Solar Cell Chapter 6: Design of Silicon Solar Cells. Nji Raden Poespawati Department of Electrical Engineering Faculty of Engineering University of Indonesia. Contents. 6.1. Optical Properties 6.2. Reducing Recombination 6.3. Top Contact Design 6.4. Solar Cell Structure.
Solar CellChapter 6: Design of Silicon Solar Cells
Nji Raden Poespawati
Department of Electrical Engineering
Faculty of Engineering
University of Indonesia
6.1. Optical Properties
6.2. Reducing Recombination
6.3. Top Contact Design
6.4. Solar Cell Structure
Basic Solar Cell Design
Solar cell design involves specifying the parameters of a solar cell structure in order to maximize efficiency, given a certain set of constraints.
Fig. 1 shows Evolution of silicon solar cell efficiency.
In designing such single junction solar cells, the principles for maximizing cell efficiency are:
Optical losses chiefly effect the power from a solar cell by lowering the short-circuit current.
Sources of optical loss in a solar cell is illustrated in Figure 2.
There are a number of ways to reduce the optical losses:
Anti-reflection coatings on solar cells are similar to those used on other optical equipment such as camera lenses.
The minimum reflection is calculated by:
n1 : a refractive index of transparent material (ARC)
l0 : a free-space wavelength and
d1: the thickness
n0 : a refractive index of glass or air
n2 : a refractive index of semiconductor
Figure 3 illustrates use of a quarter wavelength anti-reflection coating to counter surface reflection.
For photovoltaic applications, the refractive index, and thickness are chosen in order to minimize reflection for a wavelength of 0.6mm.
Comparison of surface reflection from a silicon solar cell, with and without a typical anti-reflection coating is depicted in Figure 4
Surface texturing, either in combination with an anti-reflection coating or by itself, can also be used to minimize reflection.
Surface texturing can be accomplished in a number of ways:
Figure 5 is shown the surface texturing which are used those methods
The amount of light absorbed depends on the optical path length and the absorption coefficient.
For silicon material in excess of 10 mm thick, essentially all the light with energy above the band gap is absorbed. The 100% of the total current refers to the fact that at 10 mm, all the light which can be absorbed in silicon, is absorbed.
In material of 10 microns thick, only 30% of the total available current is absorbed. The photons which are lost are the orange and red photons.
a solar cell with no light trapping features may have an optical path length of one device thickness, while a solar cell with good light trapping may have an optical path length of 50, indicating that light bounces back and forth within the cell many times.
Light trapping is usually achieved by changing the angle at which light travels in the solar cell by having it be incident on an angled surface.
the angle at which light enters the solar cell (the angle of refracted light) can be calculated:
In a textured single crystalline solar cell, the presence of crystallographic planes make the angle q1 equal to 36° as shown in Figure 6.
Lambertian Rear Reflectors
A Lambertian back reflector is a special type of rear reflector which randomizes the direction of the reflected light.
A Lambertian rear surface is illustrated in the figure 7.
Recombination losses effect :
The main areas of recombination are :
Thedepletion region is another area in which recombination can occur (depletion region recombination).
Current Losses Due to Recombination
In order for the p-n junction to be able to collect all of the light-generated carriers, both surface and bulk recombination must be minimized.
In silicon solar cells, the two conditions commonly required for such current collection are:
The quantum efficiency of a solar cell quantifies the effect of recombination on the light generation current. The quantum efficiency of a silicon solar cell is shown in Figure 8.
Figure 9 is illustrated Quantum efficiency curves for three different types of crystalline silicon solar cells.
Voltage Losses Due to Recombination
The open-circuit voltage is the voltage at which the forward bias diffusion current is exactly equal to the short circuit current.
The forward bias diffusion current is dependent on the amount recombination in a p-n junction and increasing the recombination increases the forward bias current.
high recombination the forward bias diffusion current , which in turn reduces the open-circuit voltage.
The recombination is controlled by the number of minority carriers at the junction edge, how fast they move away from the junction and how quickly they recombine.
Consequently, the dark forward bias current, an hence the open-circuit voltage is affected by the following parameters:
Effect of doping (ND) on diffusion length and open-circuit voltage assuming well passivated surfaces is shown in Figure 10.
Surface recombination can have a major impact both on the short-circuit current and on the open-circuit voltage.
Lowering the high top surface recombination is typically accomplished by reducing the number of dangling silicon bonds at the top surface by growing a "passivating" layer (usually silicon dioxide) on the top surface.
Techniques for reducing the impact of surface recombination is depicted in Figure 11
In addition to maximizing absorption and minimizing recombination, is to minimize parasitic resistive losses.
Both shunt and series resistance losses decrease the fill factor and efficiency of a solar cell.
A detrimentally low shunt resistance is a processing defect rather than a design parameter. However, the series resistance, controlled by the top contact design and emitter resistance, needs to be carefully designed for each type and size of solar cell structure in order to optimize solar cell efficiency.
The series resistance of a solar cell consists of several components as shown in Figure 12
The resistance and current of the base is assumed to be constant.
The resistance to the current of the bulk component of the cell, or the "bulk resistance", Rb, is defined as:
taking into account the thickness of the material. Where:
L = length of conducting (resistive) path
rb = "bulk resistivity" (inverse of conductivity) of the bulk cell
material (0.5 - 5.0 W cm for a typical silicon solar cell)
A =cell area,and
w = width of bulk region of cell.
The "sheet resistivity", which depends on both the resistivity and the thickness.
For a uniformly doped layer, the sheet resistance is defined as:
where r is the resistivity of the layer; and t is the thickness of the layer.
The sheet resistivity is normally expressed as ohms/square or W/
For non-uniformly doped n-type layers, ie., if r is non-uniform:
Based on the sheet resistivity, the power loss due to the emitter resistance can be calculated as a function of finger spacing in the top contact.
Idealized current flow from point of generation to external contact in a solar cell is shown in Figure 13.
Contact resistance losses occur at the interface between the silicon solar cell and the metal contact.
To keep top contact losses low, the top N-layer must be as heavily doped as possible.
Figure 14 shows points of contact resistance losses at interface between grid lines and semiconductor.
Metal Grid Pattern
The design of the top contact involves not only the minimization of the finger and busbar resistance, but the overall reduction of losses associated with the top contact.
These include resistive losses in the emitter, resistive losses in the metal top contact and shading losses.
The critical features of the top contact design which determine how the magnitude of these losses are :
These are shown in the figure 15.
for practical reasons most top surface metalization patterns are relatively simple and highly symmetrical.
A symmetrical contacting scheme can be broken down into unit cells and several broad design rules can be determined. It can be shown (Serreze, 1978) that:
Silicon Solar Cell Parameters
For silicon solar cells, the basic design constraints on :
The result in an optimum device of about 25% theoretical efficiency. A schematic of such an optimum device is shown in Figure 16.
Basic Cell Design Compromises:
SubstrateMaterial (usually silicon)
Bulk crystalline silicon dominates the current photovoltaic market, in part due to the prominence of silicon in the integrated circuit market.
CellThickness (100-500 µm)
An optimum silicon solar cell with light trapping and very good surface passivation is about 100 µm thick.
Doping of Base(1 W·cm)
A higher base doping leads to a higher Voc and lower resistance, but higher levels of doping result in damage to the crystal.
Reflection Control (front surface typically textured)
The front surface is textured to increase the amount of light coupled into the cell.
Emitter Dopant (n-type)
N-type silicon has a higher surface quality than p-type silicon so it is placed at the front of the cell where most of the light is absorbed. Thus the top of the cell is the negative terminal and the rear of the cell is the positive terminal.
A large fraction of light is absorbed close to the front surface. By making the front layer very thin, a large fraction of the carriers generated by the incoming light are created within a diffusion length of the p-n junction.
Doping Level of Emitter(100 W/ )
The front junction is doped to a level sufficient to conduct awaythe generated electricity without resistive looses. However, excessive levels of doping reduces the material's quality to the extent that carriers recombine before reaching the junction.
Grid Pattern(fingers 20 to 200mm width, placed 1 – 5mm apart)
The resistivity of silicon is too low to conduct away all the current generated, so a lower resistivity metal grid is placed on the surface to conduct away the current. The metal grid shades the cell from the incoming light so there is a compromise between light collection and resistance of the metal grid.
The rear contact is much less important than the front contact since it is much further away from the junction and does not need to be transparent. The design of the rear contact is becoming increasingly important as overall efficiency increases and the cells become thinner.
Figure 3. Use of a quarter wavelength anti-reflection coating to counter surface reflection.
Figure 5. (a) A square based pyramid which forms the surface of an appropriately textured crystalline silicon solar cell.(b)Scanning electron microscope photograph of a textured silicon surface.(c) Scanning electron microscope photograph of a textured silicon surface. (d) Scanning electron microscope photograph of a textured multicrystalline silicon surface.
Figure 7. Light trapping using a randomized reflector on the rear of the cell. Light less than the critical angle escapes the cell but light greater than the critical angle is totally internally reflected inside the cell. In actual devices, the front surface is also textured using schemes such as the random pyramids mentioned earlier.
Figure 9. Quantum efficiency curves for three different types of crystalline silicon solar cells. The buried contact and screen printed curves are internal quantum efficiencies, while the PERL is an external quantum efficiency. The PERL cell has the best response to infrared light since it has a well passivated, highly reflective rear incorporating light trapping.
Figure 13. Idealised current flow from point of generation to external contact in a solar cell. The emitter is typically much thinner than shown in the diagram.
Figure 16. Basic schematic of a silicon solar cell. The top layer is referred to as the emitter and the bulk material is referred to as the base.