Indolocarbazole based small molecules: an efficient hole transporting material for perovskite solar cells

Abstract

To date, Spiro-OMeTAD, which is an expensive organic compound, has been used as the benchmark hole transporting material (HTM) in perovskite based solid state solar cells. Development of an inexpensive HTM with competitive performance to Spiro-OMeTAD is therefore significantly important for the commercialization of perovskite cells. Herein, an indolocarbazole based small molecule derivative (C12-carbazole) has been introduced as an environmentally stable, cost effective and highly efficient HTM. In contrast to the power conversion efficiency of 9.62% exhibited by the Spiro-OMeTAD based solid state solar cell, the C12-carbazole based device under the same experimental conditions has demonstrated an enhanced power conversion efficiency of 11.26%. The improved photovoltaic performance of the C12-carbazole based device is attributed to reduced carrier recombination by a better hole extraction ability of the C12-carbazole, which has demonstrated remarkably higher hole mobility compared to Spiro-OMeTAD.

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1. Introduction

Solar cells have been considered the most promising green technology, and convert solar radiation directly into electricity, a highly demanded form of energy. Although silicon based solar panels, which command the largest market share, have been used mostly to harvest solar radiation and directly supply output to the public electricity grid, more efficient and cost effective materials which can substitute silicon are still highly demanded. Recently, perovskite, a crystalline organometal halide (i.e., CH₃NH₃PbX₃), has been the main focus of research in photovoltaics with a strategy to enhance the energy conversion efficiency and reduce the material and device fabrication costs compared to silicon based solar cells.^1–13 Starting with the solar-to-electric power conversion efficiency (η) of 3.8% demonstrated by a perovskite based liquid junction device in 2009,¹ very recently a solid state prototype based on perovskite has already demonstrated breakthrough in solid state dye-sensitized solar cells with certified η of 20.1%.¹⁴ Researchers are still pushing enormous efforts continuously to improve the cell performance further. On the basis of various results and speculations, realistic η of as high as close to the η of silicon based solar cell has been predicted from a solid-state mesoscopic solar cell based on perovskite – thus putting the material on a par with silicon.⁹ Like silicon based solar cell, perovskite solar cells can also work without a hole transport material (HTM), which demonstrated a maximum η ranging from about 8 to 12%.^15–19 Nevertheless, study shows that the most efficient charge separation is obtained when using TiO₂ and HTM together. Although large number of HTMs has been introduced for a perovskite based solar cell, Spiro-OMeTAD is still demonstrating as the benchmark model for efficient HTM.^20–29 However, in addition to its relatively low hole mobility and high air/moisture sensitivity, Spiro-OMeTAD is also one of the expansive material in perovskite based solar cells. Therefore, a cost effective and efficient HTM, which can substitute Spiro-OMeTAD is highly desirable and significantly important to commercialize the perovskite based solar cells. Herein, we introduce a noble indolocarbazole based small molecule derivative (FW = 670.84) shown in Fig. 1a, and chemically can be called as 8,16-didodecyl-8,16-dihydrobenzo[a]benzo[6,7]indolo[2,3-h]carbazole (this compound is represented here as C12-carbazole), as an efficient HTM for perovskite based solid-state solar cells, which enables us to obtain a higher η of 11.26% as compared to 9.62% of η demonstrated by the similar device fabricated in the present work based on Spiro-OMeTAD HTM. It should be worthwhile to mention that η of the perovskite based solid-state device has been suffered from large variations in device performance and reproducibility.¹⁴ In addition, generally η higher than the average value of 12 ± 2% (ref. 13) has been successfully demonstrated only by world leading research groups on perovskite based solar cells. Nevertheless, η achieved by the C12-carbazole based device of the present work is closer to the above mentioned average efficiency of the similar device based on Spiro-OMeTAD HTM, but the efficiency is still higher than that of the Spiro-OMeTAD HTM based device constructed under similar experimental conditions in our laboratory.

Fig. 1 Molecular structure of (a) C12-carbazole, and (b) Spiro-OMeTAD. Normalized UV/Visible and PL emission spectra of 9 mM solution of (c) C12-carbazole, and (d) Spiro-OMeTAD in chlorobenzene.

2. Experimental details

2.1 Materials

All chemicals were of analytical grade, and they were used as obtained from the suppliers without further purification. C12-carbazole was synthesized as described in our previous work.³⁰ Briefly, Cu(I)-catalyzed coupling reaction between 4-diiodobenzene and benzyl carbazate was carried out to obtain benzyl 1,1′-(1,4-phenylene)bis(hydrazinecarboxylate). A double Fischer indolization reaction was carried out by mixing and refluxing the above reaction product with α-tetralone and 0.4 M H₂SO₄ in EtOAc. Thus, linear and angular inseparable 3 to 1 mixture of N,N′-bis-Cbz indolocarbazole was obtained, which was then aromatized using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The mixture of linear and angular products from aromatization was separated by virtue of their solubility differences. Recrystallization of the aromatized product from EtOAc provided dibenzyl benzo[a]benzo[6,7]indolo[2,3-h]carbazole-8,16-dicarboxylate, which upon unmasking the two Cbz groups, and subsequent alkylation with dodecyl bromide yielded C12-carbazole (90% yield). The formation of C12-carbazole was confirmed using ¹H, ¹³C NMR and mass spectroscopy.

2.2 Device fabrication

Solid state perovskite solar cells were fabricated using the method reported previously.^31,32 The method is described briefly here. Approximately 60 nm of TiO₂ blocking layer was coated on patterned and cleaned fluorine-doped tin oxide-coated (FTO) glasses (Pilkington, 7 Ω sq⁻¹) by spinning 0.15 M titanium diisopropoxide bis(acetylacetonate) (75%, Sigma-Aldrich) prepared in anhydrrous 1-butanol (99.9%, Alfa Aesar) solution at 3000 rpm followed by heat treatment at 125 °C for 5 min. The procedure was repeated for 3 cycles, and finally the spin-coated FTO glasses were heated at 500 °C for 20 min.

An approximately 0.6 μm thick mesoporous layer of TiO₂ was deposited onto the blocking layer coated FTO glasses by spin coating using diluted TiO₂ paste (TTP-20N, ENB Korea) in isopropyl alcohol (IPA, ≥99.7%) (TiO₂ : IPA ratio was 1 : 3.5 by wt) at 4000 rpm. The layers were then sintered at 450 °C in air for 1 h. The mesoporous TiO₂ film coated FTO glasses were immersed in 0.02 M aqueous TiCl₄ (>98%, Sigma-Aldrich) solution at 70 °C for 60 min. After washing with deionized water and drying, the film was heated at 500 °C for 30 min.

1 M solution of PbI₂ solution was prepared by dissolving 461 mg PbI₂ (99%, Aldrich) in 1 mL DMF, (99.8%, Sigma-Aldrich) under stirring at 60 °C. 20 μL of the PbI₂ solution was spin-coated on to the mesoporous TiO₂ film at 3000 rpm for 5 s followed by repeating the cycle at 6000 rpm for 5 s (without loading time). The film was then dried at 40 °C for 3 min, and again at 100 °C for 5 min. After cooling the film down to room temperature, 200 μL of 0.063 M CH₃NH₃I perovskite solution in 2-propanol was loaded on to the PbI₂-coated TiO₂ film for 20 s, which was then spun at 4000 rpm for 20 s and dried at 100 °C for 30 min.

To use 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD) as HTM,³¹ 20 μL of Spiro-OMeTAD solution (which was prepared by dissolving 72.3 mg of Spiro-MeOTAD (SHT-263, MERCK) in 1 mL of anhydrous chlorobenzene (99.8%, Sigma-Aldrich)), to which already 28.8 μL of 4-tert-butyl pyridine (96%, Sigma-Aldrich) and 17.5 μL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, Alfa Aesar from 98%, solution containing 520 mg Li-TSFI in 1 mL acetonitrile was added) was spin-coated on to the CH₃NH₃PbI₃ perovskite layer coated TiO₂ film at 3000 rpm for 30 s.

Similarly, to use C12-carbazole as HTM,³² 70 mL 4 wt% of C12-carbazole in chlorobenzene solution mixed with 4-tert-butyl pyridine and Li-TFSI was loaded on to the CH₃NH₃PbI₃ perovskite layer coated TiO₂ film (area 2.5 × 2.5 cm⁻²) for 1 minute, and spun at 3000 rpm for 30 s. The above C12-carbazole HTM solution for spin-coating was prepared as follows. 4-tert-Butyl pyridine was added to 4 wt% C12-carbazole solution with a volume to mass ratio of 1 : 26 μL mg⁻¹. To this mixture, Li-TFSI (pre-dissolved in acetonitrile at 170 mg mL⁻¹) was added at 1 : 12 μL mg⁻¹ of Li-TFSI solution: C12-carbazole. After spin coating, the film was dried overnight in Ar-atmosphere inside a glove box.

Finally, ∼80 nm of gold was thermally evaporated on the HTM-coated films.

3. Results and discussion

C12-carbazole (Fig. 1a) was synthesized as a yellow solid with M_w = 670.84 g mol⁻¹, and its chemical composition was confirmed as described in our previous work.³⁰ Based on the molar mass, C12-carbazole is nearly two times smaller than Spiro-OMeTAD (Fig. 1b, M_w = 1225 g mol⁻¹) molecule (Fig. 1b), and as compared to the expensive Spiro-OMeTAD, the total cost of synthesis of C12-carbazole is relatively cheaper. In addition, as compared to Spiro-OMeTAD, C12-carbazole is relatively hydrophobic, and is therefore environmentally more stable. As a result, previously we could fabricate Field-Effect Transistors based on C12-carbazole nanowires at room temperature and pressure on an experimental desk outside a glove box.³⁰ The device under these open laboratory conditions did not show performance degradation up to 30 days. This report reveals that C12-carbazole is relatively less sensitive to moisture and air, and hence it is environmentally more stable. This finding suggests that C12-carbazole can be handled more easily while fabricating the devices. On the other hand, C12-carbazole has demonstrated remarkably high carrier mobility. The hole mobility of a single crystalline nanowire of C12-carbazole has been measured previously³⁰ as 1.5 cm² V⁻¹ s⁻¹, which is significantly higher than that of Spiro-OMeTAD (5.31 × 10⁻⁵ cm² V⁻¹ s⁻¹).²⁵ Considering the advantage of these perspectives of the materials, C12-carbazole is investigated in the present work as a potential HTM for solid state perovskite solar cells, and comparatively studied with the Spiro-OMeTAD based cells.

The optical absorption/emission spectra of C12-carbazole and Spiro-OMeTAD are shown in Fig. 1c and d, respectively. Fig. 1c shows the strong absorption of UV light at about 341 nm and a very weak absorption of visible light at about 421 nm. Such a weak absorption of visible light suggests that C12-carbazole would not interfere the penetration of visible light reaching to the sensitizer inside the mesoporous layer structure of the device. Meanwhile, it should be noted that the emission spectrum shown in Fig. 1c has intersected the absorption spectrum at 417.5 nm, which suggests the optical band gap of C12-carbazole equals to 2.97 eV. This value is close to those of Spiro-OMeTAD as shown in Fig. 1d, and X51 – a carbazole based HTMs²⁵ (see Fig. S1†) as summarized in Table 1. Thus, these three HTMs have more or less similar energy gap. The ground state oxidation of the HTMs, which corresponds to the highest occupied molecular orbital (HOMO) level, should be suitably situated in order to keep the optimized balance between hole-transfer yield and open-circuit voltage (V_oc) of the device.^33,34 In the present work, the ground state oxidation of C12-carbazole is determined using cyclic voltammetry. Fig. S2† shows the cyclic voltamogram of C12-carbazole film in acetonitrile containing 0.1 M tetrabutyl ammonium tetrafluoroborate as a supporting electrolyte. From the onset potential of the anodic branch of the voltamogram, the ground state oxidation is estimated at 0.79 V vs. NHE, which reveals the HOMO energy level of −5.29 eV below vacuum level. Although, this HOMO level is little lower than those of Spiro-OMeTAD, X51 and P3HT, based on the HOMO energy level of the CH₃NH₃PbI₃ perovskite sensitizer (−5.46 eV (ref. 34)), the 0.17 eV over potential is large enough driving force for the complete hole transportation (Fig. S3†), and thereby a complete regeneration of the oxidized dye can be ensured.^33,34 Apart from the HOMO level, hole mobility of the material is another key factor to be considered while designing a new HTM. In the present work, hole mobility of C12-carbazole was estimated by constructing an organic thin film transistor, and measuring drain current–gate voltage transfer curves (Fig. S4†) as reported previously.³⁰ Fig. S5a† shows the SEM top view of the spin-coated film on, which exhibits a thin fused mass of C12-carbazole in the film, and being significantly smaller than Spiro-OMeTAD and X51, the C12-carbazole can easily contact with the CH₃NH₃PbI₃. This indicates that C12-carbazole can easily infiltrate and the infiltrated mass can make interconnections together with a thin over layer of C12-carbazole on the top of the mesoporous structures of the device (Fig. S5b†), which is highly desirable and significantly important to extract holes efficiently.^7,12 The measured hole mobility of the C12-carbazole film have been comparatively summarized to those of the reported values of Spiro-OMeTAD and X51 in Table 1. The hole mobility of the C12-carbazole film is less than that for a single crystalline C12-carbazole nanowire, which we reported earlier.³⁰ The higher hole mobility of the C12-carbazole in nanowire architecture could be due to the unidirectional charge transportation. Nevertheless, the hole mobility of the spin-coated C12-carbazole film is still remarkably higher, which is higher by three and two order of magnitude as compared to those of Spiro-OMeTAD and X51, respectively.

Table 1 Optoelectronic properties of various HTMs

HTMs	C12-carbazole	Spiro-OMeTAD^a	X51^b
a Data from ref. 22.b Data from ref. 22.c Single crystal nanowire data from ref. 27.d Spin-coated film of this work.
λabs (nm)	341(max); 352; 399; 421	356; 386(max)	307(max); 365
λem (nm)	427(max); 450	423	469
Band gap (nm)	2.97	2.98	2.93
HOMO (eV)	−5.29	−5.13	−5.14
Hole mobility (cm^2 V^−1 s^−1)	^c1.5; ^d5.1 × 10^−2	5.31 × 10^−5	1.51 × 10^−4

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Based on the above optoelectronic properties, C12-carbazole has been investigated as the potentially efficient HTM in a perovskite based solid-state solar cell, and compared with the similar cell based on Spiro-OMeTAD HTM. For this, we constructed a solid-state mesoscopic device with the following configuration: compact TiO₂/400 nm mesoporous TiO₂/CH₃NH₃PbI₃/HTM-Li-TFSI/Au. The detail configuration of the device can be viewed at the SEM cross-sectional image of the cell shown in Fig. 2. The devices were constructed using the standard method as used for the Spiro-OMeTAD based cell without further any optimizations. The details about the construction of the device have been described in the ESI,† and the details on the characterization of the cells are discussed below.

Fig. 2 SEM cross-sectional view of the perovskite cell. (i) 600 nm FTO, (ii) 60 nm compact TiO₂, (iii) 400 nm mesoporous TiO₂, (iv) 180 nm perovskite over layer, (v) 140 nm C12-carbazole over layer, and (vi) 80 nm gold film.

Fig. 3 shows the photoluminescence (PL) emission spectra of the CH₃NH₃PbI₃ sensitized TiO₂ film on a glass substrate before and after addition of HTMs. The emission spectra reveal that the intense fluorescence exhibited by the CH₃NH₃PbI₃–TiO₂ film has been quenched quantitatively after the addition of both HTMs. The similar degree of PL quenching suggests that both HTMs establish an easy electron transfer path from CH₃NH₃PbI₃ to TiO₂ by extracting the holes efficiently, and thereby making an efficient charge separation. Thus, a highly competitive photovoltaic performance of the devices based on these two HTMs can be expected. Fig. 4a shows the typical characteristic current–voltage (J–V) curves obtained from the best performing devices of the present work, and the detail photovoltaic parameters obtained from the J–V curves are tabulated in Table 2. It is important to note that, in contrast to η of 9.62% exhibited by the Spiro-OMeTAD based cell, the C12-carbazole based device has demonstrated a better photovoltaic performance with a η of 11.26%. As evident in Fig. 4a, the short-circuit current densities (J_sc) of the devices based on both HTMs are similar. Therefore, their contribution to η should also be similar, which is also supported by the similar incident-photon-to-current efficiency (IPCE) of the both devices (Fig. 4b).

Fig. 3 PL emission spectra of (i) TiO₂–CH₃NH₃PbI₃, (ii) TiO₂–CH₃NH₃PbI₃–C12-carbazole, and (iii) TiO₂–CH₃NH₃PbI₃–Spiro-OMeTAD films.

Fig. 4 (a) J–V curves, and (b) incident-photon-to-current efficiency (IPCE) spectra of the perovskite cells with two different HTMs.

Table 2 Photovoltaic parameters obtained from J–V curves of the perovskite cells based on two different HTMs

HTMs	Voc (V)	Jsc (mA cm^−2)	FF	Eff. (%)
C12-carbazole	0.86	21.13	0.62	11.26
Spiro-OMeTAD	0.93	21.97	0.47	9.62

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Based on the J–V curves in Fig. 4a, the better η of the C12-carbazole based cell can be undoubtedly ascribed to the better fill factor (FF) of the device, which is mainly controlled by the carrier recombination of the devices. In the present work, the degree of carrier recombination of the two devices with different HTMs was investigated by investigating the photovoltage decay kinetics. As expected, relatively sluggish photovoltage decay kinetic of the C12-carbazole HTM based device can be observed in Fig. 5. This suggests an improved charge separation of the device from the better hole extraction ability of the C12-carbazole, and thereby a better photovoltaic performance of the device shown in Fig. 4a and Table 2 can be assured. In addition to the better power conversion efficiency, a fairly good reproducibility of the device fabrication and performance, while examining a number of devices fabricated with C12-carbazole HTM, was also obtained (Fig. S6 and Table S1†). In this case, η of as low as 8.73% was obtained, which is within the acceptable range of deviation. It should be also noted that the present C12-carbazole based device has demonstrated a better photovoltaic performance than that of the previously reported X51 (which is also a carbazole based small molecule) HTM²⁵ based device (Table 2).

Fig. 5 Photovoltage decay kinetics after stopping illumination of the perovskite solar cell based on two different HTMs.

On the other hand, apart from Spiro-OMeTAD as a model for small molecule HTM, various polymeric hole conductors such as P3HT, PTAA, and PANI have also been used in perovskite solar cells.^29,35–37 Among them, P3HT is often used as model polymeric HTM in solar cells owing to their suitable carrier mobility, conductivity and compatible HOMO energy level to the light harvesting sensitizers (Table S2†). Therefore, it would be worthy to compare the photovoltaic performance of a perovskite solar cell based on P3HT HTM. When a perovskite based solid state solar cell similar to the one described above but with P3HT as HTM was constructed in the present study, the device demonstrated a η of 7.25% (Fig. S7†). As compared to C12-carbazole and Spiro-OMeTAD based cells, the poor photovoltaic performance of the P3HT based device under similar experimental conditions could be due to the poor infiltration of the P3HT into the mesoporous structure of the device owing to its larger molecular size. This is one of the reasons for the great attraction of researchers on development of small molecular HTMs.

4. Conclusions

Based on the results of the present work, C12-carbazole can be considered as a frugal, efficient and providently competitive HTM as compared to the Spiro-OMeTAD, which is the benchmark HTM in perovskite based solar cells. In addition, as compared to X51 and P3HT, the present study demonstrates C12-carbazole as an efficient HTM for the perovskite based solid state solar cell. The photovoltage decay kinetic study reveals that as compared to the Spiro-OMeTAD HTM based device, an improved charge separation of the C12-carbazole HTM based device is obtained from the better hole extraction ability of the C12-carbazole, and thereby a better photovoltaic performance of the device can be obtained. Works to further improve the performance of the C12-carbazole HTM based perovskite solar cell is underway.

Acknowledgements

This research was supported by the KIST Institutional Program (2E23964) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013009768). One of authors (Won K. Seok) appreciates support from research program of Dongguk University 2014, and N. K. Shrestha acknowledges The Korean Federation of Science and Technology Societies for the support under Brain Pool program.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10148d

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