Thermo-cleavable fullerene materials as buffer layers for efficient polymer solar cells

A new class of thermo-cleavable fullerenes, di-tert-butyl methano[60]fullerene-61,61-dicarboxylate (DBMD), bis(2-methylhexan-2-yl) methano[60]fullerene-61,61-dicarboxylate (BMHMD), and di-tert-butyl methano[60] fullerene-61,61-dicarboxylate bis-adducts (bis-DBMD), was facilely prepared through one-step Bingel reaction from C-60. At high temperature, the tertiary alkyl ester groups of the three compounds undergo thermo-cleavage processes to produce insoluble methano[60]fullerene-61-carboxylic acids (MCAs). BMHMD and bis-DBMD were applied in inverted polymer solar cells as a cathode buffer layer. Heating BMHMD or bis-DBMD films on ZnO produced MCA1 and MCA2 interfacial layers, respectively. MCA-modified ZnO possesses a smoother and more hydrophobic surface, and shows excellent solvent-resistance. The MCA buffer layer dramatically improved the device performance by enhancing J(sc) and FF of the solar cells. The power conversion efficiency (PCE) increased from 3.44% (control) to 3.79% (MCA1) and 4.10% (MCA2) for a P3HT/PC61BM solar cell, and from 6.95% (control) to 7.13% (MCA1) and 7.57% (MCA2) for a PBDTTT-C/PC71BM solar cell.