15%-20% is for photovoltaic solar cells (http://en.wikipedia.org/wiki/Photovoltaics): where sunlight is converted directly into an electical current. The article here talks about converting solar irradiation into pure heat.
Yeah, but gus_massa calculated the upper bound of the sunlight->electric current conversion using the material we were talking about, which seems like an improvement to me. However I don't know what the ballpark efficiency of solar heat plants is, so it could turn out to be not that big of a deal if current modern plants achieve similar levels of efficiency.