Solar cells: Where is the world moving?
22nd January 2018
One of the better inventions in the human history was the development of solar cell. A solar cell works on photovoltaic effect i.e. it generatespower when light falls on it. Combination of solar cells (in series or in parallel) formulates solar module to produce desired power output. The most widespread and commercially acceptable types of solar cells are mono-crystalline and multi-crystalline silicon cell. However, with the increasing demand to produce a more efficient and high powered technology, variety of new solar cell technology were invented and (few of them) are put into commercial use. This article and its subsequent parts aim to educate its readers on all such prevalent and new cell technologies.
Figure 1: A typical mono crystalline (on left) and multi crystalline (on right) solar cell technology (source: Waaree Energies)
Back Surface Field (BSF) solar cell
The solar cell is able to generate power when electron (negative charged particle) and hole (positive charged particle) are separated and collected at their respective collectors. However, due to surface recombination, a process in which the electron and hole are lost, leads to loss of power in solar cell. In order to counter such effect BSF cells were introduced. As evident from Figure 2 below, in the normal solar cell, the electrons at P side and hole at N side are lost due to recombination at surface. The BSF cell in addition to the regular p-n junction uses n++ and p++ doping at front and back side of the cell respectively. Such heavy doping enables the reflection of holes and electrons at n++ and p++ junction respectively leading to reduction in the surface recombination. This enhances its efficiency and hence its power output.These cells are currently dominatingthe market with an efficiency of around 17% in multi crystalline and 19% in mono crystalline technology.
Figure 2: A prototype of BSF solar cell (Source:Waaree Energies)
Passivated Emitter and Rear Contact (PERC)
First developed in Australia in 1980's by famous scientist Martin Green and his team at University of New South Wales, Passivated Emitter and Rear Contact (or more commonly known as PERC) cells utilizes two extra steps while compared to manufacturing regular solar cells. This extra steps leads to formulation of small metal contacts (dielectric passivated layer with local BSF) at the rear side of the cell. This ensures that the electron reaching the back surface due to (localized BSF) have an additional change of reflecting back into the cell leading to generation of extra current. Also as evident from Figure 3 below, the localized BSF also enables increased internal light reflectivity leading to increased generation of power. This increases the overall efficiency (up to 25% lab efficiency and up to 22% commercial efficiency) of the PERC cell. PERC cells can be made in both mono and multi crystalline silicon technology. Despite having superior performances, these cells have a marginal share in the market due to light induced degradation (LID) effect. The efficiencies of these cells can be adversely affected due to LID. Few manufacturers have come with LID free solar cells, but as per our understanding this effect (may be marginal) is still prevalent in PERC solar cells.
Figure 3: A typical PERC solar cell (Source: Waaree Energies)
The history of bifacial solar cell dates back to 1960 when Japanese researcher H. Mori proposed the design of bifacial PV solar cell and had developed the working prototype by 1966. A typical structure of a bifacial solar cell is shown in the figure below. Comparing it to the typical solar cell used in current market, the bifacial solar cell has textured surface (enables more absorption of incident light) on both sides. The bifacial solar cell has an additional surface passivation (enables increased current collection) on its rear surface. Additionally, instead of full metallic back surface, the bifacial cell has finger grid at rear side. All these factors enhance power performance from the module.There is however a slight confusion with determining the efficiency of these types of cells (and modules). This is because the light falling on the rear side of the cell depends on the ground surface and location. As different ground surfaces have different light reflectivity, the actual value of irradiance falling on the rear surface of the module cannot be generalized. This makes it hard to determine the exact efficiency of the module at the moment.
Figure 4: An architectural representation of bifacial solar cell (Source: EPRI)
Black silicon cell
Black silicon solar cells have been known to have enhanced light absorption while comparing it to the traditional solar cell. This is because its manufacturing process is slightly different than traditional solar cell. The traditional (crystalline) solar cells have orderly arrangement of silicon atoms giving it a smooth surface which would result in increased reflectivity of incident light. Hence an Anti-Reflective Coating (ARC) is applied on these cells to enhance light absorption capacity. However, the black silicon cells are manufactured with high energy laser pulses which lead to formation of rod/cone particles (Figure 5) of nanometre scale. Such particles are present on the top surface of cell in random orientations causing maximum absorption of photons. Additionally, any reflected photons have better chances to be absorbed into the cell. Lab efficiency as high as 25% have been reported for black silicon technology. Black modules have now started penetrating the market and may become the new normal in mere future.
Figure 5: A typical representaion of black silicon cell vs compared to crystalline cell (on left) andfront surface SEM image of black silicon cell (on right) (Source Google images)
While this article dealt with the next immediate technologies, the next article would inculcate its readers on the futurepotential technologies. Keep looking this space for our next article.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION