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