Influence of Non-Fused Cores on the Photovoltaic Performance of

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Influence of Non-Fused Cores on the Photovoltaic Performance of Linear Triphenylamine-Based Hole-Transporting Materials for Perovskite Solar Cells Yungen Wu, Zhihui Wang, Mao Liang, Hua Cheng, Mengyuan Li, Liyuan Liu, Baiyue Wang, Jinhua Wu, Raju Prasad Ghimire, Xuda Wang, Zhe Sun, Song Xue, and Qiquan Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02090 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Applied Materials & Interfaces

Influence of Non-Fused Cores on the Photovoltaic Performance of Linear Triphenylamine-Based HoleTransporting Materials for Perovskite Solar Cells ⊥

Yungen Wu, † Zhihui Wang, Mao Liang,*,†,‡ Hua Cheng, † Mengyuan Li, † Liyuan Liu, † Baiyue Wang, † Jinhua Wu, † Raju Prasad Ghimire, § Xuda Wang, † Zhe Sun, † Song Xue*,†, Qiquan Qiao*,§ †

Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Department of

Applied Chemistry, Tianjin University of Technology, Tianjin 300384, P. R. China; ‡

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, Tianjin 300071, P. R. China; §

Center for Advanced Photovoltaics, Department of Electrical Engineering, South Dakota State

University, Brookings, South Dakota 57007, United States; ⊥

Jiangsu Provincial Key Laboratory of Palygorskite Science and Applied Technology, College of

Chemical Engineering, Huaiyin Institute of Technology, Jiangsu Province, Huaian 223003, P. R. China. KEYWORDS: Perovskite Solar Cells, Hole Transporting Materials, Non-Fused Cores, linear Molecular, Conductivity, Hole Mobility

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ABSTRACT: The core plays a crucial role in achieving high performance of linear hole transport materials (HTMs) toward the perovskite solar cells (PSCs). Most studies focused on the development of fused heterocycles as cores for HTMs. Nevertheless, non-fused heterocycles deserve to be studied since they can be easily synthesized. In this work, we reported a series of low cost of triphenylamine HTMs (M101-106) with different non-fused cores. Results concluded that the introduced core has a significant influence on conductivity, hole mobility, energy level, and solubility of linear HTMs. M103 and M104 with non-fused oligothiophene cores are superior to other HTMs in terms of conductivity, hole mobility and surface morphology. PSCs based on M104 exhibited the highest power conversion efficiency (PCE) of 16.50% under AM 1.5 sun, which is comparable to that of spiro-OMeTAD (16.67%) under the same conditions. Importantly, the employment of M104 is highly economical in terms of the cost of synthesis as compared to that of spiro-OMeTAD. This work demonstrated that non-fused heterocycles such as oligothiophene are promising cores for high performance of linear HTMs toward PSCs.


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INTRODUCTION The perovskite-based solar cells (PSCs) have attracted significant attention in photovoltaic fields due to their excellent performance.1 Rapid research progress has been achieved since the first report in 2009 by Miyasaka et al.2 PSCs consisted of an electron transport layer (ETL),3 a semiconducting perovskite material, a hole transport layer (HTL)4 and an opaque metal. Photogenerated electrons (e-) move from the perovskite layer to the ETL, while holes (h+) to the HTL, thus creating a current in PSCs. As demonstrated, the performance of the n-i-p type of PSCs highly depends on hole transporting materials (HTMs), which plays a crucial role in holes extracting and transferring.4 The state-of-art small organic HTM employed in PSCs is 2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (sprio-OMeTAD), which exhibits high power conversion efficiencies (PCEs) over 21%.5 However, high PCE values were achieved only with high purity (sublimation) of spiro-OMeTAD which increases the cost of fabrication. In addition, the conductivity of spiro-OMeTAD is moderate. To date, a variety of new organic HTMs have been developed for PSCs.4-36 For example, Petrus et al. developed a series of linear azomethine-based HTMs with low cost, high efficiency and good moisture barrier properties. Several groups synthesized star-shaped HTMs by using the truxene-core.9,29,34 Regardless of the configuration of PSCs selected (n-i-p34 or p-i-n9,29), these HTMs exhibit great potential. Xu, Sun and co-workers developed a series of spiro[fluorene-9,9′-xanthene]-Based HTMs and a very impressive PCE of 20.8% was achieved by X55.35 Han and co-workers reported a linear HTM C12-carbazole based on indolocarbazole, showing a higher hole mobility compared to SpiroOMeTAD.36 It can be found that most of organic HTMs contain a core leading to linear, starshaped and spiro structures.37 Moreover, optical, electrochemical properties and photovoltaic


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performance of HTMs were observed to be greatly affected by the core.37 Thus, our research focused on the relationship between the core and properties of organic HTMs.

Figure 1. Chemical structures of the M101-106. Thiophene-based heterocycles are particularly employed as a core for linear HTMs due to the thiophene-iodine interaction. Initially, Li et al found that heterocycle-containing material could achieve >10% efficiency in PSCs.30 Recently, Saliba et al. reported a simple dissymmetric fluorene–dithiophene (FDT) based HTM with a PCE of 20.2%,38 setting a new benchmark for spiro HTMs. Azmi et al. developed small HTMs containing di(1-benzothieno)[3,2-b:2′,3′d]pyrrole (DBTP) as a core showing a PCE of 18.09%.39 In addition, several groups have studied energy levels, optical properties and hole transfer behaviors of a series of fused thiophene derivatives by using first-principles calculations combined with the Marcus theory.40-42 Undoubtedly, development of fused-ring heterocycles as cores for HTMs is a good strategy to construct highly efficient organic HTMs.31,38,39,43,44 Nevertheless, non-fused heterocycles deserve


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to be studied since they can be synthesized with a lower cost. To the best of our knowledge, scientific studies of non-fused heterocycles as cores for linear triphenylamine HTMs are limited. To expand our knowledge of the relationship between non-fused heterocycles and chargetransport properties, we have designed and synthesized a series of triarylamine-based linear HTMs (M101-106, Figure 1). The M101 HTM contains a thiophen core while the M102 has a 2,2'-bithiophene core. Taking the prototypical HTM M102 as structural reference, we introduce one more unit to construct the M103 (inserting 3-hexylthiophene at the center), M104 (inserting 3,4-ethylenedioxythiophene (EDOT) at the center) and M105 (inserting 3,6-positions of thieno[3,2-b]thiophene at the center). M106 with a carbazole unit was synthesized for a comparison. Conductivity and hole mobility of HTMs (M101-104) gradually increase with increasing conjugated length. Moreover, M103 and M104 with the oligothiophene core show better photovoltaic performance than other HTMs including M105 and M106. Note that perovskite-sensitized solar cells with M104 exhibited the highest power conversion efficiency (PCE) of 16.50% under AM1.5 sun, which is comparable to that of spiro-OMeTAD (16.67%). It should be noted that, the synthetic cost of the M104 is only 309.65 RMB/g or 49.38 $/g (Detail cost evaluation of M104 can be found in the Supporting Information (SI)), which is about 1/3 of that of spiro-OMeTAD (1000 RMB/g or 166 $/g, purchased from the p-OLED (China)). This work demonstrated that the introduction of a non-fused oligothiophene core is a promising strategy for achieving high performance of linear HTMs toward PSCs. RESULTS AND DISCUSSION Optoelectronic properties. Normalized UV-Vis absorption spectra of the M101-106 HTMs are shown in Figure 2a-f and all optical data are shown in Table 1. The absorption peak of M101


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is located at 401 nm which is slightly larger than that of spiro-OMeTAD (383 nm). The absorption peaks of M102 and M104 are found at 425 and 456 nm, respectively, which are red shifted in comparison with M101 due to an increased conjugation backbone. Nevertheless, the absorption peak of M103 (428 nm) is slightly higher than that of M102, though the former contains one more thiophene unit. Replacing 3-hexylthiophene in M103 with 3,6-positions of thieno[3,2-b]thiophene or carbazole leads to a hypsochromic shift of the absorption peak of M105 (392 nm) and M106 (389 nm). The change in absorption peak is close related with the degree of conjugation. Among these HTMs, degrees of conjugation of M103 and M104 are higher than other HTMs reported in this work.

Figure 2. Normalized UV-Vis absorption spectra of the M101 (a), M102 (b), M103 (c), M104 (d), M105 (e) and M106 (f) in dichloromethane (solid line) and in the form of a thin film on a glass substrate (dash line). The degree of conjugation also determines band gaps (Eg) of the M101-106 HTMs which were estimated from the intersection of normalized absorption and emission spectra (Figure S1 in SI). The bandgap of the linear HTM becomes narrow with an increasing conjugated length, as is clear from the comparison of M101 with M102/M103/M104 (Table 1). Nevertheless, the introduction of 3,6-positions of thieno[3,2-b]thiophene and carbazole into the core leads to an increased the


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Eg of M105/M106 compared to that of M103/M104. This comparison indicates that, the introduction of 3,6-positions of thieno[3,2-b]thiophene or carbazole core decreased the intramolecular charge transfer (ICT) from triphenylamine to core. To study the influence of the molecular configuration on conjugation and intermolecular π−π interaction, we optimized the molecular configuration of M101-106 at the B3LYP/6-31G(d) level. As shown in Figure 3, calculation results revealed that all dihedral angles in cores are below 30o. The planarity of the non-fused oligothiophene increased in the order of M103 < M104. And we can observed from the electrostatic surface potential (ESP) that the planarity of M105 and M106 decreased as compared to those of M101-104. The calculated dihedral angle between EDOT and the thiophene unit in M104 is only 0.7o, which can be attributed to the conformational locking enabled by the intramolecule noncovalent S···O interaction.45 Yu et al. found that the dihedral angles between two thiophenes or thiazoles are very minimal.45 This locking effect caused by S···O interaction has been confirmed by crystallography. McEntee et al.46 synthesized three thiophene-based conjugated molecules, and they found that the interatomic S···O distances (being 2.87 A°) are significantly shorter than the sum of the van der Waals radii for sulfur and oxygen. As a result, the EDOT-thienothiophene core unit is essentially planar. Therefore, it can be concluded that the noncovalent S···O interaction in M104 hold the molecule in a planar conformation. In contrast, the dihedral angle between 3-hexylthiophene and thiophene in M103 slightly increased to 29.2o due to introduction of 3-hexyl. Nevertheless, the oligothiophene core of M103 could be seen as a planar conformation by comparing it with the twist triphenylamine. Overall, the influence of cores over the planarity of these linear triphenylamine HTMs is small. This conclusion could be further confirmed by UV-vis absorption of thin films and crystallography study.


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Figure 3. HOMO and LUMO orbitals, optimized ground-state molecular geometries and electrostatic surface potential (ESP, electronegative part is red and electropositive part is blue) of M101-106. We monitored the absorption of thin films based on M101-106 (Figure S2 presents the film). As shown in Figure 2a-f, M101-106 based films display a slightly red shifted spectrum if compared to the corresponding HTM in solutions, suggesting that there is a J-aggregates47 in these films due to intermolecular π−π interaction. However, it can be found that these films showed almost similar red-shift (2~10 nm) though their cores keep a planar conformation. This indicates that the highly twisted triphenyl amine prevent the π−π interaction between cores. This suggestion could be crosschecked by the crystallography study, which has been wildly used in the PSCs field.19,48 We use different solutions to prepare the crystal of M101-106. Most of these solutions gave powders or bad quality crystal after two weeks. Fortunately, the crystal of M102


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has been obtained (CCDC 1831943, the Cambridge Crystallographic Data Centre). This crystal could provide useful information on molecular packing for readers since it is a representative linear HTMs with triphenylamine and bithiophene core.

Analysis of the X-ray structures

indicates that M102 has a dumbbell-shaped configuration (Figure 4a). The center part of M102 shows a coplanar conformation. The dihedral angle between two thiophene is only 6.2o, which is smaller than that of calculation result. Therefore, we believe that most of calculated dihedral angles are close to true values but still slightly higher. This deduce could be confirmed by some references49,50, which reported crystal structure of oligothiophene compounds containing alkyl thiophene and thiophene. It can be found from these crystal data that all dihedral angles between alkyl thiophene and thiophene is below 20o. Thus, the oligothiophene in M103 should have a coplanar conformation.

Figure 4. (a) The top view and side view of M102; (b) π–π short contacts observed in the crystal of M102; (c) View down the a axis of the stacking molecules of M102. As presented in Figure 4b, M102 with a dumbbell-shaped configuration could not form a π–π stacking in directions parrallel to each other. In fact, molecules of M102 display C-H/π and C-


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C/π interaction in the orthogonal interaction. C-C/π hydrogen bonds were observed between the thiophene and triphenylamine of nearby molecule with a mean distance of 3.66 Å. Close C-H/π interaction also exists between triphenylamine and oxymethyl of nearby molecule, with a mean distance of 2.86 Å. These short contacts (Figure 4c) provides possible channels for charge transport among the HTM molecules, which is beneficial for improving hole mobility. This result clear shows that the thiophene core is involved in the π–π stacking of these linear HTMs. Therefore, it is possible that the π–π interaction will increased with the extend of cores, resulting different hole mobility.

Figure 5. (a) CV curves of M101-106 in DCM; (b) energy level diagram of M101-106, TiO2 and (FAPbI3)0.85(MAPbBr3)0.15. To compare the HOMO and LUMO level of these triphenylamine-thiophene HTMs, we performed cyclic voltammetry (CV) measurements of M101-106 under the same conditions with ferrocene as an internal standard. The values of HOMO derived from CV shown in Figure 5a are summarized in Table 1. The increase of thiophene unit shift of the HOMO value of the M101 (5.14 eV), M102 (-5.18 eV), and M103 (-5.20 eV), downward as presented in Figure 5b. In contrast, the HOMO of M104 (-5.12 eV) moves up as a result of the decreased Eg. The change of Eg also affects the HOMO of M105 which is lower than those of M101-104. This showed, all HOMO of M101-106 are higher than that of the mixed perovskite ((FAPbI3)0.85(MAPbBr3)0.15, -


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5.65 eV51) employed in this work, thus these HTMs could extract photo-generated holes at the HTM/perovskite interface and then transport holes to the metal electrode. In addition, the position of HOMO also has influence on the open-circuit voltage and charge transfer at the interface of devices. Table 1. Optical and Electrochemical Properties of M101-106. HTMs

λabs/nm a

λemi/nm b

Eg/eV c

M101

401

479

2.78

-5.14

-2.43

M102

425

509

2.59

-5.18

-2.59

M103

428

521

2.52

-5.20

-2.68

M104

456

520

2.45

-5.12

-2.67

M105

392

499

2.77

-5.26

-2.49

M106

389

479

2.82

-5.19

-2.37

HOMO/V vs NHE

d

LUMO/V vs NHE

e

The absorption a and emission b peak of M101-106 in DCM. c Eg is the energy gap between the HOMO and LUMO. d HOMO was recorded by cyclic voltammetry of HTMs. e LUMO was calculated from HOMO–E0–0.

Table 2. Conductivity, Hole Mobility, Solubility and Tg of M101-106.

HTMs

Conductivity / S.cm-1

Hole mobility / cm2.V-1.s-1

Solubility a / mmol mL-1

Tg / oC

M101

3.90×10-4

6.11×10-5

75

65.3

M102

5.21×10-4

9.31×10-5

70

79.8

M103

5.28×10-4

1.63×10-4

85

62.4

M104

6.73×10-4

1.12×10-4

80

102.5

M105

3.24×10-5

8.42×10-5

10

84.2

M106

4.16×10-4

6.71×10-5

75

89.7

a

The maximum amount of HTMs dissolve in chlorobenzene at the room temperature.

Calculation. To gain insight into geometrical and electronic properties of the M101-106 HTMs, density functional theory (DFT) calculations were conducted using a B3LYP/6-31G (d) level in the vacuum. As shown in Figure 3, the HOMO of M101-106 is delocalized throughout


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the entire molecule while the LUMO is mainly localized on the central thiophene part. This demonstrates that triphenylamines act as the donor and cores play a role of acceptor, thus the charge could move in molecule. The calculated electrostatic potential map is shown in Figure 3. All these HTMs present a red colour at the center of molecule, indicating a negative electron distribution. Meanwhile, the more electropositive part is spread out over two triphenylamine units. Apparently, the strong dipole in M101-106 is caused by thiophene-based cores and these dipoles could facilitate close molecular packing in solid state due to molecular interaction.23 Another proposed advantage is the thiophene-iodine interaction between the HTM and perovskite, which may improve hole transfer at the HTM/perovskite interface.38 Conductivity and Hole Mobility. High conductivity and hole mobility of HTMs are essential for PSCs to achieve a decent PCEs. To better understand the influence of non-fused thiophenebased cores on the final photovoltaic properties of these HTMs, we recorded the conductivity and hole mobility of M101-106 by using the methods reported by the literature.52 The conductivity of thin films of M101-106 were measured between 0.2 cm spaced interdigitated Ag electrodes (detail information can be found in the SI). The concentrations of HTMs and dopants (containing Li salt (LiTFSI), tert-butylpyridine (TBP) and cobalt salt (FK209)) used in the measurement are the same as in the photovoltaic devices. For comparison, a reference film coated with sprio-OMeTAD HTM was also fabricated using the same fabrication process. As it can be observed from Figure 6a and Table 2, the values of the conductivities of the film based on M102 (5.21×10-4 S cm−1), M103 (5.28×10-4 S cm−1) and M104 (6.73×10-4 S cm−1) are higher than that fabricated with M101 (3.90×10-4 S cm−1), demonstrating that increasing the thiophene units could improve the conductivity of HTM. Particularly, EDOT is superior to 3-hexylthiophene in


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terms of improving the conductivity of linear HTMs. In addition, it seems that carbazole is inferior to thiophene since the introduction of carbazole in M106 (4.16×10-4 S cm−1) leads to decreased conductivity as compared to M102. On the other hand, the conductivity of M105 suffers from its solubility, with a conductivity lag far from other HTMs. The maximum solubility of M105 is only about one seventh of other HTMs, being 10 mmol mL-1 in chlorobenzene. As presents in Table 2, M103 and M104 exhibited the highest solubility (over 80 mmol mL-1) and the solubility of M101, M102 and M106 exceed 70 mmol mL-1. Thus, M101, M102, M103, M104 and M106 could form a thicker film in devices than M105. This would affect the quality of the formation of film as well as the photovoltaic performance of M105 based PSCs. Noted that, except for M105, the conductivity of these HTMs are higher that of sprio-OMeTAD (3.71×10-4 S cm−1), suggesting that higher conjugation is beneficial to higher conductivity of HTMs.35

Figure 6. (a) Current-voltage characteristics of M101-106 and sprio-OMeTAD based solid films. (b) J-V curves of M101-106 and sprio-OMeTAD based PEDOT:PSS-HTM devices. The hole mobility of M101-106 (without doping) was analyzed by using space-charge-limited currents (SCLCs) method52 (detail information can be found in the SI). The fitted current– voltage (J–V) curves for M101-106 based PEDOT:PSS-HTM devices as well as the corresponding hole mobility data are depicted in Figure 6b and Table 2, respectively. M103 and


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M104 exhibit a good hole mobility of 1.63×10-4 and 1.12×10-4 cm2 V-1 s-1, respectively, which are much higher than that of M101 (6.11×10-5 cm2 V-1 s-1) and M102 (9.31×10-5 cm2 V-1 s-1). Factors such as the π-bridge conjugation length42 could affect the hole mobility of HTMs. As discussed above, M103 and M104 have a better conjugation than other HTMs, contributing to their higher hole mobility. Interestingly, the hole mobility of M103 is slightly higher than that of M104, although the latter has a better molecular planarity. One of the possible reasons for this observation is better pore filling of M103 caused by its higher solubility37 (85 mmol mL-1, Table 2). In contrast, incorporation of the 3,6-positions of thieno[3,2-b]thiophene in M105 (8.42×10-5 cm2 V-1 s-1) does not lead to enhanced hole mobility related to M102 (9.31×10-5 cm2 V-1 s-1). This result may be, again, due to the low solubility of M105. We also noted that the carbazole core has a negative effect on the hole mobility of film. M106 (6.71×10-5 cm2 V-1 s-1) showed relatively low mobilities on the order of 10-5 cm2 V-1 s-1. Overall, we proposed that hole mobility of linear HTMs can be improved by the introduction of non-fused oligothiophene cores. Thermal properties. The glass transition temperature (Tg) of HTMs is an important parameter for evaluating the stability of PSCs. The conversion of the metastable amorphous state of HTM into a thermodynamically stable polycrystalline state during the operation of the device is one cause of rapid degradation of PSCs.37 In order to investigate the effects of cores on thermal properties, differential scanning calorimetry (DSC) was used to analyze the Tg of HTMs (Figure 7a). It can be seen from Table 2 that the Tg increases in the order of M101 (65.3 oC) < M102 (79.8 oC) < M104 (102.5 oC), indicating that we could enhanced the Tg of HTM by introducing more thiophene unit as the bridge. This deduction can be checked by comparing the Tg of M104 with H101, a HTM reported by Grimsdale and co-workers.30 The replacement of EDOT by thiophene-EDOT-thiophene lead to a significant increase of Tg from 73 oC to 102.5 oC. In the


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case of M103, the Tg (62.4 oC) is decreased due to the introduction of the hexyl chain. Thus, it can be concluded that M104 is more stable in the amorphous state than other HTMs. Figure 7b shows the thermogravimetric analysis (TGA) of M101-106. It can be found that, M101 with one thiophene unit displays a relative lower decomposition temperature of 363.3 oC. In contrast, M102-106 with longer cores exhibit decomposition temperatures of over 400 oC. Therefore, expand the conjugation by introducing more thiophene units is benificial to the thermal stability of HTMs.

Figure 7. DSC (a) and TGA (b) of H101-106. Photovoltaic Performance. To further analyze the photovoltaic performance of M101-106 as HTMs,

mesoscopic

PSCs

with

a

fluorine

tin

oxide

(FTO)/compact-TiO2/meso-

TiO2/[FAPbI3]0.85[MAPbBr3]0.15/HTM/Ag structure were fabricated. Figure 8a shows the crosssection image of the device architecture based on M104. It can be observed that the perovskite layer (around 400 nm) is sandwiched in between the mesoporous layer of TiO2 (around 90 nm) and HTM. Detailed information of devices was fabricated as described in the SI. Figure 8b shows J–V curves (under reverse bias) of PSCs based on M101-106 under simulated solar illumination (AM 1.5, 100 mW cm−2) and the photovoltaic parameters are summarized in Table 3. J–V curves of PSCs based on M101-106 under forward bias can be found in Figure S3. The


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statistical analysis results for PSCs based on at least 15 individual devices are shown in Figure S4 and the average PCEs of the devices are listed in Table 3.

Figure 8. (a) SEM cross-section image of a typical device based on M104; (b) J−V curves under reverse bias measured for M101-106 in ambient atmosphere; (c) Histogram of devices PCEs based on M104; (d) IPCE action spectra of the studied devices. PCEs in the range of 6.69 to 16.59% have been achieved by devices prepared with M101-106. The best PSC based on M101 exhibits a short circuit current density (JSC) of 20.42 mA cm−2, an open-circuit voltage (VOC) of 0.97 V and a fill factor (FF) of 0.56, leading to a PCE of 10.74%. Alteration of the core by replacing thiophene with 2,2'-bithiophene substantially increases the FF of M102 based PSC to 0.70. Additionally, this change successfully realized a significant increase in VOC (90 mV). Thus, M102-based PSCs achieved a better PCE of 14.12% with respect to those of M101. Notably, an incorporation of non-fused oligothiophene in M103 and M104 further raised the PCE of PSCs. M103 demonstrated a higher VOC of 1.07 V than that of M104 due to the


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deeper HOMO level. Nevertheless, M104 shows a better JSC (22.57 mA cm−2) and FF (0.71) than those of M103. As a result, the best device employing the M104 reached a PCE of 16.50%. The high performance of M104 can be primarily ascribed to its higher conductivity and mobility,13,18 which leads to a high charge collection efficiency. It is worth noting that the PCE of M104 is on a par with the best control device employing spiro-OMeTAD (16.67%, JSC of 21.94 mA cm−2, VOC of 1.07 V, and FF of 0.71) under the same condition. The histogram of PCEs for M104 based PSCs are shown in Figure 7c, giving the average PCE of 15.85% over 20 devices (Table 3). Our results demonstrated that the non-fused oligothiophene, i.e. the thiophene-EDOT-thiophene group, is an outstanding core for constructing high performance linear HTMs. Table 3. Photovoltaic Parameters of the Best Devices based on M101-106 and spiro-OMeTAD.

a

HTMs

JSC/ mA cm–2

VOC/mV

FF

Best PCE

Averagea

M101

20.42

0.97

0.56

10.74%

10.13%

M102

19.03

1.06

0.70

14.12%

13.82%

M103

20.83

1.07

0.69

15.37%

14.76%

M104

22.57

1.03

0.71

16.50%

15.85%

M105

15.89

0.86

0.49

6.69%

5.51%

M106

17.06

1.06

0.67

12.11%

11.26%

sprio

21.94

1.07

0.71

16.67%

16.13%

Average PCE is obtained from at least 15 PSC devices. In contrast, M105 with 3,6-positions of thieno[3,2-b]thiophene shows a much lower

performance (PCE = 6.69%, Table 3) as compared to M103/M104, which should be related to its low solubility. It has been demonstrated that poor solubility of linear molecule makes the hole difficult to infiltrate and fill the pore of perovskite.53 We found that the solubility of linear HTMs is vulnerable to the core as the molecule become larger. This characteristic of linear HTM


17


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contrast sharply with spiro-linked compounds, their solubility is generally good due to the perpendicular arrangement of molecule, leading to excellent film-forming properties and thus the formation of thicker films with a high quality.54 Therefore, to extends the conjugation length of the linear HTMs but not sacrifice the solubility, it is crucial to introduce alkyl chains in HTMs as observed in M103 and M104. Unfortunately, M106 exhibits a low performance (PCE = 12.11%) despite its normal solubility. The M106 based PSC shows a low JSC of nearly 17.06 mA cm−2, which is likely ascribable to its low conductivity and hole mobility. By comparing M106 with M103/104 it can be concluded that thiophene derivatives outperforms carbazole in increasing the efficiency of PSCs based on linear HTMs. In short, our findings clearly show that non-fused oligothiophene cores are good candidates for linear HTMs toward PSCs. The corresponding incident photon-to-current efficiency (IPCE) spectrum of these cells are shown in Figure 8d. It can be seen clearly that devices with M101-103 have good IPCE response above 60% from 400 to 750 nm. The M104 based PSC displays a higher IPCE than those based on M101-103, in good agreement with the higher JSC for M104 based PSCs compared to M101103. As expected, the M105 and M106 show relatively lower IPCE response in the visible region. Devices based on M101-104 have been employed to evaluate the long-term stability of linear HTMs based on thiophen and non-fused oligothiophene cores, while devices based on M105 and M106 are not involved in the test since their efficiencies are not idea. The long-term chemical stability of the devices without encapsulation was tested at humidity of 25%. The efficiency of the M101, M102, M103, M104 and spiro-OMeTAD devices were retained 69%, 71%, 75%, 80% and 77% of their initial PCE after storage for 35 days in the dark, respectively. One reason for the decreased PCE of device is the slowly volatilization 4-Tert-butylpyridine (TBP) after


18

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storage.55 Importantly, M103 and M104 with the oligothiophene core show better stability than M101 and M102 when used as HTMs in PSCs.

Figure 9. Variations of photovoltaic parameters (JSC, VOC, FF, and PCE) with aging time for devices based on M101 (a), M102 (b), M103 (c), M104 (d) and spiro-OMeTAD (e). Surface Morphology. To investigate the film quality of these HTMs, the top view of the M101-106 based films were measured by Scanning Electron Microscopy (SEM), as shown in Figure 10. It can be found that M103 and M104 based films exhibit a relatively smooth morphology and no special features could be observed within a micrometer scale, which in turn contribute to the photovoltaic performance of PSCs. On the contrary, M101, M102, M105 and M106 based films are less homogeneous. There are small pinholes on the surface of the M101 based film. It is possible that the perovskite layer has direct contact with the counter electrode (Ag), leading to a decreased VOC due to charge-carrier losses in the device.13 Therefore, it can be found that the HOMO of M101 is deeper than that of M104, but the VOC of the former (0.97 V) is lower than the latter (1.03 V). Scrutinizing the surface of M105 layers, it has been found that film pitted with solid with small grains are most likely due to its poor solubility in chlorobenzene.


19


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Moreover, low solubility also results in thinner film, making it easy for contact between the perovskite layer and the counter electrode.

Figure 10. SEM top-images of devices fabricated with the M101-106 HTMs. Steady-State Photoluminescence. To investigate the behavior of M101-106 to extract holes at the perovskite/HTM interfacial, we monitored the steady-state photoluminescence (PL) of the FTO/perovskite/M101-106 bilayer. The films based on M101-106 were prepared with the same concentration and additives as during device preparation. The results were compared with the pristine perovskite film as shown in Figure 11. All samples show a dramatic quenching emission of over 90% with respect to the pristine perovskite except for M105, indicating that these linear triphenylamine HTMs containing the thiophene core could achieve an effective hole-transfer at perovskite/HTM interface. Scrutinizing the quenching values of M101 (93.4%), M102 (94.2%), M103 (95.1%), M104 (95.7%) and M106 (92.3%), it has been found that the hole-transfer at the M104/perovskite interface is more effective than others. The quenching trend clearly supports


20

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observed difference in the photovoltaic performance discussed above, confirming the superior character of the non-fused oligothiophene core for constructing linear HTMs.

Figure 11. Steady-state PL spectra of perovskite/M101-106 films. Electrochemical impedance spectroscopy (EIS). The charge recombination behavior of M101106 based PSCs was studied by EIS spectroscopy. Nyquist plots for PSCs with M101-106 were fitted to the appropriate equivalent circuit56 (Figure S5). The larger arc in lower frequency can be attributed to the recombination between M101-106 and electron in TiO2. As shown in Figure 12, the recombination resistance (Rrec) of M102 and M103 based PSCs are higher than M101, indicating a higher recombination of the latter. In addition, the HOMO of M101 (-5.14 eV) is higher than that of M102 (-5.18 eV) and M103 (-5.20 eV). Both factors lead to a decreased VOC of M101. Likewise, PSCs based on M104 exhibit a lower recombination compared to that of M103. Therefore, VOC over 1 V was achieved by M104, though its HOMO (-5.12 eV) is higher than other HTMs. We noted that, the M105 based device shows a significantly lower Rrec as compared to M104, indicating a higher recombination in devices. Moreover, it can be found that the M105 shows the highest recombination rate (Figure 12) and the lowest PL quenching value (Figure 11). In addition, similar trend has been observed for M102, M103 and M104. Their


21


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recombination rates are low, but their PL quenching values are high. It seems that the recombination rate is related to the PL quenching. Nevertheless, the PL quenching results of M101 and M106 are not in agreements with their results of their Nyquist plots. In theory, the PL quenching results and the results of Nyquist plots are in good agreements if only considering the perovskite/HTM interface. However, there are some differences for the PL and EIS measurements. The recombination resistance is not only related to the perovskite/HTM interface, but also the quality of HTM layer. Recently, Liu et al. found that better HTM layer could reduce the charge recombination.10 Therefore, the PL quenching results may not in good agreements with the results of Nyquist plots. It can be found that there are small pinholes on the surface of the M101 based film (Figure 10). It is possible that the perovskite layer has direct contact with the counter electrode, leading to an enhancement of charge recombination. Therefore, the recombination resistance of M101 is lower than M106, although the PL quenching value of latter is slightly higher than the former.

Figure 12. Nyquist plots for PSCs with M101-106 in the dark condition at 1.0 V forward bias.


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Apart from the PL quenching value, the recombination resistance of device is also correlated with the fill factor. Lim et al. proposed that the FF of the device is mainly controlled by the carrier recombination of the devices.36 A similar trend has been observed in this work. The M101 and M105 based devices exhibited the smallest recombination resistance among these devices, leading to the lowest fill factor (FF = 0.56 for M101, FF = 0.49 for M105). In addition, the fill factor of M102 (0.70), M103 (0.69), M104 (0.71) and M106 (0.67) are very similar and much higher than that of M101 and M105, which is consistent with the trend of recombination resistance. This comparison indicates that the recombination resistance is correlated with the FF. EXPERIMENTAL SECTION Materials and instruments The synthetic routes for the M101-106 are shown in Scheme 1. All chemicals and solvents were pure grade and used without further processing. M101 was synthesized according to the literature.57 Their final products were determined by 1H NMR, 13C NMR and HRMS.

Scheme 1. Synthetic routes to M101-106.


23


Page 24 of 35

Synthesis of product 3b To a mixture of 1 (948 mg, 2.2 mmol), compound 2b (324 mg, 1 mmol) and K2CO3 (1.38 g, 10 mmol, 2M) in THF (15 mL), Pd(PPh3)4 (58 mg, 0.05 mmol) were added under an argon atmosphere. The resulting mixture was stirred and heated at reflux for 8 hours. After cooling to room temperature, water was added to terminate the reaction and the solution was extracted with ethyl acetate. The organic layer was separated and dried over anhydrous MgSO4. After the solvent was evaporated, the remaining crude product was purified by column chromatography on silica gel eluented with DCM:PE = 1:15~2:1 (v/v) to give the desired product 3b (yellow solid, 78% yield). MP: 176-178 °C. 1H NMR (400 MHz, CDCl3): δ 7.38 (d, J = 8.4 Hz, 4H), 7.08-7.06 (m, 12H), 6.91 (d, J = 8.4 Hz, 4H), 6.84 (d, J = 8.6 Hz, 8H), 3.80 (s, 12H). 13C NMR (100 MHz, CDCl3): δ 156.0, 148.3, 143.1, 140.6, 135.6, 126.7, 126.2, 126.1, 124.1, 122.3, 120.5, 114.7, 55.5. HRMS (EIS) cacld for C48H40N2O4S2 (M+H+): 772.2429, found: 772.5864. Synthesis of product 6c,d,e,f To a mixture of 5c (0.11 g, 0.337 mmol), 4 (0.55 g, 0.82 mmol) and in toluene (10 mL), Pd(PPh3)4 (61 mg, 0.053 mmol) were added. The resulting mixture was stirred and heated at reflux for 5 hours under an argon atmosphere. After cooling to room temperature, water was added to terminate the reaction and the solution was extracted with ethyl acetate. The organic layer was separated and dried over anhydrous MgSO4. After the solvent was evaporated, the remaining crude product was purified by column chromatography on silica gel eluented with DCM:PE = 20:1~1:3 (v/v) to give the desired product 6c (red solid, 69% yield). 1H NMR (400 MHz, CDCl3): δ 7.46-7.37 (d, J = 6.2 Hz, 4H), 7.17-7.06 (m, 12H), 7.02 (s, 1H), 6.98-6.91 (d, J = 5.4 Hz,4H), 6.90-6.82 (d, J = 8.9 Hz, 8H), 3.83 (s, 12H), 2.83-2.74 (t, J = 8.8 Hz, 2H), 1.69


24

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(dd, J = 15.4, 7.9 Hz, 2H), 1.40-1.31 (m, 6H), 0.92 (t, J = 7.0 Hz, 3H).

13

C NMR (400 MHz,

CDCl3): δ 156.7, 148.3, 144.4, 143.4, 140.7, 140.6, 140.0, 135.2, 135.0, 134.0, 127.0, 126.9, 126.8, 126.8, 126.3, 126.1, 124.1, 122.3, 121.9, 120.8, 120.2, 114.7, 77.3, 55.5, 31.7, 30.9, 29.6, 26.9, 22.7, 14.1. HRMS (EIS) cacld for C58H54N2O4S3 (M+H+): 938.3241, found: 938.1106. 6d (Yellow solid, 72%). 1H NMR (400 MHz, CDCl3): δ 7.42 (d, J = 9.6 Hz, 4H), 7.11 (d, J = 6.6 Hz, 4H), 7.10 (d, J = 8.8 Hz, 8H), 6.92 (d, J = 10.4 Hz, 4H), 6.86 (d, J = 8.9 Hz, 8H), 4.44 (s, 4H), 3.83 (s, 12H).

13

C NMR (400 MHz, CDCl3): δ 155.9, 148.0, 142.8, 140.6, 137.4, 132.7,

126.6, 126.4, 126.1, 123.8, 121.9, 119.6, 114.7, 109.6, 65.0, 55.5. HRMS (EIS) cacld for C54H44N2O6S3 (M+H+):912.2361, found: 911.9879. 6e (Yellow solid, 57%). 1H NMR (400 MHz, CDCl3): δ 7.55 (s, 2H), 7.47 (d, J = 8.6 Hz, 4H), 7.36 (d, J = 3.7 Hz, 2H), 7.21 (d, J = 3.4 Hz, 2H), 7.11 (d, J = 8.6 Hz, 8H), 6.96 (d, J = 8.1 Hz, 4H), 6.87 (d, J = 8.9 Hz, 8H), 3.83 (s, 12H).

13

C NMR (400 MHz, CDCl3): δ 156.08, 148.48,

143.92,140.63, 137.34, 135.06, 128.56, 126.74, 126.48, 125.16, 122.38, 121.26, 120.51, 114.89, 55.59. 6f (Yellow solid, 48%). 1H NMR (400 MHz, CDCl3) : δ 8.37 (d, J = 4.4 Hz, 2H), 7.76 (d, J = 6.2 Hz, 2H), 7.49 (d, J = 8.6 Hz, 4H), 7.42 (d, J = 8.6 Hz, 2H), 7.33 (s, 2H), 7.07 (m, 12H), 6.88 (d, J = 7.9 Hz, 8H), 4.34 (t, J = 7.2 Hz, 2H), 3.84 (s, 12H), 1.97-1.85 (m, 2H), 1.36-1.29 (m, 6H), 0.94-0.86 (t, J = 5.2 Hz, 3H).

13

C NMR (400 MHz, CDCl3): 155.96, 148.03, 143.59, 142.71,

140.81, 140.35, 126.89, 126.60, 126.17, 126.05, 124.20, 123.28, 122.91, 122.68, 120.76, 117.54, 114.76, 55.52, 31.59, 29.73, 29.02, 26.97, 22.56, 14.04.


25


Page 26 of 35

CONCLUSIONS In summary, we have prepared a series of low costs small organic HTMs (M101-106) with different non-fused cores. Crystallography study indicates that these dumbbell-shaped molecules form a π–π stacking in the orthogonal interaction, providing possible channels for charge transport among the HTM molecules. The conductivity and hole mobility of HTMs improved as the number of thiophene rings increase due to the enhanced conjugation degree. M103 and M104 with the oligothiophene core outperforms other HTMs in terms of conductivity and hole mobility. In addition, high solubility of M103 and M104 favors the formation of homogeneous thin films. As a consequence, PCEs up to 15.37% and 16.50% were obtained for M103 and M104 based devices under AM 1.5G (100 mW cm-2), respectively. Worthy to highlight, cells based on M104 showed a higher JSC than those of sprio-OMeTAD, leading to a comparable PCE with that of the latter. In contrast, M105 based devices exhibit the lowest PCE mainly due to the low quality of HTM layer. Therefore, we propose that having alkyl chain to increase the solubility is crucial to achieve a high performance of linear HTMs containing the non-fused oligothiophene core. This work demonstrated that introduction of the non-fused heterocycles such as oligothiophene core is a promising strategy for achieving high performance of linear HTMs toward PSCs.

ASSOCIATED CONTENT Supporting Information. Details of device fabrication, instrumentation, characterization, the cost of M104, 1H NMR and

13

C NMR spectra of new compounds (Figure S6-10) are provided.

This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author


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* E-mail: [email protected] (Mao Liang) * E-mail: [email protected] (Song Xue) *E-mail: [email protected] (Qiquan Qiao) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Science Foundation of China (No. 21373007, 21376179, 21602074), 111 project (B12015), Jiangsu natural science foundation (BK20150416), NSF IGERT (DGE-0903685), NSF MRI (1428992), NASA EPSCoR (NNX15AM83A), U.S.-Egypt Science and Technology (S&T) Joint Fund, and Pakistan-US Science and Technology Cooperation Program, Training Project of Innovation Team of Colleges and Universities in Tianjin (TD13-5020).

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