A Comparative Study on Silicon and Perovskite Solar Cells
By Vishnu Tej Gunisati And Suganesh R.

Abstract:

The aim of this article is to draw the attention of the reader to the current problems and limitations associated with crystalline silicon solar cells and how the perovskite solar cells are capable of overcoming the issues in efficiency and the production costs of crystalline solar cells. In the beginning of the article we will first introduce various aspects of silicon solar cells i.e. the material introduction, method of manufacture  of both crystalline silicon solar cells and perovskite solar cell. Then we explicate the advantages of the later by comparative analysis of both types of cell and conclude with the significance of reorienting the solar energy markets towards adopting perovskite solar cell.

 

I. Silicon solar cells

Silicon as an element [1]

Silicon is an element in the periodic table, it belongs to group 14 and period 2. The atomic number of the element is 14. Silicon is comparatively abundant maybe not as much as oxygen or helium or etc. it is generally at a solid state at room temperature. There exists 2 allotropes of silicon in room temperature i.e., amorphous and crystalline. Silicon dioxide is the most common compound of silicon found in the earth crust. This compound exists in the form of quartz, rock solid etc., and it is mostly used in manufacturing of glass and bricks. Mostly, silicon is used commercially in its original form and may be with little mixing with natural minerals.

In this paper, we explain the production, manufacturing steps of silicon solar cells, the various limitations of silicon solar cells. After which, we describe the production, manufacturing and limitations of perovskite solar cells. We also discuss how the perovskite solar cells overcome the limitations of crystalline silicon solar cells. To this effect, we also show the current market dynamics and the conditions of perovskite cells in the market. In conclusion, we compare both the silicon and perovskite solar cells and lay out our suggestions as to how the future of solar cells will be highly dependent on the perovskite solar cells given its efficiency in generating solar energy.

 

Silicon in solar cells [2]

Pure silicon is the main requirement in a solar cell for a quite a number of years. At present, silicon is used in the solar cell market over 90%. Pure silicon in crystalline form is a bad conductor of electricity since it is a semiconductor at its core. Hence, to increase the conductivity of silicon in a solar cell, it is being doped i.e., addition of other atoms to the silicon atom. This thereby makes capturing of sunlight more efficient and hence the energy is converted into electricity. For instance, gallium atom and arsenic atom has one electron more and less relative to compared to an atom of silicon. Electron rich layer can be created when arsenic atom is put in between silicon atoms. Instead when gallium is used in silicon solar panels, will result in electron poor layer since it has one electron less.

 

Silicon solar cell parameters [3]

A common cell comprises the following component

  1. “Substrate material”  this material is usually silicon

  2. “Cell thickness”  this is usually 1 ohm.cm

  3. “Reflective centre” this means that the front surface of the cell is textured for better reflection.

  4. “Emitter Dopent”  this is usually n-type

  5. “Emitter thickness”  this is usually less than 1micrometer

  6. “Doping level of emitter”  this is usually 100 ohm

  7. “Grid pattern” this usually has a width of 20 to 200 micrometer and is places 1 to 5 meter

  8. “Rear contact” this is equally important in a cell since it can increase the efficiency of the cell in the end of the process.

 

Manufacture of silicon solar cells (industrial process)

  1. Cell production begins with a texturing process: An etching solution forms pyramid like structures on the surface of monocrystalline silicon wafer reducing reflectivity from 30% to almost 11%

  2. Top surface emitter diffusion (adding a layer of phosphorous containing material): The  wafers are cleaned thoroughly and is placed in a diffusion furnace. The phosphorous source is fed into the furnace through a carrier the generated phosphorous diffusing into the wafer surface at a high temperature forming a P-N junction.

  3. Edge isolation(removes the unwanted phosphorous diffusion): The unwanted phosphorous are chemically removed from the edges and rear surfaces in another patented process so then this process ensures the solar cells have excellent shunt resistances and higher efficiency under low light illumination.

  4. Anti-reflective coating (helps in increasing the sunlight absorption): In the plasma enhanced chemical vapour deposition area, the wafer is placed in a furnace and coated with a silicon nitride anti-reflective coating. This coating further reduces reflectivity to 6%, which combined with the texturing process reduces the overall surface reflection to around 1% at a special spectral range.

  5. Screen printing (metallization): Aluminum paste is applied to wafer rear surface further enhancing cell efficiency while silver paste is screen printed onto the wafers front and rear surface and then co-fired to act as the cell negative and positive metal electrode.

  6. Testing and sorting(confirms electrical output of each cell): The cells then undergo rigorous testing and sorting using a solar simulator , quality control is performed at every step of the production process from receiving the incoming wafers to packaging the completed cells are sent to solar module facility to be assembled.

  7. Cell interconnection: first the solar cells with the same characteristics are soldered in series.

  8. Encapsulation (provides protection of the cell): The interconnected cells are placed in a sandwich structure comprising a front sheet of tempered low iron glass a layer on EVA , the solar cells of other layers of EVA and a back sheet this sandwiched structure is laminated to protect the solar cell from moisture and mechanical shock.

 

Working of silicon solar cells (General working)

  1. When the light waves hit the top surface of the silicon solar cell, the only light with wavelength from a specific window of the solar spectrum (350-1140 micrometre) are absorbed into middle layer of the solar cell.

  2. The light wave knocks an electron off a silicon atom, setting the electron loose and leaving an area of positive charge (a hole) where the electron used to be.

  3. The loose electron then moves towards the top and reaches the top n-type, which readily accepts electron.

  4. Similarly, the loose hole moves towards the bottom and reaches the bottom p-type layer, which readily accepts holes.

  5. Now that the electrons and the holes have been separated, connecting a wire between the top and the bottom metal electrodes provides a pathway for the electrons to move towards the holes.

  6. Flow of electrons leads to electric current.

 

Challenges of silicon in solar cells [4]

To increase the efficiency issues, mostly solar cells are built with “single crystalline silicon”. Due to monocrystalline solar cell’s continuous structure it does not have the grain boundary thus the excited electrons move around the silicon structure without any grain boundaries to obstruct their movement. Comparing this with the polycrystalline cell, the latter have several more grain boundaries therefore inhibiting continuous flow excited electron in the semi-conductor this issue leads to radical drop in efficiency up to 10-15%. Hence, from research they have found that to balance the efficiency and cost ‘thin film solar cells’ were made. Even though this comprises of amorphous silicon they are still not that efficient due to absence of structures with uniform crystals in place. The major problems are discussed below-

1- Cost of the process: A lot energy is required for the manufacture of monocrystalline silicon cells by the conventional czochralski method .It requires a lot of processing for the single crystalline silicon to become structurally uniform. Almost 10% of impurity concentration is the requirement of the silicon used in single crystal silicon. It needs “extremely precise control and balance of the single crystal silicon seed” so an ingot can be generated “that has mono crystalline silicon all throughout” [8]. There is an unavoidable consequence in this oxidation of the ingot as the oxygen combines with the silicon and also it combines with dopants on the surface of ingots. Then this combined oxygen further reduces the flow of charged electrons in the cell thereby reducing the efficiency. This in turn complicates the entire process thereby increasing the overall cost

2- Material loss: From single crystal silicon to the modulus , it needs to be sliced into wafers of thickness (200 to 300 micrometre) therefore an instrument called inner diameter saw is utilized here. This saw comprises of diamond particles which cuts the ingots into fine wafers that is required. There is a tendency of the wafers to break easily with the required range of thickness hence it is difficult to employ. Due to this sawing process, even though this is done in its most efficient way there is a loss of 50% of the silicon produced in the form of saw dust.

3- Issues with absorption: According to the known absorption formulas , not all rays are absorbed i.e., a huge range of wavelengths are rejected since only a very narrow range creates the electron hole pair.

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FIGURE 1. Graph of Spectral Response versus Absorption. Source: Spectral Response." PVEducation. Accessed October, 2020. https://www.pveducation.org/pvcdrom/solar-cell-operation/spectral-response.

 

Polycrystalline silicon [5]

We know that an electron hole pair is created when a light ray (photon) strikes the surface of the cell. When a hole is formed at the p-n junction, electrons coming from the top surface are captured by the holes on the top layer and the same process happens in the holes created at the bottom layer and eventually the bottom layer captures electron from the junction. Hence, the total decrease in the electron flow in turn reduces the current produced from the cell. This has become since they are thick in size. Therefore all the losses direct to the overall decrease in the efficiency of the cell. The polycrystalline cells is more thicker than mono crystalline cells , since these cells are made of many single crystal molecules hence has a lot of grain boundaries thereby causing regular line defects such as- “simple dislocations, partial dislocations, and stacking faults” [8]. I-V DE convolution and charge emission are the two methods used to measure the grain boundaries in a polycrystalline silicon. Amongst all the methods, I-V DE convolution is the most common and efficient one.

 

II. Perovskite Solar Cells (PSC’s)

 

Perovskite as a Compound

Perovskite is any material with crystalline structure same as that of calcium titanium oxide (CaTiO3). The formula for a perovskite structure is ABX3, where A is an organic ion, B is an inorganic ion and X is halogen. The common occurrence of a perovskite structures is as bridgmanite (Mg, Fe) SiO3 and it occurs in the earths lower mantle under high pressure. In the lower mantle of earth the (Mg, Fe) O rock salt structure periclase form is abundant at a given temperature and pressure. As pressure varies with the depth of the earth, the tetrahedral form (SiO4) in silica bearing minerals becomes more unstable than the octahedral unit (SiO6) and so with pressure and temperature variations in the earth’s mantle different forms of silica exist. Although most perovskite structures contain oxygen, halogenic atoms like fluorine may also be present like NaMgF3 or a combination of rare earth ion, transition metal ions and light metalloids giving the form RT3M. This showcases the large family of perovskite compounds, abundant in their availability which draws us towards researching on perovskite solar cells [6].

 

Properties of Perovskite

Perovskite exhibits many interesting and important properties like:

  1. “Magnetoresistance”: electric resistance variation in presence of magnetic field.

  2. “Ferroelectricity” : ability to reverse spontaneous electric polarization with applied electric field.

  3. “Superconductivity” : increased conductivity relative to current room temperature conductance due to vanishing of electric resistance.

  4. “Charge ordering”: ordered arrangement of charges which lead to phase transition in transition metal oxides or organic semiconductors and also leads to ferro electricity.

  5. “High Thermoelectric power”: temperature difference constituting electricity.

  6. Superconducting ceramic substances containing copper, 3 or more other metals and some vacant oxygen positions like in case of “Yttrium barium copper oxide” and having a similar structure to that of perovskite can exhibit both insulating and superconducting properties depending on its oxygen content [6].

 

Perovskite Solar Cell

In these solar cells the “hybrid organic-inorganic lead” or “tin halide based material” works as solar energy utilizing layer [6]. Methylammonium halides are a good option for a perovskite solar cell material because of their economic and manufacturing feasibility. Perovskites have certain intrinsic properties which makes them a good choice for manufacturing solid-state solar cells like:

 

  1. “Wide absorption spectrum,” [7]

  2. “Long transport distance of electrons and holes,” [7]

  3. “Long carrier separation lifetime,” [7]

  4. Fast charge separation

 

From my survey I found that the general combination being used in perovskite solar cells is of type ABX3 is with A being an organic cation is usually methylammonium (CH3NH3+), B being inorganic cation is usually lead (II) (Pb2+) [7].

 

Principle Of Operation

A PSC works on the same fundamental principle as that of a general photovoltaic cell where the incident solar radiation provides enough energy to break the electron hole pair and initiate the flow of free electrons to the opposite end which constitutes the current

table 2.jpg

FIGURE 2: Illustration of PV Operation. Source: Zhang, Huili, Tom Van Gerven, J. Bayens, and Jan Degreve.”Photovoltaics: Reviewing the European Feed-in-Tariffs and Changing PV Efficiencies and Costs.” Schematic Operating Principle of a PV Solar Cell.

 

Manufacture Process of Perovskite Solar Cells (Psc’s)

There are many techniques to manufacture a PSC’s as discussed by Kajal et. al (2018). These include,

  1. “Solution processing-based techniques,” [8]

  2. “Roll to Roll printing,” [8]”

  3. “Vapour-based techniques” [8].

 

I will just elaborate here on the most commonly used technique for perovskite solar cell manufacture including Spin Coating technique and Roll to Roll printing.:

 

Spin Coating Technique

Spin coating is used mainly for producing small area solar cells. It is simple and cost-effective solution-processed technique(where the deposition of materials is carried out in the liquid state rather than a solid or gaseous state) which leads to uniform deposition of the perovskite layers on the PSC’s. Here the fluid to be coated is placed as a drop on centre of the disk which is rotated at high speeds where the centrifugal force causes the fluid to spread and form a uniform film on the coating surface-the characteristics of which depend upon the fluid viscosity and the start acceleration, ventilation which affect the drying speed.

table 3.jpg

FIGURE 3: Illustration of Spin Coating Technique. Source: Kajal, Priyanka,Kunal Ghosh, and Satvasheel Powar. Manufacturing_Techniques_of_Perovskite_Solar_Cells in Application of Solar Energy. January 2018.

 

After spin coating the fill is generally baked leading to strong adhesion and bonding between meatal cation and the halogen anions thereby producing a good crystallized layer of perovskite. By setting the speed of the spin ,time required for spin coating etc the thickness and quality of deposition can be optimised. However, it offers limitation in terms of non-uniformity of deposited surface over large hence can hinder the efficiency of industrial scale manufacturing [8]. Hence to overcome the limitation of the above process for industrial scale manufacture alternative processes as mentioned below can be chosen.

 

Roll to Roll Printing

It is again a solution processed technique which is found to be more feasible for large scale manufacturing and faster production of a PSC. In this technique various type of roll-to-roll compatible techniques such as spray coating, ultrasonic spray coating etc are used for depositing layers on the substrate. Technique like slot die coating is used for active hole transfer layers(HTL) and active electron transfer layers (ETL) deposition [8].

table 4.jpg

FIGURE 4: Illustration of Roll to Roll Printing Technique. Source: Kajal, Priyanka, Kunal Ghosh, and Satvasheel Powar. Manufacturing_Techniques_of_Perovskite_Solar_Cells in Application of Solar Energy. January 2018.

 

View of the Current PV Market

In general, the market can be broadly classified into two categories as discussed in The Perovskite Experts blog [7]:

  • Wafer based PV’s

  • Thin film cell PV’S

Single and multi-crystalline silicon solar cells belong to the wafer-based PV’s category where “c-Si cells dominate the current PV market (about 90%) and exhibit an efficiency of ~29%” [7]. Even though thin film cells exhibit better efficiency, certain other technologies like Cadmium telluride (CdTe) have been successfully commercialised with 20% cell efficiency. Other thin film technologies like “hydrogenated amorphous silicon”[7] and “copper indium gallium selenide cells” also take approximately 2% market share [7]. But the major dominator in the market is c-Si photovoltaic cells [7].

table 5.jpg

FIGURE 5: Current PV market layout. Source: ”Home. “Perovskite. Accessed October, 2020.

 

Sillicon Vs Perovskite Which Is Better?

Andreani et al.(2018) claim that the current photovoltaic market is largely dominated by wafer-based silicon solar cells (>90%) because:

  • Silicon is an abundant element in the earth’s crust

  • Manufacture process of the c-Si PV’s is non toxic

  • Bandgap of the silicon solar cell is found to be in the optimum range for PV conversion.

 

However there are certain issues associated with silicon solar cells such as-

  • Processing and energy cost: The current method of manufacturing monocrystalline silicon solar cells (Czochralski method), is very energy intensive because single crystalline silicon has to be pure for uniformity in its crystalline structure which requires a lot of processing which of course is done through a lot of energy input [9].

  • Energy conversion efficiency: The current record efficiency of crystalline Silicon photovoltaic cell is around (~18-26%) and is very close to the theoretical limiting value of approximately 29% [9].

 

Why perovskite solar cells?

Both of the above-mentioned problems can be solved by the use of PSC’s

  1. Processing cost: The perovskite solar cells are relatively cheaper compared to crystalline silicon solar cells since the PSC manufacturing technology does not require any extraction process as the latter is manufactured by a process of “solution processing” (referring grossly) which is also used in printing out newspapers.

  2. Scalability: Courtesy of the solution processing technique which is highly scalable, there is potential to make production costs lower in comparison to other current solar panel production technologies [10].

  3. Energy conversion efficiency: perovskite solar cells can achieve a higher efficiency because of having a relatively longer carrier diffusion length when compared to the silicon solar cells.

  4. In addition to the above advantages the PSC’s like “methylammonium-lead-halide (MALI) material with a perovskite structure” that can operate with single junction with efficiency of ~25.2% and multijunction technologies (in tandem with high efficiency crystalline silicon photovoltaic cells) increases the efficiency further through integrated photovoltaics thereby increasing the efficiency beyond 29% to ~40% theoretically [10].

  5. The use of plasmonics is also being attempted in perovskite solar cells which will enhance the localised light absorption by creating hot spots in the active layers thereby widening its operating wavelength range.

  6. The all silicon multi junction solar cells are currently more efficient ~16.3% but relatively expensive compared to their single junction counterparts in c-Si solar cells [6]. These above results are more encouraging for perovskite solar cells as research in solution processed multi junction solar cells (where in place of silicon a highly volatile acetonitrile/methylamine solvent based perovskite solution is used) with all perovskite tandem or triple junction solar cells can demonstrates efficiencies of ~26%.This is, in either ways better than the silicon counterpart there by providing a possibility of a large scale and low cost perovskite multi junction solar cells in the PV market [12].

Another astounding fact is that these levels of efficiencies have been predicted in a short period of research and researchers are hoping better efficiency limits in traditional panels on further lab development.

 

Comparative Analysis Of Efficiency Of Perovskite Vs Crystallised Sillicon Solar Cell

Figure 6 clearly shows rapid growth in terms of the efficiency of a PSC over a short period of time in comparison to the silicon solar cells which are almost approaching their limiting efficiency and also not showing considerable increase in efficiency over a long period of time. But the main question arises why are perovskite solar cells showing such remarkable progress and what exactly is the main reason for them showing better efficiency? Well, in my survey, I found that the “Diffusion Length” could be used as one of the parameters to map out the reason for the observed increase in efficiency, of course other parameters also mark the increase in efficiency.

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FIGURE 6: Graph illustrating the rapid rise in efficiency for perovskites as compared to other photovoltaic materials. Source: ”Higher End, Lower Cost - Weizmann Wonder Wander - News, Features and Discoveries.” Weizmann Wonder Wander - News, Features and Discoveries from the Weizmann Institute of Science. August 29, 2018. Accessed October, 2020. https://wis-wander.weizmann.ac.il/chemistry/higher-end-lower-cost.

 

What is diffusion length?

When solar radiations fall on the surface of the solar cell, it gives rise to free carriers (which are electrons that are negatively charged) and the voids of electrons (known as holes which are positively charged due to absence of electrons). Diffusion length is the distance which these two carriers travel before recombining again to become neutral. This diffusion length is directly related to the square root charge lifetime which points to more collection of charges(electrons) and more production of electricity. Therefore, it is to be noted that the longer the diffusion length the better the performance which indirectly means better the efficiency.

 

Limitations Of Perovskite Solar Cells

Though perovskite solar cells have demonstrated competitive efficiency with a great potential for high performance over photovoltaic cells but there are certain challenges which lie in the process of replacement like

  1. not standing up well to moisture

  2. not being able to tolerate extended periods of heat and light or high heat conditions

  3. the use of lead which serves and can cause environmental impact

 

To mitigate the above issue extensive research is being done on the degradation aspects of perovskite material and contact layers of the perovskite solar cells and also effort is being made to create high efficiency lead-free perovskite structures in order to eliminate these potential issues.

 

Conclusion

Through this research article we aimed at bringing out the potential reasons why the perovskite solar cells are a chief aspect of research in the field of photovoltaic cells in terms of their low manufacturing costs and remarkable progress in the efficiencies over a short period of time. Then we further substantiated the statements of how perovskite solar cells are more efficient than both single crystalline silicon solar cells and multi-junction crystalline silicon solar cells and also explained how the limitations of perovskite are still being subjected to research for potential solutions.

At the end we want to conclude that perovskite solar cells are still under progress as a whole but since the silicon solar cells are approaching the limit in their conversion efficiency the perovskite solar cells are the potential future of photovoltaic cells.

 

References

 

Silicon. (n.d) Retrieved from https://en.wikipedia.org/wiki/Silicon#Characteristics

 

Stewart, D. (n.d.). Silicon Element Facts. Retrieved from https://www.chemicool.com/elements/silicon

 

Silicon Solar Cell Parameters. (n.d.). Retrieved from https://www.pveducation.org/pvcdrom/design-of-silicon-cells/silicon-solar-cell-parameters

 

Seager C. 1985. Grain Boundaries in Polycrystalline Silicon. www.annualreviews.org.

 

Goetzberger A, Knobloch J, Voss B, ed. 1998. Crystalline Silicon Solar Cells. England: Willey press

 

Fan, Zhen, Kuan Sun, and John Wang. "Perovskites for Photovoltaics: A Combined Review of Organic–inorganic Halide Perovskites and Ferroelectric Oxide Perovskites." Journal of Materials Chemistry A. July 16, 2015. Accessed October, 2020. https://pubs.rsc.org/en/content/articlelanding/2015/ta/c5ta04235f#!divAbstract.

 

"Home." Perovskite. Accessed October, 2020. https://www.perovskite-info.com/perovskite-solar#:~:text=Perovskites possess intrinsic properties like, for solid-state solar cells.

 

Kajal, Priyanka, Kunal Ghosh, and Satvasheel Powar. 

https://www.researchgate.net/publication/321396151_Manufacturing_Techniques_of_ Perovskite_Solar_Cells.

 

Bharam, Vishal. "Advantages and Challenges of Silicon in the Photovoltaic Cells." https://commons.trincoll.edu/vbharam/files/2013/03/Final-Paper-Vishal-Bharam-_Silicon-in-Photovoltaic-Cell.pdf.

 

Marsh, Jacob. "Perovskite Solar Cells May Be the Future of Solar Panels: EnergySage." Solar News. July 15, 2020. Accessed October 2020. https://news.energysage.com/perovskite-solar-cells/#:~:text=Based on lab calculations, scientists, lab efficiencies above 20 percent.

 

Yan, Baojie, Guozhen Yue, Laura Sivec, Jeffrey Yang, Subhendu Guha, and Chun-Sheng Jiang. "Innovative Dual Function Nc-SiOx:H Layer Leading to a 16% Efficient Multi-junction Thin-film Silicon Solar Cell." AIP Publishing. September 12, 2011. Accessed October, 2020. https://aip.scitation.org/doi/abs/10.1063/1.3638068.

 

McMeekin, David Pattrick, Suhas Mahesh, Nakita K. Noel, and Matthew T. Klug. Solution-Processed All-Perovskite Multi-Junction Solar Cells. https://www.researchgate.net/publication/330489971_Solution-Processed_All-Perovskite_Multi-Junction_Solar_Cells.