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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The fi eld of organic photovoltaics (OPVs) is attracting enormous interest due to its promise of providing a low-cost, easily processable technology for energy generation. Whilst the shortcircuit current density ( J SC ) of OPV devices can be optimized through morphology control, [ 1 ] intermixing, [ 2 , 3 ] nanostructuring [ 4 ] or templating [ 5 ] amongst other techniques, the opencircuit voltage ( V OC ) is critically dependent on the choice of photoactive materials. The relative abundance of suitable commercially available materials has led to a wealth of reports on optimization of the donor layer; [ 6 , 7 ] however the same is not true for the acceptor layer, with most work focusing on the ubiquitous fullerenes. Whilst fullerenes are undoubtedly effi cient electron transport materials, this class of acceptor material has inherent issues with long-term stability [ 8 , 9 ] and relatively low bandgaps which limit the maximum obtainable V OC in single heterojunction systems. [ 10 ] Overcoming these issues by use of alternative acceptor materials, whilst maintaining or improving overall effi ciencies, is an important challenge in the OPV fi eld.Initially, the V OC in OPVs was believed to be primarily limited by the work function difference between the two electrodes, however, it has since become clear that the difference in energy between the highest occupied molecular orbital (HOMO) of the donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A), i.e. the interface gap ( I G ), is the primary determinant of the maximum V OC obtained. [ 6 , 11 ] Importantly, the frontier orbital energies of organic materials can be readily tuned through introduction of electron-donating or electron-withdrawing substituents. For example, halogenation of conjugated molecules has been shown to lower the HOMO and LUMO levels with respect to the vacuum level whilst having only a small effect on other electronic properties such as the bandgap, or the position of the Fermi level within the bandgap. [ 12 , 13 ] As an example of the extent of modifi cation possible by this method, complete fl uorination of copper phthalocyanine (CuPc) to its hexadecafl uoro derivative (F 16 CuPc) results in a shift of the HOMO level by over 1 eV. [ 14 ] The D/A heterojunction system of boron subphthalocyanine chloride (SubPc)/fullerene (C 60 ) is amongst the current fi eld-leaders for single small molecule heterojunction devices both in terms of V OC ( ∼ 1.1 V) and overall power conversion effi ciency ( PCE ∼ 3%). [ 15 , 16 ] However, consideration of the interfacial energetics of this system reveals that C 60 is far from optimal as the acceptor (vide infra). In this communication, we report the synthesis of several new selectively chlorinated and fl uorinated SubPcs, and demonstrate that effi cient OPV devices can be constructed using these materials as electron acceptors. Moreover, it is shown that peripheral halogenation can be used to maximize the interface gap, and hence the V OC obtained. As well as a remarkably high V ...