Tandem Cells Definition
One technique for increasing the solar cell efficiency is by splitting the spectrum and using a solar cell which is optimized to every individual section of the spectrum.
Tandem solar cells can be either simple individual cells or be cells connected in series. Series connected cells are easier to manufacture, but the current is the equal through every cell so this restrains the possible band gaps which can be used. The most typical tandem cells arrangement is by growing them monolithically , so all of the cells grow as layers on the substrate and tunnel junctions connect individual cells.
So for Series connected solar tandem cells. Addition of extra devices permits the optimization of each device to a slimmer spectrum thus giving a better overall efficiency.
The need for Tandem cells
Different technologies for tandem cells are one of the main topics of interest for various research institutes and teams all around the globe. The technology has lots of potential and is quickly developing and becoming more promising, as it tries to overcome past theoretical limits of single cell efficiency and it not exceeding 30 %.
A closer look is required for such a technology and its history, before questions about the tandems market readiness, problems and issues faced and possible combinations of existing solar cells.
As scientists always look for more efficient ways to improve existing technology or some process of it, tandem cell is the result of that. Around half a century ago, William Shockley and Hans Joachim Queisser reached the remarkable discovery -which is now named the Shockley-Queisser limit – when they proved theoretically that solar cells with just one single layer suffers from limitations to efficiency as a result of not being able to absorb solar light energy to the fullest.
Tandem Cells Structure
Single junction solar cell has inefficient utilization of solar spectrum for two reasons: first, photons with lower energy than the bandgap not contributing. Second, every photon with higher energy contributing to the current with one electron, yet, all the energy above the bandgap is lost. For an improved utilization of photon energies we could try using different semiconductor materials where each photon is guided into an absorber in which the bandgap and photon energy match.
A tandem is the simplest structure of this type, in which two absorbers are stacked. For combining high and low bandgap material, the illumination should strike first the absorber with the higher bandgap as high energy light will be absorbed, producing a high output voltage. Besides, this material will have high transparency for low energy light that can transferred to the second absorber with the low bandgap.
The fabrication of tandem devices is really demanding technologically due to currents extraction not being trivial. Mainly there are two designs, one is the monolithically integrated tandem and the other is the mechanically stacked tandem.