Tandem organic photovoltaics using both solution and vacuum deposited small molecules

We demonstrate a tandem organic photovoltaic cell incorporating solution- and vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride:C-70 graded heterojunction (HJ) sub-cell is combined with a green and red-absorbing functionalized squaraine/C-70 bilayer HJ sub-cell, resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with un-annealed cells. The incorporation of C-70 in both sub-cells leads to an increase of short-circuit current by at least 30% compared to analogous cells using C-60. The optimized power conversion efficiency of the tandem cell is 6.6% +/- 0.1%, with an open-circuit voltage of 1.97 +/- 0.1 V under simulated 1 sun, AM 1.5G illumination. The tandem cell voltage is equal to the sum of the constituent sub-cells, indicating that the transparent, Ag nanoparticle/MoO3 compound charge recombination layer interposed between the cells is nearly lossless. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4742921]