http://www.digitaljournal.com/news/environment/perovskite-solar-cells-reach-20-percent-efficiency/article/503813

Perovskite solar cells reach 20 percent efficiency

Posted Sep 29, 2017 by Tim Sandle
In a significant breakthrough, scientists have enhanced perovskite solar cells, enabling the cells to reach record long-term stability and an efficiency of over 20 percent.
Toshiba s 5 cm X 5 cm Film-based Perovskite Solar Cell Mini-module.
Toshiba's 5 cm X 5 cm Film-based Perovskite Solar Cell Mini-module.
Toshiba
Perovskite solar cells, as a theoretical energy source, are attractive to businesses. The cells offer high light-conversion efficiency with low manufacturing costs. One limitation, however, is that perovskite films must also be durable and not degrade under solar light over time. Problems in rendering these affects the commercial viability of the alternative energy technology. The new study comes from the Ecole Polytechnique Fédérale de Lausanne.
The operational stability of perovskite solar cells has been enhanced by introducing cuprous thiocyanate protected by a thin layer of reduced graphene oxide to the cell structure. When the newly fashioned cells were subjected to an accelerated aging test, during which they were exposed to direct sunlight, they lost less than 5 percent performance (in other words the cells retain more than 95 percent of their initial efficiencies). The accelerated aging involved subjecting the cells to 1,000 hours to full sunlight at 60°C. As well as holding energy for longer, the new process has led to an energy efficiency rating of 20 percent (when the cells are exposed to full sunlight).
To put this success in context, standard silicon solar cells have efficiencies plateauing around 25 percent. Moreover, the production costs, on a large scale, are relatively high. In contrast, and as well as being efficient, the new technology offers low-cost and light-weight photovoltaic cells. The new additive, cuprous thiocyanate costs only around $0.5 per gram. The material acts as an electron-hole conductor, primarily accounting for the energy efficiency. Stability was boosted by adding a thin spacer layer of reduced graphene oxide to the cuprous thiocyanate.
According to one of the lead researchers, M. Ibrahim Dar, the technology "will benefit the numerous scientists in the field that have been intensively searching for a material that could replace the currently used, prohibitively expensive organic hole-transporters."
The research has been published in the journal Science. The research paper is titled "Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies >20%."