FUEL CELLS

-PROTON EXCHANGE MEMBRANE FUEL CELL

--What are they?Proton exchange membrane fuel cells, also known aspolymer electrolyte membrane(PEM) fuel cells (PEMFC), are a type offuel cellbeing developed for transport applications as well as forstationary and portable fuel cell applications. Each individual fuel cell produces about 0.7V when operating in air. Then, in order to produce a useful voltage, the electrodes of many cells must be linked in series and we must ensure that reactant gases can still reach the electrodes and that the resistance of the electrodes has a minimal effect. As each cell produces only about 0.7V, even a small reduction in this is not good, so cells are not normally wired up this way.

- Reactions:- Anode:2H2 --> 4H+ + 4e- (fast)- Cathode:Direct pathway: O2 + 4H+ + 4e- --> 2H2O (kinetically slow)Indirect pathway: O2 + 2H+ + 2e- H2O2H2O2 + 2H+ + 2e- 2H2O- Overall reaction:2H2 + O2 --> 2H2O

- Relevant parts:

--Fuel: H2---Specific energy content: 2 kWh/kg

--Membrane (electrolyte): proton-conductive polymer membrane. Its purpose is to separate the anode and the cathode, to prevent mixing of the fuel and the oxidant and to provide a conductive pathway for protons. The membrane needs to have high ionic conductivity (and zero electronic conductivity) under cell operating conditions, long-term chemical and mechanical stability at elevated temperature in oxidizing and reducing environments, good mechanical strength, with resistance to swelling, low oxidant and low fuel cross-over, interfacial compatibility with catalyst layers and low cost. In a polymer membrane, the proton conductivity is strongly dependent on membrane structure and water content.Ex: Nafion, that is a stable material, has a selective ion permeability, is compatible with current fuel cell technology and has high proton conductivity under aqueous conditions. But, on the other hand, it has low conductivity at low water content, poor mechanical strength at high temperatures and is expensive. Nafion is characterized by presence of isolated spherical ionic clusters that are modified when water is absorbed in the membrane. When water is absorbed, the membranes hydrophilic domain size increases and the swelling induces a modification of the cluster structure which become spherical water pools. As more water is absorbed, the cluster size is connected to each other through the water passage. This way, water and hydrophilic solvents can penetrate the membrane through water channel and it can also provide the passage of protons (percolation). The Nafion membrane can be modified to have its characteristics improved: addition of water-retaining additives, non-conducting inorganic particles, proton conducting inorganic salts and heteropolyacids, substitution of water with a high boiling proton solvent (if water boiled at 200C, most of the problems related to PEMFC operation at temperatures greater than 100C would be resolved).For a H2/air fuel cell, it is needed a membrane with a high proton conductivity at 120C and that operates without water at elevated temperatures.---Water management: water is produced at the cathode as a result of the electrochemical reaction and it is dragged from the anode to cathode by protons moving through the electrolyte (by electroosmotic drag). But, the water generation and electroosmotic drag create a large concentration gradient, and the water diffuses back to the anode.---Gas permeation: the membrane should be impermeable to reactant gases (H2 and O2), but, as the membrane has a porous structure and water content, and H2 and O2 are soluble in water, some gases permeate through the membrane.

--Electrode: must be porous because reactant gases are fed from the back and they must reach the electrode/membrane interface and the catalyst layers (part of porous electrode or part of the membrane, depending on the manufacturing process). Example: Multilayer Electrode Assembly (MEA), that consists in multilayer assembly of the membrane sandwiched between two electrodes.

(assembly stack of proton conductive membrane + catalyst electrodes + gas diffusion layers)The MEA is sandwiched between collector/separator plates. The collector collects and conducts electrical current and the separator separate gases in the adjacent cells (in multicell configuration). In multicell configuration it connects (physically and electrically) the cathode of one cell with the anode of an adjacent cell (this is what is called bipolar plates), provides the pathways for the flow of reactant bases (flow fields) and gives structural rigidity to the cell.

--Gas diffusion layers (GDL): are conductive and porous sheet of material also known as carbon cloth/paper, in the PEMFCs, it is crucial for it to remain free of water in order to provide a pathway for gaseous fuel transport. The GDL is a porous material composed of a dense array of carbon fibres, which also provides an electrically conductive pathway for current collection. The GDL have some important roles: provide a pathway for reactant gases from the flow field to the catalyst layer (gases need to access the entire active area, not just those adjacent to the channels), provide pathway for product water from the catalyst layer to the flow field channels, electrically connect the catalyst layer to the bipolar plate, allowing electrons to complete the circuit, conduct heat generated in the electrochemical reactions in the catalyst layer to the bipolar plate, allowing heat removal and provide mechanical support to the MEA, preventing it from sagging into the flow field channels. To perform these roles well, the GDL must be sufficiently porous (to permit the flow of the gas and water), electrically and thermally conductive and, it must be sufficiently rigid to support MEA but must have some flexibility to maintain good electrical contacts. Generally, carbon fibre based materials are used. The material is usually made hydrophobic to enhance water removal because, in practice, flooding is one of the major causes of PEMFC performance degradation.

--Bipolar plates: conductive plates that act as an anode for one cell and as a cathode for the next cell. They can be made of metal, carbon or conductive composite polymer. It must distribute reactant gases over the surface of the anode, and O2/air over the cathode. Bipolar plates may also have to carry a cooling fluid and need to keep all these gases and fluids separate. So, to connect cells electrically in series, they must be electrically conductive, to separate gasses in adjacent cells, they must be impermeable to gases, they must have adequate strength to provide structural support for the stack but must be lightweight, they also have to be thermally conductive to conduct heat from active cells to cooling cells. Besides that, they must be corrosion resistant, cheap, suitable for mass production, the electrical contacts should be as large as possible, the plate should be thin to minimise resistance and the gas needs to flow easily across the plate.

--Catalysts:Anode: usually noble metal such as Pt powder (Pt tolerates only about 50 ppm of CO and a few ppm of sulfur compounds in fuel) or alloy catalysts that can be used to increase the catalysts Co-tolerance (that enter into the cell with the hydrogen gas and poisons the catalyst thats why hydrogen fuel needs to be as pure as possible). Example: PtRu (Ruthenium increases the affinity for adsorption of oxygen containing species in less positive potentials).Pt-(CO)ads + Ru-(OH)ads Ru + Pt + CO2 + 2H+ + 2e-Cathode: usually Ni powder, but Pt and PtNi alloys can also be used. The catalyst must be reversible, must have a high oxygen adsorption capacity, stability during the adsorption and reduction, stability in electrode medium, ability to decompose H2O2, good conductivity and must be cheap.Example: Pt. Advantages: high work function, ability to catalyse the reduction of oxygen, good resistance to corrosion and dissolution, high exchange current density. Disadvantages: slow oxygen reduction reactions due to the formation of OH species at 0.8V, expensive and scarce. The reactivity of Pt depends on its size and shape. When the size of the particles is reduced, the relative amount of surface atoms in edge and corner positions increases, these atoms are electrocatalytically more active than other surface atoms. Reactions:O2+2Pt Pt2O2Pt2O2 + H+ + e- Pt2-O2HPt2-O2H Pt-OH + Pt-OPt-OH + Pt-O + H+ + e- Pt-OH + Pt-OHPt-OH + Pt-OH + 2H+ + 2e- 2Pt + 2H2O

- How it works: Hydrogen fuel is processed at the anode where electrons are separated from protons on the surface of a platinum-based catalyst. The protons pass through the membrane to the cathode side of the cell while the electrons travel in an external circuit, generating the electrical output of the cell. On the cathode side, another metal electrode combines the protons and electrons with oxygen to produce water, which is expelled as the only waste product; oxygen can be provided in a purified form, or extracted at the electrode directly from the air.

- Pros: operate at relatively low temperatures (below 100C), sustained operation at high current density, low weight, compactness, etc.

- Cons: the fuel must be pure and the water management is complex (these could be overcome by using high temperature PEMFC, where the electrolyte used is not water-based, but a mineral acid-based system and the HT PEMFC can operate up to 200C), the membrane is expensive, so as the Pt catalyst used.

- Uses and applications: the PEMFC are currently the leading technology for light duty vehicles and materials handling vehicles, also for stationary and other applications.

(type of PEMFC) DIRECT METHANOL FUEL CELL (DMFCS)

-Fuel: CH3OH (liquid). It can be supplied to the cell in two ways: passive (reactant into cell by gravitational, capillary forces or hydraulically more simple and cheaper) or active (reactant into cell by special pumps more uniform supply).-Advantages: handling CH3OH (liquid) is safer than handling H2 (gas). It can be stored in cheap plastic containers and is an excellent carrier fuel that hydrogen can be extracted from to power fuel cells. The DMFCs are seen as a candidate to eventually replace lithium-ion battery because they can produce a high amount of energy in a small space over a long period of time.-Disadvantages: anode and cathode needs Pt catalysts (Pt tolerates only about 50 ppm of CO and a few ppm of sulfur compounds in fuel), gas humidification is required, the membrane is expensive, some methanol pass through the membrane without producing electricity (methanol crossover), and efficiency is low.-Specific energy content: 6 kWh/kg (higher than H2 but less than gasoline 10 kWh/kg)-Components: polymer membrane as electrolyte (usually Nafion), peripheral equipments needed are similar to PEMFCs (bipolar plate, MEA), CCM (catalyst coated membrane, it is a proton-conducting membrane plus two electrodes), GDL (gas diffusion layer, it can be carbon cloth or paper with carbon particle filter and Teflon), anode and cathode plate (it can be done with graphite, carbon composite or metal with machined or stamped flow field), gaskets and seals (seals around edge of structure). Both the anode and cathode can have Pt particles (same usage as for PEMFCs). But, as CH3OH oxidation produces CO, Pt catalyst is poisoned. This way, Ru or Au can be added to increase the catalytic activity towards the oxygen reduction reactions and to increase CO tolerance:H2O (Ru) OH* + H+ + e- OH* + CO CO2 + H+ + e-

-Reactions: Anode:CH3OH + H2O 6H+ + 6e- + CO2Slow oxidation, with considerable polarisation at the negative electrode, what generates lower working voltage of the fuel cell.Cathode:3/2O2 + 6H+ + 6e- 3H2OOverall reaction:CH3OH + 3/2O2 2H2O + CO2The reaction occurs in several stages:CH3OH COHads + 3HadsCOHads + 3OHads CO2 + 2H2OIonization of Hads and the anodic formation of OHads from water molecules are the steps producing current:Hads H+ + e-H2O OHads + H+ + e-As we can see, H2O is needed for the reaction, so pure CH3OH cannot be used. So, H2O has to be pumped in, what limits the power achievable. The concentration of CH3OH must be between 1M and 3M because higher concentrations cause diffusion to the cathode and lower concentrations limit the maximum attainable currents. The H2O is lost at the anode and every proton formed drags from 2 to 5 water molecules.

-Efficiency: ~40%

-Operating temperature: 60-120C

-Applications: due to the relatively low range of operating temperature, this fuel cell is attractive for tiny to mid-sized applications, like mobile plants, such as small fuel cells for portable devices (PCs, mobile phones, etc) where energy and power density are more important than efficiency.

-Difference from other fuel cells: the anode catalyst itself draws the H2 from the liquid CH3OH eliminating the need for a fuel reformer. Hydrogen fuel can be supplied in two ways - either directly as pure hydrogen gas or through a "fuel reformer" that converts hydrocarbon fuels such as methanol, natural gas, or gasoline into hydrogen-rich gas. CO2 is produced at the anode and not at the cathode.

(type of PEMFC) INDIRECT (OR REFORMED) METHANOL FUEL CELL (RMFCS)

The fuel CH3OH is reformed before being fed into the fuel cell.

-Advantages: higher efficiency, smaller fuel cell stacks and no water management.-Disadvantages: operate at high temperature (200C) and high pressure (25-50 bar) and needs heat management.

DIRECT ETHANOL FUEL CELL (DEFCS)

-Reactions:AnodeCH3CH2OH + 3H2O 12H+ + 12e- + 2CO2Cathode3O2 + 12H+ + 12e- 6H2OOverall reactionCH3CH2OH + 3O2 2CO2 + 6H2O-Advantages: lower toxicity, EtOH can be obtained by fermentation (already popular in Brazil where there are EtOH infrastructures).-Disadvantages: when EtOH is produced by fermentation, the crops for fuel production competes with the crops for food production (the, arise the questions: which one is better? How to conciliate them?), and it requires higher working temperature (200C).

PHOSPHORIC ACID FUEL CELL (PAFCS)

-Applications: hospitals and all commercial purposes.-Components: electrolyte is liquid phosphoric acid saturated in a SiC matrix, electrode is carbon paper and the catalyst is platinum.-Electrical efficiency: ~50%-Advantages: uses relatively low purity H2 as fuel and 85% of the steam can be used for cogeneration, it is stable, has low electrolyte volatility and its construction is simple.-Disadvantages: uses expensive Pt as catalyst, has large size and weight, has low power and current density, has aggressive electrolyte.-Reactions:Anode2H2 4H+ + 4e-CathodeO2 + 4H+ + 4e- 2H2O(occurs at a faster rate than in PEMFCs due to the higher operating temperature)Overall reaction:2H2 + O2 2H2OThe water generated (steam) is used for water heating.-Operating temperature: 150-200 C (this higher temperature imparts a slightly higher tolerance to impurities, and so, phosphoric acid cells can function with 1-2% CO and a few ppm of sulfur in the reactant streams.-Efficiency: up to 50% with potential to reach 70%

MOLTEN CARBONATE FUEL CELL (MCFCS)

A class of fuel cells that work at high temperature (>600 C).-Components: molten carbonate salt (Na, Li, K, Mg) mixture is used as its electrolyte (at the operating temperature of about 650 C, it is liquid and good ionic conductor). The electrolyte is suspended in a porous, insulating and chemically inert ceramic matrix called BETA (beta-alumina solid electrolyte).-Reactions:AnodeH2 + CO32- H2O + CO2 + 2e-Cathode1/2O2 + CO2 + 2e- CO32-Overall reactionH2 + 1/2O2 H2OAdditional H2 is produced from gas shift reactionH2O + CO H2 + CO2The anode process a reaction between H2 and carbonate ions (CO32-) from the electrolyte. The reaction produces H2O and CO2 while releasing electrons to the anode. The cathode process combines O2 and CO2 from the oxidant stream with electrons from the anode to produce CO32- ions which enter the electrolyte. The need for CO2 in the oxidant stream requires a system for collecting CO2 from the anode exhaust and mixing it with the cathode feed stream.-Advantages: possibility to use the reaction heat to generate additional electricity, high rate of electrode reactions, little electrode polarization, does not need to use Pt catalysts and Ni catalysts are cheaper, use of H2 with CO and other impurities, possibility to use CO, natural and bio-gas and petroleum products directly through internal conversion of these fuels to H2, do not require external reformer to convert energy dense fuel to H2, and the high temperature limits protect from CO poisoning.-Disadvantages: the high temperature limit the materials and safe uses of MCFCs, molten carbonates are highly corrosive and CO32- ions are used up in the reactions, making it necessary to inject carbon dioxide o compensate.-Efficiency: ~60% but can reach 85% when wasted heat is used.

SOLID OXIDE FUEL CELL (SOFCS)

-Components: solid oxide materials (or non-porous ceramic) are used as electrolyte. The electrolyte must possess a high ionic conductivity and no electrical conductivity in order to oxygen ions migrate through the electrolyte to the fuel side of the cell. It must be dense to prevent short circuiting of reacting gases through it and it should be as thin as possible to minimize resistive losses in the cell. And it must be chemically, thermally and structural stable across a wide temperature range. The anode must meet most of the same requirements as the cathode in terms of electrical conductivity, thermal expansion compatibility and porosity, and must function in a reducing atmosphere. So, metals are attractive candidate materials, such as Ni, that is abundant and affordable and can be combined with YSZ to avoid sinterization of Ni and provide structural support. The cathode must be porous in order to allow oxygen molecules to reach the electrode/electrolyte interface, and its conductivity must be all electronic (no ionic) because, this way, the electrons from the open circuit flow back through the cell via the cathode to reduce the oxygen molecules, forcing the oxygen ions through the electrolyte.Interconnect: used when fuel cells are used in combination in order to generate enough voltage and current. The interconnect functions as the electrical contact to the cathode while protecting it from the reducing atmosphere of the anode. It must have 100% electrical conductivity, no porosity (to avoid mixing of fuel and oxygen), thermal expansion compatibility, and must be inertness with respect to the other fuel cell components. It will be exposed simultaneously to the reducing environment of the anode and the oxidizing atmosphere of the cathode.-Operating temperature: 1000 C (so, there is no need for precious-metal catalyst, what reduces costs).-Reactions:AnodeH2 + O2- H2O + 2e-Cathode1/2O2 + 2e- O2-Overall reactionH2 + 1/2O2 H2O-Advantages: high efficiency, long-term stability, fuel flexibility, low emissions, relatively low cost (because it uses currently available fossil fuel, not H2), high temperatures allow SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces cost associated with adding a reformer to the system and SOFCs are not poisoned by CO, which can even be used as fuel.-Disadvantages: the fuel cell needs to run at high temperatures in order to achieve sufficiently high current densities and power output. This high temperature also places stringent durability requirements on materials, what increases their costs.-Applications: good to big power units (utility applications), but not to transportation and small portable applications.-Designs:Planar: the components are assembled as flat stacks, with air and fuel flowing through channels built into the cathode and anode.

Tubular: components are assembled as a hollow tube, with the cell constructed in layers around a tubular cathode. The air flows through the inside of the tube and fuel flows around the exterior.

ALKALINE FUEL CELL (AFCS)

The presence of OH- ions travelling across the electrolyte allow a circuit to be made and electrical energy to be extracted. The H2O formed at the anode migrates back to the cathode to regenerate OH-. The by-products are heat and H2O.-Advantages: AFCs are the cheapest fuel cells to manufacture, the catalyst that is required on the electrode can be any of a number of different materials that are relatively inexpensive (they are non-noble metal and metal oxides as NiO, CoO)-Disadvantages: they are sensitive to CO2 because it reacts with the electrolyte to form a carbonate (K2CO3) which can decrease the conductivity, because of this, AFCs operates using pure O2 or purified air.-Efficiency: 70%- Operating temperature: 60-220 C-Reactions:AnodeH2 + 2OH- 2H2O + 2e-CathodeO2 + 2H2O + 4e- 4OH-Overall reactionH2 + O2 2H2O- Components: KOH as electrolyte.

ENZYMATIC FUEL CELL

Power density up to 40 W/m2, specific power up to 6 kW/l-Disadvantages: no full scale implementation, costs (enzymes are very expensive), the enzymes must be stable to avoid inactivation and inhibition and they have low fuel versatility (enzymes are very specific).

MICROBIAL FUEL CELL (MFCS)Converts chemical energy, available in a bio-convertible substrate, directly into electricity. To achieve this, bacteria are used as catalyst to convert substrate into electrons. Bacteria can convert a huge variety of organic substrates into CO2, H2O and energy. They use this energy to grow and to maintain their metabolism. However, by using a MFC, part of this microbial energy in the form of electricity may be harvested.-Components: anode (substrates such as carbohydrates (glusose, sucrose, cellulose), volatile fatty acids (formate, acetate), alcohols (ethanol, methanol), amino acids, proteins, etc), cathode, proton or cation exchange membrane and electrical circuit.The bacteria live in the anode and convert a substrate such as glucose, acetate, etc, into CO2, protons and electrons. In aerobic conditions, bacteria use O2 or NO3- as a final electron acceptor to produce water. However, in the anode no O2 is present, so bacteria need to switch from their natural electron acceptor to an insoluble acceptor, such as the anode. Due to the ability of bacteria to transfer electrons to an insoluble electron acceptor, we can use a MFC to collect the electrons originating from the microbial metabolism. The electron transfer can occur via different components (membrane, soluble electron shuttles, etc). In anaerobic conditions, electricity can be generated indirectly (via biogas) and directly (without biogas).-Disadvantages: the microbial nature of the process is affected by electrochemical laws and principles which generally results in a lowering of the attainable voltage. The losses are to mass transfer, ohmic losses, activation losses, electrons quenching and competition of parallel metabolic processes. The biological activity can be inhibited by the products generated, by the medium pH, etc. Biofilm can be formed on the electrode and it is difficult to control. Good mediators are often toxic. The cathode efficiency is affected by H2O2 production. And, the membranes to transport protons are costly.

-Example: Sulfate Reducing Fuel Cell

PHOTOVOLTAIC CELLS (PV CELL)

- What is it?It is a device that converts solar energy directly to electricity. The photovoltaic effect is the phenomenon that certain materials produce electric current when they are exposed to light.The sunlight is a primary source of energy, so it is a renewable, free and limitless energy that can be powered for remote locations. Solar radiation received at the surface oscillates between 275 MW/m2 in the deserts of Middle East and 75 MW/m2 in the Artic.-Advantages of solar photovoltaic: converts sunlight directly to electricity (sunlight is the most abundant renewable resource), there is no moving parts and it has a long lifetime (> 20 years).-Disadvantages: sunlight is very spread out, it is irregular and unpredictable, the electricity is difficult to store and it is an expensive technology compared to other means of power generation.

-PV modules are solar cells electrically connected to each other and mounted in a frame. The panels will supply electricity at a certain voltage, usually a 12V system. So, multiple module can be wired together to form an array. The larger the area of a panel or array, the more electricity will be produced.-The installation of PV devices can be gried-tied solar or off-grid solar. In the grid-tied solar, the PVs arrays are directly connected to the utilities grid, in other words, the array acts like a generator and produces energy that is first used by the owner and the extra is sold to the operator. And off-grid solar is when a system is installed and is no longer attached to an electrical operator. The energy generated feeds a charge controller that monitors the coupled battery bank to maintain fully charged batteries.

-Evolution of PV technology:

-The first generation of PV devices are large-area, single layer p-n junction diode, which are capable of generating electrical energy from light source with the wavelengths of sunlight. These cells are typically made using a silicon wafer.-The second generation is based on the use of thin-film deposits of semiconductors. These devices are designed to be of high efficiency and to have multiple junction. The most used materials are Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (A-Si). The most competitive and cheaper among these is the CdTe technology.-In the third generation, photovoltaics dont rely on a traditional p-n junction to separate photogenerated charge carriers.

- Classification of PV technology:

-- Silicon based PVs:

Nowadays, the most important and common semiconductor material for solar cells production is silicon. It has some important advantages: it is abundant, not poisonous, it is environment friendly, its waste does not represent any problems, it can be easily melted, handled, and it is fairly easy formed into monocrystalline form. Furthermore, its electrical properties with endurance of 125 C allow the use of Si-based semiconductor devices even in the most harsh environment and applications. Pure silicon is produced from sand (SiO2) by reduction in specially design furnaces at 1800 C. The produced material contains 98-99% of pure Si. To reduce SiO2, carbon electrodes are used: SiO2 + C Si + CO2. But, processing SiO2 to produce Si is a very high energy process, and it takes over two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains. Another disadvantage is that 1.5 tons of CO2 is emitted for each ton of Si produced.The cells have single layers of p-n diode, so there is just one excitation per photon. The lifetime of Si PVs is more than 30 years and the energy payback period is in 2-8 years.Only 77% of solar spectrum is absorbed by silicon, of this, around 30% is used as electrical energy. Silicon is transparent at wavelengths longer than 1.1 microns, so, 23% of sunlight passes right through it with no effect, excess photon energy is wasted as heat. So, all together, the maximum efficiency for a silicon PV is about 23%.-Disadvantages: Si PVs cells are made in high vacuum and using high temperature, so demands a lot of energy; the manufacturing costs are high and they are fragile, rigid and thick.The silicon has four valence electrons, and all electrons are used in bonds, so it is not very conductive. To overcome this low conductivity, the silicon can be doped by other compounds:--- p-type silicon: created by doping with compounds containing one less valence electrons than Si, such as B, Ga, In, Tl This way, only three electrons are available for bonding with 4 adjacent Si atoms. Therefore, an incomplete bond (hole) can attract an electron from a nearby atom. But, filling one hole creates another in a different Si atom. This movement of holes is available for conduction.--- n-type silicon: created by doping the Si with compounds that contain one more valence electrons than Si, such as P, As This way, since only 4 electrons are required to bond with the 4 adjacent Si atoms, the 5th valence electron is available for conduction.

(em laranja: empty conduction band; em azul: filled valence band)

-p-n junction:

Each side starts from an electrically neutral state. Then, negative charges move left and positive charges move right, thereby a charge imbalance is generated. This creates an electrical field that acts like an obstacle to further flow of electrons. When all electrons and holes in the depletion layer have combined, a voltage across the junction is created (built-in potential). So, two concomitant phenomena occur: a diffusion process that tends to generate more space charge and an electric field generated by the space charge tends to counteract the diffusion.

A PV device can be a homojunction or a heterojunction device. The homojunction device is made up of single material altered so that one side is p-type and the other side is n-type (Si PVs) and the p-n junction is located so that the maximum amount of light is absorbed near it. The heterojunction device is the one where the junction is formed by contacting two different semiconductors: top layer (high band-gap material selected for its transparency to light) and bottom layer (low band-gap material that absorbs light). Besides these two, there is the p-i-n and n-i-p junction that contains a middle intrinsic layer between n-type and p-type layer where the light generates free electrons and holes.-Photogeneration: when a photon of light is absorbed by one of the atoms in the n-type doped Si, it will dislodge an electron, creating a free electron and a hole. If a wire is connected from the cathode (n-doped Si) to the anode (p-doped Si), electrons will flow through it (electric current). The holes created by the dislodged electron are attracted to the negative charge of n-doped material and migrates to the back electrical contact. Then, as the electron enters the p-doped Si from the back electrical contact, it combines with the hole restoring the electrical neutrality.

But, sunlight contains a spectrum of photons of different energy E. If E < Eg (bandgap) the photon are useless and if E > Eg the excess of energy is heat.There are two types of Si PV technology:-Silicon Crystalline Technology: currently makes up 86% of PV market. It is very stable with module efficiencies of 10-16%. It can be mono crystalline or multi crystalline. The mono crystalline PV cells are made using saw-cut from single cylindrical crystal of Si and its operating efficiency is up to 15%, whilst multi crystalline PV cells are cast from ingot of melted and re-crystallised silicon, has an efficiency of about 12% and accounts for 90% of crystalline Si market.-Amorphous Silicon PV Cells: due to the manufacturing procedure, these modules are also known as thin-film solar cells. To make one amorphous silicon PV cell first, a glass substrate is thoroughly cleaned, then a lower contact layer is applied on it. The surface is then structured (divided into bands) and so, in vacuum, under high frequency electric field amorphous silicon layer is applied. The surface is re-banded and upper metal electrodes are fixated. This kind of PV cell has an operating efficiency of about 6%.

-- Thin Film Technology:

Thin film modules are constructed by depositing extremely thin layers of photosensitive materials onto a low-cost backing such as glass, stainless steel or plastic. This technology results in lower production costs, but lower efficiency rates (from 4% to 11%).-Advantages: low cost substrate and fabrication process.-Disadvantages: not very stable.

-- Poly-crystalline PV Cells:

This kind of PV cell can be made of copper indium gallium selenide with band gap to 1eV and have high absorption coefficient (10^5cm^-1). They have high efficiency levels (>11%). But the manufacturing process is still immature and slow. If it is made of CdTe, that has a band gap of 1.4eV and high absorption coefficient, the module efficiency is about 6-9% and the manufacturing process is again immature.

-- Gratzel cells:

A dye, adsorbed on TiO2 (that is cheap and very porous) is excited by light, injecting e- into the TiO2. The e- leaves the TiO2 and is carried through as current. The electron, then, returns to the device and reduces the electrolyte. The electrolyte, then, reduces the dye. Example of dye: ruthenium dyes in which the color changes according to the functional group and so, they can absorb different wavelengths. -How to prepare TiO2 dye sensitezed solar cells: first, cut and clean TCO (transparent and conductive oxide) electrode, usually ITO (indium tin oxide), and check the conductive side. Then a layer of titanium dioxide (TiO2) nanoparticles (in the size of 10-40 nm) must be put on the TCO glass plate. The titania layer must be sintered and re-firing if it was prepared and stored for long period. After this, the titania layer must be stained with natural (or synthetic) dyes. After it is dry, another TCO is used as cathode (carbon or Pt). So, the anode (titania) and the cathode electrodes are put together. Then, the gap between electrodes must be filled with electrolyte (iodine salt) and the filling holes must be sealed. The last procedure, then, is to test the solar cell. These solar cells have a power conversion efficiency of 11%. -Advantages: relatively inexpensive (made in non-vacuum setting mainly at room temperature, relatively easy and simple manufacturing process), thin, lightweight, flexible, has a short return on investment (~ 3 months), works even with low light conditions, high price/performance ratio.-Disadvantages: slightly lower efficiencies, the dye can breakdown, has a slightly larger bandgap than silicon and liquid electrolyte can leak or degrade (a possible solution maybe is the use of ionic liquids).

-- Organic photovoltaics:

Are PV cells that use organic molecules (conducting polymers or small organic molecules) for light absorption and charge transport. There is no p-n junctions but electron donor and electron acceptor and electron-hole pair are generated by excitons.

-How it works:

The energy difference between the HOMO and LUMO is termed the HOMOLUMO gap. Roughly, theHOMOlevel is toorganic semiconductorswhat thevalence bandmaximum is to inorganicsemiconductors. The same analogy exists between theLUMOlevel and theconduction bandminimum.Organic thin-film solar cells from low-molecular weight materials can be fabricated by vacuum deposition, spin coating, etc. Organic materials having p and n-type conductivities are successively deposited on a transparent electrode, and then, a metal electrode is deposited on them. Sunlight is absorbed by organic layers and excitons generated by light absorption are dissociated to electrons and holes at the interface between p and n-type organic layers. The electrons and holes are collected at the upper and lower electrodes, respectively, and electricity is generated. The process of energy generation on an organic PV cell can be simplified by the diagram below:

-Advantages: low-cost materials, lightweight, flexible and customizable at molecular level.-Disadvantages: 1/3 efficiency of Si solar cells (efficiency has reached 8.3% in 2011 and now is about 12%) and can suffer of photochemical degradation.

-- Other technologies: the main alternative types of fuel cells are concentrated photovoltaic and flexible cells. The concentrated photovoltaic is building into concentrating collectors that use a lens to focus the sunlight onto the cells. The main objective is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. The efficiencies are from 20-30%. The flexible cells are based on a production process similar to thin film cells, but the active material is deposited in a thin plastic, so the cell can be flexible. This open the range of applications, especially for building (roof-tiles for example) and end-consumer applications. The efficiency is around 20.3%.