Fluoranthene-based dopant-free hole transporting materials for efficient perovskite solar cells

Unreported 2,3-dicyano-fluoranthene was first prepared as an efficient electron-withdrawing building block for constructing D–A type dopant-free hole transporting materials.


Introduction
Organic-inorganic hybrid perovskite solar cells (PVSCs) have triggered worldwide attention due to their impressive research progress within a short time and their very promising market prospects. [1][2][3][4][5] Recently, the record-high power conversion efficiency (PCE) of PVSCs has reached the certied value of 22%, almost rivaling that of the crystalline silicon based photovoltaics. 6 When fabricating PVSCs, the introduction of suitable interfacial materials, i.e. electron transporting materials (ETMs) and hole transporting materials (HTMs), is critical for achieving high device performance, because they not only improve the charge carrier transport/collection efficiency, but also act as a protective layer for perovskites to enhance device stability. [7][8][9][10][11] For HTMs applied in PVSCs, organic semiconductors are more popular over their inorganic counterparts which is attributed to their milder processing conditions that are compatible with perovskites. Nevertheless, most organic HTMs exhibit relatively low hole mobility, and thus need to be improved through a chemical doping process by ionic dopants such as Li-bis(triuoromethanesulfonyl)imide (Li-TFSI). [12][13][14][15][16] However, the doping process always induces a negative effect on device stability due to the sophisticated oxidation process associated with undesired ion migration/interactions. 17,18 For example, the device PCE derived from the well-known doped HTM, 2,2 0 ,7,7 0 -tetrakis(N,N-bis(p-methoxyphenyl)amino)-9,9 0 -spirobiuorene (spiro-OMeTAD), oen vanished aer 30 days of ambient storage, even though the maximum of its initial PCE was close to 19%. 19 Thus, development of efficient dopant-free HTMs is urgently needed. However few dopant-free HTMs can show comparable PCEs to the doped spiro-OMeTAD. 18,[20][21][22][23][24][25][26] Normally, HTMs would not need an additional doping process if they exhibit hole mobility up to 10 À4 -10 À3 cm 2 V À1 s À1 . 27 To this end, two molecular design strategies, donoracceptor (D-A) type and star-shaped structure, have been mainly used for designing dopant-free HTMs with better intermolecular interactions to ensure sufficient hole mobility. 17,28-34 By integrating these two strategies, Nazeeruddin et al. prepared a new class of star-shaped D-A molecules that served as dopantfree HTMs towards $19% PCE with enhanced device stability. 19,35 Nonetheless, most of the reported D-A type HTMs come from relatively complicated scaffolds with multistep syntheses and purication processes. We recently reported a new dipolar chromophore based dopant-free HTM via a facile synthesis, which afforded a high PCE of 16.9%. 36 Following this success, it is crucial to exploit new D-A combinations towards high performance dopant-free HTMs with low synthetic complexity.
One of the key factors for designing high performance HTMs is designing a suitable core structure. Fluoranthene is one typical cyclopentene-fused polycyclic aromatic hydrocarbon, which exhibits a rigid planarized structure that can favor enhanced p-p stacking. More importantly, uoranthene, as a substructure of fullerene, shows a typical electron-decient character due to the cyclopenta-fused non-alternant structure. [37][38][39][40] Thus, uoranthene may be an ideal electronwithdrawing building block to construct D-A type HTMs for PVSCs, which have not yet been reported, possibly due to a lack of efficient molecular design strategy. In this study, we describe a new and facile synthetic method to prepare an unreported 2,3dicyano-uoranthene moiety as an efficient core towards high performance dopant-free HTMs.

Results and discussion
The synthetic route of the HTMs is shown in Scheme 1, while the details are provided in the Experimental section in the ESI. † First, we prepared uoranthene-cored BTF1-2 via a simple Buchwald-Hartwig coupling reaction between the readily accessible 3,8-dibromo-uoranthene (2) 41 and diphenylamine units (3)(4). So far, facile synthesis of functionalized uoranthene derivatives remains a challenge. One of the most widely used methods of affording uoranthenes is through a typical Diels-Alder (D-A) reaction based on acenaphthenequinone derivatives (Scheme 2a), but the functionalized positions of this method are always limited, mainly to the C7-C10 positions. 42,43 Another typical method proposed by Stubbs and Tucker et al. employs uorene derivatives as the starting materials (Scheme 2b), but the total synthetic efficiency is too poor due to the tedious multistep reactions. 44 Through careful searching, we encouragingly found that in 1949, dibenzofulvene and maleic anhydride were reported as the starting materials by Wang et al. to yield 2,3-substituted uoranthene based on the D-A reaction followed by dehydrogenation (Scheme 2c). 45 Although the yield of this reaction is low, only $10%, the simple and facile procedure is very attractive for us. Thus, with a purpose of synthesizing a more electron-decient uoranthene core, tetracyanoethylene (TCNE) was selected as the dienophile to react with dibenzofulvene (Scheme 2d) through the D-A reaction but followed by dehydrocyanic acid. Excitingly, 2,3-dicyano-uoranthene was successfully obtained as we expected, in spite of a low isolated yield of 3.5%. With an indirect method, we prepared diphenylamine moieties substituted on dibenzofulvene (8-9) rst, and then used them as the critical intermediates to react with TCNE as shown in Scheme 1, which successfully obtained our desired products, BTF3-4, with enhanced isolated yields (29.1% for BTF3 and 43.0% for BTF4). The electron-rich substitutions were found to activate the reactivity of fulvene. This can be further evidenced by the higher yield of BTF4, as methoxyl groups in 9 make diene more electron-rich. Moreover, due to the facile synthesis, all synthetic costs, 7.1 $ g À1 , 11.4 $ g À1 , 53.1 $ g À1 and 62.8 $ g À1 for BTF1-4 (Tables S1-S4 †) respectively, are much lower than that of spiro-OMeTAD, making them good material candidates suitable for large scale production.
The ground-state geometric structures of BTF3 (triclinic, P 1 space group) and BTF4 (monoclinic, P2 1 /c space group) were rened using X-ray crystallography, and the detailed data are shown in Tables S5-S6 † in the ESI. † Fig. 1 shows the packing arrangements of BTF3-4. According to the relative arrangement between molecules and contact positions, four types of molecular packing mode are found ( Fig. 1a and S3-S4 †), including Htype dimeric stacking through dipole-dipole interactions due to dicyano-substitutions (mode 1), H-type dimeric stacking through p-p interactions between two adjacent molecules (mode 2), J-type dimeric stacking through p-p interactions between two adjacent diphenylamine units (mode 3), and herringbone stacking through p-p interactions between two adjacent molecules (mode 4).
As shown in Fig. 1b, BTF3 molecules pack into highly ordered and extended H-aggregates along the (0-11) plane, wherein two types of molecular stacking (mode 1 and 2) appear in an alternating fashion. In stacking mode 1, the stacking distance between two dicyano-uoranthene units is 3.515Å, while that in stacking mode 2 is 3.435Å. Except for short stacking distance, the contact area in mode 2 is much larger than that in mode 1, as the diphenylamine units also show a close contact with each other with a distance of 3.593Å. Moreover, it is encouragingly found that BTF3 molecules in two adjacent H-aggregates can pack into highly ordered J-like aggregates along the (100) plane based on stacking mode 3 with a p-p-distance of 3.451Å. Thus, the solid state structure of BTF3 can be classied as a two-dimensional "brick-wall"-like assembly.
BTF4 molecules interestingly show a highly ordered herringbone assembly with partial edge-to-face packing along the (101) plane (stacking mode 4, Fig. 2c). Due to the asymmetric nature of cyano-uoranthene, they make contact in three different positions (Fig. S4 †) with a p-p-distance of 3.464Å, 3.471Å, and 3.617Å respectively, making the molecular packing very dense. Differently from those observed in BTF3, the neighboring herringbone assemblies interact with each other along both the (110) and (101) planes, based on stacking mode 3 and 1 respectively. Thus, the packing structure of BTF4 can be classied as a quasi-three-dimensional herringbone assembly. The edge-to-face p-p-distance within the dimer in stacking mode 3 for BTF4 is slightly longer (3.669Å) than that in BTF3. However, a much shorter dipole-dipole interaction distance of 3.37Å is obtained for BTF4 in stacking mode 4, attributed to the  close edge-to-face p-contact with a distance of 3.564Å between diphenylamine units that also occurs. In contrast, we only obtained a low-quality single crystal of BTF1 for structure analysis, which indicates an absence of strong p-p interactions (Fig. S5 †). Based on these results, we can conclude that cyanosubstitution plays a critical role in enabling strong intermolecular interactions in solid states, which is expected to be benecial for achieving efficient charge transport.
The absorption spectra of BTF1-4 in diluted solutions and as thin lms are shown in Fig. S6 † and 2a respectively, with related data listed in Table 1. All spectra show two distinct absorption bands, ascribed to localized p-p* transitions and intramolecular charge transfer (ICT). By comparing the lm spectra with their corresponding solution spectra, it is found that the low-energy absorption bands of BTF3-4 are more red-shied than those of BTF1-2, suggesting that aggregation of the former is more pronounced, which is consistent with the results from the crystal structure analysis. Both methoxy-and cyanosubstitutions result in a red-shi in the absorption bands, while the red-shi degree for the latter is much larger, up to 150 nm of the absorption maximum for BTF4, ascribed to more effective ICT. The absorption edges of BTF1-4 in thin lms are determined to be 554, 575, 731 and 781 nm respectively, corresponding to the optical bandgaps (E opt , Table 1) of 2.24, 2.16, 1.70, and 1.59 eV respectively.
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were determined by cyclic voltammetry (CV), and the CV curves in DCM solutions are given in Fig. 2b, with data summarized in Table 1. BTF3-4 display reversible oxidative and reductive processes, while only oxidative processes are observed for BTF1-2. The HOMO levels of BTF1-4 are obtained as À4.94, À4.80, À5.19, and À5.02 eV respectively, versus Fc/Fc + . As shown, the cyano-substitution results in a decrease in the HOMO levels for BTF3-4, making them more compatible with perovskites to facilitate hole extraction ( Fig. S10 and S17 †). Moreover, we noted that the introduction of electron-rich methoxy groups can induce an increase in the HOMO levels. The LUMO levels of BTF3-4 are calculated from the onset of reduction potential, À3.46 and À3.38 eV respectively, while those of BTF1-2 are determined as À2.68 and À2.59 eV respectively, by subtracting E opt from the HOMO levels.
The hole mobilities (m) of BTF1-4 without adding any dopants were measured by the space-charge-limited-current (SCLC) method (Fig. 2c). As shown in Table 1, the m values of BTF3-4 are 6.36 Â 10 À5 and 1.17 Â 10 À4 cm 2 V À1 s À1 respectively, much higher than those of BTF1-2 and spiro-OMeTAD (2.36 Â 10 À5 cm 2 V À1 s À1 ). The high mobility of BTF4 would fulll the needs of dopant-free HTMs. This can be attributed to its quasi-three-dimensional supramolecular assembly associated with much closer molecular stacking. We also characterized the steady-state photoluminescence (PL) spectra (Fig. 2d) of bilayered perovskite/non-doped HTL lms to check their ability to extract holes. As seen, the PL of all bilayered lms can be quenched by the introduction of hole transporting layers (HTLs), most effectively for BTF4, followed by BTF3, BTF2 and then BTF1. It is worth noting that the quenching effectiveness of dopant-free BTF3-4 is even comparable to that of doped spiro-OMeTAD (Fig. 2d), suggesting that holes are extracted efficiently from perovskites for BTF3-4 even without adding any dopants.
We The current density-voltage (J-V) curves of the bestperforming conventional PVSCs using dopant-free HTMs are shown in Fig. 3a, with related photovoltaic parameters listed in Table 2. The champion devices based on dopant-free BTF1 and BTF2 showed limited PCEs of 9.97% and 10.45% respectively, which are only slightly enhanced compared to that of spiro-OMeTAD-based dopant-free control devices (9.33%) under the same fabrication conditions. In contrast, the device performance of dopant-free BTF3-4 can be signicantly improved, due to their more suitable HOMO levels and enhanced hole mobilities. The champion PVSC based on dopant-free BTF4 delivered a very impressive PCE of 18.03% with an open-circuit voltage (V oc ) of 1.06 V, a short-circuit photocurrent (J sc ) of 22.5 mA cm À2 , and a FF of 75.6%, which is among the best for dopant-free devices reported so far. 25 For BTF3, its dopant-free devices exhibited a slightly decreased device PCE of 16.34%, mainly due to a reduction in the J sc value caused by its inferior hole mobility compared to that of BTF4. To understand the charge transfer behaviors of these dopant-free HTMs, the electrochemical impedance spectroscopy (EIS) of fabricated cells was further measured at open circuit with a frequency ranging from 1 Hz to 1 MHz, and the Nyquist plots are shown in Table 1 Optical, electrochemical, and hole mobility properties of BTF1-4 HTMs  Fig. S14. † Similar to hole mobility, the obtained recombination resistance (R rec ) also shows an order of R rec (BTF4) > R rec (BTF3) > R rec (BTF2) $ R rec (spiro-OMeTAD) > R rec (BTF1). This therefore suggests that BTF3 and BTF4 can efficiently reduce the recombination process with higher recombination resistance to produce higher FFs. Doped conventional devices were also fabricated using Li-TFSI as the dopant with an additive of 4-tert-butyl pyridine (TBP). As shown in Fig. S16, † the device PCEs of doped BTF1-2 can be enhanced by 40% due to an effective improvement on both V oc and FF. Nonetheless, such enhancement is still poor in comparison with that of doped spiro-OMeTAD (18.8%). A slight enhancement on PCEs was observed for the doped BTF3-4 based devices, while the highest PCE of 18.94% was achieved for BTF4. These results, on the other hand, suggest that our dicyano-uoranthene-cored molecules can serve as highly efficient HTMs without needing dopants.
Today, most HTMs used in inverted p-i-n devices are polymer HTMs such as poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and polytriarylamine (PTAA). [46][47][48][49] A doping process is also required for PTAA to improve device performance. 50 PEDOT:PSS belongs to a class of self-doped polymers, but always suffers a large potential loss and inherent acidity-induced stability problem. 51,52 Therefore, it is also important to develop dopant-free HTMs suitable for inverted PVSCs. 53,54 However, few molecular HTMs have been reported to be applied in both conventional and inverted planar PVSCs. To this end, inverted PVSCs with a conguration of ITO/ HTL/mixed perovskite/PCBM/LiF/Ag were also tested using BTF1-4 as the dopant-free HTMs, while a PEDOT:PSS-based  control device was fabricated for comparison. The J-V curves of the best-performing devices are shown in Fig. 3b. From BTF1 to BTF4, the resulting PCEs of the inverted devices increase gradually (Table 2), a similar trend to that observed in conventional devices. The champion PCE from BTF4-based inverted PVSCs is 17.01%, higher than that of the PEDOT:PSSbased control device (16.42%). The stabilized PCE and photocurrent of the champion devices of BTF4 near the maximum power point (Fig. 3c and S18b †) were tested to evaluate the efficacy of the J-V curves. The results clearly show the high reliability of our J-V curve with an absence of current hysteresis, when combined with those J-V curves with forward and reverse scanning directions at different scan rates (Fig. S12b and S18c †). We also measured the incident photon-to-electron conversion efficiency (IPCE) spectra of all champion conventional and inverted devices based on dopantfree BTF1-4 ( Fig. S15 and S20 †), wherein the integrated J sc values are consistent with those obtained from the experimental J-V measurements ( Table 2).
To check if removing the doping process can improve the device stability of PVSCs over doped spiro-OMeTAD, we compared the environmental stability of PVSCs derived from dopant-free BTF4 and doped spiro-OMeTAD without encapsulation. The PCE decay curves are presented in Fig. 3d, wherein it is easily seen that the performance of both devices decreased quickly at the beginning when being stored in ambient air under a relative humidity of 40-50%. However, the dopant-free BTF4 based device can still retain over 50% of its original PCE aer being stored for 30 days, while the PCE of the doped spiro-OMeTAD based device almost vanishes. Moreover, BTF4-based inverted devices also showed an enhanced stability and can retain 55% of their original PCE aer being stored in air for 7 days, while 90% of the PCE of the PEDOT:PSS-based control device was lost (Fig. S18e †). Thus, our designed uoranthenecored dopant-free HTMs not only deliver a high PCE comparable to doped spiro-OMeTAD, but also show an enhanced device stability, suggesting that they are very promising material candidates towards efficient PVSCs.

Conclusions
In summary, 2,3-dicyano-uoranthene was rst prepared through a new and facile synthetic method based on a Diels-Alder reaction. We found that uoranthene could be an ideal building block for designing D-A type dopant-free HTMs with low synthetic cost and compatible energy levels with perovskites through rational molecular design. The detailed crystal structure analysis indicates that the resulting molecules with dicyano-substituted uoranthene as the core present highly ordered and strong molecular packing in solid states, and in particular, BTF4 forms a quasi-three-dimensional herringbone assembly, leading to a much higher hole mobility (up to 10 À4 cm 2 V À1 s À1 ) than that of spiro-OMeTAD. Encouragingly, our designed molecules can be applied in planar PVSCs as efficient dopant-free HTMs yielding high device performance, including efficiency and stability. For BTF4, impressive PCEs of 18.03% and 17.01% have been achieved for conventional and inverted cells respectively. Therefore, our work not only develops a general material design strategy to achieve efficient dopantfree HTMs for PVSCs, but also provides a new synthetic method to obtain functionalized uoranthenes.

Conflicts of interest
There are no conicts to declare.