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Questions and Answers
Which of the following can be classified as semiconductor materials?
Which of the following can be classified as semiconductor materials?
The band gap of a semiconductor is the minimum energy required to move an electron from a free state to a bound state.
The band gap of a semiconductor is the minimum energy required to move an electron from a free state to a bound state.
False
In an insulator, the conduction band and valence band are far away from each other.
In an insulator, the conduction band and valence band are far away from each other.
True
What is the name given to the concentration of negative and positive charges in an intrinsic semiconductor?
What is the name given to the concentration of negative and positive charges in an intrinsic semiconductor?
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What is the range of energy gap values for semiconductors?
What is the range of energy gap values for semiconductors?
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What is the unit of energy commonly used in semiconductor physics?
What is the unit of energy commonly used in semiconductor physics?
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What is the term used to describe the process of adding impurities to a semiconductor to alter its conductivity?
What is the term used to describe the process of adding impurities to a semiconductor to alter its conductivity?
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What is the name of the energy level at which the probability of an electron occupying a specific energy state in a semiconductor is 1/2?
What is the name of the energy level at which the probability of an electron occupying a specific energy state in a semiconductor is 1/2?
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What is the term used to describe the junction formed when a p-type semiconductor is joined with an n-type semiconductor?
What is the term used to describe the junction formed when a p-type semiconductor is joined with an n-type semiconductor?
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The internal electric field in a p-n junction always facilitates the flow of electrons from the n-type to the p-type region.
The internal electric field in a p-n junction always facilitates the flow of electrons from the n-type to the p-type region.
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Name the equation that describes the relationship between current and voltage in a diode under forward bias.
Name the equation that describes the relationship between current and voltage in a diode under forward bias.
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In a solar cell, the absorption of photons with energy less than the band gap can generate electron-hole pairs.
In a solar cell, the absorption of photons with energy less than the band gap can generate electron-hole pairs.
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What is the maximum wavelength of solar radiation that can be absorbed by silicon for generating electron-hole pairs?
What is the maximum wavelength of solar radiation that can be absorbed by silicon for generating electron-hole pairs?
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What is the term used to describe the phenomenon where the energy of the incoming photon is partially lost as heat when generating electron-hole pairs?
What is the term used to describe the phenomenon where the energy of the incoming photon is partially lost as heat when generating electron-hole pairs?
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Higher band gap materials are generally more efficient at converting solar energy into electricity.
Higher band gap materials are generally more efficient at converting solar energy into electricity.
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What is the name of the device that produces an internal electric field capable of separating the electrical charges created by the absorption of light in a solar cell?
What is the name of the device that produces an internal electric field capable of separating the electrical charges created by the absorption of light in a solar cell?
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In a solar cell, what type of current is generated when electron-hole pairs are created by the absorption of light?
In a solar cell, what type of current is generated when electron-hole pairs are created by the absorption of light?
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What is the term used to describe the maximum voltage that a solar cell can produce when no current is flowing through it?
What is the term used to describe the maximum voltage that a solar cell can produce when no current is flowing through it?
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The maximum power that a solar cell can produce is achieved at what specific operating point?
The maximum power that a solar cell can produce is achieved at what specific operating point?
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The fill factor of a solar cell is a measure of how closely the actual power output matches the ideal power output.
The fill factor of a solar cell is a measure of how closely the actual power output matches the ideal power output.
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What is a common cause of energy loss in a solar cell that can significantly impact its overall efficiency?
What is a common cause of energy loss in a solar cell that can significantly impact its overall efficiency?
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What is the primary impact of series resistance in a solar cell?
What is the primary impact of series resistance in a solar cell?
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What are the standard conditions (STC) used for comparing the performance of different solar cells?
What are the standard conditions (STC) used for comparing the performance of different solar cells?
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What is the term used to describe the temperature reached by cells within a module under specific operating conditions?
What is the term used to describe the temperature reached by cells within a module under specific operating conditions?
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What is the name of the type of solar cell that consists of multiple layers of different semiconductor materials, each optimized to absorb a specific range of wavelengths?
What is the name of the type of solar cell that consists of multiple layers of different semiconductor materials, each optimized to absorb a specific range of wavelengths?
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Multi-junction solar cells are typically less expensive to produce than single-junction solar cells.
Multi-junction solar cells are typically less expensive to produce than single-junction solar cells.
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What is the primary purpose of the 'charge controller' in a stand-alone solar system?
What is the primary purpose of the 'charge controller' in a stand-alone solar system?
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A grid-connected photovoltaic system can sell excess energy back to the power grid.
A grid-connected photovoltaic system can sell excess energy back to the power grid.
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What type of energy storage system is commonly used with a photovoltaic system to store excess energy for use during periods of low sunlight?
What type of energy storage system is commonly used with a photovoltaic system to store excess energy for use during periods of low sunlight?
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What is the main function of an inverter in a photovoltaic system?
What is the main function of an inverter in a photovoltaic system?
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MPPT (Maximum Power Point Tracking) is a technique used by inverters to ensure that the solar panels are operating at their maximum power output.
MPPT (Maximum Power Point Tracking) is a technique used by inverters to ensure that the solar panels are operating at their maximum power output.
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In a grid-connected system, why is it important to synchronize the frequency of the inverter output with the frequency of the power grid?
In a grid-connected system, why is it important to synchronize the frequency of the inverter output with the frequency of the power grid?
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What is the term used to describe the arrangement of solar panels in a row that are connected in series?
What is the term used to describe the arrangement of solar panels in a row that are connected in series?
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What are some of the benefits of using a solar tracking system?
What are some of the benefits of using a solar tracking system?
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Study Notes
Renewable Energy Technologies
- PV Conversion Theory is covered.
- Bibliography: Documents are available in Moodle and on pveducation.org.
Basics
- Peripheral electrons are grouped into conduction electrons and valence electrons.
- Conduction electrons carry electricity in the conduction band and are free to move in an electric field.
- Valence electrons have limited mobility and form chemical bonds with other atoms.
- The band gap of a semiconductor is the minimum energy required to excite an electron.
- The gap between energy bands dictates if a material is a metal, insulator, or semiconductor.
Energy Gap
- Conducting materials have overlapping conduction and valence bands.
- Semiconductors have a small energy gap between the bands (approximately 1-2 eV).
- Insulating materials have a large energy gap between the bands.
Other Topics
- Conductive materials have a very small bandgap that overlaps or are already in the conduction band.
- Other elements have a wide bandgap preventing conduction electrons at room temperature, which makes them insulators.
- The bandgap for semiconductors is in the range of 1 to 1.5 eV.
- In semiconductors, electrons in the conduction band and holes in the valence band are responsible for electrical conduction.
- The energy required to break bonds corresponds to the width of the bandgap.
- Silicon energy gap value: 1.12 eV
- Gallium arsenide energy gap value: 1.42 eV
- Intrinsic carriers concentration (n₁) is equal to the concentration of both electrons and holes in pure intrinsic semiconductors.
- The concentration of intrinsic carriers in silicon is 1.5x10^10 cm-3.
- The concentration of intrinsic carriers in gallium arsenide is 1.8x10^6 cm-3.
- Activation energy E₂ is approximately equal to Eg / 2, where k = 1.38 x 10^-23 J/K and T is the absolute temperature.
- Calculation to determine the concentration of silicon atoms in a crystalline solid, using the given data.
- Doping: Adding impurities to a crystal to increase the number of electrons or holes in the semiconductor to improve electrical conductivity.
- Phosphorus is a donor impurity, adding extra electrons.
- Boron is an acceptor impurity, creating holes.
- The ionization energy of phosphorus in silicon is smaller than the energy gap, which allows electrons to move to the conduction band.
- The ionization energy of an acceptor in silicon is also smaller.
- In semiconductors, both electrons in the conduction band and holes in the valence band take part in electrical conductivity.
- For pure semiconductors, the concentration of negative charges (conduction electrons) equals that of positive charges (conduction holes), which is represented as n₁.
- Factors impacting intrinsic carrier concentration: bandgap and temperature.
Doping
- Adding impurities changes the number of electrons in the conduction band or holes in the valence band.
- Extrinsic semiconductors are formed after introducing impurities.
- Impurity atoms are either donors (extra electrons) or acceptors (missing electrons/holes).
Energy Level
- Probability that an energy level is occupied by an electron, which depends on Fermi energy and temperature.
- In intrinsic materials the number of energy states is approximately the same for both bands.
- By doping with n-type atoms, the electron concentration increases leading to an increase in Fermi energy level.
- Conversely, doping with p-type atoms leads to a decrease in Fermi energy level.
Junction
- P-N junction formation: one side of semiconductor material is doped with p-type atoms (e.g boron) and the other side with n-type atoms (e.g. phosphorus).
- Two layers of material that are originally neutral, form an electric field through the contact forming the P-N junction.
- Charge concentration gradient leads to electron diffusion in the p-type and holes in the n-type region.
- This diffusion generates a potential barrier between regions, with the p-type becoming negatively charged and the n-type positively charged.
- The internal electric field distorts the energy bands where the Fermi level is constant across the junction.
I-V Characteristics
- I-V characteristics show the relationship between current and voltage across the junction.
- With no voltage, there is no current.
- With direct polarization, an exponential increase in current (diode effect) occurs.
- With reverse polarization, current is very low (ideally zero), acting as a diode.
- The curve depends on incident light.
Photon Energy
- Only photons with energy equal to or greater than the bandgap of semiconductor material can excite electrons into the conduction band.
- When a photon has enough energy, it's absorbed and creates an electron-hole pair.
- Amount of photons reaching the surface each second depends on the wavelength.
- The wavelength of solar radiation that can be fully converted is determined from the energy gap.
Preliminary Evaluation of Electric Power
- Calculation of the electric current generated in ideal conditions under solar radiation.
Solar Cell
- Photovoltaic cell converting light into electricity by exploiting the electric field separating charges generated during light absorption.
- Excess energy is converted to heat which is a major loss mechanism in the PV cell.
- Semiconductor materials that use different processes to convert solar energy into electricity are the main subjects of the topic.
- PV cell consists of p-type and n-type semiconductor materials joined together forming a junction.
Efficiency of a Solar Cell
- Comparison of the power output to the amount of power input considering standard conditions.
Conversion Efficiency of a Cell
- Relationship between the maximum power (Pstc) from the cell under standard test conditions.
Main Inefficiencies in Solar Cells
- Impurities or defects may lead to the annihilation of electron-hole pairs.
- Surface reflections and shading affect top contacts.
- Series resistances through emitter and base of the solar cell, contact resistance, top and rear metal contacts increase resistances.
- Impact on fill factor (reduces) and short circuit current.
Factors Affecting Performance
- Efficiency, peak power, irradiance, cell temperature, and air mass.
NOCT - Nominal Operating Cell Temperature
- Standardized conditions to calculate cell temperature under operational conditions.
Single Junction Cell
- The material for solar cells affects their efficiency as high Eg implies less photons in the solar spectrum with E > Eg.
Multi-Junction Cell
- Efficiency roughly 32% today with high costs and used in aerospace and similar applications
- Stacked materials to absorb different frequencies of sunlight are the most efficient.
Different Approaches to PV
- Conventional approach: uses silicon.
- Thin film approach: uses materials like cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon in a thin film form.
- Concentrator approach: uses lenses to focus sunlight onto a smaller area.
Solar Cell Components
- Structures for multi-layers and different materials used.
- Descriptions and diagrams for cells, panels, modules, and photovoltaic generator.
Photovoltaic Modules
- Modules have various parts: cells panels, modules, and photovoltaic generator etc.
- Silicon based components used.
Silicon Ingots
- Silicon refinement into single crystal polysilicon versus multi-crystalline polysilicon.
- Ingot growth and wafer processing.
Costs and Energy Sharing in Silicon PV
- Division of costs among different system elements (e.g., system, installation, BOS, modules etc.)
Costs of PV
- Historical price trend for crystalline silicon PV cells. This involves tracking the cost per watt over time.
- Implementation of Swanson's law: demonstrates a relationship between cumulative installations and cost per watt.
PV Module Production by Region
- Global productions across different regions.
Different Approaches to PV
- Approaches to developing PV materials with lower cost: conventional, thin film, and concentrator approaches.
BIPV (Building Integrated Photovoltaics)
- Technologies for integrating PV into buildings.
- Categories according to their level of integration: least, more, and fully integrated.
Potential Applications
- Low and high-rise buildings considering available space, facades, and technical restrictions.
- Important advantage: heat reduction, reducing temperatures in urban areas by 20 to 30 degrees.
AgriVoltaics
- Combination of agriculture and solar.
- Descriptions of advantages and constraints for agrivoltaics.
- Crop yield increase and lower temperatures.
- Potential for increased harvest sizes.
- High investment and shade tolerance needed.
Floating PV
- PV technology that is built over water systems.
- Global trends/growth and examples of applications.
PV Software
- Freeware software for PV energy performance estimation.
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Description
Test your knowledge on the fundamentals of photovoltaic conversion theory and energy gap concepts in renewable energy technologies. Explore the differences between conductors, semiconductors, and insulators, and understand the significance of electron mobility in materials.
