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Photovoltaic Cells

Introduction


Photovoltaic Cell is the heart of a module what produce Electricity by Solar Energy conversion. All we are utilizing today as Photovoltaic systems are indebted to Albert Einstein paper in 1905 that based upon Max Planck Theory of Black Body radiation.

However, the first attempt defined Photovoltaic effect as a research, but today we are interested in more efficient Photovoltaic systems as a routine way of idea to industry and then business. The challenge is more significant if consider nowadays Environmental crisis what conduct us to stable-feasible Energy from Solar Energy to Electricity. Hence, more efficient but feasible photovoltaic cell as the most important part of a module will be bolded as new line of research.

There are three major directions in efficient improvement as:

Each of the items has a share in cell performance improvement.

Materials


There are different materials that could be used as semiconductors:


There are some factors that present pros and cons of the materials as:


Current market accepted Silicone as favorite because of cheap raw material. Silicone abundance in the Earth curst (figure 1) shows why it is cheap. However, some other material provides more efficient semiconductors, but public semiconductors applications comes out from Silicone base.

Toxicity could be another parameter if recycling refer to Environmental consideration is important; what seems to be an important parameter item in near future, and fortunately Silicone is Non-toxic material.


Structures

Second item is Semiconductor Crystal structure that categorized in three major directions:


To understand better structure and its role in Semiconductors, let us back to education. In the semiconductor’s realm in Physics, two matter factors are important:


Basically, semiconductor means not fully conductor, not fully insulator and in short depends on number of last outer shell electrons in the Element atom. Then, 4 Electrons at the last outer shell from 8 maximum possibility, provides Semiconductor base material. Therefore, column number 14 (figure 2) will be suitable choice and Silicone (Si) between them is our target.

Now, assume Semiconductor base (Si) as a football field. Now you need players and a Ball. Ball could be Electron and players from each side could be other materials (Boron and Phosphorus) what doped to Silicone. Now, there are Field (Si), Ball (Electron), Team + (Boron doping) and Team – (Phosphorus doping) and need to start by referee whistle what Solar irradiance plays its role. Better arrangement of team members and then better playing give more opportunity to winner. This is as Semiconductor structure what makes different character of Photovoltaic Cells.

Basically, 50% of physical properties comes out from matter variety and 50% rest depends on matter’s atom and molecules arrangement what is called as Structure. Some physical properties like toughness, Elasticity, conductivity, insulation and so on depends on Element atom and molecules structure what represents atoms and molecules arrangement in the matter.


More regular arrangement is called as Monocrystalline, less regularity represents Polycrystalline and random crystallization is called Amorphous.


There are two major Energy production aspects:


However more generating shows more Energy production capability but less Energy lost represents more efficient structure. In words, assume a cell able to produce 1 Wh pure Electricity but 0.5 Wh heat, then we will achieve 0.5 Wh Electricity at the end of process. Therefore, both more Energy generating, and Less Energy waste conduct us to more efficient system. Of course, refer to principles whenever a system is working, lost of Energy is inevitable but waste of Energy controlling provides more efficient system. As an example, assume a car what moves by converting Chemical Energy of fuel to wheel motion. No body can expect all Energy convert to wheel motion; because different parts as Engine must move and transfer force to wheel. Then, waste of Energy in conversion as heat in reasonable. Same concept in Photovoltaic cells with light Energy conversion to Electricity.

Another parameter is Crystal degradation. As a matter of fact, more perfect cell crystal provides more Electricity in a longer time but like every matter in the world cell crystal has long life and little by little during working, its perfect initial crystal structure demolish and then cell efficiency goes down.

Next time if you are dealing with Photovoltaic cells consider efficiency, heat lost and degradation beside voltage and current (IV curve).

Photovoltaic Cell Technologies

According to above, more efficient cell means:


More Power generating is achievable by:


Less Degradation is in access by better cell crystallization.

Each of the goals could be result of a specific technology and here we speak about the most famous in a practical simple way.

Silicone standard cells in two types Monocrystalline and Polycrystalline are base of routine Photovoltaic cells. Basically, Monocrystalline cells with almost 1% more efficiency versus Polycrystalline are the best type of standard cells but cheaper price of Polycrystalline made it favorite. Of course, generally Polycrystalline degradation is more than Monocrystalline too. Therefore, however initial investment by Polycrystalline is cheaper than Monocrystalline but Monocrystalline by less degradation and then longer life produce more Electricity for investors. Of course, different manufacturer provide different numbers in catalogues and exact judge must be case by case, but you can keep in mind this instruction as a general concept. Normally, known cells in the market are 6 inches (156 x156 mm) with 14% as minimum efficiency what improve by technologies. Then, different technologies try to manipulate standard Silicone cells and find better result but today as a rule of thumb 20% efficiency is well known.

Basically, there are a few main methods for manipulation:

Heterojunction Silicone cell and less heat generating (HJT, HIT)



More Irradiance Energy conservation and more light Energy conversion (Perk)


Smaller cell surface and less heat generating (Half cut cell, Shingled)




More conversion cell area as double side cell (Bifacial, HDT)



Summary

Exact speaking about the best technology is impossible; because efficiency is not the only item what defines better Photovoltaic system. Reviewing technical specification beside project conditions, initial investment, Area of the Array and then simulation what conduct us to feasibility study, show which technology is better for you.

AFSHIN BAKHTIARI

AE Solar Senior Engineer



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