Solution-Processed Bulk-Heterojunction Solar Cells containing Self-Organized Disk-Shaped Donors

Two molecular disks 1 and 2 composed of a central pyrene core, four oligothiopenes, and peripheral alkyl chains were synthesized and characterized with respect to optical and redox properties in solution and in solid films. It was found that the lowest unoccupied molecular orbital (LUMO) energy levels of 1 and 2 were ideal for achieving efficient electron transfer to fullerene derivatives PC₆₀BM and PC₇₀BM, and that 1 and 2 can function as electron donor components in solution-processed bulk-heterojunction (BHJ) solar cells. Disk-shaped molecules 1 and 2 organized ordered structures through intermolecular π-π interactions as monitored by temperature-controlled polarized optical microscope (TPOM), differential scanning calorimetry (DSC), and powder X-ray diffraction (XRD). Solution-processed BHJ solar cells using 1 or 2 as electron donor materials and fullerene derivatives as acceptor materials were fabricated and investigated. The oligothiophene lengths were reflected in the performance characteristics of solar cell devices fabricated using disk-shaped donors 1 and 2. Power conversion efficiency (PCE) of 2.6% was achieved for small-molecule BHJ solar cells containing self-organized crystals of 2 in the active layer under one sun condition.