Perovskite Solar Cells: How They Work and Why They Degrade

Perovskite solar cells are a class of thin-film photovoltaic devices that have attracted intense research interest because of their rapidly rising efficiency and low-cost potential. The term “perovskite” refers to a crystal structure, not a specific material. The most common perovskite used in solar cells is a hybrid organic-inorganic lead halide, such as methylammonium lead iodide (CH3NH3PbI3). These materials have a unique combination of properties that make them excellent light absorbers: a high absorption coefficient, a tunable bandgap, and long carrier diffusion lengths. As a result, perovskite solar cells have achieved laboratory efficiencies exceeding 25% for single-junction cells, rivaling established technologies like crystalline silicon. However, their commercial deployment has been hindered by stability issues, particularly sensitivity to moisture, heat, and ultraviolet light. This page explains the operating principle, the reasons for their instability, and the concept of perovskite-based tandem cells.

How a Perovskite Solar Cell Works

Like any solar cell, a perovskite device converts light into electricity through the photovoltaic effect. The cell consists of a thin layer of perovskite material sandwiched between charge-transport layers and electrodes. When light strikes the perovskite, photons with energy greater than the bandgap are absorbed, exciting electrons from the valence band to the conduction band, leaving behind holes. The key to efficient operation is the ability to extract these charge carriers before they recombine.

Perovskite materials have high carrier mobilities and long diffusion lengths, meaning electrons and holes can travel relatively far without recombining. This allows for a simple planar device structure, where the perovskite layer is just a few hundred nanometers thick. In a typical architecture, the perovskite is placed between an electron-transport layer (ETL) and a hole-transport layer (HTL). The ETL, often made of titanium dioxide or a fullerene derivative, selectively conducts electrons, while the HTL, such as spiro-OMeTAD, conducts holes. This selective extraction creates a voltage difference, driving current through an external circuit.

One of the reasons perovskites are so efficient is their high open-circuit voltage. The bandgap of methylammonium lead iodide is about 1.55 eV, which is close to the ideal for a single-junction solar cell. The voltage loss, which is the difference between the bandgap and the open-circuit voltage, is relatively small in high-quality perovskite cells, leading to high efficiencies.

The Stability Problem

Despite their impressive performance, perovskite solar cells have a critical weakness: they degrade over time. The degradation is primarily driven by environmental factors, especially moisture and heat. The organic component in the perovskite is hygroscopic, meaning it absorbs water, which breaks down the crystal structure. Exposure to moisture can cause the material to decompose into lead iodide and other byproducts, destroying the cell’s photovoltaic properties.

Heat is another major stressor. Perovskite materials can undergo phase transitions at moderate temperatures, and prolonged exposure to temperatures above 85°C can accelerate degradation. Additionally, ultraviolet light can cause defects and ion migration, further reducing performance. These stability issues are the main reason why perovskite solar cells have not yet entered the mainstream market, despite their high efficiency.

Researchers are addressing stability through various approaches, including encapsulation, compositional engineering (e.g., using cesium and formamidinium instead of methylammonium), and the use of 2D/3D hybrid structures. These methods aim to improve the material’s resistance to moisture and heat, but long-term stability comparable to silicon (which lasts 25-30 years) remains a challenge.

Perovskite Tandem Cells

One of the most promising applications of perovskite technology is in tandem solar cells, where a perovskite cell is stacked on top of a silicon cell. The idea is to capture a broader range of the solar spectrum: the perovskite layer, with a higher bandgap, absorbs high-energy photons, while the silicon cell, with a lower bandgap, absorbs lower-energy photons. This reduces thermalization losses and can push efficiencies beyond the Shockley-Queisser limit for a single-junction cell.

Tandem cells can be configured in two ways: mechanically stacked (four-terminal) or monolithically integrated (two-terminal). In a four-terminal tandem, each subcell has its own electrical contacts, allowing independent operation. In a two-terminal tandem, the cells are connected in series, requiring current matching between the subcells. Two-terminal tandems are simpler to integrate but more sensitive to spectral variations.

Perovskite-silicon tandems have achieved efficiencies above 30% in the lab, surpassing the best single-junction silicon cells. The perovskite layer can be deposited directly on a textured silicon cell using solution processing or vacuum deposition. However, the stability of the perovskite layer remains a concern, as the tandem must last as long as the silicon underneath.

Comparison of Perovskite and Silicon Cells

The table below compares key properties of perovskite solar cells and crystalline silicon cells. Note that the “when not applicable” column indicates situations where a particular property does not apply.

What People Get Wrong About Perovskite Solar Cells

A common misconception is that perovskite solar cells are inherently unstable and therefore useless. While it is true that early perovskite cells degraded quickly, significant progress has been made. Modern perovskite cells can pass damp-heat tests (85°C and 85% relative humidity) for thousands of hours when properly encapsulated. The stability issue is not insurmountable; it is an engineering challenge that is being actively addressed.

Another mistake is to assume that all perovskites are the same. The term covers a wide range of compositions, and the stability and performance vary greatly. For example, replacing methylammonium with formamidinium or cesium can improve thermal stability. The bandgap can also be tuned by altering the halide composition, which is crucial for tandem applications.

People also often confuse laboratory efficiency with commercial viability. A record efficiency in a small cell does not guarantee that a large module will perform similarly. Scaling up perovskite manufacturing introduces challenges such as uniformity and defect control. Additionally, the stability data from lab tests may not reflect real-world conditions, which include temperature cycling, UV exposure, and mechanical stress.

What to Check in a Proposal

If a proposal includes perovskite technology, the following points should be examined:

• Stability data: Look for accelerated aging tests, such as damp-heat and thermal cycling, with results showing minimal degradation.
• Encapsulation: The quality of encapsulation is critical for protecting the perovskite from moisture.
• Warranty: A standard solar panel warranty is 25 years; if the perovskite product offers a shorter warranty, that indicates lower confidence in longevity.
• Certifications: Check for IEC 61215 and IEC 61730 certifications, which are the international standards for PV module reliability and safety.
• Tandem configuration: If it is a tandem, ask about the current matching and how it performs under different light conditions.

Conclusion

Perovskite solar cells offer a promising path to higher efficiency and lower cost, but they are not yet a proven technology for long-term outdoor use. The stability problem is the main hurdle, and while solutions are emerging, they have not been validated over decades. For now, perovskite is most relevant in research and pilot production, and in tandem with silicon to push efficiency limits. As with any emerging technology, it is wise to monitor developments and rely on established technologies for critical applications.