Additional optical and electrical losses in solar cells


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.

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