Heterojunction Device

Heterojunction Device

An example of this type of device structure is a CIS cell,where the junction is formed by contacting two different semiconductors Cds and CuInse.This structure is often chosen for producing cells made of thin-film materials that absorb light much better than silicon.The top and bottom layers in heterojunction device have different roles.The top layer,or "window" layer,is a material with a high bandgap selected for its transparency to light.The window allows almost all incident light to reach the bottom layer,wich is a material with low bandgap that readily absorbs light.This light then generates electrons and holes very near the junction, which helps to effectively separate the electrons and holes before they can recombine .

Heterojunction devices have an inherent advantage over homojunction devices,which require materials that can be doped both p- and n- type.Many PV materials can be doped either p-type or n-type,but not both.Again, because heterojunction don't have this constrain,many promising pv materials can be investigated to produce optimal cells.

Also, a high-bandgap window layer reduces the cell's series resistance,the window material can be made highly conductive , and the thickness can ba increased without reducing the transmittance of light.As a result, light-generated electrons can easily flow laterally in the window layer to reach an electrical contact.

From:http://www1.eere.energy.gov/solar/pv_batteries.html