المــــركــز الوطنــــــي لبحــــــوث الطـــاقــــــة National Energy Research Center Introduction to Photovoltaic (PV) Technology Sponsored by

Introduction to Photovoltaic (PV) Technology Eng. Firas Alawneh Head of Photovoltaics (PV) Division National Energy Research Center (NERC) Royal Scientific Society (RSS) Amman-Jordan

Outline History Semiconductors From Sand to Solar Cells Semiconductors & Photovoltaic phenomenon Silicon PV Cell Operation Properties of the PV Cells Standard Test Conditions (STC) of PV Cells & performance parameters Types of PV Cells

History 1839 – photovoltaic effect first recognized 1883 – first solar cell created 1946 – modern pn junction solar cell 1954 – doped silicon first used in solar cells 1958 – first spacecraft to use solar panels 1970 – GaAs solar cells created 1989 – first dual junction cell created 1993 – first dual junction cells for spacecraft 1995 – 30% efficiency barrier broken 2004 – terrestrial solar cell production exceeds 1 GW 2009 – 40% efficiency barrier broken Source: NREL Feb. 2010

Semiconductors Solar cells are fabricated using semiconductors. Semiconductors are made from crystal and can act as conductors or insulators in different circumstances, according to the amount of energy that is given to the material. Silicon is the most common semiconductor crystal. Silicon

From Sand to Silicon Solar Cells semiconductor silicon (hyper pure) reduction Solar cell processing slicing purification (several steps) Cast ingot wafers SiO 2 Quartz Sand Metallurgical Silicon Solar cell °C300 °C 1100°C for ~200 – 300 hours

Photovoltaic Technology Photovoltaic (PV) is the technology of converting light directly to electrical energy (photo = light, voltaic = electricity). Commonly known as “solar cells.” The simplest systems power the small calculators we use every day. More complicated systems will provide a large portion of the electricity in the near future. PV represents one of the most promising means of maintaining our energy intensive standard of living while not contributing to global warming and pollution.

Photon Energy

Silicon Chemical Properties Melting Point: 1410 ºC Boiling Point: 2355 ºC

Energy Bands for Materials Conduction Band Valence Band Metal Conduction Band Valence Band Semiconductor EgEg Conduction Band Valence Band Insulator EgEg E Photon E = hc/ λ e - E g (eV) Element 1.14Silicon 0.67Germanium 0.1Tin 0Copper All at 20ºC The semiconductors in general lies between metal and insulator properties, it needs a small energy related to insulator to be in conduction band.

The response of the silicon due to the incident Photons e- e+ e- e+ e- e+ e- e+ Conclusion: we have to reengineer the material, so that we can separate the electrons (e⁻) from the holes(e⁺) to prevent the recombination inside the material.

e+e- e+e- e+e- e+e- Photosensitivity?

2. Doping of Silicon : positive (p) and negative (n) layers

What is Doping? Answer: Adding foreign atoms to the silicon crystal to produce negative or positive free charge carries (electrons or holes). Why Doping? Answer: As mentioned before, electrons freed and energized by photons will wander for a short time and then recombine with a wandering hole. The energy originally transferred to the electron from the photon is simply lost as heat. The key to producing usable output current is to sweep the freed electrons out of the material before they recombine with holes.

Doping the Silicon Pure silicon wafer is doped with a small amount of another atoms at temperature ( )°C, which creates a valence bond between it and the silicon. The most common impurity atoms are the Boron (B5) and the Phosphorus (P15). The Boron has three electrons in its outer level (less than the silicon by one electron). The Phosphorus has a five electrons in its outer level (more than the silicon by one electron). The Boron is doped by one atom for every 10,000,000 silicon atoms to form the P-type silicon. The Phosphorus is doped by one atom for 1000 silicon atoms, to form the N-type silicon.

The P-type silicon The silicon atom creates four covalent bonds with other neighboring atoms in the pure silicon crystal. When the crystal is doped with Boron atoms, the silicon will make three covalent bonds with it with the forth bond missing, which represents a hole (e + ), so this type of semiconductor is called P-type. This hole is waiting for a free electron to fill its location to create the forth bond, so the impurity atoms then is referred to it as acceptor atoms.

The N-type silicon Silicon is doped with Phosphorus which has five electrons in its outer orbit. So one electron (e-) will be free. This type of semiconductor is called N-type. Phosphorous atoms (P) can donate this electron to another bond that needs it, so it is referred to as donor atoms.

Doping in 3D view P-type N-type

Doping in 2D view N-type semiconductor P-type semiconductor

3. Photovoltaic Effect: p-n junction operation and its parts

e- e+ e- e+ Voltage Difference Depletion region Built-in electric field e- e+ E e-e+ e- e+ The p-n junction Conclusion : The goal of doping is to create the depletion region to create the electric field that separates the electrons from the holes to produce the potential difference.

Depletion Region

e- e+ Solar Cell Operation

Solar Cell Parts (n+) & (p+) diffusions (heavily doped silicon) used to connect the layers with the metal to decrease the series resistance. The top metal grid N layer P layer Top view of the cell Bottom view of the cell Bottom metal

Silicon Solar Cell Packaging

CZ Crystallization Method Mono c-Si Si liquid seed Mono-crystalline vs. Poly-crystalline Silicon There are two types of crystalline silicon depending on its purity and crystals orientation obtained during the crystal growth process: Poly-crystalline: Non-uniform crystals orientation Mono-crystalline: Uniform crystals orientation (purer and more expensive and efficient) The mono-crystalline silicon ingots are prepared by the exacting Czochralski (CZ) crystal growth process (crystal pulling). While the poly-crystalline silicon ingots are prepared by a simpler casting (or, more generally, directional solidification). Simple Crystallization Insulation Electric Heaters Poly c-Si Si liquid

How to distinguish between polycrystalline and monocrystalline silicon solar cells by visual inspection? Poly-crystallineMono-crystalline

4. Equivalent circuit of the solar cell and characteristic curve

Equivalent Circuit for Solar Cell Real Solar cell Standard Solar cell

Equivalent Circuit for Solar Cell Where: I ss : Reverse saturation current (depends on: Material, Geometry, & temperature) q : Electron charge (1.6* C) A : Diode quality factor (1 for ideal diodes and >1 up to 2 for real diodes( k : Boltzmann constant (1.38* J/K) T: Absolute cell temperature in Kelvin degrees For real solar cells with finite values for R S and R sh :

Characteristics and Power for Solar Cell I = I PH - I D

Operating Point & Maximum Power Point

5. Standard test conditions (STC) and main performance parameters and factors

Standard Test Conditions (STC) Global Solar Irradiance (G): 1000 W/m 2 Cell Temperature (T): 25 °C Air Mass (AM): 1.5

PV Performance Parameters Open-circuit voltage (V oc ) Short-circuit current (I sc -( I ph) ) Maximum power voltage (V mp ) Maximum power current (I mp ) Maximum power (P mp ) Maximum Power Efficiency (η max ) Fill factor ( FF )

Solar Cell Fill Factor

Solar Cell Efficiency The electrical output depends on the operating point of the solar cell and the incident radiant power depends on the solar radiation (perpendicular to the surface of the solar cell) and cell surface area. The maximum efficiency of the solar cell is calculated at MPP, which is:

Efficiency of Solar Cell at MPP Input Power = G [W/m 2 ] x Area [m 2 ] Output Power = V mp [V] x I mp [A] G “Global Solar Irradiance” Area + - V I Resistor Solar Cell The efficiency of the solar cell is the ratio of electrical power output to the incident radiant power :

PV Efficiency Losses Optical losses: Not all of the light is absorbed because of finite reflectivity. Use antireflective coating. Use multilayer coating with different indices of refraction. Further reduction is caused by light blocked by the metal grid which is needed for electrical contacts. Recombination losses: Many charge carriers recombine before they can diffuse to the device terminals. Series and Shunt resistance: The bulk resistance of the semiconductor contributes some series resistance. The shunt resistance can be caused by crystal lattice defects in the depletion region and/or leakage currents around the edges of the cell.

Temperature Effect on Solar Cells The parameter most affected by an increase in temperature is the open-circuit voltage (Voc). Accordingly, the power of the solar cell at the Maximum Power Point (MPP) decreases by increasing the cell’s temperature.

Temperature Effect on Solar Cells

6. Solar Cells Types

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