So far, we have discussed the Shockley-Queisser

limit to the maximal conversion efficiency of a single-junction solar cell. However, to derive the Shockley-Queisser limit

we used a number of assumptions that hold for an ideal solar cell. In reality, there are always additional losses

in solar cells that we, engineers, try to overcome to get as close as possible to the

maximal efficiency. We will explore these losses in this video. The purpose of THIS video is to present all

the additional losses together and to understand how to overcome them. Let’s start by recapping the additional

optical losses. You see an example of a crystalline silicon

solar cell in this slide. The metal grid placed on the front side of

the solar cell as the electrode is not transparent to the incident light. Thus, the area under the grid is shaded. The active areas where light can penetrate

into the solar cell is reduced due to this shading. The second optical loss results from front-side

reflection. Third, not all the light that enters the solar

cell will be absorbed in the absorber layer because part of it is absorbed by supporting

layers. This is called parasitic absorption. Lastly there are transmission losses The metal

front electrode is opaque to the incident light, so the area that is covered by the

front electrode decreases the active area of a solar cell. We can denote the total area of a solar cell

as A_tot and the cell area that is not covered by the electrode as A_f. We define the active area coverage factor,

C_F, using the ratio of A_f to A_tot. Different patterns of front electrode are

used to overcome the shading loss. There has been a lot of work to minimize the

metalization at the front of the solar cell. This is a schematic figure of a standard silicon

solar cell with an emphasis on the front metal electrode. The front metal pattern is made up of thicker

busbars, used to make connections with adjacent cells, and thinner fingers. This cell uses two bus bars and numerous fingers

. As discussed in the previous section, the

design of the bus bars and density of the fingers is a tradeoff between the contact

resistance and shading loss. This can be seen if we look at R, the resistance

of a metal. It is proportional to the length of the given

metal finger and inversely proportional to the cross-sectional area. So you can see if we reduce our cross sectional

area, we minimize shading losses but we increase resistance. There is a trade off between minimizing the

shading and resistance losses. The tradeoff between shading and electrical

losses is shown here. If we increase the spacing between fingers

in a solar cell, this means there will a larger active area, this results in lower shadow

losses. However, it increases the resistance between

the contact electrode and emitter since the carriers have to travel, on average, a larger

distance in the emitter to reach the contacts. Hence, to achieve the minimum power losses,

an optimal spacing distance should be determined. A similar relation is present with the finger

width. From the right figure we conclude, that the

larger the finger width is, the higher the shading losses but the lower the resistances. An optimum finger width exists, at which the

power losses are minimal. This illustrates one of the many tradeoffs

that engineers have to work with when designing a solar cell. One way designers are getting around this

issue is to use interdigitated back-contacted solar cells, or IBC solar cells. In these cells there is absolutely no front

shading from metallization, since there is no metal grid at the front of the solar cell. You may ask, how do they collect the carriers? Well, they actually have two different contacts

at the back. One for hole collection and one for electron

collection. These contacts are interdigitated so that

carrier transport to these contacts is minimal. This is a very advanced concept and will be

explained it further in the next course, Photovoltaic Technologies. When light arrives at any interfaces, part

of light will be transmitted and part of it will be reflected due to the different refractive

indices of materials that form interfaces. Therefore, when a silicon solar cell is illuminated

a part of the incident light is reflected. We can find out how big is the fraction of

incident light that is reflected. from the silicon/air interface. We use the Fresnel equation and refractive

indices of air and silicon. We easily find out that at the wavelength

of 500 nm more than 30% of incident light is reflected from the solar cell. Reflection losses can be overcome by adding

an antireflective coating. Reflection can further be reduced by adding

front texturing. You should remember, however, that there is

a tradeoff with front surface texturing. When you do this, you essentially add surface

area to the solar cell which increases the surface recombination. However, excellent passivation techniques

essentially minimize this effect, so texturing is still a very worthwhile method for reducing

front reflection. You can learn more about passivation techniques

in the next course on PV Technologies. Not all the light entering a solar cell can

be absorbed. The light that passes through a solar cell

without being absorbed causes the transmission loss. The transmission loss depends on two factors:

the absorption coefficient and the optical path length through a material. In the figure, the absorption coefficient

of four different semiconductor materials is plotted as a function of wavelength. The transmittance of an absorber layer can

be estimated by the Lambert-Beer law, which describes the attenuation of the intensity

of an electromagnetic wave as it travels through a certain medium. For c-Si material, the absorption coefficient

at wavelength of 400 nm is 10 to the power of 5 per centimeter. While the absorption coefficient at 800 nm

drops down to 10 to the power of 3 per centimeter. In a one micrometer thick crystalline silicon

without any light-enhancement techniques, blue light can be fully absorbed while 90%

of red light passes through c-Si solar cell without being absorbed. Therefore, to absorb as much light as possible,

light management techniques and appropriate absorber layer thickness must be adopted. Apart from additional optical losses, which

generally affect the current density of a solar cell, solar cells also suffer from additional

electrical losses. These electrical losses affect the voltage

and fill factor of the cell. Let’s take a look at how the fill factor

can be affected. The resistances of bulk semiconductor material,

metal electrodes and contact between semiconductor and metal result in the series resistance

in a solar cell. On the right, the effect of the series resistance

on the J-V curve and the fill factor is shown. The J-V curve of an ideal solar cell is represented

by the blue curve in the figure. The larger the series resistance, the less

steep the slope at the open-circuit voltage and the smaller the maximum power point. Hence, the series resistance should be as

small as possible to have high fill factor. The other type of resistance in a solar cell

is the shunt resistance or sometimes called parallel resistance. The shunt resistance is the result of a macroscopic

defects in a solar cell, which offers an alternative low resistance path for the generated photocurrent. The effects of the shunt resistance on the

J-V curve and fill factor is illustrated in the figure. The J-V curve of an ideal solar cell is represented

by the purple curve in the figure. The shunt resistance in an ideal solar cell

is infinite so that there is not any leakage of photocurrent. If the shunt resistance is reduced, we see

the slope at the short-circuit current density becomes steeper and the maximum power point

is reduced. The smaller the shunt resistance, the smaller

fill factor will be. Now let’s take a look at factors that influence

the open-circuit voltage of the solar cell. To utilize solar energy as electricity, we

need to separate and collect photo-generated electrons and holes. As we already discussed, recombination process

can destroy or annihilate the photogenerated carriers before we separate them. Therefore recombination strongly affects the

performance of solar cells. Solar engineer do their best to minimize recombination

of photogenerated charge carriers in the bulk of the absorber and at the interfaces of a

solar cell. The bulk recombination is in general the contribution

of three different recombination processes: radiative, Shockley-Read-Hall and auger recombination. We also need to take into account surface

recombination. The lifetime of minority carriers related

to the surface recombination is denoted as tau_s and depends on the surface recombination

velocity. We can sum up contributions of all recombination

processes in a solar cell and obtain an effective minority carrier lifetime, tau. The figure presents the efficiency of a solar

cell as a function of bulk minority carrier lifetime and the surface recombination velocity. The highest efficiency is obtained at a high

bulk minority carrier lifetime and low surface recombination velocity. We can see that when the surface recombination

is high the efficiency of a solar cell is almost independent of the bulk minority carrier

lifetime. Therefore, good passivation techniques are

necessary to reduce the surface recombination velocity and to increase the efficiency. So, for obtaining a high efficiency solar

cell, a high quality absorber material and extremely good passivation of surfaces is

required. To summarize. We presented that in a practical solar cell

there are losses due to shading, total reflection from different interfaces, parasitic absorption

and transmission, drop in fill factor connected to the series and shunt resistances and collection

losses because of the bulk and surface recombination. In the next video, we will derive the mathematical

expression for the conversion efficiency of a practical solar cell by including all optical

and electrical losses.