Molecular engineering of the hole-transporting material spiro-OMeTAD via manipulation of alkyl groups

Abstract

Aliphatic substituent effects on the HOMO energy levels and the ability to transport charge and form stable molecular glasses of systematically modified spiro-OMeTAD analogues were investigated. It was determined that the thermal properties, energy levels and hole mobility values are dependent on the number of alkyl substituents and their position in the investigated spirobifluorene-based hole transporting materials (HTMs). The charge mobility of HTM3 possessing a seemingly insignificant m-methyl group in the diphenylamino moieties is the highest with a value of 2.8 × 10⁻³ cm² V⁻¹ s⁻¹ at 6.4 × 10⁵ V cm⁻¹ field strength. It was found that moving one methoxy group into the m-position in the diphenylamino fragment ensured a stable amorphous phase of HTM1. Moreover, the long-term stability of a solid state dye-sensitized solar cell (ssDSSC) device comprising HTM1 was significantly enhanced over a cell with spiro-OMeTAD, in lifetime tests. The findings described in this publication could be applicable to hybrid solar cell research as a number of well-performing architectures rely heavily upon doped spiro-OMeTAD as a HTM.

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Introduction

Continual growth of the human population and improving living standards result in increasing energy consumption, while the combustion of fossil fuels, which currently is the main way to satisfy global energy needs, causes environmental pollution and climate change.^1,2 Thus development of sustainable energy solutions is urgently needed to manage these challenges. Among various renewable sources solar energy has several advantages and is believed to have the largest potential of satisfying future global energy needs.^3,4 Solar energy can reduce dependence on imported energy as it is widely available throughout the world, and it enhances the security of supply because no fuel price risks or constraints are involved.⁵ Besides, solar power could eventually contribute to stabilize the costs of electricity generation by increasing energy variety. Solar photovoltaics (PV) do not generate greenhouse gases and other pollutants (such as sulphur and nitrogen oxides) during operation and, in addition, little or no water is consumed.⁶ These benefits of solar cells (SC) are especially significant in dry and hot areas where the large-scale use of fresh water for cooling of thermal power plants and local air pollution have become severe problems.⁵ About 90 percent of the photovoltaic market is currently occupied by crystalline silicon solar cells;⁵ however, relatively high production costs resulting in a long financial payback time¹ encourages the search for cheaper alternatives. Due to their potential low cost, ease of processing and good performance, dye-sensitized solar cells (DSSCs) based on nanocrystalline semiconductors are expected to become a promising alternative photovoltaic technology. Among the various hole conductors employed in solar cells, low molecular weight organic compounds have advantages of plentiful resources, low cost, easy film formation and simple tuning of their properties by structure modification.⁷ On the other hand, such p-type semiconductors, forming stable amorphous phases, are also attractive materials to be used in organic light-emitting diodes, photorefractive applications or electrophotography.⁸

Recently, organometal halide perovskites (e.g., methyl-ammonium lead halides CH₃NH₃PbX₃, where X = halogen) have emerged as promising light harvesters for high-efficiency nanostructured photovoltaic devices.^9–12 In these cells, the hole transporting material is the key component. HTMs used for perovskite solar cells (PSCs) are expected to have good hole-drift mobility and high morphological stability as well as meeting certain HOMO and LUMO energy requirements. Although numerous materials can form amorphous films during the vitrification process, it is not rare that they crystallize over time in the device thus resulting in a decrease of device efficiency.^13,14 From the class of small molecule HTMs, 2,2′,7,7′-tetrakis(4,4′-dimethoxydiphenylamino)-9,9′-spirobi-fluorene (spiro-OMeTAD) (Fig. 1a) is the most often used hole-transporting material in solid-state dye-sensitized solar cells (ssDSSCs)^15,16 as well as in PSCs.^9,17–20

Fig. 1 (a) Spiro-OMeTAD; (b) TPD molecule with methyl groups at different positions.

Alternative low molecular weight p-type semiconductors such as 3,4-ethylenedioxythiophene-,²¹ pyrene-,²² linear π-conjugated-,²³ azomethine-,²⁴ butadiene-,²⁵ methoxydiphenylamine-substituted carbazole-,²⁶ and triphenylamine-based^27,28 structures pave the way towards low-cost, environmentally friendly, and efficient HTMs. However, the majority of these compounds fail to show performance similar to that of spiro-OMeTAD. Unfortunately, spiro-OMeTAD is found to exist in a semicrystalline state and has a tendency to crystallize in the device, thus impairing the cell’s performance.¹⁵ Therefore, the strategy for modification of the HTM in order to obtain truly non-crystallizable molecular glasses with retained outstanding processability and tuneable electronic properties could be a great achievement in improving the performance and stability of their corresponding photovoltaic devices.

Until now only a few concrete recommendations have been made with regard to molecular structure modification. Jeon et al. reported that PSCs’ performance was dependent on the position of the p-, m-, and o-OMe substituents.²⁹ The PSCs based on modified spiro-OMeTAD, i.e. possessing 2,4′-dimethoxydiphenylamino fragments, showed highly improved performances, which resulted in an overall power conversion efficiency (PCE) of 16.7%, compared to a PCE (∼15%) with conventional p-OMe positions. Recently, we have demonstrated that the introduction of local asymmetry into the spiro-OMeTAD molecule suppresses crystal growth in the HTM layer and significantly increases solar cell resistance to thermal stress and the overall lifetime of the ssDSSC device.³⁰

Our current research was inspired by the study of Nukada, Sato and Akasaki³¹ who reported how seemingly insignificant methyl groups at different positions of the N,N,N′,N′-tetraarylbenzidine (TPD derivative) molecule (Fig. 1b) can influence the hole-drift mobility. They have found that the introduction of methyl groups into the side phenyl moieties increases the hole drift mobility from o- to p-positions. It was also revealed that the presence of two methyl groups is superior to that of a single one. Taking into consideration that spiro-OMeTAD (Fig. 1a) is conceptually derived mostly from the respective TPD compounds,³² we have applied these findings for our research.

In this study, we systematically modified spiro-OMeTAD by either inserting additional methyl group(s) or by moving the methoxy group(s) to the meta-position and investigated their structure–property relationship. Methyl groups were selected because of the commercial availability of the starting materials; moreover there is a very slight difference between the methyl and ethyl groups in this case.³¹ A methoxy group was replaced by an ethoxy group in some cases in order to increase the local asymmetry of the molecule. Thus, we report the synthesis and characterization of seven spirobifluorene derivatives HTM1–HTM7 (Fig. 2) as well as their photoelectric and morphological properties.

Fig. 2 Structures of modified spiro-molecules HTM1–HTM7.

Results and discussion

Synthesis and morphological properties of spiro-OMeTAD analogues

Various substituted diphenylamines 1–7 were synthesized from their respective aniline and an appropriate aryl halide, employing the Buchwald–Hartwig reaction, as precursors for the synthesis of their corresponding spirobifluorenes HTM1–7. Water-mediated catalyst preactivation was necessary for the success of this reaction. It was performed by mixing palladium(II) acetate with the ligand in a mixture of 1,4-dioxane and water at 80 °C for about 1.5 min.³³ The reaction was over in 15–30 minutes and the products were isolated in 85–96% yield (Scheme 1). Cross-coupling of 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene with the synthesized diphenylamines 1–7 in the presence of Pd(II) acetate, tri-tert-butylphosphonium tetrafluoroborate and sodium tert-butoxide in dry toluene at 110 °C afforded the target compounds HTM1–7 in 70–87% yields (Scheme 2).

Scheme 1 Synthesis of various substituted diphenylamines 1–7.

Scheme 2 Synthetic route to spirobifluorene derivatives HTM1–HTM7.

The results of NMR spectroscopy, MS and elemental analysis confirmed the structures of compounds HTM1–7. All these compounds are soluble in common organic solvents (e.g. tetrahydrofuran, chloroform, dichloromethane etc.) which makes them suitable for solution processing. A detailed description of the synthesis procedures and spectroscopic data is presented in the ESI.†

Thermal properties were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. It was found that moving one of the methoxy groups into the m-position, or incorporating additional methyl(s) or m-methoxy groups in the diphenylamino fragments did not significantly change the thermal stability of the hole transporting material compared to spiro-OMeTAD. The materials HTM1–5 start to decompose at temperatures in the range between 400 and 451 °C (Fig. 3 and S1,† Table 1), confirming their good thermal stability and demonstrating practical utilization.

Fig. 3 Thermogravimetric heating curve of HTM1.

Table 1 Thermal and photoelectrical properties of compounds HTM1–7

Compound	Tm, °C	Tg, °C	Tdec^a, °C	Ip^b, eV	μ0^c, cm^2 V^−1 s^−1	μ^d, cm^2 V^−1 s^−1
a Thermal decomposition temperature (Tdec) was registered at 5% weight loss.b Ionization potential (Ip) was measured by the photoemission in air method from films.c Hole drift mobility (μ0, μ) was measured by the xerographic time of flight (XTOF) technique.d Mobility value at 6.4 × 10^5 V cm^−1 field strength.
Spiro-OMeTAD	245	124	449	5.00	4.1 × 10^−5	5 × 10^−4
HTM1	—	93	451	5.12	1.4 × 10^−5	5.6 × 10^−4
HTM2	237	107	430	5.08	2.2 × 10^−6	4.4 × 10^−5
HTM3 (ref. 28)	237	121	440	4.95	5.3 × 10^−5	2.8 × 10^−3
HTM4	256	138	430	4.91	3.0 × 10^−5	1.2 × 10^−3
HTM5	233, 285	140	400	5.02	3.0 × 10^−5	5.7 × 10^−4
HTM6	271	120	385	4.98	1.0 × 10^−5	5.0 × 10^−4
HTM7	220, 233, 237	125	375	5.00	8.0 × 10^−5	2.0 × 10^−4

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Meanwhile, changing the methoxy groups to ethoxy groups decreases the thermal stability of HTM6 and HTM7 (Table 1, Fig. S1†) more significantly. Slight weight loss (∼2%) around 100 °C in the case of compound HTM7 is attributed to the evaporation of the traces of solvents that remained after purification of the material. DSC was used to characterize the morphology of the synthesized HTMs. As seen from the results, the introduction of some local asymmetry by moving one of the methoxy groups into the m-position ensured a stable amorphous phase of compound HTM1; all DSC scans demonstrate that this HTM is fully amorphous and shows only a glass transition temperature (T_g) at 93 °C (Fig. 4a, Table 1). This is desired to form homogeneous HTM films in SC devices. Meanwhile, the second methoxy group at m-position (HTM2) caused the material to exist in both amorphous and crystalline states (Fig. 4b, Table 1). This could probably be explained by the higher degree of symmetry in the molecule.

Fig. 4 DSC first and second heating curves of: (a) HTM1 and (b) HTM2 (heating rate 10 °C min⁻¹).

However, in this case only slight crystallization is observed. HTM4 and HTM5 are also molecular glasses but with more expressed endothermic melting peaks in the first heating run compared to HTM2. Apparently the introduction of one additional methyl group into the m-position in HTM3 was not enough to inhibit the tendency to crystallize, and both glass transition (120 °C) and melting of crystals (237 °C) were observed during the DSC experiment as reported elsewhere.³⁰ Compounds HTM4–7 also appeared to be mixtures of amorphous and crystalline materials (Table 1). Most likely, in the case of HTM4 and HTM5, the reason was a high degree of local symmetry in their structure. The incorporation of the slightly heavier ethoxy group tends to hinder translation, rotational, and vibrational motions of the molecule, leading to an increase in the crystallinity of compounds HTM6 and HTM7 (Fig. 5a and b).³⁴ Moreover, replacement of the methoxy group with an ethoxy substituent decreased the thermal stability as was mentioned above. The crystallization of samples above the glass transition temperature was induced by the presence of crystalline material, and the crystallization exothermic peaks of HTM6 and HTM7 were observed at 153 °C and 159 °C correspondingly. Melting of the crystals was detected at 271 °C for compound HTM6, while polymorphism was observed in the case of HTM7 (i.e. melting was registered at 220 °C, 233 °C and 237 °C).

Fig. 5 DSC first and second heating curves of: (a) HTM6 and (b) HTM7 (heating rate 10 °C min⁻¹).

Optical and photoelectrical properties

The UV-vis absorption bands of the spirobifluorene-based HTM1–7 compounds and the parent spiro-OMeTAD were measured in THF and are presented in Fig. 6. All investigated HTMs are based on the similar principal structure as spiro-OMeTAD, and therefore, their UV-vis spectra are quite similar as well.

Fig. 6 Light absorption spectra of HTM1–7 and spiro-OMeTAD in THF (c = 10⁻⁴).

Two main photon absorption maxima at ca. 303 nm and 387 nm are assigned to π–π* transitions of the conjugate system in the analyzed compounds. Comparison of the UV/vis spectra of the new HTMs with spiro-OMeTAD shows that the movement of the methoxy group to the m-position (HTM1) or an additional m-methoxy fragment (HTM2) resulted in a slight hypsochromic (4 nm) shift. Most likely, due to the presence of methoxy groups at the m-positions in one of the benzene rings in the diphenylamine moieties, the methoxydiphenylamine-substituted fluorene fragments are slightly more twisted out-of-plane compared with spiro-OMeTAD. Moreover, the molar absorptivity of all the synthesized compounds is slightly larger compared to spiro-OMeTAD (69 670 M⁻¹ cm⁻¹ at 387 nm), varying from 76 492 (HTM3) to 95 833 M⁻¹ cm⁻¹ (HTM2), which is typical of compounds in this class.^32,35,36 The largest hyperchromic shift is observed for HTM2 possessing two m-methoxy fragments on one of the benzene rings in the diphenylamine moieties.

When considering the use of an organic material for hole-transport applications it is important to have an understanding of its solid state ionization energies (I_p). This understanding can help in identifying suitable partner organic transport and inorganic electrode materials. The ionization potential was measured by the photoelectron spectroscopy in air (PESA) method (Fig. S2†) and results are presented in Table 1. The I_p values of all of the investigated materials were quite similar and close to that of spiro-OMeTAD (5.00 eV). Movement of the methoxy group to the m-position or introduction of an additional m-methoxy group in compounds HTM1 and HTM2 resulted in the increase of I_p by 0.12 and 0.08 eV correspondingly compared to spiro-OMeTAD. Additional methyl group(s) in the m-position of the diphenylamino fragment(s) of HTM3 and HTM4 decreased the ionization potential to 4.95 eV and 4.91 eV accordingly. However, compound HTM5, possessing two m-methyl groups on one of the benzene rings, showed an ionization potential very similar to that of spiro-OMeTAD. Replacement of one methoxy group with an ethoxy group in transporting materials HTM6 and HTM7 also did not have significant influence on I_p.

To estimate the charge transport properties of the synthesized materials, the hole drift mobility of HTM1–7 was measured by the xerographic time of flight (XTOF) technique from the films of the neat samples (Fig. 7a and b). The corresponding layers of spiro-OMeTAD were also prepared in order to compare the charge-transporting ability of the new HTMs.

Fig. 7 Transient photocurrents in the layers of: (a) HTM6 and (b) HTM5 at different applied sample voltages, insets show the one transient curve in the linear plot.

Nevertheless, well-defined transit times (t_t) established from intersection points of two asymptotes of double-logarithmic plots provided hole-drift mobility at respective applied fields. Moreover, hole transport is Gaussian and transit time is defined in the linear plot of the transients only for the HTM5 sample (Fig. 7b). Examples of the mobility field dependencies are given in Fig. 8. In all cases the investigated mobility μ may be well approximated by the formula:

where μ₀ is the zero field mobility, α is the Poole–Frenkel parameter and E is the electric field strength. Such mobility dependence is explainable in terms of the Borsenberger and Weiss,³⁷ and Borsenberger et al.³⁸ disorder formalism. The mobility defining parameter μ₀ and the mobility value at 6.4 × 10⁵ V cm⁻¹ field strength are given in Table 1. As seen from the results, the mobility of HTM3 possessing an additional m-methyl group is higher in comparison to spiro-OMeTAD, while moving the methoxy group to the m-position did not have much influence on the mobility of HTM1 but it was slightly decreased at the low electric field. However, the presence of two m-methoxy groups resulted in a significant decrease of the hole mobility in compound HTM2.

Fig. 8 Field dependencies of the hole drift mobility: (a) of compounds HTM1–3 and spiro-OMeTAD; (b) of compounds HTM4–7 and spiro-OMeTAD.

This is most probably due to the electron-withdrawing effect of the m-OMe substitution by an inductive effect in the material’s chemical structure leading to a reduction in the hole mobility.^39–41 Two additional m-methyl groups did not significantly influence the mobility except for compound HTM7 which showed an increased value of μ₀ compared to spiro-OMeTAD.

Strong dependence of the mobility on the electric field was observed for the hole transporting materials HTM5 and HTM4 possessing additional methyl groups at the meta-position of one or both of the benzene rings respectively. The results of the photoelectrical measurements are presented in Table 1.

Photovoltaic performance

The synthesized compounds were also used as hole-transporting materials in ssDSSCs. The photovoltaic properties of the devices constructed with the solid hole transporters were measured using a perylene monoimide dye ID504 as a sensitizer.⁴² The J–V characteristics of the cells with spirobifluorene derivatives HTM1–7 under AM 1.5G illumination are shown in Fig. 9 and Table 2.

Fig. 9 J/V curves of solar cells comprising HTM1–7 as the solid HTMs, a glass substrate with a transparent conductive layer (FTO), a dyed mesoporous TiO₂ layer, and a silver counter electrode.

Table 2 Photovoltaic performance of the ssDSSCs comprising HTM1–7

Compound	JSC, mA cm^−2	VOC, mV	FF, %	η, %
Spiro-OMeTAD	9.76	760	64	4.8
HTM1	6.19	880	37	2.0
HTM2	1.84	800	43	0.6
HTM3	9.34	820	63	4.8
HTM4	4.54	880	73	2.9
HTM5	7.89	820	43	2.9
HTM6	8.09	840	40	2.8
HTM7	4.62	900	70	2.9

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It is noteworthy that all devices with the synthesized transporting materials demonstrated a higher open-circuit voltage (800–900 mV) compared to spiro-OMeTAD (760 mV). However, structure modifications resulted in a lower short-circuit photocurrent density (J_SC) of the new compounds in comparison to spiro-OMeTAD. As expected from the charge mobility measurements, the introduction of two m-methoxy groups (HTM2) in one of the benzene rings had a particularly negative influence on the short-circuit current density (1.84 mA cm⁻² in comparison to 9.76 mA cm⁻² of the solar cell comprising spiro-OMeTAD) and the overall conversion efficiency (0.6% compared to 4.8% in the device with spiro-OMeTAD). Compound HTM3 with an additional m-methyl group has shown similar J_SC and FF values and an increase in the V_OC by 60 mV with respect to spiro-OMeTAD. Finally, the overall conversion efficiency of the solar cell with HTM3 was equal to that of the device with spiro-OMeTAD. A small difference in the J_SC and V_OC values, and a large difference in FF value is observed when comparing HTM6 with HTM3. This can be explained by a stronger tendency of HTM6 to crystallize as was revealed by the DSC analysis. Most probably the crystallization of the HTM inside the pores or on the interface affects the contact between the light absorber and the hole transporter.³⁰

The decrease of the J_SC values was registered in the solar cells with organic semiconductors HTM4–7 although the FF values of HTM4 and HTM7 were higher in comparison to spiro-OMeTAD. The device comprising HTM7 demonstrated the best open-circuit voltage value (900 mV) of all the investigated samples. Overall conversion efficiency values in these cases were very similar, though lower compared to spiro-OMeTAD and reached 2.9%. The decrease in the short-circuit current density values in the case of HTM4–7 most likely could be attributed to the larger charge recombination at the HTM/dye interface, which arises from the alterations in the shape of these molecules and subsequent changes in packing and charge transport properties.

Previously we reported that the introduction of local asymmetry into the spiro-OMeTAD molecule suppressed crystal growth in the HTM layer and significantly increased the solar cell resistance to thermal stress and overall lifetime of the ssDSSC device.³⁰ This inspired us to investigate the stability of the device comprising HTM1 as this hole transporting material is fully amorphous and shows only a glass transition temperature. Devices based on HTM1 and spiro-OMeTAD were investigated to determine their stability (Fig. 10). Both solar cells were sealed and constantly kept at 60 °C and 30% humidity after fabrication. This revealed that despite its initial overall conversion efficiency being more than twice as low, the sample with HTM1 was more stable compared to the DSSC based on spiro-OMeTAD. A sharp decrease of the conversion efficiency of the device with spiro-OMeTAD was registered at the beginning of the experiment and was equal to ca. 50% after 120 hours. Only 40% and 30% of its initial efficiency were observed after 170 hours and at the end of the experiment, accordingly. In the case of compound HTM1, the efficiency of the device remained almost unchanged and was equal to 85% of the initial cell’s performance at the end of investigation after 300 hours.

Fig. 10 Lifetime of solar cells possessing hole-transporting materials HTM1 and spiro-OMeTAD at 60 °C.

Conclusions

In summary, seven systematically modified spiro-OMeTAD analogues were synthesized and investigated as HTMs. The I_p values of all the investigated materials were quite similar and close to that of spiro-OMeTAD. The charge mobility of HTM3 possessing an additional m-methyl group in the diphenylamino moieties is the highest with a value of 2.8 × 10⁻³ cm² V⁻¹ s⁻¹ at 6.4 × 10⁵ V cm⁻¹ field strength. This illustrates the statement that photoelectric properties can be improved by introducing seemingly insignificant methyl groups. Moreover, it was found that moving one of the methoxy groups into the m-position in the diphenylamino fragment ensured a stable amorphous phase of the spiromolecule HTM1. For this reason the long-term stability of the ssDSSC device comprising HTM1 was significantly enhanced over the cell with spiro-OMeTAD, in lifetime tests.

Acknowledgements

The authors acknowledge funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement no. 604032 of the MESO project and BASF SE. We thank E. Kamarauskas for his help with the ionization potential measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedures, device construction and characterisation details. See DOI: 10.1039/c6ra09878a

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