Solar cells with Cu2ZnSnS4 absorber thin films have a potential for high
energy conversion efficiencies with earth-abundant and non-toxic elements. In
this work the formation of CZTSSe from CuxSnSy nanoparticles (NPs) deposited
on ZnO nanorod (NR) arrays as precursors for zinc is investigated. The NPs are
prepared using a chemical route and are dispersed in toluene. The ZnO NRs are
grown on fluorine doped SnO2 coated glass substrates by electro deposition
method. A series of samples are annealed at different temperatures between 300
°C and 550 °C in selenium containing argon atmosphere. To investigate the
products of the reaction between the precursors the series is analyzed by
means of X-ray diffraction (XRD) and Raman spectroscopy. The morphology is
recorded by scanning electron microscopy (SEM) images of broken cross
sections. The XRD measurements and the SEM images show the disappearing of ZnO
NRs with increasing annealing temperature. Simultaneously the XRD and Raman
measurements show the formation of CZTSSe. The formation of secondary phases
and the optimum conditions for the preparation of CZTSSe is discusse

Ennaoui, Ahmed

Kavalakkatt, Jaison

Kornhuber, Kai

Kusch, Patryk

Lin, Xianzhong

Lux-Steiner, Martha Ch.

Reich, Stephanie

Jaison Kavalakkatt

Xianzhong Lin

Kai Kornhuber

Patryk Kusch

Ahmed Ennaoui

Stephanie Reich

Martha Ch. Lux-Steiner

Crossref

Thin Solid Films

Cu2ZnSn(S,Se)4 from CuxSnSy nanoparticle precursors on ZnO nanorod arrays

Solar cells with Cu2ZnSnS4 absorber thin films have a potential for high energy conversion efficiencies with earth abundant and non toxic elements. In this work the formation of CZTSSe from CuxSnSy nanoparticles  NPs  deposited on ZnO nanorod  NR  arrays as precursors for zinc is investigated. The NPs are prepared using a chemical route and are dispersed in toluene. The ZnO NRs are grown on fluorine doped SnO2 coated glass substrates by electro deposition method. A series of samples are annealed at different temperatures between 300  C and 550  C in selenium containing argon atmosphere. To investigate the products of the reaction between the precursors the series is analyzed bymeans of X ray diffraction  XRD  and Raman spectroscopy. Themorphology is recorded by scanning electron microscopy  SEM  images of broken cross sections. The XRDmeasurements and the SEM images show the disappearing of ZnO NRs with increasing annealing temperature. Simultaneously the XRD and Ramanmeasurements show the formation of CZTSSe. The formation of secondary phases and the optimum conditions for the preparation of CZTSSe is discusse

Kavalakkatt, J.

Lin, X.

Kornhuber, K.

Kusch, P.

Ennaoui, A.

Reich, S.

Lux Steiner, M.Ch.

HZB Repository

Cu2ZnSn S,Se 4 from CuxSnSy nanoparticle precursors on ZnO nanorod arrays

Institutional Repository of the Freie Universität Berlin

* Corresponding author: e-mail: jaison.kavalakkatt@helmholtz-berlin.de, jai.k@web.de Phone: +49 30 8062 42319, Fax: +49 30 8062 43199, Mobile: +49 179 3572588 Cu2ZnSn(S,Se)4 from CuxSnSy nanoparticle precursors on ZnO nanorod arrays 1 Jaison Kavalakkatt*1,2, Xianzhong Lin1, Kai Kornhuber1, Patryk Kusch2, Ahmed 2 Ennaoui1, Stephanie Reich2, Martha Ch. Lux-Steiner1,2 3 1) Helmholtz-Zentrum Berlin fuer Materialien und Energie, Hahn-Meitner-Platz 1, D-4 14109 Berlin, Germany 5 2) Freie Universitaet Berlin, Berlin, Germany 6  7 Abstract 8 Solar cells with Cu2ZnSnS4 absorber thin films have a potential for high energy 9 conversion efficiencies with earth-abundant and non-toxic elements. In this work the 10 formation of CZTSSe from CuxSnSy nanoparticles (NPs) deposited on ZnO nanorod 11 (NR) arrays as precursors for zinc is investigated. The NPs are prepared using a 12 chemical route and are dispersed in toluene. The ZnO NRs are grown on fluorine 13 doped SnO2 coated glass substrates by electro deposition method. A series of 14 samples are annealed at different temperatures between 300 °C and 550 °C in 15 selenium containing argon atmosphere. To investigate the products of the reaction 16 between the precursors the series is analyzed by means of x-ray diffraction (XRD) 17 and Raman spectroscopy. The morphology is recorded by scanning electron 18 microscopy (SEM) images of broken cross sections. The XRD measurements and 19 the SEM images show the disappearing of ZnO NRs with increasing annealing 20 temperature. Simultaneously the XRD and Raman measurements show the 21 formation of CZTSSe. The formation of secondary phases and the optimum 22 conditions for the preparation of CZTSSe is discussed. 23  24  25 Keywords: kesterite; CZTS; nanoparticle; nanorod; Raman; X-ray diffraction  26  27  28  29  30  31  32  33  34 1 Introduction 1 Cu2ZnSnS4 (CZTS) is a promising nontoxic absorber for solar cells made from earth-2 abundant elements. With a direct band gap energy of 1.5 eV and a high absorption 3 coefficient (104 cm-1) in the visible range CZTS fulfils the requirements for thin film 4 solar cells. Cu2ZnSn(S,Se)4 (CZTSSe) based devices have already reached an 5 energy conversion efficiency of 10.1 % [1]. With absorber layers made from CZTS 6 nanoparticle (NP) precursors solar cells have reached an energy conversion 7 efficiency of 7.2 % [2]. The NPs disperse in a solution can be deposited by non 8 vacuum methods. The films are then annealed to obtain a compact layer with large 9 grains. For solar cells in superstrate configuration, the absorber thin film can also be 10 deposited on transparent conducting oxide (TCO) layer, e.g. ZnO. With 11 nanostructured TCO the surface can increase many times over. In this work CZTSSe 12 absorber is produced in two steps: (1) ZnO nanorod (NR) arrays are electrodeposited 13 on fluorine doped SnO2 coated glass substrates (FTO) with a compact ZnO seed 14 layer, followed by a dip coating process of CuxSnSy (CTS) NP. (2) Both serve as 15 precursors during an annealing process in a Se containing atmosphere. A series of 16 samples are prepared at different annealing temperatures. The resulting absorber 17 layers are explored by means of X-ray diffraction and Raman spectroscopy. 18  19  20 2 Experimental details 21 2.1 Preparation 22 The method for the preparation of CTS NPs, which are dispersed in toluene, is 23 reported elsewhere [3]. The size of the NPs extracted by means of transmission 24 electron microscopy is 16 ± 3 nm. 25 The ZnO NR arrays (Fig. 1a) are prepared at 75 °C in an aqueous solution of 7 mM 26 Zn(NO3)2 and 5 mM NH4NO3 by the electro deposition route. The deposition is 27 performed in a three-electrode electrochemical cell with Pt counter and another Pt 28 electrode as reference. FTO is used as working electrode. The deposition is carried 29 at a polarization potential of -1.3 V versus Pt. Further information about the 30 preparation and characteristics of ZnO NR arrays can be found in Ref. [4]. 31 To coat the ZnO NR array with the CTS NPs (Fig. 1b), the substrates with the NRs 32 are dipped in the CTS NP solution. The dipping is performed in three steps with a dip 33 robot. The substrates are dipped into the solution with a velocity of 8 mm/s, held in 1 the solution for 30 s. The sample is pulled out with 2 mm/s to complete one cycle. 2 To investigate the formation of CZTSSe during annealing a series of 6 samples is 3 heated between 300 and 550 °C in a tube furnace under Se/Ar atmosphere. The 4 furnace is heated up with 10 °C/min to the set temperature. The given temperature is 5 held for 30 minutes and afterwards the furnace is switched off to cool down to room 6 temperature. 7  8 2.2 Analysis 9 The structural properties of the CZTSSe are investigated by means of X-ray 10 diffraction (XRD) and Raman spectroscopy. The XRD measurements are performed 11 in grazing incidence mode with an incidence angle of 0.9° in a Bruker AXS D8 12 ADVANCE. A copper X-ray tube is used to measure with the copper Kα line 13 (λKα=1.542 Å). 14 For the Raman measurements a Ti:Sa-ring-laser is used as an excitation. The laser 15 is fully tunable from 690 nm to 1050 nm. To avoid laser heating the beam power is 16 kept below 3.5 mW. Raman spectra are recorded with a Horiba T64000 triple 17 monochromator system in backscattering configuration with a microscope and a 18 motorized XY stage. The micro-Raman spectroscopy with a 100× objective is 19 performed at room temperature with a wavelength of 800 nm.  20 The morphologies and the compositions of the layers are analyzed in a LEO 1530 21 GEMINI scanning electron microscope (SEM) of Zeiss. The SEM images are 22 recorded at an acceleration voltage of 10 kV. The energy dispersive X-ray (EDS) 23 measurement are performed in the SEM at an acceleration voltage of 10 kV by a 24 Thermo Noran X-ray silicon drift detector (acquisition and evaluation software Noran 25 System Seven). 26 Optical measurements are performed with a LAMBDA 950 UV/Vis/NIR 27 Spectrophotometer of PerkinElmer with wavelengths between 250 and 2500 nm. 28  29  30 3 Results and discussion 31 Cross section images of the ZnO NR array before and after dipping in the CTS 32 solution are shown in Fig. 1a and 1b. The NRs are growing perpendicular to the 33 rough FTO surfaces. The mean length of the NRs is around 350 ± 50 nm. From top 1 view SEM images a packing density of 76×108 cm-2 is determined.  2 After the dipping process, the CTS NPs have filled the space between the NRs, 3 which are completely covered after 4 dips. The NPs form a 300 nm thick top layer. 4 The thickness of this NP layer cannot increase indefinitely, which can also be 5 observed on flat and compact substrates with the same NPs. With the length of the 6 NRs it is possible to control the thickness of the whole layer system. Cross section 7 images of the annealed samples are shown in Fig. 1c - h. During the annealing 8 remaining solvent of the NPs is evaporating and leads to a compact top layer of 9 around 200 nm (Fig. 1c). With increasing annealing temperature the ZnO NR are 10 disappearing. They seem to transform to small particles. After annealing at 500 °C 11 the cross section image cannot provide evidence of ZnO NRs (Fig. 1g). Instead, a 12 double layer with larger crystals on the top of the NPs is obvious. The size of these 13 crystals is increasing after annealing at 550 °C in Se/Ar atmosphere (Fig. 1h). An 14 energy dispersive x-ray measurement in the SEM at the last cross section shows, 15 that Se, S, Zn and Cu are distributed homogeneous in the double layer and Sn has a 16 higher concentration in the bottom layer. The compositions in atom % extract from 17 this measurement is for the top layer: Cu ≈ 20 %, Zn ≈ 30 %, Sn ≈ 6 %, Se ≈ 30%; 18 and S ≈ 10 %; and for the bottom layer: Cu ≈ 16%, Zn ≈ 26%, Sn ≈ 20 %, Se ≈ 26%; 19 and S ≈ 8%. Both layers contain additional carbon (≈ 2 %) and oxygen (≈ 2 %).  20 Fig. 2 shows the XRD pattern of ‘as deposited’ CTS NPs on ZnO NRs and the 21 annealed samples. The measured peaks are allocated by using the database of the 22 International Centre for Diffraction Data (ICDD) [5]. 23 We observe an evolution of ZnO peaks, which are decreasing with higher annealing 24 temperature.  After annealing at 550 °C the ZnO signals disappear. The SnO2 signals 25 from the FTO are also decreasing slightly with the higher annealing temperatures, but 26 are still present after annealing at 550 °C. 27 With the ICDD database [5] we allocate Cu2SnS3 as well as Cu3SnS4 (tetragonal 28 structure), because the peaks are overlapping, as possible CTS phases in our NPs. 29 The Intensity of the CTS main peak (at 2θ = 28.43 °) is increasing slightly up to 30 annealing after 400 °C. This peak is shifting to a smaller 2θ = 28.14°. Since the 31 diffraction pattern of CTS, ZnS, and CZTS differ a little from each other [5], we 32 conclude that the shift indicates the formation of ZnS or CZTS during the annealing. 33 As the diffraction pattern after annealing at 450 °C shows peaks of CZTSe, the shift 34 may also caused by the diffusion of Se atoms in the CZTS lattice, which leads to the 1 formation of CZTSSe. This is supported by the fact the intensities of the CZTSe 2 peaks are increasing with higher annealing temperature. 3 The diffraction pattern after annealing at 550 °C indicates still some CTS, ZnS or 4 CZTS. It can also not be excluded, that ZnSe is formed. The diffraction positions of 5 the ZnSe peaks are similar to the peaks of CZTSe [5]. Other indentified phases 6 shown in Fig. 2 are CuSeO4 [5] after annealing at 350 °C and CuSe [5] after 7 annealing at 500 °C. We observed that the CuSeO4 phase disappears after 8 annealing at 400 °C. 9  10 The measured Raman spectra are shown in Fig. 3 for the ‘as deposited’ as well as 11 the annealed samples between 300 °C and 550 °C. The grey colored circles show 12 the measured and the black solid lines the multi lorentzian fitted spectra. Raman 13 measurements are also performed on a ZnO NR array grown on FTO without NPs. 14 But neither the ZnO NR nor the FTO glass substrate are Raman active at least with 15 an excitation energy of 1.55 eV. The CTS NPs of the ‘as deposited’ sample show 16 broad Raman peaks at 294 cm-1, 323 cm-1 and 349 cm-1. A plateau between 280 and 17 260 cm-1 is also present. By comparison with the work of Fernandes et al. [6] the 18 peaks is allocated to the Raman shifts of Cu2SnS3 with cubic (F-43m) and tetragonal 19 (I-42m) structures as well as to Cu3SnS4 with an orthorhombic (Pmn21) structure. We 20 conclude that the main phase in our CTS NPs is Cu2SnS3 since the XRD diffractions 21 indicates no orthorhombic and the Raman spectra no tetragonal Cu3SnS4 [7]. 22 With increasing annealing temperature these peaks are disappearing in favor of the 23 appearing of CZTS and CZTSe in the Raman spectra. The characteristic main peaks 24 (A1) of CZTS with a Raman shift of 336 cm-1 and of CZTSe with a Raman shift of 198 25 cm-1 are clearly identified after annealing at 450 °C. With increasing temperature next 26 to the main peaks other peaks appear at 235 cm-1, 250 cm-1, 276 cm-1, 298 cm-1, 352 27 cm-1 and 372 cm-1. After annealing at 550°C also a shoulder at the CZTSe peak at 28 198 cm-1 is identified with a Raman shift of 172 cm-1. 29 The Raman spectra of CZTS layers grown by sulfurization of metallic precursor 30 layers show the main peak at 339 cm-1 [8]. These layers also show Raman peaks of 31 CZTS at 287 cm-1, 306 cm-1, 367 cm-1 and 375 cm-1. In our layers the peaks at 367 32 cm-1 and 375 cm-1 are present in one broad peak at 372 cm-1 (Fig. 3). The other 33 broad peak at 298 cm-1 is allocated to the other two peaks showed in Ref. [8].  34 CZTSe thin films prepared by single-stage coevaporation technique show the A1 peak 1 at 195 cm-1 and further peaks at 172 cm-1 and 231 cm-1 [9]. The small shoulder (172 2 cm-1) after annealing at 550 °C and the broad peak at 235 cm-1 (Fig. 3) are allocated 3 to CZTSe. Since the spectra could not fit with only the CZTS and CZTSe peaks, we 4 conclude that secondary phases are present, indicated by the peaks at 250 cm-1, 276 5 cm-1 and 352 cm-1. These phases are identified as ZnSe (253 cm-1) [10] and ZnS 6 (271 cm-1, 352 cm-1) [11]. 7 Our results on XRD and the Raman measurements at different annealing 8 temperatures confirm the formation of CZTS and CZTSe. From a comparison of our 9 Raman spectra and the one of CZTSSe, prepared with different sulfur to selenium 10 ratios [12], we conclude that CZTSSe is more likely present in our layers. There are 11 several reactions which lead to the formation of CZTSSe. A reaction of the sulfur in 12 the CTS NPs with our ZnO NRs can form ZnS. In the next step CZTS is formed by a 13 reaction of CTS with ZnS, which is described with electroplated precursors to form 14 CZTS [13, 14]. The diffusion of selenium in the CZTS layers can form CZTSSe. The 15 presence of selenium transforms ZnO and ZnS to ZnSe [15, 16]. The ZnSe can also 16 react with the CTS NPs to CZTSSe crystals.  17 Additional optical measurements of the sample series show an increasing absorption 18 (from approximately 40 to 80 %) of the visible light in favor of a decreased 19 transmission (from approximately 45 to 16 %) . The reflection shows no significant 20 changes in the visible range. The absorption coefficient extract from these 21 measurements is in order of 104 to 105 cm-1. 22  23 4. Conclusions 24 We prepared CZTSSe by annealing of ZnO NRs coated with CTS NPs. XRD pattern 25 and the Raman spectra show that the CTS NPs react with the ZnO NRs in Se 26 containing atmosphere, resulting in the formation of intermediate phases such as 27 ZnS, ZnSe, CuSeO4. After annealing at 550 °C the dominant phase is CZTSSe. But 28 still secondary phases such as ZnS, ZnSe and CuSe are present in the layer. ZnSe 29 and ZnS are identified by Raman and CuSe by XRD. We believe that the solid state 30 reaction is not completed due the lack of tin to form CZTSSe. To resolve this problem 31 another formulation of the CTS precursor with increasing amount of tin or a better 32 control of the annealing process are required. 33   34 Acknowledgement 1 The work was supported by the BMBF (grant: 03SF0363E). 2  3  4  5  6  7 References 8  9 [1] D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K. Todorov, D. B. Mitzi, Prog. 10 Photovolt: Res. Appl. 20 (2011) 6. 11  12 [2] Q. Guo, G. M. Ford, W.-C. Yang, B. C. Walker, E. A. Stach, H. W. Hillhouse, R. 13 Agrawal, J. Am. Chem. Soc. 132 (2010) 17384. 14  15 [3] X. Lin, J. Kavalakkatt, K. Kornhuber, S. Levcenco, M. Ch. Lux-Steiner, A. 16 Ennaoui, EMRS 2012 Symposium B, Strasbourg. 17  18 [4] J. Chen, L. Aé, Ch. Aichele, M. Ch. Lux-Steiner, Appl. Phys. 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The images show a) ZnO NR array deposited 3 via electro deposition, b) ZnO NR array covered with CTS NP after dip coating, c) - h) 4 Layers after annealing at temperatures between 300 and 550 °C of ZnO NR arrays 5 covered with CTS NP. 6  7  1 Figure 2:  2 XRD patterns of not annealed ZnO NR covered with CTS NP (as deposited) and after 3 annealing between 300 and 550°C in selenium containing atmosphere. With 4 increasing annealing temperature CZTSe is present in the layers and ZnO is 5 disappearing. Secondary phases of CuSe (after annealing at 500 and 550 °C) and 6 CuSeO4 (after annealing at 350 °C) are identified. 7  1 Figure 3: 2 Raman spectra of not annealed ZnO NR covered with CTS NP (as deposited) and 3 after annealing between 300 and 550°C in selenium containing atmosphere. The 4 grey colored circles show the measured and the black solid lines the fitted spectra. 5 With increasing temperature CZTS and CZTSe are present in the layers. Secondary 6 phases of ZnS and ZnSe are indicated. 7 

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Cu2ZnSn(S,Se)4 from CuxSnSy nanoparticle precursors on ZnO nanorod arrays

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