Materials in the Active Layer in Organic Solar Cells

When a polymer is designed for the active layer, there are 3 factors to consider.  It has to be highly stable, it has to provide high efficiency, and it should be produced cheaply on a large scale (by processing in solution with cheap starting materials).  A P3HT PCBM mixture provides only 3-4% efficiency, but it's easy to process, so it's the choice for this description.  The active layer is made up of a donor polymer, or a donor small molecule, and an acceptor material.

Acceptor materials accept electrons from the donor materials (which are transported to the Cathode) and are usually fullerenes (see diagram above).  The use of fullerenes started with the C60 (see diagram below), which was not very soluble in common organic solvents, so it was evaporated onto the active layer to form bilayers, not a BHJ.  Hence, the PCBM was developed to enable BHJ to be formed with the donor material.

The C70 fullerene can be used to prepare the derivative of PCBM.  The derivative is more expensive, but its band gap (acceptor LUMO level) can be tuned.  Another example is the Bis-adduct (ICBA), which has a higher LUMO level (see diagrams below).

Polymers can also be used as acceptor materials.  This enables absorption in both donor and acceptor materials.  The energy level of polymers is easier to tuned, and there is easier levelling of the HOMO and LUMO levels of both polymers.  Both polymers are also formed with more solubility than PCBM, so there is easier control of solution viscosity.  However, these polymer blends lead to lower efficiencies because the mixability of polymers is poor.

The donor material is the polymer P3HT.  A polymer is formed from a combination of monomers (see diagram above), which are repeated units.  For example, thiophene is combined to form polythiophene.  Side chains (indicated by R in the diagram above) are also included to make the polymer soluble (see diagram below for the side chain of P3HT).  This is required for polymer solar cell production by printing and coating.  The side chains can be either alkyl or ester chains, which can be cleaved off to create more stable polymers.

The most important property of conducting polymers is that they should be conjugating (see diagram below for the difference between an unconjugated polyalkane and a conjugated polyacetylene).  Conjugation refers to the alternating of single and double bonds along a chain of carbon atoms.  The Pi electrons of the double bonds can diffuse (be delocalised) over the entire polymer molecule.  Upon the absorption of light, these Pi electrons are the ones that are excited.

Upon excitation, when electrons are shifted from the HOMO to the LUMO level, a pi to pi* transfer has occurred.  The conjugation length also affects the absorption.  If the conjugation length is shorter, the absorption will blue shift because there is less overlap of pi orbitals.  Hence, the different conjugation lengths will determine the visible polymer colours, which are the complementary of the wavelengths absorbed.  A longer polymer backbone/chain will also have higher absorption.

Lastly, the absorption spectra of a polymer film can be different from that of being in solution, because of the ordering of the molecules.  In the P3HT example, it is ordered in a lamella structure, which results in an absorption spectra different from the spectra in solution.



Reference:
Materials in the Active Layer, https://www.coursera.org/learn/solar-cell/lecture/omqzo/materials-in-the-active-layer

Layers in the Organic/Polymer Solar Cell

There are many types of organic/polymer solar cells, but basically, there are 2 main types: normal geometry devices, and inverted geometry devices.  For normal geometry devices, holes are extracted from the front electrode (anode) where light enters, while electrons are extracted from the back electrode (see diagram above).  This extraction is reversed for inverted geometry devices.  The naming convention is because the inverted geometry was invented after the normal geometry.  The inverted geometry allows the use of other electrode materials.

The diagram above names the layout of a polymer solar cell in sequence from top to bottom.  The main materials for substrates are glass (stiff material) and plastic (flexible material).  Glass is often used in research due to its suitability with techniques such as spin coating and evaporation.  It is very heat stable, and a can be a very good barrier to oxygen and moisture.  However, the advantage of polymer solar cells is that they can be mass produced cheaply by simple printing and coating technologies.  Hence, plastic foil, mainly based on PET (polyethylene terephthalate), is also used in research.  Its barrier properties also had much improvements, resulting in very stable solar cells.

Due to the requirement of the front electrode to be transparent, ITO (indium doped tin oxide) is often used with a good hole conductor PEDOT:PSS.  However, indium is a scarce material and ITO is very brittle, so alternatives such as FTO are being researched into.  An alternative technique to ITO deposition is to use a very thin metal grid.  This has been described in High-Efficiency Concepts of c-Si Wafer Based Solar Cells.

Intermediate layers - HTL (hole transport layer) and ETL (electron transport layer) - are required to facilitate charge carriers transport to the electrodes.  HTL such as PEDOT:PSS, MoOx (molybdenum oxide) and V2O5 (vanadium(V) oxide), and ETL such as TiO2, ZnO, LiF (lithium fluoride) and CeCO3 (caesium carbonate), have been used.

The active layer, a BHJ (Bulk Heterojunction), is created quite randomly by mixing 2 components together in a solvent.  The components have a tendency to microphase separate once the solvent evaporates.  Much research is done into the generation and maintenance of better BHJ structures, such as by using heat treatment or exposure to solvent vapours to help phase separation.

Finally, there is the difficulty of creating a tandem solar cell, where efficiency is possibly doubled due to the addition of voltages from both individual solar cells.  The difficulty arises from the stacking of many layers of materials (see diagram above), the selection of polymer materials with approximately the same current production and with different absorption spectra, and the development of an intermediate layer that can form an ohmic contact to both individual cells to transport charge carriers between them.

Reference:
Layers in the Solar Cell, https://www.coursera.org/learn/solar-cell/lecture/smOjn/layers-in-the-solar-cell

Continuation of Blog from http://maxloosolarenergy.blogspot.sg/

This Blog continues from http://maxloosolarenergy.blogspot.sg/

Perovskite Solar Cells (PSC) - a first history as of early 2014 - summarising the developments

The later few developments in the historical recounting were based on the different ways of structuring the PV device - either one or both optional layers (see diagram above).  They also include using 3 different mixed halide perovskites and 2 different HTM.  Similarities with OPV technology, especially the ease of fabrication, has increased the amount of research done in PSC.

There are other important developments.  The first was depositing PbI₂ from solution, with on site conversion to perovskite using vapour phase CH₃NH₃I reaction.  The second was the use of different organic cations: CH₃NH₃⁺, CH₃CH₂NH₃⁺ (ethylammonium), and NH₂CH=NH₂⁺ (formamidinium).  The latter 2 cations with larger ionic radii will increase the tolerance factor t, so crystal structures are more symmetrically cubic.  In addition to varying the organic cations, the inorganic cations and halide anions proportions can also be varied in mixed PSC, thereby adjusting their properties.  The most common adjustment is through halides.

The third important developments pertains to HTM and ETM (Electron Transport Media).  The FTO and compact TiO₂ combination has been replaced by another TCO (transparent conducting oxide) called ITO (indium tin oxide) with another thin ZnO nanoparticle layer (25nm).  This resulted in an efficiency of 15.7% for planar cells on glass.  It is also quite effective to use low temperature processing on the flexible polyethylene terephthalate.  Similarly, it is also quite effective to use inorganic HTM: CuI and CuSCN.  OPV that are quite effective are the use of (6,6)-phenyl C₆₁-butyric acid methyl ester as ETM, and the use of poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulphonate) as HTM.  The efficiencies are up to 12%.

These different technologies improve the potential uses, eg. flexible solar cells only need low processing temperatures < 150°C.  Compact TiO₂ will need 500°C to process.  The use of graphene nanoflakes in compact TiO₂ layer reduces the processing temperature.  A PSC structure with all optional layers and Al₂O₃ scaffolding, which is 0.6% graphene/TiO₂ by weight, achieved an efficiency of 15.6%.  Another PSC structure with scaffolding that combines TiO₂ nanoparticles with a titanium diisopropoxide bis(acetylacetonate) binder achieved an efficiency of 15.9%.

Another important development is the use of non-uniformity in solution deposition to produce semi-transparent PSC with neutral colour.  This produces small invisible islands instead of continuous perovskite, and should be less expensive than using lasers to fabricate semi-transparent a-Si solar cells.



Reference:

"The emergence of perovskite solar cells" by Martin A. Green, Anita Ho-Baillie and Henry J. Snaith, published online 27 June 2014, https://www.researchgate.net/publication/280388277

Organic PV Technology 3

Continuing from Organic PV Technology 2:

DSSC is a different kind of organic solar cell.  It is a photoelectrochemical system consisting of TiO₂ nanoparticles, dye particles/sensitiser, an electrolyte, and a platinum contact.  The photoactive dye sensitiser is the electron donor, while the TiO₂ nanoparticles are the electron acceptors.  They are mixed together to form an organic bulk heterojunction solar cell.  The dye photosensitiser is ruthenium polypyridine.

When a photon is absorbed by the dye, an electron is excited from its ground state S (the HOMO) to an excited state S* (the LUMO).  The S* state is at a higher energy level compared with the energy level of the conduction band of TiO₂.  This causes the light excited electrons to be injected into the TiO₂ nanoparticles.  The dye molecules (see diagram above) will remain positively charged, while the electrons in the TiO₂ will diffuse to the TCO based back contact, and across the external electric circuit to the other contact/electrode (the counter electrode).

There is an electrolyte between the counter electrode and dye.  The usual electrolyte contains iodine, where the positively charged oxidised dye molecule is neutralised by a negatively charged iodide.  3 negatively charged iodides neutralise 2 dye molecules to create 1 negatively charged triiodide.  This triiodide will move to the counter electrode and be reduced by 2 electrons into 3 negatively charged iodides.

A DSSC requires an expensive platinum (Pt) back contact to catalyse the reactions.  Hence, it depends on the HOMO and LUMO level of the dye, the Fermi level of the TiO₂, and the redox potential of the iodide and triiodide reactions.  The best efficiency of DSSC is 14.1%.  DSSCs are relatively low cost, but the electrolyte may freeze at low temperatures, stopping power production and possibly damaging the DSSC.  Higher temperatures will expand the electrolyte significantly, so encapsulation of DSSC is more difficult.

To improve the DSSC, materials cheaper than Pt must be discovered.  More stable and resistive electrolyte materials must be developed.  Finally, dyes with improved spectral and band gap utilisation must be developed.



Reference:
5.5 Organic PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=jCtgMm55nBA

Organic PV Technology 2

Continuing from Organic PV Technology 1:

Organic materials are different from inorganic semiconductors when it comes to light excitation.  In inorganic semiconductors, excitation will move an electron from the valence to conduction band, leaving behind a hole, and this electron-hole pair is weakly bound and can easily separate and diffuse away from each other.  In organic materials, excitation creates excitons, which are electron-hole pairs that are excited but still bound together due to mutual coulombic force between the electron and hole.  The excitons can move around through diffusion, but their diffusion lengths are short (about 10nm) because they recombine easily back to the ground state in a few ns.

To build an organic solar cell, a heterojunction based on 2 types of intrinsic materials can be used, where one type is an electron donor, and the other is an electron acceptor.  The materials used can be either semiconductor or conjugated compounds.  As an example, when we join an electron donor conjugated polymer to an electron acceptor conjugated polymer, the HOMO and LUMO levels can be seen in the diagram above.  The difference between the HOMO and LUMO leads to the creation of an electrostatic force between the 2 layers.  The excitons will hence diffuse to the interface between the 2 materials.  When the materials are chosen to enable the difference between the HOMO and LUMO to be sufficiently large, the local electric fields may be strong enough to break up the excitons.  Hence, electrons are injected into the electron acceptor, while the holes remain in the electron donor.

Due to the short diffusion lengths of excitons and the required absorption length of at least 100nm, the electron donor and acceptor materials are mixed together to form bulk heterojunction photovoltaic devices (see diagram above).  The short diffusion lengths can be reached in the bulk heterojunction blend.  Hence, a large proportion of the excitons can reach an interface to separate into electrons and holes.  The electrons will move through the acceptor material to the metal back electrode, while the holes will move through the donor material and collect at a TCO electrode like ITO.

The best efficiency reached by tandem organic solar cells is 12%.  The stability of these cells is unknown, but the production cost is low.  These cells can also be chemically engineered to have a wide variance in band gaps, and can be integrated into flexible substrates.  However, they have low efficiencies, stability and strength as compared with inorganic PV cells.  There is also a need for more expensive encapsulation materials to stabilise the organic PV.



Reference:

5.5 Organic PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=jCtgMm55nBA

Organic PV Technology 1

Organic solar cells (OPV) include polymer cells and dye-sensitised solar cells (DSSC).  The materials used can be considered large conjugated systems, with compounds based on carbon, including P3HT, phthalocyanine, PCBM, and ruthenium dye N3.  A conjugated system is a system that has carbon atoms in a chain with alternating single or double bonds, and every atom in the chain contains a p-orbital that can be delocalised.  Delocalisation is the merging of all individual valence electrons of p-orbitals in the chain in a shared space, such that all electrons belong to the chain of atoms.  The compounds may be cyclic, acyclic, linear or mixed conjugated.  An example is the benzene ring (see diagram below).

Another example is the ethene molecule (see diagram below) consisting of 2 atoms.  It has 3 sp² hybrid bonds with bond angles of 120° per atom, and an electron in the p_z-orbital.  Both electrons in both p_z-orbitals will form a pi bond, making a molecular pi orbital.  The pi bond will consist of bonding and/or anti-bonding states.  Hence, conjugated molecules have similar properties to semiconductor materials.

Most electrons at room temperature will be in the bonding state, also known as the HOMO (highest occupied molecular orbital).  The anti-bonding state is known as the LUMO (lowest unoccupied molecular orbital).  Since conjugated molecules are quite long, the HOMO and LUMO will broaden and become similar to the valence and conduction band respectively.

To distinguish between p- and n-types of an organic material, the vacuum level must be considered.  The vacuum level is the energy of a free stationary electron that is not in any material, which means it's in a vacuum.  It is a reference energy level to align energy levels between different materials.  Ionisation energy is the energy required to excite an electron from the HOMO (valence band) to the vacuum level, and so would be for the formation of positive ions.  Electron affinity is the energy released when an electron moves from the vacuum level to the LUMO (conduction band), and so would be for negative ions.

When an organic material has low ionisation potential, less ionisation energy is required to excite an electron.  Hence, this type of material can be electron donors.  When an organic material has high electron affinity, it can attract additional electrons more easily, which means it can be an electron acceptor.



Reference:

5.5 Organic PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=jCtgMm55nBA

BONDING IN ETHENE, http://www.chemguide.co.uk/basicorg/bonding/ethene.html

ELECTRON AFFINITY, http://www.chemguide.co.uk/atoms/properties/eas.html