2層構造のセル(ウエハー)を2枚重ね合わせる構造で、これはSoitec社が得意とする「Smart Stacking Technology」と呼ばれる技術の応用。
下図の様に、違うバンドギャップ(0.7eV, 1.0eV, 1.4eV, 1.8eV)に対応する層を重ねる。

この記事が分かりやすいが、下にその抜粋を。319 sunsで43.6%の変換効率との事。

The result of the French-German collaboration is a four-junction solar cell device with efficiency of 43.6% at a concentration level of 319 suns, as confirmed by the Fraunhofer ISE Calibration Laboratory. The efficiency remains above 43% in a concentration range of 250-500 suns. It is the first time that such a high efficiency has been obtained for a solar cell with four pn-junctions in series. The further development of this new four-junction solar cell offers the opportunity to improve conversion efficiencies even further to 50% in the future.