Third Generation Concepts Flashcards
Why are commercially available tandem solar cells either typically low
efficiency ( ~ 10%) or very high efficiency ( > 30%) devices?
thin-film Si tandem solar cells exhibit a record efficiency of 12.3%.
A major challenge is the short diffusion/drift length, limiting the maximum thickness of the absorber layers.
Why is the efficiency of a solar cell under concentrated sunlight higher?
Concentrated sunlight increases the ratio of the current produced when the device is illuminated compared to when it is dark.
Therefore, boosting the output voltage and increasing the efficiency.
Concept of current matching
In a two-terminal tandem solar cell, the same current must flow through both cells.
Advantages / disadvantages of other 3rd gen PV concepts
x
Explain the principle behind (i) UC, (ii) DC, (iii) LDS,
and (iv) LSC.
2 low-energy photons -> 1 high-energy photon
1 high-energy photon -> 2 low-energy photons
the conversion of 1 high-energy photon, which is inefficiently absorbed by the PV material, to 1 photon that can be efficiently absorbed
A device for concentrating radiation, solar radiation in particular, to produce electricity.
Which fundamental loss is addressed by (i) UC, (ii) DC, and (iii) LDS,
respectively?
- sub-bandgap losses
- lattice thermalisation losses
- Enhance performance of solar cells with poor external quantum effiency
What are the challenges hampering the deployment of
(i) UC
- For UC to work efficiently, need to >100x enhancement in generation rate G
- Also need concentrated sunlight, 1000 suns. But receive losses due to Auger recombination making operating c-Si solar cells at such high concentration impractical
- Plasmonic structures -> increase weak absorption
- Inorganic materials -> nanocrystals, open up new device structures but challenging due to surface recombination
What are the fundamental losses reduced in a tandem solar cell
architecture? How?
Transmission and thermalisation losses are reduced.
Most conventional solar cells, it’s not sensitive to most of the infrared spectrum. Conversely, if the energy of the photon is higher than the bandgap energy, than it is absorbed but energy gets wasted in the form of heat.
Having different P-N junction that contain different semiconductor materials; each material will produce electric current in response to different wavelengths of light.
Explain the difference between a monolithic tandem solar cell and a
multi-terminal tandem solar cell.
Monolithic are PN junctions which are connected in series; but need current matching
Thin-film silicon tandem cells, an a-Si:H top cell is stacked onto an nc-Si:H bottom cell.
To include current matching, the top cell is much thinner than the bottom cell.
What is the maximum efficiency for an infinite number of junctions? For two junctions? Compare to the SQ limit of a single junction solar cell.
infinity number of cells; 86.8%
multijunction cells with 2-subcells; 42%
single junction solar cell; 34%
What is the optimal bandgap (range) for a wide bandgap top solar cell in combination with a low bandgap Si solar cell.
The max efficiency for a monolithic tandem solar cell under the AM1.5G spectrum and without concentration is 47%.
At peak efficiency, the top cell has a bandgap of 1.63eV and bottom cell has a bandgap of 0.96eV
Which solar cell needs to face the sun in a tandem solar cell: (i) the
wide bandgap solar cell or (ii) the low bandgap solar cell? Why?
The high band gap is stacked on top of the low band gap, as the high band gap will absorb the short wavelength (blue) light.
The bottom cell has the lowest bandgap and absorbs the long wavelength (red and near-infrared) light.
What are the targeted applications for III/V-multijunction solar cells?
Why?
III-V materials have very sharp bandgap edges and high absorption coefficients.
Germanium - BG 0.67eV
Gallium Arsenide - 1.4eV
GaInP - 1.86eV
What are the challenges hampering the deployment of (ii) DC?
- Very hard to achieve strong absorption (Pr3+ ion PLQY = 140%, but no 185nm in solar spectrum)
- Low phonon energy -> materials required to minimise non-radiative recombination
What are the challenges hampering the deployment of LDS?
- Need to enhance life of organic PV devices
- Development of stable, large Stokes shift and high PLQY materials
