Multi-Junction Solar Cells: Pushing Efficiency Limits

Introduction

Solar cells convert sunlight into electricity, but conventional single-junction cells cannot use every part of the solar spectrum equally effectively. This creates a fundamental limit on how much energy can be converted into electricity from sunlight.

A multi-junction solar cell addresses this limitation by combining multiple semiconductor layers with different energy bandgaps. Each layer is designed to absorb a different portion of the solar spectrum, allowing the cell to use sunlight more effectively.

This approach has enabled some of the highest solar conversion efficiencies demonstrated in laboratories and has particular relevance for applications where maximising power from a limited area is important.

What is a Multi-Junction Solar Cell?

So, what is a multi-junction solar cell? It is a photovoltaic device containing two or more semiconductor junctions, with each junction designed to respond to a different range of light wavelengths.

A conventional silicon solar cell has one primary light-absorbing junction. In contrast, a multi-junction solar cell stacks different semiconductor materials so that incoming sunlight can be divided across multiple energy ranges.

The basic concept can be represented as:

Sunlight → Top cell → Middle cell → Bottom cell → Combined electrical output

Higher-energy photons can be absorbed by the upper layers, while lower-energy photons pass through to layers underneath.

How Does Multi-junction Technology Work?

The meaning of multi-junction solar cell becomes clearer when looking at the role of bandgaps.

A semiconductor's bandgap determines the minimum photon energy it can absorb. A single material therefore cannot convert all incoming sunlight with equal effectiveness.

In a multi-junction structure, materials with different bandgaps are combined. The higher-bandgap material can absorb higher-energy photons, while lower-bandgap materials can capture lower-energy photons that pass through the upper layers.

This reduces some of the energy losses associated with conventional single-junction cells.

Why are Multi-junction Cells More Efficient?

A major source of energy loss in a single-junction solar cell occurs because sunlight contains photons with a wide range of energies.

Two situations can occur:

• Photons Below the Cell's Bandgap: They may pass through without generating electricity.
• Photons with Energy Significantly Above the Bandgap: Excess energy is released as heat after the photon creates an electron-hole pair.

Using multiple semiconductor junctions with different bandgaps allows more of the solar spectrum to be matched with suitable absorbing materials.

This is why a multi-junction solar cell can achieve efficiencies beyond those possible with conventional single-junction architectures.

How Efficient Can Multi-junction Solar Cells Be?

A multi-junction solar cell can achieve significantly higher conversion efficiency than conventional silicon cells under certain conditions.

Laboratory records are particularly impressive for multi-junction devices. The National Renewable Energy Laboratory (NREL) maintains a chart of research-cell efficiency records showing multi-junction and III-V technologies among the highest-performing photovoltaic devices.

Multi-junction solar cells have achieved efficiencies exceeding 40% under concentrated sunlight in laboratory demonstrations. However, these figures should not be directly compared with the efficiency ratings of ordinary rooftop solar modules because testing conditions, concentration, and cell configurations can differ.

Where are Multi-junction Solar Cells Used?

The technology is particularly valuable where high power output relative to available surface area justifies greater complexity or cost.

• Space Applications

  Spacecraft have limited surface area for generating electricity. Multi-junction cells can provide high power from relatively small areas, making them useful for satellites and other space applications.

• Concentrated Photovoltaics

  Multi-junction cells can also be used with concentrated photovoltaic systems. Optical equipment focuses sunlight onto a small, highly efficient cell.

  This approach can increase the amount of electricity generated from the cell but requires specialised equipment and thermal management.

• Specialised Terrestrial Applications

  Certain high-performance terrestrial applications can benefit from the technology when efficiency has a higher priority than low manufacturing cost.

  For conventional residential rooftops, however, established silicon-based technologies remain much more common.

What are the Challenges of Multi-junction Technology?

Despite their efficiency potential, multi-junction solar cells are more complex to manufacture than conventional silicon cells.

Key challenges include:

• Higher manufacturing costs
• Complex semiconductor fabrication
• Material availability
• Thermal management
• Difficulty in scaling production
• More complicated module design

The different semiconductor layers must also work together reliably over long operating periods.

For this reason, a technology achieving a very high laboratory efficiency does not automatically become a practical replacement for mainstream solar panels.

Multi-junction vs Conventional Silicon Cells

The comparison shows why a multi-junction solar cell can offer exceptional efficiency while still remaining a specialised technology.

Understanding the Next Generation of Solar Cells

The development of the multi-junction solar cell demonstrates how photovoltaic research is addressing the fundamental efficiency limitations of single-junction technology. By combining semiconductor materials with different bandgaps, these cells can make more effective use of the solar spectrum.

The technology has already demonstrated remarkable efficiency in specialised applications, while tandem architectures are opening new possibilities for terrestrial solar generation. However, manufacturing complexity, durability and cost remain important considerations.

For today's rooftop systems, established silicon technologies continue to offer a practical foundation, while multi-junction and tandem technologies represent an important direction for future photovoltaic development.