Band Gap Engineering of Alloyed Cu2ZnGexSn1âxQ4 (Q = S,Se

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Band Gap Engineering of Alloyed CuZnGeSn Q (Q=S,Se) Films for Solar Cell Dhruba B. Khadka, and Junho Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510877g • Publication Date (Web): 05 Jan 2015 Downloaded from http://pubs.acs.org on January 12, 2015

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The Journal of Physical Chemistry

Band Gap Engineering of Alloyed Cu2ZnGexSn1-xQ4 (Q=S,Se) Films for Solar Cell Dhruba B. Khadka, JunHo Kim* Department of Physics, Incheon National University, 12-1 Songdo-dong Yeonsu-gu, 406-772 Incheon, Republic of Korea

*Corresponding Author: JunHo Kim Email: [email protected] T: 82-32-835-8221

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Abstract We have fabricated polycrystalline Cu2ZnGexSn1-xQ4 (Q= S or Se) thin films by using spray-based deposition. The effects of Ge alloying were studied by X-ray diffraction (XRD), Raman spectroscopy and ultraviolet-visible spectroscopy. XRD and Raman spectroscopy revealed that lattice parameters decreased linearly and characteristic Raman peaks shifted to higher frequency with increasing Ge alloying. The band gap energies of post-sulfurized CZGTS films (1.51± 0.05 eV - 1.91± 0.05 eV) and postselenized CZGTSe films (1.07± 0.05 eV - 1.44 ± 0.05 eV) were found to be increased almost linearly with increase of Ge alloying in the respective films. Analysis of band gap bowing model showed a small bowing constant b~0.1± 0.02 eV, indicating high miscibility of alloyed elements. The band gap tuning of CZGTS(Se) thin films can be utilized for tuning band gap of subcell in multi-junction cell and for band gap graded photo absorber of high efficient solar cell.

Keywords: Cu2ZnGexSn1-xQ4; Post-annealing; Band gap tuning; Photo-absorber


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1. Introduction Quaternary chalcogenide Cu2ZnSnS(Se)4 (CZTS(Se)) thin films have been investigated extensively as a promising absorber layer of thin film solar cell owing to earth-abundant constituents and good optical properties i.e. high absorption coefficient (~104 cm-1) in visible spectrum range and tunable band gap from ~1.1 eV (CZTSe) to ~1.5 eV (CZTS).1,2 The power conversion efficiency (PCE) of kesterite CZTSSe solar cell reached 12.6%, in which absorber layer was realized by liquid coating of hydrazine-based solution.3 Several research groups have fabricated CZTSe thin film solar cells by various techniques such as sputtering,4 co-evaporation,5 spin coating,3 doctor blading,6 spray pyrolysis7 and electrodeposition technique.8 The devices fabricated by above approaches have demonstrated best PCE of 5% ~ 12.6%.3-8 However, the reported efficiency is still much lower compared to the predicted value of ~32 % (31%) for kesterite CZTS(CZTSe) solar cell in theory.9 In addition, the best efficiency of CZTSe solar cell (12.6%) is also quite lower than that of the CuIn1-xGaxSe2 (CIGSe) thin film solar cell which achieved record PCE of 21.7%.10 Precise controlling of high quality kesterite phase with suppressing of secondary phases,2 back contact barrier formation4,11,12 and detrimental defect levels13 as well as efficient light harvesting14 are the issues to be addressed for high efficiency CZTSe solar cell. The graded band gap of absorber layer should be required to enhance the PCE of CZTSe solar cell considering its beneficial role in the CIGSe solar cell. The success of CIGSe solar cells is mainly based on the band graded absorber layer, where stoichiometry gradient of gallium (Ga) through the absorber layer has been implemented to get larger open circuit voltage and higher short circuit current.15,16 A similar approach can be adopted to get band gap graded kesterite Cu2ZnGexSn1-x(S,Se)4 (CZGTSSe) absorber for better



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performance of photovoltaic devices. The band gap of CZTSSe is tunable by alloying Sn with Ge17-19 and Zn with Fe.20 To date, the incorporation of Ge into CZTS crystal lattice has been reported as a potential method to improve the photovoltaic performance.6,21-24 The alloying of Sn with Ge atoms in the CZTSe crystal can tune the band gap of the material from 1.1 eV (Eg(CZTSe)) to 1.45eV (Eg(CZGSe)) with controlling x=Ge/(Ge+Sn) from 0 to 1,25 and similar trend can also be expected in alloyed CZGTS material. In addition, Ge4+ also plays beneficial role, being stable against oxidation compared to the Sn4+ which can undergo a change to oxidation state (Sn2+) during device operation. Thus, the Ge-alloyed CZTSe crystal lattice leads to the reduction of recombination centers associated with multivalent Sn atoms, which potentially improve stability of absorber material.23,26 In recent progress, the solar cell with Ge-alloyed CZTSe absorber layer have achieved record efficiency of 9.4% by widening its band gap.6 However, there is still lacking of the theoretical and experimental understanding of fundamental problems of the alloyed quaternary chalcogenide. Therefore, more systematic studies on film growth, electrical and optical properties for alloyed CZGTS(Se) are necessary, which provide a solid foundation to realize a high efficient solar cell. Herein, we report the fabrication of Cu2ZnGexSn1-xS(Se)4 (CZGTS(Se)) thin films using spray-based deposition for the first time. Spray pyrolysis is one of most costeffective fabrication techniques without using high-cost vacuum facilities. In spray-based deposition, the cation stoichiometry is easily adjustable just by dissolving corresponding chemicals in the precursor solution according to the designed ratios, which is successfully employed in our earlier reports on fabrication of quaternary chalcogenide materials.20,25,2729

In this article, we report on fabrication method and film properties of alloyed CZGTS(Se)

films. The Ge alloying effects on crystal lattice and band gap have been discussed mainly in terms of solar cell application.


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2. Experiments 2.1. Materials Copper (II) chloride (CuCl2; ≥97%), zinc chloride (ZnCl2; ≥98%), germanium oxide (GeO2; 99.99%), tin (IV) chloride pentahydrate (SnCl4·5H2O; 98%), thiourea (SC(NH2)2; 99.0%) and elemental sulfur and selenium powder (99.99%, 75 μ m) were used as precursor chemicals. We used all chemicals from Sigma-Aldrich.

2.2. Fabrication of Films Ge alloyed CZTS thin films were fabricated by spray pyrolysis technique. For the deposition of films, aqueous precursor solutions were made by dissolving CuCl2, ZnCl2, GeO2,

SnCl4·5H2O

and

SC(NH2)2

with

molar

ratios

as

CuCl2:

ZnCl2:

GeO2:SnCl4·5H2O:SC(NH2)2 = 1.8:1.2:x:1-x:14, where Ge content is 0≤ x ≤1. We used de-ionized water (DIW) as solvent, which is cheap, safe and environmentally benign solvent. The sulfur source i.e. thiourea (SC(NH2)2) was used excessively in order to avoid sulfur deficiency and some oxidation in the sprayed films. The precursor solutions were sprayed onto soda lime glass (SLG) and molybdenum coated soda lime glass (Mo-SLG) at substrate temperature of 450 °C using home-made spray facility.20,25,28,29 The sprayed films were sulfurized and selenized under sulfur and selenium vapor ambient at ~ 510 °C for 25 min and 530 ° C for 35 min, respectively. The details of annealing experiments are described in the previous reports.20,25,28,29 After the heat treatment, the films were cool down naturally to room temperature. We followed the same annealing pattern as our previous reports.20,25

2.3. Characterization We characterized morphology and stoichiometry of CZGTS thin films by using FESEM (JSM-7001F, Jeol) with EDS (INCA, Oxford). The crystal lattice structure was



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studied by using XRD system (Smart Lab, Rigaku) equipped with Cu-Kα source ( =1.5412 Å ) operated at 45 kV and 200 mA. All XRD results were obtained in 2θ scan mode with scan speed of 3o/min. We used Raman spectroscopy to detect possible secondary phases in the fabricated films. The wavelength of excitation laser was 532 nm, and the irradiation power was kept below 1mW to avoid any thermal shift or broadening of Raman peak. We

investigated energy band gaps of fabricated films by measuring transmission and absorption spectra with ultraviolet-visible (UV-Vis) spectrometer (Lambda 40, PerkinElmer).

3. Results and Discussions 3.1. Elemental Composition and Morphology The analyzed chemical compositions of all fabricated films are summarized in Figure 1, where atomic percentages of constituent elements Cu, Zn, Ge, Sn, S, and Se are also added. Figures 1a and 1b display the elemental compositions of post-annealed CZGTS and CZGTSe films, respectively. In both sulfurized and selenized films, we could not sense oxygen atom within limit of the EDS detection. The atomic percentage ratios i.e. Cu/P, Zn/Q, Ge/Q, Sn/Q and S(Se)/R, where P=Zn+Ge+Sn, Q=Ge+Sn and R=Cu+Zn+Ge+Sn, reflect the stoichiometry of respective films, which are very close to the chemical compositions of spray precursor solutions. These results are similar to our earlier reports on spray pyrolyzed quaternary chalcogenide thin films.20,25,27 Some cation ratios show negligible fluctuations, which are usually observed in post-annealed films due to possible elemental loss during chalcogenization.6,20 It is to be noted that the post-annealing in chalcogen (sulfur/selenium) vapor atmosphere is necessary to improve crystalline texture and stoichiometry of the fabricated films.


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Figure 2 shows surface SEM images of post-annealed CZGTS (a1-e1) and CZGTSe (a2e2) films. All post-sulfurized CZGTS films show comparatively rough surface textures while post-selenized CZGTSe films show better crystalline surfaces with enlarged grains. Post-selenized CZGTSe films exhibit large grain structure with increase of Ge content, indicating that Ge-alloying promotes the grain growth in the film texture. The Ge-alloyed CZTSe film with large grain is also seen in the previous report.23 The promotion of crystallization was also reported in antimony assisted growth of CZTSe films.30 Our Gealloyed CZTSe films show grains with micrometer size, which is close to those of best CZTSSe solar cell (12.6%).3 The absorber layer having larger grain size reduces the grain boundaries which is advantageous to minimize the number of defects along grain boundaries and hence enhances the solar cell performance.13,23,31 Thus, the quality of CZGTSe thin film can be further improved for larger and compact grain texture by optimizing selenization, which is desirable for high charge carrier collections.

3.2. Analysis of Crystal Structure We characterized crystal structure and quality of fabricated films by XRD and Raman spectroscopy. Figures 3(a) and 3(b) show XRD patterns of post-chalcogenized CZGTS (Se) (0≤ x ≤ 1) films. The post-sulfurized CZGTS (0≤ x ≤ 1) films (Figure 3a) exhibit sharp and large diffraction peak of (112) orientation and other small peaks of (002), (004), (220), (204), (312), and (116) planes and Mo peak at 2θ of 40.4° from Mo substrate, which are assigned with ICCD references.32 For the alloyed CZGTS (0

Band Gap Engineering of Alloyed Cu2ZnGexSn1âxQ4 (Q = S,Se

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Band Gap Engineering of Alloyed Cu2ZnGexSn1âxQ4 (Q = S,Se