Design Strategy for Improving Optical and Electrical Properties and

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A Design Strategy for Improving Optical and Electrical Properties and Stability of Lead-Halide Semiconductors Cai Sun, Gang Xu, Xiao-Ming Jiang, Guan-E Wang, Pei-Yu Guo, Ming-Sheng Wang, and Guo-Cong Guo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10101 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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Journal of the American Chemical Society

A Design Strategy for Improving Optical and Electrical Properties and Stability of Lead-Halide Semiconductors Cai Sun, Gang Xu, Xiao-Ming Jiang, Guan-E Wang, Pei-Yu Guo, Ming-Sheng Wang,* and GuoCong Guo* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100039, People's Republic of China

ABSTRACT: Broad absorption, long-lived photogenerated carriers, high conductance, and high stability are all required for a light absorber toward its real application on solar cells. Inorganic–organic hybrid lead-halide materials have shown tremendous potential for application in solar cells. This work offers a new design strategy to improve the absorption range, conductance, photoconductance, and stability of these materials. We synthesized a new photochromic leadchloride semiconductor, by incorporating a photoactive viologen zwitterion into a lead-chloride system in the coordinating mode. This semiconductor has a novel inorganic–organic hybrid structure, where 1-D semiconducting inorganic leadchloride nanoribbons covalently bond to 1-D semiconducting organic π-aggregates. It shows high stability against light, heat and moisture. After photoinduced electron transfer (PIET), it yields a long-lived charge-separated state with a broad absorption band covering the 200–900 nm region while increasing its conductance and photoconductance. This work is the first to modify photoconductance of semiconductors by PIET. The observed increasing times of conductivity reached three orders of magnitude, which represents a record for photoswitchable semiconductors. The increasing photocurrent comes mainly from the semiconducting organic π-aggregates, which indicates a chance to improve the photocurrent by modifying the organic component. These findings contribute to the exploration of light absorbers for solar cells.

INTRODUCTION Inorganic–organic hybrid lead-halide semiconductors1 have shown great promise for use in field-effect transistors,2 light-emitting diodes,3-5 lasers,6 and photodetectors.7,8 Recently, methylammonium lead-iodide perovskite, first reported in 1978, attracted significant attention for the fabrication of low-cost solar cells as light absorbers.9 To realize highly efficient and stable solar cells, considerable human, material and financial resources as well as thousands of work have been poured into the studies of innovative architectures, chemical compositions and deposition processes for such perovskite materials.10-13 As a result, the power conversion efficiency of perovskite solar cells has increased rapidly from 3.8% to over 22.1% in only five years.14-17 Nevertheless, the poor long-term stability against light,18 heat19 and moisture20 remains a critical issue for perovskite materials. Broad absorption, longlived photogenerated carriers, high conductance, and high stability are all required for a light absorber toward its real application.10,21 Aside from improving the stability of perovskite materials, developing new design strategies to explore light absorbers that can fulfill these requirements is necessary. Electron-transfer (ET) photochromic materials can form long-lived charge-separated states with characteristic absorption bands covering the visible region.22 Moreo-

ver, ET triggered by external stimuli is also an effective approach to modify the electrical properties of semiconductors.23,24 Recently, we reported that a viologentemplated lead bromide semiconductor (viologen = N,N’disubstituted bipyridinium) showed conductance switch through heat-induced ET between lead-bromide lattice and viologen cations, and demonstrated that the change value of conductivity had a clear positive correlation with the ET degree.24 In another work, Roy et al. found that photoinduced electron transfer (PIET) from an electron donor to a semiconducting organic π-aggregate was beneficial in generating more stabilized free radicals and more carriers, which resulted in the increase of conductivity.25 Moreover, the poor long-term stability of lead-halide perovskite materials is predominantly caused by heatinduced phase change at low temperatures (for example, ∼55 °C for methylammonium lead iodide perovskite),19 light-induced dissociation of the ammonium cations,18 or water-induced rearrangement of the inorganic skeleton.20 The stability can be expected to improve after enhancing the structural rigidity of an entire material through covalent bonding between the inorganic skeleton and organic component. With these in mind, we aim to design an inorganic–organic hybrid lead-halide semiconductor, where the organic component is active for ET photochromism and has the ability to form a semiconducting π-aggregate that covalently bonds to the inorganic skeleton.

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Figure 1. Crystal structure of 1.

N,N’-4,4’-bipyridiniodipropionate (CV) is an ET photochromic viologen zwitterion with a π-conjugated 4,4’bipyridinium (bpy) cation and two terminal carboxylate anions (Figure S1–3 in the Supporting Information (SI)). Numerous previous examples have demonstrated that cation–π interactions are in favor of the formation of organic semiconductors.25,26 Likewise, the presence of a πconjugated cation in CV offers a good probability to generate a semiconducting π-aggregate, if the intermolecular separation is less than 3.8 Å.27 Moreover, carboxylate is a multidentate coordination group that easily covalently bonds to lead halide.28 Through a mild solution processing procedure, we successfully incorporated the CV ligand into the lead-chloride system in the coordinating mode. The obtained crystalline lead-chloride semiconductor, {[Pb3Cl6(CV)]H2O]}n (1), has an inorganic–organic hybrid nanoribbon array structure,29 where 1-D semiconducting inorganic [Pb3Cl6]n nanoribbons covalently bond to 1-D semiconducting organic π-aggregates (Figure 1). It has good stability against light, heat, and moisture with a photochromic behavior due to the ET between the inorganic and organic components. After PIET, it forms a long-lived charge-separated state with a broad absorption band covering 200–900 nm range while increasing its conductance and photoconductance. The observed conductivity ratio between the colored and as-synthesized samples reached three orders of magnitude. This value exceeds the recently reported values for a photochromic viologen-based organic semiconductor (~16 times)25 and a photochromic diarylethene-based single-molecule junction switch (~100 times),30 and represents a record for photoswitchable semiconductors. Furthermore, previous examples on modification of photoconductance by external stimuli, such as electric field,31 are available. This work shows the first example of modifying photoconductance of semiconductors through PIET.

EXPERIMENTAL SECTION Materials and instrumentation. PbCl2, 4,4’bipyridine, and acrylic acid in AR grade were purchased commercially. They were directly used without further purification. Water was deionized and distilled before use. Elemental analyses of C, H, and N were measured on an Elementar Vario EL III microanalyzer. 1H NMR spectra were measured at room temperature (RT) by an Advance

III NMR spectrometer operating at 400 MHz. FT-IR spectra were measured on a PerkinElmer Spectrum One FT-IR spectrometer using KBr pellets. Electronic absorption spectra were measured in the diffuse reflectance mode on a PerkinElmer Lambda 900 UV/vis/near-IR spectrophotometer equipped with an integrating sphere, and a BaSO4 plate was used as the reference. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Desktop MiniFlexII diffractometer using Cu Kα radiation (λ = 1.54056 Å) powered at 30 kV and 15 mA. Simulated PXRD pattern was derived from the Mercury Version 3.5.1 software using the X-ray single crystal diffraction data. Thermogravimetric analysis was conducted on a Mettler TOLEDO simultaneous TGA/DSC apparatus. ESI-MS was obtained using a ThermoFinnigan LCQ Deca XP MAX LC/MS system. Electron spin resonance (ESR) spectra were recorded on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in the X band at RT. X-ray photoelectron spectroscopy (XPS) studies were performed in a ThermoFisher ESCALAB250 X-ray photoelectron spectrometer (powered at 150 W) using Al Kα radiation (λ = 8.357 Å). Temperature-dependent electrical conductivities and I–V curves were measured in a Keithley 4200-SCS semiconductor parameter analyzer using pellet samples by the two probe method using silver paste. Temperature and vacuum control for electric tests. We placed the sample onto a piece of sapphire, and put the lateral device architecture onto the sample stage of a probe station (Lake Shore CRX-VF) with thermally conductive and electrically insulating grease (DuPont Krytox). To avoid an increase in temperature during the electric measurements, an IR filter was used in all experiments. The temperature of the sample was monitored by a GaAs diode (Lake Shore, top model 336) and was kept constant within 0.01 °C to ensure that the magnitudes of the current changes upon light irradiation are not due to temperature increase. Moreover, the pressure of vacuum chamber was kept at ~ 1.4 × 10–6 torr to ensure the reliability and quality of data during the electric measurements. Given the strict control of temperature and pressure conditions, we obtained the experimental data of conductivity cycle (Figure 3c) through a long period of time. Synthesis of CV. The viologen dicarboxylate was prepared by reacting 1.6 g (10.0 mmol) of 4,4’-bipyridine with


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12 mL (150.0 mmol) of acrylic acid in 15 mL chloroform at RT for 24 h. The precipitated solid was filtered off, washed ultrasonically with acetone and methanol in sequence, and finally dried at 75 °C to yield a pale yellow powder of CV. 1H NMR (400 MHz, D2O): δ = 2.88 (t, J = 6.52 Hz, 4H), 4.82 (t, J = 6.52 Hz, 4H), 8.42 (d, J = 6.96 Hz, 4H), 9.03 (d, J = 6.96 Hz, 4H) (Figure S1). ESI-MS (H2O): 301 [M+H]+ (Figure S2). Synthesis of {[Pb3Cl6(CV)]H2O]}n (1). A mixture of PbCl2 (280 mg, 1 mmol) and CV (300 mg, 1 mmol) in 20 mL water was stirred at 70 °C for several min to yield a bright yellow clear liquid, which was filtered and allowed to volatilize in the dark for 3 d. In most cases, the yielded crystals were exclusively pale yellow needle-like crystals of 1 (ca. 51% yield based on Pb). In some times, several yellow block crystals of the other compound {[PbCl(CV)]Cl}n were included. The crystals of 1 for all testing and characterization were carefully picked with the help of a microscope. Elemental analysis: Found: C, 16.86; H, 1.61; N, 2.31%; Calc. for C16H18O5N2Cl6Pb3: C, 16.67; H, 1.57; N, 2.43%. The PXRD study demonstrates the phase purity of the obtained crystalline sample of 1 (Figure S5). FT-IR (KBr, 4000–400 cm–1): 3445 (m), 3089 (w), 3043 (w), 2963(m), 1630 (m), 1539 (m), 1498 (w), 1430 (m), 1402 (m), 1349 (w), 1306 (w), 1261 (s), 1197(w), 1163 (w), 1097 (s), 1020 (s), 948 (w), 865 (w), 800 (s), 676 (w), 586 (w), 561 (w), 533 (w) (Figure S6). X-ray crystallographic study. Single-crystal X-ray diffraction measurements were performed on a Rigaku Pilatus 200K diffractometer, using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Intensity data sets were collected using ω scan techniques, and corrected for Lp effects. The structures were solved by the direct method and refined by full-matrix least squares on F2 using the Siemens SHELXTLTM Version 5 package of crystallographic software,32 with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms of the lattice water molecules were included from the difference Fourier maps, and their temperature factors were set as 1.2 times those of the parent atoms. No higher space groups for 1 were found using the Platon software from the IUcr website (http://www.iucr.org/). Crystal data and structure refinement results are summarized in Table S1. The entry of CCDC 1569616 contains the supplementary crystallographic data for 1. The data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, U.K. Fax: (Internet) +441223/336-033. E-mail: [email protected].

wave (PAW) potential, which is implemented in the Vienna ab initio Simulation Package (VASP).33,34 The Perdew−Burke−Ernzerhof (PBE) exchange-correlation functionals with an optB86b-vdW correction was used considering the weak van der Waals like interaction between inorganic and organic components.35,36 The crystallographic data of 1 was applied to build calculation models with a plane-wave cutoff energy of 400 eV and a 7 × 2 × 2 Monkhorst–Pack grid of k-points. Calculations of linear optical properties described in terms of the complex dielectric function ε = ε1 + iε2 were also made in this work.37 The imaginary part of the dielectric function ε2(ω) was given as follows: πe ∑, 2 = , 2 | · | − − ℏ

4

where Σ is the sum over the valence bands (v) and conduction bands (c), ω is the optical frequency, m denotes electron mass, ∫ is an integration over k vectors in Brillouin zone (BZ), e·Mcu(k) is an electron transition moment between the conduction and valence bands at k point, and δ function is the energy difference between the conduction and valence bands at k point with absorption of a quantum ћω. The Kramers–Kronig transform was used to obtain the actual part ε1(ω) of the dielectric function: # = 1 +

* ε 2 &' d , + ′ −

where p in front of the integral represents the principal value. The calculated absorbance α(ω) shown in Figure 4a can be expressed as - =

, ./

where C and n(ω) are the velocity of light and refractive index, respectively. The n(ω) can be expressed as follows: . =

1

√2

1# +

#2

+ # 3

#2

.

2) Calculations of reduced density gradient (RDG) and electronic coupling (V). All calculations were based on the DFT level using the M06-2X functional and 6-31G* basis set by Gaussian 09 D01 version software.38 The calculation model of a CV dimer was taken from the single crystal structures of 1.

Computational approaches. 1) Calculations of band structure (BS), partial density of states (PDOS) and linear optical properties. All calculations were based on the density functional theory (DFT) in conjunction with the projector-augmented-

RDG (s) is a fundamental dimensionless quantity in DFT used to describe the deviation from a homogeneous electron distribution. It is defined as s = |∇7|/ 23 #⁄ /7 ;⁄ , where ρ is the electronic density, and



∇7 is the first derivative of ρ.39 The results were analyzed by the Multiwfn software.40 According to a simple tight-binding model, as reported in the literature,41 the absolute value of the V related to hole transport can be obtained from the energy difference, V = (EHOMO – EHOMO–1)/2, where EHOMO and EHOMO–1 are the energies of HOMO and HOMO–1 orbitals taken from the closed-shell configuration of the dimer, respectively. Similarly, the absolute value of the V of electron transport can be obtained from the energy difference, V = (ELUMO+1 – ELUMO)/2, where ELUMO+1 and ELUMO are the energies of LUMO+1 and LUMO orbitals of the dimer, respectively.

RESULTS AND DISCUSSION PXRD patterns (Figure S5) and elemental analysis data indicated that the as-synthesized crystalline sample was a pure phase. Thermogravimetric analysis (Figure S4), PXRD data (Figure S5) and FT-IR spectra (Figure S6) showed that compound 1 was thermally stable at least up to 120 °C. PXRD and FT-IR data also revealed that compound 1 was stable after placing in an environment with a relative humidity level of 100% for a week or being irradiated by UV light for one month (Figures S5 and S6).

Figure 2. a) Color change in a photochromic process. b) ESR spectra of 1A, 1B and decolored samples. c) Time-dependent electron absorption spectra upon irradiation. Inset: Firstorder kinetic plot for change in absorbance at λ = 600 nm, where A0, At, and A∞ are the absorbance values at time zero, time t, and infinite time of the reaction, respectively.

Single-crystal X-ray diffraction analysis revealed that three crystallographically independent Pb atoms (Pb1, Pb2, Pb3) exist in the 3-D inorganic–organic hybrid framework of 1 (Figures 1 and S7). Every Pb1 atom was coordinated by five Cl atoms and four O atoms from three CV ligands to form a tricapped trigonal prism polyhedron. These polyhedra shared one face with each other to yield an infinite chain extending along the a direction. Likewise, Pb2-

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centered [PbCl6] octahedra and Pb3-centered [PbCl5O3] bicapped trigonal prism octahedra linked themselves in the edge-sharing and face-sharing manners, respectively, to form infinite chains extending along the a direction. Each Pb1-centered chain bridged one Pb2-centered chain and one Pb3-centered chain through edge-sharing the polyhedra to generate a V-shaped chain. Such V-shaped chain further grew into a zigzag-like nanosized ribbon through the 21 symmetry, where the two Pb3-centered chains were bridged by edge-sharing. The CV ligands were stacked to yield a nanosized ribbon by strong cation–π interactions between adjacent pyridinium rings, with the distance between each N+ atom and its neighboring pyridinium plane being 3.51(2) or 3.57(2) Å. As shown in Figure 2a, the as-synthesized pale yellow crystalline sample (1A) turned blue upon irradiation by a 300 W Xe lamp (ca. 50 mW/cm2) under ambient conditions. IR and PXRD data revealed no clear structural change during the coloration process (Figures S5 and S6). The colored sample (1B) exhibited a new electron absorption band covering the 380–900 nm region (Figure 2c). This absorption band was a characteristic of viologen radicals. As depicted in Figure 2b, no electron spin resonance (ESR) signals existed before coloration; however, a radical signal at g = 2.0018 with a linewidth of 14 Gauss appeared after coloration. After annealing at 60 °C in air for 20 min or placing in dark for 1 d, the radical signal disappeared (Figure 2b) and the initial pale yellow color was regained (Figure S8). XPS data provided us insight into the photochromic behavior of 1 (Figure S9, S10 and Table S2). After coloration, the core-level spectra of Pb 4f and N 1s remained almost unchanged, whereas those of Cl 2p, O 1s, and C 1s varied. Before coloration, two normal Cl 2p3/2 and 2p1/2 peaks for negatively charged Cl atoms existed, lying at approximately 197.57 and 199.16 eV, respectively. After coloration, these peaks slightly moved to regions with higher binding energies. The initial OPb–O 2p spectrum peaked at 530.99 eV; however, a new peak with higher binding energy appeared at 531.49 eV after coloration. At the same time, the C sp2 1s peak at binding energies of ∼284.96 eV increased in intensity after coloration. Therefore, the Cl and O atoms lost the electrons and the bpy cation received electrons upon irradiation. That is to say, the photoinduced coloration of 1 originated from a PIET process from the Cl and carboxylate O atoms to the bpy cations and the formation of stable radicals. Timedependent electron absorption spectra monitored at λ = 600 nm indicated that the PIET process follows first-order reaction kinetics, with the rate constant kabs of 2.324 × 10–4 s–1.42 Bulk electric tests of 1 were investigated in vacuum using a lateral device architecture by the two probe method using silver paste (Figure 3a) at RT,43 and the abovementioned Xe lamp was used as the light source (∼50 mW/cm2). The pellet samples 1A and 1B (obtained by in situ illumination) all exhibited Ohmic current–voltage (I–


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Figure 3. a) Schematic of the experimental set-up. b) First-order kinetic plot for change in conductivity. σ0, σt, and σ∞ are conductivities at time zero, time t, and infinite time of accumulated irradiation, respectively. c) Photo-switching of the conductivity in four cycles. d) Temperature-dependent conductivities of 1A and 1B. lnσ = –Ea/kBT + constant, where Ea is the activation energy. e) Irradiation time-dependent change in current (I) under a constant bias of 3 V. f) Linear relationship between Iirr – Idark and accumulated irradiation time. Idark and Iirr indicate current in the absence and presence of irradiation, respectively. g) Linear relationship between σirr – σdark and accumulated irradiation time. σdark and σirr indicate conductivities in the absence and pres2 ence of irradiation, respectively. The irradiance of the Xe lamp is ∼50 mW/cm for all tests.

V) characteristics (Figure S12). The intrinsic conductivity σ increased as the irradiation time prolonged (Figure 3b), and increased about 1720 times from 7.16 (±0.86) × 10–11 to 1.23 (±0.72) × 10–7 S⋅cm–1 when the saturation was reached (based on four samples, Figure S13, S14). To the best of our knowledge, this conductance contrast was the largest value hitherto for photoswitchable semiconductors.25,30 The switching of electrical conductivities between 1A and 1B could be cycled at least four times (Figure 3c). The linear relationship of natural logarithm of σ versus 1/T (Arrhenius law; T, temperature) showed that both 1A and 1B behave as intrinsic semiconductors, with calculated activation energies (Ea) of 0.58 and 0.29 eV over the temperature range of 270–320 K, respectively (Figure 3d). Figure 3e shows the typical variations in the current when the sample was exposed to the Xe lamp at regular intervals. Upon irradiation, the current increased approximately linearly, changing from 10 to 122 pA within 5 min. A sudden current jump, attributed to photogenerated carriers, was gradually observed as accumulative irradiation time increased, and the degree of the current jump varied linearly with the accumulative irradiation time (Figure 3f). The change of conductivity (Figure 3b) and quantity of radicals (Figure S11) follows first-order reaction kinetics. Thus, the conductance had a clear positive correlation with the quantity of radicals, as reported in the literature.24 The total conductivity can be described as σ(t) = σ0 + σ1(t) + σ2(t), where σ0 is the initial conductivity; and σ1(t) and σ2(t) are increased conductivities due to the PIET process and the photogenerated carriers at accumulated irradiation time t, respectively. According to the linear relationship of photoconductance jump versus accumulating irradiated time (Figure 3g) and the good fitting of first-order reaction kinetics for conductivity (Figure 3b), we can obtain the following two equations: σ0 +

σ1(t) = (σ0 – σ∞)e–kt + σ∞ and σ2(t) = k’t, where σ∞ is the conductivity when the reaction ends, k is the first-order rate constant and < is the fitted slope of photoconductance jump. The total conductivity can be expressed as a parabola form σ(t) = 1/2(σ0 – σ∞)k2t2 + [(σ∞ – σ0)k + k’]t + σ0 after the second order approximation of the Taylor Series: e–kt = 1 – kt + k2/2 when t is short. It describes well the initial evolution of conductivity (proportional to current) over accumulated irradiation time (Figures 3e and S16). The calculated imaginary ε2(ω) part of dielectric function can describe the actual electron transitions between occupied and unoccupied bands.44 Based on the calculated ε2(ω) (Figure 4a) and BS (Figure 4b), the observed absorption around 450 nm for 1A corresponded to the electron transitions from the valence band around Γ0 to the conduction band around Γ2 and Γ3. The valence band maximum Γ0 was mainly dominated by the inorganic lattice. The Γ2 and Γ3 positions were, contributed by the bpy π-aggregate and the inorganic lattice, respectively. Thus, the absorption band around 450 nm was mainly caused by the electron transition in the inorganic lattice and that between the inorganic lattice and the bpy πaggregate. Likewise, the absorption band around 340 nm corresponded to the electron transition within the inorganic lattice with the maximum assigned to Γ0→Γ4 in the BS. Similar to the case found in the literature,45 the emerged absorption band in the 380–900 nm range after coloration was assigned to the electron transition of the bpy cation radicals. To understand the reason for the increase of the σ value after coloration, RDG isosurfaces39 and intermolecular electronic coupling were calculated, and valence band XPS spectra data were measured. In the semiconducting



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Figure 4. a) Experimental (exp) electronic absorption spectra (top) and calculated (calc) anisotropic electronic absorption spectra (middle) and dielectric spectra (imaginary part; bottom). b) BS (left) and PDOS for all Pb atoms, Cl atoms, carboxylate O atoms, bpy and water (right). The Fermi level is set to zero. Γ0–Γ4 denote the maxima of density of states in the corresponding regions. c) Color-filled RDG map of a CV dimer with the isovalue set to 0.5 au. The surfaces are colored on a blue-green-red scale according to values of sign(λ2)ρ, ranging from –0.04 to 0.04 au. Blue indicates strong attractive interactions, and red indicates strong nonbonded overlap. d) Valence band XPS spectra for 1A and 1B (circle: experimental data; blue/red solid lines: the sum of the resolved peaks). e) Linear response of Iirr – Idark to applied voltage U at accumulated irradiation time of 60 min. Inset: plot of relative current change versus U. f) Change in relative current of 1A under different irradiation wavelengths. g) Change in rela2 tive current of 1B under different irradiation wavelengths. Irradiance of the Xe lamp with each bandpass filter is ∼10 mW/cm for all electric tests.

1-D organic π-aggregates, strong cation–π charge transfer interactions among the CV components can be clearly observed from the dispersive bandwidths (X → Γ, approximately 229 meV) in the band structure (Figure 4b) and the gradient isosurfaces (Figure 4c). Electronic coupling can describe the strength of electronic interactions. The 1D organic π-aggregate benefitted transport of electrons instead of holes, because electronic couplings between two π-stacking CV molecules (a CV dimer) were 231.6 and 553.2 meV for holes and electrons, respectively. After accepting an electron, the electronic couplings in the CV dimer increased to 444.1 and 664.6 meV for holes and electrons, respectively, consistent to the increase of the σ value (Figure 3d). A comparison of valence band XPS spectra data46,47 (Figure 4d) with the calculated PDOS (Figure 4b) showed that the density of states near the top of the valence band increased after coloration. The carriers from intrinsic thermal excitation originate mainly from electron states near the top of the valence band. The increase of the density of states near the top of the valence band after coloration indicated that the carrier density increased,48 in accordance with the increase of the σ value. The fact that the activation energy Ea (Figure 3d) decreased noticeably after coloration also supported this point, because a smaller activation energy indicates a larger charge density.49 To gain further insight into the origin of photocurrent of 1, studying the change in current was necessary. The magnitude of the relative current changes, ∆I = (Iirr – Idark)/Idark was related to the wavelength (λ) and irradiance (M) of the light source and the applied voltage (U). For different λ, the differences between the light-induced

current Iirr and the ‘dark’ current Idark were linear in U (Figure 4e), such that ∆IM,λ(U) was basically independent of U.23 Furthermore, at constant λ, the change of ∆IU,λ(M) has positively correlated with the irradiance M (Figure S15). As shown in Figure 4f, at constant U and M, the rate of increase in ∆IU,M(λ = 300–380 nm) was faster than those of ∆IU,M(λ = 410–490 nm) and ∆IU,M(λ = 550–630 nm) at the initial stage of coloration. Thus, the contribution of increased conductance by the PIET process mainly originated from electron transition within inorganic lattice, instead of that between the inorganic lattice and bpy cations. As the PIET process proceeded, the absorption bands in the 400–900 nm range assigned to bpy cation radicals became stronger (Figure 4a). At last, the ∆IU,M(λ = 550– 630 nm) was close to ∆IU,M(λ = 410–490 nm), and both ∆IU,M(λ = 550–630 nm) and ∆IU,M(λ = 410–490 nm) were larger than ∆IU,M(λ = 300–380 nm) (Figure 4g). Therefore, the contribution of increased photocurrent jump mainly originated from the absorption transition of bpy cation radicals.

CONCLUSIONS In summary, we successfully design and synthesize a novel photochromic semiconductor with an inorganic– organic hybrid nanoribbon array structure, by incorporating a photoactive viologen zwitterion into the lead-halide system in the coordinating mode. This semiconductor has high stability against light, heat, and moisture. After PIET, it yields a long-lived charge separated state with a broad absorption band covering the 200–900 nm region while increasing its conductance and photoconductance. This work is the first to modify the photoconductance of semi-


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conductors by PIET, and the increasing times of conductance represents a record for photoswitchable semiconductors. The increase of photocurrent comes mainly from the semiconducting organic π-aggregates, which indicates a chance to improve photoconductance of lead-halide semiconductors by modifying the organic component. This work offers a new design strategy to improve absorption range, conductance, photoconductance and stability of lead-halide semiconductors, which may help in the exploration of more light absorbers for solar cells.

ASSOCIATED CONTENT Supporting Information Supplementary information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant numbers 21373225, 21221001, and 21471149), the Natural Science Foundation of Fujian Province (grant number 2014J07003), Key Scientific Research Project of Chinese Academy of Sciences (grant numbers QYZDBSSW-SLH020, QYZDJ-SSW-SLH028), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.

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Journal of the American Chemical Society SYNOPSIS TOC. A Design Strategy for Improving Optical and Electrical Properties and Stability of Lead-Halide Semiconductors Cai Sun, Gang Xu, Xiao-Ming Jiang, Guan-E Wang, Pei-Yu Guo, Ming-Sheng Wang,* and Guo-Cong Guo*


9


Graphic for TOC 42x24mm (300 x 300 DPI)


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Figure 1 in the manuscript 49x13mm (300 x 300 DPI)





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Design Strategy for Improving Optical and Electrical Properties and

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