Review: Organic Photovoltaics: Ubiquitous Solar Energy?by Oxford Energy Network Coordinator
View the associated event for this review
Moritz Riede is Professor of soft functional nanomaterials in the Department of Physics at Oxford University. He has been at the University since 2013 and his research areas are renewable energies with a particular interest in organic solar cells. He is also the Co-Director for thin film cluster for advanced functional materials solar cells, organic solar cells and functional materials.
Moritz began his seminar by giving an outline of organic photovoltaics and how research and development have progressed over the decades. He gives an overview of how the process has been commercialised and an insight into how he sees the future of organic photovoltaics.
There are many photovoltaics, both in research and in commercial use. Silicon is mainly used commercially. Organics and perovskites are the newcomers to the market. The market is huge and growing rapidly.
Photovoltaics and what they can do was the main focus of the seminar. Solar panels have been around as rooftops for some time now and some rooftop areas in India contain panels that are so large, they are visible from space. The panels are a proven process made from rigid and brittle discs.
Organic photovoltaics offers bendable and lightweight features that can be added to tent like and curved roof structures. They can also be used on windows, such as sky scrapers. It offers the same technology as previously developed photovoltaics, ie it is a semi-conductor that converts sunlight into energy. Its main difference is that it’s made from carbon-based materials such as plastic bottles including hydrogen, oxygen, sulphur and others. These come in different forms and usually in strong pigments used in applications such as blue road signs on motorways. They have already been used in Organic Light Emitting Diodes (OLED) such as mobile phones and TV screens and organic solar cells are the focus of this seminar.
How are organic solar cells made?
Earth abundant raw materials are used in the production of solar cells. There are two main ways to produce organic solar cells:
– Solution processing
– Vacuum processing
Both processes are compatible with large roll to roll production using flexible plastic substrates. Manufacturing processes are low temperature, fast and scalable with very little material input.
Vacuum processing is the chosen focus of Oxford’s research using the National Thin Film Cluster Facility for advanced function materials, co-directed by Henry Snaith and Moritz Riede with financial support from the EPSRC, The Wolfson Foundation and the University. Vacuum processing produces OLED. It is commercially successful and the production doesn’t involve any solvents so there is no difficult disposal of materials.
History of development
In the 1970s, a simple system of using one organic material between two electrodes dyes that were already known, was developed, but the performance was not good. Ching Tang, ground-breaking inventor of several electronic devices, was working at Kodak at the time and came up with the idea of using two organic materials and developed the heterojunction concept.
It was quite a journey through the 1980s to develop a reasonable output. Silicon also had a low percentage output when it was first developed.
A major breakthrough occurred in 1986 with the development of the Donor Acceptor Heterojunction, followed by a second breakthrough in the early 1990s when it was discovered that two materials could be mixed together. This was deemed to be way too simplified.
A typical OPV device structure takes the following form:
Flexible substrate > anode > transparent hole transport layer > mixed layer (dyes) > transparent electron transport layer > cathode
The benefit is that these materials are very thin and have good light absorption. Simply place the solar cell into the sun, apply a current and see what voltage is released.
Solar Cell Optimisation
There are lots of options for doing this – millions of organic semiconductors are available for use. Some are solution processable materials and some are vacuum processable materials. Synthetism organic chemistry offers so much opportunity to design materials. There are lots of options and challenges – but which are the best to investigate?
Light absorption – one of the benefits of organic solar cells is the ability to have absorption bands which are good for windows that can generate electricity, but also for greenhouses as adjustments can be made to allow the light in that the plants need to grow.
There are a lot of molecules and a lot of processing parameters involved but what is the right combination?
It is necessary to understand how the molecules pack and what can be done to make them pack better as they have different properties depending on how they pack. Research has been conducted with collaborators including Diamond Light Source to test what has an effective molecular arrangement. Many factors are involved such as life time and light absorption.
Open Circuit Voltage – how much energy can be generated – the higher the better. It is possible to play with the energy levels to improve performance. Comparisons have been made of charge transfer states to analyse different materials. Over the past few years, new molecules have been discovered which have allowed significant increase in efficiency.
Lifetime – it’s not just efficiency of light absorption that counts, lifetime is also a factor. Devices need to last 20 plus years which is hard to emulate in a lab. Experiments rely on microstructure stability. Intrinsically stable solar cells can last over 20 years. Stability depends on the device architecture, encapsulation and use of materials. Organic solar cells get better the warmer they get, proving excellent thermal behaviour. Silicon solar cells tend to worsen the warmer they get.
There are several companies working on OPVs using both solution processing and vacuum processing, including companies that have a traditional background in printing. Processing is done on a large scale with the first applications being curved buildings which are completely covered in OPVs, semi-transparent facades for office buildings and greenhouses, mobile electronics, ie portable solar cell units and lightweight structures which are particularly important in lower income countries. Installation is very easy using double sided sticky tape which saves both costs and energy.
Global market projection
There are two markets – the distributed market and the centralised market. Both are growing rapidly. The overall market in 2020 was around US$20 billion. A global study has been conducted which has identified 193m2 of suitable rooftop area around the world that is applicable to solar. Industry can now provide the first data on organic solar cell use. Sales price for the modules are around €2 per watt peak. They are constructed from 98% hydrocarbons, making recycling very easy. When a solar cell is purchased, electricity is paid for the next 20 years. The longer they are run for, the less the kilowatt per hour charge. There is still room for improvement with organic solar cells and this is achievable.
Speculation is risky but the overall thought is that switching to organic solar cells now has the capacity to save billions or even trillions. They use less materials and low temperature processing resulting in less energy use. There is no need to increase the temperature of production as is necessary when creating glass.
Organic solar cells:
are made from earth abundant non-toxic materials using fast, scalable and inexpensive production processes.
require only 40% of the material and energy input currently used by silicon solar cells.
are light weight and flexible making them open to new ways of deployment.
have the lowest life cycle of greenhouse gas emissions.
are more expensive at the moment but have a huge, global market capacity, and use cheaper materials.
have a more environmentally friendly recycling capacity due to use of raw materials.