Cadmium Telluride (CdTe) Solar Technology

Cadmium Telluride (CdTe) Solar Technology is a leading thin-film photovoltaic (PV) technology that uses cadmium telluride, a compound semiconductor, as the primary light-absorbing material. It represents one of the most commercially successful alternatives to conventional crystalline silicon (c-Si) solar cells, offering advantages in cost, manufacturing efficiency, and performance under varying environmental conditions. CdTe technology has played a crucial role in the expansion of renewable energy, particularly in large-scale solar installations across the world.

Composition and Structure

Cadmium telluride is a II–VI compound semiconductor, composed of cadmium (Cd) from group II and tellurium (Te) from group VI of the periodic table. It has a direct band gap of approximately 1.45 electron volts (eV), which is close to the optimum value for converting sunlight into electricity according to the Shockley–Queisser limit. This allows CdTe to efficiently absorb a broad range of the solar spectrum while using only a thin layer of material.
A typical CdTe solar cell consists of several layers:

• Substrate: Usually glass or flexible polymer that supports the cell.
• Transparent conducting oxide (TCO): Such as tin oxide (SnO₂) or cadmium stannate (Cd₂SnO₄), which allows light to pass while conducting electricity.
• Cadmium sulphide (CdS) buffer layer: Acts as a window layer forming a p–n junction with CdTe.
• CdTe absorber layer: The main layer that captures photons and generates electron–hole pairs.
• Back contact: A metallic layer (e.g., copper or graphite) to collect charge carriers and complete the circuit.

The entire active region of a CdTe solar cell is typically less than 10 micrometres thick, significantly thinner than the 150–200 micrometre wafers used in silicon PV cells.

Working Principle

CdTe solar cells operate on the same basic photovoltaic principle as other semiconductor-based solar technologies. When sunlight strikes the CdTe layer:

1. Photon absorption excites electrons from the valence band to the conduction band, creating electron–hole pairs.
2. The p–n junction formed between CdS (n-type) and CdTe (p-type) establishes an internal electric field.
3. This field drives electrons towards the front contact and holes towards the back contact, generating an electrical current.
4. The collected current is directed through external circuitry to produce usable electricity.

Because of its direct band gap and high absorption coefficient, CdTe requires only a thin absorber layer to capture most of the incident sunlight, making it ideal for lightweight and flexible solar modules.

Development and Historical Background

The development of CdTe photovoltaic technology began in the mid-20th century, but significant commercial success was achieved only in the late 1990s and early 2000s. The U.S.-based company First Solar pioneered large-scale manufacturing of CdTe modules, establishing the technology as a cost-competitive alternative to silicon.
Initially used for small-scale and off-grid applications, CdTe modules rapidly advanced in efficiency and scalability. By the 2010s, CdTe technology accounted for a substantial share of global thin-film solar installations. Today, First Solar remains the largest producer, with module efficiencies exceeding 22% in laboratory settings and about 18–20% in commercial production.

Advantages of CdTe Solar Technology

CdTe photovoltaic technology offers several notable advantages over traditional crystalline silicon:

• Lower manufacturing cost: CdTe modules can be produced using continuous thin-film deposition methods such as close-spaced sublimation or vapour transport deposition, which require less material and energy.
• High absorption efficiency: A CdTe layer just a few micrometres thick can absorb over 90% of incident sunlight, reducing raw material usage.
• Better temperature performance: CdTe modules have a lower temperature coefficient (~-0.25%/°C) than silicon, maintaining efficiency in hot climates.
• Good low-light response: They perform well under diffuse or low-irradiance conditions, making them suitable for diverse environments.
• Short energy payback time: Typically, CdTe modules recover the energy used in their manufacture within one year of operation.
• Scalability: Thin-film manufacturing allows for large-area modules and easy integration into utility-scale installations.

Efficiency Trends and Research Innovations

The efficiency of CdTe solar cells has improved markedly due to advances in materials science and process optimisation. Early modules achieved efficiencies around 10%, but modern laboratory cells have surpassed 22%, approaching the theoretical limit for this material.
Key research advancements include:

• Band-gap engineering: Fine-tuning the electronic properties by alloying CdTe with selenium (CdTeSe) to enhance carrier mobility.
• Passivation techniques: Surface and grain-boundary passivation reduce recombination losses.
• Transparent conductive layers: Development of improved front-contact materials such as aluminium-doped zinc oxide (AZO).
• Back-contact optimisation: Introduction of copper-free or stable contacts that improve long-term durability.
• Tandem configurations: Combining CdTe with perovskite or other semiconductors to create high-efficiency multi-junction cells.

Environmental and Safety Considerations

One of the main concerns associated with CdTe technology is the toxicity of cadmium, a heavy metal known to cause environmental and health hazards. However, several factors mitigate this risk:

• In CdTe, cadmium is chemically bound with tellurium, forming a stable compound that is far less toxic and non-volatile under normal conditions.
• CdTe modules are encapsulated between protective glass layers, preventing leakage during operation.
• Manufacturers operate closed-loop recycling systems, recovering up to 90–95% of semiconductor materials and glass at the end of a module’s life cycle.
• The overall cadmium used in CdTe PV systems is significantly lower than cadmium released by coal-fired power generation per unit of electricity produced.

Environmental lifecycle assessments consistently show that CdTe modules have one of the lowest carbon footprints and environmental impacts among all solar technologies.

Comparison with Other Solar Technologies

While crystalline silicon dominates global solar markets, CdTe remains competitive in utility-scale applications due to its lower production cost, robust field performance, and shorter energy payback period.

Applications

CdTe solar modules are widely used in:

• Utility-scale solar power plants: Large installations across the United States, India, and the Middle East rely on CdTe modules for cost-effective electricity generation.
• Building-integrated photovoltaics (BIPV): Due to their thin, lightweight structure, CdTe panels can be incorporated into facades or rooftops.
• Remote and off-grid systems: Their durability and performance in harsh environments make them suitable for rural electrification and telecommunication towers.
• Hybrid and floating solar projects: CdTe’s low weight and good temperature resilience enhance performance in water-based systems.

Limitations and Challenges

Despite its advantages, CdTe solar technology faces certain limitations:

• Material scarcity: Tellurium is a relatively rare element, potentially constraining large-scale expansion.
• Recycling and end-of-life management: While feasible, recycling infrastructure must expand to handle growing deployment volumes.
• Public perception of toxicity: Misconceptions about cadmium safety require ongoing education and transparency.
• Efficiency gap: Although improving, CdTe modules still lag slightly behind the highest-efficiency silicon and perovskite technologies.

Future Prospects

Research continues to focus on improving material utilisation, increasing efficiency, and ensuring sustainable scaling. Emerging innovations include tandem CdTe-perovskite cells, nanostructured interfaces, and flexible thin-film substrates, which promise to enhance performance and broaden applications.
With companies like First Solar investing in next-generation manufacturing facilities and recycling systems, CdTe technology is expected to remain a major contributor to the global renewable energy mix. Its combination of cost efficiency, proven reliability, and rapid scalability positions it as a cornerstone of the transition towards a sustainable, low-carbon energy future.