Organic Solar cells: Construction and operation in Photo-Voltaic systems

Abdullah Rehan Electrical and Computer Engineering Department,

University of Victoria,

Victoria, BC

arehan@uvic.ca

Abstract- The world is currently dependant on an energy infrastructure that heavily relies on fossil

fuels which have high chances of getting depleted. Such energy resources are major contributors of

green house gases and carbon dioxide. If current practices of energy generation are continued,

excessive Carbon dioxide emissions will degrade the environment. Carbon emission rates need to be

maintained at present levels by implementing carbon neutral energy resources immediately in a cost

efficient manner. Optimized energy systems with minimal production, transmission and maintenance

costs are required which provide constant energy with minimal losses. This paper will give a technical

review of how solar energy utilized by photovoltaic systems and solar fuel cells in addition to biomass,

wind, and other types of renewable energy resources can be relied on for meeting clean energy

requirements.

Keywords- Renewable energy, flexible solar cells, organic solar cells, Photo-Voltaic systems, Construction

1. Introduction to Photovoltaic Systems

Solar cells are an essential part of the Photo-Voltaic system. The system’s operation rests on the fact of

the direct conversion that takes place between electromagnetic radiation that comes directly from the

sun which is then turned into electricity that can be used all following the principle of the photovoltaic

effect. The radiation that falls on the OSC surface largely constitutes of visible light which falls in the

wavelength range of 400 nanometers to 750 nanometers. The photovoltaic effect is known to be

following the basic principle of energy conversion from photons that are incident into electrical

potential energy. This newly converted energy is then transferred to charge carriers inside a

semiconductor material. This process allows the charge carriers to transfer between various voltage

bands inside the material. This process causes accumulation of voltage between two electrodes and this

is the output voltage of the solar cell recorded.

A PV system consists of a solar module, voltage converter/charge controller, a storage unit and an

inverter that converts AC into DC. The output might be connected directly to a load or to a power grid. A

solar panel consists of solar cells connected in series and parallel. Typically, the cells in one panel should

match well otherwise a big difference reduces the overall efficiency and even destroys the weakest cell.

Group cells are sometimes connected in parallel so that reverse current does not become an issue

because of shadowing of a high number of cells. A large number of cells can be used, provided it is a cost

plausible option. Bypass diodes can be added to the circuit if the probability of cells breaking down is

high and the additional costs are not very expensive. Optical concentrators can be incorporated so that

the cells get maximum light raising the output power and efficiency. This is an option as long as the

concentrator is cheaper than the cell itself since using the concentrators eliminates the need of rare

materials for solar cell construction.

A solar generator will then finally have PV modules connected in series and parallel. Each module

should be regulated actively and connected in series. Conversion of input power to output power can be

done by choosing linear regulators, buck, buck-boost, boost or fly-back converters. Each converter has a

different power input to output ratio and efficiency so the choice is application sensitive. The storage of

this power can be done by using big loads of lead acid accumulators or by storing the power directly into

a grid where it can be shared with other users immediately after generation.

2. Organic Photovoltaics:

Organic Photovoltaic (OPV) devices are responsible for the conversion of solar energy into electrical

energy. An Organic Photovoltaic device mainly constitutes of either a single or multiple photoactive

materials that are sandwiched between two electrodes. Figure 1 shows what a particular bi-layer

organic photovoltaic device looks like. In this bi-layer OPV cell, the process starts by absorbing sunlight

in the photoactive layers which contain donors and acceptors organic materials that are semiconducting

so they are able to generate photocurrents.

Figure 1: Structure of a bi-layer organic photovoltaic device [2]

The donor material (D) has the ability to donate electrons and is responsible for transporting holes

whereas the acceptor material (A) has the ability to adopt electrons and is largely responsible for

transportation of electrons. As seen in figure 2, the photoactive layers yield photons from the incident

sunlight in order to form excitons. Excitons have electrons that get excited from the valence band to the

conduction band and this process can also be called (Light Absorption). Because of the presence of a

concentration gradient, these particular excitons are able to diffuse to the donor/acceptor interface and

the process is called exciton diffusion. This process leads to the separation into free holes that can also

be called positive charge carriers and electrons which can also be called negative charge carriers. This

process is widely known as Charge Separation. The photovoltaic is a result of holes and electrons moving

to their corresponding electrodes depending on which phase they follow; the donor or acceptor phase

in the process of Charge Extraction. [2]

Figure 2: Functional mechanism of a bi-layer organic photovoltaic (D = donor, A = acceptor) [2]

The main advantage of the technology of Organic Photo-Voltaic over other inorganic technologies is its

ability to be used in a large area and capacity, also allowing flexible solar modules to be constructed

which cater to the particular need of roll-to-roll (R2R) production. Since OPV solar cells are easier to be

manufactured, their cost is effectively lesser than the silicon based materials solar cells. In order to have

a performance that is good enough when compared to silicon solar cells, these organic solar cells need

both the donor and acceptor materials to have good extinction coefficients possessing high stabilities

and excellent film morphologies. Because the donor has an important role to play in the absorption of

solar photon flux, the donor materials need to have a wide optical absorption so that it matches the

solar spectrum. The ideal donor/acceptor should also possess large hole/electron mobility so that the

charge transport is maximum. The major step up of OPV device performance has been consummate by

introducing a variety of OPV architectures, such as bulk-hetrojunction (BHJ), inverted device structures,

and utilizing low band gap conjugated polymers and innovative organic small molecules as donor

materials. [2]

3. Construction of Organic Solar Cells and basic working principle

In organic devices, a photovoltaic current can be produced even when there is a symmetrical device

which is basically a device that only contains a single photoactive material and the electrodes that it

contains are constructed out of the same material at both top and bottom. In this device, the excitons

are required to remain intact/contracted and not relaxed for a period that is long enough to get to an

electrode and undergo dissociation. Because the electrons and holes are very firmly bound, only the

excitons which can reach an electrode and undergo dissociation are able to cause a charge flow. This

shows the advantage of having a donor–acceptor hetero-junction in the active layer of an organic

device. Figure 3 shows the band structure of a device which only constitutes of a single material in the

active layer, whereas Figure 4 shows the band structure of a device which has a donor–acceptor blend.

The figures basically show how exciton dissociation is able to produce free charge carriers either at the

electrode or at the hetero-junction.

Moreover, it is known that the structure of the hetero-junction bi-layer or bulk is very essential towards

the characteristics of this device. In a bi-layer device, usually the junction between donor and acceptor

materials is in a planar manner. When a bulk hetero-junction device is being talked about, it is always

better to maximize and optimize the crossing point between phases. The organic solar cells that are

made of blends of conjugated polymers which are also donors and fullerenes which are also acceptors,

the conjugated polymer is responsible for absorbing the incident light. As a result of the absorption

process, an exciton is generated which can either relax back to the ground state or it can dissociate back

up into an electron and a hole. Because usually in organic cells, the lengths of exciton diffusion are small,

the dissociation process only occurs at the donor/acceptor interface ultimately having the ability to

control the structure of the active layer which is extremely important in the construction of efficient

devices. It is seen that an organic device is usually inverted as compared with a conventional device. The

organic device usually has a cathode that is transparent so that the light can enter through. When we

see the working principle of the conventional solar cells, light particularly enters from the anode side

when the anode itself consists of a grid of conductive material. The organic solar cell is known to be

consisting of at least four different layers excluding the substrate. The substrate may be glass or another

flexible, transparent polymer. The cathode comes on top of the substrate. Usually a popular choice is

Indium tin oxide (ITO) as a cathodic material because of its transparency and glass substrate that is

coated with ITO. It is also commercially available widely. A layer of the conductive polymer mixture

poly(3,4 ethylenedioxythiophene)/poly (styrenesulfonate) (PEDOT–PSS) can be employed between the

cathode and the active layer. The PEDOT–PSS layer provides various functions. It serves as a hole

transporter as well as an exciton blocker. It is also responsible for smoothening out the ITO surface and

sealing the active layer from oxygen. It also makes sure that the cathode material does not diffuse into

the active layer because that leads to unwanted trap sites. On top of the PEDOT–PSS, a single or

multiple active layers are employed. This particular layer also performs the function of light absorption,

exciton generation/dissociation, and charge carrier diffusion. The active layer that is present in a

heterojunction device is constructed from two materials which include a donor and an acceptor. Poly

(phenylene vinylene) derivatives and poly-(alkylthiophenes) are the common choices among donors;

fullerene and its derivatives are common choices among acceptors. The other materials that can

possibly be used include phthalocyanines (donors) and perylene bisimides (acceptors). The anode is

deposited on top of the active layer, and this anode is typically made of aluminum. Calcium, silver, or

gold can also be used. Moreover, a very thin layer of lithium fluoride (5–10 A ˚) is normally put in

between the active layer and the anode made of aluminum. The lithium fluoride usually does not react

chemically, but has the ability to serve as a protective layer between the metal and the organic material.

The structures of some commonly used materials are shown in Figure 6. [4]

Fig 3: Schematic diagram of the band structure of an organic solar cell having only one material in the active layer and different types of metal electrodes [4]

Fig 4: Schematic diagram of the band structure of a hetero-junction organic solar cell. The active layer in this type of device contains a donor and an acceptor. Also, here the electrodes are short-circuited, which equalizes their work functions. [4]

4. Organic Solar cells and their properties:

The output of PV system depends on the fill factor of a solar cell. The higher the fill factor, higher the

efficiency of the solar cell. These relationships can be seen from equations 1 and 2.

….(1)

….(2)

where is the fill factor, is the current at maximum point, is the voltage at maximum point,

is short circuit current, is open circuit voltage, is the efficiency and is the input power.

Some of the terms that are important in understanding the organic solar cells and their working

principle are the following:

“Air Mass (AM) – A measure of how much atmosphere sunlight must travel through to reach the

earth’s surface. This is denoted as AM(x), where x is the inverse of the cosine of the zenith angle

of the sun. A typical value for solar cell measurements is AM 1.5, which means that the sun is at

an angle of about 48. Air mass describes the spectrum of radiation, but not its intensity. For

solar cell purposes, the intensity is commonly fixed at 100 W/cm2.

Quantum Efficiency (QE) – The efficiency of a device as a function of the energy or wavelength

of the incident radiation. For a particular wavelength, it specifically relates the number of charge

carriers collected to the number of photons shining on the device. QE can be reported in two

ways: internal QE and external QE.

External Quantum Efficiency – This type of quantum efficiency includes losses by reflection and

transmission. External quantum efficiency is also called IPCE (Incident Photon to Current

Efficiency).

Internal Quantum Efficiency – This quantum efficiency factors out losses due to reflection and

transmission of photons such that it considers processes only involving absorbed photons. By

accounting for transmission and reflection processes, external QE can be transformed into

internal QE.” [4]

Figure 5: Graph of current versus voltage for photovoltaic devices. [3]

5. Dye Sensitized Organic Solar Cells:

In a dye-sensitized solar cell, the presence of an organic dye which is adsorbed at the surface of an

inorganic wide-band gap semiconductor is responsible for the absorption of light and injection of the

photo-excited electron into the conduction band of the semiconductor. In the recent years, research

projects on dye-sensitized solar cells have gained considerable importance and speed, when Grätzel and

his co-workers largely enhanced the interfacial area that lies between the organic donor and inorganic

acceptor by the use of nano-porous titanium dioxide (TiO2). Currently, ruthenium dye-sensitized nano-

crystalline TiO2 (nc-TiO2) solar cells are known to have an energy conversion efficiency of about 10%. In

the Grätzel cell, the ruthenium dye is responsible for light absorption and electron injection into the

TiO2 conduction band. A redox couple, which is contained in an organic solvent, is used for the

regeneration or reduction of photo-oxidized dye molecules. In these cells, the positive charge is

undergoes transportation due to the liquid electrolyte to a metal electrode, where I3 is responsible for

taking up an electron from the external circuit which is also called the counter electrode, whereas the

negative charges are injected in nc-TiO2 which are collected at the TCO electrode. [3]

Figure 6: The dye-sensitized solar cell. [3]

6. Double Layer Cells:

The double-layer configuration of the cells consists of photo-generated excitons present in the

photoactive material which have to reach the p-n interface so that charge transfer is allowed to occur,

before the excitation energy of the exciton is lost because of intrinsic radiative processes and non

radiative decay processes to be able to return to ground state. Because the length of the diffusion of the

exciton of the organic material is generally confined to about 5-10 nm, the absorption of light within a

very specific thin region around the interface is the only reason to the photovoltaic effect happening.

This also means that the performance of double-layer devices is now limited the reason being that such

thin layers are not able to absorb incident light. One way to improve the efficiency of the double-layer

cell is to improve it structurally. The structural organization of the organic material should be able to

extend the exciton diffusion length in turn creating a thicker photoactive interfacial area.

Figure 7: Molecular structures of copper phthalocyanine (CuPc) and a perylene diimide derivative. [3]

7. Conclusion

Organic solar cells (OSC) are a rapidly advancing technology that is beginning to show commercial

viability. Tailored materials, morphology control and optical optimization have led to promising

efficiencies and further improvements are expected soon. Flexible solar cells are gaining popularity in

the PV industry because of the product diversity and cost reduction. Moreover, they also have several

applications which the rigid counterparts do not offer (discussed shortly), different flexible substrates

are developed and optimized. Current thin-film solar cells using flexible substrates (metal foil, and

plastic) are under disadvantages in terms of cost reduction due to the complex manufacturing process

and lower conversion efficiency. This is the reason that crystalline semiconductor and conventional rigid

solar cells are still leading the market share but technologies such as the organic photovoltaic, fiber

based dye sensitized and nano-wire photovoltaic technologies are emerging which are flexible and

researchers propose that at some point in the future they can exceed the photovoltaic conversion

efficiencies of conventional solar cells and probably even replace them thereby changing the PV market

completely.

References:

[1] “OPTIMIZATION OF ORGANIC TANDEM SOLAR CELLS BASED ON SMALL MOLECULES”, Moritz Riede\ Christian Uhrich2, Ronny Timmreck\ Johannes Widmer1, David Wynands\ Marieta Levichkova\ Mauro Furno\ Gregor Schwartz2, Wolf Gnehr, Martin Pfeiffer2 and Karl Leo1, 2010 IEEE [2] “Organic Photovoltaics”, http://www.sigmaaldrich.com/materials-science/organic-electronics/opv tutorial.html, Sigma Aldrich [3] “Organic Solar Cells”, Tom J. Savenjie. [4] “Organic solar cells: An overview focusing on active layer morphology”, Travis L. Benanti & D. Venkataraman, 10th March 2005.