Solar CellChapter 6: Design of Silicon Solar Cells

Nji Raden PoespawatiDepartment of Electrical Engineering

Faculty of EngineeringUniversity of Indonesia

Contents

6.1. Optical Properties

6.2. Reducing Recombination

6.3. Top Contact Design

6.4. Solar Cell Structure

Optical Properties

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.

Optical Properties(continued)

In designing such single junction solar cells, the principles for maximizing cell efficiency are:

1. increasing the amount of light collected by the cell that is turned into carriers;

2. increasing the collection of light-generated carriers by the p-n junction;

3. minimizing the forward bias dark current; 4. extracting the current from the cell

without resistive losses.

Optical Properties(continued)

Optical LossesOptical 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: Top contact coverage of the cell surface can be minimized

(although this may result in increased series resistance); Anti-reflection coatings can be used on the top surface of

the cell. Reflection can be reduced by surface texturing. The solar cell can be made thicker to increase absorption The optical path length in the solar cell may be increased by

a combination of surface texturing and light trapping.

Optical Properties(continued)

Anti-Reflection CoatingsAnti-reflection coatings on solar cells are similar to those used on other optical equipment such as camera lenses.

The minimum reflection is calculated by:

Where :n1 : a refractive index of transparent material (ARC)

0 : a free-space wavelength and

d1 : the thickness

n0 : a refractive index of glass or air

n2 : a refractive index of semiconductor

……………………………(6.1)

Optical Properties(continued)

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 0.6m.

Comparison of surface reflection from a silicon solar cell, with and without a typical anti-reflection coating is depicted in Figure 4

Optical Properties(continued)

Surface TexturingSurface 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:1. A single crystalline substrate can be textured by etching

along the faces of the crystal planes. (random pyramid)2. the pyramids are etched down into the silicon surface rather

than etched pointing upwards from the surface (inverted pyramid)

3. using a photolithographic technique as well as mechanically sculpting the front surface using dicing saws or lasers to cut the surface into an appropriate shape (multicrystalline wafers).

Figure 5 is shown the surface texturing which are used those methods

Optical Properties(continued)

Material ThicknessThe 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.

Optical Properties(continued)

Light Trappinga 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:

………………………………(6.2)

Optical Properties(continued)

In a textured single crystalline solar cell, the presence of crystallographic planes make the angle 1 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.

Reducing RecombinationRecombination LossesRecombination losses effect :1.the current collection (the short-circuit current)2.the forward bias injection current (open-circuit voltage).

The main areas of recombination are :1.at the surface (surface recombination)2.the bulk of the solar cell (bulk recombination)

The depletion region is another area in which recombination can occur (depletion region recombination).

Reducing Recombination (continued)

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:

1. the carrier must be generated within a diffusion length of the junction, so that it will be able to diffuse to the junction before recombining; and

2. in the case of a localized high recombination site, the carrier must be generated closer to the junction than to the recombination site. For less severe localized recombination sites, carriers can be generated closer to the recombination site while still being able to diffuse to the junction and be collected without recombining.

Reducing Recombination (continued)

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.

Reducing Recombination (continued)

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:

1. the number of minority carriers at the junction edge. Minimizing the equilibrium minority carrier concentration reduces recombination. Minimizing the equilibrium carrier concentration is achieved by increasing the doping;

2. the diffusion length in the material. The diffusion length depends on the types of material. High doping reduces the diffusion length;

3. the presence of localized recombination sources within a diffusion length of the junction. A high recombination source close the the junction will allow carriers to move to this recombination source very quickly and recombine, thus dramatically increasing the recombination current. The impact of surface recombination is reduced by passivating the surfaces.

Reducing Recombination (continued)

Effect of doping (ND) on diffusion length and open-circuit voltage assuming well passivated surfaces is shown in Figure 10.

Surface Recombination

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

Top Contact Design Series Resistance

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

Top Contact Design (continued)Base Resistance

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:

………………………………..(6.3)

taking into account the thickness of the material. Where:

L = length of conducting (resistive) pathb = "bulk resistivity" (inverse of conductivity) of the bulk cell material (0.5 - 5.0 cm for a typical silicon solar cell)A =cell area,andw = width of bulk region of cell.

Top Contact Design (continued)Sheet ResistivityThe "sheet resistivity", which depends on both the resistivity and the thickness.

For a uniformly doped layer, the sheet resistance is defined as:

……………………………………..(6.4)

where is the resistivity of the layer; and t is the thickness of the layer.

The sheet resistivity is normally expressed as ohms/square or / For non-uniformly doped n-type layers, ie., if is non-uniform:

…………………..(6.5)

Top Contact Design(continued)

Emitter Resistance

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.

Top Contact Design (continued)

Contact Resistance

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.

Top Contact Design (continued)

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 :

1. the finger and busbar spacing,2. the metal height-to-width aspect ratio, 3. the minimum metal line width and 4. the resistivity of the metal. These are shown in the figure 15.

Top Contact Design (continued)

Design Rules

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:

1. the optimum width of the busbar, WB, occurs when the resistive loss in the busbar equals its shadowing loss;

2. a tapered busbar has lower losses than a busbar of constant width; and

3. the smaller the unit cell, the smaller finger width, WF , and the smaller the finger spacings, S, the lower the losses.

Solar Cell Structure Silicon Solar Cell ParametersFor silicon solar cells, the basic design constraints

on :1. surface reflection, 2. carrier collection, 3. recombination and 4. parasitic resistances

The result in an optimum device of about 25% theoretical efficiency. A schematic of such an optimum device is shown in Figure 16.

Solar Cell Structure(continued)

Basic Cell Design Compromises:

Substrate Material (usually silicon)Bulk crystalline silicon dominates the current photovoltaic market, in part due to the prominence of silicon in the integrated circuit market.

Cell Thickness (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 ·cm)A higher base doping leads to a higher Voc and lower resistance, but higher levels of doping result in damage to the crystal.

Solar Cell Structure(continued)

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.

Emitter Thickness (<1m)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.

Solar Cell Structure(continued)

Doping Level of Emitter (100 / ) The front junction is doped to a level sufficient to conduct away the 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 200m width, placed 1 – 5 mm 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.

Rear Contact.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.

Thank You

Figure 1. Evolution of silicon solar cell efficiency.

Figure 2. Sources of optical loss in a solar cell.

Figure 3. Use of a quarter wavelength anti-reflection coating to counter surface reflection.

Figure 4. Comparison of surface reflection from a silicon solar cell, with and without a typical anti-reflection coating.

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.

(a) (b)

(c) (d)

Figure 6. Reflection and transmission of light for a textured silicon solar cell.

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 8. Typical quantum efficiency in an ideal and actual solar cell, illustrating the impact of optical and recombination losses.

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 10. Effect of doping (ND) on diffusion length and open-circuit voltage assuming well passivated surfaces.

Figure 11. Techniques for reducing the impact of surface recombination.

Figure 12. Resistive components and current flows in a solar cell.

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 14. Points of contact resistance losses at interface between grid lines and semiconductor.

Figure 15. Key features of a top surface contacting scheme.

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.