Theoretical,spectroscopical,and experimental investigations of small azomethine molecules for organic solar cells

Abstract

Small azomethine molecules (4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenylmethane (BP-DPM) and 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenyl ether (BP-DPE)) for photovoltaic applications were synthesized by condensation of appropriate arylaldehydes and arylendiamines and characterized using Fourier-transform infrared spectroscopy, ¹H NMR, ¹³C NMR, and liquid chromatography–mass spectrometry. Azomethine molecules are additives in organic solar cells. The effect of a possible energy transfer between BP-DPE and P3HT on the photovoltaic performance of devices employing ternary blends of BP-DPE:P3HT: phenyl-C61-butyric acid methyl ester (PCBM) was investigated by absorption and emission spectra. The devices employing BP-DPE:P3HT:PCBM with 1:4 ratio exhibited a J_sc of 4.2 mA cm⁻², V_oc of 575 mV, and FF of 0.27 which led to a power conversion efficiency (PCE) of 0.65%. In addition, density functional theory calculations (DFT/B3LYP/6-31G(d)) were used to determine the optimized molecular geometry, highest occupied molecular orbital–lowest unoccupied molecular orbital energies, electronic structures, and the molecular electrostatic potential surfaces of the molecules.

Introduction

The conjugation in organic molecules is the basis of many applications, including fluorescent sensors, organic lasers, fluorescent biological labels, organic light-emitting diodes (OLEDs), photovoltaic cells, organic field-effect transistors (OFET), and so on.^1–4 The use of azomethine linkers in conjugated structures has allowed the assembly of highly useful compounds.^5–9 The use of these compounds as photovoltaic materials has increased due to the fact that the electron transfer is good and the azomethine group provides the electron donor.^10–12

Azomethines are also very suitable components of conjugated compounds for reasons such as their easy synthesis under mild reaction conditions. Compounds containing polyazomethines have also been studied as organic photovoltaic materials, and their power conversion capacities were found to be in the range of about 0.1%–0.3% in these studies. Besides, the results of recent studies have shown that the photovoltaic performance in non-polymer small molecules is able to compete with that of polymers. Moreover, small molecules have many advantages in photovoltaics because their molecular structures can be determined precisely and easily, and their solubility is high.^13–16

Many azomethines have been studied for organic optoelectronic applications. Indeed, due to the ease of obtaining azomethines and their useful photovotaic performance, a great deal of work has been done on these compounds.^17–21 Thus, two new azomethines (4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenylmethane (BP-DPM) and 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenyl ether (BP-DPE)) were synthesized and characterized by spectroscopic methods. The effect of the central atom in the molecule on the photovoltaic efficiency was investigated theoretically and experimentally, for the first time, using these novel molecules. In this way, attention was drawn to the difference caused by the change of one atom. The optimized molecular geometries, highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) energies, UV-Vis spectra, and molecular electrostatic potential (MEP) of compounds were determined by using density functional theory (DFT) calculations. Also, additive behavior in the active layer of the device and photovoltaic performance of BP-DPE and BP-DPM were investigated. The results are discussed and compared with each other.

Results and discussion

UV-Vis and fluorescence spectroscopic analysis

The UV-Vis and fluorescence spectra of BP-DPM, BP-DPE, and P3HT were measured experimentally. Furthermore, the UV-Vis properties of BP-DPM and BP-DPE were investigated theoretically. All computations were performed using the time-dependent density functional theory (TD-DFT) method at the B3LYP/6-31G(d) level. The experimental absorbance and emission spectra of BP-DPM, BP-DPE, and P3HT and the theoretical absorbance spectra of BP-DPM and BP-DPE are illustrated in Figure 1.

Figure 1.

Experimental UV-Vis and fluorescence spectra of (a) BP-DPM, (b) BP-DPE, (c) P3HT, and (d) the theoretical UV-Vis spectra of BP-DPM and BP-DPE.

As can be seen from Figure 1(a), BP-DPM has a strong absorption maxima at about 347 nm. Its fluorescence spectrum spans between 400 and 580 nm and has a maximum at about 434 nm. BP-DPE has an absorption maxima which is a strong peak about 354 nm. Its fluorescence spectrum spans between 400 and 580 nm and has a maximum at about 434 nm, see Figure 1(b).

The experimental and computed UV-Vis spectral parameters and the related electronic transitions of BP-DPM and BP-DPE are listed in Table 1.

Table 1.

The experimental and calculated UV-Vis parameters of BP-DPM and BP-DPE.

Compounds	Exp. λ (nm)	Transitions	The calculated UV-Vis parameters at the DFT-B3LYP/6-31 G(d) level
			λmax (nm)	Major transition	Excitation energy (eV)	f (oscillator strength)
BP-DPM	347	π → π*	349.25	H → L	3.5500	1.1825
	–	π → π*	336.58	H → L + 1	3.6836	0.2569
	–	π → π*	328.72	H-1 → L	3.7717	0.2629
BP-DPE	354	π → π*	371.75	H → L	3.3351	1.3140
	–	π → π*	345.95	H → L + 1	3.5838	0.1684
	–	π → π*	327.95	H-1 → L	3.7806	0.2262

DFT: density functional theory; BP-DPM: 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenylmethane; BP-DPE: 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenyl ether.

The strong absorption wavelengths for BP-DPM and BP-DPE that were observed at 347 and 354 nm in their experimental UV-Vis spectra can be assigned to π → π* electronic transitions. The calculated spectra, see Figure 1(d), show bands corresponding to these experimental bands at 349.25 and 371.75 nm, respectively. It is known that the most likely transition in a compound is from its HOMO (H) to its LUMO (L). Thus, the highest oscillator strength values for BP-DPM and BP-DPE were observed at strong spectral peaks that corresponded to H→L electronic transitions. Both the experimental and theoretical results for the UV-Vis spectra of BP-DPM and BP-DPE show that the strong absorption of BP-DPE shifted compared with that in the spectrum of BP-DPM. This change in the absorption spectrum may be attributed to the oxygen atom which is in the center of the BP-DPE molecule.

As can be seen from Figure 1(c), P3HT has two absorption maxima, one of which is about 275 nm and the other is about 450 nm. Its fluorescence spectrum spans between 500 and 800 nm and has a maximum about 580 nm.

When Figure 1(a) and (b) are compared, P3HT’s absorption and BP-DPE’s fluorescent spectra maxima overlap, which means that P3HT absorbs in the region where BP-DPE emits which may give rise to an energy transfer. Therefore, we blended BP-DPE into P3HT:PCBM. As a comparison, we also blended BP-DPM into P3HT:PCBM.

Theoretical HOMO-LUMO calculation

The molecular structures, the HOMO energies, and the LUMO energies of BP-DPM and BP-DPE have been investigated. All calculations were performed by using the Gaussian 09W program^22,23 at the DFT/B3LYP level with the 6-31 G(d) basis set. The optimized molecular structures of the compounds are shown in Figure 2.

Figure 2.

Optimized molecular structures of (a) BP-DPM and (b) BP-DPE.

HOMO-LUMO orbitals are important tools to understand the chemical reactivity and kinetic stability of molecules. ²⁴ From photovoltaic point of view, the HOMO-LUMO relationship between the donor and the acceptor is significant for an effective charge transfer as well. There should be an energy difference of at least 0.3 eV between the LUMOs of the acceptor and donor for efficient charge transfer. The frontier orbital and HOMO-LUMO energy gaps of the azomethines are presented in Figure 3.

Figure 3.

The plots of the frontier molecular orbitals and the HOMO-LUMO energy diagrams for (a) BP-DPM and (b) BP-DPE.

According to these values, charge transfer interactions may take place in these molecules. Due to the existence of electron-withdrawing azomethine groups and the central linker atoms in the chemical structures of BP-DPM and BP-DPE, they were evaluated as acceptor additives in devices. The theoretical HOMO-LUMO levels of compounds BP-DPM and BP-DPE were compared with those of the most commonly used donor and acceptor materials in organic photovoltaics. The theoretical HOMO and LUMO levels of P3HT are −4.32 and −2.3 eV, respectively, whereas the theoretical values of PCBM are −5.66 and −3.02 eV, respectively. ²⁵ In this work, the HOMO and LUMO energy levels of the small molecules were calculated to be (−5.31) to (−5.15) eV and (−1.36) to (−1.39) eV, respectively. The LUMO levels of the compounds were found to be higher than P3HT, and there is an energy difference of 0.91 − 0.94 eV between their LUMOs which is more than 0.3 eV,^26–28 and so a charge transfer can be possible between P3HT and BP-DPM or BP-DPE.

MEP surface analysis

The chemical reactivity, electronegativity, and structure activity of compounds can be investigated using their MEP which is a method for mapping electrostatic potential on the iso-electron density surface. ²⁹ The MEP surfaces were simulated at the B3LYP/6-31G(d) level for BP-DPM and BP-DPE. The electrostatic potential regions of BP-DPM and BP-DPE are depicted in Figure 4 with three-dimensional charge distribution from MEP. As can be seen in Figure 4, negative electrostatic potential regions regarded as nucleophilic centers are represented with a red color, while the positive electrostatic potential regions are electrophilic sites and are represented with a blue color.

Figure 4.

The MEP surfaces of (a) BP-DPM and (b) BP-DPE molecules.

The negative electrostatic potential (nucleophilicity) of BP-DPM is situated mainly around the O48 atom, whereas the positive sites of the compound (electrophilic centers) are placed around the H51 atom. The calculated values of the electron densities on O48 and H51 are found as 0.04900 and 0.07314 a.u., respectively. Similarly, the nucleophilic sites of BP-DPE are placed around the O45 atom, whereas the positive sites (electrophilic centers) are placed around the H48 atom. The calculated values of electron densities on O45 and H48 are −0.04830 and 0.07325 a.u., respectively. As can be seen from the MEP results for BP-DPM and BP-DPE, it might be said that the electronegativity and chemical reactivity of these compounds are close to each other.

Photovoltaic studies

Device fabrication

Initially, all indium tin oxide (ITO)-coated glass substrates were patterned by etching with a mixture of HCl:HNO₃:H₂O (4.6:0.4:5) for 40 min. Then, the glass substrates were cleaned in an ultrasonic bath with iso-propanol and acetone at each stage for 20 min, respectively. An aqueous solution of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate; PEDOT:PSS, Clevios PH1000), to be used as the hole transport layer, was deposited on the ITO-coated glass substrates by spin coating at 1000 r min⁻¹ for 45 s and annealed at 120 °C for 30 min under ambient conditions to remove any residual moisture. BP-DPE of 5 mg was dissolved in 1-mL chlorobenzene. A total of 12 mg of P3HT and 6.5 mg of PCBM were dissolved in 1-mL chlorobenzene, and later, BP-DPE was added into the P3HT: PCBM solution with 1:2 and 1:4 volume ratios. The same route was followed for BP-DPM. The resulting blend solution was cast onto PEDOT:PSS-coated ITO substrates at 1500 r min⁻¹. For the top electrode, 100 nm of aluminum (Al) was thermally evaporated.

Current density–voltage (J–V) characterization

Figure 5 shows the current density–voltage (J–V) graph of the fabricated devices consisting of BP-DPM:P3HT:PCBM blends with different BP-DPM ratios. The photovoltaic data are summarized in Table 2.

Figure 5.

Current density–voltage (J–V) graphs of P3HT:PCBM and BP-DPM:P3HT:PCBM blends with different volume ratios.

Table 2.

PV data of solar cells employing BP-DPM.

Cell	Jsc (mA cm−2)	Voc (mV)	FF	η (efficiency %)
BP-DPM:P3HT:PCBM (1:2)	2.70	475	0.28	0.35
BP-DPM:P3HT:PCBM (1:4)	3.50	500	0.27	0.47
P3HT:PCBM	3.32	476	0.31	0.49

PV data: photovoltaic data; BP-DPM: 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenylmethane.

As shown in Figure 5, the devices employing BP-DPM:P3HT:PCBM with 1:2 ratio showed J_sc of 2.7 mA cm⁻², V_oc of 475 mV, and an FF of 0.28 which led to a PCE of 0.35%. The devices employing BP-DPM:P3HT:PCBM with 1:4 ratio exhibited a J_sc of 3.5 mA cm⁻², V_oc of 500 mV, and an FF of 0.27 which led to a PCE of 0.47%.

Figure 6 shows the current density–voltage (J–V) graph of the devices consisting of BP-DPE:P3HT:PCBM blended with different BP-DPE ratios. The photovoltaic result is summarized in Table 3.

Figure 6.

Current density–voltage (J–V) graphs of P3HT:PCBM and BP-DPE:P3HT:PCBM blends with different volume ratios.

Table 3.

PV data of solar cells employing BP-DPE.

Cell	Jsc (mA cm−2)	Voc (mV)	FF	η (efficiency %)
BP-DPE: P3HT: PCBM (1:2)	3.80	580	0.28	0.62
BP-DPE: 3HT:PCBM (1:4)	4.20	575	0.27	0.65
P3HT: PCBM	3.32	476	0.31	0.49

FF: fill factor; PV data: photovoltaic data; BP-DPE: 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenyl ether.

As can be seen from Figure 6, P3HT:PCBM-based devices in which BP-DPE is not employed exhibited a J_sc of 3.32 mA cm⁻², V_oc of 476 mV, and an FF of 0.31 which led to a PCE of 0.49%. For the devices employing BP-DPE:P3HT:PCBM in a 1:2 ratio were obtained a J_sc of 3.8 mA cm⁻², V_oc of 580 mV, and an FF of 0.28 which led to a PCE of 0.62%. The devices employing BP-DPE:P3HT:PCBM with 1:4 ratio exhibited a J_sc of 4.2 mA cm⁻², V_oc of 575 mV, and an FF of 0.27 which led to a PCE of 0.65%.

In literature,^21,26–32 P3HT:PCBM solar cells exhibit PCEs of 3.5%. However, these efficiencies are achieved for the devices fabricated in a glovebox where the oxygen and humidity effects are omitted and also in which thermal annealing is not applied. ³³ In this study, the devices have been fabricated in an ambient atmosphere. On the other hand, partial crystallization of P3HT upon postproduction annealing over 120 °C improves the device performance. Here, a postproduction treatment was not applied to either P3HT:PCBM or ternary blends.

Conclusion

Absorption–emission spectra, HOMO-LUMO energies, mo-lecular structures, and MEP surface properties of BP-DPM and BP-DPE have been investigated to evaluate their potentials in organic photovoltaic devices. Organic solar cells employing ternary compounds are of great interest. The presence of the –CH2– and –O– linkers in the designed molecular structures made a significant difference to performance. The higher photovoltaic performance of BP-DPE is attributed to the continuous electron transfer due to the existence of an electron-rich atom such as oxygen at the center. An increase of 15% in PCE for the devices involving a BP-DPE:P3HT:PCBM ternary blend was achieved in this study. But no increase in PCE was observed for the devices comprising BP-DPM:P3HT:PCBM ternary blends.

Here, ternary blend systems were studied whose components’ absorption and fluorescence spectra are complementary which in turn may favor an energy transfer. Further investigation will be performed by changing the volume ratio of the blends, the choice of the solvent, and film thicknesses.

Experimental

Materials

2,4-Dihydroxybenzaldehyde, n-octylbromide, 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenylmethane were purchased from Merck (Merck KGaA, Darmstadt, Germany). P3HT and PCBM were purchased from Rieke Metals (Rieke Metals Inc., Lincoln, Nebraska) and Nano-C. Other reagents used were obtained from commercial suppliers and used without further treatment. The solvents were purified and dried according to standard procedures. All solvents were purchased from Merck. All reactions were performed under the inert atmosphere of nitrogen.

Instruments and measurements

UV-Vis spectra were recorded using PerkinElmer Lambda 25 UV-Vis spectrometers in CHCI₃. The fluorescence spectra were determined using a Jobin Yvon Horiba FluoroMax-P fluorescence spectrophotometer. The IR spectra were determined on a NICOLET iS10 Fourier-transform infrared spectroscopy (FTIR) spectrometer. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer (in CDCl₃). Mass spectra (liquid chromatography–mass spectrometry (LC-MS)) were obtained on an Agilent 1200 Infinity HPLC-Agilent 6460 Jet-Stream TripleQuad spectrometer. Melting points were determined with an Electrothermal 9100 Melting Point Apparatus.

The devices were performed on ITO-coated glass substrates with a sheet resistance of 12 Ω cm⁻² from Kintec Company, Hong Kong. The ITO was patterned by etching of the glass sheet with the mixture of HCl:HNO₃:H₂O (4.6:0.4:5) for 30 min. The part of the substrate which forms the contact was covered with a scotch tape to prevent etching. The tape was removed after etching and the substrate was then cleaned by sonication with iso-propanol and acetone. The blends for the active layer with 1:2 wt ratio was prepared by 12.5 mg of P3HT, 6 mg of PCBM, and 5 mg of BP-DPE or BP-DPM in 1 mL of chlorobenzene.

Organic bulk heterojunction solar cells were prepared by spin coating the PEDOT:PSS layer on top of the ITO-coated glass surface. The PEDOT:PSS layers were tempered at 140 °C for 10 min. Then, active layers of P3HT: BP-DPE or BP-DPM was spin cast onto PEDOT:PSS-coated ITO substrates at 1500 r min⁻¹ in a glovebox. The devices were finished by evaporating of Al as top contact.

The current–voltage (J–V) characteristics of the prepared devices were measured by Keithley 2400 SourceMeter under inert atmosphere. Devices were characterized on an Abet solar simulator, simulating AM1.5 conditions, which was used as the excitation source with an input power of 100 mW cm⁻² white-light illumination.

Synthesis and characterization

General experimental procedure for small molecules

The azomethine compounds BP-DPE and BP-DPM were prepared by the reaction of appropriate reactants according to known procedures^2,24,34 (Figure 7).

Figure 7.

Synthesis of new azomethine molecules.

2-Hydroxy-4-octyloxybenzaldehyde (1.0 mmol), which was synthesized by alkylation of 2,4-dihydroxybenzaldehyde with n-octylbromide, and the diamine (1.0 mmol; either 4,4′-diaminodiphenyl ether or 4,4′-diaminodiphenylmethane) were dissolved dry ethanol (10 mL). Then, the p-toluenesulfonic acid (0.01 g) used as a catalyst was added to the reaction mixture. The solution was stirred and refluxed for 8 h. The precipitated yellow solid was filtered, washed with cold ethanol, and recrystallized from chloroform.

The compounds were characterized by using UV-Vis, FTIR, ¹H NMR, ¹³C NMR, and LC-MS data. All the spectroscopic data of the target compounds were fully in accordance with their assigned structures.

Synthesis of 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenylmethane ( BP-DPM ). Yellow solid, yield: 89%, m.p. 195–197 °C. IR(ATR) v_max, 3074, 3055, 2955, 1615, 1572, 1191 cm⁻¹; ¹H NMR (500 MHz, CDCl₃): δ 8.59 (s, 2H, 2HC=N), 7.34-7.27 (m, 10 ArH), 6.57-6.54 (m, 4 ArH),4.09 (s, 2H, –CH₂–), 4.07 (t, 4H, J = 7.0 Hz, 2OCH₂), 1.90-1.84 (m, 4H, 2 CH₂), 1.56-1.50 (m, 4H, 2 CH₂), 1.46-1.33 (m, 16H, 8 CH₂), 0.97 (t, 6H, J = 7.0 Hz, 2 CH₃). ¹³C NMR (125 MHz, CDCl₃) δ 163.9, 163.5, 161.0, 146.6, 139.3, 133.4, 129.8, 121.1, 112.9, 107.6, 101.6, 68.2, 40.9, 31.8, 29.3, 29.2, 29.1, 25.9, 22.6, 14.1. LC-MS m/z: 661.10 [M − 1]⁺ (calculated for C₄₂H₅₂N₂O₃, 662.89).

Synthesis of 4,4′-bis((2-hydroxy-4-octyloxyphenyl)methylimino)diphenyl ether (BP-DPE) . Yellow solid, yield: 83%, m.p. 203–206 °C. IR(ATR) v_max, 3096, 3056, 2931, 1620, 1597, 1192 cm⁻¹; ¹H NMR (500 MHz, CDCl₃): δ 8.55 (s, 2H, 2HC=N), 7.29-7.27 (m, 6 ArH), 7.08 (d, J = 8.5 Hz, 2 ArH), 6.82 (d, J = 9.0 Hz, 2 ArH), 6.66 (d, J = 9.0 Hz, 2 ArH), 6.51 (d, J = 8.5 Hz, 2 ArH), 4.02 (t, 4H, J = 7.0 Hz, 2 OCH₂), 1.84-1.79 (m, 4H, 2 CH₂), 1.51-1.45 (m, 4H, 2 CH₂), 1.40-1.27 (m, 16H, 8 CH₂), 0.91 (t, 6H, J = 7.0 Hz, 2 CH₃). ¹³C NMR (125 MHz, CDCl₃) δ 163.7, 163.5, 160.8, 144.0, 133.4, 122.3, 119.6, 116.3, 112.9, 107.6, 101.6, 68.2, 31.7, 29.3, 29.2, 29.1, 25.9, 22.6, 14.0. LC-MS m/z: 663.10 [M − 1]⁺ (calculated for C₄₂H₅₂N₂O₃, 664.87).