Effect of 6 MeV electron Irradiation on Nano-Cu 2 ZnSnS 4


 Microwave synthesized nano sized Cu2ZnSnS4 (CZTS) powder was irradiated with 6 MeV electrons, to investigate stability under radiation. The structural, optical, vibrational and morphological properties were explored using X-ray diffraction, UV-Visible spectroscopy, Raman spectroscopy and Scanning Electron Microscope (SEM).The irradiated sample shows significant change in properties when compared to the pristine sample. X ray peak broadening analysis has been used to estimate the crystallite size and lattice strain. Raman spectroscopy analysis confirms the transition of ordered kesterite to disordered kesterite phase after electron irradiation at electron fluence of 4 x1015 e-/cm2. CZTS nano-particles having hierarchical flower like morphology starts agglomerating after electron irradiation as observed from SEM images. The sample did not amorphize upto the highest fluence 4 x 1015 e-/cm2 employed in this study.


Introduction
All the spacecrafts and satellites use photovoltaic solar cell power in space as an energy source [1]. For many decades in conventional spacecrafts Si [2][3][4][5], multi junction solar cell [6][7][8][9] and GaAs/Ga are used [10][11][12][13] which provide high efficiency and are also quite robust to space environment. Researchers are trying to replace it with an alternate. Photovoltaic thin film solar cell array which offers high reliability, lower production cost, proven manufacturing and scalability of required power levels have been shown as useful for space mission [14].The report from M. R. Reddy suggests that conventional solar cell can be replaceable with flexible thin film solar cell array of CIGS based solar cell which reduces not only mass but also lowers the cost by ~30% compared to the conventional space solar cell [15].
One of the most important necessities of space solar cell is the ability to withstand harsh space radiation environment. The energetic particles in space environments such as proton, electron, gamma etc. cause lattice damage in an active area of solar cell which degrades the performance of the solar cell used as a power sources in space satellite [10][11][12][13][14][15][16].This puts a limit on the use of solar cell for longer duration. Therefore, the stability of material which is used for making the solar cell for space application is very important. A study by Jaseneket al., shows that CuInGaSe2 (CIGS) solar cell can tolerate more than 10 times larger fluence of high energy electrons than any other solar cell [17]. This study was further strengthened by Karsten Otte et al., which reported that flexible CIGS thin film solar cells are superior to that of GaAs with respect to radiation hardness [18].However, indium and gallium are rare earth metals, due to which cost of solar cell is high. Therefore, the necessity of alternative material to CIGS is needed which can stand harsh radiation environment.
Very recently Sauli et al., reported that after gamma irradiation the structural and optical properties of Cu2ZnSnS4 (abbreviated as CZTS hereafter) thin films improves, making them favourable in order to use them in outer space where gamma radiation is abundant [19]. In another study, Sugiyama et al. reported effect of electron and proton irradiation on CZTS solar cell. In their study they reported the effect of electron irradiation using constant energy of 2 MeV and the fluence was varied from 1 x 10 14 e -/cm 2 to 2 x 10 17 e -/cm 2 , while for proton irradiation, the energy was kept 380 keV from a 400 KV ion implanter with fluence ranging from 1× 10 12 to 3× 10 16 cm −2 . They claimed CZTS solar cell performance improves up to small amount for both electron and proton irradiations, which shows that CZTS solar cell tolerates electron and proton irradiation analogous to CIGS solar cell [20]. Similar study was also done by Suvanam et al., where thin films of Cu2ZnSnS4/Se4 (CZTSSe) and CZTS were investigated after 3 MeV proton irradiation; it showed that CZTSSe and CZTS are high radiation hardened materials compared to CIGS solar cell [21]. In continuation with these studies, we have carried out an experimental study in which our objective was to explore the effect of 6 MeV electron irradiation on CZTS nano-flower like structures. Further we carried out detailed analysis using micro-Raman spectroscopy to investigate how the structure changes or modifies upon irradiation. There are no reports available on structural transition of CZTS due to electron irradiation. Earlier Parkin et. al. reported, how the presence of electron beam induces sulphur vacancies in monolayer of MoS2 and Raman spectroscopy was used as nondestructive method to estimate defect concentration in MoS2 [22].
In our study we report 6 MeV electron irradiation induced changes on nano-crystalline CZTS (size ~14.4nm).The structural, vibrational, optical and morphological properties were studied using X-ray diffraction, Raman spectroscopy, UV visible spectroscopy and Scanning Electron Microscope (SEM).

Materials and Methods
One step Microwave assisted synthesis method was used to synthesize the material as mentioned in previous reports [23][24] with a slight modification that instead of cyclic microwave irradiation, single step microwave irradiation was carried out. In a typical synthesis process, all the required precursors were taken in an appropriate concentration and exposed to microwaves for 10 min. After that, the precipitate was washed with ethanol, centrifuged and dried to get the final Cu2ZnSnS4nano-powder. Indigenously developed Race-Track Microtron accelerator having 6 MeV energy electron beam with faraday cup attached to a counter (1count=10 11 electrons) was used [25] for irradiation. To determine crystal structure and crystallite size of the sample X-ray diffractometer D8 Advance BRUKER AXS was used with Cu Kα ray of wavelength 1.54 Å.
Laboratory based X-Ray Diffractometer is not sufficient to differentiate the two forms of CZTS crystal structures [26,27]. To identify various impurity phases, quantifying disorder and also to estimate the quality of crystallinity in this class of compounds Raman spectroscopy is particularly used. Renishaw Raman spectrometer was used in backscattering configuration at the excitation wavelength of 532 nm for Raman spectroscopy measurements. For Raman spectra analysis, Lorentzian function was used to fit Raman spectra to determine mode frequencies, area under the curve, FWHM and integrated intensity. Fitting was carried out with the minimum number of peaks which provides peak position, FWHM and integrated intensity. The same standard procedure is followed for the data for all electron irradiated samples.

Results and Discussion
Where θ is the diffraction angle, λ is the wavelength of X-ray (1.54 Å), D is the crystallite size and K (0.9) is shape factor.
Similarly, the X-ray diffraction peak broadening due to strain is given by, where, βε is the broadening due to strain, ε is the micro strain and θ is the diffraction angle.
Putting equation (2) and (3) in (1) we get, Rearranging above equation we get (5) Equation 5 represents Uniform Deformation Model (UDM) where it was predicted strain is uniform in all crystallographic directions [28]. βTcosθ is plotted as a function of 4sinθ for peaks of Cu2ZnSnS4. Strain (ε) is estimated from slope of the fit, and crystallite size D is estimated from the Y intercept of the fit.  Figure 4 shows strain due to electron irradiation on of Cu2ZnSnS4.We can clearly see that change in strain on Cu2ZnSnS4 with electron irradiation is marginal. Figure   5shows peak position as a function of electron fluence which clearly indicates there is no change in peak position due to electron fluence, whereas area under the curve (integrated intensity) is decreasing for (112), (220) and (312) planes with increase in electron fluence. Considering pristine sample intensity as 100 % for (112) peak intensity decreases to ~ 30%, for (220) peak intensity decreases to ~ 26% and for (312) it decreases to ~ 16% for the 4 x10 15 e -/cm 2 electron fluence as shown in Figure 6, indicating decrease in crystallinity as decrease in XRD peak intensity generally implies decrease in crystallinity [29]. Figure 7 shows UV absorption spectra of pristine and electron irradiated samples of Cu2ZnSnS4 and the corresponding Tauc's plot is shown in Figure 8. The band gap energy was calculated from the absorption spectra by extrapolating the linear portion of the graph of (αhν) 1/2 as a function of (hν) to the energy axis using Tauc's relation. band gap cannot be attributed to the change in particle size and we believe that this could be due to decrease in crystallinity of Cu2ZnSnS4 because of electron irradiation. Figure 9 shows Raman spectra of CZTS for pristine and irradiated samples. Bands at 280 cm -1 and 331cm -1 are the signature peaks of nano Cu2ZnSnS4 [30][31][32]. From figure 9 we can clearly observe that up to 2 x10 15 e -/cm 2 Raman mode frequencies of Cu2ZnSnS4 are stable, while for further irradiation at 4 x10 15 e -/cm 2 the relative intensity of band at 280 cm -1 increases compared to 331 cm -1 mode. Our earlier high temperature study on nano-CZTS indicated that beyond 550 K there is a considerable variation in relative intensities of 280 cm -1 mode and 331 cm -1 Raman mode frequencies indicating phase transition from ordered kesterite to disordered kesterite structure [33].The transition that occurs at higher irradiation dose is exactly same as transition at high temperature. Figure 10 shows electron fluence dependence of Raman mode frequencies of nano-CZTS. Here, we see that Raman mode at 331cm -1 is stable with increase in electron fluence, while Raman mode frequency of the 280 cm -1 mode decreases marginally with electron fluence. Electron irradiation dependent FWHM of Raman mode frequencies of nano-CZTS is plotted in Figure 11. It shows that the width of mode at 280 cm -1 increases with electron fluence, but width of 331cm -1 is stable which shows irrespective of the electron fluence the strongest mode of CZTS is stable. Intensity ratio of mode at 280 cm -1 to mode at 331 cm -1 is plotted in Figure   12, which shows a clear increase beyond 4x10 15 e -/cm 2 , indicating a phase transition to a disordered kesterite phase.
The SEM images of CZTS before irradiation are shown in Figure 13(a); it consists of hierarchical flower like morphology having around 1 µm particle size clearly visible in magnified image fig 13(b) . Figure   13(c) shows SEM images after different irradiation. We can clearly see for 1 x10 15 e -/cm 2 there is no change in morphology but beyond this, particles start agglomerating and become irregular. For 4 x10 15 e -/cm 2 irradiation fluence, the morphology is completely changed as shown in SEM image in Figure 13 (d).

Conclusions
In conclusion, the effect of electron irradiation on Cu2ZnSnS4 was investigated, to explore its radiation stability.
The samples were characterized for their structural, optical, vibrational and morphological properties using X ray diffraction, UV visible spectroscopy, Raman spectroscopy and Scanning Electron Microscope                  Estimated crystallite size of Cu2ZnSnS4 samples using XRD at different irradiation uences.

Figure 4
Strain on Cu2ZnSnS4 samples calculated using XRD patterns at different irradiation uences.

Figure 5
Dependence of X-ray diffraction peak positions of Cu2ZnSnS4samples for different electron irradiation uences.

Figure 10
Electron irradiation uence dependence of Raman mode frequencies of CZTS samples.

Figure 11
Dependence of FWHM of Raman modes of nano-CZTS on electron irradiation uence.

Supplementary Files
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