are too small will not be absorbed and will go straight through the band gap. So, in general, photons need to have enough energy to excite an electron from the valence band up to the conduction band.

The Solar SpectrumThe entire spectrum is not available to single junction solar cell

Basic Physics of Solar Photovoltaic CellsThis is the Solar Spectrum. The graph shows the flux of photons that come down to the Earth vs. the wavelength of the photons. The Orange line represents the solar spectrum outside the atmosphere. The spectrum reaches a peak in the visible as expected from black body radiation at 5800K (the surface temperature of the sun). The missing regions of flux (black line) at the earths surface are due to absorption from water vapor and carbon dioxide. As you can clearly see, the solar energy is not just a single energy (wavelength or frequency of light). If it were, we would have solar cells based on current technology with efficiencies over 95%. So why do we lose energy at different wavelengths? Part of it is because we are not absorbing the whole spectrum of a single photon. If we put in photons with larger energies than the bandgap we only capture the energy equal to Eg we lose the rest to waste heat. Photons with energies lower than the bandgap do not contribute to carriers (electrons and holes) in a solar cell.

Source: http://ess.geology.ufl.edu/ess/Notes/040-Sun/spectrum.GIF

Generation of Electron Hole Pairs with LightPhoton enters, is absorbed, and lets electron from VB get sent up to CB Therefore a hole is left behind in VB, creating absorption process: electron-hole pairs.Because of this, only part of solar spectrum can be converted. The photon flux converted by a solar cell is about 2/3 of total flux.

Basic Physics of Solar Photovoltaic CellsSo what happens to a photon that is absorbed in a semiconductor?The photon enters and is absorbed and thereby excites an electron from the valence band up to the conduction band. The extra energy of the photon is then lost and turned into heat. Because of that, only 2/3 of the total flux can be turned into electricity. Even after that, more energy flux can be lost from thermal process.

Generation CurrentGeneration Current = light induced electrons across bandgap as electron currentElectron current:= Ip=qNAN = # of photons in highlighted area of spectrumA = surface area of semiconductor thats exposed to lightBecause there is current from light, voltage can also occur.Electric power can occur by separating the electrons and holes to the terminals of device. Electrostatic energy of charges occurs after separation only if its energy is less than the energy of the electron-hole pair in semiconductorTherefore Vmax=Eg/qVmax= bandgap of semiconductor is in EVs, therefore this equation shows that wide bandgap semiconductors produce higher voltage.

Basic Physics of Solar Photovoltaic CellsWhen a photon is absorbed it generates current. Generation current is just light induced electrons across the band gap as electron current. The equation for electron current is Ip=qNA. Where q is the charge, N is the number of photons in the highlighted area of the spectrum and A is the surface area of semiconductor thats exposed to light. Because the of this the maximum Voltage it will ever get to is just the band gap energy in electron-volts, or eVs. Electrostatic energy of charges occurs after separation only if its energy is less than the energy of the electron-hole pair in semiconductor. Therefore Vmax=Eg/q, where q is the charge on an electron if Eg is measured in electron volts eV, you are left with voltage. Vmax= bandgap of semiconductor is in EVs, therefore this equation shows that wide bandgap semiconductors produce higher voltage.

Direct vs Indirect BandgapEverything just talked about, where all energy in excess of bandgap of photons are absorbed, are called direct-bandgap semiconductors.More complicated absorption process is the indirect-gap seriesquantum of lattice vibrations, of crystalline silicon, are used in the conversion of a photon into electron-hole pair to conserve momentum there hindering the process and decreasing the absorption of light by semiconductor.

Basic Physics of Solar Photovoltaic CellsWhat we have seen so far is the direct band gap approach. In reality, a lot of semiconductors, for example silicon, are not direct band gaps. Semiconductors have to have the right energy and momentum to be able to capture photons. Indirect gap semiconductors also demand a quantum of lattice vibrations to absorb light. The photons in the crystal have to be able to create the pair.

In addition to structural and electron space there is also momentum space. Momentum has to be conserved in any type of process. In turns out that for a perfect crystal, there are resonance vibrations and when a photon is excited it not only has to conserve energy but also conserve momentum.

The Solar CellElectric current generated in semiconductor is extracted by contacts to the front and rear of cell. Widely spaced thin strips (fingers) are created so that light is allowed through. these fingers supply current to the larger bus bar.

Antireflection coating (ARC) is used to cover the cell to minimize light reflection from top surface. ARC is made with thin layer of dielectric material.

Basic Physics of Solar Photovoltaic CellsElectric current generated in semiconductor is extracted by contacts to the front and rear of cell. Widely spaced thin strips (fingers) are created so that light is allowed through - These fingers supply current to the larger bus bar. In this blue area in the figure you have the two doped layers that make up the p-n junction. On top of that there are metal contacts which are connected to a bus bar. There is metal on the back of the solar cell, a semiconductor (p-n junction) in the middle, and another method of collecting electrons on the front. As shown in the diagram, you have the bus bar that is connected to the small fingers. On top of the solar cell there is an antireflection coating, or ARC. Antireflection coating (ARC) is used to cover the cell to minimize light reflection from top surface. ARCs are made with thin layer of dielectric material. When light comes in, you want to collect as much of it as possible and if you can see light reflecting off the solar cell, the ARC is not good. The ideal solar cell does not reflect much light and has as small shadowing as possible (due to the top metal contacts). All of this helps with trapping light in the solar cell.

Different Types of Photovoltaic Solar CellsDiffusionDriftExcitonic

Basic Physics of Solar Photovoltaic Cells There are three different types of solar cells. The most established is diffusion. The other two are drift and excitonic and represent 2nd and 3rd generation types of solar cells, respectively.

Diffusionn-type and p-type are aligned by the Fermi-levelWhen a photon comes in n-type, it takes the place of a hole, the hole acts like an air bubble and floats up to the p-typeWhen the photon comes to the p-type, it takes place of an electron, the electron acts like a steel ball and rolls down to the n-type

Basic Physics of Solar Photovoltaic CellsIn diffusion-based silicon solar cell there is the p-n junction. Light is absorbed and excites electrons up to the conduction band. You can think of the electrons in the conduction band as little balls rolling down a hill. So once the electron is excited up to the conduction band, it rolls down the hill. The holes are like air bubbles, they float up inside the valence band. The movement of electrons and holes creates a current. Take note that the direction the current is moving is the opposite of the direction the electrons are moving.

This diagram is an example of diffusion. For an electron to get to the hill to fall down, it has to diffuse. You have to have a very good crystal for this to work. If not, the defects in the crystal will cause the electrons to combine with the holes and mutually annihilate.

Diagram of p-n Junction and Resultant Band Structure

Basic Physics of Solar Photovoltaic CellsThis is what is happening when we create a diffusion based p-n junction. On the left is the p-type material (purple) with a lot of holes and on the right the n-type material (orange) with a lot of electrons. If you are putting these together, the Fermi level has to remain flat, which creates the bend in the bands as seen in the band diagram on the lower right.

DriftThere is an intrinsic gap where the photon is absorbed in and causes the electron hole pair to form.The electron rises up to the top and drifts downwards (to n-type)The hole drifts upwards (to p-type)

Basic Physics of Solar Photovoltaic CellsThe second type of solar cell is called the drift solar cell. A good example is amorphous silicon solar cells. If you put a p and n junction of amorphous silicon right next to each other, you create the electron-hole pairs with light absorption but because the material is so defective they will instantly be annihilated. So to make an amorphous silicon solar cell work you put in an intrinsic layer that had less defects, in between the p and n layers. They also made the p and n layers very thin, approximately 10-20nm thick. The intrinsic layer is much thicker at thousands of nm thick.

The thick layer absorbs the vast majority of the photons and instantly creates electron-hole pairs. Because of the band bending, the electron will automatically roll down the hill and the hole with float up. Notice the current moves right because the electron moves left. In addition, the p and n layers are thin enough that you dont have to worry about recombination.

Excitonic Solar CellDye moleculeelectron hole pair splits because it hits the dyethe electron shifts over to the electric conductor and the hole shifts to the hole conductor

Basic Physics of Solar Photovoltaic CellsOne of the newest types of solar cell, which is causing a lot of excitement is the dye-sensitized excitonic solar cell. As you can see there is a hole conductor and an electron conductor and a dye in between them. The dye has a color associated with it and absorbs the photons. The absorption of the photon creates and electron-hole pair. If you pick the right hole and electron conductors, with appropriate energy levels the electron will jump into the electron conductor and the hole will jump into the hole conductor. Now we have a solar cell. By moving this excitation from dye to the conductors, it creates electrical current. Dye molecule type is relatively new and can be done with cheap materials. The first commercial production has already begun in Europe over the last few years.

Power Losses in Solar Cells

Basic Physics of Solar Photovoltaic CellsThere are physical constraints that create power losses in real solar cells. This diagram shows these constraints. For 100mW of power coming from the sun are coming in, 21 mW is the below the band gap, so its not enough energy to excite an electron. 31mW of it is excess of the band gap so its lost through heat. So already, more than half of your power is lost just because of the limitations of a single bandgap.

The Voc available in silicon is 1.1V and because of all the above losses, all you have left are 44 mAmps. Recombination losses, which are electrons smashing into holes, drops the 1.1V to approximately 0.6V. As you can see, recombination cuts the maximum voltage almost in half. Now the current, already being cut down earlier, will be cut down once again due to collection efficiency and incomplete absorption. Collection efficiency is when some of the photons cannot excite electron-hole pairs. Incomplete absorption means some of the photons pass through the cell. This demonstrates one of the technical challenges of designing solar cells: If you make your cell thicker, its easier to absorb photons but harder to collect electrons because they have to diffuse over longer distances. However, if you make the solar cell thinner, it would be easier to collect electrons but you will lose a lot of photons that do not absorb. In addition, there is top surface reflection on a cell and no absorption where the metal is located due to shadowing. The current of 44milliAmps is now decreased to 21 milliAmps. The fill factor is reduced from 1 to .7. The final power is about 14 mW, which means this solar cell is 14% efficient.

RecombinationOpposite of carrier generation, where electron-hole pair is annihilatedMost common at:impuritiesdefects of crystal structuresurface of semiconductorReducing both voltage and current

Basic Physics of Solar Photovoltaic CellsNow we will go through each one of these losses in detail. Recombination is when an electron-hole pair is created and then is smashed back into each other. Recombination usually happens when there is an impurity. For example, if you have a perfect silicon lattice except for the addition of a few sodium atoms. The sodium atoms create defects in the gap which means you end up with nothing instead of an electron-hole pair. There can also be defects in the crystal structure. No matter how perfect the bulk material is, on the surface of a semiconductor there are silicon atoms that are not fully bonded which then cause surface states. The more surface states you have on the silicon solar cell, such as is created at grain boundaries in a polycrystalline silicon solar cell, the worse off you are. This explains why polycrystalline silicon solar cells do not have as high efficiencies as seen for single crystal silicon solar cells.

Series ResistanceLosses of resistance caused by transmission of electric current produced by the solar cell.I-V characteristic of device: I = Il-I0 [exp(qV+IRs / mkT) 1] m= nonideality factor

Basic Physics of Solar Photovoltaic CellsSeries resistance losses are caused by transmission of electric current produced by the solar cell. This equation (I = Il-I0 [exp(qV+IRs / mkT) 1] ) is for the IV characteristic of a typical device but now includes the recombination losses. The m term is a nonideality factor.

Other LossesCurrent losses- called collection efficiency, ratio b/w number of carriers generated by light by number that reaches the junction.Temperature dependence of voltageV decreases as T increases Other losses light reflection from top surfaceshading of cell by top contactsincomplete absorption of light

Basic Physics of Solar Photovoltaic CellsOther losses include collection efficiency, which we talked about earlier. It is the ratio between the number of carriers generated by light, and the number that reach the junction. Another loss is the temperature dependence of voltage. The voltage decreases as the temperature increases. Addition losses are from light reflection from top surface, shading of the solar cell by top contacts, and the incomplete absorption of light.

Minimize Recombination Lossesby Adapting the Device

Basic Physics of Solar Photovoltaic CellsHow can we eliminate or at least reduce some of these loss mechanisms? First, manufacturers already texture the top surface of the solar cell and create rough areas. Why would we want a rough surface? If the surface is textured, it forces the light to take contorted pathways through the absorption material (your semiconductor). It increases the optical path length (and thus increases the chances that the photon will be absorbed) while keeping the electrical path length the same (and thus not adding any recombination). You can see the roughening in the details of the two cells in the figure.

Second, you can use a reflective back surface so the light needs to go through the absorber twice once on the way in and once on the way out.

If we want to make the best solar cell possible using conventional technologies and materials we need to use some tricks. Two of these tricks come from The University of New South Wales (NSW) and Stanford University, respectively. Both NSW and Stanford use textured front surfaces and a back surface reflector. They also realized that losses occur due to the grid, so both universities reduced the grid on the front of the device.

NSW applied top contacts, put a really doped n+ spot contact directly under the metal and had a thin layer of n material underneath it then p and finally a really high doped p-layer. So what they are doing is adding more doping on a normal p-n junction, and added more fingers so it can collect more light. The Stanford group went one step further. They got rid of the grid on the top entirely, enlarged the n type, maximized doping and put the contacts on the back. This is basically like having a p-i-n junction.

The Stanford point contact high-efficiency silicon solar cells designed for optimum operation under concentrated sunlightUniv. of New South Wales passivated emitter solar cell (PESC) operates under ordinary sunlight

Other high performance cells use more expensive materials like gallium arsenide solar cells. They are used for satellites and concentration systems. For some application, such as going into space, you want the most efficient solar cell as possible. It may be expensive but using these tricks would help create a higher efficiency.

Tandem CellsTandem cell- several cells, Top cell has large bandgap Middle cell mid eV bandgapBottom cell small bandgap.

Indium Tin Oxidep-a-Si:H

Blue Cell i-a-Si:Hn-a-Si:HpGreen Celli-a-SiGe:H (~15%)

npRed Celli-a-SiGe:H (~50%)

nTextured Zinc Oxide

SilverStainless Steel SubstrateSilver Grid

Schematic diagram of state-of-the-art a-Si:H based substrate n-i-p triple junction cell structure.

Basic Physics of Solar Photovoltaic CellsAnother method to increase the efficiencies of solar cells is to use the tandem concept pictured here. This is where several cells are stacked on top of each other. In the example here of a triple junction amorphous silicon solar cell. In order to fabricate a tandem cell you do the following. First, you have stainless steel substrate covered with silver as a back reflector on the bottom. Here when light comes into the cell it is reflected and goes back through for another chance at absorption. Also the back is textured so that the light will scatter and must take a longer path through the material. Next is the red cell (which absorbs red light). It is an p-i-n structure. On top of that is green and then blue cells, which each absorb their respective color of light. Both of those also have the p-i-n structure. Finally, the top surface is made up of a textured transparent conductive layer. The green and red cells are actually silicon germanium. Germanium has a smaller band gap than silicon, so that when you combine those, you get a small gap, then bigger, and bigger, increasing on the way up. The blue cell is on top of the red because blue has a larger band gap it will absorb the highest energy photons and let the photons with less energy (green and red) pass through it to be absorbed by the lower cells. If the tandem cell was turned upside down, the green and blue energy of the photon would get lost because red has the smallest band gap and would absorb them but lose a large percentage of the energy to waste heat.

Solar Photovoltaics is the Future

Basic Physics of Solar Photovoltaic CellsSolar photovoltaics is the future and the future is here now!!!

AcknowledgementsThis is the first in a series of presentations created for the solar energy community to assist in the dissemination of information about solar photovoltaics. This work was supported from a grant from the Pennsylvania State System of Higher Education.The author would like to acknowledge assistance in creation of this presentation from Heather Zielonka, Scott Horengic and Jennifer Rockage.

Basic Physics of Solar Photovoltaic Cells

This presentation will cover the fundamental semiconductor physics that makes solar photovoltaics possible and review the materials needed to make solar cells.

Notes for InstructorFeel free to use all or parts of this presentation but please follow the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License which can be found here:http://creativecommons.org/licenses/by-nc-sa/2.5/

For a basic understanding of solar photovoltaic physics see Solar Electricity, 2nd Edition by Tomas Markvart (Editor), 2000.

For a more advanced discussion of solar photovoltaic physics see The Physics of Solar Cells by Jenny Nelson, Imperial College Press, 2003.

For a detailed discussion of the future of solar photovoltaic physics see Third Generation Photovoltaics : Advanced Solar Energy Conversion by Martin Green, Springer, 2003.Photovoltaic (PV) systems convert light energy directly into electricity. They are commonly known as solar cells. The simplest systems power the small calculators we use every day. More complicated systems will provide a large portion of the electricity in the near future. PV represents one of the most promising means of maintaining our energy intensive (with appropriate and technically viable efficiency gains) standard of living while not contributing to global warming and pollution. Solar Photovoltaics is the future of energy!Here is the timeline of photovoltaic technology. Solar cell technology is actually pretty old. The photovoltaic effect was discovered in the late 1830s. An explanation of the photoelectric effect won Einstein the Nobel prize. There was a big break through in 1954 because the first crystalline silicon solar cell was developed. Four years later it was used on a space satellite. The good news is that it worked, the bad news is that the solar cells kept working past when NASA expected so then the satellite kept sending data to earth when it was no longer needed and took up E and M bandwidth.

In the mid 1970s people were getting worried because of the oil crisis and OPEC. Because people were worried, more thought and money was given to find new and cheaper ways of making solar cells. One development that came out of this research push was the first amorphous silicon cell developed by Chris Wronski and Dave Carlson. The discovery of the amorphous silicon solar cells caused a lot of excitement because it was a fundamentally cheaper material than standard crystalline silicon. This material is also used for the thin film transistors that drive modern flat panel displays. They have been mass produced real cheap, and over the years the price for a flat panel has gone down significantly. The same is happening for solar cell prices.

In the 1980s the solar photovoltaic community made steady progress towards higher efficiency and many new types of solar cells were introduced so that by the 1990s there were large scale production of solar cells more than 10% efficient with the following materials: Ga-As and other III-Vs, CuInSe2 and CdTe, TiO2 Dye-sensitized, and finally the largest producers Crystalline, Polycrystalline, and Amorphous Silicon solar cells. Today prices continue to drop and new 3rd generation solar cells are researched.

This graph show the record efficiency of different types of solar cells as a function of time. What is important to see here is that all the different types of solar cells follow the same trend as more research is undertaken the efficiencies go up which will drive down the cost of solar electricity relative to fossil fuel fired electricity.Solar cells are made from semiconductors - the most important being silicon. Semiconductors have special electronic properties which allow them to be insulating or conducting depending on their composition. In photovoltaic materials you are dealing with the semiconductors (yellow). Most of the doping comes from boron and phosphorous for silcion solar cellsate. There are different types of solar cells such as cadmium telluride (CdTe) made from Cadmium and Tellurium or copper indium diselenide or gallium aresenide. Source:http://www.mse.cornell.edu/courses/engri111//semicon.htmMost solar cells are made from silicon. Silicon has the diamond structure -a perfect tetrahedral coordinated crystal. Each silicon atom in the lattice is in an identical position to every other atom and each has four nearest neighbors. The 4 electrons in Si form bonds to the nearest neighbors (as seen by the red bonds in the figure). Silicon is a semiconductor and is the element largely responsible for the integrated circuit, which makes modern computers possible.Lets take a look at silicon technology first because it makes up the majority of the current photovoltaic market. Here is the graphical difference between crystalline silicon and amorphous silicon. The crystalline silicon lattice is perfect, like the swan. The amorphous is messed up with defects (strained and broken bonds), like the ugly duckling. These physical defects lead to electronic defects and lower solar cell performances. There are some advantages and disadvantages of crystalline and polycrystalline silicon. Advantages include high efficiency, very established technology and it is stable. The technology for the crystalline silicon is the same technology that drives the semiconductor industry. The disadvantages for crystalline technology include expensive production, low absorption (so you need a lot of material), and a large amount of highly purified feedstock, which of course is expensive. Polycrystalline silicon is cheaper but the electric properties are not as good so the efficiency is lower. The other kind of silicon solar cell material is amorphous silicon. It has a very high absorption coefficient because it acts like a direct band gap semiconductor instead of an indirect gap like crystalline silicon. It can also come like a sticker, such as the solar cell in the image, where all you do is peal off and stick it to the building and it lasts virtually forever (30 year warranty) . It also does not need any building materials to put it up and it is cheaper than glass, metal or plastic. The disadvantages include efficiency is only 7-10% and it degrades when light hits it this is known as the Staebler-Wronski effect or SWE. All amorphous silicon solar cells sold now have warrantees for the degraded steady state where SWE has stopped. For example you might buy a solar cell with a warrantee of 10% efficient when you first install it your will be happy to find that it will produce electricity at 11% efficient. After a few months it will degrade down to 10% and then stay there the rest of its lifetime.

Band theory models the behavior of electrons in solids, by postulating the existence of energy bands, continuous ranges of energy which electrons may occupy, and gaps, which they may not. It successfully explains many physical properties of solids, such as electric resistivity and optical absorption. For the conductor, the valence band is above conduction so there are free electrons to conduct electricity. In the insulator, the two bands are far apart from each other and its very hard to get charge carried from the bottom (valence) to top band (conduction band). For the semiconductor there is a gap between the valence and conduction bands this is known as the bandgap and is denoted Eg (Energy of the gap). The Fermi Energy (Ef) is also shown for the insulator and semiconductor. The ground state of a non-interacting fermion system (like one made up of electrons here) is constructed by starting with an empty system and adding particles one at a time, consecutively filling up the lowest-energy unoccupied quantum states. The Fermi-energy is the level (if there is one) where half of the states are occupied with electrons. Energy of an electron in a semiconductor must fall between two defined bands. The valence band are energies of valence orbitals which form bonds between atoms. The conduction band, next higher up, is separated from the valence band by and energy gap, or bandgap. The width of the energy gap, Eg, is defined by the difference in energy of the conduction and valence bands: Ec- Ev. This is the energy band structure for a typical semiconductor in detail like silicon. This is your valence band and conduction band. There is an energy gap noted Eg between the two. The valence band is filled with electrons the energy of electrons is going up. A hole is an absence of an electron. The conduction band is empty at absolute zero but at room temperature some valence electrons are excited into it and free to travel. For the remainder of the presentation, if something is full (almost full) the color will be dark and if something is empty (or almost empty) the color will be light. For example, dark green means full valence band and light green means empty conduction band.There are three types of semiconductors. There is the intrinsic, which we just discussed (green). There are no extra electrons in conduction band and the valence band is full at absolute zero. Then there is the n-type (orange). It has extra electrons, in the conduction band, which are denoted by the blue circles. The extra electrons pull the Fermi level (black line) up towards the conduction band. For clarity the Fermi energy was not shown for the intrinsic semiconductor although in this case it would be in the middle of the gap. The p-type (purple) is when you have extra holes in valence band and the Fermi level is closer to the valence band. You can create n and p type semiconductors by placing atoms of a different element inside the lattice. For example, you can place elements in row three and row five (B and P) and remove electrons or give extra ones respectively for silicon.The pure intrinsic type contains the right number of electrons to fill the valence band, therefore, the conduction band is empty. Because of that you cannot have any movement of electrons in the valence band. The pure intrinsic type basically acts like an insulator. You have to dope it if you want useful electronic properties.For the n-type you have the group 5 impurities and you are adding an electron. It is also doping with a donor because now there are extra electrons in the conduction band which have room to move. This acts like a conductor, for example, a copper wire. In n-type the current is carried by negatively charged electrons - How? By using group 5 impurity atoms added to silicon melt from which is crystal is grown so 4/5 of outer electrons used to fill valence band and 1/5 left is then put into conduction band. These impurity atoms are called donors.In the p-type semiconductor electrons are removed by adding group 3 elements to the silicon. This creates holes or positive charges. There is now room for charge carriers to move around so now you have a hole conductor. Holes are the moving absence of electrons. What carries the current?In the n-type material its the electrons. In the p-type material its the holes. This is true for all semiconductors. You can create devices by putting two similar semiconductors together. The device is called a junction and there are four different types. There is the p-n junction, p-i-n junction, Schottcky barrier, and the heterojunction. Each has a built in potential (a built in voltage) associated with them.Here are the first two types of junctions. The p-n and p-i-n junctions. You have the conduction band and valence band. No electrons can exist in band gap, however, semiconductors are not perfect. In thermodynamic equilibrium at room temperature, you actually populate some of the states in this band gap. The solid line is the Fermi level. You can think of it as electrons as a liquid being poured into a bucket (the gap) and being filled up to the Fermi level. In semiconductors, when you put two dissimilar semiconductors together, the Fermi level must stay flat. This is what created the bending of these bands (both the conduction and valence band).

So in the p-n junction you have the p layer and an n layer. In the p-type the Fermi level is very low in the gap an pulled to the valence band. In the n-type semiconductor, the Fermi level is very high and pulled toward the conduction band. There is an enormous bending of the conduction and valence bands due to trying to keep Fermi level flat when they are combined. The bending creates the voltage and the electric field observed when putting two dissimilar (p and n) semiconductors together. The energy between the conduction band in the p-layer and the n-layer is the built in potential.

What happens when you put an intrinsic layer of semiconductor between the p and n types? Well now, this bending of the bands is spaced out. The Fermi level still flat.Here are the next two types of junctions that can be used for solar cells: Schottcky Barriers and Heterojunctions. For the Schottcky barrier, there is a metal, filled with electrons, placed right beside the semiconductor. This creates the bend in the band. This type of junction is a way to test semiconductors and is generally not technically practical for solar cells in production.

The heterojunction has two different semiconductors put together that have different energies. When these two semiconductors are put together, they create a bend in the band as well. All the junctions already shown contain strong electric fields. What is actually happening is that if you have n and p semiconductors, that come into contact, electrons near the interface in the n-type material will be transferred over to the p-type material. What you do is deplete a region of electrons in the n-type right beside the p-type. The same works for holes. The holes move from p-type to n-type and it ends up that there is a region where the negative electrons become positive and positive holes become negative. The changing of charges gives the barrier. Electric field pulls electrons and holes in opposite directions.

The barrier can be altered by a forward or reverse bias. The motion of the electrons and holes in the far left picture is equal and opposite. If you put on a forward bias, by attaching a battery for example, it forces the electrons to move in the opposite direction which weakens electric field. You can strengthen it by reverse bias. Equilibrium means there is no net current. A reduced barrier height is called forward bias (positive voltage applied to p-side) and results in increases in current through diode. Increased barrier height is called reverse bias.

The figure is a typical current (I) and voltage (V) plot for a solar cell known as an IV curve. Applying a voltage from negative to positive and record the current. For the dark current, it is not quite zero and it starts to go up and pushes electrons over creating a massive current. When illuminated, you instantly end up with a current. The power is calculated as the current multiplied by the voltage (P=IV).We will talk about extracting power from a semiconductor p-n junction in a moment.In order to calculate current in a solar cell use the following formula: I = Il-Io [ exp(qV/kT)-1]. Where Il=light generated current. Io= dark generated current, q = electric charge, V = voltage, and k = Boltzmans constant = 1.3807 10-23 J/K. Just by looking at this equation you can see one of the problems that occur in solar cells. As temperature increases, the current in the solar cell decreases. You dont get as much energy output from a solar cell if the temperature is higher. This occurs in all types but it hurts the diffusion type of solar cell the most. In amorphous silicon drift type of solar cells, however, you decrease the defects by increasing the temperature and that effect somewhat balances the loss in performance due to temperature increases. The open circuit is defined when I=0. Short circuit conditions occur is when V=0. Rearranging the output current equation and solving for voltage gives the open circuit equation. Voc = (kT/q) ln( Il/Io +1). In addition, no power is generated under short or open circuit but, Pmax = VmIm=FFVocIsc.

This is the same graph as before only flipped around to highlight the 4th quadrant of the I-V plot (Current density vs. Voltage). As V=0, you have maximum current, which is called the short circuit current. If you increase voltage, the current decreases. If you continue to increase voltage, the current goes to zero, and this is called an open circuit. The dashed line signifies power density and is shown on the right axis. The maximum current density and voltage is at the maximum power point, because after that, the power drops off, as well as the current. Maximum power for current= Jmp. Maximum power for voltage= Vmp. Fill factor is defined as: FF= (Imax)( Vmax) / short circuit open circuit. A 100% fill factor would have a square angle in the figure.In order for a solar cell to work, it has to absorb light. Light is basically a stream of photons, which are quantized packets of light energy. There is a huge amount of photons in light. For example, on a clear day 4.4x1017 photons hit 1 m2 of Earths surface every second. Each photon has energy associated with it that depends on the wavelength, or the color of the light. This is shown in the Energy equation for a photon: Eph()=hc/=hf, where h = planks constant = 6.625 x 10-34 J-s, = wavelength, c = speed of light =3 x 108 m/s, and f = frequency. However, only photons with energy in excess of bandgap energy can be converted into electricity by solar cells. This is because, when light shines on a semiconductor the only photons that are absorbed have at least the energy of the bandgap. Photons that are too small will not be absorbed and will go straight through the band gap. So, in general, photons need to have enough energy to excite an electron from the valence band up to the conduction band.This is the Solar Spectrum. The graph shows the flux of photons that come down to the Earth vs. the wavelength of the photons. The Orange line represents the solar spectrum outside the atmosphere. The spectrum reaches a peak in the visible as expected from black body radiation at 5800K (the surface temperature of the sun). The missing regions of flux (black line) at the earths surface are due to absorption from water vapor and carbon dioxide. As you can clearly see, the solar energy is not just a single energy (wavelength or frequency of light). If it were, we would have solar cells based on current technology with efficiencies over 95%. So why do we lose energy at different wavelengths? Part of it is because we are not absorbing the whole spectrum of a single photon. If we put in photons with larger energies than the bandgap we only capture the energy equal to Eg we lose the rest to waste heat. Photons with energies lower than the bandgap do not contribute to carriers (electrons and holes) in a solar cell.

Source: http://ess.geology.ufl.edu/ess/Notes/040-Sun/spectrum.GIF

So what happens to a photon that is absorbed in a semiconductor?The photon enters and is absorbed and thereby excites an electron from the valence band up to the conduction band. The extra energy of the photon is then lost and turned into heat. Because of that, only 2/3 of the total flux can be turned into electricity. Even after that, more energy flux can be lost from thermal process.

When a photon is absorbed it generates current. Generation current is just light induced electrons across the band gap as electron current. The equation for electron current is Ip=qNA. Where q is the charge, N is the number of photons in the highlighted area of the spectrum and A is the surface area of semiconductor thats exposed to light. Because the of this the maximum Voltage it will ever get to is just the band gap energy in electron-volts, or eVs. Electrostatic energy of charges occurs after separation only if its energy is less than the energy of the electron-hole pair in semiconductor. Therefore Vmax=Eg/q, where q is the charge on an electron if Eg is measured in electron volts eV, you are left with voltage. Vmax= bandgap of semiconductor is in EVs, therefore this equation shows that wide bandgap semiconductors produce higher voltage. What we have seen so far is the direct band gap approach. In reality, a lot of semiconductors, for example silicon, are not direct band gaps. Semiconductors have to have the right energy and momentum to be able to capture photons. Indirect gap semiconductors also demand a quantum of lattice vibrations to absorb light. The photons in the crystal have to be able to create the pair.

In addition to structural and electron space there is also momentum space. Momentum has to be conserved in any type of process. In turns out that for a perfect crystal, there are resonance vibrations and when a photon is excited it not only has to conserve energy but also conserve momentum. Electric current generated in semiconductor is extracted by contacts to the front and rear of cell. Widely spaced thin strips (fingers) are created so that light is allowed through - These fingers supply current to the larger bus bar. In this blue area in the figure you have the two doped layers that make up the p-n junction. On top of that there are metal contacts which are connected to a bus bar. There is metal on the back of the solar cell, a semiconductor (p-n junction) in the middle, and another method of collecting electrons on the front. As shown in the diagram, you have the bus bar that is connected to the small fingers. On top of the solar cell there is an antireflection coating, or ARC. Antireflection coating (ARC) is used to cover the cell to minimize light reflection from top surface. ARCs are made with thin layer of dielectric material. When light comes in, you want to collect as much of it as possible and if you can see light reflecting off the solar cell, the ARC is not good. The ideal solar cell does not reflect much light and has as small shadowing as possible (due to the top metal contacts). All of this helps with trapping light in the solar cell.

There are three different types of solar cells. The most established is diffusion. The other two are drift and excitonic and represent 2nd and 3rd generation types of solar cells, respectively.In diffusion-based silicon solar cell there is the p-n junction. Light is absorbed and excites electrons up to the conduction band. You can think of the electrons in the conduction band as little balls rolling down a hill. So once the electron is excited up to the conduction band, it rolls down the hill. The holes are like air bubbles, they float up inside the valence band. The movement of electrons and holes creates a current. Take note that the direction the current is moving is the opposite of the direction the electrons are moving.

This diagram is an example of diffusion. For an electron to get to the hill to fall down, it has to diffuse. You have to have a very good crystal for this to work. If not, the defects in the crystal will cause the electrons to combine with the holes and mutually annihilate. This is what is happening when we create a diffusion based p-n junction. On the left is the p-type material (purple) with a lot of holes and on the right the n-type material (orange) with a lot of electrons. If you are putting these together, the Fermi level has to remain flat, which creates the bend in the bands as seen in the band diagram on the lower right.The second type of solar cell is called the drift solar cell. A good example is amorphous silicon solar cells. If you put a p and n junction of amorphous silicon right next to each other, you create the electron-hole pairs with light absorption but because the material is so defective they will instantly be annihilated. So to make an amorphous silicon solar cell work you put in an intrinsic layer that had less defects, in between the p and n layers. They also made the p and n layers very thin, approximately 10-20nm thick. The intrinsic layer is much thicker at thousands of nm thick.

The thick layer absorbs the vast majority of the photons and instantly creates electron-hole pairs. Because of the band bending, the electron will automatically roll down the hill and the hole with float up. Notice the current moves right because the electron moves left. In addition, the p and n layers are thin enough that you dont have to worry about recombination.

One of the newest types of solar cell, which is causing a lot of excitement is the dye-sensitized excitonic solar cell. As you can see there is a hole conductor and an electron conductor and a dye in between them. The dye has a color associated with it and absorbs the photons. The absorption of the photon creates and electron-hole pair. If you pick the right hole and electron conductors, with appropriate energy levels the electron will jump into the electron conductor and the hole will jump into the hole conductor. Now we have a solar cell. By moving this excitation from dye to the conductors, it creates electrical current. Dye molecule type is relatively new and can be done with cheap materials. The first commercial production has already begun in Europe over the last few years. There are physical constraints that create power losses in real solar cells. This diagram shows these constraints. For 100mW of power coming from the sun are coming in, 21 mW is the below the band gap, so its not enough energy to excite an electron. 31mW of it is excess of the band gap so its lost through heat. So already, more than half of your power is lost just because of the limitations of a single bandgap.

The Voc available in silicon is 1.1V and because of all the above losses, all you have left are 44 mAmps. Recombination losses, which are electrons smashing into holes, drops the 1.1V to approximately 0.6V. As you can see, recombination cuts the maximum voltage almost in half. Now the current, already being cut down earlier, will be cut down once again due to collection efficiency and incomplete absorption. Collection efficiency is when some of the photons cannot excite electron-hole pairs. Incomplete absorption means some of the photons pass through the cell. This demonstrates one of the technical challenges of designing solar cells: If you make your cell thicker, its easier to absorb photons but harder to collect electrons because they have to diffuse over longer distances. However, if you make the solar cell thinner, it would be easier to collect electrons but you will lose a lot of photons that do not absorb. In addition, there is top surface reflection on a cell and no absorption where the metal is located due to shadowing. The current of 44milliAmps is now decreased to 21 milliAmps. The fill factor is reduced from 1 to .7. The final power is about 14 mW, which means this solar cell is 14% efficient. Now we will go through each one of these losses in detail. Recombination is when an electron-hole pair is created and then is smashed back into each other. Recombination usually happens when there is an impurity. For example, if you have a perfect silicon lattice except for the addition of a few sodium atoms. The sodium atoms create defects in the gap which means you end up with nothing instead of an electron-hole pair. There can also be defects in the crystal structure. No matter how perfect the bulk material is, on the surface of a semiconductor there are silicon atoms that are not fully bonded which then cause surface states. The more surface states you have on the silicon solar cell, such as is created at grain boundaries in a polycrystalline silicon solar cell, the worse off you are. This explains why polycrystalline silicon solar cells do not have as high efficiencies as seen for single crystal silicon solar cells.

Series resistance losses are caused by transmission of electric current produced by the solar cell. This equation (I = Il-I0 [exp(qV+IRs / mkT) 1] ) is for the IV characteristic of a typical device but now includes the recombination losses. The m term is a nonideality factor.

Other losses include collection efficiency, which we talked about earlier. It is the ratio between the number of carriers generated by light, and the number that reach the junction. Another loss is the temperature dependence of voltage. The voltage decreases as the temperature increases. Addition losses are from light reflection from top surface, shading of the solar cell by top contacts, and the incomplete absorption of light.

How can we eliminate or at least reduce some of these loss mechanisms? First, manufacturers already texture the top surface of the solar cell and create rough areas. Why would we want a rough surface? If the surface is textured, it forces the light to take contorted pathways through the absorption material (your semiconductor). It increases the optical path length (and thus increases the chances that the photon will be absorbed) while keeping the electrical path length the same (and thus not adding any recombination). You can see the roughening in the details of the two cells in the figure.

Second, you can use a reflective back surface so the light needs to go through the absorber twice once on the way in and once on the way out.

If we want to make the best solar cell possible using conventional technologies and materials we need to use some tricks. Two of these tricks come from The University of New South Wales (NSW) and Stanford University, respectively. Both NSW and Stanford use textured front surfaces and a back surface reflector. They also realized that losses occur due to the grid, so both universities reduced the grid on the front of the device.

NSW applied top contacts, put a really doped n+ spot contact directly under the metal and had a thin layer of n material underneath it then p and finally a really high doped p-layer. So what they are doing is adding more doping on a normal p-n junction, and added more fingers so it can collect more light. The Stanford group went one step further. They got rid of the grid on the top entirely, enlarged the n type, maximized doping and put the contacts on the back. This is basically like having a p-i-n junction.

The Stanford point contact high-efficiency silicon solar cells designed for optimum operation under concentrated sunlightUniv. of New South Wales passivated emitter solar cell (PESC) operates under ordinary sunlight

Other high performance cells use more expensive materials like gallium arsenide solar cells. They are used for satellites and concentration systems. For some application, such as going into space, you want the most efficient solar cell as possible. It may be expensive but using these tricks would help create a higher efficiency.

Another method to increase the efficiencies of solar cells is to use the tandem concept pictured here. This is where several cells are stacked on top of each other. In the example here of a triple junction amorphous silicon solar cell. In order to fabricate a tandem cell you do the following. First, you have stainless steel substrate covered with silver as a back reflector on the bottom. Here when light comes into the cell it is reflected and goes back through for another chance at absorption. Also the back is textured so that the light will scatter and must take a longer path through the material. Next is the red cell (which absorbs red light). It is an p-i-n structure. On top of that is green and then blue cells, which each absorb their respective color of light. Both of those also have the p-i-n structure. Finally, the top surface is made up of a textured transparent conductive layer. The green and red cells are actually silicon germanium. Germanium has a smaller band gap than silicon, so that when you combine those, you get a small gap, then bigger, and bigger, increasing on the way up. The blue cell is on top of the red because blue has a larger band gap it will absorb the highest energy photons and let the photons with less energy (green and red) pass through it to be absorbed by the lower cells. If the tandem cell was turned upside down, the green and blue energy of the photon would get lost because red has the smallest band gap and would absorb them but lose a large percentage of the energy to waste heat.

Solar photovoltaics is the future and the future is here now!!!