Copper Indium Gallium Selenide (CIGS) Photovoltaic Cells

Copper Indium Gallium Selenide (CIGS) Photovoltaic Cells represent one of the most advanced and promising thin-film solar technologies in the field of renewable energy. Distinguished by their high efficiency, flexibility, and adaptability, these cells are built from a compound semiconductor material composed of copper (Cu), indium (In), gallium (Ga), and selenium (Se). The unique electronic and optical properties of this material make CIGS solar cells an effective and versatile alternative to traditional crystalline silicon solar panels.

Introduction and Background

Since the 1970s, researchers have sought alternatives to silicon-based solar cells that could reduce material costs, allow flexible applications, and maintain high energy conversion efficiency. Thin-film technologies emerged from this pursuit, with CIGS standing out as a leading candidate due to its excellent light-absorbing characteristics.
CIGS belongs to a class of chalcopyrite semiconductor materials, characterised by a direct bandgap that enables strong absorption of sunlight even in extremely thin layers—typically between 1 and 2 micrometres thick. This contrasts with crystalline silicon wafers, which are around 200 micrometres thick. The result is a lightweight, efficient, and potentially low-cost solar cell suitable for a wide range of applications, from rooftop modules to flexible, portable energy systems.

Material Composition and Structure

The defining feature of CIGS solar cells is the absorber layer made of Cu(In,Ga)Se₂, a compound semiconductor whose properties can be tuned by adjusting the indium-to-gallium ratio. By varying this ratio, researchers can change the bandgap energy from about 1.0 electron volt (eV) (for pure CuInSe₂) to 1.7 eV (for pure CuGaSe₂). This tunability allows the cell to optimise absorption across different portions of the solar spectrum, improving efficiency.
A standard CIGS solar cell consists of several thin layers stacked in sequence, each serving a specific function:

1. Substrate: Provides mechanical support and determines the cell’s flexibility. Common materials include soda-lime glass, stainless steel, or polyimide for flexible modules.
2. Back Contact Layer: Usually made of molybdenum (Mo), this layer acts as the electrical contact to the absorber and provides stability under high-temperature processing.
3. CIGS Absorber Layer: The active layer where sunlight is absorbed and electron-hole pairs are generated. This is the most critical component of the cell.
4. Buffer Layer: A thin film of cadmium sulfide (CdS) or non-toxic alternatives such as zinc oxysulfide (Zn(O,S)) forms the p–n junction, enabling efficient charge separation.
5. Transparent Conductive Oxide (TCO): A layer of zinc oxide (ZnO) or aluminium-doped zinc oxide (AZO) allows light to pass into the absorber while conducting current to the external circuit.
6. Front Contacts and Encapsulation: Metal grids and protective coatings ensure electrical collection, durability, and resistance to environmental degradation.

This layered structure makes CIGS cells both efficient and mechanically robust, with the potential for high performance in compact or flexible forms.

Working Principle

The operation of CIGS photovoltaic cells follows the same fundamental principles as other semiconductor-based solar technologies:

1. Light Absorption: When sunlight hits the CIGS absorber layer, photons with sufficient energy excite electrons from the valence band to the conduction band, creating electron-hole pairs.
2. Charge Separation: The built-in electric field at the p–n junction drives electrons toward the n-type TCO layer and holes toward the p-type back contact.
3. Charge Collection: These charge carriers are collected at their respective electrodes, generating an electric current when the circuit is closed.
4. Power Generation: The flow of current through an external load converts solar energy into usable electrical power.

The direct bandgap and high absorption coefficient of CIGS allow it to capture most of the solar spectrum with a very thin absorber layer, reducing material consumption while maintaining high efficiency.

Manufacturing Techniques

Several deposition techniques are used to create the CIGS absorber layer, each with its advantages and challenges:

• Co-evaporation: The elements Cu, In, Ga, and Se are evaporated simultaneously under vacuum. This method produces high-quality films with excellent control over composition, leading to record efficiencies exceeding 23%.
• Two-step Process (Selenisation or Sulphurisation): Metal precursors are first deposited by sputtering or electroplating, then reacted with selenium or sulphur vapour to form the CIGS compound.
• Sputtering: A cost-effective, scalable technique that allows deposition over large areas. However, achieving precise composition control requires advanced process monitoring.
• Solution Processing and Printing: Emerging methods involve using nanoparticle inks or soluble precursors that can be printed onto flexible substrates and then annealed. These techniques promise low-cost, high-throughput production in roll-to-roll manufacturing systems.

Efficiency and Performance

CIGS cells have achieved world-record conversion efficiencies of over 23% in laboratory conditions, rivalling the best crystalline silicon cells. Commercial modules typically exhibit efficiencies between 14% and 18%, depending on the manufacturing process and substrate type.
One of the main strengths of CIGS technology lies in its excellent temperature coefficient—meaning performance degrades less under high temperatures compared to silicon. This characteristic makes CIGS modules ideal for deployment in hot climates. Furthermore, their high absorption efficiency allows them to perform well under low-light conditions, ensuring consistent energy output throughout the day.
CIGS cells can also be fabricated on flexible substrates without significant efficiency loss, giving them an edge in applications requiring lightweight or portable power generation.