Introduction to Solar Panel
How does a solar cell / semiconductor (SC) work?
Most solar cells are made of Silicon, which has four most outer electrons all involved in perfect covalent bonds. Hence a pure silicon crystal is an insulator.
Si can be turned into SC by “doping”, which introduces small quantity of impurities to create free electrons or holes. N-type has free electrons, P-type has free holes.
At P/N junction, electrons and holes recombine and neutralize, leaving ionized “donors” and “acceptors”. This is the “depletion layer” where no mobile charge carriers are present.
When the sun shines…
Photon can knock an electron from its bond, creating an electron-hole pair. Electrons are holes are now free to move around inside crystal.
Because of the electric field at PN junction, electrons are drawn to the N-side, while the holes are drawn to the P-side.
Electrons are collected by the thin metal fingers at the top of the cell and they flow through an external circuit, before returning through the back.
Introduction to Quantum Dot
What are “Quantum dots”?
Quantum dots (QDs) are nanostructured semiconductors. QDs can be considered as an artificial atoms with electrons are “confined” inside
This can be done by encapsulating a material with smaller ECP (electrochemical potential), with a higher ECP material.
In this case, InAs for QDs and GaAs for substrate.
Energy potential in QD
Theoretically, “well” like energy potential confines electrons into nanometer space.Energy levels are quantized and electrons can only exist at certain energy levels.
Energy level can be determined using Schrodinger’s equation: En=(π^2 h^2)/(2mL^2 )∗n^2
Intermediate Band (IB) Formation
What is an “Intermediate Band”
When QDs are in close proximity, free-electrons are able to move from one QD to another. This is the “tunneling effect”.
By tunneling, individually quantized energy states combine to form an “intermediate-band”(IB).
Two reasons why IBSC achieve higher efficiency
1. By capturing wider spectrum of solar energy
- Ability to capture photon energy smaller than the bandgap (Eg) through two-step excitation mechanism.
- Ability to minimize heat loss for larger energy by adjusting Eg.
2. By extending the lifetime of free-electron and allowing more time for the second excitation.
The next two scenes explain in detail.
* IBSC: Intermediate Band Solar Cell
Benefits of IBSC 1
~ Ability to capture wider spectrum of solar energy
Limitations of single-junction SCs
The maximum conversion efficiency of a single-junction solar cell is limited to the Shockley-Queisser limit of ~ 31%.
Here are two reasons why:
1. Transmission loss (15%):
When photons with energies smaller than Eg are not absorbed, by material characteristics.
2. Heat loss (30%):
When electron-hole pairs are created by energies larger than Eg, but quickly relax to the bandedge (thermalization) before being converted to electricity.
How does IBSC solve these issues?
With IB, free-electrons can either reach CB directly or go through IB. This reduces transmission loss.
The magnitude of Eg can be adjusted to reduce heat loss by using different combinations of QD materials.
* CB: Conduction band, VB: Valance band, IB: Intermediate band
Benefits of IBSC 2
~ Extended electron-hole lifetime
Now, the voltage is applied across the cell
When the voltage is applied, electrons and holes start to move across the depletion layer.
Note, in the physical space, electrons move towards N-terminus (holes toward P-terminus).
How does IB extend electron-hole lifetime?
See electrons moving inside IB? This is the key.
Because of IB, electrons/holes can move to neighboring QDs. This slows down the recombination process. (i.e. reduced probability of recombination as potential difference separates electrons/holes into separate direction)
Slower recombination means more time for 2nd excitation.
Recap | Superlattice at Work
Here’s how photon energy is converted into electricity in QDs
Electrons are first confined in QDs by the energy potential (in VB). When photons strike, free-electrons are created.
They can be converted into current by two possible scenarios.
Scenario 1:
Gain energy larger than Eg (directly reaches CB), flow out into the circuit.
Scenario 2:
Energy is insufficient to reach Eg (reaches IB), then drift through neighboring QDs towards N-terminus. Then receives 2nd excitation energy to flow out into the circuit (reaches CB)
* CB: Conduction band, VB: Valance band, IB: Intermediate band
MBE: Molecular Beam Epitaxy
Molecular Beam Epitaxy (MBE) is a thin film growth method widely used for fabricating semiconductor crystal.
How does a MBE system work?:
Crystals are grown by depositing vaporized metals onto the substrate in high vacuum.
Metals are prepared in evaporation “guns” (effusion cell). The guns are heated so metals are in gas form. Then molecular “beams” are fired to the wafer(substrate). In this case, In+As makes QDs and Ga+As makes spacers.
Due to the long mean free paths of the atoms, evaporated atoms do not interact with each other until reaching the wafer.
Controlling the Crystal Growth
Depending on the substrate temperature and the deposition rate, crystal growth can be precisely controlled. This allows us to manipulate the composition and the doping-concentration by atomic level.
Controlling crystal growth :
The “flux” of molecular beams are dictated by the temperature of each guns, which translates to the deposition rate. The rate can be controlled as precisely as 0.1nm/s.
By controlling the shutters for each guns, the thickness and of each layers can be manipulated, down to a single layer of atoms.
QD fabrication: S-K growth mode
The S-K growth mode is induced by growing a QD layer (InAs) on a substrate(GaAs) with a significantly different lattice constant.
(InAs and GaAs have 7.2% difference)
“Stranski-Krastanov” growth mode:
QDs are fabricated through the following:
1.Deposited InAs form a highly strained monolayer. This is “Wetting Layer” (WL).
2.On top of the WL, InAs atoms construct 3D islanded pyramids. WL reduces elastic constraint, which explains this transformation.
3.Pyramids (=QD) grow larger as incoming atoms “stick” to make new layers.
Our Approach
Fabrication of highly efficient IBSC is achieved by 1. good size uniformity and 2. ordered alignment of QDs.
1. Good size uniformity
-> “Strain-compensation technique”
The lattice constant for InAs is 7.2% larger than GaAs. Thus, strain energy accumulates when GaAs is used for spacers, which enlarges QD sizes in higher layers. Our approach is called “strain-compensation” technique. By using a spacer (GaNAs), which exerts opposite strain energy to QD growth, total strain energy is reverted to zero at each layer.
2. Ordered alignment
-> High-index GaAs(311) base layer
For the base layer, traditionally used GaAs(100) is substituted by GaAs(311)B, which promotes more ordered QD growth. Higher surface free energy of GaAs(311)B induces “mass transport” of atoms along with SK growth. As a result, QDs grow orderly in high density covering the surface.
Where We Are / Where We Are Headed
IBSC vs conventional technologies
This is the NREL “Best Research-Cell Efficiency Chart”.
Efficiencies of conventional single crystal technologies are bound by the Shockley-Queisser limit of ~ 31%.
On the other hand, QD solar cell is an up-and-coming technology theoretically capable of achieve over 60% efficiency.
At Okada lab, we have achieved 32% efficiency in 2018.
Applications and future outlook
High efficiency = less surface area required. This also achieves weight reduction.
IBSC is highly anticipated for “bendable” and “light-weight” application.
