Squaraine donors for high efficiency small molecule solar cells

It has been a challenge to achieve high efficiency organic photovoltaics (OPV) that absorb long wavelength solar radiation without incurring unacceptable reductions in open circuit voltage (V_oc) or charge separation efficiency. Based on the parent structure of the 2, 4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ), we have increased V_oc using a family of highly near-infrared absorbing SQs, achieving values as high as 0.94 V. These SQ donors are: 2, 4-bis[4-(N-Phenyl-1-naphthylamino)-2,6-dihydroxyphenyl] squaraine (1-NPSQ),2,4-bis[4-(N, N-diphenylamino)-2,6 dihydroxyphenyl] squaraine (DPSQ), 2,4-bis[4-(N, N-diphenylamino)-2,6-dihydroxyphenyl] asymmetric squaraine (DPASQ). The spin-cast SQ, 1-NPSQ, DPSQ and DPASQ donors are then coated with the acceptor C₆₀ to form bulk heterojunction (BHJ) solar cells that take advantage of their exceptionally high absorption coefficient and nanocrystalline morphology to overcome the short diffusion length characteristic of these materials. Combined with a high short-circuit current density (J_sc=10.6 mA/cm²) and high fill factor (FF=0.64), the optimized 1-NPSQ/C₆₀ photovoltaic cells with 1-NPSQ annealed at elevated temperature have a power conversion efficiency of η_p as high as 6.0% (correcting for solar mismatch) at 1 sun (AM 1.5G) simulated solar illumination, which to our knowledge is the highest efficiency reported to date for small molecule OPVs.