P3HT PCBM – In Quest of Low Cost Sources of Renewable Energy
P3HT PCBM is the most researched polymer blend materials in the world. The polymer solar cells (PSCs) have been in the spotlight of the photovoltaic community for the last twenty years. And there is a reason for it.
A growing concern about global warming and also the fact that we are running out of the fossil fuels reserves have led to an urgent need for low cost sources of renewable energy.
Organic photovoltaic cells (OPVs) are one of the possible solutions, because they are offering several promising advantages over the conventional semiconductors.
In fact poly (3-hexythiophene) (P3HT) and 1-(3-methoxycarbonyl) propyl-1-phenyl [6, 6] C 61 (PCBM) plays an important role as it increases the energy efficiency.
Do you want to know how does it work? Are you interested what are the benefits and drawbacks as well? This article will provide you with all important information you need to know, in order to fully understand the purpose and potential of P3HT PCBM.
P3HT PCBM Solar Cell Efficiency – Why Does It Matter So Much?
We are living in a world that is in a desperate need for a clean energy. This is actually one of the most emerging problems humanity will face in the 21st century.
It is clear that the development of the solar cells is extremely important.
Solar cells or Photovoltaic cells convert the sun radiation to electricity through the photovoltaic effect.
The sun is an unlimited source of clean energy, but the challenge is to find a way how to efficiently utilize it.
History of Solar Cells
- The idea of photovoltaic effect was discovered already in 19th century
- The first modern solar cell (Si p-n junction) was invented at bell lab in 1954
- In 1970 the team around Zhores Alferov in USSR made the first highly effective GaAs heterostructure cell
- The efficiency of 10 % was reached in 1980 at the University of Delaware. They used Cu2S/CdS and developed the first thin film cells
- Another milestone in the solar cell development came in 1991. This is the year when was the first dye-sensitized cell made
- Just three years later in 1994, they reached the efficiency of 30 % by using GaInP/GaAs
- The barrier of 40 % was exceeded in 2006
There is no doubt these efforts have brought satisfying results. The main aim is to increase the sunlight concentration, carrier collection and also cell stability.
Therefore there have been done numerous material researches and what is more important the interest have gone beyond inorganic chemistry.
Although the organic polymers show low efficiency, they posses unique set of properties that are worth thorough investigation.
P3HT PCBM is the most effective fullerene derivative and that is why it is one of the most promising organic solar cell materials.
What is PCBM P3HT?
Let’s begin with a brief introduction of the two main components.
PCBM
This abbreviation stands for the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester.
To be more precise it is a fullerene derivative of the C₆₀ buckyball. It was synthesized for the first time in 1990s.
Thanks to its unique structure, it works as an electron acceptor in many organic cells and it is also used in plastic solar cells and electronics in conjunction with electron donor materials (polymers). In this particular case it is P3HT.
P3HT
Poly(3-hexylthiophene) is a kind of conducting polymer that belongs to the Polythiopene family. It is mainly used in organic electronics, because of its structure (regular end-to-end arrangements of side chains).
The photovoltaic effect in the blend is caused by the excitation of the π-orbit electron in P3HT.
Why You Should Be More Interested in PCBM Solar Cells?
The main reason is definitely the fact, that it could be more effective and cheaper alternative to normal silicon solar cells.
How Do Silicon Solar Cells Operate?
The process has four major steps.
- During the first step light strikes the solar cells. They absorb the incident photons. The efficiency depends on the macroscopic properties of the surface
- What happens next is the production of the exciton. Basically it means that the electrons are removed from the atoms of the semiconductor material
- The third step is the separation of the pairs. This action is determined by the distribution of the charge inside the cells
- And in the end an electric current takes place. The generated charge is collected at electrodes
Silicon solar cells pretty much dominate the market since the very beginning. The silicon as a material is non-toxic and there is plenty of it available in the Earth’s crust. However, comparing to P3HT PCBM it is less efficient and more expensive. So what is the issue here?
What Are the Benefits and Drawbacks of P3HT and PCBM?
There are many researches in progress investigating the use of P3HT in active layer of solar cells.
This active layer is used to convert the solar energy into electrical energy. They consider not only the efficiency, but also the production costs.
The Main Benefits
- Cheaper productio
- Higher energy efficienc
- Lightweigh
- More accurate allocation of resource
- Better physical durability
The Drawbacks
The P3HT reacts with water vapor present in the air. This means that such solar cells would have to be changed after being decomposed. Due to these reactive properties they do not have long life span.
However, this is the only disadvantage of this polymer. The scientists currently research how to solve this problem.
Following chapters are dedicated to a detailed description of its physical and chemical properties.
What Is P3HT PCBM Refractive Index?
The refractive index or in other words the index of refraction of a material is a number (dimensionless) that describes how fast light goes through a material.
It is determined as: n = c / v where c is the speed of light in the vacuum and v is phase velocity of light in the medium.
The refractive index of P3HT PCBM is 2.1564. This means that the light is 2.1564 times as fast in vacuum as in this fullerene derivative.
Bulk Heterojunctions – P3HT PCBM Morphology
What are bulk heterojunctions? They have an absorption layer which consists of a blend of electron acceptor and donor materials. What is characteristic for it is that the domain sizes are in nanometers. Thanks to that the excitons which have short lifetime can reach an interface and separate due to the huge donor-acceptor interfacial area.
As you already know PCBM is an electron acceptor used in bulk heterojunction photovoltaic cells. P3HT is an electron donor material.
Thermal Annealing
This is a method commonly used to improve the material morphology. The aim is to enhance the cell’s efficiency.
In research they prepared solar cells based on P3HT PCBM bulk heterojunction. Then they were exposed to the post annealing at different temperatures (specifically at 100°C, 120°C, 140°C, 160°C and 180°C).
P3HT PCBM AFM, SEM and Optical Images Disclosure
What happens to P3HT PCBM structure after the post annealing process? If you look at these images you could notice that this post deposition heat treatment has provoked considerable phase separation between P3TH and PCBM.
This has resulted into a growth of PCBM clusters on the films surface.
What Role Plays P3HT PCBM Phase Separation in Solar Cell Efficiency?
As it was mentioned above, experiments have showed that after thermal annealing P3HT PCBM blends have a better organized structure. Therefore, morphology and also other approaches have been tried in order to achieve higher solar cell’s efficiency.
In fact the ideal outcome would be the vertical separation of conjugated polymer and fullerenes.
It was discovered that the organic solvents with high boiling points can enhance the vertical phase separation of polymer mixture.
Exciton Generation – P3HT PCBM Energy Level
What happens with the photon when it incidents certain material? There are two options, either it is scattered or absorbed. When absorbed the photon should have a minimum energy. In this case an electron would be excited to a higher energy state.
The exciton is an electron-hole pair. We can take a semiconductor as an example. When the photon incidents it creates certain amount of energy. An electron in the valence band would be excited to the conducting band, which means it leaves the hole in the valence band.
It is clear that the efficient solar cell should have a wide absorb spectrum. This means it is desirable to create as many pairs as possible.
Extra energy is wasted in heat. The efficiency would be also increased, if the energy waste in the form of heat was minimized.
Other Factors that Influence the Solar Cells Efficiency – P3HT PCBM Electron Mobility
It should be emphasized that the conjugated fullerenes and polymers have different electrical properties.
The fullerene is n-type conductor where the negative electrons are the main charge carriers. The polymer is a p-type conductor. In this case the major charge carriers are the positive holes.
Interesting Fact
The carrier mobility of PCBM in polymer blends depends on the type of polymer.
For example, in annealed P3HT PCBM is the electron mobility higher then in PTB7 PCBM.
P3HT PCBM Solution – Very Promising Organic Material in Deed
There is absolutely no doubt about it. Organic solar cells have a huge potential.
There are several factors that could be improved and thus achieve efficient solar cells.
Now you already know what the main benefits are and what the only real drawback of this material is.
You have also learned more elaborate information about its structure, electron mobility and energy level.
Last but not least you are aware of the importance of the vertical phase separation.
Are organic solar cells a solution of the energy crisis? Can you also recognize its potential or you think it is still just a speculation? Please share your opinion with us.