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Dopant-free hole transport materials processed with green solvent for efficient perovskite solar cells
Chemical Engineering Journal 385 (2020) 123976
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Dopant-free hole transport materials processed with green solvent for efficient perovskite solar cells
T
Huiqiang Lua, Bizu Hea, Yu Jia, Yahan Shana, Cheng Zhongb, Jing Xua, Junxia LiuYanga, ⁎ ⁎ Fei Wua, , Linna Zhua, a b
Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, School of Materials & Energy, Southwest University, Chongqing 400715, PR China Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430000, PR China
H I GH L IG H T S
was developed as dopant-free hole • F23 transport material (HTM) in per-
G R A P H I C A L A B S T R A C T
A new green-solvent-processable dopant-free hole transport material F23 was developed for efficient and stable perovskite solar cells, with a high power conversion efficiency of 17.6%.
ovskite solar cells (PSCs).
green solvent tetra• Nonhalogenated hydrofuran (THF) is used for F23 film
• •
fabricating and only small amount is required. F23 film exhibits uniform and smooth morphology, with excellent hole transport property. High efficiency (17.60%) with enhanced stability was achieved in F23based PSCs.
A R T I C LE I N FO
A B S T R A C T
Keywords: Dopant-free Green solvent Hole transport materials Perovskite solar cells Power conversion efficiency
At present, dopant-free hole transport materials (HTMs) have been largely explored to improve the performance and stability of perovskite solar cells (PSCs), and significant progresses have been made. In current reports, chlorobenzene is the most commonly used solvent to dissolve HTMs in PSCs. Dopant-free HTMs based on green solvent were rarely reported yet. Here in this work we synthesized a new dopant-free HTM F23, which could be processed with the nonhalogenated and environmental-friendly green solvent tetrahydrofuran (THF). Interestingly, F23 film prepared using THF as solvent exhibits uniform and smooth morphology, with excellent hole transport property. In addition, the process requires only a small amount of F23 (3 mg/mL), thus further lowering the costs in device fabrication. As a result, device using dopant-free F23 prepared in THF as HTM exhibited a high power conversion efficiency (PCE) of 17.60%, with enhanced stability.
1. Introduction The growing demand for energy and concerns about global warming have sparked a strong interest in renewable energy such as solar energy around the world [1,2]. As the third generation solar cell technology,
⁎
the organic–inorganic hybrid perovskite solar cells (PSCs) have great potential to solve the energy shortage issue due to their impressive photovoltaic performance and low production cost [3–6]. They have been extensively studied by academia and caused wide interests [7–10]. PSCs have many unique and excellent optical and electronic properties
Corresponding authors at: School of Materials & Energy, Southwest University, Chongqing, PR China. E-mail addresses: [email protected] (F. Wu), [email protected] (L. Zhu).
https://doi.org/10.1016/j.cej.2019.123976 Received 22 October 2019; Received in revised form 7 December 2019; Accepted 27 December 2019 Available online 28 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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polarity and intermolecular interactions, thus facilitate charge transfer. The carbazol-diphenylamine group was modified on different positions on bipyridine mioety, making F22 and F23 show different molecular configuration, charge delocalization and energy levels. Compared with F22, F23 has smaller reorganization energy and a higher hole mobility (1.18 × 10−4 cm2 V−1 s−1, undoped), which guarantees F23 as a potential dopant-free HTM [45]. More interestingly, the non-halogen solvent THF is successfully applied to prepare F23 film and more uniform morphology is obtained in comparison with F22 film deposited from THF. In addition, the green solvent process has great significance, and is promising for future practical application of PSCs [46,47]. It is exciting that the device utilizing dopant-free F23 processed with THF exhibits excellent photovoltaic performance, with the PCE reaching 17.60%. By contrast, the device based on undoped F22 shows a much inferior PCE, of 15.31%. The PCE of control device based on doped Spiro-OMeTAD is 17.57% and that of dopant-free Spiro-OMeTAD device is 16.94% measured with the same device structure. Therefore, F23 as a dopant-free HTM has a remarkable potential for fabricating low cost and environment-friendly PSCs.
in comparison with common inorganic semiconductors, such as strong light harvesting ability [11–13], high charge carrier mobility [14] and long charge diffusion length [15,16], which contributes to its rapid PCE increase from 3.8% [17] to 25.2% [18] in just 10 years. Planar PSCs have been favored by researchers for its high efficiency and easy preparation since it was proposed in 2013 [19]. Nowadays, it is reported that the n-i-p type planar devices have exhibited excellent performance over 20% [20–22]. In n-i-p type device structure, the hole transport layer (HTL) is usually sandwiched between the bottom perovskite and the top metal electrodes. HTL plays an important role in promoting hole extraction and hole transporting, protecting perovskite from the adverse effects of external factors such as moisture and oxygen, thus improving device performance and device stability [23–26]. At present, the state-of-art molecule 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is the most widely used hole transport material (HTM), and device based on Spiro-OMeTAD has achieved excellent performance exceeding 20% [27–31]. However, the complex synthetic procedure, difficult purification and high cost limit its large-scale production and application [32,33]. More important, due to the poor conductivity of pristine SpiroOMeTAD, dopants such as tert-butylpyridine (t-BP) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) are usually added to improve its conductivity and hole mobility [34,35]. However, the oxidized HTMs formed during doping will interact with the t-BP additive, forming new pyridinated products which could decrease the device performance and has a negative effect on the long-term stability of PSCs [36,37]. The hygroscopicity of Li-TFSI also accelerates the device degradation and is detrimental to device stability [38]. Moreover, it will introduce complexity during device fabrication, increase total costs of the device fabrication, and hinder commercialization of PSCs [2]. To solve these problems, it is thus urgent to develop efficient dopant-free HTMs. To date, a series of novel dopant-free HTMs have been devised and investigated. For example, Liu et al [39] recently reported three new dopant-free HTMs based on 6,12-dihydroindeno[1,2-b]fluorine for PSCs and the device based on IDF-SFXPh obtained a PCE of 17.6%. Feng et al [40] reported some star-shaped oligo-arylamines as dopant-free HTMs, and the Ph-TPA-4A-based device shows the highest efficiency of 17.4%. Cai et al [41] have developed two new HTMs, and the undoped device using C202 based on indolo[3,2-b]carbazole as HTM exhibited a high PCE(17.7%). All of these HTMs exhibit high efficiency, however, they were processed with chlorinated solvent chlorobenzene (CB). As an ideal dopant-free HTM, an environmental-friendly strategy for HTL preparation is preferential like green solvent processing [42]. In 2017, a green-solvent-processable polymeric HTM asy-PBTBDT was reported for the first time by Lee et al. The device based on dopant-free asyPBTBDT processed with green solvent 2-methylanisole shows a high PCE (18.3%) [42]. Quite recently, our group reported a green solvent processable dopant-free Spiro-OMeTAD, which could be processed from a non-halogenated solvent tetrahydrofuran (THF), and the corresponding device showed a high performance approaching 17% [43]. Whereas as a whole, dopant-free HTM fabricated using green solvent is rarely seen in literatures. In this work, we synthesized a new dopant-free green-solvent-processable HTM F23 for PSCs with high efficiency and improved stability. In F23, the carbazole-diphenylamine is modified on 4,4′-position of the 2, 2′-bipyridine group, while in F22 the carbazole-diphenylamine group was substituted on the 5, 5′-position of the bipyridine core structure (Fig. 1) [44]. Bipyridine has a planar structure with the electronwithdrawing ability, which works to adjust the energy level of HTMs. Carbazol-diphenylamine is an excellent module for building new HTMs due to the superior electron-donating ability. In F23 or F22, bipyridine as electron acceptor could regulate their energy level due to its electron-withdrawing ability. Moreover, the electron-withdrawing ability of bipyridine could also increase the intramolecular charge transfer from carbazole-diphenylamine to bipyridine, enhancing the molecular
2. Results and discussion Fig. 1 shows the molecular structure of F23. F23 was synthesized from N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole-3,6-diamine (compound 1) [44] and 4,4′-dibromo-2,2′-bipyridine via a palladiumcatalyzed Buchwald–Hartwig cross coupling reaction [48]. The synthesis details of F23 are described in the support information. Then, the structure of F23 was characterized by 1H NMR, 13C NMR and high resolution MALDI-TOF spectrum, the results are shown in the Supporting Information. UV–Vis spectra of F23 and F22 were measured in CH2Cl2 solution. As Fig. 2a shows, the absorption peaks of F23 and F22 between 280 and 300 nm are caused by π-π* transition, and the shoulder bands around 380 nm are attributed to the intramolecular charge transfer from carbazole rings to bipyridine ring. In contrast, the absorption bands of F23 are blue-shifted relative to that of F22, which should come from the reduced effective conjugation length in F23. The energy level of F23 was estimated by cyclic voltammetry (CV), with Spiro-OMeTAD measured as the reference (Fig. S4) [49]. The energy level diagrams of F22 and F23 are depicted in Fig. 2b. The highest occupied molecular orbital (HOMO) level of F23 is estimated to be −5. 20 eV, which is derived from the onset of the first oxidation peak of CV (EHOMO = −(4.74 + (Eonset. ox vs Fc+ /Fc)). The lowest unoccupied molecular orbital (LUMO) level of F23 is estimated to be −2.41 eV, according to the equation of ELUMO = EHOMO + Eg, in which Eg is calculated from the onset of the UV absorption spectrum (Eg = 1240/ λonset). The results show that F23 has a suitable HOMO level to extract holes from perovskite, which matches well with the valence band edge of perovskite. The LUMO level of F23 is much higher than the conduction band edge of perovskite, which can block the electrons back transfer from perovskite to the metal electrode. The thermal stability of F23 was determined by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), as shown in Fig. S5. The thermal decomposition temperature (Td) of F23 is 328 °C, while recorded at the weight loss of 5% under N2 atmosphere. The glass transition temperature (Tg) of F23 is 110 °C from DSC analysis. The results indicate that F23 has excellent thermal stability and is suitable as HTM for PSCs. Compared to the thermal properties of F22, F23 shows lower Td (332 °C) than that of F22 (442 °C), and the Tg of F23 (110 °C) is also lower compared with that of F22 (162 °C). It is most likely due to the different substitution positions of the periphery carbazole moiety, leading to more twisted molecular structure in F23 than F22 [32]. Density functional theory (DFT) calculations were carried out at the B3LYP/def2SVP level with Grimme’s D3BJ empirical dispersion 2
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Fig. 1. The chemical structures of F22 and F23.
(100 nm)) were fabricated. F22 was also studied as dopant-free HTM using the same device structure. Here, C60 was evaporated on the ITO substrate as the electron transport material. Furthermore, the thin MoO3 layer is employed as the cathode buffer layer to reduce contact resistance at the interface of HTM/Ag electrode [52] and prevent Ag diffusion. The details of device fabrication are described in Supporting Information. The schematic diagram of device structure and the crosssectional scanning electron microscopy (SEM) image of a typical device configuration are shown in Fig. 4a and 4b. It is noteworthy that the green solvent THF was used to dissolve F23, and the F23 film was prepared by dynamic spin-coating. The use of THF can avoid the toxic halogen-containing CB solvent, which has great significance for future industrialization of PSCs in an environmental-friendly way. To obtain the optimal photovoltaic performance of the device, F23 in different concentrations were studied. The results are exhibited in Fig. S7, and the related data are summarized in Table S1 and Table S2. The device based on F23 achieves the optimal photovoltaic performance when the concentration of F23 in THF is 3 mg/mL. It should be pointed out that the amount of F23 is much less than the amount of HTMs used in conventional devices. For example, in doped Spiro-OMeTAD-based
correction (Fig. 3) [50]. It is observed that the HOMO orbitals of F23 and F22 have similar distributions, that is, they are mainly distributed on the periphery carbazole–diarylamine parts. The LUMO orbitals of F22 and F23 are mainly distributed on the central bipyridine rings, notably, there are more LUMO orbitals distributed on the adjacent carbazole ring in F22 than in F23. This might be due to the more distorted configuration of F23 than F22. Moreover, the calculated reorganization energy of F23 (0.092 eV) is smaller than that of F22 (0.095 eV), which is conducive to obtaining higher hole mobility [51]. Hole mobility is also one of the key factors affecting the hole transport performance of HTMs. Therefore, hole mobilities of pristine F23 and F22 film fabricated using THF as solvent were measured by the space charge limitation of current (SCLC) method (show in Fig. S6). The hole mobility of pristine F23 and F22 films are 1.18 × 10-4 cm2 V−1 s−1 and 6.45 × 10−5 cm2 V−1 s−1 respectively. Compared with F22 film, F23 has higher hole mobility, which is conducive to more efficient hole transporting. To evaluate the performance of F23 as potential dopant-free HTMs for PSCs, planar n-i-p type devices (ITO/C60 (20 nm)/ (CH3NH3PbI3−xClx) (410 nm)/F23 (29.2 nm)/MoO3 (10 nm)/Ag
Fig. 2. a) UV/Vis absorption of F22 and F23 measured in CH2Cl2 solution; b) Energy level diagram of PSC with F22, F23 or Spiro-OMeTAD as HTM. 3
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Fig. 3. The frontier molecular orbitals distribution of F22 and F23.
exhibits slightly obvious hysteresis effect with a higher hysteresis index (0.039). Fig. 4e shows the IPCE spectra of the devices based on F22 and F23. The devices based on F23 and F22 show strong photoelectric conversion intensity in the visible range from 350 nm to 750 nm. The integrated current densities of F22 and F23-based devices are 20.07 mA cm−2 and 20.49 mA cm−2, respectively, which are consistent with the Jsc values determined from the J-V curves. In order to confirm the accuracy of the photovoltaic performance test, we measured the stabilized output of the devices in 200 s at the maximum power point (Fig. 4f). It turned out that devices based on F23 and F22 have stable output with PCE of 17.40% and 15.10%, Jsc of 19.77 mA cm−2 and 18.42 mA cm−2 respectively. In addition, the statistical analysis of PCE, Voc, Jsc and FF based on 20 independent devices in Fig. S9 show a narrow distribution, indicating that the devices fabricated in this way have a good reproducibility. The surface coverage of perovskite crystal and the quality of HTM film above it are very important for interfacial charge transfer dynamics, which determine the photovoltaic properties of PSCs [54]. Therefore, the surface morphology of perovskite active layer and HTL
devices, usually 70 mg/mL of Spiro-OMeTAD is required [27–31]. Usually, for dopant-free HTMs, a low concentration around 10 mg/mL is needed [4,26,39–41,45,53]. It is very important for reducing the device fabrication costs. The optimized J–V curves for F23 and F22based devices are presented in Fig. 4c and the corresponding data are collected in Table 1. PSCs based on dopant-free F23 produces a promising PCE of 17.60%, with a short circuit current density (Jsc) of 21.62 mA cm−2, an open circuit voltage (Voc) of 1.070 V, and a fill factor (FF) of 76.08%, which are comparable to the doped SpiroOMeTAD-based device with the same configuration (17.57%) (Fig. S8). In contrast, the undoped F22-based PSCs yields a poor PCE of 15.31%. In addition, the device using dopant-free F23 as HTM exhibits a higher performance than the dopant-free Spiro-OMeTAD-based device (Jsc = 21.29 mA cm−2, Voc = 1.023 V, FF = 77.78%, PCE = 16.94%; shown in Fig. S8) under the same fabrication and working conditions. The hysteresis effect of the devices are measured (Fig. 4d and Table S3). There are neglectable differences between the forward and reverse scanning of F23-based device, and the hysteresis index is 0.027, demonstrating it has little hysteresis. In contrast, device based on F22
Fig. 4. a) Illustration of the device structure studied in this work; b) Cross-sectional SEM image of the device with F23 as HTM; c) J–V curves of PSC devices employing F22 or F23 as dopant-free HTMs; d) J–V curves of F22 or F23-based PSCs under different scan directions; e) IPCE spectra of PSC devices based on F22 or F23 and the integrated Jsc curve; f) Stabilized efficiency and Jsc of F22 and F23-based PSCs under a bias of 0.82 V and 0.88 V, respectively. 4
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Table 1 Photovoltaic parameters of devices based on dopant-free F22 and F23. HTM
Voc (mV)
Jsc (mA/cm2)
FF (%)
PCE (%)
Undoped F22 Undoped F23
1007 (997 ± 10) 1070 (1061 ± 9)
21.11 (20.90 ± 0.27) 21.62 (21.39 ± 0.26)
72.02 (70.21 ± 1.24) 76.08 (74.48 ± 1.26)
15.31 (14.64 ± 0.52) 17.60 (16.90 ± 0.56)
The data in parentheses is the average value based on 20 devices.
Fig. 5. a) AFM images of pristine perovskite film and perovskite with F22 or F23 on top. b) Steady-state PL spectra of perovskite film and that coated with F22, F23 or Spiro-OMeTAD; c) Time-resolved PL decay curves of perovskite, and that with F22, F23 or Spiro-OMeTAD on top.
deposited on top. The perovskite with F23 has a shortest PL decay lifetime (13.84 ns) in comparison with that coated with F22 (21.43 ns) or Spiro-OMeTAD (15.61 ns). This indicates the fastest hole collection at the perovskite/F23 interface relative to the perovskite/F22 interface or perovskite/Spiro-OMeTAD interface. The excellent hole extraction capability of dopant-free F23 provides the corresponding device with high Jsc and FF values [39]. Compared to F22, the device based on dopant-free F23 shows an enhanced photovoltaic efficiency although they have similar structures. The increased PCE of F23 device is mainly due to the higher FF and Voc. Structurally speaking, the different substitution position of carbazoldiphenylamine on bipyridyl group leads to different electronic properties in F23 and F22. The more twisted structure of F23 resulted in decreased reorganization energy, which facilitates increased hole mobility in F23 film. In addition, F23 film has a higher hole extraction ability and a more uniform surface morphology than F22 film. These factors promote hole extraction and hole transporting in F23 film, and resulting in efficient Ohmic contact between perovskite layer and the Ag electrode, thus reducing charge recombination at the perovskite/Ag interface. Therefore, F23-based devices show enhanced photovoltaic performances compared to F22-based device [10,55]. Stability is one of the important indicators of excellent PSCs. To study the stability of the device based on undoped F23, the tests were
were explored by Atomic force microscope (AFM), as shown in Fig. 5a. The results show that the perovskite film has a rough surface morphology with a root mean square (RMS) of 21.42 nm. After deposition of F22 or F23, smoother and more uniform film with small RMS of 9.53 nm for F22 and 8.92 nm for F23 was observed. The results indicate that the HTMs dissolved in the green solvent THF could also form a good coverage of the perovskite layer. Moreover, the F23 film shows less roughness than F22, which helps to form an effective selective contact between the perovskite active layer and the metal electrode, and helps to reduce the charge recombination and enhance charge transfer at the interface [54]. The steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were further investigated to have a good insight into the hole extraction capability of HTMs F23 and F22 with the Spiro-OMeTAD as reference. Fig. 5b shows the steady-state PL spectra of pristine perovskite and that coated with undoped F22, F23 or Spiro-OMeTAD. The pristine perovskite has a strong fluorescence signal, and when it is covered by F22, F23 or Spiro-OMeTAD, the emission signal has a significant decrease. Compared with F22 and Spiro-OMeTAD, PL quenching ability of F23 is more efficient, denoting that F23 has best hole extraction ability. Fig. 5c displays the TRPL spectra of perovskite with or without HTMs. The PL lifetime of perovskite decreases obviously when F22, F23 or Spiro-OMeTAD is
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Fig. 6. Stability test of doped Spiro-OMeTAD and undoped F23 based PSCs in N2 and ambient condition (air, 25–30% relative humidity).
measured in both N2 and ambient condition (air, 25%-30% relative humidity). PSCs based on Spiro-OMeTAD was also studied for comparison (Fig. 6). The device based on pristine F23 remains 90% of the original PCE when it is placed in N2 atmosphere for 500 h, while the PCE of the device based on doped Spiro-OMeTAD is less than 80% of the original PCE. When stored under 20−25% relative humidity in the air, the device based on pristine F23 still retains 80% of the original efficiency in 200 h, while PCE of the doped Spiro-OMeTAD-based device decreases significantly to less than 60% of its original value. The contact angles of doped and dopant-free Spiro-OMeTAD and pristine F23 were also measured to elucidate the distinctions in devices stability (Fig. 6, Fig. S10). Pristine F23 film has the largest water contact angle (97°), and a relatively smaller contact angle is obtained for doped (79°) or dopant-free Spiro-OMeTAD (94°), indicating that the pristine F23 has the highest hydrophobicity among these HTMs studied. The above results demonstrate that the device based on dopant-free F23 has better stability than that incorporating doped-Sprio-OMeTAD, which could be attributed to the higher hydrophobicity of F23. 3. Conclusions In conclusion, we developed a new compound F23, which was successfully applied as dopant-free HTM in PSCs. F23 shows small molecular reorganization energy, high hole mobility, strong hole extraction ability and smooth film morphology. Of particular interesting is that F23 film was prepared using nonhalogenated green solvent THF and only small amount of F23 is required. As a result, PCE of device based on F23 achieves 17.6%. Moreover, the device also exhibits good stability due to the absence of the hygroscopic dopants. The green solvent process we used provides a new strategy toward the preparation of PSCs with reduced costs and environmental-friendly solvent. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work described in this paper was supported by the National Natural Science Foundation of China (No. 51703183, 51873160), the “Project supported by Basic research and Frontier Exploration of Chongqing Municipal Science and Technology Commission 6
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Dopant-free hole transport materials processed with green solvent for efficient perovskite solar cells
T
Huiqiang Lua, Bizu Hea, Yu Jia, Yahan Shana, Cheng Zhongb, Jing Xua, Junxia LiuYanga, ⁎ ⁎ Fei Wua, , Linna Zhua, a b
Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, School of Materials & Energy, Southwest University, Chongqing 400715, PR China Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430000, PR China
H I GH L IG H T S
was developed as dopant-free hole • F23 transport material (HTM) in per-
G R A P H I C A L A B S T R A C T
A new green-solvent-processable dopant-free hole transport material F23 was developed for efficient and stable perovskite solar cells, with a high power conversion efficiency of 17.6%.
ovskite solar cells (PSCs).
green solvent tetra• Nonhalogenated hydrofuran (THF) is used for F23 film
• •
fabricating and only small amount is required. F23 film exhibits uniform and smooth morphology, with excellent hole transport property. High efficiency (17.60%) with enhanced stability was achieved in F23based PSCs.
A R T I C LE I N FO
A B S T R A C T
Keywords: Dopant-free Green solvent Hole transport materials Perovskite solar cells Power conversion efficiency
At present, dopant-free hole transport materials (HTMs) have been largely explored to improve the performance and stability of perovskite solar cells (PSCs), and significant progresses have been made. In current reports, chlorobenzene is the most commonly used solvent to dissolve HTMs in PSCs. Dopant-free HTMs based on green solvent were rarely reported yet. Here in this work we synthesized a new dopant-free HTM F23, which could be processed with the nonhalogenated and environmental-friendly green solvent tetrahydrofuran (THF). Interestingly, F23 film prepared using THF as solvent exhibits uniform and smooth morphology, with excellent hole transport property. In addition, the process requires only a small amount of F23 (3 mg/mL), thus further lowering the costs in device fabrication. As a result, device using dopant-free F23 prepared in THF as HTM exhibited a high power conversion efficiency (PCE) of 17.60%, with enhanced stability.
1. Introduction The growing demand for energy and concerns about global warming have sparked a strong interest in renewable energy such as solar energy around the world [1,2]. As the third generation solar cell technology,
⁎
the organic–inorganic hybrid perovskite solar cells (PSCs) have great potential to solve the energy shortage issue due to their impressive photovoltaic performance and low production cost [3–6]. They have been extensively studied by academia and caused wide interests [7–10]. PSCs have many unique and excellent optical and electronic properties
Corresponding authors at: School of Materials & Energy, Southwest University, Chongqing, PR China. E-mail addresses: [email protected] (F. Wu), [email protected] (L. Zhu).
https://doi.org/10.1016/j.cej.2019.123976 Received 22 October 2019; Received in revised form 7 December 2019; Accepted 27 December 2019 Available online 28 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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polarity and intermolecular interactions, thus facilitate charge transfer. The carbazol-diphenylamine group was modified on different positions on bipyridine mioety, making F22 and F23 show different molecular configuration, charge delocalization and energy levels. Compared with F22, F23 has smaller reorganization energy and a higher hole mobility (1.18 × 10−4 cm2 V−1 s−1, undoped), which guarantees F23 as a potential dopant-free HTM [45]. More interestingly, the non-halogen solvent THF is successfully applied to prepare F23 film and more uniform morphology is obtained in comparison with F22 film deposited from THF. In addition, the green solvent process has great significance, and is promising for future practical application of PSCs [46,47]. It is exciting that the device utilizing dopant-free F23 processed with THF exhibits excellent photovoltaic performance, with the PCE reaching 17.60%. By contrast, the device based on undoped F22 shows a much inferior PCE, of 15.31%. The PCE of control device based on doped Spiro-OMeTAD is 17.57% and that of dopant-free Spiro-OMeTAD device is 16.94% measured with the same device structure. Therefore, F23 as a dopant-free HTM has a remarkable potential for fabricating low cost and environment-friendly PSCs.
in comparison with common inorganic semiconductors, such as strong light harvesting ability [11–13], high charge carrier mobility [14] and long charge diffusion length [15,16], which contributes to its rapid PCE increase from 3.8% [17] to 25.2% [18] in just 10 years. Planar PSCs have been favored by researchers for its high efficiency and easy preparation since it was proposed in 2013 [19]. Nowadays, it is reported that the n-i-p type planar devices have exhibited excellent performance over 20% [20–22]. In n-i-p type device structure, the hole transport layer (HTL) is usually sandwiched between the bottom perovskite and the top metal electrodes. HTL plays an important role in promoting hole extraction and hole transporting, protecting perovskite from the adverse effects of external factors such as moisture and oxygen, thus improving device performance and device stability [23–26]. At present, the state-of-art molecule 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is the most widely used hole transport material (HTM), and device based on Spiro-OMeTAD has achieved excellent performance exceeding 20% [27–31]. However, the complex synthetic procedure, difficult purification and high cost limit its large-scale production and application [32,33]. More important, due to the poor conductivity of pristine SpiroOMeTAD, dopants such as tert-butylpyridine (t-BP) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) are usually added to improve its conductivity and hole mobility [34,35]. However, the oxidized HTMs formed during doping will interact with the t-BP additive, forming new pyridinated products which could decrease the device performance and has a negative effect on the long-term stability of PSCs [36,37]. The hygroscopicity of Li-TFSI also accelerates the device degradation and is detrimental to device stability [38]. Moreover, it will introduce complexity during device fabrication, increase total costs of the device fabrication, and hinder commercialization of PSCs [2]. To solve these problems, it is thus urgent to develop efficient dopant-free HTMs. To date, a series of novel dopant-free HTMs have been devised and investigated. For example, Liu et al [39] recently reported three new dopant-free HTMs based on 6,12-dihydroindeno[1,2-b]fluorine for PSCs and the device based on IDF-SFXPh obtained a PCE of 17.6%. Feng et al [40] reported some star-shaped oligo-arylamines as dopant-free HTMs, and the Ph-TPA-4A-based device shows the highest efficiency of 17.4%. Cai et al [41] have developed two new HTMs, and the undoped device using C202 based on indolo[3,2-b]carbazole as HTM exhibited a high PCE(17.7%). All of these HTMs exhibit high efficiency, however, they were processed with chlorinated solvent chlorobenzene (CB). As an ideal dopant-free HTM, an environmental-friendly strategy for HTL preparation is preferential like green solvent processing [42]. In 2017, a green-solvent-processable polymeric HTM asy-PBTBDT was reported for the first time by Lee et al. The device based on dopant-free asyPBTBDT processed with green solvent 2-methylanisole shows a high PCE (18.3%) [42]. Quite recently, our group reported a green solvent processable dopant-free Spiro-OMeTAD, which could be processed from a non-halogenated solvent tetrahydrofuran (THF), and the corresponding device showed a high performance approaching 17% [43]. Whereas as a whole, dopant-free HTM fabricated using green solvent is rarely seen in literatures. In this work, we synthesized a new dopant-free green-solvent-processable HTM F23 for PSCs with high efficiency and improved stability. In F23, the carbazole-diphenylamine is modified on 4,4′-position of the 2, 2′-bipyridine group, while in F22 the carbazole-diphenylamine group was substituted on the 5, 5′-position of the bipyridine core structure (Fig. 1) [44]. Bipyridine has a planar structure with the electronwithdrawing ability, which works to adjust the energy level of HTMs. Carbazol-diphenylamine is an excellent module for building new HTMs due to the superior electron-donating ability. In F23 or F22, bipyridine as electron acceptor could regulate their energy level due to its electron-withdrawing ability. Moreover, the electron-withdrawing ability of bipyridine could also increase the intramolecular charge transfer from carbazole-diphenylamine to bipyridine, enhancing the molecular
2. Results and discussion Fig. 1 shows the molecular structure of F23. F23 was synthesized from N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole-3,6-diamine (compound 1) [44] and 4,4′-dibromo-2,2′-bipyridine via a palladiumcatalyzed Buchwald–Hartwig cross coupling reaction [48]. The synthesis details of F23 are described in the support information. Then, the structure of F23 was characterized by 1H NMR, 13C NMR and high resolution MALDI-TOF spectrum, the results are shown in the Supporting Information. UV–Vis spectra of F23 and F22 were measured in CH2Cl2 solution. As Fig. 2a shows, the absorption peaks of F23 and F22 between 280 and 300 nm are caused by π-π* transition, and the shoulder bands around 380 nm are attributed to the intramolecular charge transfer from carbazole rings to bipyridine ring. In contrast, the absorption bands of F23 are blue-shifted relative to that of F22, which should come from the reduced effective conjugation length in F23. The energy level of F23 was estimated by cyclic voltammetry (CV), with Spiro-OMeTAD measured as the reference (Fig. S4) [49]. The energy level diagrams of F22 and F23 are depicted in Fig. 2b. The highest occupied molecular orbital (HOMO) level of F23 is estimated to be −5. 20 eV, which is derived from the onset of the first oxidation peak of CV (EHOMO = −(4.74 + (Eonset. ox vs Fc+ /Fc)). The lowest unoccupied molecular orbital (LUMO) level of F23 is estimated to be −2.41 eV, according to the equation of ELUMO = EHOMO + Eg, in which Eg is calculated from the onset of the UV absorption spectrum (Eg = 1240/ λonset). The results show that F23 has a suitable HOMO level to extract holes from perovskite, which matches well with the valence band edge of perovskite. The LUMO level of F23 is much higher than the conduction band edge of perovskite, which can block the electrons back transfer from perovskite to the metal electrode. The thermal stability of F23 was determined by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), as shown in Fig. S5. The thermal decomposition temperature (Td) of F23 is 328 °C, while recorded at the weight loss of 5% under N2 atmosphere. The glass transition temperature (Tg) of F23 is 110 °C from DSC analysis. The results indicate that F23 has excellent thermal stability and is suitable as HTM for PSCs. Compared to the thermal properties of F22, F23 shows lower Td (332 °C) than that of F22 (442 °C), and the Tg of F23 (110 °C) is also lower compared with that of F22 (162 °C). It is most likely due to the different substitution positions of the periphery carbazole moiety, leading to more twisted molecular structure in F23 than F22 [32]. Density functional theory (DFT) calculations were carried out at the B3LYP/def2SVP level with Grimme’s D3BJ empirical dispersion 2
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Fig. 1. The chemical structures of F22 and F23.
(100 nm)) were fabricated. F22 was also studied as dopant-free HTM using the same device structure. Here, C60 was evaporated on the ITO substrate as the electron transport material. Furthermore, the thin MoO3 layer is employed as the cathode buffer layer to reduce contact resistance at the interface of HTM/Ag electrode [52] and prevent Ag diffusion. The details of device fabrication are described in Supporting Information. The schematic diagram of device structure and the crosssectional scanning electron microscopy (SEM) image of a typical device configuration are shown in Fig. 4a and 4b. It is noteworthy that the green solvent THF was used to dissolve F23, and the F23 film was prepared by dynamic spin-coating. The use of THF can avoid the toxic halogen-containing CB solvent, which has great significance for future industrialization of PSCs in an environmental-friendly way. To obtain the optimal photovoltaic performance of the device, F23 in different concentrations were studied. The results are exhibited in Fig. S7, and the related data are summarized in Table S1 and Table S2. The device based on F23 achieves the optimal photovoltaic performance when the concentration of F23 in THF is 3 mg/mL. It should be pointed out that the amount of F23 is much less than the amount of HTMs used in conventional devices. For example, in doped Spiro-OMeTAD-based
correction (Fig. 3) [50]. It is observed that the HOMO orbitals of F23 and F22 have similar distributions, that is, they are mainly distributed on the periphery carbazole–diarylamine parts. The LUMO orbitals of F22 and F23 are mainly distributed on the central bipyridine rings, notably, there are more LUMO orbitals distributed on the adjacent carbazole ring in F22 than in F23. This might be due to the more distorted configuration of F23 than F22. Moreover, the calculated reorganization energy of F23 (0.092 eV) is smaller than that of F22 (0.095 eV), which is conducive to obtaining higher hole mobility [51]. Hole mobility is also one of the key factors affecting the hole transport performance of HTMs. Therefore, hole mobilities of pristine F23 and F22 film fabricated using THF as solvent were measured by the space charge limitation of current (SCLC) method (show in Fig. S6). The hole mobility of pristine F23 and F22 films are 1.18 × 10-4 cm2 V−1 s−1 and 6.45 × 10−5 cm2 V−1 s−1 respectively. Compared with F22 film, F23 has higher hole mobility, which is conducive to more efficient hole transporting. To evaluate the performance of F23 as potential dopant-free HTMs for PSCs, planar n-i-p type devices (ITO/C60 (20 nm)/ (CH3NH3PbI3−xClx) (410 nm)/F23 (29.2 nm)/MoO3 (10 nm)/Ag
Fig. 2. a) UV/Vis absorption of F22 and F23 measured in CH2Cl2 solution; b) Energy level diagram of PSC with F22, F23 or Spiro-OMeTAD as HTM. 3
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Fig. 3. The frontier molecular orbitals distribution of F22 and F23.
exhibits slightly obvious hysteresis effect with a higher hysteresis index (0.039). Fig. 4e shows the IPCE spectra of the devices based on F22 and F23. The devices based on F23 and F22 show strong photoelectric conversion intensity in the visible range from 350 nm to 750 nm. The integrated current densities of F22 and F23-based devices are 20.07 mA cm−2 and 20.49 mA cm−2, respectively, which are consistent with the Jsc values determined from the J-V curves. In order to confirm the accuracy of the photovoltaic performance test, we measured the stabilized output of the devices in 200 s at the maximum power point (Fig. 4f). It turned out that devices based on F23 and F22 have stable output with PCE of 17.40% and 15.10%, Jsc of 19.77 mA cm−2 and 18.42 mA cm−2 respectively. In addition, the statistical analysis of PCE, Voc, Jsc and FF based on 20 independent devices in Fig. S9 show a narrow distribution, indicating that the devices fabricated in this way have a good reproducibility. The surface coverage of perovskite crystal and the quality of HTM film above it are very important for interfacial charge transfer dynamics, which determine the photovoltaic properties of PSCs [54]. Therefore, the surface morphology of perovskite active layer and HTL
devices, usually 70 mg/mL of Spiro-OMeTAD is required [27–31]. Usually, for dopant-free HTMs, a low concentration around 10 mg/mL is needed [4,26,39–41,45,53]. It is very important for reducing the device fabrication costs. The optimized J–V curves for F23 and F22based devices are presented in Fig. 4c and the corresponding data are collected in Table 1. PSCs based on dopant-free F23 produces a promising PCE of 17.60%, with a short circuit current density (Jsc) of 21.62 mA cm−2, an open circuit voltage (Voc) of 1.070 V, and a fill factor (FF) of 76.08%, which are comparable to the doped SpiroOMeTAD-based device with the same configuration (17.57%) (Fig. S8). In contrast, the undoped F22-based PSCs yields a poor PCE of 15.31%. In addition, the device using dopant-free F23 as HTM exhibits a higher performance than the dopant-free Spiro-OMeTAD-based device (Jsc = 21.29 mA cm−2, Voc = 1.023 V, FF = 77.78%, PCE = 16.94%; shown in Fig. S8) under the same fabrication and working conditions. The hysteresis effect of the devices are measured (Fig. 4d and Table S3). There are neglectable differences between the forward and reverse scanning of F23-based device, and the hysteresis index is 0.027, demonstrating it has little hysteresis. In contrast, device based on F22
Fig. 4. a) Illustration of the device structure studied in this work; b) Cross-sectional SEM image of the device with F23 as HTM; c) J–V curves of PSC devices employing F22 or F23 as dopant-free HTMs; d) J–V curves of F22 or F23-based PSCs under different scan directions; e) IPCE spectra of PSC devices based on F22 or F23 and the integrated Jsc curve; f) Stabilized efficiency and Jsc of F22 and F23-based PSCs under a bias of 0.82 V and 0.88 V, respectively. 4
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Table 1 Photovoltaic parameters of devices based on dopant-free F22 and F23. HTM
Voc (mV)
Jsc (mA/cm2)
FF (%)
PCE (%)
Undoped F22 Undoped F23
1007 (997 ± 10) 1070 (1061 ± 9)
21.11 (20.90 ± 0.27) 21.62 (21.39 ± 0.26)
72.02 (70.21 ± 1.24) 76.08 (74.48 ± 1.26)
15.31 (14.64 ± 0.52) 17.60 (16.90 ± 0.56)
The data in parentheses is the average value based on 20 devices.
Fig. 5. a) AFM images of pristine perovskite film and perovskite with F22 or F23 on top. b) Steady-state PL spectra of perovskite film and that coated with F22, F23 or Spiro-OMeTAD; c) Time-resolved PL decay curves of perovskite, and that with F22, F23 or Spiro-OMeTAD on top.
deposited on top. The perovskite with F23 has a shortest PL decay lifetime (13.84 ns) in comparison with that coated with F22 (21.43 ns) or Spiro-OMeTAD (15.61 ns). This indicates the fastest hole collection at the perovskite/F23 interface relative to the perovskite/F22 interface or perovskite/Spiro-OMeTAD interface. The excellent hole extraction capability of dopant-free F23 provides the corresponding device with high Jsc and FF values [39]. Compared to F22, the device based on dopant-free F23 shows an enhanced photovoltaic efficiency although they have similar structures. The increased PCE of F23 device is mainly due to the higher FF and Voc. Structurally speaking, the different substitution position of carbazoldiphenylamine on bipyridyl group leads to different electronic properties in F23 and F22. The more twisted structure of F23 resulted in decreased reorganization energy, which facilitates increased hole mobility in F23 film. In addition, F23 film has a higher hole extraction ability and a more uniform surface morphology than F22 film. These factors promote hole extraction and hole transporting in F23 film, and resulting in efficient Ohmic contact between perovskite layer and the Ag electrode, thus reducing charge recombination at the perovskite/Ag interface. Therefore, F23-based devices show enhanced photovoltaic performances compared to F22-based device [10,55]. Stability is one of the important indicators of excellent PSCs. To study the stability of the device based on undoped F23, the tests were
were explored by Atomic force microscope (AFM), as shown in Fig. 5a. The results show that the perovskite film has a rough surface morphology with a root mean square (RMS) of 21.42 nm. After deposition of F22 or F23, smoother and more uniform film with small RMS of 9.53 nm for F22 and 8.92 nm for F23 was observed. The results indicate that the HTMs dissolved in the green solvent THF could also form a good coverage of the perovskite layer. Moreover, the F23 film shows less roughness than F22, which helps to form an effective selective contact between the perovskite active layer and the metal electrode, and helps to reduce the charge recombination and enhance charge transfer at the interface [54]. The steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were further investigated to have a good insight into the hole extraction capability of HTMs F23 and F22 with the Spiro-OMeTAD as reference. Fig. 5b shows the steady-state PL spectra of pristine perovskite and that coated with undoped F22, F23 or Spiro-OMeTAD. The pristine perovskite has a strong fluorescence signal, and when it is covered by F22, F23 or Spiro-OMeTAD, the emission signal has a significant decrease. Compared with F22 and Spiro-OMeTAD, PL quenching ability of F23 is more efficient, denoting that F23 has best hole extraction ability. Fig. 5c displays the TRPL spectra of perovskite with or without HTMs. The PL lifetime of perovskite decreases obviously when F22, F23 or Spiro-OMeTAD is
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Fig. 6. Stability test of doped Spiro-OMeTAD and undoped F23 based PSCs in N2 and ambient condition (air, 25–30% relative humidity).
measured in both N2 and ambient condition (air, 25%-30% relative humidity). PSCs based on Spiro-OMeTAD was also studied for comparison (Fig. 6). The device based on pristine F23 remains 90% of the original PCE when it is placed in N2 atmosphere for 500 h, while the PCE of the device based on doped Spiro-OMeTAD is less than 80% of the original PCE. When stored under 20−25% relative humidity in the air, the device based on pristine F23 still retains 80% of the original efficiency in 200 h, while PCE of the doped Spiro-OMeTAD-based device decreases significantly to less than 60% of its original value. The contact angles of doped and dopant-free Spiro-OMeTAD and pristine F23 were also measured to elucidate the distinctions in devices stability (Fig. 6, Fig. S10). Pristine F23 film has the largest water contact angle (97°), and a relatively smaller contact angle is obtained for doped (79°) or dopant-free Spiro-OMeTAD (94°), indicating that the pristine F23 has the highest hydrophobicity among these HTMs studied. The above results demonstrate that the device based on dopant-free F23 has better stability than that incorporating doped-Sprio-OMeTAD, which could be attributed to the higher hydrophobicity of F23. 3. Conclusions In conclusion, we developed a new compound F23, which was successfully applied as dopant-free HTM in PSCs. F23 shows small molecular reorganization energy, high hole mobility, strong hole extraction ability and smooth film morphology. Of particular interesting is that F23 film was prepared using nonhalogenated green solvent THF and only small amount of F23 is required. As a result, PCE of device based on F23 achieves 17.6%. Moreover, the device also exhibits good stability due to the absence of the hygroscopic dopants. The green solvent process we used provides a new strategy toward the preparation of PSCs with reduced costs and environmental-friendly solvent. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work described in this paper was supported by the National Natural Science Foundation of China (No. 51703183, 51873160), the “Project supported by Basic research and Frontier Exploration of Chongqing Municipal Science and Technology Commission 6
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