Boron Subphthalocyanine Chloride Research Articles

Redetermination of the crystal structure of boron subphthalocyanine chloride (Cl-BsubPc) enabled by slow train sublimation.

The crystal structure of boron subphthalocyanine chloride [systematic name: chlorido(subphthalocyaninato)boron], C24H12BClN6, a material of widespread interest in organic electronic device applications, has been redetermined with a higher precision using large single crystals obtained via slow train sublimation. Details are given for the construction and operation of the train sublimation system, which has been designed to reproducibly yield single crystals suitable for diffraction experiments in a manner which approximates the vacuum deposition conditions commonly used to fabricate organic electronic devices. Diffraction experiments were conducted using two crystal samples and four temperatures (90, 123, 147 and 295 K), enabling a discussion of changes in the unit cell and intermolecular interactions with respect to temperature and in comparison to two previously published structures of Cl-BsubPc. The redetermined structure confirms the original structure published 41 years ago [Meller & Ossko (1972). Monatsh. Chem. 103, 150-155], with significantly improved precision for the geometric parameters. Analysis of the crystal structure revealed three intersecting ribbon motifs formed through a combination of π-π and halogen-π (specifically B-Cl...π) interactions. H atoms were refined independently in order to facilitate a thorough discussion of these intermolecular interactions using Hirshfeld surface analysis.

Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell

The interfacial band alignment among boron subnaphthalocyanine chloride (SubNc), boron subphthalocyanine chloride (SubPc), and α-sexithiophene (α-6T) is explored using ultraviolet, inverse, and X-ray photoemission spectroscopies (UPS, IPES, and XPS, respectively). With these tools, the ionization energy (IE) and electron affinity (EA) for each material are determined. Layer-by-layer deposition of SubPc and SubNc on α-6T as well as SubPc on SubNc, combined with UPS and IPES, allows for the direct determination of the energy level alignment at the interfaces of interest. A small dipole is found at the α-6T/SubNc/SubPc interface, expanding the donor-LUMO to acceptor-HOMO gap and explaining the large open circuit voltage obtained with these devices. However, there is a small electron barrier between SubNc and SubPc, which may limit the efficiency of electron extraction in the current device configuration. Excess chlorine may be responsible for the high IE and EA found for SubNc and could potentially be remedied with improved synthetic methods or further purification.

Evaluating the role of energetic disorder and thermal activation in exciton transport

Temperature dependent measurements of the exciton diffusion length (LD) are performed for three archetypical small-molecule, organic semiconductors: aluminum tris-(8-hydroxyquinoline) (Alq3), dicyanovinyl-terthiophene (DCV3T), and boron subphthalocyanine chloride (SubPc).

Axially diethylaminophenoxypropanoxy substituted new subphthalocyanines: synthesis and electropolymerization properties.

The treatment of boron(III) subphthalocyanine chloride with 1,3-bis[3-(diethylamino)phenoxy]propan-2-ol 2 and 2,3-bis[3-(diethylamino)phenoxy]propan-1-ol 4 in toluene gave the corresponding axially substituted boron(III) subphthalocyanine compounds 3 and 5. The novel axially diethylaminophenoxypropanoxy substituted subphthalocyanines were characterized by standard spectroscopy methods. The electropolymerization properties of new axially diethylaminophenoxypropanoxy substituted subphthalocyanines were also characterized by using cyclic and square wave voltammetry techniques. This study is the first example of electropolymerization of axially substituted subphthalocyanines in the literature.

Aluminum Chloride Mediated Alkynylation of Boron Subphthalocyanine Chloride Using Trimethylsilyl-Capped Acetylenes.

A mild and versatile procedure is presented for functionalization of boron chloride subphthalocyanine at the axial boron position with trimethylsilyl-protected alkyne nucleophiles in the presence of aluminum chloride. The method allows a large variety of substituents on the alkyne units, including electron-donating/withdrawing aryl groups, silyl-protected alkynyl groups, as well as ferrocenyl and azulenyl groups. In addition, ferrocene itself reacts smoothly under these conditions allowing for directly anchoring it to the boron of the subphthalocyanine.

Synthesis of phthalonitrile derivatives by photoinduced reactions. New unsymmetrical substituted zinc phthalocyanines

The synthesis of new phthalonitrile derivatives by photoinduced reaction is described. Novel unsymmetrically substituted Zn(II) phthalocyanine bearing an aryl alcohol group (ArZnPc) was synthesized by the ring expansion reaction of boron(III) subphthalocyanine chloride with an appropriated phthalonitrile. The spectroscopic and photodynamic properties of these ArZnPc were studied.

Highly efficient tandem organic light-emitting diodes based on SubPc:C60 bulk heterojunction as charge generation layer

High efficiency tandem organic light-emitting diodes (OLEDs) were realized using an organic bulk heterojunction (BHJ) as charge generation layer (CGL) consisted of boron subphthalocyanine chloride (SubPc) and fullerene (C60). The results showed that the SubPc:C60 based CGL is a promising connecting unit for the fabrication of high efficiency tandem OLEDs with 2.64 folds enhancement in a current efficiency of 63.6cd/A, compare to the corresponding single-unit OLEDs. The efficiency enhancement of tandem OLEDs is attributed to better charge carrier balance derived from the optimized charge transport pathways and reduced interface energy barrier. It showed that the charge generation ability and charge transport characteristics of CGL were strongly dependent on SubPc:C60 mixing ratio and corresponding film morphology, which was verified by the charge extracting analysis. Additionally, the highest occupied molecular orbital energy level of SubPc is compatible for most of hole transporting layer to minimize the hole injection barrier and suppress the hole accumulation and charge recombination in tandem OLEDs.

Utilising solution processed zirconium acetylacetonate as an electron extracting layer in both regular and inverted small molecule organic photovoltaic cells

Interfacial layers are commonly employed in organic photovoltaic (OPV) cells in order to improve device performance. These layers must be transparent, stable, be compatible with the photo-active materials and provide efficient charge extraction with a good energetic match to the relevant organic material. In this report we demonstrate the compatibility of zirconium acetylacetonate (ZrAcac) electron extracting layers in both regular and inverted small molecule OPV cells. When the ZrAcac was processed in both air and under N2, low work function (3.9 and 3.7eV respectively), highly transparent layers were formed, with good energetic alignment to both C60 and hexachlorinated boron subphthalocyanine chloride (Cl6-SubPc) acceptors. Initial measurements indicate similar stabilities when using the ZrAcac in either device architecture. These results indicate that the ZrAcac layer can be used as a direct replacement for the commonly used bathocuproine (BCP) in small molecule OPV cells.

The Role of Exciton Ionization Processes in Bulk Heterojunction Organic Photovoltaic Cells

To realize efficient photoconversion in organic semiconductors, photogenerated excitons must be dissociated into their constituent electronic charges. In an organic photovoltaic (OPV) cell, this is most often accomplished using an electron donor–acceptor (D–A) interface. Interestingly, recent work on MoOx/C60 Schottky OPVs has demonstrated that excitons in C60 may also undergo efficient bulk‐ionization and generate photocurrent as a result of the large built‐in field created by the MoOx/C60 interface. Here, it is demonstrated that bulk ionization processes also contribute to the short‐circuit current density (JSC) and open‐circuit voltage (VOC) in bulk heterojunction (BHJ) OPVs with fullerene‐rich compositions. Temperature‐dependent measurements of device performance are used to distinguish dissociation by bulk‐ionization from charge transfer at the D–A interface. In optimized fullerene‐rich BHJs based on the D–A pairing of boron subphthalocyanine chloride (SubPc)–C60, bulk‐ionization is found to be responsible for ≈16% of the total photocurrent, and >30% of the photocurrent originating from C60. The presence of bulk‐ionization in C60 also impacts the temperature dependence of VOC, with fullerene‐rich SubPc:C60 BHJ OPVs showing a larger VOC than evenly mixed BHJs. The prevalence of bulk‐ionization processes in efficient, fullerene‐rich BHJs underscores the need to include these effects when engineering device design and morphology in OPVs.

The improvement in organic photovoltaic response by inserting an interlayer between MoO3 and mixed layer of C60:5 wt% TAPC

We demonstrate organic photovoltaic (OPV) cells with structure of ITO/MoO3 (5nm)/boron subphthalocyanine chloride (SubPc) (d nm)/C60:5wt% 1,1-bis-(4-methyl-phenyl)-aminophenyl-cyclohexane (TAPC) (40nm)/BCP (8nm)/Al, where d=0, 1, 3, 5, 7 and 13nm. We found that a 5-nm-thick SubPc based cell had a peak power conversion efficiency (PCE) of 3.75% and a voltage (Voc) of 1.03V, which was an increase of 35.4% and 0.19V, respectively, compared with the reference cell without SubPc. We found that the spectral response of cells with 1–5nm SubPc mostly corresponded to the C60 absorption and the cell with 5nm SubPc had the highest responsivity. The spectral response of the cell with 13nm SubPc corresponded to both C60 and SubPc absorptions, which is analogous to a previously reported flat heterojunction cell. The excellent PV parameter of the cell with 5nm SubPc was attributed to a built-in field induced by the Schottky barrier contact of MoO3/5-nm SubPc. To confirm this hypothesis, another two series of OPV cells with different interlayers and donors instead of SubPc and TAPC were developed.

Carrier injection dynamics in heterojunction solar cells with bipolar molecule

A boron subphthalocyanine chloride (SubPc) is a bipolar molecule and is used in hetero-junction organic solar cells. Here, we investigated the carrier injection dynamics from the donor α-sexithiophene (6T) or acceptor C60 layers to the bipolar SubPc layer by means of the femtosecond time-resolved spectroscopy. We observed gradual increase of the SubPc– (SubPc+) species within ≈300 ps. The increases are interpreted in terms of the exciton diffusion within the 6T (C60) layer and subsequent electron (hole) injection at the interface. In 6T/SubPc heterojunction, the electron injection is observed even at 80 K. The robust electron injection is ascribed to the efficient charge separation within the 6T layer under photo exciation at 400 nm.

Evaluating thiophene electron-donor layers for the rapid assessment of boron subphthalocyanines as electron acceptors in organic photovoltaics: solution or vacuum deposition?

In this study, we consider the choice of a standard electron-donating material to be paired with boron subphthalocyanines (BsubPcs) to rapidly assess the viability of new BsubPc derivatives as electron-accepting materials within organic photovoltaic devices (OPVs). Specifically, we evaluate the effectiveness of solution-cast poly(3-hexylthiophene-2,5-diyl) (P3HT) as an electron donor paired with BsubPc derivatives relative to vacuum-deposited sexithiophene (α-6T). By using fullerene (C60 ), boron subphthalocyanine chloride (Cl-BsubPc), and hexachloro boron subphthalocyanine chloride (Cl-Cl6 BsubPc) as electron acceptors, we find that devices made with α-6T outperform those with P3HT. However, the two thiophene-based materials show the same performance trends. Given the preservation of these trends, we can recommend either option for assessing the potential of new BsubPc derivatives; P3HT as a solution-cast electron-donor layer or α-6T as a vacuum-deposited alternative.

Degradation of interface between boron subphthalocyanine chloride and fullerene

Apart from external environmental factors, we herein show with ultra-violet photoemission spectroscopy (UPS) studies that degradation in organic photovoltaic (OPV) devices can also be caused by internal deterioration of the donor/acceptor interface. Albeit with impressive initial open circuit voltage (Voc), boron subphthalocyanine chloride (SubPc)/fullerene (C60) device shows a fast Voc decade upon operation. UPS results show that the energy offset between the highest occupied molecular orbit (HOMO) of SubPc and the lowest unoccupied molecular orbit (LUMO) of C60 is reduced from 1.66 to 1.45eV after aging in ultra-high vacuum (UHV) condition. This result is consistent with the change in built-in voltage of the corresponding device upon operation. The related charge interaction and degradation mechanism in the SubPc/C60 device are discussed.

A new anodic buffer layer material for non-mixed planar heterojunction chloroboron subphthalocyanine organic photovoltaic achieving 96% internal quantum efficiency

Nonmixed planar heterojunction (PHJ) small-molecule organic photovoltaics (OPVs) with 96% internal quantum efficiency (at 595nm) and 4.77% power conversion efficiency (PCE) have been demonstrated. In addition to boron subphthalocyanine chloride (SubPc) and C60 as electron donor and acceptor materials, respectively, PHJ OPVs contain an ultrathin (2nm) buffer layer of bis-(naphthylphenylaminophenyl)fumaronitrile (NPAFN) between the indium tin oxide (ITO) anode and the donor layer (SubPc). Compared with copper phthalocyanine (CuPc) or α-naphthylphenylbiphenyl diamine (NPB) buffer layers, the NPAFN buffer layer blocks the exciton diffusion from the SubPc electron donor layer to the ITO anode more effectively and considerably improves the short circuit current (JSC) from 5.96 (without an NPAFN layer) to 7.70mA/cm2 (with a 4-mm-thick NPAFN layer ). In addition, experimental results indicated that the NPAFN buffer layer reduces the crystallization, or stacking, of the SubPc electron donor, thereby limiting the reverse saturation current and elevating the open circuit voltage (VOC) from 1.01 (without an NPAFN layer) to 1.08V (with a-2-nm thick NPAFN layer). However, series resistance (RS) of the OPV monotonically increases with increasing NPAFN layer thickness. The performance of the OPV is optimized when the NPAFN buffer layer thickness is 2nm. Compared with a SubPc–C60 PHJ OPV without an NPAFN buffer layer, the PCE of a OPV with a buffer layer increases by 22% from 3.96% to 4.77%, with a concurrent increase in JSC (from 5.96 to 7.02mA/cm2) and VOC (from 1.01 to 1.08V). However, a decrease in RS (from 10.21 to 14.95Ωcm2) and in fill factor (from 65% to 63%) is also observed.

Energy-cascade organic photovoltaic devices incorporating a host-guest architecture.

In planar heterojunction organic photovoltaic devices (OPVs), broad spectral coverage can be realized by incorporating multiple molecular absorbers in an energy-cascade architecture. Here, this approach is combined with a host-guest donor layer architecture previously shown to optimize exciton transport for the fluorescent organic semiconductor boron subphthalocyanine chloride (SubPc) when diluted in an optically transparent host. In order to maximize the absorption efficiency, energy-cascade OPVs that utilize both photoactive host and guest donor materials are examined using the pairing of SubPc and boron subnaphthalocyanine chloride (SubNc), respectively. In a planar heterojunction architecture, excitons generated on the SubPc host rapidly energy transfer to the SubNc guest, where they may migrate toward the dissociating, donor-acceptor interface. Overall, the incorporation of a photoactive host leads to a 13% enhancement in the short-circuit current density and a 20% enhancement in the power conversion efficiency relative to an optimized host-guest OPV combining SubNc with a nonabsorbing host. This work underscores the potential for further design refinements in planar heterojunction OPVs and demonstrates progress toward the effective separation of functionality between constituent OPV materials.

Effect of organic solar cells using various sheet resistances of indium tin oxide and different cathodes: Aluminum, silver

The effect of different electrodes on organic solar cells (OSCs) performance was studied. Various sheet resistances of indium tin oxide (ITO) were used as an anode to find out which is suitable for OSCs application. The efficiency of OSCs based on small molecule material using different ITO substrates is almost the same due to the low absorption efficiency of active layer. In contrast, the efficiency of device based on polymer material using ITO (15Ω/sq) has a better efficiency, which is attributed to reaching the optimal balance between the light transmittance and the carrier extraction. In cathode, the commonly used for OSCs are aluminum (Al) and silver (Ag), respectively. When substituting Ag for Al as a cathode, the efficiency of device based on copper phthalocyanine and C60 increased from 0.71% to 0.86% and the efficiency of device based on boron subphthalocyanine chloride and C60 increased from 2.61% to 2.96%. The performance enhancement is mainly ascribed to the current density (Jsc) improvement which is resulted from the difference of optical characteristic between the Al and Ag. The reflectance of Ag is higher than that of Al above wavelength of 405nm, indicating that the light reflected from the Ag is more than that of Al. As a result, the more light can be absorbed in the active layer, leading to the efficiency of device using Ag as a cathode have above 10% improvement. However, the most commonly used cathode in OSCs is Al rather than Ag, which is also investigated.

Fully organic dye based tandem photovoltaic cells

We report on fabrication and testing of an all-organic-dye-based photovoltaic cell using a tandem architecture. Proceeding from the well-known concept of stacked molecular p/n-junctions, we developed a device with two stacked Schottky barrier subcells. Both subcells are based on p-type organic dyes: boron subphthalocyanine chloride (SubPc/Al barrier) for the front cell and vanadyl phthalocyanine (PcVO/Mg:Ag barrier) for the back one. These two dyes are complementary to each other in terms of optical activity, so the newly formed tandem cell absorbs a broad range of wavelengths from 300 to 1100nm. Introduction of ultrathin intermediate layer of a third conductive dye, vanadyl naphthalocyanine (NcVO), at the Al/PcVO interface makes possible a serial connection of subcells, resulting in summation of open circuit voltages.

Study of junction charge transport properties of boron subphthalocyanine chloride thin film

The charge carrier transport properties and conduction mechanism of boron subphthalocyanine chloride (Cl-BsubPc) thin film based junction was analyzed by using current-voltage (I-V) characteristics and dielectric spectroscopy technique. I-V characteristic of ITO/Cl-BsubPc/Al junction confirmes the domination of space charge limited conduction (SCLC) in the high voltage region (7–9 V). The AC conductivity study indicates the presence of frequency induced hopping conduction mechanism in Cl-BsubPc thin film with hopping relaxation time of 16.6–4.1 ms. The temperature dependence of AC conductivity suggests that conduction in the films dominated by hopping of carriers between the localized states at low temperature and movements of thermally excited carriers from energy levels within the band gap at higher temperature. The activation energy of the charge carriers responsible for conduction was found to lie between 0.35–0.40 eV.

Effect of open-circuit voltage in organic solar cells based on various electron donor materials by inserting molybdenum trioxide anode buffer layer

The effect of inserting a molybdenum oxide (MoO3) anode buffer layer into organic solar cells based on various electron donor materials was studied. The device was composed of an indium tin oxide (ITO)/MoO3/donor material/C60/bathocuproine/aluminum structure, and the donor materials were boron subphthalocyanine chloride (SubPc), rubrene, 2-[(7-{4-[N,N-bis(4-methylphenyl)amino]phenyl}-2,1,3-benzothiadiazol-4-yl)methylene]propanedinitrile (DTDCPB) and copper phthalocyanine (CuPc). Inserting a MoO3 buffer layer between ITO and donor layer resulted in the efficiency of device improvement from 1.04% to 2.49%, from 0.27% to 1.01%, and from 0.13% to 1.12% for the donor material of SubPc, rubrene, and DTDCPB, respectively. The result was ascribed to a greatly enhancement of open-circuit voltage (Voc), which originated from the work function improvement of ITO by covering MoO3 layer. However, the function of MoO3 was not evident in the device using CuPc as donor material. The interaction between MoO3 and CuPc was detected using UV–visible absorption and X-ray photoelectron spectroscopy. The electron transfer between MoO3 and CuPc caused the formation of an interface state at the MoO3/CuPc interface, resulting in Fermi-level pinning at the interface. Consequently, the efficiency of the CuPc/C60 heterojunction device cannot be improved by inserting a MoO3 anode buffer layer.

Tandem small molecule organic photovoltaic cells with broad spectral response up to 1 μm and a high open-circuit voltage

We report a series-connected small molecule tandem photovoltaic cell utilizing two donors with complementary photovoltaic characteristics, lead phthalocyanine (PbPc) in the front subcell and boron subphthalocyanine chloride (SubPc) in the back subcell, to achieve both near infrared (NIR) response up to 1μm and high open-circuit voltage (VOC) of more than 1.5V in the same device. We find that the C60 layer thickness in the front subcell has a critical impact on the overall optical structure and photovoltaic performance of the tandem device. By combining transfer matrix calculations with subcell-selective spectral measurements, we are able to tune the optical field distribution inside the active layers and increase the photocurrent outputs from both subcells, leading to EQE>30% over the wavelength range 400nm<λ<900nm. This optimized tandem cell exhibits JSC=(5.5±0.1)mA/cm2, fill factor=0.54, VOC=1.53V, and a power conversion efficiency of (4.5±0.2)%.