In this article, we shall have a glance about various Design Rules for Solar Cells and understand the basics of Band Gap Utilization.
Design Rules for Solar Cells
- Band gap energy Utilization
- Spectral Utilisation
- Light trapping
Band Gap Utilisation (BGU):
Solar Cell BGU affects —à Voc —à Hence influences the Power drawn from Solar Cell. How ?
Voc = (kT/q) ln (((Iph-Ish)/Io)+1)
The answer for the above snapshot is illustrated as below:
Examining the Expression 1 for open circuit voltage, Voc was dependent on the parameters such as:
- Photon Current, Iph in turn affected by the Irradiance W/Sq.mtr
- Dark Current or Leakage Current, Io in turn affected by the Temperature
- Shunt or Recombination Current, Ish in turn affected by different recombination mechanisms.
Voc = (2kT/q) ln ((Glτo)/ ni)
*Expression 1 can also be written as Expression 2
Examining the Expression 2 for open circuit voltage, Voc was dependent on the parameters such as:
- Generation rate of Charge Carriers, Gl in turn affected by the Irradiance W/Sq.mtr
- Life Time of minority Charge Carriers, τo in turn affected by different recombination mechanisms.
Solar Cell Open circuit voltage ‘Voc’ is always lower than the Band Gap voltage ‘Vg’ this can be understood by bandgap utilisation efﬁciency ηV, ηV is dependent on Shockley Quessier Limit.
Further from the above two expressions i.e., 1 & 2 the Open circuit voltage is limited by charge carrier recombination mechanisms.
As the life time of minority charge carriers were shorter due to higher recombination rate results in reduced Voc or in other words
Shorter the life time, the smaller the possible splitting between the quasi-Fermi Levels and the smaller the fraction of the bandgap energy that can be utilized results in reduced Voc.
Various PV materials possess different recombination mechanisms that limit the utilisation of the band gap energy.
Figure 1: shows a p-n-junction, with quasi-Fermi levels, p-doped region on left and n-doped region
There exist three different recombination mechanisms:
- Shockley-Read-Hall (SRH) and
- Auger recombination.
While Radiative and Auger recombination affected by semiconductor itself, we distinguish between direct and indirect bandgap semiconductors.
Auger Recombination is the mechanism when an electron & hole pair is recombined; an energy equivalent to the Band Gap Energy is transferred to the neighbouring electron in the CB. This electron is excited to the higher energy level in the Conduction Band ‘CB’ this hot electron will lose its energy as heat to the lattice of the semiconductor.
Auger recombination dominates for Indirect Band Gap Materials, under very high illumination conditions.
Radiative Recombination is the mechanism when an electron & hole pair is recombined; an energy equivalent to the Band Gap energy is released in the form of Photon.
Radiative recombination dominates for Direct Band Gap Materials, under moderate illumination conditions.
Shockley-Read-Hall (SRH) recombination is dependent on the density of traps or impurities in the semiconductor. Semiconductor materials have various types of defects in the lattice, these defects have energy states in the forbidden band gap like the donor & acceptor energy levels during doping.
An electron in the CB can be trapped in such defect states i.e., intermittent energy levels which makes life easier for the electron to recombine with the hole in the VB.
Shockley-Read-Hall (SRH) recombination
*Source: all pictorial representations were taken from TU Delft
In the three recombination mechanisms energy and momentum are transferred from charge carriers to phonons or photons.
The efﬁciency of the different recombination processes depends on the nature of the band gap of the used semiconductor material used.
Crystalline silicon is an indirect band gap material, Auger recombination dominates for C-Si and radiative recombination is inefficient.
For direct band gap materials such as GaAs under moderate illumination conditions, radiative recombination dominates.
In the defect rich solar cells, the open circuit voltage is limited by the SRH recombination.
So during Modules Selection, Designer/Client needs to make sure the solar cells are less defective to limit SRH recombination and to achieve this Modules Vendor need to follow quality Production Process. Designer need to check for micro cracks and randomly selected Modules by Client representative during Production Lot supplied for the respective project need to be sent for EL Test to TUV/UL approved Third Party Laboratories.
TUV Certified BOM need to be used without fail and TUV/Third Party/Client approved QA-QC need to be strictly followed by the Module Manufacturer.
Depending on the area of Selection where the illumination conditions vary the choice between Direct & Indirect Band Gap Semi-Conductor Material Technology can be taken.
In low-defect solar cells based on indirect band gap materials, the open circuit voltage is limited by Auger recombination.
In low-defect solar cells based on direct band gap materials, the open circuit voltage is limited by radiative recombination.