As sunlight bathes every inch of Earth’s surface, humanity continually contemplates how to harness this cosmic gift with greater efficiency. Solar cells stand as pivotal vehicles for achieving this objective. Among numerous photovoltaic materials, a newcomer named ‘perovskite’ has surged to prominence over the past decade, rapidly becoming the most sought-after ‘top performer’ in the global solar industry. Its ability to spark a new technological wave hinges on a key factor: its theoretically optimal 1.5 eV bandgap.

I. Perovskite: A Primitive-Sounding Name, Yet Futuristic Performance

Do not be misled by this distinctly mineral-sounding name. While genuine perovskite (CaTiO₃) is indeed a mineral discovered in the Ural Mountains during the 19th century, the term ‘perovskite’ now widely discussed in the photovoltaic community no longer refers to a specific crystal. Instead, it denotes a family of materials sharing the ABX₃ structural framework.

This structure can be conceptualised as a ‘modular building block’ system:

• A site: Large cations (e.g., MA, FA, Cs)
• B site: Metal cations (e.g., Pb, Sn)
• X site: Halide anions (e.g., I, Br, Cl)

Any combination of these yields distinct bandgaps and stability, making it the ‘free-form assembly’ of the materials world.

Particularly noteworthy are organic-inorganic hybrid perovskites. Since Miyasaka’s team first demonstrated their photovoltaic application in 2009, conversion efficiency has soared from 3.8% to over 26% as recorded by NREL, with tandem cells surpassing the 30% threshold.

Crucially, their fabrication is mild, low-cost, and straightforward, requiring neither high-temperature sintering nor ultra-clean facilities, making them highly industrialisable.

Consequently, they are regarded as the ‘third-generation photovoltaic technology’ succeeding silicon cells.

II. Why is 1.5 eV termed the ‘golden bandgap’ for solar absorption?

Sunlight resembles a ‘waterfall of light energy’ composed of photons of varying energies. The task of solar cells is to utilise these photons to excite electrons and generate an electric current. However, if the bandgap lies outside the optimal range, energy is wasted.

If the bandgap is too large (> 2 eV):

Red and infrared light lack sufficient energy, passing directly through the material and causing transparency losses.

If the bandgap is too narrow (< 1 eV),

excess energy from high-energy photons dissipates as heat, causing thermal losses.

Neither extreme is optimal.

Scientists have theoretically calculated that

a bandgap of approximately 1.34–1.5 eV maximises the balance between absorption efficiency and thermal losses, forming the ideal ‘sweet spot’ for solar cells.

At this bandgap, the theoretical upper limit of photovoltaic efficiency (Shockley–Queisser Limit) can exceed 33%.

The remarkable property of perovskite materials lies in their ability to adjust the bandgap through simple element substitution:

Replacing Pb with Sn reduces the bandgap

Adjusting halogens (I → Br → Cl) increases the bandgap

Blending FA/MA/Cs finely tunes crystal stability and energy levels

For instance, the master material MAPbI₃ possesses a bandgap of approximately 1.55 eV. Introducing Sn or FA readily adjusts this to 1.35–1.4 eV—closer to the theoretical optimum—rendering its band structure almost tailor-made for sunlight.

III. Perovskite: Beyond Remarkable Efficiency, Its Applications Defy Imagination

Perovskite’s strengths extend beyond efficiency to its remarkable plasticity:

• It can be formed into transparent solar glass
• Fabricated into flexible films for building facades
• Integrated into wearable devices and compact electronics
• Combined with silicon in tandem cells to overcome efficiency bottlenecks

By 2025, China established the world’s first perovskite all-scenario green electricity demonstration park. Major research institutions continue to break efficiency records, signalling this technology’s accelerated transition from laboratory to industrialisation.

Though stability, weather resistance, and large-scale production remain challenges, perovskite has undeniably become the focal point of global energy technology competition.

IV. 1.5 eV Unlocks the Door to a New Era

In the future, you may witness:

Transparent power-generating windows as clear as glass

Ultra-thin photovoltaic films as lightweight as stickers

Flexible solar textiles as pliable as fabric

Roofs, walls, and even electronic casings silently ‘absorb light to generate power’

These seemingly science-fiction scenarios all stem from that perfect 1.5 eV bandgap and the perovskite material’s flexible, tunable structural advantages.

This photovoltaic revolution, sparked by the bandgap’s “golden point”, has already quietly commenced amidst the wave of energy transformation.