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All-sputtered CdTe solar cell activated with a novel method
Solar Energy 193 (2019) 31–36
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All-sputtered CdTe solar cell activated with a novel method ⁎
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E. Camacho-Espinosa , A. López-Sánchez, I. Rimmaudo, R. Mis-Fernández, J.L. Peña Centro de Investigación y de Estudios Avanzados del IPN Unidad Mérida, Applied Physics department, Km. 6 Antigua Carretera a Progreso, A.P. 73-Cordemex, C.P 97310 Mérida, Yucatán, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Sputtering CdTe Solar cells Activation
CdTe thin film solar cells are usually fabricated depositing the absorber layer by Close Space Sublimation (CSS) and Vapor Transport Deposition (VTD), mainly due to the high deposition rate allowed by these techniques. However, most of the solar cell films can be deposited by sputtering, making to deposit also the CdTe by this technique attractive. Indeed, an all-sputtered solar cell could be deposited in one sequential chamber at a relatively low temperature and without breaking the vacuum, resulting in a sensible decrease in the production cost at the industrial level. Usually, treatments based on CdCl2, also known as activations, are applied to the CdTe solar cells to achieve high power conversion efficiency (PCE). However, it was observed that the usual activation methods could be aggressive for all-sputtered CdTe solar cells, damaging the films and resulting in low performance. In this work, we propose a novel activation method based on Nitrogen/Oxygen/CHCIF2 gas mixture, which is more suitable for this kind of device. Morphological, structural, optical and electrical properties were analyzed and compared to our CSS-CdCl2 baseline. A PCE of 12% was achieved for all-sputtered CdTe solar cells.
1. Introduction CdTe-based solar cells are the second most common photovoltaic (PV) technology, representing 5% of the world market (Lee and Ebong, 2017). The success of CdTe thin-film solar cells is based on their low production cost (Reinders et al., 2017). During many years the CdTe solar cell was fabricated with the classical structure TCO/buffer/CdS/ CdTe/back contact, which present several disadvantages and a limited performance. However, the focus has been recently moved to TCO/ MZO/CdSexTe1-x/CdTe/CuTe structure, reporting efficiencies around 19.1% (Munshi et al., 2017). In this work, the classical structure was studied to understand the effect of different activations on CdTe deposited by different techniques. Most of the films that compose the classical CdS/CdTe solar cell can be deposited by the RF-Sputtering technique. On the other hand, the CdTe layer can also be deposited by screen printing, electrodeposition, pulsed laser deposition (PLD), physical vapor deposition (PVD), close space sublimation (CSS), etc. (Ikegami, 1988; Woodcock et al., 1991; Li et al., 2012; Compaan et al., 2004; Luo et al., 2016; Rios-Flores et al., 2012). Among these, the most commonly used are CSS and VTD, because allow the deposition of thick, high-quality films, which usually result in high PCE solar cells. However, at an industrial level, it would be attractive to use the same deposition method for all the layers,
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avoiding the use of different chambers. One sequential chamber was previously proposed by Swanson et al. (2016) for CSS, demonstrating better process repeatability, versatility and reliability. In the case of Sputtering, the use of one sequential chamber would have additional advantages such as smaller heating and cooling times, due to the lower deposition temperature. Furthermore, the throughput would increase avoiding breaking the vacuum. These advantages could compensate the R.F. sputtering low deposition rate. Alternatively, the deposition time would be dramatically reduced by using pulsed DC magnetron Sputtering (Kaminski et al., 2015; Kaminski et al., 2016). All-sputtered (AS) CdTe solar cells have been fabricated before for other groups (Ablekim et al., 2014; Paudel et al., 2014a,b; Kim et al., 2016; Treharne et al., 2012). These groups have carried out the activation treatment using CdCl2, annealing the samples in an air atmosphere, achieving the world record value for sputtered CdTe of 14.4% (Li et al., 2015). However, when the treatment is performed by CdCl2, the solution leaves waste, causing a sensible increase in the solar cellś series resistance, and consequently, a reduction in solar cell PCE (Valdna et al., 2001; Hiie, 2003; Valdna, 1999; Hiie and Valdna, 2001; Bai et al., 2011). In a previous study (Camacho-Espinosa et al., 2017), we observed the presence of various residues, which are difficult to remove, and its removal adds steps to the process. An alternative for CdCl2 activation is the use of difluorochloromethane (CHClF2) gas, which was proposed by Romeo
Corresponding author. E-mail address: [email protected] (E. Camacho-Espinosa).
https://doi.org/10.1016/j.solener.2019.09.023 Received 12 June 2019; Received in revised form 29 August 2019; Accepted 7 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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et al. (2006). The use of a gas, instead of a solution or vapor, could imply some advantages at an industrial level (e.g. storage facilities and waste management). In this paper, we propose the fabrication of allsputtered CdTe solar cell, activated using a N2/O2/CHCIF2 gas mixture, which can be considered as a combination of difluorochloromethane gas with dry air. Activating the solar cell in this way, the air moisture is avoided, allowing a better experimental control. Furthermore, the use of N2 has been reported to have beneficial effects for CdTe (Peña et al., 2014; Romeo et al., 1986; Oehling et al., 1996; Akbarnejad et al., 2016). It should be noted that the use of N2/O2/CHClF2 in the activation process for AS CdTe solar cells has not been studied before, nor its role in structural and opto-electrical changes. Thus, the effects of this novel activation treatment will be addressed by the comparison with our baseline (BL) solar cells, which are made depositing CdTe with CSS and activating with a CdCl2/MeOH solution. The study of the intermixing between CdS and CdTe caused by the N2/O2/CHClF2 activation process was made with X-ray Photoelectron Spectroscopy (XPS). Structural changes were analyzed with grazing incident X-ray diffraction (GIXRD). Field Emission Scanning Electron Microscopy (FE-SEM) was used to study the morphological changes of the CdTe due to the activation. The finished solar cells were characterized by current-voltage (J-V) and external quantum efficiency (EQE).
Fig. 1. FE-SEM micrograph of micro and nano defects observed in sputtered CdTe films, (a, b) pinhole and (c, d) blister.
radiation (λ = 1.5418 Å); the grazing incident angle was 3°. The measurements were recorded for a diffraction angle 2θ ranging from 20° to 80° in steps of 0.02°/3s. The surface morphology was analyzed by a Jeol 7600F FE-SEM in SEI mode, with an energy of 15 kV. The J-V measurements were performed with a Keithley 2420 source-meter under simulated 100 mW/cm2 light intensity generated with halogen lamps. The light intensity calibration was obtained by an Oriel Sol2A solar simulator and its Si reference cell. The photovoltaic parameters open circuit voltage (Voc), short circuit current (Isc), fill factor (FF) and PCE were determined. The external quantum efficiency (EQE) was measured by an USHIO UXL150 Xenon lamp in a Newport 67005 lamp housing, a monochromator Yvon-Jobin H20VIS, and a Newport 71650 calibrated Si diode.
2. Experimental The CdS/CdTe cell structure was prepared in a superstrate configuration, using commercial ITO-coated Corning® glasses as substrates. All the films mentioned below were deposited by RF-sputtering. In particular, the ZnO films (200 nm) were deposited from a metallic Zn target using an Ar/O2 reactive atmosphere, at the power, pressure and temperature conditions of 80 W, 2.6x10-2 mbar and 400 °C respectively. The CdS films (120 nm) were grown in a pure Ar atmosphere from a CdS target at the power, pressure and temperature conditions of 35 W, 3.3 × 10−2 mbar and 300 °C respectively. The CdTe film (~4 µm) was grown from a target with 4 N purity and 1 in. in diameter. The deposition was carried out in two steps varying the deposition power, during the first 90 min the power was fixed at 20 W and in the last 90 min the power was increased to 40 W. The working pressure was maintained constant at 2.3 × 10−2 mbar in an Ar atmosphere, while temperature was fixed at 200 °C, obtaining an average deposition rate of 22 nm/min. For AS CdS/CdTe, the activation process was carried out introducing the sample into a quartz tube, which was evacuated until 1.3 × 10−5 mbar. Afterwards, the activation gases were introduced into the quartz tube with the following percentage ratios: 6.25% CHClF2, 18.75% O2 and 75% N2, reaching a total pressure of 800 mbar. Once the gas mixture was inside the tube, the temperature was ramped, and then kept constant at 400 °C for five minutes. Finally, the heat source was turned off, the gases were evacuated, and the vacuum was restored and maintained for ten additional minutes, allowing the samples to cool down. To address the sputtered CdTe properties, we compare it with our BL solar cells in this paper. These were deposited exactly as described above except for the CdTe and the activation. The absorber was deposited by CSS, which is considered a high temperature deposition. In our case, the source of CdTe was heated to 570 °C and the sample to 530 °C. The activation was performed by a CdCl2/MeOH saturated solution and heated to 400 °C in air atmosphere for 30 min. The Cu/Mo back-contact deposition process has been described in our previous works (Camacho-Espinosa et al., 2017). The solar cells’ area was delimited by mechanical scribing and fixed at 20 mm2. XPS measurements were performed using a K-Alpha equipment by Thermo Scientific with an Al X-ray source. For elemental depth profiling of the CdS/CdTe interface, an Ar+ gun of 3 keV was used; spectra were calibrated by using the C 1s emission at 284.6 eV. The film’s crystalline structure was analyzed by grazing incident X-ray diffraction by a Siemens D-5000 diffractometer, using CuKα monochromatic
3. Results and discussion 3.1. Field-Emission Scanning Electron Microscopy (FE-SEM) A critical issue in sputtered CdTe is the pinhole formation (Fig. 1), which was attributed to film stress (Plotnikov et al., 2011). Moreover, an additional problem is the appearance of blisters (Kaminski et al., 2017). Kaminski et al., suggest that the blister formation is caused by Ar trapped into the CdTe lattice, which tends to form bubbles. We also observed the formation of blisters in our samples as shown in Fig. 1c and d. In order to reduce the pinhole formation, we deposited the CdTe film in two stages changing the R.F. Sputtering power as described in experimental section (increasing from 20 W to 40 W). In this way, the sputtered atoms have a lower kinetic energy at the beginning of the deposition, probably causing less defects in the lattice (i.e. less stress). Once a robust layer has formed, we increase the sputtering power to increase the deposition rate. On the other hand, the blisters were almost eliminated mainly by increasing the substrate-target distance and decreasing the activation temperature, avoiding in both cases the gas incorporation. Additionally, we report beneficial effects by improving the substrate cleaning procedure and reducing the sample transfer time between chambers. In Fig. 2c the grains of the sputtered CdTe appear compact and free of pin-holes. Furthermore, to address the effect of the activation, we compared the samples before (as-deposited) and after the activation, both for AS and BL. In Fig. 2a the FE-SEM superficial micrograph of the as-deposited AS sample shows compact grains with sizes ranging from 80 nm to 200 nm. On the contrary, the CdTe grain size of the BL sample (Fig. 2b) is in the range of 3–5 µm (the magnification of the picture has been reduced for clarity). This difference in grain size could be 32
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Hädrich et al., 2009); it is known as an intermixing layer and its presence is considered critical for good performance of CdS/CdTe solar cells (Romeo et al., 2007). The intermixing is a consequence of S and Te interdiffusion, which generates wider interface regions. We registered a similar effect of the activation in our samples, both in AS and BL. Comparing Fig. 3c and d, the sputtered CdTe appears to be more sensitive than BL to the activation process. Although no film damage was generated by the N2/O2/CHClF2 activation, the interface region of the AS sample appears much more extended than the BL sample activated by CdCl2. Fig. 3c shows that the S and In diffused deep into the CdTe and ZnO. On the other hand, the rapid increase in oxygen content near to the glass side in AS activated sample can be reasonably attributed to voids at the CdS/CdTe interface, which were observed in our samples. In the BL sample the same diffusions were registered, but in Fig. 3d they appear less pronounced. This confirms that the optimization of the activation process is even more critical for AS solar cells and, in general, it can be considered to be the bottle neck of this technology. Another interesting consideration is the Cd and Te atomic concentration, which appears improved by the activation process both in the AS and BL samples. In the latter case, the bulk CdTe is almost stoichiometric, as expected to be. Similar discrepancies in the CdTe composition have also been observed by (Kobyakov et al., 2011). Finally, it should be noted that the Cl introduced by the activation processes was detected in small traces only at the interface of the BL sample. On the contrary, in the AS sample the Cl is not detected this might be attributed to the gaseous state of the CHClF2, which could affect the diffusion mechanism through the grain boundaries (Poplawsky et al., 2014; Major, 2016; Humphreys and Hatherly, 2004).
Fig. 2. FE-SEM micrograph of as-deposited (a) AS and (b) BL and after the activation, (c) AS and (d) BL. The magnification of images has been adjusted for clarity.
reasonably attributed to the different deposition temperatures of Sputtering and CSS, as described in the previous section. In both cases the film looks compact, meaning without voids. After the activation, both samples presented clear morphology changes (Fig. 2c and d). Comparing Fig. 2b and d, we can conclude that for the BL sample the activation induced a slight increase in the grain size and soft superficial erosion. However, their shape is similar. On the other hand, for the AS sample (see Fig. 2a and c), a dramatic increase of the grain size can be observed, changing from the nanometer to micrometer range. Moreover, in this case, the grain shape is also different, and the surface appears less compact.
3.3. XRD analysis The Grazing Incident X-Ray Diffraction (GI-XRD) spectra of the AS and BL samples before and after activation are compared in Fig. 4. The AS sample before activation (Fig. 4a) shows a peak corresponding to a cubic CdTe (1 1 1) diffraction plane (PDF 15-0770). The absence of other peaks indicates that the sample has a high preferential orientation along this plane. On the contrary, our as-deposited BL presents most of the peaks associated with CdTe; nevertheless, the preferential orientation along the (1 1 1) plane is maintained, but it is not as pronounced as in the AS. This could be explained by the sensible grain size difference observed in the FE-SEM images. On the other hand, it is observed that the N2/O2/CHClF2 activation promotes a structural change in the allsputtered sample (Fig. 4c), since the spectrum presents a multi-plane orientation with preferential orientation along the (2 2 0) plane. This change in preferential orientation has been associated with twinning
3.2. X-ray photoemission Spectroscopy (XPS) XPS depth profile composition measurements are shown in Fig. 3 They were carried out to study the interdiffusion of the elements through the CdS/CdTe interface, addressing the differences before (Fig. 3a and b) and after (Fig. 3c and d) the activation for AS and BL samples. The AS as-deposited sample presents a well-defined CdS/CdTe interface region, while in the as-deposited BL the elements close to the interface appear more mixed. This could be attributed to the deposition temperature, which is almost 3 times higher in the latter case. Moreover, the CdTe in both cases present a variation from the stoichiometric composition with a sensible excess of Cd in CdTe and in CdS. However, in the first case (AS), this is clearly more pronounced. After the activation, a ternary compound with uniform composition CdS1−xTex is usually observed (Mathew et al., 2012; Potter et al., 2000;
Fig. 3. XPS depth-profile along CdTe/CdS junction (a) AS and (b) BL, and after activation, (c) AS and (d) BL.
Fig. 4. GI-XRD spectrum of ITO/ZnO/CdS/CdTe structure as-deposited: (a) AS and (b) BL and after activation, (c) AS and (d) BL. 33
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Fig. 6. EQE curve obtained for AS and BL CdS/CdTe solar cells activated by N2/ O2/CHClF2 gas mixture and CdCl2/MeOH solution.
Fig. 5. Lattice parameters of AS and BL samples before and after activation with N2/O2/CHClF2 and CdCl2/MeOH respectively.
3.4. Electrical analysis during the recrystallization process, which is common in face-centered cubic materials (Nowell et al., 2015). Moreover, peaks associated with the underlying film such as CdS and ITO were observed. This was probably registered due to uncovered areas of the sample or voids on the CdTe film, like those presented in Fig. 1a. Additionally, small peak was detected at 40.5°. However, its attribution is inconclusive. In Fig. 4d, the structural changes due to the CdCl2 activation of the BL sample were not as noticeable as in the sputtered case, confirming that for the AS sample the chlorine plays a significant role in the recrystallization. Using the peaks’ position in the XRD spectra, the lattice parameter of each was calculated as in (Barrett and Massalski, 1980). When multiple peaks are present, those can be plotted as a function of the Nelson-Riley-Sinclair-Taylor function (NRST) to obtain a lattice parameter of greater precision (a-CdTe) (Abrikosov et al., 1969). Fig. 5 shows the lattice parameter versus the NRST function for AS and BL samples, before and after the activation process. The as-deposited AS spectrum presents only one peak (Fig. 4a). Thus, a constant lattice parameter was determined using the simple relation a = d h2 + k 2 + l 2 , equal to 6.478 Å. Instead, the presence of other peaks in the as-deposited BL spectrum (Fig. 4b), allowed the calculation of the lattice parameter as a function of NRST. From the intercept of the linear regression, the lattice value of 6.488 Å was calculated. It can be noted that this value is greater than CdTe powder (6.481 Å) value (Spalatu et al., 2014), and is in line with values frequently reported for the CSS deposition technique (Spalatu et al., 2015; Moutinho et al., 1998; Gordillo et al., 1995; Moutinho et al., 1999; Romeo et al., 2009; Ikhmayies, 2014; Salavei et al., 2013), suggesting that the films are subjected to strong compressive stress along the plane parallel to the substrate. This has been attributed to the typical CSS high deposition rate, generating a lattice mismatch with CdS (Romeo, 2018). On the contrary, the AS-sample lattice parameter is smaller than the powder value, indicating that in this case the stress in the CdTe lattice is no longer compressive but tensile. Applying the same calculation to the activated sample spectra after the activation, a decrease of the lattice parameter is observed. In the AS-sample, it reduced from 6.478 Å to 6.463 Å, while for the BL-sample, it reduced from 6.488 Å to 6.482 Å. Therefore, we can conclude that the CdCl2 activation of CSS samples relaxes the compressive stress generated by the lattice mismatch (Romeo et al., 1986; Paudel et al., 2011), while the N2/O2/CHClF2 activation increases the tensile stress on the sputtered CdTe.
The normalized external quantum efficiency (EQE) of the AS and BL CdS/CdTe solar cell are shown in Fig. 6. For the AS-sample, a partial collection in short wavelength region (350–520 nm) can be observed. Since the CdS band gap is around 2.45 eV (i.e. 520 nm), this behavior is typical of the CdS/CdTe solar cells when its thickness is small (< 100 nm) (Potter et al., 2000). On the other hand, the gradual EQE increase between 490 nm and 520 nm can be attributed to the interdiffusion of sulphur and tellurium forming the intermixing layer (CdSyTe1−y), which has a lower bandgap than the CdS (Lane, 2006). In the case of the BL solar cell, practically no photons are collected for a wavelength < 475 nm, which can be interpreted as a strong CdS absorption (Hädrich et al., 2011). This is confirmed in the CdS intermix region, where a sharp cut-off is observed. In the region between 550 nm and 835 nm the AS-sample EQE signal is sensibly smaller than that of the BL. Losses in this region were attributed to a combination of effects such as reflection, inherent glass/TCO absorption and photons’ deep penetration (Demtsu and Sites, 2005). In our case, reflection is likely the dominant loss effect, since the EQE presents fringe pattern. Indeed, sputtered CdTe usually presents very smooth surface (Islam et al., 2013; Paudel et al., 2013) and consequently more reflective interfaces can be generated. The good EQE response for the BL sample indicates that the photo-generated carriers in the CdTe can diffuse and be efficiently collected even away from the interface. In the long wavelength region, the AS solar cell exhibited a shift to higher wavelength with respect to the BL. Since for a small interval of x, the band gap of the CdTe1−xSx is lower than that of the CdTe (Lane, 2006; Compaan et al., 1996), this shift is usually considered as a further signature of the CdTe1−x Sx alloy formation (Rejón et al., 2013; Mc Candless, 2001). Therefore, the lower intermix between the CdS and the CdTe for BL sample is confirmed. Considering the enhanced absorption at short wavelengths and the shift on the infrared wavelengths, a higher Jsc is expected for the ASsample. However, Fig. 7 shows that the Jsc for AS-sample (24.6 mA/ cm2) exceed the BL sample (23.4 mA/cm2) only by 1.1 mA/cm2. This suggests that the beneficial improvement of the light absorption is partially compensated by the detrimental reflection effect. Considering the Jsc values reported in the literature for all-sputtered solar cells (Park et al., 2013; Shao et al., 1996a,b; Guo et al., 2015; Paudel et al., 2014a,b; Gupta et al., 2006), we can assume that, activating them with the N2/O2/CHClF2 gas mixture, allows a competitive photogeneration rate. On the other hand, the Voc of our AS is slightly lower than the values reported for this kind of solar cells (Park et al., 2013; Shao et al., 1996a,b; Guo et al., 2015; Paudel et al., 2014a,b; Gupta et al., 2006).
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off region, being sharp for BL, while it is soft for AS. Furthermore, the AS sample showed a shift in longer wavelengths characteristic of intermixing, supporting XPS results. It should be noted that EQE demonstrated the CdTe bulk recombination in AS samples, which is probably connected to film defects. The current density–Voltage curve of the AS solar cell showed good performance in JSC, while voltage is smaller than the average. However, achieved PCE is near to the record values in these kinds of cells (14.4%). Thus, we considered that activation using a N2/O2/CHClF2 gas mixture is promising for future applications in this kind of solar cell technology. Acknowledgements This work was financially supported by CONACYT-SENER LENERSE (Grant no. 254667). Measurements were performed at LANNBIO CINVESTAV-Mérida. Adolfo López Sánchez acknowledges to CONACYT-México for scholarship no. 556160. Authors thank D. Huerta and Adriana Gabriela Gonzalez for their technical help and secretarial assistance.
Fig. 7. Current density-voltage (J-V) curve measured for AS and BL CdS/CdTe solar cells activated by N2/O2/CHClF2 gas mixture and CdCl2/MeOH solution.
The Voc of the AS is also lower than the BL, which is, on the contrary, in the range of the commonly reported values (800–850 mV), indicating a poor diode for AS solar cell (Sites and Pan, 2007; Becker et al., 2017; Mei et al., 2018; Burst et al., 2016). The AS also presents a low FF, which is usually attributed to low shunting and/or high series resistance. In Fig. 7, it is visible that the two JV curves are almost parallel at high forward bias, whereas they have slightly different slopes close to 0 V, that is, AS has a lower shunting resistance (see the inset table in Fig. 7). Another detrimental effect on the FF, is the photocurrent collection voltage dependency, attributed to recombination within the space charge region, which could be attributed to higher defects density at the CdS/CdTe interface (Hegedus et al., 2007). All these considerations are in line with the results presented above in this paper. We can conclude that the N2/O2/CHClF2 activation on AS likely promotes an excess of S diffusion (XPS, EQE), enhancing the tensile stress at the CdS/CdTe interface (XRD), and even damaging the films somewhere (FE-SEM) or increasing the density of recombination centers (EQE and JV). The possible micro-shunts result in poor junction with low Voc, FF (JV). However, the PCE of the all-sputtered solar cell presented here (12%) is not so far from the BL solar cell PCE (13%) and close to the world record values for sputtered CdTe (Compaan et al., 2004; Li et al., 2015). Moreover, considering the effect of the CdCl2 activation on allsputtered solar cells reported in (Dhakal et al., 2012; Islam et al., 2015; Shao et al., 1996a,b), we conclude that the proposed N2/O2/CHClF2 could be the best compromise to fabricate good quality all-sputtered solar cells.
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4. Conclusions Efficiencies over 12% have been obtained in AS CdTe/CdS solar cells with the proposed activation method. After activation, AS samples showed significant structural, chemical and morphological changes. Baseline samples instead showed softer changes. SEM analysis revealed that activation in AS samples promotes grain growth and at the same time loss of compactness. XRD analysis suggests that drastic changes in AS samples are the result of increased film stress after activation, promoting the formation of nano defects. On the other hand, activation in BL samples produces slight morphology changes and no grain growth. In this case, XRD analysis shows that film stress is released after activation. In general, both activation processes promote the interdiffusion between the CdS and CdTe. After the activation intermix region expands in both samples, being larger the region for the AS sample, indicating that CdTe deposited by sputtering is more sensitive to activation. Differences in intermix length are reflected in slope of the EQE cut35
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Islam, M.A., Huda, Q., Hossain, M.S., Aliyu, M.M., Karim, M.R., Sopian, K., Amin, N., 2013. High quality 1 μm thick CdTe absorber layers grown by magnetron sputtering for solar cell application. Curr. Appl. Phys. 13, S115–S121. Islam, M.A., Khandaker, M.U., Amin, N., 2015. Effect of deposition power in fabrication of highly efficient CdS:O/CdTe thin film solar cell by the magnetron sputtering technique. Mater. Sci. Semicond. Proc. 40, 90–98. Kaminski, P.M., Abbas, A., Chen, C., Yilmaz, S., Bittau, F., Bowers, J.W., Walls, J.M., 2015. Internal strain analysis of CdTe thin films deposited by pulsed DC magnetron sputtering. In: Proc. 42nd IEEE Photovoltaic Specialist Conference (PVSC), pp. 1–6. Kaminski, P.M., Abbas, A., Yilmaz, S., Bowers, J.W., Walls, J.M., 2016. High rate deposition of thin film CdTe solar cells by pulsed dc magnetron sputtering. MRS Adv. 1 (14), 917–922. Kaminski, P.M., Yilmaz, S., Abbas, A., Bittau, F., Bowers, J.W., Greenhalgh, R.C., Walls, J.M., 2017. Blistering of magnetron sputtered thin film CdTe devices. In: Proc. 44th IEEE Photovoltaic Specialist Conference (PVSC), pp. 3430–3434. Kim, M., Kim, D., Shim, J.P., Kim, D., Lee, J., 2016. Influence of deposition conditions on properties of all sputtered CdS/CdTe thin film solar cells. J. Nanosci. Nanotechnol. 16 (5), 5308–5311. Kobyakov, P.S., Kephart, J.M., Sampath, W.S., 2011. Sublimation of Mg onto CdS/CdTe films fabricated by advanced deposition system. In: Proc. 37th IEEE Photovoltaic Specialists Conference (PVSC), pp. 002740–002745. Lane, D.W., 2006. A review of the optical band gap of thin film CdSxTe1x. Sol. Energy Mater. Sol. Cells 90, 1169–1175. Lee, T.D., Ebong, A.U., 2017. A review of thin film solar cell technologies and challenges. Renew. Sust. Energy Rev. 70, 1286–1297. Li, B., Liu, J., Xu, G., Lu, R., Feng, L., Wu, J., 2012. Development of pulsed laser deposition for CdS/CdTe thin film solar cells. Appl. Phys. Lett. 101, 1539031–153903-4. Li, H., Liu, X., Du, Z., Yan, B., 2015. Twin boundaries enhanced current transport in 14.4%-efficient CdTe solar cells by RF sputtering. In: Proc. 42th IEEE Photovoltaic Specialist Conference (PVSC), pp. 1–4. Luo, R., Liu, B., Yang, X., Bao, Z., Li, B., Zhang, J., Li, W., Wu, L., Feng, L., 2016. The large-area CdTe thin film for CdS/CdTe solar cell prepared by physical vapor deposition in medium pressure. Appl. Surf. Sci. 360, 744–748. Major, J.D., 2016. Grain boundaries in CdTe thin film solar cells: a review. Semicond. Sci. Tech. 31 093001(1–10). Mathew, X., Cruz, J.S., Coronado, D.R., Millán, A.R., Segura, G.C., Morales, E.R., Martínez, O.S., Garcia, Ch.C., Landa, E.P., 2012. CdS thin film post-annealing and Te–S interdiffusion in a CdTe/CdS solar cell. Sol. Energy 86, 1023–1028. Mc Candless, B.E., 2001. Thermochemical and kinetic aspects of cadmium telluride solar cell processing. MRS Online Proc. Library Arch. 668. Mei, X., Wu, B., Guo, X., Liu, X., Rong, Z., Liu, S., Chen, Y., Qin, D., Xu, W., Hou, L., Chen, B., 2018. Efficient CdTe nanocrystal/TiO2 hetero-junction solar cells with open circuit voltage breaking 0.8 V by incorporating a thin layer of CdS nanocrystal. Nanomaterials 8 (8), 614. Moutinho, H.R., Al-Jassim, M.M., Levi, D.H., Dippo, P.C., Kazmerski, L.L., 1998. Effects of CdCl2 treatment on the recrystallization and electro-optical properties of CdTe thin films. J. Vac. Sci. Technol. A 16 (3), 1251–1257. Moutinho, H.R., Dhere, R.G., Al-Jassim, M.M., Levi, D.H., Kazmerski, L.L., 1999. Investigation of induced recrystallization and stress in close-spaced sublimated and radio-frequency magnetron sputtered CdTe thin films. J. Vac. Sci. Technol. A 17 (4), 1793–1798. Munshi, A.H., Kephart, J., Abbas, A., Raguse, J., Beaudry, J.N., Barth, K., Sampath, W., 2017. Polycrystalline CdSeTe/CdTe absorber cells with 28 mA/cm2 short-circuit current. IEEE J. Photovolt. 8 (1), 310–314. Nowell, M.M., Scarpulla, M.A., Paudel, N.R., Wieland, K.A., Compaan, A.D., Liu, X., 2015. Characterization of sputtered CdTe thin films with electron backscatter diffraction and correlation with device performance. Microsci. Microanal. 21 (4), 927–935. Oehling, S., Lugauer, H.J., Schmitt, M., Heinke, H., Zehnder, U., Waag, A., Becker, C.R., Landwehr, G., 1996. p-type doping of CdTe with a nitrogen plasma source. J. Appl. Phys. 79, 2343–2346. Park, Y., Lee, S., Yi, J., Byung-Duck, Ch., Doyoung, K., Jaehyeong, L., 2013. Sputtered CdTe thin film solar cells with Cu2Te/Au back contact. Thin Solid Films 546, 337–341. Paudel, N.R., Wieland, K.A., Compaan, A.D., 2011. High efficiency sputtered CdS/CdTe cells without CdCl2 activation. MRS Online Proc. Library Arch. 1323. Paudel, N.R., Compaan, A.D., Yan, Y., 2013. Sputtered CdS/CdTe solar cells with
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Solar Energy journal homepage: www.elsevier.com/locate/solener
All-sputtered CdTe solar cell activated with a novel method ⁎
T
E. Camacho-Espinosa , A. López-Sánchez, I. Rimmaudo, R. Mis-Fernández, J.L. Peña Centro de Investigación y de Estudios Avanzados del IPN Unidad Mérida, Applied Physics department, Km. 6 Antigua Carretera a Progreso, A.P. 73-Cordemex, C.P 97310 Mérida, Yucatán, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Sputtering CdTe Solar cells Activation
CdTe thin film solar cells are usually fabricated depositing the absorber layer by Close Space Sublimation (CSS) and Vapor Transport Deposition (VTD), mainly due to the high deposition rate allowed by these techniques. However, most of the solar cell films can be deposited by sputtering, making to deposit also the CdTe by this technique attractive. Indeed, an all-sputtered solar cell could be deposited in one sequential chamber at a relatively low temperature and without breaking the vacuum, resulting in a sensible decrease in the production cost at the industrial level. Usually, treatments based on CdCl2, also known as activations, are applied to the CdTe solar cells to achieve high power conversion efficiency (PCE). However, it was observed that the usual activation methods could be aggressive for all-sputtered CdTe solar cells, damaging the films and resulting in low performance. In this work, we propose a novel activation method based on Nitrogen/Oxygen/CHCIF2 gas mixture, which is more suitable for this kind of device. Morphological, structural, optical and electrical properties were analyzed and compared to our CSS-CdCl2 baseline. A PCE of 12% was achieved for all-sputtered CdTe solar cells.
1. Introduction CdTe-based solar cells are the second most common photovoltaic (PV) technology, representing 5% of the world market (Lee and Ebong, 2017). The success of CdTe thin-film solar cells is based on their low production cost (Reinders et al., 2017). During many years the CdTe solar cell was fabricated with the classical structure TCO/buffer/CdS/ CdTe/back contact, which present several disadvantages and a limited performance. However, the focus has been recently moved to TCO/ MZO/CdSexTe1-x/CdTe/CuTe structure, reporting efficiencies around 19.1% (Munshi et al., 2017). In this work, the classical structure was studied to understand the effect of different activations on CdTe deposited by different techniques. Most of the films that compose the classical CdS/CdTe solar cell can be deposited by the RF-Sputtering technique. On the other hand, the CdTe layer can also be deposited by screen printing, electrodeposition, pulsed laser deposition (PLD), physical vapor deposition (PVD), close space sublimation (CSS), etc. (Ikegami, 1988; Woodcock et al., 1991; Li et al., 2012; Compaan et al., 2004; Luo et al., 2016; Rios-Flores et al., 2012). Among these, the most commonly used are CSS and VTD, because allow the deposition of thick, high-quality films, which usually result in high PCE solar cells. However, at an industrial level, it would be attractive to use the same deposition method for all the layers,
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avoiding the use of different chambers. One sequential chamber was previously proposed by Swanson et al. (2016) for CSS, demonstrating better process repeatability, versatility and reliability. In the case of Sputtering, the use of one sequential chamber would have additional advantages such as smaller heating and cooling times, due to the lower deposition temperature. Furthermore, the throughput would increase avoiding breaking the vacuum. These advantages could compensate the R.F. sputtering low deposition rate. Alternatively, the deposition time would be dramatically reduced by using pulsed DC magnetron Sputtering (Kaminski et al., 2015; Kaminski et al., 2016). All-sputtered (AS) CdTe solar cells have been fabricated before for other groups (Ablekim et al., 2014; Paudel et al., 2014a,b; Kim et al., 2016; Treharne et al., 2012). These groups have carried out the activation treatment using CdCl2, annealing the samples in an air atmosphere, achieving the world record value for sputtered CdTe of 14.4% (Li et al., 2015). However, when the treatment is performed by CdCl2, the solution leaves waste, causing a sensible increase in the solar cellś series resistance, and consequently, a reduction in solar cell PCE (Valdna et al., 2001; Hiie, 2003; Valdna, 1999; Hiie and Valdna, 2001; Bai et al., 2011). In a previous study (Camacho-Espinosa et al., 2017), we observed the presence of various residues, which are difficult to remove, and its removal adds steps to the process. An alternative for CdCl2 activation is the use of difluorochloromethane (CHClF2) gas, which was proposed by Romeo
Corresponding author. E-mail address: [email protected] (E. Camacho-Espinosa).
https://doi.org/10.1016/j.solener.2019.09.023 Received 12 June 2019; Received in revised form 29 August 2019; Accepted 7 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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et al. (2006). The use of a gas, instead of a solution or vapor, could imply some advantages at an industrial level (e.g. storage facilities and waste management). In this paper, we propose the fabrication of allsputtered CdTe solar cell, activated using a N2/O2/CHCIF2 gas mixture, which can be considered as a combination of difluorochloromethane gas with dry air. Activating the solar cell in this way, the air moisture is avoided, allowing a better experimental control. Furthermore, the use of N2 has been reported to have beneficial effects for CdTe (Peña et al., 2014; Romeo et al., 1986; Oehling et al., 1996; Akbarnejad et al., 2016). It should be noted that the use of N2/O2/CHClF2 in the activation process for AS CdTe solar cells has not been studied before, nor its role in structural and opto-electrical changes. Thus, the effects of this novel activation treatment will be addressed by the comparison with our baseline (BL) solar cells, which are made depositing CdTe with CSS and activating with a CdCl2/MeOH solution. The study of the intermixing between CdS and CdTe caused by the N2/O2/CHClF2 activation process was made with X-ray Photoelectron Spectroscopy (XPS). Structural changes were analyzed with grazing incident X-ray diffraction (GIXRD). Field Emission Scanning Electron Microscopy (FE-SEM) was used to study the morphological changes of the CdTe due to the activation. The finished solar cells were characterized by current-voltage (J-V) and external quantum efficiency (EQE).
Fig. 1. FE-SEM micrograph of micro and nano defects observed in sputtered CdTe films, (a, b) pinhole and (c, d) blister.
radiation (λ = 1.5418 Å); the grazing incident angle was 3°. The measurements were recorded for a diffraction angle 2θ ranging from 20° to 80° in steps of 0.02°/3s. The surface morphology was analyzed by a Jeol 7600F FE-SEM in SEI mode, with an energy of 15 kV. The J-V measurements were performed with a Keithley 2420 source-meter under simulated 100 mW/cm2 light intensity generated with halogen lamps. The light intensity calibration was obtained by an Oriel Sol2A solar simulator and its Si reference cell. The photovoltaic parameters open circuit voltage (Voc), short circuit current (Isc), fill factor (FF) and PCE were determined. The external quantum efficiency (EQE) was measured by an USHIO UXL150 Xenon lamp in a Newport 67005 lamp housing, a monochromator Yvon-Jobin H20VIS, and a Newport 71650 calibrated Si diode.
2. Experimental The CdS/CdTe cell structure was prepared in a superstrate configuration, using commercial ITO-coated Corning® glasses as substrates. All the films mentioned below were deposited by RF-sputtering. In particular, the ZnO films (200 nm) were deposited from a metallic Zn target using an Ar/O2 reactive atmosphere, at the power, pressure and temperature conditions of 80 W, 2.6x10-2 mbar and 400 °C respectively. The CdS films (120 nm) were grown in a pure Ar atmosphere from a CdS target at the power, pressure and temperature conditions of 35 W, 3.3 × 10−2 mbar and 300 °C respectively. The CdTe film (~4 µm) was grown from a target with 4 N purity and 1 in. in diameter. The deposition was carried out in two steps varying the deposition power, during the first 90 min the power was fixed at 20 W and in the last 90 min the power was increased to 40 W. The working pressure was maintained constant at 2.3 × 10−2 mbar in an Ar atmosphere, while temperature was fixed at 200 °C, obtaining an average deposition rate of 22 nm/min. For AS CdS/CdTe, the activation process was carried out introducing the sample into a quartz tube, which was evacuated until 1.3 × 10−5 mbar. Afterwards, the activation gases were introduced into the quartz tube with the following percentage ratios: 6.25% CHClF2, 18.75% O2 and 75% N2, reaching a total pressure of 800 mbar. Once the gas mixture was inside the tube, the temperature was ramped, and then kept constant at 400 °C for five minutes. Finally, the heat source was turned off, the gases were evacuated, and the vacuum was restored and maintained for ten additional minutes, allowing the samples to cool down. To address the sputtered CdTe properties, we compare it with our BL solar cells in this paper. These were deposited exactly as described above except for the CdTe and the activation. The absorber was deposited by CSS, which is considered a high temperature deposition. In our case, the source of CdTe was heated to 570 °C and the sample to 530 °C. The activation was performed by a CdCl2/MeOH saturated solution and heated to 400 °C in air atmosphere for 30 min. The Cu/Mo back-contact deposition process has been described in our previous works (Camacho-Espinosa et al., 2017). The solar cells’ area was delimited by mechanical scribing and fixed at 20 mm2. XPS measurements were performed using a K-Alpha equipment by Thermo Scientific with an Al X-ray source. For elemental depth profiling of the CdS/CdTe interface, an Ar+ gun of 3 keV was used; spectra were calibrated by using the C 1s emission at 284.6 eV. The film’s crystalline structure was analyzed by grazing incident X-ray diffraction by a Siemens D-5000 diffractometer, using CuKα monochromatic
3. Results and discussion 3.1. Field-Emission Scanning Electron Microscopy (FE-SEM) A critical issue in sputtered CdTe is the pinhole formation (Fig. 1), which was attributed to film stress (Plotnikov et al., 2011). Moreover, an additional problem is the appearance of blisters (Kaminski et al., 2017). Kaminski et al., suggest that the blister formation is caused by Ar trapped into the CdTe lattice, which tends to form bubbles. We also observed the formation of blisters in our samples as shown in Fig. 1c and d. In order to reduce the pinhole formation, we deposited the CdTe film in two stages changing the R.F. Sputtering power as described in experimental section (increasing from 20 W to 40 W). In this way, the sputtered atoms have a lower kinetic energy at the beginning of the deposition, probably causing less defects in the lattice (i.e. less stress). Once a robust layer has formed, we increase the sputtering power to increase the deposition rate. On the other hand, the blisters were almost eliminated mainly by increasing the substrate-target distance and decreasing the activation temperature, avoiding in both cases the gas incorporation. Additionally, we report beneficial effects by improving the substrate cleaning procedure and reducing the sample transfer time between chambers. In Fig. 2c the grains of the sputtered CdTe appear compact and free of pin-holes. Furthermore, to address the effect of the activation, we compared the samples before (as-deposited) and after the activation, both for AS and BL. In Fig. 2a the FE-SEM superficial micrograph of the as-deposited AS sample shows compact grains with sizes ranging from 80 nm to 200 nm. On the contrary, the CdTe grain size of the BL sample (Fig. 2b) is in the range of 3–5 µm (the magnification of the picture has been reduced for clarity). This difference in grain size could be 32
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Hädrich et al., 2009); it is known as an intermixing layer and its presence is considered critical for good performance of CdS/CdTe solar cells (Romeo et al., 2007). The intermixing is a consequence of S and Te interdiffusion, which generates wider interface regions. We registered a similar effect of the activation in our samples, both in AS and BL. Comparing Fig. 3c and d, the sputtered CdTe appears to be more sensitive than BL to the activation process. Although no film damage was generated by the N2/O2/CHClF2 activation, the interface region of the AS sample appears much more extended than the BL sample activated by CdCl2. Fig. 3c shows that the S and In diffused deep into the CdTe and ZnO. On the other hand, the rapid increase in oxygen content near to the glass side in AS activated sample can be reasonably attributed to voids at the CdS/CdTe interface, which were observed in our samples. In the BL sample the same diffusions were registered, but in Fig. 3d they appear less pronounced. This confirms that the optimization of the activation process is even more critical for AS solar cells and, in general, it can be considered to be the bottle neck of this technology. Another interesting consideration is the Cd and Te atomic concentration, which appears improved by the activation process both in the AS and BL samples. In the latter case, the bulk CdTe is almost stoichiometric, as expected to be. Similar discrepancies in the CdTe composition have also been observed by (Kobyakov et al., 2011). Finally, it should be noted that the Cl introduced by the activation processes was detected in small traces only at the interface of the BL sample. On the contrary, in the AS sample the Cl is not detected this might be attributed to the gaseous state of the CHClF2, which could affect the diffusion mechanism through the grain boundaries (Poplawsky et al., 2014; Major, 2016; Humphreys and Hatherly, 2004).
Fig. 2. FE-SEM micrograph of as-deposited (a) AS and (b) BL and after the activation, (c) AS and (d) BL. The magnification of images has been adjusted for clarity.
reasonably attributed to the different deposition temperatures of Sputtering and CSS, as described in the previous section. In both cases the film looks compact, meaning without voids. After the activation, both samples presented clear morphology changes (Fig. 2c and d). Comparing Fig. 2b and d, we can conclude that for the BL sample the activation induced a slight increase in the grain size and soft superficial erosion. However, their shape is similar. On the other hand, for the AS sample (see Fig. 2a and c), a dramatic increase of the grain size can be observed, changing from the nanometer to micrometer range. Moreover, in this case, the grain shape is also different, and the surface appears less compact.
3.3. XRD analysis The Grazing Incident X-Ray Diffraction (GI-XRD) spectra of the AS and BL samples before and after activation are compared in Fig. 4. The AS sample before activation (Fig. 4a) shows a peak corresponding to a cubic CdTe (1 1 1) diffraction plane (PDF 15-0770). The absence of other peaks indicates that the sample has a high preferential orientation along this plane. On the contrary, our as-deposited BL presents most of the peaks associated with CdTe; nevertheless, the preferential orientation along the (1 1 1) plane is maintained, but it is not as pronounced as in the AS. This could be explained by the sensible grain size difference observed in the FE-SEM images. On the other hand, it is observed that the N2/O2/CHClF2 activation promotes a structural change in the allsputtered sample (Fig. 4c), since the spectrum presents a multi-plane orientation with preferential orientation along the (2 2 0) plane. This change in preferential orientation has been associated with twinning
3.2. X-ray photoemission Spectroscopy (XPS) XPS depth profile composition measurements are shown in Fig. 3 They were carried out to study the interdiffusion of the elements through the CdS/CdTe interface, addressing the differences before (Fig. 3a and b) and after (Fig. 3c and d) the activation for AS and BL samples. The AS as-deposited sample presents a well-defined CdS/CdTe interface region, while in the as-deposited BL the elements close to the interface appear more mixed. This could be attributed to the deposition temperature, which is almost 3 times higher in the latter case. Moreover, the CdTe in both cases present a variation from the stoichiometric composition with a sensible excess of Cd in CdTe and in CdS. However, in the first case (AS), this is clearly more pronounced. After the activation, a ternary compound with uniform composition CdS1−xTex is usually observed (Mathew et al., 2012; Potter et al., 2000;
Fig. 3. XPS depth-profile along CdTe/CdS junction (a) AS and (b) BL, and after activation, (c) AS and (d) BL.
Fig. 4. GI-XRD spectrum of ITO/ZnO/CdS/CdTe structure as-deposited: (a) AS and (b) BL and after activation, (c) AS and (d) BL. 33
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Fig. 6. EQE curve obtained for AS and BL CdS/CdTe solar cells activated by N2/ O2/CHClF2 gas mixture and CdCl2/MeOH solution.
Fig. 5. Lattice parameters of AS and BL samples before and after activation with N2/O2/CHClF2 and CdCl2/MeOH respectively.
3.4. Electrical analysis during the recrystallization process, which is common in face-centered cubic materials (Nowell et al., 2015). Moreover, peaks associated with the underlying film such as CdS and ITO were observed. This was probably registered due to uncovered areas of the sample or voids on the CdTe film, like those presented in Fig. 1a. Additionally, small peak was detected at 40.5°. However, its attribution is inconclusive. In Fig. 4d, the structural changes due to the CdCl2 activation of the BL sample were not as noticeable as in the sputtered case, confirming that for the AS sample the chlorine plays a significant role in the recrystallization. Using the peaks’ position in the XRD spectra, the lattice parameter of each was calculated as in (Barrett and Massalski, 1980). When multiple peaks are present, those can be plotted as a function of the Nelson-Riley-Sinclair-Taylor function (NRST) to obtain a lattice parameter of greater precision (a-CdTe) (Abrikosov et al., 1969). Fig. 5 shows the lattice parameter versus the NRST function for AS and BL samples, before and after the activation process. The as-deposited AS spectrum presents only one peak (Fig. 4a). Thus, a constant lattice parameter was determined using the simple relation a = d h2 + k 2 + l 2 , equal to 6.478 Å. Instead, the presence of other peaks in the as-deposited BL spectrum (Fig. 4b), allowed the calculation of the lattice parameter as a function of NRST. From the intercept of the linear regression, the lattice value of 6.488 Å was calculated. It can be noted that this value is greater than CdTe powder (6.481 Å) value (Spalatu et al., 2014), and is in line with values frequently reported for the CSS deposition technique (Spalatu et al., 2015; Moutinho et al., 1998; Gordillo et al., 1995; Moutinho et al., 1999; Romeo et al., 2009; Ikhmayies, 2014; Salavei et al., 2013), suggesting that the films are subjected to strong compressive stress along the plane parallel to the substrate. This has been attributed to the typical CSS high deposition rate, generating a lattice mismatch with CdS (Romeo, 2018). On the contrary, the AS-sample lattice parameter is smaller than the powder value, indicating that in this case the stress in the CdTe lattice is no longer compressive but tensile. Applying the same calculation to the activated sample spectra after the activation, a decrease of the lattice parameter is observed. In the AS-sample, it reduced from 6.478 Å to 6.463 Å, while for the BL-sample, it reduced from 6.488 Å to 6.482 Å. Therefore, we can conclude that the CdCl2 activation of CSS samples relaxes the compressive stress generated by the lattice mismatch (Romeo et al., 1986; Paudel et al., 2011), while the N2/O2/CHClF2 activation increases the tensile stress on the sputtered CdTe.
The normalized external quantum efficiency (EQE) of the AS and BL CdS/CdTe solar cell are shown in Fig. 6. For the AS-sample, a partial collection in short wavelength region (350–520 nm) can be observed. Since the CdS band gap is around 2.45 eV (i.e. 520 nm), this behavior is typical of the CdS/CdTe solar cells when its thickness is small (< 100 nm) (Potter et al., 2000). On the other hand, the gradual EQE increase between 490 nm and 520 nm can be attributed to the interdiffusion of sulphur and tellurium forming the intermixing layer (CdSyTe1−y), which has a lower bandgap than the CdS (Lane, 2006). In the case of the BL solar cell, practically no photons are collected for a wavelength < 475 nm, which can be interpreted as a strong CdS absorption (Hädrich et al., 2011). This is confirmed in the CdS intermix region, where a sharp cut-off is observed. In the region between 550 nm and 835 nm the AS-sample EQE signal is sensibly smaller than that of the BL. Losses in this region were attributed to a combination of effects such as reflection, inherent glass/TCO absorption and photons’ deep penetration (Demtsu and Sites, 2005). In our case, reflection is likely the dominant loss effect, since the EQE presents fringe pattern. Indeed, sputtered CdTe usually presents very smooth surface (Islam et al., 2013; Paudel et al., 2013) and consequently more reflective interfaces can be generated. The good EQE response for the BL sample indicates that the photo-generated carriers in the CdTe can diffuse and be efficiently collected even away from the interface. In the long wavelength region, the AS solar cell exhibited a shift to higher wavelength with respect to the BL. Since for a small interval of x, the band gap of the CdTe1−xSx is lower than that of the CdTe (Lane, 2006; Compaan et al., 1996), this shift is usually considered as a further signature of the CdTe1−x Sx alloy formation (Rejón et al., 2013; Mc Candless, 2001). Therefore, the lower intermix between the CdS and the CdTe for BL sample is confirmed. Considering the enhanced absorption at short wavelengths and the shift on the infrared wavelengths, a higher Jsc is expected for the ASsample. However, Fig. 7 shows that the Jsc for AS-sample (24.6 mA/ cm2) exceed the BL sample (23.4 mA/cm2) only by 1.1 mA/cm2. This suggests that the beneficial improvement of the light absorption is partially compensated by the detrimental reflection effect. Considering the Jsc values reported in the literature for all-sputtered solar cells (Park et al., 2013; Shao et al., 1996a,b; Guo et al., 2015; Paudel et al., 2014a,b; Gupta et al., 2006), we can assume that, activating them with the N2/O2/CHClF2 gas mixture, allows a competitive photogeneration rate. On the other hand, the Voc of our AS is slightly lower than the values reported for this kind of solar cells (Park et al., 2013; Shao et al., 1996a,b; Guo et al., 2015; Paudel et al., 2014a,b; Gupta et al., 2006).
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off region, being sharp for BL, while it is soft for AS. Furthermore, the AS sample showed a shift in longer wavelengths characteristic of intermixing, supporting XPS results. It should be noted that EQE demonstrated the CdTe bulk recombination in AS samples, which is probably connected to film defects. The current density–Voltage curve of the AS solar cell showed good performance in JSC, while voltage is smaller than the average. However, achieved PCE is near to the record values in these kinds of cells (14.4%). Thus, we considered that activation using a N2/O2/CHClF2 gas mixture is promising for future applications in this kind of solar cell technology. Acknowledgements This work was financially supported by CONACYT-SENER LENERSE (Grant no. 254667). Measurements were performed at LANNBIO CINVESTAV-Mérida. Adolfo López Sánchez acknowledges to CONACYT-México for scholarship no. 556160. Authors thank D. Huerta and Adriana Gabriela Gonzalez for their technical help and secretarial assistance.
Fig. 7. Current density-voltage (J-V) curve measured for AS and BL CdS/CdTe solar cells activated by N2/O2/CHClF2 gas mixture and CdCl2/MeOH solution.
The Voc of the AS is also lower than the BL, which is, on the contrary, in the range of the commonly reported values (800–850 mV), indicating a poor diode for AS solar cell (Sites and Pan, 2007; Becker et al., 2017; Mei et al., 2018; Burst et al., 2016). The AS also presents a low FF, which is usually attributed to low shunting and/or high series resistance. In Fig. 7, it is visible that the two JV curves are almost parallel at high forward bias, whereas they have slightly different slopes close to 0 V, that is, AS has a lower shunting resistance (see the inset table in Fig. 7). Another detrimental effect on the FF, is the photocurrent collection voltage dependency, attributed to recombination within the space charge region, which could be attributed to higher defects density at the CdS/CdTe interface (Hegedus et al., 2007). All these considerations are in line with the results presented above in this paper. We can conclude that the N2/O2/CHClF2 activation on AS likely promotes an excess of S diffusion (XPS, EQE), enhancing the tensile stress at the CdS/CdTe interface (XRD), and even damaging the films somewhere (FE-SEM) or increasing the density of recombination centers (EQE and JV). The possible micro-shunts result in poor junction with low Voc, FF (JV). However, the PCE of the all-sputtered solar cell presented here (12%) is not so far from the BL solar cell PCE (13%) and close to the world record values for sputtered CdTe (Compaan et al., 2004; Li et al., 2015). Moreover, considering the effect of the CdCl2 activation on allsputtered solar cells reported in (Dhakal et al., 2012; Islam et al., 2015; Shao et al., 1996a,b), we conclude that the proposed N2/O2/CHClF2 could be the best compromise to fabricate good quality all-sputtered solar cells.
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