ISAT 436 Micro-/Nanofabrication and Applications Photovoltaic Cells David J. Lawrence Spring 2004

Properties of Light (1) H f = frequency (Hz)  o = wavelength in vacuum or air [usually measured in  m, nm, or Angstroms (Å)]  c = speed of light in vacuum = 3  10 8 m/s  c = f o H n = refractive index of a material (“medium”) H v = c / n = speed of light in material  = o / n = wavelength in material  v = f

Properties of Light (2) H E = h f = energy of a photon  h = Planck’s constant =  J-s =  eV- s  E = (h c) / o  h c = 1240 eV-nm = 1.24 eV-  m  1 eV =  J   = h / 2  =  J-s

Properties of Light (3) H A useful equation for the energy of a photon: H Rearranged, this gives

Properties of Light (4) For the visible portion of the electromagnetic spectrum, the wavelength in vacuum (or in air) ranges from: Color o (nm) f (Hz)E photon (eV) red ~4.5 x ~1.9 orange ~4.9 x ~2.0 yellow ~5.2 x ~2.15 green ~5.7 x ~2.35 blue ~6.3 x ~2.6 violet ~7.1 x ~ nm 700 nm Å to Å

Properties of Light (5)  Light with wavelength o  < 400 nm is called ultraviolet (UV).  Light with wavelength o  > 700 nm is called infrared (IR).  We cannot see light of these wavelengths, however, we can sense it in other ways, e.g., through its heating effects (IR) and its tendency to cause sunburn (UV).

Optical Generation of Free Electrons and Holes H Recall that light can generate free electrons and holes in a semiconductor. H See Photovoltaic Fundamentals, p.12 and p.16. H The energy of the photons (hf) must equal or exceed the energy gap of the semiconductor (E g ). H If hf > E g, a photon can be absorbed, creating a free electron and a free hole.

Optical Generation of Free Electrons and Holes - - Bond Model H See Photovoltaic Fundamentals, p.12 and p.16. Si free electron free hole Photons

Optical Generation of Free Electrons and Holes - - Band Model  If a photon has an energy larger than the energy gap, the photon will be absorbed by the semiconductor, exciting an electron from the valence band into the conduction band, where it is free to move.  A free hole is left behind in the valence band.  This absorption process underlies the operation of photoconductive light detectors, photodiodes, photovoltaic (solar) cells, and solid state camera “chips”. Electron Energy “Conduction Band” (Nearly) Empty “Valence Band” (Nearly) Filled with Electrons “Forbidden” Energy Gap

Photoconductive Light Detectors  Photons having energy greater than the energy gap of the semiconductor are absorbed, creating free electrons and free holes, and thus the resistivity, , of the semiconductor decreases. semi- conductor hf I V out

Photoconductive Light Detectors  Since R semiconductor =  / A, the resistance of the semiconductor sample also decreases. semi- conductor hf I V out H Recall that:

Photovoltaic Cells H Photovoltaic cells, also called solar cells, convert sunlight directly into electricity. H A p-n junction is the key element of all efficient photovoltaic cells. H See Photovoltaic Fundamentals, pages 8 and n-side p-side E junction

Photovoltaic Cells -- Bond Model H Recall that there is an electric field, E, in the depletion region of a p-n junction. H This electric field causes optically generated carriers to move, enabling a solar cell to generate an electric current n-side p-side E depletion region neutral here

Photovoltaic Cells -- Bond Model H If light generates free electrons and holes in the depletion region, the electric field makes these carriers move. H Which way do they go? H What direction does the current flow? n-side p-side E depletion region neutral here

Photovoltaic Cells -- Band Model H Recall that a p-n junction can also be described by an energy band diagram. conduction band n-side p-side valence band E          EgEg depletion region

P-N Junction Diode  Electrons behave like marbles  they tend to go downhill.  Holes behave like helium-filled balloons  they tend to float uphill. conduction band n-side p-side valence band E          EgEg depletion region

P-N Junction Diode H The bent energy bands are a barrier to electron motion. H The bent energy bands are a barrier to hole motion. conduction band n-side p-side valence band E          EgEg depletion region

Photovoltaic Cells -- Band Model H Photons with energy hf > E g will be absorbed by the semiconductor. H If a photon is absorbed in the depletion region, a free electron and a free hole are generated there. conduction band n-side p-side valence band E          EgEg depletion region  

Photovoltaic Cells -- Band Model H The optically generated free electron and hole will move in response to the electric field. H Which way do they go? H What direction does the current flow? conduction band n-side p-side valence band E          EgEg depletion region  

Photovoltaic Cells -- Band Model H In order for current to flow, we must form a complete circuit. H Electrons flow counterclockwise in this circuit. H Current flows clockwise in this circuit. n-side p-side E            metal contact metal contact I  V + “load”, e.g., motor

Photovoltaic Cells -- Band Model H Light energy is converted to electrical energy. n-side p-side E            metal contact metal contact I  V + “load”, e.g., motor

Photovoltaic Cells H Notice that the “photocurrent” flows opposite the diode symbol arrow. I  V + “load”, e.g., motor

Photovoltaic Cells -- Band Model H Photons absorbed outside the depletion region can contribute to the “photocurrent”. H The electrons and holes that are generated must diffuse to the depletion region before they recombine. conduction band n-side p-side valence band E          EgEg depletion region      

Photovoltaic Cells H Photovoltaic (solar) cells are designed for energy conversion, so they usually have a large (> 5 cm 2 ) surface area. H Smaller light detecting p-n junctions, called photodiodes, have numerous other applications, e.g., G light measurement G scientific instruments G light detection in fiber optic communications systems G light detection in reading “heads” in optical disc systems (e.g., CD, CD-ROM, DVD) G light sensitive elements in solid state camera “chips”.

Photovoltaic Cells H Next, let’s consider some practical solar cell structures. H Photovoltaic Fundamentals is a good reference. H An essential feature that all efficient solar cells have is a p-n junction. H All solar cells also have metal electrical contacts to conduct the photogenerated current to the outside world. H Solar cells can be made from G single crystal semiconductors G polycrystalline (and semicrystalline) semiconductors G amorphous semiconductors.

Silicon Photovoltaic Cell H Single crystal silicon solar cell. H Key features to observe: G p-n junction G front contact G back contact G antireflection coating G cross section not to scale   (Greek “nu”)  f  frequency  h  hf  photon energy Larger diagram on next slide!

Silicon Photovoltaic Cell

H Starting material G Single crystal silicon wafer (2” to 6” diameter)  p-type  boron-doped   1  -cm  p  ? G Wafer is cleaned to remove contaminants. G Surface may be “textured” to reduce the reflection of incident sunlight (see Photovoltaic Fundamentals, page 22). This is done with a chemical etching solution. G We will begin by considering the fabrication of a cell without texturing.

Silicon Photovoltaic Cell  The top ~ 0.3  m of the wafer must be converted from p-type to n-type.  This is usually done by introducing phosphorus from the wafer surface so that the phosphorus concentration greatly exceeds the background boron concentration from the surface down to a depth of about 0.3  m. H The concentration of added phosphorus is typically to /cm 3. H The process by which phosphorus is introduced is called diffusion. H Diffusion is described in detail in Chapter 4 of Jaeger.

Silicon Photovoltaic Cell H Essentials of the diffusion process: G The wafer is heated to 900 to 1200 ° C in a furnace with gas (typically N 2 or a mixture of N 2 and O 2 ) flowing over the wafer (Jaeger, p. 96). G Phosphorus is delivered to the wafer surface by adding a phosphorus-containing compound (e.g., POCl 3 ) to the gas or by maintaining a solid source containing P 2 O 5 near or in contact with the wafer (Jaeger, p ). 1  -cm p-type Si  p  1.5  /cm 3 PPPPPPPPPPPPPPPPPP

Silicon Photovoltaic Cell H Diffusion process (continued): G The depth to which phosphorus diffuses is controlled by adjusting the temperature ( ° C) and duration (minutes to hours) of the diffusion process.  Typical diffusion depths are 0.2 to 1.0  m. G Since the phosphorus concentration in the diffused layer (10 19 to /cm 3 ) greatly exceeds the background boron concentration, the diffused layer is converted to n-type. 1  -cm p-type Si  p  1.5  /cm 3 phos.-doped n-type Si,  n  /cm 3     -cm

Silicon Photovoltaic Cell H We now have the required p-n junction. H We need a metal electrical contact to the p-side >> the back contact. H We need a metal electrical contact to the n-side. >> the front contact. 1  -cm p-type Si  p  1.5  /cm 3 phos.-doped n-type Si,  n  /cm 3     -cm

Silicon Photovoltaic Cell H PV cell is a large area p-n junction.   of most semiconductors (e.g., silicon) is substantially greater than for a metal (  metal ~ to  -cm). H A small wire contact to each side is insufficient. H Metal must extend over much of both surfaces in order to collect the photocurrent efficiently. H A metal grid front contact on the n-side allows light to enter the semiconductor, where it is absorbed. 1  -cm p-type Si  p  1.5  /cm 3 phos.-doped n-type Si,  n  /cm 3     -cm

Silicon Photovoltaic Cell H In the process of diffusing phosphorus into a p-type silicon wafer to form a p-n junction, the surface may have been oxidized or otherwise “contaminated”. H Before metal contacts are deposited, any SiO 2 or surface contamination is removed by etching. H The etching process consists of immersion in a liquid solution containing hydrofluoric acid (HF). 1  -cm p-type Si  p  1.5  /cm 3 phos.-doped n-type Si,  n  /cm 3     -cm

Silicon Photovoltaic Cell H A metal back contact can be deposited over the entire p- type substrate using a process called evaporation. H See Jaeger, pp H For example, aluminum in a ceramic crucible is heated by a tungsten filament until it evaporates. H The silicon wafer is placed above the crucible and the aluminum vapor condenses on the p-type side, forming a thin film, nm thick. H In order to ensure the purity of the deposited metal, evaporation is carried out in an evacuated chamber. (If any oxygen were present in the chamber, it would immediately react with the aluminum vapor.)

Silicon Photovoltaic Cell H Evaporation Aluminum Vapor Aluminum Film Silicon Wafer

Silicon Photovoltaic Cell H The metal front contact is usually in the form of a grid pattern, as shown on the next slide and on pages 21 and 23 of Photovoltaic Fundamentals. H A grid contact on the n-side allows light to enter the semiconductor, through the spaces between narrow metal “fingers”. H The metal fingers must extend over every part of the cell’s surface in order to collect the photocurrent efficiently. H The front contact can be produced by evaporation of silver or aluminum.

Silicon Photovoltaic Cell H Metal grid pattern on top surface of a photovoltaic cell:

Silicon Photovoltaic Cell H In order to produce the grid pattern, the metal is evaporated through a “shadow mask”. H See page 23 of Photovoltaic Fundamentals.

Silicon Photovoltaic Cell H The shadow mask is in contact with the wafer. Silicon Wafer Metal Vapor Metal Film Shadow Mask

Silicon Photovoltaic Cell H The metal contacts are usually annealed in an inert atmosphere at a temperature of 400 to 500°C. H This causes the metal and silicon to interdiffuse, reducing the contact resistance (the electrical resistance of the interface between metal and semiconductor). H Processes other than evaporation are frequently used to apply metal contacts to solar cells. H The most common process is screen printing, which doesn’t require a vacuum and is far less expensive to implement. H Shadow masks and screen printing cannot produce the small features required for integrated microelectronic circuits. H A patterning process called photolithography is used. >> More about this later.

Silicon Photovoltaic Cell H An antireflection coating (silicon monoxide = SiO, SiO 2, or Si 3 N 4 ) is applied by evaporation, chemical vapor deposition, or other techniques to be described. p-type silicon n-type silicon antireflection coating

P-N Junction Diode H The electrical characteristics of a p-n junction diode are given by a “current-voltage” graph -- a graph of electric current through the diode as a function of applied voltage across the diode. I V forward bias  + reverse bias +  “ reverse breakdown ”