SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $57,&/(6 mater.scichina.com link.springer.com . . . . . . . . . . . . . . . . . . . . . . . . . . . . Published online 29 August 2017 | doi: 10.1007/s40843-017-9078-1 Sci China Mater 2017, 60(9): 829–838 (QKDQFHG SHUIRUPDQFH RI VRODU FHOOV YLD DQFKRULQJ &X*D6 TXDQWXP GRWV Jinjin Zhao1,2,3† , Zhenghao Liu1,3†, Hao Tang4, Chunmei Jia1, Xingyu Zhao1, Feng Xue5, Liyu Wei1,3, Guoli Kong1, Chen Wang1 and Jinxi Liu1 ABSTRACT Ternary I–III–VI quantum dots (QDs) of chalcopyrite semiconductors exhibit excellent optical properties in solar cells. In this study, ternary chalcopyrite CuGaS2 nanocrystals (2–5 nm) were one-pot anchored on TiO2 nanoparticles (TiO2@CGS) without any long ligand. The solar cell with TiO2@CuGaS2/N719 has a power conversion efficiency of 7.4%, which is 23% higher than that of monosensitized dye solar cell. Anchoring CuGaS2 QDs on semiconductor nanoparticles to form QDs/dye co-sensitized solar cells is a promising and feasible approach to enhance light absorption, charge carrier generation as well as to facilitate electron injection comparing to conventional mono-dye sensitized solar cells. Keywords: CuGaS2, quantum dots, TiO2 nanoparticles, solar cells, photo-anode INTRODUCTION Quantum-dot-sensitized solar cells (QDSSCs) have gained more attention as a promising option for next-generation solar cells [1–5] due to the quantum confinement effect [6], large dipole moment, high molar coefficients, multiple-exciton generation (MEG), low cost and facile fabrication [7–12]. The chalcopyrite semiconductor quantum dots (QDs), such as CdS(Se) [13–20], PbS(Se) [21–24], SnSe2 [25], InAs [26,27], Sb2S3 [28], were introduced into QDSSCs as light-harvesting sensitizers via various methods. Since photons with lower energy could be absorbed, the chalcopyrite semiconductor QDs are considered as a promising candidate for the high-efficiency solar cells 1 2 3 4 5 ‚ [29]. The reported semiconductor QDs in QDSSCs were mainly attributed to the binary semiconductor materials. To date, a few ternary semiconductor QDs have been reported in the field of solar cells [30]. Inspired by these researches, the ternary I–III–VI QDs of AIBIIIC2VI (A = Cu, Ag; B = Al, Ga, In; C = S, Se, Te) chalcopyrite semiconductors are expected to exhibit excellent optical properties [31,32]. Recently, some of them have been reported applied on solar cells, such as CuInS2, CuInSe2, CuInSexS2−x and CuInxGa(1−x)S2 [33–38], which attracted great attention to serve as Pb/Cd-free light-harvesting materials in QDSSCs. Among these ternary semiconductors, CuGaS2 is the most promising ternary compound due to its conductivity, normally p-type [39], large direct band gap energy [40], facile fabrication [38,41], and environmental friendliness. CuGaS2 nanocrystals exhibit excellent activity in solar water splitting [42,43], biological and chemical sensing [44,45]. Ascribing to its wide band gap energy (2.2–2.5 eV) [46,47] corresponding to the green light region and direct transition [29,48–51], CuGaS2 nanocrystals could be an optimum candidate for the QDSSCs. However, very few studies on CuGaS2 QDSSCs were reported. A survey is essential for further experimental efforts in CuGaS2 QDSSCs in order to achieve ecofriendly processes in preparative protocols. Herein, the CuGaS2 QDs directly and homogeneously grew on TiO2 nanocrystals (named as TiO2@CGS) for QDSSCs by a vacuum one-pot nanocasting method. CuGaS2 QDs with particle size of 2–5 nm were anchored on the surface of TiO2 nanoparticles directly without organic School of Materials Science and Engineering, Department of Engineering Mechanics, Shijiazhuang Tiedao University, Shijiazhuang 050043, China Engineering Research Center of Nano-Geo Materials of Ministry of Education, China University of Geosciences, Wuhan 430074, China Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195-2120, USA Hesteel Group Technology Research Institute, Shijiazhuang 050000, China These authors contributed equally to this work. Corresponding authors (emails: jinjinzhao2012@163.com (Zhao J); liujx02@hotmail.com (Liu J)) September 2017 | Vol. 60 No. 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Science China Press and Springer-Verlag Berlin Heidelberg 2017  $57,&/(6                          SCIENCE CHINA Materials í3.0 - eí CB í4.0 eV í4.0 - eí CB í3.85 eV - eí CB í3.96 eV VB í5.45 eV + h+ í5.0 í6.0 VB í7.2 eV í7.0 VB í6.56 eV + h+ + h+ í8.0 FTO TiO2 CuGaS2 N719 Ií/I3í Ee (eV) Vacuum level Figure 1 A schematic illustration of the relative band energy levels for charge transfer in the FTO/TiO2/CGS/N719 electrolyte. E) linker molecules. The relative band energy levels for holes and electrons transfer in the FTO/TiO2/CGS/N719 electrolyte based QDSSCs are illustrated in Fig. 1. Because of the quantum confinement effects, the CuGaS2 QDs hold a higher conduction band (CB) edge, which facilitates the photoelectron injection from the excited CuGaS2 QDs into TiO2 nanoparticles [52–54]. Moreover, the CuGaS2 QDs in the TiO2@CGS nanocrystals could act as complimentary sensitizers to enhance the light harvesting properties in dye sensitized solar cell (DSSC). sequently, the solution was filtered and the products were washed with deionized water and dried at 80°C. Finally, the obtained powders were sintered at 610°C for 10 min after being kept at 300°C for 90 min and 500°C for 240 min. During the entire sintering process, the heating rate was kept at a constant of 2°C min−1. After sintering, the products were dispersed in deionized water and nitric acid (65%) until the pH of this suspension reached to about 2. The suspension was vigorously stirred at 80°C for 8 h to obtain TiO2 nanoparticles. Preparation of TiO2@CGS nanoparticles TiO2 nanoparticles were put into a sealed container and subjected to vacuum. After vacuumed for 30 min, the mixed solution of GaCl3 (0.2 mol L−1) was allowed to enter into the vacuum system, and then held for 10 min before the solvent was fully removed by evaporation. The precipitate was dried in vacuum oven at 80°C for 8 h. After drying, the mixed powder was washed twice with ethanol and dried in vacuum oven again. This entire procedure described above was repeated twice except that the GaCl3 solution used was replaced by 0.2 mol L−1 CuCl2 aqueous solution and 0.4 mol L−1 Na2S aqueous solution, respectively. Finally, the sample was calcined in Ar gas at 500°C for 1 h to obtain TiO2@CGS nanoparticles. EXPERIMENTAL SECTION Chemicals Copper chloride (CuCl2), sodium sulfide, titanium tetrachloride (99.5%), ethanol (98%), nitric acid (65%) were bought from Sinopharm Chemical Reagent Co., Ltd; F127 (CAS No. 9003-11-6) was purchased from Sigma-Aldrich, Inc. Polyethylene glycol (PEG; 20000 in molecular weight) and gallium(III) chloride (GaCl3) were purchased from J&K. The sensitizer N719 (cis-di(thiocyanato)-N,N-bis(2, 2ʹ-bipyridyl-4,4ʹ-dicarboxylate)Ru (II)bis-tetrabuty lammonium) electrolyte, surlyn film were purchased from Yingkou Opvtech New Energy Co., Ltd. Synthesis of the TiO2@CGS film and fabrication of device Synthesis of TiO2 nanoparticles The synthesis of TiO2 nanoparticles was as the following procedure [54]. 2.97 g F127 was dissolved in 36.88 g ethanol at 40°C and stirred for 30 min to obtain a clear solution. After that, 3.4 g TiCl4 was added into the prepared clear F127 solution. The precursor solution was then placed into a Teflon-lined stainless steel autoclave (100 mL in capacity) after stirring for 8 h under 40°C. The autoclave was placed in an oven at 160°C for 16 h. Sub- Preparation of mesoporous TiO2 films and TiO2@CGS film The mesoporous TiO2 films were prepared by the doctorblade method [55]. The TiO2 paste was made from the TiO2 nanoparticles synthesized in the previous section. Briefly, 0.8 g TiO2 was added into a mixed solution of ethanol/deionized water (3:1) and ultrasonicated for 30 min after the addition of PEG aqueous which acted as the pore-forming material. After that, the mixture was grinded into ropiness in agate mortar. A mask, with a window encompassed by 3M scotch tape, was used to define the 5 mm × 5 mm area which was used to spread the paste dropped on edge of the window with a glass slide on the fluorine doped tin oxide (FTO) conductive glass (SnO2:F coated glass). Subsequently, the as-prepared TiO2 films were sintered in air with a heating rate of 2°C min−1 from room temperature to 300°C for 30 min, and to 500°C for 60 min. The synthesis of mesoporous TiO2@CGS film was as the same procedure as the one described above. Fabrication of quantum dot-dye bilayer-sensitized solar cells (QDBSC) The mesoporous TiO2 film and TiO2@CGS film were sensitized with N719 dye by direct adsorption. Firstly, the                                                                      © Science China Press and Springer-Verlag Berlin Heidelberg 2017 September 2017 | Vol. 60 No. 9 SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $57,&/(6 RESULTS AND DISCUSSION To characterize the phase structures and crystal size, the XRD pattern of the TiO2@CuGaS2 nanoparticles is shown in Fig. 2, which well matches with that of anatase phase TiO2 (JCPDS 21-1272) and CuGaS2. Although some peaks of TiO2 and CuGaS2 were seen overlapped, the XRD of CuGaS2 exhibited several diffraction peaks in consistent with 2θ values of 29.12°, 48.07°, 48.60°, 57.17°, 58.15°, and 70.36°, and these peaks can be attributed to (112), (220), (204), (312), (116), and (400) planes of CuGaS2 (JCPDS 25-0279). The XRD diffraction features of TiO2@CuGaS2 sample indicated the composite materials of anatase TiO2 and gallite CuGaS2, which conformed well with the electron diffraction patterns of TEM in Fig. 3. Fig. 3 shows the TEM images of TiO2@CGS nanocrystals and bare TiO2 particles. The TEM images of bare TiO2 nanoparticles in Fig. 3a and the synthesized TiO2@ CGS nanocrystals in Fig. S1 and Fig. 3b show that the diameter of the prepared TiO2 particles is about 20–50 nm, and the CuGaS2 QDs have an average size of approximately 2–5 nm. The high resolution TEM images in Fig. 3c exhibit the crystalline structure of the synthesized CuGaS2 QDs. Fig. 3d is the spectra taken via energydispersive spectroscopy (EDS) for TiO2@CGS nanocrystals, which shows the existence of Cu, Ga, S elements in the QDs with the ratio of Cu/Ga/S to be 1.04 (±0.1):1.0:1.90(±0.2). SEM-EDS mapping of TiO2@CGS nanocrystals is shown in Fig. 4 to characterize the elemental distribution. The molar ratio of CuGaS2 in TiO2@CGS could be determined to be 5.4% by EDS analysis and the QDs are off-stoichiometric CuGaS2 with a uniform elemental distribution. 250 ◆ 200 TiO2 ◆ CuGaS2 # (112) (101) # # ◆ (220) (400) (215) ◆ ◆ (204) (211) (312) (116) (200) ◆ (105) ◆ # # (116) 50 (112) (004) 100 (204) (220) # 150 (103) Characterization X-ray diffraction (XRD) patterns of powders were obtained using D8 Advance (Germany) diffractometer with Cu Kα radiation (40 kV and 40 mA) with a scanning rate of 4° min−1 for wide angle tests. The N2 sorption measurements were performed by using Micromeritics Tristar 3000 for mesoporosity and Micromeritics ASAP 2020 for porosimeters and microporosity at 77 K, respectively. The mesoporous specific surface area and the pore size distribution were calculated using the Brunauer–Emmett– Teller (BET) method. Scanning electron microscopy (SEM) analysis was performed on a Hitachi-S-4800 electron microscope. Transmission electron microscopy (TEM) images were obtained on a JEOL-2010F electron microscope operated at 200 kV. The UV-vis absorbance spectra were measured by a Shimadzu UV-2550 spectrophotometer. Symmetric dummy cells were used for the electrochemical impedance spectroscopy (EIS) measurements. The EIS measurements were conducted by using a computer-controlled electrochemical workstation (CHI660A, Chenhua, Shanghai) in dark. Photovoltaic measurement (J-V) was recorded with a Newport Oriel class AAA solar simulator (model 92250A-1000) equipped with a class A 300 W xenon light source powered by a Newport power supply (model 69907). The power output of the lamp was calibrated to 1 Sun (AM1.5G, 100 mW cm−2) using a certified Si reference cell (VLSI standard, S/N 10510-0031). The current-voltage characteristics of each cell were measured with a Keithley2400 digital source meter. Photovoltaic performance was characterized by using a mask with an aperture area of 0.25 cm2. The incident photon-to-current efficiency (IPCE) was measured in DC mode with a 1/4 m double monochromator (Crowntech DK242), a multi-meter (Keithley 2000), and two light sources depending on the wavelength range required (300–600 nm: xenon lamp, 300 W; 600–900 nm: tungsten-halogen lamp, 150 W). The monochromatic light intensity for IPCE efficiency was calibrated with a reference silicon photodiode. All the measurements were conducted under ambient conditions. Intensity (a.u.) as-prepared TiO2 film and TiO2@CGS film were heated to 80°C and immersed into the N719 ethanol solution (0.5 mmol L−1) for 24 h. After rinsing the film in solution by ethanol and drying in air, the desired mesoporous TiO2/N719 film and TiO2@CGS/N719 electrodes were obtained. The photovoltaic cells were assembled with the mesoporous TiO2/N719 or TiO2@CGS/N719 photoelectrode, Pt coated counter electrode, and sealing material (OPV-SN-60) with a thickness of 60 μm. Commercially available electrolyte of I3−/I− was injected into the space between the photoelectrode and the counter electrode. ◆ ◆ ◆ # 0 10 20 30 40 50 2ș (°) 60 70 80 Figure 2 XRD pattern of the TiO2@CGS nanoparticles. September 2017 | Vol. 60 No. 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Science China Press and Springer-Verlag Berlin Heidelberg 2017  $57,&/(6                          SCIENCE CHINA Materials a b 10 nm 10 nm 250 d c O Ti Counts 200 CuGaS2 (112) d = 0.31 nm 150 C 100 Cu 50 Cu Ga Ti SS CuGa 0 0 5 nm 4000 6000 Energy (keV) 2000 8000 10000 Figure 3 TEM images of (a) TiO2 nanoparticles and (b) the synthesized TiO2@CGS nanocrystals. Inset of (b) is the selected-area electron diffraction pattern of TiO2@CGS. (c) HR-TEM image of TiO2@CGS taken from the area within red square in (b). (d) Simultaneous EDS spectra of TiO2@CGS. a 5 μm c b 5 μm 5 μm d e f 5 μm 5 μm 5 μm Figure 4 SEM images (a) and (b) of TiO2@CGS; SEM-EDS elemental mapping of (c) Ti, (d) Cu, (e) Ga and (f) S. Fig. 5 shows the N2 adsorption-desorption isotherms measured for TiO2@CGS nanoparticles and naked TiO2 to characterize their specific surface areas and pore volumes. Both isotherms of TiO2@CGS and TiO2 nanoparticles exhibit hysteresis loops of type-H1, with the adsorption and desorption jumps at 0.6 and 1.0, which is characteristic for mesoporous materials. According to the Barrett- Joyner-Halenda (BJH) method and derived from the desorption branch shown in Fig. 5 and Table 1, the pore size distribution of TiO2@CGS shows a smaller mesopore size of 3.6 nm compared to 10.4 nm of TiO2 nanoparticles. The BET surface area and mesopore volume of TiO2@CGS are measured to be 22.03 m2 g−1 and 0.098 cm3 g−1, respectively, which are 51.3% and 40.2%                                                                      © Science China Press and Springer-Verlag Berlin Heidelberg 2017 September 2017 | Vol. 60 No. 9 SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $57,&/(6 120 10.4 nm 100 80 60 shorter than 400 nm, which is in the region of ultraviolet. TiO2 film with CGS could absorb light with wavelength shorter than 530 nm, and the two absorption peaks of N719 dye are at 430 and 520 nm, respectively. By sensitizing CuGaS2 QDs in TiO2/N719, the absorption spectra of the TiO2@CGS/N719 films got a significant red-shift and extended to around 700 nm. Fig. 6b shows the plots of (Ahν)2 versus hν (A = absorbance, h = Planck’s constant, and ν = frequency) for all samples, from which it is possible to extrapolate the slope near the absorption onset and to extract the energy of band gap. The curves yielded band gaps of 2.69 eV for TiO2@CGS, 2.25 eV for TiO2/ N719, and 2.03 eV for TiO2@CGS/N719, respectively. Thus, the introduction of sensitized CuGaS2 QDs could enhance the light harvesting ability in TiO2/N719, which can be attributed to the following factors: a higher intensity and a red shift of light absorption edge from 600 nm to around 700 nm. Such improvement could lead to an increased electron concentration in TiO2/N719 substrate sensitized with CuGaS2 QDs [56–62]. Co-sensitization of semiconductor QDs and organic dyes has also been investigated as an effective strategy for enrichment [54,55,63,64]. The J-V curves in Fig. 7 shows that these two absorbers also have vital contributions on the overall cell performance. The bisensitized device (TiO2@CGS/N719) is revealed to have a great improvement on its photovoltaic performance, compared to the one with monosensitizers. Table 2 lists the open circuit potential (Voc), short circuit current density (Jsc), fill factor TiO2 TiO2@CGS Pore volume (cm3 gí1 nmí1) Volume adsorbed (cm3 gí1) 0.020 40 0.015 0.010 0.005 3.6 nm 0.000 3 6 9 Pore diameter (nm) 12 15 20 0 0.0 0.2 0.4 0.6 Relative pressure (P/P0) 0.8 1.0 Figure 5 N2 adsorption–desorption isotherms and the corresponding pore diameter distribution curves of different samples (inset). less than those of TiO2 nanoparticles, in accordance with previously reported studies [54]. Smaller particle size (2–5 nm) of the CGS QDs led to a severe aggregation within the mesopores, and thus decreased the mesopore volume. Due to the larger density of CGS QDs, even with an increased surface area, the specific surface area calculated via BET method got reduced. Whereas, the decreased BET surface area and mesopore volume of TiO2@CGS are still good enough to enrich the loading capacity of the N719 dye [55]. Fig. 6a shows the UV-vis absorption spectra of bare TiO2 film and the TiO2@CGS film before and after sensitized with N719. The absorption onset for TiO2 film is Table 1 Structural parameters of TiO2 nanoparticles and TiO2@CGS 2 3 BET surface area (m g ) Mesopore volume (cm g ) TiO2@CGS 22.03 0.098 3.6 TiO2 45.24 0.164 10.4 −1 a b TiO2@CGS/N719 1.0 TiO2/N719 0.24 TiO2@CGS 0.8 TiO2@CGS/N719 Eg=2.03 eV TiO2/N719 Eg=2.25 eV TiO2@CGS Eg=2.69 eV 0.18 0.6 (AhȞ)2 Abs. TiO2 0.4 0.12 0.06 0.2 0.0 400 0.30 Mesopore size (nm) −1 450 500 550 600 Wavelength (nm) 650 700 0.00 1.8 2.0 2.2 2.4 hȞ (eV) 2.6 2.8 3.0 2 Figure 6 (a) UV-vis absorption spectra of different samples; (b) plots of (Ahν) against the photon. September 2017 | Vol. 60 No. 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Science China Press and Springer-Verlag Berlin Heidelberg 2017  $57,&/(6                          SCIENCE CHINA Materials (FF) and total energy conversion efficiency (η) of these devices. The Voc increased from 704 to 756 mV (+52 mV) and Jsc increased from 14.88 to 18.36 mA cm−2 (+23.4%) after co-sensitized with CGS. However, FF reduced from 0.572 to 0.53, mainly owing to increased charge carrier recombination rate and larger shunt resistance indicated by the slope of Jsc point of the J-V curve. Consequently, in one sun illumination condition, η of the bisensitized device with TiO2@CGS/N719 as photoanode could reach up to 7.4%, which is 23% higher than that with TiO2@N719 [61,62]. 20 TiO2@CGS Current density (mA cmí2) TiO2/N719 16 TiO2@CGS/N719 12 8 4 0 0.0 0.1 0.2 0.3 0.4 0.5 Voltage (V) 0.6 0.7 0.8 Figure 7 Photocurrent density–voltage characteristic curves of different solar cells. Attributed to the red-shift visual light absorption, TiO2@CGS/N719 co-sensitized solar cell displays a higher energy conversion efficiency. The IPCE spectra shown in Fig. 8 investigate the role of CuGaS2 quantum dots. The trend of the IPCE spectra of TiO2@CGS/N719 co-sensitized solar cell and TiO2/N719 dye sensitized solar cell are in accordance with that of UV-vis absorption spectra. The calculated Jsc values of TiO2@CGS/N719, TiO2/N719, TiO2@CGS devices are 18.1, 14.2, and 5.0 mA cm−2, respectively, in consistent with the J-V curves. By introducing the CuGaS2 QDs as a co-sensitizer, the photon-toelectron conversion efficiency got significantly enhanced. To investigate the effects of QDs on charge transport and recombination at the photoanode, EIS under dark was carried out to exhibit the representative impedance plots of cells based on TiO2@CGS, TiO2/N719 and TiO2@CGS/N719 photoanodes, shown in Fig. 9. The equivalent circuit in Fig. 9 is used to fit the impedance measurements, which is composed of resistance-capacitance pairs and a distributed element to describe redefinable characteristics of the electrode and its interfaces with the electrolyte [65]. The resistance R records recombination in the solar cell, while C relates to the carrier accumulation and splitting of thΩe Fermi levels [66]. The main semicircle in Fig. 9 is relevant to the charge transfer process at the interface of anode-electrolyte. The frequency large arcs (the semicircles from 32 to 160 Ω) are Table 2 Photovoltaic parameters of different solar cells Voc (mV) Jsc (mA cm ) FF TiO2@CGS/N719 756 18.36 0.53 7.4 TiO2@CGS 672 5.26 0.649 2.29 TiO2/N719 704 14.88 0.572 6 −2 100 η (%) 60 Rs TiO2@CGS/N719 TiO2/N719 TiO2@CGS 80 TiO2@CGS/N719 Rct1 TiO2/N719 CPE1 50 TiO2@CGS íZƎ (ȍ) IPCE (%) 40 60 40 30 20 20 0 350 10 0 400 450 500 550 Ȝ (nm) 600 650 700 750 Figure 8 IPCE spectra of the solar cells fabricated with different photoanodes. 20 40 60 80 100 Zƍ (ȍ) 120 140 160 Figure 9 Impedance spectra and the inset is the corresponding equivalent circuit.                                                                      © Science China Press and Springer-Verlag Berlin Heidelberg 2017 September 2017 | Vol. 60 No. 9 SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $57,&/(6 ascribed to the anode/electrolyte interfacial charge transfer resistance or recombination (Rct1) in parallel with the capacitance (CPE1) in the TiO2-based anodes and the anodes/electrolyte interface. The diameter of the semicircle exhibits the charge transfer resistance (recombination) at the anodes/electrolyte interfaces [8,67,68]. Obviously, comparing devices with TiO2@CGS/N719 and TiO2/N719 as photoanodes, the TiO2@CGS/N719 based cell has a smaller diameter of the semicircle, which exhibits a decreased charge recombination resistance and a certain portion of the injected charges are lost by electron transfer from TiO2 to I3− in the electrolyte. Meanwhile, the TiO2@CGS/N719 photoanode with doped CGS QDs and dye shell decreased the direct contact surface area between bare TiO2 surface and the electrolyte, which reduced the charge recombination resistance. 7 8 9 10 11 12 13 CONCLUSIONS Novel visible-light induced TiO2@CuGaS2 nanocrystals have been successfully synthesized via the vacuum onepot-nanocasting process, with CuGaS2 (2–5 nm) grown uniformly on TiO2 (20–50 nm). The TiO2@CuGaS2/N719 photoanode shows an excellent absorbance on broad wavelength light due to the narrow energy band gap (Eg = 2.03 eV). Photoanodes with the TiO2 and TiO2@CuGaS2 nanocrystals were fabricated and the device with TiO2@ CuGaS2/N719 has a power conversion efficiency η of 7.4%, which is 23% higher than that of monosensitized dye solar cell. Anchoring CuGaS2 QDs on semiconductor nanoparticles to form QDs/dye co-sensitized solar cells is a promising and feasible approach to enhance light absorption, charge carrier generation as well as to facilitate electron injection comparing to conventional dye sensitized solar cells. 14 15 16 17 18 19 20 Received 23 May 2017; accepted 15 July 2017; published online 29 August 2017 21 1 2 3 4 5 6 Kamat PV. Quantum dot solar cells. Semiconductor nanocrystals as light harvesters. J Phys Chem C, 2008, 112: 18737–18753 Nozik AJ. Quantum dot solar cells. Phys E-Low-dimensional Syst Nanostruct, 2002, 14: 115–120 Sargent EH. 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Electrolyte effects on electron transport and recombination at ZnO nanorods for dye-sensitized solar cells. J Phys Chem C, 2010, 114: 17880–17888 Acknowledgements The authors thank the financial support from the National Key Research and Development Program of China (2016YFA0201001), the National Natural Science Foundation of China (11627801, 51102172 and 11772207), Science and Technology Plan of Shenzhen City (JCYJ20160331191436180), the Leading Talents of Guangdong Province Program (2016LJ06C372), the Natural Science Foundation for Outstanding Young Researcher in Hebei Province (E2016210093), the Key Program of Educational Commission of Hebei Province of China (ZD2016022), the Youth Top-notch Talents Supporting Plan of Hebei Province, the Graduate Innovation Foundation of Shijiazhuang Tiedao University, Hebei Provincial Key Laboratory of Traffic Engineering materials, and Hebei Key Discipline Construction Project. Author contributions Zhao J and Liu Z designed and engineered the samples; Liu Z, Jia C and Kong G performed the experiments; Xue F and Wei L performed the structural and J-V performance measurement; Wang C and Wei L performed the data analysis; Zhao J wrote the paper with support from Liu J and Tang H. All authors contributed to the general discussion. Conflict of interest interest. The authors declare that they have no conflict of Supplementary information Supporting materials are available in the online version of the paper. September 2017 | Vol. 60 No. 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Science China Press and Springer-Verlag Berlin Heidelberg 2017  $57,&/(6                          SCIENCE CHINA Materials Jinjin Zhao obtained her BE degree in materials science and engineering from Hebei University of Science and Technology in 2005, and her PhD in materials physics and chemistry from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2010. She did her visiting doctoral studies at Max Plank Institute of Colloids and Interfaces, Germany, from October 2007 to October 2008, and visiting scholar at the University of Washington from August 2015 to August 2016. She helds faculty appointment in Shijiazhuang Tiedao University. She is interested in probing multi-physical couplings in perovskite solar cells based on dynamic photovoltaic thermal strain. Zhenghao Liu obtained his BE degree in materials physics from Inner Mongolia University of Technology in 2013, and now he is a master degree candidate in the School of Materials Science and Engineering from Shijiazhuang Tiedao University. He is interested in performing quantum dots sensitized solar cells. Jinxi Liu received his BE degree in engineering mechanics in 1982 from Liaoning University of Engineering Technology, Fuxin, China, and his MS and PhD degrees in 1988 and 1997 from Harbin Institute of Technology. He was a visiting professor at the Department of Mechanical Engineering of the University of Hong Kong under Croucher Foundation from 2000 to 2001. He is now a professor at the Department of Engineering Mechanics, Shijiazhuang Tiedao University. His research interests are the mechanics problems of photovoltaic, piezoelectric and magneto-electric materials and structures. 㗎ⰅCuGaS2㑠䓴⮄ⰵ㳂ⷀ㲌䂕⮈⧹⺃ⴝ㾵㚽⭥䁱㈠ 䍵㆛㆓1,2,3† , 㒖䎞⼧1,3†, 㲧⼧4, シ⪛㗥1, 䍵㾨䈏1, 䁇ⴆ5, 㸛㏗䈒1,3, ㋸⺛㏗1, 㶖⧠1, 㒖㆑㻓1 䍋䄋 㧞䊋I–III–VI䔆⿧㵎㌔㑠䓴⮄䔘㸋㲌䂕⮈⧹⭥㘕⿐ア⢎㻷⨗䇦䅍⭥⺃䁈㾵䐫. 㸳㗨⤪䇤䄜⤞ⳉㅌ2–5㚪㗸⭥㧞䊋⿧㵎㌔CuGaS 2㑠䓴 ⮄㗎Ⰵ䊻TiO2㚪㗸㋦㑄㩰, ⤜㵉⺞㦯⼯䇱〛⳷䓴䔘㸋㑕ㅴ䐧⡙⨗㑬TiO 2@CGSⶕ⼰⤥㑰. 䁱㈠ⳃ㻷㑠䓴⮄⼮㦟㑰TiO2@CuGaS2/N719⹓㘕 ⿐㲌䂕⮈⧹㾈㔫⫐⭞7.4%, 㼁ⰵ䇻⭆㘕⿐㦟㑰㲌䂕⮈⧹ⱙ䁵, 㡅⮈⧹㾈㔫㳂ⷀ㑬23%. CuGaS2㑠䓴⮄㗎Ⰵ䊻⟌⭝㳆㚪㗸㋦㑄䋗㣠㑬⹓㘕⿐ 㲌䂕⮈⧹⭥⺃㹝㬶㚽㑇᱃䋗ゴ㑬⮈⼪䊹㒘䓴㭞㑠, ⪺㆙㑬⮈䓴䇱㾈䓃㧌, ㉀䇱㬏⳷⺄㎌⭥䇇䇤㋶ヅ.                                                                      © Science China Press and Springer-Verlag Berlin Heidelberg 2017 September 2017 | Vol. 60 No. 9