Solution-processed squaraine bulk heterojunction photovoltaic cells.

The donor, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) is used with the acceptor, [6,6]-phenyl C70 butyric acid methyl ester (PC70BM) to result in efficient, solution-processed, small-molecule bulk heterojunction photovoltaic cells. The distribution of the donor nanoparticles in the acceptor matrix as a function of relative concentrations results in a trade-off between exciton dissociation and hole mobility (and hence, cell series resistance). A bulk heterojunction solar cell consisting of an active region with a component ratio of SQ to PC70BM of 1:6 has a power conversion efficiency of 2.7 +/- 0.1% with a 8.85 +/- 0.22 mA/cm(2) short-circuit current density and an open-circuit voltage of 0.89 +/- 0.01 V obtained under simulated 1 sun (100 mW/cm(2)) air mass 1.5 global (AM1.5 G) solar illumination. This is a decrease from 3.3 +/- 0.3% at 0.2 sun intensity, and is less than that of a control planar heterojunction SQ/C60 cell with 4.1 +/- 0.2% at 1 sun, suggesting that the nanoparticle morphology introduces internal resistance into the solution-based thin film. The nanomorphology and hole mobility in the films is strongly dependent on the SQ-to-PC70BM ratio, increasing by greater than 2 orders of magnitude as the ratio increases from 28% to 100% SQ.