Halogenated Boron Subphthalocyanines as Light Harvesting Electron Acceptors in Organic Photovoltaics

Abstract

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 ...

Decreased Recombination Through the Use of a Non‐Fullerene Acceptor in a 6.4% Efficient Organic Planar Heterojunction Solar Cell

“…As an acceptor, we investigate the use of hexachlorinated boron subphthalocyanine chloride (Cl 6 SubPc, Figure 1 c). It was previously shown that SubPc/Cl 6 SubPc ( I G = 1.8 eV) increases the V oc compared to SubPc/C 60 ( I G = 1.3 eV), from V oc = 1.1 V to the remarkably high V oc = 1.3 V. [ 18 ] This resulted in an effi ciency of η p = 2.7%, for a fullerene-free solar cell. We here pair the acceptor material Cl 6 SubPc with SubNc.…”

Boron Subphthalocyanines as Triplet Harvesting Materials within Organic Photovoltaics

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 In general, BsubPcs hold significant promise as organic electronic materials, 18 having been demonstrate as both electron donors, 19 electron acceptors, [20][21] and ambipolar interlayers 22 in singlet fission free organic photovoltaic devices. BsubPcs have been used with the complimentary absorbers α-sexithophene and subnaphthalcyanine chloride (Cl-BsubNc) to construct a planar heterojunction photovoltaic cell that absorbed a broad portion of the visible spectrum that also featured an energy cascade (transferring excitons from Cl-BsubPc to ClBsubNc within the cell) leading to a power conversion efficiency of up to ~8 %.…”

8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer

“…Alternatives to fullerenes for vacuum-deposited small-molecule organic solar cells mainly comprise perylene derivatives 4 or halogenated (sub-) phthalocyanines [16][17][18] . However, these non-fullerene acceptors rarely show improved performance over their fullerene counterparts.…”