Tandem solar cells with two layers of perovskites aim to replace conventional silicon solar cells.
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By Robert F. Service
Silicon dominates the world of solar power. Even the newest solar cell designs, tandem devices that have a silicon solar cell below a cell made of a crystalline material called a perovskite, rely on the material. Now, researchers are doing away with silicon altogether, creating tandems from two of the best yet perovskites, each tailored to absorb a different part of the solar spectrum. Because perovskites are easier to manufacture than silicon cells, the advance could lead to less costly solar power.
“The high efficiency [of] these all-perovskite tandem solar cells is an important advance in photovoltaics and [is] likely to lead to further innovations,” Prashant Kamat, a chemist at the University of Notre Dame in South Bend, Indiana, who was not involved in the new research, wrote in an email to Science.
Silicon solar cells have already made a considerable impact on energy markets. Improvements in technology and manufacturing have dropped the price of these cells some 88% in the past decade, according to a recent analysis by Lazard, a global financial analysis firm. That has prompted, over the same period, a more than 30-fold increase in solar energy deployment around the world to more than 30 billion watts, or 30 gigawatts, of installed capacity, enough to power at least 3.7 million homes.
Perovskite solar cells aim to build on these trends. These crystalline materials, typically made from lead, iodine, bromine, and other abundant elements, are cheap to make; unlike silicon, they are easy to process into sunlight-absorbing layers. Their efficiency at converting sunlight into electricity has also risen to near the level of the best silicon solar cells: from just 3.8% to more than 24% over the past decade.
Perovskites are also better than silicon at absorbing high-energy blue photons from sunlight. That has prompted numerous research groups and companies to marry the two, topping conventional silicon cells—which are better at snagging lower-energy yellow, red, and near-infrared photons—with semitransparent perovskite cells to double up on power production. One such tandem, created by startup Oxford PV in the United Kingdom, can reportedly achieve 28% solar-to-electrical efficiency.
But to do away with silicon altogether requires replicating silicon’s low-energy light-grabbing ability. One strategy is to tailor a perovskite to do the job. In 2014, for example, researchers in Japan and the United States did so by adding tin into the standard recipe for a lead-based perovskite. That has enabled teams around the world to build tandems with two perovskites: a conventional, lead-based high-energy absorbing cell and a tin-lead perovskite that takes the place of silicon. The resulting perovskite tandems are about 23% efficient.
Problems remain. One is that tin readily reacts with oxygen from the air, creating defects in the tin-lead perovskite’s crystalline lattice. These defects disrupt electrical charges’ movement through the cell, limiting the cell’s efficiency. Now, researchers led by Joseph Berry, a physicist at the National Renewable Energy Laboratory in Golden, Colorado, report they’ve found a way to prevent tin in a perovskite from reacting with oxygen. They added a simple organic compound to their tin-lead perovskite mix called guanidinium thiocyanate, which essentially coats the perovskite crystallites that make up the solar absorbing film, preventing oxygen from seeping inside to react with the tin. As a result, the efficiency of the tin-lead perovskite layer jumped from 18% to 20%. When Berry and his team combined this material with a conventional high-energy absorbing top perovskite layer, the resulting tandem cell converted 25% of the energy in sunlight to electricity, they report today in Science.
The efficiency of the new all-perovskite tandems still lags behind the silicon-perovskite pairings from Oxford PV and others. But Berry notes that his team’s lead perovskite wasn’t as efficient as it could have been. So now they are looking to improve that layer.
Kamat says that to produce power for decades in the field, all-perovskite tandems will need to match silicon’s robustness, and they still have a long way to go. But because all-perovskite tandems are expected to be far cheaper to produce than silicon-perovskite tandems or silicon cells alone, it’s a safe bet that scientists will do everything they can to prove their worth.
Source: Science Mag