Thermochromism to Tune the Optical Bandgap of a Lead-Free Perovskite-Type Hybrid Semiconductor for Efficiently Enhancing Photocu

Journal of Materials Chemistry C

View Article Online PAPER View Journal | View Issue

Thermochromism to tune the optical bandgap of a lead-free perovskite-type hybrid semiconductor Cite this: J. Mater. Chem. C, 2017, 5, 9967 for eﬃciently enhancing photocurrent generation†‡

Weichuan Zhang, Zhihua Sun,* Jing Zhang, Shiguo Han, Chengmin Ji, Lina Li, Maochun Hong and Junhua Luo *

Thermochromic materials have recently attracted great attention due to their controllable and rich physicochemical properties. However, until now, no studies have been reported on thermochromic materials for photovoltaic and optoelectronic applications. Here we report a new lead-free hybrid

semiconductor material, (C16H20N2)SbBr5 (1), which adopts the zero-dimensional (0-D) perovskite-type inorganic framework. Strikingly, the thermochromism in 1 leads to a wide tunable bandgap and superior photoelectric properties. Three distinct color-varying stages were first observed, i.e. colorless to yellow (I), yellow to red (II), and red to black brown (III). In particular, the figure-of-merits for thin-film Received 18th June 2017, photodetectors based on II-thermochromism were greatly improved, with the dark current lowered to Accepted 23rd August 2017 one quarter and light photocurrent enhanced at least 12-fold. The photocurrent on/oﬀ switching ratio DOI: 10.1039/c7tc02721d was thus improved by B50 times through thermochromism. As a new conceptual strategy to engineer the optical bandgap and meet specific photoelectric functions, our study paves the way for building rsc.li/materials-c high-performance optoelectronic devices based on thermochromic materials.

The development of stimuli-responsive smart materials has chemical diversity exhibits vast opportunities for creating technolo- caught much attention because of their potential physicochemical gically optical and electronic properties.16 The most notable 1 2,3 4,5 properties, such as photochromic, piezochromic, halo- compounds are organometal trihalide perovskites (CH3NH3PbX3, chromic,6 and thermochromic7–10 behaviors. Thermochromic X = Cl, Br and I), which have boosted the photovoltaic conversion Published on 24 August 2017. Downloaded by Shanghai Tech University 11/12/2017 15:03:57. materials, which enable the optical bandgap to be altered in efficiency higher than 22.1%,17–21 catching up with commercial response to a temperature change, have been studied by researchers silicon-based solar cells. For these hybrids, tunable optical for centuries and have made tremendous progress.11,12 However, absorption plays an increasingly important role for their photo- controllable thermochromic materials are still rarely applied in electric application.22 For instance, the chemical substitution of

the fields of photovoltaics and optoelectronics, although diverse halogens in CH3NH3PbX3 enables tunable bandgaps in the range of thermochromic systems have been explored, such as photonic 2.97–1.53 eV, which is favorable for diverse functions.23,24 Mean- systems,13 surface plasmon absorption14 and quantum dots,15 while, the modification of cationic components in two-dimensional

etc. Recently, hybrid organic–inorganic perovskite-type semi- (2-D) perovskite hybrids, such as (RNH3)2(CH3NH3)mÀ1PbmX3m+1 conductor materials have taken a dominant position within (whereR=analkyloraromaticmoietyandX=Cl,BrorI),has the fastest growing areas of materials chemistry because their also fueled enormous interest for optoelectronic researchers.25,26 The previously-reported method for changing the optical absorption bandgap consists of controlling organic cations State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the and/or halogens.27,28 These studies, however, only extend the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. 29 E-mail: [email protected], [email protected]; Fax: +86-0591-63173126; absorption range to a specific wavelength. In this context, it still Tel: +86-0591-63173126 remains challenging to continuously tune the bandgaps of hybrid † Dedicated to Prof. Academician Xin-Tao Wu from the FJIRSM, CAS, on the perovskites to the wide visible-light range. Alternatively, pioneering occasion of his 80th birthday. studies reveal that the colors of metal-halide hybrids could be ‡ Electronic supplementary information (ESI) available: Details of the synthesis; changed by the expansion or contraction of the metal and halide structural information for the compounds (NMR, IR analysis, powder X-ray 30 diﬀraction, Raman, computational, etc.); Fig. S1–S10, and Tables S1 and S2. interatomic distances. This mechanism suggests that thermo- CCDC 1533611–1533615. For ESI and crystallographic data in CIF or other chromism might act as an effective strategy to engineer optical electronic format see DOI: 10.1039/c7tc02721d bandgaps and to improve the potential optoelectronic performance.

This journal is © The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 9967--9971 | 9967 View Article Online

Paper Journal of Materials Chemistry C

2+ In this work, we attempted to explore new thermochromic Ammonium counter-cations ([C16H20N2] ) are situated inside the 31 4À candidates in the organic–inorganic lead-free perovskite hybrids cavities and are bonded to the binuclear unit [Sb2Br10] through for potential photoelectric applications. As some of the most out- N–HÁÁÁBr hydrogen bonds (Fig. S5, ESI‡). Each organic cation was standing charge-transfer units, configuration-locked stilbazolium clamped with alternate binuclear units Sb–Br along the a-direction, chromophores have been found to show structural changes in forming a 0-D perovskite-like anionic framework. It is noteworthy which the CQC bonds generally behave as photoreactive moieties. that 1 displays rich thermochromic behaviors, changing from Herein, we report the first lead-free organic–inorganic thermo- colorless to black brown in the range of 100–500 K, which was

chromic hybrid, (C16H20N2)SbBr5 (compound 1,C16H20N2 is 1,3- observed by single crystal X-ray diffraction at different temperatures. (4-dimethylamino-styryl)-2,4-(4-methyl-pyridinium)-cyclobutane), As shown in part c of Fig. 1, the room-temperature yellow crystals of which features a 0-D perovskite-type inorganic binuclear frame- 1 gradually turned colorless upon cooling down to 100 K. Contrarily, work. It is noteworthy that 1 undergoes three distinct color- the crystal color changed to red (B450 K) and black-brown varying stages, i.e. colorless to yellow (I), yellow to red (II), and (B500 K) in the heating mode. For convenience, the thermo- red to black brown (III). This unprecedented thermochromism chromic course was approximately sorted into three stages: leads to widely tunable optical bandgaps, ranging from 2.5 to colorless to yellow (I, 100–350 K), yellow to red (II, 350–450 K)

1.5 eV, which is almost comparable with those of CH3NH3PbI3. and red to black-brown (III, 450–500 K). However, the black- More importantly, the figure-of-merits for the thin-film photo- brown crystals could not return to yellow upon cooling, but detector based on II-thermochromism are temperature-stable remained stable in the red-color state to 300 K (the cooling stage and greatly enhanced by tuning the optical bandgap; that is, the in Fig. 2a (C–RT) and Fig. S8, ESI‡). Here, the thermochromism- light photocurrent was enhanced at least 12-fold while the dark induced color variations of 1 between the yellow and red states current was lowered about 4-fold. As a result, the photocurrent provide an excellent platform for assembling potential photo- switching ratio during on/off illumination is highly improved by detectors. more than B50 times by using thermochromism. To the best of For a deeper understanding of this thermochromic phenom- our knowledge, this is the first report on photocurrent enhancement enon, a crystal wafer of 1 with a thickness of B0.8 mm was heated in thermochromic lead-free perovskite hybrids, which affords a new and kept at the corresponding temperatures for five minutes. Then conceptual strategy to achieve high-performance optoelectronic ultraviolet-visible absorption spectra were collected at room devices. temperature on a LAMBDA 950 UV/Vis Spectrophotometer. As Bulk yellow crystals of 1 were obtained from a concentrated shown in Fig. 2a and Fig. S6a (ESI‡), the curves of 300, 325 and

hydrobromic acid (HBr) solution of antimonous oxide (Sb2O3) 350 K almost overlap, which illustrates that 1 is reversible in this and trans-4-(4-dimethylanilino-styryl)-N-methyl-pyridinium iodide temperature range (stage I). Within the temperature range of (DAMS+) by slow evaporation at room temperature. The purities 350 to 450 K, the absorption edges were obviously red-shifted to were confirmed by infrared spectroscopy (IR) and 1HNMR(Fig.S1 B680 nm, corresponding to a bandgap of 1.85 eV. However, the and S2, ESI‡). Details on the sample preparation and basic red color of the sample heated at 450 K remained stable even as characterization are given in the methods section in the ESI‡ the temperature cooled down to room temperature, behaving Experimental. As shown in Fig. 1a, crystal 1 crystallizes in the as the metastable state, which reveals that thermochromism-II

monoclinic system with a space group of P21/n (Table S1, ESI‡). Published on 24 August 2017. Downloaded by Shanghai Tech University 11/12/2017 15:03:57. The asymmetric unit contains one [2+2] photodimerization of + DAMS and half of a Sb2Br10 anionic cluster. The binuclear unit 4À [Sb2Br10] clusters make a dominant contribution to charge transport in 1, resembling that of tin- and lead-based perovskites.

Fig. 2 (a) Ultraviolet-vis absorption spectra of 1 performed on samples preheated at diﬀerent temperatures (C–RT, cooling to room temperature). Fig. 1 (a) Structural illustration of 1 viewed along the c-axis. (b) Bond (b) Variation of Sb–Br bond lengths measured at 100, 295, 300, 400 and distance induced color change. (c) Variation of bond length with thermo- 295 K (preheated at 450 K). (c) Raman spectra collected at different chromic changes in three stages (thermochromism-I is sensitive to temperatures. (d) XRD patterns of the thin films, powder samples and temperature, while the II-stage and III-stage are stable.) simulated results.

9968 | J. Mater. Chem. C, 2017, 5, 9967--9971 This journal is © The Royal Society of Chemistry 2017 View Article Online

Journal of Materials Chemistry C Paper

is irreversible. A low bandgap of B1.5 eV was further achieved through thermochromism-III, as revealed by the absorption edge of the curve obtained at 500 K. Unexpectedly, the black color at 500 K was unstable, and returned to a red color as the temperature decreased to 300 K (as shown in the C–RT trace). Structural transformation or phase transition is closely associated with color changes in thermochromic compounds.32 Hence, variable-temperature PXRD patterns and dielectric constants were performed on single crystals, which showed that the thermochromic behavior was not caused by structural phase transition (Fig. S3, S4 and S9, ESI‡). Moreover, some studies disclose that thermochromism is likely induced by the variation of Sb–Br interatomic distances.33 To certify the structural evolution with temperature, crystallographic data of 1 were collected from different batch samples at 100, 295, 300, 400 and 295 K (preheated at 450 K) (Table S2, ESI‡). Fig. 1c and 2b show that the Sb–Br bond Fig. 3 Topographical AFM images (a and b) and conductive AFM images (c and d) recorded in the same film treated at 400 and 450 K, respectively. lengths contract with an increase in temperature and recover after

cooling down to 295 K. However, the Sb–Br1 and Sb–Br2 bonds could not return back to their original states. This is proposed to these conductive areas were between À9.1 and 9.3, and À5.3 and be one possible reason why crystals still display a red color at 300 K 7.1 nm, respectively. The sizes and distribution patterns are almost after heating. The bond changes were also confirmed by Raman the same for the thin films. Most strikingly, the current density of spectroscopy (Fig. 2c) using a 532 nm laser under ambient the conductivity peaks at 400 K (B4.08 pA) is obviously higher temperature. We see three peaks at 117, 154, and 176 cmÀ1,which than that at 450 K (B3.37 pA), which indicates that the dark

are similar to those reported for (C3H5NH3)3SbBr6 at 104, 166, and current was reduced through thermal-induced thermochromism 174 cmÀ1 (where the first two peaks were attributed to Sb–Br (see Fig. S7, ESI‡). Some conductive areas appear to be clustered bridged changes and the last peak was assigned to Sb–Br terminal together at the same location, whereas others appear to be alterations).34 Obviously, the main Raman peak at 175 cmÀ1 slowly completely isolated. This phenomenon might be associated disappeared upon heating, which directly shows that the thermo- with the grain boundaries on the thin-films and the intrinsic chromic phenomenon is induced by variation of the Sb–Br reasons such as a lower dark current might be relative to the interatomic distances. Additionally, our recent report on the ferro- variation of the Sb–Br interatomic distances.

electric semiconductor (N-methylpyrrolidinium)3Sb2Br9 revealed To further evaluate the effects of stable thermochromism-II that inorganic metal-halide moieties determine the optical band- on photoelectric behaviors, thin-film photodetecting devices gap of hybrids by calculation of the partial density of states.35 To were fabricated using the thermochromic crystals preheated at further study the optical absorption and electronic mechanisms of 400 and 450 K. The lateral device architecture was employed the semiconducting properties, density functional theory was used and photo-excitation was performed under a light source with a Published on 24 August 2017. Downloaded by Shanghai Tech University 11/12/2017 15:03:57. to calculate the bandgaps of 1 at 400 and 450 K. The calculated 300 mW cmÀ2 xenon lamp at room temperature (Fig. 4a). As valuesare1.93and1.76eV,whichareingoodaccordancewiththe illustrated in Fig. 4b, the device based on the 400 K sample experimental results (1.92 and 1.8 eV, Fig. S6, ESI‡). exhibited a low dark current (B10À10 A), and under illumination Owing to the temperature stability of the samples in thermo- the corresponding photocurrent rapidly increased to about chromism-II, thin-film photodetectors were fabricated via spin- 15-fold the dark current. In comparison, the light current of coating of a dimethylsulfoxide solvent followed by temperature the device based on the 450 K sample was obviously enhanced annealing (400 K, 10 min) under a nitrogen atmosphere, and about 10-fold, and the dark current became one quarter of that identified by X-ray diffraction (Fig. 2d). Most of the Bragg main of the 400 K sample. In detail, at a voltage of 10 V, the light- peaks were indexed through the simulated and PXRD patterns, induced photocurrent was enhanced from 1.575 Â 10À9 (400 K) indicating that 1 is stable and easy to crystallize in thin films. to 1.124 Â 10À8 (450 K). The corresponding dark current was The surface topography and electrical properties of the thin lowered from 1.645 Â 10À10 (400 K) to 2.376 Â 10À11 (450 K). As films were characterized by atomic force microscopy (AFM) and a result, the ‘‘on/off’’ switching ratio was improved by at least conducting atomic force microscopy (C-AFM) at the nanoscale. B50 times through thermochromism. The time-dependent Fig. 3a and b show the surface topography changes obviously photocurrent response of the device with a 520 nm laser after heating from 400 to 450 K, with a decrease in the surface operated at a bias voltage of 5 V and an incident power of roughness from 9.3 to 7.1 nm, revealing that the film quality 100 mWmmÀ2 is shown in Fig. 4c. The multi-switching measure- was improved by heating treatment. Furthermore, the C-AFM ments show that the switching was steady and reversible, which method uses a conductive tip to measure the surface conductivity could allow the device to act as a photosensitive switch. In of thin-films by the contact mode. The conductivity images shown addition, it distinctly shows that the dark current was decreased in Fig. 3c and d display that both temperature states had weak and light photocurrent was enhanced by the 520 nm laser. The electric currents over most of the sample area. The roughnesses of rise and decay times of the film device based on the 450 K

Paper Journal of Materials Chemistry C Acknowledgements

This work was supported by the NSFC (21525104, 21622108, 21601188, 91422301, 21373220, 51402296, 21571178, 51502288 and 51502290), the NSF for Distinguished Young Scholars of Fujian Province (2016J06012), the NSF of Fujian Province (2015J05040), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), the Youth Innovation Promotion of CAS (2014262, 2015240 and 2016274), and the ‘‘Chunmiao Projects’’ of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-002 and CMZX-2014-003). Z. S. acknowledges the support from the State Key Laboratory of Luminescence and Applications (SKLA-2016-09).

Notes and references

1 O. Sato, Nat. Chem., 2016, 8, 644. Fig. 4 The photodetecting performance of the thin-film device of 1. 2 P. X. Li, M. S. Wang, M. J. Zhang, C. S. Lin, L. Z. Cai, (a) Schematic diagram of the Au device architecture with an area of about 3 Â 10À6 cm2. (b) Current–voltage curves of the device measured in the dark and on S. P. Guo and G. C. Guo, Angew. Chem., Int. Ed., 2014, illumination under a 300 mW cmÀ2 xenon lamp at room temperature. (c) The 53, 11529. time-dependent photocurrent response of the device with a 520 nm laser, 3 M. Hammerich, C. Schu¨tt, C. Sta¨hler, P. Lentes, F. Ro¨hricht, À2 operated at a bias voltage of 5 V and an incident power of 100 mWmm .(d)An R. Ho¨ppnerand and R. Herges, J. Am. Chem. Soc., 2016, enlarged view of the photocurrent response time during on/off switching. 138, 13111. 4 J. Kunzelman, M. Kinami, B. R. Crenshaw, J. D. Protasiewicz sample were estimated to be 0.079 s and 0.0168 s, which are and C. Weder, Adv. Mater., 2008, 20, 119. almost equivalent to those measured on the device based on the 5 A. Jaﬀe, Y. Lin, W. L. Mao and H. I. Karunadasa, J. Am. 400 K sample (0.105 s and 0.0171 s, Fig. 4d). This remarkable Chem. Soc., 2015, 137, 1673. enhancement in the device photo-detection performance is 6 G. Qian, J. Qi, J. A. Davey, J. S. Wright and Z. Y. Wang, Chem. greatly attributed to the thermochromism-induced narrower Mater., 2012, 24, 2364. bandgaps, which are closely associated with the electronic 7 X. Wei, L. Yu, X. Jin, D. Wang and G. Z. Chen, Adv. Mater., structure of the Sb–Br binuclear frameworks. A previous report 2009, 21, 776. 17 8 A. Llorde´s, G. Garcia, J. Gazquez and D. J. Milliron, Nature, on the iodo-bismuthate hybrid [metal(1,10-phen)3][Bi3I11], showed that thermochromism enables photodetecting materials 2013, 500, 323. to broaden their absorption capability. In the case of 1,thermo- 9 L. Zhang, Y. Yuan, X. Tian and J. Sun, Chin. Chem. Lett., chromism plays a dominant role in tuning its optical bandgap, 2015, 26, 1133. which greatly enhances the photocurrent generation. To the best of 10 B. Xu, H. Wu, J. Chen, Z. Yang, Z. Yang, Y. Wu, Y. Zhang, Published on 24 August 2017. Downloaded by Shanghai Tech University 11/12/2017 15:03:57. our knowledge, this is the first report on thermochromism to C. Jin, P. Lu, Z. Chi, S. Liu, J. Xua and M. Aldredd, Chem. enhance the photodetecting performance in hybrid materials. Sci., 2017, 8, 1909. In summary, we have reported a new lead-free perovskite 11 A. Seeboth, D. Lo¨tzsch, R. Ruhmann and O. Muehling, hybrid semiconductor, of which the optical bandgap is widely Chem. Rev., 2014, 114, 3037. tuned by thermochromism ranging from 2.5 to 1.5 eV. It is 12 O. S. Wenger, Chem. Rev., 2013, 113, 3686. notable that crystal 1 exhibits three distinct thermochromic 13 A. Tikhonov, R. D. Coalson and S. A. Asher, Phys. Rev. B: stages: colorless to yellow (I), yellow to red (II) and red to black Condens. Matter Mater. Phys., 2008, 77, 235404. brown (III). Based on the stable II-thermochromism stage, we 14 D. I. Uhlenhaut, P. Smith and W. Caseri, Adv. Mater., 2006, developed a new conceptual strategy to improve the intrinsic 18, 1653. photo-responses of thin-film photodetectors for the first time. 15 O. Tagit, N. Tomczak, E. M. Benetti, Y. Cesa, C. Blum, Through thermochromism, the photocurrent was enhanced at V. Subramaniam, J. L. Herek and G. J. Vancso, Nanotechnology, least 12-fold, the dark current was lowered to about one 2009, 20, 185501. quarter, and the photocurrent on/oﬀ switching ratio was greatly 16 K. Eckhardt, V. Bon, J. Getzschmann, J. Grothe, F. M. Wisser improved by B50 times. Our study may open up a new avenue and S. Kaskel, Chem. Commun., 2016, 52, 3058. to exploit the stimuli-responsive smart candidates of perovskite 17 W. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo and hybrids for device application in the field of optoelectronics. S. I. Seok, Science, 2015, 348, 1234. 18 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050. Conflicts of interest 19 T. H. Liu, K. Chen, Q. Hu, J. Wu, D. Y. Luo, S. Jia, R. Zhu and Q. H. Gong, Chin. Chem. Lett., 2015, 26, 1518. There are no conflicts to declare. 20 G. L. Yang and H. Z. Zhong, Chin. Chem. Lett., 2016, 27, 1124.

Journal of Materials Chemistry C Paper

21 National Renewable Energy Labs (NREL) Eﬃciency 28 G. Raino, G. Nedelcu, L. Protesescu, M. I. Bodnarchuk, M. V. Chart. http://www.nrel.gov/ncpv/images/eﬃciency_chart.jpg, Kovalenko, R. F. Mahrt and T. Stoferle, ACS Nano, 2016, 10, 2485. accessed July 25, 2016. 29 S. D. Stranks and H. J. Snaith, Nat. Nanotechnol., 2015, 10,391. 22 S. Bayrammurad and B. M. David, Chem. Rev., 2016, 116, 30 M. G. Andrea, A. T. Meredith, D. S. Mark, P. J. LeRoy, G. K. 4558. Jennifer, J. I. D. William and Z. L. Hans-Conrad, J. Am. 23 Y. Liu, Y. Zhou, D. Cui, X. Ren, J. Sun, X. Liu, J. Zhang, Chem. Soc., 2011, 133, 603. Q. Wei, H. Fan, F. Yu, X. Zhang, C. Zhao and S. Liu, Adv. 31 G. Flora, D. Gupta and A. Tiwari, Interdiscip. Toxicol., 2012, Mater., 2015, 27, 5176. 5, 47. 24 M. Saliba, T. Matsuil, K. Domansk, J. Seol, A. Ummadisingu, 32 L. Li, Z. Sun, C. Ji, S. Zhao and J. Luo, Cryst. Growth Des., S. M. Zakeeruddin, J. Correa-Baena, W. R. Tress, A. Abate, 2016, 16, 6685. A. Hagfeldt and M. Gra¨tze, Science, 2016, 353, 1409. 33 E. v. H. S. Rupert, P. H. Donald and L. B. Stephen, Organo- 25 G. Hodes, Science, 2013, 342, 317. metallics, 1992, 11, 3492. 26 M. D. McGehee, Nat. Mater., 2014, 13, 845. 34 I. Płowas´, A. Białon´ska, R. Jakubas, G. Bator, B. Zarychta and 27 L. Dou, A. B. Wong, Y. Yu, M. Lai, N. Kornienko, S. W. Eaton, J. Baran, Chem. Phys., 2010, 375, 16. A.Fu,C.G.Bischak,J.Ma,T.Ding,N.S.Ginsberg,L.W.Wang, 35 Z. Sun, X. Liu, T. Khan, C. Ji, M. A. Asghar, S. Zhao, L. Li, A. P. Alivisatos and P. Yang, Science, 2015, 349, 1518. M. Hong and J. Luo, Angew. Chem., 2016, 128, 6655. Published on 24 August 2017. Downloaded by Shanghai Tech University 11/12/2017 15:03:57.