Using Self‐Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells

Surface modification of indium tin oxide (ITO)-coated substrates through the use of self-assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the anode work function and device performance in molecular solar cells based on a CuPc:C60 (CuPc: copper phthalocyanine) heterojunction. Use of SAMs increases both the short-circuit current density (Jsc) and fill factor, increasing the power-conversion efficiency by up to an order of magnitude. This improvement is attributed primarily to an enhanced interfacial charge transfer rate at the anode, due to both a decrease in the interfacial energy step between the anode work function and the highest occupied molecular orbital (HOMO) level of the organic layer, and a better compatibility of the SAM-modified electrodes with the initial CuPc layers, which leads to a higher density of active sites for charge transfer. An additional factor may be the influence of increasing electric field at the heterojunction on the exciton-dissociation efficiency. This is supported by calculations of the electric potential distribution for the structures. Work-function modification has virtually no effect on the open-circuit voltage (Voc), in accordance with the idea that Voc is controlled primarily by the energy levels of the donor and acceptor materials.

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