Boron Subphthalocyanine Research Articles

Influence of a MoOx interlayer on the open-circuit voltage in organic photovoltaic cells

Metal-oxides have been used as interlayers at the anode-organic interface in organic photovoltaic cells (OPVs) to increase the open-circuit voltage (VOC). We examine the role of MoOx in determining the maximum VOC in a planar heterojunction OPV and find that the interlayer strongly affects the temperature dependence of VOC. Boron subphthalocyanine chloride (SubPc)-C60 OPVs that contain no interlayer show a maximum VOC of 1.2 V at low temperature, while those with MoOx show no saturation, reaching VOC > 1.4 V. We propose that the MoOx-SubPc interface forms a Schottky junction that provides an additional contribution to VOC at low temperature.

Small molecule tandem organic photovoltaic cells incorporating an α-NPD optical spacer layer

We report an improvement in power conversion efficiency in a small molecule tandem organic photovoltaic (OPV) device by the optimisation of current balancing of the sub-cells using an optical spacer layer. A co-deposited layer of N,N’-bis(1-naphthyl)-N,N′-diphenyl-1,1’-biphenyl-4,4’-diamine (α-NPD) and molybdenum oxide was used as the optical spacer layer and provided a highly transparent and conductive layer. Optical simulations showed the addition of the optical spacer in a boron subphthalocyanine (SubPc)/C60 based tandem OPV device increased the SubPc absorption in the front sub-cell and resulted in current balancing through the device. Fabricated tandem OPV devices showed similar trends, with the power conversion efficiency increasing from 2.3% to 4.2% with the addition of an optimised optical spacer thickness. External quantum efficiency and total absorption efficiency measurements back up the optical model data which attribute the increased performance to improved SubPc absorption in the front sub-cell, balancing the photocurrents of the two sub-cells.

Synthesis and fluorescence properties of hexameric and octameric subphthalocyanines based cyclic phosphazenes

Abstract Hexameric and octameric subphthalocyanines bearing cyclophosphazene derivatives were synthesized by threating hexa(p-hydroxyphenoxy)cyclotriphosphazene and octa(p-hydroxyphenoxy)cyclotetraphosphazene with excess of boron subphthalocyanine chloride in toluene. The hexameric and octameric subphthalocyanines bearing cyclophosphazene derivatives were fully characterized by elemental analysis, ESI and MALDI mass spectrometry, FT-IR, 1 H, 13 C and 31 P NMR spectroscopy. The photophysical properties of title compounds were investigated by means of UV/Vis absorption and fluorescence spectroscopies in dilute 1,4-dioxane solutions. The fluorescence quenching behavior of these complexes by 1,4-benzoquinone were also reported.

Boron Subphthalocyanine Polymers by Facile Coupling to Poly(acrylic acid‐ran‐styrene) Copolymers Synthesized by Nitroxide‐Mediated Polymerization and the Associated Problems with Autoinitiation

Boron subphthalocyanines (BsubPcs) are macrocyclic aromatic small molecules containing a chelated boron atom. BsubPcs have interesting optoelectronic and physical properties, justifying their use in various organic electronic devices such as organic solar cells and organic light-emitting diodes. However, our group has only recently reported the first incorporation of a BsubPc moiety into a polymer using a two-step post-polymerization procedure. This communication outlines the use of acrylic acid as a method for obtaining carboxylic acid functional copolymers for the facile coupling to BsubPc post polymerization. In addition, the observations and the proposed mechanism of a side product unique to the copolymerization of acrylic acid and styrene due to autoinitiation are presented.

The influence of strong and weak hydrogen bonds on the solid state arrangement of hydroxy-containing boron subphthalocyanines

We have investigated the occurrence of hydrogen bonding in boron subphthalocyanine (BsubPc) derivatives by synthesizing a series of four hydroxy-containing BsubPcs and studying their solid state arrangements using X-ray diffraction. The series of derivatives comprises hydroxy-BsubPc (HO-BsubPc) as well as para-, meta-, and ortho-hydroxy-phenoxy-BsubPc (p-, m- and o-HOPhO-BsubPc). Each solid state arrangement demonstrated both strong (O–H⋯N, O–H⋯O) and weak (C–H⋯O, C–H⋯N) hydrogen bonds, with the structure for o-HOPhO-BsubPc including examples of bifurcated hydrogen bonds. Many of the C–H⋯O interactions were found to occur alongside notable perturbations to the expected intra- and intermolecular geometry of BsubPcs, providing structural evidence for the significance of weak C–H⋯O bonds. We observed that the imine nitrogen atom of the BsubPc moiety is a reliable hydrogen bond acceptor, and in most arrangements, the hydroxy moiety participated in hydrogen bonds both as a donor and an acceptor. The presence of hydrogen bonds in addition to other intermolecular interactions (π–π and CH–π, which are common in BsubPc solid state arrangements) facilitates the arrangement of BsubPc into two- and three-dimensional networks of closely associated molecules.

A New Exciton Blocking Material for Organic Solar Cell Applications

We report a new efficient exciton blocking material, tris(2,4,6-trimethyl-3-(pyridin-3-l)phenyl) (3TPYMB), for organic solar cell applications. The introduction of 3TPYMB as an exciton blocking layer instead of well known BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) results in the significant improvement of short circuit current (Jsc) and open circuit voltage (Voc) in the boron subphthalocyanine (SubPC)/C60 based organic solar cell devices. As the results, the power conversion efficiency is enhanced by 11.8%. Such improvement with this new exciton blocking layer is mainly attributed to the good electron transportation by deep penetration of Al cathode metal to 3TPYMB layer.

Modified boron subphthalocyanines with stable electrochemistry and tuneable bandgaps

The synthesis of boron subphthalocyanines (BsubPc) from modified phthalonitriles is reported. The BsubPcs have intense red-shifted absorption compared to normal BsubPcs and readily tuneable optoelectronic properties including enhanced electrochemical stability and the presence of up to two reversible electrochemical reductions.

A Comparative Study of the VOC in CuPc and SubPc Organic Solar Cells

The open-circuit voltage (VOC) with respect to the reverse dark saturation current density in planar CuPc/C60 and non-planar boron subphthalocyanine chloride (SubPc)/C60 organic solar cell (OSCs) devices is compared in the present study. The fabricated OSCs devices were: ITO/CuPc/C60/BCP(90Å)/Al(1000Å) and ITO/SubPc/ C60/BCP(90Å)/Al(1000Å) organic solar cells. We observed that the dark saturation current and the VOC values vary in the CuPc/C60 solar cells, whereas almost no variation of the dark saturation current and VOC values in SubPC/C60 solar cells occur. All SubPC devices show almost similar value of dark current. These differences may be attributed to the surface morphologies of planar CuPc and non-planar SubPC donor layers.

Dielectric spectroscopic studies of boron subphthalocyanine chloride thin films

Boron subphthalocyanine chloride (Cl-BsubPc) thin film have been deposited by thermal evaporation technique and studied for structural and dielectric properties. AFM study indicates the formation of uniform and cracks free films. To study the dielectric properties as function of both frequency and voltage, Al/Cl-BsubPc/Al heterostructure has been fabricated. The impedance-frequency study indicates the formation of space charge region in lower frequency range. The conduction mechanism in Cl-BSubPc film, under applied ac field, found to be electronic hopping. The mobility and charge carrier concentration of Cl-BsubPc films have also been measured using capacitance techniques.

Study of Small Molecule Organic Solar Cells Performance Based on Boron Subphthalocyanine Chloride and C60

The small molecule organic solar cells based on boron subphthalocyanine chloride (SubPc) and C<sub >60</sub> by varying the SubPc layer thickness from 3&#x2009;nm to 21&#x2009;nm were fabricated. The maximum power conversion efficiency (PCE) of 1.47&#x25; was obtained at the 9&#x2009;nm SubPc layer under 100&#x2009;mW/cm<sup >2</sup> AM1.5G illumination, which is attributed to reach the optimal balance between the light absorption efficiency and the carrier collection efficiency in the device. To increase the open-circuit voltage (<svg style="vertical-align:-3.39064pt;width:24.575001px;" id="M1" height="15.4" version="1.1" viewBox="0 0 24.575001 15.4" width="24.575001" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.112)"><path id="x1D449" d="M730 650l-8 -28q-52 -4 -72 -18t-64 -77q-79 -113 -321 -539h-33l-119 541q-13 59 -29.5 73t-66.5 20l7 28h245l-8 -28l-28 -5q-33 -6 -40.5 -15.5t-0.5 -38.5l102 -450h2q191 320 246 430q21 42 15.5 56t-43.5 19l-31 4l7 28h240z" /></g> <g transform="matrix(.012,-0,0,-.012,12.675,15.187)"><path id="x6F" d="M257 449q92 0 154 -65t62 -158q0 -112 -67 -175t-150 -63q-98 0 -158.5 66.5t-60.5 154.5q0 59 21 106.5t54.5 75.5t71 43t73.5 15zM244 416q-48 0 -81 -47t-33 -128q0 -96 38 -158t99 -62q51 0 82 43.5t31 139.5q0 91 -36 151.5t-100 60.5z" /></g><g transform="matrix(.012,-0,0,-.012,18.85,15.187)"><path id="x63" d="M390 111l17 -21q-34 -45 -80 -73.5t-89 -28.5q-91 0 -146 62t-55 147q0 118 101 195q74 57 149 57h1q59 0 90 -27q16 -14 16 -30q0 -15 -12 -29t-21 -14q-8 0 -19 11q-44 41 -101 41q-52 0 -87.5 -42.5t-35.5 -117.5q0 -49 15 -87t39 -58t49 -30t48 -10q33 0 60.5 12&#xA;t60.5 43z" /></g> </svg>) of device, the molybdenum oxide (MoO<sub >3</sub>) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) were inserted between the indium tin oxide and the SubPc layer, respectively. Finally, the <svg style="vertical-align:-3.39064pt;width:24.575001px;" id="M2" height="15.4" version="1.1" viewBox="0 0 24.575001 15.4" width="24.575001" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.112)"><use xlink:href="#x1D449"/></g> <g transform="matrix(.012,-0,0,-.012,12.675,15.187)"><use xlink:href="#x6F"/></g><g transform="matrix(.012,-0,0,-.012,18.85,15.187)"><use xlink:href="#x63"/></g> </svg> of device increased from 0.46&#x2009;V to 1&#x2009;V by using MoO<sub >3</sub> buffer layer, resulting in the fact that the PCE of device increased from 1.47&#x25; to 2.52&#x25;.

Halogen bonds can direct the solid state arrangement of phenoxy–boron subphthalocyanines

We report the synthesis and crystal structure analysis of a series of halo-substituted phenoxy–boron subphthalocyanine (phenoxy–BsubPc) derivatives, including meta-halogenated-phenoxy–BsubPcs, ortho-halogenated-phenoxy–BsubPcs, pentabromophenoxy–BsubPc and pentachlorophenoxy–BsubPc. We have found that the majority of the resulting solid state arrangements contain definite halogen bonds, which have not previously been seen in the solid state arrangement of phenoxy–BsubPcs. We observed that the presence and strength of halogen bonds are dependent upon the particular halogen atom. We used semi-empirical methods to calculate the approximate halogen bond strength for the meta-halogenated-phenoxy–BsubPcs. This comparative approach is useful in cases where a structurally-similar isomer exists with a solid state arrangement that is similar, but which differs by exactly one type of significant intermolecular interaction. We also observed that the tendency to form halogen bonds in phenoxy–BsubPcs depends on the position of the halogen atom, and increases in the order: para < ortho < meta. The presence of halogen bonds facilitates the arrangement of BsubPc into networks of closely associated molecules extending into two and three dimensions, resulting in solid state arrangements which are noteworthy exceptions to the typical phenoxy–BsubPc packing motifs.

The first report of the crystal structure of non-solvated μ-oxo boron subphthalocyanine and the crystal structures of two solvated forms

The first instance of the solvent-free X-ray determined single-crystal structure of the oxygen-bridged boron subphthalocyanine dimer [μ-oxo-(BsubPc)(2), C(48)H(24)B(2)N(12)O] is reported. Single crystals obtained by train sublimation were found to have μ-oxo-(BsubPc)(2) organized into a C2/c space group. The crystal structure obtained by sublimation is of particular interest as it is highly symmetric and also of notably high density when compared with other BsubPc crystals. The acquisition of this crystal structure came about from the direct chemical synthesis of μ-oxo-(BsubPc)(2) followed by a work-up which culminated in obtaining the single crystals by sublimation. Several methods for the direct chemical synthesis of μ-oxo-(BsubPc)(2) were also investigated each using dichlorobenzene as the solvent. On standing, these reaction mixtures produced a crystal of the dichlorobenzene (DCB) solvate of μ-oxo-(BsubPc)(2) [μ-oxo-(BsubPc)(2)·2DCB]. It is also reported that the conversion of bromo-boron subphthalocyanine (Br-BsubPc) to μ-oxo-(BsubPc)(2) happens on train sublimation which resulted in the acquisition of a partially hydrated crystal [μ-oxo-(BsubPc)(2)·0.25H(2)O].

Tailored exciton diffusion in organic photovoltaic cells for enhanced power conversion efficiency

Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (L(D)) that restricts migration to the dissociating electron donor/acceptor interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer L(D) by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host material, we optimize the degree of interaction between donor molecules and observe a ~50% increase in L(D). Using this approach, we construct planar-heterojunction OPVs with a power conversion efficiency of (4.4 ± 0.3)%, > 30% larger than the case of optimized devices containing an undiluted donor layer. The underlying correlation between L(D) and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.

Effect of crystal density on sublimation properties of molecular organic semiconductors

Thermal evaporation is an essential process employed in the fabrication of optoelectronic devices based on small molecular organic materials. Knowing the evaporation properties (e.g. sublimation enthalpy, vapor pressure) of archetypal compounds and being able to predict these properties of new compounds is therefore important for the design of processes and deposition apparatus. To address this lack of reliable, easily reproducible information we used thermogravimetry to characterize the sublimation properties of pentacene; tris(8-hydroxyquinolino) aluminum (Alq3); 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA); metal-free phthalocyanine (H2Pc); boron subphthalocyanine chloride (SubPc); iron phthalocyanine (FePc); copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc). A linear relationship was found between enthalpy and vapor pressure, and crystal density, allowing for the estimation of thermophysical properties of untested compounds of a similar class using a simple model.

Sulfonate pseudohalides of boron subphthalocyanine

The crystal structures of three sulfonate pseudohalide derivatives of boron subphthalocyanine (BsubPc) are described and compared with four structures of three published sulfonate derivatives. Benzenesulfonate boron subphthalocyanine [(benzenesulfonato)(subphthalocyaninato)boron, C(30)H(17)BN(6)O(3)S, (I)] crystallizes in the space group P-1 with Z = 2. The structure contains two centrosymmetric π-stacking interactions between the concave faces of the isoindoline units in the BsubPc ligands. 3-Nitrobenzenesulfonate boron subphthalocyanine [(3-nitrobenzenesulfonato)(subphthalocyaninato)boron, C(30)H(16)BN(7)O(5)S, (II)] crystallizes in the space group P2(1)/c with Z = 4. The structure contains an intermolecular S-O···π interaction from the sulfonate group to a five-membered N-containing ring of an isoindoline unit on the concave side of a neighbouring BsubPc ligand, at a distance of 3.151 (3) Å. The crystal of methanesulfonate boron subphthalocyanine [(methanesulfonato)(subphthalocyaninato)boron, C(25)H(15)BN(6)O(3)S, (III)] was produced via sublimation and it is not a solvate, in contrast with two previously published structures of the same compound. Compound (III) crystallizes in the space group P2(1)/n with Z = 2, and its structure is similar to that of the more common compound Cl-BsubPc.

Ultra‐High Voltage Multijunction Organic Solar Cells for Low‐Power Electronic Applications

AbstractLow power electronics are an ideal application for organic photovoltaics (OPV) where a low‐cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra‐high voltage small molecule multijunction devices with open circuit voltage (VOC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi‐layer stacks and ensure current balancing is achieved between sub‐cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub‐cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C60) and SubPc/Cl6‐SubPc were selected both for their high VOC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro‐energy cell type battery under both low illumination intensity white light and monochromatic illumination.

Guard flow-enhanced organic vapor jet printing of photovoltaic donor materials in air

We demonstrate the ability to rapidly deposit (local deposition rates well exceeding 1000Å/s) in air a small molecular electron donor material boron subphthalocyanine chloride (SubPc) for use in photovoltaic devices. The method employs a highly collimated jet of hot nitrogen gas carrying organic vapor toward a substrate, on which the organic molecules condense. A secondary jet of nitrogen coaxially surrounds the primary jet, shielding the organic vapor from the atmosphere. We study how the guard flow rate affects the printed film morphology and the resulting device properties. Notably, the crystallinity of the donor film can be increased while lowering the thin-film roughness, enhancing short circuit photocurrent nearly twofold over films printed without guard flow, allowing the photovoltaic power conversion efficiency of planar heterojunction SubPc/C60 cells to exceed 2% with SubPc deposited at ambient conditions.

A Boron Subphthalocyanine Polymer: Poly(4-methylstyrene)-co-poly(phenoxy boron subphthalocyanine)

Boron subphthalocyanines have been explored as functional materials in a wide range of organic electronic applications including organic light-emitting diodes and organic solar cells. They have only, however, been studied as small molecules; there have been no reports of boron subphthalocyanine polymer(s) in the literature. Herein, the synthesis of the first boron subphthalocyanine-containing polymer, poly(4-methylstyrene)-co-poly(phenoxy boron subphthalocyanine), is reported along with its basic physical characterization/properties. The synthesis of the boron subphthalocyanine polymer was only possible via a postpolymerization functionalization approach, where the boron subphthalocyanine moiety is appended to the side chain of a styrene-based prepolymer. The three-step transformation begins with the nitroxide-mediated copolymerization of 4-methylstyrene and 4-acetoxystyrene to form poly(4-methylstyrene)-co-poly(4-acetoxystyrene). The acetoxy groups are subsequently deacetylated to afford hydroxyl groups prior to phenoxylation with bromo boron subphthalocyanine to afford the title polymer(s).

Boron Subphthalocyanines as Organic Electronic Materials

Boron subphthalocyanines (BsubPcs) are an emerging class of high performing materials in organic electronics. Since the first use of chloroboron subphthalocyanine in an organic electronic device 6 years ago subphthalocyanines have shown potential as functional materials in organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Here we review the material properties of chloroboron subphthalocyanine (Cl-BsubPc) and its use as an organic semiconductor. We then highlight our efforts toward derivatives of boron subpthalocyanine beyond Cl-BsubPc and discuss the impact of molecular design on the material properties and the performance of the BsubPc. Finally, we comment on the status of BsubPcs in the field of organic electronics and discuss how we believe future progress can be made.

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:C70 graded heterojunction (HJ) sub-cell is combined with a green and red-absorbing functionalized squaraine/C70 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 C70 in both sub-cells leads to an increase of short-circuit current by at least 30% compared to analogous cells using C60. 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.