Effect of dimethylamino substituent on tetraphenylethylene-based hole transport material in perovskite solar cells

Organic Electronics 39 (2016) 323e327

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Effect of dimethylamino substituent on tetraphenylethylene-based hole transport material in perovskite solar cells Fei Wu a, b, **, Yahan Shan a, b, Xiaolong Li a, b, Qunliang Song a, b, Linna Zhu a, b, * a b

Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Southwest University, Chongqing, 400715, PR China Institute for Clean Energy & Advanced Materials, Faculty of Materials & Energy, Southwest University, Chongqing, 400715, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2016 Received in revised form 11 October 2016 Accepted 13 October 2016

A N,N-dimethylamino substituted tetraphenylethylene derivative (TPE-NMe) was synthesized and characterized, and was successfully applied as hole transport material (HTM) in perovskite solar cells. The methoxy-substituted analogue TPE-4DPA was also studied for comparison. The effect of replacing the para-methoxy substituent with N,N-dimethylamino on photophysical properties, energy levels, and hole transport properties is investigated. Photovoltaic performances of the corresponding devices using the two HTMs are studied. Compared to the methoxy substituent, the N,N-dimethylamino groups in TPENMe generates a lower Voc (0.87 V), yet it provides higher Jsc (21.69 mA/cm2) and FF (0.73) values, resulting in an overall power conversion efficiency of 13.78%. © 2016 Elsevier B.V. All rights reserved.

Keywords: Perovskite solar cells Hole transport materials N,N-dimethylamino group Power conversion efficiency Tetraphenylethylene

1. Introduction The organic-inorganic hybrid perovskite solar cells have created a big breakthrough in solar cell community in less than a decade. Their photovoltaic performances have been tremendously improved from 3.9% in 2009, to a recently certified record of 22.1% [1e3]. The organometal halide perovskites possess a lot of merits such as direct band gap, strong light harvesting characteristics within the visible solar spectrum, high charge carrier mobility and long charge diffusion length [4e6]. To date, most of the highly efficient perovskite solar cell devices composed of the hole transport materials (HTMs) [7e11]. The HTMs mainly works to extract holes from the perovskite layer and then transport holes to the counter electrode. The 2,2e7,7-tetrakis(N,N0 -diparamethoxy-phenylamine) 9,90 -spirobifluorene, abbreviated as Spiro-OMeTAD, represents the most state-of-the-art hole transport material, and the reported record PCE of 20.8% has been accomplished quite recently [12e14]. In spite of these advantages, it is widely demonstrated that the multi-step synthetic approaches and

* Corresponding author. Faculty of Materials & Energy, Southwest University, Chongqing, PR China. ** Corresponding author. Faculty of Materials & Energy, Southwest University, Chongqing, PR China. E-mail addresses: [email protected] (F. Wu), [email protected] (L. Zhu). http://dx.doi.org/10.1016/j.orgel.2016.10.022 1566-1199/© 2016 Elsevier B.V. All rights reserved.

complicated purification of Spiro-OMeTAD renders it with a high cost and limits its commercialization in the future [15e17]. In this regard, many efforts have been devoted to exploring alternatives to replace Spiro-OMeTAD. Various conjugated polymers (such as P3HT, PTAA, PCBTDPP and PCPDTBT) [17,18] and small organic molecules with different configurations [16] have emerged in large numbers, and high PCEs achieved in these years. Whereas, compared to conjugated polymers, small molecules are cheaper and easier to be purified, in addition, they show good infiltration into the nanostructure which induces reproducible device performances [8]. Therefore, various types of small organic moleculesbased HTMs have been devised and investigated during the past few years [19e21], and impressive photovoltaic performances achieved. For example, Hu et al. reported new spirobifluorenebased HTMs with different heteroatom substitution, and the methylsulfanyl substituted HTM showed the highest PCE of 15.92% [22]. A low-cost spiro[fluorene-9,90 -xanthene]-based hole transport material was reported to show remarkable PCE of 19.84%, which is comparable to the well-known Spiro-OMeTADbased device under the same test conditions [23]. A methoxydiphenylamine-substituted carbazole was synthesized and applied in perovskite solar cell, delivering an overall efficiency of 16.91% [24]. Most recently, the tetraphenylethylene derivatives have proved their potential as efficient HTMs in perovskite solar cells. PCE of ~13% was achieved when the N,N-di(4methoxyphenyl)aminophenyl substituted tetraphenylethylene

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density (Jsc) of 21.69 mA/cm2, and FF of 0.73. Compared to its methoxyl-substituted analogue, TPE-NMe shows significantly enhanced Jsc and FF. 2. Results and discussion

Scheme 1. Molecular structures of TPE-NMe and TPE-4DPA.

derivative was applied as HTM [25,26]. More excitingly, our group has just demonstrated an overall power conversion efficiency of 15.4% in meta-Methoxy substituted tetraphenylethylene [27]. In the reported cases, the methoxy group is most extensively utilized to devise small molecule HTMs, as it is widely accepted that the eOMe is electron-donating at the para-position, which is facilitate for hole transporting [28]. Although the TPE-4DPA bearing four methoxy substituents has been successfully applied as HTM in perovskite solar cells, the compound has relatively poor solubility at room temperature (~15 mg/mL in chlorobenzene). From the perspective of device fabrication, the poor solubility may lead to bad film quality of the HTM layer, thus affecting the uniformity of the film as well as the device performance. Considering the facile synthesis and purification of the TPE-based molecules, it is thus reasonable to improve the solubility of TPE-based HTM by rational structure modifications. On the other hand, in the field of optoelectronic materials, the nitrogen-substituent has also been used to tune the frontier molecular orbital energy levels [29e31]. And we could anticipate that the N,N-dimethylamino group might be an alternative to the methoxy group. Herein in this work, we report a new centrosymmetric hole transport material TPE-NMe comprising the tertraphenylethylene core and the electron-donating N,Ndimethylamine groups (Scheme 1). TPE-NMe was synthesized in a simple and cost-effective way. The N,N-dimethylamine substituents greatly improved the solubility of TPE-NMe to ~40 mg/mL in chlorobenzene (r.t.), in contrast to the ~15 mg/mL solubility of the methoxy-substituent. The photophysical properties, energy levels, and photovoltaic performances are systematically investigated and compared with the previously reported methoxy substituted analogue TPE-4DPA. It turned out that the device utilizing TPE-NMe as HTM delivers an overall power conversion efficiency of 13.78% with an open-circuit voltage (Voc) of 0.87 V, short-circuit current

TPE-NMe was prepared through a facile reaction between 1,1,2,2-tetrakis(4-bromophenyl)ethane and N1-(4-(dimethylamino) phenyl)-N4,N4-dimethylbenzene-1,4-diamine (Scheme S1) [32,33]. The product is fully characterized by 1H NMR, 13C NMR and the HRMS spectra. Detailed methods and characterizations are provided in the ESI. The UV-vis absorption spectrum of TPE-NMe in THF solution is shown in Fig. 1a, together with TPE-4DPA which has been reported before. One main absorption peak centered at 330 nm could be observed for TPE-NMe, with a relatively weak shoulder band at around 410 nm. Obviously, compared to the TPE-4DPA, the main absorption band of TPE-NMe has a slightly red shift for about 20 nm. The red shift of the absorption maximum of TPE-NMe could be attributed to the stronger electron donating ability of the nitrogen-substituent than the methoxy-substituent. The bandgap of TPE-NMe estimated from the onset of the absorption spectrum is 2.67 eV. Cyclic voltammetry measurements were carried out to analyze and compare the energy levels of TPE-NMe with SpiroOMeTAD for the favorable hole injection process from the perovskite layer. For comparison, we measured CV of both materials under the same condition with ferrocene as an internal standard. The highest occupied molecular orbital level (HOMO) is determined by the CV result, and the lowest occupied molecular orbital (LUMO) energy level is estimated according to its HOMO level and the band gap. As a result, the HOMO and LUMO energy levels of TPE-NMe were calculated to be 5.03 eV and 2.33 eV, respectively. Because of the stronger electron-donating nature of N,N-dimethylamino, the energy levels of TPE-NMe are slightly higher than that of the TPE-4DPA (HOMO: 5.17 eV, LUMO: 2.48 eV) [27]. The suitable energy level of TPE-4DPA with the perovskite layer as shown in Fig. 1b allows hole extraction from the perovskite layer to the Ag anode. Moreover, it is reasonable to anticipate that the large energy barrier (~1.60 eV) between the LUMO level of perovskite and TPE-NMe could efficiently block the electrons and thus largely reduce the recombination chance. Thermal stability of the new hole transport material TPE-NMe was measured by DSC (differential scanning calorimetry) and TGA (thermal gravimetric analysis). The curves are presented in Fig. S9. The results suggest that the decomposition temperature corresponding to a 5% weight loss (Td) is about 394  C, and the glass

Fig. 1. a) UV-vis absorption spectra of TPE-NMe and TPE-4DPA in THF solution; b) Energy diagram of each layer in perovskite solar cells utilizing TPE-NMe as HTM.

F. Wu et al. / Organic Electronics 39 (2016) 323e327

Fig. 2. The cross-section scanning electron microscopy (SEM) image of the perovskite device with TPE-NMe as HTM.

transition temperature (Tg) is about 154  C for TPE-NMe. Hole mobility is very critical for a new HTM, therefore, hole-only devices are fabricated with device structure of ITO/PEDOT:PSS/ TPE-NMe (or Spiro-OMeTAD)/MoO3/Al. Hole mobility of the HTMs is measured by space charge limitation of current (SCLC) under dark conditions, with the Spiro-OMeTAD as a reference [34,35]. Fitting the JeV curves (Fig. S4) for each material to this expression gives the mobility data. As a consequence, the hole mobility of doped TPE-NMe calculated from Mott-Gurney law is

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3.95  104 cm2 V1 s1, which is a little higher than that of the doped Spiro-OMeTAD (2.41  104 cm2 V1 s1) measured under identical condition. The results suggest that the nitrogensubstituent exhibits good hole mobility. In addition, hole extraction and hole transport ability of TPE-NMe was evaluated by steady-state photoluminescence spectra. As shown in Fig. S8, perovskite film shows a strong PL emission band from 750 to 800 nm. After the deposition of HTM layer, the PL intensity decreased significantly. It is observed that perovskite films covered by doped TPE-NMe display a slightly stronger PL quenching tendency than that covered with doped TPE-4DPA. The result indicates that hole transfer at the perovskite/TPE-NMe interface is more efficient than that at the perovskite/TPE-4DPA interface. The perovskite solar cell devices were fabricated with the planar structure of FTO/TiO2/CH3NH3PbI3-xClx/HTM/Ag, in which the HTM is either TPE-NMe or TPE-4DPA. The cross-section scanning electron microscopy (SEM) image of the perovskite device with TPE-NMe as HTM is presented in Fig. 2. It is clearly observed that a perovskite layer is sandwiched between the thin TiO2 compact layer and the HTM layer. Meanwhile, there is a clear interface boundary between the HTM layer and the perovskite layer. The large crystallites of the perovskite layer expand across the whole thickness and maintain a high coverage on the substrate. The active area of the device was 0.09 cm2, as determined by the overlap between the Ag electrode and the layers below. Photovoltaic performances of perovskite solar cells using TPENMe or TPE-4DPA as HTMs are measured under AM 1.5 G

Fig. 3. a) Current densityevoltage characteristics and b) EQE spectra for the devices with TPE-NMe or TPE-4DPA as HTMs; c) Devices measured at forward scanning and reverse scanning model without encapsulations; d) Devices measured at different scanning time: 0, 100 and 200 ms.

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Table 1 Photovoltaic parameters of perovskite solar cells with TPE-NMe or TPE-4DPA as HTMs. HTM

TPE-NMe TPE-4DPA a

VOC [V]

0.87 1.02

JSC [mA/cm2]

21.69 19.63

FF

0.73 0.65

PCE [%] Best

Averagea

13.78 13.04

12.91 ± 0.66 11.82 ± 0.95

The average value was evaluated based on 12 devices.

100 mW/cm2 simulated light illumination. The current-voltage (JV) responses of the perovskite solar cells are presented in Fig. 3a, and the corresponding photovoltaic parameters are summarized and presented in Table 1. Excitingly, the best performance of the cell with TPE-NMe generates a Voc of 0.87 V, Jsc of 21.69 mA/cm2, and FF of 0.73, resulting in an overall power conversion efficiency of 13.78%. As a reference, the device based on TPE-4DPA as HTM exhibits a PCE of 13.04% under identical conditions, which is also consistent with the previously reported value [25,26]. We also note that the Voc of TPE-NMe is relatively low (0.87 V), as the N,Ndimethylamino substituent slightly elevates the HOMO orbital energy level, thus leading to a lower open-circuit voltage. However, higher Jsc and FF values are observed in TPE-NMe, as compared to the methoxy substituent. The high Jsc and FF suggest that higher hole extraction and hole transporting ability of TPE-NME than TPE4DPA, and the result is in good agreement with the hole mobility tests measured above. Therefore, the incorporation of the N,Ndimethylamino groups for tetraphenylethylene-based HTM is effective in enhancing the device performance. The incident photon to current conversion efficiency (IPCE) spectra of perovskite solar cells based on TPE-NMe or TPE-4DPA as HTM is presented in Fig. 3b. The devices based on the two materials both exhibit a broad conversion range from 400 nm to 760 nm. And device utilizing TPENMe as HTM shows a slightly stronger spectral response compared to the TPE-4DPA. The results above well demonstrate that TPE-NMe is qualified as a hole transport material. Hysteresis behavior is frequently observed in planar perovskite solar cells. Hysteresis with TPE-NMe based device is depicted in Fig. 3c, and the measured differences between the forward scan and the reverse scan is only about 7%, suggesting little hysteresis when the TPE-NMe is used as HTM (forward scan: Jsc ¼ 21.16 mA/cm2, Voc ¼ 0.87 V, FF ¼ 0.72, PCE ¼ 13.19%; reverse scan: Jsc ¼ 21.32 mA/ cm2, Voc ¼ 0.83 V, FF ¼ 0.69, PCE ¼ 12.22%.). In addition, different scanning rates still generate similar J-V curves as shown in Fig. 3d, which could further confirm less hysteresis in the TPE-NMe based device.

3. Conclusions In summary, we have rationally designed and synthesized a new N,N-dimethylamino-substituted tetraphenylethylene derivative TPE-NMe, and demonstrated its application as hole transport material in planar-type perovskite solar cells. The new HTM was synthesized from quite simple reactions. The energy levels estimated from CV are well aligned with respect to the perovskite energy level. Compared to the methoxy-substituted analogue, TPE-NMe exhibits better hole mobility. Device fabricated using TPE-NMe results in a Voc of 0.87 V, Jsc of 21.69 mA/ cm2, and FF of 0.73, achieving an overall power conversion efficiency of 13.78%. The relatively low Voc should be ascribed to the high-lying HOMO level, whereas, the N,N-dimethylamino substituent still provides high Jsc and FF values. We believe that the nitrogen-substitution is promising in constructing hole transport materials.

Acknowledgements The authors thank the National Natural Science Foundation of China (No. 51203046 and No. 11274256) and the “Fundamental Research Funds for the Central Universities” (XDJK2016C131, XDJK2016C129, and XDJK2014A006) for financial support. X. Li thanks the National College Students Innovation and Entrepreneurship Program (No. 201610635027) for financial support. The authors also acknowledge support from “Project supported by Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011)”. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.10.022. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050e6051. [2] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gr€ atzel, Energy Environ. Sci. 9 (2016) 1989e1997. [3] Research Cell Efficiency Records, NREL, http://www.nrel.gov/ncpv/, accessed: April 2016. [4] F.D. Giacomo, S. Razza, F. Matteocci, A. D'Epifanio, S. Licoccia, T.M. Brown, A.D. Carlo, High efficiency CH3NH3PbI(3x)clx perovskite solar cells with poly(3-hexylthiophene) hole transport layer, J. Power Sources 251 (2014) 152e156. [5] S. Kazim, M.K. Nazeeruddin, M. Gr€ atzel, S. Ahmad, Perovskite as light harvester: a game changer in photovoltaics, Angew. Chem. Int. Ed. 53 (2014) 2812e2824. [6] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643e647. [7] Y.S. Kwon, J. Kim, H.-J. Yun, Y.-H. Kim, T. Park, A diketopyrrolopyrrole-containing hole transporting conjugated polymer for use in efficient stable organiceinorganic hybrid solar cells based on A perovskite, Energy Environ. Sci. 7 (2014) 1454e1460. [8] Z. Yu, L. Sun, Recent progress on hole-transporting materials for emerging organometal halide perovskite solar cells, Adv. Energy. Mater. 5 (2015) 1500213. [9] N.J. Jeon, J. Lee, J.H. Noh, M.K. Nazeeruddin, M. Gr€ atzel, S. Seok, Efficient inorganiceorganic hybrid perovskite solar cells based on pyrene arylamine derivatives as hole-transporting materials, J. Am. Chem. Soc. 135 (2013) 19087e19090. [10] S.S. Reddy, K. Gunasekar, J.H. Heo, S.H. Im, C.S. Kim, D.-H. Kim, J.H. Moon, J.Y. Lee, M. Song, S.-H. Jin, Highly efficient organic hole transporting materials for perovskite and organic solar cells with long-term stability, Adv. Mater. 28 (2016) 686e693. [11] X. Li, X. Liu, X. Wang, L. Zhao, T. Liu, J. Fang, Polyelectrolyte based holetransporting materials for high performance solution processed planar perovskite solar cells, J. Mater. Chem. A 3 (2015) 15024e15029. [12] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gr€ atzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316e319. [13] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395e398. [14] D. Bi, W. Tress, M.I. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P. Correa Baena, J.-D. Decoppet, S.M. Zakeeruddin, €tzel, A. Hagfeldt, Efficient Luminescent solar cells M.K. Nazeeruddin, M. Gra based on tailored mixed-cation perovskites, Sci. Adv. 2 (2016) e1501170. [15] H. Li, K. Fu, A. Hagfeldt, M. Gr€ atzel, S.G. Mhaisalkar, A.C. Grimsdale, A simple 3,4-ethylenedioxythiophene based hole-transporting material for perovskite solar cells, Angew. Chem. Int. Ed. 53 (2014) 4085e4088. [16] T. Swetha, S.P. Singh, Perovskite solar cells based on small molecule hole transporting materials, J. Mater. Chem. A 3 (2015) 18329e18344. [17] D. Bi, L. Yang, G. Boschloo, A. Hagfeldt, E.M.J. Johansson, Effect of different hole transport materials on recombination in CH3NH3PbI3 perovskite-sensitized mesoscopic solar cells, J. Phys. Chem. Lett. 4 (2013) 1532e1536. [18] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.-S. Lim, J.A. Chang, Y.H. Lee, H.-J. Kim, A. Sarkar, K. Nazeeruddin, M. Gr€ atzel, S.I. Seok, Efficient inorganiceorganic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors, Nat. Photonics 7 (2013) 486e491. [19] B. Xu, E. Sheibani, P. Liu, J. Zhang, H. Tian, N. Vlachopoulos, G. Boschloo, L. Kloo, A. Hagfeldt, L. Sun, Carbazole-based hole-transport materials for efficient solid-state dye-sensitized solar cells and perovskite solar cells, Adv.

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