Morphology Controlled n-Type TiO2 and Stoichiometry Adjusted p

Aug 17, 2017 - *E-mail: [email protected], [email protected] (S.V.)., *E-mail: [email protected], [email protected] ...
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Morphology Controlled n‑Type TiO2 and Stoichiometry Adjusted p‑Type Cu2ZnSnS4 Thin Films for Photovoltaic Applications S. Varadharajaperumal,*,†,§ Chinnaiyah Sripan,‡ R. Ganesan,‡ Gopalkrishna Hegde,*,† and M. N. Satyanarayana§ †

Centre for Nano Science and Engineering, and ‡Department of Physics, Indian Institute of Science, Bengaluru, 560012, India Optoelectronics Laboratory, Department of Physics, National Institute of Technology, Surathkal, Mangalore 575 025, India

§

S Supporting Information *

ABSTRACT: This paper presents the fabrication and characterization of stoichiometry adjusted Cu2Zn1.5Sn1.2S4.4 thin film (FTO/TiO2/CdS/CZTS/Au) photovoltaic (PV) devices. The PV devices were developed using the window layer of rutile TiO2 nanoarchitecture arrays, i.e., one-dimensional (1D) nanorods and three-dimensional (3D) combined/ hierarchical structures (nanorods with microspheres). Onedimensional (1D) nanorods and 3D combined structures of TiO2 window layers were synthesized by a hydrothermal method with different solvents without any assistance of surfactants and templates. We achieved two kinds of TiO2 nanostructures by tuning the precursor concentrations and volume by keeping a constant growth time and temperature. The detailed structural properties were studied using X-ray diffraction and high resolution transmission electron microscopy. Phase formation and chemical state of the prepared samples were examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The surface morphology and luminescence studies of TiO2 nanostructures were analyzed using field emission scanning electron microscopy and cathodoluminescence techniques. The current−voltage performance of fabricated devices were measured under an AM 1.5 solar simulator. It is observed that combined structure PV device shows better efficiency (1.45%) than the nanorods alone structure (0.55%). Present work is a first attempt made to construct the inverted CZTS based solar cells. This study establishes the window layer of hierarchical TiO2 nanostructures based morphology that offers a great potential for the development of high-efficiency nonstoichiometric CZTS based solar cells. and large absorption coefficient on the order of ≥10 4 cm−1.3,4,11−13 Structures with a stoichiometric ratio of Cu/ (Zn + Sn) = 0.8 and Zn/Sn = 1.2 have achieved the highest efficiency due to the formation of Cu vacancy which acts as an enhanced shallow acceptor.10,14−16 Physical vapor deposition is one of the well-established methods for the deposition of polycrystalline compound semiconductor materials. In that, thermal evaporation provides a good control on the element flux, substrate temperature, and purity of source materials.17,18 In semiconductor solar cells, to separate photogenerated electrons from the active materials, many different charge extraction strategies have been employed to design PV devices for higher efficiency. To achieve better charge separation in Cu2Zn1.5Sn1.2S4.4 (CZTS) based solar cells, many researchers have explored interfacial or buffer layers such as CdS and ZnS, etc.19 Among them, cadmium sulfide (CdS) is one of the most commonly used buffer layers due to its wider optical band gap (∼2.5 eV) and good interfacial properties with CZT(S,Se) showing the highest efficiency of 12.6%.5 Further, enhancing

1. INTRODUCTION Energy is needed for a wide range of applications such as transportation, industrial, agricultural, aerospace, household and office requirements, etc. The energy requirements are increasing every year, and current forms of available energy resources will not be able to meet the above requirements. In the future, finding proper energy sources to meet the world’s growing requirement is one of society’s foremost challenges for the next half century. Renewable energy sources, particularly solar energy, is gaining considerable interest as an alternative to other sources of energy, such as fossil fuels and nuclear energy.1 In recent years, thin film based solar cells have become one of the promising candidates for power generation and other integrated photovoltaic (PV) applications, as part of an effort to develop new renewable energy technologies. Especially, recently chalcopyrite semiconductor systems, such as CuInSe2, CuInS 2 , Cu(In,Ga)Se 2 , Cu(In,Ga)S 2 , Cu 2 ZnSnS 4 , and Cu2ZnSnSe4, have attracted a great deal of interest as potential absorber materials for thin film solar cells.2−5 This is because the constituting elements of these chalcogenide materials are less toxic, are most abundant in the Earth’s crust, have a simple fabrication process, and are inexpensive.6−10 This compound semiconductor has an optimal optical band gap of 1.4−1.5 eV © 2017 American Chemical Society

Received: May 4, 2017 Revised: August 5, 2017 Published: August 17, 2017 5154

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Figure 1. Schematic of (a) FTO/TiO2 nanorods/CdS/CZTS/Au and (b) FTO/TiO2 combined structures/CdS/CZTS/Au photovoltaic devices.

(TiO2 nanostructured films) such as microspheres with nanorods on fluorine doped tin oxide (FTO) substrate. Different nanostructured morphologies were successfully obtained at constant 180 °C for 60 min growth time by changing the precursor concentrations and volume under the more acetic conditions. Further, we synthesized a stoichiometric adjusted CZTS thin film with copper poor and zinc rich, a ratio of Cu/(Zn + Sn) = 0.74 and Zn/Sn = 1.25 deposited as an absorbing layer using the thermal evaporation technique. To the best of our knowledge, the present work is a first attempt made to construct the inverted CZTS based solar cells. We have replaced the conventional i-ZnO with n-type TiO2 1D/3D nanostructures as a window layer along with active layer of stoichiometry adjusted CZTS and explored the effect of such architecture on PV performance.

the light harvesting efficiency (LHE) with fast electron transportation and reduced charge recombination has to consider choosing an effective and excellent photoanode or window layer materials along with CdS/CZTS geometry. A wide band gap semiconductor titanium dioxide (TiO2) is one of the promising and most widely used materials in PV applications. Its good transparency in the visible region, low cost, nontoxicity, and large refractive index make it an ideal ntype material in dye sensitized solar cells (DSSCs), quantum dot (QD) solar cells, QD-DSSCs, perovskite and CZTS solar cells.20−25 Crystalline TiO2 has three polymorphs such as anatase, rutile, and brookite. In that, lower band gap (3.02 eV) rutile phase is a more stable state than anatase (3.23 eV), which means rutile has stronger absorption in the near-ultraviolet (UV) and visible regions (360−415 nm) than the anatase phase.26 In general, the rutile phase has a lower reactivity than anatase in solar cell applications, because of its higher rates of electron and hole recombination, but at the same time, rutile has better photoscattering reactivity than the anatase phase.27 It is reported that DSSCs have shown the higher significant power conversion efficiency (PCE) with the large surface area of TiO2 nanoparticles as a window layer.20,28 On the other hand, because of its large surface area which enhances the electron trap among the grain boundaries, it resulted in reduction of net electron transport and creates higher recombination rate at their interfaces.29 Hendry et al. had demonstrated that the electron mobility is strongly dependent on material morphology.30 The highly oriented 1D nanostructured TiO2 such as nanorod and nanotube are the key parameters for an uninterrupted electrical pathway for the photogenerated electrons toward the bottom electrode.31,32 A variety of synthesis methods have been well studied and explored to grow nanorods, which includes chemical vapor deposition, glancing angle deposition, oblique angle deposition, and hydrothermal technique.31,33−35 Among them, a hydrothermal method is simple, cost-effective, and suitable for a large area process. The properties of crystalline size and morphology can be tuned by changing the parameters of reaction temperature, growth time, pH, precursor’s volume, and its concentration.31 During the past few decades, lots of research activities have been going on to improve the charge separation and quick electron transportation by constructing TiO2 hierarchical like nanostructures within the solar cell architecture.36,37 In the present study, we used a single step hydrothermal method to deposit surfactant free highly oriented TiO2 1D nanorods and 3D-hierarchical or combined nanostructures

2. EXPERIMENTAL DETAILS 2.1. Preparation of TiO2 Nanostructures. The rutile phase TiO2 nanostructured films were synthesized on FTO coated glass substrate (resistivity 7 Ω/square) without any surfactants by a single step hydrothermal method. Initially the FTO substrates were cleaned by ultrasonication with 1:1:1 ratio of acetone, 2-propanol, and deionized (DI) water for 15 min, following that it was dried under N2 gas before further use. TiO2 seed layer (20 nm) was deposited on FTO substrates by DC magnetron sputtering, and then the substrates were air annealed in a furnace at 450 °C for 30 min. A mixture of a 2:1 ratio concentrated hydrochloric acid (35−38%) and DI water were sonicated at ambient temperature for 5 min. For the growth of TiO2 nanorods, 0.5 mL of titanium(IV) isopropoxide (TTIP) precursor and in the case of TiO2 combined structures a 1:1 ratio of 0.5 mL of TTIP precursor and titanium(IV) butoxide (TBO) precursors were added dropwise into the mixtures. Both solutions were further sonicated at ambient temperature for 5 min, and the resultants were poured into 50 mL Teflon lined stainless steel autoclaves. The seed layer coated TiO2 (20 nm)/FTO substrates were placed inside the autoclave with the conducting surface facing up. The sealed autoclaves were heated in oven at 180 °C with the growth time of 60 min. After completion of hydrothermal process, the autoclaves were cooled down slowly to reach room temperature, and then the films were thoroughly rinsed with DI water. 2.2. Preparation of CdS and CZTS Thin Films. A CdS buffer layer (50 nm) was deposited by the chemical bath deposition (CBD) method using the precursor of cadmium sulfide (CdSO4), thiourea (CH4N2S), and ammonium hydroxide (NH4OH). A total of 0.0015 mol of CdSO4 dissolved with 35 mL of DI water, followed by 7.8 mL of NH4OH (1.5 mol) solution was vigorously added (solution A). A total of 0.05 mol of CH4N2S dissolved in 35 mL of DI water (solution B) was added dropwise into the mixtures of solution A. The TiO2 nanostructured thin films were immersed into the prepared solution contained in a beaker. The solution was modestly stirred, and 5155

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temperature was maintained around 70 °C to form a CdS buffer layer (yellowish) for 3−4 min. The deposition lasted for 7 min (∼50 nm), and then the samples were rinsed with DI water and allowed to dry at room temperature. CZTS films of about 1 μm thick were deposited over the FTO/ TiO2 nanostructured films/CdS by the thermal evaporation method at 150 °C substrate temperature using single source material. In order to achieve Cu poor and Zn rich conditions for harvesting higher efficiency, the composition of Cu, Zn, Sn, and S was mixed with a ratio of 2:1.5:1.2:4.4 and melted at 950 °C for 24 h from our previous research work.38,39 The as-deposited films were annealed at 450 °C for 15 min, and the complete device fabrication was carried out in the sandwich geometry of FTO/TiO2 (1D and 3D)/CdS/CZTS/Au. 2.3. Characterization Techniques. The surface morphology of fabricated thin films was studied using FESEM (Carl ZEISS-ULTRA 55 FESEM), and all images were taken at 5 keV (accelerating voltage). The luminescence property of annealed TiO2 nanostructured films was characterized using cathadoluminescence (CL) (Gatan Mono CL4) which is attached with ZEISS-ULTRA 55 FESEM. The CL spectra were recorded at the condition of ∼20 nA beam current, 20 keV as an accelerating voltage and in the wavelength range from 300 to 1000 nm. Structural analysis of the TiO2 nanostructured films and CZTS thin films were carried out by XRD using Cu−Kα source (wavelength = 1.5405 Å) with a diffraction angle from 10° to 80° (Rigaku Smart lab). The detailed structural properties were analyzed using HRTEM. The samples were prepared by mixing a few milligrams of TiO 2 nanostructured films with isopropyl alcohol of concentration 0.2 g/ mL followed by ultrasonication for 20 min. The solution was dispersed in carbon coated copper grid and then was dried at 100 °C for 2 h. The phase formation of TiO2 nanostructured films and CZTS thin films were investigated by Raman spectroscopy (HORIBA Jobin YVON Lab RAM HR800 spectrometer) using 532 nm as the excitation wavelength. The chemical state analysis of TiO2 nanostructured films were performed by XPS. The XPS survey along with core-level high resolution spectra were obtained by monochromatic Al Kα X-ray (1486.6 eV) at the vacuum level of 10−9 Torr. The measurement was carried out at an applied beam current of 9 mA and acceleration voltage of 13 keV (117 W). Ultra-violet photoelectron spectroscopy (UPS) studies were carried out to find the valence band maximum (VBm) of TiO2 using the source energy of He I (21.22 eV) (Kratos Axis Ultra DLD). The diffused reflectance spectra (DRS) of TiO2 nanostructured films and absorption spectrum of CZTS films were recorded by the UV−visible spectrometer (Shimadzu MPC3600) in the range of 250−500 nm and 500−1100 nm at room temperature, respectively. Current−voltage (I−V) performance of the fabricated devices was measured using Keithley 2420 with the light source of 1000W (Newport) oriel solar simulator under AM 1.5 condition.

Figure 2. SEM images of (a,b and c,d) top and (e,f) cross sectional view of TiO2 nanostructured films.

microspheres on the FTO substrate. Figure 3a,b indicates the cross-sectional view of CZTS/CdS/TiO2 nanostructured films/

Figure 3. . Cross-sectional FE-SEM image of devices (a) annealed Cu2Zn1.5Sn1.2S4.4/CdS/TiO2 nanorods/FTO and (b) annealed Cu2Zn1.5Sn1.2S4.4/CdS/TiO2 combined structures.

3. RESULTS AND DISCUSSION The surface morphology of as-deposited TiO2 nanorods prepared by 0.5 mL of TTIP and TiO2 combined structures by 0.5 mL of TTIP and 0.5 mL of TBO with a 2:1 ratio of HCl and DI water at 180 °C for 60 min shown in Figure 2a−f, respectively. The FESEM images in Figure 2a,b clearly confirm the highly oriented TiO2 nanorods, which are of tetragonal shape with square top facets and were formed more densely and uniformly on the FTO substrate. The nanorods have an average diameter of ∼50 nm and length of ∼2.5 μm. In the case of combined structures, due to the addition of another titanium precursor of TBO with TTIP, there was a formation of a marigold-like structure on top of the nanorods resulting in an increase in their length to ∼6 μm (nanorods along with microspheres) and an average diameter of the microsphere is ∼8 μm. Figure 2c,d revels the highly dense microspheres uniformly dispersed on the surface of nanorod arrays. Figure 2e,f depicts the cross section FESEM images of highly oriented vertical TiO2 nanorods and nanorods along with

FTO films, which shows the CdS anchored on top of the TiO2 nanostructured films and uniform deposition of CZTS films over the CdS/TiO2 nanostructured films. Figure S1a−d shows FESEM images of as-prepared and 450 °C annealed devices of TiO2 nanostructured films. It revealed that due to annealing, the formation of recrystallization and increased grain size compare to the as-prepared CZTS films over the CdS/TiO2 nanostructured films. However, the deposited films were smooth, homogeneous, and uniform, but in the case of annealed films the presence of voids or cavities due to deficiency of the elements was noted. The XPS analysis was performed to study the surface and subsurface level composition and chemical states (depth information up to ∼10 nm) of rutile phase TiO2 nanostructured films (all XPS spectral peaks were fitted with Vision Processing software). Figure 4a−c shows the survey spectra of TiO2 nanorods and TiO2 combined structures, and the peak presents binding energies of 529.6 and 458.4 eV corresponding to O 1s and Ti 2p3/2, respectively. Figure 4b shows Ti 2p 5156

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Figure 4. XPS spectra of TiO2 nanostructured films (a) wide, (b) Ti 2p, (c) O 1s.

Figure 5. Raman spectra of (a) TiO2 nanostructured films (b) Cu2Zn1.5Sn1.2S4.4 thin films.

(Eg), and 608 cm−1 (A1g) in the as-prepared nanorod, and combined structures were attributed to Ti−O−Ti vibrations and are the characteristic peaks of a rutile TiO2 crystal system.24,40,42,43 The sharpness of the peaks and their intensity signify that the sample is highly crystalline and pure. We could not observe blue-shift of the Eg mode among the samples, and the TiO2 nanostructures can be ascribed to the characteristic vibrational modes of the rutile phase which is consistent with XPS analysis. Quaternary Cu based chalcogenide solar cells (Cu-poor and Zn-rich) showed the highest energy conversion, due to the formation of VCu, ZnCu, and antisite defect CuZn. These defects reduce the secondary phase formation and show enhancement in its tunable optical and electrical properties, which makes it a promising material for thin film solar cells.5,15,44−46 Figure 5b shows the Raman spectra of as-prepared and annealed CZTS thin films. The most intense mode at 338 cm−1 as well as the weaker modes at 287 and 369 cm−1 are close to the reported Raman modes of CZTS.2,10,24,39,47−49 Himmirch et al. reported the strongest peak (338 cm−1) was attributed to the A1 symmetry, and it is related with the vibration of the S atoms.50 According to that, it confirms the presence of a CZTS primary kesterite structure (338 cm−1) and also a secondary peak at 287−288 cm−1. An increase in their peak intensity indicates a slight improvement in the crystallinity with respect to the annealing temperature at 450 °C. There is no evidence of

spectrum was attributed to the binding energies of 458.4 and 464.1 eV, assignable to 2p3/2 and 2p1/2, respectively, and they were separated from each other by 5.7 eV. This indicates that the identical chemical state of Ti atoms for all rutile TiO2 samples was of Ti4+. It also confirms that there were no Ti3+ signals observed in Ti 2p spectra. For O 1s spectra (Figure 4c), a well-formed binding energy value of 529.6 eV was attributed to the oxygen in TiO2. One shoulder at 533.1 eV was observed and is attributed to hydrolysis for samples prepared by the hydrothermal method.40,41 The C 1s spectral line 284.6 eV was taken as reference for the shift correction of Ti 2p and O1s spectra (Figure 4b,c). In both TiO2 nanostructured films we could not find any peak positions shift and extra shoulder peaks, while it showed variation only in intensity. It clearly reveals that there is no change in their composition and phase or chemical state; both confirm the formation of TiO2 phase. Figure 5a shows the laser Raman spectra of TiO 2 nanostructured films which were taken at room temperature with a green (532 nm) laser in back scattering mode. On the basis of the space group D4h14 for rutile and assumed site symmetries for the Ti and O atoms within the unit cell, grouptheoretical analysis shows four Raman-active “lattice vibrations” assigned as follows: A1g (610 cm−1) + B1g (144 cm−1) + B2g (827 cm−1) + Eg (446 cm−1). The typical Raman bands of rutile phase became clear at 237 cm−1 (two phonon bands), 446 cm−1 5157

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Figure 6. XRD patterns of (a) TiO2 nanostructured films (b) Cu2Zn1.5Sn1.2S4.4 thin films.

Figure 7. HRTEM and FFT images of TiO2 (a) nanorods (b) combined structures.

Figure 8. Spectral behavior of (khν)2 versus photon energy for TiO2 (a) nanorods (b) combined structures (c) (αhν)2 as a function of photon energy (hν) for CZTS thin films (d) schematic band diagram of FTO/nanostructured TiO2/CdS/CZTS/Au devices.

TiO2 combined structures that confirm (101) and (110) oriented growth, respectively.30,36,40 No other significant peaks were detected by XRD, indicating the fabricated samples were pure-phase of rutile TiO2 with the scanning Bragg’s angles 2θ range from 20° to 90°. However, in the case of TiO2 nanorods, the (110) peak was noticeably small compared to the highest diffraction intensity (101) peak. The highly intense (101) peak along with the small (002) peak in the nanorods film suggests that the rutile crystal grows with (101) plane parallel to the FTO substrate. Because of a change in the precursor volume

impurities such as binary and ternary sulfide phases found in the Raman spectra of thin films annealed at 450 °C. The crystal structure and phase confirmation of TiO2 nanostructured films were examined by XRD. Figure 6a shows the XRD patterns of TiO2 nanostructured films deposited on FTO substrates, and it confirms that both are highly crystalline with rutile phase. Evidently, all the diffraction peaks of TiO2 nanostructured films correspond to tetragonal rutile phase TiO2 (JCPDS card no. 21-1272). Very strong rutile peaks were observed at 27.4° for TiO2 nanorods and 36.1° for 5158

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Figure 9. CL spectra of TiO2 (a) nanorods (b) combined structures.

Figure 10. UPS spectra of TiO2 (a) nanorods (b) combined structures.

(0.5 mL of TTIP and 0.5 mL of TBO) for TiO2 combined structures, the (110) peak had higher intensity than the (101) peak and it clearly shows the microspheres were grown well at the (110) direction along with nanorods. The XRD patterns of as-prepared and annealed Cu2Zn1.5Sn1.2S4.4 thin films are shown in Figure 6b. The CZTS structure is a tetrahedrally coordinated system, where each sulfur anion is bonded with four cations (Cu, Zn, and Sn), and each cation is bonded with four other sulfur anions. In general, the phase purity of kesterite layers was difficult to confirm unambiguously, because of possible secondary chalcogenide phases such as Sn2S3, Cu2S, ZnS, SnS, and Cu2SnS3, which show a very similar XRD pattern.10,38,51−54 Asdeposited Cu2Zn1.5Sn1.2S4.4 film consists of Sn2S3 impurity peaks along with the CZTS phase. After annealing in nitrogen atmosphere at 450 °C the Sn2S3 impurity phase peaks totally disappeared and improved peak intensity of the CZTS phase (see Figure 6b). It can be inferred that, at and above this annealing temperature, all the elements in CZTS combined to form a single phase. In the case of stoichiometry adjusted CZTS polycrystalline, the formation of a single kesterite phase occurring at a lower annealing temperature as well as the tunable optical bandgap with enhanced carrier concentrations makes it a suitable absorbing layer in thin film solar cell applications.39,55 Figure 7a,b shows the high resolution TEM images and fast Fourier transform (FFT) patterns of TiO2 nanostructured films petals, respectively. The upper inset image in Figure 7a shows the lattice fringe at 1.47, 2.19, and 3.25 Å, indicating that the crystal was formed along the (001), (111), and (110) planes, respectively. The corresponding FFT further verified that the nanorods are of highly oriented crystalline nature. These results

indicate a good crystalline quality of the obtained material, which is consistent with the XRD results shown in Figure 6a. The optical band gap of TiO2 nanostructured films were calculated from the Kubelka−Munk function (khν)2 vs incident photon energy plot, and the values were obtained at 3.12 and 3.2 eV respectively (Figure 8a,b). It is in good agreement with the reported value of rutile phase TiO2 nanostructures.56 The optical band gap of CdS (2.42 eV) (Figure S2) Cu2Zn1.5Sn1.2S4.4 thin films were determined (Figure 8c) by extrapolating the linear region of the plot of (αhν)2 versus photon energy (hν) and taking the intercept on the hν axis where y = 0. The band gap of the absorbers 1.75 and 1.63 eV were determined for as-prepared and 450 °C annealed thin films, respectively. The decreased band gap in 450 °C annealed film may be attributed to the reduction in the number of unsaturated defects and the consumption of binary and ternary compounds for the formation of CZTS. Room-temperature CL measurement was used to study the luminescent properties of fabricated TiO2 nanostructured films, as presented in Figure 9a,b. The curves were deconvoluted into two peaks, in the case of TiO2 nanorods at 420 and 495 nm and for TiO2 combined structures into three peaks were at 416, 458, and 525 nm. The peaks at 420 and 416 nm correspond to the emission of free excitons by titania groups near defects. The broad peak from 465 to 525 nm is attributed to oxygen vacancies on the surface of the TiO2 nanostructures.57 The presence of oxygen vacancies in TiO2 nanostructures in CL spectra may be due to the localized CL analysis of the nanostructures and large interaction volume at 20 keV accelerating voltage (1 μm depth information). At the same extent it is not in agreement with the XPS analysis, because of surface oxygen absorption and wide lateral resolution (spot size 300 μm × 700 μm). 5159

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Figure 11. I−V curve of Cu2Zn1.5Sn1.2S4.4/CdS/TiO2 nanostructured thin film solar cells (a) nanorods (b) combined structures.

enhancement of TiO2/CZTS based solar cells. Also, this work opens up a challenging task for the fabrication of promising photo electrodes for such novel 3D nanostructures toward thin film CZTS solar cells. This study establishes that the window layer of hierarchical TiO2 nanostructure based morphology offers a great potential for the development of high-efficiency nonstoichiometric CZTS based solar cells. With appropriate design, stoichiometry of materials, and theoretical understanding, high performance PV devices can be realized using the proposed architecture.

Further, in order to determine the charge transfer direction of the photogenerated electrons, ultraviolet photoelectron spectroscopy (UPS) spectra were recorded for TiO2 nanostructured films (Figure 10a,b). The upper emission onset energy (E1) and the lower emission onset energy (E2) of secondary photoelectrons appeared at 17.51 and 3.74 eV for TiO2 nanorods and 17.13 eV and the same at 3.02 eV for TiO2 combined structures, respectively. Using the relation, ϕ = hν − (E1 + E2), we calculated the valence band (VB) (Ev) energies of these TiO2 nanostructured to be 7.45 and 7.11 eV. Thus, the estimated conduction band energies (Ec) of TiO2 nanostructured films were calculated by Ev − Eg giving 4.33 and 3.91 eV respectively. Figure 11a,b shows the PV performance of FTO/TiO2 nanostructured films/CdS/CZTS/Au solar cells. The conversion efficiency (η) of TiO2 nanorods based device was found to be 0.55% with Isc = 4.2 × 10−5 A, Voc = 0.10 V, and FF = 23.1%. The device with TiO2 combined nanostructures showed an efficiency of 1.45% with Isc = 7.7 × 10−5 A, Voc = 0.14 V, and FF = 26.5% for the same active area of 0.196 mm2. Both devices are shown in the inset of Figure 11. The current increased sharply with increase of reverse bias voltage, indicating the presence of junction between the CZTS/CdS/TiO2 in both devices. The combined nanostructure TiO2 film based PV cell facilitates more light trapping and increased light harvesting ability than TiO2 nanorods due to a higher surface area and showed better PCE. Observed lower conversion efficiency of the fabricated devices may be due to the faster electron−hole recombination at the junctions, series resistance between the materials, and sulfur deficiency at higher annealing temperatures (>500 °C), which are affecting FF of the devices.

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ASSOCIATED CONTENT

* Supporting Information S

This materials is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00632. FESEM images of as-deposited and 450 °C annealed devices, XRD spectra of CZTS film at different annealing temperatures, and optical studies of CdS thin film (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected] (S.V.). *E-mail: [email protected], gopal.hegde@gmail. com (G.H.). ORCID

S. Varadharajaperumal: 0000-0002-2843-1270 Chinnaiyah Sripan: 0000-0003-1625-6580 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS In summary, we have prepared the highly oriented rutile phase of 1D-TiO2 nanorods and 3D and1D-TiO 2 combined structures films grown on FTO substrate using a single step hydrothermal process. To the best of our knowledge, this is the first report on TiO2 nanorods and hierarchical nanostructures with stoichiometry adjusted CZTS thin film based photovoltaic devices. Device performance analysis showed that FTO/TiO2 combined structures/CdS/CZTS/Au solar cell exhibits better PCE of η = 1.45% compared to the TiO2 nanorods alone solar cell exhibiting 0.55%. The improvement in efficiency can be attributed to the multiple functions of the incorporated TiO2 combined structures and sulfurization temperatures of CZTS. A detailed theoretical analysis is required to understand the charge transport mechanism in such structures. However, the present work will offer a new perspective for the performance

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ACKNOWLEDGMENTS The authors would like to thank Micro and Nano Characterization Facility (MNCF), Centre for Nano Science and Engineering (CeNSE) and Dept. of Physics, Indian Institute of Science (IISc), Bengaluru, India, for providing the fabrication and characterization facilities. We also thank DST and SERBN-PDF/2016/002112 for their financial support.

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REFERENCES

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DOI: 10.1021/acs.cgd.7b00632 Cryst. Growth Des. 2017, 17, 5154−5162