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Tandem photovoltaics for record efficiencies? By Paul Buckley Researchers from Massachusetts Institute of Technology (MIT) and Stanford University are exploring ways to create solar cells using low-cost manufacturing methods. A novel prototype device combines perovskite with silicon solar cells to form a two-terminal ‘tandem’ device. As the team reports in the journal Applied Physics Letters, from AIP Publishing, the tandem cells have the potential to achieve higher energy conversion efficiencies than standard single-junction silicon solar cells. Stacking perovskite on top of a conventional silicon solar cell forms a tandem device that has the potential to improve the cell’s overall efficiency. The research team focused on tandem solar cells because there was big room for improvement in their cost and market penetration. Tandem solar cells have only garnered a worldwide market share of 0.25 percent compared to the 90 percent market share captured by silicon solar cells. “Despite having higher efficiency, tandems are traditionally made using expensive processes - making it difficult for them to compete economically,” explained Colin Bailie, a Ph.D. student at Stanford and an author on the research paper. The team’s tandem approach focuses on keeping costs low and “integrates perovskite solar cells monolithically -- building them sequentially in layers - onto a silicon solar cell, without significant optical or electrical losses, by using commonly available semiconductor materials and deposition Schematic diagram shows the layered structure of the hybrid solar cell. The top sub-cell, made of pervoskite (blue), absorbs most of the high-energy (blue arrow) photons from the sun, while letting lower energy (red arrow) photons pass through, where they are absorbed by the lower sub-cell made of silicon (gray). The other layers serve to provide electrical connections between the two sub-cells, and to carry the electricity generated by the cells out to wires. Image courtesy of the researchers (edited by Jose-Luis Olivares/MIT) methods,” explained Jonathan P. Mailoa, a graduate student in MIT’s Photovoltaic Research Laboratory and a co-author on the paper. Before creating the tandems, the researchers first needed to design an interlayer to facilitate electronic charge carrier recombination without significant energy losses. “Fortunately, the physical concepts already exist for other types of multijunction solar cells, so we simply needed to find the best interlayer material combination for the perovskite-silicon pair,” said Mailoa. To form a connecting layer, known technically as the semiconductor ‘tunnel junction’, between the two sub-cells, the team used degenerately doped p-type and n-type silicon, which facilitates the recombination of positive charge carriers (holes) from the silicon solar cell and negative charge carriers (electrons) from the perovskite solar cell. Because the two silicon layers are highly doped, “the energy barrier between them is thin enough so that electrons and holes in the semiconductor easily pass through using quantum mechanical tunneling,” explained Mailoa. While electrons from a perovskite solar cell will not normally enter this tunnel junction layer, a titanium-dioxide (TiO2) layer commonly used in perovskite solar cells works as an electronselective contact for silicon allowing electrons to flow from the perovskite solar cell through the TiO2 layer, eventually passing into the silicon tunnel junction, where they recombine with the holes from the silicon solar cell. Once the tandem is set up, it relies on its multiple absorber layers to absorb different portions of the solar spectrum. “The perovskite absorbs all of the visible photons higher in energy, for example, while the silicon absorbs the infrared photons lower in energy,” said Bailie. Splitting the solar spectrum allows these specialized absorbing layers to convert their range of the spectrum into electrical power much more efficiently than a single absorber can convert the entire solar spectrum on its own. “This minimizes an undesirable process in solar cells called “thermalization”, in which the energy of an absorbed photon is released as heat until it reaches the energy of the absorber’s bandgap,” Bailie explained. “Using a high-bandgap absorber on top of the low-bandgap absorber recovers some of this energy in the high-energy photons that would otherwise be ‘thermalized’ if absorbed in the low-bandgap absorber.” Another key part of the tandem’s design is that it uses a serial connection, which means that the two solar cells are connected in a manner so that the same amount of current passes through each of the solar cells which means that the same amount of light is absorbed in each solar cell and their voltage is added together. The team’s tandem “demonstrated an open-circuit voltage of 1.65 V, which is essentially the sum of top and bottom cells, with very little voltage loss,” said Tonio Buonassisi, an associate professor of mechanical engineering at MIT who led the research. An open-circuit voltage of 1.65V was the highest best-case scenario the team had predicted, which indicates that their tunnel junction performs very well. But an efficiency evolution is on the horizon. The efficiency record for single-junction perovskite cells ranges from 16 to 20 percent, depending on the formula used. “By contrast, the perovskite in our tandem is based on a technology that achieves only 13 percent in our lab as a single-junction device,” said Bailie. “Improving the quality of our perovskite layer will lead to better tandem devices.” Another area for improving the tandem’s efficiency is by “reducing parasitic optical losses in other layers of the multi-junction solar cell devices and predicting their efficiency potentials through simulation to determine whether or not this approach is truly cost effective,” added Mailoa. www.electronics-eetimes.com Electronic Engineering Times Europe April 2015 23


EETE APR 2015
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