Discover how high-pressure experiments solve the fundamental dilemma in perovskite solar cell development by simultaneously narrowing band gap and prolonging carrier lifetime.
Solar energy is a cornerstone of our clean energy future, but its widescale adoption relies on making solar cells both highly efficient and inexpensive. For over a decade, a class of materials known as organic-inorganic trihalide perovskites has promised to deliver on this goal. These materials are remarkable; they are easy to produce and have seen a meteoric rise in efficiency, now competing with traditional silicon. However, a fundamental scientific challenge has stalled their progress: the inability to simultaneously narrow the material's band gap (to capture more sunlight) and prolong the carrier lifetime (to generate more electricity). This article explores a groundbreaking experiment where scientists used high pressure to solve this dilemma, mapping a new route for the future of photovoltaics 7 8 .
This is the minimum amount of energy needed for an electron in the material to jump from a bound state (the valence band) to a free state (the conduction band) where it can conduct electricity. A solar cell can only absorb sunlight with energy greater than its band gap. If the band gap is too wide, lower-energy photons (like infrared light) are wasted. If it's too narrow, higher-energy photons (like blue light) waste their excess energy as heat. The ideal band gap for a single-junction solar cell, as predicted by the Shockley-Queisser theory, is around 1.34 eV 4 .
Once sunlight frees an electron, creating an "electron-hole pair" (or carrier), that carrier must live long enough to be extracted from the material as electricity. A long carrier lifetime is crucial because it allows the electrical current to build up, directly leading to a higher voltage and efficiency from the solar cell 7 8 .
For years, scientists have faced a frustrating trade-off. Most strategies to narrow the band gap, such as chemical doping, inadvertently disrupt the delicate crystal structure of the perovskite, creating defects that trap and kill charge carriers, thus shortening their lifetime. This trade-off has been a major roadblock to pushing perovskite solar cells to their theoretical limits.
A team of researchers pioneered a clean and innovative approach to this problem: using controllable hydrostatic pressure to tune the properties of pure hybrid perovskites without introducing any adverse chemical or thermal effects 7 8 . This method acts as a "knob" to directly adjust the material's atomic structure and, consequently, its electronic properties.
The researchers prepared high-quality samples of organic-inorganic trihalide perovskites, such as MAPbI₃ (methylammonium lead iodide).
They placed these samples inside a diamond anvil cell (DAC), a sophisticated device that can generate immense, finely controlled pressure by squeezing a sample between two diamonds.
While the sample was under pressure, the team used a suite of optical techniques to measure its properties in real-time:
The results, achieved under relatively mild pressures of around 0.3 GPa (gigapascals), were unprecedented. The researchers observed a simultaneous enhancement in both key properties 7 8 :
The band gap of the perovskite reduced, allowing it to absorb a broader range of the solar spectrum.
The carrier lifetime increased dramatically by 70% to 100%.
This simultaneous improvement was a landmark achievement. Typically, these two properties were thought to be inversely related. The high-pressure experiment proved that the electronic structure of perovskites could be tuned to enhance both light absorption and charge collection at the same time.
| Pressure (GPa) | Band Gap (eV) | Color Change |
|---|---|---|
| Ambient Pressure | 2.28 eV | Orange |
| ~0.22 GPa | 2.37 eV | Yellow |
| ~2.2 GPa | ~2.06 eV | Red (partial) |
| ~27.5 GPa | 1.25 eV | Dark |
| ~35.0 GPa | <1.0 eV | Very Dark |
| Pressure (GPa) | Carrier Lifetime (Relative to Ambient) |
|---|---|
| Ambient Pressure | 1x (Baseline) |
| 9.9 GPa | 20x |
| >13 GPa | Varies, remains enhanced |
The scientists explained this phenomenon by looking at the atomic level. Pressure compresses the lead-halide (Pb-I) octahedra that form the perovskite's backbone. This compression strengthens the interactions between atoms, which can push the energy bands closer together, narrowing the band gap. Furthermore, the specific structural changes induced by pressure seem to reduce the number of charge-trapping defects, creating a cleaner pathway for electrons to travel and thereby extending their lifetime 4 7 .
Working with hybrid perovskites is challenging because they are often soluble in common polar solvents. The table below lists some key materials and methods essential for this field of research.
| Tool/Reagent | Function/Description | Key Feature |
|---|---|---|
| Hydrofluoroether (HFE) Solvents | Electrolyte for characterizing perovskite films without dissolving them 6 . | Chemically orthogonal; enables electrochemical studies on fragile films. |
| Dimethyl Sulfoxide (DMSO) | Common solvent for preparing perovskite precursor inks 2 . | Considered a green solvent with low toxicity and high purity. |
| Pre-synthesized Perovskite Powder | High-purity (>99.99%) precursor for fabricating consistent, high-quality films 2 . | Reduces impurities and defects, improving device performance and stability. |
| Diamond Anvil Cell (DAC) | Device used to generate extremely high hydrostatic pressure on a sample 4 7 . | Allows for clean tuning of material properties without chemical alteration. |
This high-pressure experiment is more than a laboratory curiosity; it provides a "pioneering route" for improving photovoltaic performance 8 . By demonstrating that band gap narrowing and carrier lifetime prolongation can happen together, it guides materials scientists toward new strategies. The goal is now to find ways to mimic the beneficial compressed state of the perovskite without the need for constant external pressure. This could be achieved through chemical engineering, such as introducing smaller atoms or applying internal strain through layered structures.
The knowledge gained is already informing the design of more efficient and stable tandem solar cells, which stack a perovskite cell with a different material (like silicon or an organic semiconductor) to capture different parts of the solar spectrum .
The ability to precisely tune a perovskite's band gap makes it an ideal partner in these next-generation architectures.
This research accelerates the search for lead-free perovskite alternatives, such as those based on bismuth (e.g., Mg₃BiI₃), by providing a deeper understanding of how atomic structure dictates optoelectronic performance 5 .
The journey of perovskite solar cells from a lab novelty to a potential commercial technology has been filled with both excitement and obstacles. The clever application of high pressure has not only solved a key scientific puzzle but has also illuminated the path forward, bringing us one step closer to harnessing the sun's power more completely than ever before.