A pairing of silicon and perovskite has the potential to achieve significantly higher solar energy conversion efficiencies than standard, single-junction silicon cells. A prototype tandem solar cell developed by researchers at MIT and Stanford University has demonstrated an open-circuit voltage of 1.65 V, which the team said was its best-case scenario. The prototype’s a 13.7 percent power-conversion efficiency still leaves much to be desired. But in this first step the researchers overcame problems around pairing silicon with perovskite that could enable efficiencies greater than the peak 25.6 percent of standard silicon cells. “Despite having higher efficiency, tandems are traditionally made using expensive processes — making it difficult for them to compete economically,” said Stanford doctoral student Colin Bailie. A 1-cm2 monolithic perovskite-silicon tandem solar cell. Courtesy of Rongrong Cheacharoen/Stanford University. Perovskite is an inexpensive crystalline material that can easily be produced in labs. It also absorbs different wavelengths of light than silicon, giving cells that combine the two materials a broader absorption range. The team’s tandem approach focused on keeping costs low and integrating perovskite onto silicon using commonly available semiconductor materials and deposition methods. Before creating their prototype, the researchers first needed to design an interlayer, or semiconductor tunnel junction, 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 MIT graduate student Jonathan P. Mailoa. The team used degenerately doped p-type and n-type silicon, which facilitates the recombination of holes from the silicon solar cell and 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,” Mailoa said. While electrons from a perovskite solar cell wouldn’t normally enter the tunnel junction layer, a titanium dioxide layer commonly used in perovskite solar cells works as an electron-selective contact for silicon. This allows electrons to flow from the perovskite through the TiO2 layer, eventually passing into the silicon tunnel junction, where they recombine with the holes from the silicon solar cell. While the work is still far from becoming commercially available, the researchers said, it frees others to focus efforts on important aspects of the multijunction device and improve its stability and efficiency in the future. One problem that must be addressed is the fact that perovskite degrades quickly when exposed to open air. This formulation may not turn out to be the most advantageous for better solar cells, the researchers said, but is one of several pathways worth exploring. “Our job at this point is to provide options to the world,” said MIT professor Dr. Tonio Buonassisi. “The market will select among them.” Funding came from the U.S. Department of Energy and the Bay Area Photovoltaic Consortium. The research was published in Applied Physics Letters (doi: 10.1063/1.4914179). For more information, visit www.mit.edu.