Perovskite solar cells: harnessing clean energy for a bright future

The budding field of perovskite solar cells holds substantial possibilities for the power and energy sector. Research into perovskite materials such as mixed halide (X site) and mixed organic cations (A site) in ABX₃ perovskites has yielded considerable results in recent years, including a rapid increase in power conversion efficiency from initial experimental reports of between 2% and 3% in 2006 to 22% in April 2017.

The number of inventions filed in this technology domain increased significantly after 2013. Research has been carried out mainly in:

• China;
• South Korea;
• the United States;
• Japan;
• Taiwan;
• Germany; and
• France.

China leads the way in perovskite solar cell technology. Analysis of these inventions reveals that Chinese inventors are less interested in Patent Cooperation Treaty applications and are filing their patents exclusively in China. Further, the legal status of these inventions shows that grant rates are significantly higher in China compared to other countries.

Breakthrough
There are many reasons why perovskite solar cells are considered an exciting breakthrough in clean energy, including the potential to develop high energy conversion efficiencies at relatively low cost compared to existing silicon-based solar cells.

The first silicon cell (reported in 1941) had an energy conversion efficiency of less than 1%. Today’s advanced silicon solar cells such as the passivated emitter with rear locally diffused cells and crystalline silicon solar cells have much higher energy conversion efficiencies (24% and 26.63%, respectively). The technology took considerable time to escalate to a point where high energy conversion efficiency was possible at an optimum cost.

However, it took perovskite solar cells less than a decade to increase their energy conversion efficiency from 2% to 22%. Notwithstanding major scientific advancements, including new manufacturing methods and materials, recent research supports the promising future of perovskite solar cells and suggests that this technology could replace existing solar cell technologies in the next few years.

The technology is also being used in basic research to ask and answer a variety of questions concerning the chemistry and structure of perovskite materials – for example, we understand how different material combinations in the perovskite layer allow us to study optical, chemical, physical and electrical properties in different ways. The technology promises broad application in the field of photovoltaics.

Advantages
Perovskite solar cell technology is attracting attention from the wider scientific community for its:

• exceptional light-absorbing properties, exhibited by hybrid organic/inorganic perovskite/organo-metallic halides (ie, broad absorption spectrum and high optical absorption coefficients);
• fast charge separation;
• long-distance electron transport (or holes) and long carrier separation lifetime;
• low exciton binding energy;
• large dielectric constant;
• relatively low-temperature processing from solutions;
• superconductivity; and
• structure (allowing many different elements or compounds to yield exceptional optical and electrical properties).

Challenges
Improving the stability and environmental compatibility of perovskite solar cells remains a key technical challenge.

Patent and non-patent literature reveal that controlling the toxic effect of lead-based perovskite solar cells without affecting the open-circuit voltage and fill factor of the cell is the major concern facing researchers. While the use of alternative materials such as tin and gallium seems promising, the results fall short of lead-based perovskite solar cell technology. Experiments using tin alloys to prepare perovskite structures (eg, CH₃NH₃SnI₃ perovskite, CsSnI₃ and CH₃NH₃SnI_3-xBr_x) demonstrated poor stability and low energy conversion efficiency and fill factors.

The perovskite layer degrades when moisture enters the solar cell structure, leading to poor device performance. While most new inventions in this field aim to improve the stability of perovskite solar cells and their environment compatibility, some focus on solving technical problems such as improving:

• thermal stability of organic materials in cells;
• manufacturing methods;
• open circuit voltage;
• cell structure (thereby improving properties such as electron mobility in the hole transport and electron transport layers); and
• perovskite material absorptivity.

Industry players
Rising energy demands, the detrimental effects of fossil fuels and initiatives by the global community to strengthen the international response to climate change (eg, the Paris Agreement) have shifted the focus of major industry players towards renewable energy. In 2015 renewable solar energy generated $160 billion worth of investment, with approximately 9% invested in thin film solar cells.

Major players involved in R&D around perovskite solar cells include:

• Fujifilm;
• Sekisui Chemical;
• LG Corporation;
• Saule Technologies;
• Fraunhofer ISE;
• Oxford Photovoltaics;
• Panasonic Corporation;
• Hunt Energy Enterprises;
• Xiamen Weihua Solar Co Ltd;
• Dyesol;
• FrontMaterials;
• Solaronix SA;
• Dyenamo;
• Peccell Technologies Inc;
• Tosoh Corporation; and
• Toshiba Corporation.

In 2017 newly formed organisations such as Suzhou GCL Energy Technology Development and Changzhou City Ruitai Materials filed their first inventions in this domain. In the past four years, many universities and institutes have contributed significant technical research in perovskite solar cells, including:

• Huazhong University of Science and Technology;
• Tianjin Vocational Institute;
• Wuhan University of Technology;
• Nanjing University of Science and Technology;
• Korea Research Institute of Chemical Technology;
• Pohang University of Science and Technology;
• North China Electric Power University;
• Soochow University;
• Bohai University;
• Ecole Polytechnique Federale de Lausanne; and
• Oxford University Innovations (previously known as Isis Innovation Ltd).

China’s R&D
China is betting big on renewable energy – the government’s promotion of solar photovoltaic production has greatly influenced China’s role in the global market. The largest number of patent publications in this technological field are being made in China (nearly 55%).

Patent data regarding published inventions in China is a useful indicator of R&D activity in perovskite solar cell technology. In China, universities and institutes carry out most of the reseach; among these, the major players are:

• Huazhong University of Science and Technology;
• Wuhan University of Technology;
• Tianjin Vocational Institute;
• North China Electric Power University;
• Nanjing University of Science and Technology; and
• Soochow University.

The focus of their inventions lies in materials and methods of manufacturing perovskite solar cells (particularly organo-metallic halide solar cells) for improving the energy conversion efficiency, environment compatibility and structure of perovskite solar cells.

Patent literature discloses the manufacture of metal halide perovskite quantum dots, organic-inorganic halide semiconductor and flexible perovskite cell structure, demonstrating the interest of academic institutes in other aspects of this technology.

As major advances have been made in this field over the past five years, it will be interesting to see how patents and their underlying technologies evolve in China.

Looking ahead
Perovskite solar cell technology is developing at breakneck speed. Among its exciting but challenging aspects are the creation of solar cells containing organic-inorganic perovskites and fine-tuning device properties through organo-metallic halide and organic-inorganic halide structures.

Further challenges include commercialising these cells and solving technical challenges relating to their stability and environmental compatibility. The basic material properties have sparked interest in using hybrid perovskite semiconductors in a broader class of energy applications, including:

• sensors and catalyst electrodes;
• fuel cells;
• solar cells;
• lasers; and
• memory devices.

The impressive range of structures and properties (especially intrinsic properties) promises major opportunities for further research into the materials science, basic physics and chemistry surrounding perovskites.

As the world switches to clean energy, perovskite solar cells will play a pivotal role in the power and energy sector. The dream to fully leverage this technology’s potential is encouraging research into the different compounds and combinations in perovskite’s structure, as well as other layers (eg, glass electrode, electron and hole transport layers and metal electrode) in perovskite solar cells.