- Email: [email protected]
Pressure effects in RF and DC sputtered Sb2Te3 thin films and its applications into solar cells
Materials Science in Semiconductor Processing 112 (2020) 104876
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Pressure effects in RF and DC sputtered Sb2Te3 thin films and its applications into solar cells �ndez-P�erez c, J. Aguilar-Herna �ndez b, J. R. Mendoza-P�erez a, *, J. Sastre-Hern� andez b, M.A. Herna a d a A. Del Oso , G. Santana-Rodríguez , J.J. Lizardi a
Universidad Aut� onoma de la Ciudad de M�exico, M�exico City, 09790, Mexico Escuela Superior de Física y Matem� aticas, Instituto Polit�ecnico Nacional, M�exico City, 07738, Mexico Instituto Polit�ecnico Nacional, DIMM-ESIQIE, M�exico City, Mexico d Instituto de Investigaciones en Materiales, UNAM, M�exico City, Mexico b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Sb2Te3 Sputtering RF and DC pþ material CdTe solar cells
In this work we developed the synthesis and characterization of Sb2Te3 thin films, which were grown by RF as well as DC sputtering as a function of the deposition pressure (Pd), 5–15 mTorr. Sb2Te3 films were characterized through X-Ray Diffraction, Energy Dispersive Spectrometry (EDS), Scanning Electron Microscopy (SEM) and electrical resistivity measurements. As a part, contact layer, of the CdTe-based Solar Cell, Current-Voltage (I–V) as well as External Quantum Efficiency (EQE) measurements were carried out. Our results indicate that the radiofrequency sputtered Sb2Te3 are polycrystalline with predominant rhombohedral crystalline structure, whereas the DC-sputtered films crystallized mainly in the monoclinical structure. Both set of samples showed a resistivity of the order of 10 4 Ω-cm. Concerning the CdTe-based solar cell, the incorporation of the Sb2Te3 as a back surface film in the back contact improves the solar cell efficiency up to 8.01%, 10 mTorr pressures into the growth chamber, as compared to the CdTe-based solar cell, 4.82% efficiency, without the Sb2Te3 layer.
1. Introduction One of the main drawbacks of the solar cells based on CdTe films is the forming of a good ohmic contact. Due to its electron affinity value (4.28 eV) is necessary a metal with a work function about 5.7 eV, for a good ohmic contact to CdTe surface. The better approach to overcome this problem is the formation of a pþ region between CdTe and the metal electrode to provide the advantageous electrical properties. Usually a cooper layer is used to form the back contacts with a low barrier high, however cooper diffuse along the grain boundaries to the main junction resulting in a degradation of the solar cell device. Sb2Te3 (antimony telluride) is a binary semiconducting compound in the forma A2B3, which is a promising candidate, as a thin film, for the back contact in photovoltaic devices because of its excellent physical properties like low thermal conductivity, low electrical resistivity, low bandgap and long term stability [1–4]. Sb2Te3 have been obtained by different growth techniques, like MOCVD [5,6], electrochemical [7,8], co-evaporation [9] among others. RF and DC sputtering offer some advantages in the growth of Sb2Te3
films for solar cell applications, i.e., desirable stoichiometry, short fabrication process, fine thickness control, among other [10,11]. The Sb2Te3 film analysis was carried out by a procedure similar to Gorji et al. [17]. In this work it is presented the synthesis, as well the structural, morphological and electrical characterization of the Sb2Te3 films, grown by RF and DC sputtering, as a function of the chamber pressure, and also the performance of the CdTe-based solar cell using the Sb2Te3 layer as a back contact. 2. Experimental section and characterization methods 2.1. Deposition of Sb2Te3 Soda lime glass (SLG) was used as substrates for films deposition. They were cleaned using alkaline biodegradable liquid soap in deionized water prior to dip a 10 min in isopropyl alcohol solution in an ultrasonic bath, thereafter they were dried with dry nitrogen. A Sb2Te3 target, 75.3 mm in diameter, was employed for the sputtering process; the distance
* Corresponding author. E-mail address: [email protected] (R. Mendoza-P�erez). https://doi.org/10.1016/j.mssp.2019.104876 Received 30 September 2019; Received in revised form 27 November 2019; Accepted 5 December 2019 Available online 29 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
Table 1 Labels, growth rate, thickness and roughness values for the Sb2Te3 samples grow at different chamber pressures by RF as well as DC Sputtering. Thin Films
Devices
Sputtering System
Label
Growth rate (nm/min)
Thickness (nm)
Roughness (nm)
Label
Deposition time (min)
RF
RF3-5 RF1-10 RF2-15 DC3-5 DC1-10 DC2-15
9.8 5.0 2.5 13.1 5.6 3.4
588 300 150 786 336 204
22 26 32 29 44 56
CSa CSb CSc CSd CSe CSf
2.6 4.8 9.9 2.0 4.5 7.4
DC
Fig. 1. XRD patterns different pressure.
of
Sb2Te3
samples
deposited
by
RF-Sp
at
Fig. 2. XRD patterns different pressure.
of
Sb2Te3
samples
deposited
by
DC-Sp
at
Fig. 3. %Sb and %Te composition for RF Sb2Te3 samples as a function of deposition pressure.
Fig. 4. %Sb and %Te composition for DC Sb2Te3 samples as a function of deposition pressure.
Table 2 EDS and resistivity measurements for Sb2Te3 samples growing at different pressures by RF and DC Sputtering techniques. Sputtering-System
RF
Sample name
RF3
RF1
RF2
DC3
DC1
DC2
Pd (mTorr) % Sb % Te Resistivity (10
5 65.79 34.21 5.87
10 65.49 34.51 6.04
15 67.28 32.72 9.05
5 68.05 31.95 5.31
10 67.47 32.53 8.19
15 66.92 33.08 9.62
4
Ω-cm)
between magnetrons (Kurt Lesker) and the substrate was of 10 cm. The substrate was kept in rotation, 5 rpm, during the growth of the films in order to enhance the uniformity of the deposited films. Sputtering pro cess was performed at a constant power of 60 W; the base pressure, in the growth chamber, was �10 6 Torr, and the target surface cleaned by pre-sputtering for 5 min under pure Ar gas, meanwhile the substrate was covered with a shutter. Once the preparation and cleaning of the target was finished, the deposition film process started. The substrate tem perature was kept constant at 200 � C, by using a power supply (Sorense), during the deposition time, 60 min. A flow rate of Ar was provided through a mass flow controller varying from 4 to 23 sccm (standard cubic centimeter per minute), into the chamber in order to maintain a
DC
2
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
complete CdTe-based solar cells were completed by growing a 4–6 μm thick CdTe film by Close Space Sublimation (CSS), thereafter this structure was submitted to thermal annealing, with sprayed layer of CdCl2, at 400 � C during 30 min [16]. Finally, two metal circulars of 0.09 cm2 layers of Cu and Mo, 15 and 500 nm thick respectively, were deposited over the same Sb2Te3 films area by DC sputtering as described in previous work [12]. For solar cells devices the configuration was glass/SnO2:F/ZnO þ CdS-TT/CdTe þ CdCl2-TT, over this final layer the Sb2Te3 thin film was grown. In this case the thickness of the Sb2Te3 layer was controlled to be approximately 25 nm for each one of the pressures in the growth chamber. The superficial of the films was in 0.09 cm2, this area coincides with the area of the Cu–Mo back contact, in order not to have additional collection carriers come from outside the bounds of the contacts.
Fig. 5. EDS measurement of the RF2 Sb2Te3 sample.
2.3. Deposition of Sb2Te3 at different chamber pressures
working pressure in the range 5–15 mTorr. Film thickness was moni tored during the deposition with a Inficon SQM-160 instrument. For RF and DC sputtering system a source power, Kurt Lesker R301 and a Mag Technology, were used, respectively.
The Sb2Te3 samples grown either by RF-sputtering or DC-sputtering were labeled according to Table 1. The second numbers in the labels correspond to the pressure (into the chamber in mTorr). The substrate temperature was fixed at 200 � C during the processing of all samples [10,11,15]. On the other hand, two sets of Sb2Te3 thin-films were used for solar cells applications and labeled as summarized in Table 1.
2.2. Processing of CdTe solar cells ZnO as well as CdS thin films were deposited onto SnO2:F/soda-lime glass substrates also by sputtering RF. These thin films semiconductor structure was annealed at 450 � C, in muffle, during 60 min. The
Fig. 6. SEM images of Sb2Te3 samples by a) RF and b) DC at 5 mTorr.
Fig. 7. SEM images of Sb2Te3 samples by a) RF and b) DC at 10 mTorr. 3
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
Fig. 8. SEM images of Sb2Te3 samples by a) RF-Sp and b) DC at 15 mTorr.
with a 2420 Keithley source meter instrument. 2.7. Measurements for photovoltaics devices (I vs. V and EQE) For the photovoltaic devices, with Sb2Te3 as back contact, the cur rent vs. voltage (I vs. V) were measured using an Oriel-Newport-Sol3A Class AAA Solar simulator (100mW/cm2 – AM1.5). Finally, a QEPVSIb Oriel Instrument was used to acquire external quantum efficiency (EQE). 3. Results and discussion 3.1. Profile measurements The values for growth rate, thickness and roughness results of Sb2Te3 samples grown by RF and DC sputtering as a function of deposition pressure are also summarized in Table 1. The experimental results show that when the pressure is increased from 5 to 15 mTorr, the layer thickness decreases as well as the grown rate decreases from 9.8 to 2.5 nm/min whereas the roughness increases from 22 to 32 nm for RF samples. For DC samples, there is a similar behavior with respect to the RF samples, the grown rate varies from 13.1 to 3.4 nm/min, and the roughness is increases respect to RF samples of 29–56 nm. On the other hand, Sb2Te3 thin films used for solar cell fabrications, the growth rate was adjusted in order to obtain a similar thickness (~25 nm) in all samples.
Fig. 9. Porosity of the Sb2Te3 samples vs. deposition pressure for RF and DC.
2.4. Thickness and roughness analysis Sb2Te3 film surface were analyzed with a step profiler (Sloan Dektak II) to measure the thickness and the roughness. The analysis was per formed in several regions for an average and the parameters used was a range of scanning of 3 mm, intensity of force 5 mg, frequency of 100 Hz and velocity of scanning 100 μm/s.
3.2. XRD results The XRD patterns of the Sb2Te3 samples deposited with the RF-Sp system are presented in Fig. 1. Films are of polycrystalline nature; all the patterns show a visible contribution from amorphous substrate being more intense as thickness diminishes. Regardless the deposition pres sure, the films present the peaks of the rhombohedral structure and preferential orientation on (0114) plane, according to ICDD-PDF files of Sb2Te3 (03-065-3678 and 01-089-6185). However, as pressure increases preferential orientation changes from (0114) to (015) plane. Preferential orientation on (0114) family planes was reported to films deposited by thermal co-evaporation [13]. In addition, the formation of a small quantity of monoclinic phase is promoted at higher deposition pressure as revealed by the low intensity XRD peaks observed at 2θ ¼ 15.7� , 20.9� and 24.7� . Fig. 2 presents the XRD patterns of the films deposited by DC. Sharp and well-defined peaks are observed to all samples indicating high crystalline quality. Rhombohedral crystalline structure is confirmed to
2.5. Crystalline structure and surface composition analysis Crystalline structure was determined by the X-ray diffraction pat terns (XRD), by means of a D8 Bruker X-ray system using the CuKa, 1.54 A, line. The composition of the films was determined by EDS studies with a JEOL JSM-6701 model microscope using an acceleration voltage of 5 kV, allowing a resolution of 2 nm with an Oxford Instrument Energy 200 EDS analytical system method and the morphological analysis was made using a SEM equipment. 2.6. Resistivity analysis of the Sb2Te3 films The resistivity of the Sb2Te3 films was measured using a four-probe, 4
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
3.3. Composition and resistivity The atomic composition (Sb and Te) and the resistivity values ob tained for RF and DC samples as a function of deposition pressure are summarized in Table 2 and Figs. 3 and 4. For RF Sb2Te3 films is possible to observed that the %Sb composition slightly decreases while the re sistivity also decreases. On the other hand, the pressure in the chamber decreases to the lowest value, when the %Te composition increases. Similar resistivity values between RF and DC Sb2Te3 films were obtained. When the deposition pressure inside of the RF sputtering system decreases the electrical resistivity also decreases (Table 2) and a large amount of Te in the composition of the film is obtained, this relation between the %Te composition and the resistivity value is expected [11]. In general, the modification of the growth parameters influences the electrical transport properties of the Sb2Te3 films processed by RF and DC sputtering systems, giving rise to a slight decrease in the resistivity values. When the pressure in the chamber of the DC system was set at 5 mTorr, the Sb2Te3 samples processed (DC3) showed the highest growth rate (~13.1 nm/min) and the lowest Te atomic composition (31.95%), see Fig. 4. For the same deposition pressure value, the RF3 sample had the highest growth rate (9.8 nm/min) and a high Te atomic composition (~34.21%). On the other hand, a high Te atomic composition, low resistivity value and a preferential orientation plane (0114) was observed for Sb2Te3 samples processed by RF at lowest deposition pressure (Fig. 3). We obtained an %At. (Te/Sb) ratio close to 0.5 and resistivity’s values around 5:3 9:6 x10 4 Ω cm (conductivity �105 (Ω-m) 1) in both sputtering system in agreement to Eliana et al. for Sb2Te3 thin films growth by co-evaporation technique [13]. In Fig. 5, EDS analysis shows the presence of the expected elements for the Sb2Te3 compound and the substrate (glass). However, the in tensity of the band associated to the oxygen (O) in relation to the in tensity of the band associated to the Silicon (Si), indicates a high amount of O in the composition of the samples, therefore, oxides could be created, which could lead to possible contamination of the films. Nevertheless, no one diffraction line associated to Sb2O5 or TeO2 in the XRD diffractograms of the Sb2Te3 samples, was observed. The EDS was taken over the full area shown in red on the SEM image of the RF2 Sb2Te3 sample, see Fig. 8a). 3.4. Morphology SEM images as a function of the deposition pressure of Sb2Te3 sam ples are shown in Figs. 6a–8a. Grain sizes about 150, 100 and 50 nm were obtained when the samples were processed at 5 mTorr, 10 mTorr and 15mTorr by RF, respectively. This means that, larger grains are obtained when the pressure in the chamber decreased; possibly due to a higher coalescence of smaller grains. SEM images of Sb2Te3 DC samples as a function of the pressure inside the chamber are shown in Figs. 6b–8b, where is possible to observe that grains are less compact and more irregular when the deposition pressure is 15 mTorr. Moreover, when the deposition pressure increases to 15 mTorr, the grain size increases from 150 to 250 nm without any direc tional orientation. A similar behavior is observed on flexible polyamide substrates, where oriented grains and a rapid accumulation of microflakes are promoted, resulting in a diversity of flake sizes [14]. Although the grain size is close to 150 nm for samples processed by DC and RF, the morphology was more compact for samples processed by RF. The porosity values as a function of the deposition pressure for Sb2Te3 samples by RF and DC are showed in Fig. 9. The inserted images show the calculated porosity using the ImageJ 1.52a software. Accord ing to Fig. 9, if the deposition pressure increases, the porosity for RF samples decreases from 8.3 to 6.7% and increases from 7.09 to 9.5% for
Fig. 10. J vs. V for thin films-solar cells device with Sb2Te3 by a) RF and b) DC at different deposition pressure.
these films from the comparison of the diffraction reflections with PDF files. In this case, only the film deposited at 5 mTorr presents a small peak of monoclinic Sb2Te3 phase (PDF 01-080-8016). The films pre pared at 10 and 15 mTorr exhibit preferential orientation to (015) plane. As deposition pressure decreases to 5 mTorr it appears a low-intensity reflection of monoclinic phase as well as it occurs a preferential orien tation change to (0014) plane [11]. On the other hand, a similar behavior for XRD results we observe for Sb2Te3 films deposited on flexible polyimide substrates by DC-Sp [13].
5
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
Table 3 Photovoltaic parameters for Sb2Te3 thin films-solar cells deposited by RF and DC Sputtering system at different pressure. Sputtering-System
Without Sb2Te3
RF
DC
Solar cell
CS
CSa
CSb
CSc
CSd
CSe
CSf
Pd (mTorr) Voc (V) Jsc (mA/cm2) Jsc-expected (mA/cm2)a FF η (%) Rs (Ω-cm) Rshunt (Ω-cm)
– 0.558 20.18 15.80 0.41 4.82 130.4 1402.3
5 0.609 22.69 18.75 0.42 6.02 133.9 1333.8
10 0.640 25.38 21.64 0.49 8.01 126.6 1990.5
15 0.550 21.82 17.69 0.31 3.82 252.6 666.2
5 0.625 23.33 20.19 0.44 6.42 120.7 1228.9
10 0.527 21.84 18.46 0.39 4.49 134.1 1251.1
15 0.582 15.03 13.98 0.36 3.15 252.9 1341.6
a
Jsc.- calculated in base of EQE curve theory.
DC samples. The porosity of Sb2Te3 samples processed by RF is lower than the porosity of samples processed by DC for higher growth pressure [10,17]. In Fig. 9, we have observed for Sb2Te3 thin films by DC that’s correct that more pressure leads to more porosity, but the stoichiometry (%At. Te/Sb ratio is close to 0.5). For other hand, for thin films of Sb2Te3 by RF more pressure leads to less porosity, but the stoichiometry% At. Te/Sb ratio is moves away from 0.5 and the resistivity’s values increase. If the porosity is the unique property that consider for CdTe solar cells with Sb2Te3, there is not an optimal pressure value, because other properties of the Sb2Te3 thin films must be considered such as: stoichi ometry between Te and Sb, grain size and resistivity.
cells (Fig. 11a) with Sb2Te3 films by RF-Sp system where the EQE for these devices is above the 70%. In Fig. 11b we observe the EQE of the devices that were finalized with Sb2Te3 films by DC-Sp system and the greatest value is around the 65%. For CSb RF solar cell is clearly seen the influence of the Sb2Te3 thin films in the large absorber wavelength, therefore the enhancement of EQE is due to a possible improvement of collection and carrier transportation. The absorption for light of the cells with RF Sb2Te3 thin films it is almost the same amount but is in a certainly smaller for cells with DC Sb2Te3 thin films. 4. Conclusions
3.5. J vs. V characterization
The influence of deposition pressure on the properties of Sb2Te3 film growth by RF and DC sputtering system and its relationship with XRD, atomic composition, electrical resistivity and morphological measure ments were studied. The thickness increased systematically with the deposition pressure decreased, so, higher growth rates were obtained as a function of the pressure in the chamber. The structural analysis shows a higher crystalline quality for RF and DC samples when de deposition pressure decrease. A Te atomic composition of 34% was determined for RF samples. More homogeneous grains for Sb2Te3 films deposited by RF were observed. The Sb2Te3 thin films grown by RF are less porous (~7%) than the DC samples (~9.5%) processed at 15 mTorr. Compact grains in the morphology of Sb2Te3 samples, implies a reduction in the quantity of pinholes, which promotes lower resistivity (~6 � 10 4 Ohm-cm), that is very desirable for a back-surface film in a typical solar cell configura tion. We show that the pressure in the chamber is an important exper imental parameter that influences the porosity and stoichiometry of the Sb2Te3 films. Six CdTe photovoltaic devices with Sb2Te3 processed by RF and DC sputtering are compared in this study. A conversion efficiency of 8.01% was obtained when a RF Sb2Te3 film processed at 10 mTorr was used in the structure of a solar cell. On the other hand, an efficiency of 6.42% was determined when a DC Sb2Te3 film processed at 5 mTorr was inserted in the back contact of a solar device. Finally, the external quantum efficiency of solar cells fabricated with RF Sb2Te3 are improved in almost all the absorber wavelength compared to a cell without Sb2Te3 back contact, possibly indicating an improvement on carrier collection and transportation at back contact in the photovoltaic device.
Fig. 10, show the J vs. V characterizations that were carried out for the solar cells finalized with Sb2Te3 thin films by RF (Fig. 10a) and DC (Fig. 10b) sputtering systems and the output electrical parameters for these devices are summarized in Table 3. The experimental results show that the optimal Sb2Te3 deposition pressure for CdTe solar cells is 10 mTorr for RF Sb2T3 films (CSb) and 5 mTorr for DC Sb2Te3 films (CSd) with efficiencies of 8.01% and 6.42%, respectively. CSb solar cell shows the highest Rshunt for RF Sb2T3 films, while the CSd has the lowest Rs for DC Sb2Te3 films. The conversion efficiency values as a function of the deposition pressure were higher for photovoltaics devices with a Sb2Te3 thin film processed by RF than the fabricated using a Sb2Te3 thin film processed by DC. When a deposition pressure of 10 mTorr is used to processed a RF Sb2Te3 thin film the open circuit voltage (VOC) of the solar cell, is increased dramatically, due to improvement in the morphology, lower percent porosity and higher %Te composition. Moreover, the JSC decreased when the deposition pressure of the DC Sb2Te3 films increased. However, the opposite happened when a RF Sb2Te3 is applying for CdTe solar cells. The behavior of the FF values for solar cells with DC Sb2Te3 thin films was expected, because an increase of FF is due to the applying of Sb2Te3 layer reduce the Schottky barrier. So that, the possible reduction of the Schottky barrier increase the collection and transportation of holes and hence an increase on the current density values was obtained. For other hand, we observed that the solar cells with the lowers re sistivity values has the better photovoltaic response. Although, the sample of Sb2Te3 grown by RF at 10 mTorr has not lowest resistivity value (with uncertainty of �8 � 10 6 Ω-cm), if it has the highest photovoltaic efficiency due to influence of other properties such as: higher stoichiometry between the atomic percentage of Te and Sb (%At. Te/Sb ratio is close to 0.5) and to a larger degree to a lower porosity (~7%), uniform and more compact grain sizes in the order of 100 nm.
CRediT authorship contribution statement �rez: Writing - original draft. Jorge SastreRogelio Mendoza-Pe � �ndez: Writing - original draft. María de los Angeles �ndez Herna Herna �rez: Writing - original draft. Jorge R. Aguilar-Herna �ndez: Writing Pe � A. Del Oso Acevedo: Writing - original draft. original draft. Jose � J. Lizardi Del Angel: Guillermo Santana: Writing - original draft. Jose Writing - original draft.
3.6. QE spectral response Fig. 11, show the External Quantum Efficiency (EQE) for the solar 6
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
�ndez-P�erez through SIP-201954558, Jorge Sastr�eAngeles Herna �ndez through SIP-20196099 and Jorge R. Aguilar Herna �ndez Herna under SIP-20195512 projects. Special thanks for technical support to �jera. Mtro. Carlos Ramos Vilchis and Ing. Benito Ortega Na References [1] A. Romeo, D.L. B€ atzner, H. Zogg, A.N. Tiwari, in: A Comparison of the Vacuum Evaporated CdTe for Substrate and Superstrate Solar Cells. 1–4. Conference: 16th European Photovoltaic Solar Energy Conference and Exhibition, May 2000. [2] D.L. B€ atzner, R. Wendt, A. Romeo, H. Zogg, A.N. Tiwari, A study of the back contacts on CdTe/CdS solar cells, Thin Solid Films 463 (7) (2000) 361–362. Available from, http://www.sciencedirect.com/science/article/pii/S0040609099 008421. [3] N. Romeo, et al., A highly efficient and stable CdTe/CdS thin film solar cell, Sol. Energy Mater. Sol. Cells 58 (1999) 209–218. [4] P. Fan, Z.-H. Zheng, G.-X. Liang, D.-P. Zhang, X.-M. Cai, Thermoelectric characterization of ion beam sputtered Sb2Te3 thin films, J. Alloys Compd. 505 (1) (2010) 278–280. Availablefrom:http://www.sciencedirect.com/science/article/p ii/S092583881001443. [5] G. Bendt, S. Schulz, S. Zastrow, K. Nielsch, Single-source precursor-based deposition of Sb2Te3 films by MOCVD**, Chem. Vap. Depos. 19 (7–8–9) (2013) 235–241. Available from, https://onlinelibrary.wiley.com/doi/abs/10.1002/cvde. 201207044. [6] B. Aboulfarah, D. Sayah, A. Mzerd, A. Giani, A. Boyer, MOCVD Growth of Bi2Te3Sb2Te3 Layers : Effect of Growth Parameters on the Electrical and Thermoelectrical Properties, Moroc. J. Of Condens. Matter 3 (2011) (0). Available from: https://re vues.imist.ma/index.php?journal¼MJCM&page¼article&op¼view&path%5B% 5D¼67. [7] R. Rostek, J. Kottmeier, M. Kratschmer, G. Blackburn, F. Goldschmidtb€ oing, M. Kr€ oner, et al., Thermoelectric characterization of electrochemically deposited Bi2Te3 films accounting for the presence of conductive seed layers [Internet], J Electrochem Soc 160 (9) (2013) D408–D416. Available from: http://jes.ecsdl.or g/content/160/9/D408.abstract. [8] I.-J. Yoo, Y. Song, D. Chan Lim, N.V. Myung, K.H. Lee, M. Oh, et al., Thermoelectric characteristics of Sb2Te3 thin films formed via surfactant-assisted electrodeposition [Internet]. The Royal Society of Chemistry, J Mater Chem A 1 (17) (2013) 5430–5435, https://doi.org/10.1039/C3TA01631E. Available from:. [9] Bin Lv Songbai Hu, Z. Lei, Preparation and characterization of Sb2Te3 thin films by coevaporation, Int J Photoenergy (2010 Jun) {hindawi} Publishing, 2010:4. Available from, https://www.hindawi.com/journals/ijp/2010/476589/cta/. [10] T. Chen, P. Fan, Z. Zheng, D. Zhang, X. Cai, G. Liang, Influence of substrate temperature on structural and thermoelectric properties of antimony telluride thin films fabricated by RF and DC, Cosputtering 41 (4) (2012) 679–683. [11] B. Fang, Z. Zeng, X. Yan, Z. Hu, Effects of annealing on thermoelectric properties of Sb2Te3 thin films prepared by radio frequency magnetron sputtering [Internet], J Mater Sci Mater Electron 24 (4) (2013) 1105–1111, https://doi.org/10.1007/ s10854-012-0888-1. Available from:. [12] J.M. Flores-Marquez, et al., Improving CdS/CdTe thin film solar cell efficiency by optimizing the physical properties of CdS with the application of thermal and chemical treatments, Thin Solid Films 582 (2015) 124–127. [13] Eliana M.F. Vieira, Joana Figueira, Ana L. Pires, Jose Grilo, Manuel F. Silva, Andre M. Pereira, Luis M. Goncalves, Enhanced thermoelectric properties of Sb2Te3 and Bi2Te3 films for flexible thermal sensors, J. Alloys Compd. 774 (2019) 1102–1116, https://doi.org/10.1016/j.jallcom.2018.09.324. [14] Shengfei Shen, Wei Zhu, Yuan Deng, Huaizhou Zhao, Yuncheng Peng, Chuanjun Wang, Enhancing thermoelectric properties of Sb2Te3 flexible thin film through microstructure control and crystal preferential orientation engineering, Appl. Surf. Sci. 341 (2015), https://doi.org/10.1016/j.apsusc.2017.04.074. [15] N.R. Paudel, K.A. Wieland, A.V. Nawarange, D.A. Sanchez, K. Gardinier and A.D. Compaan. Studies of RF Sputtered Sb2Te3 Thin Films for Back Contacts to CdS/ CdTe Solar Cells. 978-1-4244-9965-6/11/$26.00 ©2011 IEEE. [16] O. Vigil-Gal� an, A. Arias-Carbajal, R. Mendoza-P�erez, G. Santana, J. Sastr� eHern� andez, G. Contreras-Puente, A. Morales-Acevedo, M. Tufi~ no-Vel� azquez, Spectral response of CdS/CdTe solar cells obtained with different S/Cd ratios for the CdS chemical bath”, Solar Energy Materials and Solar 90 (15) (August 2006) 2221–2227, https://doi.org/10.1016/j.solmat.2006.02.020. [17] Nima E. Gorji, Rob O’Connor, Andre Mussatto, Matthew Snelgrove, P.G. Mani Gonzalez, Dermot Brabazon, Materialia, Recyclability of Stainless Steel (316L) powder within the additive manufacturing process. https://doi.org/10.1016/j.mt la.2019.100489, 2019.
Fig. 11. EQE for thin films-solar cells with Sb2Te3 by a) RF and b) DC at different deposition pressure.
Acknowledgments The research acknowledgments to IPN for the support: María de los
7
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Pressure effects in RF and DC sputtered Sb2Te3 thin films and its applications into solar cells �ndez-P�erez c, J. Aguilar-Herna �ndez b, J. R. Mendoza-P�erez a, *, J. Sastre-Hern� andez b, M.A. Herna a d a A. Del Oso , G. Santana-Rodríguez , J.J. Lizardi a
Universidad Aut� onoma de la Ciudad de M�exico, M�exico City, 09790, Mexico Escuela Superior de Física y Matem� aticas, Instituto Polit�ecnico Nacional, M�exico City, 07738, Mexico Instituto Polit�ecnico Nacional, DIMM-ESIQIE, M�exico City, Mexico d Instituto de Investigaciones en Materiales, UNAM, M�exico City, Mexico b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Sb2Te3 Sputtering RF and DC pþ material CdTe solar cells
In this work we developed the synthesis and characterization of Sb2Te3 thin films, which were grown by RF as well as DC sputtering as a function of the deposition pressure (Pd), 5–15 mTorr. Sb2Te3 films were characterized through X-Ray Diffraction, Energy Dispersive Spectrometry (EDS), Scanning Electron Microscopy (SEM) and electrical resistivity measurements. As a part, contact layer, of the CdTe-based Solar Cell, Current-Voltage (I–V) as well as External Quantum Efficiency (EQE) measurements were carried out. Our results indicate that the radiofrequency sputtered Sb2Te3 are polycrystalline with predominant rhombohedral crystalline structure, whereas the DC-sputtered films crystallized mainly in the monoclinical structure. Both set of samples showed a resistivity of the order of 10 4 Ω-cm. Concerning the CdTe-based solar cell, the incorporation of the Sb2Te3 as a back surface film in the back contact improves the solar cell efficiency up to 8.01%, 10 mTorr pressures into the growth chamber, as compared to the CdTe-based solar cell, 4.82% efficiency, without the Sb2Te3 layer.
1. Introduction One of the main drawbacks of the solar cells based on CdTe films is the forming of a good ohmic contact. Due to its electron affinity value (4.28 eV) is necessary a metal with a work function about 5.7 eV, for a good ohmic contact to CdTe surface. The better approach to overcome this problem is the formation of a pþ region between CdTe and the metal electrode to provide the advantageous electrical properties. Usually a cooper layer is used to form the back contacts with a low barrier high, however cooper diffuse along the grain boundaries to the main junction resulting in a degradation of the solar cell device. Sb2Te3 (antimony telluride) is a binary semiconducting compound in the forma A2B3, which is a promising candidate, as a thin film, for the back contact in photovoltaic devices because of its excellent physical properties like low thermal conductivity, low electrical resistivity, low bandgap and long term stability [1–4]. Sb2Te3 have been obtained by different growth techniques, like MOCVD [5,6], electrochemical [7,8], co-evaporation [9] among others. RF and DC sputtering offer some advantages in the growth of Sb2Te3
films for solar cell applications, i.e., desirable stoichiometry, short fabrication process, fine thickness control, among other [10,11]. The Sb2Te3 film analysis was carried out by a procedure similar to Gorji et al. [17]. In this work it is presented the synthesis, as well the structural, morphological and electrical characterization of the Sb2Te3 films, grown by RF and DC sputtering, as a function of the chamber pressure, and also the performance of the CdTe-based solar cell using the Sb2Te3 layer as a back contact. 2. Experimental section and characterization methods 2.1. Deposition of Sb2Te3 Soda lime glass (SLG) was used as substrates for films deposition. They were cleaned using alkaline biodegradable liquid soap in deionized water prior to dip a 10 min in isopropyl alcohol solution in an ultrasonic bath, thereafter they were dried with dry nitrogen. A Sb2Te3 target, 75.3 mm in diameter, was employed for the sputtering process; the distance
* Corresponding author. E-mail address: [email protected] (R. Mendoza-P�erez). https://doi.org/10.1016/j.mssp.2019.104876 Received 30 September 2019; Received in revised form 27 November 2019; Accepted 5 December 2019 Available online 29 February 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
Table 1 Labels, growth rate, thickness and roughness values for the Sb2Te3 samples grow at different chamber pressures by RF as well as DC Sputtering. Thin Films
Devices
Sputtering System
Label
Growth rate (nm/min)
Thickness (nm)
Roughness (nm)
Label
Deposition time (min)
RF
RF3-5 RF1-10 RF2-15 DC3-5 DC1-10 DC2-15
9.8 5.0 2.5 13.1 5.6 3.4
588 300 150 786 336 204
22 26 32 29 44 56
CSa CSb CSc CSd CSe CSf
2.6 4.8 9.9 2.0 4.5 7.4
DC
Fig. 1. XRD patterns different pressure.
of
Sb2Te3
samples
deposited
by
RF-Sp
at
Fig. 2. XRD patterns different pressure.
of
Sb2Te3
samples
deposited
by
DC-Sp
at
Fig. 3. %Sb and %Te composition for RF Sb2Te3 samples as a function of deposition pressure.
Fig. 4. %Sb and %Te composition for DC Sb2Te3 samples as a function of deposition pressure.
Table 2 EDS and resistivity measurements for Sb2Te3 samples growing at different pressures by RF and DC Sputtering techniques. Sputtering-System
RF
Sample name
RF3
RF1
RF2
DC3
DC1
DC2
Pd (mTorr) % Sb % Te Resistivity (10
5 65.79 34.21 5.87
10 65.49 34.51 6.04
15 67.28 32.72 9.05
5 68.05 31.95 5.31
10 67.47 32.53 8.19
15 66.92 33.08 9.62
4
Ω-cm)
between magnetrons (Kurt Lesker) and the substrate was of 10 cm. The substrate was kept in rotation, 5 rpm, during the growth of the films in order to enhance the uniformity of the deposited films. Sputtering pro cess was performed at a constant power of 60 W; the base pressure, in the growth chamber, was �10 6 Torr, and the target surface cleaned by pre-sputtering for 5 min under pure Ar gas, meanwhile the substrate was covered with a shutter. Once the preparation and cleaning of the target was finished, the deposition film process started. The substrate tem perature was kept constant at 200 � C, by using a power supply (Sorense), during the deposition time, 60 min. A flow rate of Ar was provided through a mass flow controller varying from 4 to 23 sccm (standard cubic centimeter per minute), into the chamber in order to maintain a
DC
2
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
complete CdTe-based solar cells were completed by growing a 4–6 μm thick CdTe film by Close Space Sublimation (CSS), thereafter this structure was submitted to thermal annealing, with sprayed layer of CdCl2, at 400 � C during 30 min [16]. Finally, two metal circulars of 0.09 cm2 layers of Cu and Mo, 15 and 500 nm thick respectively, were deposited over the same Sb2Te3 films area by DC sputtering as described in previous work [12]. For solar cells devices the configuration was glass/SnO2:F/ZnO þ CdS-TT/CdTe þ CdCl2-TT, over this final layer the Sb2Te3 thin film was grown. In this case the thickness of the Sb2Te3 layer was controlled to be approximately 25 nm for each one of the pressures in the growth chamber. The superficial of the films was in 0.09 cm2, this area coincides with the area of the Cu–Mo back contact, in order not to have additional collection carriers come from outside the bounds of the contacts.
Fig. 5. EDS measurement of the RF2 Sb2Te3 sample.
2.3. Deposition of Sb2Te3 at different chamber pressures
working pressure in the range 5–15 mTorr. Film thickness was moni tored during the deposition with a Inficon SQM-160 instrument. For RF and DC sputtering system a source power, Kurt Lesker R301 and a Mag Technology, were used, respectively.
The Sb2Te3 samples grown either by RF-sputtering or DC-sputtering were labeled according to Table 1. The second numbers in the labels correspond to the pressure (into the chamber in mTorr). The substrate temperature was fixed at 200 � C during the processing of all samples [10,11,15]. On the other hand, two sets of Sb2Te3 thin-films were used for solar cells applications and labeled as summarized in Table 1.
2.2. Processing of CdTe solar cells ZnO as well as CdS thin films were deposited onto SnO2:F/soda-lime glass substrates also by sputtering RF. These thin films semiconductor structure was annealed at 450 � C, in muffle, during 60 min. The
Fig. 6. SEM images of Sb2Te3 samples by a) RF and b) DC at 5 mTorr.
Fig. 7. SEM images of Sb2Te3 samples by a) RF and b) DC at 10 mTorr. 3
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
Fig. 8. SEM images of Sb2Te3 samples by a) RF-Sp and b) DC at 15 mTorr.
with a 2420 Keithley source meter instrument. 2.7. Measurements for photovoltaics devices (I vs. V and EQE) For the photovoltaic devices, with Sb2Te3 as back contact, the cur rent vs. voltage (I vs. V) were measured using an Oriel-Newport-Sol3A Class AAA Solar simulator (100mW/cm2 – AM1.5). Finally, a QEPVSIb Oriel Instrument was used to acquire external quantum efficiency (EQE). 3. Results and discussion 3.1. Profile measurements The values for growth rate, thickness and roughness results of Sb2Te3 samples grown by RF and DC sputtering as a function of deposition pressure are also summarized in Table 1. The experimental results show that when the pressure is increased from 5 to 15 mTorr, the layer thickness decreases as well as the grown rate decreases from 9.8 to 2.5 nm/min whereas the roughness increases from 22 to 32 nm for RF samples. For DC samples, there is a similar behavior with respect to the RF samples, the grown rate varies from 13.1 to 3.4 nm/min, and the roughness is increases respect to RF samples of 29–56 nm. On the other hand, Sb2Te3 thin films used for solar cell fabrications, the growth rate was adjusted in order to obtain a similar thickness (~25 nm) in all samples.
Fig. 9. Porosity of the Sb2Te3 samples vs. deposition pressure for RF and DC.
2.4. Thickness and roughness analysis Sb2Te3 film surface were analyzed with a step profiler (Sloan Dektak II) to measure the thickness and the roughness. The analysis was per formed in several regions for an average and the parameters used was a range of scanning of 3 mm, intensity of force 5 mg, frequency of 100 Hz and velocity of scanning 100 μm/s.
3.2. XRD results The XRD patterns of the Sb2Te3 samples deposited with the RF-Sp system are presented in Fig. 1. Films are of polycrystalline nature; all the patterns show a visible contribution from amorphous substrate being more intense as thickness diminishes. Regardless the deposition pres sure, the films present the peaks of the rhombohedral structure and preferential orientation on (0114) plane, according to ICDD-PDF files of Sb2Te3 (03-065-3678 and 01-089-6185). However, as pressure increases preferential orientation changes from (0114) to (015) plane. Preferential orientation on (0114) family planes was reported to films deposited by thermal co-evaporation [13]. In addition, the formation of a small quantity of monoclinic phase is promoted at higher deposition pressure as revealed by the low intensity XRD peaks observed at 2θ ¼ 15.7� , 20.9� and 24.7� . Fig. 2 presents the XRD patterns of the films deposited by DC. Sharp and well-defined peaks are observed to all samples indicating high crystalline quality. Rhombohedral crystalline structure is confirmed to
2.5. Crystalline structure and surface composition analysis Crystalline structure was determined by the X-ray diffraction pat terns (XRD), by means of a D8 Bruker X-ray system using the CuKa, 1.54 A, line. The composition of the films was determined by EDS studies with a JEOL JSM-6701 model microscope using an acceleration voltage of 5 kV, allowing a resolution of 2 nm with an Oxford Instrument Energy 200 EDS analytical system method and the morphological analysis was made using a SEM equipment. 2.6. Resistivity analysis of the Sb2Te3 films The resistivity of the Sb2Te3 films was measured using a four-probe, 4
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
3.3. Composition and resistivity The atomic composition (Sb and Te) and the resistivity values ob tained for RF and DC samples as a function of deposition pressure are summarized in Table 2 and Figs. 3 and 4. For RF Sb2Te3 films is possible to observed that the %Sb composition slightly decreases while the re sistivity also decreases. On the other hand, the pressure in the chamber decreases to the lowest value, when the %Te composition increases. Similar resistivity values between RF and DC Sb2Te3 films were obtained. When the deposition pressure inside of the RF sputtering system decreases the electrical resistivity also decreases (Table 2) and a large amount of Te in the composition of the film is obtained, this relation between the %Te composition and the resistivity value is expected [11]. In general, the modification of the growth parameters influences the electrical transport properties of the Sb2Te3 films processed by RF and DC sputtering systems, giving rise to a slight decrease in the resistivity values. When the pressure in the chamber of the DC system was set at 5 mTorr, the Sb2Te3 samples processed (DC3) showed the highest growth rate (~13.1 nm/min) and the lowest Te atomic composition (31.95%), see Fig. 4. For the same deposition pressure value, the RF3 sample had the highest growth rate (9.8 nm/min) and a high Te atomic composition (~34.21%). On the other hand, a high Te atomic composition, low resistivity value and a preferential orientation plane (0114) was observed for Sb2Te3 samples processed by RF at lowest deposition pressure (Fig. 3). We obtained an %At. (Te/Sb) ratio close to 0.5 and resistivity’s values around 5:3 9:6 x10 4 Ω cm (conductivity �105 (Ω-m) 1) in both sputtering system in agreement to Eliana et al. for Sb2Te3 thin films growth by co-evaporation technique [13]. In Fig. 5, EDS analysis shows the presence of the expected elements for the Sb2Te3 compound and the substrate (glass). However, the in tensity of the band associated to the oxygen (O) in relation to the in tensity of the band associated to the Silicon (Si), indicates a high amount of O in the composition of the samples, therefore, oxides could be created, which could lead to possible contamination of the films. Nevertheless, no one diffraction line associated to Sb2O5 or TeO2 in the XRD diffractograms of the Sb2Te3 samples, was observed. The EDS was taken over the full area shown in red on the SEM image of the RF2 Sb2Te3 sample, see Fig. 8a). 3.4. Morphology SEM images as a function of the deposition pressure of Sb2Te3 sam ples are shown in Figs. 6a–8a. Grain sizes about 150, 100 and 50 nm were obtained when the samples were processed at 5 mTorr, 10 mTorr and 15mTorr by RF, respectively. This means that, larger grains are obtained when the pressure in the chamber decreased; possibly due to a higher coalescence of smaller grains. SEM images of Sb2Te3 DC samples as a function of the pressure inside the chamber are shown in Figs. 6b–8b, where is possible to observe that grains are less compact and more irregular when the deposition pressure is 15 mTorr. Moreover, when the deposition pressure increases to 15 mTorr, the grain size increases from 150 to 250 nm without any direc tional orientation. A similar behavior is observed on flexible polyamide substrates, where oriented grains and a rapid accumulation of microflakes are promoted, resulting in a diversity of flake sizes [14]. Although the grain size is close to 150 nm for samples processed by DC and RF, the morphology was more compact for samples processed by RF. The porosity values as a function of the deposition pressure for Sb2Te3 samples by RF and DC are showed in Fig. 9. The inserted images show the calculated porosity using the ImageJ 1.52a software. Accord ing to Fig. 9, if the deposition pressure increases, the porosity for RF samples decreases from 8.3 to 6.7% and increases from 7.09 to 9.5% for
Fig. 10. J vs. V for thin films-solar cells device with Sb2Te3 by a) RF and b) DC at different deposition pressure.
these films from the comparison of the diffraction reflections with PDF files. In this case, only the film deposited at 5 mTorr presents a small peak of monoclinic Sb2Te3 phase (PDF 01-080-8016). The films pre pared at 10 and 15 mTorr exhibit preferential orientation to (015) plane. As deposition pressure decreases to 5 mTorr it appears a low-intensity reflection of monoclinic phase as well as it occurs a preferential orien tation change to (0014) plane [11]. On the other hand, a similar behavior for XRD results we observe for Sb2Te3 films deposited on flexible polyimide substrates by DC-Sp [13].
5
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
Table 3 Photovoltaic parameters for Sb2Te3 thin films-solar cells deposited by RF and DC Sputtering system at different pressure. Sputtering-System
Without Sb2Te3
RF
DC
Solar cell
CS
CSa
CSb
CSc
CSd
CSe
CSf
Pd (mTorr) Voc (V) Jsc (mA/cm2) Jsc-expected (mA/cm2)a FF η (%) Rs (Ω-cm) Rshunt (Ω-cm)
– 0.558 20.18 15.80 0.41 4.82 130.4 1402.3
5 0.609 22.69 18.75 0.42 6.02 133.9 1333.8
10 0.640 25.38 21.64 0.49 8.01 126.6 1990.5
15 0.550 21.82 17.69 0.31 3.82 252.6 666.2
5 0.625 23.33 20.19 0.44 6.42 120.7 1228.9
10 0.527 21.84 18.46 0.39 4.49 134.1 1251.1
15 0.582 15.03 13.98 0.36 3.15 252.9 1341.6
a
Jsc.- calculated in base of EQE curve theory.
DC samples. The porosity of Sb2Te3 samples processed by RF is lower than the porosity of samples processed by DC for higher growth pressure [10,17]. In Fig. 9, we have observed for Sb2Te3 thin films by DC that’s correct that more pressure leads to more porosity, but the stoichiometry (%At. Te/Sb ratio is close to 0.5). For other hand, for thin films of Sb2Te3 by RF more pressure leads to less porosity, but the stoichiometry% At. Te/Sb ratio is moves away from 0.5 and the resistivity’s values increase. If the porosity is the unique property that consider for CdTe solar cells with Sb2Te3, there is not an optimal pressure value, because other properties of the Sb2Te3 thin films must be considered such as: stoichi ometry between Te and Sb, grain size and resistivity.
cells (Fig. 11a) with Sb2Te3 films by RF-Sp system where the EQE for these devices is above the 70%. In Fig. 11b we observe the EQE of the devices that were finalized with Sb2Te3 films by DC-Sp system and the greatest value is around the 65%. For CSb RF solar cell is clearly seen the influence of the Sb2Te3 thin films in the large absorber wavelength, therefore the enhancement of EQE is due to a possible improvement of collection and carrier transportation. The absorption for light of the cells with RF Sb2Te3 thin films it is almost the same amount but is in a certainly smaller for cells with DC Sb2Te3 thin films. 4. Conclusions
3.5. J vs. V characterization
The influence of deposition pressure on the properties of Sb2Te3 film growth by RF and DC sputtering system and its relationship with XRD, atomic composition, electrical resistivity and morphological measure ments were studied. The thickness increased systematically with the deposition pressure decreased, so, higher growth rates were obtained as a function of the pressure in the chamber. The structural analysis shows a higher crystalline quality for RF and DC samples when de deposition pressure decrease. A Te atomic composition of 34% was determined for RF samples. More homogeneous grains for Sb2Te3 films deposited by RF were observed. The Sb2Te3 thin films grown by RF are less porous (~7%) than the DC samples (~9.5%) processed at 15 mTorr. Compact grains in the morphology of Sb2Te3 samples, implies a reduction in the quantity of pinholes, which promotes lower resistivity (~6 � 10 4 Ohm-cm), that is very desirable for a back-surface film in a typical solar cell configura tion. We show that the pressure in the chamber is an important exper imental parameter that influences the porosity and stoichiometry of the Sb2Te3 films. Six CdTe photovoltaic devices with Sb2Te3 processed by RF and DC sputtering are compared in this study. A conversion efficiency of 8.01% was obtained when a RF Sb2Te3 film processed at 10 mTorr was used in the structure of a solar cell. On the other hand, an efficiency of 6.42% was determined when a DC Sb2Te3 film processed at 5 mTorr was inserted in the back contact of a solar device. Finally, the external quantum efficiency of solar cells fabricated with RF Sb2Te3 are improved in almost all the absorber wavelength compared to a cell without Sb2Te3 back contact, possibly indicating an improvement on carrier collection and transportation at back contact in the photovoltaic device.
Fig. 10, show the J vs. V characterizations that were carried out for the solar cells finalized with Sb2Te3 thin films by RF (Fig. 10a) and DC (Fig. 10b) sputtering systems and the output electrical parameters for these devices are summarized in Table 3. The experimental results show that the optimal Sb2Te3 deposition pressure for CdTe solar cells is 10 mTorr for RF Sb2T3 films (CSb) and 5 mTorr for DC Sb2Te3 films (CSd) with efficiencies of 8.01% and 6.42%, respectively. CSb solar cell shows the highest Rshunt for RF Sb2T3 films, while the CSd has the lowest Rs for DC Sb2Te3 films. The conversion efficiency values as a function of the deposition pressure were higher for photovoltaics devices with a Sb2Te3 thin film processed by RF than the fabricated using a Sb2Te3 thin film processed by DC. When a deposition pressure of 10 mTorr is used to processed a RF Sb2Te3 thin film the open circuit voltage (VOC) of the solar cell, is increased dramatically, due to improvement in the morphology, lower percent porosity and higher %Te composition. Moreover, the JSC decreased when the deposition pressure of the DC Sb2Te3 films increased. However, the opposite happened when a RF Sb2Te3 is applying for CdTe solar cells. The behavior of the FF values for solar cells with DC Sb2Te3 thin films was expected, because an increase of FF is due to the applying of Sb2Te3 layer reduce the Schottky barrier. So that, the possible reduction of the Schottky barrier increase the collection and transportation of holes and hence an increase on the current density values was obtained. For other hand, we observed that the solar cells with the lowers re sistivity values has the better photovoltaic response. Although, the sample of Sb2Te3 grown by RF at 10 mTorr has not lowest resistivity value (with uncertainty of �8 � 10 6 Ω-cm), if it has the highest photovoltaic efficiency due to influence of other properties such as: higher stoichiometry between the atomic percentage of Te and Sb (%At. Te/Sb ratio is close to 0.5) and to a larger degree to a lower porosity (~7%), uniform and more compact grain sizes in the order of 100 nm.
CRediT authorship contribution statement �rez: Writing - original draft. Jorge SastreRogelio Mendoza-Pe � �ndez: Writing - original draft. María de los Angeles �ndez Herna Herna �rez: Writing - original draft. Jorge R. Aguilar-Herna �ndez: Writing Pe � A. Del Oso Acevedo: Writing - original draft. original draft. Jose � J. Lizardi Del Angel: Guillermo Santana: Writing - original draft. Jose Writing - original draft.
3.6. QE spectral response Fig. 11, show the External Quantum Efficiency (EQE) for the solar 6
R. Mendoza-P�erez et al.
Materials Science in Semiconductor Processing 112 (2020) 104876
�ndez-P�erez through SIP-201954558, Jorge Sastr�eAngeles Herna �ndez through SIP-20196099 and Jorge R. Aguilar Herna �ndez Herna under SIP-20195512 projects. Special thanks for technical support to �jera. Mtro. Carlos Ramos Vilchis and Ing. Benito Ortega Na References [1] A. Romeo, D.L. B€ atzner, H. Zogg, A.N. Tiwari, in: A Comparison of the Vacuum Evaporated CdTe for Substrate and Superstrate Solar Cells. 1–4. Conference: 16th European Photovoltaic Solar Energy Conference and Exhibition, May 2000. [2] D.L. B€ atzner, R. Wendt, A. Romeo, H. Zogg, A.N. Tiwari, A study of the back contacts on CdTe/CdS solar cells, Thin Solid Films 463 (7) (2000) 361–362. Available from, http://www.sciencedirect.com/science/article/pii/S0040609099 008421. [3] N. Romeo, et al., A highly efficient and stable CdTe/CdS thin film solar cell, Sol. Energy Mater. Sol. Cells 58 (1999) 209–218. [4] P. Fan, Z.-H. Zheng, G.-X. Liang, D.-P. Zhang, X.-M. Cai, Thermoelectric characterization of ion beam sputtered Sb2Te3 thin films, J. Alloys Compd. 505 (1) (2010) 278–280. Availablefrom:http://www.sciencedirect.com/science/article/p ii/S092583881001443. [5] G. Bendt, S. Schulz, S. Zastrow, K. Nielsch, Single-source precursor-based deposition of Sb2Te3 films by MOCVD**, Chem. Vap. Depos. 19 (7–8–9) (2013) 235–241. Available from, https://onlinelibrary.wiley.com/doi/abs/10.1002/cvde. 201207044. [6] B. Aboulfarah, D. Sayah, A. Mzerd, A. Giani, A. Boyer, MOCVD Growth of Bi2Te3Sb2Te3 Layers : Effect of Growth Parameters on the Electrical and Thermoelectrical Properties, Moroc. J. Of Condens. Matter 3 (2011) (0). Available from: https://re vues.imist.ma/index.php?journal¼MJCM&page¼article&op¼view&path%5B% 5D¼67. [7] R. Rostek, J. Kottmeier, M. Kratschmer, G. Blackburn, F. Goldschmidtb€ oing, M. Kr€ oner, et al., Thermoelectric characterization of electrochemically deposited Bi2Te3 films accounting for the presence of conductive seed layers [Internet], J Electrochem Soc 160 (9) (2013) D408–D416. Available from: http://jes.ecsdl.or g/content/160/9/D408.abstract. [8] I.-J. Yoo, Y. Song, D. Chan Lim, N.V. Myung, K.H. Lee, M. Oh, et al., Thermoelectric characteristics of Sb2Te3 thin films formed via surfactant-assisted electrodeposition [Internet]. The Royal Society of Chemistry, J Mater Chem A 1 (17) (2013) 5430–5435, https://doi.org/10.1039/C3TA01631E. Available from:. [9] Bin Lv Songbai Hu, Z. Lei, Preparation and characterization of Sb2Te3 thin films by coevaporation, Int J Photoenergy (2010 Jun) {hindawi} Publishing, 2010:4. Available from, https://www.hindawi.com/journals/ijp/2010/476589/cta/. [10] T. Chen, P. Fan, Z. Zheng, D. Zhang, X. Cai, G. Liang, Influence of substrate temperature on structural and thermoelectric properties of antimony telluride thin films fabricated by RF and DC, Cosputtering 41 (4) (2012) 679–683. [11] B. Fang, Z. Zeng, X. Yan, Z. Hu, Effects of annealing on thermoelectric properties of Sb2Te3 thin films prepared by radio frequency magnetron sputtering [Internet], J Mater Sci Mater Electron 24 (4) (2013) 1105–1111, https://doi.org/10.1007/ s10854-012-0888-1. Available from:. [12] J.M. Flores-Marquez, et al., Improving CdS/CdTe thin film solar cell efficiency by optimizing the physical properties of CdS with the application of thermal and chemical treatments, Thin Solid Films 582 (2015) 124–127. [13] Eliana M.F. Vieira, Joana Figueira, Ana L. Pires, Jose Grilo, Manuel F. Silva, Andre M. Pereira, Luis M. Goncalves, Enhanced thermoelectric properties of Sb2Te3 and Bi2Te3 films for flexible thermal sensors, J. Alloys Compd. 774 (2019) 1102–1116, https://doi.org/10.1016/j.jallcom.2018.09.324. [14] Shengfei Shen, Wei Zhu, Yuan Deng, Huaizhou Zhao, Yuncheng Peng, Chuanjun Wang, Enhancing thermoelectric properties of Sb2Te3 flexible thin film through microstructure control and crystal preferential orientation engineering, Appl. Surf. Sci. 341 (2015), https://doi.org/10.1016/j.apsusc.2017.04.074. [15] N.R. Paudel, K.A. Wieland, A.V. Nawarange, D.A. Sanchez, K. Gardinier and A.D. Compaan. Studies of RF Sputtered Sb2Te3 Thin Films for Back Contacts to CdS/ CdTe Solar Cells. 978-1-4244-9965-6/11/$26.00 ©2011 IEEE. [16] O. Vigil-Gal� an, A. Arias-Carbajal, R. Mendoza-P�erez, G. Santana, J. Sastr� eHern� andez, G. Contreras-Puente, A. Morales-Acevedo, M. Tufi~ no-Vel� azquez, Spectral response of CdS/CdTe solar cells obtained with different S/Cd ratios for the CdS chemical bath”, Solar Energy Materials and Solar 90 (15) (August 2006) 2221–2227, https://doi.org/10.1016/j.solmat.2006.02.020. [17] Nima E. Gorji, Rob O’Connor, Andre Mussatto, Matthew Snelgrove, P.G. Mani Gonzalez, Dermot Brabazon, Materialia, Recyclability of Stainless Steel (316L) powder within the additive manufacturing process. https://doi.org/10.1016/j.mt la.2019.100489, 2019.
Fig. 11. EQE for thin films-solar cells with Sb2Te3 by a) RF and b) DC at different deposition pressure.
Acknowledgments The research acknowledgments to IPN for the support: María de los
7