Applications of Photovoltaic Technologies

2 Solar cell structure How a solar cell should look like ?  It depends on the function it should perform, it should convert light into electricity, with high efficiency It should be a P-N junction P-type N-type There should be ohmic contact at both side It should absorb all light falling on it  It should reflect less light  Most of the light should go in It should convert all absorb light into electricity

3 Solar Cell-structure A solar cell is a P-N junction device Light shining on the solar cell produces both a current and a voltage to generate electric power. Busbar Fingers Emitter Base Rear contact Antireflection coating Antireflection texturing (grid pattern)

4 Minimizing optical losses The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping. Top contact coverage of the cell surface can be minimized Anti-reflection coatings can be used on the top surface of the cell. Reflection can be reduced by surface texturing The solar cell can be made thicker to increase absorption There are a number of ways to reduce the optical losses:.

5 Optical properties of surface What are optical losses:  Reflection  Shadowing due to metal contact  Partial absorption Photons in the spectrum can generate EHP, ideally all the sun light falling on the cell should be absorbed Short circuit current (I SC ) is usually reduced due to optical losses Design criteria for small optical losses : Mminimize optical loss

6 Air, n 0 Semiconductor, n 2 ARC, n 1 The thickness of a ARC is chosen such that the reflected wave have destructive interference  this results in zero reflected energy The thickness of the ARC is chosen so that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave (destructive interference). n 2 > n 1 > n 0 Choice of ARC

7 Reflection from various combination Multilayer structure reduces the reflection losses Index of refraction is also a function of wavelength, minimum reflection is obtained for one wavelength More than one ARC can be used, but expensive Source: PV CDROM - UNSW

8 Surface texturing Any rough surface decreases the reflection by increasing the chances of the reflected rays bouncing back on the surface Surface texturing can be obtained by selective etching  a process by which material is removed by chemical reaction Selective etching is based on the concept of different material property in different direction in crystals, Etching rate are different in dir n than in dir n

9 Surface texturing Chemical etching in KOH results in pyramid formation on the Si surface  etching is faster in direction than in direction Using photolithography, inverted pyramids can be obtained, which are more effective surface

10 Light trapping Rear side reflector or rear side texturing is used to increase the optical path length in solar cell  Increased optical path is required for thin solar cell (thin solar cell have higher V oc. It saves expensive Si) Total internal reflection (TIR) condition are used to increase the optical path length Snell’s law (  1 for Si is 36 degree) For TIR

11 Lambertian Rear Reflectors Increases the path length by 4n 2, very good in light trapping, path ;length increases by about 50 Random reflector from the rear side TIR Lambertian reflector is one which reflects the lights in a random direction  this together with the front texturing increases the optical path length

12 P-N junction Current loss due to recombination Recombination areas  Surface recombination  Bulk recombination  Depletion region recombination Recombination of carriers reduces both short circuit current as well as open circuit voltage Bulk semiconductor rear surface Front surface Design criteria: The carrier must be generated within a diffusion length of the junction, so that it will be able to diffuse to the junction before recombining

13 w w h h d Emitter  finger and busbar spacing,  the metal height-to-width, aspect ratio,  the minimum metal line width and  the resistivity of the metal Top contact One example of top metal contact design Design criteria: minimize losses (resistive, shadow)

14 Resistive Losses: Series resistance, R s 1. the movement of current through the emitter and base of the solar cell 3. resistance of the top and rear metal contacts 2. the contact resistance between the metal contact and the silicon Contributing factors to R s : Bus bar Fingers N-layer p-layer Base emitter M-S contact

15 Contact resistance N Heavy doping under contact to minimize contact resistance Metal to semiconductor contact Contact resistance losses occur at the interface between the silicon solar cell and the metal contact. To keep top contact losses low, the top N+ layer must be as heavily doped as possible. A high doping creates a "dead layer“. Ohmic contact, High doping, tunneling contact

16 Sheet resistance In diffused semiconductor layers, resistivity is a strong function of depth. It is convenient to a parameter called the "sheet resistance" (Rs). Rs is called sheet resistance with unit of ohms/square or Ω/ □ (actual unit is Ohms) The L/W ratio can be thought of as the number of unit squares (of any size) Sheet resistance of a solar cell emitter is in the range of 30 to 100 Ω/ □ W L t

17 Emitter resistance: Power loss t P N d L x dx d/2 Zero current flow exactly at midpoint of fingers Maximum current density at the finger edge Resistance dR in infinitesimally thin layer of dx