# A Solution-Processed Tetra-Alkoxylated Zinc Phthalocyanine as Hole Transporting Material for Emerging Photovoltaic Technologies

Authors: Gloria Zanotti; Giuseppe Mattioli; Anna Maria Paoletti; Giovanna Pennesi; Daniela Caschera; Nitzan Maman; Iris Visoly-Fisher; Ravi K. Misra; Lioz Etgar; Eugene A. Katz
Journal: International Journal of Photoenergy
 (2018)

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## Abstract

A tetra-n-butoxy zinc phthalocyanine (n-BuO)4ZnPc has been synthesized in a single step, starting from commercial precursors, and easily purified. The molecule can be solution processed to form an effective and inexpensive hole transport layer for organic and perovskite solar cells. These appealing features are suggested by the results of a series of chemical, optical, and voltammetric characterizations of the molecule, supported by the results of ab initio simulations. Preliminary measurements of (n-BuO)4ZnPc-methylammonium lead triiodide perovskite-based devices confirm such suggestion and indicate that the interface between the photoactive layer and the hole transporting layer is characterized by hole-extracting and electron-blocking properties, potentially competitive with those of other standards de facto in the field of organic hole transport materials, like the expensive Spiro-OMeTAD.


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## Body

## 1. Experimental and Computational Details

### 1.1. (n-BuO)4ZnPc Synthesis and Characterization

ZnCl2 and all the solvents (RPE grade) were purchased from Carlo Erba except for N,N-dimethylethanolamine (99.5%) which was purchased from Aldrich. Acetonitrile was distilled over calcium hydride. 4-n-Butoxyphthalonitrile (95%) was purchased from Tokyo Chemical Industry Co. (TCI). 1,5-Diazabicyclo[5.4.0]undec-5-ene (DBU) (≥99%) was purchased from Fluka. Reactions were monitored by thin-layer chromatography (TLC) employing a polyester layer coated with 250 mm F254 silica gel. The crude purification was performed by filtration using silica gel Carlo Erba Reactifs SDS 60A C.C. 35–70 mm and dichloromethane and THF as eluents. To perform the fluorescence measurements, a further purification has been performed by size exclusion chromatography using Bio-Beads S-X Resin (Bio-Rad) as the stationary phase and THF as the mobile phase. 1H NMR spectra were recorded in THF d8 on a Bruker AVANCE 600 NMR spectrometer operating at a proton frequency of 600.13 MHz; chemical shifts (δ) are given in ppm relative to the residual solvent peaks of the deuterated solvent (3.58, 1.73 ppm). Infrared spectra were recorded on a Shimadzu FT-IR prestige-21 spectrometer using an attenuated total reflectance (ATR) unit. All the UV-Vis measurements were performed in direct transmission mode. Absorption spectra of 10−5 M solution in acetonitrile were recorded on a PerkinElmer LAMBDA 950 UV-Vis/NIR spectrophotometer, while transparent sensitized films have been analyzed using a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies). MALDI-TOF spectra have been recorded at Toscana Life Sciences facility in a MALDI-TOF/TOF Ultraflex III (Bruker) using α-cyano-4-hydroxycinnamic acid (HCCA) as matrix. Isotopic pattern simulations have been performed by Bruker Daltonics program. Steady-state fluorescence spectra were recorded on a 10−6 M solution in acetonitrile with a Jobin Yvon Fluorolog3 spectrofluorometer, in the range 620–850 nm, using 5 nm grids for excitation and 5 nm for emission (λex=600 nm). The corresponding excitation spectrum has been collected in the range 350–700 nm, under an excitation wavelength of 750 nm, with 10 nm grids. No filters have been used. Time-resolved fluorescence measurements were carried out by a time-correlated single-photon counting (TCSPC) system (HORIBA Jobin Yvon), using a 405 nm pulsed laser diode and collecting the emission decay at the corresponding maximum emission wavelength (686 nm). The fluorescence decay profile was analyzed through decay analysis software (DAS6a HORIBA Scientific). The curve has been fitted with a monoexponential decay, with a final time decay of 2.4 ns. The quality of the fit was checked by examining the residual distribution having a χ2 value of 1.4. Voltammograms were recorded at 25°C with a potentiostat-galvanostat Metrohm PGStat 204 in a conventional three-electrode cell; a platinum disk (~1 mm) was used as working electrode together with a platinum wire as auxiliary electrode. The reference electrode was Ag/AgNO3 (0.01 M) in distilled acetonitrile (ACN) (Aldrich) (E0 vs NHE = 0.548 V), and the Fc+/Fc (ferrocenium/ferrocene) couple was used as external standard. The sample solutions were ~10−4 M in distilled anhydrous solvents, and dry tetra(n-butyl)ammonium tetrafluoroborate (TBATFB) (Aldrich) was used as the supporting electrolyte at 0.1 M. The solutions were previously purged for 10 minutes with nitrogen, and all measurements were performed under nitrogen. Cyclic voltammetry (CV) investigations were carried out at scan rates typically ranging from 0.05 to 2 Vs−1. AFM images have been acquired in contact mode using a Keysight (formerly Agilent/MI) 5500 AFM with AppNano HYDRA6R-200N probes in air at room temperature and analyzed with the freely downloadable software WSxM 5.0 develop 8.4 December2016 [31].Synthesis of (n-BuO)4ZnPc (Mixture of Regioisomers). 200 mg of 4-n-butoxyphthalonitrile (1.0 mmol, 4 eq), 34 mg of zinc chloride (0.250 mmol, 1 eq), and a catalytic amount of 1,5-diazabiciclo[5.4.0]undec-5-ene (DBU) (1%mol, 1.7 μl) have been dissolved in 1 ml of N,N-dimethylethanolamine in a round bottom flask. The mixture has been refluxed for 15 hours and then cooled down, diluted with water, and filtered to obtain a solid which has been washed with water until the pH is neutral and the washings are colorless. The desired molecule has been purified by filtration on silica gel using dichloromethane and THF as eluents, affording 114 mg (yield 53%) of the target molecule as a shiny purple crystalline solid. 1H NMR (THF-d8, ppm), δ: 1.21–1.25 (m, 12H; CH3), 1.80–1.88 (m, 8H; CH2), 2.08–2.16 (m, 8H; CH2), 4.50 (m, 8H; OCH2), 7.45–7.55 (m, 4H; macrocycle), 8.39–8.49 (m, 4H; macrocycle), 8.79–8.90 (m, 4H; macrocycle); IR (cm−1): 2954–2870 (cluster), 1604, 1236, 1089, 1047, 742, UV-Vis (acetonitrile, nm,): 675, 354, MALDI-TOF (m/z) [M+H+] 866.3, elemental analysis: calcd = C (66.54%), N (12.93%), H (5.58%), found: C (66.07), N (13.05%), H (5.70%).


### 1.2. Device Fabrication

All the chemicals used for the device fabrication were purchased from Aldrich except methylamine (40% in methanol) which was purchased from Tokyo Chemical Industry Co. (TCI). TiO2 paste was purchased from Dyesol. Methylammonium iodide (MAI) was synthesized by stirring 55.6 ml of methylamine (40% in methanol) with 60 ml of hydroiodic acid (57 wt% in water) in a 250 ml round bottom flask at 0°C for 2 h. The solvent was then evaporated at 50°C for 1 h using a rotary evaporator; the resulting solid was collected and stirred with ethanol for 30 minutes. It was then filtered and repeatedly washed with diethyl ether to obtain a white solid that has been dried at 60°C in a vacuum oven for 24 h.The fluorine-doped tin oxide-coated SnO2:F (FTO) conducting glass (15, Pilkington) substrates have been cut in 2.5 × 1.5 cm size. The substrates were then etched with Zn/HCl and cleaned properly. A hole-blocking layer was spin coated on FTO using a solution of titanium diisopropoxide bis(acetylacetonate) (TiDIP) (75% in 2-propanol) in ethanol and then annealed at 450°C for 30 minutes. A mesoporous titania layer was then deposited by spin coating using TiO2 paste (DSL-90T TiO2 NPs paste) diluted in ethanol (0.1 g of paste in 0.32 g of absolute ethanol) and calcinated at 500°C for 30 minutes. The TiO2-deposited substrates were then treated with TiCl4 for 30 minutes at 70°C, then rinsed, and annealed at 500°C for 30 minutes. The perovskite has been deposited using a two-step method. In the first step, 1 M solution of PbI2 in a DMF : DMSO mixture (85 : 15 ratio) was spin coated on the substrates and annealed at 70°C for 45 min. The PbI2-coated films were then immersed in a 10 mg/ml solution of MAI in 2-propanol for 30 seconds, resulting in the formation of dense black-brown MAPbI3. The cells were further annealed at 90°C for 30 min. 40 μl of chlorobenzene solutions of (n-BuO)4ZnPc (40 mg/ml) and Spiro-OMeTAD (72.3 mg/ml) was deposited by spin coating. 26.3 μl of Bis(trifluoromethane)sulfonimide lithium salt in acetonitrile (520 mg/ml), 29 μl of tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)) in acetonitrile (300 mg/ml), and 19.2 μl of 4-tert-butilpyridine were added as dopants. Finally, 70 nm-thick gold-back contacts have been evaporated under pressure of 5 × 10−6 Torr. The active area was 0.09 cm2, and a 0.04 cm2 screen mask has been used during the measurements.


### 1.3. Photovoltaic Characterization

Photovoltaic measurements were made on a Newport system, composed of an Oriel I–V test station with an Oriel Sol3A simulator. The solar simulator is class AAA for spectral performance, uniformity of irradiance, and temporal stability. The solar simulator is equipped with a 450 W xenon lamp. The output power is adjusted to match AM1.5 global sunlight (100 mW·cm2). The spectral match classifications are IEC60904-9 2007, JIC C 8912, and ASTM E927-05. I–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter. A 10 mV voltage step and a 40 ms photocurrent delay time were used. External quantum efficiency was calculated by measuring the Isc of both the tested cell and a reference silicon cell at every wavelength, according to the following equation:
(1)EQE=IλIrefλEQEref.


### 1.4. Computational Details

The structural, electronic, redox, and optical properties of (n-BuO)4ZnPc, (t-Bu)4ZnPc, and Spiro-OMeTAD have been investigated using ab initio simulations based on density functional theory (DFT). In detail, the calculations have been performed by using the ORCA suite of programs [20] in a localized-basis-set framework. Kohn-Sham orbitals have been expanded on a def2-TZVPP Gaussian-type basis set [32]. Fully decontracted def2-TZVPP/J has been also used as an auxiliary basis set for Coulomb fitting in a resolution-of-identity/chain-of-spheres (RIJCOSX) framework [33]. Molecular geometries have been fully optimized at the B3LYP level of theory [34, 35], including dispersion forces calculated by using the DFT-D3 approach [36]. Redox potentials of the molecules have been calculated by using the M06-2X functional [37], as discussed in detail elsewhere [21]. The molecules and their positive and negative ions have been embedded in an implicit CH3CN solvent using the conductor-like polarizable continuum model (CPCM) [38] and fully optimized. The corresponding redox potentials have been calculated as ΔG values between neutral and charged species.


## 1.1. (n-BuO)4ZnPc Synthesis and Characterization

ZnCl2 and all the solvents (RPE grade) were purchased from Carlo Erba except for N,N-dimethylethanolamine (99.5%) which was purchased from Aldrich. Acetonitrile was distilled over calcium hydride. 4-n-Butoxyphthalonitrile (95%) was purchased from Tokyo Chemical Industry Co. (TCI). 1,5-Diazabicyclo[5.4.0]undec-5-ene (DBU) (≥99%) was purchased from Fluka. Reactions were monitored by thin-layer chromatography (TLC) employing a polyester layer coated with 250 mm F254 silica gel. The crude purification was performed by filtration using silica gel Carlo Erba Reactifs SDS 60A C.C. 35–70 mm and dichloromethane and THF as eluents. To perform the fluorescence measurements, a further purification has been performed by size exclusion chromatography using Bio-Beads S-X Resin (Bio-Rad) as the stationary phase and THF as the mobile phase. 1H NMR spectra were recorded in THF d8 on a Bruker AVANCE 600 NMR spectrometer operating at a proton frequency of 600.13 MHz; chemical shifts (δ) are given in ppm relative to the residual solvent peaks of the deuterated solvent (3.58, 1.73 ppm). Infrared spectra were recorded on a Shimadzu FT-IR prestige-21 spectrometer using an attenuated total reflectance (ATR) unit. All the UV-Vis measurements were performed in direct transmission mode. Absorption spectra of 10−5 M solution in acetonitrile were recorded on a PerkinElmer LAMBDA 950 UV-Vis/NIR spectrophotometer, while transparent sensitized films have been analyzed using a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies). MALDI-TOF spectra have been recorded at Toscana Life Sciences facility in a MALDI-TOF/TOF Ultraflex III (Bruker) using α-cyano-4-hydroxycinnamic acid (HCCA) as matrix. Isotopic pattern simulations have been performed by Bruker Daltonics program. Steady-state fluorescence spectra were recorded on a 10−6 M solution in acetonitrile with a Jobin Yvon Fluorolog3 spectrofluorometer, in the range 620–850 nm, using 5 nm grids for excitation and 5 nm for emission (λex=600 nm). The corresponding excitation spectrum has been collected in the range 350–700 nm, under an excitation wavelength of 750 nm, with 10 nm grids. No filters have been used. Time-resolved fluorescence measurements were carried out by a time-correlated single-photon counting (TCSPC) system (HORIBA Jobin Yvon), using a 405 nm pulsed laser diode and collecting the emission decay at the corresponding maximum emission wavelength (686 nm). The fluorescence decay profile was analyzed through decay analysis software (DAS6a HORIBA Scientific). The curve has been fitted with a monoexponential decay, with a final time decay of 2.4 ns. The quality of the fit was checked by examining the residual distribution having a χ2 value of 1.4. Voltammograms were recorded at 25°C with a potentiostat-galvanostat Metrohm PGStat 204 in a conventional three-electrode cell; a platinum disk (~1 mm) was used as working electrode together with a platinum wire as auxiliary electrode. The reference electrode was Ag/AgNO3 (0.01 M) in distilled acetonitrile (ACN) (Aldrich) (E0 vs NHE = 0.548 V), and the Fc+/Fc (ferrocenium/ferrocene) couple was used as external standard. The sample solutions were ~10−4 M in distilled anhydrous solvents, and dry tetra(n-butyl)ammonium tetrafluoroborate (TBATFB) (Aldrich) was used as the supporting electrolyte at 0.1 M. The solutions were previously purged for 10 minutes with nitrogen, and all measurements were performed under nitrogen. Cyclic voltammetry (CV) investigations were carried out at scan rates typically ranging from 0.05 to 2 Vs−1. AFM images have been acquired in contact mode using a Keysight (formerly Agilent/MI) 5500 AFM with AppNano HYDRA6R-200N probes in air at room temperature and analyzed with the freely downloadable software WSxM 5.0 develop 8.4 December2016 [31].Synthesis of (n-BuO)4ZnPc (Mixture of Regioisomers). 200 mg of 4-n-butoxyphthalonitrile (1.0 mmol, 4 eq), 34 mg of zinc chloride (0.250 mmol, 1 eq), and a catalytic amount of 1,5-diazabiciclo[5.4.0]undec-5-ene (DBU) (1%mol, 1.7 μl) have been dissolved in 1 ml of N,N-dimethylethanolamine in a round bottom flask. The mixture has been refluxed for 15 hours and then cooled down, diluted with water, and filtered to obtain a solid which has been washed with water until the pH is neutral and the washings are colorless. The desired molecule has been purified by filtration on silica gel using dichloromethane and THF as eluents, affording 114 mg (yield 53%) of the target molecule as a shiny purple crystalline solid. 1H NMR (THF-d8, ppm), δ: 1.21–1.25 (m, 12H; CH3), 1.80–1.88 (m, 8H; CH2), 2.08–2.16 (m, 8H; CH2), 4.50 (m, 8H; OCH2), 7.45–7.55 (m, 4H; macrocycle), 8.39–8.49 (m, 4H; macrocycle), 8.79–8.90 (m, 4H; macrocycle); IR (cm−1): 2954–2870 (cluster), 1604, 1236, 1089, 1047, 742, UV-Vis (acetonitrile, nm,): 675, 354, MALDI-TOF (m/z) [M+H+] 866.3, elemental analysis: calcd = C (66.54%), N (12.93%), H (5.58%), found: C (66.07), N (13.05%), H (5.70%).


## 1.2. Device Fabrication

All the chemicals used for the device fabrication were purchased from Aldrich except methylamine (40% in methanol) which was purchased from Tokyo Chemical Industry Co. (TCI). TiO2 paste was purchased from Dyesol. Methylammonium iodide (MAI) was synthesized by stirring 55.6 ml of methylamine (40% in methanol) with 60 ml of hydroiodic acid (57 wt% in water) in a 250 ml round bottom flask at 0°C for 2 h. The solvent was then evaporated at 50°C for 1 h using a rotary evaporator; the resulting solid was collected and stirred with ethanol for 30 minutes. It was then filtered and repeatedly washed with diethyl ether to obtain a white solid that has been dried at 60°C in a vacuum oven for 24 h.The fluorine-doped tin oxide-coated SnO2:F (FTO) conducting glass (15, Pilkington) substrates have been cut in 2.5 × 1.5 cm size. The substrates were then etched with Zn/HCl and cleaned properly. A hole-blocking layer was spin coated on FTO using a solution of titanium diisopropoxide bis(acetylacetonate) (TiDIP) (75% in 2-propanol) in ethanol and then annealed at 450°C for 30 minutes. A mesoporous titania layer was then deposited by spin coating using TiO2 paste (DSL-90T TiO2 NPs paste) diluted in ethanol (0.1 g of paste in 0.32 g of absolute ethanol) and calcinated at 500°C for 30 minutes. The TiO2-deposited substrates were then treated with TiCl4 for 30 minutes at 70°C, then rinsed, and annealed at 500°C for 30 minutes. The perovskite has been deposited using a two-step method. In the first step, 1 M solution of PbI2 in a DMF : DMSO mixture (85 : 15 ratio) was spin coated on the substrates and annealed at 70°C for 45 min. The PbI2-coated films were then immersed in a 10 mg/ml solution of MAI in 2-propanol for 30 seconds, resulting in the formation of dense black-brown MAPbI3. The cells were further annealed at 90°C for 30 min. 40 μl of chlorobenzene solutions of (n-BuO)4ZnPc (40 mg/ml) and Spiro-OMeTAD (72.3 mg/ml) was deposited by spin coating. 26.3 μl of Bis(trifluoromethane)sulfonimide lithium salt in acetonitrile (520 mg/ml), 29 μl of tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)) in acetonitrile (300 mg/ml), and 19.2 μl of 4-tert-butilpyridine were added as dopants. Finally, 70 nm-thick gold-back contacts have been evaporated under pressure of 5 × 10−6 Torr. The active area was 0.09 cm2, and a 0.04 cm2 screen mask has been used during the measurements.


## 1.3. Photovoltaic Characterization

Photovoltaic measurements were made on a Newport system, composed of an Oriel I–V test station with an Oriel Sol3A simulator. The solar simulator is class AAA for spectral performance, uniformity of irradiance, and temporal stability. The solar simulator is equipped with a 450 W xenon lamp. The output power is adjusted to match AM1.5 global sunlight (100 mW·cm2). The spectral match classifications are IEC60904-9 2007, JIC C 8912, and ASTM E927-05. I–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter. A 10 mV voltage step and a 40 ms photocurrent delay time were used. External quantum efficiency was calculated by measuring the Isc of both the tested cell and a reference silicon cell at every wavelength, according to the following equation:
(1)EQE=IλIrefλEQEref.


## 1.4. Computational Details

The structural, electronic, redox, and optical properties of (n-BuO)4ZnPc, (t-Bu)4ZnPc, and Spiro-OMeTAD have been investigated using ab initio simulations based on density functional theory (DFT). In detail, the calculations have been performed by using the ORCA suite of programs [20] in a localized-basis-set framework. Kohn-Sham orbitals have been expanded on a def2-TZVPP Gaussian-type basis set [32]. Fully decontracted def2-TZVPP/J has been also used as an auxiliary basis set for Coulomb fitting in a resolution-of-identity/chain-of-spheres (RIJCOSX) framework [33]. Molecular geometries have been fully optimized at the B3LYP level of theory [34, 35], including dispersion forces calculated by using the DFT-D3 approach [36]. Redox potentials of the molecules have been calculated by using the M06-2X functional [37], as discussed in detail elsewhere [21]. The molecules and their positive and negative ions have been embedded in an implicit CH3CN solvent using the conductor-like polarizable continuum model (CPCM) [38] and fully optimized. The corresponding redox potentials have been calculated as ΔG values between neutral and charged species.



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*Source: 2473152-2018-11-13.xml*