Pyridine Bridging Diphenylamine-Carbazole with Linking Topology as Rational Hole Transporter for Perovskite Solar Cells Fabrication

Developing cost-effective and rational hole transporting materials is critical for fabricating high-performance perovskite solar cells (PSCs) and to promote their commercial endeavor. We have designed and developed pyridine (core) bridging diphenylamine-substituted carbazole (arm) small molecules, named as 2,6PyDANCBZ and 3,5PyDANCBZ . The linking topology of core and arm on their photophysical, thermal, semiconducting and photovoltaic properties were probed systematically. We found that the 2,6PyDANCBZ shows higher mobility and conductivity along with uniform film-forming ability as compared to 3,5PyDANCBZ . The PSCs fabricated with 2,6PyDANCBZ supersede the performance delivered by Spiro-OMeTAD, and importantly also gave improved long-term stability. Our findings put forward small molecules based on core-arm linking topology for cost-effective hole selective layers designing.

Various cores such as triazatruxene, 20 thiophene-based compounds, 21,22 and carbocyclic moieties 23 have been reported as HTMs, 14 but reports dealing with pyridine as a core for designing HTMs are scarce. The pyridine moiety in these HTM can act as a Lewis base and which in turn can assist defect passivation of perovskite and π-conjugation effect. [24][25][26][27][28] Besides the core of HTMs, the arm is also vital for optimizing the semiconducting properties of HTMs. Up to now, the bis(N 3 ,N 3 ,N 6 ,N 6 -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine) (DANCBZ) moiety have been demonstrated to be a valuable arm for the designing of molecular HTMs owing to their superior charge-transporting ability, stability and low-cost. [29][30][31][32][33][34][35][36] More importantly, linking topologies in molecular designing is an effective method to finetune electro-optical properties of materials. [37][38][39][40][41] For example, Sun et al. systemically studied the impact of linking topology of carbazole-based arm on basic properties, found that HTMs based on carbazole-based arm with 2,7-substitution display higher hole mobility and conductivity than that of HTMs with 3,6-substitution. 41 While linking topology of core and arm are vital to determine properties of HTMs via developing continuous π-conjugation between core and arm, and tuning dihedral angles of small molecules, trivial attention has been made to the linking topology effect of core and arm. For example, Lee and co-workers synthesized two types of deep-blue emitter: 26BTPIPy with meta-linking and 25BTPIPy with para-linking, the front, and the corresponding device showed excellent performance because of relatively planar structure leading to considerable overlapping of its frontier molecular orbitals. 38 Using core-arm type linking topology, HTMs based on thiophene-arylamine and pyrene-arylamine was investigated by Dai et al. and Wang et al., respectively. 39,40 Considering arylamine-pyrene core-arm as an example, PYP16 with 1,6-arm substitution possessed reduced dihedral angles, lower level of planarity, improved film formation and polarized structure as compared to PYP27 with 2,7-arm substitution. By doing so, the devices with PYP16 presented higher efficiency and stability. To develop pyridine based HTMs and understanding the relationship between structural and chemical properties, and its influence on device performance, we report the synthesis of two innovative molecules-based on pyridine as core which is end-capped with common DANCBZ as an arm. Such structures have not been designed and investigated to date. We further investigated the effect of the linking topology of core and arm on the properties of pyridine-based HTMs on the opto-electrical, thermal, semiconducting and photovoltaic properties in PSCs.

Opto-electrical, thermal, semiconducting and photovoltaic properties
The normalized UV-vis absorption spectra of 2,6PyDANCBZ and 3,5PyDANCBZ in dichloromethane at room temperature were investigated (Figure 1a) and the data are compiled in Table 1. The absorption of 2,6PyDANCBZ and 3,5PyDANCBZ HTMs containing pyridine displayed a peak maximum at 309 and 308 nm, respectively, which is attributed to the π-π* electron transition of the large molecular conjugated system. The molecular structures of 2,6PyDANCBZ and 3,5PyDANCBZ exhibit absorption peaks in longer wavelength regions (350-400 nm) which could be due to the intramolecular charge transfer from donor to acceptor moieties.
The relatively high intensity of shoulder band of 2,6PyDANCBZ compared to that of 3,5PyDANCBZ suggests that the 2,6PyDANCBZ has a stronger charge transfer from the donor arm to the core acceptor unit. The onset absorption wavelengths (λonset, around 440 nm) of  Table 1. [40][41][42][43][44][45][46] Both the samples 2,6PyDANCBZ and 3,5PyDANCBZ exhibited similar EHOMO level of -5.5 eV. The lowest unoccupied molecular orbital energy level (ELUMO) was calculated from the equation (ELUMO = EHOMO + Eg opt ) to be -2.7 eV. Figure S1a shows the energy level diagram of fabricated device, where valence and conduction band of mixed-cation perovskite films is -5.9 and -4.4 eV, respectively, 47 illustrating that the presented HTMs are energetically favourable for hole transportation. More importantly, the large energy barrier between the conduction band of perovskite and ELUMO of HTMs could efficiently block the undesired photo-generated electron transfer and particularly decrease charge recombination rate at the perovskite/HTM interface. The thermal stability of HTMs were evaluated by employing differential scanning calorimetry (DSC) measurements ( Figure 1c and Figure S2). The 2,6PyDANCBZ and 3,5PyDANCBZ exhibit a high glass transition temperature (Tg) of 132 and 126 °C, respectively, which are higher than that of Spiro-OMeTAD. 48 The notable high Tg shown by HTMs will prevent any phase change process, suggesting the functioning of devices in the temperature window required for commercial validation.    the B3LYP/6-31G(d) level. The calculated frontier molecular orbitals are depicted in Figure 3a and the values are compiled in Table 1. The EHOMO level is localized at donor DANCBZ unit (arm) of pyridine-based HTMs, while the ELUMO level is localized on acceptor pyridine unit (core).
Especially, the EHOMO and ELUMO energy levels of 3,5PyDANCBZ are slightly stabilized as   The film-forming abilities of the HTMs were investigated and perovskite/2,6PyDANCBZ represents uniform microstructure while perovskite/3,5PyDANCBZ showed pin-holes, which can also be visualized by the naked eye (Figure 4a)

Device performance
To elucidate the influence of side arm of HTM on the performance of PSCs, we fabricated planar devices with an architect of ITO/SnO2/perovskite/HTM/Au, introducing 2,6PyDANCBZ, 3,5PyDANCBZ and Spiro-OMeTAD as HTMs. Device schematic with HTM is shown in Figure   S1b  showed Rs and Rsh of 41.0 Ω and 12.2 kΩ, respectively. Subsequently, 2,6PyDANCBZ yielded improved PCE with high FF, which is due to the lower Rs and higher Rsh. 16,52 Besides, the rough microstructure of 3,5-Py substituted HTM leads to unfavourable interfacial contact, which also lowers the FF. The average and detailed PV parameters are summarized in Table S1  3,5PyDANCBZ-based PSC with slightly high HI (0.305) can be attributed to poor uniformity.
Notably, PSCs with 2,6PyDANCBZ presented improved performance than of Spiro-OMeTAD, suggesting its potential as a cost-effective replacement. By further interface optimization and perovskites components tuning, we believe that the PCE of 2,6PyDANCBZ-based PSCs can be substantially improved to achieve state of the art. Further, we have also fabricated the inverted PSCs with architecture (ITO/HTM/perovskite/PC61BM/BCP/Ag). The device performance with different HTMs was shown in Figure 5h and Table S3 and    Electrical impedance spectroscopy (EIS) analysis was conducted to understand the charge transport mechanisms (interface recombination, diffusion, etc.) of devices with pyridine-based HTMs. Figure S4a,b showed the typical Nyquist (Z´-Z´´) and Bode (a´ and b´) plots of impedance spectroscopy for the studied configurations under illumination for different photovoltages. An equivalent circuit was used to extract information about recombination processes as shown in Figure S4a. The resistance and capacitance values are represented in Figure 6. From the equivalent circuit used we derived the resistance and capacitance value, and noted variance in the slope in the recombination vs voltage graph. This can be explained by the calculation of the ideality factor (n=1/β). 54 The PSCs with 2,6PyDANCBZ and 3,5PyDANCBZ presented β values of 0.26 and 0.29, respectively, and the ideality factor increases from 3.45 to 3.85 when 2,6PyDANCBZ was substituted with 3,5PyDANCBZ, suggesting 2,6PyDANCBZ based PSCs reduces the recombination process as compared to 3,5PyDANCBZ. Furthermore, 2,6PyDANCBZ based PSCs illustrated lower capacitance value compared to 3,5PyDANCBZ. The relatively higher ideality factor and lower capacitance values of 2,6PyDANCBZ-based PSCs will allow to improve charge transportation ability and thus enhanced performance of PSCs. 55,56  We studied the long-term stability of PSCs with different HTMs. The un-encapsulated devices were stored in a dry box (40-50% relative humidity, room temperature) and measured in an ambient condition. The PCE values of the devices with 2,6PyDANCBZ and 3,5PyDANCBZ maintained at 79.6% and 75.6% of their initial values after 5,500 h, respectively (Figure 7). In contrast, the performance of PSCs with Spiro-OMeTAD degraded to 70.5% of its initial value Developing cost-effective HTMs with excellent properties is an effective methodology to lower the cost of PSCs modules to promote the commercialization. 60 Our results suggest that the pyridine based HTMs are competitive candidates for high-performance PSCs fabrication. The cost of laboratory synthesis and purification of 2,6PyDANCBZ and 3,5PyDANCBZ is estimated, and the detailed cost calculations are shown in the supporting information. The cost of 2,6PyDANCBZ and 3,5PyDANCBZ is about 28.1 € and 27.0 € per gram, respectively, which is significantly lower than that of Spiro-OMeTAD (~ 400 € per gram, Sigma-Aldrich). Additionally, the optimal concentration of 2,6PyDANCBZ (39.6 mg mL -1 ) was almost half of Spiro-OMeTAD solution (72.3 mg mL -1 ), this will further allow to achieve lower the material cost during device fabrication.
(1.11 € per mL of 2,6PyDANCBZ vs 21.7 € per mL of Spiro-OMeTAD solution), suggesting the usage of 2,6PyDANCBZ as a promising candidate for hole extraction materials. 13

Conclusion
We have designed pyridine bridging diphenylamine-carbazole as hole transporting layers for PSCs. We systematically investigated the core-arm linking topology on the optical and electronic properties of developed HTMs. Our results suggest that the 2,6PyDANCBZ display higher conductivity due to stronger charge transfer from the donor arm to the core of the acceptor, and uniform film-forming ability. As compared to 3,5PyDANCBZ, the fabricated PSCs with 2,6Py substitution showed improved performance when integrated into PSCs, and supersede the performance of conventional Spiro-OMeTAD. Our results put forward new design strategies for cost-effective and efficient hole transport materials, and also provide the guiding principle on the molecular design of core-arm linking topology.

Conflicts of interest
The authors declare no conflict of interest.