Axially phenoxylated aluminum phthalocyanines and their application in organic photovoltaic cells

Abstract

Novel axially phenoxylated aluminium phthalocyanines, pentafluorophenoxy aluminium phthalocyanine (F₅PhO-AlPc) and p-nitrophenoxy aluminium phthalocyanine (NO₂PhO-AlPc), were synthesized through a one-step reaction starting from the commonly used photoactive material, chloro aluminium phthalocyanine (Cl-AlPc), and the respective phenols. Reactions with other phenols did not yield corresponding AlPc derivatives. Optical, electrical, and thermal analyses were carried out on F₅PhO-AlPc and NO₂PhO-AlPc using UV-Vis measurements (solution and thin-film), cyclic voltammetry (CV), differential-pulse voltammetry (DPV), and thermogravimetric analysis (TGA). A simple thermodynamic model was used to explain the lack of reaction when Cl-AlPc was treated a variety of alkylated phenols. We noted a side reaction producing fluoro aluminium phthalocyanine (F-AlPc) when the synthesis of F₅PhO-AlPc was attempted in DMF. The model also explains this observation. F₅PhO-AlPc and NO₂PhO-AlPc were integrated into organic photovoltaic devices (OPVs) both as electron-donating and as electron-accepting materials. The phenoxy-AlPcs enable an enhancement of the open-circuit voltage (V_OC) of the OPVs when applied as either an electron donor or as an electron acceptor compared to Cl-AlPc. The results within the OPV, specifically the increased V_OC, are consistent with the steric shielding effect seen in other OPVs.

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Introduction

Phthalocyanines (Pcs) are a family of molecules initially discovered in the early 20^th century and have since been industrially produced and used extensively in the pigment industry.^1,2 Pcs possess certain characteristics which enable their use within the pigment industry including high thermal- and photo-stability, ease of synthesis through a one-step reaction (generally) and high molar extinction coefficients. More recently Pcs have demonstrated the ability to move electrical charge and be applied as organic semiconductors in a variety of organic electronic applications including organic photoreceptors,^3–5 organic photovoltaics (OPV)^6–11 and organic light emitting diodes (OLED).^12,13 Pcs have a tight connection to OPVs as one of the first functioning OPVs reported in the literature used copper phthalocyanine (CuPc) as an electrondonor material with a power conversion efficiency just under 1%.¹⁴ Since then, many OPV devices have been fabricated using different Pc derivatives whereby Pcs have most commonly been applied as electron donating mateirals^6,15,16 or as light sensitizers.^17–20 Xue et al. achieved efficiencies up to 5% using Pcs in the light-absorbing layer.²¹ There is, however, limited research on using Pcs as electron acceptor materials in OPVs. Very recently, the group of Jones has demonstrated that Cl-AlPc can function as an electron acceptor material when paired with pentacene in an OPV.²²

Research into Pcs whereby the intended application is within an OPV has divided into two discreet areas: developing new Pcs through chemical modification^9,23–25 or altering the device fabrication process and/or configuration. Each avenue is generally targeting OPVs with higher photo conversion efficiencies.^{10,11,26–28} However, achieving better basic characteristics such as enhanced OPV open circuit voltage (V_OC) through chemical modification is also highly desirable.

Regarding chemical modification in general, there has been extensive research over past decades looking at the chemistry of Pcs through changing of the central atom,¹ peripheral substitution^29–33 or through axial substitution.^34–38 Nearly all elements in the periodic table (95 elements) have been successfully incorporated into Pcs.¹ Some of the most common central atoms are divalent such as Cu, Zn, and Mg. In these divalent cases, tuning the molecular properties is only possible through peripheral substitution and has been shown to affect the solubility,^29,30 stability,²⁹ optical properties³¹ and electrical properties of the resulting Pcs.^32,33

Trivalent (e.g. Al) and tetravalent (e.g. Si) metal phthalocyanines (AlPc and SiPc respectively) have the unique ability to form axial bonds with different chemical functionalities that can be potentially used to influence the properties of the resulting Pc. However, limited reports are available detailing the chemistry of the axial position of trivalent and tetravalent Pcs and the impact that axial modifications have on the properties of the Pcs. The majority of the reports that are available detail the chemistry of axial substitution of silicon phthalocyanine (SiPc)³⁴ and boron subphthalocyanine (BsubPc)^35–40 and include μ-oxo^41–44 and polymeric Pc derivatives.^45–47

Our group has shown the axial substitution of BsubPcs with a pentafluoro phenoxy fragment can alter the frontier orbital energies and change the OPV performance characteristics.⁴⁸ We have also recently shown that axial phenoxylation can change SiPc from being largely electronically inactive to something that has potential as an electron acceptor in a fullerene free OPV.⁴⁹ Finally, we have demonstrated that changing the axial chloride of Cl-AlPc to a fluoride (F-AlPc) results in major changes in the fundamental properties of the AlPc including its solid state arrangement.⁵⁰

Beside our recently study of F-AlPc, the axial chemistry of AlPc has been relatively understudied. Kraus and Louw,⁵¹ Brasseur et al.,⁵² and Sakai et al.⁵³ have shown that silver based reagents can be used to axially phenoxylate AlPcs although in only one case was the yield reported and in no cases was the conversion reported.

In this paper, we outline our independent investigation into the axial phenoxylation of chloro aluminum phthalocyanine (Cl-AlPc) with a specific focus on finding a general, easy, inexpensive and high yielding method for phenoxylation whereby high conversion is achieved. High conversion is desirable to simplify the purification process of the material and to minimize the potentiality of impurities being present within the final materials as they are incorporated into organic electronic devices. The investigation of the resulting derivatives in planar heterojunction (PHJ) organic photovoltaic cells (OPVs) was also undertaken. Despite multiple attempts with variety of phenols, only two derivatives, pentafluorophenoxy aluminum phthalocyanine (F₅PhO-AlPc) and p-nitrophenoxy aluminum phthalocyanine (NO₂PhO-AlPc) were successfully synthesized and characterized. Thermodynamic limitations on axial phenoxylation of a range of metal Pcs were calculated. Effects on the optical properties, redox potentials, solid-state packing, and stability were observed. OPVs were fabricated employing F₅PhO-AlPc and NO₂PhO-AlPc both as an electron donor and as an electron acceptor. OPV characteristics were compared against Cl-AlPc baseline devices.

Experimental section

Materials

All ACS grade solvents (except nitrobenzene and quinoline) were purchased from Caledon Labs (Caledon, Ontario, Canada) and used without further purification unless otherwise stated. Phthalonitrile was purchased from TCI America (Portland, Oregon, USA) and used as received. Aluminum chloride, p-nitrophenol, m-cresol, 3-pentadecylphenol, 2-sec-butylphenol, 4-tert-octylphenol, nitrobenzene, quinoline, decamethyl ferrocene and tetrabutylammonium perchlorate were purchased from Sigma-Aldrich Chemical Company (Mississauga, Ontario, Canada) and used as received. Pentafluoro phenol was purchased from Oakwood Products, Inc. (Columbia, South Carolina, USA) and used as received. Bathocuproine (BCP 99.99%) from Sigma-Aldrich, Ag (99.99%) from R. D. Mathis Company, Fullerene (C₆₀) from SES Research and α-sexithiophene (α-6T) from Sigma-Aldrich were all used as received.

General methods

All reactions were performed under an atmosphere of nitrogen gas using oven-dried glassware. Low-resolution mass spectra (LRMS) and high-resolution mass spectra (HRMS) were determined using Waters GC Premier using time-of-flight spectrometer with electron ionization source (TOF-EI MS). X-ray diffraction data were collected on a Bruker Kappa APEX-DUO diffractometer using a Copper ImuS (microsource) tube with multi-layer optics (F₅PhO-AlPc) or a molybdenum sealed tube with Triumph momochromator (NO₂PhO-AlPc) and were measured using a combination of f scans and w scans. The data were processed using APEX2 and SAINT.⁵⁴ Absorption corrections were carried out using SADABS.⁵⁴ The structure was solved and refined using SHELXT & SHELXL-2103 (ref. 55) for full-matrix least-squares refinement that was based on F2. Ultraviolet-visible (UV-Vis) absorption spectra were acquired on a PerkinElmer Lambda 1050 UV/VIS/NIR spectrometer using a PerkinElmer quartz cuvette with a 10 mm path length. Cyclic voltammetry and differential-pulse voltammetry were carried out using a Bioanalytical Systems C3 electrochemical workstation. The working electrode was a 1 mm glassy carbon disk, the counter electrode was a platinum wire, and the reference electrode was Ag/AgCl saturated salt solution. Spec-grade solvents were purged with nitrogen gas at room temperature prior to their use. The scanning rate applied throughout both experiments is 100 mV s⁻¹. Tetrabutyl ammonium perchlorate (0.1 M) was used as the supporting electrolyte and decamethyl ferrocene as an internal reference. All half-wave potentials were corrected to the half-wave reduction potential (E_1/2,red) of decamethyl ferrocene. Thermogravimetric analysis was run using TA Instruments Q50 under an atmosphere of nitrogen and a heating rate of 5 °C min⁻¹. Soxhlet extractions were performed using Whatman® single thickness cellulose extraction thimbles.

Synthetic methods

Chloro aluminum phthalocyanine (Cl-AlPc). Chloro aluminum phthalocyanine (Cl-AlPc) was synthesized as described by Santerre et al.⁵⁶ 20 g (0.16 mol) of phthalonitrile was added to a solvent-mixture of quinolone (20 mL) and nitrobenzene (32 mL) in a 250 mL round-bottom flask. The solution was cooled to 0 °C in an ice-bath and 5.72 g (0.0439 mol) of AlCl₃ was added to the mixture. The reaction was brought up to reflux at 210 °C and left stirring for 4 hours. The resulting blue-greenish solution was cooled to room temperature. The crude product was filtered using a glass-frit filter and washed thoroughly with acetone and acetonitrile. A Soxhlet extraction with acetone was performed overnight on the resulting cake. After the product was dried overnight in the vacuum-oven, 11.2 g (49.9% yield) of blue powder was collected. TOF-EI MS (Fig. S1†) m/z (%): 539.1 [MPc⁺] (30), 540.1[MPc⁺] (10), 574.1 (100), 575.1 (20). The product was then purified through conventional temperature-gradient train sublimation apparatus (as describe elsewhere⁵⁷). Anal. Calcd for C₃₂H₁₆N₈AlCl: C, 66.85; H, 2.81; N, 19.49. Found: C, 66.57; H, 3.38; N, 19.37%.

Pentafluorophenoxy aluminum phthalocyanine (F₅PhO-AlPc). Pentafluorophenoxy aluminum phthalocyanine (F₅PhO-AlPc) was synthesized by an adapted method from the synthesis of F₅BsubPc.⁵⁸ 1.0 g (1.7 mmol) of Cl-AlPc was mixed with 5 molar excess of pentafluoro phenol (1.7 g) in 20 mL of toluene. The reaction mixture was heated and kept under reflux for 24 hours. After the mixture was allowed to cool down, toluene was removed using rotary evaporation. The powder was then washed successively with toluene. The resulting blue powder was dried in a vacuum oven overnight. 1.2 g (∼100% yield) of the product was collected. TOF-EI (Fig. S2†) m/z (%): 539.1 [MPc⁺] (100), 540.1[MPc⁺] (20), 722.1 (100), 723.1 (20). Further purification of the product (0.56 g) took place using the temperature gradient train sublimation apparatus (as describe elsewhere⁵⁷) yielding 49.3% of pure product. TOF-EI MS m/z (%): 539.1 [MPc⁺] (100), 540.1[MPc⁺] (20), 722.1 (100), 723.1 (20). Anal. calcd for C₃₈H₁₆N₈AlOF₅: C, 63.17; H, 2.23; N, 15.51. Found: C, 63.26; H, 2.76; N, 15.59%.

p-Nitrophenoxy aluminum phthalocyanine (NO₂PhO-AlPc). p-Nitrophenoxy aluminum phthalocyanine (NO₂PhO-AlPc) was also, synthesized by an adapted method from the synthesis of F₅BsubPc.⁵⁸ 0.5 g (1.35 mmol) of Cl-AlPc was mixed with 5 molar excess of p-nitrophenol (0.6 g) in 10 mL of chlorobenzene. The reaction mixture was heated and kept under reflux for 24 hours. After the mixture was allowed to cool down, chlorobenzene was removed using rotary evaporation. The powder was then washed with ethyl ether. The resulting blue powder was dried in a vacuum oven overnight. 0.5 g (84.9% yield) of the product was collected. TOF-EI MS (Fig. S4†) m/z (%): 539.1 [MPc⁺] (100), 540.1[MPc⁺] (20), 677.2 (50), 678.2 (10). The reaction was also run in DMF producing similar results. The product was then taken into the train sublimation apparatus at 450 °C; the yield of the sublimed product, however, was low (∼10%). Anal. calcd for C₃₈H₁₆N₉AlO₃: C, 67.36; H, 2.98; N, 18.60. Found: C, 66.78; H, 2.29; N, 18.57%.

Device fabrication

All OPVs device were fabricated on commercially available patterned ITO substrates (Thin Film Device Inc. (TFD)) with 145 nm ITO thickness and a sheet resistance of 15 Ω □⁻¹. The ITO surfaces were ultrasonically cleaned in detergent, distilled water, acetone, methanol, and finally dried in flowing nitrogen. Subsequently, ITO was treated by plasma for 5 min to remove carbon residues. After plasma treatment, a CHEMAT Technology KW-A4 spin coater was used to spin a layer of PEDOT:PSS (Clevios™ P VP AI 4083) at 500 rpm for 10 s and 4000 rpm for 30 s on the ITO electrode anode. The ITO with the PEDOT:PSS coating was annealed in a at 110 °C for 15 min. The device structure grown by thermal evaporation with a base pressures of 10⁻⁸ torr consists ITO/PEDOT:PSS/Active layer/BCP(8 nm)/Ag(80 nm), where active layer and their thickness can be found in Table 2. Between the deposition of the BCP layer and the Ag cathode, devices were transferred from the vacuum chamber to nitrogen atmosphere glove box through a gate, without a vacuum break, to attach a shadow mask consisting of 5 active pixel area of 0.2 cm² openings then transferred back to the vacuum chamber. Layers thicknesses and deposition rates were monitored using a quartz crystal microbalance (QCM). After the Ag layers were deposited, the OPV devices were transferred directly from the vacuum chamber back to the nitrogen atmosphere glove box. Silver paste (PELCO® Conductive Silver 187) was applied to the Ag and to the ITO electrodes to enhance electrical contact and left to cure for 30 minutes before testing. A 300 W ozone free xenon arc lamp with an air Mass 1.5 Global filter, fed through a Cornerstone™ 260 1/4 m monochromator and then into the glove box by way of a single branch liquid light guide was utilized as a solar simulator. The illumination levels for the test cell were kept at 100 mW cm⁻². A UV-silicon photo-detector was used to calibrate the measurements. The current densities versus voltage (J–V) characteristics and external quantum efficiency were recorded using a Keithley 2401 Low Voltage sourcemeter controlled by a custom LabView program and a Newport Optical Power Meter 2936-R controlled by TracQ Basic software, respectively, in the nitrogen atmosphere glove box.

Table 1 Oxidation and reduction potentials using CV and DPV (in dichloromethane), electrical band gap (E_EG), optical band gap (E_OG), and the estimated frontier energy levels of AlPc derivatives

	Eox (V)	Ered (V)	EEG (eV)	EOG (eV)	HOMO (eV)	LUMO (eV)
Cl-AlPc	—	—		1.40	5.3 (ref. 59)	4.0 (ref. 59)
F5PhO-AlPc	0.70	−0.71	1.70	1.51	5.52	3.83
NO2PhO-AlPc	0.60	−0.72	1.58	1.55	5.40	3.82

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Results and discussion

We began this study by surveying the reaction of chloro aluminum phthalocyanine (Cl-AlPc) with a variety of phenols by adopting the simple method our laboratory has used for phenoxylation of chloro boron subphthalocyanine (Cl-BsubPc).⁵⁸ This approach is high yielding for BsubPcs and involves nothing more than simple heating in solution with a molar excess of the appropriate phenol. Phenols that we tried included several alkylated phenols (m-cresol, 2-sec-butylphenol, 4-tert-octylphenol, and 3-pentadecylphenol) and two more ‘acidic’ phenols (pentafluoro phenol and 4-nitrophenol, Scheme 1). Compared against the reaction of Cl-BsubPc, conversion of Cl-AlPc to the corresponding phenoxylated derivatives was low and impractical when alkylated phenols were used. At low molar excesses of alkylated phenol (5 equiv) little to no conversion of Cl-AlPc was observed. Using large molar excesses (∼100–150 equiv) low conversion was at times observed (50% maximum) but was not reproducible. These observations are in line with that of Owen and Kenney.⁵⁹ We did not consider the use of the methods described by Kraus and Louw,⁵¹ Brasseur et al.⁵² or Sakai et al.⁵³ as all use expensive silver salts to activate the Al–Cl bond to reaction.

Scheme 1 Phenoxylation reaction of Cl-AlPc. Where conversion to the phenoxylated AlPc was limited, a N/a is placed in the designation cell. Changing bonds are highlighted in red (breaking) and green (forming).

The only reproducible syntheses were found to be where the ‘acidic’ phenols pentafluoro phenol (F₅PhOH) and 4-nitrophenol (NO₂PhOH) were reacted with Cl-AlPc. F₅PhO-AlPc was successfully synthesized using toluene as a solvent and NO₂PhO-AlPc with chlorobenzene. The general workup of the reaction involved rotary evaporating the crude solution to dryness. The powder was then washed with an appropriate organic solvent and dried in a vacuum-oven overnight. Both F₅PhOH and NO₂PhOH produced high yields (>80%) of the respective AlPc derivatives. Each product was then purified by train sublimation.⁵⁷

The identity of each phenoxylated AlPc was first confirmed using LR-MS and HR-MS (Fig. S2 and S3†). Additionally, the identity of F₅PhO-AlPc was confirmed by X-ray diffraction of solvated single crystals grown by solvent diffusion (heptane into a dilute solution of toluene; Fig. S4; Tables S1–S6†). Given the crystals were solvated a discussion of the solid-state arrangement, relevant to those interested in solid-state applications such as OPV is not appropriate. Conversely, we were able to obtain single crystals of NO₂PhO-AlPc grown by sublimation an analogous process to that which is used for fabrication of OPV devices (Fig. S5–S6, Tables S7–S12,†). It was found that the solid-state packing of NO₂PhO-AlPc is extremely dense (Fig. S6†) with π–π interactions between the AlPc chromaphores at distances less than 3.8 Å in all three crystallographic axis; a desirable characteristic for an organic electronic material. The obvious point of comparison would be the solid-state arrangement of Cl-AlPc. While there is a crystal structure of Cl-AlPc report by Wynne,⁴² as noted there is significant disorder within the unit cell of the single crystal. The absolute positions of two Cl-AlPc molecules were determined but the position(s) of the remaining one or two molecules could not be determined.⁴² Therefore without a point of comparison, a more in depth discussion of the solid-state arrangement of NO₂PhO-AlPc is irrelevant. We have made several attempts in our laboratory to obtain single crystals of Cl-AlPc without success.

Determination of the NMR spectra for both F₅PhO-AlPc and NO₂PhO-AlPc was not possible due to their limited solubility (common for Pcs). While each did have limited solubility in DMSO, DCM and toluene each had only enough solubility in DMSO to yield an NMR spectrum. NMR analysis in DSMO-d₆ showed that each reacted with residual water to produce the same hydrolyzed product (presumably HO-AlPc). Final purity of each material was therefore confirmed using elemental analysis (see Experimental) prior to OPV device integration.

Given the polarity of the acidic phenols, we also tried the corresponding reactions in DMF. However, in DMF the reaction of pentafluoro phenol and Cl-AlPc yielded exclusively F-AlPc as confirmed by MS (Fig. S7†). Shoute et al. has studied the defluoration of pentafluoro phenol and formation of fluoride ions in aqueous solutions.⁶⁰ We hypothesize that the residual water present in reagent grade DMF (and not in reagent grade toluene) allowed for the generation of fluoride ions and their subsequent reaction with Cl-AlPc to produce F-AlPc. NO₂PhO-AlPc could also be synthesized in toluene or DMF as well as chlorobenzene as the solvent without issues.

Thermodynamic limitations

As outlined above, only two out of the six phenols successfully formed phenoxylated AlPcs whereas multiple examples of phenoxylation of SiPcs^34,49 and BsubPcs^35–40 using similar processes are known. In an effort to explain these observations, we turned to an estimation of the enthalpy of the phenoxylation reaction (ΔH_R) for different MPcs. The phenoxylation reaction was assumed to follow Scheme 1 and we have made no assumptions regarding the mechanism by which the reaction occurs. ΔH_R was simply estimated using bond dissociation enthalpies of the bonds involved in the phenoxylation reaction for several MPcs (eqn (1)). To a first approximation, ΔH_R is assumed to depend only on the bonds being broken and formed during the reaction and not being dependent on the molecule as a whole.⁶¹

ΔH_R = (ΔH_H–X + ΔH_O–M) − (ΔH_X–M + ΔH_O–H) (1)

ΔH_A–B is the enthalpy of formation of the bond between A and B. If we assume that the entropy of the products and the reactants are similar and that the changes in entropy can be neglected (since the number of moles throughout the reaction is constant and since the reaction is run in solution without a phase change) then the sign and magnitude of ΔH_R can indicate whether the reaction will proceed to completion spontaneously or not.

As can be seen in Fig. 1, using our simple model the phenoxylation of Cl-AlPc does not seem to be thermodynamically favorable since the value of ΔH_R is estimated to be approximately zero. The value for Br-AlPc is negative and for F-AlPc positive indicating that a phenoxylation reaction on Br-AlPc might be favorable whereas reaction on F-AlPc would not be favorable. It also means that in the presence of fluoride anions, the formation of F-AlPc from Cl-AlPc is more thermodynamically favorable than formation of the phenoxylated AlPc. This supports the observed formation of F-AlPc when pentafluoro phenol was used in DMF. Using this simple model, the reaction of X-SiPc and X-BsubPc (X = F, Cl, Br) have extremely negative ΔH_R values and is therefore consistent with the ease by which each can be phenoxylated and further validates this simple model.

Fig. 1 Heat of phenoxylation reaction (ΔH_R) of different halide-MPcs.

While this basic model explains most of the observations when attempting to phenoxylate Cl-AlPc, the synthesis of F₅PhO-AlPc and NO₂PhO-AlPc did work. Given the ΔH_R for phenoxylation of Cl-AlPc with each of these ‘acidic’ phenols is calculated to be near 0, subtle differences in the ΔH_O–H and ΔH_O–M values may cause a change in the sign of ΔH_R and, hence, the spontaneity of the reaction. Subtle differences could be attributable to the acid dissociation constants (pK_a); F₅PhOH and NO₂PhOH are 5.5 (ref. 60) and 7.15,⁶² respectively, compared to pK_a's over 10 for alkylated phenols.⁶²

Properties of phenoxy-AlPcs

Optical absorption measurements were taken for Cl-AlPc, NO₂PhO-AlPc, and F₅PhO-AlPc (Fig. 2). Minimal differences between the three compounds were observed. F₅PhO-AlPc shows maximum absorption at λ_max = 676 nm while Cl-AlPc and NO₂PhO-AlPc have λ_max = 680 and λ_max = 678, respectively (DMSO solution). Solid-state absorbance spectra were taken from vacuum-deposited thin-films (∼1000 nm thick). The solid-state spectra were broader with peak absorptions slightly red-shifted from the solution spectra. Optical band gabs (E_OG) were calculated using the onset of the main solid-state absorption peak. E_OG ranged between 1.40–1.55 eV for the three AlPc compounds.

Fig. 2 UV-Vis absorption spectra, solution in DMSO (solid line) and solid-state (dashed line), of Cl-AlPc, NO₂PhO-AlPc, and F₅PhO-AlPc.

Electrochemical oxidation and reduction potentials were measured using cyclic voltammetry (CV) and differential-pulse voltammetry (DPV) and are again consistent within this series of compounds (Fig. 3). Estimations of HOMO and LUMO levels were calculated using the empirical model developed by Dandrade et al.⁶³ and the oxidation and reduction peaks from the DPV data (Table 1). Electrochemical energy gap (E_EG) were defined and calculated as the difference between the oxidation and reduction peaks of the DPV data. The calculated values of NO₂PhO-AlPc, and F₅PhO-AlPc are similar to the energy levels of Cl-AlPc that are found in the literature.⁶⁴

Fig. 3 Cyclic voltammetry (blue) and differential-pulse voltammetry (red) of (a) F₅PhO-AlPc and (b) NO₂PhO-AlPc.

The thermal stability of all AlPc derivatives was measured using thermogravimetric analysis (TGA, Fig. 4). All of the three compounds showed two mass losses. Both phenoxylated AlPcs have a drop in mass at a lower temperature than the Cl-AlPc, between 350 °C and 400 °C compared to ∼500 °C for Cl-AlPc. The amount of mass lost corresponds approximately to the mass of the axial phenoxy groups and is therefore probably attributable to the detachment of the axial functionality. This analysis establishes the temperature window for device integration by vacuum deposition.

Fig. 4 Thermogravimetric analysis (TGA) of Cl-AlPc, F₅PhO-AlPc, and NO₂PhO-AlPc.

Device integration

Multiple organic photovoltaic (OPV) devices were fabricated using Cl-AlPc, F₅PhO-AlPc, and NO₂PhO-AlPc as both electron donor and electron acceptor materials paired with either C₆₀ or sexithiophene (α6T), respectively. C₆₀ is a well-known electron acceptor material and α6-T is a well-known electron donor material. The application of each has been well documented in many OPV device structures.^65–67 Cl-AlPc is also a relatively well studied organic semiconductor material that has been used as both an electron donor^64,68 and an electron acceptor²² within OPVs, therefore is a good comparison point.

To begin, and as a point of comparison, we constructed baseline OPVs with active layer configurations of Cl-AlPc(20 nm)/C₆₀(40 nm) and α6T (60 nm)/Cl-AlPc(20 nm). For Cl-AlPc(20 nm)/C₆₀(40 nm) OPV the short-circuit current density (J_SC) was measured at 3.85 mA cm⁻², the open circuit voltage (V_OC) at 0.67 V and the fill factor (FF) at 0.48 for an overall efficiency (η) of 1.23%. This is in line with the cell characteristics reported in the literature.^64,68–71 For the α6T (60 nm)/Cl-AlPc(20 nm) cell the J_SC was 3.82 mA cm⁻², V_OC was 0.61 V, FF 0.44 for an η of 1.02%. No point of comparison is available for the α6T/Cl-AlPc cell in the literature. The results are tabulated in Table 2 and the corresponding J–V and EQE plots are illustrated in Fig. 5 and 6.

Table 2 The electrical performance and standard deviation of OPVs with Cl-AlPc, F₅PhO-AlPc, and NO₂PhO-AlPc as electron donors (paired with C₆₀) and as electron acceptors (paired with α-6T)

Device Structure	VOC (V)	JSC (mA cm^−2)	FF	η (%)
Cl-AlPc (20 nm)/C60 (40 nm)	0.67 ± 0.03	−3.85 ± 0.3	0.48 ± 0.03	1.23 ± 0.17
F5PhO-AlPc (20 nm)/C60 (40 nm)	0.8 ± 0.01	−2.97 ± 0.35	0.44 ± 0.01	1.06 ± 0.14
NO2PhO-AlPc(20 nm)C60 (40 nm)	0.82 ± 0.01	−2.63 ± 0.38	0.40 ± 0.01	0.86 ± 0.08
α-6T(60 nm)/Cl-AlPc(20 nm)	0.61 ± 0.009	−3.82 ± 0.5	0.44 ± 0.03	1.02 ± 0.07
α-6T(60 nm)/F5PhO-AlPc(20 nm)	0.67 ± 0.007	−4.2 ± 0.3	0.40 ± 0.02	1.12 ± 0.03
α-6T(60 nm)/NO2PhO-AlPc(20 nm)	0.66 ± 0.007	−2.75 ± 0.12	0.47 ± 0.02	0.84 ± 0.07

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Fig. 5 (a) The average J–V curves and (b) the average external quantum efficiencies of Cl-AlPc/C₆₀, F₅PhO-AlPc/C₆₀, and NO₂PhO-AlPc/C₆₀ OPVs.

Fig. 6 (a) The average J–V curves and (b) the average external quantum efficiencies of α-6T/Cl-AlPc, and α-6T/F₅PhO-AlPc, and α-6T/NO₂PhO- AlPc OPVs.

Moving to the phenoxylated derivatives as electron donating materials, for the F₅PhO-AlPc(20 nm)/C₆₀(40 nm) and NO₂PhO-AlPc(20 nm)/C₆₀(40 nm) OPVs, the J_SC values were 2.92 and 2.63 mA cm⁻², V_OC values were 0.80 V and 0.82 V, FF values were 0.44 and 0.40 for overall η of 1.06% and 0.86%, respectively. When the results are compared against the baseline Cl-AlPc(20 nm)/C₆₀(40 nm), the OPVs containing the phenoxylated derivatives had a V_OC for the F₅PhO-AlPc/C₆₀ device 130 mV larger than for the Cl-AlPc/C₆₀ device however, the J_SC was 0.88 mA cm⁻² smaller. The same general observations were seen for the NO₂PhO-AlPc/C₆₀ OPV with a V_OC 132 mV larger than for Cl-AlPc/C₆₀ and again a J_SC 1.22 mA cm⁻² lower than that of the Cl-AlPc/C₆₀ OPV. The electrical characteristics illustrated in Fig. 5 and tabulated in Table 2.

Considering the application of F₅PhO-AlPc, and NO₂PhO-AlPc as electron accepting materials paired with α6T, the following OPVs were constructed: α6T(60 nm)/F₅PhO-AlPc(20 nm) and α6-T(60 nm)/NO₂PhO-AlPc(20 nm). The cells J_SC values were 4.2 mA cm⁻² and 2.75 mA cm⁻², V_OC values were 0.67 V and 0.66 V and FF values were 0.40 and 0.47 resulting in overall η of 1.12% and 0.84%, respectively (Table 2, Fig. 6). Notable, when F₅PhO-AlPc and NO₂PhO-AlPc used as electron acceptors and in line with their application as electron donating materials, an increase in V_OC of 50–60 mV over α6T/Cl-AlPc was observed. The J_SC of α6T/F₅PhO-AlPc is 0.68 mA cm⁻² larger than that of α6T/Cl-AlPc, however, the J_SC of α6-T/NO₂Pho-AlPc is 1.02 mA cm⁻² smaller than that α6T/Cl-AlPc.

Based on the electrical characterization the following conclusions can be made regarding the application of phenoxylated AlPcs in OPVs. All of the AlPcs proved functional both as electron-donating and as electron-accepting materials in an OPV, therefore they are dually functional. There is an observed and consistent increase in V_OC for the resulting OPVs when the phenoxylated AlPcs are used in place of Cl-AlPc and with one exception (α6T/F₅PhO-AlPc) a decrease in the J_SC.

These observations are in line with the ‘steric shielding effect’ described in both polymer⁷² and small molecule based OPVs.^73,74 While this effect has been debated⁷⁵ the concept is that addition of sterically bulky groups out of plane to the semiconducting π-electron system of an organic material enhances the V_OC within an OPV. The V_OC enhancement is attributed to the steric bulk increasing the distance between the electron donating and electron accepting materials at their interface thus resulting in a hole–electron pair having more potential. The high potential makes the separation into a hole–electron pair more difficult and therefore OPVs typically see a net drop in J_SC.⁷³

The results of the integration of the phenoxylated AlPc seem to add positively to the idea of the steric shielding effect. However, to positively state such a conclusion, further device optimization is being undertaken with an aim to improve the FF of the OPVs and to confirm that the reduced J_SC (and overall efficiencies) cannot be overcome with more device engineering. Given our electrochemical data does indicate that there are energy differences in the HOMO and LUMO of Cl-AlPc, F₅PhO-AlPc and NO₂PhO-AlPc this could also be contributing to the differences seen in the V_OC. Experiments will be undertaken in an effort to separate the relative contributions to the observed V_OC differences.

Conclusions

We were successful in phenoxylating Cl-AlPc using pentafluoro phenol and p-nitrophenol to produce F₅PhO-AlPc and NO₂PhO-AlPc, respectively. A basic thermodynamic model was developed to explain limitations on conversion of the phenoxylation reaction when alkylated phenols are used. The optical, electrical, and thermal characteristics of the two novel AlPcs were determined. F₅PhO-AlPc and NO₂PhO-AlPcs were applied in OPVs both as electron donors and as electron acceptors paired with C₆₀ and α6T respectively. When F₅PhO-AlPc and NO₂PhO-AlPc were applied as electron donating materials an increase in the V_OC of the OPV of ∼130 mV was observed and ∼50–60 mV when applied as electron accepting materials. Given the increase in V_OC these novel phenoxylated AlPcs show promise for application within OPVs although further device optimization and engineering is required to enhance the FF and J_SC of the OPVs.

Acknowledgements

The authors would like to acknowledge financial support from Saudi Basic Industries (SABIC).

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Footnote

† Electronic supplementary information (ESI) available: High and low resolution mass spectra and crystallographic data. CCDC 1052023 and 1061523. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra04919a

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