4: PvK (highest Eg possible in bandgap)
6: PvK (mid-value Eg of bandgap)
8: PvK (lowest value Eg)
ICL can be:
p-type: Cu(In, Ga)S2 ; Cu(In, Ga)Se2
n-type: CdS ; Zn(S, O)
ARC: MgF2, Ag-SiO2-Ag
1. demand per day x days
2. divide by voltage of system (usually 12v)
3. divide by 1-DOD (depth of discharge)
Thin film crystalline GaN LEDs are grown on insulating substrates so to prevent current leakage, often there is a high lattice mismatch between the film and the substrate so intermediate layers are grown to reduce the amount of stress dislocations in the film. Why is the replication of defects in semiconductor thin films undesirable?
A solar cell made of a heavily dosed silicon based semiconductor with no observed defects begins to experience a reduction in efficiency. What could be a potential cause for this reduced efficiency, and what is one practical solution to this problem?
An orange colored LED is dipped in liquid nitrogen and then illuminated, a change to green, a smaller wavelength of light, was seen. Given that no defects were introduced, what is happening to the band to band recombination?
Dislocations and other defects in the crystal structure of a extrinsic semiconductor like GaN will alter the band structure and possibly introduce non-radiative recombination centers, thus decreasing the LED efficiency and output.
Materials with high concentrations of electrons in the conduction band are subject to Auger recombination regardless of defect presence. either choosing a different material with a lower electron donor concentration or by reducing the energy input you will lower the probability for Auger recombination and in turn increase efficiency of the solar cell.
The electron interaction in the material is changing and in this case, results in the increased size of the band gap. Therefore the electrons have a higher difference in initial and final states, thus creating higher energy photons.
1. Photons in sunlight hit the solar panel and are absorbed by semi-conducting materials.
2. Electrons (negatively charged) are knocked loose from their atoms as they are excited. Due to their special structure and the materials in solar cells, the electrons are only allowed to move in a single direction. The electronic structure of the materials is very important for the process to work, and often silicon incorporating small amounts of boron or phosphorus is used in different layers.
3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.
When a photon hits a piece of silicon, one of three things can happen:
1. The photon can pass straight through the silicon — this (generally) happens for lower energy photons.
2. The photon can reflect off the surface.
3. The photon can be absorbed by the silicon if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat depending on the band structure
Charge carrier separation
There are two causes of charge carrier motion and separation in a solar cell:
1. drift of carriers, driven by the electric field, with electrons being pushed one way and holes the other way.
2. diffusion of carriers from zones of higher carrier concentration to zones of lower carrier concentration (following a gradient of chemical potential).
These two "forces" may work one against the other at any given point in the cell. For instance, an electron moving through the junction from the p region to the n region (as in the diagram at the beginning of this article) is being pushed by the electric field against the concentration gradient. The same goes for a hole moving in the opposite direction.
It is easiest to understand how a current is generated when considering electron-hole pairs that are created in the depletion zone, which is where there is a strong electric field. The electron is pushed by this field toward the n side and the hole toward the p side. (This is opposite to the direction of current in a forward-biased diode, such as a light-emitting diode in operation.) When the pair is created outside the space charge zone, where the electric field is smaller, diffusion also acts to move the carriers, but the junction still plays a role by sweeping any electrons that reach it from the p side to the n side, and by sweeping any holes that reach it from the n side to the p side, thereby creating a concentration gradient outside the space charge zone.
In thick solar cells there is very little electric field in the active region outside the space charge zone, so the dominant mode of charge carrier separation is diffusion. In these cells the diffusion length of minority carriers (the length that photo-generated carriers can travel before they recombine) must be large compared to the cell thickness. In thin film cells (such as amorphous silicon), the diffusion length of minority carriers is usually very short due to the existence of defects, and the dominant charge separation is therefore drift, driven by the electrostatic field of the junction, which extends to the whole thickness of the cell.
Once the minority carrier enters the drift region, it is 'swept' across the junction and, at the other side of the junction, becomes a majority carrier. This reverse current is a generation current, fed both thermally and (if present) by the absorption of light. On the other hand, majority carriers are driven into the drift region by diffusion (resulting from the concentration gradient), which leads to the forward current; only the majority carriers with the highest energies (in the so-called Boltzmann tail; cf. Maxwell–Boltzmann statistics) can fully cross the drift region. Therefore, the carrier distribution in the whole device is governed by a dynamic equilibrium between reverse current and forward current.
The name monocrystalline means “one crystal”. This is because monocrystalline solar panels are made from a single unbroken silicon crystal structure. This is one of the reasons why monocrystalline solar panels have uniform color. Monocrystalline solar panels are made from a single crystal ingot that has a cylindrical shape. With the help of a laser thin slices are cut from this silicon crystal cylinder.
These small silicon wafers are later additionaly shaped and installed on an aluminium frame which will be the future solar panel.
Monocrystalline solar panels are usually in uniform dark black color and have a characteristic square form which gives them a good aesthetic look. This square shape is achieved by cutting their corners in order to place more solar panels on a frame.
CIGS: material Mo & glasssubstrate imporant choice due to thermomechanical properties to offer compatibility with the subsequent deposition and manufacturing steps of solar module.
Table T4.3.2 for Buffers.
CIGS: n = 21.7% Voc = 0.75V Vsq = 0.91V
CdTe: n = 21%
Perovskites: n = 21% Voc = 1.1 V Vsq = 1.325V FF = 0.73
CZTS: n = 12.6%
GaAs: n = 28.8% Voc = 1.12V Vsq = 1.155V
SHJ: n = 21.3% Voc 0.717V Jsc = 38.6 FF = 0.77 --> WITHOUT BSF TCO
WITH BSF TCO --> n = 25.6% JSc = 41.8 mA/cm2 Voc = 0.74V FF = 0.827
Organic-Inorganic Hybrid Perovskite: n = 22% Voc = 1.105V Jsc = 24.97 FF = 0.803
Free e (created in p region) --> pn junction --> n region
Free h (created in in region) --> pn junction --> p region
Accumulations of above allows a voltage to build up in the pn-junction. If n region is connected with p region by R then we have current
Minimizing e-h recombination (due to imperfections in silicon cyrstal):
Thin film coating reduce desnity of surface defects and double as anti-reflection. These coatings can be electrically charged --> reduce e-h recombination
Also silicon surface doping in boron/phosphorous
No shadow loss from front electrode
low series resistance
low recombination of front surface
no trade-off--> high open Voc
SQL assumed only radiative RBN. Why n falls when cell heats up?
Panel radiate much more when its hot. Eloss ~ T^4 Blackbody. 1 dg hotter is 1% loss. Resistance connectors increase
Lifetime: T increase, material failure increase, by atom integration changes bonds.