Solar cells of the future

Young researcher at FAU develops system for increasing the efficiency of organic solar cells.

UNIVERSITY OF ERLANGEN-NUREMBERG

Research News

 

Organic solar cells are cheaper to produce and more flexible than their counterparts made of crystalline silicon, but do not offer the same level of efficiency or stability. A group of researchers led by Prof. Christoph Brabec, Director of the Institute of Materials for Electronics and Energy Technology (i-MEET) at the Chair of Materials Science and Engineering at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), have been working on improving these properties for several years. During his doctoral thesis, Andrej Classen, who is a young researcher at FAU, demonstrated that efficiency can be increased using luminescent acceptor molecules. His work has now been published in the journal Nature Energy.

The sun can supply radiation energy of around 1000 watts per square metre on a clear day at European latitudes. Conventional monocrystalline silicon solar cells convert up to a fifth of this energy into electricity, which means they have an efficiency of around 20 percent. Prof. Brabec's working group has held the world record for efficiency in an organic photovoltaic module of 12.6% since September 2019. The multi-cell module developed at Energie Campus Nürnberg (EnCN) has a surface area of 26 cm². 'If we can achieve over 20% in the laboratory, we could possibly achieve 15% in practice and become real competition for silicon solar cells,' says Prof. Brabec.

Flexible application and high energy efficiency during manufacturing

The advantages of organic solar cells are obvious - they are thin and flexible like foil and can be adapted to fit various substrates. The wavelength at which the sunlight is absorbed can be 'adjusted' via the macromodules used. An office window coated with organic solar cells that absorbs the red and infrared spectrum would not only screen out thermal radiation, but also generate electricity at the same time. One criterion that is becoming increasingly important in view of climate change is the operation period after which a solar cell generates more energy than was required to manufacture it. This so-called energy payback time is heavily dependent on the technology used and the location of the photovoltaic (PV) system. According to the latest calculations of the Fraunhofer Institute for Solar Energy Systems (ISE), the energy payback time of PV modules made of silicon in Switzerland is around 2.5 to 2.8 years. However, this time is reduced to only a few months for organic solar cells according to Dr. Thomas Heumüller, research associate at Prof. Brabec's Chair.

Loss of performance for charge separation

Compared with a 'traditional' silicon solar cell, its organic equivalent has a definite disadvantage: Sunlight does not immediately produce charge for the flow of current, but rather so-called excitons in which the positive and negative charges are still bound. 'An acceptor that only attracts the negative charge is required in order to trigger charge separation, which in turn produces free charges with which electricity can be generated,' explains Dr. Heumüller. A certain driving force is required to separate the charges. This driving force depends on the molecular structure of the polymers used. Since certain molecules from the fullerene class of materials have a high driving force they have been the preferred choice of electron acceptors in organic solar cells up to now. In the meantime, however, scientists have discovered that a high driving force has a detrimental effect on the voltage. This means that the output of the solar cell decreases, in accordance with the formula that applies to direct current - power equals voltage times current.

Andrej Classen wanted to find out how low the driving force has to be to just achieve complete charge separation of the exciton. To do so, he compared combinations of four donor and five acceptor polymers that have already proven their potential for use in organic solar cells. Classen used them to produce 20 solar cells under exactly the same conditions with a driving force of almost zero to 0.6 electronvolts.

Increase in performance with certain molecules

The measurement results provided the proof for a theory already assumed in research - a 'Boltzmann equilibrium' between excitons and separated charges, the so-called charge transfer (CT) states. 'The closer the driving force reaches zero, the more the equilibrium shifts towards the excitons,' says Dr. Larry Lüer who is a specialist for photophysics in Brabec's working group. This means that future research should concentrate on preventing the exciton from decaying, which means increasing its excitation 'lifetime'. Up to now, research has only focused on the operating life of the CT state. Excitons can decay by emitting light (luminescence) or heat. By skilfully modifying the polymers, the scientists were able to reduce the heat production to a minimum, retaining the luminescence as far as possible. 'The efficiency of solar cells can therefore be increased using highly luminescent acceptor molecules,' predicts Andrej Classen.

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'Transparent solar cells' can take us towards a new era of personalized energy

Scientists design novel transparent solar cells using thin silicon films, with efficient power generation

INCHEON NATIONAL UNIVERSITY

Research News

 

IMAGE: SCIENTISTS HAVE BEEN DEVELOPING TRANSPARENT SOLAR CELLS THAT MAY SOON FIND THEIR USE IN ALL KINDS OF DEVICES, INCLUDING BUILDINGS, VEHICLES, CELL PHONES, AND SENSORS view more 

CREDIT: JOEL FILIPE ON UNSPLASH

Today, the imminent climate change crisis demands a shift from conventionally used fossil fuels to efficient sources of green energy. This has led to researchers looking into the concept of "personalized energy," which would make on-site energy generation possible. For example, solar cells could possibly be integrated into windows, vehicles, cellphone screens, and other everyday products. But for this, it is important for the solar panels to be handy and transparent. To this end, scientists have recently developed "transparent photovoltaic" (TPV) devices--transparent versions of the traditional solar cell. Unlike the conventionally dark, opaque solar cells (which absorb visible light), TPVs make use of the "invisible" light that falls in the ultraviolet (UV) range.

Conventional solar cells can be either "wet type" (solution based) or "dry type" (made up of metal-oxide semiconductors). Of these, dry-type solar cells have a slight edge over the wet-type ones: they are more reliable, eco-friendly, and cost-effective. Moreover, metal-oxides are well-suited to make use of the UV light. Despite all this, however, the potential of metal-oxide TPVs has not been fully explored until now.

To this end, researchers from Incheon National University, Republic of Korea, came up with an innovative design for a metal-oxide-based TPV device. They inserted an ultra-thin layer of silicon (Si) between two transparent metal-oxide semiconductors with the goal of developing an efficient TPV device. These findings were published in a study in Nano Energy, which was made available online on August 10, 2020 (ahead of the scheduled final publication in the December 2020 issue). Prof Joondong Kim, who led the study, explains, "Our aim was to devise a high-power-producing transparent solar cell, by embedding an ultra-thin film of amorphous Si between zinc oxide and nickel oxide."

This novel design consisting of the Si film had three major advantages. First, it allowed for the utilization of longer-wavelength light (as opposed to bare TPVs). Second, it resulted in efficient photon collection. Third, it allowed for the faster transport of charged particles to the electrodes. Moreover, the design can potentially generate electricity even under low-light situations (for instance, on cloudy or rainy days). The scientists further confirmed the power-generating ability of the device by using it to operate the DC motor of a fan.

Based on these findings, the research team is optimistic that the real-life applicability of this new TPV design will soon be possible. As for potential applications, there are plenty, as Prof Kim explains, "We hope to extend the use of our TPV design to all kinds of material, right from glass buildings to mobile devices like electric cars, smartphones, and sensors." Not just this, the team is excited to take their design to the next level, by using innovative materials such as 2D semiconductors, nanocrystals of metal-oxides, and sulfide semiconductors. As Prof Kim concludes, "Our research is essential for a sustainable green future--especially to connect the clean energy system with no or minimal carbon footprint."

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Reference

Authors: Sangho Kim (1,2), Malkeshkumar Patel (1,2), Thanh Tai Nguyen (1,2), Junsin Yi (3), Ching-Ping Wong (4 ), Joondong Kim (1,2)

Title of original paper: Si-embedded metal oxide transparent solar cells

Journal: Nano Energy

DOI: 10.1016/j.nanoen.2020.105090

Affiliations:

(1) Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute for Future Energies (MCIFE), Incheon National University, Republic of Korea

(2) Department of Electrical Engineering, Incheon National University, Republic of Korea

(3) College of Information and Communication Engineering, SungKyunkwan University, Suwon, Republic of Korea

(4) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, United States

About Incheon National University

Incheon National University (INU) is a comprehensive, student-focused university. It was founded in 1979 and given university status in 1988. One of the largest universities in South Korea, it houses nearly 14,000 students and 500 faculty members. In 2010, INU merged with Incheon City College to expand capacity and open more curricula. With its commitment to academic excellence and an unrelenting devotion to innovative research, INU offers its students real-world internship experiences. INU not only focuses on studying and learning but also strives to provide a supportive environment for students to follow their passion, grow, and, as their slogan says, be INspired.

Website: http://www.inu.ac.kr/mbshome/mbs/inuengl/index.html

About Professor Joondong Kim

Joondong Kim is a Professor at the Department of Electrical Engineering in Incheon National University, Korea, and the head of Multidisciplinary Core Institute for Future Energies (MCIFE). He majored in electrical engineering and earned his PhD in 2006 from the University at Buffalo, State University of New York, USA. His research is focused on the design of functional materials and neo-conception devices, neuromorphic memories, photosensors, and transparent photovoltaics. He has published about 220 SCI papers and holds 150 patents.