Perovskite

Definition

Perovskites are a class of naturally occurring or synthetic metal salts with a characteristic crystal structure. The crystals form a cubic lattice of two different positively charged ions (A and B cations) and negatively charged anions in the lattice spaces. The most well-known synthetic variants are metal halide perovskites (MHP), in which metals such as lead or tin are used as A cations and caesium or methylammonium as B cations. Halides such as chloride, iodide or bromide are used as anions. The specific crystal structure leads to excellent optoelectronic properties that can be flexibly modified and controlled via the composition, meaning perovskite crystals are good semiconductors. Semiconductors are materials that switch between being conductive or non-conductive depending on the conditions. They work like a switch and are a key element in the digital logic of computer technology. In addition to this property, perovskites can also specifically transmit and receive light signals, making them ideal for applications in photonics and photovoltaics.  

Current applications and opportunities

Synthetic perovskite crystals are mainly used in three forms:  

• Firstly, as polycrystalline thin films in tandem solar cells. These consist of two different perovskite layers or one perovskite layer and one silicon layer. As a result, the spectrum of light captured can be broadened and the efficiency of the solar energy conversion can be increased to more than 30 percent. Conventional solar cells based on silicon monocells achieve less than 25 percent. Thanks to their strong light absorption, thin films made of perovskite crystals are also suitable as image sensors for high-quality colour cameras both in the consumer goods market and in niche applications, such as machine monitoring or as sensors in precision agriculture.    

• Secondly, perovskite nanocrystals can be used as quantum dots. They emit individual photons at exactly the desired wavelength, making them excellent light sources for screens and light-emitting diodes. Perovskite-based quantum dot displays in TV screens, luminous films or micro LEDs are characterised by brilliant colours and energy-efficient operation. 

• Thirdly, perovskites can also be synthesised as monocrystals up to 1 centimetre in size. They are suitable as highly sensitive detectors for X-rays or gamma rays used in medical imaging or for applications during security checks, such as scanning luggage. 

At present, only a few commercial products are available. Several companies have announced they are entering the market with perovskite products for photovoltaics, but they often have an overly optimistic timeline. The start-up Perovskia is starting to market MHP solar cells for the Internet of Things sector (see Internet of Things).  

Growth in the field of photovoltaics is driven by universities alongside numerous start-ups and established companies. The other applications are currently being driven by academic research, primarily in research groups at ETH Zurich, Empa, EPFL and CSEM in Neuchâtel in Switzerland. 

Challenges

A major challenge faced in all applications is the stability and operational reliability of the products. Solar modules based on MHP thin films can degrade after just a few hundred hours of operation under harsh weather conditions or in humidity – this is far too fast compared to silicon solar cells that can work flawlessly for 25 years or more.  

Most MHP compositions also contain small amounts of lead, which pollutes the environment when it is disposed of. Developing lead-free and non-toxic alternatives and expanding the means of recycling could speed up the journey towards market maturity for all perovskite applications. Depending on the device architecture and the way in which the amount of lead is calculated, MHP-based products may need an exemption from the EU Restriction of Hazardous Substances (RoHS) Directive.  

Focus on industry

Perovskites will play an important role in producing solar modules, measuring instruments and sensors in the future. One business area is the production of perovskites, and a second is the production of components. Unlike silicon solar cells, which are manufactured from high-purity monocrystals at high temperatures, MHP-based products can be printed, vaporised or separated out of solution from chemicals, solvents and melts at relatively low temperatures of 100–150 degrees Celsius, without requiring clean room conditions, and then processed into lightweight and flexible thin films. Compared to heavy, rigid glass plates made of silicon cells, these can be cut to a special size and thus expand the possible applications of solar modules, for example in photovoltaics integrated into buildings.  

Employees working in development require in-depth background knowledge of chemistry, materials science, solid state physics and nanotechnology. On top of this, they need specific knowledge of the relevant manufacturing processes. Knowledge of microtechnology, additive manufacturing and thin-film deposition is needed in the application of this technology. Depending on the application, expertise in optical devices, camera design and software design is also required.  

Switzerland has excellent training in this area, but given the rapid development, there is a threat of an increasing shortage of skilled workers, meaning that Switzerland will be more and more dependent on foreign experts.  

International perspective

The USA, China and the EU are amongst the leading nations in the development of perovskite optoelectronics. Global commercialisation is still in its infancy, with initial field trials and cautious scaling steps now underway. Swiss research groups are heavily involved in international collaborations with higher education institutions and companies.  

Future applications

If the light sensitivity of MHP crystals can be expanded into the infrared range, they could also be used in the future for monitoring semiconductor production, medical imaging, environmental monitoring and military surveillance.  

Perovskite nanocrystals can be used as light sources in quantum physics and, as such, they are able to generate individual photons in a controlled manner. The properties of these individual photons are virtually no different in terms of wavelength, phase and polarisation and are essential for developing quantum cryptography, quantum computers and quantum communication. 

Perovskite monocrystals are efficient radiation detectors that enable more precise diagnoses with lower radiation exposure, for example in computer tomographs. Applications in advanced imaging techniques in space travel and military defence or for security checks are also conceivable.  

Perovskites offer great potential, as thanks to their unique physical and optoelectronic properties, they can take photovoltaics, the semiconductor and sensor industries and optoelectronic applications to the next level. They are driving miniaturisation in these areas and enable energy and resource-conserving production, which has a positive impact on the sustainability of the corresponding products. They also offer Swiss start-ups and SMEs the opportunity to gain an advantage in niche applications and reduce Europe’s dependency on photovoltaic modules manufactured in China.