The search for more efficient but less expensive solar panels is going. Now, a US team has found that stacking perovskites on to a conventional silicon solar cell can boost efficiency. [McGehee et al (2015) Energy Environ Sci; DOI: 10.1039/C4EE03322A]
Michael McGehee of Stanford University and colleagues point out that the dominant photovoltaic technology based on silicon got stuck at 25% efficiency about 15 years ago. However, a way to break through to higher efficiencies might be to create tandem devices that exploit silicon together with a second inexpensive photovoltaic material. "From a commercial standpoint, it makes a lot of sense to use silicon for the bottom cell," explains McGehee in terms of fabrication and the current silicon manufacturing approach instead of re-engineering an entire industry for a new material. The upper layer might exploit the properties of perovskites the photoactivity of which was revealed only in 2009, when a lead, iodide and methylammonium perovskite was shown to have a sunlight conversion efficiency of 3.8%. Since then, perovskite efficiencies higher than 20% have been made, which is on a par with some silicon photovoltaics
Perovskites harvest only visible wavelengths and, of course, for a tandem device to work the upper layer has to be transparent to the additional wavelengths that can be absorbed by the underlying silicon. The Stanford team has now worked out how to make a transparent perovskite electrode. They used a polymer sheet with embedded silver nanowires to transfer the normally unstable perovskite on to the silicon layer like a temporary tattoo. In their proof of principle experiments they layered a perovskite cell of 12.7% efficiency on to a relatively low-quality silicon cell 11.4% efficiency. The two layers then worked synergistically to give a total efficiency for the tandem device of 17%. "Such a drastic improvement in efficiency has the potential to redefine the commercial viability of low-quality silicon," McGehee explains.
In a second experiment, the team replaced the silicon layer with a 17% efficiency photovoltaic made from copper indium gallium diselenide (CIGS) and stacked the 12.7% perovskite cell on top. This gave them a conversion efficiency of 18.6%. "Since most, if not all, of the layers in a perovskite cell can be deposited from solution, it might be possible to upgrade conventional solar cells into higher-performing tandems with little increase in cost," the team says.
There is one significant issue yet to be addressed. Silicon is heat and light stable, insoluble and as such a silicon solar panel could last for a quarter of a century without degrading. Perovskites are water soluble and break down in sunlight. Materials scientists and chemists must now work to make these materials stable for 25 years before they achieve commercial viability with a tandem device. It could be worth the effort though with lower cost and greater efficiency, perhaps as high as 30% promised by perovskites.
David Bradley blogs at the Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book Deceived Wisdom.