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1 Aville online t ScienceDirect Energy Procedi 44 ( 2014 ) E-MRS Spring Meeting 2013 Symposium D - Advnced Inorgnic Mterils nd Structures for Photovoltics, My 2013, Strsourg, Frnce Influence of CdS thin films growth relted with the sustrte properties nd conditions used on CBD technique M.L. Alor Aguiler, J.M. Flores Márquez, M.A. González Trujillo, Y. Mtsumoto Kuwhr c, G. Rued Morles nd O. Vigil Glán ESFM-IPN, Depto. Físic, U.P.A.L.M., Zctenco, México D.F , México. ESCOM IPN, Formción Básic, U.P.A.L.M., Zctenco, México D.F., 07738, México. c CINVESTAV-SEES-IPN, Av. IPN 2508, Zctenco, México D.F , México. Astrct CdS polycrystlline thin films hve een used s window lyer in solr cells; the qulity of the CdS-prtner plys n importnt role in the photovoltic device performnce. CdS thin films were deposited using Chemicl Bth Deposition (CBD) nd photossisted-cbd technique. The SnO 2 :F sustrtes used in this work were treted with HCl (0.1M) nd some of them were nneled in different tmospheres (Ar nd O 2 ). The properties of CdS thin films were influenced y the HCl tretment, position nd the movement of sustrtes inside the rector glss. CdS smples were deposited in res of 4 cm 2, 50 cm 2 nd 100 cm 2 ; otining thin films with thickness of nm with good crystlline qulity nd uniform morphology. All smples presented opticl trnsmittnce round of % nd verge gp energy of 2.5 ev The Authors. Pulished y y Elsevier Elsevier Ltd. Ltd. Open ccess under CC BY-NC-ND license. Selection nd peer-review under under responsiility of The of The Europen Europen Mterils Mterils Reserch Reserch Society Society (E-MRS) (E-MRS). Key words: CdS, CBD, solr cells Introduction CdS thin films s window mteril is the most used in thin film photovoltic solr cells technology (CdTe, Cu(In,G)Se 2, Cu 2 ZnSnS 4 ). The thin films solr cells technology requires deposition methods to ensure simplicity, low production cost nd high efficiencies. Chemicl th deposition hs proven to e one of these methods. Corresponding uthor. Tel.: ; fx: E-mil ddress: llor10@yhoo.com.mx The Authors. Pulished y Elsevier Ltd. Open ccess under CC BY-NC-ND license. Selection nd peer-review under responsiility of The Europen Mterils Reserch Society (E-MRS) doi: /j.egypro

2 112 M.L. Alor Aguiler et l. / Energy Procedi 44 ( 2014 ) The efficiency is influenced y the trnsport of chrge in the hetero-junction etween oth, the window nd sorers mterils when solr cells re fricted. CdS s window mteril must e free of defects such s intergrnulr cves nd pin-holes [1]. The record efficiency into CdS/CdTe Solr cells re 18.7 % lortory level nd 16.1% in solr pnels 2. These efficiencies, however, re well elow the theoreticl limit of out 30% clculted 3. ZnO/CdS/CIGS thin-film solr cells hve new record efficiency of 20.4% nd record efficiency sed CZTSSe solr cells close to 10% vlue [4, 5]. Greter improvements in current collection cn e chieved y using thinner CdS window lyers. Chemicl th deposition (CBD) technique is well suited for producing lrgere thin films for solr energy pplictions, nd the numer of possile mterils to e produced through this technique is ound to multiply in susequent yers; this is due to the fesiility to produce multilyer films. CBD hs demonstrted to e simple nd low cost technique to prepre CdS films s opticl window for solr cells. Mny options re required to improve CBD for technologicl pplictions. Other uthors use n dditionl illumintion y hlogen lmp during the growth of films 6, 7 ; ll in order to improve the physicl properties of CdS thin films. In im this work is to study the impct on the min physicl properties of the CdS thin films deposited y chemicl th deposition technique (CBD) using sustrtes with different therml tretments nd under different growth conditions with the porpoise to otin dequte CdS thin films to e used in lrge res solr cells technology. Experiment CdS semiconductor thin films were deposited on SnO 2 : F sustrtes (TCO) y CBD nd photossisted-cbd (P-CBD) technique, for the lst we used hlogen lmp with density power of 100mW/cm 2. As precursor solution we used CdCl 2 (0.1M), NH 4 Cl (0.2 M), NH 3 (2 M) nd (NH 2 ) 2 CS (0.3 M). Some sustrtes were treted with HCl (0.1M) during 30 min, nd others were nneled t 500 C in different tmospheres (Ar, O 2 nd ir). Bilyers of CdS with deposition time of 10 min. t 75 ± 2 C were otined. Fig. 1 shows the glss rector with the sustrtes inside it nd in fig. 1 the illumintion set is shown. The sustrte were plced in four positions (nmed A, B, C nd D) respect the rdition incidence; we used distnce (x) etween the sustrtes nd UV lmp of 10 cm. CdS thin films were otined in res of 4 cm 2, 50 cm 2 nd 100 cm 2. All the CdS thin films, deposited with 4 cm 2 were i-lyers, wheres mono-lyers were deposited on re of 50 nd 100 cm 2. Fig. 1. CBD rrngement, () Glss rector with the sustrtes inside of this, () Sustrte positions with respect the light lmp incidence. The structurl chrcteristics of the smples were determined y the X-ry diffrction ptterns (XRD), y mens of D-500 Siemens X-ry system using the CuK line. The lyer thicknesses were mesured with step profiler (Slon Dektk II). Morphologicl nlysis ws mde using scnning electron microscope (SEM) JEOL JSM model with voltge of 5 kv, llowing resolution of 2nm, nd n Innov Veeco AFM, using contct mode with phosphorus tipped (n) doped with Si. Opticl properties were otined using Shimdzu UV 2401-PC spectrometer. CdS/CdTe solr cell were completed using CdTe thin films (thickness of 4 μm) deposited y Close Spce Vpor Trnsport (CSVT). The CdTe thin films were lso thermlly treted with CdCl 2 ; we used s ck contct, two evported lyers of Cu nd Ag (20 Å nd 350nm, respectively) with n re of 0.08 cm 2 onto the CdTe lter nneled t 180 C in Ar.

3 M.L. Alor Aguiler et l. / Energy Procedi 44 ( 2014 ) Results Tle 1 shows the thickness dependency of the films on the technique used nd position of the sustrte. The deposition time nd sustrte temperture were kept constnt in ll deposits. As it cn e seen, P-CBD-CdS A,C,D smples show greter thickness thn CBD-CdS smples. This ehvior cn e explined ecuse the rdition from the lmp increses sustrte temperture nd therefore the growth rte of the films. The thickness of P-CdS B smple is smller thn the smple P-CdS A, this ehviour ws repeted ll time. This chrcteristic my e ecuse the precursor solution is poured into the rector glss etween to the ottom of the A nd D position; considering tht the solution ws spinning due to mgnetic stirrer from A position to D position the solution comes in front of the TCO sustrte (A, C, D positions); y B position cse the solution rrives first t the ck of the TCO. The externl heting my e chnge the Cd nd S incorportion ions on the TCO surfce, for B position the Cd nd S ions colliding first with the ck of TCO. Chemicl quntifiction mesurements revel tht for P-CdS A,C,D smples the sulphur (S) tomic quntity incresed in 14% compred with CdS smples; ut it ws not the cse for CdS B nd P-CdS B smples, here the sulphur quntity did not chnge. Tle 1. Dependence of CdS thickness on CDB nd P-CBD nd sustrte position Position Smples Thickness (nm) A CdS A 75 P-CdS A 105 B CdS B 79 P-CdS B 59 C CdS C 87 P-CdS C 102 D CdS D 98 P-CdS D 102 Tle 2 shows the influence of different growth conditions nd sustrte tretments on the lyers thickness. The smples nmed s CdS-M 35 nd CdS-M 52 were deposited y rotting the sustrte holder t 35 nd 52 rpm, respectively, while the smples nmed s CdS-HCl, CdS-Ar nd CdS-O 2 were deposited under TCO sustrtes previously etching in HCl or treted on Ar or O 2 mient. The movement sustrte holder produces n increse on the CdS thickness. CdS thickness increses s deposit time increses. Compring the smples otined, it is oserved tht the therml nneling in oxygen cuses n incresing in the smples thickness. Tle 2. CdS thin films thickness influenced y dditionl tretment on TCO sustrtes. Smple Are(cm 2 ) Deposited Time (min) Thickness (nm) CdS CdS-HCl CdS-M CdS-M CdS 1 monolyer CdS 2 monolyer CdS 2 monolyer CdS 2 monolyer CdS-Ar CdS-O P-CdS A P-CdS A-HCl P-CdS A-Ar P-CdS A-O mgnetic stirrer of 0.8 cm; 2 mgnetic stirrer of 2 cm; Averge (see Fig. 2)

4 114 M.L. Alor Aguiler et l. / Energy Procedi 44 ( 2014 ) Fig. 2 shows the CdS thickness vrition in res of 50 cm 2 nd 100 cm 2 ; for this chrcteriztion, the thickness ws mesured 10 points long the CdS smple. Points nmed 0 nd 10 re referred with the ottom nd the smple top inside the glss rector respectively. Compring figs.2 (-c), the smples otined exhiit greter vrition in thickness when smll mgnetic stirring (0.8 cm) is used, which origin tht there is no sufficient gittion to llow the CdS solution ws homogeneous nd then there is greter movement of fluid in the ottom thn t the; previous results (see Tle 2), the dditionl movement genertes the CdS films hve greter thickness. On the other hnd, the pressure inside the glss rector increses s function of the height due to the fluid column [9]. Figs. 2(, c, d) show tht y using lrger mgnetic stirring cuses the smple thickness is more uniform long this.the results of fig. 2 re very importnt ecuse we cn determinte the growth prmeters to otin homogeneous CdS thin films in res of 50 cm 2 nd 100 cm 2 (see fig. 2d). These films should e used in the solr cells development c ~18 nm Thickness (nm) ~15 nm 50 d ~5 nm Smple Height (cm) Fig.2. () CdS 1 monolyer50 (15 min.)() CdS 2 monolyer50 (10 min.) (c) CdS 2 monolyer50 (20 min.) (d) CdS 2 monolyer100 (14 min.). Fig. 3 shows the trnsmission spectr of some different CdS smples grown under different conditions using CBD nd P-CBD. All smples hve n opticl trnsmittnce verge in the rnge of % nd present multiple reflections tht suggest good crystlline qulity nd verge gp energy of 2.5 ev. Compring Figs. 4 (, f) CdS thin films grown y P-CDB technique present etter defined sorption edge Trnsmittnce (%) c d e f g (nm) Fig. 3. () CdS, () CdS-M 35, (c) CdS-M 52, (d) CdS-HCl, (e) CdS monolyer50 (15 min), (f) P-CdS, (g) P-CdS-HCl, P-CdS-Ar nd P-CdS-O 2. Fig. 4 shows AFM imges of TCO sustrtes with different tretments previous the CdS nd P-CdS growth. Fig. 4 shows TCO without tretment with verge grin size out 35nm; fter treting the TCO sustrtes with HCl the grin size chnged in verge to 18 nm, in this lst cse we otined more cpture centres (see fig. 4). Ar tretment chnged the size grin in TCO sustrte to 29 nm nd finlly the O 2 not produces ny pprecile chnge (see fig. 4 c, d).

5 M.L. Alor Aguiler et l. / Energy Procedi 44 ( 2014 ) Fig. 4. () TCO without tretment () TCO treted with HCl (c) TCO with Ar tretment nd (d) TCO with O 2 tretment. Figs. 5(,, c) show AFM imges of CdS thin films deposited y using CBD technique nd figs. 5 (c, d) show smples otined y P-CBD technique; in this imges we cn pprecite the morphology properties of the smples. The morphology is influenced y the prmeters used during the growth. Compring these imges we otined the grin size chnged; fig. 5 shows CdS smple without tretment, this smple presents grin size out 25 nm, CdS-HCl nd CdS-M 35 hve grin size out 36 nm nd 24 nm respectively (see figs. 5, 5c); for CdS-HCl the grin size increses nd then we hve etter morphology in the smple. Finlly fig. 5d shows P-CdS A (grin size 32 nm) nd fig. 5e P-CdS A -HCl (grin size 30 nm). P-CdS A morphology compred with CdS is very different; in this cse the growth is not uniform. Anlysing the morphologicl properties, CdS-HCl is the est smple from morphology nd cheper process point of view. Fig. 5. () CdS, () CdS-HCl, c) CdS-M 35, (d) P-CdS A nd (e) P-CdS A-HCl.

6 116 M.L. Alor Aguiler et l. / Energy Procedi 44 ( 2014 ) Tle 3 show the grin size verge of TCO treted with different inert tmospheres nd CdS thin films deposited onto these TCO sustrtes using CBD nd P-CBD. P-CdS A -Ar nd P-CdS A O 2 AFM imges re not included in fig. 5 ecuse their grin size nd morphology re similr to P-CdS A -HCl. Tle 3. Grin size verges of TCO nd CdS thin films. Smple Grin size verge (nm) TCO 35 TCO-HCl 18 TCO-Ar 29 TCO-O 2 35 CdS 25 CdS-HCl 36 CdS-M CdS-Ar 27 CdS-O 2 24 P-CdS A 32 P-CdS A-HCl 30 P-CdS A-Ar 31 P-CdS A-O 2 29 Fig. 6 shows typicl XRD pttern of CdS thin film with (002), (112) nd (004) phses, identified with JCPDS str qulity. Fig 6(, ) show there no chnge in CdS phse regrdless if used or not HCl; Fig. 6 (c, d) show n dditionl pek relted with monoclinic sulphur, these results re consistent with dt otined from chemicl composition of elements. H(002) H CdS Hexgonl S Monoclinic Intensity (.u.) (Degrees) (002) H (112) H H CdS Hexgonl SnO 2 :F S Monoclinic d Intensity (.u.) H CdS Hexgonl S Monoclinic H(002) Intensity (.u.) c (Degrees) (Degrees) Fig. 6. () CdS, ()CdS-HCl, (c) CdS-M 35 nd (d) P-CdS A

7 M.L. Alor Aguiler et l. / Energy Procedi 44 ( 2014 ) CdS-HCl thin films hd the est physicl properties mking them cndidtes for use in solr cell technology. We hve processed CdS/CdTe solr cell in order to evlute the photovoltic efficiency. Figs. 7(, ) shows the IV curves of the CdS/CdTe cell; we use CdS nd CdS-HCl thin films respectively; the photovoltic efficiency increse from 5.2% to 7.7% J SC (ma/cm 2 ) V(Volts) -20 Fig. 7. IV chrcteristics of CdTe solr cells () CdS/CdTe nd () CdS-HCl/CdTe Conclusions CdS thin films were deposited y CBD nd P-CBD techniques considering different res (4 cm 2, 50 cm 2 nd 100 cm 2 ), we used TCO sustrtes treted with HCl, Ar nd O 2. In generl is not good ide rotte the sustrtes inside the rector glss ecuse the opticl trnsmittnce flls (75 80 %), the rest of CdS smples showed trnsmittnce vlues round to % with multiple reflections tht suggest good crystlline qulity. The CdS grin size is incresed when is used TCO sustrtes previously treted with HCl; proly this tretment cn reduce the lttice stress generted with the CdTe coupler where n incresing in the finl solr cell is otined. On the other hnd CdS thin films deposited y P-CBD technique pprently growth with fewer defects in comprison with CdS thin films deposited y CBD technique, ut in this cse is necessry to use n hlogen lmp. We otin CdS thin films with good physicl properties, these smples cn e used s window mteril in CdTe, CIGS nd CZTS solr cells to improve the photovoltic efficiency. Acknowledgements This work ws sponsored y IPN (SIP ) nd CONACYT ( ). References [1] B. A. Kulp, Phys Rev. 125, 1962, p [2] First Solr Sets CdTe module efficiency world record, lunches series 3 lck module (2013), [3] V.F. Gremenok, I.V. Bondr, V.Yu. Rud, Yu.V. Rud, H.W. Schock, Solr Semiconductors, V. 36, I. 3 (2002), [4] High-efficiency flexile CIGS solr cells on polyimide film developed t Emp with novel process (2013), /l=2/chngelng=true/lrtid=131438/org=/type=/theme=/estellr=/new_t=/ucc= [5] T.K. Todorov, K.B. Reuter, D.B. Mitzi, Adv. Mter. 22 (2010),1, E156 [6] E. Bcksiz, V. Novruzo, et. Al., Light-ssisted deposition of CdS thin films, Journl of Physics D: Applied Physics 34 (2001) [7] T. Armoto, M. Tsuji, et. l., Highly efficient lrge re thin film CdS/CdTe su-module, PV R&D Center, Mtsushit Bttery Industril Co., Ltd, Moriguchi, Osk , Jpn.