Printed solar cells poised for a breakthrough

Posted on February 23, 2015 by Ryan Brown | 0 Comments

Research into alternatives to silicon solar cells is making rapid progress. Viviane Richter reports.

In one hour the Sun sends enough energy to the Earth to power civilization for a year – if only we could capture it. More than 50 years after entering the market, silicon solar cells remain our leading solar technology. Could two cheap contenders finally topple silicon from its rooftop perch?

A mineral called perovskite, known to geologists for more than a century, entered the solar scene five years ago. Meanwhile, a promising jump in the performance of printed carbon-based (organic) solar cells, a more mature challenger to silicon, has reignited interest in this potentially ultra-low-cost solar material.

Each of these contenders has its own strengths and weaknesses, but share the same basic mechanism. Incoming photons dislodge electrons, sending them skittering through the material of the solar cell. The cell’s efficiency depends on how well the material captures light to set these electrons free, and how seamlessly the electrons travel through the material to funnel into an electrical circuit.

Even 60 years ago silicon could readily be processed into the different layers needed for a functioning solar cell. The element is also abundant and non-toxic, although high temperatures are needed to process it into solar wafers, which is expensive.

But although a silicon solar panel bought today might look like one from 20 or 30 years ago, its performance will be light years ahead. By refining the purity of the material, developing surface treatments to maximize light absorption, and improving the panel’s backside electronics, researchers have boosted the efficiency of silicon solar cells handcrafted in the lab from 5% in the early days to 25.6% today. The best commercial mass-produced cells hover around 20%.

By mass producing silicon wafers in large factories in India (shown) and China, and improving production techniques, manufacturers have dramatically cut the cost of silicon solar cells.

At the same time, silicon panels have plummeted in price. Prices were already falling when a wave of giant solar panel factories opened in China, just as the global financial crisis flattened demand. Germany’s Fraunhofer Institute calculated that a 10 kilowatt rooftop system now costs less than a tenth of its 1990 price. While the flooded post-GFC market was a boon for consumers, it also dried up R&D funding for silicon cells. But demand is now rising again. Richard Corkish, chief operating officer at the Australian Centre for Advanced Photovoltaics, is optimistic that as R&D starts up again, silicon’s efficiency will continue to rise with it: “We’ve come through a dark time but another boom is coming,” he says.

But silicon solar cells are facing other challengers. Solar cells made from perovskites – a mineral that typically consists of a precise mixture of lead, iodine and a simple organic component – have jumped from 3% to 20% efficiency after only five years of research, the steepest increase of any solar cell technology to date. “The rise of metal halide perovskites as light harvesters has stunned the photovoltaic community,” wrote Michael Grätzel, solar researcher at the Ecole Polytechnique in Lausanne, Switzerland, in a recent issue of the journal Nature Materials.

Perovskites are “without a doubt the biggest advance in organic solar cells,” agrees Fiona Scholes, organic photovoltaics expert at the CSIRO.

Perovskites’ promise is that they might soon match silicon’s performance, without its costly high-temperature manufacturing step. Perovskite cells can be made by simply printing a layer of perovskite on to a plastic backing.

The efficiency of perovskite solar cells, which are still in development, has risen sharply and rapidly.

The secret to its efficiency is the perovskite crystal’s neat and tidy internal structure, according to research by materials engineer Jinsong Huang and his team at the University of Nebraska published in Science in February. They found electrons can travel in a pure perovskite crystal for three millimetres, far further than they need to reach the electrode in a printed solar cell, where the perovskite layer is typically a mere 500 nanometres thick.

When printed in a thin film, perovskite dries to form separate grains. Increasing grain size is a promising way to boost solar cell performance.

To be truly competitive with silicon solar cells, printable perovskite solar cells would need around 25% efficiency, says Mohite, adding this should be achievable in three to five years.

What might stop perovskites from sweeping silicon from its perch? They are sensitive to moisture and the most common perovskite crystal contains lead, which is toxic, presenting a problem should a cell break. Perovskites are a “high-risk, high-gain game,” says Huang. If printed perovskite cells are to be rolled out on large scale, perovskite structure may need to be optimised to make the material more stable. Researchers are also working on ways to substitute lead with less hazardous elements and engineers are looking into how to seal the material in plastics which will last.

Even so, perovskites have put a rival printable solar technology, “organic” solar cells made from carbon-based light capturing materials, firmly in the shade. Interest in these materials spiked around 10 years ago, but after years of development, efficiencies of organic printed solar cells are still only at 10%.

But a new type of organic solar cell material published in Nature Communications in January has reignited interest in the field.

“It’s a really interesting accidental discovery and we don’t actually understand why it works so well”

Whereas electrons get an easy ride through perovskites, in organic solar cells they’re forced to take a long path through the grid-like molecular structure of the light-capturing layer to reach the electrode. Many of the electrons kicked free by incoming light run out of steam before completing the journey. To compensate, organic solar panel manufacturers make the light-capturing layer only 100 nanometres thick. But printing such ultra-fine layers is a challenge. One alternative would be finding a way to give the organic layer a liquid crystal structure, a jumbled arrangement that leaves plenty of shortcuts for electrons to exploit. Melbourne University chemist David Jones and his team have now discovered such a material – a sulfur-rich molecule known as BTR. “It’s a really interesting accidental discovery and we don’t actually understand why it works so well,” he admits. The material’s efficiency, at 9.3%, is comparable to other organic materials. This efficiency doesn’t drop even when the material is printed in layers four times thicker than usual.

“I was really surprised and excited by this research,” says Doojin Vak, materials scientist at CSIRO. He believes Jones’s material has huge potential.

So will silicon soon be displaced by printed cells? Fiona Scholes says silicon cells have a head start and predicts printed solar cells will form “a key part of the renewable energy mix”. She sees wind turbines in our backyards, silicon solar panels on our roofs and printed cells on our walls and roller blinds. “Every little bit counts,” she says. Vak sees a portable future of solar cells printed on umbrellas and tents where they can harvest power for trips to the beach or a camping weekend.

But even if silicon isn’t dethroned, it might soon find itself sharing power. Silicon excels at capturing red light, whereas perovskites better tap the blue. Together, they might capture far more sunlight that either could manage alone. Researchers are already investigating how to coat perovskite layers on to silicon panels to boost efficiency above 30%.

If we’re serious about ending our dependence on fossil fuels, we need to back all these solar contenders, says Jones. “It’s a very, very tight race,” he says. “It’s exciting to be working in the midst of that,” adds Scholes.

Huang’s discovery signposts a way perovskite performance could jump still further. When printed on to a surface, perovskites form small individual grains like the layer of rice in a sushi roll, and electrons can’t easily jump between grains to reach the electrode. But just a couple of weeks before Huang’s paper, materials scientist Aditya Mohite and his team at the US Los Alamos National Laboratory published in Science a method of maximizing grain size. By slowing the speed freshly applied perovskite dries, Mohite could grow grains up to two millimetres across, 100 times larger than normal. “This grain size is quite stunning,” says Klaus Weber, photovoltaics expert at the Australian National University.


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