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10 November 2016

Berkeley sets efficiency record for perovskite solar cell

Researchers at the University of California, Berkeley (UCB) and Lawrence Berkeley National Laboratory (LBNL) have reported a new design that already achieves an average steady-state efficiency of 18.4%, with a high of 21.7% and a peak efficiency of 26% (Etgen et al, Nature Materials, DOI: 10.1038/nmat4795).

"We have set the record now for different parameters of perovskite solar cells, including the efficiency," says senior author Alex Zettl, a UC Berkeley professor of physics, senior faculty member at Berkeley Lab and member of the Kavli Energy Nanosciences Institute. "The efficiency is higher than any other perovskite cell – 21.7% – which is a phenomenal number, considering we are at the beginning of optimizing this," he adds.

"This has a great potential to be the cheapest photovoltaic on the market, plugging into any home solar system," notes UC Berkeley physics graduate student Onur Ergen (lead author of the paper).

The efficiency is also better than the 10-20% efficiency of polycrystalline silicon solar cells used to power most electronic devices and homes. Even the purest silicon solar cells, which are extremely expensive to produce, peaked at about 25% efficiency more than a decade ago.

The achievement is due to a new method for combining two perovskite photovoltaic materials – each tuned to absorb a different wavelength of sunlight – into one graded-bandgap solar cell that absorbs almost the entire spectrum of visible light. Previous attempts to merge two perovskite materials have failed because the materials degrade each other's electronic performance.

"This is realizing a graded-bandgap solar cell in a relatively easy-to-control and easy-to-manipulate system," says Zettl. "It combines two very valuable features – the graded bandgap, a known approach, with perovskite, a relatively new but known material with surprisingly high efficiencies – to get the best of both worlds."

Full-spectrum solar cells

As a semiconductor material like silicon, perovskites conduct electricity only if the electrons can absorb enough energy – e.g. from a photon of light – to raise them over the forbidden energy bandgap. These materials preferentially absorb light of specific wavelengths (corresponding to the bandgap energy) but inefficiently at other wavelengths.

"In this case, we are swiping the entire solar spectrum from infrared through the entire visible spectrum," Ergen says. "Our theoretical efficiency calculations should be much, much higher and easier to reach than for single-bandgap solar cells because we can maximize coverage of the solar spectrum."

The key to mating the two materials into a tandem solar cell is a single-atom-thick layer of hexagonal boron nitride, separating the perovskite layers from each other. In this case, the perovskite materials are made of the organic molecules methyl and ammonia, but one contains the metals tin and iodine while the other contains lead and iodine doped with bromine. The former is tuned to preferentially absorb light with an energy of 1eV (infrared) while the latter absorbs photons of energy 2eV (amber color).

The monolayer of boron nitride allows the two perovskite materials to work together and make electricity from light across the whole range of colors between 1eV and 2eV.

The perovskite/boron nitride sandwich is placed atop a lightweight aerogel of graphene that promotes the growth of finer-grained perovskite crystals, serves as a moisture barrier, and helps to stabilize charge transport though the solar cell, Zettl says. Moisture makes perovskite fall apart.

The whole structure is capped at the bottom with a gold electrode and at the top by a gallium nitride (GaN) layer that collects the electrons generated within the cell. The active layer of the thin-film solar cell is about 400nm thick.

"Our architecture is a bit like building a quality automobile roadway," Zettl says. "The graphene aerogel acts like the firm, crushed rock bottom layer or foundation, the two perovskite layers are like finer gravel and sand layers deposited on top of that, with the hexagonal boron nitride layer acting like a thin-sheet membrane between the gravel and sand that keeps the sand from diffusing into or mixing too much with the finer gravel. The gallium nitride layer serves as the top asphalt layer."

It is possible to add more layers of perovskite separated by hexagonal boron nitride, although this may not be necessary, given the broad-spectrum efficiency already obtained, reckon the researchers.
"People have had this idea of easy-to-make, roll-to-roll photovoltaics, where you pull plastic off a roll, spray on the solar material, and roll it back up," Zettl says. "With this new material, we are in the regime of roll-to-roll mass production; it's really almost like spray painting."

Co-authors are S. Matt Gilbert, Thang Pham, Sally Turner Mark and Tian Zhi Tan of UC Berkeley and Marcus Worsley of Lawrence Livermore National Laboratory, who produced the graphene aerogel.

The work was supported by the US Department of Energy, the National Science Foundation (NSF) and the Office of Naval Research (ONR).

Tags: Thin-film PV

Visit: www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4795.html

Visit: www.research.physics.berkeley.edu/zettl

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