Efficiency - Page 64

A recent market report by Lux Research stated that perovskites offer several new opportunities for partnerships with universities ahead of a likely commercial deployment between 2019 and 2021. While efficiency of perovskite-based solar cells is constantly improving, the main remaining issues are stability, cost, and the feasibility of real-world efficiencies that must be addressed before commercialization can occur.

Lux Research analysts evaluated the existing state of perovskite solar cells and identified opportunities for companies to partner with academia. Among their findings are the following: partnerships are continuously emerging from labs, and opportunities are still available. Many leading researchers have clear partnerships, but opportunities are still present with various institutions and universities. Also, China is the leading publisher on perovskite solar cells, accounting for a quarter of all academic publications, but more impactful research is coming out of Israel, Switzerland, Singapore, and the UK. China is followed by the USA and South Korea. However, European countries ' the UK, Italy, Switzerland, Germany, Spain, Sweden, France, Greece and Belgium - together account for 24% (almost equivalent to China).

Researchers at the Hong Kong Polytechnic University (PolyU) have reported reaching 25.5% efficiency with a perovskite-silicon tandem solar cell. In the tandem cell, a perovskite solar cell is placed on top so that it can harvest the short wavelength photons, while the bottom layer coated with silicon absorbs the long wavelength photons. To further increase the conversion efficiency, the research team has applied three innovative approaches, involving the use of low-temperature annealing, a tri-layer of molybdenum trioxide/gold/molybdenum trioxide, and a haze film.

The PolyU team believes that the cost of solar energy can drop significantly thanks to that concept, as compared to silicon cells available in the market. The PolyU research team will continue to work on further increasing the efficiency as well as the performance of large-scale fabrication of perovskite-silicon solar cells.

A team of Cambridge scientists, in collaboration with Oxford University AMOLF FOM Institute in Amsterdam, could lead to a revolution in the efficiency of solar power, with the development of perovskite-based panels capable of 'recycling light'.

Solar cells work by absorbing photons from the sun to create electrical charges. However, the process also works in reverse, because when the electrical charges recombine, they can create a photon. The research shows perovskite cells have the extra ability to re-absorb these regenerated photons ' a process known as "photon recycling".

EPFL researchers have achieved the highest yet reproducibility for perovskite solar cells combined with 21.1% efficiency at normal operating conditions, in a cesium-containing perovskite cell. By adding cesium, the EPFL scientists made the first ever triple-cation perovskite mixture (Cs/MA/FA).

The new films are more heat-stable and less affected by changing surrounding variables such as temperature, solvent vapors or the heating protocol used for the device. More importantly, they also show stabilized power-conversion efficiencies of 21.1% and outputs at 18% under operational conditions, even after 250 hours. The researchers regard this as "an absolute breakthrough' and state that these properties are crucial for commercializing perovskite photovoltaics, especially since reproducibility and stability are the main requirements for cost-effective large-scale manufacturing of perovskite solar cells.

A team coordinated by the Centre Suisse d'Electronique et de Microtechnique (CSEM) will work on a €5 million, three-year project to produce solar cells based on perovskite at a size of at least 15x15 cm, while maintaining a conversion efficiency of at least 14%.

In addition to this up-scaling, the research team will develop high-performance cells. Such tandem cells can harvest a broader spectrum of light than a single cell, which should lead to an increase in their efficiency further, approaching the 30% range. The team states that, in the longer term, existing manufacturing methods used to make silicon cells may require only minor modification before being used to produce tandem cells, as the perovskite layer would simply be added on top of the conventional cell to act as an 'efficiency booster'.

Researchers at EPFL in Switzerland have designed a standardized way to make nanowires out of perovskite, by guiding the growth of perovskite nanowires with nanofluidic channels. The researchers used a single-step, 'slip-coating' method to produce the first ever nanowires from methylammonium lead iodide, a material that has attracted attention for its ability to absorb light and produce electrical current in response.

Chinese and Australian researchers have designed a water-resistant perovskite solar cell that can operate in a humid environment and maintain its efficiency over three weeks.

Such cells typically have a methylammonium lead iodide (CH3NH3PbI3) structure, which previous groups have attempted to modify or change completely in order to improve its stability. The researchers in this study, however, have adopted a different approach. Recognizing that water molecules are absorbed by lead in the perovskite's surface layer, the team decided to replace a portion of the surface cations with hydrophobic tetra-alkyl ammonium ions. They invented a dipping technique capable of functionalizing perovskite films in a simple and effective manner.

A team of researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) has engineered a hole-transporting material for perovskite solar cells that costs only a fifth of other existing material options, and offers an improved efficiency of 20.2%.

A 3D illustration of fluorine-dithiophene molecules on a surface of perovskite crystals.

EPFL developed a modified hole-transporting material - a simple dissymmetric fluorine-dithiophene (FDT). The researchers explain that FDT can be easily modified, meaning it could act as a blueprint for the next generation of low-cost hole-transporting materials.

Researchers at the University of Nebraska-Lincoln presented an innovation that could improve perovskite solar cells' efficiency, pushing it forward on the way to rivaling silicon-based cells. The developed process increased the perovskite solar cells' efficiency by more than 2 percentage points, to 19.4%, and the researchers also stress their hopes of achieving 25% efficiency in 3-5 years.

The process involves applying heat and a solvent to a chemical layer that transports energy absorbed by the perovskite to an electrode. Though the effects are not visible to the naked eye, this "solvent annealing" process is said to be similar to polishing a floor so that objects will move more easily across it. This process has been acknowledged by other researchers as "an important direction for further improving the efficiency of perovskite solar cells".

Researchers from an Oxford-Berlin (Helmholtz-Zentrum) collaboration reported that an ultimate efficiency of 30% should be attainable with perovskite-silicon tandem solar cells. They discovered a structurally stable perovskite composition with its band gap tuned to an optimum value of 1.75 eV.

Tandem solar cells work by absorbing the high energy photons (visible light) in a top cell which generates a high voltage, and the lower energy photons (Infra red) in a rear cell, which generates a lower voltage. This increases the theoretical maximum efficiency by about 50% in comparison to a standalone silicon cell. To maximize efficiency, the amount of light absorbed in the top cell has to precisely match the light absorbed in the rear cell. However, the band gap of ~1.6eV of the standard perovskite material is too small to fully exploit the efficiency potential of this technology.