A spotlight on how processing and material advances from next-generation solar apply to other sectors.
By Dr Mark Spratt
Arguably the most promising next-generation solar technology is the perovskite solar cell or module. This technology has the potential to exceed the efficiency of silicon modules on its own, and to greatly exceed it when paired with silicon in a tandem device.
Perovskites as a class of materials are not new, having first been discovered in 1839. But their application to solar energy is relatively recent with the first perovskite solar cell being produced in 2009 using CsSnI3. Since that time perovskites have been studied extensively, leading to rapid advances in solar cell efficiency. Commercial solar modules using perovskites are now starting to reach the market.
The intense research efforts to develop and commercialise perovskite solar over the last 15 years have required development of new processes and materials, and have also led to increased study of perovskite for other applications. Simultaneously, development of other next-generation solar technologies such as organic photovoltaics (OPV) and dye sensitised solar cells (DSSC) has not stopped, and these have opened new fields of application.
What follows is a description of some of the advances resulting from the development of next-generation solar.
1. Dye Sensitised Solar Cells (DSSCs)
DSSCs date back to at least 1991 when Graetzel and O’Regan first published in Nature. Using titanium dioxide, a molecular dye, and an electrolyte, they perhaps more closely resemble a battery than a traditional solar cell, with the exception that one surface must be transparent. During the 1990s and 2000s dye cells were
researched extensively as scientists sought lower cost solutions to the then expensive silicon modules. DSSCs met with some limited commercial success in niche applications such as powering a Logitech wireless keyboard for an iPad under indoor light.
1.1 Sensors
In the early 2010s the UK company G24 Power applied its materials and scientific and manufacturing know-how to develop sensors for VOCs (volatile organic compounds) and other gases, working with UK and European partners. Since that time researchers have continued to apply the properties of DSSCs to sensors. For example, a wearable sensor self-powered by water evaporation, and DSSCs acting as self-powered sensors in physical reservoir computing utilising their light and time response. Materials from DSSCs have also been used in low-cost biosensors and UV detectors.
1.2 Wearables
DSSCs’ ability to operate under indoor lighting and be deposited on flexible substrates made them a natural candidate for application to wearable devices. The low temperature processing and stretchable electrodes developed for this have also enabled products such as wearable health monitors, foldable displays and smart textiles.
1.3 Chemical dissociation
DSSC materials such as TiO2 and ruthenium based dyes have been modified to enable photoelectrochmical water splitting and CO2 reduction. These processes can generate hydrogen and synthetic fuels.
1.4 Electrochromism
DSSCs are electrochromic devices, involving materials that can both generate electricity and be triggered by it. Electrochromic windows are able to change their opacity when a voltage is applied to them. They arose from research into DSSCs and enable smart and dynamic shading of buildings which can lead to greater energy efficiency and occupants’ comfort.
2. Organic Photovoltaics (OPV)
The field of OPV dates back further than DSSCs. In 1958, the year the first solar powered satellite was launched (Vanguard 1), researchers at the University of California reported on “The Photovoltaic Effect and Photoconductivity in Laminated Organic Systems.” Research continued using a variety of organic materials, but really accelerated during the 2000s, with considerable commercial interest. Today the leader in OPV is arguably Germany’s Heliatek with their roll-to-roll production facility for building-integrated PV modules. Efficiencies are still well below those of silicon, Cadmium Telluride or CIGS. But they continue to improve and OPV may find application to sectors where silicon is a poor fit.
2.1 Wearables
Similarly to DSSCs, OPV has been applied to wearables and developments such as low temperature processing. Use of flexible substrates progressed in parallel with those of OPV. Companies such as Konarka actively developed and commercialised both technologies, benefitting from cross-pollination of ideas.
2.2 Sensors
OPVs can be created with high sensitivity to light and tunable bandgaps, properties which have led to them being used in low-light imaging and near-infrared photodetection. Their tunability extends to detection of UV and X-rays.
2.3 Roll to roll printed electronics
Both Konarka and Heliatek have undertaken continuous, roll-to-roll production of OPVs in an effort to realise low-cost, volume manufacturing. These processes are now used for printed electronics such as RFID (Radio Frequency Identification) tags and smart labels, bio-electronics and low-cost printed transistors.
Right: An example of an organic solar mini module (Credit: ATIP, Swansea University)
3. Perovskites
Of the three technologies described perovskites are the most recent to have been used for solar energy. Their versatility, ease of processing, and readily attained high efficiencies have led to a great deal of research. A considerable amount of this research has contributed to other applications.
3.1 Lasers
Perovskites are direct bandgap materials with a high degree of bandgap tunability. They are well suited to high optical gain, multicolour laser applications, potentially at low cost.
3.2 LEDs
The properties of perovskites that lend themselves to lasers also position them well for LEDs. Their ability to be solution processed, avoiding the high temperatures required for current high-quality LEDs and achieve high colour purity holds promise for much lower cost LEDs. These perovskite LEDs could be used in applications such as colour displays, lighting and optical communication. The versatility of perovskite LEDs also opens new fields in “hand-written” LEDs on a wide variety of substrates including textiles.
3.3 Fuel cells
Where the above use cases make use of perovskite optical properties, their electrical properties mean they have also been studied for application in fuel cells. In solid oxide fuel cells all components bar the sealant can potentially be made from perovskite ceramics.
3.4 Sensors
As with DSSCs and OPV, perovskites are well suited to a range of sensing applications. Numerous research groups are studying these materials and how devices might be applied within the IoT. Self-powering devices are particularly attractive to this emerging industry.
3.5 Detecting gunshot residue (and lead more generally)
Beyond applications related to energy and communications, perovskites have also been explored for detection of gunshot residue. In a recent paper the lead content of gunshot residue allowed it to be detected using a sprayed perovskite solution. Whilst there remains criticism of perovskite solar devices for their lead content this study suggests perovskites might be used to detect and remediate atmospheric lead from a range of sources.
4. Associated Material Developments
The applications listed above have been made possible through use of the active materials with DSSCs, OPV and perovskite solar technologies. The study and development of these technologies has also led to advancements in printed electrodes, transparent electrodes and barrier materials, finding application in uses such as printed electronics, batteries, heating elements, display screens and food packaging.
TEA@SUNRISE is part of the Transforming Energy Access platform funded by UK aid from the UK Government to support the technologies, business models and skills needed to enable an inclusive, clean energy transition.