Synthesizing Polymers

Here's a summary of the design criteria for organic/polymer solar cells: a good absorption of sunlight using low band gap polymers that provides high current and efficiency, a good interaction between donor and acceptor units in the polymer that provides high mobility and efficient charge transfer, a good match between the energy levels of the donor and acceptor that provides efficient charge transfer and a higher Voc, a stable polymer backbone with side chains that interact to ensure cell morphology, thermal and photochemical stability and processability by solution, and finally, a good scalability of polymer synthesis.

There are 3 more important polymerisation methods for polymer synthesis of low band gap polymers: the Stille cross coupling polymerisation, the Suzuki cross coupling polymerisation, and the direct arylation polymerisation (DARP).  These 3 are palladium catalysed polymerisations.

The Stille cross coupling polymerisation (see diagram above) involves a halogen activated monomer which is usually the acceptor, coupled to an organo-tin activated monomer which is usually the donor.  The halogen is usually bromine.

The Suzuki cross coupling polymerisation (see diagram above) also involves a halogen (usually bromine) activated acceptor monomer, but its donor is a boronic ester activated monomer.  The boronic ester can vary in sizes in the ester chain, and can be cyclic too.

The Stille and Suzuki cross coupling polymerisations are most common.  The DARP method (see diagram above) is a recent development.  This method couples the halogen activated monomer with a non-activated monomer, just by using hydrogen.  This method is useful, because there is one less step in the synthesis of the polymer, and is more environmentally friendly, because the toxic tin activation group is not used.  However, it gives lower efficiencies, for example, for P3HT preparation.

Polymerisation requires activation of monomers.  The acceptor unit can be treated with NBS and THF (see diagrams above).  This reaction is simple and occurs usually in high yield.

For the donor unit activated with boronic ester (see diagram above), the boronic ester cannot react directly on the donor units, so a halogen activating group is required.  So the monomer is first treated with NBS, and then reacted with nBuLi and the boronic ester to create the diboronic ester monomer unit.  For stannylation (activation by organo-tin), the monomer is treated with nBuLi and reacted with trimethyl tin chloride.

To achieve scalable synthesis, cheap starting materials are required.  There should also be fewer steps in production with high yield.  To purify the monomers, flash chromatography, solid state chromatography, or recrystallisation can be used.  After purification, the monomers are mixed in dry toluene, degassed, and the catalyst is added (see diagram above).  These reactions happen quite quickly, with the colours of the polymer solution changing from reddish to purple and bluish as the length of the polymers increase.

When the polymer is ready, Soxhlet extraction is used to purify it.  Different solvents are used to ensure a separation of the polymer with its different molecular weights.  First, methanol is used to remove the catalyst and other low molecular weight molecules.  Then hexane is used to remove the oligomers.  Lastly, chloroform is used to extract the polymer.

After polymerisation, characterisation is done, where the polymer is analysed in depth.



Reference:
Synthesizing Polymers, https://www.coursera.org/learn/solar-cell/lecture/U9Ht2/synthesis

Important Materials for the Active Layer in Organic Solar Cells - Polymers

The polymers in organic solar cells should also be considered based on the same 3 factors: efficiency, stability and processing costs.  The polymer P3HT has a band gap of about 1.9eV, which means that it absorbs light up to about 600nm, which is about 17% of the photons (see diagram above).  If EQE is 1, there will be a current of about 11mA/cm².  A low band gap polymer with a band gap of 1.2eV can absorb 53% of the photons, which means a current of 34mA/cm² if EQE is 1.

This means that current can be increased by lowering the band gap (donor HOMO and acceptor LUMO level difference), but this will also mean reducing the V_oc of the solar cell (see equation above).  Polymers that absorb light lower than 2eV, which means at wavelengths longer than 600mm, are called low band gap polymers.

The highest reported efficiency for organic/polymer solar cells is about 10%.  The band gap of this polymer is about 0.7eV if PCBM is the acceptor.  This is obtained by ensuring that the solar cell has an efficient inter-chain charge transfer.  A better inter-chain charge transfer is achieved by different combinations of the electron deficient donor and the electron rich acceptor within the polymer backbone.

If the PITN polymer is used (see diagram above), the band gap will also be affected by the quinoid structure, which is very stable.  The band gap of this polymer is 1eV.  However, this polymer is not very soluble, so it has to be combined with other materials, such as thiophene, which will vary the band gap.  See the diagram below for an example of 2 polymers, where the donor unit is the same and the acceptor units are different.  The 2nd polymer is PSBTBT, with acceptor unit benzothiadiazole.  The donor unit is 4,4-di-2-ethylhexyl-dithieno[3,2-b:2',3'-d]silole.

See the diagram below for examples of donor and acceptor units.

With regards to stability, there is photochemical stability, usually accomplished by using different kinds of stable monomers.  For example, benzothiadiazole is more stable than other kinds of acceptor units.  The donor unit thiophene is also more stable than the fluorene monomer.

The side chains of the polymer can also affect its stability by improved morphological stability.  For example, the polymers can be crosslinked, creating a stable grid in the morphology.  Heat treatment can also be used to cleave off the side chains to obtain a stable polymer backbone.

Lastly, there's processing costs, which is lowered if the side chains enable polymer solubility in common solvents, such as chloroform or chlorobenzene.  It is also possible to use side chains to make the polymer soluble in water or ethanol.  This would benefit environmental considerations.  All these can also benefit large scale production, thereby lowering costs.



Reference:
Important Materials for the Active Layer in Organic Solar Cells - Polymers, https://www.coursera.org/learn/solar-cell/lecture/tVuEM/polymers

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

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

CIGS, CdTe PV Technology

Copper indium gallium selenide sulfide (CIGS) contains the rare element indium, while cadmium telluride (CdTe) contains the rare element tellurium.  Hence, both technologies will not be discussed here.

However, there is an upcoming technology CZTS (copper zinc tin sulfide) to replace the CIGS absorber layer.

An important point to note is the 2 types of solar cell configurations: superstrate and substrate.  A superstrate configuration is such that the processed substrate of the solar cell is also the front window where light enters from.  A substrate configuration is such that the processed substrate of the solar cell is also the back contact, or that the back contact is deposited on the substrate.  For a-Si, a p-i-n junction is a superstrate configuration, while a n-i-p junction is a substrate configuration.



Reference:

5.3 CIGS PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=sX_HB4-a0Tg

5.4 CdTe PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=v58VSHoxZhQ

Thin-Film Silicon PV Technology II

Consider a tandem (double junction) cell (see diagram above), the blue and green wavelengths of light are absorbed by the top cell, while red is absorbed in the bottom cell.  The hole generated in the a-Si top cell drifts to its p-layer and collects at the front contact, while the electron generated in the nc-Si bottom cell drifts to its n-layer and collects at the back contact.  The electron generated in the a-Si top cell drifts to its n-layer and recombines with the hole generated in the nc-Si bottom cell that drifts to its p-layer.  This happens because there is a recombination tunnel junction between the n-layer of a-Si and p-layer of nc-Si.  This is often thin and full of defects, which helps in the recombination.

The J-V curves of the tandem cell is shown in the diagram above on the right.  On the left are the a-Si and nc-Si curves shown separately.  The V_oc of the tandem cell is the sum of the V_oc of the top and bottom cells.  The J_sc of the tandem cell is, however, lower than the J_sc in either top or bottom cell.

The diagram above shows the various solar cells from single junction to triple junction, and their efficiencies.  However, for the last triple junction with an efficiency of 16.3%, the a-Si:H alloys suffer from light induced degradation (the Staebler-Wronski effect - SWE), and the stable efficiency reduces to below the 13.4% efficiency of the a/nc/nc triple junction.  SWE is caused by carrier recombination that generates some metastable defects in the absorber layers, and is one of the biggest challenges for thin film solar cells.  SWE can result in 10-15% reductions in efficiency of a-Si cells.

Finally, textured surfaces are used to scatter light and improve the absorption path length.  Intermediate reflector layers are also used.  This is done by separating the top and bottom cells of a tandem cell by a thin layer of low reflective index material (doped nanocrystalline silicon oxide), which causes more light to be reflected into the top cell.  Hence, the a-Si top cell can be thinner, and is less sensitive to SWE.  The intermediate layer also adds to the generation of the built in electric field over the absorber layers.



Reference:
5.2.2 Thin-Film Silicon PV Technology II, Delft University of Technology, https://www.youtube.com/watch?v=L-vIPE6uPck

Thin-Film Silicon PV Technology I

Thin film PV technologies are usually cheaper at the cost price per Wp than crystalline silicon technology.  Thin film silicon solar cells can also be deposited on glass and flexible substrates.  There are 2 extreme phases of the usual lattice structure: amorphous (a-Si), and nanocrystalline, also known as microcrystalline (nc-Si).  The first types of alloys are the hydrogenated ones: a-Si:H, and nc-Si:H.  This means that some of the silicon atoms in the lattice have valence electrons passivated by hydrogen, whose quantity is about 5-15%.  The 2nd types of alloys are mixed with Ge: a-Si_xGe_1-x:H, and nc-Si_xGe_1-x:H.  The 3rd types are silicon carbides: a-Si_xC_1-x:H.  The 4th types are silicon oxides: nc-Si_xO_1-x:H.  These alloys can all be doped.

a-Si has a disordered lattice compared with c-Si, where on atomic length scales (short range order), atoms are still tetrahedrally coordinated, but with slight distortions to bond angles and lengths.  On larger length scales (long range order), there are volume deficiencies consisting of vacancies, multivacancies, and nanosized voids in the lattice, and the disorder becomes clear.  The surfaces of the volume deficiencies are passivated with hydrogen for a-Si:H, but not all valence electrons can make bonds with neighbouring atoms.  Hence, these dangling bonds act as defects.

nc-Si is heterogeneous, consisting of small grains of crystalline lattice, of a few tens of nm, embedded into clumps of a-Si:H (see diagram above for the phases of thin film silicon).  Fully crystalline nc-Si is slightly less crystalline than polysilicon due to more cracks and pores.  The best nc-Si bulk for solar cells has a crystalline volume fraction of about 60%.  The band gap of nc-Si is close to c-Si, while that of a-Si is larger due to the lack of crystallinity (see diagram below).

The lack of crystallinity also cause a-Si to have a direct band gap because the electron moment is poorly defined.  Hence, the absorption of a-Si:H is better for wavelengths below its band gap (see diagram below).

SRH recombination in a-Si:H is very high, so the diffusion lengths of charge carriers is only 100-300nm.  Hence, charge carriers in a thick absorber layer cannot depend on diffusion to move, and has to have a p-i-n junction, where the i is the intrinsic a-Si absorber film which is between thin layers of p- (about 10nm) and n-doped (about 20nm) a-Si (see diagram below for the solar cell in superstrate configuration).

The doped layers of a-Si create a built-in electric field over the intrinsic layer, thereby creating a broad electronic band diagram (see diagram below).  Light excited charge carriers will drift in the absorber layer due to the electric field.  There are no majority or minority charge carriers in the intrinsic absorber layer, but holes and electrons are the majority charge carriers in the p- and n-layers respectively.  The p- and n-layers cannot be thick due to the dominant diffusion transport mechanism and the low diffusion lengths.

For p-i-n junctions, the sequence of deposition is p-layer, i-layer, and n-layer.  The TCO (transparent conductive oxide) layer can be fluorine-doped tin oxide (FTO), aluminium-doped zinc oxide, boron-doped zinc oxide, hydrogen-doped indium oxide, or tin-doped indium oxide (ITO).  The films can be processed with sputtering, low pressure CVD, MOCVD, or atmospheric pressure CVD.  P-layers with higher band gap materials such as boron-doped silicon carbide or oxide layers can be used to improve the absorption of blue light.  The metal back reflector can be either aluminium or silver, which is more expensive, but has higher back reflection.

The best stabilised efficiencies of a single junction a-Si cell is 10.1%.  The best V_oc are about 1.0eV, which is much less than the average band gap of 1.75eV, so the band gap utilisation is quite low due to SRH recombination and the broad intrinsic layer.

When nc-Si is used as intrinsic absorber layer, the spectral utilisation is better when compared with a-Si due to the lower band gap of nc-Si.  The usual thickness of nc-Si is 1-3 microns in order to utilise the spectral part from 700-950nm.  The best V_oc are about 600mV, and the record efficiency for single junction nc-Si is 10.7%.

To improve the spectral utilisation, micromorph tandems are used.  It refers to a double junction with a-Si as the top cell due to its high band gap, and nc-Si (μc-Si in the diagram above) as the bottom cell.



Reference:
5.2.1 Thin-Film Silicon PV Technology I, Delft University of Technology, https://www.youtube.com/watch?v=dFL65RIeyF4

III-V PV Technology 2

Continuing from III-V PV Technology 1:

Epitaxy is a deposition method to make high quality III-V materials, where for example, the GaAs crystalline lattice is grown one layer at a time on a germanium substrate, and adopts the substrate's crystal lattice structure.  Epitaxy, which is done in high vacuum conditions, prevents impurities, but dopants can be added.  It results in compact materials without vacancy defects.  Metal-organic chemical vapour deposition (MOCVD) is the usual epitaxy method for depositing III-V semiconductor layers.  It is an expensive process.

The main difficulty of making III-V materials is to ensure a match/similarity in their lattice constants.  When there is mismatch between the interfaces of materials, not every valence electron is able to bond with neighbouring atoms.  This variance in lattice constants can be seen in the phase diagram of semiconductors (see diagram above).  It can also be seen that the triple junction with GaInP, GaAs and Ge is quite lattice matched.  GaAs has the same lattice constant as Ge, but with a higher band gap, which reduces coordination defects.  The top material GaInP is a III-V alloy designed to have a band gap of 1.8eV and having a similar lattice constant to ensure full lattice matching.

It must be noted that III-V materials have sharp band gaps and high absorption coefficients (see diagram above).  For the triple junction example, the bottom cell generates more current than the other cells, which is more ineffective than quadruple or multi-junctions of 5-6 solar cells.  Less energy is wasted as heat for higher stacked multi-junctions, but the lattice matching may become mismatched.  If mismatched, the junctions are called metamorphic multi-junctions.  Buffer layers that have profiling in the lattice constant must be used.

In general, III-V PV technologies are costly, and so are used in space applications or concentrator technology, where sun light is focused on one solar cell.



Reference:
5.1 - III-V PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=NCRoe-S17e8

III-V PV Technology 1

III-V PV technology has the highest energy conversion efficiencies for 1 sun STC and concentrated sun conditions.  It is considered a "thin" technology in reference to c-Si wafers.  It is based on 3 valence electrons elements, such as aluminium, gallium and indium, and 5 valence electrons elements, such as phosphorous and arsenic.

Taking GaAs as an example, it has a slightly larger lattice constant and is significantly heavier than silicon.  It is also a direct band gap material, with a band gap transition of about 1.42eV.  The absorption coefficient of GaAs is much larger than silicon, and hence its thickness can be much thinner.  Its band gap is also relatively sharp, which means that the absorption coefficient increases rapidly above the minimum band gap energy.  SRH recombination can also be low due to the purity of the epitaxy processes for III-V film deposition.

The key importance of III-V technology is that these solar cells are multi-junctions, where more than one material with different band gaps are used (see diagram above).  Hence, the Shockley-Queisser limit is overcome.  This is because there is less excess energy dissipated as heat for the same quantity of photons.

An example of a III-V triple junction stack is shown in the diagram above.  The lowest solar cell is made of Germanium (Ge) with a band gap of 0.67eV.  The middle cell is made of GaAs with a band gap of about 1.4eV.  The top cell is made of GaInP (Gallium Indium Phosphide) with the highest band gap of 1.86eV.  The top cell is also the front window surface for the entire triple junction solar cell and absorbs the highest energy photons belonging to blue light.  Ge is meant to absorb the red light and near infrared light, which has the largest penetration depth.

The J-V curve of the triple junction is shown in the diagram above, where each component cell of the triple junction are considered to be in series with each other.  Hence, the lowest J_sc of the top cell is the output J_sc, and the V_oc is the sum of all 3 V_oc.

The band diagram will be more complicated (see diagram above) because the triple junction stack will have 5 p-n junctions/space charge regions, where 2 of the junctions are in reverse, which will act to reduce the total V_oc.  Reverse junctions can be prevented by including tunnel junctions, as seen in the diagram of the triple junction stack.  Tunnel junctions provide low electrical resistance, has high band gaps to avoid parasitic absorption losses, and is relatively thin.  Hence, the valence band at one side is in line with the conduction band on the other side of the tunnel junction, as seen in the diagram below.  The tunnel junction's depletion zone is so narrow that the slope of the conduction and valence bands become very steep.

Hence, electrons can tunnel through the tunnel junction barrier from the n-layer to the p-layer to recombine with the holes (see diagram below).  The low resistance of the tunnel junction means that there is low voltage loss.

Due to the fact that holes of the p-layer of the top cell recombine with electrons of the n-layer of the middle cell, and that holes of the p-layer of the middle cell recombine with electrons of the n-layer of the bottom cell, the recombination currents at the tunnel junctions will determine the J_sc of the triple junction, and ultimate determinant of J_sc will be the J_sc of the top cell, which is the lowest of the series connected triple junction stack.



Reference:
5.1 - III-V PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=NCRoe-S17e8

From Solar Cells to Solar Modules

A solar array consists of many solar modules, whereas a solar module consists of many solar cells/wafers.  Solar cells can be connected in series or in parallel.  In series, the voltages add up, while in parallel, the currents add up (see diagrams above).  Physically, a series connection involves connecting the bus bars of one solar cell to the back contact of another solar cell.

There is also an additional bypass diode connected to each solar cell as shown in the diagram above (where the 6th cell is shaded).  This is to overcome the situation where one or a few solar cells are shaded and hence produce less power.  This situation will result in less current and voltage from these shaded cells, and hence, the shaded cells will operate in reverse bias from the current driven by the non-shaded cells.  The effect is shown in the diagram below (for the situation shown in the diagram above).  Continuous shading will result in power dissipation through overheating of the shaded cells, which may damage the shaded cells.

Reference:
4.5 From Solar Cells to Solar Modules, Delft University of Technology, https://www.youtube.com/watch?v=VqfcnbyPDCA