CuSbS2 thin films by rapid thermal processing of Sb2S3-Cu stack layers for photovoltaic application

Solar Energy Materials & Solar Cells 164 (2017) 19–27

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CuSbS2 thin films by rapid thermal processing of Sb2S3-Cu stack layers for photovoltaic application

MARK

V. Vinayakumar, S. Shaji, D. Avellaneda, T.K. Das Roy, G.A. Castillo, J.A.A. Martinez, ⁎ B. Krishnan Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León 66455 Mexico

A R T I C L E I N F O

A BS T RAC T

Keywords: CuSbS2 thin films Rapid thermal processing Photovoltaics Solar cells

Copper antimony sulfide (CuSbS2) thin films were prepared by annealing and rapid thermal processing (RTP) of Sb2S3–Cu precursor layers at different conditions. Sb2S3 thin films (200 nm) were deposited by chemical bath deposition from a solution containing SbCl3 and Na₂S₂O₃. Copper layers were thermally evaporated onto the Sb2S3 thin films. A systematic study was done by varying Cu layer thickness as well as the heating conditions. Cu thickness was varied from low ( < 10 nm) to 100 nm and the heating conditions were annealing at 380 °C, RTP at 500/600 °C and annealing at 380 °C followed by RTP. The thin films formed at different conditions were analyzed using different techniques to determine their crystalline structure, morphology, elemental composition, chemical state and physical properties. For the given Sb2S3 thickness, Cu 50 nm was identified as the effective Cu thickness for the formation of CuSbS2. The CuSbS2 thin films formed at different conditions were incorporated in photovoltaic structures of superstrate configuration: Glass/ITO/CdS/CuSbS2. The best photovoltaic parameters obtained were Voc=665 mV, Jsc=1.35 mA/cm2, FF=0.62 and η=0.6% measured under illumination using AM1.5 radiation from a solar simulator. Voc and FF are the highest values ever reported for the CuSbS2 based solar cells. The present work strengthens the research activities to improve CuSbS2 based photovoltaic performance, and thus PV technologies using earth abundant and non-toxic materials.

1. Introduction Chalcogenide thin films and the respective PV devices have attracted global interest from the researchers due to their versatile production and properties [1–5]. Being two acutely investigated chalcogenides, CuInGaSe2 (CIGS-Copper Indium Gallium Selenide) and CdTe (Cadmium Telluride) solar cells have achieved conversion efficiencies more than 20% [6–9]. However, concerns on the toxicity and the high price of the respective constituent elements or precursor materials have triggered to explore novel chalcogenide thin films composed of low noxious earth abundant elements. Copper oxide (Cu2O) [10,11], copper sulfide (Cu2S) [12,13], copper zinc tin sulfide (CZTS) [14], copper zinc tin sulfo selenide (CZTSSe) [15], copper antimony sulfide CuSbS2 [16–18], copper antimony selenide (CuSbSe2) [19–21] are some of the novel chalcogenide absorber materials. Among these, CuSbS2 has gained a special attention recently [22–25] due to their suitable chemical and physical properties. Antimony preserves the same chemistry as gallium and indium due to their equivalence in ionic radius as well as the oxidation states [26–30]. Further, CuSbS2 exhibits a direct optical band gap of 1.5 eV [31–33], a high absorption

⁎

coefficient of 104 cm−1 and SLME (Spectroscopic Limited Maximum Efficiency) of 22.9% [34,35]: characteristic of an ideal absorber material in solar cells. The importance of CuSbS2 and their incorporation in a PV structure was first introduced by P.K Nair et al., [17,36,37] using a method of heating chemically deposited Sb2S3-Cu2S layers in a photovoltaic structure: SnO2:F–(n)CdS: In–Sb2S3–CuSbS2–Ag, giving Voc=330 mV and Jsc=0.3 mA/cm2. Motivated by this pioneering work, there has been an accelerated development in the studies on the properties of CuSbS2 as well as its device applications. There have been many reports on the preparation and characterization of CuSbS2 thin films both physical and chemical methods. Physical methods include: thermal evaporation of powdered CuSbS2 ingot prepared using high purity Cu, Sb and S [33,38], co-evaporation of pure elements of Cu, Sb and S [39] and co-RF sputtering of Sb2S3 and Cu2S targets [40]; and chemical methods include: spin coating of Cu-S and Sb-S in hydrazine solution [34], annealing of electrodeposited Sb-Cu alloy in excess sulfur [41], chemical bath deposition (CBD) of Sb2S3-Cu2S layers followed by heating [17], spray pyrolysis of precursor solution of SbCl3, CuCl2, NH2CSNH2 in ethanol [31,42] and recently using a non-vacuum hybrid

Corresponding author. E-mail addresses: [email protected], [email protected] (B. Krishnan).

http://dx.doi.org/10.1016/j.solmat.2017.02.005 Received 28 November 2016; Received in revised form 5 February 2017; Accepted 6 February 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

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2. Experimental

ink method [16]. Most of the methods involved post-deposition treatments of sulfurization. Our group have reported various studies on CuSbS2 prepared by heating chemical bath deposited Sb2S3 and thermal evaporated Cu stack layers [23,27,43,44]. Among all these reports, a very few achieved CuSbS2 based PV device fabrication. Using spin coated CuSbS2 samples, PV structure: FTO/CuSbS2/CdS/ZnO/ZnO:Al/Au of 0.45 cm2 area using CBD CdS and magnetron sputtered ZnO/ZnO:Al buffer-window layers, leading to the photovoltaic parameters of Jsc=3.65 mA/cm2, Voc=440 mV, FF=31% and η=0.5% [34], the viability of the highly toxic hydrazine based process is being a critical issue. Similar device structures fabricated using electrodeposited CuSbS2 thin films resulted Jsc=14.73 mA/cm2, Voc=490 mV, FF=44% and η=3.13% under AM1.5 illumination, the device characterization is lacking [41]. In our laboratory, we fabricated Glass/SnO2:F/n-CdS/i-Sb2S3/p-CuSbS2/ C/Ag PV device in superstrate configuration using CuSbS2 thin films formed at 350 °C giving Voc=405 mV, Jsc=7.54 mA/cm2, FF=0.32 and the conversion efficiency of 1% [27]. In NREL, using CuSbS2 prepared by the co-RF sputtering, PV structures were fabricated by varying the substrate temperature, absorber layer thickness (0.6–1.2 µm) and back contacts (W, Ni, Mo, MoOx etc), in the substrate configuration using CBD CdS and ZnO/ZnO:Al buffer-window layers and Al as front contact. Their best results were of nearly 1% efficiency, on Mo back contact using 1.2 µm thick CuSbS2 [25]. Annealing treatments of such samples in Sb2S3 vapor and chemical treatments using KOH to remove excess Sb2S3 and Sb2O3 improved the device parameters, the reproducibility and the stability [22]. Recently, by thermal co-evaporation CuSbS2 thin films and the respective PV device Mo/CuSbS2/CdS/ZnO/ ZnO:Al/Ag have been reported, giving a high open circuit voltage of 526 mV and an efficiency of 1.9% [39]. Also, in a Cd-free buffer layer solar cell structure of p-CuSbS2/n-GaN/InGaN, 2.99% efficiency has been demonstrated where CuSbS2 thin films were synthesized on TiN coated Mo/Glass substrate by co-sputtering of (Cu + Sb2S3) targets [45]. The efforts have been in progress to improve both technology and properties of the material in analogous to that of CIGS. In such an avenue, a hybrid ink method has been developed to produce CuSbS2 thin films and the relevant PV devices [16]. Two different types of precursor inks were prepared: one with CuS nanoparticles mixed SbAc3 and the other with Cu(I)Ac mixed SbAc3. The CuSbS2 thin films were prepared by spin coating of the precursor inks followed by sulfurization at the temperature range of 400–450 °C. Solar cells of conventional Mo/CuSbS2/CdS/i-ZnO/n-ZnO/Al structure were fabricated using the CuSbS2 films formed at different conditions, leading to 3.22% efficiency, the highest so far. In this paper, we report the formation of CuSbS2 thin films by the rapid thermal processing (RTP) for 5 min. This work is a continuation of our previous work [43] to form single phase CuSbS2 thin films by heating Sb2S3 deposited by chemical bath deposition on cleaned glass substrate followed by thermal evaporation of copper. In the present work, the thin films formed by heating such Glass/Sb2S3-Cu stack layers at different temperatures in a rapid thermal processing (RTP) system and a conventional vacuum oven were analyzed. The crystal structure, chemical states, surface morphology, optical and electrical properties of the films were analyzed using different characterization techniques viz. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Scanning electron microscopy (SEM), UV–Vis–NIR spectroscopy and an electrical measurement system. Our research findings revealed that for a given Sb2S3 thickness, by adjusting the Cu thickness and the RTP conditions, phase pure CuSbS2 thin films would be formed. The CuSbS2 thin films formed at different conditions were incorporated to superstrate solar cells with structure: Glass/ITO/n-CdS/p-CuSbS2/C/Ag, using chemical bath deposited CdS as window layer. The J-V characteristics the champion device showed: Voc =665 mV, Jsc =1.4 mA/cm2 and FF=0.61 under illumination of AM 1.5 G using a solar simulator, the highest Voc and FF values ever reported.

2.1. Preparation of CuSbS2 thin films CuSbS2 thin films were synthesized by heating sequentially deposited Sb2S3-Cu stack layers. Firstly, Sb2S3 thin films were deposited by chemical bath deposition. Then, Cu was thermally evaporated on the Sb2S3 followed by heat treatment in a conventional vacuum oven and rapid thermal processing (RTP) system. 2.1.1. Chemicals Antimony trichloride (SbCl3, 99.99%) powder, sodium thiosulfate pentahydrate (Na2S2O3·5H2O, 99.99%) powder and acetone were supplied by Fermont and Copper wire (Cu, 99.99%). All the chemicals used were of analytical grade and used as supplied without any further purification. 2.1.2. Deposition of Sb2S3 thin films Thin films of antimony sulfide (Sb2S3) were deposited on cleaned glass substrates by chemical bath deposition reported previously [46]. For that, 650 mg of SbCl3 was dissolved in 2.5 ml of acetone in which 25 ml of Na2S2O3·5H2O (1 M) and 72.5 ml of deionized water were added and stirred. The glass substrates (75 mm×25 mm×1 mm) were cleaned with neutral soap solution followed by isopropyl alcohol and dried in warm air. Such cleaned substrates were placed horizontally in a Petri dish and the solution was carefully poured into it without making bubbles. The bath was kept at room temperature (25 °C) for 2 h. The films were deposited only on the lower side of the substrates facing the bottom of the Petri dish. The samples were washed gently with distilled water and dried. The thin films were orange - yellow with good adhesion and of 200 nm in thickness. 2.1.3. Thermal evaporation of Cu on Sb2S3 thin films Thermal evaporation technique (Torr International, Model No: THE2–2.5 kW-TP) was employed to deposit Cu layer of different thicknesses (varied from 1 nm to 100 nm) on Sb2S3 thin films [23]. Cu wire of purity 99.99% was used as the source material for evaporation. The evaporation process was carried out at high vacuum (10−6 Torr) at a rate of 2 Å/s keeping the substrates rotating with 20 rpm speed. The copper film thickness (1, 2, 5, 20, 50 and 100 nm) was estimated in situ using a quartz crystal thickness monitor installed in the evaporation system. 2.1.4. Heat treatments on Sb2S3-Cu thin films The Sb2S3-Cu precursor layers with different Cu thicknesses were annealed in a vacuum oven (TM Vacuum Products, Model: V/IG-80314). The annealing process was carried out at 380 °C under 10−3 Torr for 1 h. The annealed samples were labeled with prefix CAS. After identifying the copper thickness (Cu- 50 nm) to form nearly phase pure films, Sb2S3-Cu layers (Cu −50 and 100 nm) were heated in a rapid thermal processing (RTP) system (Ecopia, Model: RTP-1300) which employs halogen lamps for the instant heating of the samples. The samples were heated in the RTP system at 500 °C and 600 °C at a pressure 10−3 Torr for 5 min. Furthermore, 380 °C annealed Sb2S3-Cu layers (Cu −50 and 100 nm)) were also subjected to RTP (pre-annealed RTP) at the same conditions as described above to examine the effect of further crystallization or grain growth on the CuSbS2 films. 2.2. Fabrication of photovoltaic structures Photovoltaic devices were fabricated on ITO coated glass substrates (supplied by Vinkarola Instruments) with structure: Glass/ITO/n-CdS/ p-CuSbS2. CdS thin film was deposited on ITO/Glass by chemical bath deposition as reported previously [44]. The bath composition was cadmium chloride (10 ml), triethanolamine (5 ml), ammonium hydroxide (10 ml), thiourea (10 ml) and deionized water (65 ml preheated at 20

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70 °C). The ITO samples were immersed in the bath at 70 °C for 20 min. The as-prepared CdS films were annealed in air at 400 °C for 30 min. The heat treatment of CdS buffer layer in oxygen to produce a barrier against interlayer diffusion CdS/CdTe solar cells (grain hardening), and thus to improve the device performance is already known [47]. In our previous article on CdS/CuSbS2 [27] PV structure studies, the device performance was improved by giving pre-annealing treatment for CdS (400 °C) buffer layer prior to the absorber layer deposition. Sb2S3 layer was deposited on the annealed CdS layer by the CBD procedure mentioned earlier. Then, 50 nm Cu was thermally evaporated to glass/ITO/CdS/Sb2S3 stacked layer. The PV structures of type glass/ITO/CdS/CuSbS2 formed by heating the stacked layers at different conditions such as conventional annealing at 380 °C for 45 min, RTP at 400 °C for 5 min and pre-annealed RTP at 400 °C for 5 min. An ohmic contact of 0.25 cm2 was made using silver paint (SPI supplies) to measure the current - voltage characteristics of the devices. 2.3. Characterization XRD patterns of the thin films were recorded in the range of 2θ =10–60° using a Bruker D8 Advance diffractometer which employs a Cu-Kα radiation (λ=1.54056 Å). Elemental composition and their chemical states were analyzed using a XPS (Thermo scientific KAlpha) equipped with monochromatized Al-Kα radiation (E =1486.68 eV). Raman spectral studies were done by DXR™ 2 Raman Microscope employed with 532 nm laser wavelength to identify the phases formed. Morphology of the samples was studied using the scanning electron microscopy (SEM) (Hitachi SU 8020). Optical properties of the samples were evaluated from the absorption data taken on a Jasco V-770 UV–Vis–NIR spectrophotometer. The electrical measurements were carried out using a picoammeter/voltage source (Keithley 6487). For photoconductivity measurements, the contacts were made using conductive silver paint (SPI® supplies): two planar electrodes of 6 mm in length separated by 6 mm. The J-V characteristics of the PV structures under dark and illumination conditions were measured using silver contacts. Illumination was performed by a solar simulator (Oriel) under an AM1.5 radiation of intensity 100 mW/cm2.

Fig. 1. XRD patterns of Sb2S3-Cu thin films annealed at 380 °C for 1 h in vacuum oven (10-3 Torr) with varying Cu thickness: Sb2S3, Sb2S3 with Cu 1 nm, Sb2S3 with Cu 2 nm, Sb2S3 with Cu 5 nm. The standard pattern corresponding to Stibnite Sb2S3 is included.

50 nm, the intensity of CuSbS2 peaks increases, and the Sb2S3 peaks disappeared. Further increase in Cu to 100 nm (CAS Cu-100), all the CuSbS2 peaks grow, and also likely to change the growth orientation to (301) plane. From the figure, it can be seen the presence of very feeble peaks of Cu2S as noted. Thus, the XRD results revealed that for the Cu thickness 50 nm (CAS Cu-50) and above, for the given thickness of Sb2S3 (200 nm), nearly phase pure CuSbS2 thin films were formed. However, considering the error limit of XRD (2%), the presence of impurity phase in the thin films cannot be ruled out completely. We selected the precursor samples with Cu 50 and 100 nm for further studies on the effect of RTP at different temperatures. The effect of annealing and RTP on the crystalline nature of the CAS films with Cu 50 nm is distinguished from the diffraction patterns given in Fig. 3. The Cu2S peak disappeared in the samples of pre-annealed RTP at 500 °C. In the direct RTP 500 °C samples, major peaks of CuSbS2 along with a Cu2S minor peak can be seen; whereas in RTP 600 °C samples, phase pure CuSbS2 peaks with improved crystalline nature is evident from the figures. Also, at 600 °C the samples tend to change their preferential growth perpendicular to (410) plane in comparison with that of the other samples. Further, peaks of Cu2S as well as Sb2S3 are insignificant as evident from the patterns. From the XRD results, it is clear that the films which undergone RTP treatments are free from the impurity phase Cu2S. This can be due to the instant heating of the samples in RTP which will increase the grain growth and thereby improve the crystallinity of the thin films [48–50]. To study the effect of relatively high copper content on the crystalline nature of CuSbS2 formed by annealing, pre-annealed RTP and RTP, the XRD patterns for the respective CAS Cu-100 samples were recorded and are given in Fig. 4. From the analysis of the diffraction peaks, RTP500 °C and 600 °C samples consist of CuSbS2 as major

3. Results and discussion After heat treatments, all the thin films turned to dark brown and the average thickness of nearly 200 nm as measured by profilometry. The structural characteristics of the thin films formed at different conditions were identified using the respective XRD patterns. Figs. 1 and 2 show the patterns of Sb2S3 and Sb2S3-Cu thin films annealed in the conventional vacuum oven at 380 °C for 1 h, as marked. Fig. 1 represents the patterns of the Sb2S3 and the films with Cu low thickness range (1, 2, and 5 nm). Sb2S3 film shows reflections from (020), (120), (130), (211), (140), (301), (240), (421) and (511) planes of orthorhombic in correlation with the standard pattern for Sb2S3 (JCPDS # 42-1393) given in the figure. The films formed by diffusing Cu −1 nm thickness (Cu 1 nm), shows neither a change in the crystalline phase of Sb2S3 nor any additional peaks in the respective diffraction patterns as seen in the figure. In the case of samples of Sb2S3 with Cu thickness 2 nm (Cu-2 nm) an additional peak is observed at 2θ=28.07° which is identified as (111) plane of CuSbS2 implying that the ternary phase CuSbS2 started to grow at Cu thickness 2 nm. Similar CuSbS2 peaks with higher intensity are observed in the sample with Cu 5 nm, diminishing Sb2S3 peaks as evident in the figure. XRD patterns of the annealed Sb2S3-Cu samples with varying Cu thicknesses 20, 50, and 100 nm are shown in Fig. 2 (all the plots in the same intensity scale). For 20 nm copper diffused films (CAS Cu20), the major peaks marked by (200), (111), (410), (301), (501) and (800) are assigned to the orthorhombic CuSbS2 comparing with the respective standard pattern included in the figure, JCPDS file # 44–1417. Also, weak peaks of Sb2S3 are present in the films. For samples with Cu 21

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Fig. 2. XRD patterns of Sb2S3-Cu (CAS) thin films annealed at 380 °C for 1 h in vacuum oven (10-3 Torr) with varying Cu thickness: Sb2S3-Cu with Cu 20 nm, Sb2S3 with Cu 50 nm, and Sb2S3 with Cu 100 nm. The standard pattern for orthorhombic CuSbS2 (PDF#44–1417) is also included.

Fig. 3. XRD pattern of CuSbS2 thin films (CASCu-50) formed at different heating conditions: RTP at 500 °C for 5 min (RTP-500), pre-annealed RTP at 500 °C for 5 min (380-RTP 500), RTP at 600 °C for 5 min, annealed at 380 °C for 1 h in vacuum (10−3 Torr) and post heated in RTP, annealed at 380 °C for 1 h in vacuum (10−3 Torr) and post heated in RTP at 600 °C for 5 min, along with the standard pattern of CuSbS2.

phase and Cu2S as minor phase as marked in the figure, highly oriented film growth. Furthermore, the orientation of the grain growth changes with the RTP temperature as seen in the figure. Also, in RTP 600 °C sample one peak corresponding to the copper rich phase of Cu3SbS4 is detected as noted in the figure. The pre-annealed RTP resulted phase pure CuSbS2 films without any impurity as seen in the figure. Thus, the formation of single phase CuSbS2 was successfully proved by XRD analysis of the CAS films with Cu 50 nm and 100 nm pre-annealed RTP at 600 °C. Crystallite size (D) was evaluated using the Scherrer formula assuming the line broadening is merely due to the size effect. According to the formula,

D=

0. 9λ βcosθ

the major peak located at 332 cm−1 and minor peaks at 316 and 254 cm−1 coincide with the RRUFF data reported for CuSbS2. The low intense peaks at 100 cm−1 and 150 cm−1 are also due to the CuSbS2 phase, as reported in the literature [51,52]. The Raman shift at 254 cm−1 can also be originated from Sb2S3, according to the respective RRUFF data, correlating with our XRD analysis of the samples. X-ray photoelectron spectroscopy (XPS) analysis was carried out to estimate the elemental composition and their chemical states. Binding energies (B.E) of all the XPS spectra were corrected using adventitious carbon binding energy at 284.6 eV besides the charge compensation using the flood gun. The complete analysis of the elemental composition and their respective chemical states for the films formed at different conditions were identified using the respective high resolution spectra. Deconvolution of the high resolution spectra was done by applying a Shirley type background calculation. The high resolution spectra of Cu 2p, S 2p and Sb 3d of our typical phase pure sample (CAS Cu-50 pre-annealed RTP 600 °C) recorded from the depth after three etching cycles at a rate of 1.19 nm/s using 2 keV Ar+ ions are presented in Fig. 6a, b, and c. The core level spectrum of copper shows the 2p doublet which constitutes Cu 2p3/2 and Cu 2p1/2 peaks due to spinorbit coupling with respective binding energies of 932.68 and 952.38 eV separated by ΔE −19.7 eV. These values are in agreement with that of Cu+ in CuSbS2 [18,27,34]. The Sb 3d core level spectrum and the deconvoluted peaks along with the resultant envelop are shown in Fig. 6b. The spectrum is composed of three doublets as marked in the figure. Peaks at 528.66 eV (Sb 3d5/2) and 538.05 eV (Sb 3d3/2) correspond to Sb0 state which may be originated from the Ar+

(1)

where, λ is the X-ray wavelength, β is full width at half maximum (FWHM) of the diffraction peak at an angle of 2θ. The average value of the crystallite size was ∼15 nm. For the calculation, the line broadening analysis of (301) peak was considered. Fig. 5 shows the Raman spectra (at 532 nm excitation wave length) of CuSbS2 samples formed at different conditions of heating Sb2S3Cu50 precursor: annealing, pre-annealed RTP 600 °C and direct RTP 600 °C. The samples were Raman active and the corresponding peaks are marked in the figure. From the spectra, the thin films show the Raman active modes at 100, 150, 165, 254, 316 and 332 cm−1). Theoretically, for orthorhombic CuSbS2 (Pnma space group), the Raman active zone-center vibrational modes can be described with four Raman active modes and three infrared active modes as specified by Baker et al. [51] in a study on pressure induced structure transformation in CuSbS2. Also, the Raman spectrum for an unoriented CuSbS2 sample at 532 nm excitation wave length is published in RRUFF Project database (http://rruff.info/). For all of our samples, 22

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Fig. 4. XRD pattern of CuSbS2 thin films thin films with Cu thickness 100 nm (CAS Cu100) formed at different heating conditions: RTP at 500 °C for 5 min (RTP-500), RTP at 600 °C for 5 min, annealed at 380 °C for 1 h in vacuum (10−3 Torr) and post heated in RTP, annealed at 380 °C for 1 h in vacuum (10−3 Torr) and post heated in RTP at 600 °C for 5 min, along with the standard pattern of CuSbS2.

Fig. 6. High-resolution XPS spectra of the core levels of (a) Cu 2p core level, (b) Sb 3d core level and (c) S 2p core level of CAS Cu-50 sample after Ar+ ions etching of CuSbS2 thin film formed by pre-annealed RTP at 600 °C for 5 min (CAS Cu-50 380-RTP 600).

Fig. 5. Raman spectra of the phase pure CuSbS2 thin films (CAS Cu-50) formed at different conditions (a) annealed at 380 °C for 1 h in vacuum (b) RTP at 600 °C for 5 min (c) pre-annealed RTP at 600 °C for 5 min.

3d3/2) concur with that from CuSbS2 phase as reported in the previous studies. The deconvoluted S 2p core line consists of (i) S 2p3/2 peak at 161.91 eV and Sp 2p1/2 peak at 163.07 eV which represent S2- in Sb2S3 [53] (ii) a higher intensity peak of S 2p3/2 at 161.48 eV and S 2p1/2 at 162.64 eV in consistent with the reported range of B.E of S2- state in CuSbS2 [27,54], as noted in Fig. 6c, both doublets with separation

sputtering induced effect [27] of the film. The smallest peaks at 529.46 eV (Sb 3d5/2) and 538.85 eV (Sb 3d3/2) imply the presence of Sb2S3, unreacted precursor in the films. From the figure, spin-orbit coupled Sb 3d major peaks at 530.50 eV (Sb 3d5/2) and 539.89 eV (Sb 23

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plots based on the relation (αhυ)n =A (hυ − E g) are used. Where α is the absorption coefficient corresponding to the energy h υ , h is the plank's constant, υ is the frequency of the incident photon, and A is a constant. The value of n equals to 2 for an allowed direct, 1/2 for an allowed indirect and 2/3 for a forbidden band gap semiconductors respectively. The absorption coefficients of the films were calculated from the respective absorbance spectra knowing the thickness values. The plot of (αhυ)2 vs hυ (Tauc plot) for the phase pure CuSbS2 thin films yielding a good linear fit for n =2 as given in the inset, implying direct optical absorption in these films. The energy range was selected corresponding to the onset of absorption as shown in the inset. The band gap values were estimated from the extrapolation of linear region of the plot to the hυ axis. The Eg values obtained for the films were 1.53 eV (RTP 600 °C), 1.54 eV (pre-annealed RTP 600 °C) which is in the reported range of values 1.5–1.6 eV [17,27]. Also, the significant sub-bandgap can be observed in all the absorption spectra indicating the presence of high density of traps in the thin films. The photocurrent response measurements at room temperature were carried out for the CuSbS2 thin films (CAS Cu-50) formed by annealing, pre-annealed RTP 600 °C and direct RTP 600 °C, as given in figure. All the films are photoconductive, as seen in Fig. 10a, b and c respectively. Photoconductivity was examined by illuminating the films by a halogen lamp with an illumination intensity of 500 W/m2. For the measurements, 10 V bias was applied between a pair of silver electrodes. The current flowing through the sample was measured in an interval of 20 s, first in the dark, followed by illuminating the sample and then after turning off the illumination. For the given bias and keeping the electrode dimensions remains the same in all the samples, the dark current and the photocurrent increased as the heating conditions were changed. The conductivity values for the films undergone direct RTP at 600 °C and pre-annealed RTP at 600 °C were in the order of 10−3 and 10−4 (Ω cm)−1 respectively. The higher conductivity for the pre-annealed RTP 600 °C film can be due to its slightly higher crystallite size and the compact morphology with reduced grain boundaries as seen from the XRD and SEM results. Based on the above studies, we prepared various photovoltaic structures using CAS Cu-50 thin films as absorber and CdS thin films as window layer. Sequentially deposited glass/ITO/CdS/Sb2S3/Cu layer structures of Cu 50 nm were annealed at different conditions to form PV junctions of type glass/ITO/n-CdS/p-CuSbS2. The PV structures were formed by annealing at 380 °C for 45 min (optimized time for PV device formation based on our previous experiments not shown here), RTP 400 °C for 5 min and pre-annealed at RTP 400 °C for 5 min. When the device structures (Glass/ITO/CdS/CuSbS2) prepared on ITO were subjected to RTP above 400 °C, the samples evaporated. The optimization for RTP conditions for the devices are in progress by varying time and temperature. The PV structures were characterized using the J-V measurements, the J-V curves are given in Fig. 11. For the J-V measurements, the contacts were made of silver paint and the active device area was area 25 mm2. Both the annealed and the RTP 400 °C devices showed Voc=485 mV, Jsc=1.5 mA/cm2, FF=0.4 and the conversion efficiency (η)=0.3%. But the direct RTP grown device resulted higher Jsc (2.3 mA/cm2) and η=0.4% than those of the device formed by annealing. However, the pre-annealed RTP grown device showed an open circuit voltage (Voc) of 665 mV and a fill factor (FF) of 0.6, the highest values ever reported for CuSbS2 based PV devices, establishing the perspective of CuSbS2 for high efficiency PV devices. Jsc value for this device was 1.3 mA/cm2 and the η=0.6%, relatively low values. From the absorption coefficient of CuSbS2 (105 cm−1), 300 nm thick absorber layer can absorb almost 95% of the incident light. Based on this and assuming standard photon flux values reported by NREL, the simulation data predict Jsc of nearly 15 mA/cm2 for CuSbS2 absorber layer of 300 nm in thickness. However, our EQE result shows very poor carrier collection efficiency. From the EQE data (Fig. 11b), 10–12% in the short wavelength region 400–500 nm which may be dominated by

Fig. 7. XPS profile montage of CuSbS2 thin film (CAS Cu-50 380-RTP 500) formed by pre-annealed RTP at 600 °C for 5 min) (b) Depth profile for composition of the thin film.

ΔE=1.16 eV. Thus, the higher intense peak values of S 2p and Sb 3d along with that of Cu 2p lines in correlation with our XRD results confirm the formation CuSbS2 phase as major phase. The peaks were deconvoluted using Shirley type background calculation, and peak fitting using Gaussian-Lorentzian sum function, and also care was taken to maintain the ΔE, FWHM and the intensity ratios at their respective values during the peak assignment. The B.E profile montage and the respective composition analysis of the same sample are presented in Fig. 7a and b. Analysis of the depth profile data showed uniformity of the Cu, Sb and S elements throughout the depth of the film. In the case of CuSbS2 samples formed by RTP at 600 °C, the surface was antimony rich in composition. Formation of the ternary CuSbS2 phase by the interlayer diffusion of Sb2S3 and Cu is clearly understood from the composition figures. The morphology of the CuSbS2 thin films formed by annealing, preannealed RTP 600 °C and direct RTP 600 °C of CAS Cu-50 precursors is presented in Fig. 8a-d along with the as prepared precursor sample. The as prepared film exhibits a compact morphology with relatively smaller spherical particles as seen in Fig. 8a. After annealing at 380 °C, the morphology changes to irregular shaped particles distributed uniformly on the surface as evident from Fig. 8b. In the pre-annealed RTP 600 °C sample, the surface is composed of coalescent grains which are continuous and interconnected uniformly, as illustrated in Fig. 8d. For the direct RTP treated films, Fig. 8c shows a better coalescence and more compact grains with well defined boundaries. Fig. 9 shows the absorbance spectra of typical phase pure CuSbS2 thin films with Cu 50 nm (CAS Cu-50) formed by pre-annealed RTP and direct RTP. CAS Cu-50 annealed at 380 °C is also included in the same figure for comparison. All the films show good absorbance in the visible region. To find the optical bandgap (Eg) of the films, the Tauc 24

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Fig. 8. Scanning electron micrographs for CuSbS2 thin film (CAS Cu-50) (a) as prepared (b) annealed at 380 °C for 1 h in vacuum (c) RTP at 600 °C for 5 min (d) pre-annealed RTP at 600 °C for 5 min.

Fig. 9. Optical absorption spectra of phase pure CuSbS2 thin film (CAS Cu-50) formed at different conditions (a) pre-annealed RTP at 600 °C for 5 min. The region of onset of absorption and the Tauc plot for the evaluation of optical band gap are given in the inset.

the optical absorption due to CdS. In the longer wavelength region corresponding to the absorber layer absorption, the EQE value is less than 5% which strongly contrasts with the carrier generation by 95% absorption by the layer. The low Jsc in the range of 1.3–2.3 mA/cm2 is in correlation with the low EQE of our PV devices. The EQE results suggests that carrier recombination dominates in and out of the space charge region. These observations demand further improvements in the CuSbS2 absorber quality by reducing the grain boundaries and bulk defects including Cu2S, Sb2S3 etc. Further requirement is to find appropriate ohmic contacts for such PV cells. These aspects will be considered in our future work in the development of CuSbS2 based cells. The champion device formed by pre-annealed RTP was further

Fig. 10. Photocurrent response curves for CuSbS2 thin films (CAS Cu-50) formed at different conditions (a) annealed at 380 °C for 1 h in vacuum (b) RTP at 600 °C for 5 min (c) pre-annealed RTP at 600 °C for 5 min.

characterized using XRD, XPS and SEM as shown in Fig. 11c, d and e. The major peaks of the GIXRD pattern of the PV device are identified as reflections from (200), (210), (400), (111), (410), (301), (220), 25

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Fig. 11. Evaluation of (a) J-V characteristics of the Glass/ITO/n-CdS/p-CuSbS2 PV devices (using CAS Cu-50 film) fabricated at different conditions: annealing at 380 °C for 45 min, RTP at 400 °C for 5 min, pre- annealed RTP at 400 °C for 5 min (b, c) EQE measurement and GIXRD of the champion device (pre-annealed RTP) (d) XPS compositional spectra of preannealed RTP device (e) SEM image of the pre-annealed RTP device.

crystallinity and smoothening of the surface without change in the respective composition. In conventional annealing, heating and cooling take place in a slow rate. The rapid heat treatments in RTP make the diffusion process faster and avoid the oxidation of the films that normally takes place in slower annealing processes. High lateral grain growth upto 4 µm was reported in the case of CuInSe2 thin films based on selenization by RTP treatment [59]. Investigations on large grain growth and thus to control the orientation of the films growth towards both materials and the devices will be our future work.

(610), (321), (501), (221), (321), (421), (002), (202), (521), (212), (402) and (412) planes of orthorhombic CuSbS2, matching with that of the respective JCPDS data shown in the figure. A few minor peaks of crystallized Sb2S3 and Cu2S phases are also observed as marked in the figure. Such impurity phases are detrimental to the PV device as seen from the EQE curve. The depth profile of composition by the XPS analysis of the PV device is shown in Fig. 11d. Formation of CuSbS2 absorber layer with uniformly distributed Cu, Sb and S and the interface of CdS/CuSbS2 layer is clearly distinguished from the compositional analysis. In direct RTP CuSbS2 samples, Sb deficient near surface layer implied the insufficient time for the interlayer diffusion to achieve uniform composition, hence reducing the performance of the respective device. In the case of 380 °C annealed device, uniform composition was achieved. However, insufficient temperature may inhibit further grain growth. In the case of combined process of annealing and RTP, uniform composition and higher grain growth may be promoted which in turn can cause devices with improved performance. However, the device surface likely to show low film density after the RTP (Fig. 11e). This could also reduce the Jsc value in our devices due to poor surface contacts. RTP is proved to reduce the number of near- surface defects which is an undesirable effects of long time high temperature annealing processes [55]. In PV devices, the reduced number of defects can diminish the recombination thorough defects, and thus improving the device properties [50]. CIGS based solar cells showed enhancement in the cell properties especially in Voc due to RTP at different conditions [56–58]. In our experiments, the highest open circuit voltage of the pre-annealed RTP device can be attributed to the formation of CuSbS2 with uniform composition by the combined processes of annealing and RTP. From the literature, it has been proved that RTP is a short time thermal processing technique which provides instant heat to the samples to obtain high crystalline thin films thereby increasing the

4. Conclusions CuSbS2 thin films were prepared by a rapid thermal processing of chemical bath deposited Sb2S3 and thermal evaporated Cu stack layers. Also, a systematic study on the growth of CuSbS2 thin films due to conventional annealing by varying Cu content for a given Sb2S3 layer was. Further, comparative study on the samples formed by annealing, direct RTP and our findings showed that conventional annealing in vacuum oven lead formation of Cu2S impurity phase along with the CuSbS2 on glass bare substrates. The impurity phase Cu2S was disappeared by applying RTP above 500 °C. The CuSbS2 thin films formed at different conditions were incorporated in photovoltaic structures of superstrate configuration: Glass/ITO/CdS/CuSbS2. The best photovoltaic parameters were Voc=0.665 V and FF=0.62, the highest values ever reported for the CuSbS2 based p-n junction solar cells. The present work reinforces the scope of research activities to improve CuSbS2 based photovoltaic technology. Further investigation is in progress to produce highly oriented thin films with improved photophysical properties and thus the device performance. Acknowledgement The authors are thankful to CEMIE-SOL Project # 35, V. Vinyakumar is grateful to CONACYT, Mexico for providing doctoral 26

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