BY WORKING TO IMPROVE
CELL EFFICENCY, SCIENTISTS AT OXFORD UNIVERSITY ARE CREATING SIMPLER AND
CHEAPER SOLAR CELL
Over the
last four years, solar cells made from materials called perovskites have reached
efficiencies that other technologies took decade to achieve ,but until recently
no-one quite knew why.
Since perovskites
was first used in 2009 to produce 3% efficient photovoltaic (PV) cells,
scientists have rapidly developed the technology to achieve efficiencies of
over 15%,Overtaking other emerging solar
technologies which have yet to break the 14% barrier.
Scientists
at oxford university have revealed that the secret to perovskites ‘success lies
in a property known as the diffusion length, and worked out a way to make it
ten times better.
‘The
diffusion length gives us an indication of how thick the photovoltaic (PV) film
can be,’ explains Sam Stranks, who led
the discovery in Henry Snaith ’s group
at oxford university’s department of physics .’ if the diffusion length is too
low ,you can only use thin films so the cell can’t absorb much light.’
So why the diffusion length is
so important-
PV cells are made from two
types of materials, called p-type and n-type semiconductors. P-type materials
mainly contain positively –charged ‘holes’ and n-type materials mainly contain
negatively-charged electrons. They meet at a ‘p-n junction’, where the
difference in charge creates an electric field.
The cells generate
electricity when light particles (photons) collide with electrons, creating
‘excited’ electron and holes. The electric field of the p-n junction guides
excited electrons towards the n-side and holes towards the p-side. They are
picked up by metal contacts, electrodes, which enable them to flow around the
circuit to create an electric current.
‘The diffusion length tells
you the average distance the charge –carries (electron and holes) can travel
before they recombine,’ explains Sam. ‘Recombination happens when excited
electrons and holes meet, leaving behind a low-energy electron which has lost
the energy it gained from sunlight.
‘if the diffusion length is
less than the thickness of the material, most charge-carriers will recombine
before they reach they reach the electrodes so you only get low currents. You
want a diffusion length that is two to three times as long as the thickness to
collect almost all of the charges.’
The thickness of a solar
cell is always a compromise-if they’re too thin they won’t absorb much light, but
if they’re too thick the charge carries inside won’t be able to travel through.
Longer diffusion lengths allow for more efficient cells overall, as they can be
made thicker without losing as many charge carries .Scientists can get around
this by arranging cells into complex
structure called ‘mesostructures,’ but this is a time-consuming and complicated
process which has yet to be proven commercially.
Previously, researchers were
able to get mesostructured perovskite cells to 15% efficiency, using a perovskite
compound with a diffusion length of around 100nanometers(nm).But by adding
chloride ions to the mix, Henry’s group achieved diffusion length over 1000nm.These improve
cell can reach 15% efficiency without the need for complex structures, making
them cheaper and easier to produce.
‘Being able to make 15% efficient cells in simple
cells in simple, flat structures makes huge difference. We’ve made hundreds
just for research purposes, it’s such an easy process. I expect we’ll be seeing
perovskite cells in commercial use within the next few years. They’re
incredibly cheap to make, have proven high efficiencies and are also
semi-transparent. We can tune the color too, so you could install them in
aesthetically-pleasing ways in office window.’
No comments:
Post a Comment