Improved bulk heterojunction organic solar cells employing C70 fullerenes

We show that the fullerene C-70 is suitable to replace fullerene C-60, which is commonly used as electron transporter and acceptor in small-molecule organic solar cells. It is shown that the higher absorption of C-70 leads to high external quantum efficiencies of over 50% in the spectral range of 500-700 nm. By optimizing the energy level alignment to hole transport layers, the absorption, and the ratio of C-70:zinc phthalocyanine (ZnPc) in a bulk heterojunction solar cell, an efficiency of eta=2.87% is achieved. This is a substantial improvement over an identical solar cell employing C-60 having eta=2.27%. The efficiency increase is due to a higher photocurrent, while fill factor and open-circuit voltage for C-70 and C-60-containing organic solar cells remain comparable.