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