Around 95% of the world’s solar modules are made with silicon. It’s stable against temperature and humidity fluctuations and we’ll never run out of it. But it’s quite inefficient at absorbing sunlight, and very brittle. So the silicon layers in PV have to be quite thick to capture sunlight and resist cracking, leading to heavy and bulky solar panels. The remaining 5% of solar modules are “thin film”, opening the way for game-changing lightweight and flexible applications. But they’re made from rare elements, slow to process, and can be highly toxic. Samantha Hood at Imperial College runs through the “thin film” challenges and describes the research looking into the creation of stable, light-to-electricity efficient, non-toxic modules made from abundant materials that will make terawatt scale solar PV a commonplace so that we can increase today’s 600GW of solar photovoltaic capacity a hundredfold by 2050.
Solar power has tremendous capacity to provide society with clean energy by displacing coal and oil used for electricity generation and transport. One key technology is photovoltaics (PV), which is used in the solar panels you see on the roofs of building and in large farms. Currently there is about 600 GW of solar photovoltaic capacity installed worldwide, with estimates of up to 63 TW (100 times present day) being installed by 2050 thanks to the continuing reduction in price of solar modules, rapid technological advancement and progressive renewable energy policy in the face of climate change.
Currently, electricity and transport emit approximately 25 billion tonnes of CO2 per year. For each terawatt (TW) of PV installed, it is estimated that 1.5 billion tonnes of CO2 can be displaced from electricity and transport industries annually. If we could install a TW of PV each year over the next 16 years, this would eliminate CO2 emissions from these sectors.
Light-to-electricity conversion efficiency target: 15-20%
Improving the efficiency and reducing the energy intensity involved in making conventional solar panels will accelerate this transition towards terawatt scale PV. For a solar cell to be efficient you want to convert as much light from the sun into electricity as possible — simply put, solar cells operate by taking photons in, and letting electrons out. Commercialising a solar material generally requires light to electricity conversion efficiencies to be around 15-20%.
Silicon: pros and cons
Currently, around 95% of the solar modules installed worldwide are made with a silicon absorber layer. Silicon is stable against temperature and humidity fluctuations as well as being earth abundant (the second most common material in the earth’s crust), however silicon is actually quite inefficient at absorbing sunlight and it is very brittle. This means that silicon absorbing layers in PV have to be quite thick to raise the likelihood of capturing sunlight and resist cracking, leading to heavy and bulky solar panels.
“Thin-film” solar cells
In contrast, thin-film solar cells can have much higher absorption coefficients than silicon and can be up to hundreds of times thinner, resulting in lightweight solar cells that can be made on flexible substrates. This makes them attractive for novel applications such as building integrated PV, such as coating windows for powering offices and commercial spaces during the day without the need for structural reinforcement.
Tandem cells for terawatt scale PV
In addition, placing a thin-film of solar absorbing material on top of silicon could boost the efficiency of solar panels without hugely increasing processing costs. This tandem cell design can increase the amount of light absorbed and is a promising path towards the goal of terawatt scale PV.
Today’s “thin films” have drawbacks
Designing materials for thin film PV has been explored for decades. After silicon, second generation PV materials such as gallium arsenide (GaAs), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) were developed for use in thin-film solar cells.
These different materials make up the remaining the 5% of solar modules installed worldwide, and despite their high efficiencies (all over 20%), they aren’t more widespread commercially due to the rarity and high cost of constituent elements (Te and Ga), slow processing conditions (GaAs) or highly toxic components (Cd). Moving to a terawatt scale solar industry requires making new solar panels from materials that are not under threat of running out in the next 100 years.
In the last decade, a class of materials known as thin-film lead-halide perovskites have led to a frenzy of activity which is expected to result in tandem perovskite-silicon cells sometime this year. Perovskites are solution processable, which makes them suitable for large-area applications, but there is concern that the materials are unstable. In addition, perovskites contain lead, which may limit opportunities commercialisation in markets where lead is banned due to toxicity.
Stable, efficient, non-toxic, abundant
In my work, I am searching for stable, high efficiency thin-film materials composed of non-toxic, earth abundant materials for solar absorbers using computational materials science. To understand whether a given material will be a good solar absorber, we can use first-principles techniques to model material features such as band gap (what colour light a material absorbs), absorption strength, and charge transport properties. In particular, we want to know whether defects in a crystal will have a large impact on the possible efficiencies from an absorbing material. Having defect tolerant materials is essential for creating easily processable, efficient and inexpensive solar cells.
Kesterite solar cells
My work, supported by the H2020 project STARCELL, is primarily focussed on developing a better understanding of kesterite solar cells. These solar cells are made of copper, zinc, tin and sulfur or selenium (also known as CZTSSe). The record efficiency of CZTSSe of 12.6% was set in 2013 — a record which has proved difficult to overcome over the past few years.
Crystal defects block electrons
Our work suggests that the main obstacle in this material is the presence of defects in the crystal. In particular, sulfur vacancies (missing sulfur atoms) in the crystal act as traps, capturing charges and limiting the maximum possible efficiency of CZTS. Now, we are investigating the role of these electron trapping defects in a range of emerging earth-abundant absorbers to see whether defects may limit efficiencies of these materials. With our computational insights, we want to work with experimentalists to accelerate the development of promising defect-tolerant solar absorbers.
By screening candidates for solar absorbers with our computation materials techniques, we hope to design the next generation of thin-film earth abundant materials for a terawatt PV industry. We can enhance existing silicon modules by incorporating a thin layer of these new materials on top of silicon (which allows us to absorb in different regions of the solar spectrum), and maybe one day, we can replace silicon altogether with an ultra-thin sandwich of non-toxic, cheap thin-films to further increase efficiencies of solar panels per area. This will help us scale up our solar industry to allow for PV to generate energy for years to come and to help displace carbon emissions from alternative, unsustainable energy sources.
Samantha Hood is a Research Assistant in Photovoltaic Materials Modelling in the Department of Materials at Imperial College
This article is published with permission