Different Types of Solar Cell
There are many different kinds of solar cells and their features have many plusses and minuses. The efficiency that can be achieved is usually balanced by the complexity of the cell and therefore cost. The figure below shows several different classes of solar cell showing how our experience in making the cells has led to improvements in efficiency over time.
The purple line corresponds to multi-junction cells having a stack of semiconductor materials with band-gaps optimized to splitting the solar spectrum into segments and then absorbing the light preferentially into these three materials. An example stack would be GaxIn1-xP, GaAs, Ge – which would have band-gaps of 1.8, 1.4, and 0.7 eV, respectively. More information on multijunction cells can be found at this NREL website.
The blue lines correspond to silicon solar cells. These are based on large single crystals like those grown for integrated circuits (solid squares), or polycrystalline material, (open squares), or Si ribbon (solid diamonds). The higher crystal quality helps ensure higher efficiency and little recombination or resistive loss.
The green lines correspond to a set of thin film cells with the Copper-Indium-Gallium-diSelenide (CIGS) cells having the best efficiencies found to date. Cadmium telluride cells are similar in performance, but with the CIGS it is possible to tune the composition and arrive at an optimum band gap value and increase the efficiency. The CIGS and CdTe cells are usually heterojunction solar cells based on a p-n junction made with two compositions of semiconductor – one of the semiconductors is the primary absorber – the other acts as a window but provides the electrical contacting required to achieved good current and voltage values. Most of these cells are fabricated by large area CVD or plasma deposition techniques.
The red lines are areas where much advancement and much
research is on-going to try and improve the efficiencies. The open circles are
for dye-sensitized solar cells (DSC or DSSC – both abbreviations are used).
DSSC’s use dye molecules as the main light-absorbing medium. A class of
nanostructure materials are able to host the dye molecules and allow for the
photo-excited electrons to be donated to the structure and yield reasonable
efficiency. The pioneer for this work has been Michael Grätzel at EPFL in
None of these technologies is “best”. They all have pros and cons. Some are well established in the manufacturing side and some are still under development in research labs.
For further information about solar cell operation, design, and fabrication, you might be interested in signing up for the class that I teach on this topic. Visit this link to find out more about “MSE 405 - Solar Cell Design and Processing”
© Copyright, 2006, Dunbar P. Birnie, III,