Photovoltaic and Solar Materials

Photovoltaic and solar materials absorb sunlight and convert it into electrical or chemical energy by generating and separating charge carriers, the heart of solar cells and solar fuel devices.

Definition

Photovoltaic and solar materials are solids that absorb solar photons to create separated electron and hole charges, which are collected to deliver electrical power or used to drive chemical reactions that store energy as fuel.

Scope

This topic covers the materials chemistry of solar energy conversion: light-absorbing semiconductors and their band-gap matching to the solar spectrum; the crystalline-silicon, thin-film, dye-sensitised, and perovskite cell families; charge generation, separation, and collection; and photoelectrochemical materials that use sunlight to drive fuel-forming reactions such as water splitting. It links absorber chemistry and interface engineering to conversion efficiency.

Core questions

• How does a material absorb sunlight and generate charge carriers?
• Why must a solar absorber's band gap match the solar spectrum?
• How are photogenerated charges separated and collected?
• How do photoelectrochemical materials convert light to fuel?

Key concepts

• Band-gap matching to the solar spectrum
• Charge generation and separation
• Crystalline-silicon and thin-film cells
• Dye-sensitised and perovskite cells
• Photoelectrochemical water splitting
• Conversion efficiency

Key theories

Light absorption and charge separation

A solar absorber must have a band gap suited to the solar spectrum so that photons generate electron-hole pairs efficiently; an internal field or junction then separates the carriers and directs them to opposite contacts to deliver current.

Photoelectrochemical conversion

In a photoelectrochemical cell, a light-absorbing electrode in contact with an electrolyte generates carriers that drive redox reactions; dye-sensitised and semiconductor photoelectrodes convert sunlight to electricity or to chemical fuels such as hydrogen from water.

Mechanisms

An absorbed photon promotes an electron across the band gap, leaving a hole; a built-in field at a junction or sensitised interface separates the pair before it recombines, and the carriers are collected at contacts to produce current or to reduce and oxidise species in an electrolyte to make fuel.

Clinical relevance

Photovoltaic and solar materials provide renewable electricity at scales from rooftops to power plants, and photoelectrochemical materials offer routes to solar fuels; their development is central to decarbonising energy, with absorber cost, efficiency, and stability the key materials challenges.

History

Crystalline-silicon solar cells emerged in the 1950s, and thin-film absorbers followed. Grätzel and O'Regan's 1991 dye-sensitised cell introduced a molecular, photoelectrochemical approach, and the discovery of efficient halide-perovskite absorbers from around 2009 produced a rapid rise in laboratory efficiencies, broadening the chemistry of solar materials.

Key figures

• Michael Grätzel
• Brian O'Regan
• Akihiro Kojima

Seminal works

• gratzel2001
• chu2012

Frequently asked questions

Why does a solar cell's material have a best band gap?

If the gap is too large, low-energy sunlight passes through unabsorbed; if it is too small, high-energy photons waste their excess as heat. An intermediate gap matched to the solar spectrum captures the most usable energy, which is why absorber chemistry is chosen to hit that range.

What is a solar fuel?

A solar fuel is a chemical, such as hydrogen, made by using sunlight to drive an uphill reaction like splitting water. Photoelectrochemical materials absorb light and use the resulting charges to perform the reaction, storing solar energy in chemical bonds for later use.