Chapter 3: PV Technologies

3.1 Introduction to Solar Cells

3.1.1 Materials and Energy Levels

Solar cells are semiconductor devices made of semiconductor materials. In order to better understand the working mechanism of solar cells we need to know what is a semiconductor. In the valence band, electrons are bound to the atomic structure. In the conduction band, electrons are free to move through the material. Between the two, there is an energy gap that the electron in the valence band must overcome to become a free carrier.  This is called the bandgap energy,  Eg.

In an insulator, the bandgap energy, Eg is more than 5 eV and electrons cannot jump into the conduction band. In a conductor, the conduction and valence bands overlap. In a semi-conductor, the bandgap energy,  Eg is less than 5 eV. With a little thermal energy, electrons can jump into the conduction band.

There are two types of semiconductor materials:

  1. Some materials (Ge and Si for example) are intrinsic semiconductors, i.e. they have semiconductor properties in their pure state. the electron and hole concentrations in intrinsic semiconductors are small.
  2. Most semiconductors used in electronic components are of the extrinsic type, i.e. they are made more conducting by the doping with microscopic amounts of another element.

3.1.2 The P-N Junction

The P-N junction is essential to the operation of solar cells and many other semiconductor devices. A P-N junction is formed when a p-type semiconductor and an n-type semiconductor are placed in contact with each other. p-type semiconductor has more positive carrier concentration, while an n-type semiconductor has more negative carrier concentration. Both p and n-type materials are formed by introducing impurities into the pure (intrinsic) semiconductor materials.
After the initial surge of free electrons across the p-n junction, the depletion region expands to a point where equilibrium is established and there is no further diffusion of electrons across the junction. This process occurs because, as electrons continue to diffuse across the junction, more and more positive and negative charges are created near the junction and the depletion region forms. A point is reached where the total negative charge in the depletion region repels any further diffusion of electrons into the p-region and the diffusion stops. The depletion region has many positive charges and many negative charges on opposite sides of the p-n junction. The force between the positive and the negative charges form an electric field.

Construction of a PN Junction:

  1. P-type Semiconductor:
    • In a P-type semiconductor, such as silicon or germanium, atoms from Group III of the periodic table (trivalent, with three valence electrons) are intentionally introduced as impurities into the crystal lattice of the semiconductor material.
    • The most common dopants for P-type semiconductors include boron. These impurity atoms create “holes” or locations where an electron is missing in the semiconductor crystal structure.
  2. N-type Semiconductor:
    • In an N-type semiconductor, atoms from Group V of the periodic table (pentavalent, with five valence electrons) are introduced as impurities.
    • Common dopants for N-type semiconductors include phosphorus or arsenic. These impurity atoms introduce extra electrons into the crystal lattice.
  3. Junction Formation:
    • When a P-type semiconductor is brought into physical contact with an N-type semiconductor, a PN junction is formed at the interface between the two regions.
    • At the junction, electrons from the N-type region diffuse across the junction into the P-type region, and holes from the P-type region diffuse into the N-type region. This movement of charge carriers establishes a region near the junction known as the depletion zone.

Depletion Zone:

The interaction between the free electrons from the N-type region and the holes from the P-type region creates a depletion zone at the PN junction. In this region:

  • Electron Diffusion: Electrons diffuse from the N-type side to the P-type side, leaving behind positively charged donor ions in the N-type region.
  • Hole Diffusion: Holes diffuse from the P-type side to the N-type side, leaving behind negatively charged acceptor ions in the P-type region.

As a result, an electric field is established across the junction due to the charge separation. This electric field opposes further diffusion of charge carriers, creating a barrier potential.

 

3.2 Types of PV Technologies

  1. Crystalline Silicon: These dominate the market and come in two main forms – monocrystalline and polycrystalline. Monocrystalline cells are known for their high efficiency, while polycrystalline cells offer a cost-effective alternative.
  2. Thin-Film: This category includes amorphous silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) technologies. Thin-film cells are lightweight and flexible, allowing for versatile applications and lower production costs.
  3. Organic Photovoltaic Cells: Leveraging organic materials, these cells hold promise for flexible, transparent, and low-cost solar solutions. Though currently less efficient than traditional technologies, ongoing research aims to improve their performance.
  4. Perovskites: A relatively recent entrant, perovskite solar cells have rapidly gained attention due to their ease of manufacturing and impressive efficiency gains. Ongoing research focuses on addressing stability issues and scaling up production.
  5. Multi-Junction Cells: Commonly used in concentrated photovoltaic systems and space applications, multi-junction cells utilize multiple layers of semiconductors to capture a broader spectrum of sunlight, maximizing energy conversion efficiency.

3.3 Cell Efficiency and Fill Factor

3.3.1 Fill Factor

The fill factor (FF) is a key parameter used to assess the performance of a solar cell. It is a dimensionless value that represents the squareness of the current-voltage (I-V) curve of a solar cell. The fill factor is calculated by dividing the maximum power point (Pmax) of the solar cell by the product of the open-circuit voltage (Voc) and short-circuit current (Isc). Mathematically, the fill factor (FF) is given by:

Fill Factor (FF) = Maximum Power Point (Pmax) ÷ Open-Circuit Voltage (Voc) × Short-Circuit Current (Isc)

In simpler terms, the fill factor provides information about how closely a solar cell approaches its ideal behavior. A higher fill factor indicates that the solar cell is operating more efficiently in converting sunlight into electrical power.

The fill factor can take values between 0 and 1, with 1 representing an ideal case where the I-V curve of the solar cell is a perfect square. In reality, factors such as manufacturing imperfections, material properties, and environmental conditions can cause deviations from the ideal square shape, resulting in a fill factor less than 1.

A well-designed and well-manufactured solar cell will have a higher fill factor, contributing to overall higher efficiency in converting sunlight into electricity. It is an important parameter for assessing the quality and performance of solar cells and is often considered along with other key metrics such as efficiency, open-circuit voltage, and short-circuit current.

3.3.2 Cell Efficiency

Efficiency is the ratio of the output power over input power. PV cell efficiencies vary considerably among different PV technologies, and for the same material and technology, efficiencies vary widely between laboratory samples and commercial devices. Efficiency is expressed as a percentage and is calculated with the following formula:

ε= Efficiency

P = Maximum power (in W)

E = Solar irradiance (in W/m2)

A = Area (in m2)

Pout/Pin =  Im x Vm/ E x A = Pm/ E X A

 

Here’s a table comparing the cell efficiencies of different solar cell technologies:

Technology Efficiency Range Key Features
Monocrystalline Silicon 15% – 22% High efficiency, mature technology, commonly used
Polycrystalline Silicon 12% – 18% Cost-effective alternative to monocrystalline silicon
Thin-Film (Amorphous Silicon) 8% – 12% Lightweight, flexible, suitable for certain applications
Thin-Film (Cadmium Telluride) 9% – 18% Relatively low-cost, high absorption in the blue part of the spectrum
Thin-Film (CIGS) 10% – 20% High efficiency potential, flexibility, and scalability
Organic Photovoltaic 5% – 15% Lightweight, potentially low-cost, suitable for specific applications
Perovskite 10% – 25% Rapidly advancing technology with high efficiency potential
Multi-Junction (Concentrated PV) 30% – 40% Used in concentrated solar power systems and space applications

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Solar Photovoltaics for Design Engineers Copyright © by Bilal Shahid is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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