Indium sulfide (In2S3) thin films have been synthesized by chemical bath deposition technique onto glass substrates using In(NO3)3 as indium precursor and thioacetamide as sulfur source. X-ray diffraction studies have shown that the crystalline state of the as-prepared and the annealed films is β-In2S3. Optical band gap values between 2.27 and 2.41 eV were obtained for these films. The In2S3 thin films are photosensitive with an electrical conductivity value in the range of 10−3

–10−7 (Ω cm)−1, depending on the film preparation conditions. We have demonstrated that the In2S3 thin films obtained in this work are suitable candidates to be used as window layer in thin film solar cells. These films were integrated in SnO2:F/In2S3/Sb2S3/PbS/C–Ag solar cell structures, which showed an open circuit voltage of 630 mV and a short circuit current density of 0.6 mA/cm2

Alvarez Gallegos, Alberto

Calixto Rodriguez, María Estela

Hernández García, T.

Lugo Loredo, Shadai

Messina Fernández, S.

Peña Méndez, Yolanda

Vázquez Dimas, Alejandro

Hernández García, Tomás Constantino

Repositorio Academico Digital UANL

Thin Solid Films 550 (2014) 110–113Contents lists available at ScienceDirectThin Solid Filmsj ourna l homepage: www.e lsev ie r .com/ locate / ts fIndium sulﬁde thin ﬁlms as window layer in chemically depositedsolar cellsS. Lugo-Loredo a,1, Y. Peña-Méndez a,⁎,1, M. Calixto-Rodriguez b, S. Messina-Fernández c, A. Alvarez-Gallegos d,A. Vázquez-Dimas a,1, T. Hernández-García a,1a Universidad Autónoma de Nuevo León, UANL, Fac. de Ciencias Químicas, Av. Universidad S/N Ciudad Universitaria San Nicolás de Los Garza Nuevo León, C.P. 66451, Mexicob Universidad Tecnológica Emiliano Zapata del Estado de Morelos, Av. Universidad Tecnológica No. 1, C.P. 62760 Emiliano Zapata, Morelos, Mexicoc Universidad Autónoma de Nayarit, Ciudad de la Cultura “Amado Nervo” S/N, C.P. 63190 Tepic, Nayarit, Mexicod Universidad Autónoma del Estado de Morelos, Centro de Investigación en Ingeniería y Ciencias Aplicadas, Av. Universidad 1001, C.P. 62209, Cuernavaca Morelos, Mexico⁎ Corresponding author at: Facultad de Ciencias QuímTel.: +52 8183294000x6363; fax: +52 8183765375.E-mail address: yolapm@gmail.com (Y. Peña-Méndez)1 Tel.: +81 83294000x63630040-6090/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tsf.2013.10.115a b s t r a c ta r t i c l e i n f oArticle history:Received 30 January 2013Received in revised form 16 September 2013Accepted 18 October 2013Available online 29 October 2013Keywords:Indium sulﬁdeChemical bath depositionThin ﬁlmsSolar cellsIndium sulﬁde (In2S3) thin ﬁlms have been synthesized by chemical bath deposition technique onto glass sub-strates using In(NO3)3 as indium precursor and thioacetamide as sulfur source. X-ray diffraction studies haveshown that the crystalline state of the as-prepared and the annealed ﬁlms is β-In2S3. Optical band gap values be-tween 2.27 and 2.41 eV were obtained for these ﬁlms. The In2S3 thin ﬁlms are photosensitive with an electricalconductivity value in the range of 10−3–10−7 (Ω cm)−1, depending on theﬁlmpreparation conditions.We havedemonstrated that the In2S3 thinﬁlms obtained in thiswork are suitable candidates to beused aswindow layer inthin ﬁlm solar cells. These ﬁlms were integrated in SnO2:F/In2S3/Sb2S3/PbS/C–Ag solar cell structures, whichshowed an open circuit voltage of 630 mV and a short circuit current density of 0.6 mA/cm2.© 2013 Elsevier B.V. All rights reserved.1. IntroductionMany efforts have been done by several research groups in order toreplace CdS buffer layer in thin ﬁlm solar cells by using an alternativesemiconductor with awide band gap, mainly due to environmental rea-sons. Indium sulﬁde has been recognized as an alternative material dueto their stability, transparency, photoconductive nature and because ofthe energy band gap that can be varied between 2 and 2.75 eV depend-ing on its composition [1–6]. Several crystalline phases have been re-ported for the In2S3 thin ﬁlms (α, β and γ) and it has been found thatthemost stable phase at room temperature is the β-tetragonal [6]. Poly-crystalline In2S3 thin ﬁlms have been used as window layer in In2S3/CuInX2 (X = S or Se) solar cell structures [4].Among the chemical methods used for the preparation of In2S3 thinﬁlms are metal–organic chemical vapor deposition [7], atomic layer de-position [8], thermal evaporation technique [9], modulated ﬂux deposi-tion [10], spray pyrolysis [11], electrodeposition [12], and chemical bathdeposition (CBD) [1,13]. CBD has been recognized as the simplest andmost economical method to obtain semiconductor thin ﬁlms. In thiswork we report the deposition of In2S3 thin ﬁlms using CBD and theirapplication as window layers in thin ﬁlm solar cells.icas-UANL, C.P. 66451, Mexico..ghts reserved.2. Experimental details2.1. In2S3 window layerThe deposition of the indium sulﬁde thin ﬁlmswas done on Corningglass substrates (2.5 cm x 7.5 cm x 0.1 cm). The substrates were previ-ouslywashed outwith a soapy solution, then rinsedwith distilledwaterand dried in ﬂowing air. For depositing the indium sulﬁde thin ﬁlms, anacidic chemical bath solution composed of In(NO3)3 0.01 M, CH3COOH2 × 10−3 M and CH3CSNH2 0.16 M at a pH of 2.60 was used. Normally,the addition of CH3COOH is used to reduce the pH value of the bath andto promote the hydrolysis of thioacetamide [14]. The substrates werevertically placed into the beaker. The chemical reaction was carriedout at 35 °C, and the deposition time was varied from 12 to 39 h.2.2. Thermal treatmentsThe indium sulﬁde thin ﬁlmswere annealed in a Thermolyne 21100tubular furnace at temperatures of 350 and 400 °C for 1 h under astream of N2 ﬂowing through the tube.2.3. Solar cell structuresThe structure of the proposed solar cell is similar to that reportedpreviously in Ref. [15], however, in the present work we have sub-stituted the CdS layer by our In2S3 window layer. The Sb2S3 thin ﬁlmFig. 2. Energy band gap values for the In2S3 thin ﬁlms with 39 h of deposition, a) as-prepared, b) annealed at 350 °C, and c) annealed at 400 °C in N2 for 1 h.111S. Lugo-Loredo et al. / Thin Solid Films 550 (2014) 110–113(200 nm) was deposited onto the In2S3 thin ﬁlm previously depositedon 3 mm glass substrates coated with SnO2:F, a transparent conductiveoxide (TEC-15 by Pilkington). The SnO2:F/In2S3/Sb2S3 structure wassubsequently heated in N2 ﬂow at 350 °C for 0.5 h. After that a PbSthin ﬁlm (200 nm) was deposited as p+ absorber, as suggested in Ref.[16]. Then graphite paintwas applied on the PbS ﬁlm andﬁnally, a silverpaint electrode was applied to be used as p-side contact.3. Characterization of the In2S3 thin ﬁlmsX-ray diffraction (XRD) analysis was performed on a Rigaku D-Max2000 X-ray diffractometer using Cu-Kα radiation (λ = 1.5406 Å) inthe grazing incidence mode atΩ = 1.5°. The optical transmittance re-sponse was measured in a Shimadzu 1800 UV–Vis spectrophotometerin the wavelength range of 250–1100 nm using air as reference. Photo-current response of the ﬁlms was obtained using a tungsten–halogenlamp and a Keithley 6487 multimeter. Morphology of the ﬁlms wasstudied by atomic force microscopy (AFM) using an AA3000 atomicforce microscope from Angstrom Advanced, the measurements wereperformed in the contact mode. Field emission scanning electronmicroscopy (FE-SEM) images of the In2S3 thin ﬁlms were obtainedusing a FE-SEM JEOL JSM 6701F microscope, the operating voltage was2–3 kV.4. Results and discussionIt was found that the In2S3 thin ﬁlms chemically deposited for 12 hhave a thickness of 85 nmand those deposited for 39 h have a thicknessof ~500 nm.4.1. Structural properties of In2S3 thin ﬁlmsFig. 1 shows the XRD patterns of the In2S3 thin ﬁlms (500 nm) re-corded at a grazing incidence angle of 1.5° for the as-prepared andannealed samples at 350 and 400 °C for 1 h in N2. The XRD patternsmatch well with that of the standard pattern of the tetragonal phaseβ-In2S3 (PDF 25-0390). The as-prepared ﬁlms showed a polycrystallinenature, and after the annealing treatment at 400 °C a little improvementof the ﬁlms crystallinity was observed. The standard pattern for β-In2S3(PDF 25-0390) is given in Fig. 1 for comparison.Fig. 1.X-ray diffraction patterns of In2S3 thin ﬁlmswith 39 h of deposition, a) as-prepared,b) annealed at 350 °C, and c) annealed at 400 °C in N2 atmosphere for 1 h.4.2. Optical properties of In2S3 thin ﬁlmsThe band gap (Eg) values were calculated from the optical transmit-tance spectra results considering that the absorption coefﬁcient (α) is afunction of the photon energy (hν) using the following equation [17]:α ¼ 1=dð Þln 100=%Tð Þwhere d is the thickness of the ﬁlm and T the optical transmittance.The Eg value was determined by extrapolating the linear part of theplot (αhν)2 vs. (hν) in the abscissa (axis x), which indicates a direct op-tical transition. The (αhν)2 vs. (hν) plots are shown in Fig. 2 for as-prepared and annealed β-In2S3 ﬁlms (500 nm). For these β-In2S3 sam-ples the calculated band gap values were 2.41, 2.34 and 2.27 eV forthe as-prepared, annealed at 350 °C, and 400 °C, respectively. Thesevalues are in agreement with those reported in the literature for thismaterial [1].We observed a decrease in the Eg valuewhen the annealingtemperature increases, which can be attributed to an increase in grainsize.Fig. 3.AFM image of the β-In2S3 thin ﬁlmwith 39 h of deposition, after thermal treatmentat 400 °C in N2 atmosphere for 1 h.Fig. 4. FE-SEM images of β-In2S3 thin ﬁlms with 39 h of deposition, a) before and b) after thermal treatment at 350 °C in N2 atmosphere for 1 h.Fig. 5. FE-SEM images for In2S3 thin ﬁlmswith 16 h of deposition, a) top and b) cross-section views of the as-prepared ﬁlms; and c) top and d) cross-section views of theﬁlms after thermaltreatment at 350 °C in N2 atmosphere for 1 h.Fig. 6. Photoresponse of the β-In2S3 thin ﬁlms with 39 h of deposition, a) as-prepared,b) annealed at 350 °C in N2, and c) annealed at 400 °C in N2. Inset d): photoresponse ofthe β-In2S3 thin ﬁlm with 16 h of deposition after annealing at 400 °C.112 S. Lugo-Loredo et al. / Thin Solid Films 550 (2014) 110–1134.3. Morphological properties of In2S3 thin ﬁlmsFig. 3 shows a 2D AFM image for the β-In2S3 thin ﬁlm with an aver-age thickness of 500 nm after the annealing treatment at 400 °C in N2.The morphology of the ﬁlm surface shows grain sizes from b100 nmup to 800 nm. On the other hand, FE-SEM images of In2S3 thin ﬁlms(500 nm) revealed a cracked surface for the as-prepared and annealedsamples at 350 °C (see Fig. 4).Fig. 5a and b shows a representative FE-SEM image of the top andcross-section views of the as-prepared In2S3 ﬁlms (165 nm). Fig. 5c andd corresponds to the In2S3 thinﬁlmannealed at 350 °C for 1 h inN2 atmo-sphere. According to this ﬁgure, after the annealing process a slight differ-ence in thickness (157 nm) is observed in the ﬁlm, which can be due toshrinking. In this case, the ﬁlm surface is homogeneous and crack-free.According to these results, the 157 nm In2S3 ﬁlm shows suitable charac-teristics to be employed as window layer in thin-ﬁlm solar cells.4.4. Electrical properties of In2S3 thin ﬁlmsThe photocurrent response of as-prepared and annealed at 350 and400 °C β-In2S3 thin ﬁlms (500 nm) are shown in Fig. 6. Inset in Fig. 6shows the photocurrent response of In2S3 ﬁlm (150 nm) after the an-nealing process at 400 °C for 1 h inN2 atmosphere. The electrical conduc-tivities of the β-In2S3 thin ﬁlms (500 nm) were 4.34 × 10−8 (Ω cm)−1and 2.05 × 10−7 (Ω cm)−1 after annealing at 350 °C and400 °C, respec-tively. In the case of In2S3 thin ﬁlms with a thickness of 150 nm, an elec-trical conductivity value of 2.8 × 10−3 (Ω cm)−1 was obtained afterperforming the thermal annealing at 400 °C. This is a very important re-sult since there are no electrical conductivity values reported in the liter-ature for In2S3 thin ﬁlms obtained by CBD.4.5. J–V characteristics of the SnO2:F/In2S3/Sb2S3/PbS/C–Ag photovoltaicstructureThe current density (J) vs. voltage (V) characteristics of three solarcell structures are given in Fig. 7. The fabricated solar cells have theSnO2:F/In2S3/Sb2S3(200 nm)/PbS(200 nm)/C–Ag structure, formedusing an as-prepared In2S3 (Fig. 7a and b) and a thermally treatedIn2S3 thin-ﬁlm at 350 °C in N2 ﬂow for 1 h (Fig. 7c). In the solar cellFig. 7. J–V characteristics of the SnO2:F/In2S3/Sb2S3/PbS/C–Ag photovoltaic structuresusing a) an as-prepared In2S3 layer with a thickness of 200 nm; b) an as-prepared In2S3layer with a thickness of 165 nm; and c) an In2S3 layer with a thickness of 157 nm afterthermal treatment at 350 °C in N2.113S. Lugo-Loredo et al. / Thin Solid Films 550 (2014) 110–113structures of Fig. 7a and b we used two as-prepared In2S3 layers withthicknesses of 200 and 165 nm, respectively. While in the solar cellstructure of Fig. 7c, we used an annealed In2S3 thin ﬁlmwith a thicknessof 157 nm. The photovoltaic parameters obtained for these structureswere: Voc = 390 mV and Jsc = 0.05 mA/cm2, Voc = 490 mV and Jsc =0.15 mA/cm2, and Voc = 630 mV and Jsc = 0.60 mA/cm2 for Fig. 7a, band c, respectively. According to these results, a thinner In2S3 layer pro-duces better cell characteristics probably due to a wider band gap value.The results showed in this work have demonstrated the feasibility ofusing In2S3 ﬁlms as window layers in cadmium free thin ﬁlm solar cells.However, the deposition conditions could be further optimized in orderto improve the optoelectronic properties of In2S3 ﬁlms, so that the J–Vcharacteristics of the solar cells can also be improved.5. ConclusionsIt was shown that In2S3 thin ﬁlms can be obtained by using CBD tech-nique. XRD analysis conﬁrmed that the ﬁlms have a polycrystalline na-ture showing the β-In2S3 phase in both cases, the as-prepared andthermally treated ﬁlms. The calculated band gap values for the In2S3ﬁlms are in the range of 2.27 to 2.41 eV, these values are in agreementwith those reported in the literature for this material. The ﬁlms are pho-tosensitivewith electrical conductivity values of ~10−3–10−7 (Ω cm)−1,depending on the ﬁlm preparation conditions. In this work, it has beenshown that the In2S3 thin ﬁlms obtained by CBD have optoelectronicproperties suitable to be used as window layers in thin ﬁlm solar cells.The best J–V results obtained for the SnO2:F/β-In2S3/Sb2S3/PbS/C–Agstructurewere Voc = 630 mV and Jsc = 0.6 mA/cm2, which correspondsto the 157 nm In2S3 thin ﬁlm thermally treated at 350 °C. The results ob-tained in this work, for the Cd free solar cell structures are promising,however further work must be done in order to improve the optoelec-tronic properties of the In2S3 layers.AcknowledgmentsWe are grateful to Maria Luisa Ramon from IER-UNAM for the XRDmeasurements (IER-UNAM) and Dr. David Avellaneda from FIME-UANL for the electrical characterization of the ﬁlms.References[1] M.G. Sandoval-Paz, M. Sotelo-Lerma, J.J. Valenzuela-Jáuregui, M. Flores-Acosta, R.Ramírez-Bon, Thin Solid Films 472 (2005) 5.[2] A.M. Abdel Haleem, M. Ichimura, Thin Solid Films 516 (2008) 7783.[3] Matthew Zervos, Pola Papageorgiou, Andreas Othonos, J. Cryst. Growth 312 (2010)656.[4] Indra Puspitasari, T.P. Gujar, Kwang-Deog Jung, Oh-Shim Joo, J. Mater. Proc. Technol.201 (2008) 775.[5] R.S. Mane, C.D. Lokhande, Mater. Chem. 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Indium sulfide thin films as window layer in chemically deposited solar cells

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Indium sulfide thin films as window layer in chemically deposited solar cells

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